@article{huynh_wang_liao_tweedie_reddy_breckenridge_collazo_sitar_sierakowski_bockowski_et al._2024, title={Dissolution of Mg-enriched defects in implanted GaN and increased p-type dopant activation}, volume={135}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0179623}, DOI={10.1063/5.0179623}, abstractNote={Annealing Mg-implanted homoepitaxial GaN at temperatures above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects, in the form of inversion domains, contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. Triple-axis x-ray data reveal that implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that strain-inducing point defects formed during implantation have reconfigured and inversion domains are formed. However, annealing at temperatures of 1400–1500 °C (one GPa N2 overpressure) eliminates the presence of the inversion domains. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop density is observed after annealing at the higher temperatures and longer times. Additionally, once inversion domains are formed and the samples are cooled to room temperature, they are shown to dissolve with subsequent annealing above 1400 °C. While such defects have been observed before, the important finding that such defects can be dissolved with a short, higher temperature step is key. Earlier work [Breckenridge et al., J. Appl. Phys. Lett. 118, 022101 (2021)] addressing electrical measurements of these types of samples showed that annealing at 1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. This work complements earlier work by identifying the inversion domains, which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (>1400 °C) annealing cycles to activate Mg in GaN, even if the Mg-containing inversion domains had been formed during lower temperature annealing.}, number={2}, journal={JOURNAL OF APPLIED PHYSICS}, author={Huynh, K. and Wang, Y. and Liao, M. E. and Tweedie, J. and Reddy, P. and Breckenridge, M. H. and Collazo, R. and Sitar, Z. and Sierakowski, K. and Bockowski, M. and et al.}, year={2024}, month={Jan} }
 @article{goorsky_liao_huynh_wang_tweedie_sitar_collazo_sierakowski_bockowski_huang_2023, title={(Invited) Understanding Mg-Related Defects for Vertical GaN p-n Junction Structures Via p-Type Ion Implantation}, url={https://doi.org/10.1149/MA2023-02351691mtgabs}, DOI={10.1149/MA2023-02351691mtgabs}, abstractNote={  	Vertical GaN power devices have emerged to become promising candidates for next-generation high power applications due to superior material properties such as high breakdown voltage, low on-resistance, and high mobility compared to devices based on Si and SiC. GaN-based p-n junction switching devices enable higher voltage power with significantly higher efficiencies with added advantages of systems with reduced size and weight. A technological limitation of GaN, however, has been the inability to achieve high p-type doping in a planar, vertical device. Here, we will focus on recent developments to achieve high p-type efficiency though ion implantation, novel high temperature annealing schemes, and the importance of defects and morphology in native substrates and epitaxial layers.}, journal={ECS Meeting Abstracts}, author={Goorsky, Mark S. and Liao, Michael Evan and Huynh, Kenny and Wang, Yekan and Tweedie, James and Sitar, Zlatko and Collazo, Ramon and Sierakowski, Kacper and Bockowski, Michal and Huang, Xianrong}, year={2023}, month={Dec} }
 @article{stein_khachariya_mecouch_mita_reddy_tweedie_sierakowski_kamler_bockowski_kohn_et al._2023, title={Analysis of Vertical GaN JBS and p-n Diodes by Mg Ion Implantation and Ultrahigh-Pressure Annealing}, volume={12}, ISSN={["1557-9646"]}, url={https://doi.org/10.1109/TED.2023.3339592}, DOI={10.1109/TED.2023.3339592}, abstractNote={We report on vertical GaN junction barrier Schottky (JBS) diodes formed by Mg ion implantation and ultrahigh -pressure annealing (UHPA). The static ON-state characteristics of the diodes show an ideality factor of 1.05, a turn-on voltage of ~0.7 V, a current rectification ratio of $\sim 10^{11}$ , and a low differential specific ON-resistance that scales with Schottky stripe width in fair agreement with the analytical model. The reverse leakage dependence on Schottky stripe width also agrees well with the analytical model. Implanted p-n junction diodes fabricated on the same wafer exhibit avalanche breakdown in reverse bias with a positive temperature coefficient, but the forward current is limited by a series barrier. Temperature-dependent current–voltage measurements of th p-n diodes verify the presence of the implanted p-n junction and reveal an additional 0.43-eV barrier, which we hypothesize arises from a p-Schottky contact and forms a second diode back-to-back with the p-n junction. This interpretation is supported by analysis of the capacitance–voltage characteristics of the implanted p-n diodes, epitaxial p-n diodes fabricated with intentional p-Schottky contacts, and comparison to TCAD simulations. Ultimately, the presence of the p-Schottky contact does not hinder JBS diode operation. The use of diffusion-aware designs and/or diffusion reduction represents future directions for Mg implantation technology in GaN power devices.}, journal={IEEE TRANSACTIONS ON ELECTRON DEVICES}, author={Stein, Shane R. and Khachariya, Dolar and Mecouch, Will and Mita, Seiji and Reddy, Pramod and Tweedie, James and Sierakowski, Kacper and Kamler, Grzegorz and Bockowski, Michal and Kohn, Erhard and et al.}, year={2023}, month={Dec} }
 @article{rathkanthiwar_reddy_quinones_loveless_kamiyama_bagheri_khachariya_eldred_moody_mita_et al._2023, title={Anderson transition in compositionally graded p-AlGaN}, volume={134}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0176419}, DOI={10.1063/5.0176419}, abstractNote={Mg-doped, graded AlGaN films showed the formation of an impurity band and high, temperature-invariant p-conductivity even for doping levels well below the Mott transition. However, compensating point defects disrupted the impurity band, resulting in an Anderson transition from the impurity band to valence band conduction and a more than tenfold reduction in room-temperature conductivity. This is the first demonstration of Anderson-like localization in AlGaN films.}, number={19}, journal={JOURNAL OF APPLIED PHYSICS}, author={Rathkanthiwar, Shashwat and Reddy, Pramod and Quinones, Cristyan E. and Loveless, James and Kamiyama, Masahiro and Bagheri, Pegah and Khachariya, Dolar and Eldred, Tim and Moody, Baxter and Mita, Seiji and et al.}, year={2023}, month={Nov} }
 @article{sengupta_vaidya_szymanski_khachariya_bockowski_kamler_reddy_sitar_collazo_pavlidis_2023, title={Chemical Vapor Deposition of Monolayer MoS2 on Chemomechanically Polished N-Polar GaN for Future 2D/3D Heterojunction Optoelectronics}, volume={3}, ISSN={["2574-0970"]}, url={https://doi.org/10.1021/acsanm.3c00038}, DOI={10.1021/acsanm.3c00038}, abstractNote={The growth of monolayer MoS2 crystals on chemomechanically polished (CMP) N-polar GaN using PTAS-assisted chemical vapor deposition is demonstrated. The formation of monolayer MoS2 was initially prevented by the as-grown GaN’s large surface roughness. CMP reduces the roughness to 250 pm, enabling monolayer MoS2 triangles with edge lengths of 30 μm, a Raman peak separation of <20 cm–1, and an optical bandgap of 1.81 eV, which is on par with those obtained on smooth Ga-polar GaN. It is thus demonstrated that high-quality MoS2 monolayers can be obtained on N-polar GaN for future high-speed optoelectronic and quantum sensing applications.}, journal={ACS APPLIED NANO MATERIALS}, author={Sengupta, Rohan and Vaidya, Shipra and Szymanski, Dennis and Khachariya, Dolar and Bockowski, Michal and Kamler, Grzegorz and Reddy, Pramod and Sitar, Zlatko and Collazo, Ramon and Pavlidis, Spyridon}, year={2023}, month={Mar} }
 @article{quinones_khachariya_bagheri_reddy_mita_kirste_rathkanthiwar_tweedie_pavlidis_kohn_et al._2023, title={Demonstration of near-ideal Schottky contacts to Si-doped AlN}, volume={123}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0174524}, DOI={10.1063/5.0174524}, abstractNote={Near-ideal behavior in Schottky contacts to Si-doped AlN was observed as evidenced by a low ideality factor of 1.5 at room temperature. A temperature-independent Schottky barrier height of 1.9 eV was extracted from temperature-dependent I–V measurements. An activation energy of ∼300 meV was observed in the series resistance, which corresponded to the ionization energy of the deep Si donor state. Both Ohmic and Schottky contacts were stable up to 650 °C, with around four orders of magnitude rectification at this elevated temperature. These results demonstrate the potential of AlN as a platform for power devices capable of operating in extreme environments.}, number={17}, journal={APPLIED PHYSICS LETTERS}, author={Quinones, C. E. and Khachariya, D. and Bagheri, P. and Reddy, P. and Mita, S. and Kirste, R. and Rathkanthiwar, S. and Tweedie, J. and Pavlidis, S. and Kohn, E. and et al.}, year={2023}, month={Oct} }
 @article{rathkanthiwar_bagheri_khachariya_mita_quiñones-garcía_guan_moody_reddy_kirste_collazo_et al._2023, title={High conductivity and low activation energy in p-type AlGaN}, url={https://doi.org/10.1063/5.0141863}, DOI={10.1063/5.0141863}, abstractNote={Record-low p-type resistivities of 9.7 and 37 Ω cm were achieved in Al0.7Ga0.3N and Al0.8Ga0.2N films, respectively, grown on single-crystal AlN substrate by metalorganic chemical vapor deposition. A two-band conduction model was introduced to explain the anomalous thermal behavior of resistivity and the Hall coefficient. Relatively heavy Mg doping (5 × 1019 cm−3), in conjunction with compensation control, enabled the formation of an impurity band exhibiting a shallow activation energy of ∼30 meV for a wide temperature range. Valence band conduction associated with a large Mg ionization energy was dominant above 500 K. The apparently anomalous results deviating from the classical semiconductor physics were attributed to fundamentally different Hall scattering factors for impurity and valence band conduction. This work demonstrates the utility of impurity band conduction to achieve technologically relevant p-type conductivity in Al-rich AlGaN.}, journal={Applied Physics Letters}, author={Rathkanthiwar, Shashwat and Bagheri, Pegah and Khachariya, Dolar and Mita, Seiji and Quiñones-García, Cristyan and Guan, Yan and Moody, Baxter and Reddy, Pramod and Kirste, Ronny and Collazo, Ramón and et al.}, year={2023}, month={Feb} }
 @article{bagheri_quinones-garcia_khachariya_loveless_guan_rathkanthiwar_reddy_kirste_mita_tweedie_et al._2023, title={High conductivity in Ge-doped AlN achieved by a non-equilibrium process}, volume={122}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0146439}, DOI={10.1063/5.0146439}, abstractNote={Highly conductive Ge-doped AlN with conductivity of 0.3 (Ω cm)−1 and electron concentration of 2 × 1018 cm−3 was realized via a non-equilibrium process comprising ion implantation and annealing at a moderate thermal budget. Similar to a previously demonstrated shallow donor state in Si-implanted AlN, Ge implantation also showed a shallow donor behavior in AlN with an ionization energy ∼80 meV. Ge showed a 3× higher conductivity than its Si counterpart for a similar doping level. Photoluminescence spectroscopy indicated that higher conductivity for Ge-doped AlN was achieved primarily due to lower compensation. This is the highest n-type conductivity reported for AlN doped with Ge to date and demonstration of technologically useful conductivity in Ge-doped AlN.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Bagheri, Pegah and Quinones-Garcia, Cristyan and Khachariya, Dolar and Loveless, James and Guan, Yan and Rathkanthiwar, Shashwat and Reddy, Pramod and Kirste, Ronny and Mita, Seiji and Tweedie, James and et al.}, year={2023}, month={Apr} }
 @article{rathkanthiwar_reddy_moody_quinones-garcia_bagheri_khachariya_dalmau_mita_kirste_collazo_et al._2023, title={High p-conductivity in AlGaN enabled by polarization field engineering}, volume={122}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0143427}, DOI={10.1063/5.0143427}, abstractNote={High p-conductivity (0.7 Ω−1 cm−1) was achieved in high-Al content AlGaN via Mg doping and compositional grading. A clear transition between the valence band and impurity band conduction mechanisms was observed. The transition temperature depended strongly on the compositional gradient and to some degree on the Mg doping level. A model is proposed to explain the role of the polarization field in enhancing the conductivity in Mg-doped graded AlGaN films and the transition between the two conduction types. This study offers a viable path to technologically useful p-conductivity in AlGaN.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Rathkanthiwar, Shashwat and Reddy, Pramod and Moody, Baxter and Quinones-Garcia, Cristyan and Bagheri, Pegah and Khachariya, Dolar and Dalmau, Rafael and Mita, Seiji and Kirste, Ronny and Collazo, Ramon and et al.}, year={2023}, month={Apr} }
 @article{stein_khachariya_mita_breckenridge_tweedie_reddy_sierakowski_kamler_bockowski_kohn_et al._2023, title={Schottky contacts on ultra-high-pressure-annealed GaN with high rectification ratio and near-unity ideality factor}, volume={16}, ISSN={["1882-0786"]}, DOI={10.35848/1882-0786/acc443}, abstractNote={Abstract}, number={3}, journal={APPLIED PHYSICS EXPRESS}, author={Stein, Shane R. and Khachariya, Dolar and Mita, Seiji and Breckenridge, M. Hayden and Tweedie, James and Reddy, Pramod and Sierakowski, Kacper and Kamler, Grzegorz and Bockowski, Michal and Kohn, Erhard and et al.}, year={2023}, month={Mar} }
 @article{pavlidis_khachariya_szymanski_reddy_kohn_sitar_collazo_2022, title={(Invited, Digital Presentation) Exploring Interfaces and Polarity to Realize Vertical III-Nitride Superjunction Devices}, url={https://doi.org/10.1149/MA2022-01311313mtgabs}, DOI={10.1149/MA2022-01311313mtgabs}, abstractNote={  	In recent years, there has been a surge of research and commercial interest in gallium nitride (GaN)-based devices for power conversion applications. This is largely motivated by the wide bandgap of GaN, which offers a unipolar limit of performance that is larger than that of silicon and silicon carbide. This performance ceiling, however, can be surpassed with the use of superjunction (SJ) structures, a strategy that has now been experimentally proven in both vertical Si- and SiC-based technologies. The ability to selectively dope lateral regions of a semiconductor is a requirement for SJ fabrication, with typical schemes relying on ion implantation, epitaxial regrowth or a combination of both. However, due to the challenges associated with applying conventional selective area doping techniques to GaN, vertical GaN SJ structures have remained elusive.}, journal={ECS Meeting Abstracts}, author={Pavlidis, Spyridon and Khachariya, Dolar and Szymanski, Dennis and Reddy, Pramod and Kohn, Erhard and Sitar, Zlatko and Collazo, Ramon}, year={2022}, month={Jul} }
 @article{wang_kirste_mita_washiyama_mecouch_reddy_collazo_sitar_2022, title={<p>The role of Ga supersaturation on facet formation in the epitaxial lateral overgrowth of GaN</p>}, volume={120}, ISSN={["1077-3118"]}, DOI={10.1063/5.0077628}, abstractNote={In this paper, facet formation of (0001) {112¯0} {112¯2} facets during epitaxial lateral overgrowth (ELO) of GaN is investigated for different Ga vapor supersaturations. The ELO was conducted via metalorganic chemical vapor deposition on patterned GaN/sapphire templates with SiO2 masks aligned along the ⟨11¯00⟩ direction of GaN. Scanning electron microscopy was used to characterize the cross section shapes of the ELO GaN islands. A correlation of supersaturation, facet formation, and the shape of the ELO GaN islands is found. It is shown that {112¯2} facets are favored under high Ga vapor supersaturation, while {112¯0} facets are favored under low Ga vapor supersaturation. A qualitative model based on Wulff construction and density functional theory calculation is proposed to illustrate the mechanism of the facet formation of the ELO GaN islands.}, number={3}, journal={APPLIED PHYSICS LETTERS}, author={Wang, Ke and Kirste, Ronny and Mita, Seiji and Washiyama, Shun and Mecouch, Will and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko}, year={2022}, month={Jan} }
 @article{rathkanthiwar_graziano_tweedie_mita_kirste_collazo_sitar_2022, title={Crystallographic Tilt in Aluminum Gallium Nitride Epilayers Grown on Miscut Aluminum Nitride Substrates}, volume={10}, ISSN={["1862-6270"]}, DOI={10.1002/pssr.202200323}, abstractNote={Heteroepitaxial crystallographic tilt has been investigated as a possible strain‐relief mechanism in Al‐rich (Al>50%) AlGaN heteroepitaxial layers grown on single‐crystal (0001) AlN substrates with varying miscuts from 0.05° to 4.30°. The magnitude of the elastic lattice deformation‐induced tilt increases monotonically with the miscut angle, tightly following the Nagai tilt model. Although tilt angles as high as 0.1° are recorded, reciprocal space mapping (RSM) broadening and wafer bow measurements do not show any significant changes as a function of the heteroepitaxial tilt angle. While crystallographic tilting has been shown to be effective in controlling strain in some other heteroepitaxial systems, it does not provide any appreciable strain relief of the compressive strain in AlGaN/AlN heteroepitaxy.}, journal={PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS}, author={Rathkanthiwar, Shashwat and Graziano, Milena B. and Tweedie, James and Mita, Seiji and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2022}, month={Oct} }
 @article{bagheri_klump_washiyama_breckenridge_kim_guan_khachariya_quinones-garcia_sarkar_rathkanthiwar_et al._2022, title={Doping and compensation in heavily Mg doped Al-rich AlGaN films}, volume={120}, ISSN={["1077-3118"]}, DOI={10.1063/5.0082992}, abstractNote={Record low resistivities of 10 and 30 Ω cm and room-temperature free hole concentrations as high as 3 × 1018 cm−3 were achieved in bulk doping of Mg in Al0.6Ga0.4N films grown on AlN single crystalline wafer and sapphire. The highly conductive films exhibited a low ionization energy of 50 meV and impurity band conduction. Both high Mg concentration (>2 × 1019 cm−3) and low compensation were required to achieve impurity band conduction and high p-type conductivity. The formation of VN-related compensators was actively suppressed by chemical potential control during the deposition process. This work overcomes previous limitations in p-type aluminum gallium nitride (p-AlGaN) and offers a technologically viable solution to high p-conductivity in AlGaN and AlN.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Bagheri, Pegah and Klump, Andrew and Washiyama, Shun and Breckenridge, M. Hayden and Kim, Ji Hyun and Guan, Yan and Khachariya, Dolar and Quinones-Garcia, Cristyan and Sarkar, Biplab and Rathkanthiwar, Shashwat and et al.}, year={2022}, month={Feb} }
 @article{szymanski_khachariya_eldred_bagheri_washiyama_chang_pavlidis_kirste_reddy_kohn_et al._2022, title={GaN lateral polar junction arrays with 3D control of doping by supersaturation modulated growth: A path toward III-nitride superjunctions}, volume={131}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0076044}, DOI={10.1063/5.0076044}, abstractNote={We demonstrate a pathway employing crystal polarity controlled asymmetric impurity incorporation in the wide bandgap nitride material system to enable 3D doping control during the crystal growth process. The pathway involves polarity specific supersaturation modulated growth of lateral polar structures of alternating Ga- and N-polar GaN domains. A STEM technique of integrated differential phase contrast is used to image the atomic structure of the different polar domains and their single atomic plane boundaries. As a demonstration, 1 μm wide alternating Ga- and N-polar GaN domains exhibiting charge balanced and periodic domains for superjunction technology were grown. The challenges in characterizing the resulting 3D doping profile were addressed with atom probe tomography with atomic scale compositional resolution corroborating capacitance measurements and secondary-ion mass spectroscopy analysis.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Szymanski, Dennis and Khachariya, Dolar and Eldred, Tim B. and Bagheri, Pegah and Washiyama, Shun and Chang, Alexander and Pavlidis, Spyridon and Kirste, Ronny and Reddy, Pramod and Kohn, Erhard and et al.}, year={2022}, month={Jan} }
 @article{bagheri_quinones-garcia_khachariya_rathkanthiwar_reddy_kirste_mita_tweedie_collazo_sitar_2022, title={High electron mobility in AlN:Si by point and extended defect management}, volume={132}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0124589}, DOI={10.1063/5.0124589}, abstractNote={High room temperature n-type mobility, exceeding 300 cm2/Vs, was demonstrated in Si-doped AlN. Dislocations and CN−1 were identified as the main compensators for AlN grown on sapphire and AlN single crystalline substrates, respectively, limiting the lower doping limit and mobility. Once the dislocation density was reduced by the growth on AlN wafers, C-related compensation could be reduced by controlling the process supersaturation and Fermi level during growth. While the growth on sapphire substrates supported only high doping ([Si] > 5 × 1018 cm−3) and low mobility (∼20 cm2/Vs), growth on AlN with proper compensation management enabled controlled doping at two orders of magnitude lower dopant concentrations. This work is of crucial technological importance because it enables the growth of drift layers for AlN-based power devices.}, number={18}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bagheri, Pegah and Quinones-Garcia, Cristyan and Khachariya, Dolar and Rathkanthiwar, Shashwat and Reddy, Pramod and Kirste, Ronny and Mita, Seiji and Tweedie, James and Collazo, Ramon and Sitar, Zlatko}, year={2022}, month={Nov} }
 @article{reddy_mecouch_breckenridge_khachariya_bagheri_kim_guan_mita_moody_tweedie_et al._2022, title={Large-Area, Solar-Blind, Sub-250 nm Detection AlGaN Avalanche Photodiodes Grown on AlN Substrates}, volume={3}, ISSN={["1862-6270"]}, url={https://doi.org/10.1002/pssr.202100619}, DOI={10.1002/pssr.202100619}, abstractNote={Herein, Al‐rich AlGaN‐based avalanche photodiodes (APDs) grown on single crystal AlN substrates high ultraviolet‐C sensitivity for λ < 200 nm are fabricated, while exhibiting blindness to λ > 250 nm. A maximum quantum efficiency of 68% and peak gain of 320 000 are estimated resulting in a figure of merit of ≈220 000 in devices with ϕ = 100 μm. As expected, a decrease in gain with increase in device size is observed and a gain of ≈20 000 is estimated in devices with ϕ = 400 μm. Overall, two orders of magnitude higher performance are observed in APDs on single crystal AlN substrates compared to those on sapphire.}, journal={PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS}, publisher={Wiley}, author={Reddy, Pramod and Mecouch, Will and Breckenridge, M. Hayden and Khachariya, Dolar and Bagheri, Pegah and Kim, Ji Hyun and Guan, Yan and Mita, Seiji and Moody, Baxter and Tweedie, James and et al.}, year={2022}, month={Mar} }
 @article{rathkanthiwar_szymanski_khachariya_bagheri_kim_mita_reddy_kohn_pavlidis_kirste_et al._2022, title={Low resistivity, p-type, N-Polar GaN achieved by chemical potential control}, volume={15}, ISSN={["1882-0786"]}, DOI={10.35848/1882-0786/ac8273}, abstractNote={Abstract}, number={8}, journal={APPLIED PHYSICS EXPRESS}, author={Rathkanthiwar, Shashwat and Szymanski, Dennis and Khachariya, Dolar and Bagheri, Pegah and Kim, Ji Hyun and Mita, Seiji and Reddy, Pramod and Kohn, Erhard and Pavlidis, Spyridon and Kirste, Ronny and et al.}, year={2022}, month={Aug} }
 @article{jadhav_bagheri_klump_khachariya_mita_reddy_rathkanthiwar_kirste_collazo_sitar_et al._2022, title={On electrical analysis of Al-rich p-AlGaN films for III-nitride UV light emitters}, volume={37}, ISSN={["1361-6641"]}, url={https://doi.org/10.1088/1361-6641/ac3710}, DOI={10.1088/1361-6641/ac3710}, abstractNote={Abstract}, number={1}, journal={SEMICONDUCTOR SCIENCE AND TECHNOLOGY}, publisher={IOP Publishing}, author={Jadhav, Aakash and Bagheri, Pegah and Klump, Andrew and Khachariya, Dolar and Mita, Seiji and Reddy, Pramod and Rathkanthiwar, Shashwat and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko and et al.}, year={2022}, month={Jan} }
 @article{rathkanthiwar_bagheri_khachariya_kim_kajikawa_reddy_mita_kirste_moody_collazo_et al._2022, title={On the conduction mechanism in compositionally graded AlGaN}, volume={121}, ISSN={["1077-3118"]}, DOI={10.1063/5.0100756}, abstractNote={A two-band transport model is proposed to explain electrical conduction in graded aluminum gallium nitride layers, where the free hole conduction in the valence band is favored at high temperatures and hopping conduction in the impurity band dominates at low temperatures. The model simultaneously explains the significantly lowered activation energy for p-type conduction (∼10 meV), a nearly constant sheet conductivity at lower temperatures (200–330 K), and the anomalous reversal of the Hall coefficient caused by the negative sign of the Hall scattering factor in the hopping conduction process. A comparison between the uniform and graded samples suggests that compositional grading significantly enhances the probability of phonon-assisted hopping transitions between the Mg atoms.}, number={7}, journal={APPLIED PHYSICS LETTERS}, author={Rathkanthiwar, Shashwat and Bagheri, Pegah and Khachariya, Dolar and Kim, Ji Hyun and Kajikawa, Yasutomo and Reddy, Pramod and Mita, Seiji and Kirste, Ronny and Moody, Baxter and Collazo, Ramon and et al.}, year={2022}, month={Aug} }
 @article{rathkanthiwar_bagheri_khachariya_mita_pavlidis_reddy_kirste_tweedie_sitar_collazo_2022, title={Point-defect management in homoepitaxially grown Si-doped GaN by MOCVD for vertical power devices}, volume={15}, ISSN={["1882-0786"]}, DOI={10.35848/1882-0786/ac6566}, abstractNote={Abstract}, number={5}, journal={APPLIED PHYSICS EXPRESS}, author={Rathkanthiwar, Shashwat and Bagheri, Pegah and Khachariya, Dolar and Mita, Seiji and Pavlidis, Spyridon and Reddy, Pramod and Kirste, Ronny and Tweedie, James and Sitar, Zlatko and Collazo, Ramon}, year={2022}, month={May} }
 @article{rathkanthiwar_dycus_mita_kirste_tweedie_collazo_sitar_2022, title={Pseudomorphic growth of thick Al0.6Ga0.4N epilayers on AlN substrates}, volume={120}, ISSN={["1077-3118"]}, DOI={10.1063/5.0092937}, abstractNote={We report on the absence of strain relaxation mechanism in Al0.6Ga0.4N epilayers grown on (0001) AlN substrates for thickness as large as 3.5 μm, three-orders of magnitude beyond the Matthews–Blakeslee critical thickness for the formation of misfit dislocations (MDs). A steady-state compressive stress of 3–4 GPa was observed throughout the AlGaN growth leading to a large lattice bow (a radius of curvature of 0.5 m−1) for the thickest sample. Despite the large lattice mismatch-induced strain energy, the epilayers exhibited a smooth and crack-free surface morphology. These results point to the presence of a large barrier for nucleation of MDs in Al-rich AlGaN epilayers. Compositionally graded AlGaN layers were investigated as potential strain relief layers by the intentional introduction of MDs. While the graded layers abetted MD formation, the inadequate length of these MDs correlated with insignificant strain relaxation. This study emphasizes the importance of developing strain management strategies for the implementation of the single-crystal AlN substrate platform for III-nitride deep-UV optoelectronics and power electronics.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Rathkanthiwar, Shashwat and Dycus, J. Houston and Mita, Seiji and Kirste, Ronny and Tweedie, James and Collazo, Ramon and Sitar, Zlatko}, year={2022}, month={May} }
 @article{khachariya_mita_reddy_dangi_dycus_bagheri_breckenridge_sengupta_rathkanthiwar_kirste_et al._2022, title={Record >10 MV/cm mesa breakdown fields in Al0.85Ga0.15N/Al0.6Ga0.4N high electron mobility transistors on native AlN substrates}, volume={120}, ISSN={["1077-3118"]}, DOI={10.1063/5.0083966}, abstractNote={The ultra-wide bandgap of Al-rich AlGaN is expected to support a significantly larger breakdown field compared to GaN, but the reported performance thus far has been limited by the use of foreign substrates. In this Letter, the material and electrical properties of Al0.85Ga0.15N/Al0.6Ga0.4N high electron mobility transistors (HEMT) grown on a 2-in. single crystal AlN substrate are investigated, and it is demonstrated that native AlN substrates unlock the potential for Al-rich AlGaN to sustain large fields in such devices. We further study how Ohmic contacts made directly to a Si-doped channel layer reduce the knee voltage and increase the output current density. High-quality AlGaN growth is confirmed via scanning transmission electron microscopy, which also reveals the absence of metal penetration at the Ohmic contact interface and is in contrast to established GaN HEMT technology. Two-terminal mesa breakdown characteristics with 1.3 μm separation possess a record-high breakdown field strength of ∼11.5 MV/cm for an undoped Al0.6Ga0.4N-channel layer. The breakdown voltages for three-terminal devices measured with gate-drain distances of 4 and 9 μm are 850 and 1500 V, respectively.}, number={17}, journal={APPLIED PHYSICS LETTERS}, author={Khachariya, Dolar and Mita, Seiji and Reddy, Pramod and Dangi, Saroj and Dycus, J. Houston and Bagheri, Pegah and Breckenridge, M. Hayden and Sengupta, Rohan and Rathkanthiwar, Shashwat and Kirste, Ronny and et al.}, year={2022}, month={Apr} }
 @article{comstock_biliroglu_seyitliyev_mcconnell_vetter_reddy_kirste_szymanski_sitar_collazo_et al._2022, title={Spintronic Terahertz Emission in Ultrawide Bandgap Semiconductor/Ferromagnet Heterostructures}, volume={10}, ISSN={["2195-1071"]}, DOI={10.1002/adom.202201535}, abstractNote={Abstract}, journal={ADVANCED OPTICAL MATERIALS}, author={Comstock, Andrew and Biliroglu, Melike and Seyitliyev, Dovletgeldi and McConnell, Aeron and Vetter, Eric and Reddy, Pramod and Kirste, Ronny and Szymanski, Dennis and Sitar, Zlatko and Collazo, Ramon and et al.}, year={2022}, month={Oct} }
 @article{szymanski_wang_kaess_kirste_mita_reddy_sitar_collazo_2022, title={Systematic oxygen impurity reduction in smooth N-polar GaN by chemical potential control}, volume={37}, ISSN={["1361-6641"]}, DOI={10.1088/1361-6641/ac3638}, abstractNote={Abstract}, number={1}, journal={SEMICONDUCTOR SCIENCE AND TECHNOLOGY}, author={Szymanski, Dennis and Wang, Ke and Kaess, Felix and Kirste, Ronny and Mita, Seiji and Reddy, Pramod and Sitar, Zlatko and Collazo, Ramon}, year={2022}, month={Jan} }
 @article{hyun kim_bagheri_kirste_reddy_collazo_sitar_2022, title={Tracking of Point Defects in the Full Compositional Range of AlGaN via Photoluminescence Spectroscopy}, volume={12}, ISSN={["1862-6319"]}, url={https://doi.org/10.1002/pssa.202200390}, DOI={10.1002/pssa.202200390}, abstractNote={A comprehensive energy map as a function of AlGaN composition over the whole alloy range is presented for commonly observed point defects in nominally intrinsic, n‐, and p‐doped material. The map covers intentional and unintentional impurities (CN, MgIII), vacancies (VIII, VN), passivating complexes (H), and self‐compensating complexes. The tracking of these defects is crucial to understand their impact on optical and electrical properties as well as for their mitigation.}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Hyun Kim, Ji and Bagheri, Pegah and Kirste, Ronny and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko}, year={2022}, month={Dec} }
 @article{khachariya_stein_mecouch_breckenridge_rathkanthiwar_mita_moody_reddy_tweedie_kirste_et al._2022, title={Vertical GaN junction barrier Schottky diodes with near-ideal performance using Mg implantation activated by ultra-high-pressure annealing}, volume={15}, ISSN={["1882-0786"]}, DOI={10.35848/1882-0786/ac8f81}, abstractNote={Abstract}, number={10}, journal={APPLIED PHYSICS EXPRESS}, author={Khachariya, Dolar and Stein, Shane and Mecouch, Will and Breckenridge, M. Hayden and Rathkanthiwar, Shashwat and Mita, Seiji and Moody, Baxter and Reddy, Pramod and Tweedie, James and Kirste, Ronny and et al.}, year={2022}, month={Oct} }
 @article{collazo_szymanski_khachariya_breckenridge_guan_bagheri_reddy_kirste_mita_pavlidis_et al._2021, title={(Invited) Ion Implantation and Polarity Control: Paths Toward a III-Nitride Superjunction}, url={https://doi.org/10.1149/MA2021-0234983mtgabs}, DOI={10.1149/MA2021-0234983mtgabs}, journal={ECS Meeting Abstracts}, author={Collazo, Ramon and Szymanski, Dennis and Khachariya, Dolar and Breckenridge, Mathew Hayden and Guan, Yan and Bagheri, Pegah and Reddy, Pramod and Kirste, Ronny and Mita, Seiji and Pavlidis, Spyridon and et al.}, year={2021}, month={Oct} }
 @article{bagheri_kim_washiyama_reddy_klump_kirste_mita_collazo_sitar_2021, title={A pathway to highly conducting Ge-doped AlGaN}, volume={130}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0071791}, DOI={10.1063/5.0071791}, abstractNote={Ge doping in AlGaN was studied over a wide dopant concentration range. For high Ge concentrations, the formation of VIII–nGeIII was determined to be the main point defect limiting the conductivity. It was shown that the complex formation could be suppressed by controlling chemical potentials during growth, leading to a higher maximum achievable carrier concentration and selective stabilization of a certain complex type. Chemical potential of the growth species was varied by changing the V/III ratio and growth temperature. Free carrier concentrations as high as 4 × 1019 cm−3 were achieved in Al0.4Ga0.6N:Ge grown on sapphire substrates under “metal-rich” conditions. The ability to control the onset of self-compensation and to stabilize a certain charge state of the compensating defect is of great technological importance for application of AlGaN in various devices.}, number={20}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bagheri, Pegah and Kim, Ji Hyun and Washiyama, Shun and Reddy, Pramod and Klump, Andrew and Kirste, Ronny and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko}, year={2021}, month={Nov} }
 @article{breckenridge_tweedie_reddy_guan_bagheri_szymanski_mita_sierakowski_bockowski_collazo_et al._2021, title={High Mg activation in implanted GaN by high temperature and ultrahigh pressure annealing}, volume={118}, ISSN={["1077-3118"]}, url={http://dx.doi.org/10.1063/5.0038628}, DOI={10.1063/5.0038628}, abstractNote={We demonstrate high p-type conductivity and hole concentrations >1018 cm−3 in Mg-implanted GaN. The implantation was performed at room temperature and by post-implantation annealing at 1 GPa of N2 and in a temperature range of 1200–1400 °C. The high pressure thermodynamically stabilized the GaN surface without the need of a capping layer. We introduce a “diffusion budget,” related to the diffusion length, as a convenient engineering parameter for comparing samples annealed at different temperatures and for different times. Although damage recovery, as measured by XRD, was achieved at relatively low diffusion budgets, these samples did not show p-type conductivity. Further analyses showed heavy compensation by the implantation-induced defects. Higher diffusion budgets resulted in a low Mg ionization energy (∼115 meV) and almost complete Mg activation. For even higher diffusion budgets, we observed significant loss of Mg to the surface and a commensurate reduction in the hole conductivity. High compensation at low diffusion budgets and loss of Mg at high diffusion budgets present a unique challenge for shallow implants. A direct control of the formation of compensating defects arising from the implantation damage may be necessary to achieve both hole conductivity and low Mg diffusion.}, number={2}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Breckenridge, M. Hayden and Tweedie, James and Reddy, Pramod and Guan, Yan and Bagheri, Pegah and Szymanski, Dennis and Mita, Seiji and Sierakowski, Kacper and Bockowski, Michal and Collazo, Ramon and et al.}, year={2021}, month={Jan} }
 @article{breckenridge_bagheri_guo_sarkar_khachariya_pavlidis_tweedie_kirste_mita_reddy_et al._2021, title={High n-type conductivity and carrier concentration in Si-implanted homoepitaxial AlN}, volume={118}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0042857}, DOI={10.1063/5.0042857}, abstractNote={We demonstrate Si-implanted AlN with high conductivity (>1 Ω−1 cm−1) and high carrier concentration (5 × 1018 cm−3). This was enabled by Si implantation into AlN with a low threading dislocation density (TDD) (<103 cm−2), a non-equilibrium damage recovery and dopant activation annealing process, and in situ suppression of self-compensation during the annealing. Low TDD and active suppression of VAl-nSiAl complexes via defect quasi Fermi level control enabled low compensation, while low-temperature, non-equilibrium annealing maintained the desired shallow donor state with an ionization energy of ∼70 meV. The realized n-type conductivity and carrier concentration are over one order of magnitude higher than that reported thus far and present a major technological breakthrough in doping of AlN.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Breckenridge, M. Hayden and Bagheri, Pegah and Guo, Qiang and Sarkar, Biplab and Khachariya, Dolar and Pavlidis, Spyridon and Tweedie, James and Kirste, Ronny and Mita, Seiji and Reddy, Pramod and et al.}, year={2021}, month={Mar} }
 @article{mirrielees_dycus_baker_reddy_collazo_sitar_lebeau_irving_2021, title={Native oxide reconstructions on AlN and GaN (0001) surfaces}, volume={129}, ISSN={["1089-7550"]}, DOI={10.1063/5.0048820}, abstractNote={Properties of AlN/GaN surfaces are important for realizing the tunability of devices, as the presence of surface states contributes to Fermi level pinning. This pinning can influence the performance of high electron mobility transistors and is also important for passivation of the surface when developing high-power electronic devices. It is widely understood that both AlN and GaN surfaces oxidize. Since there are many possible reconstructions for each surface, it is a challenge to identify the relevant surface reconstructions in advance of a detailed simulation. Because of this, different approaches are often employed to down select initial structures to reduce the computational load. These approaches usually rely on either electron counting rules or oxide stoichiometry, as both of these models tend to lead to structures that are energetically favorable. Here we explore models from these approaches but also explore a reconstruction of the (0001) surface directly observed using scanning transmission electron microscopy with predictive density functional theory simulations. Two compositions of the observed surface reconstruction—one which obeys oxide stoichiometry and one which is cation deficient and obeys electron counting—are compared to reconstructions from the previous work. Furthermore, surface states are directly calculated using hybrid exchange-correlation functionals that correct for the underestimation of the bandgaps in AlN and GaN and improve the predicted positions of surface states within the gap. It is found that cation deficiency in the observed reconstruction yields surface states consistent with the experiment. Based on all of these results, we provide insight into the observed properties of oxidized AlGaN surfaces.}, number={19}, journal={JOURNAL OF APPLIED PHYSICS}, author={Mirrielees, Kelsey J. and Dycus, J. Houston and Baker, Jonathon N. and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko and LeBeau, James M. and Irving, Douglas L.}, year={2021}, month={May} }
 @article{bagheri_reddy_mita_szymanski_kim_guan_khachariya_klump_pavlidis_kirste_et al._2021, title={On the Ge shallow-to-deep level transition in Al-rich AlGaN}, volume={130}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0059037}, DOI={10.1063/5.0059037}, abstractNote={Contrary to the arsenides where donors undergo stable DX transition, we find that Ge in AlGaN does not suffer from the DX transition; instead, it undergoes a shallow donor (30 meV) to deep donor (150 meV) transition at ∼50% Al content in the alloy. This finding is of profound technological importance as it removes fundamental doping limitations in AlGaN and AlN imposed by the presumed DX−1 acceptor state. The charge state of Ge below and above the transition was determined by co-doping with Si, which remains a shallow donor in AlGaN for up to 80% Al. It was found that Ge occupied a donor state with a (0/+) thermodynamic transition for AlGaN alloys below and above the transition. Ge as a shallow donor was completely ionized at room temperature; however, the ionization of the deep donor required elevated temperatures, commensurate with its higher ionization energy. This behavior is not unique to Ge; preliminary findings show that Si and O in AlGaN may behave similarly.}, number={5}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bagheri, Pegah and Reddy, Pramod and Mita, Seiji and Szymanski, Dennis and Kim, Ji Hyun and Guan, Yan and Khachariya, Dolar and Klump, Andrew and Pavlidis, Spyridon and Kirste, Ronny and et al.}, year={2021}, month={Aug} }
 @article{khachariya_szymanski_breckenridge_reddy_kohn_sitar_collazo_pavlidis_2021, title={On the characteristics of N-polar GaN Schottky barrier contacts with LPCVD SiN interlayers}, volume={118}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0039888}, DOI={10.1063/5.0039888}, abstractNote={We study the behavior of N-polar GaN Schottky diodes with low-pressure chemical vapor deposited (LPCVD) SiN interlayers and unveil the important role of an amphoteric miniband formed in this interlayer due to a previously identified and dominating Si dangling bond defect. Through analysis of temperature-dependent current–voltage (I–V–T), capacitance–voltage (C–V), and x-ray photoelectron spectroscopy measurements, we observe that when nickel is deposited on LPCVD SiN pretreated with hydrofluoric acid, the SiN/GaN interface is responsible for determining the overall system's barrier height. By contrast, contact formation on oxidized LPCVD SiN leads to a metal/SiN-dominant barrier. We, consequently, propose band diagrams that account for an amphoteric miniband in LPCVD SiN, leading to a new understanding of LPCVD SiN as a lossy dielectric with surface barrier-dependent behavior.}, number={12}, journal={APPLIED PHYSICS LETTERS}, author={Khachariya, Dolar and Szymanski, Dennis and Breckenridge, M. Hayden and Reddy, Pramod and Kohn, Erhard and Sitar, Zlatko and Collazo, Ramon and Pavlidis, Spyridon}, year={2021}, month={Mar} }
 @article{cancellara_markurt_schulz_albrecht_hagedorn_walde_weyers_washiyama_collazo_sitar_2021, title={Role of oxygen diffusion in the dislocation reduction of epitaxial AlN on sapphire during high-temperature annealing}, volume={130}, ISSN={["1089-7550"]}, DOI={10.1063/5.0065935}, abstractNote={Recovery of epitaxial AlN films on sapphire at high temperatures is now an established process to produce pseudo-substrates with high crystalline perfection, which can be used to grow epitaxial structures for UV-light-emitting devices. To elucidate the elementary mechanisms taking place during the thermal treatment of MOVPE-grown films, we studied as-grown and annealed samples combining transmission electron microscopy techniques and secondary ion mass spectrometry (SIMS). By using SIMS, we find a temperature-dependent increase in the overall oxygen content of the films, which cannot be explained quantitatively with either simple bulk or pure pipe-diffusion from the sapphire substrate. Instead, we propose a lateral outdiffusion from the dislocation cores to explain qualitatively and quantitatively the presence of observed oxygen concentration plateaus. Based on the formation enthalpy of various atomic defects and complexes found in literature, we conclude that the di-oxygen/aluminum vacancy complex (VAl–2ON) is the dominant point defect controlling the annealing process. The formation of this defect at high temperatures promotes a dislocation core climb process, which causes the annihilation/fusion of the threading dislocation segments.}, number={20}, journal={JOURNAL OF APPLIED PHYSICS}, author={Cancellara, Leonardo and Markurt, Toni and Schulz, Tobias and Albrecht, Martin and Hagedorn, Sylvia and Walde, Sebastian and Weyers, Markus and Washiyama, Shun and Collazo, Ramon and Sitar, Zlatko}, year={2021}, month={Nov} }
 @article{washiyama_mirrielees_bagheri_baker_kim_guo_kirste_guan_breckenridge_klump_et al._2021, title={Self-compensation in heavily Ge doped AlGaN: A comparison to Si doping}, volume={118}, ISSN={["1077-3118"]}, DOI={10.1063/5.0035957}, abstractNote={Self-compensation in Ge- and Si-doped Al0.3Ga0.7N has been investigated in terms of the formation of III vacancy and donor-vacancy complexes. Both Ge- and Si-doped AlGaN layers showed a compensation knee behavior with impurity compensation (low doping regime), compensation plateau (medium doping regime), and self-compensation (high doping regime). A maximum free carrier concentration of 4–5 × 1019 cm−3 was obtained by Ge doping, whereas Si doping resulted in only half of that value, ∼2 × 1019 cm−3. A DFT calculation with the grand canonical thermodynamics model was developed to support the hypothesis that the difference in self-compensation arises from the difference in the formation energies of the VIII-n•donor complexes relative to their onsite configurations. The model suggested that the VIII-2•donor and VIII-3•donor complexes were responsible for self-compensation for both Ge- and Si-doped AlGaN. However, a lower free carrier concentration in Si-doped samples was due to a high VIII-3•Si concentration, resulting from a lower energy of formation of VIII-3•Si.}, number={4}, journal={APPLIED PHYSICS LETTERS}, author={Washiyama, Shun and Mirrielees, Kelsey J. and Bagheri, Pegah and Baker, Jonathon N. and Kim, Ji-Hyun and Guo, Qiang and Kirste, Ronny and Guan, Yan and Breckenridge, M. Hayden and Klump, Andrew J. and et al.}, year={2021}, month={Jan} }
 @article{kirste_sarkar_reddy_guo_collazo_sitar_2021, title={Status of the growth and fabrication of AlGaN-based UV laser diodes for near and mid-UV wavelength}, volume={12}, ISSN={["2044-5326"]}, DOI={10.1557/s43578-021-00443-8}, abstractNote={In this article, the development of mid-UV laser diodes based on the AlGaN materials system is reviewed. The targeted wavelength for these lasers covers the range from 200 to 350 nm. After introducing UV laser diodes and explaining their applications, the challenges in growth, design, and fabrication are discussed. In addition, recent results from optically and electrically injected UV laser diodes are presented. Finally, we will discuss possible pathways to improve performance and give an outlook on the expected development of UV laser diodes in the near future. Graphical abstract}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Kirste, Ronny and Sarkar, Biplab and Reddy, Pramod and Guo, Qiang and Collazo, Ramon and Sitar, Zlatko}, year={2021}, month={Dec} }
 @article{reddy_khachariya_mecouch_breckenridge_bagheri_guan_kim_pavlidis_kirste_mita_et al._2021, title={Study on avalanche breakdown and Poole-Frenkel emission in Al-rich AlGaN grown on single crystal AlN}, volume={119}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0062831}, DOI={10.1063/5.0062831}, abstractNote={We demonstrate that theoretical breakdown fields can be realized in practically dislocation free Al-rich AlGaN p-n junctions grown on AlN single crystal substrates. Furthermore, we also demonstrate a leakage current density in AlGaN that is independent of the device area, indicating a bulk leakage phenomenon and not surface or mesa-edge related. Accordingly, we identified the Poole–Frenkel emission from two types of point-defect traps in AlGaN as the primary source of reverse leakage before breakdown. Mg-doped AlGaN exhibited leakage currents due to a shallow trap at ∼0.16 eV in contrast with leakage currents observed in Si-doped AlGaN due to a deep trap at ∼1.8 eV.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Reddy, Pramod and Khachariya, Dolar and Mecouch, Will and Breckenridge, M. Hayden and Bagheri, Pegah and Guan, Yan and Kim, Ji Hyun and Pavlidis, Spyridon and Kirste, Ronny and Mita, Seiji and et al.}, year={2021}, month={Nov} }
 @article{bansal_hilse_huet_wang_kozhakhmetov_kim_bachu_alem_collazo_robinson_et al._2021, title={Substrate Modification during Chemical Vapor Deposition of hBN on Sapphire}, volume={13}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.1c14591}, abstractNote={A comparison of hexagonal boron nitride (hBN) layers grown by chemical vapor deposition on C-plane (0001) versus A-plane (112̅0) sapphire (α-Al2O3) substrate is reported. The high deposition temperature (>1200 °C) and hydrogen ambient used for hBN deposition on sapphire substantially alters the C-plane sapphire surface chemistry and leaves the top layer(s) oxygen deficient. The resulting surface morphology due to H2 etching of C-plane sapphire is inhomogeneous with increased surface roughness which causes non-uniform residual stress in the deposited hBN film. In contrast to C-plane, the A-plane of sapphire does not alter substantially under a similar high temperature H2 environment, thus providing a more stable alternative substrate for high quality hBN growth. The E2g Raman mode full width at half-maximum (FWHM) for hBN deposited on C-plane sapphire is 24.5 ± 2.1 cm-1 while for hBN on A-plane sapphire is 24.5 ± 0.7 cm-1. The lesser FWHM standard deviation on A-plane sapphire indicates uniform stress distribution across the film due to reduced undulations on the surface. The photoluminescence spectra of the hBN films at 300 and 3 K, obtained on C-plane and A-plane sapphire exhibit similar characteristics with peaks at 4.1 and 5.3 eV reported to be signature peaks associated with defects for hBN films deposited under lower V/III ratios. The dielectric breakdown field of hBN deposited on A-plane sapphire was measured to be 5 MV cm-1, agreeing well with reports on mechanically exfoliated hBN flakes. Thus, under the typical growth conditions required for high crystalline quality hBN growth, A-plane sapphire provides a more chemically stable substrate.}, number={45}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Bansal, Anushka and Hilse, Maria and Huet, Benjamin and Wang, Ke and Kozhakhmetov, Azimkhan and Kim, Ji Hyun and Bachu, Saiphaneendra and Alem, Nasim and Collazo, Ramon and Robinson, Joshua A. and et al.}, year={2021}, month={Nov}, pages={54516–54526} }
 @article{kim_bagheri_washiyama_klump_kirste_mita_reddy_collazo_sitar_2021, title={Temperature dependence of electronic bands in Al/GaN by utilization of invariant deep defect transition energies}, volume={119}, ISSN={["1077-3118"]}, DOI={10.1063/5.0055409}, abstractNote={We show experimentally that deep point defect levels in GaN, AlN, and AlGaN are constant with respect to the vacuum level and can be used as invariant internal energy references. This offered a convenient and quick way to assess band shifts and impurity levels as a function of temperature via photoluminescence. For AlGaN, we determined that the band shift in the temperature range of 3–600 K occurred primarily in the valence band and that the lowering of the conduction band edge was comparatively small. The valence band shift (as a fraction of the Varshni bandgap shift) in AlGaN varies from ∼70% in AlN to ∼90% in GaN.}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Kim, Ji Hyun and Bagheri, Pegah and Washiyama, Shun and Klump, Andrew and Kirste, Ronny and Mita, Seiji and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko}, year={2021}, month={Jul} }
 @article{gulyuk_lajeunesse_collazo_ivanisevic_2021, title={Tuning Microbial Activity via Programmatic Alteration of Cell/Substrate Interfaces}, volume={5}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202004655}, abstractNote={Abstract}, journal={ADVANCED MATERIALS}, author={Gulyuk, Alexey V. and LaJeunesse, Dennis R. and Collazo, Ramon and Ivanisevic, Albena}, year={2021}, month={May} }
 @article{stein_robbins_reddy_collazo_pavlidis_2021, title={UV illumination effects on AlGaN/GaN HEMTs for tunable RF oscillators}, ISSN={["2164-2958"]}, DOI={10.1109/RWS50353.2021.9360392}, abstractNote={We present the first investigation of AlGaN/GaN HEMTs as optically-controlled microwave semiconductor devices for use in next-generation, high-power microwave photonics systems. Measurements show a modest change in S21 in the presence of UV illumination that induces internal photoconductive and photovoltaic effects. This contrasts with the significant shift in the measured gate capacitance, which can be used to tune future oscillators. This is investigated through the design of a 2.4 GHz Pierce oscillator with an optical tuning range of 3 MHz.}, journal={2021 IEEE RADIO AND WIRELESS SYMPOSIUM (RWS)}, author={Stein, Shane and Robbins, Max and Reddy, Pramod and Collazo, Ramon and Pavlidis, Spyridon}, year={2021}, pages={168–170} }
 @article{al-tawhid_shafe_bagheri_guan_reddy_mita_moody_collazo_sitar_ahadi_2021, title={Weak localization and dimensional crossover in compositionally graded AlxGa1-xN}, volume={118}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0042098}, DOI={10.1063/5.0042098}, abstractNote={The interaction between the itinerant carriers, lattice dynamics, and defects is a problem of long-standing fundamental interest for developing quantum theory of transport. Here, we study this interaction in the compositionally and strain-graded AlGaN heterostructures grown on AlN substrates. The results provide direct evidence that a dimensional crossover (2D–3D) occurs with increasing temperature as the dephasing scattering events reduce the coherence length. These heterostructures show a robust polarization-induced 3D electron gas and a metallic-like behavior down to liquid helium temperature. Using magnetoresistance measurements, we analyze the evolution of the interaction between charge carriers, lattice dynamics, and defects as a function of temperature. A negative longitudinal magnetoresistance emerges at low temperatures, in line with the theory of weak localization. A weak localization fit to near zero-field magneto-conductance indicates a coherence length that is larger than the elastic mean free path and film thickness (lφ>t>lel), suggesting a 2D weak localization in a three-dimensional electron gas. Our observations allow for a clear and detailed picture of two distinct localization mechanisms that affect carrier transport at low temperature.}, number={8}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Al-Tawhid, Athby and Shafe, Abdullah-Al and Bagheri, Pegah and Guan, Yan and Reddy, Pramod and Mita, Seiji and Moody, Baxter and Collazo, Ramon and Sitar, Zlatko and Ahadi, Kaveh}, year={2021}, month={Feb} }
 @article{khachariya_szymanski_reddy_kohn_sitar_collazo_pavlidis_2020, title={(Invited) A Path Toward Vertical GaN Superjunction Devices}, volume={98}, url={https://doi.org/10.1149/09806.0069ecst}, DOI={10.1149/09806.0069ecst}, abstractNote={GaN devices offer exciting competition to incumbent technologies to meet the growing demand for high power electronic devices. The wide bandgap of GaN makes it possible to achieve higher breakdown voltages and reduced on-resistances compared to traditional Si in unipolar devices, as predicted by the classical Baliga figure of merit (BFOM). However, unipolar performance limits can be circumvented using the superjunction (SJ), which has been demonstrated experimentally in both Si and SiC. Due to the current difficulties with selective area doping in GaN, experimental reports of vertical GaN SJs are lacking. In response, we propose the use of the lateral polar junction (LPJ), which is unique to III-Nitrides, to create next-generation vertical GaN SJ devices. We develop a model that provides first order design equations for such a device, and validate it using TCAD simulations of a 1.2 kV diode. A proposed manufacturing approach for LPJ-based GaN SJ is provided.}, number={6}, journal={ECS Transactions}, publisher={The Electrochemical Society}, author={Khachariya, Dolar and Szymanski, Dennis and Reddy, Pramod and Kohn, Erhard and Sitar, Zlatko and Collazo, Ramon and Pavlidis, Spyridon}, year={2020}, month={Sep}, pages={69–79} }
 @article{hauwiller_stowe_eldred_mita_collazo_sitar_lebeau_2020, title={Cathodoluminescence of silicon doped aluminum nitride with scanning transmission electron microscopy}, volume={8}, ISSN={["2166-532X"]}, DOI={10.1063/5.0019863}, abstractNote={Here, we apply cathodoluminescence in scanning transmission electron microscopy to infer the influence of dislocation strain fields on the formation of point defect complexes in Si doped AlN. In addition to identifying non-radiative recombination centers, tracking Si related defect emission energies reveals a red-shift at threading dislocations. We discuss these results in the context of multiple Si-vacancy defect complexes that can form and the influence of local strain on their formation energies. By correlating the electronic and structural properties at the nanoscale, cathodoluminescence elucidates the inhomogeneity of defect complexes in Si doped AlN and offers the potential for strain engineering to control the defect energy formation landscape.}, number={9}, journal={APL MATERIALS}, author={Hauwiller, Matthew R. and Stowe, David and Eldred, Timothy B. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and LeBeau, James}, year={2020}, month={Sep} }
 @article{baker_bowes_harris_collazo_sitar_irving_2020, title={Complexes and compensation in degenerately donor doped GaN}, volume={117}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0013988}, DOI={10.1063/5.0013988}, abstractNote={Gallium nitride is an increasingly technologically relevant material system. While donor doping GaN to low and intermediate dopant concentrations using silicon and germanium has become routine, compensation mechanisms activate under very high donor doping, limiting the maximum electron concentration achievable with either dopant in the degenerate doping regime. This effect, and how it differs between the two dopants, is investigated by hybrid functional density functional theory calculations and grand canonical thermodynamics models and is found to be due to the onset of multi-member Ga vacancy-donor substitutional complexes under degenerate doping conditions. The differing energetics of Ge- and Si-related complexes leads to different responses, ultimately making Ge the more effective donor in degenerate conditions.}, number={10}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Baker, Jonathon N. and Bowes, Preston C. and Harris, Joshua S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2020}, month={Sep} }
 @article{reddy_breckenridge_guo_klump_khachariya_pavlidis_mecouch_mita_moody_tweedie_et al._2020, title={High gain, large area, and solar blind avalanche photodiodes based on Al-rich AlGaN grown on AlN substrates}, volume={116}, url={http://dx.doi.org/10.1063/1.5138127}, DOI={10.1063/1.5138127}, abstractNote={We demonstrate large area (25 000 μm2) Al-rich AlGaN-based avalanche photodiodes (APDs) grown on single crystal AlN substrates operating with differential (the difference in photocurrent and dark current) signal gain of 100 000 at 90 pW (<1 μW cm−2) illumination with very low dark currents <0.1 pA at room temperature under ambient light. The high gain in large area AlGaN APDs is attributed to a high breakdown voltage at 340 V, corresponding to very high breakdown fields ∼9 MV cm−1 as a consequence of low threading and screw dislocation densities < 103 cm−2. The maximum charge collection efficiency of 30% was determined at 255 nm, corresponding to the bandgap of Al0.65Ga0.35N, with a response of 0.06 A/W. No response was detected for λ > 280 nm, establishing solar blindness of the device.}, number={8}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Reddy, Pramod and Breckenridge, M. Hayden and Guo, Qiang and Klump, Andrew and Khachariya, Dolar and Pavlidis, Spyridon and Mecouch, Will and Mita, Seiji and Moody, Baxter and Tweedie, James and et al.}, year={2020}, month={Feb}, pages={081101} }
 @article{yamamoto_takekawa_goto_nagashima_dalmau_schlesser_murakami_collazo_monemar_sitar_et al._2020, title={Hydride vapor phase epitaxy of Si -doped AlN layers using SiCl 4 as a doping gas}, volume={545}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2020.125730}, abstractNote={Growth of Si-doped AlN layers by hydride vapor phase epitaxy on AlN/sapphire templates prepared by metalorganic vapor phase epitaxy and bulk AlN substrates prepared by physical vapor transport was investigated using silicon tetrachloride (SiCl4) as a doping gas. On the AlN/sapphire templates, when the SiCl4 supply was low, the incorporation ratio of Si decreased due to the influence of O impurities resulting from the decomposition of the sapphire. In addition, pits were formed on the surfaces, and the pit density increased significantly at Si concentrations exceeding 2 × 1019 cm−3. In contrast, on the bulk AlN substrates, the Si concentration increased linearly with increasing SiCl4 supply, and a pit- and stress-free layer could be grown with a Si concentration as high as 6.5 × 1019 cm−3. Hall effect measurements revealed that this layer exhibited n-type conductivity with a donor activation energy of 253 meV. However, the carrier density at room temperature (RT) was as low as 3.6 × 1013 cm−3 because of the high compensation ratio due to the presence of acceptors. A broad peak centered at 3.3 eV was observed in the RT photoluminescence spectra of the Si-doped AlN layers grown on the bulk AlN substrates, indicating that Al vacancies formed by the Fermi level effect due to Si doping acted as acceptors that compensated for carriers.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Yamamoto, Reo and Takekawa, Nao and Goto, Ken and Nagashima, Toru and Dalmau, Rafael and Schlesser, Raoul and Murakami, Hisashi and Collazo, Ramon and Monemar, Bo and Sitar, Zlatko and et al.}, year={2020}, month={Sep} }
 @article{gleco_reddy_kirste_collazo_lajeunesse_ivanisevic_2020, title={Modulating the Stress Response of E. coli at GaN Interfaces Using Surface Charge, Surface Chemistry, and Genetic Mutations}, volume={3}, ISSN={["2576-6422"]}, url={https://doi.org/10.1021/acsabm.0c01007}, DOI={10.1021/acsabm.0c01007}, abstractNote={The surface properties of inorganic materials can be used to modulate the response of microorganisms at the interface. We used the persistent photoconductivity properties of chemically treated gallium nitride substrates to evaluate the stress response of wild-type, ΔfliC, and ΔcsgG mutant E. coli exposed to charged surfaces. Substrate surface characterization and biological assays were used to correlate the physiological response to substrate surface charge. The physiological response was evaluated by measuring the intracellular levels of reactive oxygen species (ROS) and Ca2+ cations using fluorescent probes. We evaluated the response 1, 2, and 3 h after a short exposure to the surfaces to determine generational effects of the initial exposure on the physiology of the bacteria. In general, the ROS levels 1 h after exposure were not different. However, there were differences in Ca2+ levels in E. coli 1 h after the initial exposure to charged GaN surfaces, primarily in the wild-type E. coli. The differences in Ca2+ levels depended on the substrate surface chemistry and genetic mutation that suggests the involvement of multiple factors for modulating the interactions of bacteria at interfaces.}, number={10}, journal={ACS APPLIED BIO MATERIALS}, publisher={American Chemical Society (ACS)}, author={Gleco, Sara and Reddy, Pramod and Kirste, Ronny and Collazo, Ramon and LaJeunesse, Dennis and Ivanisevic, Albena}, year={2020}, month={Oct}, pages={7211–7218} }
 @article{washiyama_guan_mita_collazo_sitar_2020, title={Recovery kinetics in high temperature annealed AlN heteroepitaxial films}, volume={127}, ISSN={["1089-7550"]}, DOI={10.1063/5.0002891}, abstractNote={Based on the experimental dislocation annihilation rates, vacancy core diffusion-controlled dislocation climb was found as a dominant recovery mechanism in high temperature annealing of AlN heteroepitaxial films. Dislocation annihilation mechanisms via dislocation glide (with or without kinks) and vacancy bulk diffusion were found to be less significant. Cross-slip was also ruled out as a possible mechanism as a majority of dislocations in heteroepitaxial AlN films are threading edge dislocations. While dislocation climb through both vacancy bulk and core diffusion could offer a plausible explanation of the recovery process, the activation energy for the vacancy core diffusion-controlled dislocation climb was relatively low (4.3 ± 0.1 eV), as estimated from an Arrhenius plot. The validity of the vacancy core diffusion mechanism was also supported by a large vacancy mean free path (∼240 nm), which was comparable to the sample thickness and thus the average dislocation length. Finally, the experimentally observed dislocation density reduction as a function of the annealing temperature and time was in good agreement with the vacancy core diffusion mechanism.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Washiyama, Shun and Guan, Yan and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko}, year={2020}, month={Mar} }
 @article{reddy_khachariya_szymanski_breckenridge_sarkar_pavlidis_collazo_sitar_kohn_2020, title={Role of polarity in SiN on Al/GaN and the pathway to stable contacts}, volume={35}, url={http://dx.doi.org/10.1088/1361-6641/ab7775}, DOI={10.1088/1361-6641/ab7775}, abstractNote={Despite being the most widely used dielectric for passivation of GaN-based lateral devices, amorphous silicon-nitride still faces many stability challenges, which arise from its complex bulk electronic and interface properties on the polar (Al)GaN surfaces. In this investigation, SiN has been applied as an ultra-thin interlayer (∼3–5 nm) in vertical contact structures on Ga-polar and N-polar GaN templates to study the metal–insulator–semiconductor- (MIS-) like system and better understand the interaction between the polar surface and its dielectric overlayer. We describe the role of amphoteric ≡Si centers in SiN in passivating and providing the polarization countercharge to Al/GaN of different polarities. The consequent requirements of the concentration profile of the amphoteric defects and the corresponding chemical profile of SiN is discussed. The importance of SiN surface termination and their influence on the interface potential on Al/GaN that determines device performance and reliability is also shown. Finally, a pathway to highly stable and reliable ohmic contacts to n-type Ga-polar GaN without instabilities associated with metal directly alloying with GaN as in the case of traditional contacts is proposed.}, number={5}, journal={Semiconductor Science and Technology}, publisher={IOP Publishing}, author={Reddy, Pramod and Khachariya, Dolar and Szymanski, Dennis and Breckenridge, M Hayden and Sarkar, Biplab and Pavlidis, Spyridon and Collazo, Ramón and Sitar, Zlatko and Kohn, Erhard}, year={2020}, month={May}, pages={055007} }
 @article{breckenridge_guo_klump_sarkar_guan_tweedie_kirste_mita_reddy_collazo_et al._2020, title={Shallow Si donor in ion-implanted homoepitaxial AlN}, volume={116}, url={http://dx.doi.org/10.1063/1.5144080}, DOI={10.1063/1.5144080}, abstractNote={We demonstrate Si as a shallow donor in aluminum nitride (AlN) with an ionization energy of ∼70 meV. The shallow state was achieved by ion implantation of Si into homoepitaxial AlN and a low thermal budget damage recovery and activation process. These results demonstrate that the DX formation may be a kinetically limited process, though being a non-equilibrium process, preventing the Si donor from relaxing to the deep donor state. The room temperature conductivity was measured to be ∼0.05 Ω−1 cm−1, which is one order of magnitude higher than what has been reported for the epitaxially doped or implanted AlN.}, number={17}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Breckenridge, M. Hayden and Guo, Qiang and Klump, Andrew and Sarkar, Biplab and Guan, Yan and Tweedie, James and Kirste, Ronny and Mita, Seiji and Reddy, Pramod and Collazo, Ramón and et al.}, year={2020}, month={Apr}, pages={172103} }
 @article{wang_huynh_liao_yu_bai_tweedie_breckenridge_collazo_sitar_bockowski_et al._2020, title={Strain Recovery and Defect Characterization in Mg-Implanted Homoepitaxial GaN on High-Quality GaN Substrates}, volume={257}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.201900705}, abstractNote={The evolution of defects due to high‐pressure annealing of magnesium ion‐implanted epitaxial GaN grown on high‐quality GaN substrates is investigated. Changes in the implant‐induced strain are quantified as a function of annealing temperature and time. After annealing at 1300 °C for 10 min, the implant‐induced strain is fully relieved and accompanied by the presence of extended defects such as basal plane stacking faults and prismatic loops. Approximately one‐third of the original implant‐induced strain remains after annealing at 700 °C, and 5% of the original strain remains at 1000 °C for 100 min. In all cases, nearly all of the recovered strain occurs within first few minutes of annealing. A prominent increase in the asymmetric (104) triple axis X‐ray rocking curve full width at 0.01 maximum (FW0.01M) is observed after annealing at 1300 °C for 10 min. After annealing at 1300 °C for 100 min, a subsequent decrease in FW0.01M is correlated with a reduction of the extended defect density from 4 × 108 to 3 × 107 cm−2, determined through transmission electron microscope (TEM) measurements. Further reduction in the density of the extended defects by optimizing annealing temperature and time is expected to improve the performance of GaN‐based vertical power devices.}, number={4}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Wang, Yekan and Huynh, Kenny and Liao, Michael E. and Yu, Hsuan-Ming and Bai, Tingyu and Tweedie, James and Breckenridge, Mathew Hayden and Collazo, Ramon and Sitar, Zlatko and Bockowski, Michal and et al.}, year={2020}, month={Apr} }
 @article{goto_takekawa_nagashima_yamamoto_pozina_dalmau_schlesser_collazo_monemar_sitar_et al._2020, title={Study of Dislocations in Homoepitaxially and Heteroepitaxially Grown AlN Layers}, volume={217}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.202000465}, abstractNote={Dislocation densities in AlN layers grown on c‐plane sapphire and physical vapor transport‐grown AlN (PVT‐AlN) (0001) substrates, by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE), respectively, are evaluated from the density of etch pits formed in sodium hydroxide (NaOH)/potassium hydroxide (KOH) eutectic heated to 450 °C. In the heteroepitaxial layers grown by MOVPE on the sapphire substrates, etch pits with different sizes are formed. Cross‐sectional transmission electron microscopy (TEM) observations reveal that the large, medium, and small pits, with densities of 1.4 × 106, 2.6 × 107, and 6.9 × 109 cm−2, respectively, correspond to screw, mixed, and edge dislocations, respectively. In contrast, in the homoepitaxial layers grown by HVPE on the PVT‐AlN substrates, only one kind of etch pit with uniform size corresponding to the edge dislocations is formed with a density of 103–104 cm−2. Cross‐sectional TEM observation confirms that the edge dislocations in the homoepitaxial layer propagate from the substrate through the interface, which indicates that the dislocation density does not increase during homoepitaxial growth by HVPE. The HVPE‐AlN homoepitaxial layers grown on the PVT‐AlN substrates are found to have very low dislocation density.}, number={24}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Goto, Ken and Takekawa, Nao and Nagashima, Toru and Yamamoto, Reo and Pozina, Galia and Dalmau, Rafael and Schlesser, Raoul and Collazo, Ramon and Monemar, Bo and Sitar, Zlatko and et al.}, year={2020}, month={Dec} }
 @article{amano_collazo_santi_einfeldt_funato_glaab_hagedorn_hirano_hirayama_ishii_et al._2020, title={The 2020 UV emitter roadmap}, volume={53}, ISSN={["1361-6463"]}, DOI={10.1088/1361-6463/aba64c}, abstractNote={Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.}, number={50}, journal={JOURNAL OF PHYSICS D-APPLIED PHYSICS}, author={Amano, Hiroshi and Collazo, Ramon and Santi, Carlo De and Einfeldt, Sven and Funato, Mitsuru and Glaab, Johannes and Hagedorn, Sylvia and Hirano, Akira and Hirayama, Hideki and Ishii, Ryota and et al.}, year={2020}, month={Dec} }
 @article{harris_gaddy_collazo_sitar_irving_2019, title={Oxygen and silicon point defects in Al0.65Ga0.35N}, volume={3}, ISSN={["2475-9953"]}, DOI={10.1103/PhysRevMaterials.3.054604}, abstractNote={The formation energies of oxygen and silicon impurities have been examined explicitly in ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$ using hybrid exchange-correlation density-functional theory simulations. Both impurities were initialized in on-site substitutional and off-site DX configurations in a range of charge states. The ${\mathrm{O}}_{\mathrm{N}}^{+1}$ donor was found to always relax into an on-site configuration, and its formation energy is relatively independent of local chemistry (the configuration of Al and Ga atoms surrounding the defect). By contrast, the ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ acceptor almost always relaxes into a DX configuration, with a formation energy that is strongly dependent on local chemistry. The differences in formation energy of distinct ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ defect configurations are understood through the interplay of two qualitative trends in the types of nearest-neighbor bonds (O-Al or O-Ga), as well as the subtler influence of the lengths of the O-Al bonds. Knowledge of ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$ formation energies as well as the relative frequencies of sites with different local chemistry allows one to compute the relative site occupancies of ${\mathrm{O}}_{\mathrm{N}}^{\ensuremath{-}1}$. Because the thermodynamic transition levels associated with different defect configurations are unique, the ${\mathrm{O}}_{\mathrm{N}}$ DX transition is associated with multiple defect levels. ${\mathrm{Si}}_{\mathrm{III}}$, where III represents the group III cation of Al or Ga, provides an interesting counterexample. ${\mathrm{Si}}_{\mathrm{III}}^{+1}$ is predicted to be the dominant charge state across the entire band gap of ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$, and little dependence of the formation energy on the composition of nearby cation sites was found. This is explained by the fact that the first-nearest neighbors are all of the same species (N), so the local environment is similar to a bulk III nitride, in which on-site ${\mathrm{Si}}_{\mathrm{III}}^{+1}$ is stable across the same Fermi level range (i.e., below the band gap of ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$). Thus, the trends in the energetics of ${\mathrm{O}}_{\mathrm{N}}$ and ${\mathrm{Si}}_{\mathrm{III}}$ in ${\mathrm{Al}}_{0.65}{\mathrm{Ga}}_{0.35}\mathrm{N}$ are both determined by the chemistry of the four nearest-neighbor sites surrounding the defect site.}, number={5}, journal={PHYSICAL REVIEW MATERIALS}, author={Harris, Joshua S. and Gaddy, Benjamin E. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2019}, month={May} }
 @article{alden_troha_kirste_mita_guo_hoffmann_zgonik_collazo_sitar_2019, title={Quasi-phase-matched second harmonic generation of UV light using AlN waveguides}, volume={114}, ISSN={["1077-3118"]}, DOI={10.1063/1.5087058}, abstractNote={As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is periodically changed commensurate with the coherence wavelength. QPM also allows the use of the highest nonlinear susceptibility, and therefore, higher conversion efficiencies are possible. In this work, the QPM SHG of UV light in AlN lateral polar structure-based waveguides is demonstrated. The peak intensity of the frequency doubled laser light was measured at 344 nm and 472 nm wavelengths, in agreement with dispersion-based theoretical predictions. These results confirm the potential of III-nitride-based lateral polar structures for quasi-phase-matched nonlinear optics and for frequency doubling media for UV light generation.}, number={10}, journal={APPLIED PHYSICS LETTERS}, author={Alden, Dorian and Troha, Tinkara and Kirste, Ronny and Mita, Seiji and Guo, Qiang and Hoffmann, Axel and Zgonik, Marko and Collazo, Ramon and Sitar, Zlatko}, year={2019}, month={Mar} }
 @article{graziano_bryan_bryan_kirste_tweedie_collazo_sitar_2019, title={Structural characteristics of m-plane AlN substrates and homoepitaxial films}, volume={507}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2018.07.012}, abstractNote={Homoepitaxial non-polar AlN films were realized on m-plane (101-0)-oriented AlN single crystals by metalorganic chemical vapor deposition (MOCVD). The microstructural properties of m-plane AlN substrates and homoepitaxial films were assessed by means of atomic force microscopy and high resolution x-ray diffraction characterization. Results indicated that both m-plane AlN substrates and films possessed exceptional structural quality, with some anisotropic mosaic distributions due to the quasi-bulk nature of the non-polar single crystals. An increase in the MOCVD growth temperature was noted to minimize the degree of inherited mosaic anisotropy without altering the m-plane AlN film growth rate, indicating that high temperature growth is critical to produce optimal film crystallinity. A dramatic change in the film surface morphology from heavily faceted “slate-like” features to monolayer steps was observed as the growth temperature was increased. The “slate-like” surface morphology produced low intensity cross-streaks in symmetric (101-0) reciprocal space maps, tilted about 18° away from the (101-0) crystal truncation rod. The orientation of these diffuse streaks corresponds to the physical alignment of the slates with respect to the substrate surface normal. X-ray line scans and defect-selective reciprocal space mapping confirmed that these low intensity streaks are solely dependent on this peculiar surface structure produced at low MOCVD growth temperatures and unrelated to basal plane stacking faults or other extended defects. All observations confirm that high quality III-nitride epitaxial structures on m-plane AlN substrates are attainable with controllable MOCVD growth processes, as demanded for future high performing AlN-based non-polar devices.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Graziano, Milena Bobea and Bryan, Isaac and Bryan, Zachary and Kirste, Ronny and Tweedie, James and Collazo, Ramon and Sitar, Zlatko}, year={2019}, month={Feb}, pages={389–394} }
 @article{guo_kirste_mita_tweedie_reddy_washiyama_breckenridge_collazo_sitar_2019, title={The polarization field in Al-rich AlGaN multiple quantum wells}, volume={58}, ISSN={["1347-4065"]}, DOI={10.7567/1347-4065/ab07a9}, abstractNote={Abstract}, journal={JAPANESE JOURNAL OF APPLIED PHYSICS}, author={Guo, Qiang and Kirste, Ronny and Mita, Seiji and Tweedie, James and Reddy, Pramod and Washiyama, Shun and Breckenridge, M. Hayden and Collazo, Ramon and Sitar, Zlatko}, year={2019}, month={Jun} }
 @article{dycus_washiyama_eldred_guan_kirste_mita_sitar_collazo_lebeau_2019, title={The role of transient surface morphology on composition control in AlGaN layers and wells}, volume={114}, ISSN={["1077-3118"]}, DOI={10.1063/1.5063933}, abstractNote={The mechanisms governing “compositional pulling” during the growth of AlxGa1−xN wells are investigated. Gallium-rich AlxGa1−xN wells grown on high dislocation density AlN/sapphire templates exhibit asymmetric and diffuse composition profiles, while those grown on low dislocation density native AlN substrates do not. Furthermore, strain in all AlxGa1−xN wells is found to be pseudomorphic, ruling it out as the dominating driving force. Rather, the high threading dislocation density of the AlN template is considered to play the defining role. We propose that a transient surface morphology is introduced during dislocation mediated spiral growth, which, in conjunction with process supersaturation, determines the Ga incorporation. These findings provide insights into compositional pulling in high Ga content AlxGa1−xN grown on AlN and provide a route to grow thicker wells with very abrupt interfaces on native AlN substrates.}, number={3}, journal={APPLIED PHYSICS LETTERS}, author={Dycus, J. Houston and Washiyama, Shun and Eldred, Tim B. and Guan, Yan and Kirste, Ronny and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon and LeBeau, James M.}, year={2019}, month={Jan} }
 @article{washiyama_reddy_kaess_kirste_mita_collazo_sitar_2018, title={A thermodynamic supersaturation model for the growth of aluminum gallium nitride by metalorganic chemical vapor deposition}, volume={124}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/1.5045058}, DOI={10.1063/1.5045058}, abstractNote={A thermodynamic supersaturation model for growth of AlGaN by metalorganic chemical vapor deposition was developed for experimentally accessible growth parameters. The derived non-linear relationships enabled us to estimate Ga and Al supersaturation during AlGaN growth for given growth conditions. Calculations revealed that the GaN phase was close to chemical equilibrium, while the Al supersaturation was as high as 1010 for typical growth conditions. Such a disparity in the supersaturation of reaction species plays a significant role in the stability of the growth of the resulting ternary alloy. The agreement between experiment and simulation suggests that the parasitic gas phase reactions between trimethylaluminum and NH3 were not significant at low NH3 flow rates/partial pressures, indicating that, under these conditions, the AlGaN growth was thermodynamically limited.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Washiyama, Shun and Reddy, Pramod and Kaess, Felix and Kirste, Ronny and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko}, year={2018}, month={Sep} }
 @article{snyder_lajeunesse_reddy_kirste_collazo_ivanisevic_2018, title={Bioelectronics communication: encoding yeast regulatory responses using nanostructured gallium nitride thin films}, volume={10}, ISSN={2040-3364 2040-3372}, url={http://dx.doi.org/10.1039/C8NR03684E}, DOI={10.1039/C8NR03684E}, abstractNote={Baker's yeast, S. cerevisiae, is a model organism that is used in synthetic biology.}, number={24}, journal={Nanoscale}, publisher={Royal Society of Chemistry (RSC)}, author={Snyder, Patrick J. and LaJeunesse, Dennis R. and Reddy, Pramod and Kirste, Ronny and Collazo, Ramon and Ivanisevic, Albena}, year={2018}, pages={11506–11516} }
 @misc{snyder_reddy_kirste_collazo_ivanisevic_2018, title={Bulk and Surface Electronic Properties of Inorganic Materials: Tools to Guide Cellular Behavior}, volume={2}, ISSN={["2366-9608"]}, DOI={10.1002/smtd.201800016}, abstractNote={Abstract}, number={9}, journal={SMALL METHODS}, author={Snyder, Patrick J. and Reddy, Pramod and Kirste, Ronny and Collazo, Ramon and Ivanisevic, Albena}, year={2018}, month={Sep} }
 @article{gulyuk_lajeunesse_collazo_ivanisevic_2018, title={Characterization of Pseudomonas aeruginosa Films on Different Inorganic Surfaces before and after UV Light Exposure}, volume={34}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.8b02079}, abstractNote={The changes of the surface properties of Au, GaN, and SiO x after UV light irradiation were used to actively influence the process of formation of Pseudomonas aeruginosa films. The interfacial properties of the substrates were characterized by X-ray photoelectron spectroscopy and atomic force microscopy. The changes in the P. aeruginosa film properties were accessed by analyzing adhesion force maps and quantifying the intracellular Ca2+ concentration. The collected analysis indicates that the alteration of the inorganic materials' surface chemistry can lead to differences in biofilm formation and variable response from P. aeruginosa cells.}, number={36}, journal={LANGMUIR}, author={Gulyuk, Alexey V and LaJeunesse, Dennis R. and Collazo, Ramon and Ivanisevic, Albena}, year={2018}, month={Sep}, pages={10806–10815} }
 @article{bryan_bryan_washiyama_reddy_gaddy_sarkar_breckenridge_guo_bobea_tweedie_et al._2018, title={Doping and compensation in Al-rich AlGaN grown on single crystal AlN and sapphire by MOCVD}, volume={112}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5011984}, DOI={10.1063/1.5011984}, abstractNote={In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the “knee behavior” in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.}, number={6}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bryan, Isaac and Bryan, Zachary and Washiyama, Shun and Reddy, Pramod and Gaddy, Benjamin and Sarkar, Biplab and Breckenridge, M. Hayden and Guo, Qiang and Bobea, Milena and Tweedie, James and et al.}, year={2018}, month={Feb} }
 @article{klump_zhou_stevie_collazo_sitar_2018, title={Improvement in detection limit for time-of-flight SIMS analysis of dopants in GaN structures}, volume={36}, ISSN={["2166-2746"]}, DOI={10.1116/1.5013001}, abstractNote={Secondary ion mass spectrometry (SIMS) has been used extensively to monitor dopant levels in semiconductor materials. The preponderance of these measurements has been made with magnetic sector or quadrupole analyzers. Use of time-of-flight (ToF) analyzers has been limited because of an inability to match the detection limit of the other analyzers. Optimization of the ToF-SIMS analysis beam pulse width and analysis frames per cycle is shown to provide as much as an order of magnitude improvement in detection limit. The magnesium dopant in GaN structures was used for the study and analysis was made with Cs+ sputtering source and Bi3+. The count rate for CsMg+ increased by a factor of 11.3 with both improvements applied. This was evidenced by a detection limit improvement for magnesium from 7.5 × 1017 atoms/cm3 to low 1017 atoms/cm3. Increasing the number of analysis frames from one to ten causes cycle time to increase by a factor of five. Hence, there is a tradeoff between improved detection limit and analysis time.}, number={3}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Klump, Andrew and Zhou, Chuanzhen and Stevie, Frederick A. and Collazo, Ramon and Sitar, Zlatko}, year={2018}, month={May} }
 @article{snyder_reddy_kirste_lajeunesse_collazo_ivanisevic_2018, title={Noninvasive Stimulation of Neurotypic Cells Using Persistent Photoconductivity of Gallium Nitride}, volume={3}, ISSN={2470-1343 2470-1343}, url={http://dx.doi.org/10.1021/ACSOMEGA.7B01894}, DOI={10.1021/ACSOMEGA.7B01894}, abstractNote={The persistent photoconductivity (PPC) of the n-type Ga-polar GaN was used to stimulate PC12 cells noninvasively. Analysis of the III-V semiconductor material by atomic force microscopy, Kelvin probe force microscopy, photoconductivity, and X-ray photoelectron spectroscopy quantified bulk and surface charge, as well as chemical composition before and after exposure to UV light and cell culture media. The semiconductor surface was made photoconductive by illumination with UV light and experienced PPC, which was utilized to stimulate PC12 cells in vitro. Stimulation was confirmed by measuring the changes in intracellular calcium concentration. Control experiments with gallium salt verified the stimulation of neurotypic cells. Inductively coupled plasma mass spectrometry data confirmed the lack of gallium leaching and toxic effects during the stimulation.}, number={1}, journal={ACS Omega}, publisher={American Chemical Society (ACS)}, author={Snyder, Patrick J. and Reddy, Pramod and Kirste, Ronny and LaJeunesse, Dennis R. and Collazo, Ramon and Ivanisevic, Albena}, year={2018}, month={Jan}, pages={615–621} }
 @inproceedings{sarkar_reddy_klump_rounds_breckenridge_haidet_mita_kirste_collazo_sitar_2018, title={On Contacts to III-nitride deep-UV emitters}, url={http://dx.doi.org/10.1109/icmap.2018.8354575}, DOI={10.1109/icmap.2018.8354575}, abstractNote={Although contacts to III-nitride visible and UV-A based emitters has been well explored, understanding the contacts to III-nitride deep-UV emitters have attracted research attention recently. Owing to the wide bandgap, both n-type and p-type contact metallization techniques result in a Schottky barrier at the metal-semiconductor interface. A way to reduce the contact resistance is to achieve a higher free carrier concentration in the epitaxial layer. As a result, growth of III-nitride epitaxial layers on native substrates are providing pathways for significant performance improvement. However, understanding the contacts to deep-UV emitters grown on native substrates are necessary to allow further performance improvement.}, booktitle={2018 3rd International Conference on Microwave and Photonics (ICMAP)}, author={Sarkar, B. and Reddy, P. and Klump, A. and Rounds, R. and Breckenridge, M. R. and Haidet, B. B. and Mita, S. and Kirste, R. and Collazo, Ramon and Sitar, Z.}, year={2018} }
 @article{harris_baker_gaddy_bryan_bryan_mirrielees_reddy_collazo_sitar_irving_2018, title={On compensation in Si-doped AlN}, volume={112}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5022794}, DOI={10.1063/1.5022794}, abstractNote={Controllable n-type doping over wide ranges of carrier concentrations in AlN, or Al-rich AlGaN, is critical to realizing next-generation applications in high-power electronics and deep UV light sources. Silicon is not a hydrogenic donor in AlN as it is in GaN; despite this, the carrier concentration should be controllable, albeit less efficiently, by increasing the donor concentration during growth. At low doping levels, an increase in the Si content leads to a commensurate increase in free electrons. Problematically, this trend does not persist to higher doping levels. In fact, a further increase in the Si concentration leads to a decrease in free electron concentration; this is commonly referred to as the compensation knee. While the nature of this decrease has been attributed to a variety of compensating defects, the mechanism and identity of the predominant defects associated with the knee have not been conclusively determined. Density functional theory calculations using hybrid exchange-correlation functionals have identified VAl+nSiAl complexes as central to mechanistically understanding compensation in the high Si limit in AlN, while secondary impurities and vacancies tend to dominate compensation in the low Si limit. The formation energies and optical signatures of these defects in AlN are calculated and utilized in a grand canonical charge balance solver to identify carrier concentrations as a function of Si content. The results were found to qualitatively reproduce the experimentally observed compensation knee. Furthermore, these calculations predict a shift in the optical emissions present in the high and low doping limits, which is confirmed with detailed photoluminescence measurements.}, number={15}, journal={APPLIED PHYSICS LETTERS}, author={Harris, Joshua S. and Baker, Jonathon N. and Gaddy, Benjamin E. and Bryan, Isaac and Bryan, Zachary and Mirrielees, Kelsey J. and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L.}, year={2018}, month={Apr} }
 @article{reddy_washiyama_mecouch_hernandez-balderrama_kaess_breckenridge_sarkar_haidet_franke_kohn_et al._2018, title={Plasma enhanced chemical vapor deposition of SiO2 and SiNx on AlGaN: Band offsets and interface studies as a function of Al composition}, volume={36}, ISSN={["1520-8559"]}, DOI={10.1116/1.5050501}, abstractNote={In this work, the authors characterized the interface of plasma enhanced chemical vapor deposition (PECVD) dielectrics, SiO2 and SiNx with AlGaN as a function of Al composition. SiO2 is found to exhibit type I straddled band alignment with positive conduction and valence band offsets for all Al compositions. However, the interface Fermi level is found to be pinned within the bandgap, indicating a significant density of interface states. Hence, SiO2 is found to be suitable for insulating layers or electrical isolation on AlGaN with breakdown fields between 4.5 and 6.5 MV cm−1, but an additional passivating interlayer between SiO2 and AlGaN is necessary for passivation on Al-rich AlGaN. In contrast, Si-rich PECVD SiNx is found to exhibit type II staggered band alignment with positive conduction band offsets and negative valence band offsets for Al compositions <40% and type I straddled band alignment with negative conduction and valence band offsets for Al compositions >40% and is, hence, found to be unsuitable for insulating layers or electrical isolation on Al-rich AlGaN in general. In contrast to passivating stoichiometric LPCVD Si3N4, no evidence for interface state reduction by depositing SiNx on AlGaN is observed.}, number={6}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Reddy, Pramod and Washiyama, Shun and Mecouch, Will and Hernandez-Balderrama, Luis H. and Kaess, Felix and Breckenridge, M. Hayden and Sarkar, Biplab and Haidet, Brian B. and Franke, Alexander and Kohn, Erhard and et al.}, year={2018}, month={Nov} }
 @article{alden_harris_bryan_baker_reddy_mita_callsen_hoffmann_irving_collazo_et al._2018, title={Point-Defect Nature of the Ultraviolet Absorption Band in AIN}, volume={9}, ISSN={["2331-7019"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85047735459&partnerID=MN8TOARS}, DOI={10.1103/physrevapplied.9.054036}, abstractNote={We present an approach, where point defects and defect complexes are identified using power dependent photoluminescence excitation spectroscopy, impurity data from SIMS and DFT-based calculations accounting for the total charge balance in the crystal. Employing the capabilities of such experimental-computational approach, in this work, the UVC absorption band at 4.7 eV, as well as the 2.7 eV and 3.9 eV luminescence bands in AlN single crystals grown via physical vapor transport (PVT) are studied in detail. Photoluminescence excitation spectroscopy measurements demonstrate the relationship between the defect luminescent bands centered at 3.9 eV and 2.7 eV to the commonly observed absorption band centered at 4.7 eV. Accordingly, the thermodynamic transition energy for the absorption band at 4.7 eV and the luminescence band at 3.9 eV is estimated at 4.2 eV, in agreement with the thermodynamic transition energy for the C N– point defect. Finally, the 2.7 eV PL band is the result of a donor-acceptor pair transition between the V N and C N point defects since nitrogen vacancies, is predicted to be present in the crystal in concentrations similar to carbon employing charge balance constrained DFT calculations. Power dependent photoluminescence measurements reveal the presence of the deep donor state with a thermodynamic transition energy of 5.0 eV, which we hypothesize to be nitrogen vacancies in agreement with predictions based on theory. The charge state, concentration and type of impurities in the crystal is calculated considering a fixed amount of impurities and using a density functional theory (DFT) based defect solver, which considers their respective formation energies and the total charge balance in the crystal. The presented results show that nitrogen vacancies are the most likely candidate for the deep donor state involved in the donor acceptor pair transition with peak emission at 2.7 eV for the conditions relevant to PVT growth.}, number={5}, journal={PHYSICAL REVIEW APPLIED}, author={Alden, D. and Harris, J. S. and Bryan, Z. and Baker, J. N. and Reddy, P. and Mita, S. and Callsen, G. and Hoffmann, A. and Irving, D. L. and Collazo, R. and et al.}, year={2018}, month={May} }
 @article{dhall_vigil-fowler_houston dycus_kirste_mita_sitar_collazo_lebeau_2018, title={Probing collective oscillation ofd-orbital electrons at the nanoscale}, volume={112}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.5012742}, DOI={10.1063/1.5012742}, abstractNote={Here, we demonstrate that high energy electrons can be used to explore the collective oscillation of s, p, and d orbital electrons at the nanometer length scale. Using epitaxial AlGaN/AlN quantum wells as a test system, we observe the emergence of additional features in the loss spectrum with the increasing Ga content. A comparison of the observed spectra with ab-initio theory reveals that the origin of these spectral features lies in excitations of 3d-electrons contributed by Ga. We find that these modes differ in energy from the valence electron plasmons in Al1–xGaxN due to the different polarizabilities of the d electrons. Finally, we study the dependence of observed spectral features on the Ga content, lending insights into the origin of these spectral features, and their coupling with electron-hole excitations.}, number={6}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Dhall, Rohan and Vigil-Fowler, Derek and Houston Dycus, J. and Kirste, Ronny and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon and LeBeau, James M.}, year={2018}, month={Feb}, pages={061102} }
 @article{dycus_mirrielees_grimley_kirste_mita_sitar_collazo_irving_lebeau_2018, title={Structure of Ultrathin Native Oxides on III-Nitride Surfaces}, volume={10}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.8b00845}, abstractNote={When pristine material surfaces are exposed to air, highly reactive broken bonds can promote the formation of surface oxides with structures and properties differing greatly from bulk. Determination of the oxide structure is often elusive through the use of indirect diffraction methods or techniques that probe only the outermost layer. As a result, surface oxides forming on widely used materials, such as group III-nitrides, have not been unambiguously resolved, even though critical properties can depend sensitively on their presence. In this study, aberration corrected scanning transmission electron microscopy reveals directly, and with depth dependence, the structure of ultrathin native oxides that form on AlN and GaN surfaces. Through atomic resolution imaging and spectroscopy, we show that the oxide layers are comprised of tetrahedra-octahedra cation-oxygen units, in an arrangement similar to bulk θ-Al2O3 and β-Ga2O3. By applying density functional theory, we show that the observed structures are more stable than previously proposed surface oxide models. We place the impact of these observations in the context of key III-nitride growth, device issues, and the recent discovery of two-dimensional nitrides.}, number={13}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Dycus, J. Houston and Mirrielees, Kelsey J. and Grimley, Everett D. and Kirste, Ronny and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon and Irving, Douglas L. and LeBeau, James M.}, year={2018}, month={Apr}, pages={10607–10611} }
 @article{rounds_sarkar_alden_guo_klump_hartmann_nagashima_kirste_franke_bickermann_et al._2018, title={The influence of point defects on the thermal conductivity of AlN crystals}, volume={123}, ISSN={["1089-7550"]}, DOI={10.1063/1.5028141}, abstractNote={The average bulk thermal conductivity of free-standing physical vapor transport and hydride vapor phase epitaxy single crystal AlN samples with different impurity concentrations is analyzed using the 3ω method in the temperature range of 30–325 K. AlN wafers grown by physical vapor transport show significant variation in thermal conductivity at room temperature with values ranging between 268 W/m K and 339 W/m K. AlN crystals grown by hydride vapor phase epitaxy yield values between 298 W/m K and 341 W/m K at room temperature, suggesting that the same fundamental mechanisms limit the thermal conductivity of AlN grown by both techniques. All samples in this work show phonon resonance behavior resulting from incorporated point defects. Samples shown by optical analysis to contain carbon-silicon complexes exhibit higher thermal conductivity above 100 K. Phonon scattering by point defects is determined to be the main limiting factor for thermal conductivity of AlN within the investigated temperature range.}, number={18}, journal={JOURNAL OF APPLIED PHYSICS}, author={Rounds, Robert and Sarkar, Biplab and Alden, Dorian and Guo, Qiang and Klump, Andrew and Hartmann, Carsten and Nagashima, Toru and Kirste, Ronny and Franke, Alexander and Bickermann, Matthias and et al.}, year={2018}, month={May} }
 @article{rounds_sarkar_sochacki_bockowski_imanishi_mori_kirste_collazo_sitar_2018, title={Thermal conductivity of GaN single crystals: Influence of impurities incorporated in different growth processes}, volume={124}, ISSN={["1089-7550"]}, DOI={10.1063/1.5047531}, abstractNote={The thermal conductivity of GaN crystals grown by different techniques is analyzed using the 3ω method in the temperature range of 30 K to 295 K. GaN wafers grown by the ammonothermal method show a significant variation in thermal conductivity at room temperature with values ranging between 164 W m−1 K−1 and 196 W m−1 K−1. GaN crystals produced with the sodium flux and hydride vapor phase epitaxy methods show results of 211 W m−1 K−1 and 224 W m−1 K−1, respectively, at room temperature. Analysis using secondary ion mass spectrometry indicates varying amounts of impurities between the respective crystals and explains the behavior of thermal conductivity trends in the samples. The observed difference between thermal conductivity curves suggests that scattering of phonons at point defects dominates the thermal conductivity of GaN within the investigated temperature range. Deviations of model curves from thermal conductivity measurements and disparities between modelled characteristic lengths and actual sample thicknesses indicate that phonon resonances are active in GaN.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Rounds, Robert and Sarkar, Biplab and Sochacki, Tomasz and Bockowski, Michal and Imanishi, Masayuki and Mori, Yusuke and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2018}, month={Sep} }
 @article{rounds_sarkar_klump_hartmann_nagashima_kirste_franke_bickermann_kumagai_sitar_et al._2018, title={Thermal conductivity of single-crystalline AIN}, volume={11}, ISSN={["1882-0786"]}, DOI={10.7567/apex.11.071001}, abstractNote={The thermal conductivity of AlN single crystals grown by physical vapor transport (PVT) and hydride vapor phase epitaxy (HVPE) was measured in the range of 30 to 325 K by the 3ω method. The measured room-temperature thermal conductivity ranged from 268 to 374 W m−1 K−1. Higher thermal conductivity correlated with higher transparency at 265 nm and lower total impurity levels.}, number={7}, journal={APPLIED PHYSICS EXPRESS}, author={Rounds, Robert and Sarkar, Biplab and Klump, Andrew and Hartmann, Carsten and Nagashima, Toru and Kirste, Ronny and Franke, Alexander and Bickermann, Matthias and Kumagai, Yoshinao and Sitar, Zlatko and et al.}, year={2018}, month={Jul} }
 @article{snyder_reddy_kirste_lajeunesse_collazo_ivanisevic_2018, title={Variably doped nanostructured gallium nitride surfaces can serve as biointerfaces for neurotypic PC12 cells and alter their behavior}, volume={8}, ISSN={["2046-2069"]}, DOI={10.1039/c8ra06836d}, abstractNote={Neurotypic PC12 cells behavior was studied on nanostructured GaN and rationalized with respect to surface charge, doping level, and chemical functionalization.}, number={64}, journal={RSC ADVANCES}, author={Snyder, Patrick J. and Reddy, Pramod and Kirste, Ronny and LaJeunesse, Dennis R. and Collazo, Ramon and Ivanisevic, Albena}, year={2018}, pages={36722–36730} }
 @article{sarkar_reddy_kaess_haidet_tweedie_mita_kirste_kohn_collazo_sitar_et al._2017, title={(Invited) Material Considerations for the Development of III-nitride Power Devices}, volume={80}, ISBN={["978-1-62332-476-6"]}, ISSN={["1938-6737"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85033580489&partnerID=MN8TOARS}, DOI={10.1149/08007.0029ecst}, abstractNote={With advancement in growth of native III-nitride substrates, remarkable progress has been made to extend the functionality of GaN based power electronic devices. The low dislocation epitaxial films grown on native substrates outperforms the films grown on foreign substrates. However, several material considerations has to be incorporated in order to exploit the full potential of GaN and AlxGa1-xN (0<x≤1) based power devices. This paper presents a review report on the development of III-nitride power devices grown on foreign and native (GaN and AlN) substrates. Discussion on state-of-the art epitaxial material quality, contact formation and surface treatments films are presented.}, number={7}, journal={GALLIUM NITRIDE AND SILICON CARBIDE POWER TECHNOLOGIES 7}, author={Sarkar, B. and Reddy, Pramod and Kaess, F. and Haidet, B.B. and Tweedie, J. and Mita, S. and Kirste, R. and Kohn, E. and Collazo, R. and Sitar, Z. and et al.}, year={2017}, pages={29–36} }
 @article{reddy_kaess_tweedie_kirste_mita_collazo_sitar_2017, title={Defect quasi Fermi level control-based C-N reduction in GaN: Evidence for the role of minority carriers}, volume={111}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5000720}, DOI={10.1063/1.5000720}, abstractNote={Compensating point defect reduction in wide bandgap semiconductors is possible by above bandgap illumination based defect quasi Fermi level (dQFL) control. The point defect control technique employs excess minority carriers that influence the dQFL of the compensator, increase the corresponding defect formation energy, and consequently are responsible for point defect reduction. Previous studies on various defects in GaN and AlGaN have shown good agreement with the theoretical model, but no direct evidence for the role of minority carriers was provided. In this work, we provide direct evidence for the role of minority carriers in reducing point defects by studying the predicted increase in work done against defect (CN−1) formation with the decrease in the Fermi level (free carrier concentration) in Si doped GaN at a constant illumination intensity. Comparative defect photoluminescence measurements on illuminated and dark regions of GaN show an excellent quantitative agreement with the theory by exhibiting a greater reduction in yellow luminescence attributed to CN−1 at lower doping, thereby providing conclusive evidence for the role of the minority carriers in Fermi level control-based point defect reduction.}, number={15}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Reddy, Pramod and Kaess, Felix and Tweedie, James and Kirste, Ronny and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko}, year={2017}, month={Oct} }
 @article{reddy_sarkar_kaess_gerhold_kohn_collazo_sitar_2017, title={Defect-free Ni/GaN Schottky barrier behavior with high temperature stability}, volume={110}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4973762}, DOI={10.1063/1.4973762}, abstractNote={In this work, we report on defect-free homogeneous behavior of Ni Schottky contacts patterned on surface treated n-GaN by photolithography with unity ideality factor, high temperature stability, and low reverse leakage. The barrier height (0.7 eV) and ideality factor (1.02) are found to be independent of temperature, indicating a highly homogeneous contact. The contacts are found to be stable with no significant change in ideality factor or leakage current up to an annealing temperature of 600 °C. Temperature dependence of the reverse leakage current shows no evidence for the existence of surface defects that would provide leakage paths, and the behavior was modeled by ATLAS simulations with an ideal homogeneous barrier of 0.7 eV. Consequently, the forward and reverse bias characteristics are explained by a common set of parameters. The surface treatment after the development and prior to metallization included an acid-based surface treatment. X-ray photoelectron spectroscopy (XPS) studies indicate that the hydroxide-based development process during photolithography changes the nitride surface composition by introducing excess C that degrades the ideality factor and introduces barrier inhomogeneity and high leakage currents. XPS studies further demonstrate that the restoration of a stable, Ga-rich surface, similar to as-grown surface, occurs due to the acid-based surface treatment, which is responsible for the observed unity ideality factor, homogeneous barrier, low leakage current, and high temperature stability.}, number={1}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Reddy, Pramod and Sarkar, Biplab and Kaess, Felix and Gerhold, Michael and Kohn, Erhard and Collazo, Ramon and Sitar, Zlatko}, year={2017}, month={Jan} }
 @article{sarkar_mita_reddy_klump_kaess_tweedie_bryan_bryan_kirste_kohn_et al._2017, title={High free carrier concentration in p-GaN grown on AlN substrates}, volume={111}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4995239}, DOI={10.1063/1.4995239}, abstractNote={A high free hole concentration in III-nitrides is important for next generation optoelectronic and high power electronic devices. The free hole concentration exceeding 1018 cm−3 and resistivity as low as 0.7 Ω cm are reported for p-GaN layers grown by metalorganic vapor phase epitaxy on single crystal AlN substrates. Temperature dependent Hall measurements confirmed a much lower activation energy, 60–80 mV, for p-GaN grown on AlN as compared to sapphire substrates; the lowering of the activation energy was due to screening of Coulomb potential by free carriers. It is also shown that a higher doping density (more than 5 × 1019 cm−3) can be achieved in p-GaN/AlN without the onset of self-compensation.}, number={3}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Sarkar, Biplab and Mita, Seiji and Reddy, Pramod and Klump, Andrew and Kaess, Felix and Tweedie, James and Bryan, Isaac and Bryan, Zachary and Kirste, Ronny and Kohn, Erhard and et al.}, year={2017}, month={Jul} }
 @article{lamprecht_jmerik_collazo_sitar_ivanov_thonke_2017, title={Model for the deep defect-related emission bands between 1.4 and 2.4 eV in AlN}, volume={254}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.201600714}, abstractNote={Abstract}, number={8}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Lamprecht, M. and Jmerik, V. N. and Collazo, R. and Sitar, Z. and Ivanov, S. V. and Thonke, K.}, year={2017}, month={Aug} }
 @article{haidet_sarkar_reddy_bryan_bryan_kirste_collazo_sitar_2017, title={Nonlinear analysis of vanadium- and titanium-based contacts to Al-rich n-AlGaN}, volume={56}, ISSN={["1347-4065"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85030640149&partnerID=MN8TOARS}, DOI={10.7567/jjap.56.100302}, abstractNote={In this work, we report on voltage dependent contact characteristics of Al-rich n-AlGaN due to the presence of a Schottky barrier at the metal–AlGaN interface. Current–voltage characteristics appear to be linear at low voltages, but a clear nonlinearity is evident at high voltages. Unlike Ga-rich n-AlGaN where V-based contact marginally outperforms the Ti-based contact, V-based contact to Al-rich n-AlGaN outperforms the Ti-based contact by ∼2 orders of magnitude for all voltage ranges. The improved performance of V-based contacts is likely due to a lower Schottky barrier and partly due to ease of formation of a low barrier V–AlGaN interface.}, number={10}, journal={JAPANESE JOURNAL OF APPLIED PHYSICS}, publisher={Japan Society of Applied Physics}, author={Haidet, Brian B. and Sarkar, Biplab and Reddy, Pramod and Bryan, Isaac and Bryan, Zachary and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2017}, month={Oct} }
 @article{sarkar_reddy_klump_kaess_rounds_kirste_mita_kohn_collazo_sitar_2017, title={On Ni/Au Alloyed Contacts to Mg-Doped GaN}, volume={47}, ISSN={0361-5235 1543-186X}, url={http://dx.doi.org/10.1007/S11664-017-5775-3}, DOI={10.1007/S11664-017-5775-3}, number={1}, journal={Journal of Electronic Materials}, publisher={Springer Nature}, author={Sarkar, Biplab and Reddy, Pramod and Klump, Andrew and Kaess, Felix and Rounds, Robert and Kirste, Ronny and Mita, Seiji and Kohn, Erhard and Collazo, Ramon and Sitar, Zlatko}, year={2017}, month={Sep}, pages={305–311} }
 @article{majkic_franke_kirste_schlesser_collazo_sitar_zgonik_2017, title={Optical nonlinear and electro-optical coefficients in bulk aluminium nitride single crystals}, volume={254}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.201700077}, abstractNote={Bulk aluminium nitride single crystals grown by physical vapour transport were characterised for their nonlinear optical properties. The two independent nonzero optical nonlinear coefficients were determined by the Maker Fringe method, with a LiNbO3 plate as a reference. At a wavelength of 1030 nm, the measured coefficient |d33| = (4.3 ± 0.3) pm V−1 and |d31| = |d33|/(45 ± 5). Michelson interferometer was used to measure two unclamped electro‐optic coefficients to give r33 = (1.16 ± 0.13) pm V−1 and r13 = (0.11 ± 0.02) pm V−1 at a wavelength of 633 nm. By calculating the dispersion relation for d33, we show that AlN is a very promising material for quasi phase‐matched frequency conversion into UV.}, number={9}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Majkic, Aleksej and Franke, Alexander and Kirste, Ronny and Schlesser, Raoul and Collazo, Ramon and Sitar, Zlatko and Zgonik, Marko}, year={2017}, month={Sep} }
 @article{thonke_lamprecht_collazo_sitar_2017, title={Optical signatures of silicon and oxygen related DX centers in AlN}, volume={214}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.201600749}, abstractNote={Bulk AlN crystals typically contain high concentrations of oxygen, silicon, and carbon − as also state‐of‐the art epitaxial layers typically do, depending on the specific growth conditions. In optical spectroscopy, such crystals show broad bands in the region from 2–5 eV in absorption and emission. We investigated several emission bands in the range from 1.4 to 1.9 eV, especially those centred at 2.0 and 2.4 eV, which under below‐bandgap excitation with a 325 nm laser dominate the photoluminescence (PL) spectra both at low and at room temperature. We find clear indications that all these transitions occur between different states of the shallow donors Si or O, and deep acceptors. The donors undergo lattice relaxation and form DX centres. Depending on temperature, the initial state is either a long‐lived (S = 1) DX‐centre state of the donors, a shallow EMT‐like conventional donor state, or a free electron − with markedly different relaxation times for the optical transitions under below‐bandgap excitation. The acceptor involved in these PL bands is likely linked to aluminum vacancies. Based on our data, we develop configuration coordinate diagrams and a combined level scheme for multiple transitions in the range from 1.4 to 2.4 eV.}, number={9}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Thonke, Klaus and Lamprecht, Matthias and Collazo, Ramon and Sitar, Zlatko}, year={2017}, month={Sep} }
 @article{sarkar_haidet_reddy_kirste_collazo_sitar_2017, title={Performance improvement of ohmic contacts on Al-rich n-AlGaN grown on single crystal AlN substrate using reactive ion etching surface treatment}, volume={10}, ISSN={["1882-0786"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85021839341&partnerID=MN8TOARS}, DOI={10.7567/apex.10.071001}, abstractNote={Four orders of magnitude improvement in specific contact resistivity of Al-rich n-AlGaN grown on single crystal AlN substrates is achieved by surface treatment based on reactive ion etching (RIE). The ohmic contacts to as-grown Al-rich n-AlGaN/AlN exhibit a high contact resistance and nonlinearity due to a large Schottky barrier and low dislocation density. The RIE surface treatment reduces the barrier height at the free surface by ∼0.5 eV and is also expected to introduce a defective surface required for ohmic contact formation.}, number={7}, journal={APPLIED PHYSICS EXPRESS}, publisher={Japan Society of Applied Physics}, author={Sarkar, Biplab and Haidet, Brian B. and Reddy, Pramod and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2017}, month={Jul} }
 @article{snyder_kirste_collazo_ivanisevic_2017, title={Persistent Photoconductivity, Nanoscale Topography, and Chemical Functionalization Can Collectively Influence the Behavior of PC12 Cells on Wide Bandgap Semiconductor Surfaces}, volume={13}, ISSN={["1613-6829"]}, DOI={10.1002/smll.201700481}, abstractNote={Wide bandgap semiconductors such as gallium nitride (GaN) exhibit persistent photoconductivity properties. The incorporation of this asset into the fabrication of a unique biointerface is presented. Templates with lithographically defined regions with controlled roughness are generated during the semiconductor growth process. Template surface functional groups are varied using a benchtop surface functionalization procedure. The conductivity of the template is altered by exposure to UV light and the behavior of PC12 cells is mapped under different substrate conductivity. The pattern size and roughness are combined with surface chemistry to change the adhesion of PC12 cells when the GaN is made more conductive after UV light exposure. Furthermore, during neurite outgrowth, surface chemistry and initial conductivity difference are used to facilitate the extension to smoother areas on the GaN surface. These results can be utilized for unique bioelectronics interfaces to probe and control cellular behavior.}, number={24}, journal={SMALL}, author={Snyder, Patrick J. and Kirste, Ronny and Collazo, Ramon and Ivanisevic, Albena}, year={2017}, month={Jun} }
 @article{reddy_washiyama_kaess_kirste_mita_collazo_sitar_2017, title={Point defect reduction in MOCVD (Al)GaN by chemical potential control and a comprehensive model of C incorporation in GaN}, volume={122}, url={https://doi.org/10.1063/1.5002682}, DOI={10.1063/1.5002682}, abstractNote={A theoretical framework that provides a quantitative relationship between point defect formation energies and growth process parameters is presented. It enables systematic point defect reduction by chemical potential control in metalorganic chemical vapor deposition (MOCVD) of III-nitrides. Experimental corroboration is provided by a case study of C incorporation in GaN. The theoretical model is shown to be successful in providing quantitative predictions of CN defect incorporation in GaN as a function of growth parameters and provides valuable insights into boundary phases and other impurity chemical reactions. The metal supersaturation is found to be the primary factor in determining the chemical potential of III/N and consequently incorporation or formation of point defects which involves exchange of III or N atoms with the reservoir. The framework is general and may be extended to other defect systems in (Al)GaN. The utility of equilibrium formalism typically employed in density functional theory in predicting defect incorporation in non-equilibrium and high temperature MOCVD growth is confirmed. Furthermore, the proposed theoretical framework may be used to determine optimal growth conditions to achieve minimum compensation within any given constraints such as growth rate, crystal quality, and other practical system limitations.}, number={24}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Reddy, Pramod and Washiyama, S. and Kaess, F. and Kirste, R. and Mita, S. and Collazo, Ramon and Sitar, Z.}, year={2017}, pages={245702} }
 @article{lamprecht_grund_bauer_collazo_sitar_thonke_2017, title={Slow decay of a defect-related emission band at 2.05eV in AlN: Signatures of oxygen-related DX states}, volume={254}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.201600338}, abstractNote={Abstract}, number={5}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Lamprecht, M. and Grund, C. and Bauer, S. and Collazo, R. and Sitar, Z. and Thonke, K.}, year={2017}, month={May} }
 @article{shelton_bryan_paisley_sachet_ihlefeld_lavrik_collazo_sitar_maria_2017, title={Step-free GaN surfaces grown by confined-area metal-organic vapor phase epitaxy}, volume={5}, ISSN={["2166-532X"]}, DOI={10.1063/1.4993840}, abstractNote={A two-step homoepitaxial growth process producing step-free surfaces on low dislocation density, Ga-polar ammonothermal GaN single crystals is described. Growth is conducted under very low supersaturation conditions where adatom incorporation occurs predominantly at step edges, and lateral growth is strongly preferred. The achievable step-free area is limited by the substrate dislocation density. For ammonothermal crystals with an average dislocation density of ∼1 × 104 cm−2, step-free mesas up to 200 × 200 μm2 in size are achieved. These remarkable surfaces create a unique opportunity to study the effect of steps on the properties and performance of semiconductor heterostructures.}, number={9}, journal={APL MATERIALS}, author={Shelton, Christopher T. and Bryan, Isaac and Paisley, Elizabeth A. and Sachet, Edward and Ihlefeld, Jon F. and Lavrik, Nick and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2017}, month={Sep} }
 @article{dycus_mirrielees_grimley_dhall_kirste_mita_sitar_collazo_irving_lebeau_2017, title={Structure and Chemistry of Oxide Surface Reconstructions in III-Nitrides Observed using STEM EELS}, volume={23}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927617007887}, DOI={10.1017/S1431927617007887}, abstractNote={.}, number={S1}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Dycus, J. Houston and Mirrielees, Kelsey J. and Grimley, Everett D. and Dhall, Rohan and Kirste, Ronny and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon and Irving, Douglas L. and LeBeau, James M.}, year={2017}, month={Jul}, pages={1444–1445} }
 @article{tsao_chowdhury_hollis_jena_johnson_jones_kaplar_rajan_van de walle_bellotti_et al._2017, title={Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges}, volume={4}, ISSN={2199-160X}, url={http://dx.doi.org/10.1002/AELM.201600501}, DOI={10.1002/aelm.201600501}, abstractNote={Abstract}, number={1}, journal={Advanced Electronic Materials}, publisher={Wiley}, author={Tsao, J. Y. and Chowdhury, S. and Hollis, M. A. and Jena, D. and Johnson, N. M. and Jones, K. A. and Kaplar, R. J. and Rajan, S. and Van de Walle, C. G. and Bellotti, E. and et al.}, year={2017}, month={Dec}, pages={1600501} }
 @article{huang_li_gurarslan_yu_kirste_guo_zhao_collazo_sitar_parsons_et al._2016, title={Atomically Thin MoS2 Narrowband and Broadband Light Superabsorbers}, volume={10}, ISSN={["1936-086X"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000381959100030&KeyUID=WOS:000381959100030}, DOI={10.1021/acsnano.6b02195}, abstractNote={We present a combined theoretical and experimental effort to enable strong light absorption (>70%) in atomically thin MoS2 films (≤4 layers) for either narrowband incidence with arbitrarily prespecified wavelengths or broadband incidence like solar radiation. This is achieved by integrating the films with resonant photonic structures that are deterministically designed using a unique reverse design approach based on leaky mode coupling. The design starts with identifying the properties of leaky modes necessary for the targeted strong absorption, followed by searching for the geometrical features of nanostructures to support the desired modes. This process is very intuitive and only involves a minimal amount of computation, thanks to the straightforward correlations between optical functionality and leaky modes as well as between leaky modes and the geometrical feature of nanostructures. The result may provide useful guidance for the development of high-performance atomic-scale photonic devices, such as solar cells, modulators, photodetectors, and photocatalysts.}, number={8}, journal={ACS NANO}, author={Huang, Lujun and Li, Guoqing and Gurarslan, Alper and Yu, Yiling and Kirste, Ronny and Guo, Wei and Zhao, Junjie and Collazo, Ramon and Sitar, Zlatko and Parsons, Gregory N. and et al.}, year={2016}, month={Aug}, pages={7493–7499} }
 @article{kaess_mita_xie_reddy_klump_hernandez-balderrama_washiyama_franke_kirste_hoffmann_et al._2016, title={Correlation between mobility collapse and carbon impurities in Si-doped GaN grown by low pressure metalorganic chemical vapor deposition}, volume={120}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/1.4962017}, DOI={10.1063/1.4962017}, abstractNote={In the low doping range below 1 × 1017 cm−3, carbon was identified as the main defect attributing to the sudden reduction of the electron mobility, the electron mobility collapse, in n-type GaN grown by low pressure metalorganic chemical vapor deposition. Secondary ion mass spectroscopy has been performed in conjunction with C concentration and the thermodynamic Ga supersaturation model. By controlling the ammonia flow rate, the input partial pressure of Ga precursor, and the diluent gas within the Ga supersaturation model, the C concentration in Si-doped GaN was controllable from 6 × 1019 cm−3 to values as low as 2 × 1015 cm−3. It was found that the electron mobility collapsed as a function of free carrier concentration, once the Si concentration closely approached the C concentration. Lowering the C concentration to the order of 1015 cm−3 by optimizing Ga supersaturation achieved controllable free carrier concentrations down to 5 × 1015 cm−3 with a peak electron mobility of 820 cm2/V s without observing the mobility collapse. The highest electron mobility of 1170 cm2/V s was obtained even in metalorganic vapor deposition-grown GaN on sapphire substrates by optimizing growth parameters in terms of Ga supersaturation to reduce the C concentration.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, publisher={AIP Publishing}, author={Kaess, Felix and Mita, Seiji and Xie, Jingqiao and Reddy, Pramod and Klump, Andrew and Hernandez-Balderrama, Luis H. and Washiyama, Shun and Franke, Alexander and Kirste, Ronny and Hoffmann, Axel and et al.}, year={2016}, month={Sep} }
 @article{alden_guo_kirste_kaess_bryan_troha_bagal_reddy_hernandez-balderrama_franke_et al._2016, title={Fabrication and structural properties of AlN submicron periodic lateral polar structures and waveguides for UV-C applications}, volume={108}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4955033}, DOI={10.1063/1.4955033}, abstractNote={Periodically poled AlN thin films with submicron domain widths were fabricated for nonlinear applications in the UV-VIS region. A procedure utilizing metalorganic chemical vapor deposition growth of AlN in combination with laser interference lithography was developed for making a nanoscale lateral polarity structure (LPS) with domain size down to 600 nm. The Al-polar and N-polar domains were identified by wet etching the periodic LPS in a potassium hydroxide solution and subsequent scanning electron microscopy (SEM) characterization. Fully coalesced and well-defined vertical interfaces between the adjacent domains were established by cross-sectional SEM. AlN LPSs were mechanically polished and surface roughness with a root mean square value of ∼10 nm over a 90 μm × 90 μm area was achieved. 3.8 μm wide and 650 nm thick AlN LPS waveguides were fabricated. The achieved domain sizes, surface roughness, and waveguides are suitable for second harmonic generation in the UVC spectrum.}, number={26}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Alden, D. and Guo, W. and Kirste, R. and Kaess, F. and Bryan, I. and Troha, T. and Bagal, A. and Reddy, P. and Hernandez-Balderrama, Luis H. and Franke, A. and et al.}, year={2016}, month={Jun} }
 @article{rohrbaugh_hernandez-balderrama_kaess_kirste_collazo_ivanisevic_2016, title={HgNO3 sensitivity of AlGaN/GaN field effect transistors functionalized with phytochelating peptides}, volume={6}, ISSN={["2158-3226"]}, DOI={10.1063/1.4953806}, abstractNote={This study examined the conductance sensitivity of AlGaN/GaN field effect transistors in response to varying Hg/HNO3 solutions. FET surfaces were covalently functionalized with phytochelatin-5 peptides in order to detect Hg in solution. Results showed a resilience of peptide-AlGaN/GaN bonds in the presence of strong HNO3 aliquots, with significant degradation in FET ID signal. However, devices showed strong and varied response to Hg concentrations of 1, 10, 100, and 1000 ppm. The gathered statistically significant results indicate that peptide terminated AlGaN/GaN devices are capable of differentiating between Hg solutions and demonstrate device sensitivity.}, number={6}, journal={AIP ADVANCES}, author={Rohrbaugh, Nathaniel and Hernandez-Balderrama, Luis and Kaess, Felix and Kirste, Ronny and Collazo, Ramon and Ivanisevic, Albena}, year={2016}, month={Jun} }
 @article{franke_hoffmann_kirste_bobea_tweedie_kaess_gerhold_collazo_sitar_2016, title={High reflectivity III-nitride UV-C distributed Bragg reflectors for vertical cavity emitting lasers}, volume={120}, ISSN={["1089-7550"]}, DOI={10.1063/1.4963831}, abstractNote={UV-C distributed Bragg reflectors (DBRs) for vertical cavity surface emitting laser applications and polariton lasers are presented. The structural integrity of up to 25 layer pairs of AlN/Al0.65Ga0.35N DBRs is maintained by balancing the tensile and compressive strain present between the single layers of the multilayer stack grown on top of an Al0.85Ga0.35N template. By comparing the structural and optical properties for DBRs grown on low dislocation density AlN and AlGaN templates, the criteria for plastic relaxation by cracking thick nitride Bragg reflectors are deduced. The critical thickness is found to be limited mainly by the accumulated strain energy during the DBR growth and is only negligibly affected by the dislocations. A reflectance of 97.7% at 273 nm is demonstrated. The demonstrated optical quality and an ability to tune the resonance wavelength of our resonators and microcavity structures open new opportunities for UV-C vertical emitters.}, number={13}, journal={JOURNAL OF APPLIED PHYSICS}, author={Franke, A. and Hoffmann, M. P. and Kirste, R. and Bobea, M. and Tweedie, J. and Kaess, F. and Gerhold, M. and Collazo, R. and Sitar, Z.}, year={2016}, month={Oct} }
 @article{reddy_washiyama_kaess_breckenridge_hernandez-balderrama_haidet_alden_franke_sarkar_kohn_et al._2016, title={High temperature and low pressure chemical vapor deposition of silicon nitride on AlGaN: Band offsets and passivation studies}, volume={119}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/1.4945775}, DOI={10.1063/1.4945775}, abstractNote={In this work, we employed X-ray photoelectron spectroscopy to determine the band offsets and interface Fermi level at the heterojunction formed by stoichiometric silicon nitride deposited on AlxGa1-xN (of varying Al composition “x”) via low pressure chemical vapor deposition. Silicon nitride is found to form a type II staggered band alignment with AlGaN for all Al compositions (0 ≤ x ≤ 1) and present an electron barrier into AlGaN even at higher Al compositions, where Eg(AlGaN) > Eg(Si3N4). Further, no band bending is observed in AlGaN for x ≤ 0.6 and a reduced band bending (by ∼1 eV in comparison to that at free surface) is observed for x > 0.6. The Fermi level in silicon nitride is found to be at 3 eV with respect to its valence band, which is likely due to silicon (≡Si0/−1) dangling bonds. The presence of band bending for x > 0.6 is seen as a likely consequence of Fermi level alignment at Si3N4/AlGaN hetero-interface and not due to interface states. Photoelectron spectroscopy results are corroborated by current-voltage-temperature and capacitance-voltage measurements. A shift in the interface Fermi level (before band bending at equilibrium) from the conduction band in Si3N4/n-GaN to the valence band in Si3N4/p-GaN is observed, which strongly indicates a reduction in mid-gap interface states. Hence, stoichiometric silicon nitride is found to be a feasible passivation and dielectric insulation material for AlGaN at any composition.}, number={14}, journal={JOURNAL OF APPLIED PHYSICS}, publisher={AIP Publishing}, author={Reddy, Pramod and Washiyama, Shun and Kaess, Felix and Breckenridge, M. Hayden and Hernandez-Balderrama, Luis H. and Haidet, Brian B. and Alden, Dorian and Franke, Alexander and Sarkar, Biplab and Kohn, Erhard and et al.}, year={2016}, month={Apr} }
 @article{tojo_yamamoto_tanaka_thieu_togashi_nagashima_kinoshita_dalmau_schlesser_murakami_et al._2016, title={Influence of high-temperature processing on the surface properties of bulk AlN substrates}, volume={446}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2016.04.030}, abstractNote={Deep-level luminescence at 3.3 eV related to the presence of Al vacancies (V Al ) was observed in room temperature photoluminescence (RT-PL) spectra of homoepitaxial AlN layers grown at 1450 °C by hydride vapor-phase epitaxy (HVPE) and cooled to RT in a mixture of H 2 and N 2 with added NH 3 . However, this luminescence disappeared after removing the near surface layer of AlN by polishing. In addition, the deep-level luminescence was not observed when the post-growth cooling of AlN was conducted without NH 3 . Secondary ion mass spectrometry (SIMS) studies revealed that although the point defect density of the interior of the AlN layers remained low, the near surface layer cooled in the presence of NH 3 was contaminated by Si impurities due to both suppression of the surface decomposition by the added NH 3 and volatilization of Si by decomposition of the quartz reactor walls at high temperatures. The deep-level luminescence reappeared after the polished AlN wafers were heated in presence of NH 3 at temperatures above 1400 °C. The surface contamination by Si is thought to generate V Al near the surface by lowering their formation energy due to the Fermi level effect, resulting in deep-level luminescence at 3.3 eV caused by the shallow donor (Si) to V Al transition. • The origin of deep-level luminescence in RT-PL spectra of HVPE-AlN was studied. • Deep-level luminescence was disappeared after surface polishing of AlN. • AlN surfaces grown by HVPE at 1450 °C and cooled with NH 3 are contaminated by Si. • Surface contamination by Si generates Al vacancies (V Al ) near the AlN surface. • Recombination of the shallow donor (Si) and V Al resulted in an emission at 3.3 eV.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Tojo, Shunsuke and Yamamoto, Reo and Tanaka, Ryohei and Thieu, Quang Tu and Togashi, Rie and Nagashima, Toru and Kinoshita, Toru and Dalmau, Rafael and Schlesser, Raoul and Murakami, Hisashi and et al.}, year={2016}, month={Jul}, pages={33–38} }
 @article{snyder_kirste_collazoa_ivanisevic_2016, title={Nanoscale topography, semiconductor polarity and surface functionalization: additive and cooperative effects on PC12 cell behavior}, volume={6}, ISSN={["2046-2069"]}, DOI={10.1039/c6ra21936e}, abstractNote={This work compares the behavior of PC12 cells on planar and patterned III-nitride materials with nanostructured topographies.}, number={100}, journal={RSC ADVANCES}, author={Snyder, Patrick J. and Kirste, Ronny and Collazoa, Ramon and Ivanisevic, Albena}, year={2016}, pages={97873–97881} }
 @article{abate_seidlitz_fali_gamage_babicheva_yakovlev_stockman_collazo_alden_dietz_2016, title={Nanoscopy of Phase Separation in InxGa1-xN Alloys}, volume={8}, ISSN={["1944-8244"]}, DOI={10.1021/acsami.6b06766}, abstractNote={Phase separations in ternary/multinary semiconductor alloys is a major challenge that limits optical and electronic internal device efficiency. We have found ubiquitous local phase separation in In1-xGaxN alloys that persists to nanoscale spatial extent by employing high-resolution nanoimaging technique. We lithographically patterned InN/sapphire substrates with nanolayers of In1-xGaxN down to few atomic layers thick that enabled us to calibrate the near-field infrared response of the semiconductor nanolayers as a function of composition and thickness. We also developed an advanced theoretical approach that considers the full geometry of the probe tip and all the sample and substrate layers. Combining experiment and theory, we identified and quantified phase separation in epitaxially grown individual nanoalloys. We found that the scale of the phase separation varies widely from particle to particle ranging from all Ga- to all In-rich regions and covering everything in between. We have found that between 20 and 25% of particles show some level of Ga-rich phase separation over the entire sample region, which is in qualitative agreement with the known phase diagram of In1-xGaxN system.}, number={35}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Abate, Yohannes and Seidlitz, Daniel and Fali, Alireza and Gamage, Sampath and Babicheva, Viktoriia and Yakovlev, Vladislav S. and Stockman, Mark I. and Collazo, Ramon and Alden, Dorian and Dietz, Nikolaus}, year={2016}, month={Sep}, pages={23160–23166} }
 @article{bain_kirste_johnson_ghashghaei_collazo_ivanisevic_2016, title={Neurotypic cell attachment and growth on III-nitride lateral polarity structures}, volume={58}, ISSN={["1873-0191"]}, DOI={10.1016/j.msec.2015.09.084}, abstractNote={III-nitride materials have recently received increasing levels of attention for their potential to successfully interface with, and sense biochemical interactions in biological systems. Expanding on available sensing schemes (including transistor-based devices,) a III-N lateral polarity structure capable of introducing quasi-phase matching through a periodic polarity grating presents a novel platform for second harmonic generation. This platform constitutes a non-linear optical phenomenon with exquisite sensitivity to the chemical state of a surface or interface. To characterize the response of a biological system to the nanostructured lateral polarity structures, we cultured neurotypic PC12 cells on AlGaN with varying ratios of Al:Ga - 0, 0.4, 0.6, and 1 - and on surfaces of varying pitch to the III-polar vs. N-polar grating - 5, 10, 20 and 50 μm. While some toxicity associated with increasing Al is observed, we documented and quantified trends in cell responses to the local material polarity and nanoscale roughness. The nitrogen-polar material has a significantly higher nanoscale roughness than III-polar regions, and a 80-200 nm step height difference between the III-polar and N-polar materials in the lateral polarity configuration generates adequate changes in topography to influence cell growth, improves cell adhesion and promotes cell migration along the direction of the features. As the designed material configuration is further explored for biochemical sensing, the lateral polarity scheme may provide a route in assessing the non-specific protein adsorption to this varying nano-topography that drives the subsequent cell response.}, journal={MATERIALS SCIENCE & ENGINEERING C-MATERIALS FOR BIOLOGICAL APPLICATIONS}, author={Bain, L. E. and Kirste, R. and Johnson, C. A. and Ghashghaei, H. T. and Collazo, R. and Ivanisevic, A.}, year={2016}, month={Jan}, pages={1194–1198} }
 @article{berg_franke_kirste_collazo_ivanisevic_2016, title={Photoluminescence changes of III-Nitride lateral polarity structures after chemical functionalization}, volume={3}, ISSN={["2053-1591"]}, DOI={10.1088/2053-1591/3/12/125906}, abstractNote={The photoluminescence changes of a III-Nitride semiconductor with various surface topographies were studied after chemical functionalization. AlxGa1−xN with a composition of 70% aluminum was used and the surfaces were functionalized with a fluorophore dye-terminated peptide using a linker molecule. The stability of the wafers in water was studied using inductively coupled plasma mass spectrometry prior to modifying the material. The leaching data demonstrated that the AlGaN material in highly stable in biological conditions over 7 d. The attachment of the dye to the wafer was investigated using x-ray photoelectron spectroscopy and photoluminescence spectroscopy (PL). The PL spectrum showed a clear signature of the dye with a pronounced emission peak at approximately 260 nm, indicating a successful attachment to the surface.}, number={12}, journal={MATERIALS RESEARCH EXPRESS}, author={Berg, Nora G. and Franke, Alexander and Kirste, Ronny and Collazo, Ramon and Ivanisevic, Albena}, year={2016}, month={Dec} }
 @article{reddy_hoffmann_kaess_bryan_bryan_bobea_klump_tweedie_kirste_mita_et al._2016, title={Point defect reduction in wide bandgap semiconductors by defect quasi Fermi level control}, volume={120}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/1.4967397}, DOI={10.1063/1.4967397}, abstractNote={A theoretical framework for a general approach to reduce point defect density in materials via control of defect quasi Fermi level (dQFL) is presented. The control of dQFL is achieved via excess minority carrier generation. General guidelines for controlling dQFL that lead to a significant reduction in compensating point defects in any doped material is proposed. The framework introduces and incorporates the effects of various factors that control the efficacy of the defect reduction process such as defect level, defect formation energy, bandgap, and excess minority carrier density. Modified formation energy diagrams are proposed, which illustrate the effect of the quasi Fermi level control on the defect formation energies. These formation energy diagrams provide powerful tools to determine the feasibility and requirements to produce the desired reduction in specified point defects. An experimental study of the effect of excess minority carriers on point defect incorporation in GaN and AlGaN shows an excellent quantitative agreement with the theoretical predictions. Illumination at energies larger than the bandgap is employed as a means to generate excess minority carriers. The case studies with CN in Si doped GaN, H and VN in Mg doped GaN and VM-2ON in Si doped Al0.65Ga0.35N revealed a significant reduction in impurities in agreement with the proposed theory. Since compensating point defects control the material performance (this is particularly challenging in wide and ultra wide bandgap materials), dQFL control is a highly promising technique with wide scope and may be utilized to improve the properties of various materials systems and performance of devices based upon them.}, number={18}, journal={JOURNAL OF APPLIED PHYSICS}, publisher={AIP Publishing}, author={Reddy, P. and Hoffmann, M. P. and Kaess, F. and Bryan, Z. and Bryan, I. and Bobea, M. and Klump, A. and Tweedie, J. and Kirste, R. and Mita, S. and et al.}, year={2016}, month={Nov} }
 @article{mohn_stolyarchuk_markurt_kirste_hoffmann_collazo_courville_di felice_sitar_vennegues_et al._2016, title={Polarity Control in Group-III Nitrides beyond Pragmatism}, volume={5}, ISSN={["2331-7019"]}, DOI={10.1103/physrevapplied.5.054004}, abstractNote={$P\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y$ $e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$ for applications in electronics and nonlinear optics requires the reliable, controllable growth of uniformly polar layers of a compound semiconductor on a nonpolar substrate. So far, this has been developed on a purely empirical basis. The authors use transmission electron microscopy and density-functional theory to see how exchange of aluminum, oxygen, and nitrogen within a sapphire substrate mediates the polarity of an epitaxial film of group-III nitride. Their results shed new light on familiar concepts like substrate nitridation and low-temperature buffers, and may further help to understand polarity control in oxides and other materials.}, number={5}, journal={PHYSICAL REVIEW APPLIED}, author={Mohn, Stefan and Stolyarchuk, Natalia and Markurt, Toni and Kirste, Ronny and Hoffmann, Marc P. and Collazo, Ramon and Courville, Aimeric and Di Felice, Rosa and Sitar, Zlatko and Vennegues, Philippe and et al.}, year={2016}, month={May} }
 @article{losego_paisley_craft_lam_sachet_mita_collazo_sitar_maria_2016, title={Selective area epitaxy of magnesium oxide thin films on gallium nitride surfaces}, volume={31}, ISSN={["2044-5326"]}, DOI={10.1557/jmr.2015.332}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Losego, Mark D. and Paisley, Elizabeth A. and Craft, H. Spalding and Lam, Peter G. and Sachet, Edward and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2016}, month={Jan}, pages={36–45} }
 @article{franke_hoffmann_hernandez-balderrama_kaess_bryan_washiyama_bobea_tweedie_kirste_gerhold_et al._2016, title={Strain engineered high reflectivity DBRs in the deep UV}, volume={9748}, ISSN={["1996-756X"]}, DOI={10.1117/12.2211700}, abstractNote={The maximum achievable reflectivity of current III-nitride Bragg reflectors in the UV-C spectral range is limited due to plastic relaxation of thick multilayer structures. Cracking due to a large mismatch of the thermal expansion and lattice constants between AlxGa1-xN/AlyGa1-yN alloys of different composition and the substrate at the heterointerface is the common failure mode. Strain engineering and strain relaxation concepts by the growth on a strain reduced Al0.85Ga0.15N template and the implementation of low temperature interlayers is demonstrated. A significant enhancement of the maximum reflectivity above 97% at a resonance wavelength of 270 nm due to an increase of the critical thickness of our AlN/Al0.65Ga0.35N DBRs to 1.45 μm (25.5 pairs) prove their potential. By comparing the growth of identical Bragg reflectors on different pseudo-templates, the accumulated mismatch strain energy in the DBR, not the dislocation density provided by the template/substrate, was identified to limit the critical thickness. To further enhance the reflectivity low temperature interlays were implemented into the DBR to partially relief the misfit strain. Relaxation is enabled by the nucleation of small surface domains facilitating misfit dislocation injection and glide. Detailed structural and optical investigations will be conducted to prove the influence of the LT-AlN interlayers on the strain state, structural integrity and reflectivity properties. Coherent growth and no structural and optical degradation of the Bragg mirror properties was observed proving the fully applicability of the relaxation concept to fabricate thick high reflectivity DBR and vertical cavity laser structures.}, journal={GALLIUM NITRIDE MATERIALS AND DEVICES XI}, author={Franke, A. and Hoffmann, P. and Hernandez-Balderrama, L. and Kaess, F. and Bryan, I. and Washiyama, S. and Bobea, M. and Tweedie, J. and Kirste, R. and Gerhold, M. and et al.}, year={2016} }
 @article{bryan_bryan_mita_rice_tweedie_collazo_sitar_2016, title={Surface kinetics in AlN growth: A universal model for the control of surface morphology in III-nitrides}, volume={438}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2015.12.022}, abstractNote={AlN epitaxial thin films were grown on both vicinal (0001)-oriented native single crystal AlN substrates and AlN templates grown on vicinal (0001)-oriented sapphire to develop a surface kinetic framework for the control of surface morphology. A Burton, Cabrera, and Frank (BCF) theory-based model is formulated and utilized to understand the dependence of the surface kinetics on the vapor supersaturation, σ, and substrate misorientation angle, α. The surface energy of the Al-polar surface of AlN was experimentally determined using BCF theory to be 149±8 meV/Å2. The critical misorientation angle for the onset of step-bunching was determined to be ~0.25° for a growth rate of 500 nm/h and temperature of 1250 °C. Transitioning from a surface with 2D nuclei to one with bilayer steps required a decrease in σ or an increase in α, whereas the suppression of step-bunching required an increase in σ or a decrease in α.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Bryan, Isaac and Bryan, Zachary and Mita, Seiji and Rice, Anthony and Tweedie, James and Collazo, Ramon and Sitar, Zlatko}, year={2016}, month={Mar}, pages={81–89} }
 @article{kaess_reddy_alden_klump_hernandez-balderrama_franke_kirste_hoffmann_collazo_sitar_et al._2016, title={The effect of illumination power density on carbon defect configuration in silicon doped GaN}, volume={120}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/1.4972468}, DOI={10.1063/1.4972468}, abstractNote={A study of efficacy of point defect reduction via Fermi level control during growth of GaN:Si as a function of above bandgap illumination power density and hence excess minority carrier density is presented. Electrical characterization revealed an almost two-fold increase in carrier concentration and a three-fold increase in mobility by increasing the illumination power density from 0 to 1 W cm−2, corroborating a decrease in compensation and ionic impurity scattering. The effect was further supported by the photoluminescence studies, which showed a monotonic decrease in yellow luminescence (attributed to CN) as a function of illumination power density. Secondary ion mass spectroscopy studies showed no effect of illumination on the total incorporation of Si or C. Thus, it is concluded that Fermi level management changed the configuration of the C impurity as the CN−1 configuration became energetically less favorable due to excess minority carriers.}, number={23}, journal={JOURNAL OF APPLIED PHYSICS}, publisher={AIP Publishing}, author={Kaess, Felix and Reddy, Pramod and Alden, Dorian and Klump, Andrew and Hernandez-Balderrama, Luis H. and Franke, Alexander and Kirste, Ronny and Hoffmann, Axel and Collazo, Ramón and Sitar, Zlatko and et al.}, year={2016}, month={Dec} }
 @article{bryan_bryan_mita_rice_hussey_shelton_tweedie_maria_collazo_sitar_2016, title={The role of surface kinetics on composition and quality of AlGaN}, volume={451}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2016.06.055}, abstractNote={Metal–polar, Al-rich AlGaN films were grown on both single crystalline AlN and sapphire substrates. The role of surface morphology and surface kinetics on AlGaN composition is presented. With the reduced dislocation density of the films grown on AlN substrates, atomically smooth bilayer stepped surfaces are achieved with RMS roughness of less than 50 pm for a 5×5 µm2 AFM scan area. By controlling the surface supersaturation through adjusting the growth rate, a transition from 2D nucleation to step flow was observed. The critical misorientation angle for step-bunching in nominal Al0.70Ga0.30N grown with a growth rate of 600 nm/h on AlN substrates was found to be 0.4°. The composition of bilayer stepped AlGaN was strongly dependent on substrate misorientation angle, where a compositional variation by a factor of two for a change in misorientation angle from 0.05 to 0.40° was observed; this is explained by the different surface diffusion lengths of Ga and Al. Step-bunching resulted in strong compositional inhomogeneity as observed by photoluminescence and scanning transmission electron microscopy studies.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Bryan, Isaac and Bryan, Zachary and Mita, Seiji and Rice, Anthony and Hussey, Lindsay and Shelton, Christopher and Tweedie, James and Maria, Jon-Paul and Collazo, Ramon and Sitar, Zlatko}, year={2016}, month={Oct}, pages={65–71} }
 @article{lamprecht_grund_neuschl_thonke_bryan_collazo_sitar_2016, title={Very slow decay of a defect related emission band at 2.4 eV in AlN: Signatures of the Si related shallow DX state}, volume={119}, ISSN={["1089-7550"]}, DOI={10.1063/1.4946828}, abstractNote={We report on a defect related luminescence band at 2.4 eV in aluminum nitride bulk crystals, for which we find strong indications to be related to silicon DX centers. Time resolved photoluminescence spectroscopy using a sub-bandgap excitation reveals two different recombination processes with very long decay times of 13 ms and 153 ms at low temperature. Based on the results of temperature and excitation dependent photoluminescence experiments, the process with the shorter lifetime is assigned to a donor-acceptor pair transition involving the shallow silicon donor state, which can be emptied with a thermal dissociation energy of 65 meV. The slower process with a thermal quenching energy of 15 meV is assigned to the slightly deeper Si DX state known from electron paramagnetic resonance experiments, which is transferred back to the shallow donor state.}, number={15}, journal={JOURNAL OF APPLIED PHYSICS}, author={Lamprecht, M. and Grund, C. and Neuschl, B. and Thonke, K. and Bryan, Z. and Collazo, R. and Sitar, Z.}, year={2016}, month={Apr} }
 @article{haidet_bryan_reddy_bryan_collazo_sitar_2015, title={A conduction model for contacts to Si-doped AlGaN grown on sapphire and single-crystalline AlN}, volume={117}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/1.4923062}, DOI={10.1063/1.4923062}, abstractNote={Ohmic contacts to AlGaN grown on sapphire substrates have been previously demonstrated for various compositions of AlGaN, but contacts to AlGaN grown on native AlN substrates are more difficult to obtain. In this paper, a model is developed that describes current flow through contacts to Si-doped AlGaN. This model treats the current through reverse-biased Schottky barriers as a consequence of two different tunneling-dependent conduction mechanisms in parallel, i.e., Fowler-Nordheim emission and defect-assisted Frenkel-Poole emission. At low bias, the defect-assisted tunneling dominates, but as the potential across the depletion region increases, tunneling begins to occur without the assistance of defects, and the Fowler-Nordheim emission becomes the dominant conduction mechanism. Transfer length method measurements and temperature-dependent current-voltage (I-V) measurements of Ti/Al-based contacts to Si-doped AlGaN grown on sapphire and AlN substrates support this model. Defect-assisted tunneling plays a much larger role in the contacts to AlGaN on sapphire, resulting in nearly linear I-V characteristics. In contrast, contacts to AlGaN on AlN show limited defect-assisted tunneling appear to be only semi-Ohmic.}, number={24}, journal={JOURNAL OF APPLIED PHYSICS}, publisher={AIP Publishing}, author={Haidet, Brian B. and Bryan, Isaac and Reddy, Pramod and Bryan, Zachary and Collazo, Ramon and Sitar, Zlatko}, year={2015}, month={Jun} }
 @article{bain_hoffmann_bryan_collazo_ivanisevic_2015, title={Adsorption and adhesion of common serum proteins to nanotextured gallium nitride}, volume={7}, ISSN={["2040-3372"]}, DOI={10.1039/c4nr06353h}, abstractNote={Morphology can be used to modulate the adsorption of proteins onto gallium nitride.}, number={6}, journal={NANOSCALE}, author={Bain, Lauren E. and Hoffmann, Marc P. and Bryan, Isaac and Collazo, Ramon and Ivanisevic, Albena}, year={2015}, pages={2360–2365} }
 @article{reddy_bryan_bryan_tweedie_washiyama_kirste_mita_collazo_sitar_2015, title={Charge neutrality levels, barrier heights, and band offsets at polar AlGaN}, volume={107}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4930026}, DOI={10.1063/1.4930026}, abstractNote={In this work, the Fermi level and band alignment at c-plane surfaces and interfaces of AlGaN thin films grown on sapphire and native single crystalline AlN substrates were analyzed via x-ray photoelectron spectroscopy. The dependence of charge neutrality level (CNL) on Al composition is found to be linear with n-type Schottky barrier heights (Φbn) exhibiting an overall quadratic behavior due to bandgap bowing. A general theoretical expression for Schottky barrier height on AlGaN is determined as a function of Al composition and metal electronegativity utilizing the interface induced gap states (IFIGS) model and is corroborated with current-voltage (I-V) characterization on Ni-based Schottky diodes. The measured CNLs were used to determine the conduction and valence band offsets in AlGaN hetero-junctions according to the IFIGS and were found to be split approximately 2/3 and 1/3 of the bandgap, respectively, at interfaces with AlGaN having comparable Al and Ga concentrations. Nonlinearities in Φbn result in an increase (>2/3) and decrease (<2/3) of the conduction band split at AlN/AlGaN and GaN/AlGaN interfaces, respectively. Characterization of core level binding energies revealed a composition-independent surface work function. Consequently, electron affinity is found to be a linear function of the barrier height at the CNL and band offsets determined via IFIGS and Anderson's rule were found to be identical. The origin of the bandgap bowing arising in the conduction band is proposed.}, number={9}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Reddy, Pramod and Bryan, Isaac and Bryan, Zachary and Tweedie, James and Washiyama, Shun and Kirste, Ronny and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko}, year={2015}, month={Aug} }
 @article{kirste_rohrbaugh_bryan_bryan_collazo_ivanisevic_2015, title={Electronic Biosensors Based on III-Nitride Semiconductors}, volume={8}, ISSN={["1936-1327"]}, DOI={10.1146/annurev-anchem-071114-040247}, abstractNote={ We review recent advances of AlGaN/GaN high-electron-mobility transistor (HEMT)-based electronic biosensors. We discuss properties and fabrication of III-nitride-based biosensors. Because of their superior biocompatibility and aqueous stability, GaN-based devices are ready to be implemented as next-generation biosensors. We review surface properties, cleaning, and passivation as well as different pathways toward functionalization, and critically analyze III-nitride-based biosensors demonstrated in the literature, including those detecting DNA, bacteria, cancer antibodies, and toxins. We also discuss the high potential of these biosensors for monitoring living cardiac, fibroblast, and nerve cells. Finally, we report on current developments of covalent chemical functionalization of III-nitride devices. Our review concludes with a short outlook on future challenges and projected implementation directions of GaN-based HEMT biosensors. }, journal={ANNUAL REVIEW OF ANALYTICAL CHEMISTRY, VOL 8}, author={Kirste, Ronny and Rohrbaugh, Nathaniel and Bryan, Isaac and Bryan, Zachary and Collazo, Ramon and Ivanisevic, Albena}, year={2015}, pages={149–169} }
 @article{kinoshita_nagashima_obata_takashima_yamamoto_togashi_kumagai_schlesser_collazo_koukitu_et al._2015, title={Fabrication of vertical Schottky barrier diodes on n-type freestanding AIN substrates grown by hydride vapor phase epitaxy}, volume={8}, ISSN={["1882-0786"]}, DOI={10.7567/apex.8.061003}, abstractNote={Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 × 1014 cm−3 was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2 V was observed. The ideality factor of the diode at room temperature was estimated to be ∼8. The reverse breakdown voltage, defined as the leakage current level of 10−3 A/cm2, ranged from 550 to 770 V.}, number={6}, journal={APPLIED PHYSICS EXPRESS}, author={Kinoshita, Toru and Nagashima, Toru and Obata, Toshiyuki and Takashima, Shinya and Yamamoto, Reo and Togashi, Rie and Kumagai, Yoshinao and Schlesser, Raoul and Collazo, Ramon and Koukitu, Akinori and et al.}, year={2015}, month={Jun} }
 @article{hoffmann_kirste_mita_guo_tweedie_bobea_bryan_bryan_gerhold_collazo_et al._2015, title={Growth and characterization of AlxGa1-xN lateral polarity structures}, volume={212}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.201431740}, abstractNote={Abstract}, number={5}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Hoffmann, Marc Patrick and Kirste, Ronny and Mita, Seiji and Guo, Wei and Tweedie, James and Bobea, Milena and Bryan, Isaac and Bryan, Zachary and Gerhold, Michael and Collazo, Ramon and et al.}, year={2015}, month={May}, pages={1039–1042} }
 @article{bryan_bryan_xie_mita_sitar_collazo_2015, title={High internal quantum efficiency in AlGaN multiple quantum wells grown on bulk AlN substrates}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4917540}, abstractNote={The internal quantum efficiency (IQE) of Al0.55Ga0.45N/AlN and Al0.55Ga0.45N/Al0.85Ga0.15N UVC MQW structures was analyzed. The use of bulk AlN substrates enabled us to undoubtedly distinguish the effect of growth conditions, such as V/III ratio, on the optical quality of AlGaN based MQWs from the influence of dislocations. At a high V/III ratio, a record high IQE of ∼80% at a carrier density of 1018 cm−3 was achieved at ∼258 nm. The high IQE was correlated with the decrease of the non-radiative coefficient A and a reduction of midgap defect luminescence, all suggesting that, in addition to dislocations, point defects are another major factor that strongly influences optical quality of AlGaN MQW structures.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Bryan, Zachary and Bryan, Isaac and Xie, Jinqiao and Mita, Seiji and Sitar, Zlatko and Collazo, Ramon}, year={2015}, month={Apr} }
 @article{guo_kirste_bryan_bryan_hussey_reddy_tweedie_collazo_sitar_2015, title={KOH based selective wet chemical etching of AlN, AlxGa1-xN, and GaN crystals: A way towards substrate removal in deep ultraviolet-light emitting diode}, volume={106}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4913705}, DOI={10.1063/1.4913705}, abstractNote={A controllable and smooth potassium hydroxide-based wet etching technique was developed for the AlGaN system. High selectivity between AlN and AlxGa1−xN (up to 12×) was found to be critical in achieving effective substrate thinning or removal for AlGaN-based deep ultraviolet light emitting diodes, thus increasing light extraction efficiency. The mechanism of high selectivity of AlGaN as a function of Al composition can be explained as related to the formation and dissolution of oxide/hydroxide on top of N-polar surface. Cross-sectional transmission electron microscopic analysis served as ultimate proof that these hillocks were not related to underlying threading dislocations.}, number={8}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Guo, W. and Kirste, R. and Bryan, I. and Bryan, Z. and Hussey, L. and Reddy, P. and Tweedie, J. and Collazo, R. and Sitar, Z.}, year={2015}, month={Feb} }
 @article{rohrbaugh_bryan_bryan_collazo_ivanisevic_2015, title={Long-term stability assessment of AlGaN/GaN field effect transistors modified with peptides: Device characteristics vs. surface properties}, volume={5}, ISSN={["2158-3226"]}, DOI={10.1063/1.4930192}, abstractNote={AlGaN/GaN Field Effect Transistors (FETs) are promising biosensing devices. Functionalization of these devices is explored in this study using an in situ approach with phosphoric acid etchant and a phosphonic acid derivative. Devices are terminated on peptides and soaked in water for up to 168 hrs to examine FETs for both device responses and surface chemistry changes. Measurements demonstrated threshold voltage shifting after the functionalization and soaking processes, but demonstrated stable FET behavior throughout. X-ray photoelectron spectroscopy and atomic force microscopy confirmed peptides attachment to device surfaces before and after water soaking. Results of this work point to the stability of peptide coated functionalized AlGaN/GaN devices in solution and support further research of these devices as disposable, long term, in situ biosensors.}, number={9}, journal={AIP ADVANCES}, author={Rohrbaugh, Nathaniel and Bryan, Isaac and Bryan, Zachary and Collazo, Ramon and Ivanisevic, Albena}, year={2015}, month={Sep} }
 @article{guo_kirste_bryan_bryan_gerhold_collazo_sitar_2015, title={Nanostructure surface patterning of GaN thin films and application to AlGaN/AlN multiple quantum wells: A way towards light extraction efficiency enhancement of III-nitride based light emitting diodes}, volume={117}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4915903}, DOI={10.1063/1.4915903}, abstractNote={Enhanced light extraction efficiency was demonstrated on nanostructure patterned GaN and AlGaN/AlN Multiple-Quantum-Well (MQW) structures using mass production techniques including natural lithography and interference lithography with feature size as small as 100 nm. Periodic nanostructures showed higher light extraction efficiency and modified emission profile compared to non-periodic structures based on integral reflection and angular-resolved transmission measurement. Light extraction mechanism of macroscopic and microscopic nanopatterning is discussed, and the advantage of using periodic nanostructure patterning is provided. An enhanced photoluminescence emission intensity was observed on nanostructure patterned AlGaN/AlN MQW compared to as-grown structure, demonstrating a large-scale and mass-producible pathway to higher light extraction efficiency in deep-ultra-violet light-emitting diodes.}, number={11}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Guo, Wei and Kirste, Ronny and Bryan, Zachary and Bryan, Isaac and Gerhold, Michael and Collazo, Ramón and Sitar, Zlatko}, year={2015}, month={Mar}, pages={113107} }
 @article{rigler_buh_hoffmann_kirste_bobea_mita_gerhold_collazo_sitar_zgonik_2015, title={Optical characterization of Al- and N-polar AlN waveguides for integrated optics}, volume={8}, ISSN={["1882-0786"]}, DOI={10.7567/apex.8.042603}, abstractNote={Dispersion of the extraordinary and ordinary refractive indices of Al- and N-polar AlN waveguides is measured by multiple angle-of-incidence and spectroscopic ellipsometry techniques. The polarity-controlled AlN layers are grown by metal–organic chemical vapor deposition on (0001)-sapphire substrates. Taking into consideration the different surface morphologies of the Al- and N-polar AlN waveguides, we propose two optical models to describe the measured ellipsometry data. The results indicate that there is no difference between the refractive indices of the AlN grown in opposite directions, which confirms the potential of the AlN lateral polar structures for use in nonlinear optical applications based on quasi phase matching.}, number={4}, journal={APPLIED PHYSICS EXPRESS}, author={Rigler, Martin and Buh, Joze and Hoffmann, Marc P. and Kirste, Ronny and Bobea, Milena and Mita, Seiji and Gerhold, Michael D. and Collazo, Ramon and Sitar, Zlatko and Zgonik, Marko}, year={2015}, month={Apr} }
 @article{majkic_puc_franke_kirste_collazo_sitar_zgonik_2015, title={Optical properties of aluminum nitride single crystals in the THz region}, volume={5}, ISSN={["2159-3930"]}, DOI={10.1364/ome.5.002106}, abstractNote={We report on measurements of the refractive indices and the absorption in bulk single crystals of aluminum nitride, in the region from 1 to 8 THz. The birefringence is approximately 0.2 and is larger than in the optical frequency range. Both indices exhibit normal dispersion with no pronounced absorption resonances. Optical power loss coefficients are approximately 2 cm−1 and 4 cm−1 and the estimated static dielectric constants are 7.84 and 9.22, for the ordinary and extraordinary polarization, respectively.}, number={10}, journal={OPTICAL MATERIALS EXPRESS}, author={Majkic, Aleksej and Puc, Uros and Franke, Alexander and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko and Zgonik, Marko}, year={2015}, month={Oct}, pages={2106–2111} }
 @article{bryan_bryan_mita_tweedie_sitar_collazo_2015, title={Strain dependence on polarization properties of AlGaN and AlGaN-based ultraviolet lasers grown on AlN substrates}, volume={106}, ISSN={["1077-3118"]}, DOI={10.1063/1.4922385}, abstractNote={Since the band ordering in AlGaN has a profound effect on the performance of UVC light emitting diodes (LEDs) and even determines the feasibility of surface emitting lasers, the polarization properties of emitted light from c-oriented AlGaN and AlGaN-based laser structures were studied over the whole composition range, as well as various strain states, quantum confinements, and carrier densities. A quantitative relationship between the theoretical valence band separation, determined using k•p theory, and the experimentally measured degree of polarization is presented. Next to composition, strain was found to have the largest influence on the degree of polarization while all other factors were practically insignificant. The lowest crossover point from the transverse electric to transverse magnetic polarized emission of 245 nm was found for structures pseudomorphically grown on AlN substrates. This finding has significant implications toward the efficiency and feasibility of surface emitting devices below this wavelength.}, number={23}, journal={APPLIED PHYSICS LETTERS}, author={Bryan, Zachary and Bryan, Isaac and Mita, Seiji and Tweedie, James and Sitar, Zlatko and Collazo, Ramon}, year={2015}, month={Jun} }
 @article{rohrbaugh_bryan_bryan_arellano_collazo_ivanisevic_2014, title={AlGaN/GaN field effect transistors functionalized with recognition peptides}, volume={105}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4896962}, DOI={10.1063/1.4896962}, abstractNote={Recognition peptides are used to modify AlGaN/GaN field effect transistors. The recognition sequence, SVSVGMKPSPRP, was compared to other biomolecules and subsequently the device stability was examined. Changes in the electrical characteristic were recorded using current-voltage measurements at a VD of 1 V and VG of −1 V. The recognition sequence coatings yielded an average increase in ID of 96.43% compared to initial values. Exposure to solution removed the peptides from the devices indicating a weak interaction between adsorbate and the semiconductor surfaces. The peptide coatings are suitable for simple device modification for short-term recognition studies.}, number={13}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Rohrbaugh, N. and Bryan, I. and Bryan, Z. and Arellano, C. and Collazo, R. and Ivanisevic, A.}, year={2014}, month={Sep}, pages={134103} }
 @article{rohrbaugh_bryan_bryan_arellano_collazo_ivanisevic_2014, title={AlGaN/GaN field effect transistors functionalized with recognition peptides}, volume={105}, number={13}, journal={Applied Physics Letters}, author={Rohrbaugh, N. and Bryan, I. and Bryan, Z. and Arellano, C. and Collazo, R. and Ivanisevic, A.}, year={2014} }
 @article{hussey_bryan_kirste_guo_bryan_mita_collazo_sitar_2014, title={Direct Observation of the Polarity Control Mechanism in Aluminum Nitride Grown on Sapphire by Aberration Corrected Scanning Transmission Electron Microscopy}, volume={20}, ISSN={1431-9276 1435-8115}, url={http://dx.doi.org/10.1017/S1431927614002530}, DOI={10.1017/S1431927614002530}, abstractNote={.}, number={S3}, journal={Microscopy and Microanalysis}, publisher={Cambridge University Press (CUP)}, author={Hussey, Lindsay and Bryan, Isaac and Kirste, Ronny and Guo, Wei and Bryan, Zachary and Mita, Seiji and Collazo, Ramón and Sitar, Zlatko}, year={2014}, month={Aug}, pages={162–163} }
 @article{rohrbaugh_bryan_bryan_collazo_ivanisevic_2014, title={Effects of Environmental Exposure on Stability and Conductance Poly-l-lysine Coated AlGaN/GaN High Electron Mobility Transistors}, volume={61}, ISSN={["1938-6737"]}, DOI={10.1149/06104.0147ecst}, abstractNote={AlGaN/GaN HEMTs have been shown promise to be used as in-situ biosensors. An inquiry was done into the stability of physisorption as a method of adhering surface bioactive molecules to the surface of the HEMT gate contacts. Poly-l-lysine (30,000 MW) was used to coat AlGaN/GaN field effect transistors as a representative charged biomolecule for biosensing applications. Coated FETs were then exposed to 1 and 4 hour soaks in DI water at ambient temperature. Atomic force microscopy (AFM) and conductance measurements (IV) were used to quantify current decay and surface morphology changes. Analysis of the results showed that charged analytes readily adsorbed onto the surface.  Upon adsorption an immediate signal decay was observed. The changes after soaking in solution were variable.  The results point to the need to develop specific chemistry that will result in stable immobilization of biomolecules onto the device surface.  Such stability is necessary for long- term reliable device operation.}, number={4}, journal={WIDE BANDGAP SEMICONDUCTOR MATERIALS AND DEVICES 15}, author={Rohrbaugh, N. and Bryan, I. and Bryan, Z. and Collazo, R. and Ivanisevic, A.}, year={2014}, pages={147–151} }
 @article{bryan_bryan_bobea_hussey_kirste_sitar_collazo_2014, title={Exciton transitions and oxygen as a donor in m-plane AlN homoepitaxial films}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4870284}, abstractNote={High-resolution photoluminescence studies on m-plane (1-100) homoepitaxial films grown by metalorganic chemical vapor deposition on AlN revealed several sharp donor-bound exciton (DBX) peaks with a full width at half maximum as narrow as 550 μeV. Power dependent photoluminescence distinguished DBXs tied to the Γ5 free exciton (FX) from those tied to the Γ1 FX. Both the n = 2 and n = 1 excited states of the Γ5 and Γ1 were resolved, giving binding energies of 52 meV and 55 meV, respectively. The DBX transition at 6.006 eV was identified as originating from the neutral-donor-oxygen (O0X). This assignment was based on secondary ion mass spectroscopy measurements, peak position with respect to the Si0X, and deep defect luminescence peaks located at 3.25 eV and 3.58 eV.}, number={13}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bryan, Zachary and Bryan, Isaac and Bobea, Milena and Hussey, Lindsay and Kirste, Ronny and Sitar, Zlatko and Collazo, Ramon}, year={2014}, month={Apr} }
 @article{bryan_bryan_gaddy_reddy_hussey_bobea_guo_hoffmann_kirste_tweedie_et al._2014, title={Fermi level control of compensating point defects during metalorganic chemical vapor deposition growth of Si-doped AlGaN}, volume={105}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4903058}, DOI={10.1063/1.4903058}, abstractNote={A Fermi-level control scheme for point defect management using above-bandgap UV illumination during growth is presented. We propose an extension to the analogy between the Fermi level and the electrochemical potential such that the electrochemical potential of a charged defect in a material with steady-state populations of free charge carriers may be expressed in terms of the quasi-Fermi levels. A series of highly Si-doped Al0.65Ga0.35N films grown by metalorganic chemical vapor deposition with and without UV illumination showed that samples grown under UV illumination had increased free carrier concentration, free carrier mobility, and reduced midgap photoluminescence all indicating a reduction in compensating point defects.}, number={22}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bryan, Zachary and Bryan, Isaac and Gaddy, Benjamin E. and Reddy, Pramod and Hussey, Lindsay and Bobea, Milena and Guo, Wei and Hoffmann, Marc and Kirste, Ronny and Tweedie, James and et al.}, year={2014}, month={Dec} }
 @article{sochacki_bryan_amilusik_bobea_fijalkowski_bryan_lucznik_collazo_weyher_kucharski_et al._2014, title={HVPE-GaN grown on MOCVD-GaN/sapphire template and ammonothermal GaN seeds: Comparison of structural, optical, and electrical properties}, volume={394}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2014.02.020}, abstractNote={Crystallization of GaN by the HVPE technique on an MOCVD-GaN/sapphire template with photo-lithographically patterned Ti mask and ammonothermally-grown GaN crystals (Am-GaN) was studied and compared. Structural and optical properties of the obtained free-standing HVPE-GaN revealed that Am-GaN seeds produced material of much higher quality in terms of etch pit density (EPD), X-ray rocking curves, and excitonic emission than the sapphire-based templates. The crystallization run on the Am-GaN seed, annealed before the growth in H2+NH3 atmosphere, resulted in the HVPE-GaN of an average EPD of 5×104 cm−2, (002) rocking curve width of 22″, and photo-luminescence as narrow as 130 µeV. However, small change in quality was observed when the Am-GaN seed was annealed in N2+NH3 atmosphere prior to growth. In turn, the Hall effect measurements showed that the HVPE-GaN grown on MOCVD-GaN/sapphire template possessed lower free carrier concentration and higher mobility than the HVPE-GaN grown on the Am-GaN seeds. These results were associated to the higher growth rate of the examined HVPE-GaN sample.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Sochacki, Tomasz and Bryan, Zachary and Amilusik, Mikolaj and Bobea, Milena and Fijalkowski, Michal and Bryan, Isaac and Lucznik, Boleslaw and Collazo, Ramon and Weyher, Jan L. and Kucharski, Robert and et al.}, year={2014}, month={May}, pages={55–60} }
 @article{bryan_bryan_bobea_hussey_kirste_collazo_sitar_2014, title={Homoepitaxial AlN thin films deposited on m-plane (1(1)over-bar00) AlN substrates by metalorganic chemical vapor deposition}, volume={116}, ISSN={["1089-7550"]}, DOI={10.1063/1.4897233}, abstractNote={AlN homoepitaxial films were grown by metalorganic chemical vapor deposition on chemo-mechanically polished (11¯00)-oriented single crystalline AlN substrates. The dependence of the surface morphology, structural quality, and unintentional impurity concentrations on the growth temperature was studied in order to determine the most appropriate growth conditions for high quality (11¯00) AlN epitaxial layers. Optically smooth surfaces (RMS roughness of 0.4 nm) and high crystalline quality, as demonstrated by the presence of FWHM values for (101¯0) rocking curves along [0001] of less than 25 arc·sec, were achieved for films grown above 1350 °C. Furthermore, sharp and intense near band edge luminescence was observed in these high quality films. A reduction in unintentional oxygen impurity levels was seen with an increase in growth temperature. These high crystalline quality films are suitable for device applications and hold great potential for providing an ideal platform for deep UV emitters with high Al content AlGaN without polarization related effects.}, number={13}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bryan, Isaac and Bryan, Zachary and Bobea, Milena and Hussey, Lindsay and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2014}, month={Oct} }
 @article{callsen_wagner_reparaz_nippert_kure_kalinowski_hoffmann_ford_phillips_dalmau_et al._2014, title={Phonon pressure coefficients and deformation potentials of wurtzite AlN determined by uniaxial pressure-dependent Raman measurements}, volume={90}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.90.205206}, abstractNote={We studied bulk crystals of wurtzite AlN by means of uniaxial pressure-dependent Raman measurements. As a result, we derive the phonon pressure coefficients and deformation potentials for all zone center optical phonon modes. For the A1 and E1 modes, we further experimentally determined the uniaxial pressure dependence of their longitudinal optical‐transverse optical (LO-TO) splittings. Our experimental approach delivers new insight into the large variance among previously reported phonon deformation potentials, which are predominantly based on heteroepitaxial growth of AlN and the ball-on-ring technique. Additionally, the measured phonon pressure coefficients are compared to their theoretical counterparts obtained by density functional theory implemented in the SIESTA package. Generally, we observe a good agreement between the calculated and measured phonon pressure coefficients but some particular Raman modes exhibit significant discrepancies similar to the case of wurtzite GaN and ZnO, clearly motivating the presented uniaxial pressure-dependent Raman measurements on bulk AlN crystals.}, number={20}, journal={PHYSICAL REVIEW B}, author={Callsen, G. and Wagner, M. R. and Reparaz, J. S. and Nippert, F. and Kure, T. and Kalinowski, S. and Hoffmann, A. and Ford, M. J. and Phillips, M. R. and Dalmau, R. F. and et al.}, year={2014}, month={Nov} }
 @article{shelton_sachet_paisley_hoffmann_rajan_collazo_sitar_maria_2014, title={Polarity characterization by anomalous x-ray dispersion of ZnO films and GaN lateral polar structures}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4863120}, abstractNote={We demonstrate the use of anomalous x-ray scattering of constituent cations at their absorption edge, in a conventional Bragg-Brentano diffractometer, to measure absolutely and quantitatively the polar orientation and polarity fraction of unipolar and mixed polar wurtzitic crystals. In one set of experiments, the gradual transition between c+ and c− polarity of epitaxial ZnO films on sapphire as a function of MgO buffer layer thickness is monitored quantitatively, while in a second experiment, we map the polarity of a lateral polar homojunction in GaN. The dispersion measurements are compared with piezoforce microscopy images, and we demonstrate how x-ray dispersion and scanning probe methods can provide complementary information that can discriminate between polarity fractions at a material surface and polarity fractions averaged over the film bulk.}, number={4}, journal={JOURNAL OF APPLIED PHYSICS}, author={Shelton, Christopher T. and Sachet, Edward and Paisley, Elizabeth A. and Hoffmann, Marc P. and Rajan, Joseph and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2014}, month={Jan} }
 @article{kirste_mita_hoffmann_hussey_guo_bryan_bryan_tweedie_gerhold_hoffmann_et al._2014, title={Properties of AlN based lateral polarity structures}, volume={11}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201300287}, abstractNote={Abstract}, number={2}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 11, NO 2}, author={Kirste, Ronny and Mita, Seiji and Hoffmann, Marc P. and Hussey, Lindsay and Guo, Wei and Bryan, Isaac and Bryan, Zachary and Tweedie, James and Gerhold, Michael and Hoffmann, Axel and et al.}, year={2014}, pages={261–264} }
 @article{hussey_white_kirste_mita_bryan_guo_osterman_haidet_bryan_bobea_et al._2014, title={Sapphire decomposition and inversion domains in N-polar aluminum nitride}, volume={104}, ISSN={["1077-3118"]}, DOI={10.1063/1.4862982}, abstractNote={Transmission electron microscopy (TEM) techniques and potassium hydroxide (KOH) etching confirmed that inversion domains in the N-polar AlN grown on c-plane sapphire were due to the decomposition of sapphire in the presence of hydrogen. The inversion domains were found to correspond to voids at the AlN and sapphire interface, and transmission electron microscopy results showed a V-shaped, columnar inversion domain with staggered domain boundary sidewalls. Voids were also observed in the simultaneously grown Al-polar AlN, however no inversion domains were present. The polarity of AlN grown above the decomposed regions of the sapphire substrate was confirmed to be Al-polar by KOH etching and TEM.}, number={3}, journal={APPLIED PHYSICS LETTERS}, author={Hussey, Lindsay and White, Ryan M. and Kirste, Ronny and Mita, Seiji and Bryan, Isaac and Guo, Wei and Osterman, Katherine and Haidet, Brian and Bryan, Zachary and Bobea, Milena and et al.}, year={2014}, month={Jan} }
 @article{reddy_bryan_bryan_tweedie_kirste_collazo_sitar_2014, title={Schottky contact formation on polar and non-polar AlN}, volume={116}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/1.4901954}, DOI={10.1063/1.4901954}, abstractNote={The interfaces of m- and c-plane AlN with metals of different work functions and electro-negativities were characterized and the Schottky barrier heights were measured. The Schottky barrier height was determined by measuring the valence band maximum (VBM) with respect to the Fermi level at the surface (interface) before (after) metallization. VBM determination included accurate modeling and curve fitting of density of states at the valence band edge with the XPS data. The experimental behavior of the barrier heights could not be explained by the Schottky-Mott model and was modeled using InterFace-Induced Gap States (IFIGS). A slope parameter (SX) was used to incorporate the density of surface states and is a measure of Fermi level pinning. The experimental barriers followed theoretical predictions with a barrier height at the surface Fermi level (Charge neutrality level (CNL)) of ∼2.1 eV (∼2.7 eV) on m-plane (c-plane) and SX ∼ 0.36 eV/Miedema unit. Slope parameter much lower than 0.86 implied a surface/interface states dominated behavior with significant Fermi level pinning and the measured barrier heights were close to the CNL. Titanium and zirconium provided the lowest barriers (1.6 eV) with gold providing the highest (2.3 eV) among the metals analyzed on m-plane. It was consistently found that barrier heights decreased from metal polar to non-polar surfaces, in general, due to an increasing CNL. The data indicated that charged IFIGS compensate spontaneous polarization charge. These barrier height and slope parameter measurements provided essential information for designing Schottky diodes and other contact-based devices on AlN.}, number={19}, journal={JOURNAL OF APPLIED PHYSICS}, publisher={AIP Publishing}, author={Reddy, Pramod and Bryan, Isaac and Bryan, Zachary and Tweedie, James and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2014}, month={Nov} }
 @article{paisley_gaddy_lebeau_shelton_biegalski_christen_losego_mita_collazo_sitar_et al._2014, title={Smooth cubic commensurate oxides on gallium nitride}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4861172}, abstractNote={Smooth, commensurate alloys of ⟨111⟩-oriented Mg0.52Ca0.48O (MCO) thin films are demonstrated on Ga-polar, c+ [0001]-oriented GaN by surfactant-assisted molecular beam epitaxy and pulsed laser deposition. These are unique examples of coherent cubic oxide|nitride interfaces with structural and morphological perfection. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100× reduction in leakage current density for the surfactant-assisted samples. HAADF-STEM images of the MCO|GaN interface show commensurate alignment of atomic planes with minimal defects due to lattice mismatch. STEM and DFT calculations show that GaN c/2 steps create incoherent boundaries in MCO over layers which manifest as two in-plane rotations and determine consequently the density of structural defects in otherwise coherent MCO. This new understanding of interfacial steps between HCP and FCC crystals identifies the steps needed to create globally defect-free heterostructures.}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Paisley, Elizabeth A. and Gaddy, Benjamin E. and LeBeau, James M. and Shelton, Christopher T. and Biegalski, Michael D. and Christen, Hans M. and Losego, Mark D. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and et al.}, year={2014}, month={Feb} }
 @article{guo_bryan_xie_kirste_mita_bryan_hussey_bobea_haidet_gerhold_et al._2014, title={Stimulated emission and optical gain in AlGaN heterostructures grown on bulk AlN substrates}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4868678}, abstractNote={Optical gain spectra for ∼250 nm stimulated emission were compared in three different AlGaN-based structures grown on single crystalline AlN substrates: a single AlGaN film, a double heterostructure (DH), and a Multiple Quantum Well (MQW) structure; respective threshold pumping power densities of 700, 250, and 150 kW/cm2 were observed. Above threshold, the emission was transverse-electric polarized and as narrow as 1.8 nm without a cavity. The DH and MQW structures showed gain values of 50–60 cm−1 when pumped at 1 MW/cm2. The results demonstrated the excellent optical quality of the AlGaN-based heterostructures grown on AlN substrates and their potential for realizing electrically pumped sub-280 nm laser diodes.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Guo, Wei and Bryan, Zachary and Xie, Jinqiao and Kirste, Ronny and Mita, Seiji and Bryan, Isaac and Hussey, Lindsay and Bobea, Milena and Haidet, Brian and Gerhold, Michael and et al.}, year={2014}, month={Mar} }
 @article{bryan_akouala_tweedie_bryan_rice_kirste_collazo_sitar_2014, title={Surface preparation of non-polar single-crystalline AlN substrates}, volume={11}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201300401}, abstractNote={Abstract}, number={3-4}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 11, NO 3-4}, author={Bryan, Isaac and Akouala, Christer-Rajiv and Tweedie, James and Bryan, Zachary and Rice, Anthony and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2014}, pages={454–457} }
 @article{bain_collazo_hsu_latham_manfra_ivanisevic_2014, title={Surface topography and chemistry shape cellular behavior on wide band-gap semiconductors}, volume={10}, ISSN={1742-7061}, url={http://dx.doi.org/10.1016/J.ACTBIO.2014.02.038}, DOI={10.1016/J.ACTBIO.2014.02.038}, abstractNote={The chemical stability and electrical properties of gallium nitride make it a promising material for the development of biocompatible electronics, a range of devices including biosensors as well as interfaces for probing and controlling cellular growth and signaling. To improve the interface formed between the probe material and the cell or biosystem, surface topography and chemistry can be applied to modify the ways in which the device interacts with its environment. PC12 cells are cultured on as-grown planar, unidirectionally polished, etched nanoporous and nanowire GaN surfaces with and without a physisorbed peptide sequence that promotes cell adhesion. While cells demonstrate preferential adhesion to roughened surfaces over as-grown flat surfaces, the topography of that roughness also influences the morphology of cellular adhesion and differentiation in neurotypic cells. Addition of the peptide sequence generally contributes further to cellular adhesion and promotes development of stereotypic long, thin neurite outgrowths over alternate morphologies. The dependence of cell behavior on both the topographic morphology and surface chemistry is thus demonstrated, providing further evidence for the importance of surface modification for modulating bio-inorganic interfaces.}, number={6}, journal={Acta Biomaterialia}, publisher={Elsevier BV}, author={Bain, Lauren E. and Collazo, Ramon and Hsu, Shu-han and Latham, Nicole Pfiester and Manfra, Michael J. and Ivanisevic, Albena}, year={2014}, month={Jun}, pages={2455–2462} }
 @article{reddy_bryan_bryan_guo_hussey_collazo_sitar_2014, title={The effect of polarity and surface states on the Fermi level at III-nitride surfaces}, volume={116}, ISSN={["1089-7550"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907611759&partnerID=MN8TOARS}, DOI={10.1063/1.4896377}, abstractNote={Surface states and their influence on the Fermi level at the surface of GaN and AlN are studied using x-ray photoelectron spectroscopy (XPS). The effect of polarity on surface electronic properties was studied. Accurate modeling of the valence band edge and comparison with XPS data revealed the presence of donor surface states at 1.4 eV and acceptor states at energies >2.7 eV from the valence band in GaN. Al polar AlN showed acceptor states at energies >3.3 eV. Density of acceptor surface states was estimated to be between 1013 and 1014 eV−1 cm−2 in both GaN and AlN. The shift in charge neutrality levels and barrier heights due to polarity and the density of surface states on AlN and GaN were estimated from XPS measurements. Theoretical modeling and comparison with XPS data implied full compensation of spontaneous polarization charge by charged surface states. Barrier height measurements also reveal a dependence on polarity with ϕmetal-polar > ϕnon-polar > ϕnitrogen-polar suggesting that the N-polar surface is the most suitable for Ohmic contacts.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Reddy, Pramod and Bryan, Isaac and Bryan, Zachary and Guo, Wei and Hussey, Lindsay and Collazo, Ramon and Sitar, Zlatko}, year={2014}, month={Sep} }
 @article{gaddy_bryan_bryan_xie_dalmau_moody_kumagai_nagashima_kubota_kinoshita_et al._2014, title={The role of the carbon-silicon complex in eliminating deep ultraviolet absorption in AlN}, volume={104}, ISSN={["1077-3118"]}, DOI={10.1063/1.4878657}, abstractNote={Co-doping AlN crystals with Si is found to suppress the unwanted 4.7 eV (265 nm) deep ultraviolet absorption associated with isolated carbon acceptors common in materials grown by physical vapor transport. Density functional theory calculations with hybrid functionals demonstrate that silicon forms a stable nearest-neighbor defect complex with carbon. This complex is predicted to absorb at 5.5 eV and emit at or above 4.3 eV. Absorption and photoluminescence measurements of co-doped samples confirm the presence of the predicted CN-SiAl complex absorption and emission peaks and significant reduction of the 4.7 eV absorption. Other sources of deep ultraviolet absorption in AlN are also discussed.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Gaddy, Benjamin E. and Bryan, Zachary and Bryan, Isaac and Xie, Jinqiao and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and Kubota, Yuki and Kinoshita, Toru and et al.}, year={2014}, month={May} }
 @article{kuittinen_tuomisto_kumagai_nagashima_kinoshita_koukitu_collazo_sitar_2014, title={Vacancy defects in UV-transparent HVPE-AlN}, volume={11}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/PSSC.201300529}, DOI={10.1002/PSSC.201300529}, abstractNote={Abstract}, number={3-4}, journal={physica status solidi (c)}, publisher={Wiley}, author={Kuittinen, Tanja and Tuomisto, Filip and Kumagai, Yoshinao and Nagashima, Toru and Kinoshita, Toru and Koukitu, Akinori and Collazo, Ramón and Sitar, Zlatko}, year={2014}, month={Feb}, pages={405–407} }
 @article{foster_collazo_sitar_ivanisevic_2013, title={Aqueous Stability of Ga- and N-Polar Gallium Nitride}, volume={29}, ISSN={["0743-7463"]}, DOI={10.1021/la304039n}, abstractNote={The stability of III-nitride semiconductors in various solutions becomes important as researchers begin to integrate them into sensing platforms. This study quantitatively compares the stability of GaN surfaces with different polarities. This type of quantification is important because it represents the first step toward designing semiconductor material interfaces compatible with solution conditions. A stability study of Ga- and N-polar GaN was conducted by immersion of the surfaces in deionized H(2)O, pH 5, pH 9, and H(2)O(2) solutions for 7 days. Inductively coupled plasma mass spectrometry of the solutions was conducted to determine the amount of gallium leached from the surface. X-ray photoelectron spectroscopy and atomic force microscopy were used to compare the treated surfaces to untreated surfaces. The results show that both gallium nitride surface types exhibit the greatest stability in acidic and neutral solutions. Gallium polar surfaces were found to exhibit superior stability to nitrogen polar surfaces in the solutions studied. Our findings highlight the need for further research on surface passivation and functionalization techniques for polar III-nitride semiconductors.}, number={1}, journal={LANGMUIR}, author={Foster, Corey M. and Collazo, Ramon and Sitar, Zlatko and Ivanisevic, Albena}, year={2013}, month={Jan}, pages={216–220} }
 @article{foster_collazo_sitar_ivanisevic_2013, title={Cell Behavior on Gallium Nitride Surfaces: Peptide Affinity Attachment versus Covalent Functionalization}, volume={29}, ISSN={["0743-7463"]}, DOI={10.1021/la401503b}, abstractNote={Gallium nitride is a wide band gap semiconductor that demonstrates a unique set of optical and electrical properties as well as aqueous stability and biocompatibility. This combination of properties makes gallium nitride a strong candidate for use in chemical and biological applications such as sensors and neural interfaces. Molecular modification can be used to enhance the functionality and properties of the gallium nitride surface. Here, gallium nitride surfaces were functionalized with a PC12 cell adhesion promoting peptide using covalent and affinity driven attachment methods. The covalent scheme proceeded by Grignard reaction and olefin metathesis while the affinity driven scheme utilized the recognition peptide isolated through phage display. This study shows that the method of attaching the adhesion peptide influences PC12 cell adhesion and differentiation as measured by cell density and morphological analysis. Covalent attachment promoted monolayer and dispersed cell adhesion while affinity driven attachment promoted multilayer cell agglomeration. Higher cell density was observed on surfaces modified using the recognition peptide. The results suggest that the covalent and affinity driven attachment methods are both suitable for promoting PC12 cell adhesion to the gallium nitride surface, though each method may be preferentially suited for distinct applications.}, number={26}, journal={LANGMUIR}, author={Foster, Corey M. and Collazo, Ramon and Sitar, Zlatko and Ivanisevic, Albena}, year={2013}, month={Jul}, pages={8377–8384} }
 @article{guo_xie_akouala_mita_rice_tweedie_bryan_collazo_sitar_2013, title={Comparative study of etching high crystalline quality AlN and GaN}, volume={366}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2012.12.141}, abstractNote={High-quality AlN and GaN bulk crystals were etched in a KOH aqueous solution or a KOH/H2O2 mixture. As etched surfaces were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS). It was found that the N-polar surfaces had approximately 900 and 20 times higher etch rate than the III-polar surface for AlN and GaN, respectively. AlN had a higher total etch rate than GaN at the same condition. Hexagonal hillocks were observed on N-polar face with {10−1−1} boundary planes for both AlN and GaN, while metal polar surfaces remained smooth. Formation of aluminum oxide/hydroxide AlOx(OH)y was confirmed by XPS on as etched N polar AlN surface; the addition of H2O2 resulted in a higher total surface oxygen concentration. The smoothening effect by adding H2O2 oxidizer was explained by the formation and dissolution of aluminum oxide/hydroxide.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Guo, W. and Xie, J. and Akouala, C. and Mita, S. and Rice, A. and Tweedie, J. and Bryan, I. and Collazo, R. and Sitar, Z.}, year={2013}, month={Mar}, pages={20–25} }
 @article{kirste_hoffmann_tweedie_bryan_callsen_kure_nenstiel_wagner_collazo_hoffmann_et al._2013, title={Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements}, volume={113}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4794094}, DOI={10.1063/1.4794094}, abstractNote={Compensation effects in metal organic chemical vapour deposition grown GaN doped with magnesium are investigated with Raman spectroscopy and photoluminescence measurements. Examining the strain sensitive E2(high) mode, an increasing compressive strain is revealed for samples with Mg-concentrations lower than 7 × 1018 cm−3. For higher Mg-concentrations, this strain is monotonically reduced. This relaxation is accompanied by a sudden decrease in crystal quality. Luminescence measurements reveal a well defined near band edge luminescence with free, donor bound, and acceptor bound excitons as well as a characteristic donor acceptor pair (DAP) luminescence. Following recent results, three acceptor bound excitons and donor acceptor pairs are identified. Along with the change of the strain, a strong modification in the luminescence of the dominating acceptor bound exciton and DAP luminescence is observed. The results from Raman spectroscopy and luminescence measurements are interpreted as fingerprints of compensation effects in GaN:Mg leading to the conclusion that compensation due to defect incorporation triggered by Mg-doping already affects the crystal properties at doping levels of around 7 × 1018 cm−3. Thereby, the generation of nitrogen vacancies is introduced as the driving force for the change of the strain state and the near band edge luminescence.}, number={10}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Kirste, Ronny and Hoffmann, Marc P. and Tweedie, James and Bryan, Zachary and Callsen, Gordon and Kure, Thomas and Nenstiel, Christian and Wagner, Markus R. and Collazo, Ramón and Hoffmann, Axel and et al.}, year={2013}, month={Mar}, pages={103504} }
 @article{neuschl_thonke_feneberg_goldhahn_wunderer_yang_johnson_xie_mita_rice_et al._2013, title={Direct determination of the silicon donor ionization energy in homoepitaxial AlN from photoluminescence two-electron transitions}, volume={103}, DOI={10.1063/1.4821183}, abstractNote={We report on the identification of a two-electron transition for the shallow donor silicon in homoepitaxial aluminum nitride (AlN). One c-oriented sample was analyzed by low temperature photoluminescence spectroscopy on multiple excitation spots. We find a unique correlation of one single emission band, 76.6 meV below the free excitonic emission, with the luminescence of excitons bound to neutral silicon proving the identity as a two-electron transition. The assignment is confirmed by temperature dependent photoluminescence investigations. We find a donor ionization energy of (63.5 ± 1.5) meV for silicon in AlN.}, number={12}, journal={Applied Physics Letters}, author={Neuschl, B. and Thonke, K. and Feneberg, M. and Goldhahn, R. and Wunderer, T. and Yang, Z. and Johnson, N. M. and Xie, J. and Mita, S. and Rice, A. and et al.}, year={2013} }
 @article{paisley_craft_losego_lu_gruverman_collazo_sitar_maria_2013, title={Epitaxial PbxZr1-xTiO3 on GaN}, volume={113}, ISSN={["1089-7550"]}, DOI={10.1063/1.4792599}, abstractNote={Epitaxial integration of PbxZr1−xTiO3 (PZT) (111) with GaN (0002) presents the possibility of polarity coupling across a functional-oxide/nitride heteropolar interface. This work describes the synthesis and characterization of such thin film heterostructures by magnetron sputtering, with specific attention given to process optimization. Using x-ray diffraction and electrical characterization, the growth of epitaxial PZT (∼250 nm) on GaN and PZT on MgO/GaN stacks was verified. A two-stage growth process was developed for epitaxial PZT with a deposition temperature of 300 °C and an ex-situ anneal at 650 °C, which was effective in mitigating interfacial reactions and promoting phase-pure perovskite growth. Electrical analysis of interdigital capacitors revealed a nonlinear and hysteretic dielectric response consistent with ferroelectric PZT. Piezoresponse force microscopy (PFM) characterization shows clear evidence of ferroelectric switching, and PFM hysteresis loop analysis shows minimal evidence for direct polarity coupling, but suggests that band offsets which accompany the oxide-nitride heterostructures influence switching.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Paisley, E. A. and Craft, H. S. and Losego, M. D. and Lu, H. and Gruverman, A. and Collazo, R. and Sitar, Z. and Maria, J. -P.}, year={2013}, month={Feb} }
 @article{paisley_craft_losego_lu_gruverman_collazo_sitar_maria_2013, title={Epitaxial lead zirconate titanate on gallium nitride (vol 113, 074107, 2013)}, volume={114}, number={23}, journal={Journal of Applied Physics}, author={Paisley, E. A. and Craft, H. S. and Losego, M. D. and Lu, H. and Gruverman, A. and Collazo, R. and Sitar, Z. and Maria, J. P.}, year={2013} }
 @article{makowski_bryan_sitar_arellano_xie_collazo_ivanisevic_2013, title={Erratum: “Kinase detection with gallium nitride based high electron mobility transistors” [Appl. Phys. Lett. 103, 013701 (2013)]}, volume={103}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4819200}, DOI={10.1063/1.4819200}, abstractNote={[This corrects the article on p. 013701 in vol. 103.].}, number={8}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Makowski, Matthew S. and Bryan, Isaac and Sitar, Zlatko and Arellano, Consuelo and Xie, Jinqiao and Collazo, Ramon and Ivanisevic, Albena}, year={2013}, month={Aug}, pages={089902} }
 @article{chichibu_hazu_ishikawa_tashiro_ohtomo_furusawa_uedono_mita_xie_collazo_et al._2013, title={Excitonic emission dynamics in homoepitaxial AlN films studied using polarized and spatio-time-resolved cathodoluminescence measurements}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4823826}, abstractNote={Excitonic emission dynamics in homoepitaxial AlN films grown on a freestanding substrate prepared by the physical-vapor-transport method were examined. Reflecting the low threading dislocation density (<104 cm−2), room-temperature cathodoluminescence intensity images mapped at the free A-exciton energy exhibited homogeneous contrasts. Low-temperature cathodoluminescence peaks at 6.0415 and 6.0287 eV, which were polarized parallel and perpendicular, respectively, to the c-axis, exhibited identical risetimes and short lifetimes; the latter coincided with the temporal delay of neutral donor-bound exciton emissions. These results support the assumption that the two peaks originate from the recombination of free A-excitons of irreducible representations Γ1 and Γ5, respectively.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Chichibu, S. F. and Hazu, K. and Ishikawa, Y. and Tashiro, M. and Ohtomo, T. and Furusawa, K. and Uedono, A. and Mita, S. and Xie, J. and Collazo, R. and et al.}, year={2013}, month={Sep} }
 @article{bryan_hoffmann_tweedie_kirste_callsen_bryan_rice_bobea_mita_xie_et al._2013, title={Fermi Level Control of Point Defects During Growth of Mg-Doped GaN}, volume={42}, ISSN={["1543-186X"]}, DOI={10.1007/s11664-012-2342-9}, number={5}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Bryan, Zachary and Hoffmann, Marc and Tweedie, James and Kirste, Ronny and Callsen, Gordon and Bryan, Isaac and Rice, Anthony and Bobea, Milena and Mita, Seiji and Xie, Jinqiao and et al.}, year={2013}, month={May}, pages={815–819} }
 @article{kirste_hoffmann_sachet_bobea_bryan_bryan_nenstiel_hoffmann_maria_collazo_et al._2013, title={Ge doped GaN with controllable high carrier concentration for plasmonic applications}, volume={103}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4848555}, DOI={10.1063/1.4848555}, abstractNote={Controllable Ge doping in GaN is demonstrated for carrier concentrations of up to 2.4 × 1020 cm−3. Low temperature luminescence spectra from the highly doped samples reveal band gap renormalization and band filling (Burstein-Moss shift) in addition to a sharp transition. Infrared ellipsometry spectra demonstrate the existence of electron plasma with an energy around 3500 cm−1 and a surface plasma with an energy around 2000 cm−1. These findings open possibilities for the application of highly doped GaN for plasmonic devices.}, number={24}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Kirste, Ronny and Hoffmann, Marc P. and Sachet, Edward and Bobea, Milena and Bryan, Zachary and Bryan, Isaac and Nenstiel, Christian and Hoffmann, Axel and Maria, Jon-Paul and Collazo, Ramón and et al.}, year={2013}, month={Dec}, pages={242107} }
 @article{makowski_bryan_sitar_arellano_xie_collazo_ivanisevic_2013, title={Kinase detection with gallium nitride based high electron mobility transistors}, volume={103}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4812987}, DOI={10.1063/1.4812987}, abstractNote={A label-free kinase detection system was fabricated by the adsorption of gold nanoparticles functionalized with kinase inhibitor onto AlGaN/GaN high electron mobility transistors (HEMTs). The HEMTs were operated near threshold voltage due to the greatest sensitivity in this operational region. The Au NP/HEMT biosensor system electrically detected 1 pM SRC kinase in ionic solutions. These results are pertinent to drug development applications associated with kinase sensing.}, number={1}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Makowski, Matthew S. and Bryan, Isaac and Sitar, Zlatko and Arellano, Consuelo and Xie, Jinqiao and Collazo, Ramon and Ivanisevic, Albena}, year={2013}, month={Jul}, pages={013701} }
 @article{xie_mita_bryan_guo_hussey_moody_schlesser_kirste_gerhold_collazo_et al._2013, title={Lasing and longitudinal cavity modes in photo-pumped deep ultraviolet AlGaN heterostructures}, volume={102}, ISSN={["1077-3118"]}, DOI={10.1063/1.4803689}, abstractNote={To unambiguously distinguish lasing from super luminescence, key elements of lasing such as longitudinal cavity modes with narrow line-width, polarized emission, and elliptically shaped far-field pattern, need to be demonstrated at the same time. Here, we show transverse electric polarized lasing at 280.8 nm and 263.9 nm for AlGaN based multi-quantum-wells and double heterojunction structures fabricated on single crystalline AlN substrates. An elliptically shaped far-field pattern was recorded when pumped above threshold. With cavities shorter than 200 μm, well-defined, equally spaced longitudinal modes with line widths as narrow as 0.014 nm were observed. The low threshold pumping density of 84 kW/cm2 suggests that the electrically pumped sub-300 nm ultraviolet laser diodes are imminent.}, number={17}, journal={APPLIED PHYSICS LETTERS}, author={Xie, Jinqiao and Mita, Seiji and Bryan, Zachary and Guo, Wei and Hussey, Lindsay and Moody, Baxter and Schlesser, Raoul and Kirste, Ronny and Gerhold, Michael and Collazo, Ramon and et al.}, year={2013}, month={Apr} }
 @article{makowski_kim_gaillard_janes_manfra_bryan_sitar_arellano_xie_collazo_et al._2013, title={Physisorption of functionalized gold nanoparticles on AlGaN/GaN high electron mobility transistors for sensing applications}, volume={102}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4791788}, DOI={10.1063/1.4791788}, abstractNote={AlGaN/GaN high electron mobility transistors (HEMTs) were used to measure electrical characteristics of physisorbed gold nanoparticles (Au NPs) functionalized with alkanethiols with a terminal methyl, amine, or carboxyl functional group. Additional alkanethiol was physisorbed onto the NP treated devices to distinguish between the effects of the Au NPs and alkanethiols on HEMT operation. Scanning Kelvin probe microscopy and electrical measurements were used to characterize the treatment effects. The HEMTs were operated near threshold voltage due to the greatest sensitivity in this region. The Au NP/HEMT system electrically detected functional group differences on adsorbed NPs which is pertinent to biosensor applications.}, number={7}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Makowski, M. S. and Kim, S. and Gaillard, M. and Janes, D. and Manfra, M. J. and Bryan, I. and Sitar, Z. and Arellano, C. and Xie, J. and Collazo, R. and et al.}, year={2013}, month={Feb}, pages={074102} }
 @article{kirste_mita_hussey_hoffmann_guo_bryan_bryan_tweedie_xie_gerhold_et al._2013, title={Polarity control and growth of lateral polarity structures in AlN}, volume={102}, DOI={10.1063/1.4804575}, abstractNote={The control of the polarity of metalorganic chemical vapor deposition grown AlN on sapphire is demonstrated. Al-polar and N-polar AlN is grown side-by-side yielding a lateral polarity structure. Scanning electron microscopy measurements reveal a smooth surface for the Al-polar and a relatively rough surface for the N-polar AlN domains. Transmission electron microscopy shows mixed edge-screw type dislocations with polarity-dependent dislocation bending. Raman spectroscopy reveals compressively strained Al-polar and relaxed N-polar domains. The near band edge luminescence consists of free and bound excitons which are broadened for the Al-polar AlN. Relaxation, better optical quality, and dislocation bending in the N-polar domains are explained by the columnar growth mode.}, number={18}, journal={Applied Physics Letters}, author={Kirste, R. and Mita, S. and Hussey, L. and Hoffmann, M. P. and Guo, W. and Bryan, I. and Bryan, Z. and Tweedie, J. and Xie, J. Q. and Gerhold, M. and et al.}, year={2013} }
 @article{sochacki_bryan_amilusik_collazo_lucznik_weyher_nowak_sadovyi_kamler_kucharski_et al._2013, title={Preparation of Free-Standing GaN Substrates from Thick GaN Layers Crystallized by Hydride Vapor Phase Epitaxy on Ammonothermally Grown GaN Seeds}, volume={6}, ISSN={["1882-0786"]}, DOI={10.7567/apex.6.075504}, abstractNote={Crystallization of GaN by hydride vapor phase epitaxy (HVPE) on ammonothermally grown GaN seed crystals is described. The initial growth conditions for HVPE are determined and applied for further bulk growth. Smooth GaN layers up to 1.1 mm thick and of excellent crystalline quality, without cracks, and with low dislocation density are obtained. Preparation of the free-standing HVPE-GaN crystal by slicing and structural and optical quality of the resulting wafer are presented.}, number={7}, journal={APPLIED PHYSICS EXPRESS}, author={Sochacki, Tomasz and Bryan, Zachary and Amilusik, Mikolaj and Collazo, Ramon and Lucznik, Boleslaw and Weyher, Jan L. and Nowak, Grzegorz and Sadovyi, Bogdan and Kamler, Grzegorz and Kucharski, Robert and et al.}, year={2013}, month={Jul} }
 @article{paisley_craft_losego_lu_gruverman_collazo_sitar_maria_2013, title={Publisher's Note: “Epitaxial lead zirconate titanate on gallium nitride” [J. Appl. Phys. 113, 074107 (2013)]}, volume={114}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4842135}, DOI={10.1063/1.4842135}, number={23}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Paisley, E. A. and Craft, H. S. and Losego, M. D. and Lu, H. and Gruverman, A. and Collazo, R. and Sitar, Z. and Maria, J.-P.}, year={2013}, month={Dec}, pages={239901} }
 @article{rigler_zgonik_hoffmann_kirste_bobea_collazo_sitar_mita_gerhold_2013, title={Refractive index of III-metal-polar and N-polar AlGaN waveguides grown by metal organic chemical vapor deposition}, volume={102}, ISSN={["1077-3118"]}, DOI={10.1063/1.4800554}, abstractNote={Optical waveguides of III-metal-polar and N-polar AlGaN are grown on sapphire substrates in order to test their use in integrated optics. The dispersion of the ordinary and extraordinary indices of refraction for films with aluminum mole fraction between 0.0 and 0.30 at four discrete wavelengths has been determined by the prism coupling method. The wavelength dependence of the refractive indices is described well by the first-order Sellmeier dispersion formula. The measurements show a small difference in the refractive indices between the two polarities, which is more pronounced at longer wavelengths.}, number={22}, journal={APPLIED PHYSICS LETTERS}, author={Rigler, Martin and Zgonik, Marko and Hoffmann, Marc P. and Kirste, Ronny and Bobea, Milena and Collazo, Ramon and Sitar, Zlatko and Mita, Seiji and Gerhold, Michael}, year={2013}, month={Jun} }
 @article{bryan_rice_hussey_bryan_bobea_mita_xie_kirste_collazo_sitar_2013, title={Strain relaxation by pitting in AlN thin films deposited by metalorganic chemical vapor deposition}, volume={102}, ISSN={["0003-6951"]}, DOI={10.1063/1.4792694}, abstractNote={Strain relaxation mechanisms were investigated in epitaxial AlN layers deposited on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition. It was revealed that epitaxial AlN layers under tensile strain can exhibit micro-cracks and nano-pits. A correlation existed between the amount of strain and number of pits in localized areas. Pit densities as high as 1010 cm−2 were observed in areas where the tensile strain reached ∼0.4%, while unstrained areas of the film showed step flow growth. These nano-pits occurred as a strain relaxation mechanism and were not related to intrinsic defects, such as threading dislocations or inversion domains.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Bryan, I. and Rice, A. and Hussey, L. and Bryan, Z. and Bobea, M. and Mita, S. and Xie, J. and Kirste, R. and Collazo, R. and Sitar, Z.}, year={2013}, month={Feb} }
 @article{gaddy_bryan_bryan_kirste_xie_dalmau_moody_kumagai_nagashima_kubota_et al._2013, title={Vacancy compensation and related donor-acceptor pair recombination in bulk AlN}, volume={103}, ISSN={["1077-3118"]}, DOI={10.1063/1.4824731}, abstractNote={A prominent 2.8 eV emission peak is identified in bulk AlN substrates grown by physical vapor transport. This peak is shown to be related to the carbon concentration in the samples. Density functional theory calculations predict that this emission is caused by a donor-acceptor pair (DAP) recombination between substitutional carbon on the nitrogen site and a nitrogen vacancy. Photoluminescence and photoluminescence-excitation spectroscopy are used to confirm the model and indicate the DAP character of the emission. The interaction between defects provides a pathway to creating ultraviolet-transparent AlN substrates for optoelectronics applications.}, number={16}, journal={APPLIED PHYSICS LETTERS}, author={Gaddy, Benjamin E. and Bryan, Zachary and Bryan, Isaac and Kirste, Ronny and Xie, Jinqiao and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and Kubota, Yuki and et al.}, year={2013}, month={Oct} }
 @article{bobea_tweedie_bryan_bryan_rice_dalmau_xie_collazo_sitar_2013, title={X-ray characterization techniques for the assessment of surface damage in crystalline wafers: A model study in AlN}, volume={113}, ISSN={["0021-8979"]}, DOI={10.1063/1.4798352}, abstractNote={A high-resolution X-ray diffraction method with enhanced surface sensitivity has been used to investigate the effects of various polishing steps on the near-surface region of single crystal substrates. The method involves the study of a highly asymmetric reflection, observable under grazing incidence conditions. Analysis of rocking curve measurements and reciprocal space maps (RSMs) revealed subtle structural differences between the polished substrates. For aluminum nitride wafers, damage induced from diamond sawing and mechanical polishing was readily identifiable by on-axis rocking curves, but this method was unable to distinguish between sample surfaces subjected to various degrees of chemical mechanical polishing (CMP). To characterize sufficiently these surfaces, (10.3) RSMs were measured to provide both qualitative and quantitative information about the near-surface region. Two features present in the RSMs were utilized to quantitatively assess the polished wafers: the magnitude of the diffuse scatter in the omega-scans and the elongation of the crystal truncation rod. The method is able to distinguish between different degrees of CMP surface preparation and provides metrics to quantify subsurface damage after this polishing step.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bobea, M. and Tweedie, J. and Bryan, I. and Bryan, Z. and Rice, A. and Dalmau, R. and Xie, J. and Collazo, R. and Sitar, Z.}, year={2013}, month={Mar} }
 @article{railsback_singh_pearce_mcknight_collazo_sitar_yingling_melechko_2012, title={Gold Nanoparticles: Weakly Charged Cationic Nanoparticles Induce DNA Bending and Strand Separation (Adv. Mater. 31/2012)}, volume={24}, ISSN={0935-9648}, url={http://dx.doi.org/10.1002/adma.201290188}, DOI={10.1002/adma.201290188}, abstractNote={On page 4261, A. V. Melechko, Y. G. Yingling, and co-workers show that weakly charged cationic nanoparticles cause structural changes including local denaturing and compaction to DNA under mild conditions. The foreground of the cover image shows molecular dynamics simulations of the collective action of weakly charged cationic nanoparticles, which results in DNA strand separation. In the background is an electrophoresis photograph in which a new band (lower left) appears upon addition of gold nanoparticles capped with a mixed monolayer containing cationic ligands.}, number={31}, journal={Advanced Materials}, publisher={Wiley}, author={Railsback, Justin G. and Singh, Abhishek and Pearce, Ryan C. and McKnight, Timothy E. and Collazo, Ramón and Sitar, Zlatko and Yingling, Yaroslava G. and Melechko, Anatoli V.}, year={2012}, month={Aug}, pages={4221–4221} }
 @article{hussey_mita_xie_guo_akouala_rajan_bryan_collazo_sitar_2012, title={Lateral epitaxial overgrowth of nitrogen polar GaN on smooth nitrogen polar GaN templates by metalorganic chemical vapor deposition}, volume={112}, ISSN={["1089-7550"]}, DOI={10.1063/1.4768526}, abstractNote={Nitrogen (N)-polar GaN lateral epitaxial overgrowth (LEO), with a reduced dislocation density, is grown by metalorganic chemical vapor deposition on patterned, smooth N-polar templates. For LEO growth conditions identical to that of smooth N-polar GaN on sapphire, mask orientation dependent morphologies and faceting of the overgrowth regions are observed. N-polar LEO oriented along the 〈1-100〉 direction exhibit flat sidewall morphologies while LEO oriented along the 〈11-20〉 direction exhibit inclined sidewalls. A lateral to vertical growth ratio for the 〈1-100〉 and 〈11-20〉 oriented LEO was found to be ∼0.3 and 0.2, respectively. Transmission electron microscope observations reveal a reduction of dislocations of up to three orders of magnitude in the overgrowth regions from ∼1010 cm−2 in the template. Additionally, dislocation bending likely initiated from interfacial tension between the N-polar GaN and SiO2 mask is observed, resulting in a reduction of dislocation density to ∼109 cm−2 in the window region of re-growth.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Hussey, Lindsay and Mita, Seiji and Xie, Jinqiao and Guo, Wei and Akouala, Christer-Rajiv and Rajan, Joseph and Bryan, Isaac and Collazo, Ramon and Sitar, Zlatko}, year={2012}, month={Dec} }
 @article{collazo_xie_gaddy_bryan_kirste_hoffmann_dalmau_moody_kumagai_nagashima_et al._2012, title={On the origin of the 265 nm absorption band in AlN bulk crystals}, volume={100}, ISSN={["1077-3118"]}, DOI={10.1063/1.4717623}, abstractNote={Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes. An absorption band centered around 4.7 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates. Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Collazo, Ramon and Xie, Jinqiao and Gaddy, Benjamin E. and Bryan, Zachary and Kirste, Ronny and Hoffmann, Marc and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and et al.}, year={2012}, month={May} }
 @article{neuschl_thonke_feneberg_mita_xie_dalmau_collazo_sitar_2012, title={Optical identification of silicon as a shallow donor in MOVPE grown homoepitaxial AlN}, volume={249}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.201100381}, abstractNote={Abstract}, number={3}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Neuschl, Benjamin and Thonke, Klaus and Feneberg, Martin and Mita, Seiji and Xie, Jinqiao and Dalmau, Rafael and Collazo, Ramon and Sitar, Zlatko}, year={2012}, month={Mar}, pages={511–515} }
 @article{callsen_wagner_kure_reparaz_buegler_brunnmeier_nenstiel_hoffmann_hoffmann_tweedie_et al._2012, title={Optical signature of Mg-doped GaN: Transfer processes}, volume={86}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.86.075207}, abstractNote={Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states that are involved in the donor-acceptor pair band transitions situated at 3.26–3.29 eV in GaN:Mg. The presented determination of the donor-acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature-dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies, which is put into a broader context based on Haynes’s rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN band edge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states, we determine binding energies of 164 ± 5 and 195 ± 5 meV, which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.}, number={7}, journal={PHYSICAL REVIEW B}, author={Callsen, G. and Wagner, M. R. and Kure, T. and Reparaz, J. S. and Buegler, M. and Brunnmeier, J. and Nenstiel, C. and Hoffmann, A. and Hoffmann, M. and Tweedie, J. and et al.}, year={2012}, month={Aug} }
 @article{tweedie_collazo_rice_mita_xie_akouala_sitar_2012, title={Schottky barrier and interface chemistry for Ni contacted to Al0.8Ga0.2N grown on c-oriented AlN single crystal substrates}, volume={9}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201100435}, abstractNote={Abstract}, number={3-4}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 3-4}, author={Tweedie, James and Collazo, Ramon and Rice, Anthony and Mita, Seiji and Xie, Jinqiao and Akouala, Rajiv-Christer and Sitar, Zlatko}, year={2012}, pages={584–587} }
 @article{nagashima_kubota_kinoshita_kumagai_xie_collazo_murakami_okamoto_koukitu_sitar_2012, title={Structural and Optical Properties of Carbon-Doped AlN Substrates Grown by Hydride Vapor Phase Epitaxy Using AlN Substrates Prepared by Physical Vapor Transport}, volume={5}, ISSN={["1882-0778"]}, DOI={10.1143/apex.5.125501}, abstractNote={Freestanding AlN substrates with various carbon (C) concentrations were prepared from C-doped thick layers grown by hydride vapor phase epitaxy (HVPE) on bulk AlN substrates prepared by physical vapor transport (PVT). The structural properties of the AlN substrates up to a C concentration of 3×1019 cm-3 were the same as those of the nominally undoped substrates, while the absorption coefficient α at 265 nm was increased by C doping from 6.6 to 97 cm-1, when C concentration changed from <2×1017 to 1×1019 cm-3, respectively. Photoluminescence (PL) below 4.0 eV also increased by C doping.}, number={12}, journal={APPLIED PHYSICS EXPRESS}, author={Nagashima, Toru and Kubota, Yuki and Kinoshita, Toru and Kumagai, Yoshinao and Xie, Jinqiao and Collazo, Ramon and Murakami, Hisashi and Okamoto, Hiroshi and Koukitu, Akinori and Sitar, Zlatko}, year={2012}, month={Dec} }
 @article{paisley_shelton_mita_collazo_christen_sitar_biegalski_maria_2012, title={Surfactant assisted growth of MgO films on GaN}, volume={101}, ISSN={["0003-6951"]}, DOI={10.1063/1.4748886}, abstractNote={Thin epitaxial films of 〈111〉 oriented MgO on [0001]-oriented GaN were grown by molecular beam epitaxy and pulsed laser deposition using the assistance of a vapor phase surfactant. In both cases, surfactant incorporation enabled layer-by-layer growth and a smooth terminal surface by stabilizing the {111} rocksalt facet. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100× reduction in leakage current density for the surfactant-assisted samples. These data verify numerous predictions regarding the role of H-termination in regulating the habit of rocksalt crystals.}, number={9}, journal={APPLIED PHYSICS LETTERS}, author={Paisley, E. A. and Shelton, T. C. and Mita, S. and Collazo, R. and Christen, H. M. and Sitar, Z. and Biegalski, M. D. and Maria, J. -P.}, year={2012}, month={Aug} }
 @article{railsback_singh_pearce_mcknight_collazo_sitar_yingling_melechko_2012, title={Weakly Charged Cationic Nanoparticles Induce DNA Bending and Strand Separation}, volume={24}, ISSN={["0935-9648"]}, url={https://publons.com/wos-op/publon/5454547/}, DOI={10.1002/adma.201104891}, abstractNote={Weakly charged cationic nanoparticles cause structural changes including local denaturing and compaction to DNA under mild conditions. The charged ligands bind to the phosphate backbone of DNA and the uncharged ligands penetrate the helix and disrupt base pairing. Mobility shifts in electrophoresis, molecular dynamics, and UV-vis spectrophotometry give clues to the details of the interactions.}, number={31}, journal={ADVANCED MATERIALS}, publisher={Wiley-Blackwell}, author={Railsback, Justin G. and Singh, Abhishek and Pearce, Ryan C. and McKnight, Timothy E. and Collazo, Ramon and Sitar, Zlatko and Yingling, Yaroslava G. and Melechko, Anatoli V.}, year={2012}, month={Aug}, pages={4261-+} }
 @inproceedings{collazo_mita_xie_rice_tweedie_dalmau_moody_schlesser_kirste_hoffmann_et al._2011, title={265 nm light emitting diodes on AlN single crystal substrates: Growth and characterization}, DOI={10.1364/cleo_si.2011.ctuu2}, abstractNote={High crystalline quality A1N and pseudomorphic AlGaN films were deposited by metalorganic chemical vapor deposition on single crystal A1N substrates for the fabrication of a 265 nm light emitting diode with estimated IQE above 40%.}, booktitle={2011 Conference on Lasers and Electro-Optics (CLEO)}, author={Collazo, Ramon and Mita, S. and Xie, J. Q. and Rice, A. and Tweedie, J. and Dalmau, R. and Moody, B. and Schlesser, R. and Kirste, R. and Hoffmann, A. and et al.}, year={2011} }
 @article{dalmau_moody_xie_collazo_sitar_2011, title={Characterization of dislocation arrays in AlN single crystals grown by PVT}, volume={208}, ISSN={1862-6300}, url={http://dx.doi.org/10.1002/pssa.201000957}, DOI={10.1002/pssa.201000957}, abstractNote={Abstract}, number={7}, journal={physica status solidi (a)}, publisher={Wiley}, author={Dalmau, Rafael and Moody, Baxter and Xie, Jinqiao and Collazo, Ramón and Sitar, Zlatko}, year={2011}, month={May}, pages={1545–1547} }
 @article{dalmau_moody_schlesser_mita_xie_feneberg_neuschl_thonke_collazo_rice_et al._2011, title={Growth and Characterization of AlN and AlGaN Epitaxial Films on AlN Single Crystal Substrates}, volume={158}, ISSN={["0013-4651"]}, DOI={10.1149/1.3560527}, abstractNote={AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.}, number={5}, journal={JOURNAL OF THE ELECTROCHEMICAL SOCIETY}, author={Dalmau, R. and Moody, B. and Schlesser, R. and Mita, S. and Xie, J. and Feneberg, M. and Neuschl, B. and Thonke, K. and Collazo, R. and Rice, A. and et al.}, year={2011}, pages={H530–H535} }
 @article{buegler_gamage_atalay_wang_senevirathna_kirste_xu_jamil_ferguson_tweedie_et al._2011, title={Growth temperature and growth rate dependency on reactor pressure for InN epilayers grown by HPCVD}, volume={8}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.201001067}, DOI={10.1002/pssc.201001067}, abstractNote={Abstract}, number={7-8}, journal={physica status solidi (c)}, publisher={Wiley}, author={Buegler, Max and Gamage, Sampath and Atalay, R. and Wang, J. and Senevirathna, M. K. I. and Kirste, R. and Xu, T. and Jamil, M. and Ferguson, I. and Tweedie, J. and et al.}, year={2011}, month={May}, pages={2059–2062} }
 @article{mita_collazo_rice_tweedie_xie_dalmau_sitar_2011, title={Impact of gallium supersaturation on the growth of N-polar GaN}, volume={8}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.201001063}, DOI={10.1002/pssc.201001063}, abstractNote={Abstract}, number={7-8}, journal={physica status solidi (c)}, publisher={Wiley}, author={Mita, Seiji and Collazo, Ramón and Rice, Anthony and Tweedie, James and Xie, Jinqiao and Dalmau, Rafael and Sitar, Zlatko}, year={2011}, month={Jun}, pages={2078–2080} }
 @article{xie_mia_dalmau_collazo_rice_tweedie_sitar_2011, title={Ni/Au Schottky diodes on AlxGa1-xN (0.7<x<1) grown on AlN single crystal substrates}, volume={8}, ISSN={1862-6351}, url={http://dx.doi.org/10.1002/pssc.201001009}, DOI={10.1002/pssc.201001009}, abstractNote={Abstract}, number={7-8}, journal={physica status solidi (c)}, publisher={Wiley}, author={Xie, Jinqiao and Mia, Seiji and Dalmau, Rafael and Collazo, Ramón and Rice, Anthony and Tweedie, James and Sitar, Zlatko}, year={2011}, month={Jun}, pages={2407–2409} }
 @article{acharya_buegler_atalay_dietz_thoms_tweedie_collazo_2011, title={Observation of NH2 species on tilted InN (01(1)over-bar1) facets}, volume={29}, ISSN={["1520-8559"]}, DOI={10.1116/1.3596619}, abstractNote={The structural properties and surface bonding configuration of InN layers grown by high-pressure chemical vapor deposition have been characterized using Raman spectroscopy, x-ray diffraction (XRD), and high resolution electron energy loss spectroscopy. The appearance of the A1(TO) mode at 447 cm−1 in unpolarized z(·)z− Raman spectrum indicates distortions in the crystal lattice due to the growth of tilted plane crystallites. A Bragg reflex in the x-ray diffraction spectrum at 2Θ ≈ 33° has been assigned to tilted InN facets in the polycrystalline InN layer. The high resolution electron energy loss spectrum for this InN layer features vibration modes assigned to NH2 species indicating a surface orientation consistent with the crystalline properties observed in Raman spectroscopy and XRD. The appearance of tilted planes is suggested to be due to the effects of high V–III ratio and lattice mismatch on the growth mechanism.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Acharya, A. R. and Buegler, M. and Atalay, R. and Dietz, N. and Thoms, B. D. and Tweedie, J. S. and Collazo, R.}, year={2011}, month={Jul} }
 @article{xie_mita_hussey_rice_tweedie_lebeau_collazo_sitar_2011, title={On the strain in n-type GaN}, volume={99}, ISSN={["0003-6951"]}, DOI={10.1063/1.3647772}, abstractNote={It was demonstrated that Ge has the same effect as Si on the strain evolution in n-type GaN as measured by x-ray diffraction. Dislocation inclination, which causes tensile strain in n-type GaN, was clearly observed by transmission electron microscopy where Ge doping was introduced during epitaxial growth. This result is explained by the Fermi level effect model that indicates dislocation inclination due to the climbing process through Ga vacancies. Therefore, there is no dependence of dislocation inclination on dopant species.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Xie, Jinqiao and Mita, Seiji and Hussey, Lindsay and Rice, Anthony and Tweedie, James and LeBeau, James and Collazo, Ramon and Sitar, Zlatko}, year={2011}, month={Oct} }
 @article{collazo_mita_xie_rice_tweedie_dalmau_sitar_wetzel_khan_2011, title={Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications}, volume={8}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201000964}, abstractNote={Abstract}, number={7-8}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 7-8}, author={Collazo, Ramon and Mita, Seiji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and Dalmau, Rafael and Sitar, Zlatko and Wetzel, C and Khan, A}, year={2011} }
 @article{feneberg_neuschl_thonke_collazo_rice_sitar_dalmau_xie_mita_goldhahn_2011, title={Sharp bound and free exciton lines from homoepitaxial AlN}, volume={208}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.201000947}, abstractNote={Abstract}, number={7}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Feneberg, Martin and Neuschl, Benjamin and Thonke, Klaus and Collazo, Ramon and Rice, Anthony and Sitar, Zlatko and Dalmau, Rafael and Xie, Jinqiao and Mita, Seiji and Goldhahn, Ruediger}, year={2011}, month={Jul}, pages={1520–1522} }
 @article{craft_rice_collazo_sitar_maria_2011, title={Spectroscopic measurements of the surface stoichiometry of chemical vapor deposited GaN}, volume={98}, ISSN={["0003-6951"]}, DOI={10.1063/1.3554762}, abstractNote={We report on the surface stoichiometry of Ga-polar GaN films grown by metalorganic chemical vapor deposition as studied by x-ray photoelectron spectroscopy. GaN film surfaces are found to be Ga-rich, with Ga:N ratios ranging from 1.3:1 to 3.2:1. In vacuo ion-beam sputter/annealing studies show that these treatments drive the apparent Ga:N surface composition farther from unity, either through a decrease in surface contamination, oxidation of the surface, or both. Simple annealing experiments decrease the Ga:N ratio. The measured GaN ratio is correlated with the GaN growth time, suggesting that residual Ga precursor after growth interacts with the GaN surface as it cools.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Craft, H. S. and Rice, A. L. and Collazo, R. and Sitar, Z. and Maria, J. -P.}, year={2011}, month={Feb} }
 @article{xie_mita_rice_tweedie_hussey_collazo_sitar_2011, title={Strain in Si doped GaN and the Fermi level effect}, volume={98}, ISSN={["0003-6951"]}, DOI={10.1063/1.3589978}, abstractNote={Using high resolution x-ray diffraction and Hall effect measurements, we found that the tensile strain caused by dislocation inclination in Si doped GaN became immeasurable when carbon codoping was used to compensate the free carriers. This result suggested that the tensile strain is related to free carrier concentration instead of Si concentration. Such an effect could be explained by the Fermi level effect on the surface-mediated dislocation climb governed by Ga vacancies, whose concentration is strongly influenced by the Fermi level position. This phenomenon is possibly similar to the well-known Fermi level effect in GaAs and GaP systems.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Xie, Jinqiao and Mita, Seiji and Rice, Anthony and Tweedie, James and Hussey, Lindsay and Collazo, Ramon and Sitar, Zlatko}, year={2011}, month={May} }
 @article{paisley_losego_gaddy_tweedie_collazo_sitar_irving_maria_2011, title={Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions}, volume={2}, ISSN={["2041-1723"]}, DOI={10.1038/ncomms1470}, abstractNote={Property coupling at interfaces between active materials is a rich source of functionality, if defect densities are low, interfaces are smooth and the microstructure is featureless. Conventional synthesis techniques generally fail to achieve this when materials have highly dissimilar structure, symmetry and bond type—precisely when the potential for property engineering is most pronounced. Here we present a general synthesis methodology, involving systematic control of the chemical boundary conditions in situ, by which the crystal habit, and thus growth mode, can be actively engineered. In so doing, we establish the capability for layer-by-layer deposition in systems that otherwise default to island formation and grainy morphology. This technique is demonstrated via atomically smooth {111} calcium oxide films on (0001) gallium nitride. The operative surfactant-based mechanism is verified by temperature-dependent predictions from ab initio thermodynamic calculations. Calcium oxide films with smooth morphology exhibit a three order of magnitude enhancement of insulation resistance. Property coupling by heteroepitaxy is severely limited in material combinations with highly dissimilar bonding. This report presents a chemical boundary condition methodology to actively engineer two-dimensional film growth in such systems that otherwise collapse into island formation and rough morphologies.}, journal={NATURE COMMUNICATIONS}, author={Paisley, Elizabeth A. and Losego, Mark. D. and Gaddy, Benjamin E. and Tweedie, James S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L. and Maria, Jon-Paul}, year={2011}, month={Sep} }
 @article{kirste_collazo_callsen_wagner_kure_sebastian reparaz_mita_xie_rice_tweedie_et al._2011, title={Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN}, volume={110}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.3656987}, DOI={10.1063/1.3656987}, abstractNote={We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescence measurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracted to the IDB. The results are successfully used to confirm a theoretical model for GaN based lateral polarity junctions. Due to the strong and pronounced emission of IDBs even at elevated temperatures, it is demonstrated that lateral polarity junctions exhibit a strong potential for future high efficiency devices.}, number={9}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Kirste, Ronny and Collazo, Ramón and Callsen, Gordon and Wagner, Markus R. and Kure, Thomas and Sebastian Reparaz, Juan and Mita, Seji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and et al.}, year={2011}, month={Nov}, pages={093503} }
 @article{losego_craft_paisley_mita_collazo_sitar_maria_2010, title={Critical examination of growth rate for magnesium oxide (MgO) thin films deposited by molecular beam epitaxy with a molecular oxygen flux}, volume={25}, ISSN={["0884-2914"]}, DOI={10.1557/jmr.2010.0096}, abstractNote={The authors report a study of molecular beam deposition of MgO films on amorphous SiO2 and (0001) GaN surfaces over a large range of temperatures (25–400 °C) and molecular oxygen growth pressures (10−7–10−4 Torr). This study provides insight into the growth behavior of an oxide with volatile metal constituents. Unlike other materials containing volatile constituents (e.g., GaAs, PbTiO3), all components of MgO become volatile at normal epitaxial growth temperatures (≥250 °C). Consequently, defining which species is the adsorption controller becomes ambiguous. Different growth regimes are delineated by the critical substrate temperature for Mg re-evaporation and the Mg:O flux ratio. These regimes have impact on phase purity, quartz crystal microbalance calibration, and film microstructure. The universal decay in deposition rate above growth 10−5 Torr O2 is also considered. By introducing a third flux of inert argon gas, rate reduction is attributed to increased molecular scattering and not oxidation of the metal source.}, number={4}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Losego, Mark D. and Craft, H. Spalding and Paisley, Elizabeth A. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2010}, month={Apr}, pages={670–679} }
 @article{collazo_mita_xie_rice_tweedie_dalmau_sitar_2010, title={Implementation of the GaN lateral polarity junction in a MESFET utilizing polar doping selectivity}, volume={207}, ISSN={["1862-6300"]}, DOI={10.1002/pssa.200982629}, abstractNote={Abstract}, number={1}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Collazo, Ramon and Mita, Seiji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and Dalmau, Rafael and Sitar, Zlatko}, year={2010}, month={Jan}, pages={45–48} }
 @article{rice_collazo_tweedie_xie_mita_sitar_2010, title={Linear dependency of Al-mole fraction with group-III precursor flows in AlxGa1-xN (0 <= x <= 1) deposition by LP OMVPE}, volume={312}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2009.09.011}, abstractNote={AlxGa1−xN/GaN heterostructures (0≤x≤1) were deposited on (0 0 0 1)-sapphire by low-pressure (20 Torr) organometallic vapor phase epitaxy utilizing a range of group-III precursor and NH3 flow rates and a deposition temperature of 1050 °C. The Al-mole fraction of AlxGa1−xN layers was controlled by variations in the molar flows of triethylgallium (fTEG) and trimethylaluminum (fTMA). Characterization by X-ray diffraction confirmed that increasing the fTMA/fTEG ratio during deposition resulted in increased Al-mole fraction of AlxGa1−xN layers. A linear relationship between the experimental Al-mole fraction of AlxGa1−xN layers and the group-III precursor flows was obtained if a correction factor γ was introduced such that x=γfTMA/(γfTMA+fTEG). This correction factor was found to be independent of group-III precursor flows but dependent on NH3 partial pressure. The value of γ for these experiments decreased from 1.4 to 1.0 when the NH3 partial pressure was increased from 0.8 to 16.0 Torr during AlxGa1−xN deposition. The growth rate of AlxGa1−xN layers was found to be a linear combination of the independent AlN and GaN growth rates when equivalent fTMA and fTEG were utilized under the same deposition conditions.}, number={8}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Rice, A. and Collazo, R. and Tweedie, J. and Xie, J. and Mita, S. and Sitar, Z.}, year={2010}, month={Apr}, pages={1321–1324} }
 @article{kirste_wagner_schulze_strittmatter_collazo_sitar_alevli_dietz_hoffmann_2010, title={Optical properties of InN grown on templates with controlled surface polarities}, volume={207}, ISSN={["1862-6300"]}, DOI={10.1002/pssa.201026086}, abstractNote={Abstract}, number={10}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Kirste, Ronny and Wagner, Markus R. and Schulze, Jan H. and Strittmatter, Andre and Collazo, Ramon and Sitar, Zlatko and Alevli, Mustafa and Dietz, Nikolaus and Hoffmann, Axel}, year={2010}, month={Oct}, pages={2351–2354} }
 @article{rice_collazo_tweedie_dalmau_mita_xie_sitar_2010, title={Surface preparation and homoepitaxial deposition of AlN on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition}, volume={108}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.3467522}, DOI={10.1063/1.3467522}, abstractNote={Chemical surface treatments were conducted on mechanically polished (MP) and chemomechanically polished (CMP) (0001)-oriented single crystalline aluminum nitride (AlN) substrates to determine a surface preparation procedure for the homoepitaxial deposition of AlN epitaxial layers by metalorganic chemical vapor deposition. MP AlN substrates characterized by atomic force microscopy exhibited 0.5 nm rms roughness and polishing scratches, while CMP AlN substrates exhibited 0.1 nm rms roughness and were scratch-free. X-ray photoelectron spectroscopy analysis of MP and CMP AlN substrates indicated the presence of a surface hydroxide layer composed of mixed aluminum oxide hydroxide and aluminum trihydroxide. Wet etching with sulfuric and phosphoric acid mixtures reduced the amount of surface hydroxide. Ammonia annealing at 1250 °C converted the substrate hydroxide layer to AlN and increased the rms roughness of MP and CMP AlN substrates to 2.2 nm and 0.2 nm, respectively. AlN epitaxial layers were deposited at 1100–1250 °C under 20 Torr total pressure with a V/III ratio of 180–300 in either N2 or H2 diluent. High-resolution x-ray diffraction measurements revealed that AlN epitaxial layers deposited on MP substrates were strained due to nucleation and coalescence of AlN grains on the mechanically damaged surfaces. AlN deposited on CMP substrates was epitaxial and strain-free. Thermodynamic models for nitridation and AlN deposition were also proposed and evaluated.}, number={4}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Rice, A. and Collazo, R. and Tweedie, J. and Dalmau, R. and Mita, S. and Xie, J. and Sitar, Z.}, year={2010}, month={Aug}, pages={043510} }
 @article{xie_mita_collazo_rice_tweedie_sitar_2010, title={The effect of N-polar GaN domains as Ohmic contacts}, volume={97}, ISSN={["0003-6951"]}, DOI={10.1063/1.3491173}, abstractNote={Transfer line method measurements revealed that if the Ohmic contact regions were replaced by N-polar GaN, the contact resistance could be reduced from 0.71 Ω mm (or ρc=4×10−6 Ω cm2) to 0.24 Ω mm for a ∼200 nm thick Si-doped GaN layer. The reduction in contact resistance was largely due to the ∼1019 cm−3 free carriers in N-polar source/drain regions as measured by Hall effect. Secondary ion mass spectroscopy confirmed that oxygen doping in the N-polar region was more than three orders of magnitude greater than that in the Ga-polar region that was explained by the large difference in the adsorption energy for oxygen (∼1.3 eV/atom) between the N- and Ga-polar surfaces during the metalorganic chemical vapor deposition.}, number={12}, journal={APPLIED PHYSICS LETTERS}, author={Xie, J. and Mita, S. and Collazo, R. and Rice, A. and Tweedie, J. and Sitar, Z.}, year={2010}, month={Sep} }
 @article{tweedie_collazo_rice_xie_mita_dalmau_sitar_2010, title={X-ray characterization of composition and relaxation of Al(x)Ga(1-x)N(0 <= x <= 1) layers grown on GaN/sapphire templates by low pressure organometallic vapor phase epitaxy}, volume={108}, ISSN={["0021-8979"]}, DOI={10.1063/1.3457149}, abstractNote={A characterization procedure was developed to determine the alloy composition and strain state of AlxGa1−xN/GaN(0<x<1) heterostructures deposited on c-plane sapphire substrates by low pressure organometalic vapor phase epitaxy. Motivated by a method suggested by Bowen and Tanner for separating the contributions of strain and composition in cubic crystals, we extended the technique to the case of hexagonal crystals by first principles derivation from elastic strain theory. The technique was evaluated using double-axis and triple-axis reciprocal space maps of 200 and 30 nm AlxGa1−xN layers. The procedure did not require absolute lattice parameter measurements and relied instead on relative measurements of the layer and substrate peak positions. Symmetric and asymmetric reflections of the film and substrate were measured in the double-axis configuration with ω–2θ scans. From the peak separation, the strained lattice parameters were determined. Assuming biaxial strain and linear variation of the relaxed lattice parameter with alloy composition, an equation was derived for composition in terms of the strained lattice parameters. The relaxation distribution in the AlGaN was found to be bimodal. Films with mole fraction of Al greater than 0.4 were completely relaxed and thoroughly cracked, while films with mole fraction of Al less than 0.4 exhibited compositionally dependent relaxation about the fully strained state.}, number={4}, journal={JOURNAL OF APPLIED PHYSICS}, author={Tweedie, James and Collazo, Ramon and Rice, Anthony and Xie, Jinqiao and Mita, Seiji and Dalmau, Rafael and Sitar, Zlatko}, year={2010}, month={Aug} }
 @article{losego_fitting kourkoutis_mita_craft_muller_collazo_sitar_maria_2009, title={Epitaxial Ba0.5Sr0.5TiO3–GaN heterostructures with abrupt interfaces}, volume={311}, ISSN={0022-0248}, url={http://dx.doi.org/10.1016/j.jcrysgro.2008.11.085}, DOI={10.1016/j.jcrysgro.2008.11.085}, abstractNote={Abstract Epitaxial heterostructures incorporating the complex ferroelectric oxide Ba 0.5 Sr 0.5 TiO 3 (BST) and GaN were prepared using a combination of RF magnetron sputtering and metalorganic chemical vapor deposition for the respective layers. The heterostructures were grown on c -plane sapphire substrates and were characterized using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM). This analysis showed that at substrate temperatures of 650 °C, BST films grow epitaxially on GaN with a {1 1 1} orientation and that the GaN–BST interface is smooth and abrupt, with disorder confined to grain boundaries and the interface plane. The film morphology is grainy indicating a 3-D growth mode. High-temperature post-deposition annealing studies suggest no interface reactions up to 900 °C. These results demonstrate that complex oxides like BST can be integrated with wide bandgap semiconductors like GaN and open exciting possibilities for new multifunctional devices.}, number={4}, journal={Journal of Crystal Growth}, publisher={Elsevier BV}, author={Losego, M.D. and Fitting Kourkoutis, L. and Mita, S. and Craft, H.S. and Muller, D.A. and Collazo, R. and Sitar, Z. and Maria, J.-P.}, year={2009}, month={Feb}, pages={1106–1109} }
 @article{mita_collazo_sitar_2009, title={Fabrication of a GaN lateral polarity junction by metalorganic chemical vapor deposition}, volume={311}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2009.01.075}, abstractNote={A fabrication process for growth of GaN lateral polarity junctions consisting of Ga-polar and N-polar domains grown simultaneously side-by-side on c-plane sapphire was developed using the polarity control scheme. An ammonia-annealing step following deposition and patterning of a thin low-temperature AlN nucleation layer played a crucial role in avoiding mixed-polarity growth of the remaining AlN nucleation layer, as well as in nitriding the bare sapphire surface to facilitate growth of N-polar GaN. The achievement of both polar domains, free from inversion domains within a contiguous domain, led to Ga-polar domain exhibiting featureless morphology with highly resistive characteristics, while N-polar domains exhibited hexagonally faceted morphology and were highly conductive.}, number={10}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Mita, S. and Collazo, R. and Sitar, Z.}, year={2009}, month={May}, pages={3044–3048} }
 @article{lu_collazo_dalmau_durkaya_dietz_raghothamachar_dudley_sitar_2009, title={Seeded growth of AlN bulk crystals in m- and c-orientation}, volume={312}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2009.10.008}, abstractNote={Seeded growth of AlN boules was achieved on m-(1 0 1¯ 0) and c-(0 0 0 1¯) orientations by physical vapor transport (PVT). The single crystalline m- and c-plane seeds were cut from freestanding AlN single crystals. AlN boules 12 mm in diameter and 7 mm in height were grown at source temperatures around 2280 °C in N2 atmosphere at 500 Torr of total pressure. Under identical process conditions, the m- and c-plane boules exhibited the same growth rates, 150–170 μm/h, and similar expansion angles, 22–27°, which indicated that the growth was controlled by the thermal profile inside the crucible rather than by crystallographic differences. X-ray rocking curve analysis and Raman spectroscopy confirmed that both m- and c-plane grown crystals possessed high crystalline quality. The dislocation density in both crystals was non-uniform and in the range 102–105 cm−2, as characterized by X-ray topography.}, number={1}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Lu, P. and Collazo, R. and Dalmau, R. F. and Durkaya, G. and Dietz, N. and Raghothamachar, B. and Dudley, M. and Sitar, Z.}, year={2009}, month={Dec}, pages={58–63} }
 @article{ihlefeld_losego_collazo_borland_maria_2008, title={Defect chemistry of nano-grained barium titanate films}, volume={43}, ISSN={["1573-4803"]}, DOI={10.1007/s10853-007-2135-3}, number={1}, journal={JOURNAL OF MATERIALS SCIENCE}, author={Ihlefeld, Jon F. and Losego, Mark D. and Collazo, Ramon and Borland, William J. and Maria, Jon-Paul}, year={2008}, month={Jan}, pages={38–42} }
 @article{lu_collazo_dalmau_durkaya_dietz_sitar_2008, title={Different optical absorption edges in AlN bulk crystals grown in m- and c-orientations}, volume={93}, ISSN={["1077-3118"]}, DOI={10.1063/1.2996413}, abstractNote={AlN single crystals were grown on m-plane (101¯0) and c-plane (0001¯) AlN seeds under identical growth conditions. The m-plane AlN crystals exhibited substantially lower oxygen incorporation, ∼1018cm−3, than the c-plane crystals, ∼1019cm−3. By investigating optical transmission spectra, m-plane AlN had absorption bands at 4.05 and 4.35eV, while c-plane AlN had an absorption band edge at 4.85eV. These below bandgap absorption bands strongly correlate with the reported transitions related to Al vacancy-impurity complexes, such as the complex of an Al vacancy and two oxygen atoms, (VAl–2ON)1− and the complex of an Al vacancy and one oxygen atom, (VAl–ON)2−, becoming the major cause for the poor, below bandgap optical transparency (α>200cm−1) of these crystals.}, number={13}, journal={APPLIED PHYSICS LETTERS}, author={Lu, P. and Collazo, R. and Dalmau, R. F. and Durkaya, G. and Dietz, N. and Sitar, Z.}, year={2008}, month={Sep} }
 @article{liu_collazo_mita_sitar_pennycook_duscher_2008, title={Direct Observation of Inversion Domain Boundaries of GaN onc-Sapphire at Sub-ångstrom Resolution}, volume={20}, ISSN={0935-9648 1521-4095}, url={http://dx.doi.org/10.1002/adma.200702522}, DOI={10.1002/adma.200702522}, abstractNote={Wide-bandgap III–nitrides have seen enormous success in modern electronic, optoelectronic, and even spintronic devices. Recently, interest has grown in manipulating the crystal polarity of GaN having a wurtzite structure, which provides a new degree of freedom for investigating III–nitrides and their novel devices. These studies include work on the inversion domain boundaries (IDBs) of GaN, which separate adjacent domains of different polarity. Ten years ago, Northrup et al. performed first-principles calculations of domain-boundary energies and proposed the structure shown in Figure 1, based on its very low energy (25 meV A ). Since then, no direct, indisputable test has been carried out to determine the exact boundary structure. At the same time, many unique properties have been observed at the IDBs. Among these is the remarkable effect, observed by Stutzmann et al. that the IDB can act as a rectifying junction when biased by two electrodes placed on adjacent Gaand N-face regions. The boundary between two adjacent domains with different polarity has been shown to be a very efficient radiative recombination center, which may have potential application for novel light-emitting devices. Meanwhile, it was found that the rectifying behavior of the IDBs can be explained by ab initio density functional calculations, assuming the IDBs have a structure as shown in Figure 1 (where a thin AlN layer is used to invert the polarity of GaN). However, the IDB structure has never been determined directly and their real structure remains unconfirmed. Here we show that we have determined directly the IDB structure, including the determination of GaN polarity, by aberration-corrected scanning transmission electron microscopy (STEM) at sub-Angstrom}, number={11}, journal={Advanced Materials}, publisher={Wiley}, author={Liu, Fude and Collazo, Ramon and Mita, Seiji and Sitar, Zlatko and Pennycook, Stephen J. and Duscher, Gerd}, year={2008}, month={Jun}, pages={2162–2165} }
 @article{losego_mita_collazo_sitar_maria_2008, title={Epitaxial growth of the metastable phase ytterbium monoxide on gallium nitride surfaces}, volume={310}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2007.10.002}, abstractNote={Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O2 system to access the metastable phase ytterbium monoxide (YbO). Experiments exploring the growth of polycrystalline YbOx films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O2 fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (0 0 1) GaN surface leads to the growth of epitaxial (1 1 1) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50 nm lateral dimensions. Rocking curves in ω and φ have full-width half-maximum values of 1.77° and 4.1°, respectively; further improvements in crystal quality appear to be limited by the thermal stability of the YbO phase.}, number={1}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Losego, Mark D. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2008}, month={Jan}, pages={51–56} }
 @article{collazo_mita_rice_dalmau_wellenius_muth_sitar_2008, title={Fabrication of a GaN p/n lateral polarity junction by polar doping selectivity}, volume={5}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200778624}, DOI={10.1002/pssc.200778624}, abstractNote={Abstract}, number={6}, journal={physica status solidi (c)}, publisher={Wiley}, author={Collazo, Ramón and Mita, Seiji and Rice, Anthony and Dalmau, Rafael and Wellenius, Patrick and Muth, John and Sitar, Zlatko}, year={2008}, month={May}, pages={1977–1979} }
 @article{mita_collazo_rice_dalmau_sitar_2008, title={Influence of gallium supersaturation on the properties of GaN grown by metalorganic chemical vapor deposition}, volume={104}, ISSN={["0021-8979"]}, DOI={10.1063/1.2952027}, abstractNote={A thermodynamic supersaturation model for gallium (Ga) was developed to describe GaN growth characteristics in low-pressure metalorganic chemical vapor deposition. The model takes into account the simplified GaN chemical reaction that occurs at the growth interface, Ga+NH3=GaN+3/2H2. The supersaturation was varied in two ways: (1) by the V/III ratio and (2) by the choice of the diluent gas. Two diluent gases were considered: H2, a commonly used diluent gas, and N2, a reaction inert gas. The choice of the diluent played a role in the degree of Ga supersaturation; since H2 is the product in the GaN formation, the addition of hydrogen significantly lowered the supersaturation. Atomic force microscopy revealed that surface morphology was associated with the different Ga supersaturation and the Burton–Cabrera–Frank model was used to relate it to the observed spiral size and terrace width. In addition to growth morphology, the degree of Ga supersaturation also influenced the carrier compensation level in n-type GaN. Secondary ion mass spectrometry studies identified that the trend of carbon incorporation followed that of the compensation level and inversely that of the Ga supersaturation. For samples intentionally doped with Si, it was found that GaN became semi-insulating when the carbon concentration exceeded that of silicon. In general, it has been shown that two growth processes at seemingly different conditions will be equivalent if the corresponding Ga supersaturation is the same. This finding showed that the supersaturation is a much more universal parameter than any other growth parameter alone.}, number={1}, journal={JOURNAL OF APPLIED PHYSICS}, author={Mita, S. and Collazo, R. and Rice, A. and Dalmau, R. F. and Sitar, Z.}, year={2008}, month={Jul} }
 @article{craft_collazo_losego_mita_sitar_maria_2008, title={Spectroscopic analysis of the epitaxial CaO (111)-GaN (0002) interface}, volume={92}, ISSN={["0003-6951"]}, DOI={10.1063/1.2887878}, abstractNote={We report an x-ray photoelectron spectroscopy (XPS) study of the CaO∕GaN interface. Epitaxial films of CaO (111) were grown on GaN (0002) and analyzed in situ using XPS. We observe Stranski–Krastanov growth, in which CaO coalesces rapidly, then converts to a three-dimensional mode. Data suggest coalescence within the first nanometer of film growth, indicating growth behavior different from published reports of the analogous MgO–GaN system. We find 1.0±0.2eV for the valence band offset and a 2.5±0.2eV conduction band offset. The results are discussed in terms of their utility in oxide-nitride electronic devices.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Craft, H. S. and Collazo, R. and Losego, M. D. and Mita, S. and Sitar, Z. and Maria, J. -P.}, year={2008}, month={Feb} }
 @article{craft_collazo_losego_sitar_maria_2008, title={Surface water reactivity of polycrystalline MgO and CaO films investigated using x-ray photoelectron spectroscopy}, volume={26}, ISSN={["0734-2101"]}, DOI={10.1116/1.3000058}, abstractNote={The authors report a study comparing the surface reactivities of the alkaline earth oxides MgO and CaO with respect to water vapor under ultrahigh and high vacuum conditions. Using x-ray photoelectron spectroscopy and a series of in vacuo exposures spanning ∼10−10Torr vacuum, to 10−6Torr flowing oxygen, the extent of surface hydroxylation was investigated by monitoring the O 1s photoelectron line. After the most aggressive exposures, the MgO surface reacted to form a maximum hydroxyl coverage of ∼1.3 ML (monolayer), while the CaO surface reached a maximum coverage of ∼3.0 ML. Both surface hydroxides could be removed by in vacuo thermal anneals; exposure to 250 and 500°C were required for MgO and CaO, respectively. These results are framed in terms of the suitability of these oxides in complex oxide-compound semiconductor heteroepitaxial multifunctional devices and with respect to understanding the development of surface morphology in epitaxial 111-oriented CaO and MgO thin films.}, number={6}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Craft, H. S. and Collazo, R. and Losego, M. D. and Sitar, Z. and Maria, J. -P.}, year={2008}, month={Nov}, pages={1507–1510} }
 @article{dietz_alevli_atalay_durkaya_collazo_tweedie_mita_sitar_2008, title={The influence of substrate polarity on the structural quality of InN layers grown by high-pressure chemical vapor deposition}, volume={92}, ISSN={["0003-6951"]}, DOI={10.1063/1.2840192}, abstractNote={The influence of substrate polarity on the properties of InN layers grown by high-pressure chemical vapor deposition has been studied. The 2Θ-ω x-ray diffraction scans on InN layers deposited on polar GaN epilayers revealed single-phase InN(0002) with a full width at half maximum (FWHM) of around 200arcsec. InN layers grown on N-polar GaN exhibit larger FWHMs. Rocking curve analysis confirmed single-phase InN for both growth polarities, with FWHM values for ω-RC(002) at 2080arcsec for InN grown on Ga-polar templates. The A1(LO) Raman mode analysis shows higher free carrier concentrations in InN grown on N-polar templates, indicating that polarity affects the incorporation of impurities.}, number={4}, journal={APPLIED PHYSICS LETTERS}, author={Dietz, N. and Alevli, M. and Atalay, R. and Durkaya, G. and Collazo, R. and Tweedie, J. and Mita, S. and Sitar, Z.}, year={2008}, month={Jan} }
 @article{liu_collazo_mita_sitar_duscher_2008, title={Three-dimensional geometry of nanometer-scale AlN pits: A new template for quantum dots?}, volume={20}, DOI={10.1002/adma.200701288}, abstractNote={The exact 3D geometry of nanometer-scale AlN pits is determined by Z-contrast imaging. The figure shows the 3D geometry of an AlN nano-pit and its corresponding GaN quantum dot. An atomic-resolution Z-contrast image is displayed in false color to clearly show the Z-contrast of the image, while the other panel displays a schematic 3D view.}, number={1}, journal={Advanced Materials}, author={Liu, F. D. and Collazo, Ramon and Mita, S. and Sitar, Z. and Duscher, G.}, year={2008}, pages={134-} }
 @article{craft_collazo_losego_mita_sitar_maria_2007, title={Band offsets and growth mode of molecular beam epitaxy grown MgO (111) on GaN (0002) by x-ray photoelectron spectroscopy}, volume={102}, ISSN={["1089-7550"]}, DOI={10.1063/1.2785022}, abstractNote={MgO is a proposed dielectric for use as a tunneling barrier in devices integrating GaN and ferroelectric oxides. In this study, we present data regarding the growth mode and band offsets of MgO grown epitaxially on GaN (0002) surfaces using molecular beam epitaxy. Using in situ x-ray photoelectron spectroscopy (XPS) and molecular beam epitaxy, we determine, from sequential growth experiments, that the growth of MgO proceeds via the Volmer-Weber (three-dimensional) mode, and full coalescence of the film does not occur until approximately 12nm of MgO has been deposited. The observation of a three-dimensional growth mode is in agreement with previously published data. For the valence band offset, we find a value of 1.2±0.2eV, which corresponds to a 3.2eV conduction band offset. XPS measurements suggest a chemically abrupt interface and no effect on band lineup due to the slow coalescence behavior.}, number={7}, journal={JOURNAL OF APPLIED PHYSICS}, author={Craft, H. S. and Collazo, R. and Losego, M. D. and Mita, S. and Sitar, Z. and Maria, J.-P.}, year={2007}, month={Oct} }
 @article{losego_mita_collazo_sitar_maria_2007, title={Epitaxial calcium oxide films deposited on gallium nitride surfaces}, volume={25}, ISSN={["1071-1023"]}, DOI={10.1116/1.2710243}, abstractNote={Solid solutions of rocksalt oxides are proposed for lattice-matched dielectrics in gallium nitride (GaN) electronics. This article explores the epitaxial growth of the rocksalt oxide calcium oxide (CaO) by molecular beam epitaxy on gallium nitride surfaces. As a possible end member to a rocksalt oxide solid solution, it is important to understand the processing space that allows for epitaxial CaO growth. Exposing the GaN surface to the oxidant flux prior to the metal flux is shown to be critical in eliminating polycrystalline growth. The effect of deposition temperature, metal flux, and oxidant flux on the film’s epitaxial crystalline quality is also examined. Optimal epitaxial quality is found for growth temperatures of ⩾600°C and near oxygen pressures of 10−6Torr. Thermal stability of the CaO∕GaN interface is experimentally evaluated. No reaction phases are observed by x-ray diffraction up to 850°C in air and >1100°C in a reducing atmosphere. However, CaO films are found to be extremely reactive with H2O, forming Ca(OH)2 within a few hours when exposed to ambient atmosphere at room temperature.}, number={3}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Losego, Mark D. and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2007}, pages={1029–1032} }
 @article{mita_collazo_dalmau_sitar_2007, title={Growth of highly resistive Ga-polar GaN by LP-MOVPE}, volume={4}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200674837}, DOI={10.1002/pssc.200674837}, abstractNote={Abstract}, number={7}, journal={physica status solidi (c)}, publisher={Wiley}, author={Mita, Seiji and Collazo, Ramón and Dalmau, Rafael and Sitar, Zlatko}, year={2007}, month={Jun}, pages={2260–2263} }
 @article{collazo_mita_dalmau_sitar_2007, title={Impact of polarity control and related defects on the electrical properties of GaN grown by MOVPE}, volume={4}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200674874}, DOI={10.1002/pssc.200674874}, abstractNote={Abstract}, number={7}, journal={physica status solidi (c)}, publisher={Wiley}, author={Collazo, R. and Mita, S. and Dalmau, R. and Sitar, Z.}, year={2007}, month={Jun}, pages={2597–2600} }
 @article{collazo_mita_rice_dalmau_sitar_2007, title={Simultaneous growth of a GaN p/n lateral polarity junction by polar selective doping}, volume={91}, ISSN={["1077-3118"]}, DOI={10.1063/1.2816893}, abstractNote={Differences in surface energy between the Ga-polar orientation and the N-polar orientation of GaN translate into a completely different behavior for the incorporation of intentional and unintentional impurities. Oxygen is found to be an impurity with higher concentrations in N-polar films than in Ga-polar films and is the cause for the high n-type carrier concentration observed in N-polar films. We fabricated a lateral p∕n junction in GaN by the simultaneous growth of p- and n-type regions, utilizing the doping selectivity of the two different polar domains. The N-polar domains were n type with a carrier concentration of 1.7×1019cm−3, predominantly due to high oxygen incorporation, while Ga-polar domains were p type with a carrier concentration of 1.7×1017cm−3. No significant difference in Mg incorporation was observed between the two polar domains. This junction showed the characteristics that define a p∕n junction: current rectification, electroluminescence, and the photovoltaic effect.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Collazo, R. and Mita, S. and Rice, A. and Dalmau, R. F. and Sitar, Z.}, year={2007}, month={Nov} }
 @article{liu_collazo_mita_sitar_duscher_pennycook_2007, title={The mechanism for polarity inversion of GaN via a thin AlN layer: Direct experimental evidence}, volume={91}, ISSN={["1077-3118"]}, DOI={10.1063/1.2815748}, abstractNote={Lateral-polarity heterostructures of GaN on c sapphire were prepared by deposition and patterning of a thin low-temperature AlN nucleation layer. Adjacent macroscopic domains were found to have opposite polarity; domains grown on the AlN nucleation layer were Ga polar while those grown on the nitrided sapphire were N polar, as confirmed by convergent-beam electron diffraction and Z-contrast images. We directly determined the atomic interface structure between the AlN and c sapphire with an aberration-corrected scanning transmission electron microscope at ∼1.0Å resolution. This is the direct experimental evidence for the origin of the polarity control in III nitrides. This understanding is an important step toward manipulating polarity in these semiconductors.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Liu, Fude and Collazo, Ramon and Mita, Seiji and Sitar, Zlatko and Duscher, Gerd and Pennycook, Stephen J.}, year={2007}, month={Nov} }
 @article{dalmau_collazo_mita_sitar_2007, title={X-ray photoelectron spectroscopy characterization of aluminum nitride surface oxides: Thermal and hydrothermal evolution}, volume={36}, DOI={10.1007/s11664-006-0044-x}, number={4}, journal={Journal of Electronic Materials}, author={Dalmau, R. and Collazo, Ramon and Mita, S. and Sitar, Z.}, year={2007}, pages={414–419} }
 @article{govindaraju_aleksov_li_okuzumi_wolter_collazo_prater_sitar_2006, title={Comparative study of textured diamond films by thermal conductivity measurements}, volume={85}, ISSN={["1432-0630"]}, DOI={10.1007/s00339-006-3697-7}, number={3}, journal={APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING}, author={Govindaraju, N. and Aleksov, A. and Li, X. and Okuzumi, F. and Wolter, S. D. and Collazo, R. and Prater, J. T. and Sitar, Z.}, year={2006}, month={Nov}, pages={331–335} }
 @article{aleksov_collazo_mita_schlesser_sitar_2006, title={Current-voltage characteristics of n/n lateral polarity junctions in GaN}, volume={89}, ISSN={["1077-3118"]}, DOI={10.1063/1.2244046}, abstractNote={Lateral Si:N-polar/Si:Ga-polar GaN homojunctions were fabricated using metal organic chemical vapor deposition with nitrogen as the carrier and dilution gas. Nominally undoped N-polar areas are n-type conductive, while nominally undoped Ga-polar areas are insulating with carrier densities below 1×1015cm−3. This allows for the fabrication of selectively doped areas within one growth step that can be used to fabricate novel lateral device structures or enhance existing III-N-device structures. In this letter we investigated the electrical properties of the simplest case of these junctions, namely, when both sides are n-type conductive. The results of the IV measurements show a linear characteristic both for the measurement of a N-polar/Si:Ga-polar junction and a N-polar/Si:Ga-polar/N-polar double junction. This result indicates that, as expected, there are no energy barriers between the N-polar and the Ga-polar material and that these structures can be used to achieve laterally selective doped areas in Ga–N for electronic device applications.}, number={5}, journal={APPLIED PHYSICS LETTERS}, author={Aleksov, Aleksandar and Collazo, Ramon and Mita, Seiji and Schlesser, Raoul and Sitar, Zlatko}, year={2006}, month={Jul} }
 @article{collazo_mita_aleksov_schlesser_sitar_2006, title={Growth of Ga- and N- polar gallium nitride layers by metalorganic vapor phase epitaxy on sapphire wafers}, volume={287}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2005.10.080}, abstractNote={Following an already established polarity control scheme for GaN thin films, we developed a process to simultaneously grow Ga- and N-polarity layers side by side on c-plane sapphire. The simultaneous growth is achieved by properly treating the AlN nucleation/buffer layer and subsequent substrate annealing. During this process, the growth is mass-transfer-limited, permitting the same growth rate for both types of polarity domains. Smooth domains of both polarity types (RMS roughness ∼1–2 nm) were obtained.}, number={2}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Collazo, R and Mita, S and Aleksov, A and Schlesser, R and Sitar, Z}, year={2006}, month={Jan}, pages={586–590} }
 @article{li_perkins_collazo_nemanich_sitar_2006, title={Investigation of the effect of the total pressure and methane concentration on the growth rate and quality of diamond thin films grown by MPCVD}, volume={15}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2006.09.008}, abstractNote={The influence of total gas pressure (50–125 Torr) and methane concentration (0.75%–10%) on diamond growth by microwave plasma chemical vapor deposition (MPCVD) was investigated. Within the regimes studied, the growth rate was proportional to the methane concentration in the source gas while it exhibited a super-linear dependence on total pressure. For a fixed methane concentration, characterization by Raman spectroscopy, scanning electron microscopy and X-ray diffraction indicated there was a minimum pressure required for the growth of large grain diamond, and conversely, for a fixed pressure, there was a maximum methane concentration that yielded diamond deposition. Higher pressures and higher carbon concentrations yielded diamond growth rates more than 10 times higher than achieved by the conventional low pressure MPCVD process.}, number={11-12}, journal={DIAMOND AND RELATED MATERIALS}, author={Li, Xianglin and Perkins, James and Collazo, Ramon and Nemanich, Robert J. and Sitar, Zlatko}, year={2006}, pages={1784–1788} }
 @article{craft_ihlefeld_losego_collazo_sitar_maria_2006, title={MgO epitaxy on GaN (0002) surfaces by molecular beam epitaxy}, volume={88}, ISSN={["1077-3118"]}, DOI={10.1063/1.2201041}, abstractNote={We report on the epitaxial deposition of magnesium oxide films with [111] crystallographic orientation on (0002) GaN by molecular beam epitaxy. Specifically, we use an adsorption controlled growth mechanism to initiate the growth process. Electron diffraction shows a spotty intense pattern without intensity fluctuations during growth and evidence of in-plane twinning. X-ray diffraction reveals the films to be epitaxial with full width at half maximum values of 0.3°, 0.5°, and 1° in 2θ, ϕ, and χ circles, respectively. Wet etching of the GaN surface with a HCl:HF mixture prior to growth is critical for achieving high crystalline quality. Epitaxial growth is observed between room temperature and 650°C, with negligible changes in crystalline quality with increased temperature. Atomic force microscopy analysis shows grainy surfaces with feature sizes near 10nm and rms roughness values of 1.4Å over 1μm2 areas. X-ray diffraction analysis suggests MgO film stability up to 850°C in ex situ air annealing.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Craft, H. S. and Ihlefeld, J. F. and Losego, M. D. and Collazo, R. and Sitar, Z. and Maria, J. -P.}, year={2006}, month={May} }
 @article{craft_collazo_sitar_maria_2006, title={Molecular beam epitaxy of Sm2O3, Dy2O3, and Ho2O3 on Si (111)}, volume={24}, ISSN={["2166-2746"]}, DOI={10.1116/1.2216721}, abstractNote={We report on the epitaxial deposition of Sm2O3, Dy2O3, and Ho2O3 films on Si (111) substrates as progress towards developing a predictive model for gate oxide selection. To date, films have been characterized by reflection high-energy electron diffraction, x-ray diffraction (XRD), and x-ray photoemission spectroscopy (XPS). In the case of Sm2O3, difficulty was encountered in achieving the growth of phase pure films; growth of the desired bixbyite oxide phase was accompanied by significant amounts of monoclinic oxide as detected by XRD. Dy2O3 and Ho2O3 films can be produced free of unwanted phases when deposited using a background pressure of 1×10−6torr O2∕O3 and a substrate temperature between 425 and 550°C. Dy2O3 films with an x-ray peak width of 0.6° in ω were obtained. XPS studies of the Dy2O3∕Si interface are in progress, and verify the phase purity of the films.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Craft, H. S. and Collazo, R. and Sitar, Z. and Maria, J. P.}, year={2006}, pages={2105–2110} }
 @article{herro_zhuang_schlesser_collazo_sitar_2006, title={Seeded growth of AlN on N- and Al-polar < 0 0 0 1 > AlN seeds by physical vapor transport}, volume={286}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2005.10.074}, abstractNote={We demonstrated seeded growth of AlN on large-area Al- and N-polar <0 0 0 1>-oriented AlN seeds using the physical vapor transport method (PVT). In both cases, crystals having a diameter of 15 mm were obtained from 5 mm seeds. Based on growth step and terrace width analyses, it was found that the N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation.}, number={2}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Herro, ZG and Zhuang, D and Schlesser, R and Collazo, R and Sitar, Z}, year={2006}, month={Jan}, pages={205–208} }
 @article{schlesser_dalmau_zhuang_collazo_sitar_2005, title={Crucible materials for growth of aluminum nitride crystals}, volume={281}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2005.03.014}, abstractNote={The growth of aluminum nitride (AlN) bulk crystals by sublimation of an AlN source requires elevated temperatures, typically in a range of 1800–2300 °C. These temperature requirements, combined with the chemically aggressive nature of the Al vapor, severely limit the choice of reactor hot-zone materials, and most notably, the selection of reaction crucibles. Aside from refractory elements, potentially promising compound materials include refractory nitrides, carbides, and borides. In this work, TaC crucibles were fabricated using a binderless sintering process and were tested in AlN bulk growth experiments. Elemental analysis of crystals grown in these crucibles revealed extremely low Ta contamination, below the analytical detection limit of 1 ppm by weight and C contamination levels as low as 50 ppm by weight; C contamination likely originated from sources unrelated to the crucible material. Crucibles were re-used in several consecutive growth runs; average crucible lifetimes exceeded 200 h at growth temperatures exceeding 2200 °C.}, number={1}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Schlesser, R and Dalmau, R and Zhuang, D and Collazo, R and Sitar, Z}, year={2005}, month={Jul}, pages={75–80} }
 @article{collazo_mita_schlesser_sitar_2005, title={Polarity control of GaN thin films grown by metalorganic vapor phase epitaxy}, volume={2}, ISSN={1610-1634 1610-1642}, url={http://dx.doi.org/10.1002/pssc.200461546}, DOI={10.1002/pssc.200461546}, abstractNote={Control of the polarity of GaN thin films grown on c-plane sapphire by low-pressure metalorganic vapor phase epitaxy has been achieved. Growth of either polarity was realized by identifying and selecting appropriate substrate pre-treatments and AlN LT-buffer growth conditions. N-face (–c) GaN films were only obtained by pre-nitridation of the sapphire substrate after a H2 anneal. Smooth films of each type of polarity were obtained by using N2 as a transport and dilution gas and a V/III ratio of less than 500. The same growth rate of 1.2 µm/h was observed for both types of polarities under the same growth conditions. Further analysis of the growth conditions demonstrated that by using N2 as the dilution gas the transport was mass-transfer limited. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={7}, journal={physica status solidi (c)}, publisher={Wiley}, author={Collazo, R. and Mita, S. and Schlesser, R. and Sitar, Z.}, year={2005}, month={May}, pages={2117–2120} }
 @article{brenner_schlesser_sitar_dalmau_collazo_li_2004, title={Model for the influence of boron impurities on the morphology of AIN grown by physical vapor transport}, volume={560}, ISSN={["1879-2758"]}, DOI={10.1016/j.susc.2004.05.003}, abstractNote={We propose that the reduction of the Schwoebel barrier by trace B impurities is responsible for the relatively flat c-plane morphology observed in AlN crystals that are deposited via physical vapor transport in BN crucibles relative to crystals grown in boron-free conditions. The model is supported by molecular statics calculations that predict that B substitutional impurities are energetically preferred at steps on the (0 0 0 1) surface, and that these impurities enhance binding and incorporation of growth species onto steps.}, number={1-3}, journal={SURFACE SCIENCE}, author={Brenner, DW and Schlesser, R and Sitar, Z and Dalmau, R and Collazo, R and Li, Y}, year={2004}, month={Jul}, pages={L202–L206} }
 @article{chang_chattopadhyay_chen_chen_chen_chen_collazo_sitar_2003, title={Band-gap dependence of field emission from one-dimensional nanostructures grown on n-type and p-type silicon substrates}, volume={68}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.68.125322}, abstractNote={Field emission of electrons from narrow-band-gap and wide-band-gap one-dimensional nanostructures were studied. N-type silicon substrates enhanced the emission from the low-band-gap silicon nanowires and carbon nanotubes, whereas p-type substrates were a better choice for field emission from wide-band-gap silicon carbon nitride nanocrystalline thin films and nanorods. The role of the substrate-nanostructure interface was modeled based on different junction mechanisms to explain, qualitatively, the fundamentally different emission behavior of these nanostructures when n- and p-type silicon substrates were used. The results could be explained on the basis of simple carrier transport across the silicon-silicon nanowire interface and subsequent tunneling of electrons for the silicon nanowires. Schottky barrier theory can explain the better field emission of electrons from the n-type silicon supported carbon nanotubes. The decreased barrier height at the interface of the silicon-silicon carbon nitride heterojunction, when p-type silicon substrate was used, could explain the superior field emission in comparison to when n-type silicon substrates were used.}, number={12}, journal={PHYSICAL REVIEW B}, author={Chang, CS and Chattopadhyay, S and Chen, LC and Chen, KH and Chen, CW and Chen, YF and Collazo, R and Sitar, Z}, year={2003}, month={Sep} }
 @article{collazo_schlesser_roskowski_miraglia_davis_sitar_2003, title={Electron energy distribution during high-field transport in AlN}, volume={93}, ISSN={["0021-8979"]}, DOI={10.1063/1.1543633}, abstractNote={The energy distribution of electrons transported through intrinsic AlN heteroepitaxial films grown on SiC was directly measured as a function of applied field and AlN film thickness. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through films thicker than 95 nm at an applied field between 200 and 350 kV/cm occurred as steady-state hot electron transport following a Maxwellian energy distribution with a characteristic carrier temperature. At higher fields (470 kV/cm), intervalley scattering was evidenced by a multicomponent energy distribution featuring a second peak at the energy position of the first satellite valley. Velocity overshoot was observed in films thinner than 95 nm and at fields greater than 550 kV/cm. In this case, a symmetric energy distribution centered at an energy above the conduction band minimum was measured, indicating that the drift component of the electron velocity was on the order of the “thermal” component. A transient transport length of less than 80 nm was deduced from these observations.}, number={5}, journal={JOURNAL OF APPLIED PHYSICS}, author={Collazo, R and Schlesser, R and Roskowski, A and Miraglia, P and Davis, RF and Sitar, Z}, year={2003}, month={Mar}, pages={2765–2771} }
 @article{wolter_borca-tasciuc_chen_govindaraju_collazo_okuzumi_prater_sitar_2003, title={Thermal conductivity of epitaxially textured diamond films}, volume={12}, ISSN={["0925-9635"]}, DOI={10.1016/S0925-9635(02)00248-0}, abstractNote={The in-plane thermal conductivity of epitaxially textured, (1 0 0) diamond has been evaluated in comparison to (1 0 0) diamond possessing a random fiber texture. The diamond films were bias-enhanced nucleated using an alternating current source and grown on (1 0 0) silicon using microwave plasma chemical vapor deposition. Thermal conductivity values of ∼1120 (±11%) W/m K and ∼550 (±11%) W/m K were determined for the epitaxial versus non-epitaxial diamond, respectively, using Joule heating thermometry. This twofold improvement in the thermal conductivity of the epitaxially textured diamond is ascribed to a reduction in the grain boundary defects within the film.}, number={1}, journal={DIAMOND AND RELATED MATERIALS}, author={Wolter, SD and Borca-Tasciuc, DA and Chen, G and Govindaraju, N and Collazo, R and Okuzumi, F and Prater, JT and Sitar, Z}, year={2003}, month={Jan}, pages={61–64} }
 @article{collazo_schlesser_sitar_2002, title={Experimental observation of electron velocity overshoot in AlN}, volume={81}, ISSN={["0003-6951"]}, DOI={10.1063/1.1534407}, abstractNote={The energy distribution of electrons transported through intrinsic AlN heteroepitaxial films grown on 6H-SiC was directly measured as a function of the applied electric field. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through 80-nm-thick layers indicated the onset of quasiballistic transport for fields greater than 510 kV/cm. This was evidenced by a symmetric energy distribution centered at energies above the conduction band minimum. Drifted Fermi–Dirac energy distribution was fitted to the measured energy distribution, with the energy scale referenced to the bottom of the AlN conduction band. The drift energy and the carrier temperature were obtained as fitting parameters. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80 nm was deduced. In addition, the velocity-field characteristic was derived from these observations.}, number={27}, journal={APPLIED PHYSICS LETTERS}, author={Collazo, R and Schlesser, R and Sitar, Z}, year={2002}, month={Dec}, pages={5189–5191} }
 @article{collazo_schlesser_sitar_2002, title={Role of adsorbates in field emission from nanotubes}, volume={11}, ISSN={["0925-9635"]}, DOI={10.1016/S0925-9635(01)00585-4}, abstractNote={Two field-emission states of single-walled carbon nanotubes were identified according to their respective emission current levels. The state yielding increased emission current was attributed to the presence of adsorbates on the nanotubes as confirmed by electron emission measurements at different background pressures. Application of high electric fields induced large emission currents and a transition between the two states. During this transition, a current drop to 1/10 of the original value was observed. For the high current state, field-emitted electrons originated from states located below the Fermi level, as was determined by field-emission energy distribution measurements. This suggested that adsorbates introduced a resonant state on the surface that enhanced the tunneling probability of electrons. The adsorbed states are removed at high applied electric fields, presumably due to thermal effects caused by large emission currents. This adsorption/desorption process is completely reversible.}, number={3-6}, journal={DIAMOND AND RELATED MATERIALS}, author={Collazo, R and Schlesser, R and Sitar, Z}, year={2002}, pages={769–773} }
 @article{yushin_wolter_kvit_collazo_stoner_prater_sitar_2002, title={Study of fusion bonding of diamond to silicon for silicon-on-diamond technology}, volume={81}, ISSN={["0003-6951"]}, DOI={10.1063/1.1516636}, abstractNote={Diamond films grown on silicon were polished to a root-mean-square roughness of 15 nm and bonded to (100) silicon in a dedicated ultrahigh vacuum bonding chamber. Successful bonding was observed at temperatures as low as 950 °C under a uniaxial mechanical stress of 32 MPa. Scanning acoustic microscopy indicated complete bonding at fusion temperatures above 1150 °C, and some cracking of the diamond film. Cross section transmission electron microscopy of the same specimens revealed an intermediate layer consisting of silicon, carbon, and oxygen. This layer was approximately 30 nm thick and had an amorphous structure.}, number={17}, journal={APPLIED PHYSICS LETTERS}, author={Yushin, GN and Wolter, SD and Kvit, AV and Collazo, R and Stoner, BR and Prater, JT and Sitar, Z}, year={2002}, month={Oct}, pages={3275–3277} }
 @article{collazo_schlesser_sitar_2001, title={Two field-emission states of single-walled carbon nanotubes}, volume={78}, ISSN={["0003-6951"]}, DOI={10.1063/1.1361089}, abstractNote={Two field-emission states of single-walled carbon nanotubes have been identified according to their respective emission current levels. The state yielding increased emission current has been attributed to the presence of adsorbates on the nanotubes. It was realized that, by application of high electric fields inducing large emission currents, a transition between the two states could be induced. For the high current state, field-emitted electrons originated from states located 1 eV below the Fermi level, as was determined by field-emission energy distribution measurements. This suggested that adsorbates introduced a resonant state on the surface that enhanced the tunneling probability of the electrons. These states are removed when the nanotubes are cleaned by application of a large electric field, thus, decreasing the field-emitted current.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Collazo, R and Schlesser, R and Sitar, Z}, year={2001}, month={Apr}, pages={2058–2060} }
 @article{schlesser_collazo_bower_zhou_sitar_2000, title={Energy distribution of field emitted electrons from carbon nanotubes}, volume={9}, ISSN={["1879-0062"]}, DOI={10.1016/s0925-9635(99)00277-0}, abstractNote={Field emission properties of single-walled carbon nanotubes (SWNTs), deposited on n-type Si substrates, were investigated by a combination of classical I–V characterization and field emission energy distribution (FEED) measurements. I–V characterization showed that current densities on the order of 1 mA/cm2 could be obtained at nominal electric fields as low as 3 V/μm. A current density of up to 25 mA/cm2 was observed, and long-term current stability and reproducibility were excellent. FEED measurements revealed that at low current densities, the field emitted electrons originated from energy levels close to the Fermi level of the Si substrate. At larger applied voltages and larger current densities, I–V characterization of the field emitters showed primarily an ohmic behavior; FEED data taken under the same experimental conditions showed a shift of the main spectral peak toward lower kinetic energies with increasing applied voltages. This energy shift was found to be linear with the applied voltage and emission current over a wide range. Both FEED and I–V data thus indicated that the field emission current was primarily limited by the ohmic resistance of the nanotubes and/or the contact resistance between the nanotubes and the Si substrate; typical measured values for the cathode surface resistances were 100 kΩ cm2.}, number={3-6}, journal={DIAMOND AND RELATED MATERIALS}, author={Schlesser, R and Collazo, R and Bower, C and Zhou, O and Sitar, Z}, year={2000}, pages={1201–1204} }
 @article{collazo_schlesser_roskowski_davis_sitar_2000, title={Hot electron transport in AlN}, volume={88}, ISSN={["0021-8979"]}, DOI={10.1063/1.1318386}, abstractNote={The energy distribution of electrons that were transported through a thin intrinsic AlN film was directly measured as a function of the applied field. The measurements were realized by extracting the electrons into vacuum through a semitransparent Au contact and measuring their energies using an electron spectrometer. At moderate applied fields (100 kV/cm), the energy distribution was found to follow a Maxwellian model corresponding to a temperature of 2700 K and a drift component below the spectrometer resolution. At higher fields, intervalley scattering was evidenced by the presence of a second peak at 0.7 eV. This coincides well with the energy position of the LM valleys in AlN.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Collazo, R and Schlesser, R and Roskowski, A and Davis, RF and Sitar, Z}, year={2000}, month={Nov}, pages={5865–5869} }
 @inproceedings{hoffmann_gerhold_kirste_rice_akouala_xie_mita_collazo_sitar, title={Fabrication and characterization of lateral polar GaN structures for second harmonic generation}, volume={8631}, booktitle={Quantum sensing and nanophotonic devices x}, author={Hoffmann, M. P. and Gerhold, M. and Kirste, R. and Rice, A. and Akouala, C. R. and Xie, J. Q. Q. and Mita, S. and Collazo, R. and Sitar, Z.} }
 @article{collazo_schlesser_sitar, title={Field emission from carbon nanotubes}, volume={13}, number={5}, journal={New Diamond and Frontier Carbon Technology}, author={Collazo, R. and Schlesser, R. and Sitar, Z.}, pages={297–306} }
 @inproceedings{seidlitz_fali_kankanamge_alden_collazo_hoffmann_dietz_abate, title={Infrared nanoscopy of In-rich InGaN epilayers (Conference Presentation)}, volume={9954}, booktitle={Fifteenth international conference on solid state lighting and led-based illumination systems}, author={Seidlitz, D. and Fali, A. and Kankanamge, I. S. M. and Alden, D. and Collazo, R. and Hoffmann, A. and Dietz, N. and Abate, Y.} }
 @article{guo_kirste_bryan_bryan_gerhold_collazo_sitar, title={Nanostructure surface patterning of GaN thin films and application to AlGaN/AlN multiple quantum wells: A way towards light extraction efficiency enhancement of III-nitride based light emitting diodes}, volume={117}, number={11}, journal={Journal of Applied Physics}, author={Guo, W. and Kirste, R. and Bryan, Z. and Bryan, I. and Gerhold, M. and Collazo, R. and Sitar, Z.} }
 @inproceedings{alden_bryan_gaddy_bryan_callsen_koukitu_kumagai_hoffmann_irving_sitar_et al., title={On the origin of the 4.7 eV absorption and 2.8 eV emission bands in bulk AlN substrates}, volume={72}, number={5}, booktitle={Wide bandgap semiconductor materials and devices 17}, author={Alden, D. and Bryan, Z. and Gaddy, B. E. and Bryan, I. and Callsen, G. and Koukitu, A. and Kumagai, Y. and Hoffmann, A. and Irving, D. L. and Sitar, Z. and et al.}, pages={31–40} }
 @inproceedings{hoffmann_tweedie_kirste_bryan_bryan_gerhold_sitar_collazo, title={Point defect management in GaN by Fermi-level control during growth}, volume={8986}, booktitle={Gallium nitride materials and devices ix}, author={Hoffmann, M. P. and Tweedie, J. and Kirste, R. and Bryan, Z. and Bryan, I. and Gerhold, M. and Sitar, Z. and Collazo, R.} }
 @misc{tsao_chowdhury_hollis_jena_johnson_jones_kaplar_rajan_walle_bellotti_et al., title={Ultrawide-bandgap semiconductors: Research opportunities and challenges}, volume={4}, number={1}, journal={Advanced Electronic Materials}, author={Tsao, J. Y. and Chowdhury, S. and Hollis, M. A. and Jena, D. and Johnson, N. M. and Jones, K. A. and Kaplar, R. J. and Rajan, S. and Walle, C. G. and Bellotti, E. and et al.} }
 @misc{tsao_chowdhury_hollis_jena_johnson_jones_kaplar_rajan_walle_bellotti_et al., title={Ultrawide-bandgap semiconductors: Research opportunities and challenges}, volume={4}, number={1}, journal={Advanced Electronic Materials}, author={Tsao, J. Y. and Chowdhury, S. and Hollis, M. A. and Jena, D. and Johnson, N. M. and Jones, K. A. and Kaplar, R. J. and Rajan, S. and Walle, C. G. and Bellotti, E. and et al.} }