@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{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{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{loveless_kirste_moody_reddy_rathkanthiwar_almeter_collazo_sitar_2023, title={Performance and reliability of state-of-the-art commercial UVC light emitting diodes}, url={https://doi.org/10.1016/j.sse.2023.108775}, DOI={10.1016/j.sse.2023.108775}, abstractNote={Commercially-available, sapphire- and AlN-based ultraviolet-C light emitting diodes (UVC LEDs) emitting in the range of 265–275 nm were evaluated and compared. Both AlN- and sapphire-based devices produced clean emission lines without parasitics and well-exceeded the industry-required L70 lifetimes of 10,000 h. Electrical and optical characterization revealed that devices grown on native aluminum nitride (AlN) substrates sustained up to 2.6 times greater current density and 6 times higher output power densities than their counterparts grown on sapphire substrate. Interestingly, the cost-performance ($/mW) was similar for both platforms despite the significantly pricier AlN substrates.}, journal={Solid-State Electronics}, author={Loveless, James and Kirste, Ronny and Moody, Baxter and Reddy, Pramod and Rathkanthiwar, Shashwat and Almeter, Jack and Collazo, Ramon and Sitar, Zlatko}, year={2023}, month={Nov} }
 @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{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{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{ryu_reddy_collazo_dickey_2022, title={Effects of temperature and oxygen partial pressure on electrical conductivity of Fe-doped β-Ga2O3 single crystals}, volume={120}, url={https://doi.org/10.1063/5.0093588}, DOI={10.1063/5.0093588}, abstractNote={In this work, we measure DC and AC conductivity and Hall voltage to determine the origin of electrical insulating properties of Fe-doped β-Ga2O3 single crystals, which are measured perpendicular to the 2¯01 crystallographic plane. We find that electrical conduction is predominantly controlled by free electrons in the temperature range 230–800 °C with the mutual compensation of the impurity donor (Si) and acceptor dopant (Fe), explaining the low concentration of free electrons and Fermi level pinning over a wide range of temperatures. Furthermore, the negative temperature-dependence of the carrier mobility indicates that it is limited by optical phonon scattering. Importantly, we find electrical conductivity to be largely independent of oxygen partial pressure (pO2) from air to 10−4 atm at 600 °C, but it becomes slightly dependent on pO2 at 800 °C, as intrinsic non-stoichiometric point defects begin to influence the charge balance.}, number={18}, journal={Applied Physics Letters}, author={Ryu, Gyunghyun and Reddy, Pramod and Collazo, Ramón and Dickey, Elizabeth C.}, year={2022}, month={May} }
 @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_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{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{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={https://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{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{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{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_sengupta_reddy_kohn_sitar_collazo_pavlidis_2020, title={Chemical treatment effects on Schottky contacts to metalorganic chemical vapor deposited n-type N-polar GaN}, url={https://doi.org/10.1063/5.0015140}, DOI={10.1063/5.0015140}, abstractNote={In this work, we investigate the effect of different chemical treatments, such as solvents, bases, and acids, on the surface properties and electrical behavior of Schottky diodes fabricated on metalorganic chemical vapor deposition-grown, n-type, N-polar GaN. The I–V and C–V barrier heights of the as-grown Schottky diodes are found to be 0.40 eV and 0.60 eV, respectively, with an ideality factor n = 1.07. It is found that the solvent treatments neither change the surface nor the electrical performance of the Schottky diodes, as expected. However, the treatments by the alkaline photolithography developer and acid—the latter of which is often used to clean the surface of Ga-polar GaN films prior to metal contact deposition—degrade the performance of N-polar GaN. These base and acid treatments severely roughen the surface by creating triangular/hexagonal crystallographic facets. The I–V and C–V barrier heights of these base- and acid-treated diodes are increased to 0.63 eV and 1.00 eV, respectively, with ideality factor values n < 1.2. X-ray photoelectron spectroscopy studies indicate that base- and acid-treated surfaces have lower oxygen content as compared to the as-grown sample surface. It is understood that the increment in the barrier height for base- and acid-treated diodes could be due to the change in polarity, from N-polar to semi-polar/non-polar, on these crystallographic features. All these results demonstrate that, unlike for Ga-polar GaN, the N-polar GaN surface is highly reactive to bases and acids.}, journal={Journal of Applied Physics}, author={Khachariya, Dolar and Szymanski, Dennis and Sengupta, Rohan and Reddy, Pramod and Kohn, Erhard and Sitar, Zlatko and Collazo, Ramón and Pavlidis, Spyridon}, year={2020}, month={Aug} }
 @article{klump_hoffmann_kaess_tweedie_reddy_kirste_sitar_collazo_2020, title={Control of passivation and compensation in Mg-doped GaN by defect quasi Fermi level control}, url={https://doi.org/10.1063/1.5126004}, DOI={10.1063/1.5126004}, abstractNote={A defect quasi Fermi level (dQFL) control process based on above bandgap illumination was applied to control H and VN-complexes, which are the main contributors to the passivation and self-compensation, respectively, in Mg:GaN grown via metalorganic chemical vapor deposition. Secondary ion mass spectrometry measurements confirmed that the total Mg incorporation was unaffected by the process. However, the total H concentration was reduced to similar levels obtained by post-growth thermal activation prior to any annealing treatment. Similarly, the 2.8 eV emission in the photoluminescence spectra, attributed to compensating VN and its complexes, was reduced for the dQFL-process samples. After thermal activation and Ni/Au contact deposition, Hall effect measurements revealed lower resistivities (increased mobilities and free hole concentrations) for dQFL-grown samples with Mg doping concentrations above and below 2 × 1019 cm−3. All these results demonstrate that the dQFL process can effectively reduce the H-passivation and self-compensation of the Mg:GaN films.}, journal={Journal of Applied Physics}, author={Klump, A. and Hoffmann, M. P. and Kaess, F. and Tweedie, J. and Reddy, P. and Kirste, R. and Sitar, Z. and Collazo, R.}, year={2020}, month={Jan} }
 @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={https://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{bagheri_reddy_kim_rounds_sochacki_kirste_bockowski_collazo_sitar_2020, title={Impact of impurity-based phonon resonant scattering on thermal conductivity of single crystalline GaN}, url={https://doi.org/10.1063/5.0018824}, DOI={10.1063/5.0018824}, abstractNote={The impact of impurities on the thermal conductivity of halide vapor phase epitaxy gallium nitride (GaN) was studied. Phonon resonances with impurities, modeled as Lorentz oscillators, were used to explain the much lower thermal conductivity than predicted by the Debye–Callaway model. The resonance energies for the oscillators were determined by Raman spectroscopy for Mn and by mass difference approximation for C and Fe. Employing the obtained resonance energies and proportionality factors extracted as fitting parameters, the modified model showed a good agreement with the experimental data. While the doping decreased thermal conductivity for all temperatures, the room temperature values started decreasing significantly once the doping levels approached ∼1019 cm−3. Consequently, required doping levels to achieve certain GaN-based devices may reduce the thermal conductivity of GaN by as much as 1/3.}, journal={Applied Physics Letters}, author={Bagheri, Pegah and Reddy, Pramod and Kim, Ji Hyun and Rounds, Robert and Sochacki, Tomasz and Kirste, Ronny and Bockowski, Michał and Collazo, Ramón and Sitar, Zlatko}, year={2020}, month={Aug} }
 @article{guo_kirste_reddy_mecouch_guan_mita_washiyama_tweedie_sitar_collazo_2020, title={Impact of the effective refractive index in AlGaN-based mid-UV laser structures on waveguiding}, volume={7}, url={https://doi.org/10.35848/1347-4065/abab44}, DOI={10.35848/1347-4065/abab44}, abstractNote={The effective refractive index in optically-pumped 265 nm AlGaN-based lasers is assessed from the spacing of the longitudinal cavity modes in short laser cavities. It is found that the effective refractive index is significantly higher than the value estimated from the Sellmeier equation (n = 2.5) and reaches values of 2.9 and 3.2 for structures with 3 and 15 quantum wells, respectively. These results indicate that the Sellmeier equation underestimates the effective refractive index in AlGaN-based laser structures and that a different approach is needed for successful mid-UV laser modeling and design.}, journal={Japanese Journal of Applied Physics}, publisher={IOP Publishing}, author={Guo, Qiang and Kirste, Ronny and Reddy, Pramod and Mecouch, Will and Guan, Yan and Mita, Seiji and Washiyama, Shun and Tweedie, James and Sitar, Zlatko and Collazo, Ramón}, 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{vetter_biliroglu_seyitliyev_reddy_kirste_sitar_collazo_gundogdu_sun_2020, title={Observation of carrier concentration dependent spintronic terahertz emission from n-GaN/NiFe heterostructures}, url={https://doi.org/10.1063/5.0011009}, DOI={10.1063/5.0011009}, abstractNote={The development of terahertz (THz) spintronics has created a paradigm shift in the generation of THz radiation through the combination of ultrafast magnetism and spin-based electronics. However, research in this area has primarily focused on all-metallic devices comprising a ferromagnetic thin film adjacent to a non-magnetic heavy metal. Here, we report the experimental observation of spintronic THz emission from an n-doped wide bandgap semiconductor, n-GaN. We found that the amplitude of THz emission strongly depends on the carrier concentration of the semiconductor layer, which could be attributed to the tunable Rashba state occurring at the n-GaN/ferromagnet interface. Our work offers exciting prospects for pursuing wide bandgap semiconductor-based spintronic THz devices and demonstrating their intriguing spin Hall physics at the ultrafast timescale.}, journal={Applied Physics Letters}, author={Vetter, Eric and Biliroglu, Melike and Seyitliyev, Dovletgeldi and Reddy, Pramod and Kirste, Ronny and Sitar, Zlatko and Collazo, Ramón and Gundogdu, Kenan and Sun, Dali}, year={2020}, month={Aug} }
 @article{gleco_noussi_jude_reddy_kirste_collazo_lajeunesse_ivanisevic_2020, title={Oxidative Stress Transcriptional Responses of Escherichia coli at GaN Interfaces}, volume={3}, url={https://doi.org/10.1021/acsabm.0c01299}, DOI={10.1021/acsabm.0c01299}, abstractNote={Microorganisms regulate their interactions with surfaces by altering the transcription of specific target genes in response to physicochemical surface cues. To assess the influence of surface charge and surface chemistry on the transcriptional oxidative stress response, we evaluated the expression of three genes, oxyS, katE, and sodB from the Gram-negative bacterium, Escherichia coli, after a short exposure to GaN interfaces. We observed that both surface charge and surface chemistry were the factors regulating the transcriptional response of the target genes, which indicates that reactive oxygen species (ROS) generation and the ROS response at the GaN interfaces were affected by changing surface properties. The changes in transcription did not correlate to the surface charge in all cases, indicating that there was an influence from multiple interfacial properties on the interactions. Alteration of the bacterial morphology also was a critical factor in these transcriptional responses to the surface cues. When compared to wild-type E. coli bacteria, bacteria missing either flagella or curli exhibited altered transcriptional profiles of the three oxidative stress genes when exposed to GaN materials. These results indicate that the bacterial flagella and curli modulated the oxidative stress response in different ways. The results of this work add to our understanding of the interactions of microbes at interfaces and will be useful for guiding the development of electronic biointerfaces.}, number={12}, journal={ACS Applied Bio Materials}, publisher={American Chemical Society (ACS)}, author={Gleco, Sara and Noussi, Theophraste and Jude, Akamu and Reddy, Pramod and Kirste, Ronny and Collazo, Ramón and LaJeunesse, Dennis and Ivanisevic, Albena}, year={2020}, month={Dec}, pages={9073–9081} }
 @article{reddy_bryan_bryan_kim_washiyama_kirste_mita_tweedie_irving_sitar_et al._2020, title={Pinning of energy transitions of defects, complexes, and surface states in AlGaN alloys}, url={https://doi.org/10.1063/1.5140995}, DOI={10.1063/1.5140995}, abstractNote={In this work, we determine the dependence of the defect transition energies, electronic bands, and surface charge neutrality levels in AlGaN. With Vacuum level as reference, we show that energy transitions of localized defects and the surface Fermi level are independent of the alloy composition as electronic bands diverge with the increase in the bandgap as a function of alloy composition. The invariance of localized states on the alloy composition creates a convenient internal reference energy with respect to which other energy states may be measured. We demonstrate a higher generality to the universality rule with the independence of deep transition states of otherwise shallow donor type defects [(+1/+3) transition for VN] and defect complexes (CN+SiIII) in addition to the earlier predicted independent nature of mid-gap states when they are either the antibonding state between cationic impurities and host anion or acceptors at anion sites.}, journal={Applied Physics Letters}, author={Reddy, Pramod and Bryan, Zachary and Bryan, Isaac and Kim, Ji Hyun and Washiyama, Shun and Kirste, Ronny and Mita, Seiji and Tweedie, James and Irving, Douglas L. and Sitar, Zlatko and et al.}, year={2020}, month={Jan} }
 @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={2}, url={https://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={https://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{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{bagheri_kirste_reddy_washiyama_mita_sarkar_collazo_sitar_2020, title={The nature of the DX state in Ge-doped AlGaN}, url={https://doi.org/10.1063/5.0008362}, DOI={10.1063/5.0008362}, abstractNote={Electrical conductivity in high Al-content AlGaN has been severely limited, presumably due to a DX transition forming an acceptor state and subsequent self-compensation, which imposed an upper limit on the achievable free carrier concentration. To elucidate this idea, this paper examines Ge doping as a function of Al-content in AlGaN and finds a different behavior: for Al compositions below 40%, Ge behaved as a shallow donor with an ionization energy below 20 meV, while for Al compositions above 40%, above DX transition, it emerged as a deep donor. The ionization energy of this deep state increased with increasing Al content and reached 150 meV for 60% AlGaN. Around the DX transition, a continuous change from the shallow to deep donor was observed. In contrast to the density functional theory predictions, acceptor-type states corresponding to a DX-type transition were not observed. This finding may have profound technological consequences for the development of AlGaN- and AlN-based devices as it offers a feasible pathway to high n-conductivity in these compounds.}, journal={Applied Physics Letters}, author={Bagheri, Pegah and Kirste, Ronny and Reddy, Pramod and Washiyama, Shun and Mita, Seiji and Sarkar, Biplab and Collazo, Ramón and Sitar, Zlatko}, year={2020}, month={Jun} }
 @article{washiyama_reddy_sarkar_breckenridge_guo_bagheri_klump_kirste_tweedie_mita_et al._2020, title={The role of chemical potential in compensation control in Si:AlGaN}, url={https://doi.org/10.1063/1.5132953}, DOI={10.1063/1.5132953}, abstractNote={Reduction in compensation in Si-doped Al-rich AlGaN is demonstrated via chemical potential control (CPC). The chemical potentials and the resulting formation energies of carbon on the nitrogen site (CN) and cation vacancy complex with Si (VIII + nSiIII) were related to growth variables through a thermodynamic supersaturation model, which quantitatively predicted the incorporation of CN and the generation of the VIII + nSiIII complex. The compensation “knee” behavior, i.e., decreasing conductivity with increasing Si incorporation beyond a certain concentration, was successfully controlled. The maximum free carrier concentration was improved by impeding the formation of VIII + nSiIII complexes under III-richer conditions, while the impurity compensation by CN was reduced by making the growth environment N-richer. The results of Hall effect measurement and photoluminescence agreed well with quantitative theoretical predictions of the CPC model. Based on the developed model, the highest conductivity of 160 Ω−1 cm−1 with free carrier concentration of 3 × 1019 cm−3 in Al0.7Ga0.3N ever reported was achieved on single crystal AlN substrates. The demonstrated predictive power of the CPC model should greatly reduce the empirical analysis or iterative experimentation that would otherwise be necessary.}, journal={Journal of Applied Physics}, author={Washiyama, Shun and Reddy, Pramod and Sarkar, Biplab and Breckenridge, Mathew H. and Guo, Qiang and Bagheri, Pegah and Klump, Andrew and Kirste, Ronny and Tweedie, James and Mita, Seiji and et al.}, year={2020}, month={Mar} }
 @inproceedings{al rich algan based apds on single crystal aln with solar blindness and room temperature operation_2019, url={http://dx.doi.org/10.1109/rapid.2019.8864417}, DOI={10.1109/rapid.2019.8864417}, abstractNote={We demonstrate Al rich AlGaN based APDs grown on AlN substrates capable of high sensitivity at room temperature with ambient lighting rejection showcasing the advantage over Si and Ge based detectors. APDs are operated in linear gain region with maximum gain exceeding 1100.}, booktitle={2019 IEEE Research and Applications of Photonics in Defense Conference (RAPID)}, year={2019}, month={Aug} }
 @article{iyer_gulyuk_reddy_kirste_collazo_lajeunesse_ivanisevic_2019, title={Behavior of E. coli with Variable Surface Morphology Changes on Charged Semiconductor Interfaces}, volume={2}, url={https://doi.org/10.1021/acsabm.9b00573}, DOI={10.1021/acsabm.9b00573}, abstractNote={Bacterial behavior is often controlled by structural and composition elements of their cell wall. Using genetic mutant strains that change specific aspects of their surface structure, we modified bacterial behavior in response to semiconductor surfaces. We monitored the adhesion, membrane potential, and catalase activity of the Gram-negative bacterium Escherichia coli (E. coli) that were mutant for genes encoding components of their surface architecture, specifically flagella, fimbriae, curli, and components of the lipopolysaccharide membrane, while on gallium nitride (GaN) surfaces with different surface potentials. The bacteria and the semiconductor surface properties were recorded prior to the biofilm studies. The data from the materials and bioassays characterization supports the notion that alteration of the surface structure of the E. coli bacterium resulted in changes to bacterium behavior on the GaN medium. Loss of specific surface structure on the E. coli bacterium reduced its sensitivity to the semiconductor interfaces, while other mutations increase bacterial adhesion when compared to the wild-type control E. coli bacteria. These results demonstrate that bacterial behavior and responses to GaN semiconductor materials can be controlled genetically and can be utilized to tune the fate of living bacteria on GaN surfaces.}, number={9}, journal={ACS Applied Bio Materials}, publisher={American Chemical Society (ACS)}, author={Iyer, Divya and Gulyuk, Alexey V. and Reddy, Pramod and Kirste, Ronny and Collazo, Ramon and LaJeunesse, Dennis R. and Ivanisevic, Albena}, year={2019}, month={Sep}, pages={4044–4051} }
 @article{guo_kirste_mita_tweedie_reddy_moody_guan_washiyama_klump_sitar_et al._2019, title={Design of AlGaN-based quantum structures for low threshold UVC lasers}, url={https://doi.org/10.1063/1.5125256}, DOI={10.1063/1.5125256}, abstractNote={The influence of the polarization field on the emission properties of the AlGaN-based quantum structures grown on AlN substrates was investigated as a function of well width, barrier width, and barrier height. A thin AlGaN well and a thin AlN barrier design reduced the polarization field to ∼0.5 MV/cm, resulting in an ultralow laser threshold of 3 kW/cm2 in an optically pumped configuration. These experimental results were used to validate the simulation. In the next step, a structure with Al0.7Ga0.3N barriers was designed to support carrier injection with a minimal loss in optical performance. This structure showed a threshold of 7 kW/cm2 under optical pumping and an estimated threshold current of 8 kA/cm2 for the electric injection.}, journal={Journal of Applied Physics}, author={Guo, Qiang and Kirste, Ronny and Mita, Seiji and Tweedie, James and Reddy, Pramod and Moody, Baxter and Guan, Yan and Washiyama, Shun and Klump, Andrew and Sitar, Zlatko and et al.}, year={2019}, month={Dec} }
 @inproceedings{development of near uv laser diodes_2019, url={http://dx.doi.org/10.1109/rapid.2019.8864425}, DOI={10.1109/rapid.2019.8864425}, abstractNote={The development of near ultraviolet laser diodes based on the AlGaN materials system on single crystal GaN substrates is presented. This includes growth of relaxed Ga-rich AlGaN layers, design of UV laser diodes, as well as discussion of the electrical and optical properties. We show that with the demonstration of optically pumped lasers, a pathway toward electrically injected laser diodes is available.}, booktitle={2019 IEEE Research and Applications of Photonics in Defense Conference (RAPID)}, year={2019}, month={Aug} }
 @article{gulyuk_lajeunesse_reddy_kirste_collazo_ivanisevic_2019, title={Interfacial Properties of Doped Semiconductor Materials Can Alter the Behavior of Pseudomonas aeruginosa Films}, volume={1}, url={https://doi.org/10.1021/acsaelm.9b00347}, DOI={10.1021/acsaelm.9b00347}, abstractNote={A simple treatment of UV light exposure can change the interfacial properties of variably doped GaN substrates. The changes in surface charge and chemistry after exposure to UV light were studied as way to alter the behavior of Pseudomonas aeruginosa films. The properties of GaN surfaces were characterized by atomic force microscopy, Kelvin probe force microscopy, and X-ray photoelectron spectroscopy. The Pseudomonas aeruginosa film responses were quantified by analyzing changes in the amount of catalase, reactive oxygen species, and intracellular Ca2+ concentrations. The comprehensive analysis supports the notion that the response of P. aeruginosa biofilms can be controlled by the properties of the interface and the amount of time the film is in contact with it.}, number={8}, journal={ACS Applied Electronic Materials}, publisher={American Chemical Society (ACS)}, author={Gulyuk, Alexey V. and LaJeunesse, Dennis R. and Reddy, Pramod and Kirste, Ronny and Collazo, Ramon and Ivanisevic, Albena}, year={2019}, month={Aug}, pages={1641–1652} }
 @article{reddy_kumar_2019, title={Modified approach to modeling barrier inhomogeneity in Schottky diodes}, volume={1}, url={https://doi.org/10.1088/1361-6641/aafcc2}, DOI={10.1088/1361-6641/aafcc2}, abstractNote={The conventional model describing barrier inhomogeneities in Schottky diodes with a single series resistance leads to many anomalies including crossing over effect in current–voltage–temperature (I–V–T) plots. A new model is therefore proposed here which entails a parallel combination of several diodes, each with its own series resistance. Further, the barrier heights follow a Gaussian distribution function ρ(φ) with a mean barrier height φ ¯ and standard deviation σ. The series resistance is believed to vary inversely with the ρ(φ) value of the concerned barrier height and hence the area occupied. The occurrence of anomalies with the conventional model is examined in depth, and their effective elimination with the proposed model is discussed with the undertaken current simulations. In addition to predicting the increase in apparent barrier height with increase in temperature, decrease in ideality factor with increase in temperature, and elimination of modeling anomalies observed in the conventional model, the model is successful in predicting that apparent barrier height as a function of inverse thermal energy is a quadratic behavior even with a single distribution. This previously required unlikely scenarios involving multiple mean barrier heights and standard deviations with the conventional model. Finally, the description also explains the observed increase in the apparent barrier height with decrease in temperature of a Schottky diode when the values are deduced from the C–V–T data under reverse bias.}, journal={Semiconductor Science and Technology}, publisher={IOP Publishing}, author={Reddy, Pramod and Kumar, Jitendra}, year={2019}, month={Mar} }
 @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{6 kw/cm2 uvc laser threshold in optically pumped lasers achieved by controlling point defect formation_2018, url={http://dx.doi.org/10.7567/apex.11.082101}, DOI={10.7567/apex.11.082101}, abstractNote={Optically pumped lasing from AlGaN/AlN multiple quantum wells grown on single-crystalline AlN substrates with lasing thresholds as low as 6 kW/cm2 is demonstrated via the reduction of unintentional point defects in the active region and waveguide, which reduces the non-radiative recombination by 2 orders of magnitude. A higher lasing threshold of 11 kW/cm2 is observed for AlGaN barriers, owing to the reduced localization of electrons and holes in the wells. It is shown that for electrically injected UVC laser diodes, AlGaN barriers are essential.}, journal={Applied Physics Express}, year={2018}, month={Aug} }
 @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} }
 @inproceedings{au:ga alloyed clusters to enhance al contacts to p-type gan_2018, url={http://dx.doi.org/10.1109/rapid.2018.8508914}, DOI={10.1109/rapid.2018.8508914}, abstractNote={Deposition, annealing, and subsequent removal of Au on p-type GaN films reduced the resistivity of subsequently deposited Al metal contacts. The reduction is explained by formation of Au:Ga alloys which remove Ga from the surface, and create Ga-vacancies that surround the electrically active alloy clusters.}, booktitle={2018 IEEE Research and Applications of Photonics In Defense Conference (RAPID)}, year={2018}, month={Aug} }
 @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} }
 @inproceedings{design challenges for mid-uv laser diodes_2018, url={http://dx.doi.org/10.1109/rapid.2018.8508945}, DOI={10.1109/rapid.2018.8508945}, abstractNote={An optimized 270 nm UV laser structure is proposed with a predicted turn-on current density of 5 kA/cm2. The possible loss mechanisms are discussed, including p-GaN contact layer absorption, impact of a graded AlGaN layer on hole injection, and loss due to Mg doping.}, booktitle={2018 IEEE Research and Applications of Photonics In Defense Conference (RAPID)}, year={2018}, month={Aug} }
 @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} }
 @inproceedings{electrical and structural characterization of si implanted homoepitaxially grown aln_2018, url={http://dx.doi.org/10.1109/rapid.2018.8508915}, DOI={10.1109/rapid.2018.8508915}, abstractNote={AlN is an attractive material for UV optoelectronics and high-power device applications; however, obtaining high n-type conductivity is still a challenge. Ion implantation may provide an avenue to realize electrical conductivities suitable for device operation. A novel annealing procedure to recover lattice damage is presented.}, booktitle={2018 IEEE Research and Applications of Photonics In Defense Conference (RAPID)}, year={2018}, month={Aug} }
 @inproceedings{improving the conductivity limits in si doped al rich algan_2018, url={http://dx.doi.org/10.1109/rapid.2018.8508932}, DOI={10.1109/rapid.2018.8508932}, abstractNote={We report point defect control of two primary compensating defects in AlGaN: Cn and Viii+nSiAl, based on their dependence on chemical potentials. Reasonable control over the knee behavior of the conductivity and the low doping limit in Al0.65Ga0.35N thin films grown on sapphire is achieved.}, booktitle={2018 IEEE Research and Applications of Photonics In Defense Conference (RAPID)}, year={2018}, month={Aug} }
 @article{n- and p- type doping in al-rich algan and aln_2018, url={http://dx.doi.org/10.1149/08612.0025ecst}, DOI={10.1149/08612.0025ecst}, abstractNote={Attaining a high conductivity in both p-type and n-type Al-rich AlGaN epitaxial films is necessary for highly efficient deep-UV emitters. While reliable n-type conductivity has been demonstrated in AlxGa1-xN up to x < 0.8, achieving a reasonable p-type conductivity is a challenge even in Ga-rich AlGaN films. As one increases the x in AlxGa1-xN, several point defects and charge compensators appear in the epitaxial film. This report reviews recent observations on doping, conductivity, point defect control of Al-rich AlGaN films. Discussions on activation energy, state-of-the-art epitaxial material quality, contact formation and surface treatments are also presented.}, journal={ECS Transactions}, year={2018}, month={Jul} }
 @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} }
 @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} }
 @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}, month={Feb} }
 @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{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{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{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{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, Shun and Kaess, Felix and Kirste, Ronny and Mita, Seiji and Collazo, Ramon and Sitar, Zlatko}, year={2017}, month={Dec}, pages={245702} }
 @phdthesis{reddy_2016, title={A Study of Surfaces, Interfaces and Point Defect Control in III-Nitrides}, journal={Ph.D. Thesis}, author={Reddy, Pramod}, year={2016} }
 @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{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{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{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{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{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{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{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{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{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{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}, url={http://scitation.aip.org/content/aip/journal/jap/116/12/10.1063/1.4896377}, DOI={http://dx.doi.org/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}, pages={-} }
 @inproceedings{devi_reddy_singh_singla_ghanshyam_paul pawan kapur_vishal_2012, title={Isotherm behavior studies of silica nanoparticles: Role of surfactant concentration and particle size}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84866369056&partnerID=MN8TOARS}, DOI={10.1109/ISPTS.2012.6260884}, abstractNote={In the present work, the effect of the surfactant concentration and particle size on the 2D assembly and corresponding isotherms of the silica nanoparticles at the air/water interface is studied and reported using Langmuir-Blodgett (LB) technique. Silica nanoparticles (NPs) of different sizes (30 nm & 150 nm) were synthesized by Stöber's method and were characterized to determine their sizes and dispersity using scanning electron microscopy. As-synthesized NPs were hydrophobized to varying extents through addition of varying amounts (0.5 mg to 1.5 mg) of cationic surfactant, cetyltrimethylammoniumbromide (CTAB). The lift off area (area/particle) and corresponding yield was found to be very low for 30 nm sized particles. The yield in case of 150 nm sized particles participating in monolayer formation is found to be much higher implying a more optimal interaction with surfactant in the studied concentration range. The low yield at small particle size can be attributed to the partial surface modification of particles.}, booktitle={Proceedings - ISPTS-1, 1st International Symposium on Physics and Technology of Sensors}, author={Devi, P. and Reddy, P. and Singh, S. and Singla, M.L. and Ghanshyam, C. and Paul Pawan Kapur, A.K. and Vishal}, year={2012}, pages={79–82} }
 @inproceedings{optical and i-v studies on au-zno-ito based uv-sensing devices_2012, url={http://dx.doi.org/10.1117/12.925526}, DOI={10.1117/12.925526}, abstractNote={Being a II-VI semiconductor material with a wide direct band gap corresponding to the U-V region, ZnO finds important applications in U-V light sensors. In this work, we have developed and characterized Au-ZnO-ITO based UV photosensitive devices whose I-V characteristics show p+-i-n type behaviour and show an increased current under UV illumination. ZnO is employed as the active region. Both ZnO and gold were deposited via rf magnetron sputtering. The I-V characteristics of the fabricated UV sensor indicated a knee voltage of 0.69V. The resistance was observed to decrease by a factor of 3.5 under illumination. Further, we have optically characterized ZnO thin films deposited at different power levels to determine the dependency of various optical constants on deposition process parameters. These thin films were characterized using VASE (Variable Angle Spectroscopic Ellipsometer) and their optical properties including refractive index dispersion, band gap along with film thicknesses were extracted and modeled using WVASE modeling software.}, booktitle={16th International Workshop on Physics of Semiconductor Devices}, year={2012}, month={Oct} }
 @article{sensing behavior study of silica-coated ag nanoparticles deposited on glassy carbon toward nitrobenzene_2012, DOI={10.1007/s11051-012-1172-2}, journal={Journal of Nanoparticle Research}, year={2012} }
 @article{synthesis and optical characterization of zno quantum dots and nanorods_2012, url={http://dx.doi.org/10.1166/asem.2012.1175}, DOI={10.1166/asem.2012.1175}, abstractNote={ZnO quantum dots were synthesized via precipitation using millimolar solutions of zinc acetate and sodium hydroxide in methanol. The synthesized quantum dots were characterized via absorption and fluorescence measurements. The onset of absorption observed at a lower wavelength of ∼350 nm as compared to bulk (380 nm) indicates an increase in the band gap through quantum confinement and confirms the presence of quantum dots. In order to calculate the size and distribution of ZnO quantum dots, fluorescence measurements were made. The synthesized quantum dots were drop casted on gold coated silicon substrates and were used as seeds to grow nanorods by using aqueous solutions of zinc nitrate (hexahydrate) and hexamine. The FESEM micrographs revealed good quality hexagonal shaped nanorods with a relatively uniform diameter of ∼75 nm.}, journal={Advanced Science, Engineering and Medicine}, year={2012}, month={Jun} }
 @article{kohli_devi_reddy_raina_singla_2012, title={Synthesis and electrical behavior study of Mn<inf>3</inf>O<inf>4</inf>nanoceramic powder for low temperature NTC thermistor}, volume={23}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84867255839&partnerID=MN8TOARS}, DOI={10.1007/s10854-012-0680-2}, number={10}, journal={Journal of Materials Science: Materials in Electronics}, author={Kohli, P.S. and Devi, P. and Reddy, P. and Raina, K.K. and Singla, M.L.}, year={2012}, pages={1891–1897} }