@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{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{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{rathkanthiwar_graziano_tweedie_mita_kirste_collazo_sitar_2022, title={Crystallographic Tilt in Aluminum Gallium Nitride Epilayers Grown on Miscut Aluminum Nitride Substrates}, volume={10}, ISSN={["1862-6270"]}, DOI={10.1002/pssr.202200323}, abstractNote={Heteroepitaxial crystallographic tilt has been investigated as a possible strain‐relief mechanism in Al‐rich (Al>50%) AlGaN heteroepitaxial layers grown on single‐crystal (0001) AlN substrates with varying miscuts from 0.05° to 4.30°. The magnitude of the elastic lattice deformation‐induced tilt increases monotonically with the miscut angle, tightly following the Nagai tilt model. Although tilt angles as high as 0.1° are recorded, reciprocal space mapping (RSM) broadening and wafer bow measurements do not show any significant changes as a function of the heteroepitaxial tilt angle. While crystallographic tilting has been shown to be effective in controlling strain in some other heteroepitaxial systems, it does not provide any appreciable strain relief of the compressive strain in AlGaN/AlN heteroepitaxy.}, journal={PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS}, author={Rathkanthiwar, Shashwat and Graziano, Milena B. and Tweedie, James and Mita, Seiji and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2022}, month={Oct} }
 @article{bagheri_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_bagheri_khachariya_mita_pavlidis_reddy_kirste_tweedie_sitar_collazo_2022, title={Point-defect management in homoepitaxially grown Si-doped GaN by MOCVD for vertical power devices}, volume={15}, ISSN={["1882-0786"]}, DOI={10.35848/1882-0786/ac6566}, abstractNote={Abstract}, number={5}, journal={APPLIED PHYSICS EXPRESS}, author={Rathkanthiwar, Shashwat and Bagheri, Pegah and Khachariya, Dolar and Mita, Seiji and Pavlidis, Spyridon and Reddy, Pramod and Kirste, Ronny and Tweedie, James and Sitar, Zlatko and Collazo, Ramon}, year={2022}, month={May} }
 @article{rathkanthiwar_dycus_mita_kirste_tweedie_collazo_sitar_2022, title={Pseudomorphic growth of thick Al0.6Ga0.4N epilayers on AlN substrates}, volume={120}, ISSN={["1077-3118"]}, DOI={10.1063/5.0092937}, abstractNote={We report on the absence of strain relaxation mechanism in Al0.6Ga0.4N epilayers grown on (0001) AlN substrates for thickness as large as 3.5 μm, three-orders of magnitude beyond the Matthews–Blakeslee critical thickness for the formation of misfit dislocations (MDs). A steady-state compressive stress of 3–4 GPa was observed throughout the AlGaN growth leading to a large lattice bow (a radius of curvature of 0.5 m−1) for the thickest sample. Despite the large lattice mismatch-induced strain energy, the epilayers exhibited a smooth and crack-free surface morphology. These results point to the presence of a large barrier for nucleation of MDs in Al-rich AlGaN epilayers. Compositionally graded AlGaN layers were investigated as potential strain relief layers by the intentional introduction of MDs. While the graded layers abetted MD formation, the inadequate length of these MDs correlated with insignificant strain relaxation. This study emphasizes the importance of developing strain management strategies for the implementation of the single-crystal AlN substrate platform for III-nitride deep-UV optoelectronics and power electronics.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Rathkanthiwar, Shashwat and Dycus, J. Houston and Mita, Seiji and Kirste, Ronny and Tweedie, James and Collazo, Ramon and Sitar, Zlatko}, year={2022}, month={May} }
 @article{khachariya_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{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{graziano_bryan_bryan_kirste_tweedie_collazo_sitar_2019, title={Structural characteristics of m-plane AlN substrates and homoepitaxial films}, volume={507}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2018.07.012}, abstractNote={Homoepitaxial non-polar AlN films were realized on m-plane (101-0)-oriented AlN single crystals by metalorganic chemical vapor deposition (MOCVD). The microstructural properties of m-plane AlN substrates and homoepitaxial films were assessed by means of atomic force microscopy and high resolution x-ray diffraction characterization. Results indicated that both m-plane AlN substrates and films possessed exceptional structural quality, with some anisotropic mosaic distributions due to the quasi-bulk nature of the non-polar single crystals. An increase in the MOCVD growth temperature was noted to minimize the degree of inherited mosaic anisotropy without altering the m-plane AlN film growth rate, indicating that high temperature growth is critical to produce optimal film crystallinity. A dramatic change in the film surface morphology from heavily faceted “slate-like” features to monolayer steps was observed as the growth temperature was increased. The “slate-like” surface morphology produced low intensity cross-streaks in symmetric (101-0) reciprocal space maps, tilted about 18° away from the (101-0) crystal truncation rod. The orientation of these diffuse streaks corresponds to the physical alignment of the slates with respect to the substrate surface normal. X-ray line scans and defect-selective reciprocal space mapping confirmed that these low intensity streaks are solely dependent on this peculiar surface structure produced at low MOCVD growth temperatures and unrelated to basal plane stacking faults or other extended defects. All observations confirm that high quality III-nitride epitaxial structures on m-plane AlN substrates are attainable with controllable MOCVD growth processes, as demanded for future high performing AlN-based non-polar devices.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Graziano, Milena Bobea and Bryan, Isaac and Bryan, Zachary and Kirste, Ronny and Tweedie, James and Collazo, Ramon and Sitar, Zlatko}, year={2019}, month={Feb}, pages={389–394} }
 @article{guo_kirste_mita_tweedie_reddy_washiyama_breckenridge_collazo_sitar_2019, title={The polarization field in Al-rich AlGaN multiple quantum wells}, volume={58}, ISSN={["1347-4065"]}, DOI={10.7567/1347-4065/ab07a9}, abstractNote={Abstract}, journal={JAPANESE JOURNAL OF APPLIED PHYSICS}, author={Guo, Qiang and Kirste, Ronny and Mita, Seiji and Tweedie, James and Reddy, Pramod and Washiyama, Shun and Breckenridge, M. Hayden and Collazo, Ramon and Sitar, Zlatko}, year={2019}, month={Jun} }
 @article{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{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{franke_hoffmann_hernandez-balderrama_kaess_bryan_washiyama_bobea_tweedie_kirste_gerhold_et al._2016, title={Strain engineered high reflectivity DBRs in the deep UV}, volume={9748}, ISSN={["1996-756X"]}, DOI={10.1117/12.2211700}, abstractNote={The maximum achievable reflectivity of current III-nitride Bragg reflectors in the UV-C spectral range is limited due to plastic relaxation of thick multilayer structures. Cracking due to a large mismatch of the thermal expansion and lattice constants between AlxGa1-xN/AlyGa1-yN alloys of different composition and the substrate at the heterointerface is the common failure mode. Strain engineering and strain relaxation concepts by the growth on a strain reduced Al0.85Ga0.15N template and the implementation of low temperature interlayers is demonstrated. A significant enhancement of the maximum reflectivity above 97% at a resonance wavelength of 270 nm due to an increase of the critical thickness of our AlN/Al0.65Ga0.35N DBRs to 1.45 μm (25.5 pairs) prove their potential. By comparing the growth of identical Bragg reflectors on different pseudo-templates, the accumulated mismatch strain energy in the DBR, not the dislocation density provided by the template/substrate, was identified to limit the critical thickness. To further enhance the reflectivity low temperature interlays were implemented into the DBR to partially relief the misfit strain. Relaxation is enabled by the nucleation of small surface domains facilitating misfit dislocation injection and glide. Detailed structural and optical investigations will be conducted to prove the influence of the LT-AlN interlayers on the strain state, structural integrity and reflectivity properties. Coherent growth and no structural and optical degradation of the Bragg mirror properties was observed proving the fully applicability of the relaxation concept to fabricate thick high reflectivity DBR and vertical cavity laser structures.}, journal={GALLIUM NITRIDE MATERIALS AND DEVICES XI}, author={Franke, A. and Hoffmann, P. and Hernandez-Balderrama, L. and Kaess, F. and Bryan, I. and Washiyama, S. and Bobea, M. and Tweedie, J. and Kirste, R. and Gerhold, M. and et al.}, year={2016} }
 @article{hoffmann_kirste_mita_guo_tweedie_bobea_bryan_bryan_gerhold_collazo_et al._2015, title={Growth and characterization of AlxGa1-xN lateral polarity structures}, volume={212}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.201431740}, abstractNote={Abstract}, number={5}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Hoffmann, Marc Patrick and Kirste, Ronny and Mita, Seiji and Guo, Wei and Tweedie, James and Bobea, Milena and Bryan, Isaac and Bryan, Zachary and Gerhold, Michael and Collazo, Ramon and et al.}, year={2015}, month={May}, pages={1039–1042} }
 @article{bryan_bryan_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{kirste_mita_hoffmann_hussey_guo_bryan_bryan_tweedie_gerhold_hoffmann_et al._2014, title={Properties of AlN based lateral polarity structures}, volume={11}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201300287}, abstractNote={Abstract}, number={2}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 11, NO 2}, author={Kirste, Ronny and Mita, Seiji and Hoffmann, Marc P. and Hussey, Lindsay and Guo, Wei and Bryan, Isaac and Bryan, Zachary and Tweedie, James and Gerhold, Michael and Hoffmann, Axel and et al.}, year={2014}, pages={261–264} }
 @article{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{bryan_akouala_tweedie_bryan_rice_kirste_collazo_sitar_2014, title={Surface preparation of non-polar single-crystalline AlN substrates}, volume={11}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201300401}, abstractNote={Abstract}, number={3-4}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 11, NO 3-4}, author={Bryan, Isaac and Akouala, Christer-Rajiv and Tweedie, James and Bryan, Zachary and Rice, Anthony and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2014}, pages={454–457} }
 @article{guo_xie_akouala_mita_rice_tweedie_bryan_collazo_sitar_2013, title={Comparative study of etching high crystalline quality AlN and GaN}, volume={366}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2012.12.141}, abstractNote={High-quality AlN and GaN bulk crystals were etched in a KOH aqueous solution or a KOH/H2O2 mixture. As etched surfaces were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS). It was found that the N-polar surfaces had approximately 900 and 20 times higher etch rate than the III-polar surface for AlN and GaN, respectively. AlN had a higher total etch rate than GaN at the same condition. Hexagonal hillocks were observed on N-polar face with {10−1−1} boundary planes for both AlN and GaN, while metal polar surfaces remained smooth. Formation of aluminum oxide/hydroxide AlOx(OH)y was confirmed by XPS on as etched N polar AlN surface; the addition of H2O2 resulted in a higher total surface oxygen concentration. The smoothening effect by adding H2O2 oxidizer was explained by the formation and dissolution of aluminum oxide/hydroxide.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Guo, W. and Xie, J. and Akouala, C. and Mita, S. and Rice, A. and Tweedie, J. and Bryan, I. and Collazo, R. and Sitar, Z.}, year={2013}, month={Mar}, pages={20–25} }
 @article{kirste_hoffmann_tweedie_bryan_callsen_kure_nenstiel_wagner_collazo_hoffmann_et al._2013, title={Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements}, volume={113}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4794094}, DOI={10.1063/1.4794094}, abstractNote={Compensation effects in metal organic chemical vapour deposition grown GaN doped with magnesium are investigated with Raman spectroscopy and photoluminescence measurements. Examining the strain sensitive E2(high) mode, an increasing compressive strain is revealed for samples with Mg-concentrations lower than 7 × 1018 cm−3. For higher Mg-concentrations, this strain is monotonically reduced. This relaxation is accompanied by a sudden decrease in crystal quality. Luminescence measurements reveal a well defined near band edge luminescence with free, donor bound, and acceptor bound excitons as well as a characteristic donor acceptor pair (DAP) luminescence. Following recent results, three acceptor bound excitons and donor acceptor pairs are identified. Along with the change of the strain, a strong modification in the luminescence of the dominating acceptor bound exciton and DAP luminescence is observed. The results from Raman spectroscopy and luminescence measurements are interpreted as fingerprints of compensation effects in GaN:Mg leading to the conclusion that compensation due to defect incorporation triggered by Mg-doping already affects the crystal properties at doping levels of around 7 × 1018 cm−3. Thereby, the generation of nitrogen vacancies is introduced as the driving force for the change of the strain state and the near band edge luminescence.}, number={10}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Kirste, Ronny and Hoffmann, Marc P. and Tweedie, James and Bryan, Zachary and Callsen, Gordon and Kure, Thomas and Nenstiel, Christian and Wagner, Markus R. and Collazo, Ramón and Hoffmann, Axel and et al.}, year={2013}, month={Mar}, pages={103504} }
 @article{bryan_hoffmann_tweedie_kirste_callsen_bryan_rice_bobea_mita_xie_et al._2013, title={Fermi Level Control of Point Defects During Growth of Mg-Doped GaN}, volume={42}, ISSN={["1543-186X"]}, DOI={10.1007/s11664-012-2342-9}, number={5}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Bryan, Zachary and Hoffmann, Marc and Tweedie, James and Kirste, Ronny and Callsen, Gordon and Bryan, Isaac and Rice, Anthony and Bobea, Milena and Mita, Seiji and Xie, Jinqiao and et al.}, year={2013}, month={May}, pages={815–819} }
 @article{kirste_mita_hussey_hoffmann_guo_bryan_bryan_tweedie_xie_gerhold_et al._2013, title={Polarity control and growth of lateral polarity structures in AlN}, volume={102}, DOI={10.1063/1.4804575}, abstractNote={The control of the polarity of metalorganic chemical vapor deposition grown AlN on sapphire is demonstrated. Al-polar and N-polar AlN is grown side-by-side yielding a lateral polarity structure. Scanning electron microscopy measurements reveal a smooth surface for the Al-polar and a relatively rough surface for the N-polar AlN domains. Transmission electron microscopy shows mixed edge-screw type dislocations with polarity-dependent dislocation bending. Raman spectroscopy reveals compressively strained Al-polar and relaxed N-polar domains. The near band edge luminescence consists of free and bound excitons which are broadened for the Al-polar AlN. Relaxation, better optical quality, and dislocation bending in the N-polar domains are explained by the columnar growth mode.}, number={18}, journal={Applied Physics Letters}, author={Kirste, R. and Mita, S. and Hussey, L. and Hoffmann, M. P. and Guo, W. and Bryan, I. and Bryan, Z. and Tweedie, J. and Xie, J. Q. and Gerhold, M. and et al.}, year={2013} }
 @article{bobea_tweedie_bryan_bryan_rice_dalmau_xie_collazo_sitar_2013, title={X-ray characterization techniques for the assessment of surface damage in crystalline wafers: A model study in AlN}, volume={113}, ISSN={["0021-8979"]}, DOI={10.1063/1.4798352}, abstractNote={A high-resolution X-ray diffraction method with enhanced surface sensitivity has been used to investigate the effects of various polishing steps on the near-surface region of single crystal substrates. The method involves the study of a highly asymmetric reflection, observable under grazing incidence conditions. Analysis of rocking curve measurements and reciprocal space maps (RSMs) revealed subtle structural differences between the polished substrates. For aluminum nitride wafers, damage induced from diamond sawing and mechanical polishing was readily identifiable by on-axis rocking curves, but this method was unable to distinguish between sample surfaces subjected to various degrees of chemical mechanical polishing (CMP). To characterize sufficiently these surfaces, (10.3) RSMs were measured to provide both qualitative and quantitative information about the near-surface region. Two features present in the RSMs were utilized to quantitatively assess the polished wafers: the magnitude of the diffuse scatter in the omega-scans and the elongation of the crystal truncation rod. The method is able to distinguish between different degrees of CMP surface preparation and provides metrics to quantify subsurface damage after this polishing step.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bobea, M. and Tweedie, J. and Bryan, I. and Bryan, Z. and Rice, A. and Dalmau, R. and Xie, J. and Collazo, R. and Sitar, Z.}, year={2013}, month={Mar} }
 @article{callsen_wagner_kure_reparaz_buegler_brunnmeier_nenstiel_hoffmann_hoffmann_tweedie_et al._2012, title={Optical signature of Mg-doped GaN: Transfer processes}, volume={86}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.86.075207}, abstractNote={Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states that are involved in the donor-acceptor pair band transitions situated at 3.26–3.29 eV in GaN:Mg. The presented determination of the donor-acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature-dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies, which is put into a broader context based on Haynes’s rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN band edge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states, we determine binding energies of 164 ± 5 and 195 ± 5 meV, which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.}, number={7}, journal={PHYSICAL REVIEW B}, author={Callsen, G. and Wagner, M. R. and Kure, T. and Reparaz, J. S. and Buegler, M. and Brunnmeier, J. and Nenstiel, C. and Hoffmann, A. and Hoffmann, M. and Tweedie, J. and et al.}, year={2012}, month={Aug} }
 @article{tweedie_collazo_rice_mita_xie_akouala_sitar_2012, title={Schottky barrier and interface chemistry for Ni contacted to Al0.8Ga0.2N grown on c-oriented AlN single crystal substrates}, volume={9}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201100435}, abstractNote={Abstract}, number={3-4}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 3-4}, author={Tweedie, James and Collazo, Ramon and Rice, Anthony and Mita, Seiji and Xie, Jinqiao and Akouala, Rajiv-Christer and Sitar, Zlatko}, year={2012}, pages={584–587} }
 @inproceedings{collazo_mita_xie_rice_tweedie_dalmau_moody_schlesser_kirste_hoffmann_et al._2011, title={265 nm light emitting diodes on AlN single crystal substrates: Growth and characterization}, DOI={10.1364/cleo_si.2011.ctuu2}, abstractNote={High crystalline quality A1N and pseudomorphic AlGaN films were deposited by metalorganic chemical vapor deposition on single crystal A1N substrates for the fabrication of a 265 nm light emitting diode with estimated IQE above 40%.}, booktitle={2011 Conference on Lasers and Electro-Optics (CLEO)}, author={Collazo, Ramon and Mita, S. and Xie, J. Q. and Rice, A. and Tweedie, J. and Dalmau, R. and Moody, B. and Schlesser, R. and Kirste, R. and Hoffmann, A. and et al.}, year={2011} }
 @article{dalmau_moody_schlesser_mita_xie_feneberg_neuschl_thonke_collazo_rice_et al._2011, title={Growth and Characterization of AlN and AlGaN Epitaxial Films on AlN Single Crystal Substrates}, volume={158}, ISSN={["0013-4651"]}, DOI={10.1149/1.3560527}, abstractNote={AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.}, number={5}, journal={JOURNAL OF THE ELECTROCHEMICAL SOCIETY}, author={Dalmau, R. and Moody, B. and Schlesser, R. and Mita, S. and Xie, J. and Feneberg, M. and Neuschl, B. and Thonke, K. and Collazo, R. and Rice, A. and et al.}, year={2011}, pages={H530–H535} }
 @article{acharya_buegler_atalay_dietz_thoms_tweedie_collazo_2011, title={Observation of NH2 species on tilted InN (01(1)over-bar1) facets}, volume={29}, ISSN={["1520-8559"]}, DOI={10.1116/1.3596619}, abstractNote={The structural properties and surface bonding configuration of InN layers grown by high-pressure chemical vapor deposition have been characterized using Raman spectroscopy, x-ray diffraction (XRD), and high resolution electron energy loss spectroscopy. The appearance of the A1(TO) mode at 447 cm−1 in unpolarized z(·)z− Raman spectrum indicates distortions in the crystal lattice due to the growth of tilted plane crystallites. A Bragg reflex in the x-ray diffraction spectrum at 2Θ ≈ 33° has been assigned to tilted InN facets in the polycrystalline InN layer. The high resolution electron energy loss spectrum for this InN layer features vibration modes assigned to NH2 species indicating a surface orientation consistent with the crystalline properties observed in Raman spectroscopy and XRD. The appearance of tilted planes is suggested to be due to the effects of high V–III ratio and lattice mismatch on the growth mechanism.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Acharya, A. R. and Buegler, M. and Atalay, R. and Dietz, N. and Thoms, B. D. and Tweedie, J. S. and Collazo, R.}, year={2011}, month={Jul} }
 @article{xie_mita_hussey_rice_tweedie_lebeau_collazo_sitar_2011, title={On the strain in n-type GaN}, volume={99}, ISSN={["0003-6951"]}, DOI={10.1063/1.3647772}, abstractNote={It was demonstrated that Ge has the same effect as Si on the strain evolution in n-type GaN as measured by x-ray diffraction. Dislocation inclination, which causes tensile strain in n-type GaN, was clearly observed by transmission electron microscopy where Ge doping was introduced during epitaxial growth. This result is explained by the Fermi level effect model that indicates dislocation inclination due to the climbing process through Ga vacancies. Therefore, there is no dependence of dislocation inclination on dopant species.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Xie, Jinqiao and Mita, Seiji and Hussey, Lindsay and Rice, Anthony and Tweedie, James and LeBeau, James and Collazo, Ramon and Sitar, Zlatko}, year={2011}, month={Oct} }
 @article{collazo_mita_xie_rice_tweedie_dalmau_sitar_wetzel_khan_2011, title={Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications}, volume={8}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201000964}, abstractNote={Abstract}, number={7-8}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 7-8}, author={Collazo, Ramon and Mita, Seiji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and Dalmau, Rafael and Sitar, Zlatko and Wetzel, C and Khan, A}, year={2011} }
 @article{xie_mita_rice_tweedie_hussey_collazo_sitar_2011, title={Strain in Si doped GaN and the Fermi level effect}, volume={98}, ISSN={["0003-6951"]}, DOI={10.1063/1.3589978}, abstractNote={Using high resolution x-ray diffraction and Hall effect measurements, we found that the tensile strain caused by dislocation inclination in Si doped GaN became immeasurable when carbon codoping was used to compensate the free carriers. This result suggested that the tensile strain is related to free carrier concentration instead of Si concentration. Such an effect could be explained by the Fermi level effect on the surface-mediated dislocation climb governed by Ga vacancies, whose concentration is strongly influenced by the Fermi level position. This phenomenon is possibly similar to the well-known Fermi level effect in GaAs and GaP systems.}, number={20}, journal={APPLIED PHYSICS LETTERS}, author={Xie, Jinqiao and Mita, Seiji and Rice, Anthony and Tweedie, James and Hussey, Lindsay and Collazo, Ramon and Sitar, Zlatko}, year={2011}, month={May} }
 @article{paisley_losego_gaddy_tweedie_collazo_sitar_irving_maria_2011, title={Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions}, volume={2}, ISSN={["2041-1723"]}, DOI={10.1038/ncomms1470}, abstractNote={Property coupling at interfaces between active materials is a rich source of functionality, if defect densities are low, interfaces are smooth and the microstructure is featureless. Conventional synthesis techniques generally fail to achieve this when materials have highly dissimilar structure, symmetry and bond type-precisely when the potential for property engineering is most pronounced. Here we present a general synthesis methodology, involving systematic control of the chemical boundary conditions in situ, by which the crystal habit, and thus growth mode, can be actively engineered. In so doing, we establish the capability for layer-by-layer deposition in systems that otherwise default to island formation and grainy morphology. This technique is demonstrated via atomically smooth {111} calcium oxide films on (0001) gallium nitride. The operative surfactant-based mechanism is verified by temperature-dependent predictions from ab initio thermodynamic calculations. Calcium oxide films with smooth morphology exhibit a three order of magnitude enhancement of insulation resistance.}, journal={NATURE COMMUNICATIONS}, author={Paisley, Elizabeth A. and Losego, Mark. D. and Gaddy, Benjamin E. and Tweedie, James S. and Collazo, Ramon and Sitar, Zlatko and Irving, Douglas L. and Maria, Jon-Paul}, year={2011}, month={Sep} }
 @article{kirste_collazo_callsen_wagner_kure_sebastian reparaz_mita_xie_rice_tweedie_et al._2011, title={Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN}, volume={110}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.3656987}, DOI={10.1063/1.3656987}, abstractNote={We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescence measurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracted to the IDB. The results are successfully used to confirm a theoretical model for GaN based lateral polarity junctions. Due to the strong and pronounced emission of IDBs even at elevated temperatures, it is demonstrated that lateral polarity junctions exhibit a strong potential for future high efficiency devices.}, number={9}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Kirste, Ronny and Collazo, Ramón and Callsen, Gordon and Wagner, Markus R. and Kure, Thomas and Sebastian Reparaz, Juan and Mita, Seji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and et al.}, year={2011}, month={Nov}, pages={093503} }
 @article{collazo_mita_xie_rice_tweedie_dalmau_sitar_2010, title={Implementation of the GaN lateral polarity junction in a MESFET utilizing polar doping selectivity}, volume={207}, ISSN={["1862-6300"]}, DOI={10.1002/pssa.200982629}, abstractNote={Abstract}, number={1}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Collazo, Ramon and Mita, Seiji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and Dalmau, Rafael and Sitar, Zlatko}, year={2010}, month={Jan}, pages={45–48} }
 @article{rice_collazo_tweedie_xie_mita_sitar_2010, title={Linear dependency of Al-mole fraction with group-III precursor flows in AlxGa1-xN (0 <= x <= 1) deposition by LP OMVPE}, volume={312}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2009.09.011}, abstractNote={AlxGa1−xN/GaN heterostructures (0≤x≤1) were deposited on (0 0 0 1)-sapphire by low-pressure (20 Torr) organometallic vapor phase epitaxy utilizing a range of group-III precursor and NH3 flow rates and a deposition temperature of 1050 °C. The Al-mole fraction of AlxGa1−xN layers was controlled by variations in the molar flows of triethylgallium (fTEG) and trimethylaluminum (fTMA). Characterization by X-ray diffraction confirmed that increasing the fTMA/fTEG ratio during deposition resulted in increased Al-mole fraction of AlxGa1−xN layers. A linear relationship between the experimental Al-mole fraction of AlxGa1−xN layers and the group-III precursor flows was obtained if a correction factor γ was introduced such that x=γfTMA/(γfTMA+fTEG). This correction factor was found to be independent of group-III precursor flows but dependent on NH3 partial pressure. The value of γ for these experiments decreased from 1.4 to 1.0 when the NH3 partial pressure was increased from 0.8 to 16.0 Torr during AlxGa1−xN deposition. The growth rate of AlxGa1−xN layers was found to be a linear combination of the independent AlN and GaN growth rates when equivalent fTMA and fTEG were utilized under the same deposition conditions.}, number={8}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Rice, A. and Collazo, R. and Tweedie, J. and Xie, J. and Mita, S. and Sitar, Z.}, year={2010}, month={Apr}, pages={1321–1324} }
 @article{rice_collazo_tweedie_dalmau_mita_xie_sitar_2010, title={Surface preparation and homoepitaxial deposition of AlN on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition}, volume={108}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.3467522}, DOI={10.1063/1.3467522}, abstractNote={Chemical surface treatments were conducted on mechanically polished (MP) and chemomechanically polished (CMP) (0001)-oriented single crystalline aluminum nitride (AlN) substrates to determine a surface preparation procedure for the homoepitaxial deposition of AlN epitaxial layers by metalorganic chemical vapor deposition. MP AlN substrates characterized by atomic force microscopy exhibited 0.5 nm rms roughness and polishing scratches, while CMP AlN substrates exhibited 0.1 nm rms roughness and were scratch-free. X-ray photoelectron spectroscopy analysis of MP and CMP AlN substrates indicated the presence of a surface hydroxide layer composed of mixed aluminum oxide hydroxide and aluminum trihydroxide. Wet etching with sulfuric and phosphoric acid mixtures reduced the amount of surface hydroxide. Ammonia annealing at 1250 °C converted the substrate hydroxide layer to AlN and increased the rms roughness of MP and CMP AlN substrates to 2.2 nm and 0.2 nm, respectively. AlN epitaxial layers were deposited at 1100–1250 °C under 20 Torr total pressure with a V/III ratio of 180–300 in either N2 or H2 diluent. High-resolution x-ray diffraction measurements revealed that AlN epitaxial layers deposited on MP substrates were strained due to nucleation and coalescence of AlN grains on the mechanically damaged surfaces. AlN deposited on CMP substrates was epitaxial and strain-free. Thermodynamic models for nitridation and AlN deposition were also proposed and evaluated.}, number={4}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Rice, A. and Collazo, R. and Tweedie, J. and Dalmau, R. and Mita, S. and Xie, J. and Sitar, Z.}, year={2010}, month={Aug}, pages={043510} }
 @article{xie_mita_collazo_rice_tweedie_sitar_2010, title={The effect of N-polar GaN domains as Ohmic contacts}, volume={97}, ISSN={["0003-6951"]}, DOI={10.1063/1.3491173}, abstractNote={Transfer line method measurements revealed that if the Ohmic contact regions were replaced by N-polar GaN, the contact resistance could be reduced from 0.71 Ω mm (or ρc=4×10−6 Ω cm2) to 0.24 Ω mm for a ∼200 nm thick Si-doped GaN layer. The reduction in contact resistance was largely due to the ∼1019 cm−3 free carriers in N-polar source/drain regions as measured by Hall effect. Secondary ion mass spectroscopy confirmed that oxygen doping in the N-polar region was more than three orders of magnitude greater than that in the Ga-polar region that was explained by the large difference in the adsorption energy for oxygen (∼1.3 eV/atom) between the N- and Ga-polar surfaces during the metalorganic chemical vapor deposition.}, number={12}, journal={APPLIED PHYSICS LETTERS}, author={Xie, J. and Mita, S. and Collazo, R. and Rice, A. and Tweedie, J. and Sitar, Z.}, year={2010}, month={Sep} }
 @article{tweedie_collazo_rice_xie_mita_dalmau_sitar_2010, title={X-ray characterization of composition and relaxation of Al(x)Ga(1-x)N(0 <= x <= 1) layers grown on GaN/sapphire templates by low pressure organometallic vapor phase epitaxy}, volume={108}, ISSN={["0021-8979"]}, DOI={10.1063/1.3457149}, abstractNote={A characterization procedure was developed to determine the alloy composition and strain state of AlxGa1−xN/GaN(0<x<1) heterostructures deposited on c-plane sapphire substrates by low pressure organometalic vapor phase epitaxy. Motivated by a method suggested by Bowen and Tanner for separating the contributions of strain and composition in cubic crystals, we extended the technique to the case of hexagonal crystals by first principles derivation from elastic strain theory. The technique was evaluated using double-axis and triple-axis reciprocal space maps of 200 and 30 nm AlxGa1−xN layers. The procedure did not require absolute lattice parameter measurements and relied instead on relative measurements of the layer and substrate peak positions. Symmetric and asymmetric reflections of the film and substrate were measured in the double-axis configuration with ω–2θ scans. From the peak separation, the strained lattice parameters were determined. Assuming biaxial strain and linear variation of the relaxed lattice parameter with alloy composition, an equation was derived for composition in terms of the strained lattice parameters. The relaxation distribution in the AlGaN was found to be bimodal. Films with mole fraction of Al greater than 0.4 were completely relaxed and thoroughly cracked, while films with mole fraction of Al less than 0.4 exhibited compositionally dependent relaxation about the fully strained state.}, number={4}, journal={JOURNAL OF APPLIED PHYSICS}, author={Tweedie, James and Collazo, Ramon and Rice, Anthony and Xie, Jinqiao and Mita, Seiji and Dalmau, Rafael and Sitar, Zlatko}, year={2010}, month={Aug} }
 @article{dietz_alevli_atalay_durkaya_collazo_tweedie_mita_sitar_2008, title={The influence of substrate polarity on the structural quality of InN layers grown by high-pressure chemical vapor deposition}, volume={92}, ISSN={["0003-6951"]}, DOI={10.1063/1.2840192}, abstractNote={The influence of substrate polarity on the properties of InN layers grown by high-pressure chemical vapor deposition has been studied. The 2Θ-ω x-ray diffraction scans on InN layers deposited on polar GaN epilayers revealed single-phase InN(0002) with a full width at half maximum (FWHM) of around 200arcsec. InN layers grown on N-polar GaN exhibit larger FWHMs. Rocking curve analysis confirmed single-phase InN for both growth polarities, with FWHM values for ω-RC(002) at 2080arcsec for InN grown on Ga-polar templates. The A1(LO) Raman mode analysis shows higher free carrier concentrations in InN grown on N-polar templates, indicating that polarity affects the incorporation of impurities.}, number={4}, journal={APPLIED PHYSICS LETTERS}, author={Dietz, N. and Alevli, M. and Atalay, R. and Durkaya, G. and Collazo, R. and Tweedie, J. and Mita, S. and Sitar, Z.}, year={2008}, month={Jan} }
 @inproceedings{hoffmann_tweedie_kirste_bryan_bryan_gerhold_sitar_collazo, title={Point defect management in GaN by Fermi-level control during growth}, volume={8986}, booktitle={Gallium nitride materials and devices ix}, author={Hoffmann, M. P. and Tweedie, J. and Kirste, R. and Bryan, Z. and Bryan, I. and Gerhold, M. and Sitar, Z. and Collazo, R.} }