@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{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{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{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{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{reddy_khachariya_mecouch_breckenridge_bagheri_guan_kim_pavlidis_kirste_mita_et al._2021, title={Study on avalanche breakdown and Poole-Frenkel emission in Al-rich AlGaN grown on single crystal AlN}, volume={119}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0062831}, DOI={10.1063/5.0062831}, abstractNote={We demonstrate that theoretical breakdown fields can be realized in practically dislocation free Al-rich AlGaN p-n junctions grown on AlN single crystal substrates. Furthermore, we also demonstrate a leakage current density in AlGaN that is independent of the device area, indicating a bulk leakage phenomenon and not surface or mesa-edge related. Accordingly, we identified the Poole–Frenkel emission from two types of point-defect traps in AlGaN as the primary source of reverse leakage before breakdown. Mg-doped AlGaN exhibited leakage currents due to a shallow trap at ∼0.16 eV in contrast with leakage currents observed in Si-doped AlGaN due to a deep trap at ∼1.8 eV.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Reddy, Pramod and Khachariya, Dolar and Mecouch, Will and Breckenridge, M. Hayden and Bagheri, Pegah and Guan, Yan and Kim, Ji Hyun and Pavlidis, Spyridon and Kirste, Ronny and Mita, Seiji and et al.}, year={2021}, month={Nov} }
 @article{bansal_hilse_huet_wang_kozhakhmetov_kim_bachu_alem_collazo_robinson_et al._2021, title={Substrate Modification during Chemical Vapor Deposition of hBN on Sapphire}, volume={13}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.1c14591}, abstractNote={A comparison of hexagonal boron nitride (hBN) layers grown by chemical vapor deposition on C-plane (0001) versus A-plane (112̅0) sapphire (α-Al2O3) substrate is reported. The high deposition temperature (>1200 °C) and hydrogen ambient used for hBN deposition on sapphire substantially alters the C-plane sapphire surface chemistry and leaves the top layer(s) oxygen deficient. The resulting surface morphology due to H2 etching of C-plane sapphire is inhomogeneous with increased surface roughness which causes non-uniform residual stress in the deposited hBN film. In contrast to C-plane, the A-plane of sapphire does not alter substantially under a similar high temperature H2 environment, thus providing a more stable alternative substrate for high quality hBN growth. The E2g Raman mode full width at half-maximum (FWHM) for hBN deposited on C-plane sapphire is 24.5 ± 2.1 cm-1 while for hBN on A-plane sapphire is 24.5 ± 0.7 cm-1. The lesser FWHM standard deviation on A-plane sapphire indicates uniform stress distribution across the film due to reduced undulations on the surface. The photoluminescence spectra of the hBN films at 300 and 3 K, obtained on C-plane and A-plane sapphire exhibit similar characteristics with peaks at 4.1 and 5.3 eV reported to be signature peaks associated with defects for hBN films deposited under lower V/III ratios. The dielectric breakdown field of hBN deposited on A-plane sapphire was measured to be 5 MV cm-1, agreeing well with reports on mechanically exfoliated hBN flakes. Thus, under the typical growth conditions required for high crystalline quality hBN growth, A-plane sapphire provides a more chemically stable substrate.}, number={45}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Bansal, Anushka and Hilse, Maria and Huet, Benjamin and Wang, Ke and Kozhakhmetov, Azimkhan and Kim, Ji Hyun and Bachu, Saiphaneendra and Alem, Nasim and Collazo, Ramon and Robinson, Joshua A. and et al.}, year={2021}, month={Nov}, pages={54516–54526} }
 @article{kim_bagheri_washiyama_klump_kirste_mita_reddy_collazo_sitar_2021, title={Temperature dependence of electronic bands in Al/GaN by utilization of invariant deep defect transition energies}, volume={119}, ISSN={["1077-3118"]}, DOI={10.1063/5.0055409}, abstractNote={We show experimentally that deep point defect levels in GaN, AlN, and AlGaN are constant with respect to the vacuum level and can be used as invariant internal energy references. This offered a convenient and quick way to assess band shifts and impurity levels as a function of temperature via photoluminescence. For AlGaN, we determined that the band shift in the temperature range of 3–600 K occurred primarily in the valence band and that the lowering of the conduction band edge was comparatively small. The valence band shift (as a fraction of the Varshni bandgap shift) in AlGaN varies from ∼70% in AlN to ∼90% in GaN.}, number={2}, journal={APPLIED PHYSICS LETTERS}, author={Kim, Ji Hyun and Bagheri, Pegah and Washiyama, Shun and Klump, Andrew and Kirste, Ronny and Mita, Seiji and Reddy, Pramod and Collazo, Ramon and Sitar, Zlatko}, year={2021}, month={Jul} }