@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{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={In this work, an alternative scheme to estimate the resistivity and ionization energy of Al-rich p-AlGaN epitaxial films is developed using two large-area ohmic contacts. Accordingly, the resistivities measured using current–voltage measurements were observed to corroborate the Hall measurements in the Van der Pauw configuration. A free hole concentration of ∼1.5 × 1017 cm−3 and low ionization energy of ∼65 meV in Mg-doped Al0.7Ga0.3N films is demonstrated. Nearly an order of magnitude lower hydrogen concentration than Mg in the as-grown AlGaN films is thought to reduce the Mg passivation and enable higher hole concentrations in Al-rich p-AlGaN films, compared to p-GaN films. The alternate methodology proposed in this work is expected to provide a simpler pathway to evaluate the electrical characteristics of Al-rich p-AlGaN films for future III-nitride ultraviolet light emitters.}, 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{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{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{kelley_runnerstrom_sachet_shelton_grimley_klump_lebeau_sitar_suen_padilla_et al._2019, title={Multiple Epsilon-Near-Zero Resonances in Multilayered Cadmium Oxide: Designing Metamaterial-Like Optical Properties in Monolithic Materials}, volume={6}, ISSN={["2330-4022"]}, DOI={10.1021/acsphotonics.9b00367}, abstractNote={In this Letter, we demonstrate a new class of infrared nanophotonic materials based on monolithic, multilayered doped cadmium oxide (CdO) thin films, where each CdO layer is individually tuned to support a separate epsilon-near-zero (ENZ) resonance. Infrared reflectivity measurements reveal that the optical response of the multilayered stack combines multiple discrete absorption events, each associated with an individual ENZ plasmonic polaritonic mode. Structural and chemical characterization confirm that the multilayers are homoepitaxial and monolithic, with internal interfaces defined by discrete steps in dopant density and carrier concentration. Structurally, the layers are indistinguishable as they differ from their neighbors by only ∼1 in 10000 constituent atoms. The optoelectronic property contrast, however, is pronounced, as each layer maintains an independent electron concentration, as corroborated by secondary ion mass spectroscopy and numerical solutions to Poisson’s equation. It is this electro...}, number={5}, journal={ACS PHOTONICS}, author={Kelley, Kyle P. and Runnerstrom, Evan L. and Sachet, Edward and Shelton, Christopher T. and Grimley, Everett D. and Klump, Andrew and LeBeau, James M. and Sitar, Zlatko and Suen, Jonathan Y. and Padilla, Willie J. and et al.}, year={2019}, month={May}, pages={1139–1145} }
 @article{klump_zhou_stevie_collazo_sitar_2018, title={Improvement in detection limit for time-of-flight SIMS analysis of dopants in GaN structures}, volume={36}, ISSN={["2166-2746"]}, DOI={10.1116/1.5013001}, abstractNote={Secondary ion mass spectrometry (SIMS) has been used extensively to monitor dopant levels in semiconductor materials. The preponderance of these measurements has been made with magnetic sector or quadrupole analyzers. Use of time-of-flight (ToF) analyzers has been limited because of an inability to match the detection limit of the other analyzers. Optimization of the ToF-SIMS analysis beam pulse width and analysis frames per cycle is shown to provide as much as an order of magnitude improvement in detection limit. The magnesium dopant in GaN structures was used for the study and analysis was made with Cs+ sputtering source and Bi3+. The count rate for CsMg+ increased by a factor of 11.3 with both improvements applied. This was evidenced by a detection limit improvement for magnesium from 7.5 × 1017 atoms/cm3 to low 1017 atoms/cm3. Increasing the number of analysis frames from one to ten causes cycle time to increase by a factor of five. Hence, there is a tradeoff between improved detection limit and analysis time.}, number={3}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Klump, Andrew and Zhou, Chuanzhen and Stevie, Frederick A. and Collazo, Ramon and Sitar, Zlatko}, year={2018}, month={May} }
 @inproceedings{sarkar_reddy_klump_rounds_breckenridge_haidet_mita_kirste_collazo_sitar_2018, title={On Contacts to III-nitride deep-UV emitters}, url={http://dx.doi.org/10.1109/icmap.2018.8354575}, DOI={10.1109/icmap.2018.8354575}, abstractNote={Although contacts to III-nitride visible and UV-A based emitters has been well explored, understanding the contacts to III-nitride deep-UV emitters have attracted research attention recently. Owing to the wide bandgap, both n-type and p-type contact metallization techniques result in a Schottky barrier at the metal-semiconductor interface. A way to reduce the contact resistance is to achieve a higher free carrier concentration in the epitaxial layer. As a result, growth of III-nitride epitaxial layers on native substrates are providing pathways for significant performance improvement. However, understanding the contacts to deep-UV emitters grown on native substrates are necessary to allow further performance improvement.}, booktitle={2018 3rd International Conference on Microwave and Photonics (ICMAP)}, author={Sarkar, B. and Reddy, P. and Klump, A. and Rounds, R. and Breckenridge, M. R. and Haidet, B. B. and Mita, S. and Kirste, R. and Collazo, Ramon and Sitar, Z.}, year={2018} }
 @article{rounds_sarkar_alden_guo_klump_hartmann_nagashima_kirste_franke_bickermann_et al._2018, title={The influence of point defects on the thermal conductivity of AlN crystals}, volume={123}, ISSN={["1089-7550"]}, DOI={10.1063/1.5028141}, abstractNote={The average bulk thermal conductivity of free-standing physical vapor transport and hydride vapor phase epitaxy single crystal AlN samples with different impurity concentrations is analyzed using the 3ω method in the temperature range of 30–325 K. AlN wafers grown by physical vapor transport show significant variation in thermal conductivity at room temperature with values ranging between 268 W/m K and 339 W/m K. AlN crystals grown by hydride vapor phase epitaxy yield values between 298 W/m K and 341 W/m K at room temperature, suggesting that the same fundamental mechanisms limit the thermal conductivity of AlN grown by both techniques. All samples in this work show phonon resonance behavior resulting from incorporated point defects. Samples shown by optical analysis to contain carbon-silicon complexes exhibit higher thermal conductivity above 100 K. Phonon scattering by point defects is determined to be the main limiting factor for thermal conductivity of AlN within the investigated temperature range.}, number={18}, journal={JOURNAL OF APPLIED PHYSICS}, author={Rounds, Robert and Sarkar, Biplab and Alden, Dorian and Guo, Qiang and Klump, Andrew and Hartmann, Carsten and Nagashima, Toru and Kirste, Ronny and Franke, Alexander and Bickermann, Matthias and et al.}, year={2018}, month={May} }
 @article{rounds_sarkar_klump_hartmann_nagashima_kirste_franke_bickermann_kumagai_sitar_et al._2018, title={Thermal conductivity of single-crystalline AIN}, volume={11}, ISSN={["1882-0786"]}, DOI={10.7567/apex.11.071001}, abstractNote={The thermal conductivity of AlN single crystals grown by physical vapor transport (PVT) and hydride vapor phase epitaxy (HVPE) was measured in the range of 30 to 325 K by the 3ω method. The measured room-temperature thermal conductivity ranged from 268 to 374 W m−1 K−1. Higher thermal conductivity correlated with higher transparency at 265 nm and lower total impurity levels.}, number={7}, journal={APPLIED PHYSICS EXPRESS}, author={Rounds, Robert and Sarkar, Biplab and Klump, Andrew and Hartmann, Carsten and Nagashima, Toru and Kirste, Ronny and Franke, Alexander and Bickermann, Matthias and Kumagai, Yoshinao and Sitar, Zlatko and et al.}, year={2018}, month={Jul} }
 @article{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{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{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} }