@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{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{gacevic_grandal_guo_kirste_varela_sitar_sanchez garcia_2021, title={Structural and optical properties of self-assembled AlN nanowires grown on SiO2/Si substrates by molecular beam epitaxy}, volume={32}, ISSN={["1361-6528"]}, DOI={10.1088/1361-6528/abe2c7}, abstractNote={Abstract}, number={19}, journal={NANOTECHNOLOGY}, author={Gacevic, Z. and Grandal, J. and Guo, Q. and Kirste, R. and Varela, M. and Sitar, Z. and Sanchez Garcia, M. A.}, year={2021}, month={May} }
 @article{alden_troha_kirste_mita_guo_hoffmann_zgonik_collazo_sitar_2019, title={Quasi-phase-matched second harmonic generation of UV light using AlN waveguides}, volume={114}, ISSN={["1077-3118"]}, DOI={10.1063/1.5087058}, abstractNote={As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is periodically changed commensurate with the coherence wavelength. QPM also allows the use of the highest nonlinear susceptibility, and therefore, higher conversion efficiencies are possible. In this work, the QPM SHG of UV light in AlN lateral polar structure-based waveguides is demonstrated. The peak intensity of the frequency doubled laser light was measured at 344 nm and 472 nm wavelengths, in agreement with dispersion-based theoretical predictions. These results confirm the potential of III-nitride-based lateral polar structures for quasi-phase-matched nonlinear optics and for frequency doubling media for UV light generation.}, number={10}, journal={APPLIED PHYSICS LETTERS}, author={Alden, Dorian and Troha, Tinkara and Kirste, Ronny and Mita, Seiji and Guo, Qiang and Hoffmann, Axel and Zgonik, Marko and Collazo, Ramon and Sitar, Zlatko}, year={2019}, month={Mar} }
 @article{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{bryan_bryan_washiyama_reddy_gaddy_sarkar_breckenridge_guo_bobea_tweedie_et al._2018, title={Doping and compensation in Al-rich AlGaN grown on single crystal AlN and sapphire by MOCVD}, volume={112}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.5011984}, DOI={10.1063/1.5011984}, abstractNote={In order to understand the influence of dislocations on doping and compensation in Al-rich AlGaN, thin films were grown by metal organic chemical vapor deposition (MOCVD) on different templates on sapphire and low dislocation density single crystalline AlN. AlGaN grown on AlN exhibited the highest conductivity, carrier concentration, and mobility for any doping concentration due to low threading dislocation related compensation and reduced self-compensation. The onset of self-compensation, i.e., the “knee behavior” in conductivity, was found to depend only on the chemical potential of silicon, strongly indicating the cation vacancy complex with Si as the source of self-compensation. However, the magnitude of self-compensation was found to increase with an increase in dislocation density, and consequently, AlGaN grown on AlN substrates demonstrated higher conductivity over the entire doping range.}, number={6}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Bryan, Isaac and Bryan, Zachary and Washiyama, Shun and Reddy, Pramod and Gaddy, Benjamin and Sarkar, Biplab and Breckenridge, M. Hayden and Guo, Qiang and Bobea, Milena and Tweedie, James and et al.}, year={2018}, month={Feb} }
 @article{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} }