@article{bryan_bryan_mita_rice_hussey_shelton_tweedie_maria_collazo_sitar_2016, title={The role of surface kinetics on composition and quality of AlGaN}, volume={451}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2016.06.055}, abstractNote={Metal–polar, Al-rich AlGaN films were grown on both single crystalline AlN and sapphire substrates. The role of surface morphology and surface kinetics on AlGaN composition is presented. With the reduced dislocation density of the films grown on AlN substrates, atomically smooth bilayer stepped surfaces are achieved with RMS roughness of less than 50 pm for a 5×5 µm2 AFM scan area. By controlling the surface supersaturation through adjusting the growth rate, a transition from 2D nucleation to step flow was observed. The critical misorientation angle for step-bunching in nominal Al0.70Ga0.30N grown with a growth rate of 600 nm/h on AlN substrates was found to be 0.4°. The composition of bilayer stepped AlGaN was strongly dependent on substrate misorientation angle, where a compositional variation by a factor of two for a change in misorientation angle from 0.05 to 0.40° was observed; this is explained by the different surface diffusion lengths of Ga and Al. Step-bunching resulted in strong compositional inhomogeneity as observed by photoluminescence and scanning transmission electron microscopy studies.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Bryan, Isaac and Bryan, Zachary and Mita, Seiji and Rice, Anthony and Hussey, Lindsay and Shelton, Christopher and Tweedie, James and Maria, Jon-Paul and Collazo, Ramon and Sitar, Zlatko}, year={2016}, month={Oct}, pages={65–71} }
 @article{guo_kirste_bryan_bryan_hussey_reddy_tweedie_collazo_sitar_2015, title={KOH based selective wet chemical etching of AlN, AlxGa1-xN, and GaN crystals: A way towards substrate removal in deep ultraviolet-light emitting diode}, volume={106}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4913705}, DOI={10.1063/1.4913705}, abstractNote={A controllable and smooth potassium hydroxide-based wet etching technique was developed for the AlGaN system. High selectivity between AlN and AlxGa1−xN (up to 12×) was found to be critical in achieving effective substrate thinning or removal for AlGaN-based deep ultraviolet light emitting diodes, thus increasing light extraction efficiency. The mechanism of high selectivity of AlGaN as a function of Al composition can be explained as related to the formation and dissolution of oxide/hydroxide on top of N-polar surface. Cross-sectional transmission electron microscopic analysis served as ultimate proof that these hillocks were not related to underlying threading dislocations.}, number={8}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Guo, W. and Kirste, R. and Bryan, I. and Bryan, Z. and Hussey, L. and Reddy, P. and Tweedie, J. and Collazo, R. and Sitar, Z.}, year={2015}, month={Feb} }
 @article{bryan_bryan_bobea_hussey_kirste_sitar_collazo_2014, title={Exciton transitions and oxygen as a donor in m-plane AlN homoepitaxial films}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4870284}, abstractNote={High-resolution photoluminescence studies on m-plane (1-100) homoepitaxial films grown by metalorganic chemical vapor deposition on AlN revealed several sharp donor-bound exciton (DBX) peaks with a full width at half maximum as narrow as 550 μeV. Power dependent photoluminescence distinguished DBXs tied to the Γ5 free exciton (FX) from those tied to the Γ1 FX. Both the n = 2 and n = 1 excited states of the Γ5 and Γ1 were resolved, giving binding energies of 52 meV and 55 meV, respectively. The DBX transition at 6.006 eV was identified as originating from the neutral-donor-oxygen (O0X). This assignment was based on secondary ion mass spectroscopy measurements, peak position with respect to the Si0X, and deep defect luminescence peaks located at 3.25 eV and 3.58 eV.}, number={13}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bryan, Zachary and Bryan, Isaac and Bobea, Milena and Hussey, Lindsay and Kirste, Ronny and Sitar, Zlatko and Collazo, Ramon}, year={2014}, month={Apr} }
 @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{bryan_bryan_bobea_hussey_kirste_collazo_sitar_2014, title={Homoepitaxial AlN thin films deposited on m-plane (1(1)over-bar00) AlN substrates by metalorganic chemical vapor deposition}, volume={116}, ISSN={["1089-7550"]}, DOI={10.1063/1.4897233}, abstractNote={AlN homoepitaxial films were grown by metalorganic chemical vapor deposition on chemo-mechanically polished (11¯00)-oriented single crystalline AlN substrates. The dependence of the surface morphology, structural quality, and unintentional impurity concentrations on the growth temperature was studied in order to determine the most appropriate growth conditions for high quality (11¯00) AlN epitaxial layers. Optically smooth surfaces (RMS roughness of 0.4 nm) and high crystalline quality, as demonstrated by the presence of FWHM values for (101¯0) rocking curves along [0001] of less than 25 arc·sec, were achieved for films grown above 1350 °C. Furthermore, sharp and intense near band edge luminescence was observed in these high quality films. A reduction in unintentional oxygen impurity levels was seen with an increase in growth temperature. These high crystalline quality films are suitable for device applications and hold great potential for providing an ideal platform for deep UV emitters with high Al content AlGaN without polarization related effects.}, number={13}, journal={JOURNAL OF APPLIED PHYSICS}, author={Bryan, Isaac and Bryan, Zachary and Bobea, Milena and Hussey, Lindsay and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2014}, month={Oct} }
 @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{hussey_white_kirste_mita_bryan_guo_osterman_haidet_bryan_bobea_et al._2014, title={Sapphire decomposition and inversion domains in N-polar aluminum nitride}, volume={104}, ISSN={["1077-3118"]}, DOI={10.1063/1.4862982}, abstractNote={Transmission electron microscopy (TEM) techniques and potassium hydroxide (KOH) etching confirmed that inversion domains in the N-polar AlN grown on c-plane sapphire were due to the decomposition of sapphire in the presence of hydrogen. The inversion domains were found to correspond to voids at the AlN and sapphire interface, and transmission electron microscopy results showed a V-shaped, columnar inversion domain with staggered domain boundary sidewalls. Voids were also observed in the simultaneously grown Al-polar AlN, however no inversion domains were present. The polarity of AlN grown above the decomposed regions of the sapphire substrate was confirmed to be Al-polar by KOH etching and TEM.}, number={3}, journal={APPLIED PHYSICS LETTERS}, author={Hussey, Lindsay and White, Ryan M. and Kirste, Ronny and Mita, Seiji and Bryan, Isaac and Guo, Wei and Osterman, Katherine and Haidet, Brian and Bryan, Zachary and Bobea, Milena and et al.}, year={2014}, month={Jan} }
 @article{guo_bryan_xie_kirste_mita_bryan_hussey_bobea_haidet_gerhold_et al._2014, title={Stimulated emission and optical gain in AlGaN heterostructures grown on bulk AlN substrates}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4868678}, abstractNote={Optical gain spectra for ∼250 nm stimulated emission were compared in three different AlGaN-based structures grown on single crystalline AlN substrates: a single AlGaN film, a double heterostructure (DH), and a Multiple Quantum Well (MQW) structure; respective threshold pumping power densities of 700, 250, and 150 kW/cm2 were observed. Above threshold, the emission was transverse-electric polarized and as narrow as 1.8 nm without a cavity. The DH and MQW structures showed gain values of 50–60 cm−1 when pumped at 1 MW/cm2. The results demonstrated the excellent optical quality of the AlGaN-based heterostructures grown on AlN substrates and their potential for realizing electrically pumped sub-280 nm laser diodes.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Guo, Wei and Bryan, Zachary and Xie, Jinqiao and Kirste, Ronny and Mita, Seiji and Bryan, Isaac and Hussey, Lindsay and Bobea, Milena and Haidet, Brian and Gerhold, Michael and et al.}, year={2014}, month={Mar} }
 @article{reddy_bryan_bryan_guo_hussey_collazo_sitar_2014, title={The effect of polarity and surface states on the Fermi level at III-nitride surfaces}, volume={116}, ISSN={["1089-7550"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907611759&partnerID=MN8TOARS}, DOI={10.1063/1.4896377}, abstractNote={Surface states and their influence on the Fermi level at the surface of GaN and AlN are studied using x-ray photoelectron spectroscopy (XPS). The effect of polarity on surface electronic properties was studied. Accurate modeling of the valence band edge and comparison with XPS data revealed the presence of donor surface states at 1.4 eV and acceptor states at energies >2.7 eV from the valence band in GaN. Al polar AlN showed acceptor states at energies >3.3 eV. Density of acceptor surface states was estimated to be between 1013 and 1014 eV−1 cm−2 in both GaN and AlN. The shift in charge neutrality levels and barrier heights due to polarity and the density of surface states on AlN and GaN were estimated from XPS measurements. Theoretical modeling and comparison with XPS data implied full compensation of spontaneous polarization charge by charged surface states. Barrier height measurements also reveal a dependence on polarity with ϕmetal-polar > ϕnon-polar > ϕnitrogen-polar suggesting that the N-polar surface is the most suitable for Ohmic contacts.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Reddy, Pramod and Bryan, Isaac and Bryan, Zachary and Guo, Wei and Hussey, Lindsay and Collazo, Ramon and Sitar, Zlatko}, year={2014}, month={Sep} }
 @article{xie_mita_bryan_guo_hussey_moody_schlesser_kirste_gerhold_collazo_et al._2013, title={Lasing and longitudinal cavity modes in photo-pumped deep ultraviolet AlGaN heterostructures}, volume={102}, ISSN={["1077-3118"]}, DOI={10.1063/1.4803689}, abstractNote={To unambiguously distinguish lasing from super luminescence, key elements of lasing such as longitudinal cavity modes with narrow line-width, polarized emission, and elliptically shaped far-field pattern, need to be demonstrated at the same time. Here, we show transverse electric polarized lasing at 280.8 nm and 263.9 nm for AlGaN based multi-quantum-wells and double heterojunction structures fabricated on single crystalline AlN substrates. An elliptically shaped far-field pattern was recorded when pumped above threshold. With cavities shorter than 200 μm, well-defined, equally spaced longitudinal modes with line widths as narrow as 0.014 nm were observed. The low threshold pumping density of 84 kW/cm2 suggests that the electrically pumped sub-300 nm ultraviolet laser diodes are imminent.}, number={17}, journal={APPLIED PHYSICS LETTERS}, author={Xie, Jinqiao and Mita, Seiji and Bryan, Zachary and Guo, Wei and Hussey, Lindsay and Moody, Baxter and Schlesser, Raoul and Kirste, Ronny and Gerhold, Michael and Collazo, Ramon and et al.}, year={2013}, month={Apr} }
 @article{bryan_rice_hussey_bryan_bobea_mita_xie_kirste_collazo_sitar_2013, title={Strain relaxation by pitting in AlN thin films deposited by metalorganic chemical vapor deposition}, volume={102}, ISSN={["0003-6951"]}, DOI={10.1063/1.4792694}, abstractNote={Strain relaxation mechanisms were investigated in epitaxial AlN layers deposited on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition. It was revealed that epitaxial AlN layers under tensile strain can exhibit micro-cracks and nano-pits. A correlation existed between the amount of strain and number of pits in localized areas. Pit densities as high as 1010 cm−2 were observed in areas where the tensile strain reached ∼0.4%, while unstrained areas of the film showed step flow growth. These nano-pits occurred as a strain relaxation mechanism and were not related to intrinsic defects, such as threading dislocations or inversion domains.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Bryan, I. and Rice, A. and Hussey, L. and Bryan, Z. and Bobea, M. and Mita, S. and Xie, J. and Kirste, R. and Collazo, R. and Sitar, Z.}, year={2013}, month={Feb} }
 @article{merkle_sutorik_sanamyan_hussey_gilde_cooper_dubinskii_2012, title={Fluorescence of Er(3+):AlN polycrystalline ceramic}, volume={2}, number={1}, journal={Optical Materials Express}, author={Merkle, L. D. and Sutorik, A. C. and Sanamyan, T. and Hussey, L. K. and Gilde, G. and Cooper, C. and Dubinskii, M.}, year={2012}, pages={78–91} }
 @article{hussey_mita_xie_guo_akouala_rajan_bryan_collazo_sitar_2012, title={Lateral epitaxial overgrowth of nitrogen polar GaN on smooth nitrogen polar GaN templates by metalorganic chemical vapor deposition}, volume={112}, ISSN={["1089-7550"]}, DOI={10.1063/1.4768526}, abstractNote={Nitrogen (N)-polar GaN lateral epitaxial overgrowth (LEO), with a reduced dislocation density, is grown by metalorganic chemical vapor deposition on patterned, smooth N-polar templates. For LEO growth conditions identical to that of smooth N-polar GaN on sapphire, mask orientation dependent morphologies and faceting of the overgrowth regions are observed. N-polar LEO oriented along the 〈1-100〉 direction exhibit flat sidewall morphologies while LEO oriented along the 〈11-20〉 direction exhibit inclined sidewalls. A lateral to vertical growth ratio for the 〈1-100〉 and 〈11-20〉 oriented LEO was found to be ∼0.3 and 0.2, respectively. Transmission electron microscope observations reveal a reduction of dislocations of up to three orders of magnitude in the overgrowth regions from ∼1010 cm−2 in the template. Additionally, dislocation bending likely initiated from interfacial tension between the N-polar GaN and SiO2 mask is observed, resulting in a reduction of dislocation density to ∼109 cm−2 in the window region of re-growth.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Hussey, Lindsay and Mita, Seiji and Xie, Jinqiao and Guo, Wei and Akouala, Christer-Rajiv and Rajan, Joseph and Bryan, Isaac and Collazo, Ramon and Sitar, Zlatko}, year={2012}, month={Dec} }