@article{franke_hoffmann_kirste_bobea_tweedie_kaess_gerhold_collazo_sitar_2016, title={High reflectivity III-nitride UV-C distributed Bragg reflectors for vertical cavity emitting lasers}, volume={120}, ISSN={["1089-7550"]}, DOI={10.1063/1.4963831}, abstractNote={UV-C distributed Bragg reflectors (DBRs) for vertical cavity surface emitting laser applications and polariton lasers are presented. The structural integrity of up to 25 layer pairs of AlN/Al0.65Ga0.35N DBRs is maintained by balancing the tensile and compressive strain present between the single layers of the multilayer stack grown on top of an Al0.85Ga0.35N template. By comparing the structural and optical properties for DBRs grown on low dislocation density AlN and AlGaN templates, the criteria for plastic relaxation by cracking thick nitride Bragg reflectors are deduced. The critical thickness is found to be limited mainly by the accumulated strain energy during the DBR growth and is only negligibly affected by the dislocations. A reflectance of 97.7% at 273 nm is demonstrated. The demonstrated optical quality and an ability to tune the resonance wavelength of our resonators and microcavity structures open new opportunities for UV-C vertical emitters.}, number={13}, journal={JOURNAL OF APPLIED PHYSICS}, author={Franke, A. and Hoffmann, M. P. and Kirste, R. and Bobea, M. and Tweedie, J. and Kaess, F. and Gerhold, M. and Collazo, R. and Sitar, Z.}, year={2016}, month={Oct} }
 @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{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{rigler_buh_hoffmann_kirste_bobea_mita_gerhold_collazo_sitar_zgonik_2015, title={Optical characterization of Al- and N-polar AlN waveguides for integrated optics}, volume={8}, ISSN={["1882-0786"]}, DOI={10.7567/apex.8.042603}, abstractNote={Dispersion of the extraordinary and ordinary refractive indices of Al- and N-polar AlN waveguides is measured by multiple angle-of-incidence and spectroscopic ellipsometry techniques. The polarity-controlled AlN layers are grown by metal–organic chemical vapor deposition on (0001)-sapphire substrates. Taking into consideration the different surface morphologies of the Al- and N-polar AlN waveguides, we propose two optical models to describe the measured ellipsometry data. The results indicate that there is no difference between the refractive indices of the AlN grown in opposite directions, which confirms the potential of the AlN lateral polar structures for use in nonlinear optical applications based on quasi phase matching.}, number={4}, journal={APPLIED PHYSICS EXPRESS}, author={Rigler, Martin and Buh, Joze and Hoffmann, Marc P. and Kirste, Ronny and Bobea, Milena and Mita, Seiji and Gerhold, Michael D. and Collazo, Ramon and Sitar, Zlatko and Zgonik, Marko}, year={2015}, month={Apr} }
 @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{sochacki_bryan_amilusik_bobea_fijalkowski_bryan_lucznik_collazo_weyher_kucharski_et al._2014, title={HVPE-GaN grown on MOCVD-GaN/sapphire template and ammonothermal GaN seeds: Comparison of structural, optical, and electrical properties}, volume={394}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2014.02.020}, abstractNote={Crystallization of GaN by the HVPE technique on an MOCVD-GaN/sapphire template with photo-lithographically patterned Ti mask and ammonothermally-grown GaN crystals (Am-GaN) was studied and compared. Structural and optical properties of the obtained free-standing HVPE-GaN revealed that Am-GaN seeds produced material of much higher quality in terms of etch pit density (EPD), X-ray rocking curves, and excitonic emission than the sapphire-based templates. The crystallization run on the Am-GaN seed, annealed before the growth in H2+NH3 atmosphere, resulted in the HVPE-GaN of an average EPD of 5×104 cm−2, (002) rocking curve width of 22″, and photo-luminescence as narrow as 130 µeV. However, small change in quality was observed when the Am-GaN seed was annealed in N2+NH3 atmosphere prior to growth. In turn, the Hall effect measurements showed that the HVPE-GaN grown on MOCVD-GaN/sapphire template possessed lower free carrier concentration and higher mobility than the HVPE-GaN grown on the Am-GaN seeds. These results were associated to the higher growth rate of the examined HVPE-GaN sample.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Sochacki, Tomasz and Bryan, Zachary and Amilusik, Mikolaj and Bobea, Milena and Fijalkowski, Michal and Bryan, Isaac and Lucznik, Boleslaw and Collazo, Ramon and Weyher, Jan L. and Kucharski, Robert and et al.}, year={2014}, month={May}, pages={55–60} }
 @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{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{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_hoffmann_sachet_bobea_bryan_bryan_nenstiel_hoffmann_maria_collazo_et al._2013, title={Ge doped GaN with controllable high carrier concentration for plasmonic applications}, volume={103}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4848555}, DOI={10.1063/1.4848555}, abstractNote={Controllable Ge doping in GaN is demonstrated for carrier concentrations of up to 2.4 × 1020 cm−3. Low temperature luminescence spectra from the highly doped samples reveal band gap renormalization and band filling (Burstein-Moss shift) in addition to a sharp transition. Infrared ellipsometry spectra demonstrate the existence of electron plasma with an energy around 3500 cm−1 and a surface plasma with an energy around 2000 cm−1. These findings open possibilities for the application of highly doped GaN for plasmonic devices.}, number={24}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Kirste, Ronny and Hoffmann, Marc P. and Sachet, Edward and Bobea, Milena and Bryan, Zachary and Bryan, Isaac and Nenstiel, Christian and Hoffmann, Axel and Maria, Jon-Paul and Collazo, Ramón and et al.}, year={2013}, month={Dec}, pages={242107} }
 @article{rigler_zgonik_hoffmann_kirste_bobea_collazo_sitar_mita_gerhold_2013, title={Refractive index of III-metal-polar and N-polar AlGaN waveguides grown by metal organic chemical vapor deposition}, volume={102}, ISSN={["1077-3118"]}, DOI={10.1063/1.4800554}, abstractNote={Optical waveguides of III-metal-polar and N-polar AlGaN are grown on sapphire substrates in order to test their use in integrated optics. The dispersion of the ordinary and extraordinary indices of refraction for films with aluminum mole fraction between 0.0 and 0.30 at four discrete wavelengths has been determined by the prism coupling method. The wavelength dependence of the refractive indices is described well by the first-order Sellmeier dispersion formula. The measurements show a small difference in the refractive indices between the two polarities, which is more pronounced at longer wavelengths.}, number={22}, journal={APPLIED PHYSICS LETTERS}, author={Rigler, Martin and Zgonik, Marko and Hoffmann, Marc P. and Kirste, Ronny and Bobea, Milena and Collazo, Ramon and Sitar, Zlatko and Mita, Seiji and Gerhold, Michael}, year={2013}, month={Jun} }
 @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{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} }