@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{mohn_stolyarchuk_markurt_kirste_hoffmann_collazo_courville_di felice_sitar_vennegues_et al._2016, title={Polarity Control in Group-III Nitrides beyond Pragmatism}, volume={5}, ISSN={["2331-7019"]}, DOI={10.1103/physrevapplied.5.054004}, abstractNote={$P\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y$ $e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$ for applications in electronics and nonlinear optics requires the reliable, controllable growth of uniformly polar layers of a compound semiconductor on a nonpolar substrate. So far, this has been developed on a purely empirical basis. The authors use transmission electron microscopy and density-functional theory to see how exchange of aluminum, oxygen, and nitrogen within a sapphire substrate mediates the polarity of an epitaxial film of group-III nitride. Their results shed new light on familiar concepts like substrate nitridation and low-temperature buffers, and may further help to understand polarity control in oxides and other materials.}, number={5}, journal={PHYSICAL REVIEW APPLIED}, author={Mohn, Stefan and Stolyarchuk, Natalia and Markurt, Toni and Kirste, Ronny and Hoffmann, Marc P. and Collazo, Ramon and Courville, Aimeric and Di Felice, Rosa and Sitar, Zlatko and Vennegues, Philippe and et al.}, year={2016}, month={May} }
 @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{bain_hoffmann_bryan_collazo_ivanisevic_2015, title={Adsorption and adhesion of common serum proteins to nanotextured gallium nitride}, volume={7}, ISSN={["2040-3372"]}, DOI={10.1039/c4nr06353h}, abstractNote={Morphology can be used to modulate the adsorption of proteins onto gallium nitride.}, number={6}, journal={NANOSCALE}, author={Bain, Lauren E. and Hoffmann, Marc P. and Bryan, Isaac and Collazo, Ramon and Ivanisevic, Albena}, year={2015}, pages={2360–2365} }
 @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_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{shelton_sachet_paisley_hoffmann_rajan_collazo_sitar_maria_2014, title={Polarity characterization by anomalous x-ray dispersion of ZnO films and GaN lateral polar structures}, volume={115}, ISSN={["1089-7550"]}, DOI={10.1063/1.4863120}, abstractNote={We demonstrate the use of anomalous x-ray scattering of constituent cations at their absorption edge, in a conventional Bragg-Brentano diffractometer, to measure absolutely and quantitatively the polar orientation and polarity fraction of unipolar and mixed polar wurtzitic crystals. In one set of experiments, the gradual transition between c+ and c− polarity of epitaxial ZnO films on sapphire as a function of MgO buffer layer thickness is monitored quantitatively, while in a second experiment, we map the polarity of a lateral polar homojunction in GaN. The dispersion measurements are compared with piezoforce microscopy images, and we demonstrate how x-ray dispersion and scanning probe methods can provide complementary information that can discriminate between polarity fractions at a material surface and polarity fractions averaged over the film bulk.}, number={4}, journal={JOURNAL OF APPLIED PHYSICS}, author={Shelton, Christopher T. and Sachet, Edward and Paisley, Elizabeth A. and Hoffmann, Marc P. and Rajan, Joseph and Collazo, Ramon and Sitar, Zlatko and Maria, Jon-Paul}, year={2014}, month={Jan} }
 @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{kirste_hoffmann_tweedie_bryan_callsen_kure_nenstiel_wagner_collazo_hoffmann_et al._2013, title={Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements}, volume={113}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4794094}, DOI={10.1063/1.4794094}, abstractNote={Compensation effects in metal organic chemical vapour deposition grown GaN doped with magnesium are investigated with Raman spectroscopy and photoluminescence measurements. Examining the strain sensitive E2(high) mode, an increasing compressive strain is revealed for samples with Mg-concentrations lower than 7 × 1018 cm−3. For higher Mg-concentrations, this strain is monotonically reduced. This relaxation is accompanied by a sudden decrease in crystal quality. Luminescence measurements reveal a well defined near band edge luminescence with free, donor bound, and acceptor bound excitons as well as a characteristic donor acceptor pair (DAP) luminescence. Following recent results, three acceptor bound excitons and donor acceptor pairs are identified. Along with the change of the strain, a strong modification in the luminescence of the dominating acceptor bound exciton and DAP luminescence is observed. The results from Raman spectroscopy and luminescence measurements are interpreted as fingerprints of compensation effects in GaN:Mg leading to the conclusion that compensation due to defect incorporation triggered by Mg-doping already affects the crystal properties at doping levels of around 7 × 1018 cm−3. Thereby, the generation of nitrogen vacancies is introduced as the driving force for the change of the strain state and the near band edge luminescence.}, number={10}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Kirste, Ronny and Hoffmann, Marc P. and Tweedie, James and Bryan, Zachary and Callsen, Gordon and Kure, Thomas and Nenstiel, Christian and Wagner, Markus R. and Collazo, Ramón and Hoffmann, Axel and et al.}, year={2013}, month={Mar}, pages={103504} }
 @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{skuridina_dinh_lacroix_ruterana_hoffmann_sitar_pristovsek_kneissl_vogt_2013, title={Polarity determination of polar and semipolar (11(2)over-bar2) InN and GaN layers by valence band photoemission spectroscopy}, volume={114}, number={17}, journal={Journal of Applied Physics}, author={Skuridina, D. and Dinh, D. V. and Lacroix, B. and Ruterana, P. and Hoffmann, M. and Sitar, Z. and Pristovsek, M. and Kneissl, M. and Vogt, P.}, year={2013} }
 @article{collazo_xie_gaddy_bryan_kirste_hoffmann_dalmau_moody_kumagai_nagashima_et al._2012, title={On the origin of the 265 nm absorption band in AlN bulk crystals}, volume={100}, ISSN={["1077-3118"]}, DOI={10.1063/1.4717623}, abstractNote={Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes. An absorption band centered around 4.7 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates. Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.}, number={19}, journal={APPLIED PHYSICS LETTERS}, author={Collazo, Ramon and Xie, Jinqiao and Gaddy, Benjamin E. and Bryan, Zachary and Kirste, Ronny and Hoffmann, Marc and Dalmau, Rafael and Moody, Baxter and Kumagai, Yoshinao and Nagashima, Toru and et al.}, year={2012}, month={May} }
 @article{callsen_wagner_kure_reparaz_buegler_brunnmeier_nenstiel_hoffmann_hoffmann_tweedie_et al._2012, title={Optical signature of Mg-doped GaN: Transfer processes}, volume={86}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.86.075207}, abstractNote={Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states that are involved in the donor-acceptor pair band transitions situated at 3.26–3.29 eV in GaN:Mg. The presented determination of the donor-acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature-dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies, which is put into a broader context based on Haynes’s rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN band edge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states, we determine binding energies of 164 ± 5 and 195 ± 5 meV, which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.}, number={7}, journal={PHYSICAL REVIEW B}, author={Callsen, G. and Wagner, M. R. and Kure, T. and Reparaz, J. S. and Buegler, M. and Brunnmeier, J. and Nenstiel, C. and Hoffmann, A. and Hoffmann, M. and Tweedie, J. and et al.}, year={2012}, month={Aug} }
 @inproceedings{hoffmann_gerhold_kirste_rice_akouala_xie_mita_collazo_sitar, title={Fabrication and characterization of lateral polar GaN structures for second harmonic generation}, volume={8631}, booktitle={Quantum sensing and nanophotonic devices x}, author={Hoffmann, M. P. and Gerhold, M. and Kirste, R. and Rice, A. and Akouala, C. R. and Xie, J. Q. Q. and Mita, S. and Collazo, R. and Sitar, Z.} }
 @inproceedings{hoffmann_tweedie_kirste_bryan_bryan_gerhold_sitar_collazo, title={Point defect management in GaN by Fermi-level control during growth}, volume={8986}, booktitle={Gallium nitride materials and devices ix}, author={Hoffmann, M. P. and Tweedie, J. and Kirste, R. and Bryan, Z. and Bryan, I. and Gerhold, M. and Sitar, Z. and Collazo, R.} }