@article{huang_li_gurarslan_yu_kirste_guo_zhao_collazo_sitar_parsons_et al._2016, title={Atomically Thin MoS2 Narrowband and Broadband Light Superabsorbers}, volume={10}, ISSN={["1936-086X"]}, url={http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=ORCID&SrcApp=OrcidOrg&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=WOS:000381959100030&KeyUID=WOS:000381959100030}, DOI={10.1021/acsnano.6b02195}, abstractNote={We present a combined theoretical and experimental effort to enable strong light absorption (>70%) in atomically thin MoS2 films (≤4 layers) for either narrowband incidence with arbitrarily prespecified wavelengths or broadband incidence like solar radiation. This is achieved by integrating the films with resonant photonic structures that are deterministically designed using a unique reverse design approach based on leaky mode coupling. The design starts with identifying the properties of leaky modes necessary for the targeted strong absorption, followed by searching for the geometrical features of nanostructures to support the desired modes. This process is very intuitive and only involves a minimal amount of computation, thanks to the straightforward correlations between optical functionality and leaky modes as well as between leaky modes and the geometrical feature of nanostructures. The result may provide useful guidance for the development of high-performance atomic-scale photonic devices, such as solar cells, modulators, photodetectors, and photocatalysts.}, number={8}, journal={ACS NANO}, author={Huang, Lujun and Li, Guoqing and Gurarslan, Alper and Yu, Yiling and Kirste, Ronny and Guo, Wei and Zhao, Junjie and Collazo, Ramon and Sitar, Zlatko and Parsons, Gregory N. and et al.}, year={2016}, month={Aug}, pages={7493–7499} }
 @article{alden_guo_kirste_kaess_bryan_troha_bagal_reddy_hernandez-balderrama_franke_et al._2016, title={Fabrication and structural properties of AlN submicron periodic lateral polar structures and waveguides for UV-C applications}, volume={108}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/1.4955033}, DOI={10.1063/1.4955033}, abstractNote={Periodically poled AlN thin films with submicron domain widths were fabricated for nonlinear applications in the UV-VIS region. A procedure utilizing metalorganic chemical vapor deposition growth of AlN in combination with laser interference lithography was developed for making a nanoscale lateral polarity structure (LPS) with domain size down to 600 nm. The Al-polar and N-polar domains were identified by wet etching the periodic LPS in a potassium hydroxide solution and subsequent scanning electron microscopy (SEM) characterization. Fully coalesced and well-defined vertical interfaces between the adjacent domains were established by cross-sectional SEM. AlN LPSs were mechanically polished and surface roughness with a root mean square value of ∼10 nm over a 90 μm × 90 μm area was achieved. 3.8 μm wide and 650 nm thick AlN LPS waveguides were fabricated. The achieved domain sizes, surface roughness, and waveguides are suitable for second harmonic generation in the UVC spectrum.}, number={26}, journal={APPLIED PHYSICS LETTERS}, publisher={AIP Publishing}, author={Alden, D. and Guo, W. and Kirste, R. and Kaess, F. and Bryan, I. and Troha, T. and Bagal, A. and Reddy, P. and Hernandez-Balderrama, Luis H. and Franke, A. and et al.}, year={2016}, month={Jun} }
 @article{troha_rigler_alden_bryan_guo_kirste_mita_gerhold_collazo_sitar_et al._2016, title={UV second harmonic generation in AlN waveguides with modal phase matching}, volume={6}, ISSN={["2159-3930"]}, DOI={10.1364/ome.6.002014}, abstractNote={AlN waveguides on sapphire substrates were fabricated from AlN films grown by metalorganic chemical vapor deposition. By tuning the wavelength of the pump light we demonstrated a second harmonic generation in the UV at 306, 331, and 356 nm using the d33 coefficient and modal dispersion phase matching (MDPM). A theoretical model for MDPM with two- and three-mode interaction in planar waveguides was used to explain the results. Its essential component describes the interaction of two, possibly different, waveguide modes at the fundamental frequency that excite the third mode at doubled frequency. The experimental results were found to agree well with the theoretical model. This work confirmed the application potential of high-quality AlN films as waveguides for nonlinear processes.}, number={6}, journal={OPTICAL MATERIALS EXPRESS}, author={Troha, T. and Rigler, M. and Alden, D. and Bryan, I. and Guo, W. and Kirste, R. and Mita, S. and Gerhold, M. D. and Collazo, R. and Sitar, Z. and et al.}, year={2016}, month={Jun}, pages={2014–2023} }
 @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 AlGaN lateral polarity structures (LPS) with varying Al composition, have been fabricated by metalorganic chemical vapour deposition on LPS templates containing µm size domains. III‐metal and N‐polar AlGaN domains have been grown on LT‐AlN and c‐sapphire domains, respectively. LPS characterization by SEM and AFM reveals two major effects when the Al composition has been varied: (a) A high Al content leads to columnar growth within N‐polar domains and (b) a height difference between the polar domains can be observed for low Al compositions towards III‐polarity. The surface quality of the III‐polar domains is not effected by the Al composition. The surface roughness of N‐polar domains decreases for lower Al compositions. The high periodicity of the polar domains and the high quality of the boundary between domains suggest a high potential of AlGaN LPS to be used as lateral wave guides for quasi phase matching.}, 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{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_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{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={Growth and characterization of AlN based lateral polarity structures (LPS) are presented. The LPS were grown by MOCVD using patterned low temperature AlN/sapphire substrates: the Al-polar and N-polar AlN grew on AlN buffer layer and nitrided sapphire, respectively. AFM images showed a height difference between the two adjacent domains of different polarity on the order of 30 nm, which was equivalent to the low temperature AlN buffer layer thickness. SEM images provided an insight in the growth mode of the two polarities. It was shown that Al-polar AlN grew two-dimensionally, leading to a smooth well coalesced layer, while N-polar AlN grew primarily three-dimensionally, leading to a columnar structure. This difference in the growth mode led to different properties of domains with opposite polarity, including strain and defect incorporation, as observed by X-ray diffraction, photoluminescence, and Raman spectroscopy measurements. Finally, an outlook on the applicability and future development of AlN-based LPS is given. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)}, 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{yang_zhang_bagal_guo_chang_2013, title={Antireflection effects at nanostructured material interfaces and the suppression of thin-film interference}, volume={24}, ISSN={["1361-6528"]}, DOI={10.1088/0957-4484/24/23/235202}, abstractNote={Thin-film interference is a well-known effect, and it is commonly observed in the colored appearance of many natural phenomena. Caused by the interference of light reflected from the interfaces of thin material layers, such interference effects can lead to wavelength and angle-selective behavior in thin-film devices. In this work, we describe the use of interfacial nanostructures to eliminate interference effects in thin films. Using the same principle inspired by moth-eye structures, this approach creates an effective medium where the index is gradually varying between the neighboring materials. We present the fabrication process for such nanostructures at a polymer-silicon interface, and experimentally demonstrate its effectiveness in suppressing thin-film interference. The principle demonstrated in this work can lead to enhanced efficiency and reduce wavelength/angle sensitivity in multilayer optoelectronic devices.}, number={23}, journal={NANOTECHNOLOGY}, author={Yang, Qiaoyin and Zhang, Xu A. and Bagal, Abhijeet and Guo, Wei and Chang, Chih-Hao}, year={2013}, month={Jun} }
 @article{guo_xie_akouala_mita_rice_tweedie_bryan_collazo_sitar_2013, title={Comparative study of etching high crystalline quality AlN and GaN}, volume={366}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2012.12.141}, abstractNote={Abstract High-quality AlN and GaN bulk crystals were etched in a KOH aqueous solution or a KOH/H 2 O 2 mixture. As etched surfaces were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS). It was found that the N-polar surfaces had approximately 900 and 20 times higher etch rate than the III-polar surface for AlN and GaN, respectively. AlN had a higher total etch rate than GaN at the same condition. Hexagonal hillocks were observed on N-polar face with {10−1−1} boundary planes for both AlN and GaN, while metal polar surfaces remained smooth. Formation of aluminum oxide/hydroxide AlO x (OH) y was confirmed by XPS on as etched N polar AlN surface; the addition of H 2 O 2 resulted in a higher total surface oxygen concentration. The smoothening effect by adding H 2 O 2 oxidizer was explained by the formation and dissolution of aluminum oxide/hydroxide.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Guo, W. and Xie, J. and Akouala, C. and Mita, S. and Rice, A. and Tweedie, J. and Bryan, I. and Collazo, R. and Sitar, Z.}, year={2013}, month={Mar}, pages={20–25} }
 @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{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} }
 @article{guo_kirste_bryan_bryan_gerhold_collazo_sitar, title={Nanostructure surface patterning of GaN thin films and application to AlGaN/AlN multiple quantum wells: A way towards light extraction efficiency enhancement of III-nitride based light emitting diodes}, volume={117}, number={11}, journal={Journal of Applied Physics}, author={Guo, W. and Kirste, R. and Bryan, Z. and Bryan, I. and Gerhold, M. and Collazo, R. and Sitar, Z.} }