@article{bryan_bryan_mita_rice_tweedie_collazo_sitar_2016, title={Surface kinetics in AlN growth: A universal model for the control of surface morphology in III-nitrides}, volume={438}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2015.12.022}, abstractNote={AlN epitaxial thin films were grown on both vicinal (0001)-oriented native single crystal AlN substrates and AlN templates grown on vicinal (0001)-oriented sapphire to develop a surface kinetic framework for the control of surface morphology. A Burton, Cabrera, and Frank (BCF) theory-based model is formulated and utilized to understand the dependence of the surface kinetics on the vapor supersaturation, σ, and substrate misorientation angle, α. The surface energy of the Al-polar surface of AlN was experimentally determined using BCF theory to be 149±8 meV/Å2. The critical misorientation angle for the onset of step-bunching was determined to be ~0.25° for a growth rate of 500 nm/h and temperature of 1250 °C. Transitioning from a surface with 2D nuclei to one with bilayer steps required a decrease in σ or an increase in α, whereas the suppression of step-bunching required an increase in σ or a decrease in α.}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Bryan, Isaac and Bryan, Zachary and Mita, Seiji and Rice, Anthony and Tweedie, James and Collazo, Ramon and Sitar, Zlatko}, year={2016}, month={Mar}, pages={81–89} } @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{bryan_akouala_tweedie_bryan_rice_kirste_collazo_sitar_2014, title={Surface preparation of non-polar single-crystalline AlN substrates}, volume={11}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201300401}, abstractNote={Abstract}, number={3-4}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 11, NO 3-4}, author={Bryan, Isaac and Akouala, Christer-Rajiv and Tweedie, James and Bryan, Zachary and Rice, Anthony and Kirste, Ronny and Collazo, Ramon and Sitar, Zlatko}, year={2014}, pages={454–457} } @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={High-quality AlN and GaN bulk crystals were etched in a KOH aqueous solution or a KOH/H2O2 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 AlOx(OH)y was confirmed by XPS on as etched N polar AlN surface; the addition of H2O2 resulted in a higher total surface oxygen concentration. The smoothening effect by adding H2O2 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{neuschl_thonke_feneberg_goldhahn_wunderer_yang_johnson_xie_mita_rice_et al._2013, title={Direct determination of the silicon donor ionization energy in homoepitaxial AlN from photoluminescence two-electron transitions}, volume={103}, DOI={10.1063/1.4821183}, abstractNote={We report on the identification of a two-electron transition for the shallow donor silicon in homoepitaxial aluminum nitride (AlN). One c-oriented sample was analyzed by low temperature photoluminescence spectroscopy on multiple excitation spots. We find a unique correlation of one single emission band, 76.6 meV below the free excitonic emission, with the luminescence of excitons bound to neutral silicon proving the identity as a two-electron transition. The assignment is confirmed by temperature dependent photoluminescence investigations. We find a donor ionization energy of (63.5 ± 1.5) meV for silicon in AlN.}, number={12}, journal={Applied Physics Letters}, author={Neuschl, B. and Thonke, K. and Feneberg, M. and Goldhahn, R. and Wunderer, T. and Yang, Z. and Johnson, N. M. and Xie, J. and Mita, S. and Rice, A. and et al.}, year={2013} } @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{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} } @article{tweedie_collazo_rice_mita_xie_akouala_sitar_2012, title={Schottky barrier and interface chemistry for Ni contacted to Al0.8Ga0.2N grown on c-oriented AlN single crystal substrates}, volume={9}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201100435}, abstractNote={Abstract}, number={3-4}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 3-4}, author={Tweedie, James and Collazo, Ramon and Rice, Anthony and Mita, Seiji and Xie, Jinqiao and Akouala, Rajiv-Christer and Sitar, Zlatko}, year={2012}, pages={584–587} } @inproceedings{collazo_mita_xie_rice_tweedie_dalmau_moody_schlesser_kirste_hoffmann_et al._2011, title={265 nm light emitting diodes on AlN single crystal substrates: Growth and characterization}, DOI={10.1364/cleo_si.2011.ctuu2}, abstractNote={High crystalline quality A1N and pseudomorphic AlGaN films were deposited by metalorganic chemical vapor deposition on single crystal A1N substrates for the fabrication of a 265 nm light emitting diode with estimated IQE above 40%.}, booktitle={2011 Conference on Lasers and Electro-Optics (CLEO)}, author={Collazo, Ramon and Mita, S. and Xie, J. Q. and Rice, A. and Tweedie, J. and Dalmau, R. and Moody, B. and Schlesser, R. and Kirste, R. and Hoffmann, A. and et al.}, year={2011} } @article{dalmau_moody_schlesser_mita_xie_feneberg_neuschl_thonke_collazo_rice_et al._2011, title={Growth and Characterization of AlN and AlGaN Epitaxial Films on AlN Single Crystal Substrates}, volume={158}, ISSN={["0013-4651"]}, DOI={10.1149/1.3560527}, abstractNote={AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.}, number={5}, journal={JOURNAL OF THE ELECTROCHEMICAL SOCIETY}, author={Dalmau, R. and Moody, B. and Schlesser, R. and Mita, S. and Xie, J. and Feneberg, M. and Neuschl, B. and Thonke, K. and Collazo, R. and Rice, A. and et al.}, year={2011}, pages={H530–H535} } @article{xie_mita_hussey_rice_tweedie_lebeau_collazo_sitar_2011, title={On the strain in n-type GaN}, volume={99}, ISSN={["0003-6951"]}, DOI={10.1063/1.3647772}, abstractNote={It was demonstrated that Ge has the same effect as Si on the strain evolution in n-type GaN as measured by x-ray diffraction. Dislocation inclination, which causes tensile strain in n-type GaN, was clearly observed by transmission electron microscopy where Ge doping was introduced during epitaxial growth. This result is explained by the Fermi level effect model that indicates dislocation inclination due to the climbing process through Ga vacancies. Therefore, there is no dependence of dislocation inclination on dopant species.}, number={14}, journal={APPLIED PHYSICS LETTERS}, author={Xie, Jinqiao and Mita, Seiji and Hussey, Lindsay and Rice, Anthony and Tweedie, James and LeBeau, James and Collazo, Ramon and Sitar, Zlatko}, year={2011}, month={Oct} } @article{collazo_mita_xie_rice_tweedie_dalmau_sitar_wetzel_khan_2011, title={Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications}, volume={8}, ISSN={["1862-6351"]}, DOI={10.1002/pssc.201000964}, abstractNote={Abstract}, number={7-8}, journal={PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 7-8}, author={Collazo, Ramon and Mita, Seiji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and Dalmau, Rafael and Sitar, Zlatko and Wetzel, C and Khan, A}, year={2011} } @article{feneberg_neuschl_thonke_collazo_rice_sitar_dalmau_xie_mita_goldhahn_2011, title={Sharp bound and free exciton lines from homoepitaxial AlN}, volume={208}, ISSN={["1862-6319"]}, DOI={10.1002/pssa.201000947}, abstractNote={Abstract}, number={7}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Feneberg, Martin and Neuschl, Benjamin and Thonke, Klaus and Collazo, Ramon and Rice, Anthony and Sitar, Zlatko and Dalmau, Rafael and Xie, Jinqiao and Mita, Seiji and Goldhahn, Ruediger}, year={2011}, month={Jul}, pages={1520–1522} } @article{craft_rice_collazo_sitar_maria_2011, title={Spectroscopic measurements of the surface stoichiometry of chemical vapor deposited GaN}, volume={98}, ISSN={["0003-6951"]}, DOI={10.1063/1.3554762}, abstractNote={We report on the surface stoichiometry of Ga-polar GaN films grown by metalorganic chemical vapor deposition as studied by x-ray photoelectron spectroscopy. GaN film surfaces are found to be Ga-rich, with Ga:N ratios ranging from 1.3:1 to 3.2:1. In vacuo ion-beam sputter/annealing studies show that these treatments drive the apparent Ga:N surface composition farther from unity, either through a decrease in surface contamination, oxidation of the surface, or both. Simple annealing experiments decrease the Ga:N ratio. The measured GaN ratio is correlated with the GaN growth time, suggesting that residual Ga precursor after growth interacts with the GaN surface as it cools.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Craft, H. S. and Rice, A. L. and Collazo, R. and Sitar, Z. and Maria, J. -P.}, year={2011}, month={Feb} } @article{kirste_collazo_callsen_wagner_kure_sebastian reparaz_mita_xie_rice_tweedie_et al._2011, title={Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN}, volume={110}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.3656987}, DOI={10.1063/1.3656987}, abstractNote={We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescence measurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracted to the IDB. The results are successfully used to confirm a theoretical model for GaN based lateral polarity junctions. Due to the strong and pronounced emission of IDBs even at elevated temperatures, it is demonstrated that lateral polarity junctions exhibit a strong potential for future high efficiency devices.}, number={9}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Kirste, Ronny and Collazo, Ramón and Callsen, Gordon and Wagner, Markus R. and Kure, Thomas and Sebastian Reparaz, Juan and Mita, Seji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and et al.}, year={2011}, month={Nov}, pages={093503} } @article{collazo_mita_xie_rice_tweedie_dalmau_sitar_2010, title={Implementation of the GaN lateral polarity junction in a MESFET utilizing polar doping selectivity}, volume={207}, ISSN={["1862-6300"]}, DOI={10.1002/pssa.200982629}, abstractNote={Abstract}, number={1}, journal={PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE}, author={Collazo, Ramon and Mita, Seiji and Xie, Jinqiao and Rice, Anthony and Tweedie, James and Dalmau, Rafael and Sitar, Zlatko}, year={2010}, month={Jan}, pages={45–48} } @article{rice_collazo_tweedie_xie_mita_sitar_2010, title={Linear dependency of Al-mole fraction with group-III precursor flows in AlxGa1-xN (0 <= x <= 1) deposition by LP OMVPE}, volume={312}, ISSN={["0022-0248"]}, DOI={10.1016/j.jcrysgro.2009.09.011}, abstractNote={AlxGa1−xN/GaN heterostructures (0≤x≤1) were deposited on (0 0 0 1)-sapphire by low-pressure (20 Torr) organometallic vapor phase epitaxy utilizing a range of group-III precursor and NH3 flow rates and a deposition temperature of 1050 °C. The Al-mole fraction of AlxGa1−xN layers was controlled by variations in the molar flows of triethylgallium (fTEG) and trimethylaluminum (fTMA). Characterization by X-ray diffraction confirmed that increasing the fTMA/fTEG ratio during deposition resulted in increased Al-mole fraction of AlxGa1−xN layers. A linear relationship between the experimental Al-mole fraction of AlxGa1−xN layers and the group-III precursor flows was obtained if a correction factor γ was introduced such that x=γfTMA/(γfTMA+fTEG). This correction factor was found to be independent of group-III precursor flows but dependent on NH3 partial pressure. The value of γ for these experiments decreased from 1.4 to 1.0 when the NH3 partial pressure was increased from 0.8 to 16.0 Torr during AlxGa1−xN deposition. The growth rate of AlxGa1−xN layers was found to be a linear combination of the independent AlN and GaN growth rates when equivalent fTMA and fTEG were utilized under the same deposition conditions.}, number={8}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Rice, A. and Collazo, R. and Tweedie, J. and Xie, J. and Mita, S. and Sitar, Z.}, year={2010}, month={Apr}, pages={1321–1324} } @article{rice_collazo_tweedie_dalmau_mita_xie_sitar_2010, title={Surface preparation and homoepitaxial deposition of AlN on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition}, volume={108}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.3467522}, DOI={10.1063/1.3467522}, abstractNote={Chemical surface treatments were conducted on mechanically polished (MP) and chemomechanically polished (CMP) (0001)-oriented single crystalline aluminum nitride (AlN) substrates to determine a surface preparation procedure for the homoepitaxial deposition of AlN epitaxial layers by metalorganic chemical vapor deposition. MP AlN substrates characterized by atomic force microscopy exhibited 0.5 nm rms roughness and polishing scratches, while CMP AlN substrates exhibited 0.1 nm rms roughness and were scratch-free. X-ray photoelectron spectroscopy analysis of MP and CMP AlN substrates indicated the presence of a surface hydroxide layer composed of mixed aluminum oxide hydroxide and aluminum trihydroxide. Wet etching with sulfuric and phosphoric acid mixtures reduced the amount of surface hydroxide. Ammonia annealing at 1250 °C converted the substrate hydroxide layer to AlN and increased the rms roughness of MP and CMP AlN substrates to 2.2 nm and 0.2 nm, respectively. AlN epitaxial layers were deposited at 1100–1250 °C under 20 Torr total pressure with a V/III ratio of 180–300 in either N2 or H2 diluent. High-resolution x-ray diffraction measurements revealed that AlN epitaxial layers deposited on MP substrates were strained due to nucleation and coalescence of AlN grains on the mechanically damaged surfaces. AlN deposited on CMP substrates was epitaxial and strain-free. Thermodynamic models for nitridation and AlN deposition were also proposed and evaluated.}, number={4}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Rice, A. and Collazo, R. and Tweedie, J. and Dalmau, R. and Mita, S. and Xie, J. and Sitar, Z.}, year={2010}, month={Aug}, pages={043510} } @article{xie_mita_collazo_rice_tweedie_sitar_2010, title={The effect of N-polar GaN domains as Ohmic contacts}, volume={97}, ISSN={["0003-6951"]}, DOI={10.1063/1.3491173}, abstractNote={Transfer line method measurements revealed that if the Ohmic contact regions were replaced by N-polar GaN, the contact resistance could be reduced from 0.71 Ω mm (or ρc=4×10−6 Ω cm2) to 0.24 Ω mm for a ∼200 nm thick Si-doped GaN layer. The reduction in contact resistance was largely due to the ∼1019 cm−3 free carriers in N-polar source/drain regions as measured by Hall effect. Secondary ion mass spectroscopy confirmed that oxygen doping in the N-polar region was more than three orders of magnitude greater than that in the Ga-polar region that was explained by the large difference in the adsorption energy for oxygen (∼1.3 eV/atom) between the N- and Ga-polar surfaces during the metalorganic chemical vapor deposition.}, number={12}, journal={APPLIED PHYSICS LETTERS}, author={Xie, J. and Mita, S. and Collazo, R. and Rice, A. and Tweedie, J. and Sitar, Z.}, year={2010}, month={Sep} } @article{tweedie_collazo_rice_xie_mita_dalmau_sitar_2010, title={X-ray characterization of composition and relaxation of Al(x)Ga(1-x)N(0 <= x <= 1) layers grown on GaN/sapphire templates by low pressure organometallic vapor phase epitaxy}, volume={108}, ISSN={["0021-8979"]}, DOI={10.1063/1.3457149}, abstractNote={A characterization procedure was developed to determine the alloy composition and strain state of AlxGa1−xN/GaN(0