@article{preble_miraglia_roskowski_vetter_dudley_davis_2003, title={Domain structures in 6H-SiC wafers and their effect on the microstructures of GaN films grown on AlN and Al0.2Ga0.8N buffer layers}, volume={258}, ISSN={["0022-0248"]}, DOI={10.1016/S0022-0248(03)01515-X}, abstractNote={Silicon carbide wafers contain domains with varying sizes and degrees of tilt. The present research has shown that this microstructure is mimicked in GaN films deposited on AlN-containing buffer layers and that it masks most variations in the FWHM of the X-ray rocking curves of the former. The shape and FWHM in the GaN curves are determined both by domain tilting and dislocation broadening; the latter was dominant in areas of reduced tilt. Analyses of the on- and off-axis X-ray data acquired from these regions of lower tilt revealed the marked effect of the higher density of edge dislocations on broadening. This effect decreased with increasing GaN thickness due to dislocation annihilation. The densities of edge dislocations in GaN films deposited at 1010°C on pitted, less pitted and very smooth AlN layers of the same thickness grown at 1010°C, 1130°C and 1220°C, respectively, were lowest and highest in those films grown on the last two respective layers. Additional studies showed that GaN films grow on Al0.2Ga0.8N layers via step-flow and possess a lower edge dislocation density than films grown via the Stranski–Krastanov mode on AlN because of the reduced misfit and the absence of boundaries between coalesced islands.}, number={1-2}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Preble, EA and Miraglia, PQ and Roskowski, AM and Vetter, WM and Dudley, M and Davis, RF}, year={2003}, month={Oct}, pages={75–83} }
 @article{collazo_schlesser_roskowski_miraglia_davis_sitar_2003, title={Electron energy distribution during high-field transport in AlN}, volume={93}, ISSN={["0021-8979"]}, DOI={10.1063/1.1543633}, abstractNote={The energy distribution of electrons transported through intrinsic AlN heteroepitaxial films grown on SiC was directly measured as a function of applied field and AlN film thickness. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through films thicker than 95 nm at an applied field between 200 and 350 kV/cm occurred as steady-state hot electron transport following a Maxwellian energy distribution with a characteristic carrier temperature. At higher fields (470 kV/cm), intervalley scattering was evidenced by a multicomponent energy distribution featuring a second peak at the energy position of the first satellite valley. Velocity overshoot was observed in films thinner than 95 nm and at fields greater than 550 kV/cm. In this case, a symmetric energy distribution centered at an energy above the conduction band minimum was measured, indicating that the drift component of the electron velocity was on the order of the “thermal” component. A transient transport length of less than 80 nm was deduced from these observations.}, number={5}, journal={JOURNAL OF APPLIED PHYSICS}, author={Collazo, R and Schlesser, R and Roskowski, A and Miraglia, P and Davis, RF and Sitar, Z}, year={2003}, month={Mar}, pages={2765–2771} }
 @article{smith_mecouch_miraglia_roskowski_hartlieb_davis_2003, title={Evolution and growth of ZnO thin films on GaN(0001) epilayers via metalorganic vapor phase epitaxy}, volume={257}, ISSN={["1873-5002"]}, DOI={10.1016/S0022-0248(03)01469-6}, abstractNote={Zinc oxide thin films have been grown via metalorganic vapor phase epitaxy at 450°C and 250 Torr total pressure on O-terminated areas and micro-regions of native oxide formed on GaN(0 0 0 1) epilayers during exposure to the oxygen reactant. Analyses of the Ga3d core level spectra acquired from films grown for progressively longer times and comparisons of these data with both thermodynamic models of growth processes and associated atomic force micrographs revealed that the ZnO nucleated and grew via the Stranski–Krastanov mode. Considerations of (1) differences in surface energetics of the polar (0 0 0 1) surfaces of ZnO and GaN and that of the Ga-based oxide as well as (2) the relatively low stress generated by the moderate lattice mismatches in the a-axis lattice parameters of these compounds supported the observed growth mode. The shifts in the position of the Ga3d core level with increasing deposition time and decreasing peak intensity were caused by surface charging and increasing sampling volumes of the Ga-based oxide as well as the growing ZnO layer. The island/ocean morphology of the Ga-based oxide controlled the morphology of the initial ZnO layer, which, in turn controlled the locations and the morphologies of the subsequently nucleated ZnO islands. Scanning electron microscopy of thicker ZnO films revealed a highly textured microstructure.}, number={3-4}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Smith, TP and Mecouch, WJ and Miraglia, PQ and Roskowski, AM and Hartlieb, PJ and Davis, RF}, year={2003}, month={Oct}, pages={255–262} }
 @article{miraglia_preble_roskowski_einfeldt_davis_2003, title={Helical-type surface defects in GaN thin films epitaxially grown on GaN templates at reduced temperatures}, volume={253}, ISSN={["1873-5002"]}, DOI={10.1016/S0022-0248(03)00970-9}, abstractNote={Surface pits in the form of v-shaped defects and resulting surface roughening, previously associated solely with InGaN films, were observed and investigated using atomic force microscopy on GaN films grown at 780°C via metalorganic vapor phase epitaxy on conventionally and pendeo-epitaxially deposited GaN thin film templates. The density of the v-shaped defects was similar to the density of threading dislocations of ∼3×109 cm−2 (that originate from the heteroepitaxial interface between the GaN template layer and the SiC substrate). Moreover, the v-defect density was diminished with decreases in the dislocation density via increases in the template layer thickness or the use of pendeo-epitaxial seed layers. A concomitant reduction in the full-width half-maxima of the X-ray rocking curves was also observed. A qualitative model is presented that describes the formation of v-shaped defects as a result of interactions between the movement of surface steps, screw-type dislocation cores, and clusters of atoms on the terraces that form under conditions of high surface undercooling.}, number={1-4}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Miraglia, PQ and Preble, EA and Roskowski, AM and Einfeldt, S and Davis, RF}, year={2003}, month={Jun}, pages={16–25} }
 @article{miraglia_preble_roskowski_einfeldt_lim_liliental-weber_davis_2003, title={Helical-type surface defects in InGaN thin films epitaxially grown on GaN templates at reduced temperatures}, volume={437}, ISSN={["1879-2731"]}, DOI={10.1016/S0040-6090(03)00611-4}, abstractNote={Abstract The surface morphologies of InGaN films grown at 780 °C by metalorganic vapor phase epitaxy were determined using atomic force microscopy. A qualitative model was developed to explain the observed instabilities in the step morphology of these films, namely, the formation of hillocks and v-defects that give rise to surface roughening. V-defects, observed at a surface density greater than 2×10 8 /cm 2 , are a result of interactions between moving surface steps, cores of screw-type dislocations, and two-dimensional islands of atoms that form on the terraces during growth at high surface undercooling. A delay in the formation of v-defects in InGaN to a nominal thickness of 10 nm was observed and associated with the ammonia partial pressure and the interactions between steps associated with hillock islands and cores of screw-type dislocations. Hillock formation was attributed to a transition in the thermodynamic mode of film growth, as three-dimensional islands nucleated on the cores of screw-type dislocations at a density of 2×10 8 /cm 2 . Explanations for the foregoing observations are based on growth model theory previously developed by Burton, Cabrera and Frank and on changes in the surface kinetics with temperature, In composition, and gas phase composition.}, number={1-2}, journal={THIN SOLID FILMS}, author={Miraglia, PQ and Preble, EA and Roskowski, AM and Einfeldt, S and Lim, SH and Liliental-Weber, Z and Davis, RF}, year={2003}, month={Aug}, pages={140–149} }
 @article{preble_mclean_kiesel_miraglia_albrecht_davis_2002, title={Application of Nomarski interference contrast microscopy as a thickness monitor in the preparation of transparent, SiG-based, cross-sectional TEM samples}, volume={92}, ISSN={["0304-3991"]}, DOI={10.1016/S0304-3991(02)00142-0}, abstractNote={Reflected light optical microscopy using a Nomarski prism and a differential interference contrast filter have been employed in concert to achieve a technique that provides an accurate color reference for thickness during the dimpling and ion milling of transparent transmission electron microscopy samples of 6H-SiC(000 1) wafers. The samples had thin films of AIN, GaN, and Au deposited on the SiC substrate. A sequence of variously colored primary and secondary interference bands was observed when the SiC was thinner than 20 microm using an optical microscope. The color bands were correlated with the TEM sample thickness as measured via scanning electron microscopy. The interference contrast was used to provide an indication of the dimpling rate, the ion milling rate, and also the most probable location of perforation, which are useful to reduce sample breakage. The application of pressure during the initial cross-sectional preparation reduced the separation of the two halves of the sample sandwich and resulted in increased shielding of the film surface from ion milling damage.}, number={3-4}, journal={ULTRAMICROSCOPY}, author={Preble, EA and McLean, HA and Kiesel, SM and Miraglia, P and Albrecht, M and Davis, RF}, year={2002}, month={Aug}, pages={265–271} }
 @article{preble_tracy_kiesel_mclean_miraglia_nemanich_davis_albrecht_smith_2002, title={Electrical, structural and microstructural characteristics of as-deposited and annealed Pt and Au contacts on chemical-vapor-cleaned GaN thin films}, volume={91}, ISSN={["0021-8979"]}, DOI={10.1063/1.1432127}, abstractNote={Schottky contacts of Pt(111) and Au(111) were deposited on chemical-vapor-cleaned, n-type GaN(0001) thin films. The growth mode of the deposition, as determined by x-ray photoelectron spectroscopy analysis, followed the two-dimensional Frank–van der Merwe growth model. The resulting as-deposited metal films were monocrystalline and epitaxial with a (111)//(0002) relationship with the GaN. Selected samples were annealed for three minutes at 400 °C, 600 °C or 800 °C. The rectifying behavior of both contacts degraded at 400 °C; they became ohmic after annealing at 600 °C (Au) or 800 °C (Pt). High-resolution transmission electron micrographs revealed reactions at the metal/GaN interfaces for the higher temperature samples. X-ray diffraction results revealed an unidentified phase in the Pt sample annealed at 800 °C. A decrease in the room temperature in-plane (111) lattice constant for both metals, ranging from −0.1% to −0.5%, was observed as the annealing temperature was increased from 400 to 800 °C. This plastic deformation was caused by tensile stresses along the [111] direction that exceeded the yield strength as a result of the large differences in the coefficients of thermal expansion between the metal contacts and the GaN film.}, number={4}, journal={JOURNAL OF APPLIED PHYSICS}, author={Preble, EA and Tracy, KM and Kiesel, S and McLean, H and Miraglia, PQ and Nemanich, RJ and Davis, RF and Albrecht, M and Smith, DJ}, year={2002}, month={Feb}, pages={2133–2137} }
 @article{shin_arkun_thomson_miraglia_preble_schlesser_wolter_sitar_davis_2002, title={Growth and decomposition of bulk GaN: role of the ammonia/nitrogen ratio}, volume={236}, ISSN={["0022-0248"]}, DOI={10.1016/S0022-0248(02)00825-4}, abstractNote={Gallium nitride crystals grown via vapor-phase transport processes that incorporate ammonia as the only source of nitrogen below atmospheric pressures experience significant surface roughening and the eventual cessation of growth. Investigations of these phenomena in this research, and in the context of the discovery of a non-ceasing process route to larger GaN crystals, showed that the RMS values of the surface roughness of single crystal GaN (0 0 0 1) films exposed to pure ammonia flowing at 60 sccm for 2 h at 1130°C increased from the as-received value of 3.7–6.8 nm, 21.4 and 32.6 nm at 100, 430 and 760 Torr, respectively. Quadrupole mass spectrometry revealed that the concentrations of H2 and N2 measurably increased at pressures above 400 Torr. The primary reason for the increased roughness above 430 Torr was the enhanced etching of GaN via reaction with atomic and molecular hydrogen derived from the dissociation of the ammonia. At lower pressures, the decomposition of the GaN via the formation and evaporation of N2 and Ga increased in importance relative to etching for enhancing surface roughness. Dilution with nitrogen reduced the amount of hydrogen generated from the dissociation of the ammonia. The GaN surface annealed at 1130°C and 430 Torr in ammonia diluted with 33 vol% N2 maintained the smoothest surface with a nominal RMS value of 10.4 nm. Mixtures with higher and lower percentages of N2 showed enhanced roughness under the same conditions. Use of this optimum gas mixture also allowed the seeded growth of a 1.5×1.5×2.0 mm3 GaN crystal and a 2.3×1.8×0.3 mm3 thick platelet with neither observable decomposition nor cessation of the growth over periods of 36 and 48 h, respectively.}, number={4}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Shin, H and Arkun, E and Thomson, DB and Miraglia, P and Preble, E and Schlesser, R and Wolter, S and Sitar, Z and Davis, RF}, year={2002}, month={Mar}, pages={529–537} }
 @article{roskowski_preble_einfeldt_miraglia_davis_2002, title={Investigations regarding the maskless pendeo-epitaxial growth of GaN films prior to coalescence}, volume={38}, ISSN={["0018-9197"]}, DOI={10.1109/JQE.2002.801005}, abstractNote={Pendeo-epitaxy employs lateral growth from etched seed forms to achieve a marked reduction in dislocation density in a material. In this research, high-resolution X-ray diffraction and atomic force microscopy of GaN stripes and the laterally grown wings confirmed transmission electron microscopy results regarding the reduction in dislocations in the latter regions. Micro-Raman and X-ray diffraction measurements showed the wings to be tilted /spl les/0.15/spl deg/ due to tensile stresses in the stripes induced primarily by the mismatch in the coefficients of thermal expansion between the GaN stripe and the SiC substrate. A strong, low-temperature D/spl deg/X peak at /spl ap/3.466 eV with a FWHM of /spl les/300 /spl mu/eV was measured in the wing material by micro-photoluminescence. Films grown at 1020/spl deg/C exhibited similar vertical [0001] and lateral [112~0] growth rates. Increasing the growth temperature increased the latter due to the higher thermal stability of the (112~0) GaN and initiated growth of spiral hillocks on the (0001) surface of the stripes. The latter were due to adatom diffusion to heterogeneous steps previously nucleated at the intersections of pure screw or mixed dislocations. The (112~0) surface was atomically smooth under all growth conditions with a root mean square roughness value of 0.17 nm.}, number={8}, journal={IEEE JOURNAL OF QUANTUM ELECTRONICS}, author={Roskowski, AM and Preble, EA and Einfeldt, S and Miraglia, PM and Davis, RF}, year={2002}, month={Aug}, pages={1006–1016} }
 @article{roskowski_preble_einfeldt_miraglia_schuck_grober_davis_2002, title={Reduction in dislocation density and strain in GaN thin films grown via maskless pendeo-epitaxy}, volume={10}, number={4}, journal={Opto-electronics Review}, author={Roskowski, A. M. and Preble, E. A. and Einfeldt, S. and Miraglia, P. M. and Schuck, J. and Grober, R. and Davis, R. F.}, year={2002}, pages={261–270} }
 @article{roskowski_miraglia_preble_einfeldt_davis_2002, title={Surface instability and associated roughness during conventional and pendeo-epitaxial growth of GaN(0001) films via MOVPE}, volume={241}, ISSN={["0022-0248"]}, DOI={10.1016/S0022-0248(02)00943-0}, abstractNote={Investigations of the origins of surface roughening in GaN(0 0 0 1) have resulted in the development of a growth process route having an optimum temperature of 1020°C and a film thickness beyond 2.5 μm which results in films with the smoothest surface morphology. Atomic force microscopy (AFM) revealed uncoalesced GaN islands and hillocks for non-optimum temperatures below and above 1020°C, respectively. Uncoalesced islands were a result of insufficient lateral growth. Hillocks were a result of the rotation of heterogeneous steps formed at pure screw or mixed dislocations which terminated on the (0 0 0 1) surface. Growth of the latter features was controlled kinetically by temperature through adatom diffusion. The 106 cm−2 density of hillocks was reduced through growth on thick GaN templates and regions of pendeo-epitaxy (PE) overgrowth with lower pure screw or mixed dislocations. Smooth PE surfaces were obtained at temperatures that reduced the lateral to vertical growth rate but also retarded hillock growth that originated in the stripe regions. The (112̄0) PE sidewall surface was atomically smooth, with a root mean square roughness value of 0.17 nm, which was the noise limited resolution of the AFM measurements.}, number={1-2}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Roskowski, AM and Miraglia, PQ and Preble, EA and Einfeldt, S and Davis, RF}, year={2002}, month={May}, pages={141–150} }
 @article{jia_yu_keogh_asbeck_miraglia_roskowski_davis_2001, title={Polarization charges and polarization-induced barriers in AlxGa1-xN/GaN and InyGa1-yN/GaN heterostructures}, volume={79}, ISSN={["0003-6951"]}, DOI={10.1063/1.1412594}, abstractNote={Polarization charges are measured and the formation of large electrostatic barriers arising primarily as a consequence of the presence of polarization-induced charge densities is deduced from capacitance–voltage analysis of n-type AlxGa1−xN/GaN and InyGa1−yN/GaN heterostructures. In structures consisting of 5–10 nm AlxGa1−xN or InyGa1−yN surrounded by n-GaN, capacitance–voltage profiling studies combined with elementary electrostatic analysis yield experimental estimates of polarization charge densities, which are compared with values expected based on the combined effects of spontaneous and piezoelectric polarization. These results imply the existence of electrostatic barriers that are due primarily to the large polarization charge densities at each heterojunction interface and the resulting potential difference maintained across the thin AlxGa1−xN or InyGa1−yN layers. The electrostatic barriers formed in these structures are large in comparison to the heterojunction conduction-band offsets, demonstrating the utility of polarization-based engineering of electrostatic barriers in nitride semiconductor heterostructures.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Jia, L and Yu, ET and Keogh, D and Asbeck, PM and Miraglia, P and Roskowski, A and Davis, RF}, year={2001}, month={Oct}, pages={2916–2918} }
 @article{roskowski_miraglia_preble_einfeldt_stiles_davis_schuck_grober_schwarz_2001, title={Strain and dislocation reduction in maskless pendeo-epitaxy GaN thin films}, volume={188}, DOI={10.1002/1521-396x(200112)188:2<729::aid-pssa729>3.0.co;2-w}, abstractNote={Measurement of strain in GaN films grown via pendeo-epitaxy (PE) indicates that the overgrowth, or wing, material is crystallographically relaxed. An increase of ≈0.02% in the c-axis lattice parameter of the wing material was measured via high-resolution X-ray diffraction (HRXRD); additional evidence for this increase was indicated by an upward shift of the E2 Raman line frequency. Atomic force microscopy studies revealed a reduction in the density of mixed-type dislocations in the wing. A reduction in screw-type dislocations in the wings with respect to the stripes is indicated by a reduction in HRXRD rocking curve FWHM of the (0002) reflections from 646 to 354 arcsec. The off-axis FWHM of the wing area was 126 arcsec compared to 296 arcsec for the stripe indicating a reduction in the edge-type dislocations as well. Pendeo-epitaxy growth of wings off the (110) surface of a GaN stripe produced a material that is crystallographically relaxed, contains fewer defects compared to the stripe and is atomically smooth on the (110) surface.}, number={2}, journal={Physica Status Solidi. A, Applications and Materials Science}, author={Roskowski, A. M. and Miraglia, P. Q. and Preble, E. A. and Einfeldt, S. and Stiles, T. and Davis, R. F. and Schuck, J. and Grober, R. and Schwarz, U.}, year={2001}, pages={729–732} }
 @article{murty_miraglia_mathew_shah_haggag_1999, title={Characterization of gradients in mechanical properties of SA-533B steel welds using ball indentation}, volume={76}, ISSN={["0308-0161"]}, DOI={10.1016/S0308-0161(99)00006-X}, abstractNote={Gradients in mechanical and fracture properties of SA-533B steel welds were studied using ball indentation technique. The local stress–strain behaviors of different microstructural zones of the weld were determined at various temperatures. Gradients in the strength of the base metal, weld metal and the different positions in the heat affected zone were observed to be consistent with the changes in the microstructure. The maximum in yield and the corresponding minimum in indentation energy to fracture occurred at around 1 mm from the fusion line.}, number={6}, journal={INTERNATIONAL JOURNAL OF PRESSURE VESSELS AND PIPING}, author={Murty, KL and Miraglia, PQ and Mathew, MD and Shah, VN and Haggag, FM}, year={1999}, month={May}, pages={361–369} }