@article{barabash_ice_liu_einfeldt_roskowski_davis_2005, title={Local strain, defects, and crystallographic tilt in GaN(0001) layers grown by maskless pendeo-epitaxy from x-ray microdiffraction}, volume={97}, number={1}, journal={Journal of Applied Physics}, author={Barabash, R. I. and Ice, G. E. and Liu, W. and Einfeldt, S. and Roskowski, A. M. and Davis, R. F.}, year={2005} }
 @article{barabash_ice_liu_einfeldt_hommel_roskowski_davis_2005, title={White X-ray microbeam analysis of strain and crystallographic tilt in GaN layers grown by maskless pendeoepitaxy}, volume={202}, number={5}, journal={Physica Status Solidi. A, Applications and Materials Science}, author={Barabash, R. I. and Ice, G. E. and Liu, W. and Einfeldt, S. and Hommel, D. and Roskowski, A. M. and Davis, R. F.}, year={2005}, pages={732–738} }
 @article{paskova_valcheva_paskov_monemar_roskowski_davis_beaumont_gibart_2004, title={HVPE-GaN: comparison of emission properties and microstructure of films grown on different laterally overgrown templates}, volume={13}, ISSN={0925-9635}, url={http://dx.doi.org/10.1016/j.diamond.2003.10.072}, DOI={10.1016/j.diamond.2003.10.072}, abstractNote={We report on a comparative study of defect and emission distributions in thick hydride vapor phase epitaxial (HVPE) GaN films grown on two different patterned template structures separately produced by multi-step procedures using metalorganic vapor phase epitaxy (MOVPE). The observed differences in the microstructures and emission distributions at the early stages of the growth in both cases were related to the change of the dominating growth mode sequence and point defects incorporation. Both template structures were found to favor formation of voids in the coalescence regions, which leads to a partial strain relaxation and allows overgrowth of thicker films without cracks.}, number={4-8}, journal={Diamond and Related Materials}, publisher={Elsevier BV}, author={Paskova, T. and Valcheva, E. and Paskov, P.P. and Monemar, B. and Roskowski, A.M. and Davis, R.F. and Beaumont, B. and Gibart, P.}, year={2004}, month={Apr}, pages={1125–1129} }
 @article{hartman_roskowski_reitmeier_tracy_davis_nemanich_2003, title={Characterization of hydrogen etched 6H-SiC(0001) substrates and subsequently grown AlN films}, volume={21}, ISSN={["0734-2101"]}, DOI={10.1116/1.1539080}, abstractNote={Wafers of n-type, 6H–SiC(0001) with (ND–NA)=(5.1–7.5)×1017 and 2.5×1018 were etched in a flowing 25%H2/75%He mixture within the range of 1500–1640 °C at 1 atm. Equilibrium thermodynamic calculations indicated that the presence of atomic hydrogen is necessary to achieve etching of SiC. Atomic force microscopy, optical microscopy, and low energy electron diffraction of the etched surface revealed a faceted surface morphology with unit cell and half unit cell high steps and a 1×1 reconstruction. The latter sample also exhibited a much larger number of hexagonal pits on the surface. Annealing the etched samples under ultrahigh vacuum (UHV) at 1030 °C for 15 min resulted in (1) a reduction of the surface oxygen and adventitious hydrocarbons below the detection limit of Auger electron spectroscopy, (2) a (√3×√3)R30° reconstructed surface and (3) a Si-to-C peak-to-peak height ratio of 1.2. By contrast, using a chemical vapor cleaning (CVC) process consisting of an exposure to 3000 Langmuir (L) of silane at 1030 °C for 10 min under UHV conditions resulted in a (3×3) surface reconstruction, a Si-to-C ratio of 3.9, and islands of excess silicon. Continued annealing of the latter material for an additional 10 min at 1030 °C resulted in a (1×1) LEED pattern with a diffuse ring. Films of AlN grown via MOCVD at a sample platter temperature of 1274 °C for 15 min on hydrogen etched wafers having a doping concentration of 8.7×1017 cm−3 and cleaned via annealing had a rms roughness value of ≈0.4 nm.}, number={2}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Hartman, JD and Roskowski, AM and Reitmeier, ZJ and Tracy, KM and Davis, RF and Nemanich, RJ}, year={2003}, pages={394–400} }
 @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{schwarz_schuck_mason_grober_roskowski_einfeldt_davis_2003, title={Microscopic mapping of strain relaxation in uncoalesced pendeoepitaxial GaN on SiC}, volume={67}, number={4}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Schwarz, U. T. and Schuck, P. J. and Mason, M. D. and Grober, R. D. and Roskowski, A. M. and Einfeldt, S. and Davis, R. F.}, year={2003}, pages={045321–1} }
 @article{hartlieb_roskowski_davis_platow_nemanich_2003, title={Response to "Comment on 'Pd growth and subsequent Schottky barrier formation on chemical vapor cleaned p-type GaN surfaces' [J. Appl. Phys. 91, 732 (2002)]"}, volume={93}, ISSN={["0021-8979"]}, DOI={10.1063/1.1542911}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Hartlieb, PJ and Roskowski, A and Davis, RF and Platow, W and Nemanich, RJ}, year={2003}, month={Mar}, pages={3679–3679} }
 @article{hartlieb_roskowski_davis_nemanich_2002, title={Chemical, electrical, and structural properties of Ni/Au contacts on chemical vapor cleaned p-type GaN}, volume={91}, ISSN={["1089-7550"]}, DOI={10.1063/1.1471578}, abstractNote={Chemical vapor cleaned, Mg-doped, p-type GaN(0001) surfaces and Ni/Au contacts deposited on these surfaces have been studied using several characterization techniques. Stoichiometric surfaces without detectable carbon and an 87% reduction in the surface oxygen to 2±1 at. % were achieved. The binding energies of the Ga 3d and N 1s core level photoelectron peaks were reduced by 0.5±0.1 eV following the chemical vapor clean. The band bending at the clean surface was measured to be 0.8±0.1 eV. As-deposited Ni/Au contacts on chemical vapor cleaned surfaces exhibited significantly less rectification in the low voltage region (<2 V) compared to identical contact structures on conventional HCl treated surfaces. The specific contact resistance of these contacts deposited on chemical vapor cleaned surfaces and subsequently annealed at 450 °C for 30 seconds was 3±2 Ω cm2. Improved ohmic behavior and a specific contact resistance of 4±2 Ω cm2 was obtained for contacts deposited on HCl treated surfaces and annealed using the same schedule. The formation of Au:Ga and Au:Ni solid solutions was observed for contacts on HCl treated surfaces following the 450 °C anneal. There were significantly less interfacial reactions for annealed contacts on chemical vapor cleaned surfaces. The values of specific contact resistance, sheet resistance, and transfer length of the annealed contacts deposited on both chemical vapor cleaned and HCl treated surfaces and measured from room temperature to 140 °C did not change during three successive thermal cycles within this range.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Hartlieb, PJ and Roskowski, A and Davis, RF and Nemanich, RJ}, year={2002}, month={Jun}, pages={9151–9160} }
 @article{schuck_grober_roskowski_einfeldt_davis_2002, title={Cross-sectional imaging of pendeo-epitaxial GaN using continuous-wave two-photon microphotoluminescence}, volume={81}, ISSN={["0003-6951"]}, DOI={10.1063/1.1506948}, abstractNote={A technique utilizing continuous-wave two-photon absorption has been developed for optically sectioning and imaging deep into GaN structures. Imaging at depths greater than 20 μm below the surface of a coalesced pendeo-epitaxial GaN sample is demonstrated. Free and donor-bound excitonic emission in this sample appears to originate at the surface, acceptor-bound exciton transitions are strongest in the top bulk portion of the sample, and subgap luminescence is most intense deep in the sample. The depth resolution of the imaging system is measured to be 1.75 μm near the GaN surface.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Schuck, PJ and Grober, RD and Roskowski, AM and Einfeldt, S and Davis, RF}, year={2002}, month={Sep}, pages={1984–1986} }
 @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_davis_2002, title={Maskless pendeo-epitaxial growth of GaN films}, volume={31}, ISSN={["1543-186X"]}, DOI={10.1007/s11664-002-0095-6}, number={5}, journal={JOURNAL OF ELECTRONIC MATERIALS}, author={Roskowski, AM and Preble, EA and Einfeldt, S and Miraglia, PM and Davis, RF}, year={2002}, month={May}, pages={421–428} }
 @article{hartlieb_roskowski_davis_platow_nemanich_2002, title={Pd growth and subsequent Schottky barrier formation on chemical vapor cleaned p-type GaN surfaces}, volume={91}, ISSN={["1089-7550"]}, DOI={10.1063/1.1424060}, abstractNote={Characterization of chemical vapor cleaned, Mg-doped, p-type GaN(0001) surfaces and Pd contacts sequentially deposited on these surfaces has been conducted using x-ray and ultraviolet photoelectron spectroscopies and low-energy electron diffraction. The band bending and the electron affinity at the cleaned p-GaN surface were 1.4±0.1 eV and 3.1±0.1 eV, respectively. A previously unidentified band of surface states was observed at ∼1.0 eV below the Fermi level on this surface. The Pd grew epitaxially on the cleaned surface in a layer-by-layer mode and formed an abrupt, unreacted metal–semiconductor interface. The induced Fermi level movement with Pd deposition has been attributed to a complex interaction between extrinsic and intrinsic surface states as well as metal induced gap states. The final Schottky barrier height at the Pd/p-GaN interface was 1.3±0.1 eV; the interface dipole contribution was 0.4±0.1 eV.}, number={2}, journal={JOURNAL OF APPLIED PHYSICS}, author={Hartlieb, PJ and Roskowski, A and Davis, RF and Platow, W and Nemanich, RJ}, year={2002}, month={Jan}, pages={732–738} }
 @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{einfeldt_roskowski_preble_davis_2002, title={Strain and crystallographic tilt in uncoalesced GaN layers grown by maskless pendeoepitaxy}, volume={80}, ISSN={["0003-6951"]}, DOI={10.1063/1.1448145}, abstractNote={The strain in thin GaN layers grown by maskless pendeoepitaxy has been investigated using high-resolution x-ray diffraction and finite-element simulations. The crystallographic tilt of the free-hanging wings was determined to result from the strain relaxation of the seed stripes along [0001]. The impact of the dimensions of the pendeostructure and of the formation of crystal defects on the expected wing tilt is discussed.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Einfeldt, S and Roskowski, AM and Preble, EA and Davis, RF}, year={2002}, month={Feb}, pages={953–955} }
 @article{roskowski_felder_bullard_2002, title={Student Use (and Non-Use) of Instructional Technology}, volume={2}, number={2002}, journal={Journal of SMET Education: Innovations and Research}, author={Roskowski, A. M. and Felder, R. M. and Bullard, L.}, year={2002}, pages={41–45} }
 @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} }
 @inproceedings{roskowski_felder_bullard_2001, title={Instructional software: if you build It, they may or may not come}, booktitle={2001 ASEE Annual Conference Proceedings, ASEE, June 2001}, publisher={Washington, D.C.: American Society for Engineering Education}, author={Roskowski, A. M. and Felder, R.M. and Bullard, L. G.}, year={2001} }
 @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} }