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The electrical properties, presence of deep electron and hole traps and photoluminescence spectra were measured for undoped a-GaN films grown by metal-organic chemical vapor deposition (MOCVD) in a two-stage process using a high V/III ratio at the first stage and low V/III ratio at the second stage. Growth was performed on r-sapphire substrates with a high temperature GaN nucleation layer. The films showed a full width at half maximum of 450-470 arcseconds for the (11-20) x-ray rocking curve with little anisotropy with respect to the sample rotation around the growth direction. The stacking fault (SF) density determined by selective etching was ∼5 × 104 cm-1. The residual donor concentration was 1014-1015 cm-3, with a very low density (2.5 × 1013 cm-3) of electron traps located at Ec - 0.6 eV, which are believed to be one of the major non-radiative recombination centers in nonpolar GaN. Consequently, the films showed a high intensity of bandedge luminescence with negligible contribution from defect bands associated with SFs. In contrast to previously studied nonpolar GaN films, the a-GaN layers showed a high concentration of gallium-vacancy-related acceptors near Ev + 1 eV and a strong yellow luminescence band, both indicating that growth conditions were effectively N-rich. a-AlGaN/GaN heterojunctions with thin heavily Si doped AlGaN barriers made on a-GaN substrates showed two-dimensional electron gas (2DEG) concentrations of 1.2 × 1013 cm-3, with 2DEG mobility of 80 cm2/Vs. Capacitance-voltage profiling of Schottky diodes on these HJs suggest that the 2DEG is fully depleted by the built-in voltage of the Schottky diode. [ABSTRACT FROM AUTHOR]

Structural and electrical properties of nonpolar m-GaN films grown on m-SiC using standard metalorganic chemical vapor deposition (MOCVD) and two versions of sidewall epitaxial lateral overgrowth were studied. It is shown that lateral overgrowth allows one to dramatically reduce the dislocation density from over 109 cm-2 to ∼107 cm-2. In good correlation with that we observed a strong reduction in the density of electron traps Ec-0.25 eV and Ec-0.6 eV from over 1015 cm-3 to ∼1014 cm-3, respectively, in MOCVD m-GaN and in laterally overgrown m-GaN. Preliminary studies of the effects of changing the V/III ratio and of Si doping were performed. The MOCVD m-GaN films grown with high V/III ratio of 1000 were semi-insulating, with the Fermi level pinned near the 0.6 eV traps. Decreasing the V/III value to 250 shifted the Fermi level upward, close to the level of the 0.25 eV traps. Si doping in laterally overgrown samples strongly suppressed the formation of major electron traps but enhanced the formation of hole traps near Ev+0.9 eV. We also report on electrical properties of the GaN interface with the AlN buffer used to facilitate good quality growth on SiC. These properties are dominated by a high concentration of 0.15 eV traps. [ABSTRACT FROM AUTHOR]

Thick a-plane GaN films were grown by hydride vapor phase epitaxy on a-plane GaN templates prepared by metalorganic chemical vapor deposition (MOCVD) and also on a-plane MOCVD templates using in situ nitridized Ti underlayers. The growth on a-GaN showed improved crystalline quality with increasing hydride vapor phase epitaxy thickness, while MOCVD template quality had little effect. With 30 nm Ti films deposited on the templates and converted to TiN islands by nitridation during growth, the authors obtained thick (350 μm), freestanding a-GaN films detached from the template. Microcathodoluminescence spectra of the growth surface showed intense band edge luminescence at 3.47 eV at 90 K with no defect bands. Spectra taken from the surface turned to the substrate were dominated by stacking fault-related bands at 3.42, 3.3, and 3.0 eV, similar to the spectra of the a-GaN templates. X-ray measurements showed the freestanding a-GaN layers consisted of misoriented large grains of a-GaN with halfwidth for individual grains close to 300-400 arc sec and halfwidth anisotropy with respect to sample rotation around the [11-20] direction. [ABSTRACT FROM AUTHOR]

Growth of GaN on polycrystalline chemical vapor deposition (CVD) diamond prepared on Si was achieved by hydride vapor phase epitaxy (HVPE). If the polycrystalline CVD diamond is separated from the Si substrate and the side turned to Si is covered by either TiN or anodic Al oxide, this is found to facilitate deposition of good crystalline quality GaN films. Porous TiN in these experiments was formed by Ti evaporation and subsequent in situ nitridation in the HVPE reactor during pregrowth heating. The films showed double crystal x-ray (0002) rocking curve half width of 245 in. and a strong bandedge luminescence. Thick films self-separated from the substrate. When growing GaN on the Si side of the diamond substrate is covered with porous Al anodic oxide, the authors observed the formation of well defined (0001) GaN texture with the maximum angle between the c-axes of various grains lower than 6.5°, a very low (120 in.) half-width of the (0002) rocking curves of individual grains and an intense bandedge luminescence. [ABSTRACT FROM AUTHOR]

Nonpolar a-GaN films were grown by hydride vapor phase epitaxy on r-sapphire. As the thickness of the grown films was increased from 50 to over 250 μm, the width of the double-crystal rocking curve for the (11–20) reflection decreased from 2000 to 500 arc sec, indicating a strong decrease in the dislocation density. Microcathodoluminescence mapping of the thick films suggests that the dislocation density is ∼102 cm-2, which is more than two orders of magnitude lower than for thin films. The authors also observe a corresponding decrease in the density of residual donors from 1020 to 1018 cm-3, with respective mobility increase from (10–20) to 150 cm2/V s. The luminescence spectra of the thin films show the presence of intense defect bands attributed in literature to stacking faults. These bands are very strongly suppressed in thick films, which also show about an order of magnitude higher band edge luminescence intensity. [ABSTRACT FROM AUTHOR]