The polarity of AlN epitaxial layers grown on (0001) sapphire, SiC, and nitrided sapphire substrates was examined by convergent beam electron diffraction, and the morphology and microstructure were characterized by atomic force microscopy and scattering contrast transmission electron microscopy (TEM). The AlN films grown on sapphire and SiC without a buffer layer or nitridation of the substrate were flat and had Al polarity. From TEM studies in both cross section and plan view, it was found that the threading dislocation (TD) density was similar to1x10(8) and 2x10(10) cm(-2) for screw-component and pure edge component dislocations, respectively. N-face AlN was realized by pregrowth nitridation of sapphire substrates, but these films contained a small volume fraction of Al-face inversion domains which were related to hexagonal pyramids defined by {1(2) over bar 02} facets. The density of screw-component TDs was significantly reduced due to nitridation. Cross-sectional TEM showed that the film grown on nitrided sapphire was almost free of screw-component TDs and that the density of pure edge TDs was similar to4x10(9) cm(-2). (C) 2004 American Institute of Physics.

This letter discusses the structural and morphological characteristics of planar, nonpolar (11 (2) over bar0) a-plane GaN films grown on (1 (1) over bar 02) r-plane sapphire by hydride vapor phase epitaxy. Specular films with thicknesses over 50 mum were grown, eliminating the severely faceted surfaces that have previously been observed for hydride vapor phase epitaxy-grown a-plane films. Internal cracks and crack healing, similar to that in c-plane GaN films, were observed. Atomic force microscopy revealed nanometer-scale pitting and steps on the film surfaces, with rms roughness of similar to2 nm. X-ray diffraction confirmed the films are solely a-plane oriented with on-axis (11 (2) over bar0) and 30degrees off-axis (10 (1) over bar0) rocking curve peak widths of 1040 and 3000 arcsec, respectively. Transmission electron microscopy revealed a typical basal plane stacking fault density of 4x10(5) cm(-1). The dislocation content of the films consisted of predominately edge component (b(edge)=+/-[0001]) threading dislocations with a density of 2x10(10) cm(-2), and mixed-character Shockley partial dislocations (b=1/3(1) over bar 00>) with a density of 7x10(9) cm(-2). (C) 2003 American Institute of Physics.

This letter reports on the reduction in extended-defect densities in a-plane (11 (2) over bar0) GaN films achieved via lateral epitaxial overgrowth (LEO) by hydride vapor phase-epitaxy. A variety of dielectric mask patterns was used to produce 8-125-mum-thick, fully coalesced nonpolar GaN films. The nanometer-scale pit densities in the overgrown regions were less than 3x10(6) cm(-2) compared to similar to10(10) cm(-2) in the direct-growth a-plane GaN. Cathodoluminescence revealed a fourfold increase in luminous intensity in the overgrown material compared to the window material. X-ray rocking curves indicate the films were free of wing tilt within the sensitivity of the measurements. Whereas non-LEO a-plane GaN exhibits basal plane stacking fault and threading dislocation densities of 10(5) cm(-1) and 10(9) cm(-2), respectively, the overgrown LEO material was essentially free of extended defects. The basal plane stacking fault and threading dislocation densities in the wing regions were below the detection limits of similar to5x10(6) cm(-2) and 3x10(3) cm(-1), respectively. (C) 2003 American Institute of Physics.