Magnesium incorporation during the molecular beam epitaxy growth of wurtzite GaN is found to invert the Ga-polar (0001) face to the N-polar face. The polarity is identified based on the two different sets of reconstructions seen on the film prior to and after about 1 monolayer Mg exposure. The inversion boundary is seen to lie on the (0001) plane from transmission electron microscopy images, and a structural model is presented for the inversion. On the Ga-polar face, Mg is also seen to stabilize growth in the N-rich regime.

Large-scale wurtzite GaN nanowires and nanotubes were grown by direct reaction of metal gallium vapor with flowing ammonia in an 850–900 °C horizontal oven. The cylindrical structures were as long as 500 μm with diameters between 26 and ∼100 nm. Transmission electron microscopy, scanning electron microscopy, and x-ray diffraction were used to measure the size and structures of the samples. Preliminary results show that the size of the nanowires depends on the temperature and the NH3 flow rate. The growth mechanism is discussed briefly. The simple method presented here demonstrates that GaN nanowires can be grown without the use of a template or catalyst, as reported elsewhere.

AlGaN samples grown by plasma-assisted molecular-beam epitaxy on sapphire (0001) substrates, with 20%–50% Al content and without the use of indium, show intense room-temperature photoluminescence that is significantly redshifted, 200–400meV, from band edge. This intense emission is characterized by a long room-temperature lifetime (∼375ps) comparable to that seen in low defect density (∼108cm−2) GaN. Room-temperature monochromatic cathodoluminescence images at the redshifted peak reveal spatially nonuniform emission similar to that observed in In(Al)GaN alloys and attributed to compositional inhomogeneity. These observations suggest that spatial localization enhances the luminescence efficiency despite the high defect density (>1010cm−2) of the films by inhibiting movement of carriers to nonradiative sites.

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GaN films are grown by plasma-assisted molecular-beam epitaxy on SiC substrates. The width of the x-ray rocking curve for the (101̄2) reflection exhibits a distinct minimum for Ga/N flux ratios which are only slightly greater than unity. Correlated with this minimum, the surface morphology is somewhat rough, with a hill and valley topography. Based on transmission electron micrographs, the reduction in rocking curve width is attributed to enhanced annihilation of edge dislocations due to their tendency to cluster at topographic valleys.