Reconstructions of the AlN͑0001͒ surface are studied. For moderately Al-rich surfaces, surface reconstructions with symmetry of 2)ϫ2)-R30°and 5)ϫ5)-R30°are found on the basis of scanning tunneling microscopy and low-energy electron diffraction observations. Such surfaces display a predominantly 2ϫ6 pattern in reflection high-energy electron diffraction. Auger electron spectroscopy indicates an Al coverage for such surfaces of 2-3 monolayers. Based on this result and on first-principles total energy calculations it is argued that these reconstructions involve a laterally contracted Al adlayer structure similar to that previously proposed for GaN͑0001͒. At higher Al coverages a thick, flat Al film is found to form on the surface. For Al-poor conditions, additional surface reconstructions with )ϫ)-R30°and 2ϫ2 periodicities are observed.

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.

The effect of trace arsenic on the growth and surface structure of GaN(0001) has been studied. We find that a partial pressure of only 10 -9Torr of arsenic during molecular beam epitaxial growth significantly modifies the growth kinetics. Such a small background pressure of arsenic leads to an arsenic-terminated surface displaying a 2×2 reconstruction during growth which is absent for the clean surface. First-principles theoretical calculations show that As-terminated surfaces are energetically more favorable than Ga-terminated surfaces for arsenic pressures of 10 -9 Torr, and structural models for the As-adatom 2×2 reconstruction are presented.

The morphology of the porous network in porous SiC has been studied. It has been found that pore formation starts with a few pores on the surface and then the porous network grows in a V-shaped branched structure below the surface. The hydrogen etching rates of porous and nonporous SiC have been measured. Etch rates of porous and nonporous wafers of various miscuts are found to be equal within a factor of two, indicating that the rate-limiting step in the etching process arises from the supply of active etching species from the gas phase. The porous SiC etches slightly faster than the nonporous SiC, which is interpreted simply in terms of the reduced average density of the porous material. II. Experiment The porous SiC samples studied in this work were purchased from TDI, Inc. They were prepared by anodization at current density of 7 mA/cm 2 for 3 min, with a 250 watt

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