Reconstructions of the AlN(0001) surface

Reconstructions of the AlN(0001) surface are studied. For moderately Al-rich surfaces, surface reconstructions with symmetry of 2root3x2root3-R30degrees and 5root3x5root3-R30degrees are found on the basis of scanning tunneling microscopy and low-energy electron diffraction observations. Such surfaces display a predominantly 2x6 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 GaN(0001). At higher Al coverages a thick, flat Al film is found to form on the surface. For Al-poor conditions, additional surface reconstructions with root3xroot3-R30degrees and 2x2 periodicities are observed.