Reconstructions of the AlN(0001) surface
Reconstructions of the AlN(0001) surface are studied. For moderately Al-rich surfaces, surface reconstructions with symmetry of 2√3×2√3-R30° and 5√3×5√3-R30° are found on the basis of scanning tunneling microscopy and low-energy electron diffraction observations. Such surfaces display a predominantly 2×6 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 √3×√3-R30° and 2×2 periodicities are observed.