Theoretical study of nonpolar surfaces of aluminum nitride: Zinc blende (110) and wurtzite (101-|Am0)

All-electron density-functional calculations are performed to study atomic structure and electronic properties of the nonpolar surfaces, namely zinc blende (110) and wurtzite (101-|Am0) of AlN. Both surfaces are modeled using a two-dimensional periodic slab allowing the relaxation of the first two surface layers in the calculations. The results predict a small layer rotation angle accompanied by a contraction of Al-N bond length for both surfaces. These results do not follow the well-accepted rotation-relaxation model that predicts large layer rotation angles (∼28°) with no change in the bond length for most of the III-V semiconductor surfaces. Analysis of the relaxed configurations of the AlN surfaces in terms of atomic geometry, density of states, and charge density plots shows a presence of partial double-bond character in the surface Al-N bond. A similarity of these results with an earlier study on GaN nonpolar surfaces [J. E. Jaffe, R. Pandey, and P. Zapol, Phys. Rev. B 53, R4209 (1996)] led us to suggest the contraction-relaxation model where the relaxation proceeds via strengthening of the surface bond. The primary driving force of such a type of relaxation appears to be the ability of nitrogen to form a double bond that facilitates redistribution of the charge density associated with anion dangling bond to the surface bond.