Theoretical study of nonpolar surfaces of aluminum nitride: Zinc blend (110) and wurtzite (10(1)over-bar-0)

All-electron density-functional calculations are performed to study atomic structure and electronic properties of the nonpolar surfaces, namely zinc blende (110) and wurtzite (10 (1) over bar 0) 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 (similar to 28 degrees) 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.