Bond-rotation versus bond-contraction relaxation of (110) surfaces of group-III nitrides

The rotation-relaxation model with large layer-rotation angles of about 30 degrees is well accepted for the (110) cleavage surfaces of ordinary m-V compounds. We examine this model for the group-III nitrides BN, AlN, GaN, and InN with larger covalency and polarity of the bonds. Well converged ab initio calculations using a plane-wave pseudopotential method suggest a tendency for a bond-contraction relaxation where the buckling and the zigzag character of the III-N chains parallel to [1 (1) over bar 0] are reduced. The resulting geometries, energetics, and electronic structures are analyzed in terms of bond strength and bond ionicity.