Atomic geometry of cleavage surfaces of tetrahedrally coordinated compound semiconductors

Analyses of low-energy electron diffraction intensities are utilized to determine the surface atomic geometries of GaAs(110), ZnO(101̄0), and ZnO(112̄0). The geometry of GaAs(110) is rippled, with the As atoms protruding upwards and the Ga atoms downwards relative to the truncated bulk (zincblende) geometry in a fashion such that no nearest-neighbor bond lengths are altered. The geometries of ZnO(101̄0) and ZnO(112̄0) are nearly those of the truncated bulk (wurtzite) lattice. Small contractions of the upper-layer spacings of both the oxygen (Δd=−0.1 Å) and the zinc (Δd=−0.3 Å) sublattices appear probable, however, for ZnO(101̄0). The qualitatively different nature of the surface reconstruction on GaAs(110) from those which may occur on ZnO(101̄0) is interpreted in terms of the covalent character of the bonding in GaAs as contrasted with a more ionic bonding in ZnO. Both types of surface reconstruction are consistent with a model in which the cation-to-anion charge transfer in the surface layer is less than in the bulk.