Trends in surface atomic geometries of compound semiconductors

Abstract

A brief synopsis is presented of low-energy electron diffraction structure analyses reported for the cleavage faces of zincblende and wurtzite compound semiconductor surfaces, specifically GaAs(110), InSb(110), ZnTe(110), InP(110), and ZnO(101̄0). In all of these cases the surface atomic geometry is distorted from that of a truncated bulk solid, although the space group symmetries at the surface are indistinguishable from those of the corresponding bulk materials. The surface reconstructions involve large (Δd∠0.5 Å) displacements of the species in the uppermost atomic layer and often smaller displacements (Δd∠0.1 Å) in the layer beneath. The character of these reconstructions depends, however, on the nature of the chemical bonding in the bulk materials. For the most covalent materials, i.e., GaAs and InSb, the anion rotates outward and the cation inward in a fashion such that bond lengths remain unaltered to within 1% except for a 5% contraction of the bond between the uppermost anion and the cation in the layer beneath. Morever, these ’’covalent’’ reconstructions penetrate at least two atomic layers into the surface with the displacements in the second layer being in the opposite sense to those in the first. The surface reconstructions of the more ionic materials, i.e., ZnTe and ZnO, are characterized by generally smaller displacements of the anion species and the absence of definitve evidence for distortions of the second and deeper layers from the bulk geometry. Therefore available structural data suggest a systematic change in the nature of the surface reconstructions of the cleavage faces of tetrahedrally coordinated compound semiconductors with increasing ionicity. The change is most pronounced for materials with ionicities in the vicinity of those of ZnTe and InP, and seems to be associated with the transition from III–V materials (which exhibit ionicities Fi?0.5) to II–VI materials (which exhibit ionicities fi?0.5).