Surface structure and bonding of the cleavage faces of tetrahedrally coordinated II–VI compounds

Abstract

New sp3 tight-binding models for ZnO, ZnS, ZnSe, CdS, and CdSe are utilized to calculate the atomic geometries and surface-state eigenvalue spectra of the (110) surfaces of zinc-blende allotropes and the (101̄0) and (112̄0) cleavage faces of the wurtzite allotropes. The models predict that each cleavage face [i.e., zinc-blende (110), wurtzite (101̄0), and wurtzite (112̄0)] exhibits a characteristic relaxation in which the anion relaxes outward and the cation inward. The details of these relaxations depend on the local atomic connectivity and hence vary from one cleavage face to another. The dependence of all three classes of relaxed structure on the specific material considered is shown to be described by a linear scaling of the independent surface structural parameters with the bulk lattice constant. All of these structures are shown to result from a rehybridization of surface bonds created by the movement of a dangling-bond surface state from the energy gap into the top of the valence band as it becomes modified to contain some back bonding and/or surface bonding character. The relaxed cleavage face geometries correspond to new types of epitaxially constrained chemical bonding which exhibit no bulk or molecular analogs.