Cleavage faces of wurtzite CdS and CdSe: Surface relaxation and electronic structure

The atomic geometries and electronic structures of the cleavage faces, (101¯0) and (112¯0), of wurtzite-structure CdS and CdSe are calculated using an ${\mathrm{sp}}^3$ tight-binding model. The model is validated by comparison with bulk optical and x-ray photoemission data. The perpendicular displacement of the top-layer anion relative to the corresponding cation, ${\Delta }_1$,⊥, is predicted to be 0.72 Å (CdS) and 0.77 Å (CdSe) for the (101¯0) surfaces, and 0.68 Å (CdS) and 0.71 Å (CdSe) for the (112¯0) surfaces. The model also predicts a surface state near the top of the valence band, as well as at least four additional surface states and resonances lying within the valence band for both surfaces. Both the surface reconstructions and surface states are analogous to those predicted for other wurtzite-structure compound semiconductors. The values of ${\Delta }_1$,⊥ are shown to scale linearly with the bulk lattice constant just as for the (110) surfaces of zinc-blende-structure materials.