SELF-CONSISTENT ELECTRONIC-STRUCTURE CALCULATIONS OF THE (1010) SURFACES OF THE WURTZITE COMPOUNDS ZNO AND CDS

We report ab initio calculations of the surface electronic structure o f the hexagonal wurtzite semiconductors ZnO and CdS. The calculations are carried out self-consistently in the local-density approximation e mploying separable norm-conserving pseudopotentials. Localized Gaussia n orbitals are used in the basis set for an efficient description of t he strongly localized wave functions. The Zn 3d and Cd 4d electrons ar e explicitly taken into account as valence electrons since they are im portant for a quantitative description of the structural as well as th e electronic properties, as our investigations of bulk ZnO and CdS hav e shown. Cationic d-d repulsion, p-d and s-d interactions, and orthogo nalization effects are, therefore, included in our calculations of the ZnO and CdS (1010BAR) surfaces. The nonpolar cleavage face is describ ed in a supercell geometry with eight atomic and four vacuum layers in each cell. The surface atomic structure is determined by elimination of the forces. For ZnO, we find a rotation relaxation, in which both t he Zn and O surface atoms move inward towards the substrate. The calcu lated surface-perpendicular displacements of the Zn atoms relative to the O atoms in the top layer tum out to be slightly smaller than those determined experimentally by low-energy electron diffraction. The sur face electronic structure exhibits an oxygen-derived dangling-bond ban d in the fundamental gap which is essentially unaffected by the calcul ated surface relaxation. This is in marked contrast to the results of previous studies using an empirical tight-binding approach which eithe r find no surface states in the gap at all or one band of surface stat es that is shifted down into the projected bulk valence bands by the s urface relaxation. For CdS(1010BAR) we find a surface structure that n icely agrees in general with the relaxation model experimentally deter mined for the related CdSe(1010BAR) surface. The surface electronic st ructure is compared with the results of polarization- and angle-resolv ed photoemission experiments. With the help of selection rules we are able to identify two occupied surface states at the top of the project ed bulk valence bands in very good agreement with experiment.