AB-INITIO STUDY OF ZNO (101(BAR)0) SURFACE RELAXATION

Periodic Hartree-Fock total-energy calculations on two-dimensional sla bs have been used to study the symmetry-conserving relaxation of the n onpolar (1010BAR) surface of ZnO. We find that it is energetically fav orable for the Zn-O surface dimers to tilt slightly (by 2.3-degrees) a nd move downwards towards the slab, and for the dimer bond to shorten significantly. Our results agree fairly well with those of a recent de nsity-functional calculation, but disagree with empirical tight-bindin g theory which predicts surface bonds to shorten only slightly while t he surface dimers undergo a large tilt (18-degrees). The available exp erimental data lies between the ab initio and tight-binding results wi th large error bars. We have tested the effects of several refinements of our Hartree-Fock calculation, including improvements of the orbita l basis set and precision tolerances, the use of thicker slabs in appr oximating the semi-infinite crystal, and post-self-consistent-field de nsity-functional correlation corrections to the total energy. None of these refinements significantly changed our results. We discuss possib le reasons for the disagreement between our results and those of tight -binding theory.