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

Periodic Hartree-Fock total-energy calculations on two-dimensional slabs have been used to study the symmetry-conserving relaxation of the nonpolar (1010BAR) surface of ZnO. We find that it is energetically favorable for the Zn-O surface dimers to tilt slightly (by 2.3-degrees) and move downwards towards the slab, and for the dimer bond to shorten significantly. Our results agree fairly well with those of a recent density-functional calculation, but disagree with empirical tight-binding theory which predicts surface bonds to shorten only slightly while the surface dimers undergo a large tilt (18-degrees). The available experimental data lies between the ab initio and tight-binding results with large error bars. We have tested the effects of several refinements of our Hartree-Fock calculation, including improvements of the orbital basis set and precision tolerances, the use of thicker slabs in approximating the semi-infinite crystal, and post-self-consistent-field density-functional correlation corrections to the total energy. None of these refinements significantly changed our results. We discuss possible reasons for the disagreement between our results and those of tight-binding theory.