AB-INITIO STUDY OF PIEZOELECTRICITY AND SPONTANEOUS POLARIZATION IN ZNO

We demonstrate the feasibility of ab initio studies of piezoelectricit y within an all-electron scheme. The focus of our analysis is on wurtz ite ZnO; for comparison, some results are also presented for the relat ed materials BeO and ZnS. The comparative study is performed in order to understand the microscopic origin of the peculiar behavior of ZnO, whose piezoelectric response is the strongest among the tetrahedrally bonded semiconductors. In all such materials, the piezoelectric effect results from two different terms of opposite sign: these are usually referred to as the ''clamped-ion'' and the ''internal-strain'' contrib utions. Cancellation amongst them is least effective in ZnO, where the dominant effect is due to a rigid-ion-like mechanism. Furthermore, we compute the spontaneous polarization of ZnO and we discuss the puzzli ng agreement between our calculated value and a very indirect experime ntal estimate of the same quantity.