PNNL

Abstract

Vacancy clusters have been observed in ZnO, but their physical origin remains elusive. This is because under thermal equilibrium clusters typically have higher formation energies. Combining first-principles calculation and kinetic Monte Carlo (kMC) simulation, we determine the roles of non-equilibrium kinetics on the vacancy clustering. We find that clustering starts with the formation of Zn and O vacancy pairs (VZn-Vo), which further grow by attracting additional mono-vacancies. At this stage, vacancy diffusivity becomes crucial: due to the larger diffusivity of VZn compared to VO, more VZn-abundant clusters are formed than VO-abundant clusters. The large dissociation energies, e.g., 3 eV for (VZn-Vo), suggest that, once formed, it is difficult for the clusters to dissociate. As such, thermal annealing will increase the size of the clusters by promoting mono-vacancy diffusion, in agreement with experiment.