Understanding the presence of vacancy clusters in ZnO from a kinetic perspective

Vacancy clusters have been observed in ZnO by positron-annihilation spectroscopy (PAS), but detailed mechanisms are unclear. This is because the clustering happens in non-equilibrium conditions, for which theoretical method has not been well established. Combining first-principles calculation and kinetic Monte Carlo simulation, we determine the roles of non-equilibrium kinetics on the vacancies clustering. We find that clustering starts with the formation of Zn and O vacancy pairs (V-Zn - Vo), which further grow by attracting additional mono-vacancies. At this stage, vacancy diffusivity becomes crucial: due to the larger diffusivity of V-Zn compared to V-O, more V-Zn-abundant clusters are formed than V-O-abundant clusters. The large dissociation energy barriers, e. g., over 2.5 eV for (V-Zn - Vo), suggest that, once formed, it is difficult for the clusters to dissociate. By promoting mono-vacancy diffusion, thermal annealing will increase the size of the clusters. As the PAS is insensitive to V-O donor defects, our results suggest an interpretation of the experimental data that could not have been made without the in-depth calculations. (C) 2014 AIP Publishing LLC.