Oxygen vacancies: The origin of n -type conductivity in ZnO

Oxygen vacancy $({V}_{\mathrm{O}})$ is a common native point defect that plays crucial roles in determining the physical and chemical properties of metal oxides such as ZnO. However, fundamental understanding of ${V}_{\mathrm{O}}$ is still very sparse. Specifically, whether ${V}_{\mathrm{O}}$ is mainly responsible for the $n$-type conductivity in ZnO has been still unsettled in the past 50 years. Here, we report on a study of oxygen self-diffusion by conceiving and growing oxygen-isotope ZnO heterostructures with delicately controlled chemical potential and Fermi level. The diffusion process is found to be predominantly mediated by ${V}_{\mathrm{O}}$. We further demonstrate that, in contrast to the general belief of their neutral attribute, the oxygen vacancies in ZnO are actually $+2$ charged and thus responsible for the unintentional $n$-type conductivity as well as the nonstoichiometry of ZnO. The methodology can be extended to study oxygen-related point defects and their energetics in other technologically important oxide materials.