  Oxygen vacancy (VO) is a common native point defects that plays crucial roles
in determining the physical and chemical properties of metal oxides such as
ZnO. However, fundamental understanding of VO is still very sparse.
Specifically, whether VO is mainly responsible for the n-type conductivity in
ZnO has been still unsettled in the past fifty 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 VO. 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 non-stoichiometry of ZnO.
The methodology can be extended to study oxygen-related point defects and their
energetics in other technologically important oxide materials.
