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

TL;DR

This study investigates the role of oxygen vacancies in ZnO, revealing they are positively charged and primarily responsible for its unintentional n-type conductivity through oxygen self-diffusion experiments.

Contribution

It provides direct experimental evidence that oxygen vacancies in ZnO are +2 charged and mediate n-type conductivity, clarifying a long-standing debate.

Findings

Oxygen vacancies are predominantly mediated by VO in ZnO.

VO are positively charged (+2), not neutral.

Oxygen vacancies cause n-type conductivity and non-stoichiometry in ZnO.

Abstract

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.