First-principles study of oxygen vacancies in MgxZn1-xO alloys

A first-principles study of oxygen vacancies in MgxZn1-xO alloys is presented. Different types of oxygen vacancies are distinguished by their number of Mg and Zn nearest neighbors. The formation energy is found to be lowest for vacancies surrounded by four Zn nearest neighbors and is almost independent of the overall concentration in the alloy. Since this energy of formation enters the concentration via a Boltzmann factor, it implies that vacancies other than purely Zn surrounded are very unlikely even in relatively Mg-rich alloys. The defect energy level associated with the vacancy is a very deep donor level closer to the valence band than the conduction band. It does not follow the band gap with concentration but stays approximately fixed relative to the valence-band maximum. The defect level gradually increases with number of Mg neighbors.