Models for the electronic structures of various native bulk and interface defects in ZnO were constructed on the basis of self-consistent-field X-α scattered wave cluster molecular orbital calculations. The dispositions and characters of the defect states relative to the valence and conduction bands of bulk ZnO were studied for zinc and oxygen atomic clusters representing a coordinatively unsaturated zinc surface site, free and chemisorbed molecular oxygen and zinc and oxygen vacancies. The calculations indicate that coordinative unsaturation at ZnO interfaces (i.e. lattice truncation) did not give rise to defect levels which lie in the ZnO band-gap. Upon adsorption at free ZnO interface sites, molecular oxygen forms a deep acceptor. The highest occupied levels were antibonding with respect to the adsorbed O2 molecule and therefore were expected to facilitate dissociative O2 chemisorption when fully occupied at higher temperatures and/or bulk electron concentrations. Calculations of Zn-O bond-centered clusters, in which zinc and oxygen second-nearest neighbors were present, indicated that oxygen vacancies formed deep donors (ED ≈ 2eV). Zinc vacancies were predicted to form shallow acceptors (E ⩾ 0.1eV), contrary to experimental data for bulk ZnO.

Defects and Electronic Structure of Interfaces in ZnO: Cluster Molecular Orbital Calculations. Sukkar, M.H., Johnson, K.H., Tuller, H.L.: Materials Science and Engineering B, 1990, 6[1], 49–59