Mechanism of creation and destruction of oxygen interstitial atoms by nonpolar zinc oxide(100) surfaces

Abstract

Oxygen vacancies (V_O) influence many properties of ZnO in semiconductor devices, yet synthesis methods leave behind variable and unpredictable V_O concentrations. Oxygen interstitials (O_i) move far more rapidly, so post-synthesis introduction of O_i to control the V_O concentration would be desirable. Free surfaces offer such an introduction mechanism if they are free of poisoning foreign adsorbates. Here, isotopic exchange experiments between nonpolar ZnO(100) and O₂ gas, together with mesoscale modeling and first-principles calculations, point to an activation barrier for injection only 0.1–0.2 eV higher than for bulk site hopping. The modest barrier for hopping in turn enables diffusion lengths of tens to hundreds of nanometers only slightly above room temperature, which should facilitate defect engineering under very modest conditions. In addition, low hopping barriers coupled with statistical considerations lead to important qualitative manifestations in diffusion via an interstitialcy mechanism that does not occur for vacancies.