(502a) Surface-Mediated Mechanisms for Defect Engineering in Metal Oxides

Control of native oxygen defects in transition metal oxides like ZnO plays an important role in photovoltaic, catalytic, photonic and gas sensing applications. The interaction chemistry between bulk point defects and reactive sites on clean semiconductor surfaces is comparable in richness to the reactions of gases with surfaces. Surfaces of semiconducting metal oxides can be used to manipulate the concentrations and spatial distributions of oxygen defects, particularly oxygen vacancies. Such surface-based defect engineering methods should play an especially important role in nanostructured devices where the surface to volume ratio is high. The present work discusses a novel mechanism of bulk defect interaction with c-axis polar ZnO surfaces that enables control of oxygen defect injection. Oxygen diffusion rates were measured by exposing natural-abundance single-crystal c-axis wurtzite ZnO to isotopically-labeled oxygen (¹⁸O₂) gas.  The resulting diffusion profiles were measured by secondary ion mass spectrometry (SIMS) and modeled with mass transport equations for the reaction and diffusion of mobile and stagnant oxygen point defect species. The effective oxygen diffusivity is determined by fitting this model to experimental diffusion profiles. In addition, first principles calculations based on density functional theory (DFT) were performed to identify the elementary-step oxygen defect injection mechanism for active site exchange. Influence of the surface polarity on defect injection rates was also investigated. Gas-solid exchange experiments coupled with continuum and ab initio modeling will help us understand mechanistically the effect of each of the elementary steps on the defect injection rates at polar ZnO surfaces.