(152c) Electrostatic Interaction Mechanism for near-Surface Defect Redistribution

Concentration and diffusion of point defects in metal oxides strongly influence the manufacture and performance of these materials in nanoelectronic, gas sensing, photonic, and photocatalytic applications. Near-surface effects are particularly important in nanoscale devices because most of the bulk is located in the vicinity of the surface. The electric fields that naturally exist near many semiconductor surfaces influence the motion of charged defects, resulting in their redistribution. In metal oxides, the redistribution of charged oxygen defects affects the performance of nanoelectronic devices such as memory resistors and Schottky barriers. Near-surface electric fields can arise from electrostatic charge buildup due to dangling bonds, surface polarity and adsorption. These effects are controllable in principle. Another important contributor that is less controllable is the chemical potential difference between defects residing deep in the bulk compared to near the surface.  With a view toward developing improved methods for defect engineering, we have employed a combination of diffusion measurements, surface potential measurements and simulations to estimate the relative contributions of electrostatic and thermodynamic forces to the surface potential V_s.  Oxygen defect behavior was monitored indirectly via isotopic oxygen self-diffusion measurements made by exposing single-crystal rutile titania and zinc oxide to isotopically-labeled oxygen gas. The resulting profiles were measured by secondary ion mass spectrometry and subsequently simulated with continuum equations describing the reaction, Fickian diffusion and electric field-driven diffusion of the key point defects.