(38d) Control of Self-Diffusion in Titanium Dioxide for Nanoelectronics
The technologically useful properties of semiconductor oxides such as titania often depend on the concentration and motion of defects. Native point defects such as interstitial atoms and vacancies strongly affect the operation of nanoelectronic gas sensors, light emitting diodes and memory resistors. Past work in our laboratory with silicon has shown that semiconductor surfaces serve as efficient pathways for generation and annihilation of point defects in the underlying bulk. Such pathways should play an especially important role in nanoelectronics fabrication, where devices have high surface-to-volume ratios. The present work extends the findings for silicon to the oxide semiconductor titanium oxide, wherein we have identified a new pathway for interstitial-mediated diffusion of oxygen in titanium oxide. Oxygen diffusion rates were measured by exposing natural-abundance single-crystal rutile titania to isotopically labeled oxygen gas. The resulting profiles were measured by secondary ion mass spectroscopy and subsequently modeled with continuum equations for the reaction and diffusion of the key point defects that control self-diffusion. The exponential diffusion profile shapes, together with the increase of the diffusion coefficients with oxygen pressure, strongly suggest the diffusion is mediated by oxygen interstitials. The measured diffusion coefficients were nearly two orders of magnitude higher than those expected from the literature, but could be decreased substantially by the adsorption of submonolayer quantities of sulfur to saturate surface dangling bonds. These latter observations suggest that the pristine titania surface is especially efficient at creating oxygen interstitials that then sink into the bulk and mediate diffusion there.