New Methods for Defect Manipulation in Semiconducting Oxides

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

NON-TECHNICAL DESCRIPTION The technologically useful properties of a solid often depend upon the types and concentrations of the defects it contains. In oxide semiconductors, defects such as vacancies in the ordered atomic arrangement of the solid or extra atoms wedged into the structure control the effectiveness of certain catalysts for environmental remediation, the sensitivity of important kinds of sensors, and the efficiency of devices for converting sunlight to electrical power. Current methods for controlling defect behavior suffer from consumption of the solid, structural damage, or incorporation of unwanted atoms. The present research seeks to develop entirely new forms of defect manipulation in oxide semiconductors based on control of the chemical state of the surface or on illumination by light. The work employs experimental measurements of diffusion in titanium dioxide (a technologically important oxide semiconductor) supported by extensive mathematical modeling. This research takes place in parallel with development of an industry-supported laboratory course for upper-division undergraduates and graduate students called "Chemistry and Transport in Semiconductor Materials Synthesis." In addition, the notion is being examined based on scholarly literature that the advisor-protege relationship mirrors the parent-offspring relationship in important respects. Finally, several activities to promote the importance of ethics in science and engineering are being pursued. TECHNICAL DETAILS The defect manipulation relies upon three recently-discovered mechanisms: two for bulk-surface coupling and one for optical stimulation of defect formation and migration. Surface coupling to bulk defects occurs through electrostatic and surface bond insertion/generation mechanisms. Optical stimulation of defect mobilities and concentrations occurs through changes in defect charge state. Specially synthesized structures of titanium dioxide offer the possibility to measure generation and annihilation rates of vacancies and interstitial atoms at surfaces. Quantification is accomplished through detailed continuum modeling backed by characterization of near-surface electric fields through optical photoreflectance. The research examines the effects of surface crystallographic orientation, degree of surface polarity, gas adsorption, and doping level. The effects of optical stimulation are quantified by analogous experiments performed under super-bandgap illumination.