Persistent Optical Phenomena in Oxide Semiconductors

Washington State University, Pullman WA

Investigators

Abstract

Nontechnical description: The project investigates two novel phenomena involving the interaction between light and matter. Persistent photoconductivity is a dramatic increase in a material’s ability to conduct electricity when exposed to light, which continues even after the light is turned off. Photochromism occurs when a material changes color upon light exposure. The research team aims to tie these important optical phenomena into a unified defect model. Advancing fundamental knowledge about persistent optical effects could benefit technologies ranging from rewritable electronics to smart windows. To accomplish these goals, they use state-of-the-art facilities to grow novel crystals and perform measurements to uncover their atomic-scale physical properties. Students involved in this research perform hands-on experiments, present results at scientific conferences, and meet with startup companies to discuss potential applications. Connected to the research project’s theme of defects in crystals, tutorials are developed for upper-division physics courses that provide a conceptual understanding of systems with many atoms and electrons. Technical description: The project investigates persistent photoconductivity and photochromism in oxide semiconductors and ties these important optical phenomena into a unified defect model. Advancing fundamental knowledge about persistent optical effects could benefit technologies ranging from rewritable electronics to smart windows. When a semiconductor is exposed to light, electrons from the valence band or defect levels can be promoted to the conduction band, increasing electrical conductivity. In the dark, the system generally relaxes to the ground state and conductivity returns to normal. Persistent photoconductivity (PPC) occurs when the conductivity remains even after the light source is turned off. Related to PPC, photochromism is a change in color upon exposure to light. The researchers are testing the hypothesis that PPC and photochromism in oxide semiconductors arise from the same effect: light liberates a hydrogen impurity from its substitutional site, leaving behind an oxygen vacancy. Using unique facilities at Washington State University, crystals are grown with high purity and controlled defect populations. A variety of experimental methods, including Hall effect, infrared spectroscopy, and photoluminescence, are utilized to study the defects underlying PPC and photochromism. Additionally, the researchers are exploiting PPC to investigate transport over a wide range of free-electron concentrations. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.