Large Persistent Photoconductivity in Strontium Titanate at Room Temperature
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Large Persistent Photoconductivity in Strontium Titanate at Room Temperature Violet M. Poole,1 J. Dashdorj,2 Mary Ellen Zvanut,2 and Matthew D. McCluskey1 1 Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814, U.S.A. 2 Department of Physics, University of Alabama at Birmingham, 1300 University Blvd., CH 310, Birmingham, AL 35294-1170, U.S.A. ABSTRACT Strontium titanate (SrTiO3) is a wide-band-gap semiconductor with a variety of novel properties. In this work, bulk single crystal SrTiO3 samples were heated to 1200°C, resulting in the creation of point defects. These thermally treated samples showed large persistent photoconductivity (PPC) at room temperature. Illumination with sub-gap light (>2.9 eV) caused an increase in free-electron concentration by over two orders of magnitude. After the light is turned off, the conductivity persists at room temperature, with essentially zero decay over several days. The results of electron paramagnetic resonance (EPR) measurements suggest that a point defect is responsible for PPC because the photo-induced response of one of the EPR signals is similar to that seen for the PPC. Due to a large barrier for recapture, the photo-excited electron remains in the conduction band, where it contributes to the conductivity. INTRODUCTION Strontium titanate (SrTiO3, or STO) is a wide band gap (3.25 eV) transparent conducting oxide semiconductor with the perovskite crystal structure. It serves as a substrate for high temperature superconductors and is also used to make high temperature oxygen sensors [1]. Recently, it was found that STO can be made to exhibit persistent photoconductivity (PPC) by annealing at 1200°C with strontium oxide (SrO) powder under rough vacuum [2]. The crystal goes from semi-insulating to conductive upon exposure to sub-gap light (>2.9 eV). The change in conductivity is several orders of magnitude and stable at room temperature, days or weeks after the light is turned off. The samples, purchased from commercial vendors, have varying concentrations of defects and impurities. Some samples that underwent the SrO annealing treatment did not exhibit PPC. In the present work, we systematically studied point defects and correlated their presence to PPC. We found that oxygen vacancies and hydrogen impurities appear to be important actors in this phenomenon. The results enabled us to develop a sample processing scheme that “guarantees” PPC behavior. EXPERIMENT Verneuil-grown, single-crystal, oriented STO samples were purchased from MTI Corporation, MaTeck GmbH, and Marketech International Inc. Samples were sealed in an evacuated fused silica ampoule. For the SrO annealing runs, SrO powder was also placed in the ampoule, along with the sample. Annealing was performed in a three-zone horizontal tube furnace at 1200°C for 1 hr. After annealing, the ampoule was retrieved from the furnace and left
to cool in the dark. Some samples were exposed to 405 nm light from a light-emitting diode or laser diode. Infrared (IR) spectra were obtained using
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