Persistent photoconductivity in titanate semiconductor crystals

Nontechnical description:

The project investigates a novel phenomenon, persistent photoconductivity, which was recently observed at room temperature. Samples with this unique property experience a dramatic increase in electrical conductivity when exposed to light, which persists long after the light is turned off. Strontium titanate, a transparent crystal, exhibits large persistent photoconductivity at room temperature, opening possibilities for industrial applications ranging from optical memory devices to electroplating of metals on surfaces.

The research team is attempting to characterize the defects that are responsible for this unusual behavior, maximize the size of the effect, and discover it in new materials. They use state-of-the-art facilities to grow novel crystals and study their physical properties. Students involved in this research perform hands-on experiments, present results at international conferences, and meet with startup companies to discuss potential applications.

In conjunction with the optical theme of the research, lessons are developed for a required course for elementary education students seeking the middle science credential. These lessons on waves and light target the K–12 learning goals defined by the Next Generation Science Standards and Washington State Professional Educator Standards Board, Middle Level Science Endorsement Competencies.

Technical description:

The project aims to achieve large, room-temperature persistent photoconductivity in oxide semiconductors and exploit the phenomenon to advance knowledge about fundamental processes such as electronic transport. Kinetics, energy-barrier heights, and fundamental defect properties are investigated. These studies also elucidate the structure of hydrogen-related complexes in oxides.

The research team is attempting to achieve large, room temperature persistent photoconductivity in barium titanate, a ferroelectric material that could exhibit larger conductivity changes than those previously discovered in strontium titanate. This unique phenomenon is exploited to investigate electrical properties, over a well-defined range of carrier concentrations, to provide insight into scattering mechanisms and small-polaron versus free-electron transport.

Using unique facilities at Washington State University, crystals are grown with high purity and controlled defect populations. These crystals include solid solutions (alloys) that are designed to have optimal properties. A variety of experimental methods, including Hall effect, infrared spectroscopy, and photoluminescence, are used to study the persistent photoconductivity and the defects responsible for it.

Besides providing fundamental insight, establishing defect and transport properties is important because it will allow researchers to control (i.e., induce and erase) and optimize the effect. The fundamental knowledge gained in this study could provide the basis for holographic storage, optically defined electronics, and selective electroplating.

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.