Induced Persistent Photoconductivity in Strontium Titanate

Strontium titanate (STO) is a wide, direct, bandgap semiconductor which can be doped from the insulating state to a high mobility semiconductor through to a metallic state which at temperatures lower than ~ 300 mK becomes one of the lowest carrier density superconductors. As well as traditional chemical doping, carriers can also be introduced by using band-gap illumination which generates long lived electron-hole pairs, where the electron mobility is much higher than that of the holes. The density of carriers can be controlled by intensity of the light. Furthermore by varying the wavelength of the incident light near the band edge the optical penetration depth, and therefore the sheet thickness of the photo-generated electrons, can also be tuned. Photocarrier doping, therefore, offers continuous, real-time tuning of both sheet carrier density and thickness. In undoped STO these photo-generated carriers recombine rapidly, however the presence of defects can dramatically enhance their lifetime leading to photoconductivity which persists for many hours after illumination has ceased. Here we present our work on inducing tuneable persistent photoconductivity at the surface of STO by large doses of ultra-violet light at low temperatures.