Transport properties of SrTiO$_{3}$ / wide-gap insulator heterointerfaces

A field-effect transistor is a sensitive probe for the investigation of interfacial electronic properties. We have demonstrated the importance of an epitaxial interface, using SrTiO$_{3}$ (100) single crystal field-effect transistors with amorphous and epitaxial CaHfO$_{3}$ layers as gate insulators. The devices with amorphous insulator layers showed n-type transistor operation with a field-effect mobility of 0.4 to 0.5~cm$^{2}$~/~V~s at room temperature. A large threshold voltage shift was observed at low temperatures and the transistor performance was temperature independent when that shift was taken into account. The device properties were greatly affected by the interface between amorphous insulator and SrTiO$_{3}$. To improve the quality of the channel layer-insulator interface, an ultra-thin epitaxial CaHfO$_{3}$ layer was grown on the SrTiO$_{3}$ substrate surface at high temperature, followed by room-temperature deposition of an amorphous insulator layer. The devices with epitaxial interfaces exhibited a large improvement over the amorphous transistors. A field-effect mobility of around 2~cm$^{2}$~/~V~s was attained at room temperature and found to increase at low temperature, reaching 25~cm$^{2}$~/~V~s at 50~K. This result means that the carriers induced by the field effect behaved as would be expected for electron-doped SrTiO$_{3}$.