High electron mobility in epitaxial SnO2−x in semiconducting regime

Abstract

We investigated the electronic transport properties of epitaxial SnO2−x thin films on r-plane sapphire substrates. The films were grown by pulsed laser deposition technique and its epitaxial growth direction was [101] and the in-plane alignment was of SnO2−x [010]//Al2O3[12̄10]. When the SnO2−x films were grown in the oxygen pressure of 30 mTorr, we have found the electron mobility of the 30 nm thick SnO2−x thin films strongly dependent on the thicknesses of the fully oxidized insulating SnO2 buffer layer. When the buffer layer thickness increased from 100 nm to 700 nm, the electron mobility of values increased from 23 cm2 V−1 s−1 to 106 cm2 V−1 s−1 and the carrier density increased from 9 × 1017 cm−3 to 3 × 1018 cm−3, which we attribute to reduction of large density of dislocations as the buffer layer thickness increases. In addition, we studied the doping dependence of the electron mobility of SnO2−x thin films grown on top of 500 nm thick insulating SnO2 buffer layers. The oxygen vacancy doping level was controlled by the oxygen pressure during deposition. As the oxygen pressure increased to 47.5 mTorr, the carrier density was found to decrease to 9.1 × 1016 cm−3 and the electron mobility values to 13 cm2 V−1 s−1, which is consistent with the dislocation limited transport properties. We also checked the conductance change of the SnO2−x during thermal annealing cycles, demonstrating unusual stability of its oxygen. The correlation between the electronic transport properties and microstructural defects investigated by the transmission electron microscopy was drawn. The excellent oxygen stability and high electron mobility of low carrier density SnO2−x films demonstrate its potential as a transparent oxide semiconductor.