Tuning optical and electrical properties of TixSn1−xO2 alloy thin films with dipole-forbidden transition via band gap and defect engineering

Abstract

We investigated the optical, electrical properties and electronic band structures of band gap tunable TixSn1−xO2 alloy with dipole-forbidden transition. TixSn1−xO2 alloy thin films with various Ti concentrations (0 ≤ x ≤ 0.36) were fabricated on quartz substrate by sol-gel method. As Ti is alloyed into SnO2, the narrowing of optical band gap of TixSn1−xO2 alloy was observed in the optical absorption spectra and was supported by first-principles calculations, which differs from the widening of fundamental band gap. Hall effect measurements indicate that the electrical resistivity of the TixSn1−xO2 alloy thin films increases with the increase of Ti concentration. Comparing with the pure SnO2, the TixSn1−xO2 alloy thin films exhibit a strong deep-level emission associated to oxygen vacancy (VO) defects in the photoluminescence spectra. First-principles calculation results suggest that the formation energy of TiSn-VO complex defect in a TixSn1−xO2 alloy is lower than that of the single VO in a perfect SnO2 under both oxygen-rich and oxygen-poor conditions. Especially, under the oxygen-poor limit, the TiSn-VO complex defect has the lowest formation energy among several major point defects of SnO2. Our results suggest that the band gap engineering of TixSn1−xO2 can not only control the band gap, but also modulate the defects, thus tuning the electrical and optical properties, which is of great significance for the understanding of the nature of electronic band structure of TixSn1−xO2 alloys.