Thickness induced modifications in the valence, conduction bands and optoelectronic properties of undoped and Nb-doped anatase TiO2 thin films

Abstract

Nb-doped anatase TiO2 (NTO) is a promising wide band gap transparent semiconductor as it is an inexpensive, environment-friendly and chemically stable compound. However, its overall electronic structure and optoelectronic properties are tailored by tuning the microstructure, which are effected by varying process conditions. To this end, here series of thickening NTO and undoped anatase TiO2 (TO) films are deposited on quartz substrate in RF magnetron sputtering, followed by vacuum annealing at 823 K. Following fabrication, detailed electronic structure, defect states, microstructure and optoelectronic characterization of all these films are performed. Here, energy positions of the valence and conduction band edges are correlated with the oxygen and titanium content of these films. Whereas, TO films are found to be highly insulating, NTO films show highly conducting characteristics. The lowest electrical resistivity of 4.80 × 10−3 Ω cm is obtained for ~74 nm thick NTO film. Additionally, relative positions and intensities of the shallow donor level defect states of anatase are analyzed and compared with ZnO to gain fundamental insight into the differences in their electrical behavior. Along with visible region, these films also exhibit high transmittance in NIR range, thereby making these promising transparent conductors in the entire spectral range.