Effect of thickness on the structural, morphological, electrical and optical properties of Nb plus Ta co-doped TiO2 films deposited by RF sputtering

Abstract

Transparent conductive Nb plus Ta co-doped TiO2 films with various thicknesses were deposited on quartz substrates by RF magnetron sputtering with vacuum annealing. The effect of film thickness on structural, morphological, electrical and optical properties of the films was investigated in detail. X-ray diffraction measurements indicate that the post-annealed films are polycrystalline anatase structure, which is further confirmed by Raman spectroscopy. The average crystal size and surface roughness of the annealed films gradually increase as thickness increases from 50 to 500 nm. X-ray photoelectron spectroscopy results show that the substitution of Nb5+ and Ta5+ ions in Ti4+ sites can be significantly promoted by annealing treatment. Meanwhile, Raman spectroscopy results indicate that with the increase of film thickness, the peak position of Eg (1) mode shifts to higher band frequencies, implying the increase of carrier concentration. The average visible transmittance of all these films is in the range of 75–81%. The lowest resistivity of 8.9 × 10−4 Ω cm with carrier concentration of 1.1 × 1021 cm−3 and Hall mobility of 6.2 cm2 V−1 s−1 can be obtained at the film thickness of 500 nm, indicating that the optical and electrical properties of optimized NTTO films can be compared to those of Nb or Ta single-doped anatase TiO2 films. However, co-doping with Nb and Ta provides one possibility to complement the limitations of each dopant.