Structural characterization and optoelectrical properties of Ti–Ga co-doped ZnO thin films prepared by magnetron sputtering

The Ti–Ga co-doped zinc oxide (TGZO) thin films were deposited on glass substrates by radio frequency magnetron sputtering technique in an argon atmosphere. The effect of working pressure on the crystallinity, microstructure, morphology and optoelectrical properties of thin films was investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, four-point probe and spectrophotometer. It is observed that all the deposited films are polycrystalline with a hexagonal wurtzite structure and highly textured along the c-axis perpendicular to the substrate surface. As the working pressure increases, the dislocation density, stress and resistivity decrease initially and then increase, while the crystallite size, average visible transmittance and figure of merit exhibit the reverse variation trend. The TGZO thin film deposited at the working pressure of 0.4 Pa possesses the best optoelectronic properties, with the largest crystallite size of 85.66 nm, the lowest dislocation density of 1.363 × 1014 lines m−2, the minimum compressive stress of 0.232 GPa, the lowest resistivity of 6.715 × 10−4 Ω cm, the highest average visible transmittance of 88.24 % and the maximum figure of merit of 1.314 × 103 Ω−1 cm−1. The optical energy-gaps of thin films were estimated by Tauc’s law and observed to be in the range of 3.515–3.548 eV which are hardly affected by the working pressure. Furthermore, the optical parameters such as refractive index, extinction coefficient, dielectric constant and nonlinear optical susceptibility were determined by the method of optical spectrum fitting, and the dispersion behavior of refractive index was studied in terms of the Sellmeier’s dispersion model. The results show that the microstructure and optoelectrical properties of the TGZO thin films are subjected to the working pressure.