Microstructural and optoelectronic properties of rf magnetron sputtered ZnO:(Ga,Ti) semiconductor thin films

Applying radio-frequency (rf) magnetron sputtering technique, Ga-Ti co-doped ZnO [ZnO:(Ga,Ti)] transparent conductive oxide films were deposited onto glass substrates. The films were characterized by X-ray diffraction, four-point probe and UV-visible spectrophotometer. The influence of sputtering pressure on microstructure and optoelectronic properties of the films was investigated. The results show that all the films are polycrystalline with a hexagonal wurtzite structure and grow preferentially in the (002) direction. The ZnO:(Ga,Ti) films deposited at sputtering pressure of 0.4 Pa exhibit the maximum grain size of 86.6 nm, the highest transmittance of 85.9 %, the lowest resistivity of 1.67 × 10^-3 Ω cm, and the highest figure of merit of 1.38 × 10^-2 Ω^-1. The optical constants such as refractive index, extinction coefficient, dielectric constant and dissipation factor were determined using the method of whole optical spectrum fitting. Meanwhile, the dispersion behaviour of the films was studied by the single electronic oscillator model. The oscillator parameters and optical energy gaps were achieved. The results demonstrate that the microstructure and optoelectronic properties of the films are closely related to the sputtering pressure.