Microstructure and optoelectronic properties of gallium–magnesium codoped zinc oxide thin films by magnetron sputtering technique

The gallium–magnesium codoped zinc oxide (GMZO) thin films were deposited on glass substrates by radio frequency magnetron sputtering technique in an argon atmosphere. The influence of substrate temperature on the microstructure, morphology and optoelectronic properties of thin films was investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, four-point probe and spectrophotometry. It is found that all the deposited films have a hexagonal structure and a preferred orientation along the c-axis perpendicular to the substrate. As the substrate temperature increases, the dislocation density, lattice strain and electrical resistivity decrease initially and then increase, while the average crystallite size, average visible transmittance and figure of merit exhibit the reverse variation trend. The GMZO thin film deposited at the substrate temperature of 570 K possesses the best optoelectronic properties, with the largest average crystallite size of 52.05 nm, the lowest dislocation density of 3.69 × 10¹⁴ lines m^{− 2}, the minimum lattice strain of 1.10 × 10^{− 3}, the lowest electrical resistivity of 1.62 × 10^{− 3} Ω cm, the highest average visible transmittance of 88.63% and the maximum figure of merit of 5.11 × 10³ Ω⁻¹ cm^{− 1}. The optical energy gaps of the films were evaluated by extrapolation method and observed to be in the range of 3.34–3.55 eV. Furthermore, the complex refractive index, the complex dielectric constant and the dissipation factor were determined by optical characterization methods, and the dispersion behaviour of refractive index was studied in terms of the single electronic oscillator model. The results show that the microstructure and optoelectronic properties of the GMZO thin films are dependent on substrate temperature.