Investigation of the annealing effects on the structural and optoelectronic properties of RF-sputtered ZnO films studied by the Drude–Lorentz model

Zinc oxide films were deposited on glass substrates by RF reactive magnetron sputtering and post-annealed in vacuum at 100, 200, and 300 ºC. Structural and optical properties of films were obtained using X-ray diffraction and UV–visible spectroscopy. Optical parameters were extracted from transmittance curves using the single-oscillator Drude–Lorentz model. The evolution of the optical and structural properties of films with the annealing process was investigated. The films crystallized into the hexagonal würzite lattice structure, with preferential growth along the c-axis [0002]. The results indicate that the crystalline quality of films improved with annealing, whereas transparency was reduced from 90 to 80 % at 300 ºC. With post-annealing, the absorption edge shifted to the red, while the optical band gap decreased from E_g=3.28 to E_g=3.26 eV because of the Burstein–Moss effect. Calculated values of plasma frequency, w_p, fall within the IR range and decrease with temperature, from w_p=5.56×10¹⁴ rad/s (2950cm^-1) to w_p=1.1×10¹⁴ rad/s (587cm^-1).