Structural and optical properties of ZnO:Al thin films prepared by RF magnetron sputtering

Abstract

ZnO:Al (AZO) films have potential applications in ultraviolet detecting devices. The structural and optical properties of the AZO films are presented. Highly c axis oriented wurtzite phase AZO films are prepared on quartz substrate by rf sputtering method. The optical constants and the thickness of the AZO films are determined by fitting the measured transmission spectra with Tauc-Lorentz (TL) model. The refractive index n increases as the photon energy increases, and reaches the maximum of 2.50 at 3.66 eV, beyond which the refractive index n decreases with further increasing of photon energy. The peak of the refractive index n corresponds to the optical band gap of the AZO films, which is associated with interband transition between the valence and conduction bands. The extinction coefficient k also increases with the enhancement of the photon energy, and a strong absorption peak with maximum of 1.10 is prominent. The absorption peak due to an electronic transition accords with the peak transition energy E₀ (3.79 eV) is obtained by TL model. The energy E₀ of this model corresponds to the Penn gap, where the strong absorption of the material took place. By fitting the absorption coefficient, the optical band gap 3.62 eV of the film is evaluated. Based on the Tauc's power law, the optical band gap of the films is proved as a direct interband transition between the valence and conduction bands. This enhanced band gap compared with ZnO (3.37 eV) correlates to the Burstein-Moss band filling effect due to Al doping.