The Effect of RF Power on the Properties of Gallium and Aluminium Co-doped Zinc Oxide (GAZO) Thin Films

Abstract

X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and spectrophotometry were used to study the effect of RF power on the properties of gallium and aluminium co-doped zinc oxide (GAZO) thin films for optoelectronic device fabrication. Two peaks appeared in the XPS spectra of the Zn 2p core-level at 1045 and 1022 eV, and these were assigned to ${\text{Zn}}~2{{\text{p}}_{1/2}}$ and ${\text{Zn}}~2{{\text{p}}_{3/2}}$, respectively. The O 1s core-level revealed peaks at 530 and 531 eV which indicated the presence of two different forms of oxygen. Raman spectroscopy confirmed the films’ hexagonal wurtzite crystal structure and revealed the presence of few defects and negligible residual tensile stress. Spectral dependence of the refractive index was analyzed on the basis of the Cauchy’s dispersion model and the Wemple and DiDomenico (WDD) single oscillator model. Low refractive indices (1.6–2.0) and nearly zero extinction coefficients were obtained in the visible region (400–700 nm), indicating the high transparency nature of the GAZO thin films. The optical band gap decreased with increasing RF power, in accordance with the Burstein–Moss effect. Low Urbach energy values were obtained at low RF power, indicating less structural disorder. The free carrier concentration to effective mass ratio $({N_c}{\text{/}}{m^*})$, plasma frequency $({\omega _P})$ and zero frequency dielectric constant $({\varepsilon _\infty })$ were determined. Films deposited at 150 W exhibited the optimum optical properties, desirable for optoelectronic application.