Investigating the simultaneous effect of bias voltage and annealing on structural and optical properties of ZnO layer prepared by RF magnetron sputtering

In this study, the effects of different bias voltages (0, −100, −200 V) with an annealing temperature of 450 °C have been investigated on the structural and optical properties of zinc oxide (ZnO) thin film as a transparent conductive oxide (TCO) deposited by radio frequency (RF) magnetron sputtering method. The investigation on effects uniformity or the double effects of annealing on the layers prepared in different biases is the aim of this work. The prepared thin films were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–visible (Uv–Vis) spectroscopy, and four-point probe resistance measurement. The XRD patterns analysis confirmed the formation of crystalline c-axis (002) orientated hexagonal wurtzite structure of ZnO thin films. The transmittance spectra of ZnO thin films exhibited a transparency of nearly 90 % in the visible region. The direct optical band gap of ZnO thin films was calculated using Tauc’s plot, showing about 3.9 eV for the as-deposited (or before annealing) samples while post-annealing (or after annealing) reduced the band gap by 17 % to about 3.2 eV in all samples. Moreover, by applying a bias voltage of −100 V, not only the lowest sheet resistance was observed compared to that of the 0 V and −200 V, but also an increase in grain size and a reduction in surface roughness were observed. Moreover, annealing has shown that at a bias voltage of −100 V the structure and electro-optical properties of the thin films are improved, while the opposite occurs for −200 V.