Annealing dependence of structural and optical properties of Zr-doped ZnO films deposited by radio frequency magnetron co-sputtering

Abstract

To examine the effects of annealing temperatures, Zr-doped ZnO (Zn1-xZrxO1+δ; ZZO) thin films with x = ~0.067 were deposited at room temperature by radio frequency co-sputtering on quartz substrates and annealed in air at 300 °C, 400 °C, 500 °C, respectively. Annealing dependence of ZZO films was investigated by using X-ray diffraction, atomic force microscope X-ray absorption fine structure (XAFS), ultraviolet-visible (UV–Vis) spectroscopy, and resistivity. The results show that ZZO films maintain a strong preferred orientation toward c-axis, but the size of crystallites increased and the surfaces of the ZZO films were roughened with increasing annealing temperature. The Zn atoms were in divalent state and slightly affected by annealing, but Zr atoms were tetravalent and the spectra were slightly different from that of ZrO2. The variance of the Zn-O distance distribution, probed by polarization-dependent K-edge XAFS, in the ab-plane was stronger than that of the c-axis. The optical transmission spectra at 200 ≤ λ ≤ 900 nm showed high transparency of ≥90%, decrease of optical band gap, and red-shift as temperature increased. The resistivity decreased with enormously with increasing temperature. These results are ascribed to the increase of grain size.