Influence of Annealing Temperature on Structural and Optical Properties of Undoped and Al-Doped Nano-ZnO Films Prepared by Sol-Gel Method

Abstract

Undoped and Al-doped nano-ZnO thin films are grown on sapphire substrates by a sol–gel method. Their structural, morphological, and optical properties have been investigated with respect to post-annealing temperatures of 500 °C and 700 °C. The undoped and Al-doped nano-ZnO films exhibit a dominant (002) XRD peak. By increasing the annealing temperature of ZnO from 500 °C to 700 °C, the homogeneously distributed granular nano-grains with 20–40 nm diameter are changed into faceted grains with 30–80 nm. The photoluminescence spectra show strong emission peak around 3.26 eV indicating a direct band gap recombination ascribed to free exiton emission. The Raman spectra exhibit a strong A ₁(LO) peak and its shift with increasing annealing temperature due to strain of undoped and Al-doped ZnO thin films concurs with the broadening of XRD peak.