A simple process step for tuning the optical emission and ultraviolet photosensing properties of sol–gel ZnO film

Abstract

A simple process step involving controlled cooling action was successfully established for defect control in a sol–gel ZnO film. In this study, a sol–gel process was adapted to form two sets of ZnO films with two different pre-heat treatment temperatures (set A and set B), but with the same post-heat treatment temperatures subjected to three different cooling rates (0.12, 0.25 and 90 °C s⁻¹) for each set in order to control the defect formation. The structural and surface morphological results showed that ZnO films, for both the sets, exhibited a wurtzite structure, whereas the crystallite orientations, FWHM of the (002) peak and the surface roughness, were found to vary with the cooling rate irrespective of the pre-heat treatment temperature. The films showed phenomenal changes in the photoluminescence (PL) and ultraviolet photoconductivity (PC) results. Irrespective of the set type, the rapidly cooled (90 °C s⁻¹) films showed an improved ultraviolet-to-visible PL intensity ratio as compared to others as a result of reduced defect emission in the visible region. The dark and photo current measurement results indicated the existence of higher adsorbed O₂ molecules-related trap states and an enhanced UV photoresponse in rapidly cooled films for both the sets, providing strong evidence in support of defect control. X-ray photoelectron spectroscopy results confirmed the presence of larger adsorbed O₂ molecules at the strained oxygen vacancy sites in the rapidly cooled films. These results demonstrated that the post-heat treatment cooling rate could be a brilliant and easy pathway to control the defects for tuning the optical emission and ultraviolet photosensing properties of sol–gel ZnO films.