Deposition of ZnO ultrathin films by sol-gel route

Crack free, uniform and high quality ultrathin films (∼ 2–3 nm) of zinc oxide were achieved by cost effective and simplest wet chemical sol-gel route. The films were deposited at different spinning speed (1500 to 6000 rpm) on the silicon substrates. The average thickness per layer for each sample was measured by using surface profiler. The thickness of the film is reduced linearly with the spinning speed but the uniformity of the films was degraded for deposition at higher RPM (≤ 3500). The uniformity of deposited layers was observed by Photoluminescence mapping and it was found that the cracks density enhanced for higher spinning. In AFM images for the deposited films the grains were appeared with outlying distribution and the average grain size was increased with spinning speed. The films possessed high uniformity with lowest thickness (∼ 5 nm) at 3000 rpm. Further reduction in thickness was achieved by decreasing the concentration of zinc in the prepared sol. It was observed that the films were still smooth but the thickness extensively reduced up to 2 nm. The effect of substrate size on the quality of thin film also observed. To study this effect the films were deposited on 1 × 1, 2 × 2, 3 × 3 and 5 × 5 cm
²
substrates at 3000 rpm. It was observed by PL mapping that for substrates with large area the density of cracks was enhanced.