Morphology, microstructure and optical properties of nanostructured ultrasonically sprayed ZnO thin films with different thicknesses

Abstract

Two sets of nanostructured ZnO thin films were deposited by Ultrasonic Spray Pyrolysis (USP) at 350 °C and annealed at 450 °C for 2 h in air, one on crystalline silicon to measure refractive index by Ellipsometry and the other on transparent (glass) substrates to measure transmittance. A homemade USP set-up was developed with specific features to avoid traditional difficulties with this deposition technique. The spraying solution was prepared by dissolving zinc acetate dihydrate in distilled water. All deposition parameters were fixed, but the deposition time, thus obtaining films with different thicknesses. The measurements were directed to follow the growth process through time. The films are polycrystalline with preferential growth in the direction normal to (101) planes and exhibit a single phase hexagonal wurtzite microstructure. According to SEM observations, surface reactions of the gas-solid type seems to dominate the growth in all cases; however, as thickness increases, a change occurs. Initially, the films exhibit a granular (nanostructured) growth, film surfaces are specular to naked eye, the grain size increases and the refractive index at λ = 632.8 nm varies in the atypically wide range from 1.51 to 1.96. These experimental values fit a curve that nears the value n ≈ 1.99, close to that of bulk ZnO single crystal. As thickness further increases, granular growth ceases and the formation of grain aggregates begins, thus transforming the film surface from specular to opaque. Therefore, film morphology and optical properties depend on thickness for both types of substrate.