Structural and optical properties of γ-alumina thin films prepared by pulsed laser deposition

Preparation of γ-alumina thin films by pulsed laser deposition from a sintered α-alumina target is investigated. The films were deposited on (100) silicon substrates at 973 K with varying oxygen partial pressures in the range 2.0 × 10−5–3.5 × 10− 1 mbar. X-ray diffraction results indicated that the films were polycrystalline γ-Al2O3 with cubic structure. The films prepared in the oxygen partial pressure range 2.0 × 10− 5–3.5 × 10− 2 mbar contained nanocrystals of sizes in the range 10–16 nm, and became amorphous at pressures > 3.5 × 10− 1 mbar. Topography of the films was examined by atomic force microscopy using contact mode and it showed the formation of nanostructures. The root-mean square surface roughness of the film prepared at 2.0 × 10− 5 mbar and 3.5 × 10− 1 mbar were 1.4 nm and 3.5 nm, respectively. The thickness and optical properties were studied using ellipsometry in the energy range 1.5–5.5 eV for three different angles of incidence. The refractive index was found to decrease from 1.81 to 1.73 with the increase of oxygen partial pressures from 2.0 × 10− 5 to 3.5 × 10− 2 mbar. The variation in the refractive index has been found to be influenced by the microstructure of the films obtained as a function of oxygen partial pressure.