Optical properties of amorphous Al2O3 thin films prepared by a sol-gel process

Using Al(NO3)3·9H2O as a precursor, optically transparent Al2O3 thin films have been successfully synthesized by combining a sol-gel spin-coating process with one of two different annealing methods. One set of samples was subjected to a conventional annealing process (C.A) at 700 °C for 2 h in a muffle furnace, while the other set was exposed to a microwave annealing process (M.A) at 700 °C for 30 min in a microwave oven at 2.45 GHz. The structure, morphology, composition, stoichiometry, chemical bonding and optical properties of the films were investigated by X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and UV-vis-NIR spectrophotometry. XRD, FTIR and XPS investigations confirm that both the C.A and M.A films consist of amorphous Al2O3 with a certain amount of chemisorbed oxygen, and both Al2O3 films have a transmittance of at least 92% in the wavelength range of 400-1200 nm. However, compared to the C.A film, the M.A films have an even higher transmittance, a wider optical band gap and a lower refractive index.