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

Abstract Using Al(NO 3 ) 3 ·9H 2 O as a precursor, optically transparent Al 2 O 3 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 Al 2 O 3 with a certain amount of chemisorbed oxygen, and both Al 2 O 3 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.