Amorphous and nanocrystalline SrTiO3 thin films

The interdependence of stoichiometry, structure and optical properties is discussed for SrTiO3 thin films prepared by chemical sol-gel process and physical vapour deposition (PVD). Both sol-gel and PVD coatings have a thickness of 50 to 500 nm and a homogeneous structure with an average grain size ranging from 15 to 30 nm and increasing to 50-250 nm after heat treatment to 700 degrees C. The molar ratio Sr to Ti to O is close to 1:1:3. The films are amorphous but, after annealing at temperatures above 450 degrees C, they have a polycrystalline structure. Optical properties were measured and data were compared with bulk material values. The films have a lower refractive index and absorption index in a 300-800 nm wavelength range by comparison with SrTiO3 single crystal data. This is due to the different preparation techniques resulting in changes in structure and lower film density.