Optical properties of sol-gel processed amorphous and crystalline SrTiO3 thin films

Transparent strontium titanate thin films were prepared on fused silica sub strate by spin-on sol-gel process using strontium 2-ethyl hexanoate and tit anium isopropoxide as precursors in ethanol medium. The lattice constants o f crystalline films were a = b = c = 3.91 Angstrom. Optical constants of th e films in amorphous and polycrystalline forms were measured in the wavelen gth range of 300 nm to 900 nm with the aid of transmission spectra. The obs erved band gaps for amorphous and crystalline films were 4.07 eV and 3.62 e V respectively. The refractive index (n) and extinction coefficient (k) at 610 nm for the amorphous films are 1.72 and 0.0022 and for the crystalline films the corresponding values are 1.88 and 0.0062. Packing density of the amorphous and crystalline films were 0.65 and 0.75 respectively. The polycr ystalline films have a lower n and k in a 350-850 nm wavelength range and h igher band gap as compared to single crystal data. The dispersion data of t he refractive index is interpreted with the single oscillator model. Using this model, the values obtained for oscillator strength (S-0) and oscillato r energy (epsilon(0)) were 0.64 x 1014m(-2) and 7.31 eV respectively for th e amorphous film and 0.43 x 1014m(-2) and 5.69 eV for crystalline films.