Optical Properties of Sol–Gel Processed Amorphous and Crystalline SrTiO₃ Thin Films

Abstract

Transparent strontium titanate thin films were prepared on fused silica substrate by spin-on sol–gel process using strontium 2-ethyl hexanoate and titanium isopropoxide as precursors in ethanol medium. The lattice constants of crystalline films were a = b = c =3.91 Å. Optical constants of the films in amorphous and polycrystalline forms were measured in the wavelength range of 300 nm to 900 nm with the aid of transmission spectra. The observed band gaps for amorphous and crystalline films were 4.07 eV and 3.62 eV 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 polycrystalline films have a lower n and k in a 350–850 nm wavelength range and higher band gap as compared to single crystal data. The dispersion data of the refractive index is interpreted with the single oscillator model. Using this model, the values obtained for oscillator strength ( S 0 ) and oscillator energy (ε 0 ) were 0.64 ×1014 m -2 and 7.31 eV respectively for the amorphous film and 0.43 ×1014 m -2 and 5.69 eV for crystalline films.