Optical and electrical properties of BaTiO 3 thin films prepared by chemical solution deposition

Abstract

Barium titanate sol was prepared using barium ethyl hexanoate and titanium isopropoxide. The sol was then spin coated on p-type single crystal silicon (100) wafers, stainless steel and fused silica substrates and annealed to give polycrystalline, transparent, and crack-free films. The surface morphology and structural properties of the films were studied using scanning electron microscopy and X-ray diffraction respectively. Crystalline phase could form only at an annealing temperature of 650°C and above. The effect of post deposition annealing on the optical and structural properties as well as on the band gap were analysed. Transmission spectra were recorded and from this, refractive index, extinction coefficient and thickness were calculated for films on fused silica annealed at different temperatures. The dispersion curve for the refractive index n of 650°C annealed film is fairly flat beyond 450 nm and rises sharply towards the shorter wavelength region, showing the typical shape of a dispersion curve near an interband transition. The present study indicates the validity of the DiDomenico model for the interband transition with a single electronic oscillator. The refractive indices lie in the range 1.75–2.5 for films annealed in the range 300–750°C. The electrical measurements were conducted on metal-ferroelectric-semiconductor (MFS) and metal-ferroelectric-metal (MFM) capacitors. The typical measured small signal dielectric constant was 66 and 140 at 1 MHz for the MFS and MFM capacitors respectively. Debye type dispersion was observed for films on stainless steel substrates with an activation energy of about 0.34 eV. The low field ac conduction was found primarily due to hopping of electrons through the trap centres. The I-V characteristics of the MFS capacitor were found to be ohmic at low fields and space charge limited at high fields. I-V characteristics of the MFM capacitor showed a strange behaviour, linear dependence of current on voltage up to 10 6 V/m and V 5/2 dependence beyond 10 6 V/m.