Insight into the structural and optoelectronic properties Of e-beam evaporated nanostructured tio2 thin films Annealed in air

Abstract

About 480 nm thick titanium oxide (TiO2) thin films have been deposited by electron beam evaporation followed by annealing in air at 300—600oC with a step of 100oC for a period of two hours. Optical, electrical and structural properties are studied as a function of annealing temperature. All the films are crystalline (having tetragonal anatase structure) with small amount of amorphous phase. Crystallinity of the films improves with annealing at elevated temperatures. XRD and FESEM results suggest that the films are composed of nanoparticles of 25-35 nm. Raman analysis and optical measurements suggest quantum confinement effects since Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk TiO2. Optical band gap energy of the asdeposited TiO2 film is 3.21 eV, which decreases to about 3.06 eV after annealing at 600 oC. Refractive index of the as-deposited TiO2 film is 2.26, which increases to about 2.32 after annealing at 600 oC. However the films annealed at 500 oC represent peculiar behavior as its band gap increases to highest value of 3.24 eV whereas refractive index, RMS roughness and dc-resistance illustrate a drop as compared to all other films. Illumination to sunlight decreases the dc-resistance of the as-deposited and annealed films as compared to dark measurements possibly due to charge carrier enhancement by photon absorption.