Effect of annealing on microstructural and optoelectronic properties of nanocrystalline TiO_2 thin films

摘要

In this study, semi-transparent nanostructured titanium oxide (TiO_2) thin films have been prepared by sol-gel technique. The titanium isopropoxide was used as a source of TiO_2 and methanol as a solvent and heat treated at 60℃. The as prepared powder was sintered at various temperatures in the range of 400-700℃ and has been deposited onto a glass substrate using spin coating technique. The effect of annealing temperature on structural, morphological, electrical and optical properties was studied by using X-ray diffraction (XRD), high resolution transmittance electron microscopy (HRTEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), dc resistivity measurement and optical absorption studies. The XRD measurements confirmed that the films grown by this technique have good crystalline nature with tetragonal-mixed anatase and rutile phases and a homogeneous surface. The HRTEM image of TiO_2 thin film (annealed at 700℃) showed grains of about 50-60 nm in size with aggregation of 10-15 nm crystallites. Electron diffraction pattern shows that the TiO_2 films exhibited a tetragonal structure. SEM images showed that the nanoparticles are fine and varies with annealing temperature. The optical band gap energy decreases with increasing annealing temperature. This means that the optical quality of TiO_2 films is improved by annealing. The dc electrical conductivity lies in the range of 10~(-6) to 10~(-5) Ωcm~(-1) and it decreases by the order of 10 with increase in annealing temperature from 400℃ to 700℃. It is observed that the sample Ti 700℃ has a smooth and flat texture suitable for different optoelectronic applications.