Effects of annealing temperature on structural, optical, and electrical properties of antimony-doped tin oxide thin films

Antimony-doped tin oxide (ATO) films, approximately 320 nm in thickness, have been prepared by electron beam evaporation onto glass substrates. The films were annealed at temperatures between 400 degrees C and 550 degrees C in air and their structure and surface morphologies were observed by X-ray diffraction (XRD) and atomic force microscopy (AFM) after the different annealing treatments. XRD patterns of the ATO thin films as-deposited and annealed at 400 degrees C showed that they were amorphous, but annealing beyond 400 degrees C caused the films to become polycrystalline with tetragonal structure and orientated in the (1 1 0) direction. The grain size in the annealed films, obtained from the XRD analysis, was in the range 146-256 angstrom and this increased with the annealing temperature. The dislocation density, cell volume and strain were found to decrease gradually with increasing annealing temperature. Photoluminescence spectra revealed an intensive blue/violet peak at 420 nm, which increased gradually in height with annealing. It is suggested that an increase in the population of Sb+5 ions might be the reason for the enhancement of the blue/violet emission. The optical properties of the films were also investigated in the UV-visible-NIR region (300-1000 nm). The optical constants, namely the refractive index n and the extinction coefficient k in the visible region were calculated. The optical energy band gap, as determined by the dependence of the absorption coefficient on the photon energy at short wavelengths, was found to increase from 3.59 to 3.76 eV with annealing temperature.