Modulation of Physical Properties of Sprayed ZnO Thin Films by Substrate Temperature for Optical Applications

We investigated the structural, electrical and optical properties of zinc oxide thin films as the n-type semiconductor. In the present paper, the effect of substrate temperature on the synthesis of ZnO thin films was carried out from 250 degrees C to 500 degrees C. ZnO thin films were deposited on glass substrates via ultrasonic spray technique with 0.2 mol/L of zinc acetate dehydrate. The crystal quality of the thin films was analyzed by X-ray diffraction which results in modified substrate temperature. The optical transmittance and electrical conductivity measurements were carried out by Ultraviolet-visible spectrophotometer and four-point methods, respectively. Polycrystalline films with a hexagonal wurtzite structure with (100) and (002) preferential orientation corresponding to ZnO films were observed at high temperature. The optimal values of the average crystallite size of the ZnO films under consideration are observed beginning with 350 degrees C of substrate temperature. All films exhibit an average optical transparency of about 85% in the visible region. The shift of optical transmittance toward higher wavelength can be shown by the increase of bandgap energy from 3.245 eV to 3.281 eV with increasing substrate temperature of 250-500 degrees C. The observed Urbach energy of ZnO thin films decreases from 0.11311 eV to 0.04974 eV. At a high temperature, the electrical conductivity of ZnO films was increased from 387 x 10(-3) to 41.58 (Omega.cm)(-1) with the increasing substrate temperature from 350 degrees C to 500 degrees C.