Effect of annealing and UV illumination on properties of nanocrystalline ZnO thin films

Abstract

ZnO thin films with preferred orientation along the (002) plane were prepared onto the glass substrates by the sol-gel spin coating method for different post- annealing temperatures. The XRD study confirms that the thin films grown by this method have good crystalline hexagonal wurtzite structure. The optical band gap of the samples was determined from UV-visible spectra. It is found that the size of the ZnO grains increase and the optical band gap of the films decrease as the annealing temperature increases from 350^°C to 550^°C. The transmittance of the film in the visible range was about 75%, 85% and 65% for 350^°C, 450^°C and 550^°C annealed samples respectively. SEM images of the films show worm shaped structures at 350^°C and distinct grains in films annealed above 350⁰C. Photoluminescence spectral study reveals that all the samples exhibit violet luminescence. The resistivity of the ZnO films found to reduce drastically from mega ohm to ohm range as a result of further vacuum annealing treatment of samples. The variation of electrical resistivity with respect to intensity of UV illumination over the samples has been reported.