Annealing temperature tuned structural and impedance properties of ZnO thin films

Zinc oxide thin films were synthesized using sol-gel spin coating technique. The films were annealed at 250 °C, 350 °C, 450 °C and 600 °C for 1 h. X-ray diffraction pattern of films showed transition from amorphous to crystalline nature with rise in annealing temperature. Field emission scanning electron microscopy of ZnO films showed absence of granular nature for films with annealing temperature 250 °C and 350 °C. High resolution transmission electron microscopy of single grain revealed lattice spacing of 2.8 Å corresponding to (100) plane of hexagonal ZnO. The impedance spectra of ZnO films was analyzed in terms of parallel RC element which indicated high resistive nature for films annealed at 250 °C and 350 °C. The relaxation time was found to be five orders of magnitude higher for films annealed at 250 °C and 350 °C compared to films with annealing temperature 450 °C and 600 °C, indicating the improvement in crystallinity resulted in improved conduction.