The role of annealing temperature on the optical and electrical transport properties of NiOx films

In this paper, we studied the effects of the annealing temperature of nickel oxide (NiOx) thin films on the structural, optical, and electrical transport properties. Thin NiOx was synthesized by the sol-gel method, followed by annealing at different temperatures varying from 200 to 450 °C. From the obtained AFM image data, an increase in the surface Ra value is observed with rising film annealing temperature. The decrease in film thickness with increasing annealing temperature is associated with significant densification of NiOx. During thermal annealing, a change in the redistribution of Ni/O elements is seen in the films. The increase in the intensity of the (1P)LO scattering peak in NiOx with rising annealing temperature is associated with a growth in the density of defects due to Ni vacancies. When the annealing temperature reaches 250 °C, an increase in Eg = 3.49 eV is detected; a further rise of the annealing temperature leads to a decrease in Eg = 2.92 eV. The increase in the index n of films with rising annealing temperature is associated with the formation of a denser and more ordered NiOx film. A rise in the annealing temperature of NiOx films leads to an increase in the value of the total resistance of the films, a decrease in the resistance of hole recombination at the NiOx/electrode interface, and the value of the conditional mobility of holes.