Influence of annealing temperature on nickel oxide thin films grown by chemical bath deposition

Nickel oxide thin films were prepared with a simple formulation using nickel sulfate and triethanolamine aqueous solutions via chemical bath deposition. After deposition, the films were subjected to annealing in an O₂/N₂ atmosphere for two hours at 200, 300, and 400 °C. X-ray diffraction patterns indicated that the as-deposited NiO_x films were amorphous. After annealing, the NiO_x crystallizes into a cubic phase. The X-ray photoelectron spectroscopy analysis confirms the presence of NiO_x with an FCC phase that yields a chemical composition of NiO_1.17, without the appearance of other Ni compounds. The band gap for the NiO_x films is determined to lie between 4.0 and 3.4 eV. Scanning electron micrographs exhibit a compact deposition and worm-like structure morphology. Hall effect measurements indicate a p-type conductivity and the resistivity is found to vary from 1.73 × 10³ to 0.89 × 10⁶ Ω cm due to oxygen incorporation. Employing Kelvin probe microscopy and photoemission spectroscopy, the NiO_x films present a work function between 4.70 and 5.48 eV and an ionization energy of ~ 5.6 eV. From the work function, ionization energy, and the band gap results, we propose a band diagram for the films at different annealing temperatures.