Dopant concentration influence on the optical properties of NiO thin films doped by Al and Nb deposited by DC and RF magnetron sputtering

Optical characteristics of nickel oxide (NiO) can be fine-tuned to match specific application prerequisites. Doping NiO with Al or Nb influences the structure of its thin films, as they have d-orbital ionic radii of 142.2 and 181.9 pm, far from the ionic radius of Ni2+ of 33.8 pm. Al- and Nb-doped NiO thin films were deposited on silicon and quartz substrates using RF (Radio Frequency) and DC (Direct Current) reactive magnetron sputtering. The deposition powers on the dopant targets varied between 30 and 60 W in steps of 10 W. The XRD (X-Ray Diffraction) analysis revealed no phase changes in the NiO structure. However, a shift toward a smaller angle was observed. The bandgap decreased to 3.30 eV. The metal-doped NiO grains retained an average size of a few nanometers. The minimum resistivity of the Al-doped NiO reached 0.0130 omega.m, while it decreased to 0.0058 omega.m for the Nb-doped NiO. Using ellipsometry and effective medium approximation (EMA), it was possible to determine the refractive indices of the composites and dopants. The amount of dopants determined using this method was consistent with the deposition power. The influence of the dopant percentage on the optical properties of the doped NiO met the expectations. The optical and electrical properties of Al- and Nb-doped NiO make them to be good candidates for electrochromic and optical sensor applications.