Optical and electrical properties of NiO and Cu-doped NiO thin films synthesized by spray pyrolysis

Abstract

Spray pyrolysis technique was employed to synthesize NiO and NiO:Cu thin films (0.1–25% Cu) on soda-lime glass substrate. Optical properties, as well as electrical resistivity of these films, were investigated. UV–Visible spectrophotometer was employed to measure the optical properties. The absorption edge of the films was located within the UV range i.e. λ = 280–380 nm. The average reflectance of the films was 16.9% in the range λ = 280–900 nm. On increasing the Cu/Ni ratio, the optical band gap decreased whereas the refractive index and Urbach energy increased. Quantitatively, the measured average value of the refractive index and the value calculated from the Reddy-Ahammed empirical formula, which connects refractive index with optical band gap, were in good agreement. Optical conductivity was found to be in the range 10 14 –10 15 s −1 while the dielectric loss was rather very low; it points to the good optical response of the films. The average oscillator energy as well as dispersion energy evaluated from Wemple–DiDomenico model decreased rapidly to begin with as Cu/Ni ratio was increased up to 5% but later on decreased rather slowly. Optical band gap values calculated by the Wemple-DiDomenico model and by Tauc relation agreed very well. The PL spectra of all the films displayed an intense peak at 403 nm and two secondary peaks at 426 nm and 365 nm. The PL intensity of each peak decreased monotonically with an increase in Cu content in NiO:Cu thin film from 0 to 3%. However, it remained almost independent of Cu content in the range of 4–25%. With an increase in Cu/Ni ratio, electrical resistivity and carrier mobility decreased whereas carrier concentration increased. • Optical band gap of NiO:Cu thin films (0–25% Cu) was in the range 3.57–3.21 eV. • Refractive index (2.15–2.42) increased on doping and average reflectance was 16.9%. • Optical band gap calculated by Tauc relation and Wemple-DiDomenico model agree well. • PL spectra of all the films displayed three peaks at 403, 426, and 365 nm. • Intensity of each PL peak decreased up to 3% Cu and then remained nearly constant.