Properties of nickel doped In₂S₃ thin films deposited by spray pyrolysis technique

In this work, nickel (Ni) doped indium sulfide (In₂S₃) films have been prepared by the spray pyrolysis (CSP) technique on glass substrates at 350 °C. The Ni doping level was changed with Ni:In (0, 2 and 4% in solution). The structural studies reveal that the deposited films are polycrystalline in nature exhibiting cubic structure. The crystallite size decreases from 27.5 to 23 nm and the root mean square roughness values increase from 13 to 18 nm. The transmission coefficient is about 70–55% in the visible region and 85–75% in near-infrared region. The band gap energy increases with nickel content from 2.74 to 2.82 eV for direct transitions. The refractive index values of In₂S₃:Ni thin films decrease from 2.43 to 2.40 and the extinction coefficient values are in the range 0.01–0.20. Besides, the AC conductivity contribution is interpreted using the universal Jonscher’s power law and it is found thermally activated and it can be described by the correlated barrier-hopping models. These studies help to form significant correlation between temperature and activation energy. Nyquist plots show that the electrical response is accurately fitted by the Cole–Cole model and represented by an equivalent electrical circuit which consists of a parallel combination of a resistance and a constant phase element. From this analysis, the evidence of grain boundary conduction has been observed.