Influence of substrate temperature on the optical and vibrational properties of RF sputter coated NiO:WO3 thin films for optoelectronic applications

Mixed nickel –tungsten oxide thin films denoted as NiO:WO 3 were prepared at 100 W rf power with varying substrate temperatures (T sub = 100, 200, and 300 °C) using radio frequency (RF) magnetron sputtering technique. The influence of substrate temperature (T sub ) of NiO:WO 3 thin films was studied in detail. All the prepared samples were found amorphous in nature with very fine smooth morphology. The average transmittance decreases from 88% to 83% with increasing substrate temperature. The energy band gap (E g ) decreases from 3.43 to 3.21 eV with increasing substrate temperature. AFM study revealed the roughness value of 0.875 nm. The detected binding energy values of Ni, O, and W observed from XPS study were, 853.7 and 871.8 eV with their respective satellite peaks at 859.88 and 878.2 eV corresponding to Ni 2p 3/2 and Ni 2p 1/2 , O 1 s peaks at 529.51 eV due to Ni 2+ ions and the higher binding energy peak at 531.13 eV due to the Ni 3+ ions, peaks at 35.2 and 37.3 eV, corresponds to W 4f 7/2 and W 4f 5/2 . The relationship between the refractive index and energy band gap proposed by different models was discussed. The refractive index (n) evaluated from these models shows an increase in n values with increasing T sub owing to an increase in interaction among the molecules of NiO:WO 3 and effect of these interactions on light. The derived optical constant k decreases from 0.024 to 0.022 indicating the decreased optical loss. The dielectric constants vary between 5.262 and 5.439 for ε 1 and 0.113-0.106 for ε 2 with variation in T sub . The room temperature photoluminescence (RTPL) spectroscopy showed emission bands around 368 nm and 423 nm corresponding to the transition of 3 d 8 Ni 2+ ions. Micro-Raman analysis shows two broad peaks due to Ni–O bond vibrations at 570 cm −1 and 1100 cm −1 , while the peak found at 870 cm −1 is due to a W–O bond vibration. We have aimed to discuss these results and correlate them with possible mechanisms behind the phenomena. • Influence of substrate temperature on optical properties and optical parameters of RF sputtered NiO:WO 3 thin film is reported. • The relation between band-gap energies and refractive indices using different proposed models were studied for NiO:WO 3 thin film in the UV region and reported for the first time. • The transmittance and energy band gap (Eg) decreases with increasing substrate temperature. • Effective changes in the optical and vibrational properties were observed with respect to change in substrate temperatures, which confirms the deposited films can be used for optoelectronic applications. • We have discussed the results and correlated them with possible mechanisms behind the phenomena.