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

Abstract

Mixed nickel -tungsten oxide thin films denoted as NiO:WO3 were prepared at 100 W rf power with varying substrate temperatures (Tsub = 100, 200, and 300 degrees C) using radio frequency (RF) magnetron sputtering technique. The influence of substrate temperature (Tsub) of NiO:WO3 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 (Eg) 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 2p3/2 and Ni 2p1/2, O 1 s peaks at 529.51 eV due to Ni2+ ions and the higher binding energy peak at 531.13 eV due to the Ni3+ ions, peaks at 35.2 and 37.3 eV, corresponds to W 4f7/2 and W 4f5/2. 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 Tsub owing to an increase in interaction among the molecules of NiO:WO3 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 epsilon 1 and 0.113-0.106 for epsilon 2 with variation in Tsub. The room temperature photoluminescence (RTPL) spectroscopy showed emission bands around 368 nm and 423 nm corresponding to the transition of 3 d8 Ni2+ 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.