Correlation between Thickness and Optical Properties in Nanocrystalline γ-Monoclinic WO3 Thin Films

Abstract

Results from the analysis of the variation of structural defects, such as oxygen vacancies indicate that by adjusting the thickness of the WO3 films, fabricated by DC reactive sputtering, it is possible to modulate the oxygen vacancies concentration. This has a tremendous influence on the applications of these semiconductor materials. The thicknesses analyzed here are 42, 66, and 131 nm. After the annealing process at 500 °C, films were directly transformed to a stable γ-monoclinic crystal structure with P21/n space group, with a preferential orientation in the (200) plane. Atomic force microscopy exhibits nanometer range particle size with the highest roughness and higher surface area for the thinner film. FTIR analysis shows the presence of characteristic bands of the double bond stretching vibrational modes (W=O) and stretching vibrations of the γ(W-O-W) bonds corresponding to the monoclinic WO3. Raman bands located at 345, and 435 cm−1 are ascribed to the presence of W5+ species that induces the formation of oxygen vacancies VO. The thinner film shows a decrease in the optical indirect band gap attributed to the formation of oxygen vacancies in combination with W5+ species that induce the formation of energy states within the forbidden band gap range.