Tungsten oxide (WO 3 ) thin films are being studied for a wide variety of sensing applications. This paper discusses structural, morphological, and optical characteristics of RF sputtered WO 3 films deposited on Si (100) substrates and subsequently annealed at 600 °C for 1 h in air. The films are named as T1 (450 nm thick), T2 (2000 nm thick), and T3 (3000 nm thick). All the annealed WO 3 films showed the presence of monoclinic and orthorhombic mixed phases. The WO 3 films exhibited multiple Raman peaks due to the W-W vibration modes (134 cm − 1 , 187 cm − 1 ), O-W-O bending modes (274 cm − 1 , 326 cm − 1 ), and W-O-W stretching modes (717 cm − 1 , 806 cm − 1 ). Increased Raman peak intensities with the film thickness arise due to the improved crystalline quality. Optical band gaps of the films are found to be 3.4 eV (T1), 2.88 eV (T2), and 2.84 eV (T3). The variation of the band gaps seems to be due to the presence of different proportions of monoclinic and orthorhombic phases. The films showed coupling of IR-photons and surface phonons in the 800–980 cm − 1 regions in the infrared (IR) attenuated total reflectance (IR-ATR) spectroscopy measurement. The WO 3 films exhibited the surface phonon polariton (SPP) absorption dip at 920 cm − 1 (T1), 875 cm − 1 (T2), and 897 cm − 1 (T3). Corresponding broadening (full width half maxima) of the SPP dip are found to be 99 cm − 1 (T1), 125 cm − 1 (T2), and 57 cm − 1 (T3). Improved texture coefficient (TC) corresponding to the (002) plane in the sample T3 (3.6) can be attributed to the underlying reason for sharper SPP absorption in the T3 sample compared to TC of the other two films (T1: 1.6, and T2: <1.0) as the Evanescent wave come out normally from the film surface during the IR-ATR spectroscopy measurement.
Singh et al. (Tue,) studied this question.