Physical Investigations on (MoO₃)_x-(WO₃)_1-x Composite Thin Films

Abstract

Thin films of (MoO3)x-(WO3)1-x (x = 0.4, 0.6, 0.8) were deposited on glass and silicon (100) substrates by flash evaporation technique. The purpose of the flash evaporation is to prevent the decomposition of composite into individual species during thin film deposition. The films were deposited at the oxygen partial pressures of 2x10-5, 2x10-4 mbar and substrate temperatures of 150 0C, 350 0C. The deposited films were characterized for their structure by Graging Incidence X-ray Diffraction (GIXRD), microstructure by Field Emission Scanning Electron Microscopy (FESEM), optical property by UVVis spectra. The X-ray diffraction reveals that the (MoO3)x-(WO3)1-x composite thin film crystallizes in orthorhombic and monoclinic phases. At lower oxygen partial pressures of 2x10-5 mbar and lower substrate temperatures of 150 0C the film crystallizes in orthorhombic and tetragonal phases. Whereas at higher substrate temperatures of 350 0C both orthorhombic and monoclinic mixed phases are present. The optical transmittance spectra of the films were recorded in the wavelength range 300-1100 nm. The optical energy gap of the films is 3.05 eV and increases to 3.21 eV with increase in MoO3 concentration. The width of localized states is 0.47 eV and decreasing with increasing MoO3 concentration. The oxide materials in thin film form exhibit the change in the transmittance when exposed to electro magnetic waves (EM) of visible region. In this respect the estimation of color centre concentration will give the information regarding the response of the films to change their transmittance when exposed to EM waves in the visible region. The colourcenter concentration of the films (for x = 0.4) deposited at 150 0C and irradiation time of 120 minutes, is 3.02 x 1017/cm3 and reaches to maximum value of 4.94 x 1017/cm3, (for x = 0.8) when deposited at 350 0C and irradiated for 150 minutes.