Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films

Optical, structural and electronic properties of amorphous and crystalline molybdenum oxide thin films have been investigated. As-deposited amorphous films got crystallized into a layered orthorhombic phase on annealing at 350°C. Refractive index (n) and extinction coefficient (k) of as-deposited films and films annealed at 150°C, 240°C and 350°C have been calculated using reflectance and transmittance data. Spectral dependence of absorption coefficient has been explained on the basis of charge transfer transition mechanism. Optical band gap of amorphous MoO_3−x is 3.16 eV and it has increased by 0.11 eV on crystallization. XPS core level analysis reveals the presence of Mo⁺⁴, Mo⁺⁵ and Mo⁺⁵ oxidation states in amorphous films, proving oxygen deficiency in as-deposited films. Same studies on crystalline films show the presence of only Mo⁺⁶ states. Valence band spectrum of amorphous films reveal emission from Mo4d levels, which is absent in crystalline films. Complete correlation is seen between the optical properties and XPS data.