Effects of substrate temperature on nanostructure and band structure of sputtered Co3O4 thin films

Effects of substrate temperature (Ts) on nanostructure and band structure of sputtered Co3O4 thin films were investigated. The lattice constant of Co3O4 sputtered at room temperature was longer than the reported bulk Co3O4 value. It decreased with increasing Ts and approached the reported value. Average grain size of the film formed at Ts=918 K was 13.6 nm which was twice as big as that of the film obtained at Ts=293 K. Intensities of optical absorption peaks increased with increasing Ts. Band gaps corresponding to the charge transfer from Co2+(π*e) to Co2+(π*t2)(0.8 eV), from Co3+(π2t2) to Co2+(σ*t2)(1.3 eV), and from O2−(π*Γ) to Co2+(σ*t2) (2.1 eV) increased as a function of Ts. Detailed analysis of the optical absorption spectra in the infrared region indicated that there were many kinds of defects in the Co3O4 thin film formed at Ts=293 K and there were fewer defects in the film formed at Ts=918 K. The defects, low density, and increasing interface area at intergrains of nanoparticles caused a lowerin...