Optical properties of titanium trisulphide (TiS3) thin films

Titanium trisulphide thin films have been grown on quartz substrates by sulphuration of electron-beam evaporated Ti layers (d similar to 300 nm) in a vacuum sealed ampoule in the presence of sulphur powder at 550 degrees C for different periods of time (1 to 20 h). Thin films were characterized by X-ray diffraction, energy dispersive analyses of X-ray and scanning electron microscopy. Results demonstrate that films are composed by monoclinic titanium trisulphide. Films show n-type conductivity with a relatively high resistivity (rho similar to 4 +/- 2 Omega.cm) and high values of the Seebeck coefficient (-600 mu V/K) at room temperature. Values of the optical absorption coefficient about alpha similar to 10(5) cm(-1), determined from reflectance and transmittance measurements, have been obtained at photon energies h upsilon > 2 eV. The absorption coefficient dependence on the photon energy in the range of 1.6-3.0 eV hints the existence of a direct transition with an energy gap between 1.35 and 1.50 eV. By comparing these results with those obtained from bulk TiS3, a direct transition with lower energy is also found which could have been hidden due to the low value of the absorption coefficient in this energy range. (C) 2012 Elsevier B.V. All rights reserved.