Optical and crystallisation behaviour of TiO2 and V/TiO2 thin films prepared by plasma and ion beam assisted methods

This paper reports the synthesis of TiO2 and V/TiO2 thin films, prepared by plasma enhanced and ion beam induced chemical vapour deposition procedures. Thin films with different contents of vanadium from a few to some tenth percents of this element have been prepared. Vanadium oxide thin films were also prepared for comparison. Morphological, structural and chemical characterisation of the samples was carried out by means of several techniques including scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. Optical properties of these samples were studied by ellipsometry and ultraviolet-visible absorption spectroscopy. It has been shown that the absorption threshold of V/TiO2 thin films shifts continuously towards the visible by increasing the V content. On the basis of this behaviour it is proposed the use of these thin films as optical filters in the UV-vis region of the spectrum. In the 'as prepared' samples vanadium is homogeneously distributed within an amorphous Ti-O structure. After annealing in air at T>673 K, both segregation of vanadium as vanadium oxide and crystallisation of the TiO2 into the rutile structure take place. It is realised that infrared spectroscopy can be a complementary technique of XRD to determine the crystallisation behaviour of the samples. The observed structural modifications are accompanied by a change in the light absorption properties of the films that can now be interpreted as due to the superposition of the absorption spectra of TiO2 and vanadium oxide. In the ion beam assisted films, only slight changes in their refraction index (n) are found as a function of the vanadium content and/or the annealing treatments. The changes were greater for the less compact films obtained by plasma deposition, indicating that the main parameter controlling the value of n is the density of the films. (C) 2003 Elsevier Science B.V. All rights reserved.