IR SPECTROSCOPY OF Fe-TiO2, FILMS PREPARED BY MAGNETRON SPUTTERING

Abstract

The doping of TiO2 fi lms with various metals, including Fe, allows measuring the TiO2 band gap, which makes it possible to use the compounds for photovoltaic applications. The discussed research was aimed at studying the formation of complex oxides by IR spectroscopy on the surface and inside Fe-TiO2 fi lms. The fi lms were prepared by magnetron sputtering of iron on titanium oxide, followed by annealing in a diffractometer vacuum chamber at P = 1.33·10-2 Pa, vacuum reduction annealing at P = 10-4 Pa, and thermooxidation in fl owing oxygen at atmospheric pressure. The fi lms were deposited on single-crystal silicon substrates with a Pt sublayer. Fe magnetron sputtering was conducted in a modernized vacuum unit UVN-1: the discharge was excited in 13.3·10-2 Pa high-purity argon. The discharge voltage was 420 V. A metal iron target with an impurity level lower than 0.01 at.% was used as a cathode. The sputtering rate was 7.2 nm/s. The thickness of the iron fi lm was determined by the time of spluttering and was 120 nm. The phase composition of Fe-TiO2 fi lms was studied by X-ray powder diffraction using ARL X’TRA diffractometer. The refl ectance-absorption (RAS) and total internal refl ection (ATR) spectra of the fi lms were measured with the help of the Vertex-70 infrared Fourier spectrometer by Brooker (Germany). The refl ectance-absorption spectra were measured with a mirror refl ection attachment with a 13-83° variable incidence angle. ATR spectra were measured with a single-pass Platinum-ATR with a diamond prism. Films obtained under different annealing regimes were heterophase and contained: TiO2 in the rutile structure, two iron oxides – Fe3O4 and Fe2O3, and phases of complex oxides based on titanium and iron oxides: FeTiO3 (ilmenite) and Fe2TiO4 (ulvospinels). The research demonstrated that the shift in the characteristic vibrations of the Ti-O bond toward lower frequencies can be explained by the presence of Fe+3cations in the TiO2 crystal structure. It was established that the absorption bands caused by the deformation vibrations of the Fe-OH bond can only be observed in ATR spectra and are shifted toward higher frequencies (1.045 cm-1) compared to individual iron oxides, which indicates the formation of complex oxides in the Fe-TiO2 fi lms. The shift of the characteristic vibrations of Ti-O bond toward the lower frequencies during the modifi cation of rutile by iron is caused by distortions in the symmetry of the Ti+4 coordination environment and indicates the presence of Fe+3 cations in the crystal structure.

Keywords

• IR spectroscopy
• magnetron sputtering
• Fe-TiO2 fi lm.