Anatase phase evolution and its stabilization in ion beam sputtered TiO2 thin films

Thin films of titanium oxide (TiO2) were prepared by ion beam sputtering at room temperature under various oxygen partial pressure and annealed at 350 °C and higher. Complete target oxidation is observed at O2/Ar pressures much lower than the conventional sputtering. The films are analyzed using X-ray diffraction, Raman spectroscopy, optical transmittance, and soft X-ray absorption spectroscopy. As-deposited thin films are all amorphous. Their Ti coordination number is less than that of crystalline phases, anatase and rutile. Upon post-heating above 350 °C in vacuum, films prepared with a ratio O2/Ar between 0.25 and 0.66 remain amorphous. Films prepared below this range, develop Ti6O11 and Ti3O5 grains, those prepared above this range become anatase. The band gap of the films varied between 3.12 and 3.36 eV, their refractive index between 2.07 and 2.81. Dielectric modeling of the transmittance spectra shows a broad Gaussian distribution of resonance frequency oscillators suggesting the as-deposited films to be disordered.