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

Highlights: • TiO₂ thin films deposited using reactive ion beam sputtering at room temperature. • Complete target oxidation at O₂/Ar pressures much lower than for conventional sputtering. • Ti coordination in the as-deposited films is less than anatase and rutile. • Annealed films prepared with the O₂/Ar ratio between 0.25 and 0.66 remain amorphous. • Below this range, develop Ti₆O₁₁and Ti₃O₅ grains and above this anatase occurs. - Abstract: 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.