Influence of thickness on the structural properties of radio-frequency and direct-current magnetron sputtered TiO₂ anatase thin films

Thin films of TiO₂ were deposited by reactive sputtering of a Ti target on unheated substrates and post-heated at 300 °C and 500 °C. They exhibit a granular structure. Direct current-sputtered films are amorphous as-deposited and crystallize (to pure anatase) only at 500 °C. Radio-frequency (rf)-sputtered films are already crystalline (pure anatase) as-deposited on unheated substrates. Above a thickness of 100 nm, the crystallite size, as deduced from the half-width of X-ray diffraction (XRD) peaks, is constant at 35 nm and decreases to zero when the thickness decreases to 25 nm. Below a thickness of 25 nm, the films are X-ray amorphous. Height and half-width of the XRD peaks of rf-sputtered films do not change upon post-heating at 300 or 500 °C. A larger lattice parameter ratio c/a is observed with respect to the bulk value that decreases with increasing film thickness and is about 1% larger for a film thickness larger than 100 nm. - Highlights: • Anatase films without the presence of rutile were grown on unheated substrates. • Radio-frequency sputtered films are crystalline as-deposited. • Direct-current sputtered films become crystalline only after annealing at 500 °C. • The crystallite size approaches zero when the film thickness approaches 25 nm. • There is an expansive strain in the crystallites along the c-axis