Anatase and rutile TiO₂ thin films prepared by reactive DC sputtering at high deposition rates on glass and flexible polyimide substrates

Titanium oxide thin films have been prepared by reactive DC sputtering at room temperature on soda-lime glass (SLG) and flexible polyimide (Kapton) substrates. The sputtering conditions have been adjusted in order to achieve high deposition rates between 15 and 45 nm/min. Post-deposition heating of the samples has been performed at 350 °C in nitrogen during 30 min. The crystalline structure of these films and their optical, morphological, and electric characteristics have been studied before and after annealing by X-ray diffraction, spectrophotometric measurements, atomic force microscopy, and the four-point probe method. The optical data indicate that pure anatase phase, which crystallizes after the heating process, is obtained on both SLG and Kapton substrates at the lowest deposition rate (15 nm/min). In this case, the samples exhibit high electric resistivity above 300 Ωcm. Otherwise, for the highest deposition rate (45 nm/min), crystalline rutile is identified in the as-grown and heated films, with a lower optical gap energy than anatase and also a lower resistivity that reaches 0.3 Ωcm after heating. At intermediate deposition rates, anatase and rutile coexist in the samples as has been verified by X-ray diffraction, although the overall optical and electric characteristics are dominated by the rutile phase. Analogous titanium oxide layers have been obtained on SLG and Kapton, and this opens the possibility of substituting flexible plastics for conventional glass substrates.