Sputter deposition of TiO2-based transparent conductive thin films

Transparent electrodes are widely used in photovoltaic cells which drives the need for novel cost-efficient transparent conductive materials. The epitaxially grown TiO2 films doped with Nb or Ta show electrical resistivity values comparable to those of the best In2O3:Sn and ZnO:Al films. However, for the most applications the growth of low electrical resistivity polycrystalline TiO2 films is required. In order to address this challenge, we studied the films formed on glass substrates without heating by DC magnetron sputtering of reduced TiO2:Ta ceramic targets followed by vacuum annealing. We present a systematic study of the effect of the process total pressure (Ar+O2) during the deposition on electrical and structural properties of the films after annealing. The optimum total pressure in combination with O2 fine tuning yielded the films with the best free electron mobility of 8 cm²/Vs. Our approach delivered films with an electrical resistivity in the range of 10-3  cm, optical transmittance above 80% for 400nm thick films and electrical activation of Ta dopants up to 80% that is substantially higher than that of Al in ZnO.