Thin films of SnO2:F by reactive magnetron sputtering with rapid thermal post-annealing

Abstract

Thin films of fluorine-doped tin oxide (FTO) are transparent conducting oxide (TCO) layers that exhibit excellent corrosion and mechanical resistance and are typically produced by high temperature (>400°C) spray pyrolysis. Magnetron sputtering is another industry relevant method for large area FTO deposition where a low temperature process can be used although the electrical properties of sputtered FTO are currently inferior to those of spray pyrolysis. Here reactive sputtering of FTO is investigated in order to optimize the sputter and post-treatment parameters for achieving high quality TCO layers. FTO layers are deposited by reactive DC magnetron sputtering of metallic tin targets in an atmosphere of Ar/O2/CF4 or CHF3. Post-deposition rapid thermal processing (RTP) for a short annealing time of less than 5min at 450°C is sufficient to lower electrical resistivity down to ~2×10−3Ωcm as a result of the increased carrier concentration and mobility while keeping the average optical transmittance above 79%. The free carrier concentration can be controlled by changing the fluorine doping via the CF4/CHF3 flow, as determined by Rutherford backscattering spectrometry and elastic recoil detection analysis. The electrical activation of the fluorine dopant is found to be low, on the order of 4%. Based on temperature-dependent Hall measurements we show that the electrical conductivity in RTP-annealed FTO is primarily limited by ingrain scattering.