Optical, structural and electrical properties of Mn doped tin oxide thin films

Mn doped SnOx thin films have been fabricated by extended annealing of Mn/SnO2 bilayers at 200°C in air for 110 h. The dopant concentration was varied by controlling the thickness of the metal layer. The overall thickness of the film was 115 nm with dopant concentration between 0 and 30 wt% of Mn. The films exhibit nanocrystalline size (10-20 nm) and presence of both SnO and SnO2. The highest transmission observed in the films was 75% and the band gap varied between 2.7 and 3.4 eV. Significantly, it was observed that at a dopant concentration of ∼4 wt% the transmission in the films reached a minimum accompanied by a decrease in the optical band gap. At the same value of dopant concentration the resistivity also reached a peak. This behaviour appears to be a consequence of valence fluctuation in Sn between the 2+ and 4+ states. The transparent conductivity behaviour fits into a model that attributes it to the presence of Sn interstitials rather than oxygen vacancies alone in the presence of Sn2 +.