Electrical and optical properties of wide-gap n-type Sn₂Ta₂O₇ films

Wide-gap n-type Sn₂Ta₂O₇ polycrystalline films were prepared by RF-magnetron sputtering followed by annealing in a reducing atmosphere (N₂). Sn₂Ta₂O₇ is known to show both p- and n-type conductivity. The electrical and optical properties of the films were examined as a function of annealing time. The bandgap of the film was estimated to be 2.7 or 3.2 eV assuming an indirect or direct transition, respectively. The electrical conductivity of the film at 300 K increased from 2.1 × 10⁻⁸ S cm⁻¹ (as-deposited) to a maximum of 2.1 S cm⁻¹ (annealed for 14 h). The temperature dependence of the conductivity changed from semiconducting to degenerate semiconducting behavior with increasing annealing time, suggesting that oxygen vacancies were generated by the annealing in N₂. From the temperature dependence of the mobility, it was found that neutral impurities were the dominant scattering centers for electron carriers.