Superconductivity in transparent Sn-doped In2O3 films

Superconducting thin films of Sn-doped In2O3 with high visible transmittances were prepared by electron-beam evaporation on polyester and glass substrates, followed by post-deposition annealing. Effects of annealing on electrical and optical properties were examined and optimum conditions under which superconductivity appears were determined. With increasing annealing temperature or annealing time, films change their electrical conduction from semiconducting to metallic behavior, and the superconducting state could be obtained in the transition region between the two phases. Post-oxidation at a low temperature (∼140 °C) is important to produce superconducting films, which have transition temperatures Tc of 2–4 K for resistivities of the order of 10−3 Ω cm and carrier densities in the range of 1021 cm−3. Superconducting films with much higher transmittance (∼80% at the wavelength 550 nm) were obtained on polyester substrates, whereas glass substrates were used to produce films with higher Tc’s and lower resistivities. The temperature variations of the resistivity near Tc and of the perpendicular critical field can be reconciled with those expected for a dirty superconducting thin film. It is found that the Tc is well correlated with the resistivity and transmittance, and that the superconducting state appears in restricted ranges of these parameters. Chemical bonding and valence-band structures in films were measured by means of x-ray photoelectron spectroscopy. The present results are compared with those for other superconducting oxides including high-Tc cuprate systems. The appearance of superconductivity in our samples is discussed on the basis of the electrical, optical, and chemical properties.