Synergy of rutile SnO2 and TiO2 in optoelectronic applications: Electronic structure and doping properties of TixSn1-xO2 alloys

There is an ongoing quest to replace indium-tin-oxide (ITO), the currently best n-type conductive transparent oxide (TCO), with a more readily available and cheaper material. The most common alternative is fluorine-doped tin-oxide, but it cannot match the high carrier concentration of ITO. Based on electronic structure calculations, we propose titanium-tin-oxide as an alternative. In its anatase form, TiO2 was proposed as a TCO itself, and high carrier concentrations could be achieved by cation-site doping. Unfortunately, anatase is not the ground state of TiO2, and its stable rutile form cannot effectively be doped n-type, because of electron self-trapping in small polaron states. It is shown here that TixSn1-xO2 alloys, which are known to preserve the rutile structure, are free from polaronic trapping and preserve the SnO2-like conduction band edge up to x < 0.2, with an effective electron mass smaller than 0.4 m (0) . While cation-site doping is not favored in the mostly covalently bonded SnO2, it is shown here that alloying with Ti increases the ionicity, making cation-site doping favorable. Nb donors on Sn sites have a higher solubility and lower activation energy than F on O sites, making the achievement of higher carrier concentrations possible.