Tuning of Electrical and Optical Properties of Highly Conducting and Transparent Ta-Doped TiO2 Polycrystalline Films

We present a detailed study on polycrystalline transparent conducting Ta-doped TiO2 films, obtained by room-temperature pulsed laser deposition followed by an annealing treatment at 550 degrees C in vacuum. The effects of Ta as a dopant element and of different synthesis conditions are explored to assess the relationship between material structure and functional properties, that is, electrical conductivity and optical transparency. We show that for the doped samples, it is possible to achieve low resistivity (of the order of 5 x 10(-4) Omega cm) coupled with transmittance values exceeding 80% in the visible range, showing the potential of polycrystalline Ta:TiO2 for application as a transparent electrode in novel photovoltaic devices. The presence of trends in the structural (crystalline domain size, anatase cell parameters), electrical (resistivity, charge carrier density, and mobility), and optical (transmittance, optical band gap, effective mass) properties as a function of the oxygen background pressure and laser fluence used during the deposition process and of the annealing atmosphere is discussed, and points toward a complex defect chemistry ruling the material behavior. The large mobility values obtained in this work for Ta:TiO2 polycrystalline films (up to 13 cm(2) V-1 s(-1)) could represent a definitive advantage with respect to the more studied Nb-doped TiO2.