Carrier generation and transport properties of heavily Nb-doped anatase TiO2 epitaxial films at high temperatures

Abstract

To compare the intrinsic thermoelectric (TE) properties of heavily Nb-doped TiO2 to those of heavily Nb-doped SrTiO3 [S. Ohta et al., Appl. Phys. Lett. 87, 092108 (2005)], the electrical conductivity (σ), carrier concentration (ne), Hall mobility (μHall), and Seebeck coefficient (S) of heavily Nb-doped TiO2 (anatase) epitaxial films were measured at high temperatures (300–900K). The epitaxial films were grown on the (100)-face of LaAlO3 single-crystalline substrates by a pulsed-laser deposition technique at 800°C. The carrier effective mass (m*) of the anatase TiO2 epitaxial films was ∼1m0, which is an order of magnitude smaller than that of Nb-doped SrTiO3 (∼10m0). The estimated TE power factor (S2σ) of the ∼2%-Nb-doped anatase TiO2 film (ne∼5×1020cm−3) was ∼2.5×10−4Wm−1K−2 at 900K, which is approximately 15% of the 20%-Nb-doped SrTiO3 (1.5×10−3Wm−1K−2). The present findings will help establish a future TE material design concept for Ti-based metal oxides.