Large electron mass anisotropy in a d -electron-based transparent conducting oxide: Nb-doped anatase TiO2 epitaxial films

Electron mass anisotropy in the Nb-doped anatase TiO2 (TNO) was determined by polarized infrared spectroscopy measurements for (012)-oriented TNO epitaxial films. The electron mass along the c axis, m 001, derived by Drude analyses, was found to be 0.5-3.3 m0, which was 3-6 times larger than that along the a axis, m 100, 0.2-0.6 m0. This large anisotropy was attributed to not only the anisotropic crystal structure but also the d -orbital-dominated conduction band. These results indicate that control of crystallographic orientation is of crucial importance for realizing high conductivity in polycrystalline TNO films.