Multiple-Band Conduction inn-Type Rutile (TiO2)

The existence of multiple-band conduction in $n$-type rutile for temperatures above ≈40°K is deduced from studies of the Hall coefficients and electrical conductivities from ≈2 to 600°K. This investigation emphasizes the anisotropy of the Hall coefficients, i.e., the Hall effect measured with magnetic fields along the $a$ and the $c$ axis, respectively ($R_a$ and $R_c$). The pronounced temperature dependence of the ratio of these two coefficients can most easily be interpreted on the basis of two competing conduction processes. Measurements on samples with widely differing charge-carrier concentrations prove that the conduction processes involved are due to intrinsic energy states of the crystal. The energy separation between the lowest conduction band and the bottom of the next higher one is calculated to be 0.05 (estimated error: ±0.01 eV). These concepts are consistent with recent band-structure calculations. Impurity and surface conduction are shown to be negligible.