Electronic conduction above 4 K of slightly reduced oxygen-deficient rutile TiO_2-x

Based on electrical resistivity and Hall-coefficient measurements from 2 to 370 K on slightly reduced semiconductive rutile single crystals with various oxygen deficiencies O_d from 3.7×10¹⁸ to 1.3×10¹⁹/cm³, electronic conduction above 4 K is discussed in terms of two conduction bands separated by 0.03–0.05 eV. In this range of O_d, defects are mostly Ti interstitial ions. Ionization of Ti interstitial donors takes place above about 4 K. The ionization energy E₂ depends on the donor concentration N_D and decreases with increasing N_D for O_d<1×10¹⁹/cm³. The density of states effective mass m_d** of the lower conduction band is estimated to be (7–8)m_e (m_e is the free electron mass). The anisotropy of the effective mass is also estimated to be roughly m_c **≃(2–4)m_e and m_a **≃(10–16)m_e in the c and a directions, respectively. The mobility above 50 K of the lower conduction band electrons is discussed. © 1996 The American Physical Society.