Electronic conduction above 4 K of slightly reduced oxygen-deficient rutile TiO2-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 x 10(18) to 1.3 x 10(19)/cm(3), 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 E(2) depends on the donor concentration N-D and decreases with increasing N-D for O-d < 1 x 10(19)/cm(3). 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)** congruent to (2-4)m(e) and m(a)** congruent to (10-16)m(e) in the c and a directions,respectively. The mobility above 50 K of the lower conduction band electrons is discussed.