Electric field effect on variable-range hopping conductivity in Bi1.9Lu0.1Te3

Electric field effect on low-temperature electrical resistivity of n-type Bi1.9Lu0.1Te3 was examined and analyzed. It was found that with decreasing temperature from 35 down to 2.5 K, the conductivity mechanism changes from “metal” type to “semiconductor” one resulting in appearance of minimum in the specific electrical resistivity, ρ. The temperature of this minimum, Tm, depends on electric field strength, E. At low electric field of 0.15 Vˑm−1 this temperature is equal to ∼11 K and decreases as E increases up to 4.65 Vˑm−1. Variable-range hopping (VRH) conductivity mechanism was applied to explain the ρ(T, E) behavior below Tm. Average hopping distance estimated from the experimental ρ(T, E) dependences decreases as temperature or electric field strength increase, that is the VRH conductivity can be activated both temperature and electric field. It was also found that density of states at the Fermi level rapidly enhances as electric field increases. This enhancement could be attributed to high and sharp density of states within an impurity band formed from the electronic f-levels of Lu. The forming of this impurity band is believed to be responsible for enhancement of the thermoelectric figure-of-merit of Bi2Te3.