VALENCE-BAND ENERGY-SPECTRUM OF SOLID-SOLUTIONS OF NARROW-GAP-SEMICONDUCTOR BI2-XSNXTE3 SINGLE-CRYSTALS

Electrical resistivities along the C2 and C3 axes, Hall effect, Shubnikov-de Haas effect, and pulsed-laser-induced transient thermoelectric effect (TTE) have been measured in the temperature range 4.2-300 K for solid solutions of narrow-gap p-type Bi2-xSnxTe3 (0≤x≤0.035) single crystals. We have found systematic variations of the hole concentration, Hall mobility, Dingle temperature, and cyclotron mass with increasing Sn content. By doping Sn atoms, the hole Fermi energy of the upper valence band (UVB) is increased and a Sn-induced impurity band is formed near the top of the lower valence band (LVB) lying by about 15 meV below the top of the UVB. The observed TTE voltages decay exponentially with time, showing a multiple relaxation process with characteristic relaxation times for thermal diffusions of photogenerated carriers. The TTE data show that there are at least four different relaxation times (τ1-τ4) due to holes, and two, τ5 and τ6, due to electrons. Combined with the transport data, as well as the existing band model, we have evaluated the effective masses m1*0.033m0 and m2*0.060m0 for the many-valley hole Fermi surfaces of the UVB, as well as two effective masses m3*0.16m0 and m4*0.31m0 for the LVB along the C2 direction, corresponding to τ1-τ4, respectively. Discussion will be given for the energy spectrum and anisotropy in the UVB and LVB, as well as the lower electron band, in this material system. The thermal conductivities for holes and lattices are also evaluated from the TTE data. © 1994 The American Physical Society.