Upper and lower valence band energy spectra of narrow gap semiconductor solid solutions of Bi2- x Sn x Te3 crystals

Transport properties and pulsed laser induced 'transient thermoelectric effect' (TTE) along the C₂ axis have been measured in the temperature range 4.2-300 K for solid solutions of p-type Bi_{2- x} Sn _x Te₃ crystals (0≤x≤0.035). By doping Sn atoms, the hole Fermi energy of the upper valence band (UVB) is increased and the 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 are characterized by a multiple relaxation process with relaxation times for thermal diffusions of photogenerated carriers. The TTE data show that there are four different relaxation times (τ ₁-τ ₄) due to holes and two τ ₅ and τ ₆ due to electrons. Combined with the static transport data, as well as existing band model, we have discussed the energy spectra of these crystals.