Valence-band changes in Sb2-xInxTe3 and Sb2Te3-ySey by transport and Shubnikov-de Haas effect measurements

Measurements of galvanomagnetic effects in the temperature range 4.2-300 K and photoinduced ''transient thermoelectric effect'' (TTE) along the C2 axis at 300 K have been made for two types of solid solutions of semiconductors Sb2-xInxTe3 (0≤x≤0.4) and Sb2Te3-ySey (0≤y≤1.8). By incorporating In atoms into the Sb2Te3 lattices, Hall coefficients, Hall mobilities, and the frequencies of Shubnikov-de Haas (SdH) oscillations are varied systematically. For Sb2Te3-ySey, the Hall mobility is decreased with y up to y=0.7 and then increased appreciably in the range 0.7<y<1.8, and a frequency component of SdH oscillations is observed for y0.25. The observed TTE voltages decay exponentially with time, showing a multirelaxation process with characteristic relaxation times τi (i=1,2,...) for thermal diffusions of photoinduced conduction carriers, whose analyses give valuable information about carrier mobilities and effective masses. In the host material Sb2Te3, four relaxation times τi (i=1-4) are found, which are attributable to holes in the anisotropic upper and lower valence bands with effective-mass anisotropies of about 3. In addition, we have found two kinds of extra relaxation times τi (i=5 and 6) for y>0.6 in Sb2Te3-ySey, confirming the existence of a valence band, whose anisotropy in the effective mass along the C2 direction is evaluated to be of the order of 2-2.5. Based on these experimental data we have proposed the most probable band model for these solid solutions.