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

Abstract

Measurements of galvanomagnetic effects in the temperature range 4.2--300 K and photoinduced ``transient thermoelectric effect'' (TTE) along the ${\mathit{C}}_{2}$ axis at 300 K have been made for two types of solid solutions of semiconductors ${\mathrm{Sb}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$${\mathrm{Te}}_{3}$ (0\ensuremath{\le}x\ensuremath{\le}0.4) and ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Se}}_{\mathit{y}}$ (0\ensuremath{\le}y\ensuremath{\le}1.8). By incorporating In atoms into the ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3}$ lattices, Hall coefficients, Hall mobilities, and the frequencies of Shubnikov--de Haas (SdH) oscillations are varied systematically. For ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Se}}_{\mathit{y}}$, the Hall mobility is decreased with y up to y=0.7 and then increased appreciably in the range 0.7y1.8, and a frequency component of SdH oscillations is observed for y\ensuremath{\ge}0.25. The observed TTE voltages decay exponentially with time, showing a multirelaxation process with characteristic relaxation times ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$ (i=1,2, . . .) for thermal diffusions of photoinduced conduction carriers, whose analyses give valuable information about carrier mobilities and effective masses. In the host material ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3}$, four relaxation times ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$ (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 ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$ (i=5 and 6) for y>0.6 in ${\mathrm{Sb}}_{2}$${\mathrm{Te}}_{3\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Se}}_{\mathit{y}}$, confirming the existence of a valence band, whose anisotropy in the effective mass along the ${\mathit{C}}_{2}$ 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.