Figure 3

Temperature dependence of carrier concentration n (panel a) and electron mobility μ (panel b) for a commercial ingot and the exf8h-SPS sample.

For T < 100 K, the grain boundary and charged defect scattering dominates and for T > 100 K, the electron-phonon scattering dominates. The dotted lines in panel b represent the temperature dependence of various scattering mechanisms, viz. grain boundary (, charged defect (~T^3/2) and electron-phonon (~T^−3/2) scattering processes. A schematic depicting of the grain boundaries scattering of charge carriers in n-type Bi₂Te₃ system (panel c) shows that low kinetic energy carriers (represented by red arrow) are effectively scattered by the grain boundary potential barrier (φ_a) unlike high kinetic energy carriers (represented by green arrows). (d) The presence of a positively charged donor-like grain boundary defect (arising from Te vacancies and Te-Bi anti-sites) can inject excess charge carriers into the core of the grain resulting in an increase of carrier concentration (cf. Fig. 5a). (e) The positively charged grain boundary leads to selective scattering of holes over electrons due to increased Coulomb barriers. Such a preferential scattering mechanism is responsible for the observed temperature upshift in the bipolar contribution to transport properties of exfoliated n-type Bi₂Te₃.