Compositional effects on the low-temperature transport properties of non-stoichiometric Bi2TexSey-based crystals

Abstract

We grew a series of non-stoichiometric crystals of the ternary alloy Bi2TexSey via a direct solidification route. The temperature-dependent electrical and thermal transport properties, such as electrical resistivity (ρ), Seebeck coefficient (S), Hall coefficient (RH), and thermal conductivity (κ), of six selected crystals of Bi2TexSey with x + y ~ 3 were studied. We found that the physical properties of the Bi2TexSey-based crystals varied significantly with the Te/Se (x/y) content. This is essentially due to the modification of the electronic band structure of the crystals with the change in sample composition, which ultimately leads to the formation of a p-type material (Bi2Te2.10Se0.66) from an n-type Bi2Te1.89Se0.98. Most importantly, the feature associated with surface-state conduction at low temperatures was also observed in some of these crystals, including Bi2Te1.89Se0.98, Bi2Te2.16Se0.71, and Bi2Te2.10Se0.66. In particular, two highly resistive compounds, Bi2Te2.16Se0.71 and Bi2Te2.10Se0.66 with low carrier densities were identified as potential materials for topological applications. The observed complex transport behavior was realized in connection with the formation of impurity bands (multiband effects) in the Bi2TexSey-based systems. Analysis of the lattice thermal conductivity of the Bi2TexSey-based crystals indicates that the sample composition has a major effect on low-temperature heat conduction via phonon-boundary and phonon-point-defect scattering. Finally, the highest room temperature thermoelectric figure of merit, a ZT of about 0.46, was achieved for n-type Bi2Te1.63Se1.23.