p-type Bi₂Se₃ for topological insulator and low-temperature thermoelectric applications

The growth and elementary properties of p-type Bi₂Se₃ single crystals are reported. Based on a hypothesis about the defect chemistry of Bi₂Se₃, the p-type behavior has been induced through low-level substitutions (1% or less) of Ca for Bi. Scanning tunneling microscopy is employed to image the defects and establish their charge. Tunneling and angle-resolved photoemission spectra show that the Fermi level has been lowered into the valence band by about 400 meV in Bi_1.98Ca_0.02Se₃ relative to the n-type material. p-type single crystals with ab-plane Seebeck coefficients of +180 μV/K at room temperature are reported. These crystals show an anomalous peak in the Seebeck coefficient at low temperatures, reaching +120 μV K⁻¹ at 7 K, giving them a high thermoelectric power factor at low temperatures. In addition to its interesting thermoelectric properties, p-type Bi₂Se₃ is of substantial interest for studies of technologies and phenomena proposed for topological insulators.