Intrinsic conduction through topological surface states of insulating Bi₂Te₃ epitaxial thin films

Topological insulators represent a novel state of matter with surface charge carriers having a massless Dirac dispersion and locked helical spin polarization. Many exciting experiments have been proposed by theory, yet their execution has been hampered by the extrinsic conductivity associated with the unavoidable presence of defects in Bi₂Te₃ and Bi₂Se₃ bulk single crystals, as well as impurities on their surfaces. Here we present the preparation of Bi₂Te₃ thin films that are insulating in the bulk and the fourpoint probe measurement of the conductivity of the Dirac states on surfaces that are intrinsically clean. The total amount of charge carriers in the experiment is of the order of 10¹² cm⁻² only, and mobilities up to 4,600 cm²/Vs have been observed. These values are achieved by carrying out the preparation, structural characterization, angle-resolved and X-ray photoemission analysis, and temperature-dependent four-point probe conductivity measurement all in situ under ultra-high-vacuum conditions. This experimental approach opens the way to prepare devices that can exploit the intrinsic topological properties of the Dirac surface states.