Several small-bandgap semiconductors are now known to have protected metallicsurface states as a consequence of the topology of the bulk electronwavefunctions. The known "topological insulators" with this behavior includethe important thermoelectric materials Bi_2Te_3 and Bi_2Se_3, whose surfacesare observed in photoemission experiments to have an unusual electronicstructure with a single Dirac cone. We study in-plane (i.e., horizontal)transport in thin films made of these materials. The surface states from topand bottom surfaces hybridize, and conventional diffusive transport predictsthat the tunable hybridization-induced band gap leads to increasedthermoelectric performance at low temperatures. Beyond simple diffusivetransport, the conductivity shows a crossover from the spin-orbit inducedanti-localization at a single surface to ordinary localization.
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