Tuning the temperature domain of phonon drag in thin films by the choice of substrate.

At low temperatures, in reasonably pure conductors subjected to a thermal gradient, charge carriers (electrons and holes) are swept (dragged) by out of equilibrium phonons, giving rise to a large contribution to the Seebeck coefficient called phonon drag. We demonstrate a spectacular influence of substrate phonons on charge carriers in thin films of Bi2Te3. We show that one can control and tune the position and magnitude of the phonon-drag peak over a wide range of temperatures by depositing thin films on substrates with vastly different Debye temperatures. Our experiments also provide a way to study the nature of the phonon spectrum in thin films, which is rarely probed but clearly important for a complete understanding of thin film properties and the interplay of the substrate and films.