Nanostructured thermoelectrics: Bi2Te3 / Sb2Te3 based superlattice systems fabricated by MBE and sputtering

Abstract

Thermoelectric materials yield mutual conversion of thermal to electrical energy by the Seebeck and Peltier effect. Synthesis of thermoelectric materials with high energy conversion efficiencies is a challenge for decades since transport properties are inversely interrelated. For this, semiconducting materials with optimized charge carrier concentration and thus Seebeck coefficients and electrical conductivities, high charge carrier mobilities, and low thermal conductivities are required. It has been proposed to overcome the unfavourable interrelations between the transport properties and increase efficiencies by introducing superlattice (SL) structures in thin films, making use of electron quantum confinement effects and phonon scattering.

The aim of this work was to facilitate the fabrication of high-efficiency Bi2Te3-based thin films and SLs for room temperature applications, using new synthesis routes. Besides Molecular Beam Epitaxy growth on hot substrates, the nanoalloying method was applied, which allows to avoid re-evaporation of the volatile Te and thus control over chemical chemical composition and charge carrier concentration. For this thin element films were deposited by sputtering or evaporation on a cold substrate and annealed ex-situ, yielding the desired Bi2Te3-related compound. In addition, the nanoalloying method allowed to control texture in this highly anisotropic rhombohedral layered material without using substrates with epitaxial relation to the films. Transport properties, chemical composition and texture were measured and correlated for optimizing synthesis parameters. The nanoalloying synthesis method yielded Bi2Te3 - related thin films and SLs with controlled texture and chemical composition and improved efficiencies beyond bulk material’s efficiencies. The films were strongly c-oriented, providing enhanced stability of the SL structures against interdiffusion.