Physical origin of inertness of Ta contacts on Bi₂Te₃

Interfacial reactions and underlying atomic mechanisms between Ta contacts (space group $Im3¯m$⁠) and thermoelectric Bi₂Te₃ (space group $R3¯m$⁠) are studied experimentally and theoretically. A Ta/Bi₂Te₃ mixture is found to be inert up to the melting point of Bi₂Te₃ (∼589 °C) based on calorimetry and interfacial composition analyses. This can be understood using density functional theory. Bi and Te adatoms hop across a close-packed Ta(110) surface in the <111>, <110>, and <100> directions with the highest dwelling time on equilibrium (fourfold hollow) sites, but they do not exchange with Ta surface atoms. To identify the electronic structure fingerprint of Ta(110) inertness, the adsorption energies and electron density distributions are calculated for the Bi₂Te₃ constituting atoms and possible dopants (15 elements) stemming from C, N, and O groups. C, N, O, and S strongly adsorb to Ta(110), exhibiting enhanced reactivity. We propose that these four species can initiate exchange diffusion with Ta due to ionic interactions between Ta and the adsorbates. Our results imply that elements with a high electronegativity should be avoided in Bi₂Te₃ doping because interfacial interactions may occur, degrading its stability and transport properties.