Design of additives with different physical properties to control nanostructures of n-type Bi₂Te₃ thermoelectric thin films grown by a sputtering process

We designed sputter process parameters to produce three distinct types of n-type Bi₂Te₃ thin films (Ni-doped, Ti-doped, and undoped) to compare the nanostructures and the thermoelectric properties. The three films with different additive conditions (Ni-, Ti-, and undoped) resulted in different nanostructures; a connected column structure, a fibrous-grain nanostructure with void grain boundaries, and a dense columnar nanostructure, respectively. The additives were also considered to modify the carrier concentration of the films by controlling defect chemistry of the films. The amount of Te-evaporation was considered to be tuned by the different location between two additives, Ni at van der Waals bonding layer, Ti at Bi site, respectively. We tried to discuss the results using the structure zone model (SZM) and the difference of electronegativity of the additives. The maximum power factor value 400 μWK²/m was obtained from the Ni 1.12 at% doped film (Ni-11) owing to the large Seebeck coefficient and the high density (the large electronic mobility).