Thermoelectric properties of Bi-Te films with controlled structure and morphology

A study of the thermoelectric transport properties of Bi-Te thin films with different structures and morphologies is here presented. Films were grown by pulsed laser deposition (PLD), which permits to control the composition, phase and crystallinity of the deposited material, and the morphology at the micrometer/nanometer scale. The carrier density and mobility at room temperature and the in plane electrical resistivity and Seebeck coefficient in the temperature range 300-400 K have been measured both for films characterized by a compact morphology and by the presence of different phases (Bi₂ Te₃, BiTe, and Bi₄ Te₃) and for Bi₂ Te₃ films with different morphologies at the micrometer/nanometer scale (from a compact structure to a less connected assembly of randomly oriented crystalline grains). The correlation among thermoelectric and structural properties has been investigated, showing the potential of PLD to produce n -type Bi-Te thin films with desired properties for peculiar applications. Films with a layered Bi₂ Te₃ structure show the best properties, with Seebeck coefficient in the range from -175 to -250 μV/K and power factor in the range 20-45 μW/cm K², with expected ZT values greater than 1.5. Also films composed by partially randomly oriented submicrometer crystals look promising, since the smaller power factor (∼10 μW/cm K²) can in principle be compensated by a strong reduction of the phonon thermal conductivity via proper engineering of grain boundaries.