Bi2Te3, (Bi1−xSbx )2Te3 and layered Bi2Te3/(Bi1−xSbx)2Te3 superlattices fabricated by nanoalloying. Our approach is based on the sequential sputtering of nanoscale layers of the elements and subsequent annealing in order to induce a solid state reaction. While conventionally Bi2(SexTe1−x )3 compounds are used as n-type V2VI3 material system, the deposition of Se proves to be problematic especially for sputtering deposition and is therefore replaced by (Bi1−xSbx )2Te3. A superlattice consisting of 25 nm Bi2Te3/25 nm (Bi0:9Sb0:1)2Te3 – ML (periodicity of 50 nm) was synthesized and annealed at temperatures of 150, 200, 225, and 250°C. The layers are slightly rough and polycrystalline, and the grain sizes increase with increasing annealing temperature. The XRD analysis shows a pronounced (00l) texture of the sputtered layers. SIMS depth profiles reveal that the chemical separation into layers is present, yet smeared out to some degree after annealing at 200°C. High Seebeck coefficients of up to ~−190 μV/K were achieved. A high maximum power factor of 22 μW/cmK2 can be attained after annealing at 250 °C for 12 h. The superlattice system Bi2Te3 / (Bi1−xSbx )2Te3 can compete with Bi2Te3 / Bi2(SexTe1−x )3 in terms of electrical properties while representing a good practical alternative for the sputter deposition due to the substitution of problematic Se with Sb. Cross-plane thermal conductivities are in the range of 0.55 to 0.6 W/mK. The thermal conductivity is generally reduced due to the nanocrystallinity of the material, however, there seems to be no measurable reduction of the thermal conductivity by the superlattice-type 2D nanostructuring.
Financed by the National Centre for Research and Development under grant No. SP/I/1/77065/10 by the strategic scientific research and experimental development program:
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