TEM investigation of Compact Tellurium Thin Films with Bismut Atomic Doping

Compact tellurium (Te) thin films show important applications in micro-thermoelectric modules, which are able to convert waste heat to electrical energy (μTEG) or vice versa to use electricity to generate cooling (μTEC). The as-fabricated μTECs, which are based on electrochemically deposited n-type Bi₂(Te_xSe_1-x)₃ (in short BiTeSe) and p-type tellurium, demonstrate a rapid response time of 1ms, a high cycling reliability of up to 10 million cycles and a long-term cooling stability of more than 1 month at constant electric current. However, a mismatch of electrical conductivity between pure Te and BiTeSe often leads to some difficulties in the geometry design of micro-thermoelectric modules. In order to enhance the electrical property of Te, we have introduced an atomic bismuth (Bi) doping.

In this report, we have performed transmission electron microscopy (TEM, FEI Tecnai G2/ 200 kV) analysis to observe the presence and distribution of Bi within Te. To this end, cross-sectional TEM specimens were prepared using the Focused Ion Beam technique (FIB, FEI Helios NanoLab 600i). The Bi-doped Te samples have a columnar grain structure. Selected-area electron diffraction proves the presence of crystalline Te. Results of Nanodiffraction in numerous areas also suggest Te (hexagonal, space group 152, a = b = 0,4458 nm, c = 0,5925 nm, α = ß = 90°, γ = 120°) but could be explained with Bi (rhomboedral, space group 166, a = b = 0,4550 nm, c = 1,1850 nm, α= ß = 90°, γ = 120°), too. Unfortunately, the geometry of the Bi and Te unit cells differ only in the length of the c-axis. In addition, HRTEM images show lattice fringes, which could belong to Bi or Te. To unambiguously distinguish between both elements, chemical analysis is necessary.

Performing energy-dispersive X-ray spectroscopy (EDXS) analysis with a conventional Si(Li) detector, no Bi counts appear in the EDX spectra during reasonable measuring times of several minutes. To finally get the distribution of Bi in the Te matrix, we employed a FEI Talos F200X microscope operated at 200 kV and equipped with an X-FEG electron source and a Super-X EDX detector system and performed spectrum imaging analysis based on EDXS.