Epitaxial growth and thermoelectric properties of Mg3Bi2 thin films deposited by magnetron sputtering

Abstract

Mg 3 Sb 2 -based thermoelectric materials attract attention for applications near room temperature. Here, Mg-Bi films were synthesized using magnetron sputtering at deposition temperatures from room temperature to 400 C. Single-phase Mg 3 Bi 2 thin films were grown on c-planeoriented sapphire and Si(100) substrates at a low deposition temperature of 200 C. The Mg 3 Bi 2 films grew epitaxially on c-sapphire and fiber-textured on Si(100). The orientation relationships for the Mg 3 Bi 2 film with respect to the c-sapphire substrate are (0001) Mg 3 Bi 2 k(0001) Al 2 O 3 and [1120] Mg 3 Bi 2 k[1120] Al 2 O 3 . The observed epitaxy is consistent with the relatively high work of separation, calculated by the density functional theory, of 6.92 J m À2 for the Mg 3 Bi 2 (0001)/Al 2 O 3 (0001) interface. Mg 3 Bi 2 films exhibited an in-plane electrical resistivity of 34 lX m and a Seebeck coefficient of þ82.5 lV K À1 , yielding a thermoelectric power factor of 200 lW m À1 K À2 near room temperature.

Tunable p- and n-Type Tellurium-Free Mg3Bi2 Thermoelectric Thin Films by Thermal Coevaporation

“…The positive Seebeck and Hall coefficients indicate that the Mg-poor Mg 3 Bi 2 thin film has p-type conductivity, as expected from the literature for Mg-deficient compositions. ,,, The low thermal conductivity (κ) of around 1 W m –1 K –1 at RT is comparable to those of other related Mg 3 Bi 2 -based compounds, , although, to the best of our knowledge, an analogous value has not been reported so far for undoped Mg 3 Bi 2 . Regarding the electrical conductivity (σ), we find a value of ∼80 × 10 3 S m –1 , which is higher than the electrical conductivity of Mg 3 Bi 2 thin films deposited with magnetron sputtering. − The Seebeck coefficient across the selected temperature range has values (40–60 μV K –1 ) comparable to those found for the above-mentioned magnetron-sputtered films and increases linearly with temperature. Such a linear increase of S with constant σ has also been observed in other inorganic semiconductor thin films .…”

“…This represents, to the best of our knowledge, the highest reported ZT on Te-free Mg 3 Bi 2 materials and the first full thermoelectric characterization of these alloys in the form of thin films. Compared to previous results reported on films deposited by magnetron sputtering, 28 it is important to highlight that our method based on thermal evaporation does not use substrate heating during deposition, allows for tunable p-and n-type films, and yields higher PFs of up to 300 μW m −1 K −1 , making it a promising approach for flexible miniaturized TEGs.…”

“…These studies also mention that thermal vacuum deposition of Mg 3 Bi 2 thin films is challenging due to the high vapor pressure of Mg and that therefore sputtering is the preferred deposition method for these alloys. 28 However, already in 1979 Long et al had reported on the thermal coevaporation of Mg and Bi and the formation of a two-component alloy in which the compositional variations were below 0.5%. 31 The reliability of thermal evaporation of Mg is supported by other studies focusing on the coevaporation of Mg and B to form MgB thin films.…”

“…This is consistent with a potential Mg deficiency, as noted by others (we have taken this into account in the fit presented in Figure S2). , It is interesting to note that this Bi phase vanishes when the Mg evaporation rate is increased (Figure b). The Mg-rich-deposited film requires a higher annealing temperature to crystallize than the Mg-poor film does and remains less crystalline (broader diffraction peaks translating to smaller crystallites) even at 300 °C.…”

Experimental Realization of Spin‐Polarized States on Mg3Bi2 Surface

“…The bulk Brillouin zone and the projection on (001) surface with high-symmetry points are plotted in Figure 1b. X-ray diffraction (XRD) was used to verify the quality and crystal structure of Mg 3 Bi 2 single crystals [20,[22][23][24]29] as shown in Figure 1c. According to the theoretical calculations, the electronic band dispersions differ significantly with Mg and Bi surface terminations as shown in Figure 1d,e.…”