Thermoelectric Properties of Mg₂Si Thin Films by Computational Approaches

Abstract

The semiclassical Boltzmann approach coupled with density functional theory calculations has been used to investigate the structural, electronic, and thermoelectric properties of Mg2Si thin films with 001, 111, and 110 orientations. The most stable slab is found to be that with the 110 orientation. The electronic band gap vanishes for the 001 and 111 thin films whereas for the 110 orientation the film is semiconducting with a band gap ranging from 0.27 to 0.36 eV depending on the number of atomic planes used to model the thin film. The energy gap decreases when the number of planes increases. As a consequence of the electronic band structure, the 110 semiconducting thin film exhibits the highest thermoelectric performance, especially the Seebeck coefficient (−350 μV K–1 at 600 K). The lower the number of atomic planes the larger the Seebeck coefficient. By comparing experimental data for the electrical conductivity σ to our calculated value of σ/τ at 600 K we have determined the electron relaxation time τ to be about 5 × 10–16 s. Using this value, and assuming a thermal conductivity of 2–3 W m–1 K–1 for the 110 Mg2Si thin film, we estimate that the figure of merit ZT at 600 K lies in the range of 0.4–0.6.

Polycrystalline Mg2Si thin films: A theoretical investigation of their electronic transport properties

“…Using these data, and the values 100 μV/K, 150 μV/K and 200 μV/K for S at 350 K, 600 K and 900 K, respectively (see Fig. 7), we obtain for ZT around 0.02 at 350 K, 0.07 at 600 K and 0.2 at 900 K. These values, when compared to those calculated for the monocrystalline thin film [16], are about one order of magnitude smaller. The degradation of ZT for the present thin film is probably caused by the disorder observed in the structure.…”

“…In a previous work [16] we have estimated the relaxation time of electrons in Mg 2 Si thin films from experimental data of the electrical conductivity. The corresponding value was τ ¼ 5 Â 10 À 16 s, which we use hereafter.…”

“…Only a few theoretical studies have been reported on the calculations of Mg 2 Si thin films, either dealing with the structural and electronic characteristics [15], or the thermoelectric properties of the films [16]. Whereas experimentally grown thin films are always polycrystalline with preferential orientations (001, 110 and 111), the theoretical studies deal only with monocrystalline thin films.…”

Outstanding thermoelectric performances for both p- and n-type SnSe from first-principles study

“…This approximation assumed that the scattering time determining the electrical conductivity would not vary strongly with energy on the scale of kT. This method has been widely used to calculate transport properties of thermoelectric materials [11][12][13][14][15][16][17][18][19][20][21].…”

Electrical properties of Mg₂Si thin films on flexible polyimide substrates fabricated by radio-frequency magnetron sputtering

“…However, the grain growth and restructuring into thermodynamically stable structures occur with increasing Ts. Balout et al 22) have recently reported a theoretical result that the surface energy of the (110) surface of Mg 2 Si is the lowest among the (100), (110), and (111) surfaces by performing first-principles calculations based on density functional theory. Therefore, it is considered that the degree of orientation along the (220) plane increases with increasing Ts.…”