SiGe is promising candidates for the excellent thermoelectric device material with low thermal conductivity by alloy effect. In order to improve thermoelectric performance, it needs to realize high Seebeck coefficient of SiGe. From the above, we focused on the laterally graded SiGe wires fabricated by rapid melting growth (RMG) [1]. It has been reported that laterally graded energy-band-gap structures in SiGe can enhance thermoelectric performance using simulation [2]. However, the laterally graded SiGe wire structures for thermoelectric devices has not been investigated. In this study, the RMG laterally graded SiGe wire structures for thermoelectric devices were evaluated to achieve high thermoelectric performance.

2.Experiment

The RMG laterally graded SiGe wires were grown on a silicon-on-insulator (SOI) wafer. The SOI wafer with 45 nm thick was patterned to form the Si island seeds. Then, amorphous Si_0.15Ge_0.85 film was deposited by molecular beam deposition. After the patterning of wires, 1 μm thick capping SiO₂ layer was deposited by plasma-enhanced chemical vapor deposition (PECVD), and the RMG were performed at 1100 ℃ in N₂ ambient. The size of the SiGe wires are 10 mm in length, 3 mm in width and 100 nm in thickness, respectively. Finally, both end parts of the SiGe wires were connected to 1 μm thick Al thermal and electrical conductive pads.

The RMG laterally graded SiGe wires were evaluated by Raman spectroscopy and electron back-scattering pattern (EBSP). A quasi-line excitation (approximately 100 μm) was used for Raman spectroscopy. Excitation source of visible laser (λ = 532 nm) was used and focal length of the Raman spectrometer was 2,000 mm. Wavenumber resolution is approximately 0.1 cm^-1. One dimensional measurements with a quasi-line excitation source were performed at the same time to evaluate Ge concentration x in the Si_1-xGe_x wires. Moreover, the change of crystal orientation in the SiGe wires was measured by EBSP.

3. Results and Discussion

Figure 1 shows one-dimensional distribution of Raman spectra for Ge-Ge mode obtained by Raman spectroscopy using a quasi-line excitation. As shown in Fig. 1, an apparent change of Raman shift was observed along the wire. As an increase in distance from the Si seed, the Raman peak positions in the SiGe wires for Ge-Ge modes shifted toward higher wavenumbers.

Figure 2 shows Ge concentration x distribution using the Raman shifts for Ge-Ge mode. Ge concentration x and the theoretical values were calculated by using Raman shift equation for Ge-Ge mode in Ref. 3 on the basis in strain-free assumption and Scheil equation in Ref. 4, respectively [3, 4]. As a result, we confirmed that the calculated Ge concentration x is lower than the theoretical values. It is considered that the Raman peak positions for Ge-Ge mode are shifted not only by Ge concentration x but also by a strain in the SiGe wire.

Crystal orientation mapping obtained by EBSP was shown in Fig. 3. Here, the rotating angles are defined as the changes in the crystal orientations from those of the Si seed. The angles increase with distance from the Si seed. This rotation of crystal orientation may also influence electronic and thermoelectric performance as well as graded Ge concentration.

We will report the thermoelectric properties of the RMG laterally graded SiGe wire for thermoelectric devices.

Acknowledgements

This work was supported by CREST, JST (JPMJCR15Q7), PRESTO, JST (JPMJPR15R2), and the Japan Society for the Promotion of Science through a JSPS Fellows (17J08240).

References

[1] T. Tanaka et al., Appl. Phys. Express 3, 031301 (2010).

[2] M. Wagner et al., Semicond. Sci.Technol. 22, S173 (2007).

[3] F. Pezzoli et al., Mater. Sci. Semicond. Process. 11, 279 (2008).

[4] R. Matsumura et al., Appl. Phys. Lett. 101, 241904 (2012).