Thermoelectric Properties and Morphology of Si/SiC Thin-Film Multilayers Grown by Ion Beam Sputtering

Abstract

Multilayers (MLs) of 31 bi-layers and a 10-nm layer thickness each of Si/SiC were deposited on silicon, quartz and mullite substrates using a high-speed, ion-beam sputter deposition process. The samples deposited on the silicon substrates were used for imaging purposes and structural verification as they did not allow for accurate electrical measurement of the material. The Seebeck coefficient and the electrical resistivity on the mullite and the quartz substrates were reported as a function of temperature and used to compare the film performance. The thermal conductivity measurement was performed for ML samples grown on Si, and an average value of the thermal conductivity was used to find the figure of merit, zT, for all samples tested. X-ray diffraction (XRD) spectra showed an amorphous nature of the thin films. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the film morphology and verify the nature of the crystallinity. The mobility of the multilayer films was measured to be only 0.039 to 1.0 cm 2 /Vs at room temperature. The samples were tested three times in the temperature range of 300 K to 900 K to document the changes in the films with temperature cycling. The highest Seebeck coefficient is measured for a Si/SiC multilayer system on quartz and mullite substrates and were observed at 870 K to be roughly −2600 µV/K due to a strain-induced redistribution of the states' effect. The highest figure of merit, zT, calculated for the multilayers in this study was 0.08 at 870 K.

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“…This maximum value is equal to or greater than that reported by Masuda et al in the temperature range of 400~500 • C with a Seebeck coefficient ≈ −600 µV/K [44]. The highest Seebeck coefficient was measured for the SiC 6C-4Al sample and was observed at 200 • C to be roughly −2500 µV/K due to a strain-induced redistribution of the effect states caused by interface mismatch, as informed by Cramer et al [45], obtaining high Seebeck coefficients for their Si/SiC multilayer thin-film systems and reported values of −2600 µV/K at 870 K. the sintering process provided by the alumina during heat treatment and to the conductive property of graphite. Figure 24 shows the Seebeck coefficient (S) as a function of temperature for the samples of SiC mixed with alumina and/or graphite and for the sample additive-free.…”

“…This maximum value is equal to or greater than that reported by Masuda et al in the temperature range of 400~500 °C with a Seebeck coefficient ≈ −600 µV/K [44]. The highest Seebeck coefficient was measured for the SiC 6C-4Al sample and was observed at 200 °C to be roughly −2500 µV/K due to a strain-induced redistribution of the effect states caused by interface mismatch, as informed by Cramer et al [45], obtaining high Seebeck coefficients for their Si/SiC multilayer thin-film systems and reported values of −2600 µV/K at 870 K. Figure 25 shows that the thermal conductivity (κ)of the SiC, SiC 6C and SiC 6Al samples tended to increase with an increasing temperature due to the metallic behaviour of the material [42], while in the SiC 6C-4Al and SiC 6C-6Al samples, the thermal conductivity tended to decrease with an increasing temperature due to Umklapp phonon dispersion [46], obtaining values of 0.35 W/m•K and 0.377 W/m•K at 500 • C, respectively. Figure 25 shows that the thermal conductivity (κ)of the SiC, SiC 6C and SiC 6Al samples tended to increase with an increasing temperature due to the metallic behaviour of the material [42], while in the SiC 6C-4Al and SiC 6C-6Al samples, the thermal conductivity tended to decrease with an increasing temperature due to Umklapp phonon dispersion [46], obtaining values of 0.35 W/m•K and 0.377 W/m•K at 500 °C, respectively.…”