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1. Background and purpose Silicon nanowire (SiNW) is a promising candidate for next-generation thermoelectric materials, because they maintain both low-thermal conductance and high electric conductance simultaneously [1]. It has been reported that thermoelectric power is enhanced by miniaturizing the SiNW [2], and it is very important to measure the temperature difference across the SiNW for examination of thermoelectric characteristics. However, it becomes gradually difficult to measure the temperature with miniaturization of the SiNW. From the above, we focused on the temperature measurement using Raman spectroscopy. In this study, we demonstrated the measurement of temperature difference for the SiNW thermoelectric generator using operand Raman spectroscopy, in order to evaluate of thermoelectric characteristics accurately. Experimental method We fabricated SiNWs using an SOI substrate (SOI layer 55 nm/Buried oxide 145 nm/Si substrate 750 μm) via electron beam lithography and ICP-RIE. Thermal oxidation was performed in dry oxygen ambient at 850 oC for 3h, to form 20 nm thick oxide film on the surface of the SiNWs. The height and width of the SiNW were approximately 35 nm and 100 nm, respectively. The length of SiNWs were assigned to 8, 46, and 90 μm. Phosphorus (P) ions were then implanted at an acceleration energy of 25 keV, with a dose of 1.0 × 1015 ions/cm2 followed by activation annealing at 950 °C for 10 min under vacuum conditions. Next, the SiO2 layer at the pad regions was removed via photolithography and a wet-etching process using buffered hydrofluoric acid. Then, the Si pads and both ends of the SiNWs were nickelized by depositing a Ni film followed by rapid thermal annealing at 410 °C for 20 min. Thus, the both ends of the SiNWs were connected to the NiSi pads. Then, thermally conductive AlN film was deposited onto one of the NiSi pads via RF reactive sputtering to facilitate heat conduction from the heat source, where Al and an Ar/N2 mixture are used as the sputtering target and sputtering gas, respectively. Finally, a forming gas anneal was conducted at 400 °C. The fabricated SiNW-μTEG consists of 8 arrays of 400 legs [3]. Temperature measurement was performed using Raman spectrometer equipped with custom-made microthermostat consisting of a heater and a platinum thermometer on an aluminum nitride ceramic plate. Figure 1 shows schematic of operand Raman measurement. The focal length of the Raman spectrometer was 2,000 mm and the wavenumber resolution was approximately 0.1 cm-1. The excitation source was UV (λ = 355 nm) laser with the penetration depth for bulk Si of approximately 5 nm. From obtained Raman spectra of SiNW regions, temperature was estimated. Results and Discussion Figure 2 shows time transition of Raman shift obtained by operand Raman spectroscopy. The Raman shifts were measured at the edge on the heat source side and the middle of the 46 μm-long SiNWs. As shown in Fig. 2, an apparent change in Raman shift was observed, and the Raman peak tends to shift toward lower wavenumbers over time. It has been reported that the Raman spectrum for Si-Si mode shifts toward lower wavenumbers and broadened by a thermal effect as temperature rises. To obtain thermal conductivity properties in detail, the temperature in SiNW region was estimated using the relation dω/dT = −0.024 cm−1/K [4]. Figure 3 shows time transition temperature obtained by operand Raman spectroscopy. As shown in Fig. 3, we confirmed that the temperature increases over time by operand Raman spectroscopy. Moreover, it was found that the heat transfer tends to be delayed at the SiNW region away from the heat source (L=23 μm). Based on the above, we consider that operand Raman spectroscopy is a novel promising technique that can measure the temperature in a very small region. Acknowledgements This work was supported partly by JST CREST Grant Number JPMJCR15Q7 and JPMJCR19Q5, and the Japan Society for the Promotion of Science (JSPS) through a JSPS Fellows (17J08240). References [1] A. I. Hochbaum et al., Nature 451, 163 (2008). [2] M. Tomita et al., IEEE Trans. Electron Devices, 65, 5180 (2017). [3] T. Zhan, Sci. Technol. Adv. Mater. 19, 449 (2018). [4] D. Fan et al., Phys. Rev. B 96, 115307 (2017). Figure 1 |
