Evaluation of Thermal Conductivity Characteristics in Polycrystalline Silicon Grains with Nanostructures by Raman Spectroscopy
Authors: Takeuchi Haruki , Yokogawa Ryo , Takahashi Kazuya , Komori Katsuhiko , Morimoto Tamotsu , Sawamoto Naomi , Ogura Atsushi 
Published: February 2020

1.Introduction

Polycrystalline silicon (poly-Si) is a promising candidate for channel materials such as thin film transistors (TFTs). Moreover, poly-Si is also expected for the next-generation thermoelectric device materials. It has been reported that poly-Si has low thermal conductivity due to their grain boundaries and defects, and so on [1,2]. We have previously confirmed that nanostructures exist in poly-Si grains [3], and these nanostructures are the possible phonon scatterers. In this study, we directly evaluated thermal conductivity characteristics for the poly-Si thin films, in order to clarify the influence of nanostructure in grain region.

2.Experiments

The 100 nm thick SiO­2 film was grown on Si substrates. The 180 nm thick Amorphous Si (a-Si) thin films were deposited at 510 °C by low pressure chemical vapor deposition (LPCVD) using the mono-silane (SiH4) gases under the pressure of 1.5 Torr. The a-Si thin films on SiO2/Si substrate were then annealed at 700 °C 2 hrs, 1000 °C 2hrs and 700 °C 2 hrs +1000 °C 2 hrs, respectively.
The fabricated samples were measured by UV Raman spectroscopy for the investigation of thermal conductivity characteristics. The excitation source was the UV laser (λ=355 nm), and the focal length of the spectrometer was 2,000 mm. The laser power was controlled by a variable neutral density (ND) filter in the incident light path from 1 to 10 mW in 1 mW steps. Five-point Raman spectra measurements were performed on each sample to evaluate the thermal conductivity characteristics.

3.Results and Discussion

Figure 1 shows the laser-power-dependent Raman shift from the poly-Si thin films. From Fig.1, it was confirmed that the Raman peak tends to shift toward lower wavenumbers as the laser power increased. It is considered that the behavior of these Raman spectra is due to local laser heating. The temperature was calculated from the Raman shift associated with the measured laser power, as shown in Fig. 2. The relation between Raman shift and temperature are edscribed by following equation (1) [4].
dω/dT=-0.024 cm-1/K　(1)
Table 1 shows the average grain size and the average nanostructure size of each sample. The grain size is evaluated using Electron Back Scattered Diffraction Pattern, and the nanostructure size is calculated by the peak width of the UV Raman spectroscopy [3]. Figure 2 shows that the sample of 700 °C with the smallest nanostructure size has the lowest heat conduction even though this has the largest average grain size. Therefore, it has become clear that not only the crystal grain size but also the nanostructures in the grains influences the heat conduction, and the heat conduction can be suppressed by the miniaturization of the nanostructure. In conclusion, we showed that the nanostructures in poly-Si grains contribute to suppression of thermal conductivity.

REFERENCE

Y. Nakamura, Sci. Technol. Adv. Mater. 19, 31 (2018).

K. Valalaki, N. Vouroutzis, and A. G. Nassiopoulou, J. Phys. D: Appl. Phys. 49, 315104 (2016).

H. Yamazaki, M. Koike, M. Saitoh, M. Tomita, R. Yokogawa, N. Sawamoto, M. Tomita, D. Kosemura, and A. Ogura, Sci. Rep. 7,16549 (2017).

D. Fan, H. Sigg, R. Spolenak, and Y. Ekinci, Phys. Rev. B 96, 115307 (2017).

Figure 1