Evaluation of Thermal Conductivity Characteristics in Polycrystalline Silicon - The Effect of Nanostructure in the Grains
Authors: Takeuchi Haruki , Yokogawa Ryo , Takahashi Kazuya , Komori Katsuhiko , Morimoto Tamotsu , Sawamoto Naomi , Ogura Atsushi 
Published: December 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, and these nanostructures become the phonon scatters [3]. However, the detailed effect of annealing on the nanostructure size has not been sufficiently. In this study, we focused on only the nanostructure size by changing the additional annealing conditions during poly-Si thin film fabrication with keeping the same grain size, and evaluated the thermal conductivity characteristics of the poly-Si thin film.

2.Experiments

The 100 nm thick SiO2 films were thermally grown on Si substrates. The 150 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 0.4 Torr. The a-Si thin films on SiO2/Si substrate were then annealed at 700 °C 2 hrs (Sample A), 700 °C 2hrs + 900 oC 2hrs (Sample B) and 700 °C 2 hrs +1100 °C 2 hrs (Sample C), respectively. The grain size is determined by the first annealing. Therefore, the grain size of each sample is almost the same at 600 nm.
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 spectra of Sample A. It was confirmed that the higher laser power resulted in the higher Raman spectral intensity. In addition, it was confirmed the Raman spectrum tends to shift toward lower wavenumber as the laser power increases. It has been reported that the Raman spectrum for Si-Si mode shift toward lower wavenumber due to the thermal effect as temperature increases. Therefore, it is considered that the change in the Raman shift is due to the temperature change caused by local laser heating. Table 1 shows the average nanostructure size of each sample. The nanostructure size is calculated by the full width at half maximum (FWHM) of the UV Raman spectroscopy [4]. Figure 2 shows that the laser-power-dependent Raman peak shift from the poly-Si thin films. It was confirmed that there is a difference in zero power intercept due to phonon confinement effect. It is considered that the Raman spectrum for sample A shifted due to the phonon confinement effect possibly caused by the smaller nanostructures inside the grain. In addition, it was also confirmed that the discrepancy in the Raman shifts between each sample becomes larger as the laser power increases, and it indicates that there is a difference in the temperature change for each sample. The relation between Raman shift and temperature are described by following Eq. (1) [5].
dω/dT = -0.024 cm-1/K (1)
Figure 3 shows that temperature in poly-Si thin film measured by Si Raman peak shift. It was revealed that Sample B has lower thermal conductivity despite the nanostructure size is larger. It may need to consider not only size but also density of the nanostructure to understand these phenomena. In conclusion, we clearly indicated that not only the average grain size but also the nanostructures inside grains should be carefully considered to control thermal conductivities.

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Figure 1