Enhanced Thermoelectric Efficiency of Si-Ni Nanocomposite Films Grown on SOI and SOQ Substrates

To improve the poor thermoelectric performance of Si, reduction of the high thermal conductivity k (~142 Wm^–1K^–1) is essential, and one of the effective methods to reduce the k is making Si nanostructures such as Si nanocrystals (NCs) [1, 2] and nanowires[3], where the k is dramatically reduced by enhancement of phonon scattering at the interfaces of the nanostructures. Recently, we developed semiconducting composite films of Si and Ni silicide NCs (Si-Ni nanocomposite film) deposited on SiO₂ substrates [2]with the power factor of 1.9×10^-3 W/mK² , the thermal conductivity k of 4.4 W/mK, and the dimensionless figure of merit ZT of 0.13 for the B-doped (p-type ) nanocomposite film [2], which is much higher than that of bulk Si (ZT<0.01).

In this paper, we demonstrate the further improvement of the thermoelectric performance of B doped Si-Ni nanocomposite films by growing them on silicon-on-insulator (SOI) or silicon-on-quartz (SOQ) substrates. The B doped Si-Ni nanocomposite films were synthesized from amorphous Si-Ni-B films deposited on the SOI or SOQ substrates at room temperature (RT) with Ar sputtering of the B doped Ni-Si target (Ni:Si=1:20, 2mol% B). The amorphous films were annealed at 800°C in N₂gas ambient, leading to crystallization and the phase separation of the films. After this annealing , rapid annealing of 10 sec at 1200°C was carried out to activate the B atoms.

In Fig.1, the ZT values are plotted against the k valuesof Si-Ni nanocomposite films on SOI, SOQ, SiO₂ substrates [ 2], and Si nanocomposite by Bux et al. [1] at RT. We observed low k values (< 4 W/mK) of Si-Ni nanocomposite films on SOI and SOQ, which were similar k with Si-Ni nanocomposite film on SiO₂, due to phonon scattering at interfaces on NCs. As seen, the ZTs (0.23-0.36) of the Si-Ni nanocomposite films on SOI and SOQ substrates were much higher than those of the Si-Ni nanocomposite films on SiO₂ and the Si nanocomposite. We observed with cross-sectional transmission electron microscopy that Si NCs in Si-Ni composite films were epitaxially grown from thin Si crystal layers of SOI /SOQ substrates, leading to improvement of the crystal quality of Si NCs in the nanocomposite films and increase of the carrier conductivities, the Seebeck coefficients and the ZTs.

This work was partly supported by ALCA-JST.

[1] S. K. Bux, et al., Adv. Funct. Mater. 19, 2445 (2009). [2] N. Uchida et al., J. Appl. Phys. 114, 134311 (2013).  [3] A. I. Hochbaum, et al., Nature 451,163 (2008).