Phonon transport control by nanoarchitecture including epitaxial Ge nanodots for Si-based thermoelectric materials

Abstract

Phonon transport in Si films was controlled using epitaxially-grown ultrasmall Ge nanodots (NDs) with ultrahigh density for the purpose of developing Si-based thermoelectric materials. The Si/Ge ND stacked structures, which were formed by the ultrathin SiO2 film technique, exhibited lower thermal conductivities than those of the conventional nanostructured SiGe bulk alloys, despite the stacked structures having a smaller Ge fraction. This came from the large thermal resistance caused by phonon scattering at the Si/Ge ND interfaces. The phonon scattering can be controlled by the Ge ND structure, which was independent of Si layer structure for carrier transport. These results demonstrate the effectiveness of ultrasmall epitaxial Ge NDs as phonon scattering sources, opening up a route for the realisation of Si-based thermoelectric materials.

Tunable Thermal Transport Characteristics of Nanocomposites

“…Our predicted values of the room-temperature thermal conductivity for Si/Ge nanocomposites are in the same range as reported from experimental measurements on similar size PSLs [32][33][34][35] and NDSLs [36][37][38][39]. However, it should be clarified that a detailed comparison of our predicted results for Si/Ge nanocomposite structure with experimental measurements is not possible.…”

“…With regards to equal layer thickness Si(D/2)/Ge(D/2) PSL, the conductivity takes a minimum value when the period size D lies in the range 3-12 nm, depending of course on sample size L and interface quality. Reported experimental studies [32,33,[35][36][37][38][39] and the present systematic theoretical study point out that with the right choice of sample size, period size, volume insertion fraction, and short-range interface defects in a Si/Ge nanocomposite, it is possible to achieve room-temperature conductivity below the alloy and amorphous limit of around 4 Wm −1 K −1 . This positively points in the direction of the usefulness of nanocomposites for applications such as thermoelectricity.…”

Phonon and heat transport control using pillar-based phononic crystals

“…However, the experimentally observed 20% reduction in thermal conductivity is interesting to compare with the reduction obtained in other silicon-based nanostructures. On one hand, a stronger reduction was measured in silicon with pores (>50%) [9,18,19], nanodots (>70%) [13,68], polycrystalline grains (>80%) [15,69,70], dopants (>50%) [71,72] or germanium atoms (>70%) [14,71,73]. On the other hand, the 20% reduction by the pillars [63] is comparable to the reduction by holes (20–25%) [21,74] or slits (20–30%) [75] covering the same relative area.…”

Formation of various epitaxial nanodots in Si films for thermoelectric materials

“…In this structure, epitaxial Ge NDs worked as phonon scattering centers and epitaxial Si layers worked as electric conduction layers. This result indicated the possibility of the independent control of the thermal and electric conductivities [2,3]. However, S was not controlled in this nanostructure.…”