Anomalous reduction of thermal conductivity in coherent nanocrystal architecture for silicon thermoelectric material

• Published in: Nano energy 2015-03, Vol.12, p.845-851
• Main Authors: Nakamura, Yoshiaki, Isogawa, Masayuki, Ueda, Tomohiro, Yamasaka, Shuto, Matsui, Hideki, Kikkawa, Jun, Ikeuchi, Satoaki, Oyake, Takafumi, Hori, Takuma, Shiomi, Junichiro, Sakai, Akira
• Format: Article
• Language: English
• Subjects: Molecular beam epitaxy; Nano-structures; Silicon; Thermal conductivity
• ISSN: 2211-2855
• DOI: 10.1016/j.nanoen.2014.11.029

Reduction of thermal conductivity κ while preserving high electrical conductivity σ in materials continues to be a vital goal in thermoelectric study for the reuse of exhaust heat energy. In the use of an eco-friendly and ubiquitous element, Si as thermoelectric material, high κ value in bulk Si is the essential bottleneck to achieve high dimensionless figure of merit. This is a motivation for many recent studies on reducing κ in Si, by nanostructuring, e.g., using grains/wires with size smaller than the phonon mean free path. However, κ reduction that can be achieved tends to be saturated presumably due to an amorphous limit. Here, we present a nanoarchitecture for defeating the κ amorphous limit while preserving bulk-like σ. This new nanoarchitecture is an assembly of Si nanocrystals with oriented crystals separated by a 1-monolayer amorphous layer with well-controlled nanoscale shaped interfaces. At these interfaces, novel phonon scattering occurs resulting in κ reduction below the amorphous limit. Preservation of bulk-like σ results from the coherency of the carrier wavefunctions among the oriented nanocrystals separated by the ultrathin amorphous layer. The results will bring environmentally-friendly and low-cost thermoelectric Si material compatible with mature LSI process technology and represent guidelines for optimized thermoelectric nanostructures. [Display omitted]