(Invited) Nanostructures for Reduced Lattice Thermal Conductivity — Case Studies for Nanopores and Grain Boundaries
Authors: Hao Qing , Xu Dongchao , Xiao Yue , Xiao Bo , Zhao Hongbo 
Published: February 2020

Solid-state thermoelectric (TE) devices have the ability to directly convert heat into electricity for power generation. A good TE material should possess a high electrical conductivity σ, a high Seebeck coefficient S, and a low thermal conductivity k. This requirement is hard to be satisfied within the same material. To address this issue, the nanostructuring approach has been widely used to reduce the lattice part of the thermal conductivity (kL
) but still maintain the bulk electrical properties. High TE performance has thus been achieved in various nanostructured materials, such as superlattice thin films, nanoporous thin films, and different nanostructured bulk materials.

Despite many exciting results, a fundamental understanding of the observed kL
reduction by various nanostructures is still lacking. For example, diffusive phonon scattering by pore edges was found to be insufficient to explain some measurement data for nanoporous Si films [1-5]. In some studies, the kL
reduction was further attributed to modified phonon dispersion by coherent phonon transport within a periodic nanoporous structure, known as the “phononic effect” [3-6]. On the other hand, some analysis showed that the phononic effect was negligible because the structure sizes were much larger than the dominant phonon wavelength (~1 nm) at the room temperature [7-9]. Along this line, the low kL
observed for ~10 nm porous patterns [3] was consistent with analysis assuming expanded pore sizes due to amorphous pore edges [7]. However, this still did not fully explain the inconsistency between the experiments and theoretical analysis for ~100 nm porous patterns. In this work, the in-plane kL
of nanoporous Si films with patterns of down to 150–600 nm periods were systematically measured based on the self-heating of a metal-coated film. This setup eliminated the possible errors due to the thermal contact between a microdevice and the measured thin film [2, 3]. The measurement results agreed well with simulations that only considered diffusive pore-edge phonon scattering. The measurements followed the same trend as some experimental studies on nanoporous Si thin films [1, 10, 11]. Beyond Si films, In0.1Ga0.9N films with 300-nm-diameter periodic pores were also measured for the cross-plane kL
and same conclusions were reached. These nanoporous films were directly grown by Metal-Organic Chemical Vapour Deposition using SiO2 masks. This approach minimized the pore-edge defects often observed for pores drilled by dry etching or a focused ion beam.

As another important nanofeature, nanosized grain boundaries also play an important role in suppressing the phonon transport in nanostructured bulk materials formed by hot pressing nanoparticles into a bulk sample. Due to the challenge in measuring a single grain boundary, a thin film was hot pressed onto a wafer to represent a grain boundary. Compared with similar studies using wafer-wafer bonding [12], a superflexible thin film ensured better thermal contact and thus high-quality interfaces. The focus of the study was on how the crystal misorientation and interface defects affected the resulting interfacial thermal resistance between the film and the wafer.

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