[19p-W834-1~17] 9.2 Nanowires and Nanoparticles

[19p-W834-10] 3ω Method in the Quasi-Ballistic Regime for Thermal Transport Study in Si Nanowires

Fuwei Zhuge¹, Masaki Kanai¹, Kazuki Nagashima¹, Naoki Fukata², 〇Takeshi Yanagida¹ (1.Kyushu Univ, 2.NIMS)

Keywords:nanowire,thermal transport

Nanoscale thermal engineering has attracted lots of research attention in thermoelectrics and emerging high power nanodevices, such as nanowire laser and sensors. Studying the thermal transport characteristic in nanostructured materials is of great importance to manipulate thermal relevant device performances. Si nanowires has been shown as an efficient thermoelectric material with greatly reduced thermal conductivity when compared to the bulk, due to surface related phonon (the major thermal carrier in semiconductors and dielectrics) scatterings. Though many theoretical research has been conducted previously, experimental studies of phonon transport, especially concerning the longitudinal size (length) of nanowires are very limited. In this work, we conduct 3ω method measurements of thermal conductivity of Si nanowires (~45nm in diameter) of varied length scales (from 4μm to 0.5μm) in the quasi-ballistic regime. Classically, when the length of nanowire approaches the mean free path (MFP) of phonons, reduction of effective thermal conductivity will appear due to the diffusive to ballistic transition of phonon transport. However, the transition length scale is usually unknown, especially for the nanostructured materials. Here, we found the reduction of thermal conductivity of Si nanowires appears at a surprising long length scale of ~2μm. The results are discussed using the ballistic effect induced suppression function in 3ω method, which is calculated by numerically solving the 1D Boltzmann transport equation. A fitting of the measurements results at shorter length scales yields a dominant phonon MFP of ~500nm, which is very close to the average MFP of phonon (~0.6μm) in bulk Si. Our results unambiguously indicate a persistence of long MFP phonons in Si nanostructures. Using the length controlled selective suppression function to the broadband phonons, we also discuss the MFP selective phonon scatterings by surface structural changes of Si nanowires.