title: Unusually High and Anisotropic Thermal Conductivity in Amorphous Silicon Nanostructures.
abstract: Amorphous Si (a-Si) nanostructures are ubiquitous in numerous electronic and optoelectronic devices. Amorphous materials are considered to possess the lower limit to the thermal conductivity (κ), which is ∼1 W·m-1K-1for a-Si. However, recent work suggested that κ of micrometer-thick a-Si films can be greater than 3 W·m-1K-1, which is contributed to by propagating vibrational modes, referred to as "propagons". However, precise determination of κ in a-Si has been elusive. Here, we used structures of a-Si nanotubes and suspended a-Si films that enabled precise in-plane thermal conductivity (κ∥) measurement within a wide thickness range of 5 nm to 1.7 μm. We showed unexpectedly high κ∥in a-Si nanostructures, reaching ∼3.0 and 5.3 W·m-1K-1at ∼100 nm and 1.7 μm, respectively. Furthermore, the measured κ∥is significantly higher than the cross-plane κ on the same films. This unusually high and anisotropic thermal conductivity in the amorphous Si nanostructure manifests the surprisingly broad propagon mean free path distribution, which is found to range from 10 nm to 10 μm, in the disordered and atomically isotropic structure. This result provides an unambiguous answer to the century-old problem regarding mean free path distribution of propagons and also sheds light on the design and performance of numerous a-Si based electronic and optoelectronic devices.
authors.affiliation: Department of Mechanical and Aerospace Engineering, ‡Materials Science and Engineering Program, University of California-San Diego , La Jolla, California 92093, United States.
authors.forename: Soonshin
authors.initials: S
authors.lastname: Kwon
authors.affiliation: Department of Mechanical and Aerospace Engineering, ‡Materials Science and Engineering Program, University of California-San Diego , La Jolla, California 92093, United States.
authors.forename: Jianlin
authors.initials: J
authors.lastname: Zheng
authors.affiliation: Department of Mechanical and Aerospace Engineering, ‡Materials Science and Engineering Program, University of California-San Diego , La Jolla, California 92093, United States.
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Wingert
authors.affiliation: Department of Mechanical and Aerospace Engineering, ‡Materials Science and Engineering Program, University of California-San Diego , La Jolla, California 92093, United States.
authors.forename: Shuang
authors.initials: S
authors.lastname: Cui
authors.affiliation: Department of Mechanical and Aerospace Engineering, ‡Materials Science and Engineering Program, University of California-San Diego , La Jolla, California 92093, United States.
authors.forename: Renkun
authors.identifier: 0000-0001-7526-4981
authors.initials: R
authors.lastname: Chen
keywords: amorphous limit; amorphous silicon; mean free path; nanostructures; propagon; thermal conductivity