Violation of the Wiedemann-Franz law through reduction of thermal conductivity in gold thin films

S. J. Mason, D. J. Wesenberg, A. Hojem, M. Manno, C. Leighton, B. L. Zink

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Abstract

We present measurements of in-plane thermal and electrical conductivity in thermally evaporated gold thin-film samples ranging in thickness from ≈20 to >300nm, performed using a micromachined silicon-nitride membrane thermal isolation platform. In both ≈300-nm-thick films grown in a single Au deposition and a sample built up to >300nm by many sequential depositions of thinner layers, we observe strong "violations"of the Wiedemann-Franz law that relates electrical and thermal conductivities. While electrical conductivity behaves essentially as expected, thermal conductivity first rises with growing total film thickness, and then surprisingly drops as the film becomes thicker. The sharp reduction of thermal conductivity decreases the Lorenz number L for ≈300-nm-thick samples to less than half the Sommerfeld value over the entire 78-300-K temperature range studied. Such violation near room temperature, in a metal film where electron transport should be well described by Fermi-liquid theory, is previously unreported, even in the presence of disorder introduced by grain boundaries and rough surfaces. This indicates an inelastic-scattering process that we argue, based on detailed characterization of grain size in these films, is likely driven by a combination of modified phonon density of states and structural anisotropy introduced from the strongly columnar grain structure in thicker films. This highly unusual reduction of thermal conductivity while maintaining high electrical conductivity is potentially promising for increasing thermoelectric performance of nanoscale systems.

Original languageEnglish (US)
Article number065003
JournalPhysical Review Materials
Volume4
Issue number6
DOIs
StatePublished - Jun 2020

Bibliographical note

Funding Information:
We thank D. Bassett and A. D. Avery for helpful discussions and assistance in the laboratory, we thank J. Nogan and the IL staff at Center for Integrated Nanotechnologies (CINT) for guidance and training in fabrication techniques, and we gratefully acknowledge support from the National Science Foundation (NSF) (Grants No. DMR-1410247 and No. DMR-1709646). B.L.Z. also thanks the University of Minnesota Chemical Engineering and Materials Science Department, as a portion of this work benefited from support of the George T. Piercy Distinguished Visiting Professorship. Work at the University of Minnesota was supported primarily by NSF under Grant No. DMR-1807124. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the Materials Research Science and Engineering Center (MRSEC) program. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Los Alamos National Laboratory (Contract No. DE-AC52-06NA25396) and Sandia National Laboratories (Contract No. DE-AC04-94AL85000).

Publisher Copyright:
© 2020 American Physical Society.

How much support was provided by MRSEC?

  • Shared

Reporting period for MRSEC

  • Period 7

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