No Access Submitted: 08 October 2008 Accepted: 08 December 2008 Published Online: 26 January 2009
Journal of Applied Physics 105, 023710 (2009); https://doi.org/10.1063/1.3068476
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  • Department of Mechanical and Aerospace Engineering, University of Virginia, P.O. Box 400746, Charlottesville, Virginia 22904-4746, USA
  • a)Present address: Engineering Sciences Center, Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-0346. Electronic mail: .

Electron scattering at interfaces between metals and dielectrics is a major concern in thermal boundary conductance studies. This aspect of energy transfer has been extensively studied and modeled on long time scales when the electrons and phonons are in equilibrium in the metal film. However, there are conflicting results concerning electron-interface scattering and energy transfer in the event of an electron-phonon nonequilibrium, specifically, how this mode of energy transfer affects the electron cooling during electron-phonon nonequilibration. Transient thermoreflectance (TTR) experiments utilizing ultrashort pulsed laser systems can resolve this electron-phonon nonequilibrium, and the thermophysical property relating rate of equilibration to electron-phonon scattering events G can be quantified. In this work, G in Au films of varying thicknesses are measured with the TTR technique. At large fluences (which result in high electron temperatures), the measured G is much larger than predicted from traditional models. This increase in G increases as the film thickness decreases and shows a substrate dependency, with larger values of G measured on more conductive substrates. The data suggest that in a highly nonequilibrium system, there could be some thermal energy lost to the underlying substrate, which can affect G.
P.H. greatly appreciates the financial support from the Sandia National Laboratories, Harry S. Truman Fellowship, and the National Science Foundation Graduate Research Fellowship. The authors greatly acknowledge the financial support from the Office of Naval Research MURI program, Grant No. N00014-07-1-0723. The authors would like to thank Professor A. N. Smith at the U. S. Naval Academy for insightful discussions regarding nonequilibrium thermoreflectance and Professor H. K. Chelliah at the University of Virginia for clarifying aspects of electron scattering using kinetic theory.
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