Ion irradiation, damage and interfacial mixing increases thermal boundary conductance

Caroline’s paper, “Ion irradiation of the native oxide/silicon surface increases the thermal boundary conductance across aluminum/silicon interfaces,” has been published in Physical Review B (Phys. Rev. B 90, 024301 (2014)).  In this work, we demonstrate that mixing and damage resulting from ion irradiation can INCREASE the thermal boundary conductance across Al/native oxide/Si interfaces.  This counter intuitive result shows that “less perfect” interfaces can be better conductors (i.e., have less resistance) than compositionally abrupt, more “perfect” interfaces.  We wish Caroline all the best in her PhD pursuits with Prof. Alan McGaughey at CMU.  Congrats Caroline!!

Abstract

The thermal boundary conductance across solid-solid interfaces can be affected by the physical properties of the solid boundary. Atomic composition, disorder, and bonding between materials can result in large deviations in the phonon scattering mechanisms contributing to thermal boundary conductance. Theoretical and computational studies have suggested that the mixing of atoms around an interface can lead to an increase in thermal boundary conductance by creating a region with an average vibrational spectra of the two materials forming the interface. In this paper, we experimentally demonstrate that ion irradiation and subsequent modification of atoms at solid surfaces can increase the thermal boundary conductance across solid interfaces due to a change in the acoustic impedance of the surface. We measure the thermal boundary conductance between thin aluminum films and silicon substrates with native silicon dioxide layers that have been subjected to proton irradiation and post-irradiation surface cleaning procedures. The thermal boundary conductance across the Al/native oxide/Si interfacial region increases with an increase in proton dose. Supported with statistical simulations, we hypothesize that ion beam mixing of the native oxide and silicon substrate within ∼2.2 nm of the silicon surface results in the observed increase in thermal boundary conductance. This ion mixing leads to the spatial gradation of the silicon native oxide into the silicon substrate, which alters the acoustic impedance and vibrational characteristics at the interface of the aluminum film and native oxide/silicon substrate. We confirm this assertion with picosecond acoustic analyses. Our results demonstrate that under specific conditions, a “more disordered and defected” interfacial region can have a lower resistance than a more “perfect” interface.

We appreciate support from the Office of Naval Research Young Investigator Program (Grant No. N00014-13-4-0528), the Army Research Office (Grant No. W911NF-13-1-0378), and the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. Addition- ally, the authors acknowledge John Bradley for allowing use of the 80-300 Titan S/TEM at Lawrence Livermore National Laboratory. Sandia is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94Al85000.

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