  Varying the thermal boundary conductance at metal-dielectric interfaces is of
great importance for highly integrated electronic structures such as
electronic, thermoelectric and plasmonic devices where heat dissipation is
dominated by interfacial effects. In this paper we study the modification of
the thermal boundary conductance at metal-dielectric interfaces by inserting
metal interlayers of varying thickness below 10 nm. We show that the insertion
of a tantalum interlayer at the Al/Si and Al/sapphire interfaces strongly
hinders the phonon transmission across these boundaries, with a sharp
transition and plateau within ~1 nm. We show that the electron-phonon coupling
has a major influence on the sharpness of the transition as the interlayer
thickness is varied, and if the coupling is strong, the variation in thermal
boundary conductance typically saturates within 2 nm. In contrast, the addition
of a nickel interlayer at the Al/Si and the Al/sapphire interfaces produces a
local minimum as the interlayer thickness increases, due to the similar phonon
dispersion in Ni and Al. The weaker electron-phonon coupling in Ni causes the
boundary conductance to saturate more slowly. Thermal property measurements
were performed using time domain thermo-reflectance and are in good agreement
with a formulation of the diffuse mismatch model based on real phonon
dispersions that accounts for inelastic phonon scattering and phonon
confinement within the interlayer. The analysis of the different assumptions
included in the model reveals when inelastic processes should be considered. A
hybrid model that introduces inelastic scattering only when the materials are
more acoustically matched is found to better predict the thickness dependence
of the thermal boundary conductance without any fitting parameters.
