Ultrafast thermal boundary conductance under large temperature discontinuities of ultrathin epitaxial Pb films on Si(111)

TL;DR

This study investigates ultrafast heat transfer across the interface of ultrathin epitaxial Pb films on Si(111) using electron diffraction, revealing faster cooling at large temperature differences and the influence of interface morphology on thermal conductance.

Contribution

It provides new insights into thermal boundary conductance at large temperature discontinuities in nanoscale epitaxial films, highlighting the impact of interface morphology.

Findings

Faster cooling observed with increased film excitation.

Thermal boundary conductance decreases by over three times compared to H-terminated substrates.

Interface morphology significantly affects thermal boundary conductance.

Abstract

Heat transfer is a critical aspect of modern electronics, and a deeper understanding of the underlying physics is essential for building faster, smaller, and more powerful devices with an improved performance and efficiency. In such nanoscale structures, the heat transfer between two materials is limited by the finite thermal boundary conductance across their interface. Using ultrafast electron diffraction under grazing incidence we investigated the heat transfer from ultrathin epitaxial Pb films to an Si(111) substrate under strong non-equilibrium conditions. Applying an intense femtosecond laser pulse, the 5-7 ML thin Pb film experiences a strong heat up by 10-120 K while the Si substrate remains cold at $\approx$ 10 K. At such large temperature discontinuities we observe a significantly faster cooling for stronger excited Pb films. The decrease of the corresponding cooling time constant is explained through the thermal boundary conductance in the framework of the diffuse mismatch model. The thermal boundary conductance is reduced by more than a factor of three in comparison with Pb films grown on H-terminated substrates, pointing out the importance of the morphology of substrate, heterofilm and their interface.