Thermal boundary conductance under large temperature discontinuities of ultrathin Pb(111) films on Si(111)

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 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 nonequilibrium conditions. Upon applying an intense femtosecond laser pulse, the 5-7 ML thin Pb film undergoes rapid heating by 10-120 K while the Si substrate remains cold at ≈ 10 K . At such large temperature discontinuities, a significantly faster cooling is observed for more strongly excited Pb films. The decrease in the corresponding cooling time constant is explained by variations in thermal boundary conductance, interpreted within 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, underscoring the importance of substrate, heterofilm, and interface morphologies.