Phonon confinement effects in ultrathin epitaxial bismuth films on silicon studied by time-resolved electron diffraction

Abstract

The transient temperature evolution of ultrathin bismuth films, epitaxially grown on a silicon single crystal, upon femtosecond laser excitation is studied by time-resolved electron diffraction. The exponential decay of the film temperature is explained by phonon reflection at the interface, which results in a strongly reduced thermal conduction in the cross plane of the layered system. The thermal boundary conductance is found to be as low as 1273 W / ͑K cm 2 ͒. Model calculations, including phonon confinement effects, explain the linear relationship between the observed film-temperature decay constant and the film thickness. Even for 2.5 nm thin films the phonon transmission probability across the interface is given by bulk properties. Our simulations show that phonon confinement effects are negligible for bismuth-film thicknesses larger than 1 nm.

A Review of Experimental and Computational Advances in Thermal Boundary Conductance and Nanoscale Thermal Transport across Solid Interfaces

“…Along with the thermal characterization metrologies, the ability to control growth processes with atomic resolution has also led to the realization of “more model” interfaces that have allowed for the critical assessment and validation of various theoretical models. However, there are only a handful of these studies on limited material systems that have investigated the heat transfer across epitaxially grown interfaces, leaving a considerable void to be filled with future work on these types of interfaces formed between various material systems with different energy carriers dictating heat flow across interfaces.…”

Thickness-dependent electron–lattice equilibration in laser-excited thin bismuth films

“…Bi exhibits a Peierls-distorted ground state structure (rhombohedral A7) which results in a strong coupling between the electronic and lattice degrees of freedom. This makes it very susceptible to electronic excitation [22], and ultrafast diffraction has been extensively used to study various aspects of the ensuing vibrational/structural dynamics: coherent optical [23][24][25] and acoustic [16,26] phonons, phonon squeezing [27], electron-lattice equilibration in thin films [14,15,17], nanoparticles [16], and at the surface [17], anisotropic structural effects [15,16,26,28], heat transfer processes [29][30][31], and ultrafast melting [14].…”

Thickness evolution of phonon properties in ultrathin Bi (111) films

“…As a result, Bi crystal can easily be cleaved along the (111) plane, which is terminated with an intact BL . Experimentally, ultrathin films of Bi with a thickness down to several BLs on Si substrate have been realized . The electronic structure of Bi (111) ultrathin films (thickness 7 BLs) on Si was studied by angle‐resolved photoemission spectroscopy, showing the metallic character of the Bi films, in contrast with the semimetallic nature in bulk Bi .…”