Ultrafast LEED of Bismuth on Silicon (001)

Abstract

Ultrafast dynamics of epitaxial bismuth films on Si(001) substrate when exposed to femtosec ond laser pulses is investigated using Ultrafast Low Energy Electron Diffraction (ULEED). Owing to its high temporal resolution and surface sensitivity, ULEED is a powerful tool for probing ultrathin films and interfaces. Upon laser excitation of the bismuth film, a non-equilibrium electronic system is created, subsequently transferring energy to the lattice through electron-phonon coupling, causing a rapid rise in the surface temperature. Following this, the heat transfer by phonons through the thin film-substrate interface is monitored to extract cooling times from the transient lattice temperature of the bismuth film. The interface between two materials in a heterostructure serves as a barrier to the diffusive transfer of thermal energy, thereby hindering heat flow across the boundary. To quantify these processes, the rise time and cooling time constant are systematically analyzed, with a focus on their dependence on laser fluence and film thickness. Recently, it was shown that the thermal transport from Bi(111) films into a Si(001) substrate is reduced when films are thinner than the phonon mean free path due to total internal re f lection where phonon propagation becomes complex. To study the non-equilibrium phonon dynamics in this process, ULEED is a suitable and direct means of providing ultrafast tempo ral and high-momentum resolution. By analyzing the momentum-resolved map of a transient inelastic scattering background, we aim to identify phonon trapping, depopulation, and ther malization.