Transient Cooling of Ultrathin Epitaxial Bi(111)-Films on Si(111) Upon Femtosecond Laser Excitation Studied by Ultrafast
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1172-T04-08
Transient Cooling of Ultrathin Epitaxial Bi(111)-Films on Si(111) Upon Femtosecond Laser Excitation Studied by Ultrafast Reflection High Energy Electron Diffraction Anja Hanisch-Blicharski, Boris Krenzer, Simone Möllenbeck, Manuel Ligges, Ping Zhou, Martin Kammler and Michael Horn-von Hoegen Faculty of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany ABSTRACT With time resolved ultrafast electron diffraction the cooling process across the interface between a thin film and the underlying substrate was studied after excitation with short laser pulses. From the exponential decay of the surface temperature evolution a thermal boundary conductance of 1430 W/(cm2K) is determined for a 9.7 nm thin Bi(111) film on Si(111). A linear dependence between laser fluence and initial temperature rise was measured for film-thicknesses between 2.5 nm and 34.5 nm. The ratio of initial temperature rise and laser fluence for different film-thicknesses is compared to a model taking multilayer optics into account. The data agree well with this model. INTRODUCTION The heat transport in a bulk system is much faster compared to heterosystems consisting of a thin film on a substrate [1-3]. The interface between these two materials can be understood as a barrier to the thermal heat diffusion. The temporal cooling after femtosecond laser excitation - which leads to thermal heating - of a thin film with a temperature Tf on a substrate with a temperature Ts, can be described by ∂T f (t ) c⋅d = −σ K (T f (t ) − Ts (t )) , (1) ∂t with the specific heat c and the film-thickness d [3]. The thermal boundary conductance σK relates the temperature difference between film and substrate at the interface to the temporal evolution of the film temperature [1-3]. Assuming a constant substrate temperature, Eq. (1) results in an exponential function with a decay constant [3, 4]: τ dec = c ⋅ d / σ K . (2) The assumption of a constant substrate temperature is justified, because of a high absorption length of 13 µm for 1.55 eV photons and the high thermal conductivity of the silicon substrate in comparison to the Bi-film[4]. The decay constant is linearly related to the film-thickness. Because the heat transport across the interface is given by phonons, the thermal boundary conductance can be determined by the average phonon transmission probability 〈t(ω)〉 [1, 5]. The strongly reduced heat conductance across the interface is caused by a dramatic reduction of the transmission probability to a few percent. Well accepted models for the description of the heat transport across a hetero-interface are the diffuse mismatch model (DMM) and the acoustic mismatch model (AMM) [2, 3]. If the dominant phonon wavelength is smaller than the interface roughness, phonons are diffusively scattered at the interface. The diffuse mismatch model should be applied [6]. The transmission probability depends on the phonon density of states of film and substrate [2, 3].
Applying the Debye ap
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