Study On Electron-phonon Coupling Dynamics Of Nano-multilayer Structures

The microcosmic process of heat transfer and conversion in solids involves the interaction among different heat energy carriers,which mainly include electrons and phonons.The coupling of electrons and phonons in ultra-short time plays an important role in many engineering fields such as microelectronic device thermal management and ultra-fast laser micromachining.However,the theoretical model describing the ultra-fast non-equilibrium thermal process still needs to be improved.On the other hand,there have been some relevant experiments,which mostly focused on the internal situation of monolayer materials.Only a few studies reported the results of multi-layer micro-nano films that are closer to the actual situation.Therefore,it is urgent to carry out relevant research,and the femtosecond laser time domain thermoreflectance（TDTR）experimental system is the best choice for experimentally studying ultrafast carrier interactions due to its high temporal resolution.In this paper,firstly the two-color TDTR experimental system is optimized by improving the time resolution through compressing the time width of pump laser pulses before incident sample by pulse laser compressor.Then for the theoretical models,the boundary conditions of the two-temperature model（TTM）are supplemented by considering the possible coupling between the electrons in metal film and the phonons in nonmetal side during the non-equilibrium heat transfer.After that,the non-equilibrium heat transfer process of Au films with different thicknesses were numerically simulated for a theoretical analysis example,with TTM model considering the heat transport of electrons at the interface.At the same time,the two-color TDTR experimental system was used to complete the measurements of both Au and Pt single-layer films with different thicknesses on Si,Si O₂ and sapphire substrates.Combined with the numerical simulation results,the influence of various factors on the non-equilibrium heat transfer process was summarized and analyzed,and the electron-phonon coupling coefficients of Au and Pt films as well as the electron-interface thermal conductance with Si,Si O₂ and sapphire substrates were obtained by fitting the TDTR signals.Further,the TTM was extended to the structure of bilayer metal films considering the electron-electron scattering at the metal/metal interface.The effects of electron-electron scattering and phonon-phonon scattering at the metal/metal interface of the bilayer metal film were studied via numerical methods.And the micro mechanism of the reverse heat transport from the second metal film to the top layer was revealed.Au/Pt/sapphire and Au/Al/sapphire bilayer metal film samples are tested by two-color TDTR experimental system.Then the experimental signals are fitted with the heat transfer model to obtain the interfacial thermal conductance of the bilayer metal interface caused by electron-electron scattering.The research results in this paper lay a foundation for further revealing the ultrafast non-equilibrium heat transport mechanism of multilayer thin films,and provide basic experimental data and theoretical guidance for the development of chip thermal management technology and ultrafast laser micromachining.