Experimental study of electron-phonon coupling and electron internal thermalization in epitaxially grown ultrathin copper films

Electron interaction in nanoscale metal structures is of major interest in understanding and designing nanoscale electronic devices. In this study, electron interaction in epitaxially grown 20-nm-thick copper film on Si(100) was studied using femtosecond pump-probe technique with tunable probe photon energy near the d-band to Fermi-level transition. It was found that probe-photon-energy dependence on the transient reflectivity signal can be divided into four regimes, and the borders of these regimes depend on the pump power. Based on a phenomenological model, the excited electrons were separated into nonthermalized and thermalized electrons, and it was seen that the probe-photon-energy dependence of the signal can be explained by the difference of the ratios of nonthermalized to thermalized electron components. Furthermore, it was found that each of these two components can be selectively excited by properly choosing the probe photon energy. Each electron-phonon (el-ph) coupling and electron internal thermalization time were investigated individually without interference from each other's processes. We show that el-ph coupling factor is larger in epitaxial copper film than the previously reported value in polycrystalline-copper film. In addition, the electron internal thermalization time is smaller than the reported value for the polycrystalline gold and silver films.