Band alignment and defects influence the electron-phonon heat transport mechanisms across metal interfaces

Abstract

We report on the experimental determination of electron-electron conductance at Au/TiOx interfacial regions and electron-phonon coupling of thin TiOx layers for x = 0-2.62. Our study demonstrates that the electronic energy transport mechanisms at metal/metal oxide interfaces are enhanced through metallic defects that lead to electronic band alignment between the metal and metal oxide (in our case, Au and TiOx). Electronic heat transport processes are interrogated via a pump/probe technique, utilizing sub-picosecond laser pulses to monitor the ultrafast thermoreflectance responses of Au/TiOx systems, which were analyzed using a two-temperature model to extract electron-electron conductances at Au/TiOx interfaces and the electron-phonon coupling in TiOx layers. We find that TiOx stoichiometries near TiO2 have ultrahigh electron-phonon coupling factors similar to that of pure Ti and that electronic energy transmission from Au to TiOx layers is comparable to that of Au to Ti due to the presence of Ti0 defects. For x = 2.62 in TiOx, electron-phonon coupling is reduced by more than a factor of 5. Our experimental data are corroborated by real-time time-dependent density functional theory calculations, which show that excited electrons in Au do not participate in the TiOx phonon relaxation process, resulting in lower electron-electron energy transmission from Au and electron-phonon coupling due to the difference in the Fermi energy of Au relative to the conduction band minimum of TiOx when x >2.