Ultrafast electron dynamics in Pb/Si(111) investigated by two-photon photoemission

Abstract

Ultrafast electron dynamics in Pb/Si͑111͒ quantum well structures have been investigated by femtosecond time-resolved two-photon photoemission spectroscopy. Three unoccupied quantum well states ͑or subbands͒ as well as an image potential state are identified and analyzed as a function of Pb thickness. Using 1.9 eV photon energy for optical excitation electron-hole pairs are formed in the Pb film and in the Si substrate. The hot electron population in the quantum well states exhibits a biexponential decay and both decay times increase when the energy difference to the Fermi level is reduced. The slower decay component with decay times of 130 and 900 fs is attributed to delayed filling of the states in the adlayer due to electronic relaxation in the Si͑111͒ substrate. The faster decay ͑30-140 fs͒ is caused by electron-electron scattering in the Pb film. At a first glance, the extracted decay times increase toward the Fermi level as predicted by the Fermi-liquid theory. However, by investigation of the quantized electronic structure more detailed insight into scattering among defined subbands has been gained. The rise of the excited electron population in the lowest unoccupied quantum well state is delayed by 70 fs which occurs concomitantly with a decay in the second lowest state. We describe this simultaneous decay and rise in the two adjacent subbands by coupled rate equations and attribute it to intersubband scattering. Thereby, we demonstrate the importance of intersubband decay in low-dimensional metallic systems in addition to intraband scattering processes considered in Fermi-liquid theory and derived descriptions.

Time-dependent electron phenomena at surfaces

“…Indeed, in good agreement with experimental findings, it can be shown that they oscillate in phase with the energy position of the lowest unoccupied QWS, as determined from calculations and time-resolved two-photon photoemission measurements (55). The behavior of both Γ e-e and Γ e-ph as a function of the film thickness shows that it is possible to tailor the lifetime of electronic excitations by means of thinfilm nanofabrication.…”

“…Another remarkable feature of the Pb overlayer systems is the Fermi-liquid behavior of the excited electrons starting from a single Pb monolayer (54,55). This is corroborated by relativistic calculations of the e-e contribution for bulk Pb, especially for the parent band of the Pb(111) QWSs, the bulk band which crosses E F along the ΓL direction.…”

Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering

“…However, the rich variety of unexpected phenomena which were observed has incited research also in temperature ranges far above the superconduction transition temperature T C of Pb. Examples of these observations are the strong dependence of the Schottky barrier on the exact atomic structure 6 , the formation of magic height islands 7 , anomalies in the superconductivity transition temperature [8][9][10] , oscillations of the work function and extremely long excited state life times 11,12 , and the observation of a Rashba-type spin splitting 13 . The origin of many of these phenomena lies in the peculiar electronic structure of Pb on Si(111) manifested in the high in-plane effective mass [14][15][16] .…”

Unoccupied electronic structure and momentum-dependent scattering dynamics in Pb/Si(557) nanowire arrays

“…The calculations furthermore identify a downward dispersion for the lowerlying state B, while the higher-lying state A appears to consist of different branches, one of them exhibiting a positive dispersion near k =¯ . From its binding energy with respect to the vacuum level, the uppermost signature A could in principle be originating from an n = 1 image potential state (IPS) located in front of the Pb/Si(557) surface [46,47]. This would be further supported by the free-electron-like effective masses found for this state.…”