Surface Intervalley Scattering on GaAs(110): Direct Observation with Picosecond Laser Photoemission

Abstract

Angle-resolved laser photoemission investigations of the laser-excited GaAs(llO) surface have revealed a previously unobserved valley of the C3 unoccupied surface band whose minimum is at X in the surface Brillouin zone. Electron population in this valley increases only as a result of scattering from the directly photoexcited valley at T. With high momentum resolution, we have isolated the dynamic electron population changes at both r and X and deduced the scattering time between the two valleys.PACS numbers: 73.25,+i, 78.47.+p, 79.60.Eq The two-dimensional surface electronic band structure of semiconductors has attracted great interest due to striking deviations from that observed in the bulk. Termination of the bulk crystal and surface atomic rearrangements can give rise to band-gap states and resonances in both the valence and conduction bands. In the particular case of GaAs(llO) a body of work exists in which bands of the surface have been observed and mapped via photoemission and inverse photoemission, yet little is known about surface electronic dynamics, i.e., energy relaxation and scattering rates between surface states. l " 3 Studies of electron dynamics require the population and subsequent probing of normally unoccupied states, i.e., the conduction states of the system, on time scales relevant to energy and momentum relaxation times. In the bulk of semiconductors, such as GaAs, subpicosecond optical probes have been used extensively to investigate electronic relaxation rates and intervalley scattering. 4,5 The extension of such investigations to surfaces enables the study of electronic transport in fundamentally two-dimensional systems. In this Letter we report on the first such studies of electronic energy relaxation and intervalley scattering at a laser-excited semiconductor surface. Using a novel laser photoemission system with < 1-ps time resolution to perform a series of excite-probe experiments, we have directly observed a strong electronic emission from a transiently populated band whose minimum is located at X in the surface Brillouin zone (SBZ) of GaAsG 10)._ While the existence of this surface-band minimum at X, labeled C3, has been suggested from self-consistent pseudopotential calculations, 6 it has not been previously observed. Tightbinding calculations do not predict a band within the surface gap at X. 7 Since the band gap at the X location in the SBZ is significantly larger than the exciting photon energy of our experiment this surface band can only be populated indirectly, from F. By combining a picosecond laser photoemission probe with high electron momentum resolution, we have compared, in the same experiment, the time dependence of electron populations in the two valleys. This represents the first time direct surface intervalley scattering has been observed.Our photoemission studies are carried out as a series of excite-probe experiments. The excitation is an intense ( -300 /iJ/cm 2 ) pulse of red (1.78 eV) light resulting in a carrier density of ~(l-3)x 10 19 /cm 3 at the surface...

Coherent surface phonon at aGaAs(100)−c(8×2)surface

“…Haight et al have reported the electron dynamics in the surface states of GaAs͑110͒ by time-resolved two photon photoemission spectroscopy. 25 The excited electrons in the surface states decay in a few picoseconds. Since the similar carrier dynamics in the surface states of GaAs(100)-c(8ϫ2) can be expected, the photocarriers in the surface states survive sufficiently long to affect the surface phonon frequency.…”

Time‐Resolved Photoelectron Spectroscopy at Surfaces Using Femtosecond XUV Pulses

“…The first time-resolved photoemission experiments using short-pulse XUV sources focused on the thermalization dynamics of hot carriers in semiconductor such as InP (110), GaAs, Au/GaAs (110), and As-terminated Ge(111) (for an overview of these studies, see Ref. [109]).…”

“…The higher energy probe photons in the XUV regime can remove the photoelectrons from this background to give a clear spectral distribution. Figure 21.16 summarizes the results of a time-resolved electron relaxation study from a GaAs(110) surface using high harmonic light of 10.7 eV [110]. For semiconductors, the phase space for inelastic carrier decay in the excitation energy regime close to the conducting band minimum, as addressed in this work, is quite limited and governed by the electron-lattice interaction (electron-phonon scattering).…”

“…16 Pump-probe photoemission spectra from cleaved GaAs(110). The bottom spectrum was collected at an emission angle of 34 relative to the surface normal in the CÀ X direction.…”

Two photon photoemission of deposited silver clusters

“…Time-resolved pump-probe techniques of this kind combined with 2PPE spectroscopy using ultrashort laser a e-mail: busolt@physik.fu-berlin.de pulses have been applied to study carrier dynamics in semiconductors [7][8][9][10] and metals [11][12][13][14][15], adsorbate-substrate charge transfer [16,17] and electron dynamics of imagepotential states [13,[18][19][20].…”