Dispersion and Intrinsic Width of Image Resonances Measured by Resonant Inelastic Electron Scattering: Theαphase of Pb/Ge(111)

Abstract

Evidence of image states on semiconductor surfaces has been very scanty so far. We find that resonant scattering through image resonances totally dominates the electron energy loss spectra of the a phase of Pb/Ge(111) if the energy and the parallel momentum of the scattered electron are in the gap of the bulk bands projected onto the ͑1 3 1͒ surface Brillouin zone. The high energy and momentum resolution obtainable allows a detailed study of the interaction of these excited states with the substrate.[S0031-9007(98)08135-6] PACS numbers: 73.20.At, 79.20.Kz Surface electronic states are of great importance in the physics of solid surfaces. While most of the work is usually focused on filled states, the interest on empty states is increasing. The potential that an electron feels outside a solid surface has a long-range Coulombic tail (image potential) that gives rise to a particular series of empty surface states and surface resonances, the image states [1]. They are similar to the Rydberg states of an atom or molecule, and their wave functions extend tens to hundreds of angstroms from the surface into the vacuum. Because of their simplicity, they are of general interest since they provide a useful model to study the interaction of the electrons in excited surface states with the underlying substrate [2]. This interaction governs the lifetime of the states and therefore cross-section and branching ratios of all electronically induced adsorbate reaction at surfaces (e.g., desorption and dissociation processes) and influences the surface reactivity and the growth kinetics.In this Letter we show that resonant inelastic electron scattering through image states [3] (i.e., selective adsorption of low energy electrons into an image potential state followed by inelastic scattering and desorption) can provide important information on the interaction between these states and the substrate. It allows one to measure the momentum dispersion and the intrinsic width of imagepotential resonances of semiconductor surfaces in energy and momentum regions not reachable by other techniques and with high energy resolution together with good momentum resolution. To our knowledge this scattering mechanism has been observed on Ag surfaces only [3].Energy dispersions and lifetimes of the image states and image resonances (IR) have been studied mainly by inverse photoemission and by two-photon photoelectron spectroscopy on metal surfaces [4]. On the contrary, very little is known of these states on the semiconductor surfaces. Only data or calculations for GaP(110) [5]. In͞Si͑1 3 1͒R30 ± [6], and Bi͞GaAs(110) [7] have been reported.Here we present clear data on a different semiconductor surface. Both on metal and on semiconductor surfaces there is no information on the intrinsic width of the IR near the edges of the surface Brillouin zone a few eV above the vacuum level (see, e.g., Fig. 4 below) because these states are accessible only by inverse photo-emission with an energy resolution not better than 0.25 eV. We demonstrate that our techn...

Dynamical fluctuations and the $\surd$3 $times$ $\surd$3 $\leftrightarrow$ 3 $times$ 3 transition in $\alpha$-Sn/Ge(111) and Sn/Si(111)

“…The presence of a discrete loss onset in the LT spectra was interpreted as an indication of the opening of a small band gap (E g 0.065 eV at 100 K) [13]. However, a subsequent EELS study by Petaccia et al [48] shows that the important role of image resonances in the EELS spectra of this system makes its interpretation very difficult and, thus, challenges the direct association of the discrete loss onset in the LT spectra with a metal-insulator transition. A similar study on the Sn/Ge(111) [14] shows no difference between the RT and LT EELS spectra, indicating that the system remains metallic across the transition.…”

Theory of inelastic lifetimes of low-energy electrons in metals

“…Very recently, it has been shown that selective adsorption of low-energy electrons into an image-potential state, followed by inelastic scattering and desorption, can provide information on the interaction between these states and the substrate. 170 A deep theoretical analysis of this interaction, as well as the role of the substrate/adsorbate band structure, is still lacking and is needed in order to interpret the experimental data.…”

Image resonance in the many-body density of states at a metal surface

“…[7][8][9][10] IPI states are present in systems where a bulk band gap provides a barrier, trapping electrons in the image tail of the surface potential. If no gap is present at the IPI energies, an electron is not reflected completely at the bulk barrier, and hybridization with surface truncated bulk states becomes possible.…”

“…Regarding excited states, considerable experimental interest has been devoted to image-potential induced (IPI) states 3,4,5,6 and resonances. 7,8,9,10 IPI states are present in systems where a bulk band gap provides a barrier, trapping electrons in the image tail of the surface potential. If no gap is present at the IPI energies, an electron is not reflected completely at the bulk barrier and hybridization with surface truncated bulk states becomes possible.…”