Low-energy-electron probing depths in metals

Abstract

Thin-film techniques are used in conjunction with spin-polarized electron-energy-loss spectroscopy to measure directly the probing depth of low-energy ( -30 eV) electrons in metals. The data indicate the probing depth in molybdenum is small ( -I monolayer) but that it is significantly higher for copper (-3 monolayers). These differences are consistent with a model in which inelastic scattering is attributed to electron-hole pair excitation. Effects are also observed that might be interpreted in terms of scattering at the interface between substrate and overlayer.Spin-polarized-electron spectroscopies provide very powerful probes of the electronic and magnetic properties of surfaces and thin films. ' Here we report the results of a study of electron probing depths at metal surfaces and thin films using spin-polarized electron-energy-loss spectroscopy (SPEELS). In SPEELS a monoenergetic beam of polarized electrons is directed at the target surface and the polarization of scattered electrons is measured as a function of inelastic energy loss. Spin-flip inelastic scattering results when an incident electron falls into an unoccupied state above the Fermi level with the energy released being transferred to an electron of opposite spin in an occupied state below the Fermi level; the latter electron is ejected from the surface. ' For ferromagnetic materials such creation of an electron-hole pair of opposite spin is termed Stoner excitation.Recent SPEELS studies in this laboratory demonstrate that such spin-flip scattering events in paramagnetic materials manifest themselves in scattered-electron-polarization spectra that are characteristic of the target electronic structure, as will be discussed further below. Specifically, SPEELS studies of Mo(110) revealed a prominent polarization-loss feature, centered at an inelastic energy loss of -5 eV, while no significant polarization loss over that energy range was observed from Cu(100). This very difl'erent behavior provides a means to distinguish between electron scattering from copper and molybdenum; electrons that scatter inelastically from copper do so with very little loss of polarization, whereas those that scatter from molybdenum have a readily identifiable polarization-loss signature. Here we take advantage of this difference to measure directly the probing depth of low-energy electrons in copper and molybdenum by depositing thin molybdenum (copper) overlayers on a Cu(100) [Mo(110)] substrate and observing the appearance (disappearance) of the molybdenum polarization-loss feature. The probing depth is defined here as the thickness of the near-surface region from which (1 -e '), i.e. , -63%, of the detected inelastically scattered electrons originate. The data indicate that the probing depth in molybdenum is small, -l monolayer. The probing depth in copper is found to be significantly larger, -3 monolayers. EA'ects are also observed that are attributed to scattering at the interface between the substrate and overlayer.The present apparatus is shown schematically in Fig. ...