P. Storer

Condensed matter electron momentum spectrometer with parallel detection in energy and momentum

An electron momentum spectrometer has been constructed which measures electron binding energies and momenta by fully determining the kinematics of the incident, scattered, and ejected electrons resulting from (e,2e) ionizing collisions in a thin solid foil. The spectrometer operates with incident beam energies of 20–30 keV in an asymmetric, non‐coplanar scattering geometry. Bethe ridge kinematics are used which for 20 keV incident energy has scattered electron energies of 18.8 keV at a polar angle of θs=14°and azimuthal angles φs in the range from −18° to +18° and ejected electrons of 1.2 keV and θe=76°with φe=π±6°. The technique uses transmission through the target foil, but it is most sensitive to the surface from which the 1.2 keV electrons emerge, to a depth of about 2 nm. Scattered and ejected electron energies and azimuthal angles are detected in parallel using position sensitive detection, yielding true coincidence count rates of 6 Hz from a 5.5 nm thick evaporated carbon target and an incident bea...

(e,2e) Spectroscopy of solids with improved energy resolution

Abstract A brief overview of the (e,2e) technique as applied to solids is reported, including the spectrometer used in these studies. In particular, we describe how the energy resolution of our spectrometer has been improved by the addition of an electron monochromator for production of the incident electron beam. This monochromator is also discussed in some detail. Results obtained using the monochromated beam are compared with previous data collected with a standard electron gun source.

Electronic structure of amorphous Si measured by (e,2e) spectroscopy

Energy-resolved electron momentum densities are determined for a thin Si film evaporated onto a carbon foil. This is done by transmission (e,2e) spectroscopy, a technique that does not rely on crystal momentum and is therefore ideally suited for the study of amorphous materials. Spectra were collected with an energy resolution of 2 eV and a momentum resolution of 0.15 au (0.3 AA-1). The main feature disperses in a strikingly similar way to the crystalline ones. In addition to the dispersion the intensities of the peaks are obtained. In spite of having only a qualitative understanding of the shape of the spectra, the results of the comparison of measured amorphous momentum densities with calculated crystalline ones are reasonable. The basis of this agreement between amorphous solid and crystalline theory is discussed.

Direct imaging of the valence electronic structure of solids by (e,2e) spectroscopy

Abstract The spectral momentum density ϱ(E, q ) of the valence electrons of solid thin films of annealed amorphous carbon, amorphous silicon and silicon carbide has been measured using (e,2e) spectroscopy. Substantial contrast has been observed between the images of the three momentum densitites, which show not only well-defined energy band dispersion in the three materials, but also the antisymmetric gap due to the unequal potentials between the Si and C sites in the silicon carbide. The relation between the three momentum densities is explained within the framework of the one-electron band theory of solids.

Adsorbate wavefunction mapping by (e, 2e) spectroscopy

Abstract The effect of oxygen adsorption on the (e,2e) spectra of annealed carbon films has been investigated. There are two regions of extra intensity in the measured spectral momentum density. One at 31 eV below the vacuum level and a broad one near 11 eV. No dispersion is found for either of these contributions. The dependence of intensity on electron momentum is consistent with almost exclusively s character for the deepest level and a mixture of s and p character for the level closest to the Fermi level. These results are compared with the electronic structure of CO, O and graphite. The possibilities of the (e,2e) technique for the study of adsorbates are discussed.

Surface characterization of diamond-like amorphous carbon foils by (e,2e) spectroscopy and transmission electron energy loss spectroscopy

We have measured the spectral momentum densities of thin foils of diamond-like carbon using (e,2e) spectroscopy. Transmission electron energy loss spectra and (e,2e) spectra were measured before and after annealing a thin foil at around 900 degrees C and before and after thinning the foil using reactive ion etching in an argon-oxygen plasma. The valence band spectral momentum densities are compared with spherically averaged graphite and diamond band theory calculations. After annealing the surface sensitive (e,2e) data are closer to the graphite theory for the foil. Before annealing and also after plasma etching the (e,2e) data compare more favourably with the diamond theory. Bulk-sensitive transmission energy loss spectra for the annealed sample show a weak graphitic plasmon at around 6 eV energy loss which disappears after subsequent plasma etching. These measurements show that the diamond-like carbon films become graphitic only at the surface after annealing and that the graphitic surface layer can be easily removed by reactive ion etching.

Energy-resolved momentum density of amorphous germanium and the effect of hydrogen adsorption by (e,2e) spectroscopy

The energy-resolved momentum density ϱ(ϵ,q) of evaporated amorphous germanium has been studied using a surface-sensitive solid state (e,2e) spectrometer with estimated energy and momentum resolutions of about 2.0 eV and 0.15 a.u., respectively, and has been compared with a LMTO (linear muffin-tin orbitals) calculation for crystalline germanium. The density consists of two main features: one disperses upwards from around −13 eV at zero momentum to around −6 eV at a momentum value of 0.85 a.u., the other appears throughout the momentum values investigated (0 to 1.6 a.u.) and is confined within 7 eV below the valence band maximum. The former is identified as being due to the lower valence band of the germanium and agrees well with the LMTO calculation both in dispersion and in intensity, whereas only part of the intensity of the latter feature can be attributed to the upper valence bands. Hydrogen adsorption on the germanium surface reduces the intensity of the upper feature, most noticeably at momenta from 0.65 to 0.85 a.u., and introduces features around energies of −7 to −13 eV between momenta of 0.15 and 0.85 a.u. On this basis, contributions to the momentum density from the dangling bonds on the surface and those due to hydrogen adsorption are estimated. These results are discussed in association with early photoemission studies of the same material.

The nature of the background in transmission (e,2e) experiments

Abstract We present (e,2e) data of a graphitic carbon film over a large energy range (from 0 to 330 eV binding energy). This range contains both the valence band and the C 1s core level. Below the valence band there is always intensity, due to inelastic scattering effects. We discuss the momentum characteristics of this background. Finally we compare the intensity-ratio of the valence band features and the core level with the theoretically calculated one.

Dangling-bond surface states on an amorphous germanium surface as observed by (e,2e) spectroscopy

Abstract We have studied the energy-resolved momentum density of evaporated amorphous germanium and the effect of hydrogen adsorption using a surface-sensitive solid state (e,2e) spectrometer with estimated energy and momentum resolutions of about 2.0 eV and 0.15 au, respectively. Experimental evidence has been obtained which suggests the existence of dangling-bond surface states spreading from around −2 to −10 eV within a momentum range of 0.55 to 0.95 au in the valence-band region. Hydrogen-induced features are found at energies from −7 to −13 eV between momenta of 0.15 and 0.85 au.

(e,2e) momentum spectroscopy of thin films

Recent developments in (e,2e) momentum spectroscopy of thin films have resulted in the study of a diverse range of solid targets. These studies have revealed the electronic structure of solids in much more detail than has been previously available using this technique. A summary of the developments which have led up to this is presented here. Some details of a spectrometer that represents the state of the art are given. Recent results from this spectrometer are discussed.