S. A. Canney

Electronic band structure of magnesium and magnesium oxide: experiment and theory

Electron momentum spectroscopy (EMS) has been used to measure the valence band electronic structure of thin magnesium and magnesium oxide films. The band structures have also been calculated within the linear muffin-tin orbital (LMTO) approximation. The free-electron-like parabola characteristic of metallic solids was observed for magnesium with a bandwidth of approximately 6 eV, in agreement with previous measurements. The inclusion of energy broadening due to finite hole-lifetime effects and a Monte Carlo simulation of multiple scattering events gives good agreement between calculated and measured band structures. However, we measure a much higher intensity due to plasmon excitation compared with the simulated intensity. Upon oxidation the valence structure splits into two distinct, less dispersive bands typical of an ionic solid. Intensity due to plasmon excitation was almost completely absent in the experimental spectra for magnesium oxide. The LMTO calculation reproduces the overall structure and dispersion range of the oxide. The measured and calculated energy gap between upper and lower valence bands and their relative intensities do not agree quantitatively. This discrepancy may be due to a contribution of magnesium s states to the predominantly oxygen p states in the upper band.

Determination of the energy-momentum densities of aluminium by electron momentum spectroscopy

The energy-resolved momentum densities of thin polycrystalline aluminium films have been measured using electron momentum spectroscopy (EMS), for both the valence band and the outer core levels. The spectrometer used for these measurements has energy and momentum resolutions of around 1.0 eV and 0.15 atomic units, respectively. These measurements should, in principle, describe the electronic structure of the film very quantitatively, i.e. the dispersion and the intensity can be compared directly with theoretical spectral momentum densities for both the valence band and the outer core levels. Multiple scattering is found to hamper the interpretation somewhat. The core-level intensity distribution was studied with the main purpose of setting upper bounds on these multiple-scattering effects. Using this information we wish to obtain a full understanding of the valence band spectra using different theoretical models of the spectral function. These theoretical models differ significantly and only the cumulant expansion calculation that takes the crystal lattice into account seems to describe the data reasonably well.

Preparation of a 10 nm thick single-crystal silicon membrane self-supporting over a diameter of 1 mm

Abstract We report the fabrication of a 10 nm thick, self-supporting, single-crystal silicon membrane. The fabrication process can be broken up into four major stages. First, a buried SiO2 layer was formed by implantation of oxygen at a depth of 200 nm into a (100) silicon wafer. The size of the membrane was then established by removing the bulk of the silicon over a 1 mm area using a fast acid etch. After this the sample was etched in a hot EDP solution which stops at the buried SiO2 layer. The sample was then cleaned and the SiO2 layers removed, after which it was introduced into a plasma-etching chamber. The membrane was thinned down to a final thickness of 10 nm by RF plasma etching in a gas mixture of carbon tetrafluoride and oxygen. The thickness was monitored during plasma etching by measuring the intensity of He–Ne laser light transmitted through the membrane. The electron energy loss spectrum of the membrane has been measured and shows two features due to single and double plasmon excitation. The plasmon energy was 17.05 eV, in good agreement with previous measurements. Membrane thickness has also been estimated from the area of the plasmon energy loss peak. The final sample had good crystalline quality, was of even thickness over the membrane diameter and showed only a small amount of surface contamination due to the plasma etching stage.

(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.

Energy-resolved momentum densities for the valence band of a nanoscale Si single crystal

We have measured the energy- and momentum-resolved band structure, and ground state of occupation of the bands, for a crystalline silicon sample along the 100 and 110 directions. Band structures were determined directly by the technique of electron momentum spectroscopy (EMS) for a self-supporting Si membrane with a thickness of approximately 7 nm. We compare our experimental results with ab initio calculations for bulk crystalline silicon performed within the linear muffin tin orbital approximation. Qualitative agreement is seen between experiment and theory for the main valence band peak. Additional intensity is observed in the measurement on either side of the main peak and is attributed mainly to multiple-scattering events. Satellite structure could also be present in these additional features, although there is no direct evidence for this.

Electron momentum spectroscopy studies on the oxidation of aluminium

Electron momentum spectroscopy has been used to measure the electronic structure of aluminium after exposure to increasing amounts of oxygen. The experimental valence band results showed the structural features of aluminium oxide beginning to dominate the free electron contribution of metallic aluminium as the exposure increased. The interpretation of this is that the aluminium oxide layer increases in thickness as the exposure increases, eventually saturating at a thickness of about 15 A. After subtracting the intensity of the underlying Al metal contribution from the oxidised states the valence band features of aluminium oxide were resolved even after exposure to only 600 L of oxygen. It is estimated that only about 5 A of aluminium oxide is formed at this stage. A quantitative comparison was made to a linear muffin-tin orbital (LMTO) calculation for spherically averaged α-Al2O3. Best agreement was obtained for the results of the lowest oxygen exposure (600 L). The chemical shifts of the Al 2p core level were also measured in separate experiments for two intermediate oxygen exposures between the aluminium and aluminium oxide cases for further characterisation. These measurements show that electron momentum spectroscopy is able to obtain the electron bands and the momentum densities of very thin disordered surface layers.

Preparation of ultrathin free-standing targets for (e,2e) spectroscopy

We describe in detail the procedures used for the preparation of ultrathin (∼10 nm) free-standing membranes for (e,2e) spectroscopy. Such a thin target is needed to minimize electron multiple scattering before and after an (e,2e) event. The development of a rf plasma source which allows in situ thinning and thickness monitoring is of key importance to the success of the target preparation. Materials (C, Si, Ni, Cu, Al2O3, SiO2, CuO) with different properties and structures are usually prepared in different ways. For insulating targets it is important to have a conducting sublayer to avoid the charging problem. A well prepared target usually has a thin area larger than the (e,2e) beam size (∼0.2 mm in diameter) and yields high quality (e,2e) data from which the electron energy-momentum density in a chosen direction is determined. Efforts demonstrated in this article indicate that the preparation of ultrathin free-standing films is a challenging area where significant technical development is needed.

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.

(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.