Vladimir Sashin

Electronic Band Structure of Beryllium Oxide

The energy–momentum resolved valence band structure of beryllium oxide has been measured by electron momentum spectroscopy (EMS). Band dispersions, bandwidths and intervalence bandgap, electron momentum density (EMD) and density of occupied states have been extracted from the EMS data. The experimental results are compared with band structure calculations performed within the full potential linear muffin-tin orbital approximation. Our experimental bandwidths of 2.1 ± 0.2 and 4.8 ± 0.3 eV for the oxygen s and p bands, respectively, are in accord with theoretical predictions, as is the s-band EMD after background subtraction. Contrary to the calculations, however, the measured p-band EMD shows large intensity at the Γ point. The measured full valence bandwidth of 19.4 ± 0.3 eV is at least 1.4 eV larger than the theory. The experiment also finds a significantly higher value for the p-to-s-band EMD ratio in a broad momentum range compared to the theory.

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.

The valence band structures of BeO, MgO, and CaO

We have performed direct measurements of the valence band structures of the light alkaline earth oxides BeO, MgO, and CaO using electron momentum spectroscopy (EMS). From these measurements, we have determined the band dispersions, valence bandwidths, and O(2s)–O(2p) intervalence bandgaps at the Γ point. For comparison we have also performed Hartree–Fock (HF) and density-functional (DFT) calculations in the linear combination of atomic orbitals (LCAO) approximation. Intervalence bandgaps compare reasonably well with the DFT calculations and previous experimental and theoretical studies. Our measured bandwidths, however, are significantly smaller. In particular, we find that contrary to conventional wisdom, the local density approximation of DFT overestimates the valence bandwidths of these ionic solids.

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.

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.

Conduction band electronic structure of metallic beryllium

We have measured the bulk energy-momentum-resolved density of the conduction band of metallic beryllium by means of electron momentum spectroscopy. From the data we have determined the band dispersion, occupied bandwidth, electron momentum density and density of states. The experimental results are compared with theoretical band-structure calculations performed within the full-potential linear muffin-tin orbital (FP-LMTO) approximation. There is good agreement between experiment and theory for the shape and intensity of the conduction band provided multiple-scattering and hole lifetime effects are included. The measured occupied bandwidth is 11.15±0.15 eV, which is larger than that predicted by our LMTO calculation, but agrees well with previous experimental and theoretical data. The experiment also reveals that the band dispersion is narrower in momentum compared to theory, the difference reaching as much as 0.15 au near the free-electron Fermi momentum.

Spectral momentum densities in matter determined by electron scattering.

In electron momentum spectroscopy (EMS), an incoming energetic electron (50 keV in this work) ionizes the target and the scattered and ejected electrons are detected in coincidence (at energies near 25 keV). From the energy and momentum of the detected particles, the energy omega and momentum q transferred to the target can be inferred. The observed intensity distribution I(omega, q) is proportional to the spectral momentum density of the target and hence provides a direct challenge to many-body theoretical descriptions of condensed matter. This is illustrated by comparing some many-body calculations with EMS measurements on graphite and polycrystalline aluminium.

Anisotropy and Correlation Effects in the Spectral Function of Graphite as measured by Electron Momentum Spectroscopy

Electron momentum spectroscopy measurements of graphite single‐crystals are presented. Data were taken for incoming electrons with an energy of 50 keV, and both outgoing electrons with an energy near 25 keV. Spectra are presented for the major symmetry directions of graphite. To remove the effect of inelastic multiple scattering we use in all cases an identical deconvolution procedure, which consistently removed all intensity at high energy loss values. However the intensity becomes vanishing small only for binding energies of about twice the bandwidth. The shape of the observed spectra compare well with many‐body calculations based on the cumulant expansion scheme but the intensity at high momentum is less than predicted by this theory.