Low-energy Electron Backscattering Research Articles

Electron range-energy relationships for calculating backscattering coefficients in elemental and compound semiconductors

Backscattering coefficients for electrons normally impinging on Si, Ge, GaN, GaAs and InSb targets have been calculated by using the Vicanek and Urbassek theory [M. Vicanek, H.M. Urbassek, Phys. Rev. B 44 (1991) 7234] for incident energies ≤5 keV. Electron range has been calculated from various semi-empirical analytical expressions. The cross-sections used to describe the electron transport are determined via the appropriate analytical expression given by Jablonski [A. Jablonski, Phys. Rev. B 58 (1998) 16470] whose new improved version has been recently reported by Rouabah et al. [Z. Rouabah, N. Bouarissa, C. Champion, N. Bouaouadja, Appl. Surf. Sci. 255 (2009) 6217]. The results may be seen as the first predictions for low-energy electron backscattering coefficients impinging on GaN, GaAs and InSb semiconductors. The models used in the calculation of the electron range affect both the accuracy and behaviour of the electron backscattering coefficients.

Low-energy electron transmission in a partially unzipped zigzag nanotube

Based on the nearest-neighbor tight-binding approximation, we present exact analytical expressions for electron transmission in nanotube/ribbon junctions, generated by incomplete unzipping of zigzag nanotubes. By assuming one-dimer-line difference in the widths of the leads, it is demonstrated that such a contact exhibits zero backscattering of low-energy electrons entering from the graphene side of the junction. We also show that a zigzag nanotube section sandwiched between two armchair graphene ribbons is completely transparent for incident low-energy electrons. Possible application of the results to nanosensor engineering is also included.

Low-energy electron backscattering by Ta surface

Using a hypocycloidal electron spectrometer, the processes of elastic and inelastic interaction of low-energy (0–5 eV) monoenergetic (FWHM ∼20–50 meV) electrons with polycrystalline Ta surface are studied. Energy dependences of elastic electron backscattering intensity are obtained as well as threshold spectra at various values of the residual electron energy Er(0, 0.1, 0.2, 0.7 eV). A fine structure in the threshold spectra in the range 1–3.5 eV is revealed, resulting most likely from residual gas absorption.

Kinetics of residual gas adsorption on Ge(111) surface in low-energy electron backscattering

Low-energy (0.4–1.2 eV) electron backscattering is applied for the investigation of kinetics of residual gas adsorption effect on the concentration and energy positions of surface electron states of Ge(1 1 1) surface. Chemosorption of residual gas molecules on Ge(1 1 1) at P ∼ 10 −7 Pa and room temperature is shown to be most active during the first 48 h. Low concentration of dangling valence bonds on the reconstructed Ge(1 1 1) (2 × 8) surface is shown to determine its low activity to chemosorption.

Backscattering of low energy electrons from carbon films deposited on aluminum: A Monte Carlo simulation

The low energy backscattering coefficient for the special case of film of carbon deposited on aluminum is investigated. The backscattering coefficient as a function of the primary energy shows an interesting property, i.e., the appearance—for carbon film thicknesses higher than ∼100 Å on aluminum—of relative maxima of absorption (and relative minima of backscattering). It has been noted, for aluminum substrates, that the position of the relative minima of the backscattering coefficient is increasing as the carbon film thickness increases. If experimentally confirmed and quantitatively accurately defined, the results presented—here obtained through a Monte Carlo simulation—can be used as a way to measure the carbon film thickness by a set of measures of the backscattering coefficient.

Low-energy electron spectroscopy of Si(1 0 0) and Ge(1 0 0) surfaces

Using a high-resolution (Δ E⩽40 meV) low-energy electron backscattering technique, the energy loss spectra at various energies of the primary electron beam ( E p=0.1–3 eV) for mirror-polished Si(1 0 0) and Ge(1 0 0) surfaces are studied. Our results on the bulk electron states are in good agreement with the known experimental data, obtained from photoemission spectroscopy, electron energy loss spectroscopy as well as bulk electronic structure of silicon and germanium, calculated theoretically by orthogonalized plane wave, local pseudopotential and GWA methods. The presence of the known surface electron states for Si(1 0 0) and Ge(1 0 0) is confirmed and their energy positions are specified more precisely.

The specific features of low-energy electron backscattering from different Ge surfaces

Low-energy electron backscattering technique using a hypocycloidal electron spectrometer is applied to measure energy loss spectra at different incident electron energies from mirror-polished (111), (100) and (110) surfaces of Ge. The obtained results confirm and complement the available data on the energy position of the extrema of the density of both filled (below EF) and empty (above EF) surface and bulk electron states in the reduced Brillouin zone for different surfaces of germanium.

Low-energy electron backscattering spectroscopy of Mg/Mo films

Using a low-energy (0–5 eV) electron backscattering technique based on a hypocycloidal electron spectrometer with high energy (∼50 meV) and angular (∼1–5°) analyzer resolution, the specific features of the energy distribution of the density of electron states in magnesium films are investigated. The proposed technique enables not only the details of the distribution of the filled electron states density of as-prepared films, but also the evolution of surface electron states in process of film exposure to the residual-gas atmosphere to be studied.

Investigation of surface and bulk electronic structure of Ge(1 1 1) by low-energy electron backscattering technique

Electron excitation of surface electronic transitions and bulk interband transitions for Ge(1 1 1)-c(2×8) are studied by low-energy (0–3 eV) electron backscattering technique using a hypocycloidal electron spectrometer with high-energy resolution of the analyser (∼50 meV). The obtained results are in rather good agreement with the experimental data of other authors and with theoretical calculations of germanium structure performed by orthogonalized plane wave method. Additional data on the energy distribution of surface electronic states on the Ge(1 1 1)-c(2×8) surface are obtained.

The use of electron backscattering as fast imaging technique with molecular beams

Electron backscattering has been performed using a Centaurus detector, developed especially for low-energy electron backscattering. A H 2 + beam of energy 5 MeV was used, with each proton having 2.5 MeV, and the accompanying electron having an energy of 1.3 keV. The backscattered electrons were measured simultaneously with the particle-induced X-ray emission (PIXE) and Rutherford backscattering (RBS) signals, as complementary information. Results show that the electron backscattering signals indeed offer elemental information in imaging at least an order of magnitude faster than PIXE. This may result in novel applications in imaging of geological grains, with subsequent trace element analysis.

Study of the electronic spectra of noble metals by low-energy electron backscattering

The density of filled electronic states of noble metals (Au, Ag, Cu) below the Fermi level is studied by low-energy electron backscattering spectroscopy (0–10 eV) by using a specially designed hypocycloidal electron spectrometer capable of high energy (≤50 meV) and angular (∼1°–5°) resolution. The features observed in the electron scattering spectra are established to agree well with the extrema of the theoretically calculated density-of-states distributions. The results obtained amplify substantially the information provided by UV and x-ray photoelectron spectroscopy.

Effect of tissue inhomogeneity on dose distribution of point sources of low‐energy electrons

Perturbation in dose distributions of point sources of low-energy electrons at planar interfaces of cortical bone (CB) and red marrow (RM) was investigated experimentally and by Monte Carlo codes EGS and the TIGER series. Ultrathin LiF thermoluminescent dosimeters were used to measure the dose distributions of point sources of 204Tl and 147Pm in RM. When the point sources were at 12 mg/cm2 from a planar interface of CB and RM equivalent plastics, dose enhancement ratios in RM averaged over the region 0-12 mg/cm2 from the interface were measured to be 1.08 +/- 0.03 (SE) and 1.03 +/- 0.03 (SE) for 204Tl and 147Pm, respectively. The Monte Carlo codes predicted 1.05 +/- 0.02 and 1.01 +/- 0.02 for the two nuclides, respectively. However, EGS gave consistently 3% higher dose in the dose scoring region than the TIGER series when point sources of monoenergetic electrons up to 0.75 MeV energy were considered in the homogeneous RM situation or in the CB and RM heterogeneous situation. By means of the TIGER series, it was demonstrated that aluminum, which is normally assumed to be equivalent to CB in radiation dosimetry, leads to an overestimation of backscattering of low-energy electrons in soft tissue at a CB-soft-tissue interface by as much as a factor of 2.

Electronic excitation and secondary electron emission studies by low-energy electrons backscattered from thin polystyrene film surfaces

Thin atactic polystyrene films were studied by backscattering of low-energy electrons. Energy losses corresponding to the excitation in the film of triplet and singlet electronic excitation of the pendant groups are observed. The variation of the corresponding cross sections with incident electron energy are in good agreement with theoretical calculations for benzene. The variation of the backscattered intensity integrated over electron energy in a given direction, yields a ionization threshold of about 7 eV and shows evidence for structure of the valence band.

Inelastic electron scattering in nickel

Using a 200 keV electron spectrometer, with an energy resolution of ∼0.25 eV and a momentum resolution of ∼0.2 A -1, we have measured the energy loss spectra for transmission of electrons through thin (∼600 Å) Ni films. These results address the general question of the validity of momentum transfer estimates in electron loss scattering. Using low-energy electron backscattering, we have observed the dipole forbidden M 1 transition at 112 eV. For high-energy scattering, we have observed this transition only at high momentum transfer ( q⩾ 2 A -1). These results indicates sizable contributions from high momentum transfer collisions in the low-energy experiments.

The backscattering of low-energy electrons from carbon

An experimental study of the backscattering of electrons from carbon in the incident electron energy range 0.5-10 keV, was conducted. The backscattering coefficient eta is found to decrease with increasing energy throughout the investigated energy range. It is demonstrated that Everhart's model (1960) in conjunction with an experimental range-energy relationship, leads to calculated values of eta in satisfactory agreement with experimental results.

An experimental study of low energy electron backscattering in the atmosphere

The backscatter process for low energy electrons in the upper atmosphere is studied, and it is shown how variations in the pitch-angle and energy distributions of these particles may affect the backscatter ratio and also the electron component of the upward Birkeland current. “Softening” of the electron energy spectrum combined with a field-alignment of the electron fluxes may enhance the current carried by the greater than 200 eV energy electrons significantly.

Contribution of the electrostatic potential to the leed intensities at alkali-halide distorted surfaces

The electrostatic fields which are associated with structural distortions at the free (100) surface of an alkali-halide crystal could provide an important contribution to the backscattering of low-energy electrons. A model calculation is reported here for the case of NaCl, NaBr, and NaI surfaces.

Enhanced X-ray emission from extinction contours in a single-crystal gold film

Abstract Experiment has confirmed the theoretical prediction that extinction contours in the electron microscope image of single-crystal metal films should be associated with enhanced x-ray emission. An instrument combining the facilities of an electron microscope and x-ray scanning micro-analyser has been used to produce images of a 200 A single-crystal gold film which demonstrate this correlation. Increased backscattering of low energy electrons was also observed from these regions.