Low-energy Electron Transmission Experiments Research Articles

Low-energy electron transmission experiments on graphite

Low-energy-electron transmission (LEET) spectra were measured for graphite using electron energies below 30 eV. The observed LEET spectra have broadened square-wave-like features, and comparison with the conduction-band density of states above the vacuum level measured by ultraviolet photoemission and inverse photoemission spectroscopies indicated that the conduction-band density of states was not observed in the LEET spectra except band gaps. It is concluded that band gaps are more strongly reflected in LEET spectra than other features in the density-of-states. It is expected that electron-interference effects along the surface normal dominate LEET features, even for a very thick sample, where the energy dependence of electron-transmission probability through a one-dimensional periodic potential along the surface normal does not reflect the variation of the density of states (except band gaps).

Hot-electron transport properties of rare-gas solids derived from low-energy electron transmission experiments

The results of the analysis of available experimental low-energy electron transmission (LEET) data on thin solid rare-gas films deposited on a metal substrate are presented. The analysis is carried out on the basis of a most recent semiclassical electron transport model. Transport parameters inferred from the experiments are: (i) the entrance probability of the incident electrons at the vacuum-film interface; (ii) the total (elastic and inelastic) electron scattering mean free path (MFP); and (iii) the probability of inelastic scattering in the bulk of the studied materials (namely solid argon, krypton, and xenon). Comparisons made with recent energy band-structure calculations reveal that both the entrance probability and the inverse of the total MFP (or the total scattering frequency per unit pathlength) are highly correlated with the conduction-band density of states of these materials. A comparison of two sets of experimental data obtained for solid argon under different conditions of preparation of the metallic surface illustrates the reliability of our analysis to infer electron transport parameters from LEET experiments.

Hot-electron transmission through thin amorphous films of tetratetracontane: Effects of the density of gap states on the band-gap current and its anomalous temperature dependence.

Low-energy electron transmission experiments were performed for two types of amorphous films (solid amorphous and molten amorphous films) of tetratetracontane [${\mathrm{CH}}_{3}$(${\mathrm{CH}}_{2}$${)}_{42}$${\mathrm{CH}}_{3}$]. The temperature dependence of the transmission spectra and the spectral changes due to the amorphous-to-crystal transition of the films were measured. We observed that (1) the transmitted current through the band gap did not show a large decrease by the solid-amorphous--to--crystal change, and (2) for crystalline films it decreased abruptly and almost disappeared at a temperature just below the melting point, while the electron transmission through the conduction bands existed clearly even at this temperature. These results indicate that the gap current cannot be explained by the electron inelastic scattering to the localized gap states originating in the crystalline defects followed by a diffusion through the gap states. Further, we found a large difference between the spectra of the solid amorphous and molten amorphous films. The solid amorphous film showed a negative electron affinity of -0.1 eV, while the molten amorphous film gave positive electron affinity.

Electron affinity and structure of Langmuir-Blodgett films of cadmium arachidate by means of low-energy electron transmission

The results of low-energy electron transmission experiments on Langmuir-Blodgett (LB) films of cadmium arachidate deposited on copper substrates precoated with ferric stearate are described. The energy location of the bottom of the conduction band ( V 0) for the ordered alkyl tails in the LB film was directly determined to be 0.46±0.02 eV above the vacuum level at −120 to −130°C, that is, the electron affinity ( V A = − V 0) is −0.46±0.02eV. Further, it is demonstrated that the band structure is significantly blurred by the thermal excitation of molecular vibrations and by poor crystallinity. The crystallinity of the films is dependent on the number of layers deposited, and the results are consistent with those obtained by angle-resolved photoemission experiments. The present results strongly suggest that the electron affinities (2.25 and 2.3 eV) obtained by other workers for a metal/monolayer/metal junction, using fatty acid salts, are not those of the ordered parts of the alkyl tails.

Analysis of low-energy (1–10 ev) electron transmission spectra of thin solid krypton films

We present and analyse the results of low-energy electron transmission experiments performed with solid krypton films deposited on a platinum substrate at 20 K. The film thicknesses range from about 0.3 to 10 nm. The analysis of the variation of the transmitted electron current with film thickness is done with the help of a probabilistic model of particle transport in the energy region (1–10 eV) where the electron scatterings are quasi-elastic. This analysis allows the determination of the entrance probability of electrons and their mean free path in the films. The entrance probability is found to reflect the electronic conduction-band structure of solid krypton. The electron mean free path values that we obtain are large for low electron energies (about 20 nm at 1 eV) and decrease rapidly to about 2 nm for energies larger than 3.5 eV.

Hot-electron transport through thin dielectric films: Boltzmann theory and electron spectroscopy.

The transport problem of hot quasifree electrons through thin planar dielectric films of thickness d is considered by solving the steady-state Boltzmann equation for the electron current density J(E,\ensuremath{\mu},x), 0\ensuremath{\le}x\ensuremath{\le}d, within the film. The current density of injected electrons at x=0, ${J}^{(i)}$(E,\ensuremath{\mu}), evolves to J(E,\ensuremath{\mu},d), the current density of electrons escaping the layer at x=d, which is calculated.In our approach, electron transport is assumed to be controlled simultaneously by energy-dependent elastic and inelastic scattering with scattering rates ${\ensuremath{\gamma}}^{\mathrm{el}(\mathrm{E})}$ and ${\ensuremath{\gamma}}^{\mathrm{inel}(\mathrm{E})}$, respectively. For inelastic scattering, one single inelastic scattering process with energy loss \ensuremath{\Delta}E is assumed. With this assumption, the Boltzmann equation transforms into a set of coupled integro-differential equations which describe the cascading down of hot electrons in discrete energy steps \ensuremath{\Delta}E during solid-state transport. The resulting cascade problem is then solved analytically in the two-flux approximation. The transport of zero-energy-loss electrons in the topmost energy interval \ensuremath{\Delta}E at the maximum energy ${E}_{0}$ is considered separately. In this interval, no in-scattering from higher energies occurs and the Boltzmann equation for J(${E}_{0}$,\ensuremath{\mu},x) becomes energy decoupled. An exact formal solution of the zero-energy-loss transport problem is presented and exact and approximate results for the two limiting cases of pure elastic and pure inelastic scattering are compared. Our theoretical results are then applied to typical electron transmission experiments such as internal photoemission for transport analysis (IPTA), low-energy electron transmission (LEET), and x-ray photoelectron spectroscopy (XPS) in the substrate-overlayer configuration.It is shown that energy-dependent elastic and inelastic scattering rates in wide-band-gap insulators can be extracted from IPTA and LEET experiments with our theoretical analysis. Using these results, the experimentally determined J(E,\ensuremath{\mu},d) can be reconstructed. Our analysis also allows for a rigorous definition of XPS escape lengths in terms of scattering rates.

Conduction-band density of states of solid argon

The electronic conduction-band density of states (CB-DOS) of solid argon has been calculated using the linear analytic tetrahedron method. Energy bands at 240 points defining a cubic grid of the irreducible wedge of the Brillouin zone have been calculated by the augmented-plane-wave method using a first-principles, norm-conserving pseudopotential for argon. The calculated CB-DOS has been compared with the CB-DOS spectrum obtained from the analysis of recently measured low-energy electron transmission (LEET) data for solid argon films. A very good agreement is found between the CB-DOS spectra, which provides a physical insight into the origin of the maxima and minima of current observed in LEET experiments in the elastic scattering region.

Probabilistic description of particle transport. II. Analysis of low-energy electron transmission through thin solid Xe and N2 films.

The probabilistic description of quasielastic particle transport given in a previous paper is used to analyze the results of low-energy electron transmission experiments on thin solid xenon and molecular nitrogen films deposited on a metal substrate. Values of the entrance probabilities of the incident electrons at the vacuum-film interface and of the electron scattering mean free paths in the films are extracted in the electron energy range 1.6--7.9 eV for xenon, and 2.4--7.4 eV for molecular nitrogen. The effects of anisotropy in the surface scattering and in the reflections at the two interfaces of the films are also discussed.

Experimental Determination of Energy Dependent Inelastic and Elastic Scattering Rates of Hot Electrons in Large Bandgap Insulators

Internal photoemission for transport analysis (IPTA) and low energy electron transmission (LEET) experiments allow the determination of energy dependent quasi-elastic (acoustic phonon) and inelastic (LO-phonon) scattering rates of hot electrons in wide bandgap insulators for energies smaller than the electron excitation threshold. A model for the analysis of experimental data is presented. The model is used to analyse IPTA and LEET experiments in a variety of insulators such as rare gas solids, amorphous SiO2 and organic dielectrics. The results are compared to scattering rates obtained from electron-phonon interaction theory. The implications of the energy dependence of scattering rates on high field electron transport are discussed.

Energy dependence of the mean free path of excess hot electrons in solid xenon in the elastic scattering region.

It has been found recently from the analysis of low-energy electron-transmission experiments that the scattering mean free path \ensuremath{\lambda} of excess hot electrons in solid xenon films oscillates with energy. We show in this paper that in these experiments \ensuremath{\lambda} is predominantly controlled by the combined effect of both acoustical-phonon scattering and scattering by neutral point defects, and that its energy dependence is entirely caused by the changes in electron effective mass with energy.

Electron mean free path and conduction-band density-of-states in solid methane as determined from low-energy electron transmission experiments

Recently, Plenkiewicz et al. developed a theoretical model for analyzing the current I t transmitted by a thin dielectric film as a function of incident electron energy E. The purpose of this paper is to apply this model to the analysis of recent I t ( E) results for solid methane. The analysis permits the determination of both the electron mean free path as a function of energy and the electronic conduction-band density-of-states in the quasi-elastic scattering region. The differences between our results and Kunz's solid methane band structure calculations are also discussed.

Analysis of low-energy electron transmission experiments through thin solid xenon films in the elastic scattering region.

In this paper, we derive a relation between the current ${I}_{t}$ transmitted by a thin dielectric film as a function of incident electron energy E and the electronic energy-band structure of the film, with application to solid xenon. The analysis of ${I}_{t}$(E) in the elastic scattering region allows one to determine the electronic conduction-band density of states and to calculate the electron mean free path as a function of energy. It provides also a useful tool for determining the quasi-free electron ground-state energy ${V}_{0}$ of the solid.

Analysis of the Sensitivity of Low Energy Electron Transmission Experiments

Approximate expressions are given for the fraction of scattering events which contribute to the attenuation of the transmitted electron current in cylindrically symmetric electron spectrometers, with and without an imposed axial magnetic field. The sensitivity to scattering is shown to decrease strongly with increasing magnetic field strength owing to the trapping of scattered electrons by the magnetic field and multiple scattering effects due to the path length being a function of the magnetic field strength. The effect of applying a retarding electric field to electrons leaving the interaction region is shown to leave the conclusions substantially the same.