Role of the electron forward-focusing effect in the formation of Kikuchi patterns of single-crystal silicon

Kikuchi patterns produced by quasi-elastic backscattering of electrons with energies of 0.6–2 keV from a thin Si(111)7×7 near-surface layer are studied. It is shown that experimental data obtained for silicon, just like those for metals, can be described satisfactorily by calculations made in single-scattering cluster approximation, as well as in terms of a model taking phenomenologically into account the forward-focusing of backscattered electrons as they escape from the crystal. It has thus been demonstrated that the forward-focusing effect at an energy E=2 keV plays a dominant role in the formation of Kikuchi patterns, which permits their use for visualization of the atomic structure of a surface. The dependences of the focusing efficiency on the parameters of the atomic chains along which electrons propagate have been established for the closest-packed crystal directions. Fiz. Tverd. Tela (St. Petersburg) 39, 752–757 (April 1997)     

Kikuchi-band formation in medium-energy electron-diffraction patterns

To reveal the mechanism of Kikuchi-band formation, the total Si(100) diffraction pattern produced by 2-keV quasi-elastically backscattered electrons is compared to model calculations made in the single-scattering approximation for clusters constructed with different numbers of close-packed (110) planes. The formation of the Kikuchi bands is shown to be governed by two types of electron scattering in a crystal. The dominant contribution to enhanced electron-scattering intensity within a band comes from the forward-focusing effect as the electrons move along the numerous interatomic directions in the (110) planes. The other mechanism responsible for the sharp edge regions in the Kikuchi bands involves electron scattering from the nearest planes. It is proposed to use the specific profile of the Kikuchi bands in estimating the shape and size of light-element crystallites forming during initial stages of island-film growth. Fiz. Tverd. Tela (St. Petersburg) 41, 411–417 (March 1999)     

Effect of focusing of primary electrons on their reflection from a crystal and on the associated Auger emission

The orientational dependence for different groups of secondary electrons — quasi-elastically scattered, inelastically reflected with excitation of a plasmon and with ionization of the core level M _4.5, and the Auger electrons M _4.5 VV — are measured in the primary electron energy range 0.6–1.5 keV. The data are obtained for a Nb (100) single crystal by varying the azimuthal angle of incidence of the primary beam, with complete collection of secondaries. A relationship is established between the processes of focusing and defocusing of the electrons that have penetrated into the crystal in the 〈110〉 and 〈133〉 directions, which differ substantially in the atomic packing density. Specific details of the Auger orientation effect, due to the focusinginduced variation of the flux density of the reflected electrons, are identified and explained. The contributions, both of anisotropy of ionization of the core level and of variation of the backscattering intensity, to the angular dependence of Auger emission and reflection with ionization loss are estimated. The possibilities of using such orientational dependences for an element-sensitive analysis of the local atomic structure of surfaces are assessed. Zh. Tekh. Fiz. 67, 117–123 (August 1997)     

Contribution of backscattered electrons to the total electron yield produced in collisions of 8–28 keV electrons with tungsten

It is shown experimentally that under energetic electron bombardment the backscattered electrons from solid targets contribute significantly (∼80%) to the observed total electron yield, even for targets of high backscattering coefficients. It is further found that for tungsten (Z = 74) with a backscattering coefficient of about 0.50, about 20% of the total electron yield is contributed by the total secondary electrons for impact energies in the range of 8–28 keV. The yield of true backscattered electrons at normal incidence (η ₀), total secondary electrons (δ) and the total electron yield (δ _tot) produced in collisions of 8–28 keV electrons with W have been measured and compared with predictions of available theories. The present results indicate that the constant-loss of primary electrons in the target plays a significant role in producing the secondary electrons and that it yields a better fit to the experiment compared to the power-law.      

Aharonov-Bohm effect for electrons on a liquid-helium surface

A method for obtaining finite electronic systems on a liquid-helium surface is proposed. If a thin film of liquid helium lies above a bottom capacitor plate made in the form of metal rings connected with one another, then electrons will accumulate in potential troughs near these rings. The purity of the helium surface, i.e., the absence of impurities and pinning centers on it, affords an excellent opportunity for investigating the Aharonov-Bohm effect in an ideal ring of a Wigner crystal and a Luttinger liquid Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 410–414 (25 March 1998)     

Focusing of electrons reflected from a crystal with loss of energy

A study is made of the features arising in the spatial distributions of reflected electrons as a result of a focusing effect. Experiments are conducted on single-crystal Mo (100) with primary electron energies of 0.5–2 keV and detection of electrons which lose fixed amounts of energy up to 300 eV. An analysis of the data establishes the dependence of the electron focusing efficiency on the amount of energy loss. It is shown that when electrons are reflected with single losses through plasmon excitation, the magnitude of the effect is determined mainly by the average number of scattering atoms encountered by an electron along its path to the surface. When the energy losses are high, defocusing owing to multiple elastic and inelastic scattering of the electrons is found to predominate. Zh. Tekh. Fiz. 68, 128–133 (June 1998)     

Total quantum-current yield in the soft x-ray region

The emission of slow secondary electrons excited in efficient photocathodes by fast internal x-ray electrons upon absorption of x-ray photons having energies in the range 1–10 keV is analyzed. Analytical expressions are derived for the quantum current yield of the x-ray photoelectric effect for a “point” model and a “non-point” model of energy exchange of fast internal x-ray electrons. We present some estimates for its parameters in a CsI photocathode. Fiz. Tverd. Tela (St. Petersburg) 40, 1042–1046 (June 1998)     

Light scattering by electrons in the excitonic absorption region of GaAs

A study has been made of the effect of the additional generation of photoexcited electrons on the excitonic absorption and luminescence spectra of ultrapure GaAs samples at T=2 K. The observed increase in the absorption coefficient for the ground (n=1) excitonic state is shown to originate from the polariton character of the energy spectrum of this state and to be due to an increase of polariton damping. The increased damping observed under electron generation is caused by polariton scattering from hot electrons as the latter undergo thermalization. As a result, the polaritons are heated. The changes observed in the luminescence spectra are produced by the reverse effect of electron heating and polariton cooling. Fiz. Tverd. Tela (St. Petersburg) 39, 1011–1016 (1997)     

Emission of single gamma rays by electrons with energies of hundreds of GeV in oriented crystals

Cross sections are calculated for the emission of single hard photons by electrons with energies of 150–1000 GeV as they pass through oriented crystals at small angles to the crystallographic axes. The contribution of incoherent emission at isolated atoms of the crystal is taken into account in the calculations, along with the emission in the continuum potential. The calculations are compared with the customary Bethe-Heitler spectrum for a thick amorphous target with allowance for photon absorption due to electron-positron pair production. It is shown that, in this range of energies, an oriented crystal can be more efficient than a thick amorphous target for creating a larger number of hard gamma rays with energies comparable to the energies of the emitting electrons. Zh. Tekh. Fiz. 68, 37–41 (September 1998)     

Dependence of the phonon spectrum of a metal on electron temperature in a nonequilibrium electron-phonon system

The dependence of the phonon spectrum of a crystal and the associated thermodynamic functions on electron temperature in the absence of equilibrium between the electrons and the lattice is investigated. The treatment is performed within the Thomas-Fermi approximation for a body-centered cubic crystal at high pressures. Zh. éksp. Teor. Fiz. 115, 231–242 (January 1999)     

Build-up of F and M color centers in KCl single crystals under combined electron and proton irradiation

The F and M color-center build-up kinetics in KCl crystals under combined irradiation with electrons of energy 15 and 100 keV and 100-keV protons have been studied in the flux range of 10¹³–10¹⁵ cm⁻² and at a flux density of 3×10¹¹ cm⁻² s⁻¹. It is shown that consecutive irradiation with electrons and protons produces results not obtainable under electron or proton irradiation alone. Fiz. Tverd. Tela (St. Petersburg) 40, 2015–2018 (November 1998)     

Electronic and electronic-vibrational phenomena in the new organic conductor ϰ-(ET)2[Hg(SCN)2Cl] with a metal-insulator transition

The reflection spectra and optical conductivity spectra of the new organic conductor ϰ-(ET)₂[Hg(SCN)₂Cl] with a metal-insulator transition in the spectral regions 700–5500 and 9000–40 000 cm⁻¹ have been studied in polarized light at 300 K. A comparisonis made between the spectra obtained and the corresponding spectra of related isostructural conductors based on the ET molecule, and also the properties of the crystal structure of the investigated compounds. An electronic transition between the ET molecules of the dimer (ET) ₂ ⁺ in the spectral region 700–5500 cm⁻¹ has been identified, as have the features of the electronic-vibrational structure arising as a consequence of the interaction of this transition with the completely symmetric intramolecular vibrations of the ET molecule. It is found that the conductor with the stronger dimer interaction between the ET molecules has the higher the transition temperature. Fiz. Tverd. Tela (St. Petersburg) 39, 1313–1319 (August 1997)     

Influence of the quantum size effect for grazing electrons on the electronic conductivity of metal films

The thickness dependence of the electronic conductivity of thin (5–150 nm) single-crystal (100) films of refractory metals is investigated at different temperatures ranging from 4.2 K to room temperature. Regions of square-root, quasilinear, and quadratic dependences are observed. The quasilinear thickness dependence is explained by the influence of quantum effects on the transverse motion of electrons in the case when electron scattering by the film surfaces dominates. For macroscopic film thicknesses 30–50 nm, much greater than the Fermi wavelength of an electron, quantum corrections to the electronic conductivity reach values of the order of 50%. This is a consequence of the quantum size effect for grazing electrons, which leads to an anomaly in electron scattering by the film surfaces. The region of the quadratic thickness dependence corresponds to the quantum limit, and the square-root region corresponds to the classical limit. The effect is explained in a quasiclassical two-parameter model (the effective angle α* for small-angle electrons and the parameter γ, equal to the ratio of this angle to the diffraction angle) that takes into account the diffraction angular limits for grazing electrons. The effect occurs for parameters α*≪1 and γ∼1 and differs from the “ordinary” quantum size effect. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 693–698 (10 December 1997)     

Spectra of low-energy electrons excited by soft x rays

A study is made of the x-ray photoemission spectra of low-energy electrons (E _kin=0–30 eV) and its dependence on the way in which the potentials are applied to the entrance system of the analyzer. It is shown that under certain experimental conditions the spectrum consists solely of electrons emitted from the sample. The x-ray photoemission spectrum of pure silver is found to have a fine structure, which disappears when a surface layer of carbon or oxygen is deposited on the surface. The spectrum then takes on the “cascade” form. A possible interpretation of the fine structure is suggested in the framework of band theory. Zh. Tekh. Fiz. 67, 73–77 (March 1997)     

Luminescence of mercuric iodide crystals

Low-temperature (4.2–130 K) photoluminescence spectra of HgI₂ crystals have been measured in the 540–700 nm region. An analysis of the characteristics (intensity vs temperature and excitation power relations, afterglow times, excitation spectra) of the 560, 620, and 635 nm emission bands suggests the following assignments: the 560 nm band is due to radiative annihilation of excitons bound to mercury vacancies, and the “red” emission originates from recombination of free (620 nm) and donor-localized (635 nm) electrons with a hole-filled acceptor level. The energies of the corresponding donor and acceptor levels have been estimated. New emission bands at 540, 545, and 575 nm have been discovered, and their origin discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 67–73 (January 1997)     

Suppression of the scattering of conduction electrons by lattice phonons in palladium in the presence of small hydrogen (deuterium) inclusions

An increase has been found in the electrical conductivity of electrochemically dehydrogenated palladium hydrides (deuterides) as compared with original samples of pure palladium in a wide temperature range (75–300 K). It is shown that this effect is due to the suppression of the scattering of conduction electrons by phonons in the palladium lattice for T⩾θ (θ is the Debye temperature) in the presence of clusters of quasimetallic hydrogen (deuterium). Fiz. Tverd. Tela (St. Petersburg) 39, 2113–2117 (December 1997)     

Creating primary defects in calcium fluoride crystals with various prehistories, using pulsed electron irradiation

Optical spectroscopy with nanosecond resolution has been used to study how the prehistory (the presence of impurities, heat treatment) of CaF₂ crystals affects the processes of creating autolocalized excitons (AEs) under the influence of a pulse of accelerated electrons. The breakdown of the dispersivity condition for the creation of AEs is detected in a nominally pure, nonheat-treated crystal. It is shown that the initial structural defects play a defining role in creating the initial defect density. A model of the radiation disordering of an actual fluorite crystal is constructed. An analogy is traced to the processes of impurity and thermal disordering. Fiz. Tverd. Tela (St. Petersburg) 41, 442–450 (March 1999)     

Peculiarities of crystal field effects in CeInCu2 based heavy-fermion compounds

We have studied the evolution of the inelastic neutron magnetic scattering spectra of a compound with cubic symmetry, CeInCu₂, in the temperature range 10–130 K, and also their transformation with variation of the Kondo temperature T _K due to substitution of cerium ions in the system Ce_1−x (La,Y)_xInCu₂ at T=10 K. It turns out that the energy of the transition between the ground state and excited state of the 4 f electrons (Δ_CF) in the crystal electric field in CeInCu₂ increases with growth of the population of the ground state as the temperature is reduced, with a slight change in its intensity. Such behavior is inconsistent with the notion of classical one-ion effects of the crystal electric field. We have found that the scale of the observed variations in the excitation spectra of the 4f electrons depends on the Kondo temperature T _K and is insensitive to disorder in the rare-earth sublattice. Thus, despite the fact that T _K ≪Δ_CF, hybridization with states in the conduction band has a substantial effect on all parameters of the excitation spectrum of the ground multiplet of the 4f electrons at low temperatures. Zh. éksp. Teor. Fiz. 115, 2197–2206 (June 1999) A. A. Baikov Institute of Metallurgy     

Non-self-sustained hollow-cathode glow discharge for large-aperture ion sources

A discharge system is proposed in which an auxiliary gas discharge is used to inject electrons into the cathode cavity of a hollow-cathode glow discharge. A study is made of the region of stable existence of a non-self-sustaining hollow-cathode discharge. It is shown that the injection of electrons permits a reduction to <10⁻² Pa in the minimum pressure at which a discharge can exist. It is shown experimentally that this discharge can be used to generate wide-aperture ion beams. Zh. Tekh. Fiz. 67, 27–31 (June 1997)     

Calculation of the binding energy of electrons and positrons in a dielectric cluster

An analysis is made of analytic expressions for the binding energy of electrons and positrons in dielectric clusters. Calculations are made of the sizes of critical argon and xenon clusters for which the binding energy is near-zero. Fiz. Tverd. Tela (St. Petersburg) 40, 1376–1378 (July 1998)