Attenuation length and escape depth of excited electrons in solids

Escape probability and mean escape depth λ_e of emitted electrons are determined as a function of attenuation length λ_a of excited electrons, their initial energy E_n, and the height of the surface barrier χ. A variable energy loss parameter permits to apply the proposed model to different kinds of electron emission, including the energy loss free Auger and ESCA electrons. An analytical expression was found correlating the internal energy distribution of excited electrons and the external energy distribution of emitted electrons. By means of this excitation energies of secondary electrons and exoelectrons were determined.     

Secondary electron emission from thin carbon films

For the purpose of investigating how secondary electrons are produced in carbon, the correlation between energy-loss events and secondary electrons was studied experimentally by using the coincidence method. If a secondary electron is detected in coincidence with an electron transmitted through a thin film which has lost an amount of energy E, then the process causing this energy loss results in the production of secondary electrons. We established the existence of these coincidences and have taken inelastic and coincidence spectra for films of different thickness. We found that in carbon secondary electrons are predominantly produced as a result of energy losses of about 20 eV, with an efficiency of about 5%. The escape depth of secondary electrons was also estimated to be approximately 30 Å.     

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED,Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to $\text{1}\text{2}$ monolayer on both surfaces, and $\text{1}\text{3}$ monolayer on Al(111). A hexagonal pattern corresponding to $\text{7}\text{8}$ monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at $\text{1}\text{2}$ monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.     

Angle- and energy-dependent core-level photoelectron energy loss studies in Al and In

Electron energy loss structures of Al and In core-level photoemission spectra, in particular surface and bulk plasmon losses, have been investigated as functions of photon energy (i.e., photoelectron kinetic energy). These studies utilized synchrotron radiation to provide a variable photon source in the ultra-soft X-ray region, thus allowing these loss processes to be studied at photoelectron kinetic energies for which the mean free path of the electrons is minimal. The Al plasmon loss structure was also studied with soft X-ray radiation in an angle-resolved mode, allowing the variation of effective photoelectron sampling depth with different electron take-off (collection) angles. These results for the relative intensity of the bulk and surface plasmons as a function of electron kinetic energy and electron exit angle are in qualitative agreement with the predictions of ?unji? and ?ok?evi?. The core-level binding energies of surface atoms have also been studied with the result that no significant shift has been observed with respect to bulk-atom core levels.     

Characterization of electronic structure in dielectric materials by making use of the secondary electron emission

The methodology of characterizing electronic structure in dielectric materials will be presented in detail. Energy distribution of the electrons emitted from dielectric materials by the Auger neutralization of ions is measured and rescaled for Auger self-convolution, which is restructured from the energy distribution of the emitted electrons. The Fourier transform is very effective for obtaining the density of states from the Auger self-convolution. The MgO layer is tested as an example of this new measurement scheme. The density of states in the valence band of the MgO layer is studied by measuring the energy distribution of the emitted electrons for MgO crystal with three different orientations of (111), (100) and (110). The characteristic energy of ?₀ corresponding to the peak density of the states in the band is determined, showing that the (111) orientation has a shallow characteristic energy ?₀ = 7.4 eV, whereas the (110) orientation has a deep characteristic energy ?₀ = 9.6 eV, consistent with the observed coefficient γ of the secondary electron emission for MgO crystal. Electronic structure in new functional nano-films spayed over MgO layer is also characterized. It is therefore demonstrated that secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials. This method simultaneously determines the valence band structure and the coefficient γ of the secondary electron emission, which plays the most important role in the electrical breakdown phenomena.     

Fine-structure in secondary electron emission from Cu(0 0 1)c(2 × 2)-Cl: Experiment and theory

Normal emission secondary electron spectra excited by photons of energy ?ω = 21.2 eV and by electrons of 100 eV kinetic energy have been measured with high resolution from Cu(0 0 1) and Cu(0 0 1)c(2 × 2)-Cl. Calculations were performed using a “one-step” multiple scattering formalism for the semi-infinite system. The close agreement between experimental and theoretical fine structure reveals the nature of individual features. Our results demonstrate that in general bulk-derived features in the secondary electron fine structure cannot be directly associated with critical points in the band structure.     

STUDY OF THE Al/GRAPHITE INTERFACE

Thin Al films with a thickness of 20－30nm were prepared by ultra-high vacuum deposition of Al onto a graphite surface parallel to a (0001) basal plane. The samples were annealed up to 1070K. X-ray photoelectron spectroscopy analysis has shown that for temperatures just higher than 770K, a little carbide occurs in the Al film and only an Al－C phase is present at the Al/graphite interface. After annealing at 970K, the Al₄C₃ phase can be observed, and the binding energy of the Al2p electrons increases continuously from 72.7 to 74.2eV with increasing temperature up to 1070K. Auger electron spectroscopy depth profiles are measured to investigate the phases existing in the Al film as well as at the Al/graphite interface. It is found that the Al₄C₃ phase at the interface is the final product of a series of Al carbides from the interfacial reaction between Al and graphite.     

Photoelectron Energy Loss in Al(002) Revisited: Retrieval of the Single Plasmon Loss Energy Distribution by a Fourier Transform Method

A Fourier transform (FT) algorithm is proposed to retrieve the energy loss function (ELF) of solid surfaces from experimental X-ray photoelectron spectra. The intensity measured over a broad energy range towards lower kinetic energies results from convolution of four spectral distributions: photoemission line shape, multiple plasmon loss probability, X-ray source line structure and Gaussian broadening of the photoelectron analyzer. The FT of the measured XPS spectrum, including the zero-loss peak and all inelastic scattering mechanisms, being a mathematical function of the respective FT of X-ray source, photoemission line shape, multiple plasmon loss function, and Gaussian broadening of the photoelectron analyzer, the proposed algorithm gives straightforward access to the bulk ELF and effective dielectric function of the solid, assuming identical ELF for intrinsic and extrinsic plasmon excitations. This method is applied to aluminum single crystal Al(002) where the photoemission line shape has been computed accurately beyond the Doniach–Sunjic approximation using the Mahan–Wertheim–Citrin approach which takes into account the density of states near the Fermi level; the only adjustable parameters are the singularity index and the broadening energy Г (inverse hole lifetime). After correction for surface plasmon excitations, the q-averaged bulk loss function, <Im[??1 / ε(E, q)]> _q , of Al(002) differs from the optical value Im[??1 / ε(E, q?=?0)] and is well described by the Lindhard–Mermin dispersion relation. A quality criterion of the inversion algorithm is given by the capability of observing weak interband transitions close to the zero-loss peak, namely at 0.65 and 1.65 eV in ε(E, q) as found in optical spectra and ab initio calculations of aluminum.     

Study of the excitation functions of27Al(n, α)24Na,27Al(n,p)27Mg and28Si(n,p)28Al reactions

Cross sections for the reactions²⁷Al(n, p)²⁷Mg and²⁸Si(n,p)²⁸Al were measured by activation method between 13.40 and 14.83 MeV neutron energy. An accuracy of about 4% was achieved using the²⁷Al(n, α)²⁴Na reaction as a reference at 14.1 MeV where the relative excitation function has also been measured. Results obtained were compared to a recent compilation and that calculated by the Hauser-Feshbach model. Using the back-shifted level density formula and taking into account the contribution of the separated levels, the calculations were extended to the energy range from the threshold to 18 MeV. A structure was observed in the²⁷Al(n, α)²⁴Na reaction cross section curve around 14 MeV neutron energy.     

Electronic transport properties of graphene/Al2O3 (0001) interface

The electronic structure and transport properties of a single layer of graphene (Gr) on α-Al₂O₃ surface are studied using the density functional theory (DFT). We present three models that take into account the atom at the termination of the alumina surface: a) Al atoms, with the center of the Gr hexagon directly over an Al atom; b) Al atoms, with a carbon directly positioned above an Al atom; c) oxygen atoms. Two processes of geometric optimization were used: (i) All the atoms of the supercell were allowed to move in accordance with the BFGS quasi-Newton algorithm; (ii) The atoms of the three topmost layers of the α-Al₂O₃ (0001) slab, including the C atoms, were allowed to move, whereas the atoms of the remaining layers were frozen in their respective atomic bulk positions. The first two models preserve qualitatively the electronic structure of the pristine Gr using the geometric optimization process (i) whereas, in the third model this structure was lost due to a significant charge transfer between the carbon and oxygen atoms irrespective of the optimization procedure. However, models (a) and (b) with the optimization (ii) reveal a p-type semiconducting behavior.     

Secondary electron emission control in X-ray photoelectron spectroscopy

Secondary electron emission (SEE) is a major player in surface charging during X-ray photoelectron spectroscopy (XPS); its characteristics and applicability as a current source for electrical measurements are studied. We employ sample biasing and a top retarding grid to control the photoelectron current, and further compare their I–V characteristics with direct spectroscopy of the secondary electrons. Using silica-coated gold substrates, the effect of sample work function on the emitted secondary electrons is shown and fine control over the surface potential gradients, in the range of 10–100 meV, is achieved. XPS-based chemically resolved electrical measurements (CREM) can thus be extended to the positive current regime.     

The determination of electron mean free paths in solids by depth and angular dependences of inelastically scattered electron spectra

A technique is presented of electron mean free path determination in solids using the spectra of backscattered electrons. The mean free path without energy loss (λ₀) in Ge, and the mean free paths for excitation of volume (λ_v) and surface (λ_s) plasmons in Al, are determined in the electron energy range E₀ = 0.1–10 kev. The depth d_s of surface plasmon localization is estimated for Al.     

Methods of calculating the band structure and low-energy secondary electron spectroscopy of iridium

A theoretical interpretation is put forward for the fine structure of the secondary electron emission spectra of Ir normal to the (111) surface and the total current spectrum of an Ir polycrystal. The calculations took into account the energy dependence of the broadening of the energy band levels, the electron-electron and electron-plasmon contributions to the nonequilibrium electron distribution function, and the isotropic component of the current from the electrons scattered at the surface. It is shown that the fine structure of the secondary electron emission spectrum and the total current spectrum is mainly attributable to the electron structure of the final states into which the electrons enter or from which they are emitted so that the characteristics of the band configuration in the energy band structure can be reconstructed directly from the experimental data. This method can be used to separate bulk effects from surface effects in the secondary electron emission and total current spectra. It is confirmed that the fine structure of the secondary electron emission and total current spectra depends on the geometric structure and the degree of ordering of the crystals. A reduction in the intensity of the fine structure serves as a measure of the defect structure in the surface region of the sample which can be successfully used to monitor the surface state during treatment. Zh. Tekh. Fiz. 69, 94–96 (June 1999)     

Experimental study of surface and bulk plasmon losses induced in Al(001) by low energy electrons

We have measured the relative intensity of the elastic peak, and of the surface and bulk plasmon loss peaks, for electrons backscattered from an Al(001) single crystal surface. The exciting beam has an energy between 100 and 2000 eV, and impiges on the surface in an high symmetry plane of incidence with a colatitude angle of incidence varying between 0° and 80°. In a first series of experiments called “angle integrated experiment”, all the backscattered electrons are collected in a 2π solid angle retarding field analyser as a function of the primary beam energy and colatitude angle of incidence. In a second series called “angle resolved experiment”, the acceptance angle of the detector is set at 10° or $\text{1}\text{2}\text{°}$ and the variable parameters are colatitude angles of incidence and emission. We observe mainly that very clean aluminum exhibits large plasmon loss peaks with a negligible background. The results cannot be accounted for without sophisticated models, but “angular resolved experiments” are more suitable for a simple discussion than the “angle integrated experiment” and show mainly the phonon-plasmon coupling plus a θ dependent creation probability.     

电子辐照能量对Kapton/Al热控涂层光学性能的影响

 研究了电子辐照时，电子能量与累积通量对Kapton/Al热控涂层光学性能的影响。采用原位测量的手段记录了辐照前后的光谱反射系数。试验结果表明，电子辐照后Kapton/Al热控涂层的反射性能,在太阳光谱辐射强度较大的300~1 200nm波长区间产生较大程度退化。在电子辐照作用下，作为离子导电型聚合物的 Kapton薄膜表面没有发现辐照充电效应。辐照后涂层材料存在“退火效应”，或称“漂白效应”。Kapton/Al涂层太阳吸收比的变化量与电子辐照累积通量的变化关系成幂函数形式,其系数与指数的极大值与极小值分别出现在电子能量为50keV附近。在辐照累积通量相同时，该变化量随辐照电子能量的提高而增大。     

Influence of the angular anisotropy of secondary emission on the characteristics of a two-sided multipactor

We study the influence of the thermal spread in ejection velocities of secondary electrons on the initial stage of development of a two-sided multipactor. The analysis employs a statistical method based on an exact analytical solution for the electron distribution function over times of transit through the vacuum gap. The obtained theoretical relations are used as the basis for a numerical algorithm which takes into account both the angular and energy distributions of secondary electrons and makes it possible to calculate quantitatively the main multipactor characteristics for the actual dependence of the secondary-emission ratio of the walls on the energy of bombarding electrons. Detailed numerical calculations of the multipactor zones are presented. It is shown that the presence of angular anisotropy can change significantly the conditions of production of the secondary-emission discharge as compared with the case of isotropic emission. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 5, pp. 406–415, May 2006.     

Characteristics of secondary electron emission from few layer graphene on silicon (111) surface

As a typical two-dimensional (2D) coating material, graphene has been utilized to effectively reduce secondary electron emission from the surface. Nevertheless, the microscopic mechanism and the dominant factor of secondary electron emission suppression remain controversial. Since traditional models rely on the data of experimental bulk properties which are scarcely appropriate to the 2D coating situation, this paper presents the first-principles-based numerical calculations of the electron interaction and emission process for monolayer and multilayer graphene on silicon (111) substrate. By using the anisotropic energy loss for the coating graphene, the electron transport process can be described more realistically. The real physical electron interactions, including the elastic scattering of electron—nucleus, inelastic scattering of the electron—extranuclear electron, and electron—phonon effect, are considered and calculated by using the Monte Carlo method. The energy level transition theory-based first-principles method and the full Penn algorithm are used to calculate the energy loss function during the inelastic scattering. Variations of the energy loss function and interface electron density differences for 1 to 4 layer graphene coating GoSi are calculated, and their inner electron distributions and secondary electron emissions are analyzed. Simulation results demonstrate that the dominant factor of the inhibiting of secondary electron yield (SEY) of GoSi is to induce the deeper electrons in the internal scattering process. In contrast, a low surface potential barrier due to the positive deviation of electron density difference at monolayer GoSi interface in turn weakens the suppression of secondary electron emission of the graphene layer. Only when the graphene layer number is 3, does the contribution of surface work function to the secondary electron emission suppression appear to be slightly positive.     

近程碰撞对He+离子诱发背向电子发射贡献比例的计算

用Monte Carlo方法模拟了高速He+离子入射到C，Cu和Al固体表面所诱发的电子发射.用这个程序计算了背向的电子发射产额，并且同时计算了近程碰撞对总的背向电子发射产额的贡献比例，对C，Cu和Al其值分别是05，055和0.42.对在近程碰撞中产生的高能δ电子（E＞10O eV）对背向电子发射行为的影响也进行了详尽地讨论，只有那些能量为几百个eV的δ电子对产额的贡献比例较大.对于C靶，δ电子对电子阻止本领最大值附近的二次电子发射行为会产生影响.计算所得到的电子发射产额与实验结果符合得很好. 关键词： 二次电子发射 Monte Carlo模拟 近程碰撞 δ电子