Browsing by Author "Kwei, C. M."

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Angular and energy dependences of the surface excitation parameter for electrons crossing a solid surface
(2006-09-15)
Kwei, C. M.
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Li, Y. C.
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Tung, C. J.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
When fast electrons cross a solid surface, surface plasmons may be generated. Surface plasmon excitations induced by electrons moving in the vacuum are generally characterized by the surface excitation parameter. This parameter was calculated for 200-1000 eV electrons crossing the surfaces of An, Cu, Ag, Fe, Si, Ni, Pd, MgO and SiO, with various crossing angles. Such calculations were performed based on the dielectric response theory for both incident (from vacuum to solid) and escaping (from solid to vacuum) electrons. Calculated results showed that the surface excitation parameter increased with crossing angle but decreased with electron energy. This was due to the longer time for electron-surface interaction by glancing incident or escaping electrons and by slow moving electrons. The results were fitted very well to a simple formula, i.e. P-s = aE(-b)/(cosx)(c), where P, is the surface excitation parameter, E is the electron energy, alpha is the angle between the electron trajectory and the surface normal, and a, b and c are material dependent constants. (c) 2006 Elsevier B.V. All rights reserved.
Angular and energy dependences of the surface excitation parameter for semiconducting III-V compounds
(2007-02-01)
Tu, Y. H.
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Kwei, C. M.
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Tung, C. J.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
Sum-rule-constrained extended Drude dielectric functions were used to study surface excitations generated by energetic electrons moving across surfaces of semiconducting III-V compounds. Parameters in the dielectric functions were determined from fits to experimental optical data and electron energy-loss spectra. Electron inelastic mean free paths (IMFPs) in GaN, GaP, GaAs, GaSb, InAs and InSb were calculated for electron energies between 200 and 2000 eV, and the results were found to follow the simple formula, i.e., gimel = kE(P), where A is the IMFP and E is the electron energy. Surface excitation parameters (SEPs), which describe the total probability of surface excitations by electrons crossing the surface and travelling in vacuum, were also calculated for different electron energies and crossing angles. The SEP was found to follow the simple formula, i.e., P-s = aE-b/cos(c) alpha, where P-s is the SEP and alpha is the crossing anglerelative to the surface normal. (c) 2006 Elsevier B.V. All rights reserved.
Dielectric response theory for electron energy loss in clad cylindrical systems
(2006-07-01)
Tu, Y. H.
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Kwei, C. M.
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Li, Y. C.
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Tung, C. J.
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電子工程學系及電子研究所
Inelastic interactions between an electron and a clad cylindrical system were investigated using the dielectric response theory. By solving the Poisson equation and applying the boundary conditions, these interactions were formulated in terms of the surface, interface and volume excitations. Formulas of the differential inverse inelastic mean free path (DIIMFP) were derived for electrons moving parallel to the axis of such a cylindrical structure. A sum-rule-constrained extended Drude dielectric function with spatial dispersion was applied to the calculations of the DIIMFP for a Si cylindrical tube or a Si cylinder clad in a SiO2 film. For the Si tube, it was found that surface excitations occurred as electrons moved near either the inner or outer surface of the tube and either inside or outside Si. Whereas, volume excitations arose only for electrons moving inside Si. Surface excitations increased, or volume excitations decreased, as electrons moved closer to the surface. For the Si cylinder clad in the SiO2 film, inelastic interactions were contributed from volume, surface, and interface excitations. Calculated results showed that the relative importance of these excitations depended on the electron distance from the surface or interface of the cylindrical system, the radius of the Si cylinder, and the thickness of the SiO2 film.
Inelastic interactions of low-energy electrons with biological media
(2007-09-21)
Tung, C. J.
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Chan, W. T.
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Chao, T. C.
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Tu, Y. H.
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Kwei, C. M.
A study of inelastic interactions of low-energy electrons with biological media is of fundamental importance in the understanding of radiation-induced actions leading to the biological damage. In the present work, electron energy loss properties in liquid water and solid DNA were calculated based on the dielectric response theory for valence band electrons and atomic models for inner-shell electrons. For electron interactions with the valence band, an extended Drude dielectric function was fitted to experimental optical data and then checked by sum rules. The valance band was considered as composed of many subbands with different oscillator strength, binding energy and damping coefficient. The real and imaginary parts of the dielectric function and the energy loss function were all checked. For electron interactions with inner shells, the generalized oscillator strength was calculated using the sum-rule-constrained binary-encounter model and the local-plasma approximation. The differential inverse mean free paths for the valence band and inner shells were calculated using the Born approximation. Exchange effect was estimated using an approach based on the Moller cross-section. Results of the present work were compared with similar calculations by other authors. (C) 2007 Elsevier B.V. All rights reserved.
Surface excitation parameter for electrons crossing the A1N surface
(2007-10-29)
Kwei, C. M.
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Tu, Y. H.
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Tung, C. J.
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電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Fast electrons crossing a solid surface induce surface excitations. The total probability of such excitations for electrons moving outside the solid, i.e. in vacuum, is characterized by the surface excitation parameter (SEP). In the present work, the SEP was calculated for either incident or escaping electrons with normal or glancing crossing angles over the surface of aluminum nitride (AlN), a wide-band-gap semiconductor. These calculations were performed based on the dielectric response theory using the sum-rule-constrained extended Drude dielectric function with parameters obtained from a fit of this function to experimental optical data and electron energy-loss data. Dependences of the SEP on electron energy and crossing angle were analyzed. A simple formula was proposed for the fitting of SEP as a function of electron energy and crossing angle. (C) 2007 Elsevier Ltd. All rights reserved.