Browsing by Author "Tung, CJ"

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Angular distribution of electrons elastically backscattered from amorphous overlayer systems
(2001-02-10)
Kwei, CM
;
Tsai, SS
;
Tung, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Monte Carlo calculations have been performed of the angular distribution of electrons elastically backscattered from amorphous overlayer systems composed of thin copper films and semi-infinite silicon substrates. These calculations showed that the angular distribution of the elastically reflected intensity was dependent on film thickness and electron energy. They also showed that elastically backscattered electrons were due substantially to one, two and three scatterings with single-scattering events contributing about half of the intensity. Based on these findings, we have derived a formula for the contribution from single-scattering events to the angular distribution of the elastically reflected intensity. Combining this formula and the Pi-approximation for multiple scattering, we were able to construct an analytic formula for the angular distribution of electrons elastically backscattered from overlayer systems. Results from this approach were in good agreement with those computed using Monte Carlo simulations. (C) 2001 Elsevier Science B.V, All rights reserved.
Elastic reflection of low-energy electrons from polycrystalline gold targets
(1998-01-07)
Kwei, CM
;
Chen, YF
;
Tung, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Monte Carlo calculations to study the influence of solid-state, polarization, exchange and relativistic effects on the angular distribution and the reflection coefficient for electrons elastically reflected from gold surfaces have been performed. Elastic differential cross sections were determined using relativistic Dirac equations with a solid atom potential including the polarization and exchange effects. Inelastic differential inverse mean free paths were computed using the extended Drude dielectric function for volume and surface excitations. These showed that the polarization effect made a contribution to elastic differential cross sections at small scattering angles, whereas, the exchange and relativistic effects contributed to these cross sections at Barge scattering angles. These effects are important to the angular distribution and the elastic reflection coefficient for electron energies below 400 eV, Monte Carlo results calculated in this work are in very good agreement with experimental data.
Electronic stopping powers of solids for slow atoms
(2001-10-01)
Kwei, CM
;
Chou, JJ
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Yao, J
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Tung, CJ
;
電子工程學系及電子研究所
Based on the dielectric response theory, electronic stopping powers of solids for slow atoms have been studied. Using a local-field-correction dielectric function, we have calculated these stopping powers by including the correlation and exchange effects. The screening of shellwise electron charge densities of slow atoms by the conduction electrons in solids was estimated using a Thomas-Fermi screening model. In this model, atomic shellwise electron densities were constructed by a superposition of several Yukawa potentials. Stopping powers of Al, Ni, Ag, and Au were calculated for atoms with atomic number between 6 and 18. Calculated results showed that stopping powers oscillated with atomic number consistent with experimental data. Present results also indicated that our approach is much improved over other theoretical calculations. A comparison of presently calculated results with available theoretical and experimental data was made and discussed.
Energy losses of charged particles moving parallel to the surface of an overlayer system
(2003-06-01)
Kwei, CM
;
Hwang, SJ
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Li, YC
;
Tung, CJ
;
電子工程學系及電子研究所
An energetic charged particle moving parallel to the surface of an overlayer system was studied. This system was composed of a thin film on the top of a semi-infinite substrate. Based on the dielectric response theory, the induced potentia I was formulated by solving the Poisson. equation and matching the boundary conditions. The stopping force was built-up using the energy-momentum conservation relations and the extended. Drude dielectric functions with spatial dispersion. Surface (vacuum-film) and interface (film-substrate) excitations were included in the formulations of the interaction between charged particles and the overlayer system. Results of the wake potential were presented for protons moving parallel to a vacuum-copper-silicon system. Dependences of the induced potential and the stopping force on film thickness, distance of the proton from surface, and proton velocity were investigated. (C) 2003 American Institute of Physics.
Energy spectra of electrons quasi-elastically backscattered from solid surfaces
(2004-05-07)
Kwei, CM
;
Li, YC
;
Tung, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The energy distribution of electrons quasi -elastically backscattered from solids has been investigated. Monte Carlo, (MC) simulations were performed for the study of the recoil energy shift and the broadening of this distribution for backscattered electrons from Si and Au. In these simulations, electron interaction cross sections were obtained from calculations based on the dielectric response theory for inelastic interactions, including volume and surface excitations, and elastic interactions. The depth-dependent electron inelastic mean free path for volume excitations and the probability of surface excitations were calculated using the dielectric functions derived from optical data. The relativistic partial-wave expansion method was applied to calculate the elastic scattering cross section for a potential of the atom in the solid. The Rutherford-type recoil energy was included in the MC simulations by either considering or neglecting the thermal effect of atomic vibrations. Such an effect was applied using the single scattering model. The intensity of electrons quasi -elastically backscattered from Si and Au was simulated for incident electrons of an energy distribution. The adjustment for the spectrometer energy resolution was allowed. An analytic expression for the intensity of backscattered electrons by a single scattering was derived explicitly. A, comparison of simulated results with experimental data was made and discussed.
Inelastic interactions of electrons with cylindrical interfaces
(2006-02-15)
Tu, YH
;
Kwei, CM
;
Tung, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
A theory was developed to deal with inelastic interactions for an electron moving parallel to the axis of a cylindrical structure. Formulas for the differential inverse inelastic mean free path (DIIMFP) and the total inverse inelastic mean free path (IIMFP) were derived using dielectric response theory. A sum-rule-constrained extended Drude dielectric function with spatial dispersion was applied to calculate DIIMFPs and IIMFPs for a solid wire and a cavity in solid. The calculated results showed that surface excitations occurred as the electron moved near the boundary either inside or outside the solid, whereas volume excitations arose only for electron moving inside the solid. It was found that the probability for surface excitations increases and that for volume excitations decreases for an electron moving close to the surface. Near the surface, the decrease in volume excitations is compensated by the increase in surface excitations. For a cavity in solid, the IIMFP inside the solid can be approximated by a constant value equal to the IIMFP for the infinite solid, except in the immediate vicinity of the cavity boundary. (c) 2005 Elsevier B.V. All rights reserved.
Influence of the direction of motion on the inelastic interaction between electrons and solid surfaces
(2005-09-01)
Li, YC
;
Tu, YH
;
Kwei, CM
;
Tung, CJ
;
電子工程學系及電子研究所
Theoretical derivations of the inelastic differential inverse mean free path (DIMFP) and inelastic mean free path (IMFP) for electrons crossing solid surfaces were made for different crossing angles and electron distances relative to the crossing point at the surface. Individual contributions from volume and surface excitations were separated and analyzed for electrons traveling inside and outside the solid. Extended Drude dielectric functions were employed to calculate the DIMFP and inverse IMFP for electrons incident into and escaping from Cu. It was found that the DIMFP and inverse IMFP for electrons moving inside the solid were approximately independent of crossing angle and position of electrons due to the compensation of volume and surface excitations. For electrons moving deep inside the solid, the DIMFP and inverse IMFP reduced to the values for electrons moving in an infinite solid. As electrons traveling in the vacuum, the DIMFP and inverse IMFP became greater for glancing incident and escaping angles since surface excitations were more probable. The surface excitation parameter (SEP) for electrons traveling in vacuum showed an angular dependence. The SEP of escaping electrons was found larger than that of incident electrons due to the attractive force exerted by the induced surface charges. The calculated SEP was found to follow a simple expression. (c) 2005 Elsevier B.V. All rights reserved.
Memory effect on energy losses of charged particles moving parallel to solid surface
(2006-02-01)
Kwei, CM
;
Tu, YH
;
Hsu, YH
;
Tung, CJ
;
電子工程學系及電子研究所
Theoretical derivations were made for the induced potential and the stopping power of a charged particle moving close and parallel to the surface of a solid. It was illustrated that the induced potential produced by the interaction of particle and solid depended not only on the velocity but also on the previous velocity of the particle before its last inelastic interaction. Another words, the particle kept a memory on its previous velocity, v(0), in determining the stopping power for the particle of velocity v. Based on the dielectric response theory, formulas were derived for the induced potential and the stopping power with memory effect. An extended Drude dielectric function with spatial dispersion was used in the application of these formulas for a proton moving parallel to Si surface. It was found that the induced potential with memory effect lay between induced potentials without memory effect for constant velocities v(0) and v. The memory effect was manifest as the proton changes its velocity in the previous inelastic interaction. This memory effect also reduced the stopping power of the proton. The formulas derived in the present work can be applied to any solid surface and charged particle moving with arbitrary parallel trajectory either inside or outside the solid. (c) 2005 Elsevier B.V. All rights reserved.
Memory effect on the inelastic interaction of electrons moving parallel to a solid surface
(2006-02-01)
Kwei, CM
;
Hsu, YH
;
Tu, YH
;
Tung, CJ
;
電子工程學系及電子研究所
It is generally assumed that two successive inelastic interactions between an electron and a solid are independent of each other. In other words, the electron has no memory of its previous interaction. However, the previous interaction of the electron generates a potential that should influence its succeeding inelastic interaction. The aim of this work is to establish a model to account for the memory effect of an electron between two successive inelastic interactions. On the basis of the dielectric response theory, formulae for differential inverse inelastic mean free paths (DIIMFPs) and inelastic mean free paths (IMFPs) considering the memory effect were derived for electrons moving parallel to a solid surface by solving the Poisson equation and applying suitable boundary conditions. These mean free paths were then calculated with the extended Drude dielectric function for a Cu surface. It was found that the DIIMFP and the IMFP with the memory effect for electron energy E lay between the corresponding values without the memory effect for electron energy E and previous energy Eo. The memory effect increased with increasing electron energy loss, E-0 - E-1 in the previous inelastic interaction. Copyright (c) 2005 John Wiley & Sons, Ltd.
Monte Carlo calculations of the reflection electron energy loss spectra in gold
(1997-01-07)
Kwei, CM
;
Su, P
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Chen, YF
;
Tung, CJ
;
電子工程學系及電子研究所
The reflection electron energy loss spectra in gold have been calculated using a Monte Carte approach. A description of Monte Carlo simulations based on the dielectric response theory for inelastic volume and surface excitations and the partial wave expansion method for elastic scattering was presented. The influence of surface excitations on the angular and energy spectra of reflected electrons was analysed. These excitations contributed to the energy spectra by single and plural inelastic loss peaks and by the multiple inelastic loss background. Surface plasmons were particularly important for glancing incident or escape electrons. The contribution from surface excitations to the angular spectra was significant for energy losses of a few tenths of an electronvolt but less significant for lower and higher energy losses. The angular spectra revealed single elastic scattering peaks for zero energy loss but flatter multiple elastic scattering curves for large energy losses. In all cases, Monte Carlo simulation results, including surface excitations, agreed very well with experimental data.
Monte Carlo calculations of the reflection electron energy loss spectra in gold
(1997-01-07)
Kwei, CM
;
Su, P
;
Chen, YF
;
Tung, CJ
;
電子工程學系及電子研究所
The reflection electron energy loss spectra in gold have been calculated using a Monte Carte approach. A description of Monte Carlo simulations based on the dielectric response theory for inelastic volume and surface excitations and the partial wave expansion method for elastic scattering was presented. The influence of surface excitations on the angular and energy spectra of reflected electrons was analysed. These excitations contributed to the energy spectra by single and plural inelastic loss peaks and by the multiple inelastic loss background. Surface plasmons were particularly important for glancing incident or escape electrons. The contribution from surface excitations to the angular spectra was significant for energy losses of a few tenths of an electronvolt but less significant for lower and higher energy losses. The angular spectra revealed single elastic scattering peaks for zero energy loss but flatter multiple elastic scattering curves for large energy losses. In all cases, Monte Carlo simulation results, including surface excitations, agreed very well with experimental data.
Spatial distributions of elastically backscattered electrons from copper and silver
(1999-12-21)
Kwei, CM
;
Hung, CJ
;
Su, P
;
Tung, CJ
;
電子工程學系及電子研究所
We investigated the spatial distributions regarding the pathlength, the penetration depth, and the lateral displacement of 200-2000 eV electrons elastically backscattered from copper and silver We calculated these distributions by the Monte Carlo method using elastic scattering cross sections and inelastic inverse mean free paths for volume and surface excitations. in our approach, we applied the partial wave expansion method and the finite difference technique to calculate electron elastic cross sections by the Hartree-Fock-Winger-Seitz scattering potential for solid atoms. We employed the extended Drude dielectric function to estimate electron inelastic mean free paths inside the solid and electron surface excitation parameters outside the solid. Our study was focused on the energy dependence of the pathlength distribution, the maximum depth distribution and the radial distribution. We found that both the radial displacement and the maximum depth of backscattered electrons were on the order of a few angstroms. The maximum depth and the pathlength distributions obeyed the exponential attenuation form. The ratio of the attenuation lengths for the pathlength and the maximum depth distributions saturated to a value somewhat greater than two. Considering the back and forth trajectories of backscattered electrons, it revealed that mast electrons were backscattered from the solid through a single elastic scattering or a few elastic scatterings. As electron energy decreases, this ratio became larger, indicating that the probability for smaller scattering angles increases.
Surface excitation parameter for semiconducting III-V compounds
(2005-04-01)
Kwei, CM
;
Tu, YH
;
Tung, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
In the rapid development of mesoscopic science, the study of surface excitations in solids and overlayer systems plays a crucial role. The surface excitation parameter which describes the total probability of surface plasmon excitations by an electron traveling in vacuum before impinging on or after escaping from a semiconducting III-V compound has been calculated for 200-2000 eV electrons crossing the compound surface. These calculations were performed using the dielectric response theory with sum-rule -constrained extended Drude dielectric functions established by the fits of these functions to optical data. Surface excitation parameters calculated for InSb, InAs, Gap, GaSb or GaAs III-V compounds were found to follow to a simple formula, i.e. P-s = aE(-b), where Ps is the surface excitation parameter and E is the electron energy. These surface excitation parameters were then applied to determine the elastic reflection coefficient for electrons elastically backscattered from III-V compounds using the Monte Carlo simulations. Good agreement was found for the electron elastic reflection coefficient between calculated results and experimental data. (c) 2004 Elsevier B.V. All rights reserved.
The surface effect on Au 4f X-ray photoelectron spectra
(1996-02-01)
Wang, JP
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Tung, CJ
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Chen, YF
;
Kwei, CM
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電子工程學系及電子研究所
An investigation of the surface effect on Au 4f X-ray photoelectron spectra was made for different emission angles, It was found that surface plasmon excitations made significant contributions to the spectra in single and plural energy-loss peaks and in the multiple loss background, Our approach involved Monte Carlo simulations of photoelectrons in the interior region of the solid and Poisson stochastic processes in the surface region. The partial-wave expansion method with the Hartree-Fock-Wigner-Seitz potential for the solid was used to calculate differential electron elastic scattering cross sections. An extended Drude dielectric function model was employed to compute electron differential inverse mean free paths for volume excitations. The inelastic mean free paths in the surface region became spatially-varying and were calculated. Differential probabilities for surface excitations were determined using the same model dielectric function. The computed X-ray photoelectron spectra including surface excitations agreed quite well with experimental data.