Browsing by Author "CHEN, YF"

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ANALYTIC-FUNCTIONS FOR ATOMIC MOMENTUM-DENSITY DISTRIBUTIONS AND COMPTON PROFILES OF K-SHELLS AND L-SHELLS
(1993-05-01)
CHEN, YF
;
KWEI, CM
;
TUNG, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
An analytical expression involving three parameters was proposed for atomic momentum-density distributions of K and L shells. This expression was based on the superposition of hydrogenic closed-shell momentum densities. Parameters in the expression were determined by requiring four of its moments to be equal to the corresponding Hartree-Fock results. An analytical function for the Compton profiles was then derived using the impulse approximation. Excellent agreement was found between the present results and detailed theoretical computations.
ANALYTICAL REPRESENTATION OF ATOMIC SHELLWISE ELECTRON-DENSITIES AND APPLICATIONS
(1993-03-28)
CHEN, YF
;
KWEI, CM
;
TUNG, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Analytical expressions of atomic shellwise electron densities have been constructed. Our approach was based on the superposition of Yukawa potentials for the atomic electrostatic potential. The electron density distribution was then established by the Poisson equation. Parameters characterizing the shellwise electrostatic potential and the corresponding electron density distribution were obtained by a fitting procedure utilizing the Hartree-Fock-Slater (HFS) data. These parameters were determined by the atomic number and electron occupation numbers in various states. The accuracy of constructed electron density distributions was tested by comparing their various moments with the corresponding results obtained from other methods. Applications of these atomic shellwise electron densities were made for calculations of the elastic scattering cross section and the mean excitation energy.
ANGULAR-DISTRIBUTION OF ELECTRONS ELASTICALLY BACKSCATTERED FROM NONCRYSTALLINE SOLID-SURFACES
(1995-10-14)
CHEN, YF
;
KWEI, CM
;
SU, P
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
In the energy range 100 eV-2 keV, we applied the Monte Carte method to analyse the elastic reflection coefficient and the angular distribution of electrons elastically backscattered from the solid surface of an isotropic and homogeneous medium. Results indicated that elastically backscattered electrons arose substantially from only a few scatterings with a single scattering event contributing to approximately half of these electrons. Thus, neither the multiple scattering model nor the single scattering model is sufficient to describe the angular distribution. To improve these models, we evaluated the contribution from one, two and three scatterings exactly and higher scatterings by the P-1-approximation, an approximate method to solve the Boltzmann transport equation assuming multiple elastic scattering of electrons in the solid. This approach allowed us to derive analytical formulations for the elastic reflection coefficient and the angular distribution of elastically backscattered electrons. Results calculated based on these formulations were in good agreement with those using Monte Carlo simulations and experimental data.
DEPENDENCE OF ELECTRON-MOBILITY ON DOPED IMPURITIES
(1995-09-01)
CHEN, YF
;
KWEI, CM
;
SU, P
;
TUNG, CJ
;
電子工程學系及電子研究所
The dependence of electron mobility on doped impurities was investigated. Analytical expressions of the momentum relaxation cross section and the electron mobility in n- and p-doped silicon for electron-impurity scatterings have been derived. Our approach involved the application of a screened scattering potential based on a charge density distribution for impurity ions in the semiconductor. This distribution was determined by the variational statistical method. Calculated results showed that ionized acceptor impurities in silicon scattered electron carriers less strongly than did ionized donor impurities. They also showed that majority electron mobility in n-type silicon was less than minority electron mobility in p-type silicon. These were consistent with experimental observations.
DEVELOPMENT OF OSCILLATORY ASYMMETRIC RECIRCULATING FLOW IN TRANSIENT LAMINAR OPPOSING MIXED CONVECTION IN A SYMMETRICALLY HEATED VERTICAL CHANNEL
(1993-05-01)
LIN, TF
;
CHANG, TS
;
CHEN, YF
;
交大名義發表
;
機械工程學系
Detailed flow and thermal characteristics in transient laminar opposing mixed convection in a vertical plane channel subject to a symmetric heat input are numerically investigated. First, a linear stability analysis was employed to evidence the occurrence of flow bifurcation. Then, the unsteady Navier-Stokes equations along with the continuity and energy equations were respectively integrated by a third-order upwind and power-law finite-difference scheme with the resulting matrices inverted by the Fast Fourier Transform and conjugated gradient methods. Reverse flow in the form of symmetric, elongated recirculating cells is initiated earlier and is stronger in a lower Prandtl number fluid with higher opposing buoyancy and Reynolds number and longer heated section length. A t a high opposing buoyancy, sudden flow asymmetry and oscillation occur simultaneously in a nearly steady flow after the initial transient. Periodic flow and thermal evolution are noted in space and time. An empirical equation for the condition for inducing flow oscillation is proposed.
DIFFERENTIAL CROSS-SECTIONS FOR PLASMON EXCITATIONS AND REFLECTED ELECTRON-ENERGY-LOSS SPECTRA
(1994-06-15)
TUNG, CJ
;
CHEN, YF
;
KWEI, CM
;
CHOU, TL
;
交大名義發表
Electron differential inverse mean free paths for volume-plasmon excitations and differential probabilities for surface-plasmon excitations have been calculated using dielectric response theory. A model dielectric function which satisfied sum rules and agreed with optical data was established for these calculations. In surface-plasmon calculations, we considered electron-impact emissions of the reflected electron-energy-loss spectroscopy. Formulations were made for obliquely incident electrons with the recoil effect and without the small-scattering-angle assumption. For volume-plasmon excitations, we evaluated corrections due to the exchange and Z1(3) effects. Comparison between calculated results and experimental data extracted from reflected electron-energy-loss spectra showed good agreement. Calculated differential cross sections have been used to solve the transport equation for the angular and energy flux spectra of reflected electrons. Contributions to the spectra from single and plural plasmon excitations were analyzed. It was found that calculated spectra were in good agreement with measured data.
EFFECT OF HYDROGENATION ON DEEP-LEVEL TRAPS IN INP ON GAAS
(1992-01-01)
CHEN, YF
;
SUNG, KC
;
CHEN, WK
;
LUE, YS
;
電子物理學系
Deep levels in the heteroepitaxial undoped InP layers grown directly on GaAs substrates by organometallic vapor-phase epitaxy have been investigated by deep-level transient spectroscopy. Two electron traps have been observed with activation energies of 0.4 and 0.57 eV in the temperature range between 150 and 310 K. The trap concentrations of these levels are in the order of 10(15) cm-3 for samples with a carrier concentration of 10(16) cm-3. Incorporation of atomic hydrogen into the InP layer by a photochemical vapor deposition system produces substantial decreases of the trap concentrations to 10(14) cm-3 and of the carrier concentration to 10(15) cm-3.
EFFECTS OF HYDROGENATION ON ELECTRICAL-PROPERTIES OF INP GROWN ON GAAS BY THE PHOTOCHEMICAL VAPOR-DEPOSITION SYSTEM
(1993-01-01)
LIN, TY
;
CHEN, YF
;
CHEN, WK
;
LUE, YS
;
電子物理學系
Effects of hydrogenation on electrical properties of InP epilayers grown on GaAs substrates by 'photochemical vapor deposition' have been investigated. The electrical properties were characterized by current-voltage and capacitance-voltage measurements at room temperature. Deep level transient spectroscopy technique was used as an auxiliary method to detect the deep level traps. Hydrogen plasma exposure at 250-degrees-C for 3 h increased the reverse breakdown voltage by a factor of about 1.7 and the Schottky barrier height by 0.05 eV. In addition, the shallow and deep levels were substantially passivated. These results indicate that hydrogenation by the photochemical vapor deposition system for InP on GaAs is an excellent method to improve its device application.
ELECTRON INELASTIC MEAN FREE PATHS FOR PLASMON EXCITATIONS AND INTERBAND-TRANSITIONS
(1993-08-20)
KWEI, CM
;
CHEN, YF
;
TUNG, CJ
;
WANG, JP
;
電子工程學系及電子研究所
Both plasmon excitations and interband transitions are important in the response of valence bands to fast electrons. An extended Drude dielectric function was established to describe such a response in solids of complex band structures. Parameters in this function were determined by a fit of the imaginary part of the function to experimental optical data. The real part of the dielectric function and the energy-loss function were also checked with experimental data to confirm critical-point energies in the interband transitions and plasmon energies in the collective excitations. In addition, sum-rules about the imaginary part of the dielectric function and the energy-loss function, respectively, were applied to assure the accuracy of these functions. Electron inelastic mean free paths in several solids were calculated and compared to available experimental and theoretical data.
ELECTRON INELASTIC MEAN FREE PATHS VERSUS ATTENUATION LENGTHS IN SOLIDS
(1992-02-14)
CHEN, YF
;
KEWI, CM
;
TUNG, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The electron inelastic mean free path is of basic importance in theoretical and applied radiation physics and surface physics. It can be calculated using the dielectric function for the valence band and atomic generalized oscillator strengths tor inner shells of a solid. Although the experimentally determined attenuation length is conceptually different from the theoretically calculated mean free path, they are frequently used interchangeably in a loosely defined manner. For electrons with energies below a few keV, elastic scattering plays an important role in connecting these two quantities. This work employed elastic scattering cross sections derived using the partial wave expansion method with a solid potential to evaluate the path length distribution of an electron transmitted through a solid film. Both the analytical multiple-scattering formulation and the numerical Monte Carlo simulation have been applied in this investigation. A comparison between electron inelastic mean free paths and attenuation lengths was made.
ENHANCEMENT OF BAND-EDGE LUMINESCENCE IN HYDROGENATED STRAINED SI(0.84)GE(0.16)/SI QUANTUM-WELLS BY PHOTOCHEMICAL VAPOR-DEPOSITION
(1993-12-01)
PAN, SC
;
CHEN, YF
;
CHANG, DC
;
CHANG, CY
;
WANG, PJ
Well-resolved band-edge luminescence spectra are reported for strained Si0.84Ge0.16/Si quantum wells grown by ultrahigh-vacuum chemical-vapor-deposition at 550-degrees-C. Phonon assisted transitions are also observed and the phonon structure of the spectrum is resolved. Blue shifting of the peak emission with decreasing well width is found in good agreement with theoretical calculations. The effects of hydrogenation using photochemical vapor deposition system are studied and an enhancement of the band edge luminescence intensity by a factor of three accompanied with a decrease in the intensity of the peaks coming from lattice defects is observed after hydrogenation. These results indicate that photochemical vapor deposition system is a useful tool for injecting atomic hydrogen into Si1-xGex/Si quantum wells and to enhance the band edge luminescence as well as to passivate the activity of the defects.
INFLUENCE OF SURFACE EXCITATIONS ON ELECTRONS ELASTICALLY BACKSCATTERED FROM COPPER AND SILVER SURFACES
(1994-12-15)
CHEN, YF
;
SU, P
;
KWEI, CM
;
TUNG, CJ
;
電子工程學系及電子研究所
OBSERVATION OF QUANTUM CONFINEMENT EFFECTS IN STRAINED SI0.84GE0.16/SI QUANTUM-WELLS AT ROOM-TEMPERATURE
(1993-05-24)
CHEN, YF
;
DAI, YT
;
CHOU, HP
;
CHANG, DC
;
CHANG, CY
We report the first study of quantum confinement shifts of energy gap in strained Si0.84Ge0.16/Si quantum wells at room temperature by photothermal deflection spectroscopy (PDS) technique. The experimental results obtained from the amplitude and phase of the PDS signal are in good agreement with quantum well subband calculation.
OPTICAL-CONSTANTS MODEL FOR SEMICONDUCTORS AND INSULATORS
(1993-08-15)
CHEN, YF
;
KWEI, CM
;
TUNG, CJ
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The refractive index and the extinction coefficient, i.e., the real and imaginary parts of the complex index of refraction, for semiconductors and insulators are derived as a function of photon energy. These derivations apply f-sum rules and symmetry relations to critical-point transitions from the valence band to the conduction band. Comparison with measured optical data reveals that present formulations are valid over a wide range of photon energies immediately above the band gap to the first ionization threshold for inner shells. The present work shows an improvement and extension over the theory of Forouhi and Bloomer, which applies for a narrow range of photon energies above the absorption edge.
QUANTUM CONFINEMENT EFFECTS OF SI/SIGE STRAINED-LAYER SUPERLATTICES GROWN BY AN ULTRAHIGH-VACUUM CHEMICAL-VAPOR-DEPOSITION TECHNIQUE
(1994-12-01)
CHANG, TC
;
CHANG, CY
;
JUNG, TG
;
CHEN, PA
;
TSAI, WC
Well-resolved band-edge luminescence is observed for Si0.86Ge0.14/Si strained-layer superlattices grown by an ultrahigh vacuum/chemical vapour deposition technique at 550 degrees C. High-resolution double-crystal X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM) were used to determine the strain and other parameters for these strained-layer superlattices. Quantum confinement is observed for a SiGe well as thin as 1.3 nm. The blue shift of the emission peaks with decreasing well width is found to be in good agreement with theoretical calculation.
RECONSTRUCTION OF THE SUM-RULE-CONSTRAINED CLASSICAL BINARY-COLLISION MODEL FOR INNER-SHELL IONIZATIONS
(1992-04-01)
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
Tung has constructed a sum-rule-constrained classical binary-collision model for the estimation of generalized oscillator strengths (GOS's) of the 1s and 2s subshells of atoms. He applied three sum rules to determine the momentum-dependent parameters introduced in the GOS function based on the classical binary-collision model. Several deficiencies have then been found regarding these sum rules that adopted the less accurate hydrogenic model and included the unwanted contribution from discrete excitations. In the present work, we have reconstructed this model by employing improved sum rules obtained from Hartree-Fock-Slater matrix element calculations. The refined model has been successfully applied to estimate ionization generalized oscillator strengths of atomic K and L shells.
STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF YBA2(CU1-XAGX)3O7 WITH X-LESS-THAN-OR-EQUAL-TO-0.20
(1990-12-01)
LIN, JJ
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CHEN, TM
;
CHEN, YF
;
應用化學系
;
Department of Applied Chemistry
THE LONG-TERM RELAXATION AND BUILDUP TRANSIENT OF PHOTOCONDUCTIVITY IN SI1-XGEX/SI QUANTUM-WELLS
(1995-06-05)
CHU, LH
;
CHEN, YF
;
CHANG, DC
;
CHANG, CY
;
電子工程學系及電子研究所
The long-term relaxation and build-up transient of photoconductivity has been observed in Si1-xGex/Si quantum wells. The long-term relaxation behaviour pelt) of photoconductivity can be described by a stretched-exponential function, I-pc(t) = I-pc(0) exp[-(t/tau)(beta)] (0
VALENCE-BAND CRYSTAL-FIELD SPLITTING OF HEXAGONAL DILUTED MAGNETIC SEMICONDUCTORS
(1994-02-01)
TWARDOWSKI, A
;
CHERNYU, K
;
CHEN, FR
;
KUO, SS
;
RO, CS
The valence band crystal field and spin-orbit splitting in hexagonal diluted magnetic semiconductors based on CdSe and CdS is studied. It is found that the crystal field splitting is practically magnetic ion independent for CdSe based crystals, whereas in the case of CdS based samples a strong influence of magnetic ion substitution on crystal field splitting is observed.