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Author: search.filters.author.Chou, WC

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Now showing 1 - 10 of 31
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    Orientation of adsorbed dipolar molecules: A conical well model
    (2006-03-01)
    Liao, YY
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    Chen, YN
    ;
    Chou, WC
    ;
    Chuu, DS
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    電子物理學系
    Orientation of single and two coupled polar molecules irradiated by a single laser pulse under a conical-well model is investigated theoretically. The orientation of a single hindered rotor shows a periodic behavior. In particular, the amplitude of the oscillation is sensitive to the degree of alternation of the laser field. Crossover from field-free to hindered rotation is observed by varying the hindering angle for different heights of conical wells. For a small hindering potential and angle, time-averaged orientation differs greatly from that for an infinite one. The orientation at a large hindering angle shows irregularlike behavior under strong dipole-dipole interaction. Entanglement induced by the dipole-dipole interaction is also calculated for the coupled-rotor system, in which the time-averaged entropy increases monotonically as the hindering angle is increased. The competition between the confinement effect and dipole interaction is found to dominate the behavior of the coupled-rotor system.
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    Optical absorption coefficient of quantum wires in strain and electric fields with intermixing interfaces
    (2005-09-01)
    Lee, J
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    Spector, HN
    ;
    Chou, WC
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    Huang, YS
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    電子物理學系
    We have theoretically studied the optical absorption coefficient of quantum wires in electric and strain fields with intermixing interfaces. The potential profiles are governed by the intermixing heterojunctions, internal strain due to the lattice mismatch, the external electric field and stress. The second Fick's law describes the wires' intermixing heterojunctions due to the alloy interdiffusion. We adopt the Green's function method to solve the Poisson equation for the displacement to determine the internal strain due to the lattice mismatch. The single-band Schrodinger equation in the effective mass approximation is used to describe the conduction subband structures while the four-band Kohn-Luttinger Hamiltonian is used to describe the valence subband structures. In solving the Schrodinger equation and the Kohn-Luttinger Hamiltonian, we expand the wave functions for the electrons and holes by using linear combinations of the two-dimensional harmonic oscillator wave functions to yield two matrix equations. These matrix equations are numerically solved for their eigen-energies and their corresponding eigenfunctions. We investigate some physical properties of the unstrained quantum wire (GaAs/AlAs) and the strained wire (CdSe/ZnSe) as examples. When the interdiffusion becomes stronger, on one hand, the internal strain is relaxed leading to the band gap shrinkage (if the initial strain epsilon(0)< 0) and, on the other hand, the potential profiles are deformed and the effective band gap becomes wider. Variations of the transition energies with external stress show the anticrossing effect and variations of the transition energies with external electric field show the field emission effect. The hole effective masses can be enhanced or become electronlike by applying stress to the wire. The oscillator strengths of the dipole-allowed intersubband transitions for the y and z polarizations are calculated to infer possible optical transitions. The optical absorption coefficients together with the joint density of states are calculated.
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    Fabrication and physical properties of radio frequency sputtered Cd1-xMnxS thin films
    (1996-10-15)
    Tsai, CT
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    Chen, SH
    ;
    Chuu, DS
    ;
    Chou, WC
    ;
    電子物理學系
    Highly oriented Cd1-xMnxS thin films with a wurtzite structure were grown by the radio-frequency sputtering technique. The grain size of the Cd1-xMnxS thin films was found to decrease with an increase in the Mn concentration x. Moreover, the lattice softening effect due to the quantum size effect, the band-bowing phenomena accounted for by the band-gap correction arising from the chemical disorder, and the exchange interaction between the band carrier and the d electron of the magnetic Mn ions have been investigated.
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    Raman spectroscopy study of ZnSe and Zn0.84Fe0.16Se at high pressures
    (1997-05-15)
    Lin, CM
    ;
    Chuu, DS
    ;
    Yang, TJ
    ;
    Chou, WC
    ;
    Xu, JA
    The ZnSe powder and Zn0.84Fe0.16Se crystal were studied by Raman scattering spectroscopy at pressures up to 36.0 and 32.0 GPa, respectively. For ZnSe powder at 4.7 and 9.1 GPa, two phase transitions were observed. However, the resulted phases have not been identified yet. As the pressure was increased to 14.4 GPa, the LO phonon peak disappeared while the TO phonon peak was still visible until the metallization pressure, 17.0 GPa was reached. In addition, three unidentified Raman peaks were still observable above the metallization pressure. For Zn0.84Fe0.16Se crystal, the structure transition from possible zinc blende to sodium chloride phase (B-1) was identified by the disappearance of Fe local mode and longitudinal optical (LO) phonon mode at 10.9 GPa. In addition, an unidentified phase transition at 4.7 GPa was observed. The TO phonon and the split TO phonons were still observable at a pressure above the phase transition pressure up to 32.0 GPa. The existence of Fe impurity in the ZnSe up to a concentration of 0.16 reduced the semiconductor-metal phase transition pressure to 10.9 GPa. According to the calculation of Gruneisen parameters, Zn0.84Fe0.16Se was found to have a higher ionicity than ZnSe. Reasons for the observation of Raman peaks at a pressure above the metallization pressure are still unknown.
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    High-pressure phase transitions in Zn1-xMxSe (M=Cd, Fe, and Mn)
    (1998-07-01)
    Lin, CM
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    Chuu, DS
    ;
    Xu, JA
    ;
    Huang, E
    ;
    Chou, WC
    Energy-dispersive x-ray diffraction was employed to study pressure-induced phase transitions in Zn0.84Fe0.16Se, Zn0.9Cd0.1Se, and Zn0.76Mn0.24Se crystals up to 21.0, 23.3, and 24.3 GPa, respectively. Our result shows that the B3 to B1 structure transitions for these crystals occurred at 1 1.4 +/- 0.5, 9.5 +/- 0.3, and 9.6 +/- 0.5 GPa, respectively. Compared to the phase-transition pressure (P-t) of ZnSe (14.4 GPa), a reduction of about 3-5 GPa is exhibited in these ternary compounds. This reduction in phase-transition pressure Delta P-t in the ternary compounds suggests that the fractional volume change, (Delta V/V-o), of the B3-B1 phase admixture might be the source of this reduction. Our results indicate that P-t (with respect to the phase transition pressure 14.4 GPa of ZnSe) is related to the fractional volume change (Delta V/V-o) by the expression P-t = [14.4003 + 0.1568 (Delta V/V-o)-0.0281(Delta V/V-o)(2)] GPa. [S0163-1829(98)05426-5].
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    Growth and characterization of Cd1-x-yZnxMnyTe crystals
    (1996-12-01)
    Chou, WC
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    Chen, FR
    ;
    Chiang, TY
    ;
    Shin, HY
    ;
    Sun, CY
    The Cd1-x-yZnxMnyTe crystals, having potential in the development of magnetic field sensitive devices, were grown by the temperature gradient solution method at 940 degrees C. All crystals were p-type with resistivity of 10(7) Omega . cm. They have novel magneto-optical properties, and could be substitutes for the Cd1-xZnxTe ternary compounds in certain optoelectronic/integrated-optical device applications.
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    A Raman study of ZnSe at high pressure
    (1997-06-01)
    Chou, WC
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    Lin, CM
    ;
    Ro, RS
    ;
    Ho, CS
    ;
    Hong, DY
    The ZnSe was studied by Raman scattering spectroscopy at pressures up to 36 GPa at first time. The present researchers have found splittings of transverse optical (TO) phonon at 4.65 and 7.61 GPa. In addition, the Raman signals of the longitudinal optical (LO) phonon disappeared at 14.35 GPa. The disappearance of the LO phonon is attributed to the semiconductor-metal phase transition. However, the TO phonon peaks were still visible above the metallization pressure.
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    Optical properties of Zn1-xMnxSe epilayers grown by molecular beam epitaxy
    (1998-04-01)
    Chou, WC
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    Ro, CS
    ;
    Hong, DY
    ;
    Yang, CS
    ;
    Lin, CY
    Zn1-xMnxSe (x less than or equal to 0.78) epilayers were grown on the GaAs (001) substrate with tilt angle 0, 3, 10, and 15 degrees toward [010] by the molecular beam epitaxy. The reflectivity, transmission, and photoluminescence experiments were used to measure the energy gap of the epilayers. A red shift of 4 meV was found as the substrate tilt angle increased from 0 to 15 degrees. The red shift in band gap energy is attributed to the increasing incoporation of smaller Zn ion with the increase in the kink density which results from the increase in the substrate tilt angle. In addition, the lattice vibration of the Zn1-xMnxSe (0 less than or equal to x less than or equal to 0.78) epilayers measured by the Raman scattering was found to follow the intermediate mode behavior of type Ib instead of type Ic. The force constants F-MnSe = 2.81 x 10(6) amu.cm(-2) and F-ZnSe = 3.96 x 10(6) amu.cm(-2) were obtained.
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    Energy-dispersive x-ray diffraction and Raman scattering of Zn1-xMnxSe bulk crystals at high pressure
    (1999-06-15)
    Yang, CS
    ;
    Ro, CS
    ;
    Chou, WC
    ;
    Lin, CM
    ;
    Chuu, DS
    Energy-dispersive x-ray diffraction experiments were carried out to investigate the structure of phase transitions under high pressure. It was found that the zinc blende (B-3) to rock salt (B-1) phase transition pressures of Zn0.93Mn0.07Se and Zn0.76Mn0.24Se bulk crystals are found 11.8+/-1.5 and 9.9+/-0.5 GPa, respectively. The respective bulk moduli are 61.8+/-0.8 and 60.5+/-0.8 GPa. The pressure-induced zinc blende (ZB) to rock salt (RS) structure phase transition is interpreted as a signature of the semiconductor to metal transition for Zn1-xMnxSe. The above interpretation is further corroborated by the observation of the disappearance of the longitudinal optical phonon at the pressure where the ZB to RS structure transition occurs. (C) 1999 American Institute of Physics. [S0021-8979(99)05312-8].
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    Formation of self-assembled ZnTe quantum dots on ZnSe buffer layer grown on GaAs substrate by molecular beam epitaxy
    (2002-07-01)
    Kuo, MC
    ;
    Yang, CS
    ;
    Tseng, PY
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    Lee, J
    ;
    Shen, JL
    Self-assembled ZnTe quantum dot structures were grown by molecular beam epitaxy on GaAs substrates with a 200 nm ZnSe buffer layer. Surface morphology was studied by atomic force microscopy. A three-dimensional Volmer-Weber growth mode was identified. Two types of dots were observed. Strong photoluminescence observed at 1.9-2.2 eV was attributed to emission from the large type II ZnTe quantum dots. Emission from the smaller ZnTe quantum dots was observed at an energy of around 2.26 eV. The density of the larger and smaller dots was approximately 10(8)/cm(2) and 10(9)/cm(2), respectively. (C) 2002 Elsevier Science B.V. All rights reserved.