Browsing by Author "Yang, CS"

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A Raman study of ZnSe at high pressure
(1997-06-01)
Chou, WC
;
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.
A Raman study of ZnSe at high pressure
(1997-06-01)
Chou, WC
;
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.
Analysis of size distributions of type IIZnTe/ZnSe quantum dots
(2004-12-01)
Lee, J
;
Yang, CS
;
Chang, CT
;
Liu, JR
;
Chou, WC
The photoluminescence (PL) spectrum of the ZnTe/ZnSe quantum dot (QD) structure which has a type II band alignment. was investigated. A broader structure peaking at 2.180 eV together with the features of the ZnSe buffer layer located at 2.820 eV in the PL spectrum at 10 K were observed. The broadness of the PL attributes to the spatial inhomogeneity of the QD sizes. The PL spectrum was a normal distribution with a variance of 0.002 eV(2). However, the dot size distribution was a deformed Gaussian. Using only one set of data of dot size distribution measured by the Atomic Force Microscopy (AFM), the statistical estimates with standard errors were evaluated using the bootstrap methodology. The probability distribution was inferred by calculating the means, percentiles and correlation coefficients of the base diameter and the height of the QDs. We concluded that the dot distributions were indeed a deformed Gaussian and the correlation coefficient of the diameter and the height of the QDs was 0.49 +/- 0.01. The uniformity of the dot size distribution was poor. To infer the dot size distribution from the PL spectrum, we numerically solved the Schrodinger equation by elaborating on the orthogonal periodic functions (OPF) approximation for the type II lens shaped QDs. The convergence of the probability density was discussed in a great detail. We found that the band edge discontinuities inside the dot acts as a barrier with DeltaE(v) = 315 meV for the conduction band and as a well with DeltaE(v) = 735 meV for the valence band. In the AFM measurement, the offset in dot height is 14 Angstrom and the aspect ratio is 0.04. We employed the Schrodinger equation to correlated the PL spectrum (intensity versus optical energy) to the AFM data (number of QDs versus dot size) and the agreement was excellent. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Carrier capture and relaxation of self-assembled ZnTe/ZnSe quantum dots prepared under Volmer-Weber and Stranski-Krastanow growth modes
(2005-02-01)
Lee, ME
;
Yeh, YC
;
Chung, YH
;
Wu, CL
;
Yang, CS
The carrier capture and relaxation of type II ZnTe/ZnSe quantum dots have been investigated with ultrafast time-resolved photo luminescence upconversion. The carrier capture times were 7 and 38 ps for the Volmer-Weber mode and Stranski-Krastanow mode, respectively. We found that the carrier relaxation of QDs exhibits faster decay under the Volmer-Weber growth mode than under the Stranski-Krastanow growth mode. We attribute the difference of carrier relaxation to the wetting layer formed in the Stranski-Krastanow growth mode. (C) 2004 Elsevier B.V. All rights reserved.
Charge-orbital ordering and verwey transition in magnetite measured by resonant soft X-ray scattering
(2006-03-10)
Huang, DJ
;
Lin, HJ
;
Okamoto, J
;
Chao, KS
;
Jeng, HT
We report experimental evidence for the charge-orbital ordering in magnetite below the Verwey transition temperature T-V. Measurements of O K-edge resonant x-ray scattering on magnetite reveal that the O 2p states in the vicinity of the Fermi level exhibit a charge-orbital ordering along the c axis with a spatial periodicity of the doubled lattice parameter of the undistorted cubic phase. Such a charge-orbital ordering vanishes abruptly above T-V and exhibits a thermal hysteresis, correlating closely with the Verwey transition in magnetite.
Continuous-wave high-power (320 mW) single mode operation of electronic vertically coupled InAs/GaAs quantum dot narrow-ridge-waveguide lasers
(2005-12-01)
Wang, JS
;
Lin, G
;
Hsiao, RS
;
Yang, CS
;
Lai, CM
High-power 3 mu m-wide narrow-ridge-waveguide lasers with ten stacks of electronic vertically coupled InAs/GaAs quantum dots in the active region were demonstrated. Unlike that from conventional uncoupled InAs quantum dot lasers, a narrow lasing spectrum was observed because the carriers tunneled in the vertical direction. Continuous-wave operation in single lateral mode yielded a kink-free output power of 320 mW with an efficiency of 0.46 W/A , and a sensitivity of lasing wavelength to temperature of 0.28 nm/K.
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].
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
;
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.
Growth mode transfer of self-assembled CdSe quantum dots grown by molecular beam epitaxy
(2006-01-15)
Lai, YJ
;
Lin, YC
;
Fu, CP
;
Yang, CS
;
Chia, CH
Self-assembled CdSe/ZnSe quantum dots (QDs) were grown at various growth temperatures on GaAs (0 0 1) by molecular beam epitaxy. An optimum growth temperature for CdSe/ZnSe QDs was found to be 260 degrees C. The Stranski-Krastanow (SK) growth mode was confirmed clearly by atomic force microscopy images. The coherent SK QDs were observed from 2.5 monolayers (MLs). Two types of QDs were found with the CdSe coverage of 3.0 MLs. It was attributed to a growth mode change from the coherent SK growth mode to the ripening growth mode. A schematic diagram of the growth mechanism of self-assembled CdSe QDs was presented. Moreover, the photoluminescence spectra of samples with various thicknesses were investigated. A dramatic change of optical properties confirmed that the QD structure formed with thickness above 2.5 MLs. Finally, the dot size and the density distributions were controlled after growth by in-chamber thermal annealing. For the sample grown at 3.0 MLs, the distribution of dot sizes was controlled from 6.0 to 6.5 x 10(3) nm(3) at an annealing temperature of 400 degrees C. (c) 2005 Published by Elsevier B.V.
Growth of ZnSe epilayer on Si using Ge/GexSi1-x buffer structure
(2004-06-15)
Yang, TH
;
Yang, CS
;
Luo, GL
;
Chou, WC
;
Yang, TY
The epitaxial growth of ZnSe layers on Si substrates utilizing a Ge/Ge0.95Si0.0.5/Ge0.9Si0.1 buffer structure is demonstrated. In this study, we examine the structure, optical characteristics and atomic interdiffusion of the ZnSe epilayer grown on Si. In a sample with a 2degrees off-cut Si substrate, the outdiffusion of Ge into the ZnSe epilayer is suppressed. The low-temperature PL measurements indicate that the sample with a 2degrees off-cut Si substrate improves its optical characteristic effectively. The X-ray diffraction analysis and transmission electron microscopy (TEM) results indicate that the use of a 2degrees off-cut Ge/Ge0.95Si0.05/ Ge0.9Si0.1/Si substrate markedly improves the crystallinity of and reduced the number of threading dislocations in ZnSe grown on Si.
Optical properties of self-assembled ZnTe quantum dots grown by molecular-beam epitaxy
(2005-02-01)
Yang, CS
;
Lai, YJ
;
Chou, WC
;
Chen, WK
;
Lee, MC
The morphology and the size-dependent photoluminescence (PL) spectra of the type-II ZnTe quantum dots (QDs) grown in a ZnSe matrix were obtained. The coverage of ZnTe varied from 2.5 to 3.5 monolayers (MLs). The PL peak energy decreased as the dot size increased. Excitation power and temperature-dependent PL spectra are used to characterize the optical properties of the ZnTe quantum dots. For 2.5- and 3.0-ML samples, the PL peak energy decreased monotonically as the temperature increased. However, for the 3.5-ML sample, the PL peak energy was initially blueshifted and then redshifted as the temperature increased above 40 K. Carrier thermalization and carrier transfer between QDs are used to explain the experimental data. A model of temperature-dependent linewidth broadening is employed to fit the high-temperature data. The activation energy, which was found by the simple PL intensity quenching model, of the 2.5, 3.0, and 3.5 MLs were determined to be 6.35, 9.40, and 18.87 meV, respectively. (C) 2005 American Institute of Physics.
Optical properties of Zn1-xCdxSe epilayers grown on (100)GaAs by molecular beam epitaxy
(2004-08-01)
Kuo, MC
;
Chiu, KC
;
Shih, TH
;
Lai, YJ
;
Yang, CS
Zn1-xCdxSe epilayers were grown on (100) GaAs substrates by molecular beam epitaxy. Lattice constants of the epilayers were measured by (004) rocking curve X-ray diffraction. A full width at half maximum of 475 to 2100 arcsec was obtained. The dependence of the energy gap on temperature, measured by the photoluminescence (PL) spectra, was fitted by Varshni's [Y. P. Varshni: Physica 34 149 (1967)] and O'Donnell's [R. P. O'Donnell and X. Chen: Appl. Phys. Lett. 58 2924 (1991)] models. The fitting parameters beta (161 K to 368K) and (hv) (13 meV to 24 meV), related to phonon energy, were obtained from Varshni and O'Donnell fits, respectively. The activation energies calculated from the integrated PL intensity versus inverse temperature decrease as the Cd content increases. The broadening of the PL linewidth with temperature was fitted by Gamma(T) = Gamma(0) + Gamma(a)T + Gamma(LOI)/[exp(homega(LOI)/kT) - 1] + Gamma(LO2)/[exp(homega(LO2)/kT) - 1] + Gamma(i)exp(-/kT). The impurity binding energy, (E-b), was found to decrease as the Cd composition increases.
Optical properties of Zn1-xMnxSe epilayers grown by molecular beam epitaxy
(1998-04-01)
Chou, WC
;
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.
Photoluminescence of ZnSexTe1-x/ZnTe multiple-quantum-well structures grown by molecular-beam epitaxy
(2004-12-15)
Shih, YT
;
Tsai, YL
;
Yuan, CT
;
Chen, CY
;
Yang, CS
This work investigates photoluminescence (PL) spectra from ZnSexTe1-x/ZnTe multiple-quantum-well structures grown on GaAs(001) substrates by molecular-beam epitaxy. The PL data reveal that the band alignment of the ZnSexTe1-x/ZnTe system is type II. The thermal activation energy for quenching the PL intensity was determined from the temperature-dependent PL spectra. The activation energy was found to increase initially and then decrease as the thickness of the ZnSexTe1-x layers decreases from 7 to 3 nm. The temperature-dependent broadening of the PL linewidth was also investigated. The LO-phonon scattering was found to be the dominant broadening mechanism. (C) 2004 American Institute of Physics.