Browsing by Author "Wang, JS"

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1.3-mu m InAs-InGaAs quantum-dot vertical-cavity surface-emitting laser with fully doped DBRs grown by MBE
(2006-01-01)
Yu, HC
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Wang, JS
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Su, YK
;
Chang, SJ
;
Lai, FI
We report InAs-InGaAs quantum-dot vertical-cavity surface-emitting lasers (VCSELs) grown by molecular beam epitaxy with fully doped n- and p-doped AlGaAs distributed Bragg reflectors and including an AlAs layer to form a current and waveguiding aperture. The metal contacts are deposited on a topmost p(+) (GaAs)-Ga-. contact layer and on the bottom surface of the n(+)-GaAs substrate. This conventional selectively oxidized top-emitting device configuration avoids the added complexity of fabricating intracavity or coplanar ohmic contacts. The VCSELs operate continuous-wave at room temperature with peak output powers of 0.33 mW and differential slope efficiencies up to 0.23 W/A. The peak lasing wavelengths are near 1.275 mu m, with a sidemode suppression ratio of 28 dB.
Admittance spectroscopy and thermal stimulation current for band-offset characteristics in AlAs/GaAs n(+)-p structures
(2000-01-01)
Chen, JF
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Wang, JS
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Wang, PY
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Chen, NC
;
Hsu, NC
Admittance spectroscopy is used to study a molecular-beam epitaxially grown GaAs n(+)-p diode with 100-Angstrom-thick AlAs immersed in the lightly doped p-region. The measurements clearly show two trapping effects. Upon comparison with the reference sample without the AlAs layer, an equivalent circuit for the studied sample is developed. Based on this circuit, the admittance spectra are calculated and found to be consistent with the experimental spectra. From this result, the trap at E-a = 0.52 eV with a capture cross section 1.6 x 10(-14) cm(2) is believed to result from the resistance-capacitance time constant effect due to the thermionic emission of holes over the AlAs barrier and the activation energy corresponds to the AlAs/GaAs valence-band offset. The results of the thermal stimulation current further support this conclusion.
Annealing dynamics of nitrogen-implanted GaAs films investigated by current-voltage and deep-level transient spectroscopy
(2000-04-17)
Chen, JF
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Wang, JS
;
Huang, MM
;
Chen, NC
;
電子物理學系
We present electrical data to show that, after nitrogen implantation, GaAs films become resistive after high-temperature annealing. The activation energies of the resistance are determined to be 0.34, 0.59, and 0.71 eV after annealing at 500, 700, and 950 degrees C, respectively. The increase in the activation energy with increasing annealing temperature can be explained by the results of traps detected in deep-level transient spectroscopy, where two traps at 0.32 and 0.70 eV are observed in the samples after annealing. The intensity of the trap at 0.32 eV is found to reduce by annealing. By comparing to the result of the x-ray diffraction, we suspect that this trap is related to the lattice-expansion defects. The trap at 0.70 eV is observed only in samples annealed at high temperatures. Since this trap contributes to the high-resistive effect, we believe that it is associated with the nitrogen ions. (C) 2000 American Institute of Physics. [S0003-6951(00)04716-1].
Annealing temperature dependence of electric conduction and capacitance dispersion in nitrogen-implanted GaAs
(2001-05-01)
Chen, JF
;
Huang, MM
;
Wang, JS
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電子物理學系
;
Department of Electrophysics
The annealing behavior of nitrogen-implanted GaAs samples has been investigated by secondary ion mass spectroscopy, current-voltage (I-V) and capacitance-frequency (C-F) measurements. The I-V data show that the conductivity of as-implanted samples is dominated by variable-range hopping beta een defect states below 300 K. The implanted layer becomes highly resistive after annealing. The activation energy of the resistance is found to increase from 0.2 eV for as-implanted samples to 0.71 eV for 950 degreesC-annealed samples. Significant capacitance dispersion is observed over frequency for implanted samples. Based on a proposed equivalent circuit, the high-frequency capacitance dispersion is shown to be the result. of resistance-capacitance (RC) time constant effects. The increase of activation energy of the resistance can be explained by the creation of deep traps by high temperature annealing. Traps at 0.69 eV and 0.82 eV are detected for 700 degreesC and 950 degreesC-annealing, respectively.
Capacitance dispersion in n-LT-i-p GaAs structures with the low-temperature layers grown at different temperatures
(1999-12-01)
Chen, JF
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Chen, NC
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Wang, PY
;
Wang, JS
;
Weng, CM
The electrical properties of annealed low-temperature GaAs are studied by investigating the frequency-dependent capacitance of n-LT-i-p structures with the low-temperature (LT) layers grown at different temperatures. Relative to the sample grown at 610 degrees C, the samples grown at 200, 300 and 400 degrees C show significant capacitance dispersions over frequency which is explained by the emission of carriers from traps. Based on a proposed band diagram where a dominating trap at 0.66-0.74 eV exists in the LT layers, the high-frequency dispersion is shown to be affected by resistance-capacitance (RC) time constant effects. From the mid-frequency capacitance versus voltage characteristics, the concentrations of the occupied traps are estimated to be approximate to 10(17) cm(-3) for samples grown at 200, 300 and 400 degrees C, which are found to be consistent with those obtained from analyzing the current-voltage characteristics of n(+)-LT-n(+) structures.
Carrier depletion by defects levels in relaxed In0.2Ga0.8As/GaAs quantum-well Schottky diodes
(2000-02-01)
Chen, JF
;
Wang, PY
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Wang, JS
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Tsai, CY
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Chen, NC
An increase in leakage current accompanied by a drastic carrier depletion is found for InGaAs/GaAs Schottky diodes when the InGaAs thickness is larger than its critical thickness. Due to drastic carrier depletion, free-carrier concentration around the InGaAs region for relaxed samples cannot be obtained from capacitance-voltage data but from resistance-capacitance time constant effect observed in capacitance-frequency measurement. A trap at 0.33 to 0.49 eV is observed for relaxed samples by deep-level transient spectroscopy. The resistance caused by carrier depletion has an activation energy close to that of the trap, supporting that the carrier depletion is caused by capture from the trap. (C) 2000 American Institute of Physics. [S0021-8979(00)00603-4].
Carrier distribution and relaxation-induced defects of InAs/GaAs quantum dots
(2000-11-06)
Wang, JS
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Chen, JF
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Huang, JL
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Wang, PY
;
Guo, XJ
The carrier distribution and defects have been investigated in InAs/GaAs quantum dots by cross-sectional transmission electron microscopy (XTEM), capacitance-voltage, and deep level transient spectroscopy. Carrier confinement is found for 1.1- and 2.3-monolayer-(ML)-thick InAs samples. For 2.3 ML sample, XTEM images show the presence of defect-free self-assembled quantum dots. With further increase of the InAs thickness to 3.4 ML, significant carrier depletion caused by the relaxation is observed. In contrast to 1.1 and 2.3 ML samples in which no traps are detected, two broad traps and three discrete traps at 0.54, 0.40, and 0.34 eV are observed in 3.4 ML sample. The traps at 0.54 and 0.34 eV are found to be similar to the traps observed in relaxed In0.2Ga0.8As/GaAs single quantum well structures. By comparing with the XTEM images, the trap at 0.54 eV is identified to be the relaxation-induced dislocation trap in the GaAs layer. (C) 2000 American Institute of Physics. [S0003-6951(00)02245-2].
Characterization of electron emission from relaxed InAs quantum dots capped with InGaAs
(2005-07-01)
Chen, JF
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Hsiao, RS
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Wang, CK
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Wang, JS
;
Chi, JY
The effect of strain relaxation in a relaxed InAs quantum dot (QD) capped with InGaAs is investigated by admittance and deep-level transient spectroscopy (DLTS). Strain relaxation markedly increases the emission time in the QD region and extends carrier depletion to the bottom GaAs layer. The experimental data show the presence of relaxation traps in the QD region and the neighboring bottom GaAs.layer. The electron emission from the QD region is governed by a trap located at 0.17-0.21 eV below the QD ground state. The electron-escape process is identified as thermal activation at high temperatures and direct tunneling at low temperatures from the trap. In the bottom GaAs layer near the QD, DLTS reveals a relaxation trap at 0.37-0.41 eV relative to the GaAs conduction band. The energy difference between these two traps is comparable to the QD ground-state energy relative to the GaAs conduction-band edge, suggesting that the two traps may be the same trap which is pinned to the GaAs conduction band. The considerable difference between their properties may result from different atoms surrounding the trap. (c) 2005 American Institute of Physics.
Characterization of self-assembled InAs quantum dots with InAlAs/InGaAs strain-reduced layers by photoluminescence spectroscopy
(2005-04-15)
Chang, KP
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Yang, SL
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Chuu, DS
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Hsiao, RS
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Chen, JF
The optoelectronic characteristics of self-assembled InAs quantum dots (QDs) with strain-reduced layers (SRLs) were investigated using photoluminescence (PL) spectroscopy. Various SRLs that combine In0.14Al0.86As and In0.14Ga0.86As with the same total thickness were examined to ascertain their confining effect on carriers in InAs QDs. The emission wavelength is blueshifted as the thickness of InAlAs is increased. The energy separation between the ground state and the first excited state of QDs with InAlAs SRLs greatly exceeds that of QDs with InGaAs SRLs. Atomic force microscopic images and PL spectra of the QD samples demonstrated that high-quality InAs QDs with long emission wavelengths and a large energy separation can be generated by growing a low-temperature, thin InAlAs SRL onto self-assembled QDs. (C) 2005 American Institute of Physics.
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
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Lin, G
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Hsiao, RS
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Yang, CS
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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.
Design of a mathematical expression understanding system
(1997-03-01)
Lee, HJ
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Wang, JS
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交大名義發表
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資訊工程學系
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National Chiao Tung University
A scientific document usually consists of text and mathematical expressions. In this paper, we present a system for segmenting and understanding text and mathematical expressions in a document, The system can be divided into six stages: page segmentation and labeling, character segmentation, feature extraction, character recognition, expression formation, and error correction and expression extraction. After we extract all text lines in a document, we separate all symbols in each text line and calculate direction-feature vectors and aspect ratios for those symbols. Then, a nearest-neighbor algorithm recognizes characters. In the expression formation stage, we build a symbol relation tree for each text line that represents the relationships among the symbols in the text line. Each text line is decomposed into a collection of primitive tokens: operands, operators and separators. Heuristic rules based on these primitive tokens are used to correct text recognition errors. Finally, we extract all mathematical expressions according to basic expression forms. Several pages of documents were scanned to test the method. All mathematical expressions are understood. In the expressions generated, a few symbols are misrecognized. The average recognition rate was 96.16%. (C) 1997 Elsevier Science B.V.
Differential capacitance measurements of relaxation-induced defects in InGaAs/GaAs Schottky diodes
(2001-02-01)
Chen, JF
;
Chen, NC
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Wang, JS
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Chen, YF
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電子物理學系
The use of a differential capacitance technique for characterizing the relaxation-induced defect states in Schottky diodes has been studied, Based on a proposed equivalent circuit including the effect of potential drop across the carrier-depletion layer, a simple equation of capacitance at different voltages and frequencies is derived and compared with experimental data obtained from relaxed In0.2Ga0.8As/GaAs samples, It is shown that the carrier-depletion layer will introduce capacitance dispersion over frequency like traps; from it the device's geometric parameters, the resistance of the carrier-depletion lager and the ionization energy of the deep level that gives rise to this resistance can be obtained. The relation between the low-frequency capacitance and reverse voltage can be well explained by the depletion of the free carriers between the Schottky depletion and the carrier-depletion layer The relaxation-induced traps are believed to be at 0.535 and 0.36 eV, respectively, in the GaAs and In0.2Ga0.8As regions.
Effect of growth rate on the composition fluctuation of InGaAsN/GaAs single quantum wells
(2006-06-15)
Chen, JF
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Hsiao, RS
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Hsieh, PC
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Wang, JS
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Chi, JY
Effect of growth rate on the composition fluctuation is investigated in In0.34Ga0.66As0.98N0.02/GaAs single quantum wells (QWs) by photoluminescence (PL), transmission electron microscopy, and admittance spectroscopy. In an InGaAsN layer grown at a normal growth rate, the PL spectra show a low-energy bump at the tail of an InGaAsN emission, suggesting the presence of composition fluctuation. Lowering the growth rate degrades the composition fluctuation by segregating into bimodal phases of an InGaAsN and InGaAs-rich phase. Further lowering the growth rate leads to a three-dimensional growth and enhances the InGaAs-rich phase. The carrier distribution for the InGaAsN layer grown at the normal rate shows a carrier bump at the tail of a strong accumulation peak, suggesting the presence of an electron state below the QW ground state. The admittance spectroscopy shows that the activation energy (32 meV) of this electron state is comparable to the energy separation (30 meV) between the InGaAsN emission and the low-energy bump, and thus it is possible that the composition fluctuation actually induces an electron state closely below the QW ground state. (c) 2006 American Institute of Physics.
Effect of growth temperature on the electric properties of In0.12Ga0.88As/GaAs p-i-n multiple-quantum-well diodes
(2000-06-01)
Chen, JF
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Wang, PY
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Wang, JS
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Wong, HZ
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電子物理學系
The electric properties of In0.12Ga0.88As/GaAs p-i-n multiple-quantum-well (MQW) diodes, with the MQW layer grown at different temperatures by molecular beam epitaxy, have been investigated. Temperature-dependent current-voltage studies reveal a trap-filled limit current at a low temperature and a generation-recombination current via deep levels at high temperature for a 300 degrees C-grown sample. Frequency-dependent capacitance and deep-level transient spectroscopy reveal one majority trap at 0.73 eV and two minority traps at 0.71 and 0.43 eV. The 0.73 eV trap is also detected in 550 degrees C-grown samples, suggesting that it is a common defect in relaxed InGaAs/GaAs MQWs and probably originates from the defect states related to the strain relaxation. The 0.71 eV trap is believed to be the dominating deep level that governs the current conduction due to the activation energy observed in the current-voltage characteristics. (C) 2000 American Institute of Physics. [S0021-8979(00)00711-8].
Effect of incorporating an InAlAs layer on electron emission in self-assembled InAs quantum dots
(2006-01-01)
Chen, JF
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Hsiao, RS
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Hsieh, MF
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Wang, JS
;
Chi, JY
We perform capacitance-voltage and admittance spectroscopy to investigate the effect of incorporating an InAlAs layer before an InGaAs cap layer on electron emission in self-assembled InAs quantum dots (QDs). We show that this incorporation of a high potential barrier increases the emission time of the electrons thermally activated from the QD ground to the first excited state. The energy separation between the ground and first excited states in the conduction band increases from 57.2 to 79.1, to 89.2, and to 95.6 meV with increasing the thickness of the InAlAs layer from 0 to 10, to 14, and to 20 A. Combining with photoluminescence (PL) data, the ratios of the energy separation between the ground and first excited states in the conduction band and valence band are determined to be 7.3:2.7 and 7.8:2.2 for 0 and 10 A InAlAs, respectively. In addition, this incorporation is shown to blueshift the PL first excited state much larger than the ground state.
Effect of nitrogen incorporation into InAs layer in InAs/InGaAs self-assembled quantum dots
(2005-09-01)
Chen, JF
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Hsiao, RS
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Chen, YC
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Chen, YP
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Hsieh, MT
We present the results of nitrogen. incorporation into the InAs layer in InAs/InGaAs quantum dots (QDs). We show that this incorporation causes an asymmetric photoluminescence (PL) line shape and abnormally, large redshift of the PL peak from the quantum dots as temperature increases. In addition, this incorporation causes a large series resistance and a rapid increase in reverse current at a bias corresponding to the QD region. This effect is due to the formation of a deep trap at 0.34-0.41 eV, which depletes the carriers in the QD region. This depletion gives rise to a geometric resistance-capacitance time-constant effect for the ac conductivity of the free electrons in the top GaAs layer. This trap markedly alters the emission properties of the QD structure. The ac conductivity of the QD structure is governed by the thermal activation of trapped electrons rather than the electron emission from the QD confined states. This incorporation significantly increases the electron emission time from the QD region and, thus, can be used intentionally to modify the emission properties of the QD structure.
Efg1 involved in drug resistance by regulating the expression of ERG3 in Candida albicans
(2005-03-01)
Lo, HJ
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Wang, JS
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Lin, CY
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Chen, CG
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Hsiao, TY
The ERG3 gene in Candida albicans was identified as a gene whose mRNA level was higher in the cph1/cph1 efg1/efg1 double mutant than in the wild-type cells. Further study showed that Efg1, but not Cph1, negatively regulated ERG3. Mutations in EFG1 consistently increased the susceptibility of the cells to antifungal agents.
Epidemiology of haptoglobin phenotypes in chronic diseases
(2003-03-17)
Cheng, TM
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Pan, JP
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Wang, JS
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Mao, SJT
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生物科技學系
Evolution of conduction and interface states of laterally wet-oxidized AlGaAs with oxidation time
(2006-01-15)
Chen, JF
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Hsiao, RS
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Hung, WK
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Wang, JS
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Chi, JY
The conduction and interface states of laterally wet-oxidized GaAs-AlGaAs-GaAs structures after various oxidation times are investigated. Effective current blocking is achieved after 150 min oxidation and the conduction of current through the oxidized AlGaAs layer is controlled by the Poole-Frenkel mechanism, from which a relative dielectric constant of 7.07 is obtained. At an oxidation time of 15 min, capacitance-voltage spectra exhibit capacitance dispersion over frequency, implying the presence of an interface state. The intensity of the dispersion increases with increasing the oxidation time and admittance spectroscopy reveals a significant interface state at similar to 0.28 eV at 45 min. Further increasing the oxidation time to 150 min broadens the interface state to a set of continuous interface states from 0.19-0.31 eV with decreasing densities from 3x10(11) to 0.9x10(11) eV(-1) cm(-2) and generates fixed charges of about 9.1x10(11) cm(-2) in the oxidized layer. By comparison to a similar trap in a relaxed InGaAs/GaAs, the interface state is tentatively assigned to the interaction of residual As with dislocations. (c) 2006 American Institute of Physics.
Growth mode transfer of self-assembled CdSe quantum dots grown by molecular beam epitaxy
(2006-01-15)
Lai, YJ
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Lin, YC
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Fu, CP
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Yang, CS
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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.