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Author: search.filters.author.Wang, JS

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Now showing 1 - 10 of 36
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    Transition of carrier distribution from a strained to relaxed state in InGaAs/GaAs quantum well
    (1999-03-01)
    Wang, PY
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    Chen, JF
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    Wang, JS
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    Chen, NC
    ;
    Chen, YS
    This work investigates the transition of carrier distribution from the strained to the relaxed state in In0.2Ga0.8As/GaAs quantum well by measuring capacitance voltage and analyzing x-ray diffraction. According to those results, there is carrier confinement in the In0.2Ga0.8As quantum well with well thickness less than the critical thickness. Increasing the well thickness beyond the critical thickness leads to a significant carrier depletion around the quantum well. Double-crystal x-ray rocking curves reveal that when InGaAs well thickness increases beyond the critical thickness, the interference pattern disappears and relaxation begins to occur from near the bottom InGaAs/GaAs interface while the top interface still remains strained. Results obtained from the critical thickness determined from x-ray diffraction correspond to the transition of carrier distribution, illustrating that the capacitance-voltage measurement is a rather effective means of determining the critical thickness. (C) 1999 American Institute of Physics. [S0021-8979(99)01505-4].
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    Observation of abnormal capacitance-frequency behavior in In0.12Ga0.88As/GaAs p-i-n superlattice grown at low temperature
    (1999-08-23)
    Chen, JF
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    Wang, JS
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    Wang, PY
    ;
    Wong, HZ
    ;
    電子物理學系
    Capacitance-frequency measurement is used to study In0.12Ga0.88As/GaAs p-i-n superlattice, with superlattice layer grown at 300 degrees C by molecular-beam epitaxy. Three traps are observed, and their parameters are consistent with those obtained from deep-level transient spectroscopy. Among these three traps, the trap (E-a = 0.45 +/- 0.02 eV, sigma = 6 +/- 4x10(-17) cm(2)) shows an abnormal increase of capacitance with increasing frequency, similar to that observed from the 0.66 eV trap in low-temperature grown GaAs p-i-n structure, suggesting that it is created by the low-temperature growth and is a generation-recombination center. This result also shows that the capacitance-frequency measurement is effective in studying the generation-recombination centers. (C) 1999 American Institute of Physics. [S0003-6951(99)01434-5].
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    Observation of carrier depletion and emission effects on capacitance dispersion in relaxed In0.2Ga0.8As/GaAs quantum wells
    (1999-10-18)
    Chen, JF
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    Wang, PY
    ;
    Tsai, CY
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    Wang, JS
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    Chen, NC
    Strong changes in capacitance over frequency are found for highly relaxed In0.2Ga0.8As/GaAs quantum well. The high-frequency dispersion is explained by a resistance-capacitance time constant effect due to the existence of a high resistive layer while the low-frequency dispersion is due to carrier emission from traps. The high-resistance layer is created by carrier depletion when InGaAs thickness increases beyond the critical thickness. Excellent agreement is found between the data from capacitance-frequency spectra and deep-level transient spectroscopy, permitting us to conclude that both the carrier depletion and emission effects observed in capacitance-frequency spectra are due to the existence of an acceptor trap at 0.33 eV. This trap is generated when the InGaAs thickness is beyond its critical thickness and is due to defect states associated with misfit dislocations. (C) 1999 American Institute of Physics. [S0003-6951(99)02342-6].
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    Observation of a dominant EL2-like mid-gap trap in In0.12Ga0.88As/GaAs superlattice grown at low temperature by molecular beam epitaxy
    (1999-11-01)
    Chen, JF
    ;
    Wang, PY
    ;
    Wang, JS
    ;
    Wong, HZ
    ;
    電子物理學系
    Admittance spectroscopy is used to study a low-temperature (LT)-grown relaxed In0.12Ga0.88As/GaAs p-i-n superlattice. The capacitance-frequency spectra show two step-like trapping effects which are explained by the existence of two traps (E-a = 0.73 eV, sigma = 4.6 x 10(-11) cm(2) and E-a = 0.75 eV, sigma = 3.7 x 10(-15) cm(2)) in the LT-grown superlattice region. These two traps are the dominating defects observed in deep-level transient spectroscopy with one being a majority trap and the other being a minority trap. The emission parameters and photo-capacitance quenching effect for the 0.75 eV trap agree with those known for EL2 defects, suggesting that the EL2 defect is strongly enhanced in InGaAs/GaAs by LT growth.
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    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
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    Wang, JS
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    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.
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    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
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    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.
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    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].
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    Strain relaxation in In0.2Ga0.8As/GaAs quantum-well structures by x-ray diffraction and photoluminescence
    (2000-02-01)
    Chen, JF
    ;
    Wang, PY
    ;
    Wang, JS
    ;
    Chen, NC
    ;
    Guo, XJ
    The onset of strain relaxation in In0.2Ga0.8As/GaAs quantum-well structures is investigated. X-ray diffraction shows that when the InGaAs thickness increases beyond its critical thickness, another peak on the right shoulder of the GaAs peak appears, indicating that the top GaAs layer is being compressed in the growth direction by the relaxation of the InGaAs layer. Energy shifts of 44 and 49 meV are observed, respectively, from the strains of the InGaAs and GaAs top layers when increasing the InGaAs thickness from 300 and 1000 Angstrom. These energy shifts are in agreement with theory calculated based on the relaxation process observed in x-ray diffraction, providing evidence that the relaxation occurs from near the bottom InGaAs/GaAs interface while the top interface still remains strained. This result is further corroborated by the images of cross-sectional transmission electron micrographs which show that most of the misfit dislocations are confined near the bottom interface. (C) 2000 American Institute of Physics. [S0021-8979(00)07303-5].
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    Voltage and frequency dependence of differential capacitance in relaxed In0.2Ga0.8As/GaAs Schottky diodes
    (2000-03-01)
    Chen, JF
    ;
    Chen, NC
    ;
    Wang, JS
    ;
    Wang, PY
    ;
    電子物理學系
    Capacitance dispersion over frequency is investigated for relaxed In0.2Ga0.8As/GaAs Schottky diodes. While the high-frequency capacitance is voltage-independent, the low-frequency capacitance is seen to decrease with reverse voltage. Based on a Schottky barrier combined with a high-resistance layer containing traps, a simplified equation for the differential capacitance is derived to explain the capacitance-voltage-frequency relation. It is found that the high-frequency capacitance corresponds to the total thickness of the Schottky depletion and the high-resistance layer, while the low-frequency capacitance at a small reverse voltage is the Schottky depletion capacitance and at a large reverse voltage is the high-frequency capacitance.
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    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
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    Chen, NC
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    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].