Browsing by Author "Wang, PY"

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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
;
Wang, PY
;
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.
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
;
Tsai, CY
;
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].
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
;
Wang, PY
;
Wang, JS
;
Wong, HZ
;
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].
Effects of high-resistivity, low-temperature layer in transient capacitance measurements of GaAs n-i-p structures
(1998-10-15)
Chen, JF
;
Wang, PY
;
Chen, NC
;
電子物理學系
;
Department of Electrophysics
Deep-level transient spectroscopy and transient capacitance measurements are performed on a molecular-beam-epitaxially grown GaAs n-i-p diode with a 2000-Angstrom-thick low-temperature OLT)-grown layer immersed in its intrinsic region. The transient capacitance measurements reveal that the time constant and activation energy are the same for both the emission and capture processes. An equivalent circuit based on capacitance-frequency spectra is derived and used to obtain the resistivity values of the LT layer that are in agreement with experimental results. It is concluded that the transient capacitance observed corresponds to the resistance-capacitance time constant due to the LT-layer. In addition, the value of the activation energy is explained based on the equivalent circuit.
Electrical characteristics and deep-level admittance spectroscopies of low-temperature grown GaAs p-i-n structures
(1997-02-01)
Chen, JF
;
Chen, NC
;
Wang, PY
;
Tsai, MH
;
交大名義發表
The properties of low-temperature grown GaAs are studied via the electrical characterization of p-i-n structures with part of the intrinsic layer grown at 300 degrees C. Comparisons are made between the low-temperature and normally grown samples. The current of the low-temperature sample is about two orders of magnitude higher than that of the normally grown sample in both forward and reverse bias. From temperature-dependent analysis, the leakage current of the low-temperature sample is contributed by the recombination current through defect levels around the midgap, from which a recombination lifetime of 9.4X10(-12) a was obtained. By using admittance spectroscopy we observed a dominant electron level at 0.60 eV with a corresponding capture cross section of 1.0x10(-13) cm(2) that was not observed in the normally grown sample; thus it is believed to be introduced by the As-rich low-temperature layer. (C) 1997 American Institute of Physics.
Low frequency negative capacitance behavior of molecular beam epitaxial GaAs n-low temperature-i-p structure with low temperature layer grown at a low temperature
(1998-03-02)
Chen, NC
;
Wang, PY
;
Chen, JF
;
電子物理學系
;
Department of Electrophysics
The GaAs sample under study is a n-low temperature-i-p structure grown by molecular beam epitaxy with a low-temperature (LT) layer grown at 300 degrees C and annealed at 620 degrees C for 1 h. Admittance measurements on this sample reveal a negative capacitance at low frequency. This work analyzes the origin of the negative capacitance and its corresponding frequency-dependent conductance by combining two current components: charging-discharging current and the inertial conducting current. Analysis results indicate that the activation energies and time constants of both current components closely resemble each other and should correspond to the same trap. Based on the results presented herein, we can conclude that the negative capacitance at low frequency provides evidence of a generation-recombination center with an activation energy of 0.77 eV in the LT layer. (C) 1998 American Institute of Physics.
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.
Observation of abnormal capacitance-frequency behavior in In0.12Ga0.88As/GaAs p-i-n superlattice grown at low temperature
(1999-08-23)
Chen, JF
;
Wang, JS
;
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].
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
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Tsai, CY
;
Wang, JS
;
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].
Properties of defect traps in triple-stack InAs/GaAs quantum dots and effect of annealing
(2004-09-15)
Chen, JF
;
Hsiao, RS
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Shih, SH
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Wang, PY
;
Wang, JS
The electrical and optical properties of the defect traps, with and without annealing, in InAs/GaAs quantum dots (QDs) emitting at 1.3 mum are investigated by capacitance-voltage (C-V), deep-level transient spectroscopy (DLTS) and photoluminescence (PL). When increasing the InAs thickness to 3 ML, an abnormal temperature dependence of the C-V characteristic was observed in the triple-stack InAs/GaAs QD sample. This temperature dependence is attributed to the defect levels at 0.39 and 0.54 eV observed in DLTS. The level at 0.39 eV, found in the top GaAs barrier, is probably related to the relaxation-induced dislocations. The level at 0.54 eV is found close to the QD region. Rapid thermal annealing can reduce the concentrations of both levels. Comparing with PL result, which shows a blueshift of 140 meV and linewidth narrowing in the QD emission by annealing at 800degreesC, the level at 0.54 eV is speculated to be strain- or relaxation-related defects in the interface between the QDs and the barrier. Removal of this level by high-temperature RTA is important since this level lies close to the QDs and influences the optical quality of the QDs.
Role of 0.66 eV dominant trap in annealed low-temperature grown molecular beam epitaxial GaAs
(1998-02-01)
Chen, NC
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Wang, PY
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Chen, JF
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電子物理學系
;
Department of Electrophysics
The sample considered herein is an annealed low-temperature (LT) molecular beam epitaxially grown GaAs of n-LT-i-p structure with the LT layer grown at 300 degrees C. Characteristics involving the dominant trap level located at about 0.66 eV below the conduction band are obtained by analyzing the data of the admittance spectroscopy, capacitance-voltage, current-voltage, and frequency-dependent conductance experiments. This trap pins the fermi level of the LT layer and makes the LT layer semi-insulating. In this structure, the level interacts with both the conduction band and the valence band with a hole emission time constant characterized by an activation energy of 0.77 eV and a cross section of 1.1 X 10(-13) cm(2). This level is also an effective generation-recombination center when the temperature exceeds 300 K. (C) 1998 American Institute of Physics. [S0021-8979(98)04303-5].
Strain relaxation in In0.2Ga0.8As/GaAs quantum-well structures by x-ray diffraction and photoluminescence
(2000-02-01)
Chen, JF
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Wang, PY
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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].
Structural and electrical properties of GaSb, AlGaSb and their heterostructures grown on GaAs by metalorganic chemical vapor deposition
(1996-03-01)
Wang, PY
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Chen, JF
;
Chen, WK
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電子物理學系
;
Department of Electrophysics
A systematic study of structural and electrical properties of GaSb and AlGaSb grown on GaAs by metalorganic chemical vapor deposition is reported. In general, the results obtained from surface morphologies, X-ray linewidths and Hall properties are consistent with each other and indicate that the optimal growth conditions for GaSb are at 525 degrees C around V/III = 1. A highest hole mobility of 652 cm(2)/V . s at RT (3208 cm(2)/V . s at 77 K) and a lowest concentration of 2.8 x 10(16) cm(-3) (1.2 X 10(15) cm(-3) at 77 K) were obtained for GaSb grown under this optimal condition. Compared to the GaSb growth, a smaller V/III ratio is needed for the AlGaSb growth to protect the surface morphology. When Al was incorporated into GaSb growth, mobility decreased and carrier concentration increased sharply. The AlGaSb grown at 600 degrees C had a background concentration about one order of magnitude lower than the AlGaSb grown at 680 degrees C. Room-temperature current-voltage characteristics of GaSb/AlxGa1-xSb/GaSb show a rectifying feature when Al composition x is higher than 0.3, suggesting a valence-band discontinuity at the AlGaSb/GaSb interface. A leakage current much higher than the value predicted by the thermionic emission theory is observed at 77 K, presumably due to a large number of dislocations generated by the huge lattice mismatch between GaSb and GaAs.
Temperature-dependent transport properties of n(+) GaAs/low-temperature GaAs/n(+) GaAs structures grown by molecular beam epitaxy
(1996-06-01)
Chen, JF
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Chen, NC
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Chiu, SY
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Wang, PY
;
Lee, WI
The temperature-dependent I-V characteristics of n(+) GaAs low-temperature GaAs(AlGaAs) n(+) GaAs structures in which the low-temperature layers were grown at 250, 350, and 450 degrees C were analyzed. Band conduction with an activation energy of 0.72 eV dominates at T>250 K. Hopping conduction dominates at T<250 K, where the resistivity was found to be insensitive to temperature. From this analysis, it is shown that Fermi level is pinned to an acceptorlike deep level of about 10(17) cm(-3), which lies at 0.72 eV below the conduction band. Measured capacitance can be described in terms of a parallel-plate capacitance with separation being equal to the expected growth thickness. Majority traps (electrons) were observed by deep-level transient spectroscopy with an activation energy about 0.72 eV, confirming the result of the resistivity analysis. In addition, the I-V characteristics were fitted to the simulated curves based on a simplified space-charge limited theory and the result was found to be consistent with the resistivity analysis. (C) 1996 American Institute of Physics.
Temperature-dependent transport properties of n(+) GaAs/low-temperature GaAs/n(+) GaAs structures grown by molecular beam epitaxy
(1996-06-01)
Chen, JF
;
Chen, NC
;
Chiu, SY
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Wang, PY
;
Lee, WI
The temperature-dependent I-V characteristics of n(+) GaAs low-temperature GaAs(AlGaAs) n(+) GaAs structures in which the low-temperature layers were grown at 250, 350, and 450 degrees C were analyzed. Band conduction with an activation energy of 0.72 eV dominates at T>250 K. Hopping conduction dominates at T<250 K, where the resistivity was found to be insensitive to temperature. From this analysis, it is shown that Fermi level is pinned to an acceptorlike deep level of about 10(17) cm(-3), which lies at 0.72 eV below the conduction band. Measured capacitance can be described in terms of a parallel-plate capacitance with separation being equal to the expected growth thickness. Majority traps (electrons) were observed by deep-level transient spectroscopy with an activation energy about 0.72 eV, confirming the result of the resistivity analysis. In addition, the I-V characteristics were fitted to the simulated curves based on a simplified space-charge limited theory and the result was found to be consistent with the resistivity analysis. (C) 1996 American Institute of Physics.
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].
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.