Browsing by Author "CHANG, KH"

Now showing 1 - 20 of 24

2-COLOR INFRARED PHOTODETECTOR USING GAAS/ALGAAS AND STRAINED INGAAS/ALGAAS MULTIQUANTUM WELLS
(1993-06-28)
TSAI, KL
;
CHANG, KH
;
LEE, CP
;
HUANG, KF
;
TSANG, JS
A two-color infrared detector using GaAs/A]GaAs and strained InGaAs/AlGaAs multiquantum wells is demonstrated. The response peak of the GaAs/AlGaAs quantum well is at 8 mum, and that of the InGaAs/AlGaAs quantum well is at 5.3 mum. The responsivity of the detector is 1 A/W at 8 mum and 0.27 A/W at 5.3 mum; these are the best values reported for a two-color quantum well infrared detectors (QWIPs) with peak sensitivities in the spectral regions of 3-5.3 mum and 7.5-14 mum. Single-colored 5.3 and 8 mum QWIPs were also fabricated to study the bias dependent behavior. This behavior can be explained using the concept of current continuity. Because of a higher electrical resistance, a high electric field domain is always formed first in the shorter wavelength quantum well stack.
2-DIMENSIONAL BI-PERIODIC GRATING-COUPLED ONE-COLOR AND 2-COLOR QUANTUM-WELL INFRARED PHOTODETECTORS
(1995-02-01)
TSAI, KL
;
LEE, CP
;
TSANG, JS
;
CHEN, HR
;
CHANG, KH
Multiple quantum well infrared photodetectors (QWIP's) with two-dimensional bi-periodic grating couplers have been demonstrated. For one-color QWIP's, the response linewidths of such QWIP's are found to be much wider than those of conventional grating coupled QWIP's. The bi-periodic gratings have also been applied to two color QWIP's to separately optimize the absorption at two wavelength bands. The large difference in responsivities at the two wavelength bands for devices with single period gratings can be greatly reduced with the use of bi-periodic gratings.
A NOVEL TECHNIQUE FOR LOW-THRESHOLD AND HIGH-POWER INGAAS/GAAS STRAINED-LAYER 0.98-MU-M BURIED HETEROSTRUCTURE LASER FABRICATION
(1992-02-01)
LIOU, DC
;
CHIANG, WH
;
LEE, CP
;
CHANG, KH
;
LIU, DG
A novel fabrication technique has been developed for InGaAs/GaAs strained-layer buried heterostructure lasers. Dielectric masks and Zn diffusion are not required in this technique. This novel fabrication process is much easier than the conventional approach and yields excellent laser results. A low threshold of 3 mA and high-power operation for lasing wavelength of 9800 +/- 20 angstrom have been achieved with graded index separate confinement heterostructure devices using this novel technique.
ASYMMETRIC DARK CURRENT IN QUANTUM-WELL INFRARED PHOTODETECTORS
(1994-05-02)
TSAI, KL
;
LEE, CP
;
CHANG, KH
;
LIU, DC
;
CHEN, HR
Asymmetric dark current versus voltage characteristics in quantum weil infrared photodetectors have been studied. A model based on asymmetrical potential barriers was proposed. The asymmetrical potential barriers, which are most likely due to the accumulation of oxygen impurities at one of the interfaces, cause the asymmetrical I-V characteristics. The height of the potential spike is found to increase with the Al content in the AlGaAs barriers. Calculations based on our model agree well with experimental results.
BEHAVIOR OF THE 1ST LAYER GROWTH IN GAAS MOLECULAR-BEAM EPITAXY
(1990-10-01)
LIU, DG
;
LEE, CP
;
CHANG, KH
;
WU, JS
;
LIOU, DC
BINDING-ENERGIES OF D- ION IN GAAS QUANTUM-WELL
(1994-04-01)
CHANG, YH
;
YEH, JJ
;
SHEU, YM
;
WANG, CC
;
CHEN, TC
Magnetic field dependent binding energies of the D- ion in the center of a 210 angstrom GaAs quantum well are determined by temperature magneto-transport measurments. The binding energies increase from 2.1 meV at 2T to 4 meV at 8T, and are consistently higher than the transition energies ootained from magneto-optical measurements performed on the same sample. We conclude from these data that in the magneto-optical measurement the observed transitions are between ground and excited D- states.
CHARACTERIZATION OF IMPROVED ALGAAS/GAAS RESONANT TUNNELING HETEROSTRUCTURE BIPOLAR-TRANSISTORS
(1991-02-01)
WU, JS
;
CHANG, CY
;
LEE, CP
;
CHANG, KH
;
LIU, DG
We report on the fabrication of AlGaAs/GaAs resonant tunneling heterostructure bipolar transistors (RTHBT's). The devices exhibit a current peak in the transfer characteristics, with peak-to-valley current ratios of 1.7 and 9 at 300 K and 77 K, respectively. The common-emitter small signal current gains at 300 K and 77 K reach 40 and 28, respectively. They are the best results to date for AlGaAs/GaAs resonant tunneling transistors. Because the double barriers are placed in the emitter and far from the emitter-base interface, and the heterostructure emitter suppresses the hole injection from the base to the emitter, the occurrence of the negative differential resistance (NDR) in the device characteristics is governed by the emitter current but not by the base current. The operation mechanism of the NDR behavior in the common-emitter output characteristics is discussed.
DELTA-DOPED QUANTUM-WELL STRUCTURES GROWN BY MOLECULAR-BEAM EPITAXY
(1990-10-29)
LIU, DG
;
LEE, CP
;
CHANG, KH
;
WU, JS
;
LIOU, DC
DIRECT OBSERVATION OF SI DELTA-DOPED GAAS BY TRANSMISSION ELECTRON-MICROSCOPY
(1992-05-25)
LIU, DG
;
FAN, JC
;
LEE, CP
;
TSAI, CM
;
CHANG, KH
Direct observation of the Si delta-doped layer in GaAs has been achieved by high resolution transmission electron microscopy. Samples with different Si doses, from half a monolayer to two monolayers, were studied. The observed spreading of the delta-doped layer showed that Si atoms are largely confined in five monolayers at most (in the highest dose case), indicating excellent confinements of dopants in GaAs. From the images, the Si atoms were uniformly distributed in the doped layer, no cluster formation was observed. For delta-doped GaAs grown at low temperature (480-degrees-C), stacking faults originated from the doped layers were observed. These faults were thought to be caused by the large unrelaxed strain in the low-temperature grown GaAs.
ELECTRICAL CHARACTERISTICS OF DOUBLE-BARRIER RESONANT TUNNELING STRUCTURES WITH DIFFERENT ELECTRODE DOPING CONCENTRATIONS
(1991-04-01)
WU, JS
;
CHANG, CY
;
LEE, CP
;
CHANG, KH
;
LIU, DG
The electrical characteristics of AlGaAs/GaAs double-barrier resonant tunneling structures with various electrode doping concentrations have been studied. It was found that the peak current and the valley current are not sensitive to the change in the doping level of the electrodes. The peak-current voltage and the valley-current voltage, however, increase when the electrode doping level is lowered. These behaviors are explained using the band-bending effect in the electrodes. For devices with lightly doped or undoped electrodes, the quantum size effect in the accumulation layer due to the strong band bending causes additional kinks in the current-voltage characteristics. They are attributed to the resonant tunneling of electrons from the quantized levels in the accumulation layer. The quantum effect also accounts for the improved negative differential resistance behavior.
HIGH-QUALITY ALGAAS LAYERS GROWN BY MOLECULAR-BEAM EPITAXY AT LOW-TEMPERATURES
(1992-03-01)
CHANG, KH
;
WU, JS
;
LIU, DG
;
LIOU, DC
;
LEE, CP
Low-temperature (approximately 600-degrees-C) molecular beam epitaxy (MBE) growth of AlGaAs has been studied. It was found that the quality of AlGaAs grown at low temperatures can be as good as that grown at high temperatures ( > 700-degrees-C) if the source materials and the growth chamber are very clean. The threshold currents of Al0.6Ga0.4As/Al0.15Ga0.85As/Al0.6Ga0.4As double heterostructure (DH) lasers grown at low temperatures and high temperatures are almost the same. The material quality can be further improved with a proper amount of indium doping. Photoluminescence (PL) linewidths of 3.1 meV and 1.7 meV have been measured for In-doped Al0.42Ga0.58As and Al0.18Ga0.82As at 4 K, respectively. They are the narrowest linewidths for the MBE-grown AlGaAs with comparable Al contents at any growth temperature. With a proper amount of In doping, double-barrier resonant tunnelling diodes have also shown improved peak-to-valley current ratios.
IMPROVED ALGAAS/GAAS DOUBLE-BARRIER RESONANT TUNNELING STRUCTURES USING 2-DIMENSIONAL SOURCE ELECTRONS
(1991-01-15)
WU, JS
;
LEE, CP
;
CHANG, CY
;
CHANG, KH
;
LIU, DG
We report the enhancement of peak-to-valley current ratios (PVCRs) of double-barrier resonant tunneling structures (DBRTSs) based on the AlGaAs/GaAs material system. The PVCRs as high as 25.4 and 18 have been obtained at 77 K for superlattice and alloy barrier structures with 0.2-mu-m undoped electrodes, respectively. These are the largest PVCRs to date for AlGaAs/GaAs DBRTSs. The large band bending across the undoped electrodes causes size quantization of the accumulation layer, resulting in better resonant tunneling characteristics.
INFLUENCE OF INDIUM DOPING ON ALGAAS LAYERS GROWN BY MOLECULAR-BEAM EPITAXY
(1990-10-15)
CHANG, KH
;
LEE, CP
;
WU, JS
;
LIU, DG
;
LIOU, DC
INFLUENCE OF OXYGEN ON THE PERFORMANCE OF GAAS/ALGAAS QUANTUM-WELL INFRARED PHOTODETECTORS
(1994-07-01)
TSAI, KL
;
LEE, CP
;
CHANG, KH
;
CHEN, HR
;
TSANG, JS
The influence of oxygen on the performance of GaAs/AlGaAs quantum well infrared photodetectors (QWIP) has been studied. Photoluminescence and secondary ion mass spectroscopy were used to examine the relationship between the quality of AlGaAs and the oxygen content that were then correlated to the performance of QWIPs. It was found that oxygen is the dominant impurity in the GaAs/AlGaAs superlattice. Because oxygen atoms behave likes electron traps, which effectively reduce the carrier concentration in the material, both of the responsivity and the dark current of QWIP are reduced. The detectivities of QWIPs with lower oxygen concentrations are better than those with higher oxygen concentrations.
INFLUENCE OF THIN PROTECTIVE INAS LAYERS ON THE OPTICAL-QUALITY OF ALGAAS AND QUANTUM-WELLS
(1992-09-15)
TSAI, KL
;
CHANG, KH
;
LEE, CP
;
HUANG, KF
;
CHANG, Y
Influence of a thin protective but subsequently evaporated InAs layer on the regrowth of AlGaAs has been studied. It was found tha although most of the InAs could be evaporated by thermal desorption, some would react with Al0.36Ga0.64As to form InAlGaAs. These InAlGaAs islands act as potential wells for carrier recombination and dominate the photoluminescence spectrum. Transmission electron microscopy photographs show that dislocations are formed near the islands. These defects are caused by lattice mismatch between AlGaAs and InAlGaAs. These islands and defects strongly affect the optical quality of surrounding Al0.36Ga0.64As and quantum wells.
INTRINSIC AND EXTRINSIC EFFECTS ON PERFORMANCE LIMITATION OF ALGAAS/GAAS DOUBLE-BARRIER RESONANT TUNNELING STRUCTURES
(1992-05-01)
WU, JS
;
LEE, CP
;
CHANG, CY
;
CHANG, KH
;
LIU, DG
The performance limitations of negative differential resistance (NDR) in AlGaAs/GaAs double-barrier resonant tunneling structures (DBRTSs) have been investigated by varying the barrier thickness and quantum well (QW) width, and by doping In into the barrier layers. For devices with thick barriers (greater-than-or-equal-to 70 angstrom), the scattering in the material is believed to cause the saturation of the peak-to-valley current ratio (PVCR). The dependence of PVCR on the well width, however, is found to be, to the first-order, not related to the scattering mechanisms or other material properties. It is rather an inherent property of the tunneling process under different conditions. On the other hand, it was found that a proper amount of In doping into the barrier layers can increase the PVCR, because of the lower defect concentration, resulting in lower scattering rate in the active region.
INVESTIGATION OF INDIUM DOPING AND INCORPORATION IN ALGAAS/GAAS DOUBLE-BARRIER RESONANT TUNNELING STRUCTURES
(1991-02-28)
WU, JS
;
CHANG, KH
;
LEE, CP
;
CHANG, CY
;
LIU, DG
In doping and incorporation in the barrier layers of AlGaAs/GaAs double-barrier resonant tunnelling structures (DBRTSs) have been studied. It was found that the peak-to-valley current ratio (PVCR) can be improved by the proper amount of In doping. This is attributed to the improvement in the quality of the AlGaAs barrier layers due to the high surface migration rate of In atoms that reduces group III vacancies. Also pseudomorphic In(x)(Al0.5Ga0.5)(1-x)As/GaAs (x = 0.12) strained-layer DBRTSs have been fabricated by incorporating a sufficient amount of In into the AlGaAs barrier layers. PVCRs as high as 27.5 at 77 K have been obtained. This is the first realisation of such DBRTSs with lattice-mismatched quaternary barrier layers.
PERIODIC FLUX INTERRUPTION AND SUSTAINED TWO-DIMENSIONAL GROWTH FOR MOLECULAR-BEAM EPITAXY
(1989-11-23)
LEE, CP
;
CHANG, KH
;
LIU, DG
;
WU, JS
;
交大名義發表
PHOTOREFLECTION STUDY ON THE SURFACE ELECTRIC-FIELD OF DELTA-DOPED GAAS GROWN BY MOLECULAR-BEAM EPITAXY
(1992-08-15)
LIU, DG
;
CHANG, KH
;
LEE, CP
;
HSU, TM
;
TIEN, YC
Photoreflectance spectroscopy has been used to study the surface electric-field strength and the surface potential of delta-doped GaAs. Franz-Keldysh oscillations in the reflectance spectra were clearly observed and the oscillation periods were used to calculate the internal electric fields of the delta-doped samples. Based on the measured results and the self-consistent calculation, a surface potential of 0.6 eV is obtained.
PRECISE DETERMINATION OF ALUMINUM CONTENT IN ALGAAS
(1991-11-01)
CHANG, KH
;
LEE, CP
;
WU, JS
;
LIU, DG
;
LIOU, DC
The Al composition of AlGaAs has been determined by four methods: high-resolution transmission electron microscopy (HRTEM), reflection high-energy electron diffraction (RHEED), photoluminescence (PL), and double-crystal x-ray diffraction (DCXRD). HRTEM is direct and the most accurate method because it does not involve any formula or extrapolation. Using the result obtained from HRTEM as a standard, we have calibrated the results from other methods. RHEED intensity oscillation is found to be accurate and reliable, if the growth conditions are correctly chosen. Comparing the PL results with those determined from HRTEM and RHEED, we suggest three formulas to determine the Al contents at different temperatures. We also proposed a polynomial to determine the Al concentration using the DCXRD measurement.