Search Results - (Author, Cooperation:W. H. Chang)

2015-08-01

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2012-07-28

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2011-06-17

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Amino Acid Sequence ; Amino Acids/*chemistry/*pharmacology ; Amyloid/*antagonists & inhibitors/*chemistry/metabolism ; Amyloid beta-Peptides/antagonists & inhibitors/chemistry/metabolism ; Computer-Aided Design ; *Drug Design ; HIV Infections/virology ; Hydrogen Bonding ; Kinetics ; Models, Molecular ; Peptides/*chemistry/*pharmacology ; Polylysine/pharmacology ; Protein Conformation ; tau Proteins/antagonists & inhibitors

2013-11-01

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Animals ; Antibodies, Monoclonal/*therapeutic use ; Antibodies, Neutralizing/*therapeutic use ; DNA, Viral/blood ; HIV Antibodies/immunology ; HIV-1/*immunology ; Macaca mulatta ; Simian Acquired Immunodeficiency Syndrome/*therapy ; Simian Immunodeficiency Virus/*physiology ; T-Lymphocytes/immunology ; Viremia/therapy

2018-03-14

Royal Society

2054-5703

Natural Sciences in General

genomics, bioinformatics

1089-7550

AIP Digital Archive

Physics

The microstructure and electrical properties of nonalloyed epitaxial Au-Ge contacts were studied. Ohmic behavior was obtained after a 3 h anneal at 320 °C with the lowest average contact resistance and specific contact resistivity found to be ∼0.28 Ω mm and ∼7×10−6 Ω cm2, respectively. Localized reactions in the form of islands were observed across the surface of the contact after annealing and were composed of Au, Ge, and As, as determined by secondary ion mass spectroscopy (SIMS) imaging and Auger depth profiling. Back side SIMS profiles indicate deep Ge and Au diffusion into the GaAs substrate in the island regions. Ohmic contact behavior was found to depend upon both the kinetics of the reactions (localized reactions and island growth) and the thermodynamics (substantial diffusion of both Au and Ge) of the system. A model describing the coupled Au and Ge in-diffusion with respect to the GaAs substrate is presented.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A study of chemical etching and chemo-mechanical polishing of Hg0.8Cd0.2Te (MCT) with bromine-methanol has been carried out. It was found that the etch rate could be controlled down to 0.1 nm/s when 0.001% of bromine-methanol was used. Surface analysis using x-ray photoelectron spectroscopy indicated that differential etching of the constituents and accumulation of elemental tellurium occurred even when only a few monolayers were etched from a stoichiometric MCT surface. The relative etch rates were determined to be Cd(very-much-greater-than) Hg(approximately-greater-than)Te. Nevertheless, it was found that chemo-mechanical polishing could produce smooth surfaces with no significant accumulation of elemental tellurium. The production of such surfaces, however, required a balance of chemical etching and mechanical lapping. Furthermore, quick quenching of chemical etching was also required after chemo-mechanical polishing in order to prevent further surface degradation.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Raman spectroscopy has been used to characterize anodic films grown on n-Hg0.8Cd0.2Te from alkaline media. The spectral results, when compared with those from several reference chemicals, provide unambiguous identification of the anodic film as a 1:1 mixture of amorphous cadmium and mercury tellurites. Since the anodic film is transparent to visible light, Raman spectroscopy was also used to probe nondestructively the substrate surface under the film. The results indicate that the substrate surface was depleted in cadmium.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We present the scanning electron-filling modulation reflectance (SEFR) of charged In0.5Ga0.5As self-assembled quantum dots. The SEFR is performed by applying a small constant modulation voltage and scanning the dc bias through the quantum dot energy levels. The spectral intensity of the SEFR can be related to the electron density confined in each quantum state, which shows a good agreement with the apparent carrier concentration deduced from the capacitance–voltage measurements. In this study, we find that the transition energy of quantum-dot excited state is about 10 meV smaller than the values obtained from the photoluminescence and electron-filling modulation reflectance. This phenomenon is explained by the Coulomb interactions between the optical excited electron–hole pair and the electrons occupied in the quantum dots. © 2002 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A long-wavelength λc=18 μm infrared hot-electron transistor (IHET) with low dark current is demonstrated. In order to achieve long-wavelength absorption, a low barrier height is required, which in turn results in a large dark current. Therefore, operation of a normal long-wavelength quantum-well infrared photodetector (QWIP) structure is limited to very low temperatures and biases due to the thermally activated dark current. In the IHET, a high-energy pass filter placed after 30 periods of GaAs/AlGaAs quantum wells blocks the temperature-activated dark current while allowing high-energy photoexcited electrons to pass and be collected as photocurrent. A comparison of the dark current to the 300 K background photocurrent shows that the QWIP structure without the high-energy pass filter demonstrates background-limited infrared photodetection (BLIP) only at T≤35 K. Furthermore, in order to avoid saturating a typical readout circuit, detector operation of the QWIP is restricted to biases less than 0.08 V at 35 K. In contrast, the filtered dark current in the IHET is reduced by two to four orders of magnitude such that BLIP performance can be achieved for temperatures up to T=55 K without saturating the readout circuit. Because of the preferential current filtering effect, the noise equivalent temperature difference of the IHET can be improved by a factor of 100 at T=55 K. The dark-current-limited detectivity was found to be D*=1×1010 cm Hz1/2/W at λp=15 μm, Ve=−0.2 V, and T=55 K.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Effects of 35Cl+ bombardment of Si (100) surfaces were studied with a mass-separated low energy ion beam system operated under ultrahigh vacuum and with in situ x-ray photoelectron spectroscopy (XPS). Before ion bombardment, each silicon sample was etched with a hydrofluoric acid solution and rinsed in deionized water. XPS showed that the surface received no ion bombardment was effectively hydrogen passivated and had no silicon with an oxidation number higher than 2. However, oxygen was found which was probably present in the form of Si—OH or adsorbed water. Chlorine ion bombardment at room temperature initially drove the surface oxygen to the formation of silicon oxide which was subsequently etched off by further ion bombardment. The surface oxidation was initiated by the formation of Si—Cl bonds followed by the thermodynamically favorable replacement of Si—Cl with Si—O. The removal of the surface oxygen depended critically on the bombardment energy. At a bombardment energy of 1±0.6 eV, oxygen bonding was still observed even with a dose of 1018/cm2. The critical dose for oxygen depletion was found to be about 1×1018/cm2 for 40 eV bombardment, and about 1×1016/cm2 for 100 eV bombardment. Physical sputtering was the main oxide removal mechanism. Once the surface oxygen atoms were consumed, the silicon surface was etched by the chlorine ions with an enhancement by the chlorine chemistry. However, the absence of Si—Clx (x(approximately-greater-than)1) was clearly shown by XPS. Hence, bombardment must have promoted the desorption of the surface silicon chlorides.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A quantum-well infrared photodetector (QWIP) based on the corrugated light-coupling scheme has been fabricated and tested for remote temperature sensing. The QWIP consists of two stacks of multiple quantum wells (MQWs), each sensitive in one of the atmospheric infrared transmission windows and each with a separate readout circuit. High optical coupling efficiency is obtained in both wavelength ranges, demonstrating the use of the corrugated structure for two-color detection. By monitoring the ratio of the photocurrent generated simultaneously in each MQW stack, the temperature of the object emitting the radiation can be determined, regardless of its emissivity and the geometrical factors. This temperature sensing ability is tested by using a blackbody radiator with precision temperature control as the target. The agreement between the measured and the preset temperatures indicates that the corrugated QWIP is capable of precision thermometric measurements. © 1998 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Franz–Keldysh oscillations in Si-δ-doped GaAs have been studied by the application of fast Fourier transformations to the piezoreflectance spectra. In such studies, we find that the Franz–Keldysh oscillations of the piezoreflectance and its associated Fourier transformation can be described by a model calculation which considers the energy gap modulation in a uniform electric field. Owing to the character of nonelectromodulation, the Franz–Keldysh oscillation lineshape in the piezoreflectance has no modulation electric field distortion such as that which occurs in the photoreflectance and electroreflectance experiments. We demonstrate that the piezoreflectance accuracy is better than the photoreflectance or electroreflectance for the measurement of a uniform built-in electric field. The fast Fourier transformation of piezoreflectance spectra exhibits two separate heavy- and light-hole frequencies at temperatures from 20 to 425 K. Using these separated peaks, the ratio between the heavy- and light-hole effective reduced mass, the surface Fermi level, and the broadening parameters of the Franz–Keldysh oscillations are measured and discussed. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Franz–Keldysh oscillations from GaAs and AlGaAs structures have been studied and we find that the electric field obtained from the oscillations is in agreement with that derived from electrostatic calculations. Our results show that illumination from pump and probe beams in a normal photoreflectance experiment can significantly affect the measurement and thus erroneously lead to a reduced value of the electric field. The Fermi level on the bare surface of AlGaAs with different Al mole fraction has also been determined.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The line shapes of electromodulation spectra exhibit extended Franz–Keldysh oscillations and interference beats in the presence of a uniform built-in electric field. For this work, we used photoreflectance and contactless electroreflectance to study the Franz–Keldysh oscillations in Si-delta-doped GaAs. The fast Fourier transformation taken to the photoreflectance and contactless electroreflectance spectra, produced more complicated results than were observed in previous studies, when the effect of the modulation field is nonnegligible. This indicates that the interference beats are not only due to different effective heavy-hole and light-hole mass but also to the modulation field. We propose that the Franz–Keldysh oscillations generally contain four frequencies, which correspond to the heavy-hole and light-hole splitting. A comparison between the experiments and some numerical simulations attest to the validity of our proposal. This line shape analysis could then be applied to estimate the strength of a modulation field in contactless electroreflectance and photoreflectance. © 1998 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

In this communication, we present experimental evidence of the piezoelectric-field-induced quantum-confined Stark effect on In0.23Ga0.77N/GaN multiple quantum wells. The optical transitions in In0.23Ga0.77N/GaN p-i-n multiple quantum wells were studied by using electrotransmission (ET) at room temperature. Quantum-well-related signals are well resolved in our ET spectra. Since the strong internal electric field breaks the symmetry of the quantum wells, both the allowed and the forbidden transitions are observed. Clear energy blueshifts in accordance with increasing reversed bias are observed in ET spectra. The strength of piezoelectric field is found to be 1.7–1.9 MV/cm in the In0.23Ga0.77N strain quantum well layer, which is comparable with the measurement reported in the literature. We have shown experimentally how the piezoelectric field affects the energy shift for the strained multiple quantum wells. © 2002 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

This paper presents an overview of the status of a Josephson cache chip design at IBM in the Fall of 1983. Details of the design, organized as 1 K×4 bits, and employing a 2.5-μm niobium edge-junction technology, are found in the subsequent two adjoining papers. This paper presents, in a broader perspective than the adjoining papers, the background, motivation, changes from previous designs, and general difficulties of the design. General considerations related to the fabrication process and design methodology and an overview of components and the system environment are presented. A Pb-predecessor design is used as a point of reference; the discussions emphasize changes to that work. Finally, inherent design difficulties and remaining uncertainties are discussed.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A detailed optimized design of a 1 K-bit memory cell array with drivers and reset gates has been carried out based upon a set of projections for achievable tolerances in linewidths, resistances, and Josephson critical currents in a 2.5-μm technology employing niobium edge junctions. The cell operating regions were significantly widened relative to a predecessor Pb-alloy design by adjusting gate and cell inductances, adjusting current levels, and by employing a different timing sequence for application of write controls. Much-improved control of array-line current oscillations, without loss of speed, was achieved by employing a distributed filtering scheme using distributed damping. The design employs trimming of currents to accommodate ±8% chip-to-chip differences in the average critical current. The cell size is 63×63 μm. Monte Carlo calculations of threshold curve tolerances and operating current sensitivities and tolerances lead to a design-limited yield of about 95% for 4 K bits.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Designs for peripheral and timing circuits for a Josephson cache memory chip, organized as 1 K × 4-bits, are described. The designs were carried out employing a 2.5-μm minimum-linewidth niobium edge-junction technology, in conjunction with the memory cell and driver array design described in the preceding companion paper. Significant changes in decoding, sensing, and timing, relating to widening operating margins over a predecessor all-Pb-alloy design are described in detail. The resultant nominal chip access time and power are, respectively, 970 ps and 10 mW.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The performance of GaAs/AlGaAs quantum well infrared photodetectors specified in terms of background limited temperature Tb and specific detectivity D* has been calculated based on realistic detector parameters. It is found that for a detector with an external quantum efficiency η of 6.9%, Tb is 76 K for a cutoff wavelength of 10 μm. This value of Tb agrees with the recent experimental result and is significantly higher than the previous estimation given by M. A. Kinch and A. Yariv [Appl. Phys. Lett. 55, 2093 (1989)]. If η is unity, the projected Tb can be as high as 88 K with a D* of 2.2×1011 cm(square root of)Hz/W. For a lower temperature operation, D* increases to 7.5×1011 cm(square root of)Hz/W at 77 K, comparable to that of a HgCdTe detector.

Electronic Resource