Search Results - (Author, Cooperation:Z. Guan)

1440-1797

Blackwell Publishing Journal Backfiles 1879-2005

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Electronic Resource

2018-06-03

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2013-08-31

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Bacteria/*enzymology ; Bacterial Proteins/*chemistry/genetics ; Catalytic Domain ; Cell Wall/*chemistry/enzymology ; Crystallography, X-Ray ; Cytoplasm/enzymology ; Membrane Proteins/*chemistry/genetics ; Periplasm/enzymology ; Protein Conformation ; Protein Structure, Secondary ; Transferases/*chemistry/genetics

1089-7550

AIP Digital Archive

Physics

The phenomenon of simultaneous compositional disordering and the formation of electrical resistive layers induced by oxygen implantation in InGaAs/InP superlattices has been investigated. The disordering characteristics have been studied as a function of implantation temperature and ion dose. It was found that implantation at elevated temperatures (referred to as the IM or ion mixing process) usually leads to much more efficient disordering compared to implantation at room temperature followed by annealing at the same elevated temperature (referred to as the implantation plus annealing process). Of particular interest is the observation that ion mixing at 550 °C with 1×1013 O+/cm2 leads to significantly more disordering than implantation with the same dose at room temperature followed by annealing at 550 °C for the same period of ion mixing time. In addition, the electrical resistance of the ion-mixed layer at 550 °C increases 2600 times for the p-type InGaAs/InP superlattice structure, whereas the sample implanted at room temperature and annealed at 550 °C showed only a 20 times increase in electrical resistance. These results indicate a distinct advantage for the IM process in achieving simultaneous compositional disordering and electrical isolation for optoelectronic applications.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The effect of strain-induced band-gap modulation has been studied in a GaAs/AlGaAs multiple-quantum-well structure with the wells located at various depths in the structure. The energy change in the quantum wells was calculated based on simple elasticity theory and measured using photoluminescence on the structure where a thin-film stressor array was deposited. Metallic thin-film stressors were made by conventional thin-film deposition techniques followed by photolithography. It was found that the elasticity theory describes the energy changes reasonably well in comparison with the experimental results. For stressor layers that react with the heterojunction structure, the situation was more complex and requires more detailed analysis. Based on the calculated and experimental results it appears possible to fabricate quantum wire with lateral dimensions of less than 100 nm using thin-film technology and e-beam lithography. © 1996 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

Raman scattering studies were performed on ZnSe–ZnS strained-layer superlattices with different strains. In the optical phonon regime, the Raman spectra of the ZnSe–ZnS superlattices with a superlattice axis along [001] show optical phonons confined in the ZnSe well and ZnS barrier layers, and display a two-mode behavior corresponding to ZnSe-like and ZnS-like modes. We report the experimental results, by means of Raman scattering of confined longitudinal optical phonons in ZnSe–ZnS strained-layer superlattices. The Raman frequency shift dependence on the layer thickness, superlattice period, and interface strain is presented and discussed. © 1995 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

At 77 K, stimulated emission from n=1 light-hole excitons in a ZnSe-Zn0.8Cd0.2Se superlattice is observed and the oscillation mode properties of n=1 light-hole and heavy-hole excitons are analyzed. It is noticed that the ZnSe-Zn0.8Cd0.2Se superlattice with the shorter Fabry–Pérot cavity length d has the larger threshold excitation. The threshold of each mode has been determined using its time delay curve. © 1995 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

Brillouin scattering studies were performed on ZnSe-ZnSe1−xSx strained-layer superlattices with the composition x∼0.2. We have observed scattering from longitudinal guided modes (LGM) whose velocities are determined primarily by the C11 elastic constant. From the measured velocity of the first-order LGM of the quasitransverse acoustic mode and of the quasilongitudinal acoustic mode, the C11 and C44 elastic constants in ZnSe-ZnSe1−xSx superlattice film have been observed. © 1994 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

We have investigated the use of thin film technology to introduce controllable and thermally stable stress into semiconductor heterostructures. Two simple schemes are used. The first scheme is to use interfacial reactions between a metal and the substrate, such as Ni, Co, Pd, and Pt on GaAs/AlGaAs. The induced stress in the structure is reproducible and controllable because the volumetric change for a given reaction is fixed, as long as the deposited film is fully reacted to form a compound. The stability of the stress depends on the stability of the compound. In the case of Ni and Co on GaAs/AlGaAs, the induced stress is thermally stable up to 600 °C. Evaporated films and reacted films are usually under tension. The second scheme is to use rf sputtered W or WNi alloy films where W or WNi is sputtered onto a negative dc biased substrate. This scheme effectively provides highly compressed films. The thermal stability depends on the concentration of Ni in the WNi alloy. Using the two schemes above, we have fabricated low-loss (∼1 dB/cm at 1.52 μm wavelength) photoelastic waveguides in GaAs/AlGaAs heterostructures, and explored the interrelationship between the photoelastic waveguide characteristics and the stress. © 1995 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

Longitudinal acoustic and optical phonon modes of a ZnSe/ZnSxSe1−x (x(approximately-equal-to)0.20) lattice-mismatched superlattice, prepared with atmospheric metal organic chemical vapor deposition method, have been investigated by light scattering measurements. Despite a lattice mismatch as large as 1% between the alternating layers, the measured longitudinal elastic constants are in agreement with the calculated values of an unstrained effective medium model. Furthermore, a correlative study was made by fitting the spectra to a spatial correlation model, which reproduces line shapes of the observed confined longitudinal-optical modes without incorporating the strain effects. The results demonstrate that a combination of Brillouin and Raman spectroscopy provides a good method to determine accurately the elastic constants and strain information of the lattice-mismatched superlattices and heterostructures.

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1089-7550

AIP Digital Archive

Physics

Efficient 1.3 and 1.55 μm InP-based electroabsorption waveguide modulators with planar device structures have been demonstrated. Elevated temperature oxygen ion implantation and/or the photoelastic effect induced by W metal stressor stripes deposited on the semiconductor surface have been used to produce these self-aligned planar guided-wave devices. The oxygen ion mixing process has been used to simultaneously achieve compositional disordering and electrical isolation of superlattice material while the photoelastic effect has been used to improve the lateral mode confinement. A 1.3 μm Franz–Keldysh modulator with a (approximately-greater-than)10 dB extinction ratio at 2 V and a 1.55 μm device with a (approximately-greater-than)10 dB extinction ratio at 7 V are reported. These single growth step planar processing techniques have also been used to fabricate relatively low-loss (〈4 dB/cm) double heterostructure InGaAs(P)/InP single-mode optical waveguides which demonstrate their usefulness in developing InP-based photonic integrated circuits.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The principle of solid phase regrowth (SPR)has been used to induce compositional disordering in AlGaAs/GaAs superlattice structures in the temperature range of 400 °C (30 min)–650 °C (30 s) as compared to the conventional diffusion method in the temperature range of 600–850 °C for hours. The SPR process is simple to implement, requiring only thin-film deposition and annealing. The crystal quality as well as the photoluminescence signals emerging from the disordered region generally improve with increasing processing temperature. The simplicity, the low process temperature, and the short process duration of the SPR technique are distinct advantages for optoelectronic applications, especially for self-aligned devices.

Electronic Resource

1471-4159

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Protein levels of different acetylcholinesterase (AChE) splice variants were explored by a combination of immunoblot techniques, using two different antibodies, directed against the C-terminus of the AChE-R splice variant or the core domain common to all variants. Both AChE-R and AChE-S splice variants as well as several heavier AChE complexes were detected in brain homogenates from the parietal cortex of patients with or without Alzheimer's disease (AD) as well as the cerebrospinal fluid (CSF) of AD patients, compatible with the assumption that CSF AChEs might originate from CNS neurons. Long-term changes in the composition of CSF AChE variants were further pursued in AD patients treated with rivastigmine (n = 11) or tacrine (n = 17) in comparison to untreated AD patients (n = 5). In untreated patients, AChE-R was markedly reduced as compared with the baseline level (37%), whereas the medium size AChE-S complex was increased by 32%. Intriguingly, tacrine produced a general and profound up-regulation of all detected AChE variants (up to 117%), whereas rivastigmine treatment caused a mild and selective up-regulation of AChE-R (∼10%, p 〈 0.05). Moreover, the change in the ratio of AChE-R to AChE-S (R/S-ratio) strongly and positively correlated with sustained cognition at 12 months (p 〈 0.0001). Thus, evaluation of changes in the composition of CSF AChE variants may yield important information referring to the therapeutic efficacy and/or development of drug tolerance in AD patients treated with anti-cholinesterases.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

AlGaAs/GaAs double heterostructure transverse-electric (TE) waveguide polarizers, 5 mm long, with extinction ratios exceeding 20 dB and excess losses of 〈1 dB have been fabricated using simple thin film technology. The single-mode photoelastic waveguides were made by depositing either a thin Ni film (∼1000 A(ring)) with a 5-μm-wide window stripe pattern followed by thermal annealing or a thin W film (∼900 A(ring)) with a 5-μm stripe pattern using rf sputtering with the substrate under negative dc bias. The polarization mode selection is caused by the stress-induced birefrigence of the photoelastic waveguides. The change in dielectric constant due to stresses in the thin film structure was obtained from the experimental intensity output profile using an iterative calculation method. The tensile stress is 3×109 dyn/cm2 in the fully reacted Ni3GaAs stressor layer. The stress in the as-sputtered W films was compressive and decreased from 1×1010 dyn/cm2 to 4×109 dyn/cm2 after annealing between 300 and 500 °C for 30 min.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Single-mode silicon on insulator photoelastic waveguides and polarizers have been investigated. It was found that waveguides induced by the photoelastic effect can be low loss (1.5 dB/cm at 1.53 μm) and can serve as polarizers as well. A polarization distinction ratio of 14 dB for both TE and TM mode has been obtained. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Planar, low-loss AlGaAs/GaAs waveguides have been fabricated using the solid-phase regrowth (SPR) process. Single-mode waveguide with a propagation loss as low as 1.6 dB/cm have been obtained. This process requires only thin-film deposition and low-temperature short-duration annealing (i.e., 650 °C for 30 s), thus making the SPR method a much simplified technique to induce compositional disordering. Simultaneous electrical isolation and compositional disordering are also demonstrated with the SPR process.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The fabrication of low-loss photoelastic waveguides in GaAs/AlGaAs layered structures by thin film reactions is investigated. The waveguides are formed by opening a narrow window stripe, a few microns wide, in an otherwise continuous Ni layer under tension deposited on a semiconductor structure. The local tensile stress induced by the Ni layer in the semiconductor causes the local refractive index to increase, thus providing the guiding mechanism. Annealing the sample at 250 °C for 1 h induced an interfacial reaction between the Ni film and the substrate to form Ni3GaAs. The formation of an interfacial compound stabilizes the stresses, making the stress state independent of the deposition system and technique. Single-mode waveguide propagation losses as low as 1.4 dB/cm at 1.53 μm wavelength have been obtained on annealed waveguides. Further annealing up to 600 °C did not cause degradation in the optical confinement, thus indicating a thermally stable planar waveguide fabricated by this process. Other photoelastic optical devices such as polarizers, splitters, and couplers are also demonstrated.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The ohmic contact formation of Al/Ti on AlGaN/GaN heterostructure field effect transistors (HFETs) with and without Si implantation was investigated. Direct implantation and implantation through an AlN capping layer were studied. Compared to implantation through AlN, direct implantation is more effective in reducing the contact resistance. An Al(200 Å)/Ti(1500 Å) bilayer structure, called the "advancing" metallization, was used in this investigation to take advantage of consuming nearly all the top AlGaN layers for easy carrier access to the GaN layer underneath. Combining the direct implantation and the advancing metallization, low contact resistance of the order of 0.25 Ω mm (∼5.6×10−6 Ω cm2) can be readily obtained on HFET structures with an AlGaN layer about 340 Å thick and with an Al fraction of at least 22%. © 1999 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A group of well-defined exciton transitions from the localized states were observed in ZnSe-Zn0.75Cd0.25Se double-superlattice structure. The photoluminescence and photoreflectance have been employed to study the subband transitions at low temperatures. At 1.4 K, except the two ground states and two higher subbands of n=1 light-hole and n=2 heavy-hole excitonic transitions, other four peaks (A, B, C, and D) also were observed in wider-well superlattice. Those peaks were attributed to the excitonic transitions from n=2 heavy-hole subband due to the fluctuation of well-barrier interface. Another localized excitonic transition from narrower-well superlattice appeared as increasing the modulated intensity in photoreflectance spectra. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Silicon phase modulators are important silicon optoelectronic devices. The relationship of the refractive index change with injected carrier concentration is the basis for the operation of silicon phase modulators. No direct experimental data of this relationship have been reported. We have developed a new method for the direct measurement of this relationship using a Fabry–Perot interference and optical injection of carriers. The experimental results of the refractive index change are reported for the first time in the range of injected carrier concentration between 1013 and 1015 cm−3. It should be noted that our experiment results are about 5 to 10 times larger than those predicted by theory. The reliability of our experiments is also discussed. © 1996 American Institute of Physics.

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