Search Results - (Author, Cooperation:A. L. Powell)

2012-06-30

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

Chloroplasts/*genetics/physiology ; Chromosome Mapping ; Chromosomes, Plant ; Fruit/genetics/growth & development ; Lycopersicon esculentum/*genetics/*growth & development ; Phenotype ; Plant Proteins/*genetics/physiology ; Transcription Factors/*genetics/physiology

1089-7550

AIP Digital Archive

Physics

Deep level transient spectroscopy (DLTS) and capacitance-voltage measurements at various temperatures have been used to characterize defects in Si-doped (Ga1−xAlx)1−yInyAs materials for x=0.3 and different values of y (0, 0.005, and 0.07). We only detect DX centers, those associated with the doping impurity (Si), but also others associated with Te and, eventually, Sn not introduced intentionally. When the experimental conditions are chosen to obtain exponential transients, the shape of the DLTS spectrum and its variation with the filling pulse duration can be accounted for by this contamination; i.e., no sign of the so-called alloying effect is detected.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A series of unintentionally doped Ga1−xAlxAs epitaxial layers grown by molecular beam epitaxy, having various Al compositions (0.15, 0.18, and 0.29), have been Li diffused at 300 °C. Capacitance–voltage techniques show that the initial n-type doping concentrations (of the order of 1016 cm−3) increase after diffusion by typically a factor 5 to 10 demonstrating that isolated Li interstitials do exist and behave as donors. However, secondary-ion mass spectroscopy measurements show that the material contains 1019–1020 Li cm−3, which indicates that a large fraction of the Li impurities is not electrically active. Thus Li also produces defects as revealed by the fact that free electrons are frozen in the diffused layers below 77 K. Search for the existence of Li associated DX centers deep defects related to the donor impurities has been performed by deep level transient spectroscopy. Only the DX centers present before diffusion, i.e., associated with residual donor impurities, are detected in the layer of 0.29 Al fraction. This implies that either Li donors do not induce the existence of DX centers or electron emission from Li DX centers occurs below 77 K.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The properties of deep donor states (DX centers) in III-V alloys are discussed in relation to their influence on device characteristics and performance. The techniques to avoid or minimize such deleterious effects in AlGaAs-based devices are discussed, along with their physical basis, and some guidelines for improved III-V device design are established. New results about the benefits of proper donor selection, the role of In alloying, the advantage of δ doping in layers and in modulation-doped devices, and the use of AlInAs and InGaP as alternative wide band-gap III-V alloys are presented.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The ionization processes of Se-related DX centers have been studied in AlGaAs Schottky diodes under high reverse bias conditions. A spectroscopy technique that provides directly the free-electron concentration has been used. Besides the well known thermal and optical electron emission processes, a new mechanism, attributed to an impact ionization process of DX centers, is described, and its kinetics is analyzed.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

DX centers in In-mixed AlGaAs alloys are analyzed by deep level transient spectroscopy and capacitance vs temperature measurements. The addition of In to Si-doped AlGaAs, with x=0.21 and 0.30, shifts the Si-DX center to a shallower position. Under hydrostatic pressure, DX centers deepen again into the band gap. The DX center shift, and consequently, the reduction of the DX center electron occupancy, when In is added, is due to an increase of the Γ to L energy difference. In terms of band-gap energy and DX center depth, adding 1% In is equivalent to a 1% Al reduction. Then, In mixing does not offer any new benefit to minimize DX center effects in AlGaAs-based heterojunction devices.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We report on a persistent decrease of the dark conductivity in AlGaAs/GaAs heterostructures due to illumination. The decrease was observed for photon energies between 0.7 and 1.15 eV and larger than 1.4 eV in the temperature range 170〈T〈300 K. Using proper bias conditions the dark conductivity after illumination can be 20% smaller than the dark conductivity in thermal equilibrium. The studies have been performed on samples with different doping species and compositions. A possible model for the observed behavior is discussed.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] 'Prize' is an electromagnetic shock-tube using pre-ionization in the Z direction followed by a theta pinch; its main function is to form a high-temperatur e plasma in deuterium gas by shock heating1 without the ubiquitous trapped magnetic field usually associated with theta pinches and described ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] EXTENSIVE use has been made of high-speed 1% photography in plasma research but only to record plasma geometry, brightness, and the presence of instabilities and shock-waves. We now describe a new technique for recording time variations in a longitudinal magnetic field which makes use of the ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] THE theoretical work of Kahn1 and Goldsworthy2 on the gas dynamical effects of ionized HH regions associated with the formation of O and B stars has enabled them to predict the formation of weak R-type ionization fronts in interstellar gas. Axford3 has also examined the existence of weak R-type ...

Electronic Resource

0037-7996

History

0037-7996

History

1573-6776

Springer Online Journal Archives 1860-2000

Process Engineering, Biotechnology, Nutrition Technology

Summary Acetate was inhibitory to the growth of early induced E. coli cells and their expression of fusion protein, transforming growth factor-α-Pseudomonas exotoxin 40 (TGFα-PE40), but the inhibitory level was strain dependent For E. coli JM109 (pTAC-TGF57-PE40), 2 g/L of added acetate (3 g/L of total acetate in the medium) decreased TGFa-PE40 production by 38.0%. Acetate was less inhibitory to E. coli RR1, and RR1 was not affected by adding 2 g/L of acetate. However, 5 g/L of added acetate (6.7 g/L of total acetate in the medium) decreased TGFα-PE40 production by 21.2%. These results indicate that higher acetate concentration was associated with inhibition of TGFα-PE40 expression of E. coli JM109 during late induction.

Electronic Resource

0006-3592

recombinant Escherichia coli ; microbial physiology ; TGFα-PE40 ; primary metabolites ; fermentation monitoring ; Chemistry ; Biochemistry and Biotechnology

Wiley InterScience Backfile Collection 1832-2000

Biology

Process Engineering, Biotechnology, Nutrition Technology

Physiological effects of isopropyl-thiogalactopyranoside (IPTG) induction were examined in Escherichia coli strain JM109 expressing a fusion protein composed of transforming growth factor alpha and a 40-kD portion of Pseudomonas aeruginosa exotoxin A (TGFα-PE40) under control of the tac promoter. Fermentations at the 15-L scale in complex medium compared growth and metabolite profiles of the untransformed JM109 host strain, the strain transformed with the vector lacking the TGFα-PE40 open reading frame (JM109[pKK2.7]), and the strain with the complete plasmid for TGFα-PE40 expression (JM109[pTAC-TGF57-PE40]). Metabolite and growth profiles of JM109 (pTAC-TGF57-PE40) cultures changed significantly in IPTG-induced versus uninduced cultures. Prior to induction, glucose was metabolized to acetate or completely oxidized to CO2. Following induction, pyruvate was also excreted in addition to acetate. In the absence of inducer, pyruvate was excreted by JM109 (pTAC-TGF57-PE40) only when dissolved oxygen levels fell to less than 10% of saturation (microaerobic rather than anaerobic conditions). The untransformed JM109 host strain or JM109 (pKK2.7) did not excrete pyruvate in the presence or absence of inducer, although JM109 (pKK2.7) exhibited a pattern of growth following addition of IPTG that closely resembled JM109 (pTAC-TFG57-PE40). Fermentations of JM109 (pTAC-TFG57-PE40) in a synthetic medium supported lower expression levels, but resulted in similar alterations in metabolite profiles. Induction in synthetic medium resulted in pyruvate excretion without further acetate accumulation. Taken together, these data suggest that one consequence of TGFα-PE40 expression in JM109 is altered patterns of pyruvate oxidation. © 1992 John Wiley & Sons, Inc.

3 Ill.

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