Search Results - (Author, Cooperation:M. B. Johnson)

2014-02-18

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

*Alternative Splicing ; Animals ; Base Sequence ; Biological Evolution ; Body Patterning/*genetics ; Cats ; Cell Proliferation ; Cerebral Cortex/anatomy & histology/cytology/*embryology ; Codon, Nonsense ; Frontal Lobe/anatomy & histology/cytology/embryology ; Genetic Variation ; Haplotypes ; Humans ; Mice ; Molecular Sequence Data ; Neural Stem Cells/cytology/*physiology ; Pedigree ; Promoter Regions, Genetic/genetics ; Receptors, G-Protein-Coupled/*genetics ; Sequence Deletion

2011-10-28

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Adolescent ; Adult ; Aged ; Aged, 80 and over ; Aging/*genetics ; Brain/embryology/*growth & development/*metabolism ; Child ; Child, Preschool ; Exons/genetics ; Female ; Fetus/metabolism ; *Gene Expression Profiling ; Gene Expression Regulation, Developmental/*genetics ; Gene Regulatory Networks/genetics ; Humans ; Infant ; Male ; Middle Aged ; Quality Control ; Quantitative Trait Loci/genetics ; Sex Characteristics ; Time Factors ; Transcriptome/*genetics ; Young Adult

2015-11-05

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Allosteric Regulation/drug effects ; Animals ; Antiviral Agents/*pharmacology ; Biphenyl Compounds/*pharmacology ; Cell Line ; Drug Resistance, Viral/*drug effects ; Drug Synergism ; Drug Therapy, Combination ; Hepacivirus/*drug effects/*genetics/metabolism ; Hepatitis C/virology ; Hepatocytes/transplantation ; Humans ; Imidazoles/*pharmacology ; Mice ; Models, Molecular ; Protein Conformation/drug effects ; Protein Multimerization/drug effects ; Protein Structure, Quaternary/drug effects ; Reproducibility of Results ; Viral Nonstructural Proteins/chemistry/genetics/*metabolism ; Virus Replication/drug effects

1089-7550

AIP Digital Archive

Physics

We have used cross-sectional scanning tunneling spectroscopy and cross-sectional transmission electron microscopy to perform two-dimensional profiling of shallow pn junctions in Si metal-oxide-semiconductor structures. In the tunneling spectroscopy measurements, clear differences were observed between current-voltage spectra obtained from p-type, n-type, and depleted regions of the metal-oxide-semiconductor structures; current images generated from the tunneling spectra revealed the profiles of the pn junctions with spatial resolution in both the lateral and vertical directions on the order of 10 nm. Calculated tunneling current-voltage spectra were found to be consistent with observed differences in experimental spectra obtained from p-type and n-type regions of the junctions. Junction profiles obtained by scanning tunneling spectroscopy have also been compared to transmission electron microscopy images of chemically delineated junctions and measurements of vertical dopant profiles obtained by secondary ion mass spectrometry. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the observation of interband exciton transitions in InSb/AlxIn1−xSb multi-quantum-well samples. The exciton peaks are identified with the use of a simple quantum well model. The strain present in the InSb wells alters the spectrum significantly from that for unstrained III–V materials and makes it possible to use the exciton spectrum in determining the band offset. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The tunneling time for electrons to escape from the lowest quasibound state in the quantum wells of GaAs/AlAs/GaAs/AlAs/GaAs double-barrier heterostructures with barriers between 16 and 62 A(ring) has been measured at 80 K using photoluminescence excitation correlation spectroscopy. The decay time for samples with barrier thicknesses from 16 A(ring) (≈12 ps) to 34 A(ring)(≈800 ps) depends exponentially on barrier thickness, in good agreement with calculations of electron tunneling time derived from the energy width of the resonance. Electron and heavy hole carrier densities are observed to decay at the same rate, indicating a coupling between the two decay processes.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Photoluminescence excitation correlation spectroscopy has been used to measure the dependence of carrier lifetime on the H+ ion implantation dose in GaAs. For doses greater than 1×1012 cm−2 the carrier lifetime is inversely proportional to the ion dose. The minimum lifetime measured was 0.6±0.2 ps for a dose of 1×1014 cm−2. Most important, there is no sign of saturation of carrier lifetime with ion dose down to this lifetime, thus still shorter lifetimes should be achievable with increased ion dose.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We present a study of the temporal decay of photoluminescence (PL) as detected by picosecond excitation correlation spectroscopy (PECS). We analyze the correlation signal that is obtained from two simple models; one where radiative recombination dominates, the other where trapping processes dominate. It is found that radiative recombination alone does not lead to a correlation signal. Parallel trapping type processes are found to be required to see a signal. To illustrate this technique, we examine the temporal decay of the PL signal for In-alloyed, semi-insulating GaAs substrates. We find that the PL signal indicates a carrier lifetime of roughly 100 ps, for excitation densities of 1×1016–5×1017 cm−3. PECS is shown to be an easy technique to measure the ultrafast temporal behavior of PL processes because it requires no ultrafast photon detection. It is particularly well suited to measuring carrier lifetimes.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Atomically resolved cross-sectional scanning tunneling microscope topographic images of heterostructures that include a sequence of 1, 2, 5, and 10 nm AlGaAs and GaAs layers are presented. These layers clearly show alloy fluctuations and interface roughness on an atomic scale. In the thick AlGaAs layers a mottled structure with regions of higher Al content about 2 nm wide and elongated in [1¯12] or [11¯2] directions are observed. Similarly, the interfaces are rough on a 2 nm length scale. These results suggest that, for the conditions used for the epitaxial growth of the ternary layers, Al-rich regions nucleate and grow anisotropically.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the direct measurement of dopant and carrier density profiles in GaAs (001) modulation-doped structures using cross-sectional scanning tunneling microscopy. On ultrahigh-vacuum-cleaved cross-sectional (110) GaAs surfaces, individual electrically active dopants are observed as hillocks in the top several surface layers, and the tip-sample separation is found to be sensitive to the carrier concentration. In structures where the conventionally measured p-type dopant concentration varied from 1×1018 to 1×1019 cm−3 the density of such dopant hillocks varies accordingly and the tip-sample separation changes by 0.1 nm.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the direct imaging of individual electrically active dopants on cross-sectionally cleaved GaAs using scanning tunneling microscopy and compare these results to theory. The observation of these dopants is due to an enhancement in the tunneling current in the neighborhood of an ionized dopant atom in the top several surface layers. In highly p-doped GaAs, for tunneling out of the valence band, the dopants in the top several surface layers appear as individual circular hillocks about 2 nm in diameter, superimposed on the As sublattice, as expected. From the size of the hillock and the symmetry of the As sublattice enhanced by the hillock one can infer whether the dopant lies in the top, second or deeper layers.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report on the imaging of a molecular beam epitaxially grown GaAs/(AlAs)n(GaAs)m quantum well-wire array by means of cross-sectional scanning tunneling microscopy (XSTM) and scanning tunneling-induced luminescence (STL). XSTM provides atomically resolved cross-sectional images of sets of quantum well wires with chemical sensitivity within the group III species and electrical sensitivity to single dopant atoms. This permits the precise observation of growth mechanisms and the identification of defects responsible for inhomogeneities in the growth morphology, as well as the determination of dopant incorporation throughout the structure. STL permits the relative quantum efficiency of individual quantum wires to be quantified.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The scanning surface harmonic microscope, in which a microwave signal is applied across a tip-sample tunneling gap and higher harmonics are detected, is sensitive to the capacitance/voltage characteristics of semiconductor samples on a nanometer scale. We demonstrate its sensitivity to a wide range of dopant concentrations on Si, and its applications as a dopant profiler. Depletion regions are delineated with remarkable sensitivity, and variations in dopant concentration over a 35-nm scale are discussed. Indications of a 5 nm resolution have been obtained. © 1994 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Potential distributions across Si(001)p-n junctions have been studied using cross-sectional scanning tunneling microscopy, spectroscopy, and potentiometry. A clear transition between p- and n-type material can be seen across each junction, and variations in the energy of the conduction-band edge can be detected with a spatial resolution of better than 100 A(ring). Current-voltage characteristics have been measured in both unbiased and electrically biased structures, and measurements under both conditions are consistent with calculated potential distributions.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The incorporation of In in the growth of crescent-shaped In0.12Ga0.88As quantum wires embedded in (AlAs)4(GaAs)8 superlattice barriers is studied in atomic detail using cross-sectional scanning tunneling microscopy. It is found that the In distribution in both the surface and the first subsurface layer can be atomically resolved in the empty- and filled-state images, respectively. Strong In segregation is seen at the InGaAs/GaAs interfaces, but neither an expected enhancement of the In concentration at the center of the quantum wire compared to the planar quantum well nor In clustering beyond the statistical expectation is observed. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We used local probe techniques to characterize electron beam (e-beam) induced changes in thin oxides on silicon. Primary effects of the 1 nm wide, 300 keV e beam included the formation of positive charges trapped in the SiO2, physical restructuring in the oxide, and deposition of carbonaceous compounds. Charges remained stable in thicker oxides (460 nm) and appeared as changes in the contact potential or microwave response with widths down to 100 nm. In thinner oxides (20 nm) the amount of charge was smaller and less stable; below 7 nm no charge was detected. Physical changes in the oxide, evident as a swelling of irradiated areas, accounted for the etching selectivity of these regions. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Three types of defects at the surface of InSb quantum well samples are identified: hillocks, square mounds, and oriented abrupt steps. The electron mobility in the quantum well correlates to the density of abrupt features, such that samples with a high density of anisotropic defects show anisotropy in the mobility. We propose that the dominant scattering mechanism associated with these abrupt features is a fluctuation in the quantum well morphology. © 2002 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Photoluminescence excitation correlation spectroscopy is used to space and time resolve photoluminescence (PL) intensity variations in the region of isolated dislocations in as-grown In-alloyed GaAs. Spatially resolved PL maps show an annulus of high intensity with an inner and outer dark background surrounding a dislocation. Typical inner and outer diameters of the annuli are about 200 and 400 μm, respectively, which is smaller than the average dislocation separation in In-alloyed GaAs. Temporal resolution of the PL measures the carrier lifetime in the bright and dark regions. These measurements show that the lifetime variation accounts for the PL intensity variation. The variation of the lifetime with temperature indicates that the defects governing the lifetimes in the bright and dark regions are different. Moreover, both defects are deep and neither defect is EL2.

Electronic Resource

0362-2525

Life and Medical Sciences ; Cell & Developmental Biology

Wiley InterScience Backfile Collection 1832-2000

Biology

Medicine

A morphological and cytometric analysis of the adult fat body cells and oenocytes was made on sections of abdomens from immature, mature and senescent Drosophila melanogaster of both sexes. There are about 18,000 fat body cells in abdomens of female and mature male flies. Immature and senescent males have about 12,000 and 15,000 cells, respectively. The size of the cells is almost the same for immature flies of both sexes and increases about six-fold to approximately 2600μm2, so that mature flies of both sexes have equivalent amounts of fat body tissue. The proportions of lipid, glycogen, and background cytoplasm of fat body cells also remain relatively constant throughout adult life, but dense, proteinaceous granules are observed in cells of senescent flies. The amounts of cellular components change dramatically due to change of cell size with age; the amount of lipid shows the greatest sexual difference with about 2 × more in the females at all stages studied. The oenocytes number about 6,000 in the abdomens of all but immature male flies, which have approximately 4,000. Although the cells of both sexes triple in size to about 700 μm2, the oenocytes of males reach maximum size earlier than those of females. The major features of oenocytes appear to be dense background cytoplasm, putative lipid droplets found only in mature flies, and pigmented granules first seen in the cells of mature flies which accumulate with age to 33% of the cytoplasm. The number of cells and their anticipated capacity for protein synthesis is discussed in relation to the production of yolk protein precursors.

1 Ill.

Electronic Resource