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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 6070-6072 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The growth of Sr on n-type Si(001) was studied in detail for coverages between 0 and 1 monolayer (ML) using core level photoemission spectroscopy. In a similar manner, the Sr saturation coverage was studied in the 600–925 °C temperature range. Data analysis was carried out by a method that allows accurate determination of the band-bending shifts. Using this method it is possible to pinpoint the formation and destruction of chemical species from bungled core level photoemission data without needing to know details of the chemical composition of the spectra. Through this analysis it was established that the interaction between Sr and Si breaks down the binding energy difference between upward and downward Si dimer atoms. In addition, it was found that the saturation coverage exhibits a clear plateau at 1 ML around 650 °C, and a slope change at 1/3 ML around 850 °C. The surface band bending suffers a discontinuous increase as the Sr coverage surpasses 〈fraction SHAPE="CASE"〉12 ML and as the low energy electron diffraction symmetry changes from [2×3] to [2×1]. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 57 (1985), S. 5295-5301 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Transient capacitance and photocapacitance techniques have been used to study the characteristics of two electron traps related to Te in GaAs1−xPx: Te. Levels En1 and En2 have thermal activation energies of 0.17 and 0.27 eV, respectively, and their thermal electron emission and capture rates deviate markedly from Schockley–Read–Hall theory for near band gap crossover compositions. Such centers are found for 0.3〈x≤1, are linked to the X conduction band minima, and their photoionization thresholds are 0.5 and 1 eV, respectively. Trap concentrations have been studied as a function of Te doping level, Zn diffusion temperature, and N content (x〉0.4) in GaAsP LEDs. It is suggested that both defects belong to the DX type, and they have been described by a large lattice relaxation model. Franck–Condon energies of 0.3 and 0.95 eV have been determined, respectively. The properties of present Te-related defects are quite similar to donor related centers in AlxGa1−xAs, including the nonexponential capacitance transients found in near x∼0.4 compositions. It is important to mention that both centers have very large hole capture coefficients (σp〉10−14 cm2) and behave as efficient recombination centers.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 5431-5437 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This article focuses on the interaction of water with solid polymers. A portion of the water adsorbed in hydrophilic materials is strongly bound to individual sites (bound water), while an additional amount is bound less firmly to the polymeric chain (free water). Both free and bound water have been the subject of many studies that have uncovered, in some cases, qualitative evidence of the presence of water directly bound. In this article, we report a method to determine the different fractions of water adsorbed in hydrophilic materials. One fraction corresponds to the first layer in contact with the adsorbing solid and the other corresponds to the remaining water absorbed layers. We were able to determine said quantification by means of measuring the change in the natural vibration frequency brought about by the interaction of water molecules with the solid. The measurement was done on films of methylcellulose exposed to humid environments using infrared spectroscopy and gravimetric static microclime sorption methods. Theoretical predictions for bound water were developed using multilayer adsorption models. The experimental concentration of bound water as a function of relative humidity was compared with these predictions, allowing us to judge the applicability of different adsorption models employed in the study of gas condensation. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 59 (1986), S. 2235-2243 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In AlGaAs red light-emitting diodes fabricated by liquid-phase epitaxy the presence and characteristics of deep centers located near the injecting-active junction boundaries have been studied. Transient capacitance, DLTS, photocapacitance, and photocurrent techniques have been applied. Besides the presence, in the n-type injecting layer, of centers related to the Te dopant (DX defects), deep hole traps have been detected at both sides of the n-p heterojunction. The physical origin of such hole traps, present in moderately large concentrations, is discussed in terms of Zn-related complexes. This defect pattern allows us to explain the thermal- and photocapacitance, and the freeze-out and photocurrent characteristics found in such devices.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 660-661 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Deep-center characterization by deep-level transient spectroscopy (DLTS) allows a direct determination of the trap thermal emission activation energy. However, capture barrier energy measurements, based on trap partial filling by pulses of increasing width, require a quite different experimental processing and pose some hardware difficulties. In this letter we present a new method to determine the trap capture barrier energy, one that requires constant-width filling pulses and obtains capture information from standard DLTS data. This technique has been applied to Te-, Sn-, and Si-related DX centers in AlGaAs alloys.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 383-385 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Hydrostatic pressure techniques together with deep level transient spectroscopy (DLTS) measurements have shown that the Si-DX center in GaAlAs is linked to the L-conduction-band minimum. When hydrostatic pressure is applied to a 74% Al content sample, an exponential reduction of the DLTS signal is observed. This exponential dependence with pressure arises from the reduction of the DX filling factor (electron occupancy) due to the increasing X-L energy difference with pressure. Our results, together with previous data, also show that the capture barrier height originating from the lattice relaxation is an intrinsic parameter of both the material and the donor species, that does not depend on Al content or conduction-band structure.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 1877-1879 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Deep level transient spectroscopy in Si- and Sn-doped GaAlAs reveals a fine structure of the DX center thermal emission spectra under adequate filling pulse and sampling window times. This structure is reproducible in samples with Al mode fractions near 30% but it is not detectable in samples with 85% Al content. All resolved peaks of this fine structure have the same thermal emission energy but quite different capture cross section (σ∞n). This fact indicates that the origin of the fine structure and of the nonexponential behavior of the thermal emission processes is the discrete broadening of σ∞n due to the alloy effect.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 81 (2002), S. 1014-1016 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ultrathin nitrided SiO2/Si(001) films were studied using angle-resolved x-ray photoemission spectroscopy. The structure of the oxynitride depended on the nitridation process. Under one type of nitridation the film kept the structure of the SiO2, with N assuming O sites. By taking advantage of the nonuniformity on the chemical depth profile, the Si 2p chemical shift was determined for those Si atoms bonded to three O and one N atom, and for those bonded to two O and two N atoms. The stoichiometry depth profile was recognized through a simple method that allowed the input of physical constrains. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford UK : Blackwell Science Ltd.
    Journal of neurochemistry 74 (2000), S. 0 
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: The aim of this study was to investigate the role of cyclic AMP in the regulation of tryptophan hydroxylase activity localized in retinal photoreceptor cells of Xenopus laevis, where the enzyme plays a key role in circadian melatonin biosynthesis. In photoreceptor-enriched retinas that lack serotonergic neurons, tryptophan hydroxylase activity is markedly stimulated by treatments that increase intracellular levels of cyclic AMP or activate cyclic AMP-dependent protein kinase, including forskolin, phosphodiesterase inhibitors, and cyclic AMP analogues. In contrast, cyclic AMP has no effect on tryptophan hydroxylase mRNA abundance. Experiments using cycloheximide and actinomycin D demonstrate that cyclic AMP exerts its regulatory effect via posttranslational mechanisms mediated by cyclic AMP-dependent protein kinase. The effect of cyclic AMP is independent of the phase of the photoperiod, suggesting that the nucleotide is not a mediator of the circadian rhythm of tryptophan hydroxylase. Cyclic AMP accumulation is higher in darkness than in light, as is tryptophan hydroxylase activity. Furthermore, the stimulatory effect of forskolin and that of darkness are inhibited by H89, an inhibitor of cyclic AMP-dependent protein kinase. In conclusion, cyclic AMP may mediate the acute effects of light and darkness on tryptophan hydroxylase activity of retinal photoreceptor cells.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1523-5378
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Background. ABSTExperimental evidences have suggested that a Th1 response is unable to eliminate H. pylori colonization; whereas a Th2 response, like the one induced by vaccination, reduces H. pylori infection in animal models. Some parasitic infections induce a polarized Th2 response, which theoretically would favor a reduced H. pylori prevalence. The aim of this work was to study the possible association between parasitic infections and H. pylori prevalence.Materials and Methods. The study population included 120 children and 188 adults from a low socioeconomic level village. H. pylori prevalence was determined in serum by ELISA; parasitic infections were identified in feces by microscopic examination; and total serum IgE levels, as an indirect indicator of some parasitic infections, were determined by ELISA.Results. In children, H. pylori prevalence was no different between those with and without intestinal parasitic infection. By contrast, adults with intestinal parasitic infection had a significantly lower H. pylori prevalence than adults without parasites (62.6% compared with 80.4%; p = 0.006, OR 2.45). Also in adults, but not in children, total IgE levels were significantly higher in those without H. pylori infection than in those with H. pylori infection (p 〈 0.001).Conclusions. Intestinal parasitic infections and serum IgE levels showed an age-dependent association with H. pylori prevalence. In adults, but not in children, intestinal parasitic infections and increased IgE levels where associated with a reduced H. pylori prevalence.
    Type of Medium: Electronic Resource
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