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  • 1
    ISSN: 1432-1181
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Description / Table of Contents: Zusammenfassung Die zweidimensionale, stetige, laminare freie Konvektion in unterteilten Kammern von Solarkollektoren wurde numerisch untersucht. Die Wärmeübertragung entlang dieser Kammern wurde betrachtet. Ein iteratives Finite-Differenzen-Schema wurde angewandt um die Gleichungen, welche das Strömungsfeld beschreiben, zu lösen. Der Einfluß der Rayleigh-Zahl, der thermische Leitfähigkeit, des Kammerwinkels, des Neigungswinkels und der Längenverhältnisse auf die örtlichen und durchschnittlichen Wärmeübertragungskoeffizienten des Solarkollektors wurde diskutiert. Der Bereich der Rayleigh-Zahl erstreckte sich bis zu 5 × 104, das Verhältnis der thermischen Leitfähigkeit betrug 4.5 und 30, der Kammerwinkel lag zwischen 10° und 170°, der Neigungswinkel zwischen 10° und 90° und das Längenverhältnis variierte zwischen 0.2 und 10. Die Ergebnisse beinhalten, daß die konvektive Wärmeübertragung sehr stark durch das Längenverhältnis der Kammern beeinflußt wird.
    Notes: Abstract The two-dimensional, steady, laminar natural convection phenomena in partitioned enclosure of solar collector has been studied numerically. Heat conduction along the partition is considered. An iterative finite-difference scheme is employed to solve the governing equations in the flow field. The effects of Rayleigh number, thermal conductivity ratio, partition angle, tilt angle, and aspect ratio on both the local and average heat transfer coefficients of the solar collector have been discussed. The range of Rayleigh number tested was up to 5 × 104, the thermal conductivity ratio of 4.5 and 30, partition angle from 10 deg to 170 deg, tilt angle from 10 deg to 90 deg, and aspect ratio varied between 0.2 and 10. The results indicate that the convective heat transfer is strongly affected with the aspect ratio of the enclosures.
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  • 2
    ISSN: 1432-0630
    Keywords: 76.70.Hb ; 72.20.Jv ; 71.55.−i
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract The influence of free carriers on optically detected magnetic resonance (ODMR) signals for defects in silicon is discussed. The presence of free carriers induces a strong background signal in the ODMR spectrum due to carrier heating effects in a microwave field. This background signal often obscures a possible detection of a defect-related magnetic resonance signal and is therefore highly undesirable. To avoid this problem, a delayed ODMR (D-ODMR) technique is employed. On the other hand, the presence of free carriers provides a medium for a possible ODMR detection of nonradiative defects. This is realized by a shunt pass of carrier recombination at such defects, which competes with the radiative carrier recombinations detected optically, and gives rise to negative ODMR signals for the defects responsible for the nonradiative recombination. Typical examples from recent studies of these different cases of defects in silicon are demonstrated.
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  • 3
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The mechanism for low-temperature photoluminescence (PL) emissions in GaNAs epilayers and GaAs/GaNxAs1−x quantum well (QW) structures grown by molecular-beam epitaxy is studied in detail, employing PL, PL excitation, and time-resolved PL spectroscopies. It is shown that even though quantum confinement causes a strong blueshift of the GaNAs PL emission, its major characteristic properties are identical in both QW structures and epilayers. Based on the analysis of the PL line shape, its dependence on the excitation power and measurement temperature, as well as transient data, the PL emission is concluded to be caused by a recombination of excitons trapped by potential fluctuations in GaNAs. © 1999 American Institute of Physics.
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  • 4
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Influence of postgrowth hydrogen treatments on nonradiative recombination centers in undoped and B-doped Si epilayers, grown by molecular beam epitaxy at low temperatures, are studied by optical detection of magnetic resonance. Hydrogen passivation of the dominant nonradiative defects in undoped Si is shown to be rather effective, whereas in the B-doped Si epilayers the effects of the hydrogen treatment of the same defects are found to be only marginal. Possible mechanisms for this are discussed. Information on two new nonradiative defects is provided. © 1997 American Institute of Physics.
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  • 5
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Growth of Si and SiGe layers using molecular beam epitaxy was carried out with the substrate at a floating, positive or negative bias, in order to investigate effects of ion bombardment on the crystalline quality of grown materials. Although ion energies (100–1500 eV) and ion/atom flux ratios (∼0.005) used in the experiments were quite low, significant lattice distortion along the growth direction (Δa⊥/as up to ∼300 ppm) was observed by high resolution x-ray diffraction from the Si layers grown at 420 °C. At the same time, a broadband transition was observed in photoluminescence measurements from both Si and SiGe layers. Based on results of the annihilation behavior during postgrowth treatments using thermal annealing and hydrogenation, we attribute these effects to the ion bombardment induced formation and injection of different types of pointlike defects and defect clusters, which degrade the optical and electrical properties of grown layers. © 1996 American Institute of Physics.
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  • 6
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this letter, the dielectric breakdown characteristics of thermal oxides and N2O-based oxynitrides have been studied. A direct correlation was found between dielectric breakdown and the hole current generated within the gate dielectrics. The dependence of dielectric breakdown on oxide thickness was also studied. It was found that both charge-to-breakdown and hole-fluence-to-breakdown for the N2O oxynitrides were higher than those for the thermal oxides throughout the thickness range studied (33–87 A(ring)). The results suggest that N2O oxynitrides can sustain more damage before breakdown and thus have superior dielectric integrity compared to the thermal oxides. © 1995 American Institute of Physics.
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  • 7
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct experimental evidence of excitation transfer in the recombination via deep defects in silicon is provided, by a novel approach of Fourier transform photoluminescence excitation spectroscopy. The 735 meV iron-related defect in Si is used as a model case. It is shown that this defect can be excited via two deep states of other centers that give rise to broad lines in the excitation spectrum at 976.3 and 1115.6 meV. The linewidths of these features infer extremely efficient transfer processes. Such an efficient excitation transfer is expected to play an important role in excitation and recombination processes in silicon. © 1995 American Institute of Physics.
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  • 8
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The mechanism responsible for the semi-insulating properties of low-temperature-grown GaAs is investigated by determining the concentration of arsenic antisite (AsGa)-related defects in the material. The concentrations of the defects in neutral and positively charged states, As0Ga and As+Ga, are determined by near-infrared absorption and magnetic circular dichroism of absorption, respectively. Materials grown and annealed at different temperatures are investigated. We find that the defects are abundant in all samples studied, with the concentration of As0Ga higher than that of As+Ga defects. The results indicate that the defects can account for the pinning of the Fermi energy and consequently also the semi-insulating properties of the material. © 1994 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 2687-2689 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We reveal and investigate a possible lifetime-limiting defect in as-grown 6H SiC by optical detection of magnetic resonance (ODMR). This defect is shown to be a deep level center (with an energy level at about Ec−1.1 eV), evident from the related deep photoluminescence emission and a photo-excitation spectrum of the ODMR signal. The fact that this defect has been observed in both bulk crystals and epilayers, regardless of their doping type, indicates that this must be a common and basic defect in 6H SiC. © 1994 American Institute of Physics.
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  • 10
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The first observation of cyclotron resonance in 6H-SiC by optically detected cyclotron resonance (ODCR) spectroscopy at X-band microwave frequency is reported. High purity undoped, n-type 6H-SiC layers grown by chemical vapor deposition (CVD), with residual doping concentrations in the 1014–1015 cm−3 range, were investigated. Effective mass values were determined as m*⊥=(0.42±0.02)m0 and m*(parallel)=(2.0±0.2)m0. From the fit of the ODCR line shape, a remarkably high mobility at 6 K was deduced: μ⊥≈1.1×105 cm2/V s for electrons in the basal plane. The anisotropy of the effective mass and the carrier mobility is discussed in comparison with previously reported data. © 1994 American Institute of Physics.
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