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  • 1
    Electronic Resource
    Electronic Resource
    Effect of substrate photoexcitation on channel conduction in a modulation-doped AlxGa1−xAs/GaAs heterostructure (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 2488-2494 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: If a modulation-doped AlGaAs/GaAs heterostructure is illuminated by light, photoexcitation of deep levels in the GaAs substrate leads to some interesting effects. Below 100 K, the heterostructure shows a persistent photoconductivity effect. Moreover, a strong persistent channel depletion is observed at low temperatures when a small negative voltage is applied to the substrate contact (backgate). The latter effect is explained by a double-layer model of GaAs where the GaAs side of the heterostructure consists of (1) a buffer layer and (2) a semi-insulating substrate. Under illumination, most of the applied negative voltage drops across the very thin buffer layer, and the enhanced electric field in the layer exerts a very strong influence on the conducting channel.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.341630
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  • 2
    Electronic Resource
    Electronic Resource
    Influence of rapid thermal annealing on the optical properties of gallium nitride grown by gas-source molecular-beam epitaxy (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 936-938 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Raman scattering, photoluminescence (PL), and nuclear reaction analysis (NRA) have been employed to investigate the effects of rapid thermal annealing (RTA) on GaN films grown on sapphire (0001) substrates by gas-source molecular-beam epitaxy. The Raman spectra showed the presence of the E2 (high) mode of GaN and shift of this mode from 572 to 568 cm−1 caused by annealing. The results showed that RTA has a significant effect on the strain relaxation caused by the lattice and thermal expansion misfit between the GaN epilayer and the substrate. The PL peak exhibited a blueshift in its energy position and a decrease in the full width at half maximum after annealing, indicating an improvement in the optical quality of the film. Furthermore, a green luminescence appeared after annealing and increased in intensity with increasing annealing time. This effect was attributed to H concentration variation in the GaN film, which was measured by NRA. A high H concentration exists in as-grown GaN, which can neutralize the deep level, and the H-bonded complex dissociates during RTA. This leads to the appearance of a luminescent peak in the PL spectrum. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.120878
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  • 3
    Electronic Resource
    Electronic Resource
    Enhanced hydrogen desorption from Si sites during low-temperature Si1−xGex growth by disilane and solid-Ge molecular beam epitaxy (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 6920-6922 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A phenomenological model is proposed to explain quantitatively the interesting compositional dependence on the Ge incorporation rate during low-temperature growth of Si1−xGex by disilane and solid-Ge molecular beam epitaxy, based on enhanced hydrogen desorption from Si sites due to the presence of Ge atoms. The hydrogen desorption rate constant for disilane on Si sites is fitted to an exponential function of Ge incorporation rate and a possible physical explanation is discussed. Simulated results are in excellent agreement with experimental data. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370106
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  • 4
    Electronic Resource
    Electronic Resource
    Metastable carrier concentration in GaAs/GaAlAs heterostructure under hydrostatic pressure (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 2307-2310 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Different aspects of the metastable character of Si-related localized states in modulation-doped heterostructures of GaAs/GaAlAs have been studied. Hydrostatic pressure was used to change the two-dimensional electron-gas concentration at the active interface of the system. The performed studies give evidence that below a critical temperature of Tc=135 K for a given sample, at the same temperature and pressure conditions, an arbitrary carrier concentration in the quantum well can be obtained depending on the pressure at which the sample was cooled.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.341134
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  • 5
    Electronic Resource
    Electronic Resource
    APPLICATION OF LOCAL DAMAGE MODELS TO THE NUMERICAL ANALYSIS OF DUCTILE RUPTURE (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Fatigue & fracture of engineering materials & structures 12 (1989), S. 0 
    Details
    ISSN: 1460-2695
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract— Two damage models were implemented into the finite element program ADINA to study the correlation between microscopical damage and macroscopical material failure. In the first model, based on the Gurson yield function the nucleation, growth and the coalescence of voids were incorporated into the constitutive relations. In the second model the void growth was determined according to the Rice and Tracey model using the von Mises yield function, and material failure was simulated by eliminating the elements where the critical void growth ratio was exceeded. The numerical results for the local and global behaviour of the specimens were compared with experiments. The generality of the damage parameters was checked by investigating several specimen geometries. Both damage models deliver qualitatively consistent results with regard to the influence of the stress triaxiality on the void growth and on the beginning of the material failure. However, the Gurson model gives a more accurate numerical simulation because the damage development and the stress drop continue after the onset of void coalescence while the critical void growth model causes less convergence problems in the simulation of large crack extension. The Jn-curve was estimated on the basis of both models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1460-2695.1989.tb00527.x
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  • 6
    Electronic Resource
    Electronic Resource
    Three-dimensional cell model analyses of void growth in ductile materials (1996)
    Springer
    International journal of fracture 81 (1996), S. 235-258 
    Details
    ISSN: 1573-2673
    Keywords: ductile fracture ; damage mechanics ; micromechanics ; void growth
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Three-dimensional micromechanical models were developed to study the damage by void growth in ductile materials. Special emphasis is given to the influence of the spatial arrangement of the voids. Therefore, periodical void arrays of cubic primitive, body centered cubic and hexagonal structure are investigated by analyzing representative unit cells. The isotropic behaviour of the matrix material is modelled using either v. Mises plasticity or the modified Gurson-Tvergaard constitutive law. The cell models are analyzed by the large strain finite element method under monotonic loading while keeping the stress triaxiality constant. The obtained mesoscopic deformation response and the void growth of the unit cells show a high dependence on the value of triaxiality. The spatial arrangement has only a weak influence on the deformation behaviour, whereas the type and onset of the plastic collapse behaviour are strongly affected. The parameters of the Gurson-Tvergaard model can be calibrated to the cell model results even for large porosity, emphasizing its usefulness and justifying its broad applicability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00039573
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  • 7
    Electronic Resource
    Electronic Resource
    Fracture toughness evaluation by tensile and charpy-type tests based on micromechanical models of materials (1995)
    Springer
    Materials science 30 (1995), S. 223-229 
    Details
    ISSN: 1573-885X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract With the aim of applying micromechanical approaches to fracture mechanics, we carried out dynamic tensile tests and instrumented impact tests with Charpy-type specimens both with a V-shaped notch and with a crack. We compared two micromechanical strain-rate-dependent models based on a modified Gurson flow function by simulating various dynamically loaded specimens. The results of the tests indicate that the critical void volume fractionf c and the characteristic lengthl c are practically independent of the strain rate and the geometry of the specimen. The behavior of the specimens subjected to strains and fracture can be characterized by the parameters determined in tensile tests and impact tests with the Charpy-type specimens. By the three-dimensional analysis based on the strain-rate-dependent Gurson model, we predict the onset of crack growth inside the Charpy specimen.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00558577
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