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  • 1
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electrical properties of Al0.3Ga0.7As/InxGa1−xAs modulation doped heterostructures grown on GaAs substrates were studied. We found for the normal and inverted heterostructures principal differences of the transport properties. For an InAs mole fraction of 0.2 the inverted modulation doped heterostructures show a stronger decrease in the electron mobility of the two-dimensional electron gas if the critical layer thickness of the In0.2Ga0.8As layer is exceeded, in comparison to the normal heterostructures. This behavior can be explained by the relaxation process of the In0.2Ga0.8As layer. For InxGa1−xAs heterostructures with x(approximately-greater-than)0.3 the growth mode changes from two-dimensional to three-dimensional growth, which leads to interface roughness, degrading the transport properties of the normal heterostructure. Thus for high InAs mole fractions the inverted heterostructures show better transport properties in comparison to the normal heterostructures.
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  • 2
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 59 (1991), S. 1102-1104 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The sensitivity of the threshold for stimulated emission on temperature is typically described by the T0 parameter of a heuristic exponential law. This T0 parameter has a value of about 80 K in AlxGa1−xAs heterostructures with a direct fundamental gap, while it is rather large (300 K) in indirect-gap AlxGa1−xAs. Samples with an AlAs mole fraction (here x=0.43 and 0.44) close to the direct-to-indirect crossover change the nature of their fundamental gap, and thus the dominant channel for stimulated emission, from indirect to direct with rising lattice temperature. This temperature-induced change in bandstructure is reflected in a drastic change of the T0 parameter. As a direct consequence of the differential T0 values, indirect-gap Al0.46Ga0.54As has a room-temperature threshold comparable to the standard laser material Al0.33Ga0.77As.
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  • 3
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We apply time-resolved four-wave mixing as a novel tool to study resonant tunneling of carriers in semiconductor heterostructures. The polarization decay of excitons in a quantum well is much faster when the alignment of the electron levels in adjacent wells leads to resonant tunneling and subsequent scattering of the carriers.
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  • 4
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Anisotropic electron mobilities for GaAs/In0.2Ga0.8As/Al0.3Ga0.7As inverted high electron mobility transistor structures were observed using Hall effect measurements. If the In0.2Ga0.8As quantum-well thickness is below the critical layer thickness, a higher electron mobility in the 〈01–1〉 direction is observed in comparison to the 〈011〉 direction. Exceeding the critical layer thickness of the In0.2Ga0.8As quantum well results in a change in the behavior of the anisotropy, and a highly anisotropic electron mobility with a higher electron mobility in the 〈011〉 direction, in comparison to the 〈01–1〉 direction, is observed. With increasing In0.2Ga0.8As quantum-well width, the anisotropy increases. An increase of the anisotropy was also observed if the Hall-effect measurements were carried out at lower temperatures. The anisotropy in the electron mobility can be correlated to the occurrence of a highly asymmetric-dislocation density. The asymmetry in the dislocation density was observed using wavelength-selective catholuminescence measurements.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 90 (2001), S. 5027-5031 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Vibrational modes introduced by the incorporation of N into GaAs and GaInAs have been studied by Raman spectroscopy on samples grown by molecular-beam epitaxy using a rf nitrogen plasma source. When proceeding from GaAs1−xNx to Ga1−yInyAs1−xNx with x≤0.04 and y≤0.12, the nitrogen-induced vibrational mode near 470 cm−1 observed in GaAsN was found to broaden and to split into up to three components with one component at a frequency higher than that of the Ga–N mode in GaAsN. This observation shows that the incorporation of In into GaAsN strongly affects the local bonding of the N atoms by changing the local strain distributions as well as the formation of a significant fraction of In–N bonds. The resonant enhancement in the scattering cross section of the Ga–N vibrational mode, observed in low N-content GaAs1−xNx (x(approximate)0.01) for incident photon energies matching the mostly N-related E+ transition at around 1.8 eV, was found to broaden significantly upon increasing N content as well as upon the addition of In to form GaInAsN. © 2001 American Institute of Physics.
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  • 6
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Morphological and physical properties of Al0.48In0.52As/Ga 0.47In0.53As heterostructures grown by molecular beam epitaxy, lattice relaxed on GaAs substrates and lattice matched on InP substrates, are presented. Both a quaternary linear and step graded lattice relaxed buffer concept is implemented to accomodate the lattice mismatch between the GaAs substrate and the Al0.48In0.52As/Ga 0.47In0.53As layer sequence. The surface morphology and the transport properties of Al0.48In0.52As/Ga0.47In0.53As high electron mobility transistor structures were studied by atomic force microscopy and Hall effect measurements, respectively. Optical properties were investigated by low temperature photoluminescence experiments on quantum well structures. The linear graded buffer approach was found to result in superior heterostructure properties due to the two dimensional growth mode during the whole growth process resulting in the typical cross hatched surface morphology. In contrast, the use of step graded buffer layers resulted in three dimensional layer growth and inferior layer quality. However, by increasing the number of steps, i.e., reducing the change in the lattice constant for each step and thus approaching a linear grading, two dimensional growth is recovered. © 1996 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 80 (2002), S. 2081-2083 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The bonding of nitrogen in low N-content AlxGa1−xAs1−yNy with x≤0.05 and y≤0.04 has been studied by Raman spectroscopy. Upon the addition of Al to GaAsN, additional vibrational modes are observed at around 450 cm−1, which is below the GaN-like longitudinal optical (LO) phonon mode centered at 470 cm−1. These modes are attributed to the formation of Al and N containing complexes with Al-to-N bonding. With increasing Al content the Al–N related modes gain intensity at the expense of the GaN-like mode, and they become the dominant N-related feature for an Al-content of 5% at a fixed N content of 1%. On the other hand, increasing the N content from 0% up to 4% at a constant Al concentration of 5% results first in the appearance and eventual saturation in intensity of the AlN-like modes, accompanied by a steep increase in intensity and eventual dominance of the GaN-like vibrational mode. Simultaneously the AlAs-like LO2 phonon mode shows a drastic decrease in intensity for N contents exceeding 2%. All these observations strongly indicate that there is a preferential formation of Al(Single Bond)N bonds in low N- and Al-content AlGaAsN, which is in direct contrast to GaInAsN, where even after thermal annealing the GaN-like mode remains dominant in the Raman spectrum compared to the InN-like modes. © 2002 American Institute of Physics.
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  • 8
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Quaternary pseudomorphically strained GaInAsN films and double-quantum wells were grown by plasma assisted molecular-beam epitaxy on an InP substrate. The In content ranged from 53% to 70% while the N content was varied between 0% and 2.4%. A reduction of compressive strain and a low-energy shift of photoluminescence (PL) peak position was observed with increasing N concentration, accompanied by a reduction in PL peak intensity and increase in linewidth. The net effect of N incorporation on the GaInAsN band gap energy was calculated from the measured PL peak energies. The thus obtained composition dependent GaInAsN band gap energy was fitted using the band anticrossing model, yielding values for the interaction parameter CMN for high In-containing GaInAsN being only slightly smaller than that reported for low In-content GaInAsN on GaAs. © 2002 American Institute of Physics.
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  • 9
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The anisotropic conduction of GaAs/In0.2Ga0.8As/Al0.3Ga0.7As inverted high-electron-mobility transistor (HEMT) structures has been investigated. The heterostructures were grown by molecular-beam epitaxy on (100) GaAs substrates. The thickness of the pseudomorphic layer was increased stepwise (150–300 A(ring)) beyond the critical layer thickness as determined by the appearance of misfit dislocations. These mixed 60° dislocations surrounded by depletion regions were observed as straight dark lines in cathodoluminescence. The measured resistance Rs was higher in the [01¯1] direction than in the perpendicular [011] direction. At T=30 K the conduction ratio of these two directions exceeded 105 in the 300-A(ring)-thick layer. The magnitude and anisotropy of Rs was correlated with the anisotropic dislocation patterns resulting from the preferential generation of the α dislocations (parallel) [011] as compared to the orthogonal β dislocations (parallel) [01¯1]. In both directions Rs depended exponentially on the number of dark lines perpendicular to the probing current. Simultaneously, the functional form of the temperature-dependent Rs(T) strongly varied with layer thickness. The thin, still elastically strained layers showed the usual behavior of HEMT structures. For the thicker layers a completely different temperature dependence was gradually developing, eventually leading to an exponential increase of Rs with inverse temperature between 300 and 100 K. Below this range Rs(1/T) changed more slowly and leveled off at 30 K. All these features are convincingly explained by a model assuming that the electrons can surmount the insulating depletion barriers in the conducting channel by a thermally induced tunneling mechanism.
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  • 10
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Interband transitions in GaAs1−xNx (0≤x≤0.033), involving a localized resonant N-derived energy level and the GaAs-like L conduction and valence band states, have been studied by resonant Raman scattering and spectroscopic ellipsometry, respectively. Raman scattering by the GaN-like LO2 phonon showed for x(approximate)0.01 a pronounced resonant enhancement for incident photon energies approaching the mostly N-related E+ transition at around 1.8 eV, but not at the E1 and E1+Δ1 interband transitions, reflecting the strongly localized nature of both the N-related electronic level and the Ga–N vibrational mode. Spectroscopic ellipsometry, in contrast, being sensitive to the overall dielectric function of the GaAsN, revealed the effect of N incorporation on the GaAs-like E1 and E1+Δ1 interband transitions, which is a high-energy shift with increasing N-content accompanied by a significant broadening. © 2000 American Institute of Physics.
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