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  • 1990-1994  (4)
  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 3536-3543 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Measurements of the vertical mobility of photo-excited carriers in quantum well infrared detectors are reported. The low temperature mobility is extracted by means of a photo geometrical magnetoresistance technique. This approach is most suitable for the multi quantum well structure geometry. Both rectangular and stepped well structures were studied. They exhibit asymmetry with regard to bias polarity for both gain and mobility. This asymmetry is shown to be contributed by both well structure, and growth induced inequivalent interfaces. The low field mobility is governed mainly by unscreened ionized impurity scattering in the barriers. The well interfaces act as additional anisotropic scattering centers. We observe clear correlation between the extracted drift velocity and the optical gain. It is postulated that anisotropic well recapture causes polarity dependent energy distribution above the barriers. In addition, we conclude that intervalley scattering of the hot electrons has a substantial influence on the detector performance.
    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 71 (1992), S. 4382-4384 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Energy band nonparabolicity is present in the majority of materials used in heterojunctions, quantum wells, and superlattices. In the present work we derive a simplified dispersion relation for electrons using the k⋅p model. We demonstrate that this dispersion relation can account for nonparabolicity in both narrow and wide gap isotropic semiconductors. We then calculate the carrier effective mass and densities of states for both two- and three-dimensional electron systems. Finally, we derive a simple analytical relation between the carrier concentration and the Fermi energy in a nonparabolic two-dimensional electron gas. Agreement with a numerical model is demonstrated, while the traditional, parabolic approximation results in a large error. The simplicity of the new approximation allows an intuitive understanding of the nonparabolicity effect in two-dimensional systems. Therefore, the new approach should be useful for design, characterization, and modeling of quantum semiconductor devices with nonparabolic energy bands.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 61 (1992), S. 1341-1343 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new type of asymmetric stepped GaAs/AlGaAs multiquantum well infrared detector is reported. These asymmetric detectors utilize the usual bound-to-continuum transition. The current responsivity is remarkably asymmetric with regard to the voltage polarity. In contrast with rectangular wells, in which responsivity is saturated in both bias polarities, these wells exhibit saturation only for negative bias. The responsivity increases monotonously with positive electric field. This is attributed to changes induced by the field on the transport properties of the excited electrons. In particular, the bias affects the dwell time spent by the carrier wave packet in the well region. Employing this model, we achieve a very good fit with experimental data.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Algorithmica 12 (1994), S. 409-419 
    ISSN: 1432-0541
    Keywords: Prefix codes ; Huffman codes ; Encryption ; Complexity ; NP-completeness
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mathematics
    Notes: Abstract Given a natural language cleartext and a ciphertext obtained by Huffman coding, the problem of guessing the code is shown to be NP-complete for various variants of the encoding process.
    Type of Medium: Electronic Resource
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