49,165 research outputs found

    Self-consistent energy balance simulations of hole dynamics in SiGe/Si THz quantum cascade structures

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    Analysis of hole transport in cascaded p-Si/SiGe quantum well structures is performed using self-consistent rate equations simulations. The hole subband structure is calculated using the 6×6 k·p model, and then used to find carrier relaxation rates due to the alloy disorder, acoustic, and optical phonon scattering, as well as hole-hole scattering. The simulation accounts for the in-plane k-space anisotropy of both the hole subband structure and the scattering rates. Results are presented for prototype THz Si/SiGe quantum cascade structures. © 2004 American Institute of Physic

    A normalisation procedure for biaxial bias extension tests

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    Biaxial Bias Extension tests have been performed on a plain-weave carbon fibre engineering fabric. The test results have been normalised using both the upper and lower bound method proposed by Potluri et al. and also using a novel alternative normalisation method based on energy arguments. The normalised results from both methods are compared and discussed

    Carrier scattering approach to the origins of dark current in mid- and far-infrared (terahertz) quantum-well intersubband photodetectors (QWIPs)

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    A carrier scattering approach is taken in an analysis of the affect on the dark current of extending the operating wavelength of conventional bound to continuum quantum-well intersubband photodetectors. It is found that both the sequential tunneling and the thermionic emission contributions to the dark current increase as the wavelength of the detector is extended from the mid- to far-infrared. Dark current designs rules are derived

    An expert system shell for inferring vegetation characteristics: The learning system (tasks C and D)

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    This report describes the implementation of a learning system that uses a data base of historical cover type reflectance data taken at different solar zenith angles and wavelengths to learn class descriptions of classes of cover types. It has been integrated with the VEG system and requires that the VEG system be loaded to operate. VEG is the NASA VEGetation workbench - an expert system for inferring vegetation characteristics from reflectance data. The learning system provides three basic options. Using option one, the system learns class descriptions of one or more classes. Using option two, the system learns class descriptions of one or more classes and then uses the learned classes to classify an unknown sample. Using option three, the user can test the system's classification performance. The learning system can also be run in an automatic mode. In this mode, options two and three are executed on each sample from an input file. The system was developed using KEE. It is menu driven and contains a sophisticated window and mouse driven interface which guides the user through various computations. Input and output file management and data formatting facilities are also provided

    Population-inversion and gain estimates for a semiconductor TASER

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    We have investigated a solid-state design advanced (see Soref et al, in SPIE Proceedings, vol. 3795, p, 516, 1999) to achieve a terahertz-amplification-by-the-stimulated-emision-of-radiation (TASER), The original design was based on light-to heavy-hole intersubband transitions in SiGe/Si heterostructures, This work adapts the design to electron intersubband transitions in the more readily available GaAs/Ga1-xAlxAs material system. It is found that the electric-field induced anti-crossings of the states, derived from the first excited state with the ground states of a superlattice in the Stark-ladder regime, offers the possibility of a population inversion and gain at room temperature

    A 3D beam-column element implemented within a hybrid force-based method

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    This paper describes a force-based beam-column element implemented using a hybrid force-based solution strategy. The element can accommodate elastic-plastic strain hardening material behaviour under various loadings including axial, torsion, bending and shear deformation, both in and out of the plane of the element. In order to overcome difficulties associated with conventional displacement-based and force-based methods a hybrid force based-method is proposed. This alternative approach is based on simultaneous use of the principles of minimum total potential energy and minimum complementary potential energy. Here the primary equation is the equilibrium equation rather than the compatibility equation (the latter takes precedence when following a displacement based solution strategy). The predictions of the element using this solution procedure are compared against predictions from AbaqusTM , showing excellent agreemen

    The effects of linear and non-linear diffusion on exciton energies in quantum wells

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    This paper considers the technique of investigating diffusion processes via monitoring spectroscopically the ground state energy of an exciton confined in a quantum well. It is shown that the change in the exciton energy E–E0 during linear diffusion, can be described by an empirical relationship E–E0=(E–E0)(1–exp{–Dt/lw}), where E is the band gap of the initial barrier material, D the diffusion constant and t the time. Detailed calculations accounting for the changes in the exciton binding energy have shown that the parameter ~1.5 for all wells of width lw40 Å regardless of the material system. It is proposed that this relationship could be used to determine the linear diffusion coefficient D. Once D has been determined the relationship could then be utilized as a predictive tool, e.g., to determine the annealing time necessary to produce a given energy shift for a particular quantum well width. The paper goes on to discuss the effects non-linear diffusion processes could have on exciton energies in quantum wells. In particular, it is shown how detailed spectroscopy and annealing experiments when coupled with accurate modelling could be used to distinguish between constant and concentration dependent diffusion coefficients. © 1996 American Institute of Physics

    Intersubband terahertz lasers using four-level asymmetric quantum wells

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    We demonstrate the potential for laser operation at far-infrared wavelengths (30–300 µm, 1–10 THz) by using intersubband emission in four-level GaAs/AlGaAs asymmetric (stepped) quantum wells. Achieving population inversion in these devices depends critically on the lifetimes of the nonradiative intersubband transitions, and so we have performed detailed calculations of electron–electron and electron–phonon scattering rates. Our four-subband structures show potential for the realization of room temperature lasing, unlike previously considered three-subband structures which did not give population inversions except at impractically low electron densities and temperatures. Auger-type electron–electron interactions involving the highly populated ground subband effectively destroyed the population inversion in three level systems, but in these four subband structures the inversion is maintained by strong phonon-mediated depopulation of the lower laser level. The largest population inversions are calculated at low temperatures (< 30 K), but for the structures with higher emission energies, room temperature (300 K) operation is also predicted. © 1999 American Institute of Physics
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