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

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

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)

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)

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

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

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

Population-inversion and gain estimates for a semiconductor TASER

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

Intersubband terahertz lasers using four-level asymmetric quantum wells

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

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

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

An expert system shell for inferring vegetation characteristics: Interface for the addition of techniques (Task H)

All the NASA VEGetation Workbench (VEG) goals except the Learning System provide the scientist with several different techniques. When VEG is run, rules assist the scientist in selecting the best of the available techniques to apply to the sample of cover type data being studied. The techniques are stored in the VEG knowledge base. The design and implementation of an interface that allows the scientist to add new techniques to VEG without assistance from the developer were completed. A new interface that enables the scientist to add techniques to VEG without assistance from the developer was designed and implemented. This interface does not require the scientist to have a thorough knowledge of Knowledge Engineering Environment (KEE) by Intellicorp or a detailed knowledge of the structure of VEG. The interface prompts the scientist to enter the required information about the new technique. It prompts the scientist to enter the required Common Lisp functions for executing the technique and the left hand side of the rule that causes the technique to be selected. A template for each function and rule and detailed instructions about the arguments of the functions, the values they should return, and the format of the rule are displayed. Checks are made to ensure that the required data were entered, the functions compiled correctly, and the rule parsed correctly before the new technique is stored. The additional techniques are stored separately from the VEG knowledge base. When the VEG knowledge base is loaded, the additional techniques are not normally loaded. The interface allows the scientist the option of adding all the previously defined new techniques before running VEG. When the techniques are added, the required units to store the additional techniques are created automatically in the correct places in the VEG knowledge base. The methods file containing the functions required by the additional techniques is loaded. New rule units are created to store the new rules. The interface that allow the scientist to select which techniques to use is updated automatically to include the new techniques. Task H was completed. The interface that allows the scientist to add techniques to VEG was implemented and comprehensively tested. The Common Lisp code for the Add Techniques system is listed in Appendix A