Direct determination of the ambipolar diffusion length in GaAs/AlGaAs heterostructures by cathodoluminescence

A new technique for determining carrier diffusion lengths by cathodoluminescence measurements is presented. The technique is extremely accurate and can be applied to a variety of structures. Ambipolar diffusion lengths are determined for GaAs quantum well material, bulk GaAs, Al0.21Ga0.79As, and Al0.37Ga0.63As. A large increase in the diffusion length is found for Al0.37Ga0.63As and is attributed to an order of magnitude increase in lifetime

A novel technique for the direct determination of carrier diffusion lengths in GaAs/AlGaAs heterostructures using cathodoluminescence

A new technique for determining carrier diffusion lengths in direct gap semiconductors by cathodoluminescence measurement is presented. Ambipolar diffusion lengths are determined for GaAs quantum well material, bulk GaAs, and Al_xGa_(1-x)As with x up to 0.38. A large increase in the diffusion length is found as x approaches 0.38 and is attributed to an order of magnitude increase in lifetime

Effect of Al mole fraction on carrier diffusion lengths and lifetimes in AlxGa1−xAs

The ambipolar diffusion length and carrier lifetime are measured in AlxGa1−xAs for several mole fractions in the interval 0<x<0.38. These parameters are found to have significantly higher values in the higher mole fraction samples. These increases are attributed to occupation of states in the indirect valleys, and supporting calculations are presented

Icemaker^(TM): an excel-based environment for collaborative design

The creative process of team design can be rapid and powerful when focused, yet complex designs, such as spacecrafit, can slow and quench the essential elements of this process. Concurrent Engineering techniques partially address this problem, but a fuller realization of their benefits require an approach centering on the human aspects of teamwork. ICEMaker^(TM) is a Microsoft Excel® based software tool that facilitates closer-to-ideal collaboration within teams employing the new Integrated Concurrent Engineering (ICE) methodology. ICE is a generic approach that emphasizes focused collaborative design in a single-room context, and is now employed at several aerospace organizations to increase the productivity of design teams defining complex early development-phase products. By way of introduction, this paper describes the basic elements of ICE needed to understand ICEMaker and its application. We present the design approach, philosophy, and client-server architecture of the ICEMaker system, as well as a simplified user scenario. NASA's Jet Propulsion Laboratory (JPL) has recently adopted ICEMaker for its primary early-phase space mission and system advanced project design team, Team-X. We describe Team-X's experience with ICEMaker and report on the lessons learned, and qualitative product improvements, resulting from JPL's implementation of ICEMaker

Quantum wires from coupled InAs/GaAs strained quantum dots

The electronic structure of an infinite 1D array of vertically coupled InAs/GaAs strained quantum dots is calculated using an eight-band strain-dependent k-dot-p Hamiltonian. The coupled dots form a unique quantum wire structure in which the miniband widths and effective masses are controlled by the distance between the islands, d. The miniband structure is calculated as a function of d, and it is shown that for d>4 nm the miniband is narrower than the optical phonon energy, while the gap between the first and second minibands is greater than the optical phonon energy. This leads to decreased optical phonon scattering, providing improved quantum wire behavior at high temperatures. These miniband properties are also ideal for Bloch oscillation.Comment: 5 pages revtex, epsf, 8 postscript figure

Two-dimensional plasma expansion in a magnetic nozzle: Separation due to electron inertia

A previous axisymmetric model of the supersonic expansion of a collisionless, hot plasma in a divergent magnetic nozzle is extended here in order to include electron-inertia effects. Up to dominant order on all components of the electron velocity, electron momentum equations still reduce to three conservation laws. Electron inertia leads to outward electron separation from the magnetic streamtubes. The progressive plasma filling of the adjacent vacuum region is consistent with electron-inertia being part of finite electron Larmor radius effects, which increase downstream and eventually demagnetize the plasma. Current ambipolarity is not fulfilled and ion separation can be either outwards or inwards of magnetic streamtubes, depending on their magnetization. Electron separation penalizes slightly the plume efficiency and is larger for plasma beams injected with large pressure gradients. An alternative nonzero electron-inertia model [E. Hooper, J. Propul. Power 9, 757 (1993)] based on cold plasmas and current ambipolarity, which predicts inwards electron separation, is discussed critically. A possible competition of the gyroviscous force with electron-inertia effects is commented briefly

1D Exciton Spectroscopy of Semiconductor Nanorods

We have theoretically shown that optical properties of semiconductor nanorods are controlled by 1D excitons. The theory, which takes into account anisotropy of spacial and dielectric confinement, describes size dependence of interband optical transitions, exciton binding energies. We have demonstrated that the fine structure of the ground exciton state explains the linear polarization of photoluminescence. Our results are in good agreement with the measurements in CdSe nanorods

Development of an eight-band theory for quantum-dot heterostructures

We derive a nonsymmetrized 8-band effective-mass Hamiltonian for quantum-dot heterostructures (QDHs) in Burt's envelope-function representation. The 8x8 radial Hamiltonian and the boundary conditions for the Schroedinger equation are obtained for spherical QDHs. Boundary conditions for symmetrized and nonsymmetrized radial Hamiltonians are compared with each other and with connection rules that are commonly used to match the wave functions found from the bulk kp Hamiltonians of two adjacent materials. Electron and hole energy spectra in three spherical QDHs: HgS/CdS, InAs/GaAs, and GaAs/AlAs are calculated as a function of the quantum dot radius within the approximate symmetrized and exact nonsymmetrized 8x8 models. The parameters of dissymmetry are shown to influence the energy levels and the wave functions of an electron and a hole and, consequently, the energies of both intraband and interband transitions.Comment: 36 pages, 10 figures, E-mail addresses: [email protected], [email protected]

Reflection of light and heavy holes from a linear potential barrier

In this paper we study reflection of holes in direct-band semiconductors from the linear potential barrier. It is shown that light-heavy hole transformation matrix is universal. It depends only on a dimensionless product of the light hole longitudinal momentum and the characteristic length determined by the slope of the potential and doesn't depend on the ratio of light and heavy hole masses, provided this ratio is small. It is shown that the transformation coefficient goes to zero both in the limit of small and large longitudinal momenta, however the phase of a reflected hole is different in these limits. An approximate analytical expression for the light-heavy hole transformation coefficient is found.Comment: 6 pages, 2 figure

Auger Recombination in Semiconductor Quantum Wells

The principal mechanisms of Auger recombination of nonequilibrium carriers in semiconductor heterostructures with quantum wells are investigated. It is shown for the first time that there exist three fundamentally different Auger recombination mechanisms of (i) thresholdless, (ii) quasi-threshold, and (iii) threshold types. The rate of the thresholdless Auger process depends on temperature only slightly. The rate of the quasi-threshold Auger process depends on temperature exponentially. However, its threshold energy essentially varies with quantum well width and is close to zero for narrow quantum wells. It is shown that the thresholdless and the quasi-threshold Auger processes dominate in narrow quantum wells, while the threshold and the quasi-threshold processes prevail in wide quantum wells. The limiting case of a three-dimensional (3D)Auger process is reached for infinitely wide quantum wells. The critical quantum well width is found at which the quasi-threshold and threshold Auger processes merge into a single 3D Auger process. Also studied is phonon-assisted Auger recombination in quantum wells. It is shown that for narrow quantum wells the act of phonon emission becomes resonant, which in turn increases substantially the coefficient of phonon-assisted Auger recombination. Conditions are found under which the direct Auger process dominates over the phonon-assisted Auger recombination at various temperatures and quantum well widths.Comment: 38 pages, 7 figure