70 research outputs found
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
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
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
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
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
Fiber-coupled microsphere laser
We demonstrate a 1.5-mm-wavelength fiber laser formed by placement of glass microsphere resonators along a fiber taper. The fiber taper serves the dual purpose of transporting optical pump power into the spheres and extracting the resulting laser emission. A highly doped erbium:ytterbium phosphate glass was used to form microsphere resonant cavities with large gain at 1.5 mm. Laser threshold pump powers of 60 mW and fiber-coupled output powers as high as 3 mW with single-mode operation were obtained. A bisphere laser system consisting of two microspheres attached to a single fiber taper is also demonstrated
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
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
Embedding PbS Quantum Dots (QDs) in Pb-Halide Perovskite Matrices: QD Surface Chemistry and Antisolvent Effects on QD Dispersion and Confinement Properties
Hybrid materials of metal chalcogenide colloidal quantum dots (QDs) embedded in metal halide perovskites (MHPs) have led to composites with synergistic properties. Here, we investigate how QD size, surface chemistry, and MHP film formation methods affect the resulting optoelectronic properties of QD/MHP “dot-in-matrix” systems. We monitor the QD absorption and photoluminescence throughout synthesis, ligand exchange, and transfer into the MHP ink, and we characterize the final QD/MHP films via electron microscopy and transient absorption. In addition, we are the first to globally map how PbS QDs are distributed on the micrometer scale within these dot-in-matrix systems, using three-dimensional (3D) tomography time-of-flight secondary ion mass spectrometry. The surface chemistry imparted during synthesis directly affects the optical properties of the dot-in-matrix composites. Pb-halide passivation leads to QD/MHP dot-in-matrix samples with optical properties that are well-described by a theoretical model, based on a Type I finite-barrier heterostructure between the PbS QD and the MHP matrix. Samples without Pb-halide passivation show complicated size-dependent behavior, indicating a transition from a Type I heterostructure between the PbS QD wells and MHP barriers for small-sized QDs to PbS QDs that are electronically decoupled from the MHP matrix for larger QDs. Furthermore, the choice in perovskite antisolvent crystallization method leads to a difference in the spatial QD distribution within the perovskite matrix, differences in carrier lifetime, and photoluminescence shifts of up to 180 meV for PbS in methylammonium lead iodide. This work establishes an understanding of such emerging synergistic systems relevant for technologies such as photovoltaics, infrared emitters and detectors, and other unexplored technological applications
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]
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