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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Barrier potential design criteria in multiple-quantum-well-based solar-cell structures

The barrier potential design criteria in multiple-quantum-well (MQW)-based solar-cell structures is reported for the purpose of achieving maximum efficiency. The time-dependent short-circuit current density at the collector side of various MQW solar-cell structures under resonant condition was numerically calculated using the time-dependent Schroedinger equation. The energy efficiency of solar cells based on the InAs/Ga(y)In(1-y)As and GaAs/Al(x)Ga(1-x)As MQW structues were compared when carriers are excited at a particular solar-energy band. Using InAs/Ga(y)In(1-y)As MQW structures it is found that a maximum energy efficiency can be achieved if the structure is designed with barrier potential of about 450 meV. The efficiency is found to decline linearly as the barrier potential increases for GaAs/Al(x)Ga(1-x)As MQW-structure-based solar cells.

Mohaidat, Jihad M.↗
NASA NTRS1999

Exciton Absorption in Semiconductor Quantum Wells Driven by a Strong Intersubband Pump Field

Optical interband excitonic absorption of semiconductor quantum wells (QW's) driven by a coherent pump field is investigated based on semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW's. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron-hole pairs in the QW structure is low, the pump field induces an Autler-Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron-hole pairs is increased, the split contrast (the ratio between the maximum and minimum values) is decreased because the exciton effect is suppressed at higher densities due to the many-body screening.

Liu, Ansheng↗
DOE OSTI2024

Carrier diffusion in long wavelength InGaN quantum well LEDs after injection through V-defects

The efficiency of operation of GaN-based light emitting diodes (LEDs) to a large degree relies on realization of a uniform hole distribution between multiple quantum wells (QWs) of the active region. Since the direct thermionic transport between the QWs is inefficient, the hole injection through semipolar {$10\bar{1}1$} QWs that form on the facets of V-defects has been suggested as an alternative approach. However, for an efficient LED operation, the carrier distribution should be uniform not only vertically, between the QWs but also laterally, within individual QWs. In this work, the lateral carrier distribution in long wavelength InGaN/GaN QW LEDs is studied by the scanning near-field optical microscopy. The measurements have shown that emission is concentrated around the V-defect injectors. At high currents, the diffusion length of holes in polar QWs was found to be ∼0.6–1 μm and the hole diffusion coefficient ∼0.6 cm 2 /s. The obtained data should aid design of the V-defect injectors for a laterally uniform carrier distribution in the active region QWs.

Current crowding↗
NASA NTRS1991

Extended long-wavelength lambda = 11-15-micron GaAs/Al(x)Ga(1-x)As quantum-well infrared photodetectors

Success has been achieved in extending the spectral wavelength of GaAs/Al(x)Ga(1-x)As quantum-well infrared photodetectors to significantly longer wavelengths of 11-15 micron. High responsivity of 0.5 A/W, high quantum efficiency of 12 percent, and high detectivity as well as an excellent noise equivalent temperature difference of 4 mK have been achieved at T = 50 K. High performance of 19 mK has also been achieved at an even higher temperature of 60 K.

Zussman, A.↗
NASA NTRS1993

Low threshold continuous operation of InGaAs/InGaAsP quantum well lasers at about 2.0 microns

The first low threshold continuous operation of InGaAs strained layer quantum well lasers at about 2.0 microns is reported. The threshold current density of 5-micron wide and 1.5 mm long ridge waveguide lasers was less than 380 A/sq cm. The external differential quantum efficiency of 1 mm long lasers was as high as 15 percent and laser operation was observed at temperatures as high as 50 C. The lasers are characterized by T(0) = 54 C which is the highest characteristic temperature ever achieved at this wavelength in any material system.

Forouhar, S.↗
DOE OSTI2025

Charge carrier extraction and recombination effects in GaInAs/GaAsP multi-quantum well solar cells

The carrier extraction and transport mechanisms as well as the relative contributions of radiative and non-radiative recombination processes are investigated in high-quality strain-balanced GaInAs/GaAsP multi-quantum well solar cells recently implemented in record efficiency multijunction solar cells. A comprehensive suite of complementary characterization techniques including temperature- and suns-dependent photoluminescence and photovoltaic measurements are employed to analyze thermal escape and tunneling rates, which demonstrate the need to move beyond simple drift-diffusion models of p–n junctions. This study examines the processes that best characterize the operation of these devices across varying temperatures using a simple two-diode model, incorporating multiple transport protocols, and provides insights into the performance-limiting processes and pathways for their optimization.

14 SOLAR ENERGY↗
NASA NTRS1988

Ultralow threshold graded-index separate-confinement heterostructure single quantum well (Al, Ga) As lasers

Broad area graded-index separate-confinement heterostructure single quantum well lasers grown by molecular-beam epitaxy (MBE) with threshold current density as low as 93 A/sq cm (520 microns long) have been fabricated. Buried lasers formed from similarly structured MBE material with liquid phase epitaxy regrowth had threshold currents at submilliampere levels when high reflectivity coatings were applied to the end facets. A CW threshold current of 0.55 mA was obtained for a laser with facet reflectivities of about 80 percent, a cavity length of 120 micron, and an active region stripe width of 1 micron. These devices driven directly with logic level signals have switch-on delays less than 50 ps without any current prebias. Such lasers permit fully on-off switching while at the same time obviating the need for bias monitoring and feedback control.

Derry, P. L.↗
NASA NTRS1992

Temperature independent quantum well FET with delta channel doping

A temperature independent device is presented which uses a quantum well structure and delta doping within the channel. The device requires a high delta doping concentration within the channel to achieve a constant Hall mobility and carrier concentration across the temperature range 300-1.4 K. Transistors were RF tested using on-wafer probing and a constant G sub max and F sub max were measured over the temperature range 300-70 K.

Young, P. G.↗
NASA NTRS2002

Many-body Effects in a Laterally Inhomogeneous Semiconductor Quantum Well

Many body effects on conduction and diffusion of electrons and holes in a semiconductor quantum well are studied using a microscopic theory. The roles played by the screened Hartree-Fock (SHE) terms and the scattering terms are examined. It is found that the electron and hole conductivities depend only on the scattering terms, while the two-component electron-hole diffusion coefficients depend on both the SHE part and the scattering part. We show that, in the limit of the ambipolax diffusion approximation, however, the diffusion coefficients for carrier density and temperature are independent of electron-hole scattering. In particular, we found that the SHE terms lead to a reduction of density-diffusion coefficients and an increase in temperature-diffusion coefficients. Such a reduction or increase is explained in terms of a density-and temperature dependent energy landscape created by the bandgap renormalization.

Ning, Cun-Zheng↗
NASA NTRS2003

Many-Body Effects and Lineshape of Intersubband Transitions in Semiconductor Quantum Wells

Intersubband Transition (ISBT) infrared (IR) absorption and PL in InAs/AlSb were studied for narrow Quantum Wells (QWs). A large redshift was observed (7-10 meV) as temperature increased. A comprehensive many-body theory was developed for ISBTs including contributions of c-c and c-phonon scatterings. Many-body effects were studied systematically for ISBTs. Redshift and linewidth dependence on temperature, as well as spectral features were well explained by theory.

Ning, Cun-Zheng↗
NASA NTRS2000

8-9 and 14-15 Micron Two-Color 640x486 GaAs/AlGaAs Quantum Well Infrared Photodetector (QWIP) Focal Plane Array Camera

An optimized long-wavelength two-color Quantum Well Infrared Photodetector (QWIP) device structure has been designed. This device structure was grown on a three inch semi-insulating GaAs substrate by molecular beam epitaxy (MBE). This wafer was processed into several 640x486 format monolithically integrated 8-9 and 14-15 micron two color (or dual wavelength) QWIP focal plane arrays (FPAs). These FPAs were then hybridized to 640x486 silicon CMOS readout multiplexers. A thinned (i.e., substrate removed) FPA hybrid was integrated into a liquid helium cooled dewar to perform electrical and optical characterization and to demonstrate simultaneous two-color imagery. The 8-9 micron detectors in the FPA have shown background limited performance (BLIP) at 70 K operating temperature, at 300 K background with f/2 cold stop. The 14-15 micron detectors of the FPA have reached BLIP at 40 K operating temperature at the same background conditions. In this presentation we discuss the performance of this long-wavelength dualband QWIP FPA in quantum efficiency, detectivity, noise equivalent temperature difference (NEAT), uniformity, and operability.

Gunapala, S. D.↗
NASA NTRS1990

Tunable-Quantum-Well Infrared Detector

Proposed detector of infrared photons based on photon-assisted, resonant quantum-mechanical tunneling between adjacent energy wells in its semiconductor structure. Wavelength adjusted by changing applied voltage. Device contains alternating layers of different seminconductors which form double-quantum-well electron-energy structure. Tunable-quantum-well detectors made on single chip constitutes line imager. Many such devices stacked to form two-dimensional imaging device. Signal-processing circuitry integrated into chip at its lower end. Potential barriers and wells produced by alternating layers of different semiconductor materials. Steepness of slope proportional to applied electric field. Electric field and thicknesses and heights of barriers selected to favor photon-assisted, resonant quantum-mechanical tunneling at chosen photon frequency.

Maserjian, Joseph↗
NASA NTRS1994

Optimization of two dimensional gratings for very long wavelength quantum well infrared photodetectors

We have performed a detailed study of two-dimensional grating coupling for quantum well infrared photodetectors in the very long wavelength spectral region lambda is approximately 16 - 17 microns. Using calculations based on the modal expansion method we quantitatively explain the double peaked responsivity spectrum. By optimizing the grating parameters we achieve a normal incidence responsivity and detectivity which are three times larger than the 45 deg angle of incidence geometry.

Sarusi, G.↗
NASA NTRS1997

Long-Wavelength 640 x 484 GaAs/Al(x)Ga(1-x)As Quantum Well Infrared Photodetector Focal Plane Array Camera

A 9 micrometers cutoff 640 x 484 hand-held quantum well infrared photodetector (QWIP) camera has been demonstrated. Excellent imagery, with a noise equivalent differential temperature (NE.deltaT) of 43 mK has been achieved. In this paper, we discuss the development of this very sensitive long wavelength infrared (LWIR) camera based on a GaAs/AlGaAs QWIP focal plane array (FPA) and its performance in quantum efficiency, NE.deltaT, uniformity, and operability.

Gunapala, S. D.↗
NASA NTRS1994

High Performance InGaAs/GaAs Quantum Well Infrared Photodetectors

We have measured the optical and transport properties of In(0.2)Ga(0.8)As/GaAs quantum well infrared photodetectors based on bound-to-bound, bound-to-quasibound, and bound-to-continuum intersubband transitions. Excellent hot electron transport and high detectivity D*=1.8 x 1O(exp 10) cm square root of Hz/W (at lambda(sub p)=16.7 microns) were achieved at temperature T=40 K.

Gunapala, S. D.↗