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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Optically induced excitonic electroabsorption in a periodically delta-doped InGaAs/GaAs multiple quantum well structure

Large optically induced Stark shifts have been observed in a periodically delta-doped InGaAs/GaAs multiple quantum well structure. With an excitation intensity of 10 mW/sq cm, an absolute quantum well absorption change of 7000/cm was measured with a corresponding differential absorption change as high as 80 percent. The associated maximum change in the quantum well refractive index is 0.04. This material is promising for device development for all-optical computing and signal processing.

Larsson, A.↗
NASA NTRS1998

Strained quantum well photovoltaic energy converter

An indium phosphide photovoltaic cell is provided where one or more quantum wells are introduced between the conventional p-conductivity and n-conductivity indium phosphide layer. The approach allows the cell to convert the light over a wider range of wavelengths than a conventional single junction cell and in particular convert efficiently transparency losses of the indium phosphide conventional cell. The approach hence may be used to increase the cell current output. A method of fabrication of photovoltaic devices is provided where ternary InAsP and InGaAs alloys are used as well material in the quantum well region and results in an increase of the cell current output.

Freundlich, Alexandre↗
NASA NTRS1998

Modeling and Simulation of Semiconductor Quantum Well Structures and Lasers

In this talk I will cover two aspects of modeling and simulation efforts at NASA Ames Research Center. In the quantum well structure simulation, we typically start from the quantum mechanical calculation of the quantum well structures for the confined/and unconfined eigen states and functions. A bandstructure calculation of the k*p type is then performed for the confined valence states. This information is then used to computer the optical gain and refractive index of the quantum well structures by solving the linearized multiband semiconductor Bloch equations with the many-body interactions included. In our laser simulation, we typically solve the envelope equations for the laser field in space-time domain, coupled with a reduced set of material equations using the microscopic calculation of the first step. Finally I will show some examples of both aspects of simulation and modeling.

Ning, Cun-Zheng↗
DOE OSTI2025

Impact of quantum well thickness on efficiency loss in InGaN/GaN LEDs: Challenges for thin-well designs

We investigate the impact of quantum well (QW) thickness on efficiency loss in c-plane InGaN/GaN LEDs using a small-signal electroluminescence technique. Multiple mechanisms related to efficiency loss are independently examined, including injection efficiency, carrier density vs current density relationship, phase space filling, quantum-confined Stark effect, and Coulomb enhancement. An optimal QW thickness of around 2.7 nm in these InGaN/GaN LEDs was determined for QWs having constant In composition. Despite improved control of deep-level defects and lower carrier density at a given current density, LEDs with thin QWs still suffer from an imbalance in enhancement effects on the radiative and intrinsic Auger–Meitner recombination coefficients. The imbalance in enhancement effects results in a decline in internal quantum efficiency and radiative efficiency with decreasing QW thickness at low current density in LEDs with QW thicknesses below 2.7 nm. Here, we also investigate how LED modulation bandwidth varies with QW thickness, identifying the key trends and their implications for device performance.

36 MATERIALS SCIENCE↗
NASA NTRS1999

Corrugated Quantum Well Infrared Photodetector Focal Plane Array Test Results

The corrugated quantum-well infrared photodetector (C-QWIP) uses total internal reflection to couple normal incident light into the optically active quantum wells. The coupling efficiency has been shown to be relatively independent of the pixel size and wavelength thus making the C-QWIP a candidate for detectors over the entire infrared spectrum. The broadband coupling efficiency of the C-QWIP makes it an ideal candidate for multiwavelength detectors. We fabricated and tested C-QWIP focal plane arrays (FPAs) with cutoff wavelengths of 11.2 and 16.2 micrometers. Each FPA has 256 x 256 pixels that are bump-bonded to a direct injection readout circuit. Both FPAs provided infrared imagery with good aesthetic attributes. For the 11.2-micrometers FPA, background-limited performance (BLIP) was observed at 60 K with f/3 optics. For the 16.2-micrometers FPA, BLIP was observed at 38 K. Besides the reduction of dark current in C-QWIP structures, the measured internal quantum efficiency (eta) remains to be high. The values for responsivity and quantum efficiency obtained from the FPA results agree well with those measured for single devices.

Goldberg, A.↗
NASA NTRS1989

Intersubband Auger recombination and population inversion in quantum-well subbands

The intersubband-Auger-recombination time of electrons under population-inversion conditions in a single quantum well is calculated by taking into account momentum- and energy-conservation rules, and by employing Fermi-Dirac statistics. The screened matrix element of the electron-electron interaction and the overlap integral are calculated for an infinitely deep quantum well. The results are in a good agreement with published experimental data. As a major nonradiative process, the Auger recombination is related to threshold current of infrared lasers based on intersubband transitions in quantum-well structures. The realization of these devices and other limitations to achieving population inversion are discussed. In view of the results, development of these lasers for emission wavelengths corresponding to energies below the LO-phonon energy seems feasible.

Borenstain, S.↗
NASA NTRS1991

AlGaAs-GaAs quantum-well lasers for direct solar photopumping

The paper theoretically examines the solar power requirements for low-threshold AlGaAs-GaAs quantum-well lasers directly photopumped by focused sunlight. A model of separate-confinement quantum-well-heterostructure (SCQWH) lasers was developed, which explicitly treats absorption and transport phenomena relevant to solar pumping. The model was used to identify separate-confinement single-quantum-well laser structures which should operate at photoexcitation intensities of less than 10,000 suns.

Unnikrishnan, Sreenath↗
NASA NTRS1993

Photoluminescence from narrow InAs-AlSb quantum wells

We report on photoluminescence spectra from narrow InAs-AlSb quantum wells. Strong, clearly resolved peaks for well widths from 2 to 8 monolayers were observed. Transmission electron micrographs show direct evidence for the structural quality of the quantum well structures. The transition energies of the narrowest wells suggest a strong influence of the AlSb X-barrier on the electronic states in the conduction band.

Brar, Berinder↗
NASA NTRS1994

Design Issues of GaAs and AlGaAs Delta-Doped p-i-n Quantum-Well APD's

We examine the basic design issues in the optimization of GaAs delta-doped and AlGAs delta-doped quantum-well avalanche photodiode (APD) structures using a theoretical analysis based on an ensemble Monte Carlo simulation. The devices are variations of the p-i-n doped quantum-well structure previously described in the literature. They have the same low-noise, high-gain and high-bandwidth features as the p-i-n doped quantum-well device. However, the use of delta doping provides far greater control or the doping concentrations within each stage possibly enhancing the extent to which the device can be depleted. As a result, it is expected that the proposed devices will operate at higher gain levels (at very low noise) than devices previously developed.

Wang, Yang↗
NASA NTRS1991

Absorption and photoluminescence of ultrathin pseudomorphic InAs/GaAs quantum wells

Absorption data are presented for 2-4-monolayer InAs/GaAs single quantum wells obtained at 77 K using a polarization-based measurement technique. The special contribution of the optical loss features arising from bulk GaAs was minimized using the polarization selectivity of absorption in single quantum wells. The double structure observed in the spectra is attributed to transitions involving confined heavy holes and both confined and unconfined electron states.

Ksendzov, A.↗
NASA NTRS1998

Quantum Well Infrared Photodetectors for Long Wavelength Infrared Applications

Quantum Well Infrared Photodetectors (QWIPs) offer greater flexibility than usual extrinsically doped semiconductor IR detectors because the wavelength of the peak response and cutoff can be continuously tailored by varying layer thickness (well width), barrier composition (barrier height), and carrier density (well doping density).

Quantum↗
NASA NTRS1995

Long Wavelength INGaAs/InGaAsP Quantum Well Lasers at [approximately] 2.0 micrometers

InGaAs/InGaAsP strained quantum well buried heterostructure and distributed feedback lasers emitting at wavelengths as long as 2.06 micrometers with low internal loss and threshold currents have been demonstrated for the first time. The room and high temperature characteristics and the threshold gain are discussed.

quantum well lasers indium gallium↗
NASA NTRS1994

An Acoustic Charge Transport Imager for High Definition Television Applications: Reliability Modeling and Parametric Yield Prediction of GaAs Multiple Quantum Well Avalanche Photodiodes

Reliability modeling and parametric yield prediction of GaAs/AlGaAs multiple quantum well (MQW) avalanche photodiodes (APDs), which are of interest as an ultra-low noise image capture mechanism for high definition systems, have been investigated. First, the effect of various doping methods on the reliability of GaAs/AlGaAs multiple quantum well (MQW) avalanche photodiode (APD) structures fabricated by molecular beam epitaxy is investigated. Reliability is examined by accelerated life tests by monitoring dark current and breakdown voltage. Median device lifetime and the activation energy of the degradation mechanism are computed for undoped, doped-barrier, and doped-well APD structures. Lifetimes for each device structure are examined via a statistically designed experiment. Analysis of variance shows that dark-current is affected primarily by device diameter, temperature and stressing time, and breakdown voltage depends on the diameter, stressing time and APD type. It is concluded that the undoped APD has the highest reliability, followed by the doped well and doped barrier devices, respectively. To determine the source of the degradation mechanism for each device structure, failure analysis using the electron-beam induced current method is performed. This analysis reveals some degree of device degradation caused by ionic impurities in the passivation layer, and energy-dispersive spectrometry subsequently verified the presence of ionic sodium as the primary contaminant. However, since all device structures are similarly passivated, sodium contamination alone does not account for the observed variation between the differently doped APDs. This effect is explained by the dopant migration during stressing, which is verified by free carrier concentration measurements using the capacitance-voltage technique.

Hunt, W. D.↗
NASA NTRS2004

Semiconductor Lasers Containing Quantum Wells in Junctions

In a recent improvement upon In(x)Ga(1-x)As/InP semiconductor lasers of the bipolar cascade type, quantum wells are added to Esaki tunnel junctions, which are standard parts of such lasers. The energy depths and the geometric locations and thicknesses of the wells are tailored to exploit quantum tunneling such that, as described below, electrical resistances of junctions and concentrations of dopants can be reduced while laser performances can be improved. In(x)Ga(1-x)As/InP bipolar cascade lasers have been investigated as sources of near-infrared radiation (specifically, at wavelengths of about 980 and 1,550 nm) for photonic communication systems. The Esaki tunnel junctions in these lasers have been used to connect adjacent cascade stages and to enable transport of charge carriers between them. Typically, large concentrations of both n (electron-donor) and p (electron-acceptor) dopants have been necessary to impart low electrical resistances to Esaki tunnel junctions. Unfortunately, high doping contributes free-carrier absorption, thereby contributing to optical loss and thereby, further, degrading laser performance. In accordance with the present innovation, quantum wells are incorporated into the Esaki tunnel junctions so that the effective heights of barriers to quantum tunneling are reduced (see figure).

Yang, Rui Q.↗
NASA NTRS1992

Anomalous capacitance of quantum well double-barrier diodes

The S-parameters of several different quantum well double barrier diodes have been measured. A technique has been developed for measuring whisker contacted diodes with an HP 8510B automatic network analyzer. Special coaxial mounts using K-connectors were designed to enable measurements up to 20 GHz. The voltage-dependent conductance and capacitance were derived from the measured reflection coefficient of each device. The C/V characteristics were observed to exhibit an anomalous increase at voltages corresponding to the negative differential resistance region (NDR). These are the first reported S-parameter measurements in the negative differential resistance region of quantum well double barrier diodes. A theory is presented that explains, in part, the observed results.

Boric, Olga↗
NASA NTRS1989

Photovoltaic-Driven Multiple-Quantum-Well Modulator

Proposed high-performance spatial light modulator consists of multiple-quantum-well, strain-layer superlattice similar to one described in accompanying article, "Strain-Layer-Superlattice Light Modulator" (NPO-16915). Device differs from other one, operates in transmission rather than in resonant reflection and therefore includes only one stack of GaAs/InAs layers having total thickness less than penetration depth (1 to 2 micrometer) of optical modulating signal. Alternating, semitransparent semiconductor layers contain space-charge barriers forming quantum wells. Modulating light (with photon energy greater than semiconductor band gap) induces photovoltaic action, which affects transmission of modulated light (having photon energy less than band gap).

Maserjian, Joseph↗
NASA NTRS1994

Design and performance of very long-wavelength GaAs/Al(x)Ga(1-x)As quantum-well infrared photodetectors

We present an extensive and detailed study of very long wavelength quantum well infrared photodetectors covering the spectral region between 14 and 20 micrometers. Measurements were made on seven different molecular beam epitaxy grown samples having different well widths and barrier heights. In this study we combine experimental results with theoretical analysis and focus on the relationship between the quantum well structure and detector performance, i.e., responsivity, dark current, dynamic resistance, noise current, optical-gain, and detectivity. These results provide the basis for further optimization, and the detector parameters needed for the design of the readout circuit for focal plane arrays.

Sarusi, G.↗