SEARCH · Engineering Papers
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
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).
We have fabricated GaAs rear-heterojunction solar cells with strain-balanced GaInAs/GaAsP multiple quantum wells to increase the range of absorption. The overall device architecture includes a thick n-type GaAs emitter layer, followed by undoped multiple quantum wells (QWs) and then a heterojunction with a higher bandgap GaInP p-type base. The QWs shift the band edge to from ~870 nm to 930 nm and results in >2 mA/cm 2 collection below the GaAs bandgap. Due to the heterojunction, the cell demonstrated a high open-circuit voltage of Voc=1.03V and an efficiency of 26.4% at 1000 W/m 2.
The effect of inserting a nm-scale layer of Zn(Si,Ge)N2 into an AlGaN quantum well structure designed for light emission in the wavelength range from 255 to 305 nm is investigated here. The enhanced confinement of the hole within the quantum well results in an enhancement of the overlap of the hole and electron wave functions, resulting in an enhancement of the radiative recombination rate. In this theoretical calculation, for emission at a 270 nm wavelength, the enhancement in the wavefunction overlap can reach a factor of 7 when compared to an AlGaN quantum well device specifically engineered for optimal emission at the identical wavelength. Increases of almost an order of magnitude in both the peak spontaneous emission intensity and the radiative recombination rate are predicted. The peak emission wavelength can be tuned from 255 to 305 nm by adjusting the width and/or the composition of the inserted layer. The proposed structures provide a route to higher efficiency ultraviolet practical light emitting diodes and lasers.
We demonstrate single and dual junction solar cells with up to 80 strain-balanced GaInAs/GaAsP quantum wells (QWs), in order to extend the range of photon absorption. The net average stress in the quantum well layers is =12 MPa, despite coherent stresses in the individual layers that are 35-70 times greater. The GaAs-QW cell has a certified efficiency of (27.2 ± 0.2)% at one-sun; the GaInP/GaAs-QW tandem has a certified record one-sun efficiency of (32.9 ± 0.5)%. We will discuss the architecture of the devices, the physics behind the high performance, and pathways to further improvement.
In this work, we report low-temperature magneto-transport measurements of an undoped Si/SiGe asymmetric double quantum well heterostructure. The density in both layers is tuned independently utilizing top and bottom gates, allowing the investigation of quantum wells at both imbalanced and matched densities. Integer quantum Hall states at total filling factor v T =1 and v T =2 are observed in both density regimes, and the evolution of their excitation gaps is reported as a function of the density. The v T =1 gap evolution departs from the behavior generally observed for valley splitting in the single layer regime. Furthermore, by comparing the v T =2 gap to the single particle tunneling energy, Δ SAS , obtained from Schrödinger–Poisson (SP) simulations, evidence for the onset of spontaneous interlayer coherence is observed for a relative filling fraction imbalance smaller than ~50%.
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.
We demonstrate single and dual junction solar cells with up to 80 strain-balanced GaInAs/GaAsP quantum wells (QWs), in order to extend the range of photon absorption. The net average stress in the quantum well layers is =12 MPa, despite coherent stresses in the individual layers that are 35-70 times greater. The GaAs-QW cell has a certified efficiency of (27.2 ± 0.2)% at one-sun; the GaInP/GaAs-QW tandem has a certified record one-sun efficiency of (32.9 ± 0.5)%. We will discuss the architecture of the devices, the physics behind the high performance, and pathways to further improvement.
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.
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).
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.
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).
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.
A novel approach to determining absorption coefficients in thin films using luminescence is described. The technique avoids many of the difficulties typically encountered in measurements of thin samples, Fabry-Perot effects, for example, and can be applied to a variety of materials. The absorption edge for GaAs/AlGaAs multiple quantum well structures, with quantum well widths ranging from 54 to 193 A is examined. Urbach (1953) parameters and excitonic linewidths are tabulated.
Quantum Well Infrared Photodetectors (QWIPs) afford greater flexibility than the usual extrinsically doped semiconductor IR detectors.
Metal-organic species can be designed to self-assemble in large-scale, atomically defined, supramolecular architectures. Hybrid quantum wells, where inorganic two-dimensional (2D) planes are separated by organic ligands, are a particular example. The ligands effectively provide an intralayer confinement for charge carriers resulting in a 2D electronic structure, even in multilayered assemblies. Air-stable metal organic chalcogenides hybrid quantum wells have recently been found to host tightly bound 2D excitons with strong optical anisotropy in a bulk matrix. Here, we investigate the excited carrier dynamics in the prototypical metal organic chalcogenide [AgSePh] ∞ , disentangling three excitonic resonances by low temperature transient absorption spectroscopy. Our analysis suggests a complex relaxation cascade comprising ultrafast screening and renormalization, inter-exciton relaxation, and self-trapping of excitons within few picoseconds. Finally, the ps-decay provided by the self-trapping mechanism may be leveraged to unlock the material's potential for ultrafast optoelectronic applications.
Two-dimensional (2D) carrier systems confined to modulation-doped semiconductor hetero-structures provide a nearly ideal testing ground for exploring new physical phenomena. At low temperatures and in the presence of a strong magnetic field, these systems exhibit fascinating, often unexpected, many-body states, arising from the strong electron-electron interaction. In our work, we studied various many-body states of different 2D systems. These included 2D electron and hole systems confined to GaAs and also to AlAs quantum wells, including wide quantum wells where the charge distribution is bilayer like. We observed and reported new phenomena in these systems, including new (even-denominator) fractional quantum Hall states, Wigner crystal solid phases, and composite fermion compressible states.
Equivalent gate noise voltage spectra of 1-micron gate-length modulation-doped FET's with pseudomorphic InGaAs quantum-well structure have been measured for the frequency range of 0.01 Hz to 100 MHz and commpared with the noise spectra of conventional AlGaAs/GaAs MODFET's and GaAs MESFET's. The prominent generation-recombination (g-r) noise bulge commonly observed in the vicinity of 10 kHz in conventional MODFET's at 300 K does not appear in the case of the new InGaAs quantum-well MODFET. Instead, its noise spectra indicate the presence of low-intensity multiple g-r noise components superimposed on a reduced 1/f noise. The LF noise intensity in the new device appears to be the lowest among those observed in any MODFET or MESFET. The noise spectra at 82 K in the new device represent nearly true 1/f noise. This unusual low-noise behavior of the new structure suggests the effectiveness of electron confinement in the quantum well that significantaly reduces electron trapping in the n-AlGaAs, and thus eliminates the g-r noise bulge observed in conventional MODFET's.
Abstract The density of states at the Fermi surface controls many material properties by influencing the low-energy interactions of conductive electrons. Typically, external tuning knobs generate only small perturbations to the density of states in crystals, since their band structure depends strongly on the lattice potential. In contrast, quantum well states are contingent on a localized potential extrinsic to the crystal lattice and can be easily modified leading to changes in the density of states at the Fermi level. Here, we are able to control the quantum well potential on the surface of Bi 2 Se 3 with light, driving a density of states singularity below E F at ultrafast timescales, thereby triggering a reversible Lifshitz transition. We reveal a substantial ultrafast enhancement of the electron-phonon coupling with repercussions on the relaxation dynamics in the system. We argue that the relaxation dynamics are governed largely by the interplay of the dynamic density of states of the quantum wells with c -axis scattering from the $${A}_{{{{{{{{\rm{1g}}}}}}}}}^{2}$$ A 1g 2 optical phonon mode. These results demonstrate a powerful way to enhance electron-boson interaction on ultrafast timescales providing new avenues for controlling material properties and driving novel quantum phases of matter.