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To observe the effects of polarization fields and screening, we have performed contacted electroreflectance (CER) measurements on In0.07Ga0.93N/GaN single quantum well light emitting diodes for different reverse bias voltages. Room-temperature CER spectra exhibited three features which are at lower energy than the GaN band gap and are associated with the quantum well. The position of the lowest-energy experimental peak, attributed to the ground-state quantum well transition, exhibited a limited Stark shift except at large reverse bias when a redshift in the peak energy was observed. Realistic band models of the quantum well samples were constructed using self-consistent Schrödinger–Poisson solutions, taking polarization and screening effects in the quantum well fully into account. The model predicts an initial blueshift in transition energy as reverse bias voltage is increased, due to the cancellation of the polarization electric field by the depletion region field and the associated shift due to the quantum-confined Stark effect. A redshift is predicted to occur as the applied field is further increased past the flatband voltage. While the data and the model are in reasonable agreement for voltages past the flatband voltage, they disagree for smaller values of reverse bias, when charge is stored in the quantum well, and no blueshift is observed experimentally. To eliminate the blueshift and screen the electric field, we speculate that electrons in the quantum well are trapped in localized states. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A contacted electroreflectance technique was used to investigate AlGaN/GaN heterostructures and their intrinsic electric field-induced properties. By studying variations in the electroreflectance with applied field, spectral features associated with the AlGaN barrier, the two-dimensional electron gas at the interface, and bulk GaN were identified. Barrier-layer composition and electric field were determined from the AlGaN Franz–Keldysh oscillations. For a high mobility heterostructure grown on SiC, measured AlGaN polarization electric field and two-dimensional electron gas density approached values predicted by a standard bandstructure model. The two-dimensional electron gas produced a broad, field-tunable first derivative electroreflectance feature. With a dielectric function calculation, we describe the line shape and relative amplitude of the two-dimensional electron gas electroreflectance feature for a wide range of electron density and applied field values. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Measures the radiation induced conductivity of polyethylene terephthalate produced by the electron beam from a pinched electron beam diode. Experimental techniques used to measure such radiation; Description of the pinched electron beam diode and foil pack.

Deals with a study which measured the space charge and internal electric fields in metal-poly(ethylene terephthalate)-metal samples subjected to electric field stressing, using a space-charge mapping technique. Methodology of the study; Results and discussion.

Attempts to provide experimental evidence for the existence of impact ionized holes in n-type zinc oxide varistors of several chemical compositions when they are in their breakdown condition. Information on the chemical composition of noncommercial varistors; Discussion on the luminescence measurement technique; Behavior of the band-gap electroluminescence.

Focuses on a study which discussed the kinetics of a photocarrier, radically decreasing the radiation-induced conductivity of doped polyethylene terephthalate film. Condition in which the radiation-induced conductivity increases; Result of a situation once the photocarriers are generated and thermalized; Benefits from studying electronic transport in doped dielectric films.

Presents a study in which time-of-flight photoconductivty measurements were performed on mercuric iodide using a penetrating pulsed x-ray source, simulating the operation of photoconductive x-ray detectors. Range of electron and hole mobilities observed; Difference of intrinsic photocarrier generation and recombination processes from classical Onsager and Langevin mechanisms; Analysis of the field dependence of the collected electron charge.

Deep-level transient spectroscopy measurements on a reverse-biased p-type GaAsN Schottky diode grown by metalorganic chemical vapor deposition show a minority-carrier trap signal corresponding to an electron trap having an activation energy of about 0.2 eV. The proportion of trapped electrons agrees with that of modeled defect states near the Schottky-barrier metal interface whose occupation is affected by changing reverse-bias conditions. Estimates of thermionic emission provide adequate filling of the traps with electrons from the metal. The inclusion of a GaAs layer between the metal and GaAsN layer reduces the electron-trapping signal. [ABSTRACT FROM AUTHOR]

Forward-to-reverse bias step-recovery experiments were performed on an InGaN/GaN single-quantum-well light-emitting diode grown on sapphire. With the quantum well sampling the minority-carrier hole density at a single position, the optical emission displayed a two-stage decay. Using a solution to the diffusion equation to self-consistently describe both the optical and electrical recovery data, we estimated values for the hole lifetime (758±44 ns), diffusion length (588±45 nm), and mobility (0.18±0.02 cm2/V s) in GaN grown on sapphire. This low value of the minority-carrier mobility may reflect trap-modulated transport, and the lifetime is suggestive of slow capture and emission processes occurring through deep levels. [ABSTRACT FROM AUTHOR]

A contacted electroreflectance technique was used to characterize the electronic properties of AlGaN/GaN heterostructures and field-effect transistors. By studying variations in the electroreflectance with applied electric field, spectral features associated with the AlGaN barrier, the two-dimensional electron gas at the interface, and bulk GaN were observed. The AlGaN barrier displayed Franz-Keldysh oscillations which provided estimates of barrier-layer composition and electric field. The electron gas Fermi energy was determined from analysis of the broad two-dimensional electron gas feature. [ABSTRACT FROM AUTHOR]

To investigate the effects of strain and quantum confinement, Auger rates are calculated for an n-type strained InAs0.9Sb0.1/In0.87Ga0.13As mid-infrared laser structure as a function of temperature. Compressive strain in the quantum wells reduces the mass of the holes which leads to a reduction in the Auger rate compared with an unstrained quantum well. The rate reduction is explained in terms of a qualitative analysis of the role of strain. The rate includes contributions from inter-subband Auger transitions which are found to dominate the rate at low temperature. [ABSTRACT FROM AUTHOR]

A first principles total energy self-consistent pseudopotential calculation is used to predict the band offset in the lattice-matched superlattice InAs/Al0.8Ga0.2As0.14Sb0.86. We find that inclusion of interface strain changes the character of the band offset from nominally type II to strongly type II. The predicted band offset at the minimum energy configuration is in excellent agreement with the value determined from infrared photoluminescence measurements. [ABSTRACT FROM AUTHOR]

A two-terminal, monolithic cascade solar cell with an efficiency of 27.3% is reported. The device structure consists of a Ga0.5In0.5P homojunction grown epitaxially upon a GaAs homojunction, with a GaAs tunnel diode interconnect. The tandem combination of these two materials is lattice matched, and has a theoretical efficiency of 34%. The device was grown by metalorganic chemical vapor deposition at 700 °C, using trimethylgallium, trimethylindium, arsine, and phosphine as sources. The minority-carrier transport properties of the Ga0.5In0.5P are shown to be relatively insensitive to variations of the growth temperature and phosphine overpressure. Other factors that affect the efficiency of the device are presented and discussed. [ABSTRACT FROM AUTHOR]

Both large photoconductive gain and long wavelength photoresponse were observed in lateral photodetectors constructed from type II, InAsSb, strained-layer superlattices. In a novel, four-layer superlattice, gain values as large as 90 are reported with a long wavelength cutoff of 8.7 μm at 77 K. The gain is sensitive to the structure and composition of the superlattice, and the sweepout of minority carriers is eliminated with the appropriate contacts. [ABSTRACT FROM AUTHOR]

Infrared photoluminescence measurements were performed on InAs0.13Sb0.87 /InSb strained-layer superlattices. In thick layered structures we observed very low energy transitions proving that a type II superlattice occurs in the InAsSb system. Band structures were calculated based on estimates of the band offsets and strain shifts obtained from the photoluminescence data. [ABSTRACT FROM AUTHOR]

A photodiode consisting of a p-n junction embedded in an InAs0.09Sb0.91/InSb strained-layer superlattice with equal 130-Å-thick layers was grown using molecular beam epitaxy. This nonoptimized device exhibited photoresponse out to a wavelength of 8.7 μm at 77 K. The resistance and the minority-carrier diffusion length of the photodiode result in a detectivity (3×109 cm Hz1/2/W) at 7 μm that is within one order of magnitude of the detectivity of the best HgCdTe detectors at that wavelength. [ABSTRACT FROM AUTHOR]

Strained-layer superlattices of InAsSb were grown with low densities of dislocations and microcracks for optical characterization to determine the suitability of these structures for infrared photodetectors. Infrared transmission measurements revealed absorption throughout the 8–12 μm region and extended to longer wavelengths than predicted from consideration of the tensile strain-induced band-gap shift in a type-I superlattice. We conclude that a type-II superlattice occurs in the InAsSb system for alloy compositions >60% Sb. [ABSTRACT FROM AUTHOR]

A space-charge mapping technique was used to observe the formation of x-ray-induced space charge in poly(ethylene terephthalate) and polystyrene capacitor structures. Electronic transport processes, responsible for radiation-induced conductivity in these insulators, produced trapped space charge adjacent to the electrodes. These results were consistent with the conclusions of earlier photoconductivity measurements and revealed injection processes that limit the buildup of space charge. [ABSTRACT FROM AUTHOR]

Electron-induced conductivity measurements were performed on polyethylene terephthalate to compare electron and hole contributions to the radiation-induced conductivity. The electron mobility in this material was found to be >=102 times that of the holes. This result explains an earlier observation that the radiation-induced conductivity of polyethylene terephthalate can be radically reduced by chemically doping this dielectric with electron acceptor molecules. [ABSTRACT FROM AUTHOR]

Focuses on a study which measured the diamagnetic shift of band-edge luminescence, no-phonon peak and phonon sideband from undoped silicon[sub0.83] germanium[sub0.17]/silicon quantum well using magnetoluminescence spectroscopy. Features of zero field; Variational method of estimating the diamagnetic shift of a ground-state exciton of a quantum well; Effect of confinement on the value of the effective electron mass.

Lasers and light-emitting diodes with multistage, type I InAsSb/InAsP quantum well active regions are reported. These ten stage, cascaded devices were grown by metalorganic chemical vapor deposition. The broadband light-emitting diodes produced high average powers, >2 mW (∼80 K, 3.7 μm) and >0.1 mW (∼300 K, 4.3 μm). A 3.8–3.9 μm laser structure operated up to T=180 K. At 80 K, peak power >100 mW and a slope efficiency of 48% (4.8% per stage) were observed in our gain guided lasers. The slope efficiency was strongly dependent on cavity length, and analysis of efficiency data suggests an internal differential quantum efficiency >1 and a loss coefficient >=100 cm[sup -1]. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The self-absorption of radiated photons increases the minority carrier concentration in semiconductor optoelectronic devices such as solar cells. This so-called photon recycling leads to an increase in the external luminescent efficiency, the fraction of internally radiated photons that are able to escape through the front surface. An increased external luminescent efficiency in turn correlates with an increased open-circuit voltage and ultimately conversion efficiency. We develop a detailed ray-optical model that calculates Voc for real, non-idealized solar cells, accounting for isotropic luminescence, parasitic losses, multiple photon reflections within the cell and wavelength-dependent indices of refraction for the layers in the cell. We have fabricated high quality GaAs solar cells, systematically varying the optical properties including the back reflectance, and have demonstrated Voc = 1.101 ± 0.002 V and conversion efficiencies of (27.8 ± 0.8)% under the global solar spectrum. The trends shown by the model are in good agreement with the data. [ABSTRACT FROM AUTHOR]

Temperature- and bias-dependent current measurements were performed on n+-GaAs/p-GaAs1-xNx heterojunction diodes. The samples studied are in the dilute regime and contain less than 1.7% nitrogen with respect to arsenic. Current-voltage, thermally stimulated current, and current transient (after voltage change) measurements provide unique insight into the defect participation in carrier transport within the depletion region. We will present the data obtained from these measurements, which show an activation energy of 0.21 eV and may be related to a key defect measured by capacitance transients in the literature. [ABSTRACT FROM AUTHOR]

Gain-guided, injection lasers using AlAsSb for optical confinement and a strained InAsSb/InAs multiquantum well active region were grown by metalorganic chemical vapor deposition. The semi-metal properties of a p-GaAsSb/n-InAs heterojunction are utilized as a source for injection of electrons into the active region of the laser. In pulsed mode, the laser operated up to 210 K with an emission wavelength of 3.8–3.9 μm. We also report on the two-color emission of a light-emitting diode with two different active regions to demonstrate multistage operation of these ‘‘unipolar ’’ devices. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photon management has been shown to be a fruitful way to boost the open circuit voltage and efficiency of high quality solar cells. Metal or low-index dielectric-based back reflectors can be used to confine the reemitted photons and enhance photon recycling. Gaining access to the back of the solar cell for placing these reflectors implies having to remove the substrate, with the associated added complexity to the solar cell manufacturing. In this work, we analyze the effectiveness of a single-layer reflector placed at the back of on-substrate solar cells, and assess the photon recycling improvement as a function of the refractive index of this layer. Al2O3-based reflectors, created by lateral oxidation of an AlAs layer, are identified as a feasible choice for on-substrate solar cells, which can produce a Voc increase of around 65% of the maximum increase attainable with an ideal reflector. The experimental results obtained using prototype GaAs cell structures show a greater than two-fold increase in the external radiative efficiency and a Voc increase of ~2% (~18 mV), consistent with theoretical calculations. For GaAs cells with higher internal luminescence, this Voc boost is calculated to be up to 4% relative (36 mV), which directly translates into at least 4% higher relative efficiency. [ABSTRACT FROM AUTHOR]

Objective: To define the groups of patients at risk for transfusion-transmitted cytomegalovirus infection and to define the methods to reduce this risk.Data Sources: English-language publications on transfusion medicine.Study Selection and Data Extraction: Studies were selected that described cytomegalovirus infection in transfusion-dependent patients. Special attention was paid to reports that included observations about the prevalence and clinical manifestations of cytomegalovirus infection and recommendations for the prevention of infection.Data Synthesis: Some patients with impaired immune responses who have never been exposed to cytomegalovirus are at risk for transfusion-transmitted cytomegalovirus infection. This infection, which is associated with substantial morbidity and mortality, can be avoided by additional screening of blood donors or by special processing of components for transfusion.Conclusions: Transfusion products that are unlikely to transmit cytomegalovirus infection can be prepared by filtration to remove leukocytes or can be obtained by selecting donors who are seronegative for antibodies to cytomegalovirus. These products are indicated for certain groups of immunosuppressed patients, including pregnant women who are cytomegalovirus seronegative, premature infants of low birth weight who are born to cytomegalovirus-seronegative mothers, cytomegalovirus-seronegative recipients of allogeneic bone marrow transplants from cytomegalovirus-seronegative donors, and cytomegalovirus-seronegative patients with the acquired immunodeficiency syndrome (AIDS). [ABSTRACT FROM AUTHOR]

We report micro-Raman studies of self-heating in an AlGaN/GaN heterostructure field-effect transistor using below (visible 488.0 nm) and near (UV 363.8 nm) GaN band-gap excitation. The shallow penetration depth of the UV light allows us to measure temperature rise (ΔT) in the two-dimensional electron gas (2DEG) region of the device between drain and source. Visible light gives the average ΔT in the GaN layer, and that of the SiC substrate, at the same lateral position. Combined, we depth profile the self-heating. Measured ΔT in the 2DEG is consistently over twice the average GaN-layer value. Electrical and thermal transport properties are simulated. We identify a hotspot, located at the gate edge in the 2DEG, as the prevailing factor in the self-heating. [ABSTRACT FROM AUTHOR]

Ultraviolet light emitting diodes (LEDs) have been grown using metalorganic vapor phase epitaxy, while monitoring the 550 nm reflected light intensity. During nucleation of GaN on sapphire, the transition from three-dimensional (3D) grain growth to two-dimensional (2D) coalesced growth was intentionally delayed in time by lowering the NH[sub 3] flow during the initial high temperature growth. Initially, when the reflectance signal is near zero, the GaN film is rough and composed of partly coalesced 3D grains. Eventually, the reflected light intensity recovers as the 2D morphology evolves. For 380 nm LEDs grown on 3D nucleation layers, we observe increased light output. For LEDs fabricated on GaN films with a longer recovery time an output power of 1.3 mW at 20 mA current was achieved. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence spectroscopy has been used to investigate carrier decay dynamics in a In[sub x]Ga[sub 1-x]As[sub 1-y]N[sub y] (x∼0.03, y∼0.01) epilayer grown on GaAs by metal organic chemical vapor deposition. Time-resolved photoluminescence (PL) measurements, performed for various excitation intensities and sample temperatures, indicate that the broad PL emission at low temperature is dominated by localized exciton recombination. Lifetimes in the range of 0.07-0.34 ns are measured; these photoluminescence lifetimes are significantly shorter than corresponding values obtained for GaAs. In particular, we observe an emission energy dependence of the decay lifetime at 10 K, whereby the lifetime decreases with increasing emission energy across the PL spectrum. This behavior is characteristic of a distribution of localized states, which arises from alloy fluctuations. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the construction of a three-dimensional infrared photonic crystal on a silicon wafer using relatively standard microelectronics fabrication technology. How photonic crystals provide a means for achieving the goal of confining and controlling light in three dimensions.

We demonstrate 1.81 eV GaInP solar cells approaching the Shockley-Queisser limit with 20.8% solar conversion efficiency, 8% external radiative efficiency, and 80-90% internal radiative efficiency at one-sun AM1.5 global conditions. Optically enhanced voltage through photon recycling that improves light extraction was achieved using a back metal reflector. This optical enhancement was realized at one-sun currents when the non-radiative Sah-Noyce-Shockley junction recombination current was reduced by placing the junction at the back of the cell in a higher band gap AlGaInP layer. Electroluminescence and dark current-voltage measurements show the separate effects of optical management and non-radiative dark current reduction. [ABSTRACT FROM AUTHOR]

Near-field scanning optical microscopy (NSOM) was used to study cleaved edges of GaAs solar cell devices. Using visible light for excitation, the NSOM acquired spatially resolved traces of the photocurrent response across the various layers in the device. For excitation energies well above the band gap, carrier recombination at the cleaved surface had a strong influence on the photocurrent signal. Decreasing the excitation energy, which increased the optical penetration depth, allowed the effects of surface recombination to be separated from collection by the pn junction. Using this approach, the NSOM measurements directly observed the effects of a buried minority carrier reflector/passivation layer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]