High-efficiency LEDs based on GaInAsSb solid solutions with reduced arsenic content in the active region

Light-emitting diodes (LEDs) with the active region based on Ga_{1− x} In_xAs_ySb1−y solid solutions are obtained on GaSb substrates. The active region has a composition close to the domain of immiscibility (x = 0.24) and a reduced arsenic content (y = 0.16). The diode structures exhibit a high density of misfit dislocations. Nevertheless, the room-temperature external quantum yield reaches a record-high level of 1.2% in the spectral band with a maximum wavelength of 2.42 μm. The maximum output radiation pulse power reaches 3.3 mW at a current of 600 mA. The emission is predominantly due to the interband transitions, which is evidence of a high degree of stoichiometry of the active region.     

Interaction of Cu(1 1 1) surface states with different extended defects

We study numerically the interaction of the Shockley surface state at the Γ-point of Cu(1 1 1) with periodic arrangements of adsorbed/absorbed Cu and Fe atoms. We use an spd tight-binding hamiltonian with the local basis enlarged to account for electron spillover.     

Localized spin wave states of superfluid3He in a magnetic field gradient

It is shown that in the presence of a magnetic field gradient, spin wave states in superfluid³He split into discrete levels, just as for a charged particle in an electric field near the surface. The energy levels and the intensity of the magnetic absorption due to localized states are determined for the simplest cases in superfluid³He A and B. The localized states will provide a unique way to determine the spin wave velocity and spin diffusion constant in superfluid³He.Work supported by National Science Foundation under Grant No. DMR76-17702.     

Localized single-particle excitations and the density of states for superconducting composites

The problem of localized single-particle excitations and the density of states (DOS) for an inhomogeneous system consisting of a spherical superconductor (with radius a and order parameter ₁) embedded in another superconductor (order parameter ₂) of infinite size is considered. With the assumption of constant values of ₁ and ₂, the Bogoliubov equations are solved for general values of l (the orbital angular momentum quantum number). For a fixed value of ₁/₂ and different values of ₂/E _F, the dependence of the excitation energy (l=0)/₂ on the particle sizek _F a is shown (k _F is the Fermi wave vector andE _F is the Fermi energy). Fork _F a=300, 450, and 800 and a fixed value of ₂/E _F, the variations in the DOS by changing ₁/₂ are also shown.     

Spatiotemporal structures in a transversely extended semiconductor system

A semiconductor system with an N-shaped current-voltage characteristic can generate nonstationary structures that are inhomogeneous in the direction transverse to the current flow. In samples of semiinsulating GaAs with a large cross section, this effect is explained by a loss of stability of the regime of uniform domain motion.     

Effect of extended defects in planar and pixelated CdZnTe detectors

We evaluated a spectroscopy-grade 15×15×7 mm³ CdZnTe (CZT) crystal with a high μτ-product, >10⁻² cm²/V, but impaired by microscopic extended defects, such as walls of dislocations, low-angle and sub-grain boundaries, and Te inclusions. First, we evaluated a planar detector fabricated from this crystal using a Micro-scale X-ray Detector Mapping (MXDM) technique. Then, we fabricated from the same crystal a pixel detector to study local non-uniformities of the electric field. The measured X-ray response maps confirmed the presence of non-uniformities in the charge transport, and they showed that the global- and local-distortions of the internal E-field correlated to the extended defects and space-charge buildup on the side surfaces.     

Optimization of the method of pulsed thermal defectometry of extended defects

The authors discuss results of development and optimization of a procedure and equipment for determination of the parameters of extended defects in plates based on experimental and theoretical modeling of nonstationary heat fields.Belarussian State Polytechnic Academy, Minsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 4, pp. 434–441, April, 1994.     

Phase diagram with a region of liquid carbon-diamond metastable states

Metastable cubic diamond has been found in the structure of solid carbon obtained by quenching of a liquid phase at a pressure (0.012 GPa) much lower than that corresponding to the existence of stable diamond. It is suggested that this metastable diamond is formed as a result of the recalescence of supercooled liquid carbon to the melting point (T _dm) of metastable diamond due to a lower energy barrier for the formation of diamond as compared to that of graphite. A comparison between the calculated Gibbs energies of metastable phases provided an estimate of T _dm = 4160 ± 50 K. For the first time, metastable continuations of the curve of diamond melting at pressures of up to 0.012 GPa are constructed on the phase diagrams of carbon (according to various published data) using analytical curves described by a two-parametric Simon equation.     

Modeling the thermodynamic properties of sodium chloride in steam through extended corresponding states

Recent precise data on anomalous behavior of apparent molar properties of electrolyte solutions in near-critical steam have raised important questions as to how the thermodynamic properties of these systems should be described. Current Gibbs free energy models fail for highly compressible solutions. Here, a Helmholtz free energy formulation is presented as a first step in modeling compressible dilute aqueous electrolyte solutions. Comparisons are made with the known critical line, coexistence curves, apparent molar volumes, and heat capacities of NaCl in steam, and conclusions presented on improving the model.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Ion beam nanopatterning in graphite: characterization of single extended defects

The morphology and the electronic structure of a single focused ion-beam-induced artificial extended defect is probed by several methods including micro-Raman spectroscopy, atomic force and scanning tunneling microscopies and Monte Carlo and/or semi-analytical simulation within standard codes. The efficiency of the artificial defect for deposited metallic cluster pinning is also investigated. We show a correlation between the ion dose, morphology, electronic structure and cluster trapping efficiency. At room temperature, cluster pinning is efficient when the displacement per atom is one or more. Well-ordered patterned cluster networks are considered for potential applications.     

Crystalline quality of the trigonal piezoelectric materials and effects of the extended defects

Many frequently used or promising piezoelectric materials belong to crystal classes 32 or 3m. Among them are α quartz and its crystallographic analogs (AlPO(4), GaPO(4), α-GeO(2), etc.), the numerous materials of the langasite (La-(3)Ga(5)SiO,sub>14) family and also lithium tantalate (LiTaO,sub>3) and lithium niobate (LiNbO 3). In this paper we study the present state of the art for these materials, indicate their principal point and extended defects, and present methods to reduce the dislocation density. Large concentrations of intrinsic point defects often exist in crystal grown at very high temperatures. The point defects (intrinsic or related to impurities) modify the constants and can increase the acoustic losses. This is the case for the alkali ions and the OH that induce severe losses in different temperature intervals. The extended defects also affect the performances of the piezoelectric devices. Some, such as twins, ferroelectric domains, or large solid or liquid inclusions, have very detrimental effects. Dislocations, growth bands, and planar defects are more difficult to avoid and affect the devices in a more subtle manner. In quartz and its analogs, dislocations seem to increase the nonlinear elastic effects and have a collective effect on the vibration modes, particularly in energy trapping resonators. Growth bands and stacking faults also produce similar effects.     

Optical excitation of nonidentical quantum wells in the active region of a vertical-external-cavity laser

Optical excitation of a semiconductor structure comprising two nonidentical quantum wells (QWs) separated by barriers absorbing the pump radiation has been numerically simulated. It is shown that the equalization of populations in the QWs can be achieved in two ways: (i) shift of the deeper QW to a region of weak carrier generation and (ii) introduction of a blocking layer preventing the transport of carriers.     

Spatial mapping of the electronic states of a one-dimensional system

Using low-temperature scanning tunneling microscopy and spectroscopy, we have recorded spatial maps of confined electronic states in the troughs between self-organized Pt nanowires on Ge(001) that are spaced 2.4 nm apart. Two sub-bands are resolved, which correspond to the lowest energy levels of a quantum mechanical particle in a box. As expected, the spatial dI/dV maps exhibit a maximum and a minimum in the middle of the troughs for the n = 1 and n = 2 states, respectively.     

Bending properties of creased zones of paperboard related to interlaminar defects

A parallel beam model is presented for the bending behaviour of locally delaminated zones (creased zones) of paperboard. It is shown that the bending moment and the force levels are strongly dependent on the geometry and elastic properties of the creased zone. A favourable condition for low crease stiffness is a large number of long interlaminar failures. Experimental measurements of the crease stiffness and maximum bending moment for paperboards with implanted defects of various lengths and numbers were in reasonably good agreement with the crease stiffness predicted by the parallel beam model.     

Relative concentrations of carbon related defects in silicon

Irradiation induced defects in silicon are technologically important as they impact the electronic properties. Calculations based on density functional theory employing hybrid functionals have been previously used to investigate the structures and relative energies of defect clusters formed between vacancies, self-interstitials, carbon and oxygen atoms in silicon. In this study we employ a model to calculate the relative concentrations of carbon related defects in silicon. It is calculated that the carbon content has a significant impact upon the concentration of carbon-related defects. The C_iC_s defect is the most populous for all the conditions considered followed by the C_iO_iSi_I and the C_iO_i defects. C_iO_iSi_I and the C_iO_i become increasingly important for silicon with high carbon concentrations.     

Corrector design with active vase mirrors that allows a fixed telescope to access a large region of the sky

We investigate a family of two-mirror correctors to compensate for the aberrations of a parabolic mirror observing at a large angle from the zenith. We constrain our designs to optical elements that can be built with currently available technology. The secondary and the tertiary mirrors are warped by Zernike polynomials, which we know can be generated with active vase mirrors. The performances of these corrector designs are usable for imagery.     

Electrical activity of extended defects and gettering of metallic impurities in silicon

Abstract

Electrical activities of slip dislocations and Frank type partial dislocations in Si were investigated using the temperature dependent electron beam induced current technique. It was found that both types of dislocation are recombination active in the temperature range below about 200 K only and inactive in the higher temperature range if they are free from decoration by metallic impurities. From the temperature dependence of the electron beam induced current contrasts, the energy levels of recombination centres on these dislocations were determined to be less than 0.1 ev. They are not accompanied by any deep levels. Debris of defects generated from moving dislocations has deep levels about 0.3-0.4 eV from the band edge even if there is no decoration with metals. Both slip dislocations and Frank partials become recombination active at high temperatures when they getter metallic impurities. The characteristics of impurity gettering by dislocations depend on both the species of impurities and the cooling rate of a Si crystal after contamination. As a result, the recombination activities of dislocations are influenced by a variety of external parameters.