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Cubic boron nitride (cBN) is a synthetic wide band gap material that has attracted attention due to its high thermal conductivity, optical transparency and optical emission. In this work, defects in cBN have been investigated using experimental and theoretical X-ray absorption near edge structure (XANES). Vacancy and O substitutional defects were considered, with O substituted at the N site (ON) to be the most energetically favorable. All defects produce unique signatures in either the B or N K-edges and can thus be identified using XANES. The calculations coupled with electron-irradiation / annealing experiments strongly suggest that ON is the dominant defect in irradiated cBN and remains after annealing. This defect is a likely source of optical emission in cBN.
Excavations between November 1990 and February 1992 have produced important information on the date and development of the Sarn-y-bryn-caled cursus complex between 3000–2000 BC. In particular a timber circle of 2000 BC, two penannular ring-ditches and a section across the cursus monument were excavated. A radiocarbon sequence has been obtained. The results of the excavations are described in Part I. Part II comprises a discussion of the forms, dates, functions, and reconstruction of timber circles. A corpus of and chronology for timber circles is presented.
The Dominion Radio Astrophysical Observatory (DRAO) is carrying out a survey as part of an international collaboration to image the northe, at a common resolution, in emission from all major constituents of the interstellar medium; the neutral atomic gas, the molecular gas, the ionised gas, dust and relativistic plasma. For many of these constituents the angular resolution of the images (1 arcmin) will be more than a factor of 10 better than any previous studies. The aim is to produce a publicly-available database of high resolution, high-dynamic range images of the Galaxy for multi-phase studies of the physical states and processes in the interstellar medium. We will sketch the main scientific motivations as well as describe some preliminary results from the Canadian Galactic Plane Survey/Releve Canadien du Plan Galactique (CGPS/RCPG).
The Magellanic System represents one of the best places to study the formation and evolution of galaxies. Photometric surveys of various depths, areas and wavelengths have had a significant impact on our understanding of the system; however, a complete picture is still lacking. VMC (the VISTA near-infrared YJKs survey of the Magellanic System) will provide new data to derive the spatially resolved star formation history and to construct a three-dimensional map of the system. These data combined with those from other ongoing and planned surveys will give us an absolutely unique view of the system opening up the doors to truly new science!
A survey of the Milky Way disk and the Magellanic System at the wavelengths of the 21-cm atomic hydrogen (H i) line and three 18-cm lines of the OH molecule will be carried out with the Australian Square Kilometre Array Pathfinder telescope. The survey will study the distribution of H i emission and absorption with unprecedented angular and velocity resolution, as well as molecular line thermal emission, absorption, and maser lines. The area to be covered includes the Galactic plane (|b| < 10°) at all declinations south of δ = +40°, spanning longitudes 167° through 360°to 79° at b = 0°, plus the entire area of the Magellanic Stream and Clouds, a total of 13 020 deg2. The brightness temperature sensitivity will be very good, typically σT≃ 1 K at resolution 30 arcsec and 1 km s−1. The survey has a wide spectrum of scientific goals, from studies of galaxy evolution to star formation, with particular contributions to understanding stellar wind kinematics, the thermal phases of the interstellar medium, the interaction between gas in the disk and halo, and the dynamical and thermal states of gas at various positions along the Magellanic Stream.
We have studied the etching effect of AlxGa1-xAs (0≤ x ≤ 0.5) by trisdimethylaminoarsenic (TDMAAs) at different substrate temperatures, and the quality of the resulting etched/regrown GaAs interface. We find that the etching rate of AlxGa1-x As decreases with increasing Al composition, and the interface trap density of the TDMAAs etched/regrown interface can be reduced by about a factor of 10 as deduced from capacitance-voltage carrier profiles. A smooth surface morphology of GaAs with an interface state density of 1.4×l011 cm−2 can be obtained at a lower in-situ etching temperature of 550°C. Moreover, by using this in-situ etching the I-V characteristics of regrown p-n junctions of Al0.35Ga0.65As/Al0.25Ga0.75As and Al0.35Ga0.65As/GaAs can be improved.
Selective epitaxial growth (SEG) of silicon-germanium (SiGe) films on patterned-oxide silicon substrates, using a tubular hot-wall low pressure chemical vapor deposition (LPCVD) system, is demonstrated in this study. This conventional system is proposed as a low cost alternative for SiGe epitaxial growth. Three process improvements needed to achieve quality growth are discussed. First, the hydrogen bake process is modified to eliminate Ge-outgassing. Secondly, a Si SEG buffer layer is deposited prior to SiGe SEG. Finally, a small flow of dichlorosilane is introduced during the temperature ramp-down period prior to SiGe SEG. The growth results are discussed in terms of growth selectivity, thickness uniformity, growth rate, defect density, SiGe film composition, and electrical properties.
We have studied the effect of surface orientation on the optical and morphological characteristics of coherently-strained InP islands grown on GaInP/GaAs. The differences between islands grown on the (100) orientation and the (311)A orientation are studied. Islands grown on the (311)A orientation are more dense than the islands grown on the (100) orientation. For the (100) orientation, the island height distribution is bimodal peaked at 20 Å and 220 Å. For the (311)A orientation, the island height distribution is also bimodal peaked at 15 Å and 60 Å. Photoluminescence measurements for the (311)A orientation show a peak at 1.9 eV attributed to small islands. This peak is shifted to higher energies in comparison to the corresponding peak for the (100) orientation which is at 1.77 eV. This peak shift is due to the fact that the small islands on the (311)A orientation are smaller than the corresponding islands on the (100) orientation.
Epitaxial lattice mismatched heterointerfaces between layered semiconductors and themselves and II-VI semiconductors (CdS, CdTe), respectively, have been prepared and their band lineup determined by photoemission. Different physical mechanisms, which govern the heterointerface formation, can be discriminated due to the specific properties of the van der Waals (vdW) surface. The interfaces between layered semiconductors mostly follow the electron affinity rule with a small but systematic deviation, which is assigned to the influence of interfacial quantum dipoles. However, the band lineup to the II-VI semiconductors shows a large interface dipole, which is related to a structural dipole from the polar, Cd terminated, face of the (111)- in case of Zinkblende CdTe- and the (0001)- in case of Wurtzite CdS- oriented overlayer film.
The entire soil-plant-atmosphere continuum must be analysed to elucidate how xylem anatomy relates to water flow in plants. Measurements of water potential gradients and volume of water flow per unit time are needed to obtain values of hydraulic conductance per unit length. By comparing values of hydraulic conductance per unit length along the plant, the regions where xylem structure restricts water flow can be determined. Previous studies of fern water relations demonstrated that very large water potential gradients occurring in stipes of certain ferns were closely correlated with reduced conducting area of stipe xylem. A new study on Cyrtomium falcatum showed that the water potential gradient was relatively small and constant along the stipe and rachis; however, a much larger gradient occurred from the rachies into the pinnae. Hydraulic conductance per unit length varied with the leaf area to be supplied, leading to the fairly constant water potential gradient along the rachis.. The measured hydraulic conductance per unit length was only half the value predicted from the Hagen-Poiseuille equation. Although the Hagen-Poiseuille equation overestimated the measured value by a factor of 2, it did support the assumption that conduit number and lumen diameter are the principal determinants of water conductance in the xylem.
For a smooth manifold M ⊆ ℝn, the symmetry set S(M) is defined to be the closure of the set of points u∈ℝn which are centres of spheres tangent to M at two or more distinct points. (The idea has its origin in the theory of shape recognition.) The connexion with singularities is that S(M) can be described alternatively as the levels bifurcation set of the family of distance-squared functions on M. In this paper a multi-germ version of the standard uniqueness result for versal unfoldings of potential functions is used to obtain a complete list of local normal forms (up to diffeomorphism) for the symmetry sets of generic plane curves, generic space curves, and generic surfaces in 3-space. For these cases the authors verify that M can be recovered as the envelope of a family of spheres centred at smooth points of S(M).
Local models are given for the singularities that can appear on the trajectories ofgeneral motions of the plane with more than two degrees of freedom. Versal unfoldings of these model singularities give rise to computer-generated pictures describing the family of trajectories arising from small deformations of the tracing point, and determine the local structure of the bifurcation curves.
Photovoltaic devices based on soluble conjugated polymers have gained great interest in recent years because of the potential low cost of production and ease of fabrication. PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), a fullerene derivative, has been extensively investigated as a solution processable electron acceptor for bulk-heterojunction (BHJ) photovoltaic devices blended with conjugate polymers like P3HT [poly(3-hexylthiophene)]. Here, we investigated a novel solution processable organic semiconductor, C60(CN)2, as an electron acceptor for bulk heterojunction photovoltaic applications. Optical and electrical properties of C60(CN)2 are studied and compared with PCBM. Blend devices with P3HT and C60(CN)2 have been fabricated and compared with P3HT-PCBM devices. The effect of thermal annealing on the device performance is evaluated. Open circuit voltage, short circuit current, fill factor and total efficiency data are compared with PCBM based devices.
Ionically self-assembled monolayer (ISAM) films have been shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Typically, the ISAM films for NLO response are an assemblage of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Here, we investigate the effects of replacing the NLO-active polymer layers with layers of monomeric chromophores containing ionic and covalent bonding sites. Films fabricated exclusively using polyelectrolytes contain some fraction of both randomly oriented and anti-parallel oriented chromophores. We have examined the incorporation of monomeric chromophores into ISAM films in order to increase the net polar orientation of the chromophores and reduce bilayer thickness.
A novel method has been used to obtain a direct and accurate measure of the valence-band discontinuity AlyGa1−yAs/AlAs heterojunctions in quantum-well structures. The technique takes advantage of the crossover occurring at a critical Al concentration above which the indirect X minimum in the AlAs becomes the lowest-energy conduction band in the system. Within these “staggered” band alignment structures, photoexcited electrons and holes are spatially separated, and recombination occurs across the interface. The resulting emission fixes the valence-band offset to within 1% without accurate knowledge of other system parameters, such as effective masses and exciton or dopant binding energies. These measurements represent the first direct optical confirmation of staggered band alignments in this technologically important material system.
Silicon-on-Insulator (SOI) films on SiO2 scan-melted at low velocities (20 to 300 µm/sec) with reduced thermal gradients at the melt-freezing interface are found to have qualitatively different properties from similar films melt-scanned at higher gradients and scan velocities. The transition between the two regimes appears to be abrupt. Scanning at intermediate velocities often results an admixture of patches containing one or the other type of material. The slow scan regime is characterized by long straight non-branched subboundaries having a lateral spacing 50-60 µm, and very small tilt misalignments of 0.1° or less. These slow scan subboundaries are found to consist largely of threading dislocations in contrast conventional subboundaries which are tilt boundaries of up to 3° and typically consist of edge dislocations running in the plane of the film.
A systematic study of superconducting Te, Hc and also the behavior of resistivity of Nb/Si mfltiiayers is reported. Nb and Si layers with different thicknesses were deposited alternatively in an LUIV two electron—beam evaporating installation and controlled automatically by a microcomputerquartz—monitor system. Well reproduciole results were achieved. Structural analyses show very good modulated structure with (110)textured polycrystallino Nb and] amorphous Si layers. Superconducting Tc of Nb/Simultilayers are significantly higher than that of the sputtered Nb/Ge system. Thevariation of Hc(T) in parallel field manifests a 2D–3D crossover with transformation temperature of 0.7– 0.8 Tc. The possible mechanism and implication of these phenomena were discussed.