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If an ice sheet is frozen to its bed, deep ice directly under a divide experiences low deviatoric stress and is relatively hard, because the rheology of polar ice is described by a power-law constitutive relation. In steady state, stratigraphic layers tend to form an arch (“Raymond bump”) in this region. However, when the basal ice can slide, the stresses are redistributed, and longitudinal extension due to sliding is associated with increased deviatoric stress in the deep ice under the divide. This increased deviatoric stress weakens the tendency to form a Raymond bump. To find a realistic spatial distribution of sliding under an ice divide, we incorporate a thin layer of viscous till in a finite-element plane-strain flow model. The resulting basal “sliding” velocity varies approximately linearly with distance from the ice divide. By varying the till viscosity, we can adjust the amount of basal motion. We find that the Raymond bump decays exponentially with the fraction of total ice flux carried by sliding: the arch is 50% smaller when the sliding flux is only 7% of the total ice flux. This implies that the possibility of a wet bed must be considered when inferring past ice-divide locations from radar internal layering.
Polycrystalline ice near an ice divide typically shows a crystal fabric (crystal preferred orientation) with c axes clustered vertically. We explore the effect of this fabric on the large-scale flow pattern near an ice divide. We incorporate an analytical formulation for anisotropy into a non-linear flow law within a finite-element ice-sheet flow model. With four different depth profiles of crystal fabric, we find that the effect of fabric is significant only when a profile has a minimum cone angle of less than ~25º. For a steady-state divide, the shape and size of the isochrone arch can depend as much on the crystal fabric as it does on the non-linearity of ice flow. A vertically oriented fabric tends to increase the size of the isochrone arch, never to reduce it. Also, non-random fabric has little effect on the ice-divide-flow pattern when ice is modeled as a linear (Newtonian) fluid. Finally, when we use a crystal-fabric profile that closely approximates the measured profile for Siple Dome, West Antarctica, the model predicts concentrated bed-parallel shearing 300 m above the bed.
During the period from July 1996 to Dec 1998, 189 photographic positions of the galilean satellites were obtained with the zonal astrograph (D=120 mm, F=2044mm). In the year 1998 the close approach of the satellite system with the star N 117881 from the HIPPARCOS catalogue was observed. The observations of 1996 were processed using PPM catalogue and the observations of 1997, 1998 in the ICRS using HIPPARCOS and ACTRC star catalogues. The results are not yet published, but are available upon request. Observations made since 1962 up to 1996 with the zonal astrograph were reprocessed and results are in preparation for publication. During the period from 1996 to 1998, 69 positions of the Galilean satellites were determined by photoelectric meridian observations with the Repsold meridian circle equipped by CCD-microscopes (F=2150 mm, D=150 mm).
Trypanosomes and Leishmania are vector-borne parasites associated with high morbidity and mortality. Trypanosoma lewisi, putatively introduced with black rats and fleas, has been implicated in the extinction of two native rodents on Christmas Island (CI) and native trypanosomes are hypothesized to have caused decline in Australian marsupial populations on the mainland. This study investigated the distribution and prevalence of Trypanosoma spp. and Leishmania spp. in two introduced pests (cats and black rats) for three Australian locations. Molecular screening (PCR) on spleen tissue was performed on cats from CI (n = 35), Dirk Hartog Island (DHI; n = 23) and southwest Western Australia (swWA) (n = 58), and black rats from CI only (n = 46). Despite the continued presence of the intermediate and mechanical hosts of T. lewisi, there was no evidence of trypanosome or Leishmania infection in cats or rats from CI. Trypanosomes were not identified in cats from DHI or swWA. These findings suggest T. lewisi is no longer present on CI and endemic Trypanosoma spp. do not infect cats or rats in these locations.
To report a case of sebaceoma of the auricle, and to discuss the differential diagnosis, histopathological features, surgical management and genetic associations of this entity.
Case report and review of the medical literature.
A 79-year-old man presented with a slowly growing lesion of his auricle. Excision of the mass and histopathological review revealed a benign, basaloid, adnexal neoplasm consistent with sebaceoma. Due to its association with Muir–Torre syndrome and increased risk of visceral malignancy, the patient was followed closely for signs of malignancy. At 36 months post-excision, there were no signs of recurrence; thereafter, the patient continued to receive routine cancer surveillance follow up.
Sebaceoma is a rarely encountered, benign, adnexal neoplasm which can occur in the head and neck. The treatment is surgical excision, and recurrence is rare. Sebaceoma can occur as part of Muir–Torre syndrome, and in these patients there is an increased risk of other sebaceous lesions and visceral malignancy; thus, genetic testing and surveillance should be strongly considered.
The Working Group on the Natural Planetary Satellites has been created to promote the development of high-quality ephemerides. The Working Group encourages theoretical studies, coordinated observations, and makes all data available to the users through the NSDC web site (http://www.imcce.fr/nsdc).
It has been shown recently that Au labeling can be used to profile vacancy-type defects located near half the projected range (½Rp) in MeV-implanted Si. In this work we have quantified the technique by determining the ratio of vacancies annihilated to decrease in the number of Au atoms trapped (calibration factor ‘k’) for the Au labeling technique. The 3 step experiment involved: 1) a high-energy Si-self implant (HEI) followed by an anneal to form stable vacancy clusters, 2) a controlled removal of vacancies via a medium energy Si self implant and interstitial-cluster dissolution anneal, and finally 3) Au labeling to count the change in vacancy concentration in the near surface region (0.1-1.6μm). It is seen that the Au concentration decreases linearly with increasing interstitial injection and the slope of this decrease determined the number of vacancies per trapped Au atom. The value of k was determined to be 1.2±0.2 vacancies per trapped Au atom.
The effect of ion implantation damage on energy transfer processes in Er-doped silica films prepared by MeV ion implantation is studied, using measurements of the luminescence decay of Er3+(4ƒ11) at 1.535 μm. Silica films implanted with Er and annealed at 900°C show a luminescence lifetime of 14.1 ms. Subsequent irradiation with MeV C, Si, or Ge ions at fluences as low as 1011 ions/cm2 decreases the lifetime, which eventually saturates at 6.6–7.8 ms for fluences larger than 1014 ions/cm2. The fluence required to saturate and the lifetime at saturation depend on the ion used. These results are interpreted in terms of non-radiative energy transfer processes caused by irradiation-induced defects in the silica. The ion damage effects are mainly caused by the electronic component of the energy loss along the ion trajectories.
Characteristics of MOCVD GaAs grown on Si are compared before and after a rapid thermal treatment. The GaAs-on-Si samples were prepared both with and without a Ge intermediate layer, which is used to accomodate mismatches of lattice, thermal and chemical origin between GaAs and Si. Structural, interfacial, chemical, and electrical changes have been examined. RBS and Raman characterization showed improvement of GaAs crystallinity after RTA. In most cases, the interfaces were found to become sharper after RTA, but chemical interdiffusion was observed to cause some effect on the structural and electrical properties. For gold-contacted GaAs, RTA seems to degrade the electrical and optoelectronic properties via gold interdiffusion into GaAs. Comparative studies of the GaAs/Si and GaAs/Ge/Si samples suggests that the two respond somewhat differently to RTA.
We have explored the effects of ion beam irradiation on the electrical and structural properties of superconducting thin films of YBa2Cu3O7-δ. Damage created by nuclear energy loss processes degrades the superconducting transitions. In general, the onset temperatures do not vary significantly with ion fluence, but the transition widths broaden until the resistance no longer reaches zero. This behavior is strikingly similar to that observed in granular superconducting films where islands of superconductor are progressively decoupled from each other, destroying the phase coherence. Data from resistance measurements as well as from Rutherford Backscattering and channeling measurements suggest that ion bombardment is not changing the volume fraction of superconducting material substantially.
Abstract: Several different types of GaAs-AlGaAs heterostructures were grown on Si substrates by MOCVD. The defect density in as-grown samples (~108cm−2) was similar to that of GaAs layers grown directly on Si, and the crystalline quality of the material was observed to improve slightly with post-growth annealing at 900°C. We examined the diffusion of both Si and Zn dopants during this type of annealing and found only a small amount of redistribution of both species. Laser annealing of GaAs-on-Si was also examined as a method of reducing the defect density in the material - we observed substantial improvements in surface quality, but no change in sub-surface crystalline quality.
The diffusion of Cu, Ag and Au has been measured in implanted, amorphous Si, over the range 150–600° C. The diffusion coefficients are characterized by Arrhenius relationships with activation energies for Cu, Ag and Au of 1.25, 1.6 and 1.4 eV respectively. The solubility of Au in amorphous Si was measured to be 6 orders of magnitude greater than crystalline Si at a temperature of 515°C. The Cu, Ag and Au are segregated ahead of the moving amorphous-crystalline interface. The presence of Au can increase the velocity of the interface.
Iridium has been implanted at 1MeV and 500keV in the dose range 1 to 3×1017/cm2 at 525°C to form a buried Ir-rich region ∼ 2500Å below the surface in (111)Si. Samples were annealed at temperatures in the range 600°C to 1200°C to form buried IrSi3 layers. Rutherford backscattering and channeling analysis in conjunction with transmission electron microscopy were used to study the formation and the alignment of the IrSi3 layer to the silicon host as a function of annealing. For a dose of 2×1017 Ir/cm2, stoichiometric IrSi3 formed beneath ∼ 2000Å of Si after annealing to 1100°C. The IrSi3 layer was aligned with the Si substrate with Xmin ∼12%, but a band of precipitates remained either side of the main layer.
Bombardment damage produced by Si+ ions in AlxGa1−xAs/GaAs layer structures has been studied using transmission electron microscopy and ion channeling and backscattering spectrometry. The damage resistance of A1xGa1−xAs alloy layers increases with Al concentration. In particular, by comparison of complementary Si+ ion doses yielding similar nuclear displacement densities at 150keV and 2MeV bombardment energies, it is demonstrated for the first time that the local concentration of implanted Si impurity is likely to be a significant factor in controlling lattice damage build-up, especially for the highest Si+ ion implantation doses. It is also shown that, in a manner analogous to A1As, the alloy layers can confer a significant protection from ion damage upon adjacent, epitaxially-bonded narrow zones of crystalline GaAs.
GaAs layers were grown directly on misoriented (2° off (100)→) Si substrates by Metalorganic Chemical Vapor Deposition. The threading dislocation density at the surface of 4 μm thick layers was typically 108cm−2, as determined by both preferential etching and transmission electron microscopy. Rapid thermal annealing (900°C, 10s) improved the crystalline quality of the GaAs near the heterointerface while allowing no detectable Si diffusion into this layer. Two deep electron traps were observed in the undoped GaAs, but were present at a low concentration (∼ 1013 cm−3 ). The (400) x-ray diffraction peak width from the GaAs was significantly reduced with increasing GaAs layer thickness, indicating improved material quality. This is supported by Si implant activation data, which shows higher net donor activity in thicker layers.
Artificial amorphous Si/Ge multilayers of equiatomic average composition with a repeat length around 60 Å have been prepared by ion beam sputtering. Implantation with 29Si led to a decrease in the intensity of the X-ray diffraction peaks arising from the composition modulation, which could be used for an accurate measurement of the implantation-induced mixing distance. Subsequent annealing showed no difference between the interdiffusivity in an implanted and unimplanted sample.
The microstructure of oxygen implanted silicon for use in silicon-on- insulator technology has been examined by transmission electron microscopy. A variety of buried oxide layers prepared using oxygen doses below and above that required for stoichiometric SiO2 formation have been studied. High resolution imaging in crosssection has revealed exceptionally flat Si-SiO2 interfaces, comparable to the best thermally grown Si-SiO2 interfaces. Examination of as-implanted material shows a complex interwoven crystalline/amorphous structure which evolves during high temperature (1350–1400° C) annealing into a buried oxide layer.
A comprehensive review is presented of the use of transient thermal processing for the activation of implanted dopants, the alloying of ohmic contacts, the pulse diffusion of Zn for p+ contacts, and other heat treatments of GaAs. In all cases, transient processing produces results at least as good as furnace heating, and in some instances, markedly better. The special needs of GaAs, such as encapsulation of the wafer surface, and prevention of slip and warpage are discussed, as well as recent results detailing implant damage removal processes during transient annealing.
Experimental evidence for laser melting of graphite, by irradiation with 30ns pulses from a ruby laser, is presented. RBS-channeling analysis, Raman scattering and TEM measurements reveal that the surface of graphite melts at a threshold energy density of about 0.6 J/cm2. For laser pulse energy densities above 0.6 J/cm2, the melt front penetration depth increases nearly linearly with increasing energy density. An intense emission of carbon particles during and after irradiation is observed. The thickness of the carbon layer removed in this process also increases nearly linearly with increasing pulse fluence. A dramatic redistribution of ion implanted impurities is also observed. Furthermore, the crystalline structure of the resolidified material is shown to depend on the energy density of the laser pulse. In order to explain these phenomena, a model for laser melting of graphite at high temperatures to form liquid carbon has been developed in which a free electron gas approximation is used to describe the properties of liquid carbon. The model is solved numerically to give the time and depth dependences of the temperature as a function of the laser pulse energy density. Very good agreement is found between the observed melt depth dependence on laser pulse energy density, as determined by RBS-channeling, and the model calculations. The redistribution of ion implanted impurities and the modification of the crystalline structure, caused by the pulsed laser irradiation, are also consistent with the model and permit the determination, for the first time, of interfacial segregation coefficients for impurities in liquid carbon. The model also predicts that liquid carbon at low pressure (p < 1 kbar) has metallic properties.