To send content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about sending content to .
To send content items to your Kindle, first ensure email@example.com
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about sending to your Kindle.
Note you can select to send to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
We present an in-depth study of metal-poor stars, based high resolution spectra combined with newly released astrometric data from Gaia, with special attention to observational uncertainties. The results are compared to those of other studies, including Gaia benchmark stars. Chemical evolution models are discussed, highlighting few puzzles that are still affecting our understanding of stellar nucleosynthesis and of the evolution of our Galaxy.
Sixteen lambs were divided into two groups and fed two different diets. Eight lambs were stall-fed with a concentrate-based diet (C), and the remaining eight lambs were allowed to graze on Lolium perenne (G). The antioxidant status was measured in the liver and plasma samples before and after solid-phase extraction (SPE) to probe the antioxidant effects that grass phenolic compounds may have conferred onto the animal tissues. The liver and plasma samples from grass-fed lambs displayed a greater antioxidant capacity than the tissues from C lamb group, but only if samples had not been passed through SPE cartridges. Finally, the feed and animal tissues, which had been purified by SPE, were analysed by liquid chromatography combined with mass spectrometry (LC–MS) to identify phenolic compounds present in L. perenne and to evaluate the results from the antioxidant assays. It would appear that the improvement of the antioxidant capacity of lamb liver and plasma from lambs fed ryegrass was not related to the direct transfer of phenolic compounds from grass to the animal tissues.
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!
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).
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.
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 report on the optical investigations of InAs growth on GaAs. In-situ STM/AFM studies show the presence of features 2-4 ML high, which we call quasi-3D (Q3D) clusters, well in advance of 3D island formation. Though the photoluminescence (PL) emission from these Q3D clusters is in the same wavelength regime as that from well developed 3D islands, they show characteristic differences in the PL excitation spectra and temperature dependence of PL, distinguishing them clearly from the 3D islands. Finally, we discuss the lasing observed from lasers containing single and five sets of InAs layers grown with conditions in which the in-situ STM/AFM studies show only 3D islands.
Graphite intercalation compounds (GIC's) are metal-semimetal superlattices which exhibit crystalline order, and have atomically perfect interfaces between the layers of the constituent species. From the standpoint of superconductivity, the KHg-GIC's are particularly interesting. The preparation and properties of these compounds are described, along with a series of recent experiments with hydrogen doping which have helped to elucidate their electronic properties. A density of states model suggested by the results of the hydrogen–doping experiments is presented and used to explain the variation of the superconducting transition temperature in these materials.
Magneto—optic studies on InGaAs/GaAs and GaAs/GaPAs strained—layer superlattices are used to determine the in—plane light—hole valence—band effective masses. Also, hydrostatic pressure—dependent magneto—optic studies have been performed on these samples for magnetic fields up to 65 kG andpressures to 4 kbar in the temperature range of 1.6 - 4 K. The experimental pressure coefficients of the band—gap energy and the effective mass in the InGaAs/GaAsSLS structures were determined.
The effects of implantation and annealing on an AlAs-GaAs superlattice grown by OMCVD is examined with SIMS (secondary ion mass spectrometry). Several 180 keV 28Si+ implants, with doses ranging from 3 × 1013 to 3 × 1015 cm−2, are examined before and after a three hour 850 C anneal. While the implantation by itself causes some intermixing in the vicinity of the projected range, the 850 C thermal anneal induces significant mixing at depths well beyond the implant range. In the region of maximum implant damage, however, the post-thermal mixing effect is inhibited. Depth dependent diffusion lengths of Al and Si are derived from the SIMS data. The diffusion coefficient of Si is markedly enhanced in the mixed regions.
Using high dose implantation of 200 keV Co ions followed by high temperature annealing, we have created buried layers of CoSi2 in crystalline Si of both (100) and (111) orientations. For a dose of 3 × 1017 Co/cm2, the layer that forms is ∼1100Å thick and the overlying Si is ∼600Å thick. A lower dose of 2 × 1017 Co/cm2 yields a thinner layer, 700Å thick, under 1200Å of crystalline Si. Rutherford Backscattering and channeling analysis of the layers shows that they are aligned with the substrate (χmin of the Co as low as 6.4%.) and TEM inspection of the (100) CoSi2/Si interfaces shows that they are abrupt and epitaxial (with occasional small facets). Moreover, electrical characterization of these layers yields resistance ratios that are better than epitaxial CoSi2 films grown by more conventional UHV methods.
Misfit dislocations in epitaxial layers of Si grown by MBE at 570°C on GaP(001) substrates have been studied by TEM. It is found that layers as thick as 500 Å at least reside coherently on the substrate without misfit dislocations. In 1000 Å layers of Si the misfit strain is accommodated in part by 60-degree type dislocations with their Burgers vector inclined with respect to the interface, and by stacking faults intersecting the Si layer. The dislocations are dissociated into 30- and 90-degree Shockley partial dislocations. It is shown that in the case of a biaxial strain field, which is tensile in a (001)-plane, the 90-degree partial must be nucleated first. Only then can the 30-degree partial follow on the same glide plane. This geometrical effect explains the presence of dislocations as well as stacking faults in the Si layer.
Cantilever-beam bending and RBS channeling measurements have been used to examine implantation-induced disorder and stress buildup in In0.2Ga0.8As/GaAs SLS structures. Implantation fluences from 1011 to 1015/cm2 were used for 150 keV Si, 320 keV Kr, and 250 keV Zn in SLS and GaAs bulk materials. The critical fluence for saturation of compressive stress occurs prior to amorphous layer formation and is followed by stress relief. For all the ions the maximum ion induced stress scales with energy density into atomic processes and stress relief occurs above ∼1 × 1020: keV/cm3. Stress relief is more pronounced for the SLSs than for bulk GaAs. We suggest that stress-relief may lead to slip or other forms of inelastic material flow in SLSs, which would be undesirable for active regions in device applications. Such material flow may be avoided by limiting maximum fluences or by multiplestep implantation and annealing cycles (or hot implants) at high fluences.
Thin gold films over GaAs wafers with different dopants (Cr, Si, Te, and Zn) were used to study the role of he substrate electronic properties in the electrical contact and adhesion modification induced by MeV/nucleon heavy ion bombardment. The enhanced adhesion was studied using a scratch test; the results show very different modifications of adhesion depending on the bulk electronic properties of the substrate. The sample with a Cr compensation doped substrate showed enhancement in adhesion for beam doses as low as 1012 ions/cm2, but Si and Te doped (n–type) substrates showed a sudden enhancement in adhesion for doses around 1014 ions/cm2. Samples with Si and Te doped substrates were used to sudy the bombarding ion dE/dx dependence of the induced adhesion for 19F and 35C1 ions with electronic stopping power ranging from 161 eV/Å to 506 eV/Å. In this range the dose threshold fgfjhe ops! of induced adhesion has a power law dependence, D = D0(dE/dx)− (1.90 ± 1.0)
In situ Reflection High Energy Electron Diffraction and ex-situ Transmission Electron Diffraction and Ion Channeling have been applied to a reacted Pd-GaAs interface and the results obtained are critically compared. The investigation has been done on the stabilized c(2×8) surface obtained by MBE on GaAs(100) substrates. Smooth surface epitaxial growth has been observed by RHEED as soon as a few monolayers of Pd are deposited at a substrate temperature of about 325°C. TEM diffraction studies indicate the presence of an intermetallic hexagonal structure similar to the orthorhombic Pd5Ga2 but with slightly different lattice parameters due to the possible incorporation of As. A less abundant phase was also identified as an hexagonal structure similar to Pd8As2. Ion Channeling indicates pronounced reduction in scattering yield when the  axis of the substrate was aligned with the impinging beam, thus supporting the RHEED analysis. The three techniques listed above were found to be useful for the determination of the epitaxial relationship between the identified phases and the substrate.
We have examined the optical and transport properties of In.2Ga.8As/GaAs straled-kayer superlZotices (SLS's), which have been implanted either with 5 × 1015/cm2, 250keV Zn+ or with 5 × 1014/cm2, 70keV Be+ and annealed under an arsenic overpressure at 600 °C. For both cases, electrical activation in the implantation-doped regions equalled that of similar implants and anneals in bulk GaAs, even though the Be implant retained the SLS structure, while the Zn implant intermixed the SLS layers to produce an alloy semiconductor of the average SLS composition. Photoluminescence intensities in the annealed implanted regions were significantly reduced from that of virgin material, apparently due to residual implant damage. Diodes formed from both the Be- and the Zn-implanted SLS's produced electroluminescence intensity comparable to that of grown-junction SLS diodes in the same chemical system, despite the implantation processing and the potential for vertical lattice mismatch in the Zn-disordered SLS device. These results indicate that Zn-disordering can be as useful in strained-layer superlattices as in lattice-matched systems.
The characterization of electronic devices using epitaxial CaF2 on Si is described. In addition, the growth and annealing techniques used to obtain high quality epitaxial films are discussed. In particular, the results of using rapid thermal annealing to improve the epitaxial quality of CaF2 films are presented in detail.
The electronic and electrical properties of these structures are very promising. Epitaxial CaF2 films with breakdown fields as high as 3 × 106 V/cm and interface trap densities as low as 7 × 1010cm-2eV-1 have been fabricated. In addition, minority carrier dominated trapping has been observed at the CaF2 /Si interface. Finally, the material properties of these structures, as determined by Rutherford backscattering, channeling, and electron microscopy, are discussed and correlated with their electronic properties.