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The interest in the phenomenon of islanding in a range of semiconductor systems is in part due to the fundamental importance of the Stranski-Krastanow transition but also driven by potential device applications of self-organized quantum dot arrays. However, the mechanism underlying the island formation is still to a significant degree unclear. In the present work, we focus on the epitaxial InGaAs / GaAs(001) system, with layer deposition by molecular beam epitaxy. Atomic force microscopy is used to measure the surface topography of nominally 4nm thick InxGa1-xAs films. It is shown that the growth mode switches abruptly from flat layer to island growth if a critical Indium composition of x(In)≍0.25 is reached. The structure of such layers during early stages of growth is examined using energy-filtered transmission electron microscopy. Indium gradients in the islanded layers are measured and the driving force for the islanding transition itself is considered.
The way in which the Stranski-Krastanow epitaxial islanding transition can be controlled by strain due to elemental segregation within the initially-formed flat ‘wetting’ layer is examined in detail. Experimentally measured critical ‘wetting’ layer thicknesses for the InxGa1−xAs/GaAs system (x = 0.25 - 1) are demonstrated to show good agreement with values calculated using a segregation model. The strain energy associated with the segregated surface layer is determined for the complete range of deposited In concentrations using atomistic simulations. The segregation-mediated driving force for the Stranski-Krastanow transition is considered to be important also for all other epitaxial systems exhibiting the transition.
Gaia will only achieve its unprecedented measurement accuracy requirements with detailed
calibration and correction for radiation damage. We present our Silvaco 3D engineering
software model of the Gaia CCD pixel and two of its applications for Gaia: (1) physically
interpreting supplementary buried channel (SBC) capacity measurements (pocket-pumping and
first pixel response) in terms of e2v manufacturing doping alignment tolerances; and (2)
deriving electron densities within a charge packet as a function of the number of
constituent electrons and 3D position within the charge packet as input to microscopic
models being developed to simulate radiation damage.
Photocurrent spectroscopy of InAs/GaAs self-assembled quantum dots, studied as a function of applied electric field, is used to probe the nature of the confined electronic states. A field asymmetry of the quantum confined Stark effect is observed, consistent with the dots possessing a permanent dipole moment. The sign of this dipole indicates that for zero field the hole wavefunction lies above that of the electron, in disagreement with the predictions of all recent calculations. Comparison with a theoretical model demonstrates that the experimentally determined alignment of the electron and hole can only be explained if the dots contain a nonzero and non-uniform Ga content. The role of two different carrier escape mechanisms, tunneling and thermal excitation, is studied.
Understanding the finite volume throughout which plastic deformation begins is necessary to understand the mechanics of small-scale deformation. In indentation using spherical indenters, conventional yield criteria predict that yield starts at a point on the axis and at a depth of half the contact radius. However, Jayaweera et al (Proc. Roy. Soc. 2003) concluded that yield occurs over a finite volume at least 100 nm thick. Semiconductor superlattice structures, in which the stress and thickness of individual layers can be varied and in which known internal stresses can be incorporated, open up new possibilities for investigation that cannot be achieved by varying external stresses on a homogenous specimen. We have designed samples with bands of highly strained InGaAs superlattice, which are essentially bands of low yield-stress material devoid of other metallurgical artifacts. These bands are placed at different depths in a series of samples. Spherical indenters with a range of radii were used to determine the elastic-plastic transition. The stress field from different sized indenters interacts with the low yield-stress material at different depths below the surface to map out the size of the initial yield volume.
As a signatory of the Convention on Biological Diversity, the United Kingdom (UK) Government is obliged to conserve and enhance biodiversity. One step towards addressing this goal is to determine the level of biodiversity conservation already achieved within the current array of protected areas. We used national distribution data for ten taxonomic groups, and location information for three reserve networks, to assess the contribution of nature reserves to biodiversity conservation in Great Britain at the 10 km square (=100 km2) resolution. For several taxonomic groups, 10 km squares containing nature reserves had a significantly greater overlap than random networks with both hotspots (areas of high species richness) and complementary areas (sets of sites within which all species are represented). In addition, more than 94% of species from each taxonomic group have been recorded within the 10 km squares of the three reserve networks. These results provide some encouragement in terms of the UK meeting its commitment to conserve biodiversity.
Phosphoglucomutase 1 (PGM1) deficiency is a stable characteristic
line, K562, whereas the activity of the isozymes of the other two PGM loci
(PGM2 and PGM3) is
slightly elevated. In this study the molecular basis of PGM1 deficiency
investigated by a
combined approach utilising protein electrophoresis, immunodetection,
cytogenetic techniques, and
DNA and RNA analysis. Isoelectric focusing and activity staining confirmed
that K562 has no
detectable PGM1 activity. Immunoblot analysis of extracts, separated by
isoelectric focusing, starch
gel and SDS gel electrophoresis, using monospecific anti-PGM1 antibodies
showed that K562
contained no detectable immunoreactive material. Karyotype analysis revealed
the presence of two
intact chromosomes 1 and a derivative chromosome 1,
der(1)t(1; 11), each of which carried a copy
of the PGM1 gene as demonstrated by fluorescence in situ
hybridization using a PGM1 cosmid as
probe. Southern blot analysis using a PGM1 cDNA clone as probe
that the PGM1 genes
had not been subject to any gross structural rearrangements. We were also
able to determine that K562 is type PGM1 2+1+
by restriction endonuclease analysis of genomic DNA. Very low levels
of PGM1 mRNA which appeared to be full length transcripts were detected
K562 using a reverse
transcriptase PCR technique. We conclude that the most likely cause of
enzyme deficiency in K562 is abnormal regulation of transcription.
Mast cells are a rich source of potent biologically active mediators and are found in connective tissue, associated with blood vessels in many varied inflammatory conditions. Mast cells have been described in nasal polyps and turbinates and in adenoidal tissue in the upper aerodigestive tract. As the middle ear lining is contiguous with the nose and the nasopharynx, the presence of mast cells in aural polyps is interesting.
This preliminary study investigated the presence of mast cells in inflammatory aural polyps using light microscopy. All patients presenting to the department in one year were included. Patients with previous ear disease or surgery and in whom cholesteatoma was suspected were excluded. Except for one patient mast cells were seen in all aural polyps. The implications of these findings is discussed. Further work is needed using electron microscopy.
A naturalistic study was undertaken of 36 video and audio-taped interviews undertaken by 7 different psychiatric trainees. The interviews studied were those conducted in the ordinary course of clinic work for diagnostic and therapeutic planning purposes by trainees when first seeing the parent or parents of a child newly referred to a psychiatric clinic. It was found that a variety of rather different interview techniques seemed to facilitate emotional expression. These included a low level of interview talk with few interruptions, a high rate of open rather than closed questions, direct requests for feelings, interpretations and expressions of sympathy. The issue of how far these associations reflected causal influences is discussed.
A naturalistic study was undertaken of 36 video and audio taped interviews made by 7 different psychiatric trainees. The interviews studied were those conducted in the ordinary course of clinic work for diagnostic and therapeutic planning purposes by trainees when first seeing the parent or parents of a child newly referred to a psychiatric clinic. It was found that a directive style with specific probes and requests for detailed descriptions was associated with the obtaining of better-quality factual information than that associated with a more free-style approach. Interviewers who talked less and who made more use of open questions and checks tended to have more talkative informants. Double questions were liable to result in ambiguous answers, but multiple-choice questions did not appear to cause distortion and in some circumstances might be helpful.
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