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The influence of household demographic composition on rural migration in China has received limited attention. With data from a household survey in China's Sichuan and Hubei Provinces, this paper uses Probit models to explore the influence of household structure on migration decisions. It suggests that the three-generation household encourages out-migration, with the elderly playing an important role in supporting the migration of younger members by caring for their children. In contrast with earlier findings, the serious illness of an elderly member did not encourage the return of young migrants or discourage migration decisions unless the household included young children.
This review is based on geologic surveys carried out in Australia and China as well as on more than 300 14C dates published in Radiocarbon, Evaluated are the origins and pathways of carbonate formation, stable isotopic composition, carbonate nodule growth rates and paleo-climatic effects. The three identified δ 13C abundance peaks are unrelated to environment and carbon source whilst 14C ages group themselves into periods corresponding to past humid warm climate. It is concluded that the major error in caliche dating is due to incorporation of old limestone whilst error on nodule dating is related to their slow growth rate, Thus, caliche antedates and nodules postdate the times of their deposition. Delta 13C values cannot be used to correct for limestone or atmospheric contamination effects.
Bipolar disorder is a highly heritable polygenic disorder. Recent
enrichment analyses suggest that there may be true risk variants for
bipolar disorder in the expression quantitative trait loci (eQTL) in the
We sought to assess the impact of eQTL variants on bipolar disorder risk
by combining data from both bipolar disorder genome-wide association
studies (GWAS) and brain eQTL.
To detect single nucleotide polymorphisms (SNPs) that influence
expression levels of genes associated with bipolar disorder, we jointly
analysed data from a bipolar disorder GWAS (7481 cases and 9250 controls)
and a genome-wide brain (cortical) eQTL (193 healthy controls) using a
Bayesian statistical method, with independent follow-up replications. The
identified risk SNP was then further tested for association with
hippocampal volume (n = 5775) and cognitive performance
(n = 342) among healthy individuals.
Integrative analysis revealed a significant association between a brain
eQTL rs6088662 on chromosome 20q11.22 and bipolar disorder (log Bayes
factor = 5.48; bipolar disorder P =
5.85×10–5). Follow-up studies across multiple independent
samples confirmed the association of the risk SNP (rs6088662) with gene
expression and bipolar disorder susceptibility (P =
3.54×10–8). Further exploratory analysis revealed that
rs6088662 is also associated with hippocampal volume and cognitive
performance in healthy individuals.
Our findings suggest that 20q11.22 is likely a risk region for bipolar
disorder; they also highlight the informative value of integrating
functional annotation of genetic variants for gene expression in
advancing our understanding of the biological basis underlying complex
disorders, such as bipolar disorder.
We present two-wave mixing results obtained with CdSSe:V crystals. A large photorefractive gain of 0.24 cm-1 was observed at 633 nm with an optical intensity of 60 mW/cm2 and a grating period of 1.6 μrm. At this wavelength, we measured the photorefractive gain as a function of the grating period and incident optical intensity. To our knowledge, this is the first observation of the photorefractive effect in vanadium doped CdSSe crystals. The CdSSe crystals were grown by physical vapor transport (PVT) and doped with 150 ppm (nominal) vanadium for creating trap centers. The grown crystal has a large crystal size, good optical quality and a medium resistivity of 10-5 - 108 Ω-cm. Room temperature absorption and low temperature photoluminescence spectroscopy measurements are also discussed. With a significant photorefractive effect, the CdSSe:V crystals are promising for many device applications based on photorefractive effect, including optical limiting devices in the visible region.
Since the introduction of zone-melting recrystallization (ZMR)for silicon-on-insulator (SOI) films, subboundaries (low-angle grain boundaries) have been the major crystalline defects in recrystallized films. By using an improved ZMR procedure, subboundaries have been eliminated over large areas. The improvements include the use of 1-µm-thick polycrystalline-Si films deposited on 2-µm-thick thermal SiO2 film (instead of 0.5-µm-thick Si and SiO2 films), a new encapsulation technique, and improved control of the thermal gradient during ZMR. Recrystallized SOI films without subboundaries contain isolated dislocations with densities <2 × 106 cm−2.
Two approaches to preparing oriented crystalline films on amorphous substrates are reviewed briefly: zone-melting recrystallization (ZMR) and surface-energy-driven grain growth (SEDGG). In both approaches patterning can be employed either to establish orientation or to control the location of defects. ZMR has been highly successful for the growth of Si films on oxidized Si substrates, but its applicability is limited by the high temperatures required. SEDGG has been investigated as a potentially universal, low temperature approach. It has been demonstrated in Si, Ge, and Au. Surface gratings favor the growth of grains with a specific in-plane orientation. In order for SEDGG to be of broad practical value, the mobility of semiconductor grain boundaries must be increased substantially. Mobility enhancement has been achieved via doping and ion bombardment.
When Si-on-insulator films 0.3 to 0.5 µm thick are recrystallized by zone melting under the usual conditions, the defect trails within grains are subboundaries. We report that the use of an entrainment pattern, consisting of a grating of carbonized photoresist on top of the SiO2 encapsulating layer, can lead to the replacement of subboundaries by defect trails that are spaced dislocation clusters or discrete threading dislocations. Such defects are expected to be less detrimental to submicrometer MOSFET circuits than subboundaries.
We carry out an investigation of grown-in nonradiative defects in Si and SiGe/Si heterostructures grown by molecular-beam-epitaxy (MBE). A number of such defects are observed by the optical detection of magnetic resonance (ODMR) technique, in samples with various structures and growth conditions. These defects are shown to provide efficient nonradiative shunt paths for carrier recombination, competing with and reducing radiative recombination processes. It is revealed that the dominant nonradiative defect is a low-symmetry vacancy-related complex, evident from a characteristic hyperfine structure due to 29Si ligands (with nuclear spin I=1/2 and natural abundance of 4.67 %) connected to the dangling bonds. The introduction of these defects is believed to be largely due to a low surface adatom mobility during the low temperature growth. By varying the substrate bias during the MBE growth, it is shown that the formation of these nonradiative defects can be effectively enhanced by exposure to accelerated positive ions, presumably dominated by the Si+ ions. Effects of hydrogenation on these defects are also studied.
The radiative recombination processes of the two-dimensional hole gas formed in SiGe quantum wells (QWs) due to modulation doping are studied in details by photoluminescence (PL) spectroscopy. Boron-modulation-doped Si/Sil−xGex/Si heterostructures grown by molecular beam epitaxy (MBE) are studied. It is shown that charge transfer of holes from the doped Si layers causes the filling of the SiGe QWs leading to an appearance of a broad asymmetric PL band with a characteristic sharp high energy cut-off and enhanced recombination near the Fermi edge. A reduction of this PL enhancement is observed with an increase of measuring temperature. The PL bandwidth and the high energy cut-off are found to vary with either the doping level or the spatial separation between the delta-doped layers and the QWs. This PL band is argued to arise from the recombination of the holes in the QWs and electrons confined near the QWs as a result of the band bending induced by the delta-doping. The shape of the PL band with enhanced intensity near the Fermi edge are discussed in terms of the phase space filling and many-body effects.
Inorganic and organometallic polymers are macromolecular systems in which the polymer backbone contains elements other than the carbon, oxygen and nitrogen usually found in organic polymers . To take as an example silicon-containing polymers, in the silicones the polymer backbone is composed of the Si-O repeat unit; in polysilazanes, of the Si-N unit; in polysilmethylenes, of the Si-C unit. In the polysilanes there are only silicon atoms in the polymer backbone. Many of the other metalloids and metals among the elements in the Periodic Table have been or, in principle, can be incorporated into polymeric systems, so it is clear that the field of inorganic and organometallic polymers is a very large one. Inorganic and organometallic polymers have been of interest to chemists for a long time. It was the commercial development of the silicones in the 1940's that gave this field of research its modem impetus . Once it was appreciated how useful these versatile organosilicon polymers could be, chemists became interested in the possibility of developing other organometallic (and also inorganic) polymers, ones that might complement or even surpass the silicones as far as useful applications were concerned. Research on inorganic and organometallic polymers became very active in the 1950's and 1960's. Work in this area became an international effort, prompted by the need for new materials that would meet the exacting demands of the jet age that had effectively commenced around the end of World War II. Even greater demands, in terms of materials that would still be useful under extreme conditions, came with the space age.
We report an investigation on the electronic structure of two bound exciton (BE) systems from a complex defect in S-doped Si, by optical detection of magnetic resonance (ODMR). A spin-triplet (S=1) is identified to be the lowest electronic state of the BE's, which gives rise to deep photoluminescence (PL) emissions when recombining. A weak anisotropy of the magnetic interaction of the BE’s (not possible to resolve in Zeeman data) is revealed, which leads directly to the determination of the symmetry for the excited state of the defect. A S-related complex model is suggested as the identity of the defect. A critical test of two possible metastable configurations of the constituents of a single defect is undertaken.
Subboundaries are the major crystalline defects in thin semiconductor films
produced by zone-melting recrystallization (ZMR). Using transmission
electron microscopy (TEM) and chemical etching we have analyzed the angular
discontinuity and defect structure of subboundaries in ZMR Si films.
Annealing in oxygen has resulted in the elimination of dislocation bands
from sizable regions of some films. Calculations suggest that cellular
growth due to constitutional supercooling may not occur in some Si ZMR.
The effect of post adsorbed atomic hydrogen on the adsorption, desorption, and decomposition of ethylene on Si(100)-(2×l) has been studied using high-resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and low-energy electron diffraction (LEED). Exposures to atomic hydrogen of more than 1015 atoms/cm2 convert the initial (2×l) reconstruction of sp3-hybridized, di-σ bonded ethylene to a (l×l) structure. Furthermore, after post exposure to atomic hydrogen, the thermal desorption peak of molecular ethylene is shifted up by approximately 100 K and reduced in intensity. HREEL spectra for deuterated ethylene show the formation of a C-H bond after exposure to atomic hydrogen, whereas the C-C bond remains intact. We explain our data by an atomic hydrogen-driven conversion of the di-σ bonded ethylene to a mono-σ bonded surface ethyl. Thermal activation after post exposure to atomic hydrogen leads to decomposition of about 60% of the initial ethylene in contrast to the observed molecular desorption in the absence of hydrogen.
Broad photoluminescence (PL) bands with energy position ranging from 0.70 to 1.03 eV, are shown to appear in Si epilayers and Sil−xGex/Si heterostructures grown by molecular beam epitaxy under non-optimized conditions. The presence of a particular broad band (BB) is found to be determined by the growth conditions, in particular by the bias applied to the substrate during the growth. Ion bombardment during the growth is shown to be a main factor enhancing the formation of BB-related defects. To clarify their origin, the effects of post-growth hydrogen treatments and of an external magnetic field are investigated. The BBs are shown to be related to various types of radiative centres, either point or extended defects. A correlation between X-ray diffraction and PL measurements is also observed. The spatial location of the BB-related defects is inferred by results from optically detected cyclotron resonance (ODCR) and PL excitation studies.
The understanding of the electric contact formation process is critical in exploiting the full potential of Cd1−xZnxTe(CZT) material for room temperature detection applications. The metalsemiconductor electrical characteristics were shown in this study to be strongly affected by the surface preparation steps prior to metallization (polishing, and chemical etching), the choice of the metal and contact deposition technique, and by the subsequent surface passivation of CdZnTe. In this paper, we also present the implementation new detector fabrication processing stepsresulting in a significant lowering of the dark leakage current and an improvement in the detector performance.
A theoretical analysis is presented of the electromigration-induced dynamics of transgranular voids in metallic thin films. The analysis is based on self-consistent dynamical simulations of current-driven void surface propagation coupled with the distribution of the electric field in the metallic film. The simulation predictions highlight the rich nonlinear dynamics of current-driven evolution of voids that become faceted due to the strongly anisotropic nature of surface diffusion. The numerical results are analyzed based on approximate analytical solutions to faceted void migration and a linearized theory for the morphological stability of planar void facets.
FeCo nanoparticles are synthesized in a radio frequency (RF) plasma torch from metal powder precursors and simple gases. The first precursor consists of pre-alloyed FeCo powder; the second precursor is a mixture of elemental Fe and Co powders. A protective carbon coating on the particles is achieved by injection of acetylene gas into the plasma. X-ray diffraction reveals phase purity, low carbon concentration, and minimal oxidation. Analytical electron microscopy is used to examine the nanocomposite morphology and composition of individual nanoparticles. Both synthesis routes produce alloy product, but nanoparticles produced from pre-alloyed precursors exhibit smaller variations in Fe/Co ratio than particles produced from elemental powders.
Products derived from ethylene have and will continue to replace metallic materials traditionally used for transportation, building materials, and products we use in our everyday lives. As the demand continues to increase, a more suitable material for the outlet coils of ethylene pyrolysis heaters will have to be identified. In this study, we discuss utilization of scaled up pulsed deposition technology to deposit adherent carburization resistant coatings on the inner diameter of ethylene pyrolysis tubing with the intent of extending tube life. Ablation target material selection was based primarily on elevated temperature properties and the ability of the coating to prevent transformation of the inherent protective chromium oxide surface film to metal carbides while in service. The near optimal settings of the processing parameters for pulsed laser deposition of ceramic SiC on heat resistant tubing traditionally used for ethylene service were investigated using a semi quantitative controlled random search methodology. Minimization of the objective function which was based on width, thickness and coverage of the thin film resulted in an optimal deposition time of 4.3 minutes and surface finish of 272 nm.