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Determinants of anticipated acceptance of an oral cholera vaccine (OCV) were studied in urban and rural communities of Western Kenya. An explanatory model interview administered to 379 community residents assessed anticipated vaccine acceptance at various prices from no cost to full-cost recovery, socio-cultural features of cholera and social characteristics. Nearly all (99%) residents indicated willingness to accept a no-cost OCV, 95% at a price of US$ 0·8, 73% at US$ 4·2 and 59% at US$ 8·4. Logistic regression models analysed socio-cultural determinants of anticipated OCV acceptance. Prominence of non-specific symptoms for cholera was negatively associated with acceptance. A cholera-specific symptom (thirst), self-help referring to prayer, income and education were positively associated. In the high-cost model, education was no longer significant and reliance on herbal treatment was a significant determinant of vaccine non-acceptance. Findings suggest high motivation for OCVs, if affordable. Socio-cultural determinants are better predictors of anticipated acceptance than socio-demographic factors alone.
The cultivar Ajaya (IET 8585) exhibits durable broad-spectrum resistance to bacterial blight (BB) disease of rice and is widely used as a resistance donor. The present study was carried out to decipher the genetics of BB resistance in Ajaya and map the gene(s) conferring resistance. Genetic analysis in the F2 indicated a quantitative/additive nature of resistance governed by two loci with equal effects. Linked marker analysis and allelic tests revealed that one of the resistance genes is xa5. Sequence analysis of a 244 bp region of the second exon of the gene-encoding Transcription factor IIAγ (the candidate gene for xa5) confirmed the presence of xa5. Bulked-segregant analysis (BSA) revealed the putative location of the two quantitative trait loci (QTLs)/genes associated with resistance on chromosomes 5 and 8. Composite interval mapping located the first locus on Chr. 5S exactly in the genomic region spanned by xa5 and the second locus (qtl BBR 8.1) on Chr. 8L. Owing to its differential disease reaction with a set of seven hyper-virulent isolates of Xanthomonas oryzae, a map location on Chr. 8L, which was distinct from xa13 and data from allelism tests, the second resistance locus in Ajaya was determined to be novel and was designated as xaAj. A contig map spanning xaAj was constructed in silico and the genomic region was delimited to a 13·5 kb physical interval. In silico analysis of the genomic region spanning xaAj identified four putatively expressed candidate genes, one of which could be involved in imparting BB resistance in Ajaya along with xa5.
Prior to 2009 dengue fever had not been reported in the Andaman and Nicobar archipelago. In 2009, a few patients with dengue fever-like illness were reported, some of whom tested positive for dengue antibodies. In 2010, 516 suspected cases were reported, including some with dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS); 80 (15·5%) were positive for dengue antibodies. DENV RNA was detected in five patients and PCR-based typing showed that three of these belonged to serotype 1 and two to serotype 2. This was confirmed by sequence typing. Two clones of dengue virus, one belonging to serotype 1 and the other to serotype 2 appeared to be circulating in Andaman. Emergence of severe diseases such as DHF and DSS might be due to recent introduction of a more virulent strain or because of the enhancing effect of sub-neutralizing levels of antibodies developed due to prior infections. There is a need to revise the vector-borne disease surveillance system in the islands.
Millimeter-wave thermal analysis instrumentation is being developed for characterization of high temperature materials required for diverse fuel and structural needs in extreme high temperature reactor environments. A two-receiver 137 GHz system with orthogonal polarizations for anisotropic properties resolution has been implemented at MIT and is being tested with graphite and silicon carbide specimens at temperatures up to 1300ºC. Real time measurement sensitivity to submillimeter surface displacement and simulated anisotropic surface emissivity is demonstrated.
Optical and schottky diode characteristics of unintentionally doped GaN films grown by MOCVD were reported. GaN epilayers were grown with different V/III ratio by varying the source ammonia (NH3) flowrate. It exhibit changes in the density of threading dislocations (TDs) and reduced carbon and oxygen impurity incorporation. The density of dislocations determined from hot-wet chemical etching and atomic force microscopy show that on decreasing the ammonia flowrate, threading dislocations decreases. Low energy positron beam was employed to study the Ga vacancies in the epilayers. S-parameter vs. positron beam energy curves clearly shows increase in SL on increasing the V/III ratio indicating that the point defects trapping positron increases. Corroborative HRXRD, Photoluminescence and Hall measurements confirm the reduction in trapping defects and threading edge dislocations with reducing V/III molar ratio. The effects of such variation of compensating centres and radiative centres as a function of MOCVD growth conditions on optical properties and schottky device characteristics like radiative decay lifetime, barrier height and reverse leakage current respectively were discussed.
Copper metallization in sub-0.18 μm semiconductor devices is achieved by combining the dual damascence techniques followed by chemical mechanical planarization (CMP). Tantalum and its nitride have been identified as the diffusion barrier layer for copper metallization. However, the wide differences in properties between copper and tantalum layers result in selectivity problems during CMP process. The aim of this work is to obtain a better understanding on the slurry selectivity for copper and tantalum and to develop slurries with best selectivity performance. In this work, the effect of several chemical parameters (abrasive type, oxidizer type, concentration, pH etc.) was studied through static and dynamic tests using advanced electrochemical techniques and surface analysis techniques. The surface layers of the statically etched copper and tantalum discs were investigated using X-ray photoelectron spectroscopy (XPS) and surface planarity was studied using atomic force microscopy (AFM). Polishing rates results show that alumina-based slurry polished copper very well whereas tantalum removal rate was low. However, for the silica-based slurry the tantalum shows much higher removal rate than copper and better surface planarity was obtained.
GaAs Schottky barrier diodes remain a workhorse technology for submillimeter-wave applications including radio astronomy, chemical spectroscopy, atmospheric studies, plasma diagnostics and compact range radar. This is because of the inherent speed of these devices and their ability to operate at room temperature. Although planar (flip-chip and beam-lead) diodes are replacing whisker contacted diodes throughout this frequency range, the handling and placement of such small GaAs chips limits performance and greatly increases component costs. Through the use of a novel wafer bonding process we have fabricated and tested submillimeter-wave components where the GaAs diode is integrated on a quartz substrate along with other circuit elements such as filters, probes and bias lines. This not only eliminates the cost of handling microscopically small chips, but also improves circuit performance. This is because the parasitic capacitance is reduced by the elimination of the GaAs substrate and the electrical embedding impedance seen by the diodes is more precisely controlled. Our wafer bonding process has been demonstrated through the fabrication and testing of a fundamental mixer at 585 GHz (Tmix < 1200K) and a 380 GHz subharmonically pumped mixer (Tmix < 1000K). This paper reviews the wafer bonding process and discusses how it can be used to greatly improve the performance and manufacturability of submillimeter-wave components.
The electroencephalogram (EEG) plays an important diagnostic role in epilepsy and provides supporting evidence of a seizure disorder as well as assisting with classification of seizures and epilepsy syndromes. Emerging evidence suggests that the EEG may also provide useful prognostic information regarding seizure recurrence after a single unprovoked attack and following antiepileptic drug withdrawal. Continuous EEG video telemetry monitoring has an established role in the diagnosis of non-epileptic pseudo-seizures and in localizing the seizure focus for epilepsy surgery. Newer tools such as EEG mapping and magneto-encephalogram, although still investigational, appear potentially useful for defining the seizure focus in epilepsy. This review examines the traditional concepts of clinical EEG in the light of newly available data.
Titanium-aluminum alloys are proved to be a viable candidate material in the field of structural coatings due to their high temperature creep and oxidation resistance in addition to being a lightweight material. In the present paper, we study the effect of applied power on the mechanical properties and surface chemistry of dc-magnetron sputtered Ti3AI thin films. While SEM and XRD investigate the structure and morphology of the deposited films, XPS and AES proved to be an essential tool for studying the surface chemistry and detailed chemical bonding information of the Ti3AI thin films. Glancing angle XPS and Auger depth profiling were carried out to study the chemistry at a few layers beneath the surface to monitor the changes in the stoichiometry of Ti3AI films. Microhardness measurements indicate an increase in the thin film hardness with increased sputtering power. These data were further compared to thin films deposited under liquid N2 temperatures.
Focused Ion Beam (FIB) systems have been steadily gaining acceptance as specimen preparation tools in the semiconductor industry. This is largely due to the fact that such instruments are relatively commonplace as failure analysis tools in semiconductor houses, and are commonly used in the preparation of cross-sections for imaging under the ion beam or using an electron beam in an SEM. Additionally, the ease with which cross-sectional TEM specimens of semiconductor devices can be prepared using FIB systems has been well demonstrated. However, this technology is largely unknown outside the semiconductor industry. Relatively few references exist in the literature on the preparation of cross-sectional TEM specimens of non-semiconductor materials by FIB.
This paper discusses a specific use of FIB technology in the preparation of cross-sectional TEM specimens of non-semiconductor samples that are difficult to prepare by conventional means. One example of such materials is commercial galvannealed steel sheet that is used to form corrosion resistant auto-bodies for the automobile industry. Cross-sectional TEM specimens of this material have proved difficult and time-intensive to prepare by standard polishing and ion milling techniques due to galvanneal's inherent flaking and powdering difficulties, as well as the different sputtering rates of the various Fe-Zn intermetallic phases present in the galvannealed coatings. TEM results from cross-sectional samples of commercial galvannealed steel coatings prepared by conventional ion milling and FIB techniques are compared to assess image quality, the size of the electron-transparent thin regions that can be readily prepared and the quality of samples produced by both techniques. Specimen preparation times for both techniques are reported.
Magnetooptical studies have been performed on the shallow Be acceptor confined in the central region of narrow GaAs/AlGaAs quantum wells (QWs) with the magnetic field along the growth direction. The magnetic field dependence of the acceptor transition between the 1S(Г6) hh-like ground state and the excited hh-like 2S(Г6) state has been investigated by means of two independent techniques: Two-hole transitions of the acceptor bound exciton (BE) and resonant Raman scattering. The 1S(Г6) – 2S(Г6) transition energy as a function of the magnetic field has been measured for central acceptors in QWs of widths in the range 50 – 150 Å. The energy levels for the 1S ground states and 2S excited states of the confined acceptor with a magnetic field as a perturbation have also been calculated. These calculations predict a larger splitting between the mj=+3/2 and mj=−3/2 components of the acceptor 1S(Г6) ground state in comparison with the corresponding splitting of the excited 2S(Г6) state. The experimental results are in good agreement with the theoretical predictions derived without any fitting parameters. Furthermore, the Zeeman splitting of the acceptor BE emission has been measured and it is concluded that the J = 5/2 BE state is lowest in energy, similar to shallow acceptor BEs in bulk GaAs.
A number of recent studies of GaAs/AlxGa(1-x)As quantum wells using low temperature photoluminescence have demonstrated the formation of bi-excitons. It is generally assumed that bi-excitons will only be formed in samples with low impurity concentration, we demonstrate in this work that bi-excitons can be observed in 150 Å quantum well samples with an acceptor doping density as high as 3x1017 cm-3. We discuss how bi-exciton formation is limited by impurity capture and show how this influence can partly account for the less than quadratic intensity dependence typically observed.
The study of electronic properties of GaAs/AlGaAs quantum wells (QWs) has traditionally been focused on intrinsic phenomena, in particular the free exciton behaviour. Defects and impurities have often been regarded as less relevant compared to the case of bulk semiconductors. Doping in QWs is important in many applications, however, and recently the knowledge about the structure of shallow donors and acceptors from optical spectroscopy has advanced to a level comparable to the situation in bulk semiconductors. A dramatic difference from the bulk case is the common occurrence of localisation effects due to interface roughness in QW structures. The recombination of bound excitons (BEs) differs drastically from bulk, BE lifetimes decrease with decreasing well thickness Lw, but increase with decreasing barrier thickness Lb (at constant Lw) below Lb=70Å. Exciton capture at impurities is a process which is strongly influenced by the localisation potentials from the interface roughness. The recombination process in doped QWs involves a nonradiative component, for shallow acceptors an excitonic Auger process has been identified. Deep nonradiative defects in the (MBE grown) QW as well as in the barrier material are manifested in measurements of the PL decay time vs temperature. In undoped multiple QWs the decay times vs T are consistent with thermal emission out of the well into the barrier, where nonradiative recombination via deep level defects occur. Nonradiative recombination in the well itself can be studied in electron-irradiated structures. Preliminary data also demonstrate the feasibility of hydrogen passivation of dopants as well as deep levels in the QW structures.
Using tertiarybutylarsine and trimethylgallium a GaAs MESFET structure was grown and fabricated into half and quarter micron devices. The typical transconductance and unity current gain frequency (ft) for the half micron device were 360 mS/mm and 24 GHZ respectvely. The corresponding numbers for the quarter micron devices were 510 mS/mm and 55 GHZ respectively.
Using an alternate arsenic source, namely, Tertiary Butyl Arsine, a concentrator GaAs solar cell has been grown in a low pressure metal organic chemical vapor deposition reactor. Under 72 sun, air mass 1.5 illumination, the cell had an open circuit voltage of 1.1 V, a fill factor of 83% and an overall efficiency of 21%.
Transitions from the 1s ground state to 2s excited states of the Be-acceptor confined in GaAs/AIGaAs quantum wells (QWs) have been observed via two independent spectroscopic techniques: Two-hole transitions of the bound exciton (BE) measured in selective photoluminescence and Resonant Raman Scattering. The dependence of the 1s - 2s transition energy on the QW thickness (50 Å < Lz < 140 Å) has been studied for the case of acceptors in the center of the QW. The experimentally determined 1s - 2s transition energies have then been added to recently calculated binding energies for the 2s excited state in order to obtain the total binding energies for the acceptor at different confinements. The derived binding energies are finally compared with theoretical predictions. The same kind of measurements have been performed for a QW with a given thickness, but in which the position of the acceptor has been varied from the center to the edge of the QW.
The dependencies of the binding energies of the exciton bound to the investigated acceptor on the QW thickness and the position of the acceptor in the QW have also been studied. For the case of varying QW thickness, an almost linear relationship between the binding energies of the BE and the acceptor binding the exciton is found. This fact implies that a correspondence to Haynes' rule in bulk material could be applied to these QW systems, but in this case for the same acceptor at different binding energies due to the effect of varying confinement.
We report on a study of hot-carrier effects on optical properties of GaAs/AlxGa1-xAs quantum wells, by photoluminescence (PL) spectroscopy in the presence of a microwave (MW) field. Both doped and undoped, multiple quantum wells (MQWs) and single quantum wells (SQWs), grown by molecular beam epitaxy (MBE), show a profound enhancement of free exciton (FE) and bound exciton (BE) (for the doped wells) PL emissions with MW irradiation. This is attributed to effects of hot-carriers induced by the MW electric field. The mechanism responsible for the strong enhancement in PL intensity of the QWs in the presence of hot-carriers is studied, and is discussed in terms of an enhanced carrier trapping by the QWs as a consequence of the MW-induced heating of the photo-excited free carriers in the AlxGa1-xAs barriers.
The radiative recombination of two-dimensional (2D) carriers in an n-channel GaAs/AIGaAs heterojunction has been studied with time resolved photoluminescence (PL). Two bands related to the recombination of 2D carriers, the so called H-band 1 (HB1) and H-band 2 (HB2), are observed in PL. The spectral shape and position is strongly dependent on the sample and the experimental conditions. The H-bands are e.g. found to shift within a large energy range with the excitation intensity. We report here on the dependence of the decay times of the H-bands on their spectral position. The results are consistent with a recombination process involving 2D electrons, confined in the interface notch, and three-dimensional (3D) holes either from the valence band (HB1) or from neutral acceptors (HB2) in the active GaAs layer. The decay times of HB1 are found to vary in the range of 2-100 ns, while the corresponding decay times of HB2 are in the range of 100 ns - 10 µs.