bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of N2 plasma on yttrium oxide and yttrium–oxynitride dielectrics
Authors: Niu D. , Ashcraft R. W. , Hinkle C. , Parsons G. N.
Published: May 2004

In this article, we report the effect of nitrogen plasma, during and after deposition, on nitrogen incorporation into yttrium oxide dielectric films. Films are deposited using a yttrium β-diketonate precursor (Y(tmhd)3) introduced downstream from a O2 or N2 plasma. The precursor acted as a significant source of oxygen, and only small amounts of N (<10 at. %) were incorporated in the films. Chemical bonding, concentration, and distribution of N in Y-oxide films after deposition and after high-temperature anneal were characterized using x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and Auger electron spectroscopy. C–N is the primary form of nitrogen bonding in the as-deposited films, and IR results indicate the exchange of N with O to form C–O bonds occurs during prolonged exposure to air. High-temperature annealing releases N from the surface of as-deposited films, and results in a film structure that is resistant to further N incorporation. Results suggest that yttrium oxynitride is likely inherently unstable, especially in contact with ambient moisture.
Chemical-beam deposition of GaN films on Si(111) from diethylazidogallium methylhydrazine adduct
Authors: Sung Myung M. , Kim Chang G. , Kim Yunsoo
Published: May 2004

GaN films were grown on Si(111) substrates at 400–800 °C using a single precursor diethylazidogallium methylhydrazine adduct, Et2(N3)Ga⋅MeHNNH2, under high-vacuum conditions (5×10−6 Torr). The chemical composition, crystalline structure, and morphology of the deposited films were investigated by x-ray photoelectron spectroscopy, x-ray diffraction, double crystal x-ray diffraction, x-ray pole figure analysis, and scanning electron microscopy. The results show that high-quality h-GaN films with correct stoichiometry can be deposited on Si(111) at a relatively low temperature by chemical-beam deposition (CBD). Room-temperature photoluminescence measurements have been performed to evaluate the optical properties of the GaN films. The single precursor Et2(N3)Ga⋅MeHNNH2 has been found suitable for the CBD of GaN thin films on Si substrates.
Inductively coupled plasma etching of GaAs low loss waveguides for a traveling waveguide polarization converter, using chlorine chemistry
Authors: Lu J. , Meng X. , SpringThorpe A. J. , Shepherd F. R. , Poirier M.
Published: May 2004

A traveling waveguide polarization converter [M. Poirier et al.] has been developed, which involves long, low loss, weakly confined waveguides etched in GaAs (epitaxially grown by molecular beam epitaxy), with electroplated “T electrodes” distributed along the etched floor adjacent to the ridge walls, and airbridge interconnect metallization. This article describes the development of the waveguide fabrication, based on inductively coupled plasma (ICP) etching of GaAs using Cl2 chemistry; the special processes required to fabricate the electrodes and metallization [X. Meng et al.], and the device characteristics [M. Poirier et al.], are described elsewhere. The required waveguide has dimensions nominally 4 μm wide and 2.1 μm deep, with dimensional tolerances ∼0.1 μm across the wafer and wafer to wafer. A vertical etch profile with very smooth sidewalls and floors is required to enable the plated metal electrodes to be fabricated within 0.1 μm of the ridge. The ridges were fabricated using Cl2 ICP etching and a photoresist mask patterned with an I-line stepper; He backside cooling, combined with an electrostatic chuck, was employed to ensure good heat transfer to prevent resist reticulation. The experimental results showed that the ridge profile is very sensitive to ICP power and platen rf power. High ICP power and low platen power tend to result in more isotropic etching, whereas increasing platen power increases the photoresist etch rate, which causes rougher ridge sidewalls. No strong dependence of GaAs etch rate and ridge profile were observed with small changes in process temperature (chuck temperature). However, when the chuck temperature was decreased from 25 to 0 °C, etch uniformity across a 3 in. wafer improved from 6% to 3%. Photoresist and polymer residues present after the ICP etch were removed using a combination of wet and dry processes.
Laser-induced selective area tuning of GaAs/AlGaAs quantum well microstructures for two-color IR detector operation
Authors: Dubowski J. J. , Song C. Y. , Lefebvre J. , Wasilewski Z. , Aers G. , Liu H. C.
Published: May 2004

Selective area laser annealing of GaAs/AlxGa1−xAs quantum well infrared photodetector (QWIP) material has been investigated as a possible route towards the fabrication of two-color low-cost focal plane array devices. Tuning of the wavelength response of the material has been achieved as a consequence of the quantum well intermixing (QWI) effect. A 90 s irradiation with a continuous wave Nd:yttrium–aluminum–garnet laser, at the peak temperature of 850 °C, resulted in the 40 nm blueshift of the QW photoluminescence peak from 832 to 792 nm. This corresponded to the 0.7 μm redshift of the wavelength response of the investigated QWIP microstructure in the 8 μm optical absorption region. The amplitude of this shift is consistent with the literature data obtained for similar material processed directly by rapid thermal annealing (RTA) or by a two-step process involving particle implantation and RTA. We have examined the laser-QWI approach for direct writing of arrays of a two-band gap material. The preliminary results indicate the feasibility of this approach for fabricating linear arrays with a period of 0.8 mm.
Optical switching in InGaAsP waveguides using localized index gradients
Authors: Abdalla S. , Ng S. , Celo D. , El-Mougy S. , Smy T. , Syrett B. , Barrios P. , Delâge A. , Golub I. , Janz S. , McKinnon R. , Poole P. , Raymond S.
Published: May 2004

Optical switching using a dynamic local index gradient induced in an InGaAsP/InP Y-junction waveguide switch is investigated using optical, electrical, and thermal simulations, and demonstrated experimentally. Switching is accomplished by creating a steep index gradient in the optical path near the Y junction. Both simulations and experiments indicate that either carrier injection or the thermo-optic effect can be used as the active switching mechanism. For carrier injection the induced index change under the electrode, Δn, is negative. The measured switch contrast ratio is about 20 dB, with a response time of approximately 5 ns. The thermo-optic effect gives a positive Δn, and the observed contrast ratio is about 10 dB. The thermo-optic switch response time is several hundred nanoseconds, due to the highly localized nature of the thermal gradient.
Empirical tight-binding calculations of the electronic structure of dilute III–V–N semiconductor alloys
Authors: Turcotte S. , Shtinkov N. , Desjardins P. , Masut R. A. , Leonelli R.
Published: May 2004

We present empirical tight-binding (TB) calculations of the electronic structure of GaP1−xNx and InyGa1−yAs1−xNx alloys with low nitrogen content (x<0.05) over the entire Brillouin zone. Following the method recently developed for GaAs1−xNx [Shtinkov et al., Phys. Rev. B 67, 081202 (2003)], we add to the TB basis an additional anion s orbital (sN) in order to account for the N-induced change of the electronic structure. The band structures of GaP1−xNx and InyGa1−yAs1−xNx are calculated using an sp3d5s*sN TB parametrization. Our TB results are in excellent agreement with experimental and other theoretical data without introducing any additional fitting parameters, demonstrating that the developed method is a promising tool for modeling a wide range of dilute nitride materials and heterostructures.
InGaN/AlGaN near-ultraviolet multiple quantum well light-emitting diodes with p-InGaN tunneling contact layer
Authors: Chen C. H. , Chang S. J. , Su Y. K.
Published: May 2004

InGaN/AlGaN near-ultraviolet multiple quantum well light-emitting diodes (LEDs) with p-In0.23Ga0.77N tunneling contact layers were successfully fabricated. Compared to p-GaN layers, it was found that we could achieve a much larger hole concentration in p-In0.23Ga0.77N layers. We could reduce the 20 mA operation voltage from 3.78 to 3.37 V by introducing a 5 nm thick In0.23Ga0.77N layer on top of the p-GaN layer and improve the near-ultraviolet LED output power and lifetime by employing such a p-In0.23Ga0.77N layer.
Organometallic vapor phase epitaxy of GaAs1−xNx alloy layers on GaAs(001): Nitrogen incorporation and lattice parameter variation
Authors: Beaudry J.-N. , Masut R. A. , Desjardins P. , Wei P. , Chicoine M. , Bentoumi G. , Leonelli R. , Schiettekatte F. , Guillon S.
Published: May 2004

Epitaxial GaAs1−xNx alloy layers, nominally 200-nm-thick, with x up to 0.0375 were grown on GaAs(001) at temperatures Ts varying from 500 to 650 °C to investigate nitrogen incorporation and lattice parameter variations during organometallic vapor phase epitaxy from trimethylgallium, tertiarybutylarsine, and 1,1-dimethylhydrazine. Quantitative secondary ion mass spectrometry measurements (SIMS) indicate that N incorporation decreases systematically with increasing Ts to become almost negligible at 650 °C. All films are coherent with the substrate as judged by high-resolution x-ray reciprocal lattice mapping although atomic force microscopy and cross-sectional transmission electron microscopy reveal the presence of cracks in films with x>0.02. High-resolution x-ray diffraction measurements combined with SIMS analyses indicate that the lattice constant decreases linearly with increasing x following closely the predictions of Vegard’s rule for x<0.03. At higher concentrations, the lattice constant decreases more rapidly as a significant fraction of N atoms becomes incorporated in nonsubstitutional sites as demonstrated by nuclear reaction analysis.
Dopant layer abruptness in strained Si1−xGex heterostructures
Authors: Rowell N. L. , Houghton D. C. , Berbezier I. , Ronda A. , Webb D. , Ward M.
Published: May 2004

Low-temperature photoluminescence (PL) has been used to evaluate dopant concentration and vertical location in heterostructures consisting of strained Si0.85Ge0.15 layers on Si (001). The boron dopant profile was established to be abrupt within 1 nm on samples grown by reduced pressure chemical vapor deposition. The structure had a 20 nm thick Si0.85Ge0.15 layer which was either uniformly doped or had a 10 nm thick dopant in the adjoining Si. The PL variation with dopant displacement was described using simple geometric considerations for exciton penetration from the SiGe into the adjacent Si. Screening by dopant atoms in the sheet caused exciton quenching which depended on dopant displacement and concentration. The best fit between the calculated and observed dependence was obtained for an exciton radius of 6 nm. For small dopant displacement with high boron concentration, dopant diffusion was observed from the Si to the SiGe.
Effects of thermal annealing of W/SiO2 multilayer Bragg reflectors on resonance characteristics of film bulk acoustic resonator devices with cobalt electrodes
Authors: Yim Munhyuk , Kim Dong-Hyun , Chai Dongkyu , Yoon Giwan
Published: May 2004

In this article, we present the thermal annealing effects of the W/SiO2 multilayer reflectors in ZnO-based film bulk acoustic resonator (FBAR) devices with cobalt (Co) electrodes in comparison with those with aluminum (Al) electrodes. Various thermal annealing conditions have been implemented on the W/SiO2 multilayer reflectors formed on p-type (100) silicon substrates. The resonance characteristics could be significantly improved due to the thermal annealing and were observed to depend strongly on the annealing conditions applied to the reflectors. Particularly, the FBAR devices with the W/SiO2 multilayer reflectors annealed at 400 °C/30 min have shown superior resonance characteristics in terms of return loss and quality factor. In addition, the use of Co electrodes has resulted in the further improvement of the resonance characteristics as compared with the Al electrodes. As a result, the combined use of both the thermal annealing and Co electrodes seems very useful to more effectively improve the resonance characteristics of the FBAR devices with the W/SiO2 multilayer reflectors.
Spectroscopic ellipsometry characterization of ZrO2 films on Si(100) deposited by high-vacuum-metalorganic chemical vapor deposition
Authors: Song Z. , Rogers B. R. , Theodore N. D.
Published: May 2004

The integration of high-k dielectric materials into semiconductor devices requires nondestructive, fast, and accurate characterization methods. Spectroscopic ellipsometry (SE) is an outstanding candidate for this purpose. A multisample variable-angle SE method was used to characterize ZrO2 samples deposited on Si(100) by high-vacuum chemical vapor deposition. Proper modeling of the optical properties of the interfacial layer is found to be the key to accurate characterization of ZrO2 films. Based on a stacking model consisting of an effective medium approximation surface-roughness layer, a Tauc–Lorentz (TL) layer to represent the ZrO2 layer, and a second TL layer to represent the interfacial layer, we accurately extract both thickness and optical constants of each layer. The extracted surface-roughness and thickness values were confirmed by atomic force microscopy and transmission electron microscopy results. The optical constants of the interfacial layer suggest that the interfacial layer is composed of nonstoichiometric zirconium silicate.
Analysis of step-based silicon surfaces at facet boundaries during wet anisotropic etching
Authors: Stateikina I. , Elalamy M.-Z. , Landsberger L. M. , Kahrizi M.
Published: May 2004

A step-based model describing the anisotropic etching of silicon by the advancement of steps on flat {111} planes is supported by under-etch experiments on {100} and {110} silicon wafers. However, such a model does not explain etch rate variations of surfaces having the same families of Miller Indices exposed to the same etchant. This work analyzes crystal features at intersections between facets composed of two different types of steps, one called “rows of kinks,” and the other called “periodic bond chains,” intersecting at an angle of 120°. This important configuration, called K-P-120°, is analyzed with a view to whether step movement would propagate across a boundary between facets. An under-etched surface, composed of three facets and a cavity bottom, having alternating K-P-K-P combinations, (where all of the facet boundary step interactions are K-P-120°), is analyzed. Facet roughness patterns, comparative etch rates, and analysis of boundary atom positions support the conclusion that that across-facet-boundary zipping of steps is favored from P steps to K steps, more than from K steps to P steps.
Resolution enhancement in probing of high-speed integrated circuits using dynamic electrostatic force-gradient microscopy
Authors: Weng Z. , Kaminski T. , Bridges G. E. , Thomson D. J.
Published: May 2004

Dynamic mode electrostatic force microscopy is a technique capable of measuring the internal voltage signals of high-speed integrated circuits. Circuit signals are extracted by utilizing the localized nonlinear Coulomb force acting on a microfabricated probe that is closely positioned above the circuit test point. Equivalent time sampling of repetitive high-frequency signals, that can have a bandwidth much greater than the mechanical response of the probe, is achieved by driving the probe with amplitude-modulated high-speed pulses. Currently, dynamic mode electrostatic force microscopes (DEFMs) extract circuit voltage signals through direct sensing of the electrostatic interaction which results in a poor spatial resolution and is susceptible to interference due to significant coupling to the tip sidewall and the cantilever support of the probe. This is especially true for large tip-to-sample distances such as when passivated circuits are measured. This article describes a force-gradient method to improve the spatial resolution of DEFM. The force-gradient method is implemented numerically and is based on sensing the force as the tip-sample distance is modulated. The method is shown to reduce the contribution from the tip sidewall and the cantilever. Measurements of high-speed signals up to 500 Mb/s demonstrate a significant reduction of interference signals.
Numerical simulation of the thermal-mechanical process of high current pulsed electron beam treatment
Authors: Zou Jianxin , Qin Ying , Dong Chuang , Wang Xiaogang , Wu Aimin , Hao Shengzhi
Published: May 2004

Physical models and numerical simulations are used to describe the thermal-mechanical process induced by high current pulsed electron beam (energy E=10–40 keV, energy density in a pulse S=1–6 J/cm2, pulse duration τ=0.5–5 μs) irradiation. The temperature simulation reveals ultrahigh heating and cooling rates on the order of 108–109 K/s, rapid melting and re-solidification within microseconds in the range of micrometers in depth, and evaporation of the surface material. The temperature-induced nonstationary thermal stress fields consist mainly of quasistatic stress and thermal stress waves. The thermal stress wave is a typical nonlinear wave with small amplitudes of about 0.1 MPa, which manifests, however, a strong impact on materials structure and properties far beyond the heat-affected zone. The quasistatic stress is coupled with the temperature field and the maximum compressive stress in the near surface layer reaches several hundreds of MPa, which is sufficiently high for metallic materials to deform. According to the calculation and experimental results, the strengthening mechanism of metals under the thermal impact is suggested to be the work-hardening with high deformation rates, which reach about 104–105 s−1.
Focused ion beam and transmission electron microscopy method for root cause analysis of InP ridge laser devices
Authors: Griswold E. M. , Sharma U. , Phaneuf M. W.
Published: May 2004

To remain competitive in industry, the continual assessment of product reliability is critical. This article discusses the focused ion beam preparation of transmission electron microscopy (TEM) specimens from InP ridge laser devices for observation and analysis of a wide range of microstructural and chemical phenomena (root cause) in reliability-tested lasers. A method is described that enables routine sectioning of ridge lasers mounted ridge up or down and/or beneath solder. The laser devices provided for reliability studies were fully wire bonded on ceramic carriers. Given the geometry, several specimens could be acquired from a single device. Here, it was imperative to thin the metal, dielectric and semiconductor layers in one specimen where the active region of the device was 1 to 3 μm below the metal contact. The structure and morphology of the devices were studied using analytical TEM with energy dispersive x-ray spectroscopy.
SiBCN synthesis by high-dose N++C++BF2+ ion implantation
Authors: Cervera M. , Hernández M. J. , Piqueras J. , Morant C. , Prieto P. , Elizalde E. , Sanz J. M.
Published: May 2004

High-dose implantation processes of N++C++BF2+ have been performed on Si(100) substrates at 600 °C. Additional simple (N+,C+) and double (N++C+ and N++BF2+) implantations have also been produced to sequentially study the synthesis of different semiconductor compounds. FTIR spectra of all the samples before and after thermal annealing at 1200 °C have been acquired. When N+ and C+ are co-implanted the formation of a ternary phase of SiCN nature is suggested, meanwhile in the case of N+ and BF2+ competition between BN and Si3N4 takes place. The sequential implantation of the three species produces a stable surface region as deduced from the AES profiles. IR and XPS measurements suggest the formation of a SiCBxNy mainly formed by SiC with a significant content of B and N (∼20%).
Etching of gain-coupled gratings into a multi-quantum-well laser with selective wet etches
Authors: Finlay Richard
Published: May 2004

A wet chemical etch process suitable for manufacturing the gratings in gain-coupled distributed-feedback lasers is described. The process involves two etch stages. In the first stage, the grating pattern is transferred from photoresist into InP. At this stage, the bottom of the grating teeth is the top of the separate-confining heterojunction (SCH) layers. In the second stage, the grating pattern is transferred into the SCH and quantum-well/quantum-barrier layers. The process yields more uniform gratings than conventional single-etch processes because the etching in each stage is optimized for the material being etched in that stage. The process allows grating depth to be controlled to within ± one-half the distance between quantum wells. The process has been applied to manufacturing volumes of 100 wafers per week.
Inverse modeling of delta doped pseudomorphic high electron mobility transistors
Authors: Mishra Meena , Kumar M. Jagadesh , Singh Yashvir , Shukla S. R. , Vyas H. P. , Rawal D. S. , Naik A. , Sharma H. S. , Sehgal B. K. , Gulati R.
Published: May 2004

In this article an inverse modeling technique is used to design a 0.25 μm delta doped pseudomorphic high electron mobility transistor (PHEMT). The technique is based on the results obtained from the device simulator MEDICI. The technique has been used for determining the structural and physical parameters of the device for a defined threshold voltage, maximum trans conductance and the gate voltage at which the trans conductance peaks. Empirical formulas have been obtained based on a data bank created by varying the device structural parameters. Using these formulas, a 0.25 μm delta doped PHEMT has been designed and a good agreement has been obtained between the measured data and the predicted data.
Fabrication of short channel organic thin film transistors by Si-etching method
Authors: Chen Y. , Zhu W. W. , Xiao S. , Shih I.
Published: May 2004

By using a Si etching and oxidation method, drain and source patterns for organic thin film transistors (OTFTs) have been fabricated without the need of precise mask alignment. OTFTs were fabricated on these patterns by spin coating a layer of Poly [2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]. From measurement results on devices with different channel lengths down to 0.5 μm, a pronounced short channel effect was observed.
Optical switching of vanadium dioxide thin films deposited by reactive pulsed laser deposition
Authors: Soltani M. , Chaker M. , Haddad E. , Kruzelecky R. V. , Nikanpour D.
Published: May 2004

The parameters of reactive pulsed laser deposition were successfully optimized for fabrication of vanadium dioxide thin films. It is observed that the O2 concentration in Ar gas and the total deposition pressure are critical in stabilizing the single VO2 phase. Thermochromic VO2 and V1−xWxO2 (x=0.014) thin films were synthesized on various substrates (silicon, quartz, and sapphire) at 5% of O2/Ar ratio gas and total pressure of 90 mTorr. The structural properties of the deposited films were analyzed by x-ray diffraction, while their semiconductor-to-metal phase transitions were studied by electrical resistivity using the four-point technique and infrared transmittance from room temperature up to 100 °C. The observed transition temperature was about 36 °C for W-doped VO2 compared to 68 °C for VO2 films. This transition temperature was then lowered by about 22.85 °C per 1 at. % of W added. The temperature coefficient of resistance was about 1.78%/°C for VO2 and about 1.90%/°C for W-doped VO2. Using the pump-probe experiment, the application of these thermochromic films as optical switches was demonstrated at the wavelength of 1.55 μm. The transmission switching was about 25 dB for VO2 and 28 dB for W-doped VO2. In addition, application of VO2 on optical fiber components was demonstrated by direct VO2 coating on the end faces of cleaved single mode optical fibers and optical fiber connectors.
Passivation of GaAs using P2S5/(NH4)2S+Se and (NH4)2S+Se
Authors: Fanaei S. T. , Aktik C.
Published: May 2004

We compared the passivation methods for GaAs surface using (NH4)2S+Se and P2S5/(NH4)2S+Se. The characteristics of the Schottky contacts fabricated on GaAs, passivated using these methods, were studied through current–voltage and capacitance–voltage measurements. The long term variations in these characteristics show that the method using (NH4)2S+Se provides stable Schottky barrier while the contacts fabricated using P2S5/(NH4)2S+Se present ohmic behavior.
Dry release of polymer structures with anti-sticking layer
Authors: Cheng M. C. , Gadre A. P. , Garra J. A. , Nijdam A. J. , Luo C. , Schneider T. W. , White R. C. , Currie J. F. , Paranjape M.
Published: May 2004

A dry release method using a thin Teflon™ layer for SU-8 multilayered polymeric microstructures is presented. The low surface energy of Teflon makes the adhesion of SU-8 and substrate poor, enabling the SU-8 polymer photoresist to be removed after the devices have been fully processed. The surface energy was measured using the open-crack method, and the surface roughness and deformation of the released SU-8 were minimized in our processing. The dry release technique eliminates the diffusion limited problem in wet etching and is suitable to package complex three-dimensional polymer microfluidic devices. One such example, which provided the original impetus to formulate a dry release process, is a multilayered SU-8 structure that encapsulates small quantities of fluid. This device is being developed for a biomedical application, and will be used throughout this article as an example of a complex SU-8 structure that uses the dry release process.
Modeling of mushroom waveguide photodetector
Authors: El-Batawy Yasser M. , Deen M. Jamal
Published: May 2004

Waveguide photodetectors (WGPDs) are promising candidates for applications in high-speed optical communications and interconnections. In these high-speed photodetectors, both high bandwidth and high external quantum efficiency can be achieved simultaneously. Mushroom-WGPD is proposed to overcome the trade-off between the capacitance and contact resistance of the photodetector. In this article, a physical model of the mushroom-WGPD is presented including both time and frequency responses of this photodetector and how they depend on the parameters of the photodetector. A SPICE model for mushroom-WGPD including all the parasitics is also presented, showing the dependence of the transfer function of this model on the dimensions and the material parameters of the photodetector. The effects of the parasitics are also studied for different photodetector areas. The characteristics of mushroom-WGPD are studied for two cases, first without an inductor added in series with the load resistance and second, if an inductor is added in series to the load resistor. A significant improvement in performance is achieved in the second case where an inductor is added.
Circularized Chromosome with a Large Palindromic Structure inStreptomyces griseusMutants
Authors: Uchida Tetsuya , Ishihara Naoto , Zenitani Hiroyuki , Hiratsu Keiichiro , Kinashi Haruyasu
Published: May 2004

Streptomyceslinear chromosomes display various types of rearrangements after telomere deletion, including circularization, arm replacement, and amplification. We analyzed the new chromosomal deletion mutantsStreptomyces griseus301-22-L and 301-22-M. In these mutants, chromosomal arm replacement resulted in long terminal inverted repeats (TIRs) at both ends; different sizes were deleted again and recombined inside the TIRs, resulting in a circular chromosome with an extremely large palindrome. Short palindromic sequences were found in parent strain 2247, and these sequences might have played a role in the formation of this unique structure. Dynamic structural changes ofStreptomyceslinear chromosomes shown by this and previous studies revealed extraordinary strategies of members of this genus to keep a functional chromosome, even if it is linear or circular.
The Spatial Organization of the VirR Boxes Is Critical for VirR-Mediated Expression of the Perfringolysin O Gene,
pfoA
, from
Clostridium perfringens
Authors: Cheung Jackie K. , Dupuy Bruno , Deveson Deanna S. , Rood Julian I.
Published: May 2004

The transcriptional regulation of toxin production in the gram-positive anaerobe
Clostridium perfringens
involves a two-component signal transduction system that comprises the VirS sensor histidine kinase and its cognate response regulator, VirR. Previous studies showed that VirR binds independently to a pair of imperfect direct repeats, now designated VirR box 1 and VirR box 2, located immediately upstream of the promoter of the
pfoA
gene, which encodes the cholesterol-dependent cytolysin, perfringolysin O. For this study, we introduced mutated VirR boxes into a
C. perfringens pfoA
mutant and found that both VirR boxes are essential for transcriptional activation. Furthermore, the spacing between the VirR boxes and the distance between the VirR boxes and the −35 region are shown to be critical for perfringolysin O production. Other VirR boxes that were previously identified from the strain 13 genome sequence were also analyzed, with perfringolysin O production used as a reporter system. The results showed that placement of the different VirR boxes at the same position upstream of the
pfoA
promoter yields different levels of perfringolysin O activity. In all of these constructs, VirR was still capable of binding to the target DNA, indicating that DNA binding alone is not sufficient for transcriptional activation. Finally, we show that the
C. perfringens
RNA polymerase binds more efficiently to the
pfoA
promoter in the presence of VirR, indicating that interactions must occur between these proteins. We propose that these interactions are required for VirR-mediated transcriptional activation.
Reconstitution of a Staphylococcal Plasmid-Protein Relaxation Complex In Vitro
Authors: Caryl Jamie A. , Smith Matthew C. A. , Thomas Christopher D.
Published: May 2004

The isolation of plasmid-protein relaxation complexes from bacteria is indicative of the plasmid nicking-closing equilibrium in vivo that serves to ready the plasmids for conjugal transfer. In pC221 and pC223, the components required for in vivo site- and strand-specific nicking at
oriT
are MobC and MobA. In order to investigate the minimal requirements for nicking in the absence of host-encoded factors, the reactions were reconstituted in vitro. Purified MobA and MobC, in the presence of Mg
2+
or Mn
2+
, were found to nick at
oriT
with a concomitant phosphorylation-resistant modification at the 5′ end of
nic
. The position of
nic
is consistent with that determined in vivo. MobA, MobC, and Mg
2+
or Mn
2+
therefore represent the minimal requirements for nicking activity. Cross-complementation analyses showed that the MobC proteins possess binding specificity for
oriT
DNA of either plasmid and are able to complement each other in the nicking reaction. Conversely, nicking by the MobA proteins is plasmid specific. This suggests the MobA proteins may encode the nicking specificity determinant.
Modulation of Activity of
Bacillus subtilis
Regulatory Proteins GltC and TnrA by Glutamate Dehydrogenase
Authors: Belitsky Boris R. , Sonenshein Abraham L.
Published: May 2004

The
Bacillus subtilis gltAB
operon, encoding glutamate synthase, requires a specific positive regulator, GltC, for its expression and is repressed by the global regulatory protein TnrA. The factor that controls TnrA activity, a complex of glutamine synthetase and a feedback inhibitor, such as glutamine, is known, but the signal for modulation of GltC activity has remained elusive. GltC-dependent
gltAB
expression was drastically reduced when cells were grown in media containing arginine or ornithine or proline, all of which are inducers and substrates of the Roc catabolic pathway. Analysis of
gltAB
expression in mutants with various defects in the Roc pathway indicated that
rocG
-encoded glutamate dehydrogenase was required for such repression, suggesting that the substrates or products of this enzyme are the real effectors of GltC. Given that RocG is an enzyme of glutamate catabolism, the main regulatory role of GltC may be prevention of a futile cycle of glutamate synthesis and degradation in the presence of arginine-related amino acids or proline. In addition, high activity of glutamate dehydrogenase was incompatible with activity of TnrA.
Two Oligopeptide-Permease-Encoding Genes in the Clavulanic Acid Cluster of
Streptomyces clavuligerus
Are Essential for Production of the β-Lactamase Inhibitor
Authors: Lorenzana Luis M. , Pérez-Redondo Rosario , Santamarta Irene , Martín Juan F. , Liras Paloma
Published: May 2004

orf7
(
oppA1
) and
orf15
(
oppA2
) are located 8 kb apart in the clavulanic acid gene cluster of
Streptomyces clavuligerus
and encode proteins which are 48.0% identical. These proteins show sequence similarity to periplasmic oligopeptide-binding proteins. Mutant
S. clavuligerus oppA1
::
acc
, disrupted in
oppA1
, lacks clavulanic acid production. Clavulanic acid production is restored by transformation with plasmid pIJ699-
oppA1
, which carries
oppA1
, but not with the multicopy plasmid pIJ699-
oppA2
, which carries
oppA2
. The mutant
S. clavuligerus oppA2
::
aph
also lacks clavulanic acid production, shows a bald phenotype, and overproduces holomycin (5). Clavulanic acid production at low levels is restored in the
oppA2
-disrupted mutants by transformation with plasmid pIJ699-
oppA2
, but it is not complemented by the multicopy plasmid pIJ699-
oppA1
. Both genes encode oligopeptide permeases with different substrate specificities. The disrupted
S. clavuligerus oppA2
::
aph
is not able to grow on RPPGFSPFR (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg; bradykinin), but both mutants grow on VAPG (Val-Ala-Pro-Gly) as the only nitrogen source, indicating differences in the peptide bound by the proteins encoded by both genes. The null
S. clavuligerus oppA1
::
acc
and
S. clavuligerus oppA2
::
aph
mutants are more resistant to the toxic tripeptide phosphinothricyl-alanyl-alanine (also named bialaphos) than the wild-type strain, suggesting that this peptide might be transported by these peptide-binding proteins.
Single-Step Capsular Transformation and Acquisition of Penicillin Resistance inStreptococcus pneumoniae
Authors: Trzciński Krzysztof , Thompson Claudette M. , Lipsitch Marc
Published: May 2004

The capsule (cps) locus ofStreptococcus pneumoniaeis flanked by thepbp2xandpbp1agenes, coding for penicillin-binding proteins, enzymes involved in cell wall synthesis that are targets for β-lactams. This linkage suggested to us that selection for β-lactam resistance might coselect for capsular transformants. The recombination event would then involve PBP genes, as well as thecpsoperon, and would change both the serotype and the resistance profile of the strain. We transformed β-lactam-susceptible strain TIGR4 by using whole genomic DNA extracted from multidrug-resistant strain GA71, a serotype 19F variant of pneumococcal clone Spain23F-1, and selected β-lactam-resistant transformants. Smooth colonies appearing on selective plates were subcultured, serotyped by the Quellung reaction, and genotyped to confirm the presence of the GA71pbp2x-cps19-pbp1alocus in the TIGR4 genetic background by restriction fragment length polymorphism analysis of the whole locus and its flanking regions. The results showed that a new serotype, combined with resistance to β-lactams, could emerge in a susceptible strain via a single transformation event. Quantitative analysis showed that transfer of thecpslocus had occurred at an elevated rate in β-lactam-selected transformants. This suggests that in natural settings selection by host immunity and selection by antibiotics may be interrelated because of “hitchhiking” effects due to linkage of resistance determinants and the capsule locus.
Identification and Characterization ofglxR, a Gene Involved in Regulation of Glyoxylate Bypass inCorynebacterium glutamicum
Authors: Kim Hyung-Joon , Kim Tae-Hyun , Kim Younhee , Lee Heung-Shick
Published: May 2004

A corynebacterial clone, previously isolated by scoring repression oflacZYAfused to theaceBpromoter ofCorynebacterium glutamicum, was analyzed further. In the clone, an open reading frame designatedglxR, consisting of 681 nucleotides and encoding a 24,957-Da protein, was found. The molecular mass of a native GlxR protein was estimated by gel filtration column chromatography to be 44,000 Da, suggesting that the protein formed dimers. The predicted amino acid sequence contained both cyclic AMP (cAMP)- and DNA-binding motifs and was homologous with the cAMP receptor protein family of proteins. TheaceB-repressing activity of theglxRclone was markedly relieved in anEscherichia coli cyamutant, but the activity was restored in growth medium containing cAMP. In glucose medium, the intracellular cAMP concentration ofC. glutamicumreached 22 nmol/mg of protein in the early exponential phase and then decreased further; but in acetate medium, the intracellular cAMP concentration was only 5 nmol/mg of protein and remained low throughout the growth phase. The expression ofglxRwas not affected by the carbon source. Binding of purified GlxR to the promoter region ofaceBcould be demonstrated only in the presence of cAMP. These data suggest that GlxR may form dimers which bind to theaceBpromoter region in the presence of cAMP and repress the glyoxylate bypass genes.
Lactococcal Phage Genes Involved in Sensitivity to AbiK and Their Relation to Single-Strand Annealing Proteins
Authors: Bouchard Julie D. , Moineau Sylvain
Published: May 2004

Lactococcal phage mutants insensitive to the antiviral abortive infection mechanism AbiK are divided into two classes. One comprises virulent phages that result from DNA exchanges between a virulent phage and the host chromosome. Here, we report the analysis of the second class of phage mutants, which are insensitive to AbiK as a result of a single nucleotide change causing an amino acid substitution. The mutated genes occupy the same position in the various lactococcal phage genomes, but the deduced proteins do not share amino acid sequence similarity. Four nonsimilar proteins involved in the sensitivity to AbiK (Sak) were identified. Two of these Sak proteins are related to Erf and RAD52, single-strand annealing proteins involved in homologous recombination.
The Global Arginine Regulator ArgR Controls Expression of
argF
in
Pseudomonas syringae
pv. phaseolicola but Is Not Required for the Synthesis of Phaseolotoxin or for the Regulated Expression of
argK
Authors: Hernández-Flores José Luis , López-López Karina , Garcidueñas-Piña Rogelio , Jofre-Garfias Alba E. , Alvarez-Morales Ariel
Published: May 2004

In
Pseudomonas syringae
pv. phaseolicola the enzyme ornithine carbamoyltransferase (OCTase), encoded by
argF
, is negatively regulated by
argR
, similar to what has been reported for
Pseudomonas aeruginosa
. However, production of the phaseolotoxin-resistant OCTase encoded by
argK
, synthesis of phaseolotoxin, and infectivity for bean pods occur independently of the ArgR protein.
Exposure-response relations for work related respiratory symptoms and sensitisation in a cohort exposed to α-amylase: Table 1
Authors: Brisman J , Nieuwenhuijsen M J , Venables K M , Putcha V , Gordon S , Taylor A J N
Published: May 2004

Aims:To explore relations between exposure to fungal α-amylase and the risk of new work related respiratory symptoms or sensitisation.

Methods:A prospective cohort study among 300 bakers and millers was followed up for a maximum of seven years. Exposure to α-amylase was estimated by air measurements and questionnaires and classified into three categories. Symptoms were recorded with a self-administered questionnaire and skin sensitisation assessed using skin prick test (SPT).

Results:There were 36 new cases of chest symptoms, 86 of eyes/nose symptoms, and 24 of a positive SPT to α-amylase. There were exposure-response relations for chest and eyes/nose symptoms and for sensitisation, and a significantly increased prevalence ratio for chest symptoms in the highest exposure category.

Conclusion:A reduction in α-amylase exposure is likely to reduce the risk for respiratory morbidity in bakery workers.
Evaluation of the mercury exposure of dental amalgam patients by the Mercury Triple Test
Authors: Hansen G , Victor R , Engeldinger E , Schweitzer C
Published: May 2004

Aims:To establish and analyse reference data for the mercury burden of patients with and without amalgam fillings.

Methods:Atomic absorption spectroscopy was used to quantify Hg concentrations in the scalp hair and urine (before and after application of dimercaptopropane sulphonate), and Hg release from dental amalgams (using a newly developed, amalgam specific chew test), in 2223 subjects.

Results:50th centiles were 1.3 μg Hg/g creatinine in basal urine, 32 μg Hg/g creatinine after DMPS application, 454 ng Hg/g in hair, and 27 μg Hg per g of chewing gum, which corresponds to about 1 μg Hg released per minute of chewing. Total Hg intake (from ambient air, drinking water, food, and amalgams) of most patients is well below the provisioned tolerable weekly intake (PTWI) defined by the WHO, unless extremely Hg rich food is consumed on a regular basis. However, for patients exceeding the 75thcentile in chew tests, total Hg intake exceeds the PTWI by about 50%, even at the low limit of intake from food. In the absence of occupational exposure, significant Hg release from dental amalgams is a necessary but insufficient condition to obtain a high long term body burden. After removal of dental amalgams, chew tests no longer exhibit oral Hg exposure, while basal urine Hg content and DMPS induced excretion display a exponential decrease (half life about 2 months in both cases).

Conclusions:A standardised procedure for evaluation of the magnitude and origin of the Hg burden of individuals has been developed, which, by comparison with the database presented here for the first time, can serve as a diagnostic tool.
Perceived muscular tension, job strain, physical exposure, and associations with neck pain among VDU users; a prospective cohort study
Authors: Wahlström J , Hagberg M , Toomingas A , Wigaeus Tornqvist E
Published: May 2004

Aims:To determine whether perceived muscular tension, job strain, or physical exposure are associated with increased risk of developing neck pain among VDU users.

Methods:A baseline questionnaire was answered by 1283 respondents, of whom 671 were free from neck pain at baseline. Perceived muscular tension, job strain, and physical exposure were assessed at baseline. Information about newly developed neck pain was collected in 10 follow up questionnaires and the case definition was the first report of such pain in any of the follow up questionnaires. Median follow up time was 10.9 months.

Results:Both men and women who perceived muscular tension at least a few times per week, compared to those who had not perceived muscular tension the preceding month, had an incidence rate ratio (IRR) of 1.9 (95% CI 1.25 to 2.93) for developing neck pain, when stratifying for sex. High perceived muscular tension was associated with an increased risk (IRR 1.6, 95% CI 1.02 to 1.91), even when controlling for job strain, physical exposure, and age in the model stratified by sex.

Conclusions:Perceived muscular tension was associated with an increased risk of developing neck pain among VDU users. The combination of high job strain and high perceived muscular tension was associated with higher risk of developing neck pain than the combination of high physical exposure and high perceived muscular tension. There was an indication of an excess risk due to interaction between high physical exposure and high job strain.
Summer work and injury among middle school students, aged 10–14 years
Authors: Zierold K M , Garman S , Anderson H
Published: May 2004

Background:Little information exists on injury and factors associated with injury in working youth aged 10–14 years. Most studies do not involve children younger than 15.

Methods:A cross-sectional anonymous survey was administered to middle school students in five school districts and one large urban school in October 2001.

Results:Of the 3189 working middle school students who responded to the survey, the majority were employed in informal job settings, such as working for someone in a home, newspaper delivery, and working on family farms or in family businesses. Overall, 18% of children reported being injured at work. Of those injured, 26% reported that their injury was severe enough to affect their activities for more than three days. Variables that were associated with injury included having a “near-miss” incident at work (AOR 6.61, 95% CI 4.92 to 8.89), having a co-worker injured (AOR 2.65, 95% CI 1.95 to 3.60), and being asked to do something dangerous (AOR 2.25, 95% CI 1.61 to 3.14).

Conclusions:Children are working and being injured in jobs that are not covered by existing child labour laws. Injury rates in non-covered occupations are high, warranting review of current laws.
A CRITICAL GUIDE TOBUSH V. GORESCHOLARSHIP
Authors: Hasen Richard L.
Published: May 2004

▪ Abstract  This review evaluates the emerging legal and political science scholarship created in the wake of the United States Supreme Court's decision in Bush v. Gore, the case that ended the 2000 Florida election controversy between supporters of George W. Bush and those of Al Gore. The article surveys scholars' answers to four central questions: (a) Were the Supreme Court's majority or concurring opinions legally sound? (b) Was the Supreme Court's result justified, even if the legal reasoning contained in the opinions was unsound? (c) What effects, if any, will the case and the social science research it has spurred have on the development of voting rights law? (d) What does the Court's resolution of Bush v. Gore tell us about the Supreme Court as an institution?
BAYESIAN ANALYSIS FOR POLITICAL RESEARCH
Authors: Jackman Simon
Published: May 2004

▪ Abstract  Bayesian data analysis relies on Bayes' Theorem, using data to update prior beliefs about parameters. In this review I introduce and contrast Bayesian analysis with conventional frequentist inference and then distinguish two types of Bayesian analysis in political science. First, Bayesian analysis is used to merge historical information with current data in an analysis of likely election outcomes in Florida in 2000; particular attention is paid to the sensitivity of the results to the choice of prior (i.e., how much confidence one places in the historical information). Second, a more “modern” style of Bayesian analysis is reviewed, relying on Markov chain Monte Carlo algorithms to generate computationally intensive “random tours” of the high dimensional posterior distributions that are the focus of many contemporary Bayesian analyses; the example used is a central problem in political science, the analysis of legislators' preferences using roll call data.
DOWNS AND TWO-PARTY CONVERGENCE
Authors: Grofman Bernard
Published: May 2004

▪ Abstract  We take as our starting point the insights of Downs (1957) into two-party competition. A careful reading of Downs offers a much more sophisticated and nuanced portrait of the factors affecting party differentiation than the simplistic notion that, in plurality elections, we ought to expect party convergence to the views of the median voter. Later scholars have built on Downsian ideas to see what happens vis-à-vis party differentiation when we modify key assumptions found in the basic Downsian spatial model. Recent work allows us to turn what is taken to be the Downsian view on its head: Although there are pressures in two-party competition for the two parties to converge, in general we should expect nonconvergence. Moreover, contra the negative portrait offered by Green & Shapiro (1994) of the limited or nonexistent value of research on party competition models in the Downsian tradition, we argue that, when viewed as a whole, neo-Downsian models—especially those of the past decade—do allow us to reconcile theory and data in terms of a multi-componented theory of party competition with testable implications for comparative statics.