bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulations of metal thin film thermal flow processes
Authors: Liao Hung , Cale Timothy S.
Published: July 2002

We use EVFLOW, a thin film thermal flow process simulator, to simulate the thermal flow of metal thin films in axisymmetric contacts/vias on patterned wafers, where two dimensional surface evolution is appropriate. The flow processes considered are (1) Al and Au thin film laser melting processes, for which process temperatures are higher than the metal melting points, and (2) conventional Al thermal anneal processes (400–550 °C). Surface tension induced mass transport in the flowing films is calculated by solving the Navier–Stokes and continuity equations for incompressible fluids. Our simulations of Au film profiles during laser melting process predict the experimentally observed trends in void formation and collapse with feature geometry and deposited film thickness. In laser melting processes, the void shrinks and vanishes, resulting in fully filled contacts and planarized metal surfaces. Al film evolution in the thermal anneal process is modeled using a surface layer melting model. The thickness of the liquidlike flowing layer is assumed to be 0.02 μm in this work. The trends in the formation of voids with feature geometry and deposited film thickness predicted by EVFLOW reflect experimental observations. Voids may form during the thermal anneal processes. The surface layer melting model as used in this work will not predict the collapse of voids.
Characterization of two-dimensional dopant profiles: Status and review
Authors: Diebold A. C. , Kump M. R. , Kopanski J. J. , Seiler D. G.
Published: July 2002

The National Technology Roadmap for Semiconductors calls for development of two- and three-dimensional dopant profiling methods for calibration of technology computer-aided design process simulators. We have previously reviewed 2D dopant profiling methods. In this article, we briefly review methods used to characterize etched transistor cross sections by expanding our previous discussion of scanned probe microscopy methods. We also mention the need to participate in our ongoing comparison of analysis results for test structures that we have provided the community.
In situ characterization of ZnSe/GaAs(100) interfaces by reflectance difference spectroscopy
Authors: Yasuda T. , Kuo L. H. , Kimura K. , Miwa S. , Jin C. G. , Tanaka K. , Yao T.
Published: July 2002

We discuss the structure and formation mechanism of ZnSe/GaAs(100) interfaces as probed by reflectance difference spectroscopy (RDS). First we describe a simple analytic procedure that separates the surface and interface contributions in the RD spectra obtained for a heterostructure. The procedure opens up the new possibilities of RDS as an in situ interface probe. We have applied this technique to characterize the ZnSe/GaAs interfaces prepared on various GaAs(100) surfaces. The interface-anisotropy spectra thus obtained and the cross-sectional transmission electron microscopy, show clearly that the chemical composition of the interfacial layer can be changed by controlling the reconstruction and the pregrowth Zn and Se treatments of the initial GaAs surface.
Scanned probe microscope tip characterization without calibrated tip characterizers
Authors: Villarrubia J. S.
Published: July 2002

In scanned probe microscopy the image is a combination of information from the sample and the tip. In order to reconstruct the true surface geometry, it is necessary to know the actual tip shape. It has been proposed that this shape may be reconstructed from images of ‘‘tip characterizer’’ artifacts of independently known shape. The requirements for this strategy—dimensional uncertainty and instability of the characterizer small compared to the tip size—are not trivial. An alternative is ‘‘blind reconstruction,’’ which requires no information about the characterizer geometry apart from that contained within its image, yet produces an outer bound on the tip shape which for appropriately chosen characterizers is a good approximation. With blind reconstruction dimensional instability of characterizers is less problematical, and characterizer measurability is no longer a constraint, so more complex distributed characterizer geometries may be advantageously employed. In situations where part of a characterizer has a known shape, blind reconstruction and the known-characterizer method may be combined.
Molecular-beam epitaxy of high quality lattice matched In1−x−yGaxAlyAs epitaxial layers on InP substrates
Authors: Chua S. J. , Ramam A.
Published: July 2002

The InGaAlAs epilayer is lattice matched to the InP substrate at an InAs mole fraction of 0.53. Experiments were conducted systematically to establish growth parameters for InGaAlAs epilayers with good crystalline and optical quality suitable for laser device applications. Quaternary alloy epilayers of InGaAlAs were grown on InP substrates by the conventional solid source molecular-beam epitaxy technique. The normalized In flux was varied in the range 0.25–0.47 resulting in compressive, matched, and tensile epilayers with respect to the InP substrate. Rocking curves obtained from a double crystal x-ray diffractometer showed the best half-width of 15 arcsec for the epilayer matched to the InP substrate with a mismatch of only 0.03%. Photoluminescence peaks obtained at room and 4 K temperatures indicate half-widths of 70 and 12 meV, respectively. These results are comparable with the best values reported on this material.
Field-emission processes from a liquid–metal surface
Authors: Fursey G. N. , Shirochin L. A. , Baskin L. M.
Published: July 2002

This work presents the generalization of experimental and theoretical investigations of different field-emission phenomena from liquid metal. The dynamics, emission ability, and stability of the processes are described for specific regimes of electric field. A description of the stability is possible if one considers the character of the conducting-fluid surface instabilities that always precede every type of field emission from liquid in a strong electric field. We investigated the electrohydrodynamic effects in liquid–metal emitters operating in the field–ion, field–electron, and explosive–emission modes in direct-current, nanosecond-pulse, and microsecond-pulse regimes.
On the mechanism of the hydrogen-induced exfoliation of silicon
Authors: Weldon M. K. , Marsico V. E. , Chabal Y. J. , Agarwal A. , Eaglesham D. J. , Sapjeta J. , Brown W. L. , Jacobson D. C. , Caudano Y. , Christman S. B. , Chaban E. E.
Published: July 2002

We have investigated the fundamental mechanism underlying the hydrogen-induced exfoliation of silicon, using a combination of spectroscopic and microscopic techniques. We have studied the evolution of the internal defect structure as a function of implanted hydrogen concentration and annealing temperature and found that the mechanism consists of a number of essential components in which hydrogen plays a key role. Specifically, we show that the chemical action of hydrogen leads to the formation of (100) and (111) internal surfaces above 400 °C via agglomeration of the initial defect structure. In addition, molecular hydrogen is evolved between 200 and 400 °C and subsequently traps in the microvoids bounded by the internal surfaces, resulting in the build-up of internal pressure. This, in turn, leads to the observed “blistering” of unconstrained silicon samples, or complete layer transfer for silicon wafers joined to a supporting (handle) wafer which acts as a mechanical “stiffener.”
Nitridation of the GaAs (001) surface using atomic nitrogen
Authors: Hill P. , Westwood D. I. , Haworth L. , Lu J. , Macdonald J. E.
Published: July 2002

The effect of active nitrogen, generated by a radio frequency plasma source, on clean GaAs (001) surfaces has been examined using x-ray photoemission spectroscopy (XPS) and reflection high energy electron diffraction. The “nitridation” of the surface was performed under fixed plasma conditions, compatible with the molecular beam epitaxial growth of GaN, and as a function of both temperature (in the range ∼300–600 °C) and time (up to 15 min). At low temperatures the nitridation proceeds very slowly and was characterized, in its initial stages, by the transformation of the (2×4) reconstructed GaAs surface to a high intensity “amorphous” haze, presumed to be related to the As released in the anion exchange reaction but not evaporated from the surface. At high temperatures the nitridation is much more aggressive readily forming thicker GaN films of a polycrystalline nature. Curve fitting of the XPS spectra, to reveal the nature of the reaction products indicated the probable formation of As–N species in addition to GaN.
Integrated plasma-promoted chemical vapor deposition route to aluminum interconnect and plug technologies for emerging computer chip metallization
Authors: Faltermeier Jonathan , Knorr Andreas , Talevi Robert , Gundlach Heidi , Arun Kumar Kaushik , Peterson Gregory G. , Kaloyeros Alain E. , Sullivan John J. , Loan James
Published: July 2002

A low temperature plasma-promoted chemical vapor deposition (CVD) process was developed for the formation of reliable aluminum interconnect and plug metallization schemes for applications in and beyond 0.25 μm integrated chip device technology. The process employs aluminum source precursors that are based on amine adducts of alane, such as dimethylethylamine alane, where the lack of a direct aluminum–carbon bond provides a clean chemical pathway by which to eliminate the precursor’s hydrocarbon groups at relatively low temperatures and yield pure aluminum films. The formation of dense and thick aluminum films at low temperature and that have a smooth surface morphology is achieved by combining a radio-frequency hydrogen plasma at low power densities (below 0.1 W/cm2) with a low frequency (<400 kHz) substrate bias. The use of a low power density plasma is designed to activate a uniform nucleation of aluminum grains, thus eliminating the inherent problem of surface roughness that plagues thermal CVD processing, while avoiding undesirable precursor gas phase decomposition and film contamination. Additionally, the application of a substrate bias enhances the impingement rate of precursor species on the substrate, and lead to an increase in the re-emission probability of such species inside via and trench structures, leading to conformal step coverage and complete fill. The aluminum interconnect and plug structures produced were specular to thicknesses above 1 μm, exhibited contamination levels below the 1 at. % detection limit of x-ray photoelectron spectroscopy and Auger electron spectroscopy, and displayed postanneal resistivity of 3.38 μΩ cm. Conformal fill of 0.25 μm structures was achieved at growth rates in excess of 1000 Å/min.
Effect of rapid thermal annealing on the microstructure and electrical characteristics of Au/Ni/Au/Ge/Ni multilayers deposited on n-type InGaAs
Authors: Morais J. , Fazan T. A. , Landers R. , Pereira R. G. , Sato E. A. S. , Carvalho W.
Published: July 2002

The effect of rapid thermal annealing on the microstructure and contact resistance of Au/Ni/Au/Ge/Ni multilayers deposited on InGaAs:Si layers was studied by Auger electron spectroscopy (AES) and the transmission line method. The reaction process at the interfaces after thermal annealing was monitored by Auger depth profiles (AES in conjunction with ion sputtering). We have observed that the formation of a NiGeAs layer at the interface plays an important role in obtaining contacts with low specific resistance, similar to GaAs. Based on these results, we have obtained extremely low specific contact resistance [(4±1)×10−8 Ω cm2] for annealing temperatures higher than 400 °C.
Influence of secondary electrons in proximal probe lithography
Authors: Völkel B. , Gölzhäuser A. , Müller H. U. , David C. , Grunze M.
Published: July 2002

This article describes the limitations of proximal probe lithography due to electrons that are mirrored by the electric field between the tip and the surface. The incident beam generates two kinds of electrons at the sample surface: primary electrons which are elastically backscattered and secondary electrons which are produced in the resist/substrate system. The electric field confines the electrons emanating from the surface. The electron trajectories are bent in such a way that the electrons impinge on the sample surface in the vicinity of their origin. These reflected electrons contribute to the exposure of the resist and therefore, limit the resolution. For hexadecanethiol monolayers on gold substrates, we have measured the energy distribution of the mirrored electrons and the secondary electron yield as a function of the primary energy. With near edge x-ray absorption fine structure spectroscopy, we have investigated the relevance of low energy electrons in the exposure of hexadecanethiol films. Simulations of secondary electron trajectories can explain the occurrence of triple line structures observed in field emission proximal probe lithography.
New approaches to atomic force microscope lithography on silicon
Authors: Birkelund K. , Thomsen E. V. , Rasmussen J. P. , Hansen O. , Tang P. T. , Mo/ller P. , Grey F.
Published: July 2002

We have investigated new approaches to the formation of conducting nanowires on crystalline silicon surfaces using atomic force microscope (AFM) lithography. To increase processing speed and reduce wear of the AFM tip, large-scale structures are formed with a direct laser write setup, while the AFM is used to add the finer nanostructures. Both methods are based on selective oxidation of hydrogen-passivated silicon and subsequent etching to define conducting regions on the surface. This combined technique has previously been implemented on amorphous Si on oxide. To extend the technique to form crystalline silicon nanowires, we have used an arsenic implanted crystalline silicon layer on p-type Si, where the nanostructures are isolated from the substrate electrically due to p-n junction formation. Improvements in the reliability of the AFM lithography technique were achieved by using all-metal tips, which do not wear out as rapidly as metal-coated Si3N4 tips.
Temperature dependence of conductivity and Hall carrier concentration of polycrystalline SiC deposited on fused silica by laser ablation
Authors: Abu-Ageel N. , Aslam D. M. , Ager R. , Rimai L.
Published: July 2002

The conductivity and Hall coefficient of polycrystalline SiC films deposited on quartz are studied in the temperature ranges of 13–1275 K and 17–800 K, respectively. Since conductivities were monotonically increasing with the increasing temperature, these films can be used as thermistors over the entire temperature range. While Hall mobility shows a weak temperature dependence, the electron concentration, as computed from n=1/qRH, increases exponentially with temperature from 1019 to 4×1020 cm−3. Three well-separated activation energies, computed from the measured conductivity and Hall concentration, are in the ranges of 0.4–3.8 meV, 7–20 meV, and 55.5±5 meV. Due to high electron concentration measured at temperatures as low as 17 K, impurity/defect related conduction or thermally activated hopping may be dominant at low temperatures.
Photolithography with transparent reflective photomasks
Authors: Qin Dong , Xia Younan , Black Andrew J. , Whitesides George M.
Published: July 2002

A new type of photomask was fabricated by casting a prepolymer of a transparent, elastomeric polymer (polydimethylsiloxane, PDMS) against a Si(100) master whose surface has been patterned with V-shaped trenches or pyramidal pits using anisotropic etching. The PDMS replica, when placed in contact with a film of photoresist and illuminated, acts as a photomask. The sidewalls of the trenches and pits in the silicon master meet with the plateaus in dihedral angles of 54°; as a result, the PDMS replica selectively blocks the incident light in regions where it has sloping features by total internal reflection, and acts as a reflective contact mask for photolithography. The feasibility of this new type of photomask has been demonstrated by the fabrication of micropatterns in photoresist (and in an underlying silicon substrate) with smaller feature sizes and higher complexities than those present on the original chrome mask used in patterning the silicon master. The patterns produced using these elastomeric photomasks can be changed by varying the pressure applied in contacting them.
Transmission electron microscopy observation of thin foil specimens prepared by means of a focused ion beam
Authors: Saka H.
Published: July 2002

Recent development in the application of a focused ion beam to preparation of thin foil specimens for transmission electron microscopy observation has been reviewed. It has been shown that this technique is very powerful for characterization of a variety of industrial materials.
Study of the mechanisms of GaN film growth on GaAs surfaces by thermal and plasma nitridation
Authors: Losurdo M. , Capezzuto P. , Bruno G. , Lefebvre P. R. , Irene E. A.
Published: July 2002

The kinetics of GaAs nitridation using N2 plasmas, both radio frequency and electron cyclotron resonance sources, is investigated using in situ and real time ellipsometry. A comparison of plasma nitridation with the more conventional NH3 thermal nitridation of GaAs is also reported. We report that all the GaAs nitridation processes are self-limiting yielding only very thin GaN layers. The dependence of GaN layer thickness on surface pretreatment, surface temperature and N atom density in the plasma is reported. Smooth and stoichiometric GaN layers are formed at T<600 °C, whereas nitridation at T⩾600 °C yields rough and Ga-rich GaN layers. In both cases, it is shown that As segregates at the GaAs/GaN interface, indicating that GaAs plasma nitridation kinetics is limited by outdiffusion of As and/or AsN species.
Analysis of KOH etching of (100) silicon on insulator for the fabrication of nanoscale tips
Authors: Yun M. H , Burrows V. A. , Kozicki M. N.
Published: July 2002

Anisotropic etching of Si with KOH is a well-known method of forming grooves in the Si surface. We report here on the use of KOH solutions for etching extremely sharp silicon tips on silicon on insulator (SOI) material. Etching of (100) silicon on SOI wafers was carried out over a wide range of reaction temperatures and KOH concentrations. Using statistical methods, we show that the factors important in the silicon etch rate are, in decreasing order of importance, reaction temperature, KOH concentration, and interaction between temperature and KOH concentration. The activation energy of etching at different KOH concentration was calculated from Arrhenius plots to be between 0.43 and 0.59 eV. In addition, in this article we report a qualitative study of the sharpness of Si tips formed by KOH etching. The sharpness increases with temperature to a critical point and then decreases at very high temperature and KOH concentration. Hydrogen bubbles formed during etching are very important in determining both etch rate and sharpness of the tips. The sharpest tip dimension was found to occur at 30% KOH and 70 °C reaction temperature.
Surface conditioning of active molybdenum field emission cathode arrays with H2 and helium
Authors: Chalamala Babu R. , Wallace Robert M. , Gnade Bruce E.
Published: July 2002

We studied the interaction of active molybdenum field emitter arrays with molecular hydrogen and helium. Upon impact with the field emitted electrons, H2 gas introduced into the system undergoes dissociation and ionization. The emitter surface is conditioned through the interaction of the hydrogen atoms and ions with molybdenum, resulting in the formation of volatile molybdenum hydrides. The volatile species are removed by the vacuum system. Surface conditioning of the tips is reflected in the reduced work function and increased electron emission. Emission enhancement reaches a maximum for total exposures of 1000–1500 L (1 L=1×10−6 Torr s) of H2. For small exposures of helium (<1500 L), emission enhancement is also observed. Larger exposures result in significant emission degradation. We propose that the interaction of H2 with active molybdenum field emission arrays can be used as an efficient, ultrahigh vacuum compatible method for in situ cleaning of the field emitter arrays.
Investigation of postoxidation thermal treatments of Si/SiO2 interface in relationship to the kinetics of amorphous Si suboxide decomposition
Authors: Hinds B. J. , Wang F. , Wolfe D. M. , Hinkle C. L. , Lucovsky G.
Published: July 2002

Interfacial Si suboxides (SiOx, x<2) are detrimental to transistor performance and are typically minimized during postoxidation anneals. To study the kinetics of SiOx decomposition, thick films (∼2000 Å) of amorphous Si:O:H alloys (0.7<x<1.4) were deposited by remote plasma enhanced chemical vapor deposition and subjected to rapid thermal anneals. Films were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, ellipsometry, photoluminescence (PL), and transmission electron microscopy. At temperatures >500 °C initially there is a rapid segregation into amorphous Si (a-Si) surrounded by a SiO2 shell which acts as a diffusion barrier decelerating the reaction. Phenomenological modeling of kinetics with a one-dimensional Avrami–Erofe’ve treatment gives an upper limit for a-Si lateral growth rates of 1.2 Å/s at 900 °C with an activation energy of 120 kJ/mol. PL, Raman, transmission electron microscopy and ellipsometry confirm this segregation model in the amorphous state. Due to the rapid initial decomposition and relatively large diffusion coefficients, a simple kinetic hindrance explanation for the 4–8 Å of SiOx at the SiO2/Si interface is unlikely.
Surface modified spin-on xerogel films as interlayer dielectrics
Authors: Nitta S. V. , Pisupatti V. , Jain A. , Wayner P. C. , Gill W. N. , Plawsky J. L.
Published: July 2002

SiO 2 -based xerogels are highly porous materials that may enhance the performance of microelectronic devices due to their extremely low dielectric constants (ε=1.36–2.2). Conventional xerogel and aerogel manufacturing techniques include an expensive and hazardous supercritical drying step to deposit crack free, high porosity films. Ambient drying techniques have recently been developed and in this article, we discuss how the process parameters in the ambient drying process affect the properties of a spin-coated film. Successful spin-on deposition of highly porous (>70%), thick (>1 μm), crack-free, xerogel films was accomplished using a solvent saturated atmosphere during spinning and aging. The saturated atmosphere allowed for the isolation of each processing step and a better understanding of the effects of process variable changes. The film porosity was controlled by varying the extent of silylation (surface modification), the aging time, or the initial water/silane ratio. Fourier transform infrared spectra demonstrated that silylation of xerogel films helps eliminate bound moisture in these films and renders them hydrophobic. Finally, the dielectric constants extrapolated from refractive index measurements were in good agreement with those obtained from our conventional electrical measurements.
Field emission characteristics of diamond films with different surface morphologies
Authors: Ji H. , Jin Z. S. , Wang J. Y. , Lu X. Y. , Gu C. Z. , Liu B. B. , Jin W. C. , Gao C. X. , Yuan G. , Wang W. B.
Published: July 2002

The field emission characteristics of diamond films with different surface morphologies were studied. The diamond films were grown on silicon substrates by chemical vapor deposition technique under different deposition conditions. The nucleation density and surface morphological properties were analyzed by means of scanning electron microscopy and atomic force microscopy. Results from these studies showed that the diamond film with small crystal size and high nucleation densities had better field emission characteristics. In addition, (110) and (111) oriented films exhibited better field emission properties than (100) oriented films.
AlN and GaN epitaxial heterojunctions on 6H–SiC(0001): Valence band offsets and polarization fields
Authors: Rizzi Angela , Lantier Roberta , Monti Fulvia , Lüth Hans , Sala Fabio Della , Di Carlo Aldo , Lugli Paolo
Published: July 2002

From a series of in situ photoemission experiments macroscopic electric fields are clearly demonstrated in SiC/AlN, SiC/GaN, and GaN/AlN heterostructures grown by molecular beam epitaxy on 6H–SiC(0001). A significant contribution is due to the spontaneous polarization; the piezoelectric term alone would not explain the sign of the field measured in SiC/AlN. The experimental field has lower intensity as compared to theory: the role of electronic gap states at the surface is pointed out. A self-consistent tight-binding approach which is able to describe polarization fields, dielectric screening, and free carrier screening is applied for a more consistent theoretical discussion of the experimental data. The valence band offset (VBO) has been determined for all heterojunctions under study and the apparent dependence on the overlayer thickness, due to the presence of the strong polarization fields, has been pointed out in view of a correct determination of the VBO. The VBOs at the heterojunctions obtained by extrapolation to zero overlayer thickness are (substrate/overlayer): [(1.5–1.7)±0.1] eV for SiC/AlN, [(0.7–0.9)±0.1] eV for SiC/GaN, (−0.3±0.1) eV for AlN/GaN, and [(0.15–0.4)±0.1] eV for GaN/AlN.
Field induced local oxidation of Ti and Ti/Au structures by an atomic force microscope with diamond coated tips
Authors: Vullers R. J. M. , Ahlskog M. , Cannaerts M. , Van Haesendonck C.
Published: July 2002

We have investigated field induced local oxidation of thin Ti films with the tip of an atomic force microscope. Tips, which have been coated with a diamond layer to improve their wear resistance, are shown to have a much longer lifetime than conventional uncoated Si tips. We have studied the oxidation characteristics as a function of the applied tip-sample voltage and scanning speed for both diamond coated and uncoated tips. We find that the diamond coated tips result in a thinner oxide layer for the same voltage and scanning speed. The dependence of the oxidation process on the film thickness was studied for diamond coated tips. Thin films can be completely transformed into an oxide layer for a thickness up to 15 nm. Moreover, for these sufficiently thin films the measured ratio between the oxide height and the Ti film thickness is a constant. It is also possible to completely oxidize Ti films which cover Au islands, opening the way to fabricate more complicated structures.
Fabrication of dissimilar metal electrodes with nanometer interelectrode distance for molecular electronic device characterization
Authors: Guillorn Michael A. , Carr Dustin W. , Tiberio Richard C. , Greenbaum Elias , Simpson Michael L.
Published: July 2002

We report a versatile process for the fabrication of dissimilar metal electrodes with a minimum interelectrode distance of less than 6 nm using electron beam lithography and liftoff pattern transfer. This technique provides a controllable and reproducible method for creating structures suited for the electrical characterization of asymmetric molecules for molecular electronics applications. Electrode structures employing pairs of Au electrodes and non-Au electrodes were fabricated in three different patterns. Parallel electrode structures 300 μm long with interelectrode distances as low as 10 nm, 75 nm wide electrode pairs with interelectrode distances less than 6 nm, and a multiterminal electrode structure with reproducible interelectrode distances of 8 nm were realized using this technique. The processing issues associated with the fabrication of these structures are discussed along with the intended application of these devices.
Midinfrared type-II interband cascade lasers
Authors: Bradshaw J. L. , Bruno J. D. , Pham J. T. , Wortman D. E. , Yang Rui Q.
Published: July 2002

We describe the progress made in the development of type-II interband cascade lasers emitting in the midinfrared (3.8–3.9 μm) spectral region. Three nominally similar molecular beam epitaxy growths on substrates from different vendors show qualitatively different electrical and optical properties. The devices tested show significant improvements over previously reported results with respect to differential external quantum efficiency (∼500%), peak power (>4 W/facet), peak power conversion efficiency (∼7%), maximum operating temperature (217 K), and continuous-wave operation.
A Proteolytic Transmembrane Signaling Pathway and Resistance to β-Lactams in Staphylococci
Authors: Zhang H. Z. , Hackbarth C. J. , Chansky K. M. , Chambers H. F.
Published: July 2002

β-Lactamase and penicillin-binding protein 2a mediate staphylococcal resistance to β-lactam antibiotics, which are otherwise highly clinically effective. Production of these inducible proteins is regulated by a signal-transducing integral membrane protein and a transcriptional repressor. The signal transducer is a fusion protein with penicillin-binding and zinc metalloprotease domains. The signal for protein expression is transmitted by site-specific proteolytic cleavage of both the transducer, which autoactivates, and the repressor, which is inactivated, unblocking gene transcription. Compounds that disrupt this regulatory pathway could restore the activity of β-lactam antibiotics against drug-resistant strains of staphylococci.
Single Crystals of Single-Walled Carbon Nanotubes Formed by Self-Assembly
Authors: Schlittler R. R. , Seo J. W. , Gimzewski J. K. , Durkan C. , Saifullah M. S. M. , Welland M. E.
Published: July 2002

We report the self-assembly of single crystals of single-walled carbon nanotubes (SWCNTs) using thermolysis of nano-patterned precursors. The synthesis of these perfectly ordered, single crystals of SWCNTs results in extended structures with dimension on the micrometer scale. Each crystal is composed of an ordered array of tubes with identical diameters and chirality, although these properties vary between crystals. The results show that SWCNTs can be produced as a perfect bulk material on the micrometer scale and point toward the synthesis of bulk macroscopic crystalline material.
Parent-Offspring Coadaptation and the Dual Genetic Control of Maternal Care
Authors: Agrawal Aneil F. , Brodie Edmund D. , Brown Jeremy
Published: July 2002

In many animal species, the amount of care provided by parents is determined through a complex interaction of offspring signals and responses by parents to those signals. As predicted by honest signaling theory, we show that in the burrower bug,
Sehirus cinctus
, maternal provisioning responds to experimental manipulations of offspring condition. Despite this predicted environmental influence, we find evidence from two cross-foster experiments that variation in maternal care also stems from two distinct genetic sources: variation among offspring in their ability to elicit care and variation among parents in their response to offspring signals. Furthermore, as predicted by maternal-offspring coadaptation theory, offspring signaling is negatively genetically correlated with maternal provisioning.
Predicting the Mesophases of Copolymer-Nanoparticle Composites
Authors: Thompson Russell B. , Ginzburg Valeriy V. , Matsen Mark W. , Balazs Anna C.
Published: July 2002

The interactions between mesophase-forming copolymers and nanoscopic particles can lead to highly organized hybrid materials. The morphology of such composites depends not only on the characteristics of the copolymers, but also on the features of the nanoparticles. To explore this vast parameter space and predict the mesophases of the hybrids, we have developed a mean field theory for mixtures of soft, flexible chains and hard spheres. Applied to diblock-nanoparticle mixtures, the theory predicts ordered phases where particles and diblocks self-assemble into spatially periodic structures. The method can be applied to other copolymer-particle mixtures and can be used to design novel composite architectures.
Sensing Pathogens and Tuning Immune Responses
Authors: Pulendran Bali , Palucka Karolina , Banchereau Jacques
Published: July 2002

The immune system is capable of making qualitatively distinct responses against different microbial infections, and recent advances are starting to reveal how it manages this complex task. An integral component of the immune system is a network of cells known as dendritic cells (DCs), which sense different microbial stimuli and convey this information to lymphocytes. A better understanding of DC biology has allowed a model to be constructed in which the type of immune response to an infection is viewed as a function of several determinants, including the subpopulation of DCs, the nature of the microbe, microbe recognition receptors, and the cytokine microenvironment.
Observation of Charge Transport by Negatively Charged Excitons
Authors: Sanvitto Daniele , Pulizzi Fabio , Shields Andrew J. , Christianen Peter C. M. , Holmes Stuart N. , Simmons Michelle Y. , Ritchie David A. , Maan Jan C. , Pepper Michael
Published: July 2002

We report transport of electron-hole complexes in semiconductor quantum wells under applied electric fields. Negatively charged excitons (X
–
), created by laser excitation of a high electron mobility transistor, are observed to drift upon applying a voltage between the source and drain. In contrast, neutral excitons do not drift under similar conditions. The X
–
mobility is found to be as high as 6.5 × 10
4
cm
2
V
−1
s
−1
. The results demonstrate that X
–
exists as a free particle in the best-quality samples and suggest that light emission from opto-electronic devices can be manipulated through exciton drift under applied electric fields.
Bulk-Like Features in the Photoemission Spectra of Hydrated Doubly Charged Anion Clusters
Authors: Wang Xue-Bin , Yang Xin , Nicholas John B. , Wang Lai-Sheng
Published: July 2002

We produced gaseous hydrated clusters of sulfate and oxalate anions [SO
4
2–
-(H
2
O)

n

and C
2
O
4
2–
(H
2
O)

n

, where
n
= 4 to 40]. Photoelectron spectra of these clusters revealed that the solute dianions were in the center of the water cluster, (H
2
O)

n

. For small clusters, these spectra were characteristic of the respective solutes, but beyond the first solvation shell (
n
≈ 12), features in the spectra from the solutes were diminished and a new feature from ionization of water emerged, analogous to bulk aqueous solutions. For large clusters with dimensions greater than 1 nanometer, the solute photoemission features disappeared and the spectra were dominated by the ionization of water as the solvent coverage increased. A smooth transition from gas-phase clusters to behavior of electrolyte solutions was clearly revealed, and the large solvated clusters can be used as molecular models to investigate the photophysics and chemistry of aqueous electrolyte solutions.
Calcium, Calmodulin, and CaMKII Requirement for Initiation of Centrosome Duplication in
Xenopus
Egg Extracts
Authors: Matsumoto Yutaka , Maller James L.
Published: July 2002

Aberrant centrosome duplication is observed in many tumor cells and may contribute to genomic instability through the formation of multipolar mitotic spindles. Cyclin-dependent kinase 2 (Cdk2) is required for multiple rounds of centrosome duplication in
Xenopus
egg extracts but not for the initial round of replication. Egg extracts undergo periodic oscillations in the level of free calcium. We show here that chelation of calcium in egg extracts or specific inactivation of calcium/calmodulin–dependent protein kinase II (CaMKII) blocks even initial centrosome duplication, whereas inactivation of Cdk2 does not. Duplication can be restored to inhibited extracts by addition of CaMKII and calmodulin. These results indicate that calcium, calmodulin, and CaMKII are required for an essential step in initiation of centrosome duplication. Our data suggest that calcium oscillations in the cell cycle may be linked to centrosome duplication.
Quantum Impurities in the Two-Dimensional Spin One-Half Heisenberg Antiferromagnet
Authors: Vajk O. P. , Mang P. K. , Greven M. , Gehring P. M. , Lynn J. W.
Published: July 2002

The study of randomness in low-dimensional quantum antiferromagnets is at the forefront of research in the field of strongly correlated electron systems, yet there have been relatively few experimental model systems. Complementary neutron scattering and numerical experiments demonstrate that the spin-diluted Heisenberg antiferromagnet La
2
Cu

1–
z

(Zn,Mg)

z

O
4
is an excellent model material for square-lattice site percolation in the extreme quantum limit of spin one-half. Measurements of the ordered moment and spin correlations provide important quantitative information for tests of theories for this complex quantum-impurity problem.
The Kinetic Significance of V
5+
in
n
-Butane Oxidation Catalyzed by Vanadium Phosphates
Authors: Coulston George W. , Bare Simon R. , Kung Harold , Birkeland Kari , Bethke Gregory K. , Harlow Richard , Herron Norman , Lee Peter L.
Published: July 2002

Maleic anhydride, a precursor to polyester resins, is made by oxidation of
n
-butane over vanadium phosphate catalysts. This system is of general interest because it is the only heterogeneously catalyzed, alkane-selective oxidation reaction in commercial use. Time-resolved in situ x-ray absorption spectroscopy shows that when either α
I
-VOPO
4
/SiO
2
or (VO)
2
P
2
O
7
/SiO
2
catalysts are exposed to
n
-butane, the rate of maleic anhydride formation is proportional to the rate of decay of V
5+
species in the catalyst. Thus V
5+
species are kinetically significant for the production of maleic anhydride and not just for the production of by-products. The results also suggest that V
5+
species may play a role in the initial hydrogen abstraction from
n
-butane, the rate-determining step in the reaction sequence. V
4+
sites appear to be responsible for by-product formation.
Receptor and βγ Binding Sites in the α Subunit of the Retinal G Protein Transducin
Authors: Onrust René , Herzmark Paul , Chi Patty , Garcia Pablo D. , Lichtarge Olivier , Kingsley Chris , Bourne Henry R.
Published: July 2002

Transmembrane receptors for hormones, neurotransmitters, light, and odorants mediate their cellular effects by activating heterotrimeric guanine nucleotide-binding proteins (G proteins). Crystal structures have revealed contact surfaces between G protein subunits, but not the surfaces or molecular mechanism through which Gαβγ responds to activation by transmembrane receptors. Such a surface was identified from the results of testing 100 mutant α subunits of the retinal G protein transducin for their ability to interact with rhodopsin. Sites at which alanine substitutions impaired this interaction mapped to two distinct Gα surfaces: a βγ-binding surface and a putative receptor-interacting surface. On the basis of these results a mechanism for receptor-catalyzed exchange of guanosine diphosphate for guanosine triphosphate is proposed.
Reduced Ubiquitin-Dependent Degradation of c-Jun After Phosphorylation by MAP Kinases
Authors: Musti Anna Maria , Treier Mathias , Bohmann Dirk
Published: July 2002

The proto-oncogene-encoded transcription factor c-Jun activates genes in response to a number of inducers that act through mitogen-activated protein kinase (MAPK) signal transduction pathways. The activation of c-Jun after phosphorylation by MAPK is accompanied by a reduction in c-Jun ubiquitination and consequent stabilization of the protein. These results illustrate the relevance of regulated protein degradation in the signal-dependent control of gene expression.
Atomistic Simulation of Shock Wave-Induced Melting in Argon
Authors: Belonoshko A. B.
Published: July 2002

A three-dimensional molecular dynamics simulation of shock wave loading was undertaken to investigate the Hugoniot equation of state at the transition of argon from solid to liquid. The simulated data agree with shock wave and static high-pressure experimental data. The melting transition in this simulation occurs without overshooting the argon melting temperature. There are two discontinuities that may bracket a mixed-phase region of solid and liquid along the simulated argon Hugoniot. This is an intrinsic feature of the Hugoniot crossing the argon melting curve and does not require the addition of any solid-solid phase transition.
Flexibility in DNA Recombination: Structure of the Lambda Integrase Catalytic Core
Authors: Kwon Hyock Joo , Tirumalai Radhakrishna , Landy Arthur , Ellenberger Tom
Published: July 2002

Lambda integrase is archetypic of site-specific recombinases that catalyze intermolecular DNA rearrangements without energetic input. DNA cleavage, strand exchange, and religation steps are linked by a covalent phosphotyrosine intermediate in which Tyr
342
is attached to the 3-phosphate of the DNA cut site. The 1.9 angstrom crystal structure of the integrase catalytic domain reveals a protein fold that is conserved in organisms ranging from archaebacteria to yeast and that suggests a model for interaction with target DNA. The attacking Tyr
342
nucleophile is located on a flexible loop about 20 angstroms from a basic groove that contains all the other catalytically essential residues. This bipartite active site can account for several apparently paradoxical features of integrase family recombinases, including the capacity for both cis and trans cleavage of DNA.
Regulation of Lipid A Modifications by
Salmonella typhimurium
Virulence Genes
phoP-phoQ
Authors: Guo Lin , Lim Kheng B. , Gunn John S. , Bainbridge Brian , Darveau Richard P. , Hackett Murray , Miller Samuel I.
Published: July 2002

Bacterial pathogenesis requires proteins that sense host microenvironments and respond by regulating virulence gene transcription. For Salmonellae, one such regulatory system is PhoP-PhoQ, which regulates genes required for intracellular survival and resistance to cationic peptides. Analysis by mass spectrometry revealed that
Salmonella typhimurium
PhoP-PhoQ regulated structural modifications of lipid A, the host signaling portion of lipopolysaccharide (LPS), by the addition of aminoarabinose and 2-hydroxymyristate. Structurally modified lipid A altered LPS-mediated expression of the adhesion molecule E-selectin by endothelial cells and tumor necrosis factor–α expression by adherent monocytes. Thus, altered responses to environmentally induced lipid A structural modifications may represent a mechanism for bacteria to gain advantage within host tissues.
High Critical-Current Density in the Heavily Pb-Doped Bi
2
Sr
2
CaCu
2
O
8+δ
Superconductor: Generation of Efficient Pinning Centers
Authors: Chong I. , Hiroi Z. , Izumi M. , Shimoyama J. , Nakayama Y. , Kishio K. , Terashima T. , Bando Y. , Takano M.
Published: July 2002

Critical-current density (
J
c
) is a parameter of primary importance for potential applications of high-temperature copper oxide superconductors. It is limited principally by the breakdown of zero-resistive current due to thermally activated flux flow at high temperatures and high magnetic fields. One promising method to overcome this limitation is to introduce efficient pinning centers into crystals that can suppress the flux flow. A marked increase in
J
c
was observed in Bi
2
Sr
2
CaCu
2
O
8+δ
(Bi-2212) single crystals doped with a large amount of Pb. By electron microscopy, characteristic microstructures were revealed that probably underlie the observed enhancement in
J
c
: thin (10 to 50 nanometers), platelike domains having a modulation-free structure appeared with spacings of 50 to 100 nanometers along the
b
axis.
Anomalous Behavior of Sound Velocity and Attenuation in Liquid Fe-Ni-S
Authors: Nasch Philippe M. , Manghnani Murli H. , Secco Richard A.
Published: July 2002

The compressional wave velocity in molten iron containing 5 percent nickel and 10 percent sulfur increased with increasing temperature. This anomalous behavior was determined to be attributable to the presence of sulfur, which conditions the formation of large molecular units in the liquid. Temperature-induced breakup of macromolecular units resulted in greater packing efficiency and an increased velocity. Ten percent sulfur increased attenuation by one to two orders of magnitude compared with liquid iron. Such behavior at outer core pressures and temperatures would constrain the velocity gradient in the outer core and would enable the discrimination of potential light alloying elements.
Dictyostelium
Development in the Absence of cAMP
Authors: Wang Bin , Kuspa Adam
Published: July 2002

Adenosine 3′,5′-monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) are regulators of development in many organisms.
Dictyostelium
uses cAMP as an extracellular chemoattractant and as an intracellular signal for differentiation. Cells that are mutant in adenylyl cyclase do not develop. Moderate expression of the catalytic subunit of PKA in adenylyl cyclase–null cells led to near-normal development without detectable accumulation of cAMP. These results suggest that all intracellular cAMP signaling is effected through PKA and that signals other than extracellular cAMP coordinate morphogenesis in
Dictyostelium.
The World's Smallest Gas Cylinders?
Authors: Gadd G. E. , Blackford M. , Moricca S. , Webb N. , Evans P. J. , Smith A. M. , Jacobsen G. , Leung S. , Day A. , Hua Q.
Published: July 2002

Argon gas was trapped at high pressure within hollow carbon tubes grown in vapor that have an outer diameter of between 20 and 150 nanometers. The gas was forced into the tubes by hot isostatically pressing (HIPing) the carbon material for 48 hours at 650°C under an argon pressure of 170 megapascals. Energy dispersive x-ray spectroscopy maps and line scans across the tubes show that the argon is trapped inside the bore and not in the tube walls. The room temperature argon pressure in these tubes was estimated to be about 60 megapascals, which indicates that equilibrium pressure was attained within the tubes at the HIPing temperature. These findings demonstrate the potential for storing gases in such carbon structures.
An Electrochemical Coulomb Staircase: Detection of Single Electron-Transfer Events at Nanometer Electrodes
Authors: Fan Fu-Ren F. , Bard Allen J.
Published: July 2002

The current-potential curve at a 2.8-nanometer-radius electrode immersed in a solution containing micromolar concentrations of a redox couple, [(trimethylammonio)methyl] ferrocene (Cp
2
FeTMA
+
)/Cp
2
FeTMA
2+
, shows discrete steps (a coulomb staircase) representing single electron-transfer events. Discrete potential steps are also found in the current-time curve as the electrode potential relaxes back to equilibrium after a coulostatic potential step.
Preserving Biodiversity in Korea's Demilitarized Zone
Authors: Kim Ke Chung
Published: July 2002

Amidst international tensions and military posturing, the Demilitarized Zone (DMZ) separating North and South Korea has, for 45 years, provided sanctuary to endangered and threatened animals and plants. The DMZ has been rigidly enforced: It is uninhabited by humans, and its inaccessibility has allowed damaged forests to rehabilitate and farmlands that are thousands of years old to return to a natural state. The DMZ has, in fact, become a unique nature reserve containing the last vestiges of Korea's natural heritage. The Korean Peace Bioreserves System provides a strategy to preserve the rich biodiversity of the DMZ. Joint development of the Korean Peace Bioreserves System will foster trust, understanding, and respect between the Democratic People's Republic of Korea in the north and the Republic of Korea in the south.
Multistep Control of Pituitary Organogenesis
Authors: Sheng Hui Z. , Moriyama Kenji , Yamashita Tsuyoshi , Li Hung , Potter S. Steven , Mahon Kathleen A. , Westphal Heiner
Published: July 2002

Lhx3
and
Lhx4
(
Gsh4
), two closely related LIM homeobox genes, determine formation of the pituitary gland in mice. Rathke's pouch is formed in two steps—first as a rudiment and later as a definitive pouch.
Lhx3
and
Lhx4
have redundant control over formation of the definitive pouch.
Lhx3
controls a subsequent step of pituitary fate commitment. Thereafter,
Lhx3
and
Lhx4
together regulate proliferation and differentiation of pituitary-specific cell lineages. Thus,
Lhx3
and
Lhx4
dictate pituitary organ identity by controlling developmental decisions at multiple stages of organogenesis.