bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of hydrogen during rapid vapor-phase doping analyzed by x-ray photoelectron spectroscopy and Fourier-transform infrared-attenuated total reflection
Authors: Kiyota Yukihiro , Yano Fumiko , Suzuki Seiji , Inada Taroh
Published: July 2002

The surface of boron-doped layers formed by rapid vapor-phase doping was analyzed by x-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared-attenuated total reflection (FTIR-ATR), to determine the role of the hydrogen carrier gas. Boron doping was carried out with a B2H6 source gas and a hydrogen carrier gas at 800 and 900 °C. A nitrogen carrier gas was also used for comparison. Using hydrogen carrier gas, no evidence of boron segregation was observed in the XPS spectra. FTIR-ATR analysis confirmed that the hydrogen termination of the surface was maintained during doping. Using nitrogen carrier gas, layers that included segregated boron and silicon nitride were produced on the surface, which led to poor controllability of the boron concentration. When a hydrogen carrier gas is used, the hydrogen termination should promote the surface migration of adsorbed species. The hydrogen carrier gas plays an important role in terminating the silicon dangling bonds, thus preventing excessive chemisorption of boron.
Modeling of the deposition of stoichiometric Al2O3 using nonarcing direct current magnetron sputtering
Authors: Macák K. , Nyberg T. , Macák P. , Kharrazi Olsson M. , Helmersson U. , Berg S.
Published: July 2002

dc sputter deposition of stoichiometric Al2O3 is usually difficult due to the formation of an oxidized layer on the target surface, which reduces the deposition rate drastically and causes charge buildup and arcing at the target. To avoid this situation the arrival rate ratio O2/Al must be high enough at the substrate position that a stoichiometric film can form but low enough at the target that a conducting target surface is maintained. We have utilized Monte Carlo simulations to estimate the flux distribution of sputtered particles for different geometries. These results, supplemented by Bergs’ standard steady state model for the reactive sputtering process, made it possible to predict the composition at different surfaces in the processing chamber. Experimental studies were carried out for several different target-to-substrate distances and a range of sputtering gas pressures. The results show that the process can be tailored to achieve stoichiometric Al2O3 at the substrates while keeping the target in the metallic state. This is achieved by providing a high enough inert gas pressure or large enough target-to-substrate distance. Thick stoichiometric Al2O3 films were successfully deposited at these conditions with low or no arcing during deposition. A considerable back-deposition of Al on to the noneroded part of the target keeping the surfaces conducting is the key factor for the reduction of arcing.
Structural, optical and electrical characteristics of yttrium oxide films deposited by laser ablation
Authors: Araiza J. J. , Cardenas M. , Falcony C. , Mendez-Garcia V. H. , Lopez M. , Contreras-Puente G.
Published: July 2002

Yttrium oxide films deposited by laser ablation on Si(100) substrates have been evaluated for possible application as high dielectric constant insulator layers. The structural, optical and electrical characteristics of these films have been determined by means of x-ray diffraction, atomic force microscopy, Auger electron spectroscopy, ellipsometry, infrared transmittance, and capacitance and current versus voltage (C-V and I-V) as a function of the deposition parameters. It has been found that the crystallinity of these films is strongly dependent on the substrate temperature during the deposition. At low deposition temperatures (350 °C) the films present an amorphous component and a polycrystalline part that is highly oriented in the (100) direction perpendicular to the surface of the film. As the deposition temperature is increased up to 650 °C, the amorphous part of the films is reduced drastically and the dominant orientation of the polycrystalline part becomes the (111) orientation. The refractive index at 630 nm was found to be in the range between 1.91 and 1.95, having its maximum value for films deposited at 450 °C. Similar behavior was observed for the dielectric constant κ, measured from the high frequency capacitance measurements, with a maximum value of 15 obtained for the above mentioned deposition temperature as well. The average roughness of the films decreases with substrate temperature from ∼30 Å to less than 5 Å in the range of temperatures studied.
Target compound layer formation during reactive sputtering
Authors: Jonsson L. B. , Nyberg T. , Berg S.
Published: July 2002

It is well known that a compound layer may form at the target surface during reactive sputtering. However, the significance of this layer for the response to a change in target conditions has so far not been carefully investigated. The standard model for the reactive sputtering process [S. Berg et al., J. Vac. Sci. Technol. A 5, 202 (1987)] does not allow for calculations of the compound thickness at the target surface. For simplicity it has been assumed that a single monolayer is responsible for the poisoning of the target. However, experiments clearly indicate that the compound layer thickness may be significantly thicker than one monolayer. For several reasons it is important to be able to quantify the thickness of this layer. The formation of the compound layer introduces memory effects into the system when the processing conditions are changed. The sputter erosion time for the compound layer depends on its thickness. We will present an extension of the basic reactive sputtering model that allows for the formation of an arbitrary thickness of the compound layer at the target surface. From this model it is possible to examine the layer thickness dependence on the major processing parameters (reactive gas supply, target current, etc.). Experimental studies of the transient target voltage and process pressure support the validity of the new model.
Spectrophotometric analysis of aluminum nitride thin films
Authors: Joo Han-Yong , Kim Hyeong Joon , Kim Sang June , Kim Sang Youl
Published: July 2002

The optical functions of AlN thin films deposited on the quartz substrates by the reactive radio frequency magnetron sputtering, such as refractive index, extinction coefficient, optical band gap, and film thickness were determined from the transmittance and reflectance spectra in the range of 190–820 nm. For these analyses, an inverse synthesis method was established after literature survey. The results were doublechecked with a modified envelope method, and compared with those of previous reports. Spectroscopic ellipsometry analyses were performed to confirm the accuracy of the methods. Refractive indices of AlN films in this study were in the range of 1.95–2.05 at 633 nm and 2.26–2.38 at 250 nm, depending on the preparation conditions. The extinction coefficients were small (<5×10−4) and nearly constant at low energy region (<2 eV), but exhibited various dispersion features at 2.2–3.5 eV, indicating different amount and kinds of defects of AlN films. The absorption coefficient at near-band-gap energy exhibited a strong thickness dependence, resulting in various optical band-gap energies ranging 5.34–5.71 eV.
Laser infrared photothermal radiometry of electronic solids: Principles and applications to industrial semiconductor Si wafers
Authors: Mandelis Andreas , Riopel Yan
Published: July 2002

The physical principles and application case studies of the novel diagnostic technique of laser infrared photothermal radiometry (PTR) of semiconductors are presented. Following superband gap optical excitation, the signal consists of two contributions, one due to the de-exciting carrier density (plasma wave) and another from direct absorption and heating of the lattice (thermal wave). Multiparameter fits to frequency-domain amplitude and phase data have been developed to reliably measure recombination lifetime, τ surface recombination velocities (front and back surface), electronic, and thermal diffusivities. Applications case studies are presented, which demonstrate that lifetime measurements using PTR provide a most sensitive, convenient, and nonintrusive, remote industrial semiconductor metrology. The new metrology combines the features of several laboratory and commercial techniques currently available for industrial wafer (substrate and process) characterization (e.g., thermoreflectance, microwave reflectance, and surface photovoltage). The technology is capable of being used as a sensitive control of ion implantation, contamination monitor during oxidation and wafer cleans, and photoexcited carrier recombination lifetime measurements.
Optimized structural properties of wurtzite GaN on SiC(0001) grown by molecular beam epitaxy
Authors: Ramachandran V. , Feenstra R. M. , Sarney W. L. , Salamanca-Riba L. , Greve D. W.
Published: July 2002

We have investigated the optimal conditions for molecular beam epitaxial growth of high quality GaN on 6H-SiC(0001) substrates. The quality of these films is reflected both by the narrow x-ray peak widths as well as the excellent surface morphology. In this work, it is shown that increasing growth temperature leads to an improvement in bulk quality and lower x-ray peak widths for both symmetric and asymmetric reflections. We also note a marked improvement in surface morphology, from a columnar appearance to a two-dimensional surface, under extremely Ga-rich growth conditions.
A new infrared detector using electron emission from multiple quantum wells
Authors: Smith J. S. , Chiu L. C. , Margalit S. , Yariv A. , Cho A. Y.
Published: July 2002

A new type of infrared photodetector using free electron absorption in a heavily doped GaAs/GaAlAs quantum well structure has been demonstrated. Preliminary results indicate a strong response in the near infrared with a responsivity conservatively estimated at 200 A/W. The structure can potentially be tailored during fabrication for use in several infrared bands of interest, including the 3 to 5 micron band and the 8 to 10 micron band.
Optical transmission in GaSb–AlSb superlattices
Authors: Voisin P. , Bastard G. , Voos M. , Mendez E. E. , Chang C.-A. , Chang L. L. , Esaki L.
Published: July 2002

We present near band gap optical transmission experiments in a series of GaSb–AlSb superlattices grown by molecular beam epitaxy. The spectra exhibit a steplike behavior characteristic of two-dimensional electron systems. The absorption steps are attributed to transitions between valence and conduction states confined in the GaSb quantum wells. However, the energy of the absorption edge is smaller than that expected for a simple well, probably as a result of the effect of strain.
Enhanced brightness x-ray source
Authors: Leslie B. , Neukermans A. , Simon T. , Foster J.
Published: July 2002

An improved source for x-ray lithography is described which is capable of significantly enhanced brightness through an improved cooling system. In the normal Gaines source which has an inverted cone shaped anode, the cone walls are straight, and the water flows in a narrow channel coaxial with the anode. Anode cooling occurs with water in the nucleate boiling regime, and maximum heat flux is set by the critical density of vapor bubbles at which coalescence occurs. By providing the flow of water around the anode with a concave upstream curvature, huge inertial forces create instantaneous phase separation. As a result the power density at which burnout occurs is substantially increased and moreover, significantly stabilized. In addition the use of a shaped anode allows for a self-guiding jet with no channel, which eases the mechanical design considerably. Measurements of the anode temperature excursion indicate a temperature rise of 450 °C with 6.2 kW in a 3 mm spot (25 kW/cm2). It is anticipated that fluxes two to three times higher may be accommodated. The Pd Lα line output (2.84 keV) was measured as a function of the incident power. It is found that Pd Lα generation does not follow the common power law for Kα radiation of most materials. The behavior can be explained in terms of the shallow incident beam angle and radiation take off angle. The absolute conversion efficiency of Pd Lα was measured at 20 keV and found to be 34 μW/W-sr in this gun configuration.
Heterojunction band off-sets: Variation with ionization potential compared to experiment
Authors: Kraut Edgar A.
Published: July 2002

Semiconductor valence-band edges are computed on absolute energy scales determined by Herman-Skillman and Hartree–Fock neutral atom ionization energies, respectively. These calculations use Harrison’s (1977) elementary theory of heterojunctions. Differences are compared with experimental heterojunction band discontinuities. It is shown that calculations based on Herman–Skillman neutral atom energies and Harrison’s original solid-state matrix elements agree somewhat better with experiment than later results (1981) based on Hartree–Fock neutral atom energies and modified solid-state matrix elements. Although no simple theory for heterojunction core-level discontinuities seems to exist, it is shown that the experimentally measured heterojunction core-level discontinuities are roughly equal to the binding energy differences of fully relaxed neutral atom core levels such as those calculated by Huang and co-workers.
Transition metals on GaAs(110): A case for extrinsic surface states
Authors: Hughes G. , Ludeke R. , Schäffler F. , Rieger D.
Published: July 2002

The interactions of transition metal overlayers with the GaAs(110) cleaved surface have been investigated using core level photoemission spectroscopy. The appearance of chemically shifted core level components for coverages of a fraction of a monolayer indicate that both Mn and V overlayers interact strongly with the cleaved surface. At higher coverages there is evidence for the outdiffusion of both Ga and As into the metal overlayer. Valence band spectra for the V overlayer exhibit strong emission in the semiconductor band gap associated with d-electron states of the transition metal atoms. The onset of this emission coincides with the position of the surface Fermi level, which suggests that these states are responsible for Fermi level pinning. Photoemission core level studies suggest that the origin of the interface states is associated with an exchange reaction of the metal and the gallium. The notion that a transition metal impurity state is responsible for the pinning of the Fermi level at the interface is formulated and supported by the experimental results. Fermi level shifts for both n- and p-type crystals were investigated over a broad coverage range. The pinning position in the band gap was found to be identical for both n- and p-type crystals, however the final barrier heights varied for the different transition metals. Schottky barrier heights of 0.75 (0.68)±0.05 eV for V and 0.84 (0.59)±0.05 eV for Mn were measured for n- and p-type substrates.
Trends in temperature-dependent Schottky barrier formation: The Ga/GaAs and Mn/GaAs interfaces
Authors: Stiles K. , Horng S. F. , Kahn A. , McKinley J. , Kilday D. G. , Margaritondo G.
Published: July 2002

We use soft x-ray photoemission spectroscopy to study the Ga/GaAs and Mn/GaAs interfaces formed at room temperature (RT) and at low temperature (LT=80−100 K). The LT Ga/GaAs interface exhibits the strongly inhibited clustering observed at other nonreactive metal/GaAs interfaces. The Mn–GaAs reaction is slowed somewhat. At both interfaces, the effect of temperature on EF movement is in accord with the trends previously set: initial band bending on n-GaAs is slowed at LT, whereas band bending on p-GaAs is not affected significantly. For both Ga and Mn/GaAs, EF movement at low coverages is consistent with the explanation proposed for the sp and noble metals: introduction of donor states from, e.g., the specific adsorbate levels. Studies of the noble metal interfaces with GaAs have showed that the establishment of the final EF position is related to the appearance of metallicity at the interface; Mn/GaAs provides direct evidence for that interpretation.
Electric field distributions in a molecular-beam epitaxy Ga0.83Al0.17As/GaAs/GaAs structure using photoreflectance
Authors: Shen H. , Pollak F. H. , Woodall J. M. , Sacks R. N.
Published: July 2002

We have studied the photoreflectance (PR) spectra from a molecular-beam epitaxially (MBE) grown heterostructure consisting of 200 nm of Ga0.83Al0.17As , a 800-nm GaAs buffer layer on a semi-insulating (SI) 〈100〉 (LEC) GaAs substrate. By varying both the pump beam wavelength and modulation frequency we are able to identify the component layers, their quality, and the quality of the various interfaces. In this study we find evidence for a low density of interface states between the GaAs buffer layer and the GaAlAs layer, and a relatively large density of interface states, between the substrate and buffer layers. These states, previously observed by deep-level transient spectroscopy (DLTS) of doped structures, are presumably associated with the interface produced by MBE growth on etched and air exposed substrates. However, in our experiment since the substrate is semi-insulating and the buffer layer is undoped, it is difficult to resolve these states spatially by C–V techniques. Our results show that the PR technique can be used to characterize low conductivity or semi-insulating structures such as enhancement mode metal–semiconductor field effect transistor (MESFET) and high electron mobility transistor (HEMT) type devices and it may be useful for the in situ characterization of epigrown surfaces and interfaces.
Rapid thermal processing systems: A review with emphasis on temperature control
Authors: Roozeboom Fred , Parekh N.
Published: July 2002

This paper highlights the basic principles of rapid thermal processing (RTP) systems and the important areas of concern. The basic system characteristics, the fundamental physics involved, and the techniques for temperature measurement and control are extensively reviewed. We summarize the options currently available for 15 RTP equipment manufacturers and point out the latest developments in RTP system design and temperature measurement. Some novel options for temperature control (optical, fiber optical, and photoacoustic) are included.
Force microscopy on living cells
Authors: Häberle W. , Hörber J. K. H. , Binnig G.
Published: July 2002

We have developed an underwater atomic force microscope (AFM) based on detection by tunneling for investigations of single living cells. The AFM setup allows compensation of the electrochemical potentials involved and is integrated into a high magnification optical microscope. Small living cells are sucked onto a microcapillary and are brought into contact with a cantilever for imaging. The cells are kept alive under appropriate physiological conditions while maintaining high resolution to image their molecular structures. For the first time, images of red blood cells made under these conditions with resolution down to about 10 nm are shown. In addition, changes induced by higher salt concentration and by the sticking of antibodies to the cell surface were observed. This indicates the wide range of possibilities of this method for studying dynamical processes of living organisms in situ with high spatial resolution.
Secondary ion mass spectrometry analysis strategy for shallow junctions on test and product silicon wafers
Authors: Stevie F. A.
Published: July 2002

The ability to pattern silicon with sub-micron linewidths has resulted in the need to measure junction depths less than 100 nm. Analysis of shallow junctions using secondary ion mass spectrometry (SIMS) is complicated by distortions due to primary ion beam mixing, the equilibration zone required for the primary species, and surface roughness. Abrupt dopant profiles can be obtained with low energy primary beams. The pre-equilibrium region can be moved with deposition of a silicon overlayer, and topography effects can be reduced by sample rotation during bombardment. SIMS cannot provide part-per-million to part-per-billion detectability for present feature sizes. Imaging depth profiles and SIMS test areas designed into the grid regions between chips assist in the analysis of production wafers.
Linewidth uniformity versus etch rate uniformity in refractory metal plasma etching
Authors: Franssila Sami
Published: July 2002

Etch rate, etch rate uniformity, linewidth uniformity, and microloading in tungsten, molybdenum, and niobium plasma etching have been studied. Electrical linewidth data with typically 100–300 measured lines/wafer have been used. Relation between linewidth uniformity and etched depth uniformity (as measured by profilometer) has been explored. Effects of tungsten film deposition processes (oxygen contamination) have been studied. Tungsten etch rate maximum in SF6/O2 is at 85/15 ratio for the oxygen contaminated films, but higher oxygen percentage in SF6/O2 is required for maximum etch rate of pure films. Oxygen impurities in the tungsten film affect the etch rate but neither linewidth nor linewidth uniformity. In molybdenum etching in Cl2/O2 plasma the etched depth uniformity gives an overly pessimistic uniformity value even though linewidth uniformity is acceptable 12% (3σ). Addition of 3%–6% of CHF3 to Cl2/O2 is shown to change microloading characteristics and to improve linewidth uniformity. On 100 mm wafers, 6%–12% (3σ) linewidth uniformities were obtained for the different metals. Linewidth differences between isolated and array lines were 4%–8%.
Resolution enhanced scanning force microscopy measurements for characterizing dry etching methods applied to titanium masked InP
Authors: Görtz W. , Kempf B. , Kretz J.
Published: July 2002

Scanning force microscopy was used to investigate the morphology and the microroughness of the dry etched surfaces of titanium masks and InP substrates. The influence of different ion beam etching procedures on the surface roughness has been studied. Since the observed surface structures were on the nanometer scale, sharp microtips with an improved geometry were used in order to obtain high lateral and vertical resolution. The tips were produced by electron-beam-induced deposition employing a field emission scanning electron microscope. Tip convolution effects were considered and, moreover, a scan artifact at steep surfaces features was revealed.
An advanced epoxy novolac resist for fast high-resolution electron-beam lithography
Authors: Argitis P. , Raptis I. , Aidinis C. J. , Glezos N. , Baciocchi M. , Everett J. , Hatzakis M.
Published: July 2002

Aspects of the formulation of a highly sensitive cresol epoxy novolac-based chemically amplified negative resist (epoxy resist, EPR) and optimization of critical process parameters for high-resolution electron-beam lithography are reported. The bulk resist sensitivity (E80, dose for 80% thickness retention) is 0.9 μC cm−2 at 40 kV. The effect of postapply bake and postexposure bake on resist sensitivity, contrast, and resolution are investigated and optimized for lithography up to the 0.1 μm regime. The resist process is characterized by a good exposure dose latitude and relevant insensitivity to the variation of thermal processing conditions. Postexposure bake temperature variations in the 90–130 °C range cause minimal change in sensitivity but remarkable change in contrast. Due to this behavior the resist process is not described satisfactorily by the reaction kinetic models commonly used to characterize chemically amplified resists of different chemistry.
Growth of silicon oxide on hydrogenated silicon during lithography with an atomic force microscope
Authors: Marchi F. , Bouchiat V. , Dallaporta H. , Safarov V. , Tonneau D. , Doppelt P.
Published: July 2002

We present an experimental study of growth of silicon oxide strips drawn on hydrogenated silicon under the voltage biased tip of an atomic force microscope operating in ambient atmosphere. Oxide formation was found to occur at negative tip biases above a voltage threshold around |−2|V, corresponding to the minimum electric field required for hydrogen removal from the substrate surface. We show the influence of tip-sample distance and of the chemical composition of the atmosphere on the growth. An ozone enriched atmosphere leads to a growth kinetics enhancement.
Nitrogen-incorporated distorted nanocrystalline diamond films: Structure and field emission properties
Authors: Wu Kehui , Wang E. G. , Chen J. , Xu N. S.
Published: July 2002

Nitrogen-incorporated distorted nanocrystalline diamond films were grown by microwave plasma enhanced chemical vapor deposition using N2 and CH4 as precursors. Scanning electron microscopy, transmission electron microscopy, x-ray diffraction and Raman spectroscopy were used to characterize the films, which are composed of nanosized crystals with a local distorted but long-range ordered structure. A field electron emission threshold as low as 1.3 V/μm has been obtained, and it decreases with a reduction of the methane to nitrogen ratio. The observed low emission threshold, good long-term stability and high maximum emission current (1 mA) suggest a new candidate for cold cathodes. In addition, an interesting switching phenomenon was found in the present films.
Process characterization of low-dose, threshold-voltage adjust channel implants using mercury-probe capacitance–voltage measurements
Authors: Sherbondy J. , Hillard R.
Published: July 2002

The distinct need exists, now, for the capability to accurately and quickly characterize the quality of implants used in threshold-voltage adjusts. In this article we address, in detail, highly sensitive dose and energy monitoring of low-dose implants using a nondestructive, soft-contact mercury-probe capacitance–voltage technique. Currently, wafers are monitored using a wide variety of techniques that work on either a pass–fail basis, or using costly destructive techniques. These techniques lack the sensitivity that is critical for process control of implant and annealing conditions. Data are presented in the form of electrically active carrier density profiles. Detailed information such as partial implant dose, peak carrier density, range, and substrate concentration is determined. These values are enhanced further by the ability to look at uniformity over the entire wafer. From the data collected, rapid and detailed feedback about the condition of the implanter and the annealer(s) is obtained. Quality control and statistical process control of the technique will be analyzed in great detail.
Single-Molecule Studies of Heterogeneous Dynamics in Polymer Melts Near the Glass Transition
Authors: Deschenes Laura A. , Vanden Bout David A.
Published: July 2002

Single-molecule spectroscopy was used to follow the orientation of a single probe molecule in a polymer film in real time. Broad spatially heterogeneous dynamics were observed on long time scales, which result from simple diffusive rotational motions on short time scales. This diffusive behavior persists for many rotations before the molecule's local environment changes to one characterized by a new time scale. This environmental exchange occurs instantaneously on the time scale of the experiment and may arise from large-scale collective motions. The distribution of exchange times for these environments was measured for several temperatures near the glass transition.
A High-Resolution Radiation Hybrid Map of the Human Genome Draft Sequence
Authors: Olivier Michael , Aggarwal Amita , Allen Jennifer , Almendras Annalisa A. , Bajorek Eva S. , Beasley Ellen M. , Brady Shannon D. , Bushard Jannette M. , Bustos Valerie I. , Chu Angela , Chung Tisha R. , Witte Anniek De , Denys Mirian E. , Dominguez Rakly , Fang Nicole Y. , Foster Brian D. , Freudenberg Robert W. , Hadley David , Hamilton Libby R. , Jeffrey Tonya J. , Kelly Libusha , Lazzeroni Laura , Levy Michelle R. , Lewis Saskia C. , Liu Xia , Lopez Frederick J. , Louie Brent , Marquis Joseph P. , Martinez Robert A. , Matsuura Margaret K. , Misherghi Nedda S. , Norton Jolanna A. , Olshen Adam , Perkins Shanti M. , Perou Amy J. , Piercy Chris , Piercy Mark , Qin Fawn , Reif Tim , Sheppard Kelly , Shokoohi Vida , Smick Geoff A. , Sun Wei-Lin , Stewart Elizabeth A. , Fernando J. , Tejeda  , Tran Nguyet M. , Trejo Tonatiuh , Vo Nu T. , Yan Simon C. M. , Zierten Deborah L. , Zhao Shaying , Sachidanandam Ravi , Trask Barbara J. , Myers Richard M. , Cox David R.
Published: July 2002

We have constructed a physical map of the human genome by using a panel of 90 whole-genome radiation hybrids (the TNG panel) in conjunction with 40,322 sequence-tagged sites (STSs) derived from random genomic sequences as well as expressed sequences. Of 36,678 STSs on the TNG radiation hybrid map, only 3604 (9.8%) were absent from the unassembled draft sequence of the human genome. Of 20,030 STSs ordered on the TNG map as well as the assembled human genome draft sequence and the Celera assembled human genome sequence, 36% of the STSs had a discrepant order between the working draft sequence and the Celera sequence. The TNG map order was identical to one of the two sequence orders in 60% of these discrepant cases.
Turing-Type Patterns on Electrode Surfaces
Authors: Li Yong-Jun , Oslonovitch Julia , Mazouz Nadia , Plenge Florian , Krischer Katharina , Ertl Gerhard
Published: July 2002

We report stationary, nonequilibrium potential and adsorbate patterns with an intrinsic wavelength that were observed in an electrochemical system with a specific type of current/electrode-potential (
I-
φ
DL
) characteristic. The patterns emerge owing to the interplay of a self-enhancing step in the reaction dynamics and a long-range inhibition by migration currents rather than by diffusion. Theoretical analysis revealed that this self-structuring of the electrode occurs in all electrochemical systems with an S-shaped
I-
φ
DL
characteristic in wide and well-accessible parameter ranges. This unusual pattern-forming instability in electrochemical systems has all the characteristics of the mechanism proposed by Turing in 1952 in the framework of an early theory of morphogenesis. Our finding might account for structure formation in certain biological systems that have gradients in the electric potential and may open new paths for fabricating patterned electrodes.
Deterministic Delivery of a Single Atom
Authors: Kuhr Stefan , Alt Wolfgang , Schrader Dominik , Müller Martin , Gomer Victor , Meschede Dieter
Published: July 2002

We report the realization of a deterministic source of single atoms. A standing-wave dipole trap is loaded with one or any desired number of cold cesium atoms from a magneto-optical trap. By controlling the motion of the standing wave, we adiabatically transport the atom with submicrometer precision over macroscopic distances on the order of a centimeter. The displaced atom is observed directly in the dipole trap by fluorescence detection. The trapping field can also be accelerated to eject a single atom into free flight with well-defined velocities.
Crystal Structure of Sensory Rhodopsin II at 2.4 Angstroms: Insights into Color Tuning and Transducer Interaction
Authors: Luecke Hartmut , Schobert Brigitte , Lanyi Janos K. , Spudich Elena N. , Spudich John L.
Published: July 2002

We report an atomic-resolution structure for a sensory member of the microbial rhodopsin family, the phototaxis receptor sensory rhodopsin II (NpSRII), which mediates blue-light avoidance by the haloarchaeon
Natronobacterium pharaonis.
The 2.4 angstrom structure reveals features responsible for the 70- to 80-nanometer blue shift of its absorption maximum relative to those of haloarchaeal transport rhodopsins, as well as structural differences due to its sensory, as opposed to transport, function. Multiple factors appear to account for the spectral tuning difference with respect to bacteriorhodopsin: (i) repositioning of the guanidinium group of arginine 72, a residue that interacts with the counterion to the retinylidene protonated Schiff base; (ii) rearrangement of the protein near the retinal ring; and (iii) changes in tilt and slant of the retinal polyene chain. Inspection of the surface topography reveals an exposed polar residue, tyrosine 199, not present in bacteriorhodopsin, in the middle of the membrane bilayer. We propose that this residue interacts with the adjacent helices of the cognate NpSRII transducer NpHtrII.
Reconstitution of Physiological Microtubule Dynamics Using Purified Components
Authors: Kinoshita Kazuhisa , Arnal Isabelle , Desai Arshad , Drechsel David N. , Hyman Anthony A.
Published: July 2002

Microtubules are dynamically unstable polymers that interconvert stochastically between polymerization and depolymerization. Compared with microtubules assembled from purified tubulin, microtubules in a physiological environment polymerize faster and transit more frequently between polymerization and depolymerization. These dynamic properties are essential for the functions of the microtubule cytoskeleton during diverse cellular processes. Here, we have reconstituted the essential features of physiological microtubule dynamics by mixing three purified components: tubulin; a microtubule-stabilizing protein, XMAP215; and a microtubule-destabilizing kinesin, XKCM1. This represents an essential first step in the reconstitution of complex microtubule dynamics–dependent processes, such as chromosome segregation, from purified components.
Anterior Cingulate: Single Neuronal Signals Related to Degree of Reward Expectancy
Authors: Shidara Munetaka , Richmond Barry J.
Published: July 2002

As monkeys perform schedules containing several trials with a visual cue indicating reward proximity, their error rates decrease as the number of remaining trials decreases, suggesting that their motivation and/or reward expectancy increases as the reward approaches. About one-third of single neurons recorded in the anterior cingulate cortex of monkeys during these reward schedules had responses that progressively changed strength with reward expectancy, an effect that disappeared when the cue was random. Alterations of this progression could be the basis for the changes from normal that are reported in anterior cingulate population activity for obsessive-compulsive disorder and drug abuse, conditions characterized by disturbances in reward expectancy.
A Reversible Solid-State Crystalline Transformation in a Metal Phosphide Induced by Redox Chemistry
Authors: Souza D. C. S. , Pralong V. , Jacobson A. J. , Nazar L. F.
Published: July 2002

We demonstrate low-potential intercalation of lithium in a solid-state metal phosphide. A topotactic first-order transition between different but related crystal structures at room temperature takes place by an electrochemical redox process: MnP
4
↔ Li
7
MnP
4
. The P-P bonds in the MnP
4
structure are cleaved at the time of Li insertion (reduction) to produce crystalline Li
7
MnP
4
and are reformed after reoxidation to MnP
4
, thereby acting as an electron storage reservoir. This is an unusual example of facile covalent bond breaking within the crystalline solid state that can be reversed by the input of electrochemical energy.
The Nature of the 660-Kilometer Upper-Mantle Seismic Discontinuity from Precursors to the
PP
Phase
Authors: Estabrook Charles H. , Kind Rainer
Published: July 2002

Global Seismic Network data were used to image upper-mantle seismic discontinuities. Stacks of phases that precede the
PP
phase, thought to be underside reflections from the upper-mantle discontinuities at depths of 410 and 660 kilometers, show that the reflection from 410 kilometers is present, but the reflection from 660 kilometers is not observed. A continuous Lamé's constant λ and seismic parameter at the 660-kilometer discontinuity explain the missing underside
P
reflections and lead to a
P
-wave velocity jump of only 2 percent, whereas the
S
-wave velocity and density remain unchanged with respect to previous global models. The model deemphasizes the role of Lamé's constant λ with regard to the shear modulus and constrains the mineralogical composition across the discontinuity.
Survival of Cholinergic Forebrain Neurons in Developing p75
NGFR
-Deficient Mice
Authors: Van der Zee Catharina E. E. M. , Ross Gregory M. , Riopelle Richard J. , Hagg Theo
Published: July 2002

The functions of the low-affinity p75 nerve growth factor receptor (p75
NGFR
) in the central nervous system were explored in vivo. In normal mice, approximately 25 percent of the cholinergic basal forebrain neurons did not express TrkA and died between postnatal day 6 and 15. This loss did not occur in p75
NGFR
-deficient mice or in normal mice systemically injected with a p75
NGFR
-inhibiting peptide. Control, but not p75
NGFR
-deficient, mice also had fewer cholinergic striatal interneurons. Apparently, p75
NGFR
mediates apoptosis of these developing neurons in the absence of TrkA, and modulation of p75
NGFR
can promote neuronal survival. Cholinergic basal forebrain neurons are involved in learning and memory.
Laser Rapid Prototyping of Photonic Band-Gap Microstructures
Authors: Wanke Michael C. , Lehmann Olaf , Müller Kurt , Wen Qingzhe , Stuke Michael
Published: July 2002

Three-dimensional periodic microstructures of aluminum oxide, which are important for creating photonic band-gap structures (PBGs), were fabricated by laser rapid prototyping by means of laser-induced direct-write deposition from the gas phase. The structures consisted of layers of parallel rods forming a face-centered tetragonal lattice with lattice constants of 66 and 133 micrometers. These structures showed transmission minima centered around 4 terahertz (75 micrometers) and 2 terahertz (150 micrometers), respectively. PBGs will allow precise control of the optical properties of materials, including lasers without threshold.
Phylogenetic Analysis: Getting Its Day in Court
Authors: Vogel Gretchen
Published: July 2002

A trial for attempted murder, currently scheduled for 3 May in Lafayette, Louisiana, may for the first time in the United States feature a new type of evidence: phylogenetic analysis, in which DNA samples from various sources are compared to see how closely related they are. In this case, a doctor is alleged to have tried to kill his former mistress by injecting her with HIV-infected blood from one of his patients, and the prosecution wants to use the analysis to support its case. The evidence was ruled admissible in a preliminary hearing, although the defense is now challenging that ruling.
Cell Cycle-Dependent Establishment of a Late Replication Program
Authors: Raghuraman M. K. , Brewer Bonita J. , Fangman Walton L.
Published: July 2002

DNA replication origins in chromosomes of eukaryotes are activated according to a temporal program. In the yeast
Saccharomyces cerevisiae
, activation of origins in early S phase appears to be a default state. However,
cis
-acting elements such as telomeres can delay origin activation until late S phase. Site-specific recombination was used to separate origin from telomere in vivo, thereby demonstrating that the signal for late activation is established between mitosis and START in the subsequent G
1
phase. Once set, the signal can persist through the next S phase in the absence of the telomere. Establishment of the temporal program and of initiation competence of origins may be coincident events.
Nuclear Spectroscopy in Single Quantum Dots: Nanoscopic Raman Scattering and Nuclear Magnetic Resonance
Authors: Gammon D. , Brown S. W. , Snow E. S. , Kennedy T. A. , Katzer D. S. , Park D.
Published: July 2002

Resonant Raman and nuclear magnetic resonance spectroscopies from single gallium arsenide quantum dots are demonstrated. The nuclei were probed through changes in the optical spectra of the quantum dot exciton arising from exciton-nuclear interactions. This approach allowed the application of optical spectroscopy with its extremely high sensitivity and selectivity. The experiments had a lateral spatial resolution of about 10 nanometers and probe a volume that was five orders of magnitude smaller than that of previous semiconductor nuclear spectroscopic studies.
Orbital Steering in the Catalytic Power of Enzymes: Small Structural Changes with Large Catalytic Consequences
Authors: Mesecar Andrew D. , Stoddard Barry L. , Koshland Daniel E.
Published: July 2002

Small structural perturbations in the enzyme isocitrate dehydrogenase (IDH) were made in order to evaluate the contribution of precise substrate alignment to the catalytic power of an enzyme. The reaction trajectory of IDH was modified (i) after the adenine moiety of nicotinamide adenine dinucleotide phosphate was changed to hypoxanthine (the 6-amino was changed to 6-hydroxyl), and (ii) by replacing Mg
2+
, which has six coordinating ligands, with Ca
2+
, which has eight coordinating ligands. Both changes make large (10
−3
to 10
−5
) changes in the reaction velocity but only small changes in the orientation of the substrates (both distance and angle) as revealed by cryocrystallographic trapping of active IDH complexes. The results provide evidence that orbital overlap produced by optimal orientation of reacting orbitals plays a major quantitative role in the catalytic power of enzymes.
Oncogenic Transcription Factors in the Human Acute Leukemias
Authors: Look A. Thomas
Published: July 2002

Chromosomal translocations in the human acute leukemias rearrange the regulatory and coding regions of a variety of transcription factor genes. The resultant protein products can interfere with regulatory cascades that control the growth, differentiation, and survival of normal blood cell precursors. Support for this interpretation comes from the results of gene manipulation studies in mice, as well as the sequence homology of oncogenic transcription factors with proteins known to regulate embryonic development in primitive organisms, including the nematode
Caenorhabditis elegans
and the fruit fly
Drosophila melanogaster
. Many of these genetic alterations have important prognostic implications that can guide the selection of therapy. The insights gained from studies of translocation-generated oncogenes and their protein products should hasten the development of highly specific, and hence less toxic, forms of leukemia therapy.
Abrupt Climate Events 500,000 to 340,000 Years Ago: Evidence from Subpolar North Atlantic Sediments
Authors: Oppo D. W. , McManus J. F. , Cullen J. L.
Published: July 2002

Subpolar North Atlantic proxy records document millennial-scale climate variations 500,000 to 340,000 years ago. The cycles have an approximately constant pacing that is similar to that documented for the last glacial cycle. These findings suggest that such climate variations are inherent to the late Pleistocene, regardless of glacial state. Sea surface temperature during the warm peak of Marine Isotope Stage 11 (MIS 11) varied by 0.5° to 1°C, less than the 4° to 4.5°C estimated during times of ice growth and the 3°C estimated for glacial maxima. Coherent deep ocean circulation changes were associated with glacial oscillations in sea surface temperature.
Mutation of
BCL-6
Gene in Normal B Cells by the Process of Somatic Hypermutation of Ig Genes
Authors: Shen Hong Ming , Peters Andrew , Baron Beverly , Zhu Xiangdong , Storb Ursula
Published: July 2002

Immunoglobulin (Ig) genes are hypermutated in B lymphocytes that are the precursors to memory B cells. The mutations are linked to transcription initiation, but non-Ig promoters are permissible for the mutation process; thus, other genes expressed in mutating B cells may also be subject to somatic hypermutation. Significant mutations were not observed in c-
MYC
,
S14
, or α-fetoprotein (
AFP
) genes, but
BCL-6
was highly mutated in a large proportion of memory B cells of normal individuals. The mutation pattern was similar to that of Ig genes.
Pioneer Axon Guidance by UNC-129, a
C. elegans
TGF-β
Authors: Colavita Antonio , Krishna Srikant , Zheng Hong , Padgett Richard W. , Culotti Joseph G.
Published: July 2002

The
unc-129
gene, like the
unc-6
netrin gene, is required to guide pioneer motoraxons along the dorsoventral axis of
Caenorhabditis elegans
.
unc-129
encodes a member of the transforming growth factor–β (TGF-β) superfamily of secreted signaling molecules and is expressed in dorsal, but not ventral, rows of body wall muscles. Ectopic expression of UNC-129 from ventral body wall muscle disrupts growth cone and cell migrations that normally occur along the dorsoventral axis. Thus, UNC-129 mediates expression of dorsoventral polarity information required for axon guidance and guided cell migrations in
C. elegans
.
BRCA1 Required for Transcription-Coupled Repair of Oxidative DNA Damage
Authors: Gowen Lori C. , Avrutskaya Anna V. , Latour Anne M. , Koller Beverly H. , Leadon Steven A.
Published: July 2002

The breast and ovarian cancer susceptibility gene
BRCA1
encodes a zinc finger protein of unknown function. Association of the BRCA1 protein with the DNA repair protein Rad51 and changes in the phosphorylation and cellular localization of the protein after exposure to DNA-damaging agents are consistent with a role for BRCA1 in DNA repair. Here, it is shown that mouse embryonic stem cells deficient in BRCA1 are defective in the ability to carry out transcription-coupled repair of oxidative DNA damage, and are hypersensitive to ionizing radiation and hydrogen peroxide. These results suggest that BRCA1 participates, directly or indirectly, in transcription-coupled repair of oxidative DNA damage.
Direct Allelic Variation Scanning of the Yeast Genome
Authors: Winzeler Elizabeth A. , Richards Dan R. , Conway Andrew R. , Goldstein Alan L. , Kalman Sue , McCullough Michael J. , McCusker John H. , Stevens David A. , Wodicka Lisa , Lockhart David J. , Davis Ronald W.
Published: July 2002

As more genomes are sequenced, the identification and characterization of the causes of heritable variation within a species will be increasingly important. It is demonstrated that allelic variation in any two isolates of a species can be scanned, mapped, and scored directly and efficiently without allele-specific polymerase chain reaction, without creating new strains or constructs, and without knowing the specific nature of the variation. A total of 3714 biallelic markers, spaced about every 3.5 kilobases, were identified by analyzing the patterns obtained when total genomic DNA from two different strains of yeast was hybridized to high-density oligonucleotide arrays. The markers were then used to simultaneously map a multidrug-resistance locus and four other loci with high resolution (11 to 64 kilobases).
Anthropogenic Influence on the Autocorrelation Structure of Hemispheric-Mean Temperatures
Authors: Wigley T. M. L. , Smith R. L. , Santer B. D.
Published: July 2002

It is shown that lagged correlations for and cross-correlations between observed hemispheric-mean temperature data differ markedly from those for unforced (control-run) climate model simulations. The differences can be explained adequately by assuming that the observed data contain a significant externally forced component involving both natural (solar) and anthropogenic influences and that the global climate sensitivity is in the commonly accepted range. Solar forcing alone cannot reconcile the differences in autocorrelation structure between observations and model control-run data.
Regulation of Nerve Growth Mediated by Inositol 1,4,5-Trisphosphate Receptors in Growth Cones
Authors: Takei Kohtaro , Shin Ryong-Moon , Inoue Takafumi , Kato Kunio , Mikoshiba Katsuhiko
Published: July 2002

The inositol 1,4,5-trisphosphate (IP
3
) receptor (IP
3
R) acts as a Ca
2+
release channel on internal Ca
2+
stores. Type 1 IP
3
R (IP
3
R1) is enriched in growth cones of neurons in chick dorsal root ganglia. Depletion of internal Ca
2+
stores and inhibition of IP
3
signaling with drugs inhibited neurite extension. Microinjection of heparin, a competitive IP
3
R blocker, induced neurite retraction. Acute localized loss of function of IP
3
R1 in the growth cone induced by chromophore-assisted laser inactivation resulted in growth arrest and neurite retraction. IP
3
-induced Ca
2+
release in growth cones appears to have a crucial role in control of nerve growth.