bluuebunny/crossref_metadata_2025 huggingface dataset:
Transition gas flow in drag pumps and capillary leaks
Authors: Helmer John C. , Levi Giampaolo
Published: July 2002

Modern turbomolecular pumps include a drag stage in the exhaust, operating roughly in the pressure range of 10 mTorr–10 Torr. Flow conditions range from molecular flow at the drag inlet, to viscous flow at the outlet, known as ‘‘transition’’ flow. In general, models of transition flow in drag pumps have not been developed. Moreover, the model of a Gaede pump given in journals and textbooks up to the present, gives values of compression ratio that are orders of magnitude too high. In 1913, Gaede proposed a differential equation for transition flow in a drag pump. He did not solve the general equation, and the model was incomplete. We have developed a new model that takes transition flow in a differential element and integrates it over the length of the pump. This model is modified by a ‘‘pumping leak’’ expression for the gas stripper, which separates the inlet from the outlet. The result is compared with experimental measurements, and good agreement is obtained over the entire pressure range from molecular, through transition, and into viscous flow. Up to a critical pressure in viscous flow, compression ratio is constant as a function of exhaust pressure, within a factor of 2. Within this factor, increasing compression arises from the reduced pressure drop across the inlet aperture as its conductance increases in the transition flow regime. Above the critical pressure, compression drops rapidly as laminar backflow increases. This critical pressure is controlled by the dimensions of the channel. Below the critical pressure, compression is determined by the pumping leak, and is somewhat independent of molecular weight. If the surface velocity is zero, the model reduces to a capillary leak. Predictions of our model agree with Knudsen’s data for capillary leaks in transition flow, in addition to giving a better account of the ‘‘conductance minimum.’’ ‘‘Slip flow’’ is not an obvious factor, and it cannot be distinguished from the right combination of viscous and molecular flow.
Time-resolved Fourier transform infrared emission as a plasma diagnostic
Authors: Hancock G. , Sucksmith J. P.
Published: July 2002

Time-resolved infrared emission has been observed by means of a Fourier transform technique from plasmas of fluorinated hydrocarbons under conditions similar to those used in semiconductor surface processing. Emission has been detected from vibrationally excited precursors and reaction products, and examples are shown of emission from HF, CF4, CO2, and CF2O. The time dependences of the signals on plasma extinction give information on the production and relaxation mechanisms taking place. Equilibrated rotational and translational temperatures in the plasma are derived from quantum state distributions in HF. Blackbody emission is observed from both chamber surfaces and from a Si wafer placed in the plasma, and the time-resolved emission technique is used to measure the surface temperature under conditions in which strong molecular emissions are present.
Silicon epitaxy from pulsed supersonic jets: Growth, modeling, and simulation
Authors: Malik Rajeev , Guljari Erdogan
Published: July 2002

Single crystal silicon thin films have been grown on Si(100) substrates using pulsed supersonic jets of a disilane (Si2H6)-hydrogen mixture. Digital epitaxy has been achieved at substrate temperatures of 400–500  °C, with a growth rate of 0.1 Å/pulse at Ts=450 °C. Sticking coefficients of up to 0.3 are observed implying an order of magnitude increase over conventional gas source molecular beam epitaxy. A fundamental model, based on an approach proposed in the literature, has been developed to explain experimental results. Incorporating cyclic adsorption and regeneration steps, the resulting growth rate expression is fit to data to extract rate constants. An activation energy of 2.22 eV is obtained for the kinetic rate constant for hydrogen desorption, the limiting step in low temperature growth. A Monte Carlo approach has been used for the first time to study the atomic scale roughness variations in supersonic jet growth. Simulation studies show that the high energy jets only affect short-range diffusion, whereas long-range diffusion, responsible for improving surface roughness, is still thermally driven. A true atomic layer epitaxy (growth process, independent of substrate temperature), is predicted for incident jet energy exceeding the hydrogen desorption activation energy.
Improved design for an aluminum evaporator
Authors: Keck H.-G. , Kopatzki E. , Behm R. J.
Published: July 2002

The design of a previously described aluminum evaporator is improved in several important aspects. Special attention is paid to features which permit a thorough degassing procedure. The possibility of determining the temperature of the source in the current setup is important for the reproducibility and stability of the evaporator over a wide range of flux rates.
New perspectives in vacuum high voltage insulation. II. Gas desorption
Authors: Diamond William T.
Published: July 2002

An examination has been made of gas desorption from unbaked electrodes of copper, niobium, aluminum, and titanium subjected to high voltage in vacuum. It has been shown that the gas is composed of water vapor, carbon monoxide, and carbon dioxide, the usual components of vacuum outgassing, plus an increased yield of hydrogen and light hydrocarbons. The gas desorption was driven by anode conditioning as the voltage was increased between the electrodes. The gas is often desorbed as microdischarges—pulses of a few to hundreds of microseconds—and less frequently in a more continuous manner without the obvious pulsed structure characteristic of microdischarge activity. The quantity of gas released was equivalent to many monolayers and consisted mostly of neutral molecules with an ionic component of a few percent. A very significant observation was that the gas desorption was more dependent on the total voltage between the electrodes than on the electric field. It was not triggered by field-emitted electrons but often led to field emission, especially at larger gaps. The study of gas desorption led to some important new observations about the initiation of high-voltage breakdown and the underlying processes of vacuum outgassing. The physical processes that lead to voltage-induced desorption are complex, but there is strong evidence that the microdischarges are the result of an avalanche discharge in a small volume of high-density vapor desorbed from the anode. The source of the vapor may be water or alcohol stored as a fluid in the many small imperfections of a polished metal surface. Microdischarges can then trigger field-emitted electrons which, in turn, heats a small area of the anode. As the temperature of this region of the anode reaches about 500 °C, some fraction of the desorption products are ionized positively and accelerated to the cathode, producing secondary electrons with a yield greater than unity per incident ion. The positive ions appear to originate from the bulk of the metal rather than from surface ionization and the yield increases exponentially with temperature, rapidly producing a runaway condition, i.e., electrical breakdown. These observations support a new perspective on vacuum-high-voltage insulation and produce new insight into vacuum outgassing of metals.
Heteroepitaxial growth of RuO2 thin films on α-Al2O3 substrates with CeO2 buffer layers by pulsed laser deposition
Authors: Chen C. L. , Jia Q. X. , Lu Y. C. , Smith J. L. , Mitchell T. E.
Published: July 2002

Metallic conductive ruthenium oxide thin films have been grown epitaxially on sapphire (01 1̄ 2) with a cerium oxide buffer layer by pulsed laser deposition. We used transmission electron microscopy, high-resolution electron microscopy, and x-ray diffraction to characterize the growth behavior of the films and the orientation relationship between the films and the substrate. Our electron diffraction and x-ray diffraction studies indicate that the orientation relationships are (200)RuO2 //(200)CeO2 //(011̄2)Al2O3 and 〈011〉RuO2//〈001〉CeO2//〈22̄01〉Al2O3. All of the interfaces were seen to be atomically sharp by cross-sectional, high-resolution transmission electron microscopy in the as-grown samples. No precipitates or additional phases were found in the films or at their interfaces.
Spectroscopic ellipsometric characterization of diamondlike carbon films
Authors: Jayatissa Ahalapitiya Hewage , Yamaguchi Tomuo , Kinoshita Haruhisa , Hando Takuya , Sato Fumio
Published: July 2002

Amorphous diamondlike carbon films have been investigated by means of spectroscopic ellipsometery (SE). The films were measured by a null ellipsometer in the 230–750 nm range and the measured spectra were analyzed using an empirical dielectric function suitable for amorphous materials. The fitting of SE data with empirical dielectric function shows that the measured spectra can be well explained using the empirical dielectric function. The optical constants calculated from the fitting of SE data were used to calculate the optical reflectance spectra of these films. It was found that the calculated reflectance spectra coincide with the measured spectra suggesting that the optical properties obtained from the fitting of SE data are accurate.
Initial stages of Al(111) oxidation by oxygen: Temperature and surface morphology effects
Authors: Zhukov V. , Popova I. , Yates J. T.
Published: July 2002

The initial stages of Al(111) oxidation by oxygen were investigated by using x-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy. It was found that surfaces exhibiting no impurities from the Auger electron spectroscopy and low-energy electron diffraction data may show different characteristic adsorption parameters (sticking coefficient, adsorbate vibrational modes, etc.). These parameters change rather dramatically with extended time of Ar+ bombardment followed by annealing prior to O2 adsorption. It was shown that extensive sputtering, followed by 700 K annealing, was necessary to prepare a surface with reproducible behavior. The integral reactive sticking coefficient, S(T), for the carefully prepared surface was evaluated by measuring the O(1s) XPS peak area for O2 exposures below 50 L. At surface temperatures from 95 to 773 K nonmonotonic behavior of S(T) was observed. A molecular precursor state preceding dissociative adsorption is proposed. It was found that the balance between oxidic and chemisorbed oxygen is strongly temperature dependent and influences the oxygen sticking coefficient on Al(111). A step-like formation of the oxidic phase in the temperature range 473–573 K was found to influence the adsorption kinetics. Oxidation causes the transformation of the O2 adsorption process from an activated into a nonactivated one.
Investigation of induced recrystallization and stress in close-spaced sublimated and radio-frequency magnetron sputtered CdTe thin films
Authors: Moutinho H. R. , Dhere R. G. , Al-Jassim M. M. , Levi D. H. , Kazmerski L. L.
Published: July 2002

We have induced recrystallization of small grain CdTe thin films deposited at low temperatures by close-spaced sublimation (CSS), using a standard CdCl2 annealing treatment. We also studied the changes in the physical properties of CdTe films deposited by radio-frequency magnetron sputtering after the same post-deposition processing. We demonstrated that the effects of CdCl2 on the physical properties of CdTe films are similar, and independent of the deposition method. The recrystallization process is linked directly to the grain size and stress in the films. These studies indicated the feasibility of using lower-temperature processes in fabricating efficient CSS CdTe solar cells. We believe that, after the optimization of the parameters of the chemical treatment, these films can attain a quality similar to CSS films grown using current standard conditions.
Two-dimensional CT images of two-frequency capacitively coupled plasma
Authors: Kitajima T. , Takeo Y. , Makabe T.
Published: July 2002

Two-dimensional images of two-frequency capacitively coupled plasma (2f-CCP) in Ar and Ar/CF4(5%) in an axisymmetric parallel plate reactor are investigated by using 2D-t optical emission spectroscopy. Spatially averaged electron density is obtained by microwave interferometry. Results are presented in the form of 2D profiles of the net excitation rate of Ar(3p5)(εex=14.5 eV) and Ar+(4p4D7/2)(εex=35.0 eV) used as a probe. Large area uniformity of plasma production driven at very high frequency (VHF) (100 MHz) and that driven at high frequency (HF) (13.56 MHz) at low pressure (∼25 mTorr) are compared and discussed under a low frequency (LF) (700 kHz) bias voltage on the wafer. The time modulation of the net excitation rate and the electron density indicate that the LF bias is considerably influential in the production of the plasma and in the confinement of high energy electrons at HF. Functional separation between plasma production in a gas phase and ion acceleration to the wafer is achieved in 2f-CCP excited at VHF (100 MHz). The addition of a small amount of CF4 to the Ar plasma improves the uniformity of the radial profile of the excitation at HF (13.56 MHz).
Power dissipation in turbomolecular pumps at high pressure
Authors: Cerruti R. , Spagnol M. , Helmer J. C.
Published: July 2002

The operating range of turbopumps is being extended to higher pressures as a consequence of many demanding applications. Varian VPT has specialized in the design of Gaede stages for extending the operating pressure of the pump. The present work is an extension of the activity made in the last five years in the direction of a good understanding of the behavior of the Gaede stage in viscous flow conditions. Starting from the analysis of the pressure distribution into a single stage, already presented in a previous paper, a transitional power consumption model is derived. The model allows to predict the power consumption of a single Gaede stage as a function of the pumping channel dimensions and clearances, the rotational speed, the nature of the gas pumped, and the operating pressure. The model is based on three main assumptions: (1) the pressure distribution along the channel satisfies a diffusion viscous model at zero net gas flow, (2) the gas flow boundary layer across channel is derived from a simple viscous model, and (3) the gas viscosity obeys a transition expression valid for the pumping channel geometry. Results are compared with the experimental data provided by different single Gaede stages, with different dimensions, at different speed and operating pressure. The behavior of the model with different pump stage clearances is analyzed and results are compared with the experimental evidence. A useful utilization of the model is in the optimization of the design of multistage Gaede pumps.
Characterization of thin layers on perfect crystals with a multipurpose high resolution x-ray diffractometer
Authors: Bartels W. J.
Published: July 2002

A high resolution x-ray diffractometer is described in which a compact four-crystal monochromator produces a highly parallel and monochromatic incident beam of tuneable wavelength (divergence Δθ=5″, wavelength band Δλ/λ=2.3×10−5, intensity 3×104 photons/s mm2). The instrument is very suitable for the nondestructive measurement of concentration depth profiles produced in semiconductor single crystals by epitaxy, diffusion, or implantation. The equipment is useful for measuring x-ray rocking curves of any lattice plane of any material and also lattice constants can be determined on absolute scale using the Bond procedure. A special solution has been derived from the dynamical theory of x-ray diffraction, which gives the reflectivity of an absorbing layer as a function of layer thickness. This new expression takes the effects of absorption and extinction into account and has been used to determine the layer thickness of epitaxial layers grown on [001] oriented InP.
Microstructure studies of AuNiGe Ohmic contacts to n-type GaAs
Authors: Murakami Masanori , Childs K. D. , Baker John M. , Callegari A.
Published: July 2002

Microstructure analysis and contact resistance measurements of alloyed AuNiGe contacts to GaAs were performed to assist in the development of low resistance Ohmic contacts for metal–semiconductor field-effect transistor (MESFET) devices. The contact metals were prepared by sequential deposition of 100 nm of Au–27 at. % Ge, 35 nm Ni, and 50 nm Au onto sputter-cleaned GaAs wafers in which conducting channels were formed by Si doping to a level of about 1×1018 cm−3. The contact resistances were determined by the transmission line method. Analysis of the substrate and the film microstructure was carried out by x-ray diffraction, Auger electron spectroscopy (AES), and x-ray photoelectron spectroscopy (XPS). A strong correlation between the contact resistance and the film microstructure was observed. Low resistances were observed when NiAs compounds containing Ge were in contact with GaAs and the β-AuGa phase was concentrated near the top of the contact. High resistances were measured when free Au, the α-AuGa phase, or a NiGe compound were present. The temperature dependence of the contact resistance and the kinetics of compound formation were found to be influenced by the deposition sequence. Deposition of 5 nm Ni as a first layer significantly enhanced the formation of the NiAs compounds containing Ge, resulting in low contact resistance at a lower alloying temperature. However, a further increase in the thickness of the first Ni layer to 10 nm increased the contact resistance as well as the spread in the resistance values. The present study emphasizes the importance of the distribution of compounds which directly contact the GaAs in determining the contact resistance and its spread. The contact resistance increase observed previously during thermal annealing at 400 °C, after the formation of the contact, is believed to be due to the increase in contact area between GaAs and AuGa phases as a result of grain growth.
Electronic structure and thermal stability of Ni/SiC(100) interfaces
Authors: Höchst H. , Niles D. W. , Zajac G. W. , Fleisch T. H. , Johnson B. C. , Meese J. M.
Published: July 2002

We have studied the interface formation of Ni overlayers on cubic SiC(100) by photoemission spectroscopy with synchrotron radiation in the energy range hν=40–140 eV. At room temperature deposition, Ni initially reacts with SiC and forms NiSi. At a nominal coverage of ∼20 Å Ni, the overlayer is metallic and the valence band spectra are identical to that of bulk Ni. The Ni/SiC(100) system is thermally stable up to ∼600 °C. The Ni overlayer reacts with the substrate to form Ni2Si at ∼800 °C. The reacted layer is inhomogeneous and consists of a mixture of SiC and Ni2Si. The reaction of Ni with SiC is quite different than that of Ni with the (111) surface of bulk Si. Ni reacts with Si(111) at 400 °C to form an epitaxial overlayer of the more Si-rich silicide NiSi2.
A combination electron/ion field emission source
Authors: Rao K. A. , Bell A. E. , Schwind G. A. , Swanson L. W.
Published: July 2002

A combination electron/ion field emission source will be described. For ion emission the source was operated as a conventional indium or gallium liquid metal ion source (LMIS), thereby giving a beam of principally In+ or Ga+ ions with the usual range of angular intensities and total emission currents. In the case of indium it was possible to ‘‘freeze in’’ the field stabilized Taylor cone formed during LMIS operation by carefully reducing the voltage to the threshold value and then rapidly reducing both the voltage and the emitter temperature. Field electron emission was then obtained by reversing the voltage polarity. The dc electron emission angular distribution generally overlapped that observed in the LMIS mode. This process, which was determined to be reversible and reproducible, will be discussed in detail.
Reduction of gallium-related oval defects
Authors: Schlom D. G. , Lee W. S. , Ma T. , Harris J. S.
Published: July 2002

A novel gallium effusion cell consisting of a sapphire crucible in a variable temperature profile furnace was used to grow GaAs layers. The effect of the gallium source temperature profile on gallium source-related oval defects was investigated. By minimizing gallium droplets in the orifice, the gallium-related oval defect density was reduced to <50 cm−2 for a 1-μm-thick layer. These results indicate that oval defect densities can be further reduced by eliminating gallium droplets in the crucible orifice.
Analysis of surface contaminants on gallium arsenide and silicon by high-resolution time-of-flight secondary ion mass spectrometry
Authors: Niehuis E. , Heller T. , Jürgens U. , Benninghoven A.
Published: July 2002

High-performance time-of-flight secondary ion mass spectrometry (TOF SIMS); characterized by high mass resolution, extremely high sensitivity, and high dynamic range, has been applied to surface analysis of GaAs and Si wafers. Mass separation of metal and hydrocarbon ions of identical nominal masses allows the unambiguous detection of metal contaminants, even in the presence of heavy surface contamination by hydrocarbons. In favorable cases surface sensitivities in the ppm range are achieved. Quantitative element analysis and detection of molecular surface contaminants are possible.
Characterization of novel sulfonic acid photogenerating 2-nitrobenzyl ester derivatives
Authors: Houlihan F. M. , Neenan T. X. , Reichmanis E. , Kometani J. M. , Thompson L. F. , Chin T. , Kanga R. S.
Published: July 2002

The thermal stability of 2-nitrobenzyl sulfonate esters was dramatically increased by the introduction of an electron withdrawing, sterically bulky group (Br, CF3 ) at the ortho position of the benzyl moiety. This enhancement in thermal stability allowed synthetic access to thermally stable, photogenerators of acids based on more powerful organic acids which are useful in formulating deep-UV resists having sensitivities approaching those of the more traditional onium salt-based materials.
Integrating diffusion enhanced silylated resist into a single-layer resist dynamic random access memory production line
Authors: Garza Cesar M. , Catlett David L. , Jackson Ricky A.
Published: July 2002

The manufacturing of the next generation of dynamic random access memory (DRAM) will require lithographic capability in the 0.5 μm range. Not surprisingly, their requirements push the limits of optical microlithography, thereby rapidly diminishing the process window. This problem becomes more acute at the end of the line where topography and reflectivity have the largest impact on an already strained processing budget. Surface imaging [G. Taylor and T. Wolf, J. Electrochem. Soc. 127, 2665 (1980)] is less sensitive to topography and reflectivity than conventional lithography. Thus, integrating a surface imaging process into a conventional single-resist processing line is an attractive option to extend the practical resolution limit of current optical lithography without resorting to more complex multilayer resist schemes or more expensive imaging tools. The diffusion enhanced silylated resist process is the most mature of the surface imaging processes commercially available today. This report addresses integrating this process in a high volume DRAM manufacturing facility where only single-layer, conventional resist processes are being used. In this article, in addition to presenting basic lithographic engineering data such as scanning electron micrographs and process latitude curves, the topics of discussion include other manufacturing related issues such as radiation-induced damage during dry development, and the overall reliability of the process. These are issues of concern typically raised when this type of technology has been proposed for use in a high volume manufacturing facility.
Progress in sequencing deoxyribonucleic acid with an atomic force microscope
Authors: Hansma H. G. , Weisenhorn A. L. , Gould S. A. C. , Sinsheimer R. L. , Gaub H. E. , Stucky G. D. , Zaremba C. M. , Hansma P. K.
Published: July 2002

Atomic force microscope (AFM) images of single-stranded deoxyribonucleic acid (DNA) showing nucleotide resolution have been obtained using two sample preparation methods: DNA covalently attached to a polymerized lipid monolayer and then imaged under water and DNA on mica rinsed with a Ba(NO3)2 solution and then imaged under ethanol. Some of the bases were identified in the AFM image of DNA on polymerized lipid. Current problems in sequencing DNA with the AFM include movement of the DNA during imaging and the difficulty of reproducing experiments.
Photoassisted tunneling spectroscopy: Preliminary results on tungsten diselenide
Authors: Akari S. , Lux-Steiner M. Ch. , Vögt M. , Stachel M. , Dransfeld K.
Published: July 2002

In this study we introduce a novel use of the scanning tunneling microscope as an instrument to investigate the local quantum yield of photoelectrons from illuminated semiconductor surfaces. The photoinduced tunneling current resulting from the illumination of the surface with monochromatic light is investigated as a function of the wavelength. Information on the energy dependent creation and recombination of electrons in the vicinity of surfaces are obtained by this method with high spatial resolution without the necessity of covering the surface by a conducting film. Thereby all defects associated with the electrode–semiconductor interface are avoided.
Electron-beam lithography with the scanning tunneling microscope
Authors: Marrian Christie R. K. , Dobisz Elizabeth A. , Dagata John A.
Published: July 2002

The scanning tunneling microscope (STM), operated in vacuum in the field emission mode, has been used in lithographic studies of the resist SAL-601 from Shipley. Patterns have been written by raising the tip–sample voltage above −12 V while operating the STM in the constant current mode. Resist films, 50 nm thick, have been patterned and the pattern transferred into the GaAs substrate by reactive ion etching. The variation of feature size with applied dose and tip–sample bias voltage has been studied. Comparisons have been made to lithography with a 10 nm, 50 kV electron e-beam in a JEOL JBX-5DII in the same resist thickness films. In all cases the resist films were processed in the standard fashion before and after exposure. The STM can write smaller minimum features sizes and has a greater process latitude. Proximity effects are absent due to the reduced scattering range of the low energy primary electrons. However, the writing speed is slower, being limited by the response of the piezoelectric scanner. Advances have been made recently in the construction of fast STMs which scan at video rates making the STM comparable in speed to the JEOL for nanolithography. The development of ultralow voltage e-beam lithography based on STM technology is discussed.
Atom-resolved imaging and spectroscopy on the GaAs(001) surface using tunneling microscopy
Authors: Bressler-Hill V. , Wassermeier M. , Pond K. , Maboudian R. , Briggs G. A. D. , Petroff P. M. , Weinberg W. H.
Published: July 2002

Scanning tunneling microscopy and scanning tunneling spectroscopy have been used to investigate the structure and current–voltage [I(V)] characteristics of the molecular-beam epitaxially grown, As-rich GaAs(001)-(2×4) surface. High-resolution images reveal a modulation in the topography of the individual arsenic dimers measured with the tunneling microscope in the constant current mode. The observed features are attributed to an increased tunneling probability out of the occupied electronic lone pair states of the As dimers. The I(V) spectroscopy performed on the (001) surface of low doped n-type GaAs samples differs considerably from the results obtained on the (110) surface of this semiconductor. This is attributed to band bending that is due to a lower doping concentration below the surface. The electrostatics involved in imaging with a tunneling microscope are described in a simple model, based on the depletion approximation, that accounts for the experimental results.
Ex situ scanning tunneling microscopy investigations of the modification of titanium surface due to corrosion processes
Authors: Ejov A. A. , Savinov S. V. , Yaminsky I. V. , Pan J. , Leygraf C. , Thierry D.
Published: July 2002

Scanning tunneling microscopy was implemented for the comparative surface investigation of mechanically polished and electrochemically treated titanium samples. The titanium exposed to the phosphate buffered saline solution reveal the increased topography roughness (5–15 nm) of oxide thin layer on its surface. The introduction of H2O2 into the solution leads to the enhanced dissolution of the titanium oxide film resulting in a more defective surface. The I(V) dependencies obtained from spectroscopic measurements are indicative of the semiconducting properties of the thin oxide layer.
Explanation of the origin of electrons in the unintentionally doped InAs/AlSb system
Authors: Shen Jun , Goronkin Herb , Dow John D. , Ren Shang Yuan
Published: July 2002

Theoretical studies are summarized which show that (i) anion-site antisite defects AlSb in AlSb barriers and (ii) interfacial native defects such as AsAl at AlAs-like interfaces are the origin of electrons in nominally undoped InAs quantum wells of AlSb/InAs heterojunctions. Tamm interface states, while present at the InSb-like interfaces, lie at too low energy to account for the observed carrier densities.
Scanning force microscopy characterization of individual carbon nanotubes on electrode arrays
Authors: Muster J. , Burghard M. , Roth S. , Duesberg G. S. , Hernández E. , Rubio A.
Published: July 2002

We report the controlled adsorption of individual multi- and single-walled carbon nanotubes from purified aqueous dispersions onto chemically modified silicon oxide surfaces as well as on predefined electrode patterns (100 nm electrode separation). Detailed structural investigations were performed using scanning force microscopy (SFM) without interference from other carbon materials. These studies revealed the striking flexibility of single-walled nanotubes. In contrast to comparably stiff multi-walled nanotubes (MWNTs) which bridged the electrode lines, single-walled nanotubes (SWNTs) were found to follow the profile of the underlying electrodes almost exactly. Based upon the SFM cross sectional analysis of an individual MWNT (8 nm diameter) a Young modulus of about 1 TPa was estimated. Furthermore, nanotube adsorption from the surfactant-stabilized dispersions led to flow-induced orientation processes.
Mechanisms of electron field emission from diamond, diamond-like carbon, and nanostructured carbon
Authors: Robertson J.
Published: July 2002

It is argued that the facile electron field emission from carbon systems occurs primarily because surface groups such as C–H can produce large changes in local electron affinity, so that electric fields from the anode can be focused toward unhydrogenated surface areas of high affinity, the fields ending on negative charges in an underlying depletion layer. The resulting downward band bending creates large surface fields which allow Fowler–Nordheim emission, while not exceeding the material’s breakdown field.
Diring Yuriakh: A Lower Paleolithic Site in Central Siberia
Authors: Waters Michael R. , Forman Steven L. , Pierson James M.
Published: July 2002

Lower Paleolithic artifacts have been recovered from a single occupation surface within stratified deposits at Diring Yuriakh, an archaeological site in central Siberia. Thermoluminescence age estimates from eolian sediments indicate that the cultural horizon is greater than 260,000 years old. Diring Yuriakh is an order of magnitude older than documented Paleolithic sites in Siberia and is important for understanding the timing of human expansion into the far north, early adaptations to cold climates, and the peopling of the Americas.
Equatorial X-ray Emissions: Implications for Jupiter's High Exospheric Temperatures
Authors: Waite J. H. , Gladstone G. R. , Lewis W. S. , Drossart P. , Cravens T. E. , Maurellis A. N. , Mauk B. H. , Miller S.
Published: July 2002

Observations with the High Resolution Imager on the Rüntgensatellit reveal x-ray emissions from Jupiter's equatorial latitudes. The observed emissions probably result from the precipitation of energetic (>300 kiloelectron volts per atomic mass unit) sulfur and oxygen ions out of Jupiter's inner radiation belt. Model calculations of the energy deposition by such heavy ion precipitation and of the resulting atmospheric heating rates indicate that this energy source can contribute to the high exospheric temperatures (>800 kelvin at 0.01 microbar) measured by the Galileo probe's Atmospheric Structure Instrument. Low-latitude energetic particle precipitation must therefore be considered, in addition to other proposed mechanisms such as gravity waves and soft electron precipitation, as an important source of heat for Jupiter's thermosphere.
Creep Response of the Hayward Fault to Stress Changes Caused by the Loma Prieta Earthquake
Authors: Lienkaemper James J. , Galehouse Jon S. , Simpson Robert W.
Published: July 2002

In 1996, an 18-millimeter creep event, the largest ever observed on the Hayward fault, occurred between surveys 63 days apart. This event marked the end of a period of severely reduced creep on the southern part of the fault that began after the 1989 Loma Prieta, California, earthquake. The reduction in creep was consistent with elastic models for earthquake-induced static stress changes on the Hayward fault. These data suggest that creep observations can indicate regional stress changes of about 1 bar or less.
Negative Regulation by HLA-DO of MHC Class II-Restricted Antigen Processing
Authors: Denzin Lisa K. , Sant'Angelo Derek B. , Hammond Craig , Surman Michael J. , Cresswell Peter
Published: July 2002

HLA-DM is a major histocompatibility complex (MHC) class II–like molecule that facilitates antigen processing by catalyzing the exchange of invariant chain–derived peptides (CLIP) from class II molecules for antigenic peptides. HLA-DO is a second class II–like molecule that physically associates with HLA-DM in B cells. HLA-DO was shown to block HLA-DM function. Purified HLA-DM-DO complexes could not promote peptide exchange in vitro. Expression of HLA-DO in a class II
+
and DM
+
, DO
−
human T cell line caused the accumulation of class II–CLIP complexes, indicating that HLA-DO blocked DM function in vivo and suggesting that HLA-DO is an important modulator of class II–restricted antigen processing.
Evidence for the Use of Fire at Zhoukoudian, China
Authors: Weiner Steve , Xu Qinqi , Goldberg Paul , Liu Jinyi , Bar-Yosef Ofer
Published: July 2002

Zhoukoudian is widely regarded as having the oldest reliable evidence for the controlled use of fire by humans. A reexamination of the evidence in Layer 10, the earliest archaeological horizon in the site, shows that burned and unburned bones are present in the same layer with stone tools. However, no ash or charcoal remnants could be detected. Hence, although indirect evidence for burning is present, there is no direct evidence for in situ burning.
Localization of Bacterial DNA Polymerase: Evidence for a Factory Model of Replication
Authors: Lemon Katherine P. , Grossman Alan D.
Published: July 2002

Two general models have been proposed for DNA replication. In one model, DNA polymerase moves along the DNA (like a train on a track); in the other model, the polymerase is stationary (like a factory), and DNA is pulled through. To distinguish between these models, we visualized DNA polymerase of the bacterium
Bacillus subtilis
in living cells by the creation of a fusion protein containing the catalytic subunit (PolC) and green fluorescent protein (GFP). PolC-GFP was localized at discrete intracellular positions, predominantly at or near midcell, rather than being distributed randomly. These results suggest that the polymerase is anchored in place and thus support the model in which the DNA template moves through the polymerase.
Nonequilibrium Self-Assembly of Long Chains of Polar Molecules in Superfluid Helium
Authors: Nauta K. , Miller R. E.
Published: July 2002

It is shown that in the low-temperature (0.37 kelvin) environment of superfluid helium droplets, long-range dipole-dipole forces acting between two polar molecules can result in the self-assembly of noncovalently bonded linear chains. At this temperature the effective range of these forces is on the nanometer scale, making them important in the growth of nanoscale structures. In particular, the self-assembly of exclusively linear hydrogen cyanide chains is observed, even when the folded structures are energetically favored. This suggests a design strategy for the growth of new nanoscale oligomers composed of monomers with defined dipole (or higher order) moment directions.
Chaperonin Function: Folding by Forced Unfolding
Authors: Shtilerman Mark , Lorimer George H. , Walter Englander S.
Published: July 2002

The ability of the GroEL chaperonin to unfold a protein trapped in a misfolded condition was detected and studied by hydrogen exchange. The GroEL-induced unfolding of its substrate protein is only partial, requires the complete chaperonin system, and is accomplished within the 13 seconds required for a single system turnover. The binding of nucleoside triphosphate provides the energy for a single unfolding event; multiple turnovers require adenosine triphosphate hydrolysis. The substrate protein is released on each turnover even if it has not yet refolded to the native state. These results suggest that GroEL helps partly folded but blocked proteins to fold by causing them first to partially unfold. The structure of GroEL seems well suited to generate the nonspecific mechanical stretching force required for forceful protein unfolding.
Regulation of Antigen-Specific CD8
+
T Cell Homeostasis by Perforin and Interferon-γ
Authors: Badovinac Vladimir P. , Tvinnereim Amy R. , Harty John T.
Published: July 2002

T cell memory depends on factors that regulate expansion and death of these cells after antigenic stimulation. Mice deficient in perforin and interferon-γ (IFN-γ) exhibited increased expansion, altered immunodominance, and decreased death of antigen-specific CD8
+
T cells after infection with an attenuated strain of
Listeria monocytogenes
, which was cleared from these mice. Expansion of CD8
+
T cells was controlled by perforin, whereas IFN-γ regulated immunodominance and the death phase. Thus, perforin and IFN-γ regulate distinct elements of CD8
+
T cell homeostasis independently of their role as antimicrobial effector molecules.
Quantitative Imaging of Lateral ErbB1 Receptor Signal Propagation in the Plasma Membrane
Authors: Verveer Peter J. , Wouters Fred S. , Reynolds Andrew R. , Bastiaens Philippe I. H.
Published: July 2002

Evidence for a new signaling mechanism consisting of ligand-independent lateral propagation of receptor activation in the plasma membrane is presented. We visualized the phosphorylation of green fluorescent protein (GFP)–tagged ErbB1 (ErbB1-GFP) receptors in cells focally stimulated with epidermal growth factor (EGF) covalently attached to beads. This was achieved by quantitative imaging of protein reaction states in cells by fluorescence resonance energy transfer (FRET) with global analysis of fluorescence lifetime imaging microscopy (FLIM) data. The rapid and extensive propagation of receptor phosphorylation over the entire cell after focal stimulation demonstrates a signaling wave at the plasma membrane resulting in full activation of all receptors.
Discovery of the Atomic Oxygen Green Line in the Venus Night Airglow
Authors: Slanger T. G. , Cosby P. C. , Huestis D. L. , Bida T. A.
Published: July 2002

Green line emission at 557.7 nanometers arising from the O(
1
S −
1
D) transition of atomic oxygen has been observed on the nightside of Venus with HIRES, the echelle spectrograph on the W. M. Keck I 10-meter telescope. We also observe optical emissions of molecular oxygen, consistent with the spectra from the Venera orbiters, but our green line intensity is so high that we cannot explain how it could be inconspicuous in the Venera spectra. An upper limit for the intensity of the O(
1
D −
3
P) oxygen red line at 630 nanometers has also been obtained. The large green/red ratio indicates that the source is not associated with the Venus ionosphere. An important conclusion is that observation of the green line in a planetary atmosphere is not an indicator of an atmosphere rich in molecular oxygen.
Cytochrome P-450 2C9 Sensitizes Human Prostate Tumor Cells to Cyclophosphamide via a Bystander Effect
Authors: Zhou Diansong , Lu Yi , Steiner Mitchell S. , Dalton James T.
Published: July 2002

The goal of the present study was to examine the ability of cytochrome P450-2C9 (CYP2C9) to activate cyclophosphamide (CPA) and elicit tumor cell death. A CYP2C9-deficient human lymphoblastoid cell line (AHH-1 cells) and a derivative cell line (H2C9 cells) stably transfected with a cDNA encoding CYP2C9 were used. The catalytic activity present in cell lines was examined by measuring the conversion of diclofenac, a CYP2C9-specific substrate, to its 4′-hydroxy metabolite by high-pressure liquid chromatography. Initial rate plots were constructed and the maximal rate of formation (
V
max
) and the Michaelis-Menten constant (

K
m

) for diclofenac metabolism were determined. Cytotoxicity was studied by exposing the cells to 0.01 to 4 mM CPA in the presence or absence of sulfaphenazole, a CYP2C9-specific inhibitor. Cell survival was quantitated by determination of the level of tritiated thymidine incorporation. H2C9 cells quickly metabolized diclofenac, indicating the presence of high levels of CYP2C9. Kinetic experiments demonstrated a
V
max
and

K
m

of 0.62 ± 0.012 pmol/min/10
6
cells and 6.16 ± 0.62 μM, respectively, for diclofenac metabolism. Diclofenac 4′-hydroxylase activity was undetectable in AHH-1 cells. H2C9 cells were more sensitive to the cytotoxic effects of CPA (50% inhibitory concentration [IC
50
], 0.80 ± 0.03 mM) than AHH-1 cells (IC
50
, 4.07 ± 0.35 mM). The cytotoxicity (IC
50
, 1.99 ± 0.14 mM) of CPA to H2C9 cells was blocked by sulfaphenazole, demonstrating that the chemosensitivity of these cells is a consequence of intracellular prodrug activation. H2C9 cells mediated a bystander killing effect for CYP2C9-negative PPC-1 cells, reducing the IC
50
of CPA from about 14 to 3.62 ± 0.73 mM in PPC-1 cells when they were cocultured with H2C9 cells. These results suggest that the enzyme-prodrug system of CYP2C9 and CPA may be an effective combination for gene-directed enzyme prodrug therapy. Ongoing studies are examining the utility of this system for use in prostate cancer cells.
Comparison of Human Immunodeficiency Virus Type 1 Pr55
Gag
and Pr160
Gag-Pol
Processing Intermediates That Accumulate in Primary and Transformed Cells Treated with Peptidic and Nonpeptidic Protease Inhibitors
Authors: Speck R. Renae , Flexner Charles , Tian Chun-Juan , Yu Xiao-Fang
Published: July 2002

Human immunodeficiency virus type 1 (HIV-1) produces two polyproteins, Pr55
Gag
and Pr160
Gag-Pol
, that are cleaved into mature functional subunits by the virally encoded protease. Drugs that inhibit this protease are an important part of anti-HIV therapy. We studied the ordered accumulation of Gag and Gag-Pol processing intermediates by variably blocking the protease with HIV-1 protease inhibitors (PIs). Variable protease inhibition caused accumulation of a complex pattern of processing intermediates, which was the same after incubating HIV-1-infected cells with increasing concentrations of either one of the peptidomimetic inhibitors indinavir, saquinavir (SQV), ritonavir (RTV), nelfinavir, and SC-52151 or one of the nonpeptidomimetic inhibitors DMP450, DMP323, PNU-140135, and PNU-109112 for 3 days. The patterns of Gag and Gag-Pol processing intermediate accumulation were nearly identical when the following were compared: cell- versus virion-associated proteins, HIV-1-infected transformed cell lines versus primary human peripheral blood mononuclear cells (PBMCs) and HIV-1
MN
versus HIV-1
IIIB
virus strains. RTV was a more potent inhibitor of p24 production in PBMCs than SQV by approximately 7-fold, whereas SQV was a more potent inhibitor in transformed cells than RTV by approximately 30-fold. Although the antiretroviral potency of HIV-1 PIs may change as a function of cell type, the polyprotein intermediates that accumulate with increasing drug concentrations are the same. These results support sequential processing of Gag and Gag-Pol polyproteins by the HIV-1 protease and may have important implications for understanding common cross-resistance pathways.
Effect of Metronidazole on the Pathogenicity of Resistant
Bacteroides
Strains in Gnotobiotic Mice
Authors: Diniz Claudio Galuppo , Cara Denise Carmona , Nicoli Jacques Robert , Farias Luiz De Macedo , De Carvalho Maria Auxiliadora Roque
Published: July 2002

Metronidazole is widely used to treat protozoan and fungal infections. As an antibacterial drug, it is used mainly against anaerobes. Among anaerobes, the
Bacteroides fragilis
group is the most relevant in terms of frequency of recovery and antimicrobial resistance patterns. The use of metronidazole and other antimicrobial drugs induces morphological changes in this bacterial group. The present study investigated in vivo if these morphological modifications were accompanied by changes in virulence patterns by using germfree mice experimentally challenged with metronidazole-resistant
Bacteroides
strains before and after exposure to metronidazole. It was observed that metronidazole-resistant strains were more virulent after contact with the drug, as demonstrated by anatomicopathologic data for spleen, liver, and small intestine samples. These results suggest that long-term therapy and high metronidazole concentrations could interfere with microbial pathogenicity, resulting in changes to host-bacterium relationships.
Geographical Segregation of the Neurotoxin-Producing Cyanobacterium
Anabaena circinalis
Authors: Beltran E. Carolina , Neilan Brett A.
Published: July 2002

Blooms of the cyanobacterium
Anabaena circinalis
are a major worldwide problem due to their production of a range of toxins, in particular the neurotoxins anatoxin-a and paralytic shellfish poisons (PSPs). Although there is a worldwide distribution of
A. circinalis
, there is a geographical segregation of neurotoxin production. American and European isolates of
A. circinalis
produce only anatoxin-a, while Australian isolates exclusively produce PSPs. The reason for this geographical segregation of neurotoxin production by
A. circinalis
is unknown. The phylogenetic structure of
A. circinalis
was determined by analyzing 16S rRNA gene sequences.
A. circinalis
was found to form a monophyletic group of international distribution. However, the PSP- and non-PSP-producing
A. circinalis
formed two distinct 16S rRNA gene clusters. A molecular probe was designed, allowing the identification of
A. circinalis
from cultured and uncultured environmental samples. In addition, probes targeting the predominantly PSP-producing or non-PSP-producing clusters were designed for the characterization of
A. circinalis
isolates as potential PSP producers.