bluuebunny/crossref_metadata_2025 huggingface dataset:
Heteroepitaxy of GaP on Si(100)
Authors: Bachmann K. J. , Rossow U. , Sukidi N. , Castleberry H. , Dietz N.
Published: July 2002

In this article, we analyze the kinetics of heteroepitaxial growth of GaP on Si(100) by pulsed chemical beam epitaxy on the basis of results obtained by real-time optical process monitoring. In view of the large barrier to epitaxial growth on oxygen or carbon contaminated silicon surface elements and the low stacking fault energy for GaP, residual contamination of the silicon surface contributes to defect formation in the initial phase of GaP heteroepitaxy on Si, and requires special measures, such as surface structuring, to limit the propagation of defects into the epitaxial film. The control of the supersaturation during the first 10–20 s of film formation is essential for the quality of subsequent epitaxial growth and is limited to a narrow process window between three-dimensional nucleation and overgrowth at low Ga supersaturation and gallium-cluster formation at high Ga supersaturation. Steady state heteroepitaxial growth is described by a four-layer stack substrate/epilayer/surface reaction layer (SRL)/ambient and, depending of the source vapor flux, allows for more than monolayer coverage. Under this special condition of low-temperature CBE, the kinetics of chemical reactions in the SRL is composed of homogeneous reactions creating/consuming intermediates that participate in surface reactions including the incorporation of Ga and P atoms into the epitaxial film. For the homogeneous part of the surface kinetics the dipole approximation provides an adequate correlation of the changes in the dielectric function of the SRL to the activities of randomly distributed reactants and products. No adequate correlation of the dielectric function exists to the concentrations of strongly bonded surface atoms and surface molecules. Therefore, quantitative assessments of the heterogeneous kinetics on the crystal surface cannot use real-time optical monitoring as a reliable basis.
Acoustic and dynamic force microscopy with ultrasonic probes
Authors: Murdfield Th. , Fischer U. C. , Fuchs H. , Volk R. , Michels A. , Meinen F. , Beckman E.
Published: July 2002

Ultrasonic quartz resonators with quality factors ≳5000 allow one to probe surface morphologies directly, i.e., without any additional sensors determining the vertical position of the probing tip. With tip radii of several tenths of a micrometer, hydrodynamic friction forces dominate the tip/surface interaction, while, by using sharp atomic force microscopy tips glued onto rod-shaped 1 MHz length extension quartz resonators, the conventional dynamical quasi noncontact atomic force microscopy mode is obtained. This allows one to resolve atomically sized structures on mica in air. The ultrasonic sensors with spring moduli of 3000 to 400 000 N/m exhibit an extremely high sensitivity allowing one to probe even monomolecular organic layers without destruction.
Fabrication of high-aspect-ratio InP-based vertical-cavity laser mirrors using CH4/H2/O2/Ar reactive ion etching
Authors: Schramm Jeff E. , Babić Dubravko I. , Hu Evelyn L. , Bowers John E. , Merz James L.
Published: July 2002

The addition of oxygen to methane/hydrogen/argon reactive ion etching (RIE) processes can mitigate polymer deposition, and produce vertical etched sidewalls. This work contrasts the various ways in which the oxygen may be incorporated into methane/hydrogen/argon reactive ion etching of deep (>5 μm) InGaAsP/InP multilayers. Three methods are investigated: a “continuous” process in which a fixed amount of oxygen is added to methane/hydrogen/argon for the duration of the etch, a “cyclical” process in which the methane/hydrogen/argon RIE processes alternates with oxygen RIE, and a hybrid process which incorporates the advantages of both former methods. These processes are applied to the fabrication of tall (>10 μm) InGaAsP/InP quarter-wave mirrors for long-wavelength vertical-cavity lasers; the various benefits and limitations of the various approaches are discussed. It is found that the hybrid process allows formation of deeply etched structures (15 μm) with vertical profiles.
Chemical challenge of submicron oxide etching
Authors: McNevin S. C. , Guinn K. V. , Taylor J. Ashley
Published: July 2002

In order to achieve the critical dimension control and selectivity requirements of submicron etching, industrial oxide etching tools are utilizing low pressure, high density plasma etching processes. At low pressures, the mean free path (1 cm at 5 mTorr) approaches the distance between the wafer and the chamber interior. This permits chemical interaction between the chamber and the wafer, since some of the gas species leaving the chamber interior can reach the wafer before undergoing further gas/gas collisions. The contribution from gas species leaving the wafer clamp is even greater, since the distance between the wafer edge and the clamp is smaller than the mean free path under all pressure conditions. This leads to the manufacturing problem that processing results become dependent on the previous thermal and chemical history of the chamber interior and the clamp. Results obtained using an Applied Materials high density plasma contact etcher will illustrate the generic nature of this history problem. The dependence of the etching performance on the chamber temperature and previous chamber oxygen cleans will be discussed. The manufacturing challenge is to reproducibly achieve acceptable processing results in systems where the contribution to the processing from the chamber is continuously varying. Not only must the current state of the etching tool be known, but the processes required to recover a baseline operating condition must be understood. It will be argued that this will require real time diagnostic control of the process. It is also suggested that the trend to low pressure systems may have been taken too far, and pressures in the 20–40 mTorr range might result in better process control in terms of reducing the chamber “history” effect. (This article summarizes an invited presentation given by McNevin at the 42nd Annual American Vacuum Society national meeting in the Plasma Science Division.)
Work function estimate for electrons emitted from nanotube carbon cluster films
Authors: Gulyaev Yu. V. , Sinitsyn N. I. , Torgashov G. V. , Mevlyut Sh. T. , Zhbanov A. I. , Zakharchenko Yu. F. , Kosakovskaya Z. Ya. , Chernozatonskii L. A. , Glukhova O. E. , Torgashov I. G.
Published: July 2002

Relying on the obtained theoretical and experimental results the electronic work function from a nanotube carbon film was estimated. It was shown that these structures have an electronic work function that is substantially lower than that for graphite. The influence of the film surface relief on its emission was regarded.
Kink defects and Fermi level pinning on (2×4) reconstructed molecular beam epitaxially grown surfaces of GaAs and InP studied by ultrahigh-vacuum scanning tunneling microscopy and x-ray photoelectron spectroscopy
Authors: Ishikawa Yasuhiko , Fukui Takashi , Hasegawa Hideki
Published: July 2002

The relationship between kink defects and Fermi level pinning on molecular beam epitaxially grown GaAs and InP(001)-(2×4) surfaces is studied in detail by scanning tunneling microscopy and x-ray photoelectron spectroscopy. In Si-doped GaAs, the kink density increased with doping as previously found. However, actual density depended very much on the reconstruction phases and experimental conditions. At high Si doping levels, Fermi level was strongly pinned below mid-gap, but the measured kink density was found not to be large enough to explain pinning by the previous kink-deep-acceptor model assuming that each kink forms a single discrete level. In Si-doped InP, the kink density remained constant with the increase of Si doping, although the Fermi level was pinned above mid-gap. The result cannot be explained by the kink-deep-acceptor model either.
Adsorption geometries of 1,9-decadiene on Si(111) 7×7 studied by scanning tunneling microscopy
Authors: Shachal D. , Manassen Y.
Published: July 2002

The scanning tunneling microscope has been used to characterize the adsorption sites of 1,9-decadiene on Si(111) 7×7. The molecule which reacts with each of its two ends, appears as pairs of reacted adatoms on the atomically resolved surface. The size of the molecule permits adsorption with the two ends of the molecule situated either at the same half of the unit cell or in a bridging geometry with one end situated in the faulted half and the other in the unfaulted half. Three types of bridging pairs are found. The results found are compared with that of two other dienes: 1,7-octadiene and 1,13-tetradecadiene.
Investigation of the proximity effect in amorphous AlF3 electron-beam resists
Authors: Chen G. S. , Humphreys C. J.
Published: July 2002

A proximity effect occurs when two features having a close proximity are exposed using conventional organic electron-beam resists, subsequently causing overexposure of the region between the two features and ultimate broadening of the features. In this study, we employ probes of a through-focal series to irradiate amorphous AlF3 (a-AlF3) inorganic films. A proximity effect of a very different nature is also observed while employing the a-AlF3 films as self-developing electron-beam resists. Such an effect distorts the closely spaced features and sets a limit on the proximity of those nanometer-scaled features. According to the results of electron microscopy obtained while examining the peculiar behavior of the proximity effect, mass-transport phenomena are critical in the damaging behavior of the a-AlF3 films. Besides, our results presented herein demonstrated the ability of the through-focal probes to produce aluminum nanostructures of varying sizes in thin films containing a-AlF3 self-developing resists.
Minimum emission current of liquid metal ion sources
Authors: Beckman J. C. , Chang T. H. P. , Wagner A. , Pease R. F. W.
Published: July 2002

The inability of liquid metal ion sources (LMIS) to operate at low dc emission currents limits their performance. We briefly describe a model that explains why LMIS have a minimum dc emission current (Imin) and also predicts Imin as a function of the temperature and the properties of the liquid metal. The model predicts that Imin=217+0.744T(nA) for gallium LMIS, where T is the temperature (K). Measurements of Imin for gallium LMIS between 30 and 890 °C are in reasonable agreement with the model. A better fit to this data, however, is given by Imin=1187 exp(−0.026/kT)(nA) where k is Boltzmann’s constant (eV/K). Below 30 °C, Imin drops precipitously—values as low as 380 nA have been measured at temperatures as low as 25.8 °C. This drop is attributed to a supercooling effect that is not accounted for in the model. Imin is also calculated for 17 pure-elemental LMIS at their melting points, and found to vary from 10 nA for mercury to 1.0 μA for aluminum. Imin is measured to be much lower for bismuth LMIS than for gallium LMIS, as predicted by the model, although difficulties with the bismuth LMIS have allowed only an upper limit of Imin⩽77 nA to be measured. The model also suggests possibilities for improving ion sources by reducing or eliminating Imin.
Fabrication of planar silicon nanowires on silicon-on-insulator using stress limited oxidation
Authors: Kedzierski Jakub , Bokor Jeffrey , Kisielowski Christian
Published: July 2002

A new method is proposed for the fabrication of planar single crystal silicon nanowires down to 8 nm in diameter. In this method silicon lines are defined on silicon-on-insulator with electron beam lithography followed by a metal liftoff process and a silicon plasma etch. Low temperature oxidation is then used to shrink these lines to a sub-10 nm diameter. Normal stress generated by the expansion of the viscous oxide during oxidation eventually stops the reaction, leaving a small silicon core at the center of the line. The effect of the crystallographic orientation of the line and the stress complications caused by the substrate are investigated.
Photoluminescence study on twenty GaAs/Al0.3Ga0.7As tilted T-shaped quantum wires fabricated by glancing-angle molecular beam epitaxy
Authors: Tomita N. , Takekawa K. , Ohta K. , Shimomura S. , Hiyamizu S. , Fujita K. , Egami N. , Okamoto Y.
Published: July 2002

Twenty GaAs/Al0.3Ga0.7As tilted T-shaped quantum wires (T-QWRs) on a (111)B facet were uniformly fabricated with a two-step growth of molecular beam epitaxy (MBE), which consists of a glancing-angle MBE of GaAs/Al0.3Ga0.7As multi-quantum well layer with 20 GaAs wells (a well width of Lw=6.1 nm) on reverse-mesa stripes on a (001) GaAs substrate and MBE overgrowth of a GaAs/Al0.3Ga0.7As single-quantum well with Lw=6.3 nm on a (111)B facet. Full width at half maximum of a photoluminescence peak (λ=792 nm) from the tilted T-QWRs was as small as 8.7 meV at 28 K, which is comparable with those (6 meV,Reference 5 10 meVReference 10) of conventional GaAs/Al0.3Ga0.7As T-QWRs (6.8 nm/5.8 nm, 7 nm/7 nm) fabricated by the cleaved-edge overgrowth.
Influence of data analysis and other factors on the short-term stability of vertical scanning-probe microscope calibration measurements
Authors: Edwards Hal , Jo/rgensen Jan Friis , Dagata John , Strausser Yale , Schneir Jason
Published: July 2002

We report a study of a fundamental limit to the accuracy of vertical measurements made using scanning-probe microscopes (SPM): the short-term stability of a vertical calibration using a waffle-pattern artifact. To test the instrumental component of this stability, we acquired three data series, at different humidity levels. We compare the variations in waffle-pattern depth in these three data series with the differences in depth estimates using several different analysis methods. The three methods tested are: a histogram method, the scanning-probe image processor, and the polynomial step-function fit. To clarify the importance of the analysis method, a discussion of the different leveling, averaging, and depth-estimation aspects of the various methods is presented. To understand the true repeatability limit of SPM calibration, it is necessary to treat imaging artifacts such as tilt, nonlinearities, and image bow carefully. We find that, when such care is taken, the dependence of the average waffle-cell depth on algorithm is around 0.1%. This is less than the standard deviation of the step-height estimates of around 0.5%, which may be attributed to short-term instrumental variations in vertical SPM calibration. This is a far better stability than the 5%–10% variations observed under long-term aging of the piezo scanner. However, the histogram algorithm, which does not correct image bow, gives an average waffle-cell depth estimate which is nearly 1% higher than the others, indicating that careful image analysis is necessary if the measurement accuracy is to be comparable to the short-term stability of the piezo scanner.
Residence time effects on SiO2/Si selective etching employing high density fluorocarbon plasma
Authors: Chinzei Yasuhiko , Ichiki Takanori , Ikegami Naokatsu , Feurprier Yannick , Shindo Haruo , Horiike Yasuhiro
Published: July 2002

The residence time effects on SiO2 etching characteristics using inductively coupled plasma of C4F8 alone were first studied in the range from 6 to 300 ms. It was then found that SiO2 and Si etch rates were minimum at a residence time of 25 ms, at which the CF1+ ion density and the fluorocarbon polymer deposition rate measured at 130 °C became maximum. From this good correspondence, the SiO2 etching was considered to follow a reaction model where the CF1+ ions might contribute to the polymer deposition, thus suppressing the SiO2 etching, and where the dominantly observed CF3+ ions could etch SiO2 on the assumption of elevated SiO2 surface temperature due to the ion bombardment. Next, in the condition of short residence times (<25 ms), Ar was added to C4F8 in order to allow Ar+ ions to remove the fluorocarbon polymer film that is responsible for the reduction of the SiO2 etch rate. For a residence time of 10 ms the SiO2 etch rate continuously increased with the Ar concentration up to a maximum etch rate of 0.4 μm/min for 90% Ar addition. Various plasma diagnostics demonstrated that the 90% Ar addition resulted in an increase of the CFx+ (x=1–3), C+, and Ar+ ion densities, in contrast to a decrease of the CF3 radical density. Metastable Ar atoms as well as highly elevated electron temperature are considered to be responsible for the increasing ion species. Consequently SiO2/Si contact hole features with 0.18 μm opening and 2 μm depth were successfully fabricated employing the 90% Ar/C4F8 mixture at a residence time of 10 ms. The He addition has also been investigated and showed similar changes in plasma characteristics to those observed for Ar addition, but the “etch stop” occurred. This probably resulted from the poor sputtering effect of He+ ions due to their light mass.
Properties of Er-related emission in in situ doped Si epilayers grown by molecular beam epitaxy
Authors: Buyanova I. A. , Chen W. M. , Pozina G. , Ni W.-X. , Hansson G. V. , Monemar B.
Published: July 2002

Optical properties of in situ Er-doped Si epilayers grown by molecular beam epitaxy at low temperatures (<450 °C), using oxygen and fluorine as codopants, are studied using photoluminescence (PL) and electroluminescence (EL) spectroscopies. Sharp and intense Er-related PL is observed at low temperatures from the as-grown Si epilayers with Er concentrations up to 5×1019 cm−3. The structure of the dominant optically active Er centers is shown to be dependent of the codopants used and can essentially be modified by postgrowth thermal annealing. The chemical nature of the codopants as well as postgrowth treatments have only a minor effect on the thermal quenching of Er-related emissions. Thermal quenching of the Er-related PL and EL is shown to occur with a similar activation energy suggesting the same quenching mechanism may be involved for both processes. The observed higher EL efficiency at elevated temperatures is tentatively attributed to the higher concentration of excited Er ions under impact ionization conditions (EL).
Properties and reliability of ultrathin oxides grown on four inch diameter silicon wafers by microwave plasma afterglow oxidation
Authors: Chen Cheng-Rong , Hu Shu-Fen , Chen Po-ching , Hwang Huey-liang , Hsia Liang-Choo
Published: July 2002

We have successfully grown ultrathin oxides on large area of silicon wafers by the microwave plasma afterglow oxidation method. Analysis of the Fourier transform infrared spectra indicates that the general bonding structures of the ultrathin oxides grown by microwave plasma afterglow oxidation at 700 °C could be identical to those grown by dry O2 thermal oxidation. Electrical property measurements (e.g., time-zero dielectric breakdown and time-dependent dielectric breakdown) are also investigated. Based on our results, we conclude that microwave plasma afterglow oxidation is a useful method for the preparation of large area ultrathin oxide films on silicon substrates.
Fabrication of multipurpose piezoresistive Wheatstone bridge cantilevers with conductive microtips for electrostatic and scanning capacitance microscopy
Authors: Gotszalk T. , Radojewski J. , Grabiec P. B. , Dumania P. , Shi F. , Hudek P. , Rangelow I. W.
Published: July 2002

The fabrication and performance of a microprobe with multipurpose capabilities for scanning probe microscopy is presented in this article. Atomic force microscopy (AFM), scanning capacitance microscopy, and electrostatic force microscopy measurements can be simultaneously performed with the probe in which a silicon tip is integrated with a piezoresistive cantilever. Fabrication of the microprobe is based on double side bulk/surface micromachining of silicon on insulator (SOI) substrates. The novelty of this device is a highly doped silicon tip with a curvature radius of about 20 nm which is electrically isolated from the silicon cantilever by the buried oxide layer of the SOI substrate. At the beam supporting point a piezoresistive Wheatstone bridge is fabricated to allow the deflection of the microtip to be monitored. This cantilever displacement detection system enables measurements in vacuum and simplifies the design of the AFM head. Experimental measurements agree well with theoretical estimates of the sensitivity of the microprobe.
Role of cerium dioxide in a tantalum diffusion barrier film for a Cu/Ta+CeO2/Si structure
Authors: Yoon Dong-Soo , Baik Hong Koo , Lee Sung-Man
Published: July 2002

The effects of CeO2 addition on the barrier properties of a Ta diffusion barrier were investigated by using four point probe, optical microscopy, Auger electron microscopy, x-ray diffraction spectroscopy, transmission electron microscopy, and x-ray photoelectron spectroscopy. The amorphous microstructure of the Ta+CeO2 diffusion barrier was up to high temperatures and the thermal stability of the Ta+CeO2/Si interface was higher than that of Ta/Si interface. The Cu/Ta+CeO2/Si system retained its structure up to 800 °C without any increase in resistivity, whereas the Cu/Ta/Si structure degraded after annealing at 550 °C. The cerium dioxide (CeO2) in the Ta layer plays a role in amorphizing the microstructure of the Ta+CeO2 barrier film and strongly binds the Ta–Ce–O system during deposition of the Ta layer. It appeared that the thermal stability of Cu/Ta+CeO2 interface as well as that of the Ta+CeO2/Si interface was much higher than that of both the Cu/Ta and the Ta/Si interfaces. Therefore, the Ta film prepared with the addition of CeO2 effectively prevented the interdiffusion of Cu and Si through the diffusion barrier up to 800 °C.
Studies of field emission from bias-grown diamond thin films
Authors: Ding M. Q. , Gruen D. M. , Krauss A. R. , Auciello O. , Corrigan T. D. , Chang R. P. H.
Published: July 2002

We have investigated the field emission properties of diamond films grown under substrate bias conditions in a microwave plasma chemical vapor deposition system, with a substrate temperature of 800 °C, microwave power of 600 W, and a total pressure of 11 Torr. One group of films was grown with a substrate bias of −100 V in gas mixtures of 1% N2 and 1%–20% CH4 in H2, while a second group of films was grown with a substrate bias ranging from +100 to −150 V in a gas mixture of 1%N2–10%CH4–89%H2. The field emission performance in terms of threshold field and emission current improved considerably as a function of increasing CH4 concentration and negative bias voltage. Ultraviolet Raman analysis showed that the field emission enhancement resulting from an increase in CH4 concentration from 1% to 5% correlates with a decrease in the sp3 bonding character in the diamond film. The dependence of field emission on negative bias voltage appears to be correlated with ion bombardment-induced damage in the film during growth. The scanning electron microscopy image of the film grown with −150 V bias showed: smaller surface topographic features as compared to films grown under 0 and +100 V bias. The film grown with a bias of −150 V showed the lowest threshold field (∼2.0 V/μm) corresponding to an emission current density of 12.7 μA/cm2. J vs E0 measurements across a length of 40 mm over the film showed a uniform threshold field (2.0±0.55 V/μm). The film grown with a positive bias (+100 V) showed a relatively poor field emission performance.
Scanning tunneling microscopy tip shape imaging by “shadowing”: Monitoring of in situ tip preparation
Authors: Voigtländer Bert , Kästner Martin
Published: July 2002

The shape of the scanning tunneling microscope (STM) tip was imaged with nanometer resolution using “shadow images.” When a Ge molecular beam is evaporated from the side onto a Si sample while the STM tip is stationary in tunneling position close to the sample surface, the tip shades part of the sample surface. Subsequent imaging of this shadow area with the STM results in contrast between areas on the sample where Ge was evaporated, and areas where the Ge flux was impeded by the tip. This method was applied to monitor the tip shape after in situ tip preparation processes, like sputtering and heating of the tip.
Effects of oxygen and fluorine on the dry etch characteristics of organic low-k dielectrics
Authors: Baklanov M. R. , Vanhaelemeersch S. , Bender H. , Maex K.
Published: July 2002

The etch characteristics of the low-k dielectrics DVS BCB (BCB hereafter) and SILK in oxygen/fluorine based plasmas are studied. In an O2/NF3 plasma afterglow, the etch rate of both polymers first increases linearly with increasing NF3 concentration, then decreases monotonously. A pure fluorine plasma afterglow does not practically etch either BCB nor SILK but significantly changes their chemical and optical properties. The fluorinated polymers are more chemically stable and have lower refractive indices. Reactive ion etching (RIE) of the polymers shows a different behavior. The etch rate of SILK in a pure oxygen plasma is maximal, but BCB is etched slowly. Fluorine addition results in an increased etch rate of BCB. However, the etch rate of SILK decreases with increasing fluorine concentration. RIE in a pure SF6 plasma shows good etch characteristics and can be used for some practical applications. Mechanisms of the reactions are discussed.
Formation and micromachining of Teflon (fluorocarbon polymer) film by a completely dry process using synchrotron radiation
Authors: Inayoshi Muneto , Ito Masafumi , Hori Masaru , Goto Toshio , Hiramatsu Mineo
Published: July 2002

The development of a new fabrication technique of Teflon microparts using synchrotron radiation (SR) irradiation, the SR ablation process, was described. The anisotropic micromachining and thin film formation of polytetrafluoroethylene, fluorinated ethylene propylene, and perfluoroalkoxy were demonstrated using the SR ablation process. The anisotropic micromachining of Teflon with hole pattern of 2 μm diam was successfully performed, and the micromachining of Teflon with a high aspect ratio of 50 was achieved. Moreover, Teflon films with flat surface were formed at a high rate by the SR ablation of Teflon at the substrate temperature above 200 °C.
Energy-dependent conduction band mass of SiO2 determined by ballistic electron emission microscopy
Authors: Ludeke R. , Schenk Andreas
Published: July 2002

Quantum interference oscillations in ballistic electron emission microscopy (BEEM) spectra were observed for metal–oxide–semiconductor structures with 23 and 30 Å SiO2 interlayers. Maxima in the transmission coefficients, obtained from solutions of the one-dimensional Schrödinger equation that included image force corrections, could be matched to the spectral maxima provided that the effective electron mass mox, an adjustable parameter, was increased at each of the consecutive higher energy maxima. The resulting energy dependence or dispersion of mox(E) showed a dependence on the oxide thickness. The 23 and 30 Å oxides exhibit initial (zero kinetic energy) mox values of 0.52 m0 and 0.45 m0, respectively, that disperse upward with energy by ≈0.3 m0 over a 0–2.5 eV range in kinetic energies. The range of mox values observed is substantially lower than the average mox values deduced from quantum interference in Fowler–Nordheim tunneling experiments. The origin of these differences are discussed, and it is argued that BEEM is an inherently simpler and less error prone technique to evaluate mox.
Method for compensation of extreme-ultraviolet multilayer defects
Authors: Ray-Chaudhuri A. K. , Cardinale G. , Fisher A. , Yan P.-Y. , Sweeney D. W.
Published: July 2002

We propose the use of optical proximity correction on absorber features to compensate for the effect of subresolution multilayer defects that would otherwise induce a critical error in linewidth. Initial experiments have been performed which validate this concept. Process window simulations quantify the practical limits of this technique.
Anomalous scanning tunneling microscopy images of GaAs(110) surfaces due to tip-induced band bending
Authors: Aloni S. , Haase G.
Published: July 2002

Scanning tunneling microscopy (STM) images of clean nondegenerate GaAs(110) surfaces, which lack surface states in the band gap, often show the As sublattice at low positive sample bias (empty state image) for p-type material, or conversely, the Ga sublattice at low negative sample bias (filled state image) for n-type material. This happens because as the Fermi level of the tip is positioned inside the GaAs energy gap, no current can flow between the sample and the tip. As a result, the STM feedback brings the tip very close to the surface and the tip-induced electric field is greatly enhanced, creating a subsurface accumulation layer.
Shallow junction formation by decaborane molecular ion implantation
Authors: Foad Majeed A. , Webb Roger , Smith Roger , Matsuo Jiro , Al-Bayati Amir , Sheng-Wang T- , Cullis Tony
Published: July 2002

The formation of P+/N shallow junctions for 0.18 μm technology requires the incorporation of dopant atoms, e.g., boron, at a depth of 540±180 Å below the surface of the crystal. This is done by implanting boron ions at low energies. Another approach for forming shallow junctions involves, in principle, implanting large molecular ions accelerated to higher energies but with an equivalent low energy per boron atom. Decaborane was implanted at 4 and 7 keV and doses of 1E13 and 1E14 cm−2. Junctions with depth <600 Å and sheet resistance of 480 Ω/□ have been demonstrated. Annealed samples were also examined using high-resolution cross-section transmission electron microscopy. Molecular ion implantation of Si using decaborane has been simulated using molecular dynamics at energies between 4 and 1 keV per molecule. The simulation shows local swelling resulting from individual molecular impact, and hydrogen is also implanted into silicon.
Study on nucleation and growth of Ag nanoparticles prepared by radio-frequency sputtering on highly oriented pyrolytic graphite and amorphous carbon
Authors: Lü Wei-Gang , Wu Hao , Xiong Yu-Qing , Guo Yun , Yang De-Quan , Li Hu-Lin
Published: July 2002

The initial growth process of silver nanoparticles on highly oriented pyrolytic graphite (HOPG) and amorphous carbon (a-C) was studied. Scanning tunneling microscopy was used for analysis of the nanoparticles formed on HOPG. Transmission electron microscopy was employed to study the growth procedure on a-C. The number densities (Dn) of silver nanoparticles on both HOPG and a-C remain constant at a very early stage, then both decrease with deposition time and have a saturated value. The Dn decrease suggests migration followed by impingement. We found evidence of migration of 5 nm particles on a-C. Interpretations are given to explain the impingement and shape change of the silver nanoparticles.
Bandtails and defects in microcrystalline silicon (μc-Si:H)
Authors: Fuhs W. , Kanschat P. , Lips K.
Published: July 2002

The transport and recombination mechanism of microcrystalline silicon films, μc-Si:H is determined by specific defects, Si dangling bonds, and localized bandtail states. In spite of the complex heterogeneous structure, many properties of these films can be described in terms of models that have been developed for hydrogenated amorphous silicon, a-Si:H: (1) In the high-temperature regime (T>200 K), the conductivity obeys a Meyer–Neldel relationship which suggests a statistical shift of the Fermi level due to bandtail states. (2) At low temperature (T<50 K), the photoconductivity appears to be due to hopping transport in bandtail states and the recombination occurs by distant pair recombination. The discussion addresses the question of the assignment of the defect and bandtail states to different regions in the heterogeneous structure of the films.
Submicrometer Patterning of Charge in Thin-Film Electrets
Authors: Jacobs Heiko O. , Whitesides George M.
Published: July 2002

Thin-film electrets have been patterned with trapped charge with submicrometer resolution using a flexible, electrically conductive electrode. A poly(dimethylsiloxane) stamp, patterned in bas-relief and supporting an 80-nanometer-thick gold film, is brought into contact with an 80-nanometer-thick film of poly(methylmethacrylate) supported on
n
-doped silicon. A voltage pulse between the gold film and the silicon transfers charge at the contact areas between the gold and the polymer electret. Areas as large as 1 square centimeter were patterned with trapped charges at a resolution better than 150 nanometers in less than 20 seconds. This process provides a new method for patterning; it suggests possible methods for high-density, charge-based data storage and for high-resolution charge-based printing.
Protein Design of an HIV-1 Entry Inhibitor
Authors: Root Michael J. , Kay Michael S. , Kim Peter S.
Published: July 2002

Human immunodeficiency virus type–1 (HIV-1) membrane fusion is promoted by the formation of a trimer-of-hairpins structure that brings the amino- and carboxyl-terminal regions of the gp41 envelope glycoprotein ectodomain into close proximity. Peptides derived from the carboxyl-terminal region (called C-peptides) potently inhibit HIV-1 entry by binding to the gp41 amino-terminal region. To test the converse of this inhibitory strategy, we designed a small protein, denoted 5-Helix, that binds the C-peptide region of gp41. The 5-Helix protein displays potent (nanomolar) inhibitory activity against diverse HIV-1 variants and may serve as the basis for a new class of antiviral agents. The inhibitory activity of 5-Helix also suggests a strategy for generating an HIV-1 neutralizing antibody response that targets the carboxyl-terminal region of the gp41 ectodomain.
Engineering Carbon Nanotubes and Nanotube Circuits Using Electrical Breakdown
Authors: Collins Philip G. , Arnold Michael S. , Avouris Phaedon
Published: July 2002

Carbon nanotubes display either metallic or semiconducting properties. Both large, multiwalled nanotubes (MWNTs), with many concentric carbon shells, and bundles or “ropes” of aligned single-walled nanotubes (SWNTs), are complex composite conductors that incorporate many weakly coupled nanotubes that each have a different electronic structure. Here we demonstrate a simple and reliable method for selectively removing single carbon shells from MWNTs and SWNT ropes to tailor the properties of these composite nanotubes. We can remove shells of MWNTs stepwise and individually characterize the different shells. By choosing among the shells, we can convert a MWNT into either a metallic or a semiconducting conductor, as well as directly address the issue of multiple-shell transport. With SWNT ropes, similar selectivity allows us to generate entire arrays of nanoscale field-effect transistors based solely on the fraction of semiconducting SWNTs.
Interactions of the COP9 Signalosome with the E3 Ubiquitin Ligase SCF
TIR1
in Mediating Auxin Response
Authors: Schwechheimer Claus , Serino Giovanna , Callis Judy , Crosby William L. , Lyapina Svetlana , Deshaies Raymond J. , Gray William M. , Estelle Mark , Deng Xing-Wang
Published: July 2002

The COP9 signalosome is an evolutionary conserved multiprotein complex of unknown function that acts as a negative regulator of photomorphogenic seedling development in
Arabidopsis
. Here, we show that plants with reduced COP9 signalosome levels had decreased auxin response similar to loss-of-function mutants of the E3 ubiquitin ligase SCF
TIR1
. Furthermore, we found that the COP9 signalosome and SCF
TIR1
interacted in vivo and that the COP9 signalosome was required for efficient degradation of PSIAA6, a candidate substrate of SCF
TIR1
. Thus, the COP9 signalosome may play an important role in mediating E3 ubiquitin ligase–mediated responses.
A Cellular Function for the RNA-Interference Enzyme Dicer in the Maturation of thelet-7Small Temporal RNA
Authors: Hutvágner György , McLachlan Juanita , Pasquinelli Amy E. , Bálint Éva , Tuschl Thomas , Zamore Phillip D.
Published: July 2002

The 21-nucleotide small temporal RNA (stRNA)let-7regulates developmental timing inCaenorhabditis elegansand probably in other bilateral animals. We present in vivo and in vitro evidence that inDrosophila melanogastera developmentally regulated precursor RNA is cleaved by an RNA interference-like mechanism to produce maturelet-7stRNA. Targeted destruction in cultured human cells of the messenger RNA encoding the enzyme Dicer, which acts in the RNA interference pathway, leads to accumulation of thelet-7precursor. Thus, the RNA interference and stRNA pathways intersect. Both pathways require the RNA-processing enzyme Dicer to produce the active small-RNA component that represses gene expression.
Synaptotagmin Modulation of Fusion Pore Kinetics in Regulated Exocytosis of Dense-Core Vesicles
Authors: Wang Chih-Tien , Grishanin Ruslan , Earles Cynthia A. , Chang Payne Y. , Martin Thomas F. J. , Chapman Edwin R. , Jackson Meyer B.
Published: July 2002

In the exocytosis of neurotransmitter, fusion pore opening represents the first instant of fluid contact between the vesicle lumen and extracellular space. The existence of the fusion pore has been established by electrical measurements, but its molecular composition is unknown. The possibility that synaptotagmin regulates fusion pores was investigated with amperometry to monitor exocytosis of single dense-core vesicles. Overexpression of synaptotagmin I prolonged the time from fusion pore opening to dilation, whereas synaptotagmin IV shortened this time. Both synaptotagmin isoforms reduced norepinephrine flux through open fusion pores. Thus, synaptotagmin interacts with fusion pores, possibly by associating with a core complex of membrane proteins and/or lipid.
Argonaute2, a Link Between Genetic and Biochemical Analyses of RNAi
Authors: Hammond Scott M. , Boettcher Sabrina , Caudy Amy A. , Kobayashi Ryuji , Hannon Gregory J.
Published: July 2002

Double-stranded RNA induces potent and specific gene silencing through a process referred to as RNA interference (RNAi) or posttranscriptional gene silencing (PTGS). RNAi is mediated by RNA-induced silencing complex (RISC), a sequence-specific, multicomponent nuclease that destroys messenger RNAs homologous to the silencing trigger. RISC is known to contain short RNAs (∼22 nucleotides) derived from the double-stranded RNA trigger, but the protein components of this activity are unknown. Here, we report the biochemical purification of the RNAi effector nuclease from cultured
Drosophila
cells. The active fraction contains a ribonucleoprotein complex of ∼500 kilodaltons. Protein microsequencing reveals that one constituent of this complex is a member of the Argonaute family of proteins, which are essential for gene silencing in
Caenorhabditis elegans
,
Neurospora
, and
Arabidopsis
. This observation begins the process of forging links between genetic analysis of RNAi from diverse organisms and the biochemical model of RNAi that is emerging from
Drosophila
in vitro systems.
Detection of Molecular Hydrogen in the Atmosphere of Mars
Authors: Krasnopolsky Vladimir A. , Feldman Paul D.
Published: July 2002

Four hydrogen (H
2
) lines have been detected in a spectrum of Mars observed with the Far Ultraviolet Spectroscopic Explorer. Three of those lines are excited by the solar Lyman β photons. The line intensities correspond to a column H
2
abundance of 1.17 (±0.13) × 10
13
per square centimeter above 140 kilometers on Mars. A photochemical model for the upper atmosphere that simulates the observed H
2
abundance results in an H
2
mixing ratio of 15 ± 5 parts per million in the lower atmosphere. The H
2
and HD mixing ratios agree with photochemical fractionation of D (deuterium) between H
2
O and H
2
. Analysis of D fractionation among a few reservoirs of ice, water vapor, and molecular hydrogen on Mars implies that a global ocean more than 30 meters deep was lost since the end of hydrodynamic escape. Only 4% of the initially accreted water remained on the planet at the end of hydrodynamic escape, and initially Mars could have had even more water (as a proportion of mass) than Earth.
Blocks of Limited Haplotype Diversity Revealed by High-Resolution Scanning of Human Chromosome 21
Authors: Patil Nila , Berno Anthony J. , Hinds David A. , Barrett Wade A. , Doshi Jigna M. , Hacker Coleen R. , Kautzer Curtis R. , Lee Danny H. , Marjoribanks Claire , McDonough David P. , Nguyen Bich T. N. , Norris Michael C. , Sheehan John B. , Shen Naiping , Stern David , Stokowski Renee P. , Thomas Daryl J. , Trulson Mark O. , Vyas Kanan R. , Frazer Kelly A. , Fodor Stephen P. A. , Cox David R.
Published: July 2002

Global patterns of human DNA sequence variation (haplotypes) defined by common single nucleotide polymorphisms (SNPs) have important implications for identifying disease associations and human traits. We have used high-density oligonucleotide arrays, in combination with somatic cell genetics, to identify a large fraction of all common human chromosome 21 SNPs and to directly observe the haplotype structure defined by these SNPs. This structure reveals blocks of limited haplotype diversity in which more than 80% of a global human sample can typically be characterized by only three common haplotypes.
Linking Breeding and Wintering Ranges of a Migratory Songbird Using Stable Isotopes
Authors: Rubenstein D. R. , Chamberlain C. P. , Holmes R. T. , Ayres M. P. , Waldbauer J. R. , Graves G. R. , Tuross N. C.
Published: July 2002

We used the natural abundance of stable isotopes (carbon and hydrogen) in the feathers of a neotropical migrant songbird to determine where birds from particular breeding areas spend the winter and the extent to which breeding populations mix in winter quarters. We show that most birds wintering on western Caribbean islands come from the northern portion of the species' North American breeding range, whereas those on more easterly islands are primarily from southern breeding areas. Although segregated by breeding latitude, birds within local wintering areas derive from a wide range of breeding longitudes, indicating considerable population mixing with respect to breeding longitude. These results are useful for assessing the effects of wintering habitat loss on breeding population abundances and for predicting whether the demographic consequences will be concentrated or diffuse.
Chaperone Suppression of α-Synuclein Toxicity in a
Drosophila
Model for Parkinson's Disease
Authors: Auluck Pavan K. , Chan H. Y. Edwin , Trojanowski John Q. , Lee Virginia M.-Y. , Bonini Nancy M.
Published: July 2002

Parkinson's disease is a movement disorder characterized by degeneration of dopaminergic neurons in the substantia nigra pars compacta. Dopaminergic neuronal loss also occurs in
Drosophila melanogaster
upon directed expression of α-synuclein, a protein implicated in the pathogenesis of Parkinson's disease and a major component of proteinaceous Lewy bodies. We report that directed expression of the molecular chaperone Hsp70 prevented dopaminergic neuronal loss associated with α-synuclein in
Drosophila
and that interference with endogenous chaperone activity accelerated α-synuclein toxicity. Furthermore, Lewy bodies in human postmortem tissue immunostained for molecular chaperones, also suggesting that chaperones may play a role in Parkinson's disease progression.
Evolutionary Rate in the Protein Interaction Network
Authors: Fraser Hunter B. , Hirsh Aaron E. , Steinmetz Lars M. , Scharfe Curt , Feldman Marcus W.
Published: July 2002

High-throughput screens have begun to reveal the protein interaction network that underpins most cellular functions in the yeast
Saccharomyces cerevisiae
. How the organization of this network affects the evolution of the proteins that compose it is a fundamental question in molecular evolution. We show that the connectivity of well-conserved proteins in the network is negatively correlated with their rate of evolution. Proteins with more interactors evolve more slowly not because they are more important to the organism, but because a greater proportion of the protein is directly involved in its function. At sites important for interaction between proteins, evolutionary changes may occur largely by coevolution, in which substitutions in one protein result in selection pressure for reciprocal changes in interacting partners. We confirm one predicted outcome of this process—namely, that interacting proteins evolve at similar rates.
Mannose Receptor-Mediated Regulation of Serum Glycoprotein Homeostasis
Authors: Lee Sena J. , Evers Stefan , Roeder Daniel , Parlow Albert F. , Risteli Juha , Risteli Leila , Lee Y. C. , Feizi Ten , Langen Hanno , Nussenzweig Michel C.
Published: July 2002

Carbohydrates are thought to function as tags that mark circulatory glycoproteins for rapid clearance. To examine the role of the mannose receptor (MR) in glycoprotein clearance, we generated mice genetically deficient in MR. MR
−/−
mice were defective in clearing proteins bearing accessible mannose and
N
-acetylglucosamine residues and had elevated levels of eight different lysosomal hydrolases. Proteomic analysis of MR
−/−
and control mouse sera showed that an additional 4 out of 52 proteins identified were elevated in MR
−/−
serum. Each of these is up-regulated during inflammation and wound healing. Thus, MR appears to operate as an essential regulator of serum glycoprotein homeostasis.
Human Cloning and Our Sense of Self
Authors: Brock Dan W.
Published: July 2002

This Viewpoint formulates and responds to three lines of argument concerning human reproductive cloning's potential to undermine our sense of self or identity. First, cloning would undermine our sense of individuality or uniqueness. But it could only undermine our genetic uniqueness, not our full individuality. Second, cloning would undermine the value or worth of human beings. But it would not make individuals replaceable or of any less moral worth. Third, a clone's freedom or autonomy to construct his or her own life would be undermined by the presence of an earlier twin. But only a mistaken belief in genetic determinism supports this feared loss of freedom.
Ancient Pathways Programmed by Small RNAs
Authors: Zamore Phillip D.
Published: July 2002

Double-stranded RNA can now be used in a wide variety of eukaryotes to suppress the expression of virtually any gene, allowing the rapid analysis of that gene's function, a technique known as RNA interference. But how cells use the information in double-stranded RNA to suppress gene expression and why they contain the machinery to do so remain the subjects of intense scrutiny. Current evidence suggests that RNA interference and other “RNA silencing” phenomena reflect an elaborate cellular apparatus that eliminates abundant but defective messenger RNAs and defends against molecular parasites such as transposons and viruses.
Generation of X-rays from Comet C/Hyakutake 1996 B2
Authors: Bingham R. , Dawson J. M. , Shapiro V. D. , Mendis D. A. , Kellett B. J.
Published: July 2002

The instability resulting from the relative motion of newly picked-up cometary photoions and the solar wind generates lower hybrid waves that are capable of accelerating electrons to the keV range of energies. These electrons may be responsible for the x-rays from comet C/Hyakutake 1996 B2 seen by the Röntgen X-ray Satellite. The inferred x-ray photon power depends on the electron energy, with keV electrons providing values of photon power two orders of magnitude greater than 100 eV electrons. These observations and in particular, spectral resolution of the x-rays, should provide more insight into the composition of the comet.
Information Retrieval in Digital Libraries: Bringing Search to the Net
Authors: Schatz Bruce R.
Published: July 2002

A digital library enables users to interact effectively with information distributed across a network. These network information systems support search and display of items from organized collections. In the historical evolution of digital libraries, the mechanisms for retrieval of scientific literature have been particularly important. Grand visions in 1960 led first to the development of text search, from bibliographic databases to full-text retrieval. Next, research prototypes catalyzed the rise of document search, from multimedia browsing across local-area networks to distributed search on the Internet. By 2010, the visions will be realized, with concept search enabling semantic retrieval across large collections.