bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of maskless ion beam assisted etching of InP and Si using focused ion beam
Authors: Ochiai Yukinori , Gamo Kenji , Namba Susumu , Shihoyama Kazuhiko , Masuyama Akio , Shiokawa Takao , Toyoda Koichi
Published: July 2002

Temperature dependence of maskless ion beam assisted etching for InP and Si has been investigated. Thirty-five keV Ga+ focused ion beam was irradiated on a sample surface in a chlorine gas atmosphere at various temperatures up to 200 °C. The etched surface of InP was analyzed by Auger electron spectroscopy (AES). It was observed that the etching rate for InP increased with increasing temperature and it was 20–30 times larger than that of the physical sputter etching at a temperature of 140 °C. Chlorine was detected on the InP surface etched at temperatures lower than 100 °C, but no chlorine was detected above 140 °C. For Si, 20 times enhanced etching over physical sputter etching was observed near room temperature, but the etching rate decreased with increasing sample temperature.
Proposal for a new submicron dimension reference for an electron beam metrology system
Authors: Nakayama Yoshinori , Okazaki Shinji , Sugimoto Aritoshi
Published: July 2002

A new submicron level reference for an electron beam metrology system is proposed. This reference is a fine rectangular-profile diffraction grating fabricated by laser interferometer lithography and anisotropic chemical etching of (110) crystalline silicon. The pitch size of the grating is derived from the wavelength of the laser and the angle of incidence of the holographic lithography. The optical diffraction measurement absolutely assures accurate grating pitch size. The measurement error is estimated to be smaller than 0.001 μm. A deep lamellar grating fabricated by anisotropic chemical etching of (110)Si generates a stable and high-contrast secondary electron signal in an electron beam metrology system. Reference pitch size measurement by optical diffraction and by the electron beam metrology system assures that the pitch size error is smaller than 0.02 μm.
Quantum confinement and hot-phonon effects in quantum wells
Authors: Marchetti M. C. , Pötz W.
Published: July 2002

We present a theoretical study of the dynamics of photoexcited carriers in GaAs/GaAlAs quantum-well structures. Both the effects of lattice heating and of the energy exchange between electrons and holes on the carrier relaxation are included. The dynamics of the nonequilibrium optical phonons is described by a kinetic equation for phonon wave packets that properly incorporates the effect of carrier confinement on the nonthermal lattice modes. The modifications of the phonons due to the layered lattice structure are not considered here. For the carriers, Fermi–Dirac distribution functions are used and the electron–hole interaction is included explicitly. Results of this approach are presented for different layer thicknesses and are compared with the results of a previous calculation by one of us for bulk GaAs to assess the effect of quantum confinement on the carrier cooling. Confinement in quantum–well structures is found to increase the carrier cooling rate as compared to bulk GaAs. The cooling rate also increases as the well gets narrower, in agreement with recent experimental observations.
Design concept for a high-performance positive photoresist
Authors: Hanabata Makoto , Uetani Yasunori , Furuta Akihiro
Published: July 2002

An attempt was made to design a high-performance positive photoresist from the standpoint of the image formation process. There are a lot of trade-off relationships among performance of a positive photoresist. A study was made to raise performance without the decrease in other performances. It was necessary to design a new type of novolak resin that has a molecular structure different from the existing materials. Many kinds of cresol–formaldehyde type novolak resins were synthesized and evaluated lithographically. Such items as molecular weight, molecular weight distribution, isomeric structure of cresol, the position of methylene bond, and the content of quinonediazide in a positive photoresist were investigated. Three methods were found useful: (i) to apply high-ortho novolak resins that have a high content of ortho–ortho’ methylene bonds (ii) to optimize molecular weight distribution of novolak resins, and (iii) to optimize the content of quinonediazides in a photoresist. These results are explained in terms of a simple model for positive photoresist development, which we call ‘‘stone wall model’’. This model is applicable to design a high-performance positive photoresist.
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u ellipsometry during plasma etching of SiO2 films on Si
Authors: Haverlag M. , Kroesen G. M. W. , de Zeeuw C. J. H. , Creyghton Y. , Bisschops T. H. J. , de Hoog F. J.
Published: July 2002

The etching of SiO2 films on a Si substrate in an rf plasma in CF4 has been studied with in situ ellipsometry. The etch rate was measured as a function of flow, rf power and pressure. An accurate analysis of the experimental data using numerical simulations based on multilayer models has yielded information both on the refractive index of the etched SiO2 film and on the existence of a top layer. It could be established that a layer is present on top of the SiO2 during etching, which is probably caused by roughening of the SiO2 layer. Furthermore at high pressures (>8 Pa) it was demonstrated that after the complete removal of the SiO2 film a polymer layer starts growing on the Si substrate.
Observation of internal structure of a positive photoresist image using cross-sectional exposure method
Authors: Uetani Yasunori , Hanabata Makoto , Furuta Akihiro
Published: July 2002

The internal structure of a novolak/quinonediazide type positive photoresist image was observed using a cross-sectional exposure method. On the surface of a photoresist image, a photochemically inert layer was detected which does not return to diazo and phenolic compounds with acid treatment. This shows that this layer does not consist of the azoxy compounds. Fourier transform infrared spectrum indicates that this layer includes azocoupling reaction products. It was also found that the formation of the photochemically inert layer and the dissolution of the layer takes place simultaneously at the development front and a resist image is formed during the competition between these two reactions.
Effects of molecular weights and polydispersity on the properties of poly(trifluoroethyl methacrylate) as a positive x ray and electron resist
Authors: Delaire J. A. , Lagarde M. , Broussoux D. , Dubois J. C.
Published: July 2002

Different samples of poly(trifluoro-2,2,2 ethyl methacrylate), made either by radical or by anionic copolymerization, with molecular weights ranging from 100 000 to 700 000 g mol−1 have been investigated as positive x ray and electron resist. The best sensitivity and contrast have been found for the sample with the lower polydispersity index and the higher molecular weight in optimized developing conditions. Furthermore, we have found that in optimized developers, the ratio of molecular weights before and after irradiation, with a dose corresponding to the total dissolution of the film, is nearly a constant. The best resolution (0.5 μm for line and space gratings in a film of 0.6 μm thickness) has been obtained when development is carried out at low temperature (5 °C) in an appropriate solvent–nonsolvent mixture.
Photoreflectance study of Fermi level changes in photowashed GaAs
Authors: Shen H. , Pollak Fred H. , Woodall J. M.
Published: July 2002

As a result of the photowashing of (100) n-GaAs (n≊3×1016 cm−3) a decrease of about 25% in the surface potential was found using the contactless electromodulation method of photoreflectance. This corresponds to a reduction in the surface state density by about a factor of 2.
The relation between lift-off of photoresist and the surface coverage of trimethylsiloxy groups on silicon wafers: A quantitative time-of-flight secondary ion mass spectrometry and contact angle study
Authors: Ponjeé J. J. , Marriott V. B. , Michielsen M. C. B. A. , Touwslager F. J. , van Velzen P. N. T. , van der Wel H.
Published: July 2002

In this paper the interaction between a trimethylsilylated silicon substrate and diazonaphthoquinone–novolak photoresist material has been examined. Time-of-flight secondary ion mass spectrometry (TOF SIMS) and contact angle measurements of water have been used to determine the surface coverage of trimethylsiloxy (TMS) groups on cleaned and subsequently treated silicon wafers. A linear relation is obtained between the relative surface coverage as measured by TOF SIMS and the cosine value of the contact angle. The surface coverage of the TMS groups has to exceed a value of about 50% of a monomolecular layer to assure the absence of lift-off of photoresist. Contrary to the common belief that trimethylsilylation of silicon substrates improves the adhesion between photoresist and the silicon substrate, it rather deteriorates the adhesion. This is further corroborated by experiments on silica particles. It is proposed that the low-surface energy of the trimethylsilylated surface prevents the penetration of the alkaline developer solution into the interface of the silicon substrate and the photoresist material.
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u photoreflectance study of the effects of sputter/annealing on the Fermi level at (001) n- and p-type GaAs surfaces
Authors: Yin X. , Chen H-M. , Pollak F. H. , Cao Y. , Montano P. A. , Kirchner P. D. , Pettit G. D. , Woodall J. M.
Published: July 2002

We report a photoreflectance study of the effects of Ar+ sputtering and thermal annealing on the Fermi level (VF) on (001) n- and p-type GaAs with large, uniform electric fields. The measurements were performed in situ in an ultrahigh vacuum (UHV) chamber. The effect of the sputtering was to move VF from midgap to near the conduction band for both types of material. Subsequent UHV annealing (350 °C) and air exposure restored VF its original midgap value. The implication of these observations for various models of Schottky barrier formation will be discussed. Our work also demonstrates the need to simultaneously measure both n- and p-type material in order to obtain unambiguous results.
Imaging of submicron index variations by scanning optical tunneling
Authors: Courjon Daniel , Bainier Claudine , Spajer Michel
Published: July 2002

The scanning tunneling optical microscope (SNOM, STOM, PSTM, etc.) is the equivalent of the electron scanning tunneling microscope in the electromagnetic domain. Although it was born at the same time, its actual development is more recent. Here, some new results obtained with the version working in total reflection (STOM/PSTM) are reported. A grating of a periodicity of 417 nm and a thickness of 5 nm have been imaged both in TM and TE modes. It is first noted that the optical image is well resolved. Furthermore, the difference of behavior of the field versus the polarization of the incident light has been shown. More precisely, the TM mode seems to be highly sensitive to small index and topography variations due to surface contaminants. Such effects are generally not imaged by atomic force microscopy working in attractive mode, because they affect the surface topography slightly. The SNOM could be thus a very powerful tool for detecting pollutants over the surface of objects like glasses, lenses, gratings, etc., and in the biology domain.
Defect studies on single and bilayer resist systems
Authors: Muller K. Paul , Sachdev Harbans S.
Published: July 2002

Defect detection equipment and procedures to qualify resist systems with regard to defects have been investigated. An analysis was carried out for two bilayer resist systems which were compared to a single layer resist. One of the bilayer resists was developed for the mid-UV exposure range, the other for deep-UV. It showed that the pinhole-limited yield measured by metal–oxide–semiconductor test structures is approximately 10% lower for one of the bilayer resists compared to a single layer resist. The other bilayer resist scheme was compared to a single layer resist with regard to particulates. Here the dry-developed bilayer resist scheme showed approximately four times higher additive defect densities than the wet-developed single layer resist. A short dry etch process for opening an anti reflective coating underneath a single layer resist increased the defect densities. Water rinse steps are capable of reducing these defect levels substantially. The dry-developed resist schemes had higher defect densities, but it is certainly possible to reduce these to the densities of single layer resists.
Scanning tunneling microscopy of porous silicon surfaces
Authors: Amisola G. B. , Behrensmeier R. , Galligan J. M. , Otter F. A. , Namavar F. , Kalkoran N. M.
Published: July 2002

The scanning tunneling microscope (STM) was used to examine several porous silicon based surfaces. First are samples of photoluminescent porous silicon made from single crystal wafers of 13.5 Ω cm p− (100) and 0.002 Ω cm p+ (100) silicon. These were observed in an ambient He environment. The development of the porous structure in these wafers due to different etching stages is investigated with the STM. Measurements show that the vertical surface roughness increases with etching while the lateral dimensions of the structures present remain almost constant in agreement with known etching models. The porous silicon surface density of states (DOS) was also measured using scanning tunneling spectroscopy. DOS measurements of porous silicon formed on both p− and p+ substrates do not show the expected increase in band gap energy when compared with unetched crystalline silicon. However, these measurements do show that while the unetched silicon surface remains p type, porous silicon surfaces fabricated from p− Si behaves like an n-type material whereas those formed from p+ Si remain p type after etching.
Local modification of HF-treated silicon (100) surface and its characterization by scanning tunneling microscopy and spectroscopy
Authors: Pérez-Murano F. , Barniol N. , Aymerich X.
Published: July 2002

The modification of silicon (100) surfaces on samples etched with HF and without rinsing in de-ionized H2O is produced and studied by scanning tunneling microscopy and spectroscopy. The dependence of the size of the modification with the tunnel current and the voltage between tip and sample is explained from the evaluation of the electrical field between the tip and the silicon surface, starting from the planar metal–insulator–semiconductor theory. Current image tunneling spectroscopy data allow to clarify the nature of the modification, concluding that an electrochemical change on the surface of the sample has been produced.
Grazing-incidence x-ray photoemission spectroscopy investigation of oxidized GaAs(100): A novel approach to nondestructive depth profiling
Authors: Chester M. J. , Jach Terrence , Thurgate S.
Published: July 2002

When a beam of x rays (∼1 keV) impinges on a flat surface at grazing angles (≲3°) the x rays undergo total external reflection. Under these conditions, the penetration depth of the x rays can become comparable to photoelectron escape depths and the photoelectron yields from the surface are enhanced. As the incidence angle of the x rays is increased, the x-ray penetration depth increases and the photoelectron yields contain a larger contribution from deeper layers within the sample. By exploiting this depth-dependence of photoelectron yields as a function of incident x-ray beam angle, it is possible to obtain information about the depth distribution of the photoelectron emitting atoms. We have used this novel application of grazing-incidence x-ray photoelectron spectroscopy (GIXPS) to study the ultraviolet oxidized GaAs(100) surface. This oxide/GaAs surface is particularly well suited for study with GIXPS because a variety of oxide phases are formed during oxidation and questions concerning stratification of these phases can be addressed. Much insight into the composition and depth-dependence of these oxide phases can be obtained by directly comparing the spectra collected at different x-ray incidence angles.
Relationships between the material properties of silicon oxide films deposited by electron cyclotron resonance chemical vapor deposition and their use as an indicator of the dielectric constant
Authors: Fowler B. , O’Brien E.
Published: July 2002

Silicon dioxide films deposited by electron cyclotron resonance chemical vapor deposition have been characterized using Fourier transform infrared spectroscopy, Rutherford backscattering spectrometry, and ellipsometry. A commercially available reactor was used to generate a high-density plasma at low total gas pressures (3–4 mTorr). A gas mixture of Ar, O2, and SiH4 was used to deposit SiO2 films on 200 mm Si substrates at temperatures less than 450 °C for interlayer dielectric applications. The dielectric constant of the films was measured using metal oxide semiconductor capacitors. Relationships between the dielectric constant and the refractive index, Si–OH content, and film stoichiometry were investigated using a model based on the Clausius–Mosotti equation. During the study, the O2 and SiH4 gas flows were adjusted to vary the film properties. Silicon dioxide was deposited over a wide range of O2 and SiH4 gas flow conditions. For SiO2 films, the dielectric constant was strongly correlated with the Si–OH content. At low O2/SiH4 gas flow ratios, suboxide films were deposited and the dielectric constant increased in relation to the refractive index. A low-temperature, postdeposition anneal was investigated as a means to reduce the Si–OH content and dielectric constant of the films. Modeling results indicate that the dielectric constant of SiO2 films can be predicted using measurements of the film thickness, refractive index, and Si–OH content, thus providing a nondestructive technique for monitoring the dielectric constant.
Limiting factors for secondary ion mass spectrometry profiling
Authors: Cirlin Eun-Hee , Vajo John J. , Hasenberg T. C.
Published: July 2002

Understanding the limitations of depth profiling with ion sputtering is essential for accurate measurements of atomically abrupt interfaces and ultra-shallow doping profiles. The effects of cascade mixing, sputtering statistics, ion-induced roughness, the inhomogeneity of ion beams, and sample rotation on the depth resolution of Si δ-doped, AlAs, and InAs monolayers in GaAs and an AlGaAs(5 nm)/GaAs(5 nm) superlattice were investigated. Atomic force microscopy (AFM) investigation of the ion-induced surface ripple formation on a GaAs substrate sputtered with 3 keV O+2 at angle of incidence θ=40° showed that ripples form rapidly below 200 nm depth. AFM measured root mean square roughness of Si δ-doped GaAs sputtered with 2 keV O+2 was 0.8 and 2.6 nm with and without sample rotation showing that ripples play a dominant role in depth resolution degradation at shallow depth under these conditions of bombardment. Sample rotation yielded the lowest full width at half-maximum, 4.1 nm for a Si δ layer at 120 nm depth corresponding to a depth resolution ΔZ=3.5 nm. Use of AFM enabled determination of the atomic mixing ΔZm and sputtering statistics ΔZss components of depth resolution to be identified directly for the first time. These components were 3.1 and 1.5 nm, respectively.
ZnSe(100): The surface and the formation of Schottky barriers with Al and Au
Authors: Chen W. , Kahn A. , Soukiassian P. , Mangat P. S. , Gaines J. , Ponzoni C. , Olego D.
Published: July 2002

A study of the ZnSe(100) surface and of its interfaces with Al and Au is presented. We find that the (2×1) reconstructed Se-rich surface is terminated with a full monolayer of dimerized Se, whereas the C(2×2) reconstructed Zn-rich surface corresponds to a half-monolayer of nondimerized Zn atoms. These atomic configurations and corresponding surface electron affinities are consistent with the requirement of dangling bond saturation and fully accounted for by the electron counting rule. For the metal/ZnSe interfaces, we find that Au forms an abrupt junction, whereas Al reacts and forms Al2Se3, releasing Zn in the process. The stabilized Fermi level position is 2.1 eV above the valence band maximum for Al and 1.15 eV for Au, irrespective of doping type and in qualitative agreement with the Schottky limit for the barrier heights. The Au/p-ZnSe Schottky barrier height can be reduced by 0.25 eV by introducing a 2–3 ML Se interlayer between the metal and the semiconductor.
In situ observation of anodic dissolution process of n-GaAs in HCl solution by electrochemical atomic force microscope
Authors: Koinuma Michio , Uosaki Kohei
Published: July 2002

Anodic dissolution of a GaAs(100) face was investigated by in situ electrochemical atomic force microscope (AFM). While no surface structure change was observed at −0.6 V (vs Ag/AgCl) where no current flowed, dome structure on surface was removed, and flat surface was obtained after keeping the potential at 0 V (vs Ag/AgCl) where anodic current of ∼150 μA cm−2 flowed. An atomically resolved AFM image was obtained in the flat region and shows the surface is dominated by a (111) face after the anodic dissolution.
Anomalous behavior of shallow BF3 plasma immersion ion implantation
Authors: Jones Erin C. , En William , Ogawa Shinichi , Fraser David B. , Cheung Nathan W.
Published: July 2002

Plasma immersion ion implantation (PIII) with BF3 and SiF4 plasmas is used to fabricate shallow P+/N junctions in Si. Exposure to the plasma and accelerated ions can lead to simultaneous etching and deposition on the substrate during implantation. A simple mathematical model for this process is presented and applied to the case of shallow implantation of BF3. Etching rates of SiO2 are seen to vary with power and pressure of the process gas. Etching rates of Si, SiO2, and CoSi2 are studied by spectrophotometry and Rutherford backscattering spectrometry. The roughness of Si substrates and SiO2 and CoSi2 films before and after PIII is monitored by atomic force microscopy.
Influence of various growth parameters on the interface abruptness of AlAs/GaAs short period superlattices
Authors: Smith A. R. , Chao Kuo-Jen , Shih C. K. , Shih Y. C. , Anselm K. A. , Streetman B. G.
Published: July 2002

Cross-sectional scanning tunneling microscopy has been used to investigate the effects of several key growth parameters on the resulting interfacial quality of AlAs/GaAs short period superlattices. For growth on top of AlGaAs layers, only superlattices grown with periodicity no smaller than 4 unit cells of GaAs and 2 unit cells of AlAs and grown with a minimum of 30 s of growth interrupt time are resolved. On the other hand, when grown on top of GaAs layers, superlattices as fine as 2 unit cells of GaAs and 1 unit cell of AlAs grown with only 5 s of growth interrupt time are resolved. This result suggests that the material on which the superlattice is grown is at least as important as the growth interrupt time. In particular, GaAs seems to provide a smoother starting surface than AlGaAs and hence aids in the formation of abrupt interfaces. We also compare our scanning tunneling microscopy data with some predictions based on simple atomic models of the interfacial regions.
Experimental validation of a direct simulation by Monte Carlo molecular gas flow model
Authors: Shufflebotham P. K. , Bartel T. J. , Berney B.
Published: July 2002

The Sandia direct simulation Monte Carlo (DSMC) molecular/transition gas flow simulation code has significant potential as a computer-aided design tool for the design of vacuum systems in low pressure plasma processing equipment. The purpose of this work was to verify the accuracy of this code through direct comparison to experiment. To test the DSMC model, a fully instrumented, axisymmetric vacuum test cell was constructed, and spatially resolved pressure measurements made in N2 at flows from 50 to 500 sccm. In a ‘‘blind’’ test, the DSMC code was used to model the experimental conditions directly, and the results compared to the measurements. It was found that the model predicted all the experimental findings to a high degree of accuracy. Only one modeling issue was uncovered. The axisymmetric model showed localized low pressure spots along the axis next to surfaces. Although this artifact did not significantly alter the accuracy of the results, it did add noise to the axial data.
Operation and device applications of a valved-phosphorus cracker in solid-source molecular-beam epitaxy
Authors: Chin T. P. , Chang J. C. P. , Woodall J. M. , Chen W. L. , Haddad G. I. , Parks C. , Ramdas A. K.
Published: July 2002

Solid phosphorus is successfully incorporated into a molecular-beam epitaxy system by a valved-cracker for the growth of phosphorus-based materials. The operating parameters are established through beam flux and reflection high-energy electron diffraction measurements. InP and InGaP lattice matched to GaAs were grown and characterized by Hall measurements, photoluminescence, electroreflectance, x-ray diffraction, and transmission electron microscopy. The first microwave performance (ft=44 GHz, fmax=65 GHz) of an InGaP/GaAs heterojunction bipolar transistor grown by solid-phosphorus source is reported.
Theoretical study of nanoclusters at ionic surfaces: Properties of (NaCl)n clusters (n=1–48) at the (100) MgO surface
Authors: Shluger Alexander L. , Rohl Andrew L. , Gay David H.
Published: July 2002

We have studied (NaCl)n clusters (n=1–48) on the (100) MgO surface using the atomistic simulation technique. The geometry, binding energy, and interaction of these clusters with the surface are calculated and discussed. We address the questions at which cluster size do the adsorbed molecules lose their identity and how do strained clusters accommodate the strain. The relation between the structure of initial molecular fragments adsorbed at the surface and the structure of the corresponding thick film and some general features of the structure and properties of small molecular clusters at ionic surfaces with large geometric misfit are discussed. The results are compared with the calculated structures of the free clusters and the experimental data on the molecular beam epitaxy of alkali halides.
Investigations on the topology of structures milled and etched by focused ion beams
Authors: Lipp S. , Frey L. , Lehrer C. , Frank B. , Demm E. , Ryssel H.
Published: July 2002

For high precision micromachining of micro- and nanostructures by focused ion beams, the precision of the material removal process is of great importance. In this article, the topological properties of the ion beam generated structures like slope angles of trenches, surface roughness, and induced defects are investigated. The influence of the beam current and scanning strategy on the topological properties will be discussed. In addition, transmission electron microscopy analysis of thin lamellas generated by focused ion beams will be shown.
Site-specific friction force spectroscopy
Authors: Meyer E. , Lüthi R. , Howald L. , Bammerlin M. , Guggisberg M. , Güntherodt H.-J.
Published: July 2002

The two-dimensional histogram technique is used to determine the loading dependence of friction on terrace and step sites of NaCl(001). An extended adhesion model is used to analyze the data. Both Hertz and JKR theory are incorporated for the elastic deformation. Depending on the model, shear strengths of 100–190 MPa are found at the terrace region. Increased shear strength values of 500–1800 MPa are found at the step edges that are interpreted in terms of the increased energy barriers at the steps, known as Schwoebel barriers.
Growth of silicon nitride by scanned probe lithography
Authors: Pyle Jason L. , Ruskell Todd G. , Workman Richard K. , Yao Xiaowei , Sarid Dror
Published: July 2002

Scanning probe lithography has been used for the first time to grow silicon nitride nanostructures on silicon substrates. The lithography was performed by an atomic force microscope (AFM) placed in an evacuated chamber with a partial pressure of annhydrous ammonia. The silicon nitride nanostructures were grown by negatively biasing the silicon tip with respect to the sample. By changing the environment of the AFM, both silicon oxide and silicon nitride can be grown and subsequently processed.
Nature and origins of stacking faults from a ZnSe/GaAs interface
Authors: Kuo L. H. , Kimura K. , Ohtake A. , Miwa S. , Yasuda T. , Yao T.
Published: July 2002

Existence of ∼3–4 monolayers of Ga2Se3- and Ga2Te3-like interfacial layers are suggested by transmission electron microscopy of Se- and Te-exposed (or -reacted) ZnSe/GaAs interfaces, respectively. Densities of extrinsic Shockley- and intrinsic Frank-type stacking faults are of ∼5 ×107/cm2 in samples grown on Se- or Te-exposed GaAs surfaces. Annealing on the Se- or Te-exposed GaAs generated a high density of vacancy loops (⩾1×109/cm2) with an increase of the densities of both intrinsic and extrinsic-type stacking faults (⩾5×108/cm2) after growth of the films. Formation of the intrinsic stacking faults or vacancy loops and extrinsic stacking faults may be related to the presence of cation vacancies and interstitials, respectively, on the surface of the GaAs epilayer, due to the interaction between Se or Te and the GaAs epilayer with charge unbalanced Ga–Se or Ga–Te bondings. On the other hand, ∼2 and 3–4 monolayers of Zn–As interfacial layers are recognized in samples grown on Zn-exposed GaAs-(2×4) and -c(4×4), respectively. A very low density of fault defects in the range of ∼104/cm2 was obtained in samples with Zn treatment on an As-stabilized GaAs-(2×4). However, the density of As precipitates increases as the surface coverage of c(4×4) reconstruction increased on the Zn-exposed As-stabilized GaAs-(2×4) surface and this is associated with an increase of the density of extrinsic-type stacking faults bound by partial edge dislocations with a core structure terminated on additional cations. Clustering of excess As atoms and formation of Zn interstitials due to charge unbalance of Zn–As bondings on the GaAs surface may act as nucleation sites for the generation of the high densities of As precipitates and extrinsic-type stacking faults, respectively. These local stacking errors due to the interaction between Zn and GaAs-c(4×4) is reduced by thermal annealing. In this case, thickness of the Zn–As interfacial layer is decreased with increasing annealing temperature and a very low density of the fault defects is obtained.
Design and fabrication of Fresnel zone plates with large numbers of zones
Authors: Chen Z. , Vladimirsky Y. , Brown M. , Leonard Q. , Vladimirsky O. , Moore F. , Cerrina F. , Lai B. , Yun W. , Gluskin E.
Published: July 2002

The advent of high-brightness x-ray sources in the 10–40 keV region opens new possibilities of experiments with microbeams. Techniques to form these focused beams may be based on glancing mirrors, phase elements, or diffractive optics, in particular Fresnel zone plates (FZPs). Because of the long focal length and large acceptance, FZPs designed to work in the hard x-ray region tend to have quite large diameters and large numbers of zones. For instance, the zone plate described in this article has a 1860 μm diam, a focal length of f=3 m (for 8 keV), and 1860 zones. On a standard pattern generator, circular shapes are always approximated as simpler structures. The tolerance requirement for shape and positions of zones depends on the number of zones, and it is necessary to guarantee that the circular structures are approximated to the required degree of accuracy while keeping the size of the data structure to a reasonable size for processing by the exposure system. For instance, if polygons are used to approximate circular zones, a formula for the minimum acceptable number of polygon sides can be derived. An x-ray mask for a Fresnel phase zone plate (FPZP) with 1860 zones was designed directly in Cambridge source pattern data format and fabricated using the Leica Cambridge e-beam tool installed in the CXrL. The zone plates presented in this article were designed for hard x rays, and multilevel x-ray lithography was employed as a fabrication technique to form absorber thickness sufficient to provide the necessary phase shift. Minimum gold features of 0.25 μm with thicknesses of 1.6 and 3 μm, were formed to be used with 8 and 20 keV photons, respectively. Finally, in order to estimate the quality of the zone plates during fabrication, a scanning electron microscope based moiré method was used.
Influence of developer temperature and resist material on the structure quality in deep x-ray lithography
Authors: Pantenburg F. J. , Achenbach S. , Mohr J.
Published: July 2002

Deep x-ray lithography is a fabrication method for the production of microstructures with aspect ratios of up to 100 in up to several millimeter thick resist layers. Usually, poly(methylmethacrylate) is used as the resist layer. We have measured the molecular weight and developing rates of crosslinked and noncrosslinked PMMA foils at different GG developer temperatures for dose values between 0.1 and 8 kJ/cm3. The determined developing rates cover a region of 7 orders of magnitude: With decreasing temperature the contrast of the resist-developer system and the limit of the developing dose are increased. Crosslinked PMMA has a higher contrast compared to noncrosslinked material. These effects lead to an enhanced quality of microstructures, which is demonstrated by the grating of the LIGA microspectrometer.
Quantifying surface topography and scanning probe image reconstruction
Authors: Kitching S. , Williams P. M. , Roberts C. J. , Davies M. C. , Tendler S. J. B.
Published: July 2002

Scanning probe microscopy (SPM) is used widely for the characterization of topographic structure in both material and bio-sciences. However, due to the finite size of the imaging tip, the SPM may not record all of the surface. Dimensional measurements made on such surfaces will not reflect the true nature of the surface. Simulated images have been used to assess the effects of imaging and image reconstruction methods on surface roughness and dimensional measurements. A novel combination of image reconstruction techniques and connectivity criteria is used to set bounds on the certainty of dimensional and topographic measures and to obtain more information about the spatial complexity of the sample surface. A more complete description of the sample is obtained without prior knowledge of the sample or tip geometry.
Solid-state modulators for plasma immersion ion implantation applications
Authors: Gaudreau, P. E. Marcel P. J. , Casey Jeffrey A. , Kempkes Michael A. , Hawkey Timothy J. , Mulvaney James M.
Published: July 2002

One of the key technologies in the plasma immersion ion implantation (PIII) process is the delivery of tightly regulated high voltage, high current pulsed power to the plasma implantation chamber. This requires a nearly ideal high voltage switch capable of operating at short pulse widths and high pulse repetition rates. Existing switch technologies are limited in providing the ideal switching required for consistent, effective commercial PIII processes. By using new solid-state power modulators, the required power can be delivered more effectively and reliably than with the use of older, tube-based technologies such as gridded vacuum tubes, thyratrons, and pulse forming networks. Solid-state high power technology is now available to allow PIII to be a cost-effective, commercially viable process, ready for widespread commercialization.
Role of the Sphingosine-1-Phosphate Receptor EDG-1 in PDGF-Induced Cell Motility
Authors: Hobson John P. , Rosenfeldt Hans M. , Barak Larry S. , Olivera Ana , Poulton Samantha , Caron Marc G. , Milstien Sheldon , Spiegel Sarah
Published: July 2002

EDG-1 is a heterotrimeric guanine nucleotide binding protein–coupled receptor (GPCR) for sphingosine-1-phosphate (SPP). Cell migration toward platelet-derived growth factor (PDGF), which stimulates sphingosine kinase and increases intracellular SPP, was dependent on expression of EDG-1. Deletion of
edg-1
or inhibition of sphingosine kinase suppressed chemotaxis toward PDGF and also activation of the small guanosine triphosphatase Rac, which is essential for protrusion of lamellipodia and forward movement. Moreover, PDGF activated EDG-1, as measured by translocation of β-arrestin and phosphorylation of EDG-1. Our results reveal a role for receptor cross-communication in which activation of a GPCR by a receptor tyrosine kinase is critical for cell motility.
The Ecology of Genetically Diverse Infections
Authors: Read Andrew F. , Taylor Louise H.
Published: July 2002

Microparasite infections often consist of genetically distinct clonal lineages. Ecological interactions between these lineages within hosts can influence disease severity, epidemiology, and evolution. Many medical and veterinary interventions have an impact on genetic diversity within infections, but there is little understanding of the long-term consequences of such interventions for public and animal health. Indeed, much of the theory in this area is based on assumptions contradicted by the available data.
Pressure Dependence of the Superconducting Transition Temperature of Magnesium Diboride
Authors: Monteverde M. , Núñez-Regueiro M. , Rogado N. , Regan K. A. , Hayward M. A. , He T. , Loureiro S. M. , Cava R. J.
Published: July 2002

We studied the pressure and temperature dependence of the electrical resistivity of the superconducting compound magnesium diboride (MgB
2
). The superconducting transition temperature decreases monotonically with pressure, being parabolic or linear, depending on samples. The rate of decrease under pressure is higher than in conventional superconductors. We discuss our results in terms of the semimetallic character of the electronic band structure of MgB
2
.
Stable Ordering in Langmuir-Blodgett Films
Authors: Takamoto Dawn Y. , Aydil Eray , Zasadzinski Joseph A. , Ivanova Ani T. , Schwartz Daniel K. , Yang Tinglu , Cremer Paul S.
Published: July 2002

Defects in the layering of Langmuir-Blodgett (LB) films can be eliminated by depositing from the appropriate monolayer phase at the air-water interface. LB films deposited from the hexagonal phase of cadmium arachidate (CdA
2
) at pH 7 spontaneously transform into the bulk soap structure, a centrosymmetric bilayer with an orthorhombic herringbone packing. A large wavelength folding mechanism accelerates the conversion between the two structures, leading to a disruption of the desired layering. At pH > 8.5, though it is more difficult to draw LB films, almost perfect layering is obtained due to the inability to convert from the as-deposited structure to the equilibrium one.
Allosteric Activation of a Spring-Loaded Natriuretic Peptide Receptor Dimer by Hormone
Authors: He Xiao-lin , Chow Dar-chone , Martick Monika M. , Christopher Garcia K.
Published: July 2002

Natriuretic peptides (NPs) are vasoactive cyclic-peptide hormones important in blood pressure regulation through interaction with natriuretic cell-surface receptors. We report the hormone-binding thermodynamics and crystal structures at 2.9 and 2.0 angstroms, respectively, of the extracellular domain of the unliganded human NP receptor (NPR-C) and its complex with CNP, a 22–amino acid NP. A single CNP molecule is bound in the interface of an NPR-C dimer, resulting in asymmetric interactions between the hormone and the symmetrically related receptors. Hormone binding induces a 20 angstrom closure between the membrane-proximal domains of the dimer. In each monomer, the opening of an interdomain cleft, which is tethered together by a linker peptide acting as a molecular spring, is likely a conserved allosteric trigger for intracellular signaling by the natriuretic receptor family.
Loss of Sex Discrimination and Male-Male Aggression in Mice Deficient for TRP2
Authors: Stowers Lisa , Holy Timothy E. , Meister Markus , Dulac Catherine , Koentges Georgy
Published: July 2002

The mouse vomeronasal organ (VNO) is thought to mediate social behaviors and neuroendocrine changes elicited by pheromonal cues. The molecular mechanisms underlying the sensory response to pheromones and the behavioral repertoire induced through the VNO are not fully characterized. Using the tools of mouse genetics and multielectrode recording, we demonstrate that the sensory activation of VNO neurons requires TRP2, a putative ion channel of the transient receptor potential family that is expressed exclusively in these neurons. Moreover, we show that male mice deficient in TRP2 expression fail to display male-male aggression, and they initiate sexual and courtship behaviors toward both males and females. Our study suggests that, in the mouse, sensory activation of the VNO is essential for sex discrimination of conspecifics and thus ensures gender-specific behavior.
The
E. coli
BtuCD Structure: A Framework for ABC Transporter Architecture and Mechanism
Authors: Locher Kaspar P. , Lee Allen T. , Rees Douglas C.
Published: July 2002

The ABC transporters are ubiquitous membrane proteins that couple adenosine triphosphate (ATP) hydrolysis to the translocation of diverse substrates across cell membranes. Clinically relevant examples are associated with cystic fibrosis and with multidrug resistance of pathogenic bacteria and cancer cells. Here, we report the crystal structure at 3.2 angstrom resolution of the
Escherichia coli
BtuCD protein, an ABC transporter mediating vitamin B
12
uptake. The two ATP-binding cassettes (BtuD) are in close contact with each other, as are the two membrane-spanning subunits (BtuC); this arrangement is distinct from that observed for the
E. coli
lipid flippase MsbA. The BtuC subunits provide 20 transmembrane helices grouped around a translocation pathway that is closed to the cytoplasm by a gate region whereas the dimer arrangement of the BtuD subunits resembles the ATP-bound form of the Rad50 DNA repair enzyme. A prominent cytoplasmic loop of BtuC forms the contact region with the ATP-binding cassette and appears to represent a conserved motif among the ABC transporters.
Antibacterial Agents That Inhibit Lipid A Biosynthesis
Authors: Onishi H. Russell , Pelak Barbara A. , Gerckens Lynn S. , Silver Lynn L. , Kahan Frederick M. , Chen Meng-Hsin , Patchett Arthur A. , Galloway Susan M. , Hyland Sheryl A. , Anderson Matt S. , Raetz Christian R. H.
Published: July 2002

Lipid A constitutes the outer monolayer of the outer membrane of Gram-negative bacteria and is essential for bacterial growth. Synthetic antibacterials were identified that inhibit the second enzyme (a unique deacetylase) of lipid A biosynthesis. The inhibitors are chiral hydroxamic acids bearing certain hydrophobic aromatic moieties. They may bind to a metal in the active site of the deacetylase. The most potent analog (with an inhibition constant of about 50 nM) displayed a minimal inhibitory concentration of about 1 microgram per milliliter against
Escherichia coli
, caused three logs of bacterial killing in 4 hours, and cured mice infected with a lethal intraperitoneal dose of
E. coli
.
Organic Glasses: A New Class of Photorefractive Materials
Authors: Lundquist P. M. , Wortmann R. , Geletneky C. , Twieg R. J. , Jurich M. , Lee V. Y. , Moylan C. R. , Burland D. M.
Published: July 2002

The performance of amorphous organic photorefractive (PR) materials in applications such as optical data storage is generally limited by the concentration of active molecules (chromophores) that can be incorporated into the host without forming a crystalline material with poor optical quality. In polymeric PR systems described previously, performance has been limited by the necessity of devoting a large fraction of the material to inert polymer and plasticizing components in order to ensure compositional stability. A new class of organic PR materials composed of multifunctional glass-forming organic chromophores is described that have long-term stability and greatly improved PR properties.
Tumor Infarction in Mice by Antibody-Directed Targeting of Tissue Factor to Tumor Vasculature
Authors: Huang Xianming , Molema Grietje , King Steven , Watkins Linda , Edgington Thomas S. , Thorpe Philip E.
Published: July 2002

Selective occlusion of tumor vasculature was tested as a therapy for solid tumors in a mouse model. The formation of blood clots (thrombosis) within the tumor vessels was initiated by targeting the cell surface domain of human tissue factor, by means of a bispecific antibody, to an experimentally induced marker on tumor vascular endothelial cells. This truncated form of tissue factor (tTF) had limited ability to initiate thrombosis when free in the circulation, but became an effective and selective thrombogen when targeted to tumor endothelial cells. Intravenous administration of the antibody-tTF complex to mice with large neuroblastomas resulted in complete tumor regressions in 38 percent of the mice.
Interdecadal Climate Fluctuations That Depend on Exchanges Between the Tropics and Extratropics
Authors: Gu Daifang , Philander S G. H.
Published: July 2002

The unexpected and prolonged persistence of warm conditions over the tropical Pacific during the early 1990s can be attributed to an interdecadal climate fluctuation that involves changes in the properties of the equatorial thermocline arising as a result of an influx of water with anomalous temperatures from higher latitudes. The influx affects equatorial sea-surface temperatures and hence the tropical and extratropical winds that in turn affect the influx. A simple model demonstrates that these processes can give rise to continual interdecadal oscillations.
The Atmospheric Aerosol-Forming Potential of Whole Gasoline Vapor
Authors: Odum J. R. , Jungkamp T. P. W. , Griffin R. J. , Flagan R. C. , Seinfeld J. H.
Published: July 2002

series of sunlight-irradiated, smog-chamber experiments confirmed that the atmospheric organic aerosol formation potential of whole gasoline vapor can be accounted for solely in terms of the aromatic fraction of the fuel. The total amount of secondary organic aerosol produced from the atmospheric oxidation of whole gasoline vapor can be represented as the sum of the contributions of the individual aromatic molecular constituents of the fuel. The urban atmospheric, anthropogenic hydrocarbon profile is approximated well by evaporated whole gasoline, and thus these results suggest that it is possible to model atmospheric secondary organic aerosol formation.
The Origin Recognition Complex,
SIR1
, and the S Phase Requirement for Silencing
Authors: Fox Catherine A. , Ehrenhofer-Murray Ann E. , Loo Stephen , Rine Jasper
Published: July 2002

Silencing of transcription in
Saccharomyces cerevisiae
has several links to DNA replication, including a role for the origin recognition complex (ORC), the DNA replication initiator, in both processes. In addition, the establishment of silencing at the
HML
and
HMR
loci requires cells to pass through the S phase of the cell cycle. Passage through S phase was required for silencing of
HMR
even under conditions in which ORC itself was no longer required. The requirement for ORC in silencing of
HMR
could be bypassed by tethering the Sir1 protein to the
HMR-E
silencer. However, ORC had a Sir1-independent role in transcriptional silencing at telomeres. Thus, the role of ORC in silencing was separable from its role in initiation, and the role of S phase in silencing was independent of replication initiation at the silencers.