bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermodynamic analysis of AsH3 and PH3 decomposition including subhydrides
Authors: Jordan A. S. , Robertson A.
Published: July 2002

We have determined the gas-phase composition produced by the pyrolysis of AsH3 and PH3 in chemical equilibrium over a wide range of temperatures and pressures (ptot). Although the main interest in the analysis is the distribution of species in a cracker employed in metalorganic molecular-beam epitaxy (MOMBE) the results given as a function of ptot are also applicable to metalorganic vapor-phase epitaxy (MOVPE). To perform the calculations, we have updated the thermodynamic properties (free-energy function, entropy, etc.) of PH, PH2, and PH3 via statistical thermodynamics and derived the standard enthalpies of formation for these hydrides from bond energy measurements. In a previous article we provided similar data for As hydrides. Next, using a Gibbs energy minimization technique, we evaluated the equilibrium gas-phase concentrations of P, P2, P4, PH, PH2, PH3, H, and H2 and the corresponding As-related species as a function of temperature and ptot.

In the case of MOMBE, ptot is the approximate steady-state pressure. As expected, the decomposition of AsH3 is essentially complete at all temperatures, while PH3 is more stable. The major subhydride in MOMBE operations is AsH; its mole fraction reaches ∼10−4 at a cracker temperature of 1500 K and ptot=10−5 atm. AsH2 becomes more prominent in MOVPE applications. We have found that the atomic H concentration rises with temperature; it reaches a mole fraction of ∼3×10−4 at 1300 K and 10−5 atm which translates to an estimated beam flux of 0.011 monolayers (ML)/s and equals the atomic As flux. However, for PH3 decomposition the atomic H is about an order of magnitude more abundant than P. With rising temperature there is a rapid increase in the dimer/tetramer ratio. In MOMBE P2 and As2 are the dominant species and their concentrations are relatively insensitive to temperature. The calculations suggest that a cracker pressure exceeding ∼10−5 atm is advisable to attain an As2 or P2 beam flux of 1 ML/s (i.e., growth rate of 1 μm/h) in growing GaAs or InP. As low pressure MOVPE conditions are approached the tetramers become more prominent than the dimers and reach a dominant position at atmospheric pressure.
Infrared spectroscopic study of initial stages of ultraviolet ozone oxidation of Si(100) and (111) surfaces
Authors: Niwano Michio , Kageyama Jun-ichi , Kinashi Koji , Miyamoto Nobuo , Honma Koji
Published: July 2002

We have investigated the initial stages of UV ozone oxidation of hydrogen-terminated Si(100) and (111) surfaces using infrared spectroscopy in the multiple internal reflection geometry. Spectral features due to intermediate oxidation species such as SiH2(O2) and SiH(O3), which are generated by the attack of the back bonds of a surface Si atom by oxygen, are clearly observed. Upon UV ozone oxidation the concentration of the intermediate oxidation species initially increases and then drops, while that of surface hydride species Si–Hx decreases with increasing the UV ozone exposure, which indicates that at initial stages of oxidation oxygen attacks both of the Si–H and back bonds of a Si atom in the outermost layer. We find that the generation of the intermediate oxidation species exhibits a strong crystallographic orientation dependence; both SiH2(O2) and SiH(O3) are generated on the Si(100) surface, while the generation of the SiH2(O2) species is suppressed on the Si(111) surface.
Strain relaxation induced deep levels in In1−xGaxAs thin films
Authors: Raisanen A. , Brillson L. J. , Goldman R. S. , Kavanagh K. L. , Wieder H. H.
Published: July 2002

We have observed direct optical transitions from deep levels (DLs) induced by misfit dislocations at buried In1−xGaxAs/GaAs heterojunction interfaces by means of photoluminescence (PL) and low-energy cathodoluminescence (CL) spectroscopies. Misfit dislocations such as these have been shown to be electrically active by previous authors. By utilizing a combination of laser wavelengths and electron excitation energies, we are able to produce a PL/CL ‘‘depth profile’’ through the buried interface region, with the relative exponentially attenuated sampling depth determined by comparison of the near band-gap emission from the In1−xGaxAs layer with that from the underlying GaAs layer. For partially relaxed (1000 Å thick) In0.08Ga0.92As films, we observe DLs at approximately 0.83, 0.96, and 1.22 eV photon energies. The 0.83 eV feature is not present in the emission from either pseudomorphic (strained, 200 Å thick) In0.08Ga0.92As films or the GaAs buffer layer and substrate, and thus is likely directly related to the introduction of dislocations. These DL features are most intense for photon/electron excitations corresponding to sampling volumes which include the interface region, implying that the mechanisms generating them are localized near the interface. This ‘‘fingerprint’’ of interfacial misfit dislocations represents a potentially valuable new tool for growth diagnostics as a direct, nondestructive probe of epitaxial and interface quality.
Molecular dynamics simulation of atomic layer etching of silicon
Authors: Athavale Satish D. , Economou Demetre J.
Published: July 2002

A molecular dynamics study of 50 eV Ar+ ion bombardment of a Si(100) crystal with a monolayer of adsorbed chlorine was conducted to simulate atomic layer etching (ALET) of Si. The total reaction yield (Si atoms removed per ion) was 0.172; 84% of silicon was removed as SiCl, 8% as elemental Si and 8% as SiCl2. Based on the total yield, an ion dose of 1.16×1016 ions/cm2 is necessary to remove one monolayer of silicon. Reaction occurs during the ps time scale of the ion–solid interaction. Long time-scale chemistry (100s of ms) which is possible in ion-assisted etching with simultaneous exposure to neutral and ion beams does not happen in ALET. It was further found that 93% of Si originated from the top silicon layer and 7% from the layer underneath. In addition, some structural ‘‘damage’’ was induced to the top three silicon layers. It appears that perfect ALET of silicon is not possible for an ion energy of 50 eV.
New multicomponent transparent conducting oxide films for transparent electrodes of flat panel displays
Authors: Minami Tadatsugu , Takata Shinzo , Kakumu Toshikazu
Published: July 2002

Multicomponent transparent conducting oxide films with electrical, optical, and chemical properties that varied with the composition were prepared by rf magnetron sputtering. A resistivity as low as 2.8×10−3 Ω cm was obtained for InGaO3 films prepared at substrate temperatures of room temperature and 350 °C. In deposited In2O3–InGaO3–Ga2O3 films, the refractive index gradually decreased and the etching rate in HCl solutions increased as the Ga/(Ga+In) atomic ratio was increased. A resistivity of 7.8×10−4 Ω cm was obtained for a MgIn2O4 film. The etching rate of MgIn2O4 films in HCl solutions increased as the In/(In+Mg) atomic ratio was decreased. In MgIn2O4–Zn2In2O5 films, the etching rate in a 0.2 M HCl solution gradually increased, the refractive index increased, and the band gap decreased as the Zn/(Zn+Mg) atomic ratio was increased. In addition, the transmittance of an InGaO3/Zn2In2O5 multilayer film was slightly improved in comparison with that of only a single layer Zn2In2O5 film.
Adsorption isotherms of He and H2 at liquid He temperatures
Authors: Wallén E.
Published: July 2002

The adsorption isotherms of He and H2 at 4.2 K on stainless steel and Pyrex glass and of He on stainless steel at eight different temperatures between 1.9 and 4.2 K for pressures between 10−12 and 10−5 Torr have been measured. The measured data have been compared to different theoretical models describing adsorption isotherms. The Dubinin–Radushkevich–Kaganer (DRK) equation describes the measured He isotherms well for pressures down to about 10−9 Torr. The adsorption isotherms of He on stainless steel appear to follow the DRK equation as long as the energy to compress the gas from the measured equilibrium pressure to the saturated vapor pressure at the temperature of the measurement is smaller than the isosteric heat of adsorption. At pressures lower than about 10−9 Torr, the isotherms deviate from the DRK equation. The measurements show that the empirical constants in the DRK equation for He on stainless steel have similar values to He on Cu plated stainless steel while for He on the Pyrex glass surface the values are different. The isosteric heat of adsorption for He on stainless steel has been calculated from the measured isotherms and it decreases almost linearly from 8.5 meV/atom at a surface coverage of 1.8×1014 He atoms/cm2 down to 2.5 meV/atom at a surface coverage of 1.1×1015 He atoms/cm2. In the region where the DRK equation is not valid, the He isotherms can be described by a simple power law P=βSα, where P is the equilibrium pressure, S the surface coverage, α=1.63, and β varies with temperature.
Extended-spectral-range Fourier transform infrared-attenuated total reflection spectroscopy on Si surfaces using a novel Si coated Ge attenuated total reflection prism
Authors: Rudkevich E. , Savage D. E. , Cai W. , Bean J. C. , Sullivan J. S. , Nayak S. , Kuech T. F. , McCaughan L. , Lagally M. G.
Published: July 2002

We show that it is possible to extend the lower limit of the spectral range of Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy on Si surfaces from 1500 to ∼850 cm−1 using a Ge ATR prism coated with a thin epitaxial layer of Si, making accessible the especially important fingerprint region of organic molecules. The utility of the ATR prism is demonstrated by collecting FTIR spectra of several surface terminations of Si (H, oxide, 3-aminopropyltrimethoxysilane, and DNA) that contain peaks in the newly accessible spectral region.
Origin and evolution of sculptured thin films
Authors: Messier Russell , Venugopal Vijayakumar C. , Sunal Paul D.
Published: July 2002

Sculptured thin films (STFs) are a new class of engineered columnar thin films in which their shapes can resemble oblique matchsticks, chevrons, multiple zigzags, S’s, C’s, helices, and even superhelices. They can have densities as low as 10%–30% of the bulk material and can be made of virtually any material. The origin and evolution of columnar thin films are reviewed in the context of structure zone models in which both thermally induced and ion bombardment-induced adatom mobility effects are considered. Since STFs are prepared under low adatom mobility and oblique angle of vapor incidence conditions, the columns are the result of clustering at the 1–10 nm level, with atomic self-shadowing controlling their growth evolution. The distribution functions of the cluster sizes, shapes, and relative heights are shown to be critical for any quantitative understanding of columnar growth, especially for achieving and maintaining steady state column diameters for oblique incidence where the self-shadowing is anisotropic in the plane of the film.
Photoemission studies of AlAs–GaAs alloys, heterostructures, and superlattices
Authors: Okumura H. , Yoshida I. , Misawa S. , Yoshida S.
Published: July 2002

Inner core levels and valence-band structures of molecular-beam epitaxy-grown AlxGa1−xAs alloys, AlAs/GaAs heterostructures and [(AlAs)2(GaAs)2]n superlattices were investigated by photoemission measurements. The variations in accordance with the structures were observed for the valence bands, while the binding energies of inner core levels of all the structures are constant within ±0.1 eV. The minimum layer thickness of GaAs or AlAs necessary to show its peculiar band structure was found to be 5∼6 monolayers. The valence-band offset between AlAs and GaAs was estimated to be 0.12±0.33 eV. The spectra for the valence bands of superlattices showed the different features from those of GaAs, AlAs, and AlGaAs alloys.
Dipoles at polar heterojunction interfaces
Authors: Grant R. W. , Harrison W. A.
Published: July 2002

The Ge/GaAs heterojunction valence band discontinuity (ΔEv) is observed to depend on crystallographic orientation. We explore theoretically the effect of interface geometry on ΔEv. Interface geometries with unphysical charge accumulation are excluded. Allowed geometries with the same minimum sums of independent bond energies occur with positive, negative, and zero interface dipole giving a statistical average of zero dipole. Interbond interactions (metallization) yield different energies for geometries with unequal dipoles. Metallization energies are calculated for the smallest interface cells needed to sample positive, negative, and zero dipoles. A statistical average of the interface cells at the Ge/GaAs growth temperature predicts a dipole shift for each crystallographic orientation. The relative ordering of the predicted polar interface ΔEv’s agrees with experiment [i.e., ΔEv (1̄1̄1̄)As>ΔEv(100) >ΔEv(111) Ga] while the range of values from the model calculation is about a factor of 2 smaller than is experimentally observed.
The influence of an aluminum overlayer on the interaction of tungsten films with silicon substrates
Authors: Pauleau Y. , Dassapa F. C. , Lami Ph. , Oberlin J. C. , Romagna F.
Published: July 2002

The interactions of tungsten films with silicon and aluminum during annealing treatments of W/Si and Al/W/Si structures have been investigated using sheet resistance measurements, nuclear reaction analyses, x-ray diffraction techniques, Rutherford backscattering, and Auger electron spectroscopy. W(H) and W(Si) films, 100–200 nm thick, were deposited via the H2 reduction of WF6 on undoped single-crystal Si wafers and the Si reduction of WF6 on As implanted Si wafers, respectively. Prior to the metal deposition, As implanted Si wafers were cleaned by dipping in NH4OH:H2O2 followed by HCl:H2O2 at 80 °C for 20 min (RCA cleaning treatment). The oxygen content at the metal–Si interface in W(Si)/As implanted Si structures can reach 3 at. % whereas in W(H) /mono-Si structures, the metal–Si interface is oxygen free. In W/Si samples, the formation temperature of WSi2 was found to be dependent on the interfacial oxygen concentration. For oxygen-free W/Si structures, the silicidation reaction occurred at 625 °C while for structures containing interfacial oxygen atoms, this reaction was observed above 800 °C. In Al/W/Si structures, the intermetallic compound WAl12 was formed by annealing at 450 °C for 90 min. Tungsten atoms diffused in the aluminum overlayer and islandlike structures of WAl12 surrounded by unreacted Al appeared at the surface of samples. Furthermore, the formation of WSi2 was observed in the Al/W/Si structures annealed at a temperature in the range of 550–600 °C regardless of the oxygen concentration at the W–Si interface. The mechanisms of WAl12 and WSi2 formation are analyzed and the role of interfacial oxygen atoms in the silicidation reaction is demonstrated. The origin of the diminution in thermal stability of W–Si interfaces in Al/W/Si structures is also discussed in this paper.
Top-imaged single-layer resist process
Authors: Yang Bea-Jane L. , Yang Jer-Ming , Chiong Kaolin N.
Published: July 2002

A top-imaged single-layer resist (TISLR) process has been devised to make use of the different diffusion rates of organometallic compounds into the exposed and unexposed regions of a resist, so that latent patterns can be subsequently delineated in an oxygen plasma. The distinct chemical or physical properties of a resist in the exposed and unexposed areas bring about the different diffusion rates of the organometallic reagent. Notwithstanding that the bulk diffusion is easily achievable, a more versatile and high resolution yielding process can be realized provided that the diffusion is confined to the top-surface layer alone. Both positive- and negative-tone images have been realized with conventional photoresists according to the TISLR scheme. A few examples will be discussed to illustrate its features.
Growth of Si on flat and vicinal Si(001) surfaces: A scanning tunneling microscopy study
Authors: Mo Y. W. , Kariotis R. , Swartzentruber B. S. , Webb M. B. , Lagally M. G.
Published: July 2002

Scanning tunneling microscopy is used to investigate aspects of the initial stages of molecular-beam epitaxy of Si on Si(001). A self-diffusion coefficient is extracted. Anisotropic island shapes in the growth are attributed to an anisotropic lateral accommodation coefficient. Growth on vicinal surfaces shows that the barrier for atoms to cross a step cannot be much different from the barrier for migration on the terrace itself.
An analysis of polarization mixing errors in distance measuring interferometers
Authors: Augustyn Walter , Davis Paul
Published: July 2002

Many distance measuring interferometers use orthogonally polarized beams in their reference and measurement arms. In practice, instrumental imperfections corrupt the polarized beam in each arm with a small amplitude signal from the other arm having the perpendicular polarization. The impact of this polarization mixing on the absolute accuracy of a single frequency laser interferometer is shown to be significantly smaller than on the accuracy of a two-frequency interferometer. Certain signals obtained from a single-frequency interferometer have no phase error due to polarization mixing. Other signals are affected in proportion to the ratio between the small mixing amplitudes and the primary polarization amplitudes. Roughly, those errors in the single frequency device depend on the square of the small mixing ratios while the error in the two frequency interferometer depends on a larger quantity, the sum of these ratios. For a mixing ratio of 0.1, the error in the single-frequency interferometer is about 20 times smaller than in the heterodyne interferometer.
X-ray mask repair with focused ion beams
Authors: Wagner A. , Levin J. P. , Mauer J. L. , Blauner P. G. , Kirch S. J. , Longo P.
Published: July 2002

The application of focused ion beams to the repair of defects in x-ray masks is described. An image of the defective region on the mask is obtained using the ion beam in a manner analogous to a scanning electron microscope. Opaque defects are removed by physical sputtering of extra absorber. Clear defects are repaired by ion-beam-induced decomposition of an organometallic compound to form an opaque film on the substrate. Examples illustrating the repair process and demonstrating submicron spatial resolution are presented. The effect of ion channeling on imaging and opaque repair is also described.
Scanning tunneling microscopy studies of sulfur overlayers on the Re(0001) surface
Authors: Ogletree D. F. , Hwang R. Q. , Zeglinski D. M. , Vazquez-de-Parga A. Lopez , Somorjai G. A. , Salmeron M.
Published: July 2002

The structure of sulfur adsorbed on the basal plane of rhenium has been studied in ultrahigh vacuum with Auger, low-energy electron diffraction, and scanning tunneling microscopy. At low coverages (<0.25 monolayers) single sulfur adatoms form open structures with p(2×2) and c( 7/8 ×5) rectangular lattices. In these structures both the sulfur adatoms and some of the rhenium substrate atoms are imaged simultaneously, identifying the sulfur adsorption site as a threefold hollow site. At coverages above 0.25 ML sulfur adatoms aggregate into trimers, first disordered and then ordered into a (3 7/8 ×3 7/8 )R30° lattice at a coverage of 0.45 ML. Two types of trimers form that are centered around different substrate sites. Between 0.45 and 0.5 ML diamond-shaped sulfur tetramers appear until an ordered (3113) structure forms. With further exposure to H2S a final structure is formed, consisting of hexagonal sulfur rings in a (2 7/8 ×2 7/8 )R30° lattice. All of the sulfur adatoms in the three high coverage structures occupy the same type of threefold hollow sites as in the low coverage structures.
Analysis of lateral force effects on the topography in scanning force microscopy
Authors: Grafström S. , Ackermann J. , Hagen T. , Neumann R. , Probst O.
Published: July 2002

The mechanism of how lateral forces can produce artifacts in topographical images obtained by scanning force microscopy is analyzed theoretically. It is shown that due to the detailed elastic behavior of the force sensing cantilever, a scanning force microscope based on the light beam deflection method is particularly subject to such artifacts. They are produced by lateral forces, arising not only from friction but also from the topography itself. Consequently, the images are modified by a self-distortion of the topography. The various effects contributing to the image contrast are demonstrated experimentally by images of a tribologically heterogeneous sample. The topographical contrast is compared with lateral force images. A quantitative analysis shows that both friction-induced and topography-induced topographical artifacts can be described reasonably well by the theoretical formulas.
Fifteen nanometer features by sidewall processing and pattern transfer
Authors: Randall John N. , Newell Brian L.
Published: July 2002

A technique which uses conformally deposited silicon oxynitride on e-beam written polymethylmethacrylate structures to define 15 nm linewidths is described. The sidewalls can be used simply to narrow the opening in resist to produce fine metal lines with a lift-off process. Freestanding nitride structures can be produced which may be used for pattern transfer in a variety of methods. We demonstrate the production of narrow slots in metal. Additional processing is able to produce narrow slots in resist that then can be used for pattern transfer. We demonstrate a pair of 15 nm metal lines separated by approximately 20 nm. This technique can be applied to the fabrication of quantum effect devices, and has the potential of being extended to well below 10 nm features. These techniques have the potential to be applied in manufacturing since sidewall processing is already used widely in commercial complementary metal–oxide semiconductor processes.
X-ray photoelectron spectroscopy and x-ray absorption near-edge spectroscopy study of SiO2/Si(100)
Authors: Tao Y. , Lu Z. H. , Graham M. J. , Tay S. P.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) and Si K-edge x-ray absorption near-edge spectroscopy (XANES) have been used to characterize 0.5–20 nm thick SiO2 films on Si(100). In XPS measurements, the chemical shift of the Si(-O) 2p peak increases with oxide thickness. However, by neutralizing the surface with low-energy electrons, and using the C 1s as a reference, a chemical shift of 3.9±0.1 eV is found for neutralized oxide films, and the energy difference between the O 1s and Si(-O) 2p is independent of oxide thickness. XANES measurements confirm an invariant chemical shift with oxide thickness, the Si K edge from SiO2 being 1845.50±0.05 eV for all samples. XPS and XANES data then clearly show that any change in chemical shift of the Si(-O) 2p with oxide thickness is mainly due to a surface charging rather than microscopic strain induced by the molar volume mismatch between the oxide and the substrate, as has been previously believed.
Auger electron spectroscopy, x-ray diffraction, and scanning electron microscopy of InN, GaN, and Ga(AsN) films on GaP and GaAs(001) substrates
Authors: Jenkins L. C. , Cheng T. S. , Foxon C. T. , Hooper S. E. , Orton J. W. , Novikov S. V. , Tret’yakov V. V.
Published: July 2002

Layers of InN and GaN have been successfully grown on GaAs(001) and GaP(001) substrates using a modified molecular beam epitaxy technique. X-ray diffraction data reveals that these layers are, in fact, separated into binary components of GaAs and mixtures of GaN in its hexagonal or zinc-blende form. The same composite layers have also been studied with scanning electron microscopy, and we find a close correlation between the grain size measured from the scanning electron microscopy images and those determined from the full width at half-maximum of the x-ray diffraction spectra. This key observation demonstrates for the first time that there is a direct relationship between the measured x-ray half-widths and the microstructure of the films for both hexagonal and zinc-blende polytypes. In addition, we have been able to control the growth conditions of GaN such that we can now selectively grow either exclusively hexagonal or zinc-blende material.
Fabrication of double-gated Si field emitter arrays for focused electron beam generation
Authors: Itoh Junji , Tohma Yasushi , Morikawa Kazutoshi , Kanemaru Seigo , Shimizu Keizo
Published: July 2002

Double-gated Si field emitter arrays (FEAs) capable of generating focused electron beams were fabricated and experimentally evaluated. The present field emitter array has a vertical triode structure consisting of a conical Si tip and two gate openings (upper and lower) surrounding the tip. The lower gate with a 2-μm-diam opening acts as an extraction electrode controlling the emission current, and the upper one with a 3-μm-diam opening acts as an electrostatic lens focusing the electron trajectories. The focusing property was evaluated by observing the spot size of a phosphor (ZnO:Zn) screen located about 20 mm apart from the field emitter array and biased to 1 kV. It was found from experimental results that decreasing the upper gate voltage (VF) down to a few volts was quite effective to generate focused electron beams. At VF of about 4 V, the electrons emitted from the tip were well collimated and a beam current of about 0.1 nA/tip was obtained.
Carrier capture as a mechanism for defect migration at semiconductor surfaces
Authors: Lengel G. , Weimer M. , Gryko J. , Allen R. E.
Published: July 2002

Scanning-tunneling-microscopy (STM)-induced vacancy migration on GaAs(110) is shown to result from the trapping of minority carriers in dangling-bond defect states. The energy released when an electron falls into an As vacancy level on p-type GaAs, or when a hole falls into a Ga vacancy level on n-type GaAs, is sufficient to account for the observed migration events. Other hypotheses are inconsistent with our experimental signatures and theoretical calculations. The general notion that carrier capture can promote atomic motion may also apply to other systems with localized levels, and the ramifications for atomic manipulation with the STM are briefly considered.
Multiparameter grating metrology using optical scatterometry
Authors: Raymond Christopher J. , Murnane Michael R. , Prins Steven L. , Sohail S. , Naqvi H. , McNeil John R. , Hosch Jimmy W.
Published: July 2002

Scatterometry, the analysis of light diffraction from periodic structures, is shown to be a versatile metrology technique applicable to a number of processes involved in the production of microelectronic devices. We have demonstrated that the scatterometer measurement technique is robust to changes in the thickness of underlying films. Indeed, there is sufficient information in one signature to determine four process parameters at once, namely the linewidth and thickness of the photoresist grating, and the thicknesses of two underlying film layers. Results from determining these dimensions on a 25 wafer study show excellent agreement between the scatterometry measurements and measurements made with other metrology instruments [top-down and cross-section scanning electron microscopy (SEM) and ellipsometer]. In particular, measurements of nominal 0.35 μm lines agree well with cross-section SEM measurements; the average bias is −1.7 nm. Similarly, for nominal 0.25 μm lines, the average bias is −7.3 nm. In addition, the repeatability (1σ) of this technique is shown to be subnanometer for all of the parameters measured (linewidth, resist height, antireflection coating thickness, and poly-Si thickness).
Optimization of experimental operating parameters for very high resolution focused ion beam applications
Authors: Gierak J. , Vieu C. , Schneider M. , Launois H. , Ben Assayag G. , Septier A.
Published: July 2002

We report an experimental procedure to optimize the current profile of a focused ion beam probe, with a special emphasis on high resolution applications. The optimized operating conditions are given for three specific cases: specimen thinning for electron microscopy, nanoetching, and nanolithography. We present high quality membranes for transmission electron microscopy, arrays of nanoholes with reproducible dimensions of 17 nm etched on a nickel membrane, and finally nanolithography operations with a 10 nm resolution. Due to the conventional design of our focused ion beam system, the operating conditions that we have established for each nanofabrication application, should be successfully applied to a wide variety of ion columns.
Molecular beam epitaxial growth of Bi2Se3- and Tl2Se-doped PbSe and PbEuSe on CaF2/Si(111)
Authors: Fang X. M. , Chao I-Na , Strecker B. N. , McCann P. J. , Yuan S. , Liu W. K. , Santos M. B.
Published: July 2002

We report results on the incorporation of Bi (n type) and Tl (p-type) impurity in PbSe and PbEuSe grown on CaF2/Si(111) by molecular beam epitaxy. Bi2Se3 and Tl2Se were used as sources of dopants in the growth. Electron concentrations in the low 1019 cm−3 range and hole concentrations in the middle 1018 cm−3 range have been realized in the PbSe and PbEuSe layers with Eu content up to 3%. Electron and hole mobilities are comparable to those for PbSe and PbEuSe grown on BaF2.
Growth and luminescent characteristics of ZnGa2O4 thin film phosphor prepared by radio frequency magnetron sputtering
Authors: Kim Young Jin , Jeong Young Ho , Kim Kang Duk , Kang Seung Gu , Lee Ki Gang , Han Jeong In , Park Yong Kuy , Cho Kyong Ik
Published: July 2002

ZnGa 2 O 4 and doped ZnGa2O4 thin film phosphors were prepared by the rf magnetron sputtering method. Deposition parameters were controlled to characterize the deposition mechanism and structural changes. And then the relation between the structure and luminous properties was characterized. To observe the effects of the substrates, thin films were deposited on Si(100), Si(111), and indium–tin–oxide (ITO) coated glass substrates. The orientational transition was observed by varying the substrate temperature. The grain size of ZnGa2O4:Mn thin film deposited on a Si wafer was smaller than that on an ITO/glass substrate, which resulted in higher photoluminescence (PL) intensity. By heat treatment, PL intensity was increased because it eliminated the defects in films and improved the crystallinity.
Critical role of degassing for hot aluminum filling
Authors: Proost J. , Kondoh E. , Vereecke G. , Heyns M. , Maex K.
Published: July 2002

The purpose of this work is to relate desorption studies on spin-on-glass with electrical data on via resistance and yield. This will allow for a more detailed understanding of the in situ degas process preceding hot aluminum filling. It is shown that the degas temperature and time need to be tuned for both physical and chemical water desorption. Insufficient degassing leaves the chemisorbed components unreleased and has a large impact on the electrical behavior of dense via structures, while the isolated ones are unaffected. More evidence for this density effect in the degas characteristic is provided by focused ion beam techniques.
Low temperature plasma-assisted chemical vapor deposition of tantalum nitride from tantalum pentabromide for copper metallization
Authors: Chen Xiaomeng , Frisch Harry L. , Kaloyeros Alain E. , Arkles Barry , Sullivan John
Published: July 2002

In this article, the authors report the development of a new low temperature plasma-assisted chemical vapor deposition (PACVD) process for the growth of low resistivity, cubic tantalum nitride (TaNx) for incorporation as a diffusion barrier/adhesion promoter in emerging ultralarge-scale integrated (ULSI) multilevel metallization (MLM) schemes. TaNx films were produced in a low density plasma using tantalum pentabromide, hydrogen, and nitrogen as coreactants. The films were grown at substrate temperatures of 350–450 °C, reactor working pressures of 0.9–1.6 Torr, hydrogen flow rates between 250 and 1500 sccm, nitrogen flow rates of 100–600 sccm, and plasma power ranging from 10 to 60 W, corresponding to a power density of 0.06–0.33 W/cm2. The films were subsequently characterized by Auger electron spectroscopy, Rutherford backscattering spectrometry, x-ray diffraction, atomic force microscopy, four-point resistivity probe, and cross-sectional scanning electron microscopy. These studies indicated that the TaNx films produced were stoichiometric and carbon and oxygen free, contained bromine concentrations below 3 at. %, and exhibited resistivities as low as 150 μΩ cm. The conformality was higher than 95% in the nominally 0.3 μm, 4.5:1 aspect ratio structures. These results indicate that in the case of halide-based Ta chemistries, PACVD in a (N2+H2) plasma might be more viable than thermal CVD in a NH3 atmosphere for the deposition of TaNx for ULSI MLM applications.
Contrast limitations in electron-beam lithography
Authors: Crandall Richard , Hofmann Uli , Lozes Richard L.
Published: July 2002

State-of-the-art proximity effect correction (PEC) schemes show a degradation in contrast when they are applied to small features. It seems impossible to achieve a contrast better than that of the uncorrected pattern. In this article we provide mathematical proof that it is impossible to improve the contrast for the class of linear PEC kernels. For this class, higher resolution can only be achieved at the expense of contrast. This fundamental limitation implies a trade-off between the desired minimum feature size (related to critical dimension linearity) and contrast (related to critical dimension uniformity).
Consistent refractive index parameters for ultrathin SiO2 films
Authors: Wang Y. , Irene E. A.
Published: July 2002

Refined values are obtained in the present study for the refractive index parameter (n) for ultrathin SiO2 films (less than 8 nm in film thickness, Lox) and summarized as n=2.139−8.991×10−2×Lox+1.872×10−3×Lox2. This formula was obtained starting from variable angle of incidence ellipsometry data on ultrathin SiO2 films, and a previously derived refractive index parameter formula that was based on independently measured thickness’ using Fowler–Nordheim electron tunneling measurements. The new n values are useful for extracting more accurate ultrathin SiO2 film thicknesses from ellipsometry measurements.
Presynaptic Kainate Receptor Mediation of Frequency Facilitation at Hippocampal Mossy Fiber Synapses
Authors: Schmitz Dietmar , Mellor Jack , Nicoll Roger A.
Published: July 2002

Inhibition of transmitter release by presynaptic receptors is widespread in the central nervous system and is typically mediated via metabotropic receptors. In contrast, very little is known about facilitatory receptors, and synaptic activation of a facilitatory autoreceptor has not been established. Here we show that activation of presynaptic kainate receptors can facilitate transmitter release from hippocampal mossy fiber synapses. Synaptic activation of these presumed ionotropic kainate receptors is very fast (<10 ms) and lasts for seconds. Thus, these presynaptic kainate receptors contribute to the short-term plasticity characteristics of mossy fiber synapses, which were previously thought to be an intrinsic property of the synapse.
Ablation, Flux, and Atmospheric Implications of Meteors Inferred from Stratospheric Aerosol
Authors: Cziczo D. J. , Thomson D. S. , Murphy D. M.
Published: July 2002

Single-particle analyses of stratospheric aerosol show that about half of the particles contain 0.5 to 1.0 weight percent meteoritic iron by mass, requiring a total extraterrestrial influx of 8 to 38 gigagrams per year. The sodium/iron ratio in these stratospheric particles is higher and the magnesium/iron and calcium/iron ratios are lower than in chondritic meteorites, implying that the fraction of material that is ablated must lie at the low end of previous estimates and that the extraterrestrial component that resides in the mesosphere and stratosphere is not of chondritic composition.
Ventroptin: A BMP-4 Antagonist Expressed in a Double-Gradient Pattern in the Retina
Authors: Sakuta Hiraki , Suzuki Ryoko , Takahashi Hiroo , Kato Akira , Shintani Takafumi , Iemura Shun-ichiro , Yamamoto Takamasa S. , Ueno Naoto , Noda Masaharu
Published: July 2002

In the visual system, the establishment of the anteroposterior and dorsoventral axes in the retina and tectum during development is important for topographic retinotectal projection. We identified chick Ventroptin, an antagonist of bone morphogenetic protein 4 (BMP-4), which is mainly expressed in the ventral retina, not only with a ventral high–dorsal low gradient but also with a nasal high–temporal low gradient at later stages. Misexpression of
Ventroptin
altered expression patterns of several topographic genes in the retina and projection of the retinal axons to the tectum along both axes. Thus, the topographic retinotectal projection appears to be specified by the double-gradient molecule Ventroptin along the two axes.
Regional-Scale Assembly Rules and Biodiversity of Coral Reefs
Authors: Bellwood David R. , Hughes Terry P.
Published: July 2002

Tropical reef fishes and corals exhibit highly predictable patterns of taxonomic composition across the Indian and Pacific Oceans. Despite steep longitudinal and latitudinal gradients in total species richness, the composition of these key taxa is constrained within a remarkably narrow range of values. Regional-scale variation in reef biodiversity is best explained by large-scale patterns in the availability of shallow-water habitat. Once habitat area is accounted for, there is surprisingly little residual effect of latitude or longitude. Low-diversity regions are most vulnerable to human impacts such as global warming, underscoring the urgent need for integrated management at multinational scales.
Control of Glutamate Clearance and Synaptic Efficacy by Glial Coverage of Neurons
Authors: Oliet Stéphane H. R. , Piet Richard , Poulain Dominique A.
Published: July 2002

Analysis of excitatory synaptic transmission in the rat hypothalamic supraoptic nucleus revealed that glutamate clearance and, as a consequence, glutamate concentration and diffusion in the extracellular space, is associated with the degree of astrocytic coverage of its neurons. Reduction in glutamate clearance, whether induced pharmacologically or associated with a relative decrease of glial coverage in the vicinity of synapses, affected transmitter release through modulation of presynaptic metabotropic glutamate receptors. Astrocytic wrapping of neurons, therefore, contributes to the regulation of synaptic efficacy in the central nervous system.
Uniform Binding of Aminoacyl-tRNAs to Elongation Factor Tu by Thermodynamic Compensation
Authors: LaRiviere Frederick J. , Wolfson Alexey D. , Uhlenbeck Olke C.
Published: July 2002

Elongation factor Tu (EF-Tu) binds all elongator aminoacyl–transfer RNAs (aa-tRNAs) for delivery to the ribosome during protein synthesis. Here, we show that EF-Tu binds misacylated tRNAs over a much wider range of affinities than it binds the corresponding correctly acylated tRNAs, suggesting that the protein exhibits considerable specificity for both the amino acid side chain and the tRNA body. The thermodynamic contributions of the amino acid and the tRNA body to the overall binding affinity are independent of each other and compensate for one another when the tRNAs are correctly acylated. Because certain misacylated tRNAs bind EF-Tu significantly more strongly or weakly than cognate aa-tRNAs, EF-Tu may contribute to translational accuracy.
Taking Cell-Matrix Adhesions to the Third Dimension
Authors: Cukierman Edna , Pankov Roumen , Stevens Daron R. , Yamada Kenneth M.
Published: July 2002

Adhesions between fibroblastic cells and extracellular matrix have been studied extensively in vitro, but little is known about their in vivo counterparts. Here, we characterized the composition and function of adhesions in three-dimensional (3D) matrices derived from tissues or cell culture. “3D-matrix adhesions” differ from focal and fibrillar adhesions characterized on 2D substrates in their content of α
5
β
1
and α
v
β
3
integrins, paxillin, other cytoskeletal components, and tyrosine phosphorylation of focal adhesion kinase (FAK). Relative to 2D substrates, 3D-matrix interactions also display enhanced cell biological activities and narrowed integrin usage. These distinctive in vivo 3D-matrix adhesions differ in structure, localization, and function from classically described in vitro adhesions, and as such they may be more biologically relevant to living organisms.
A Carbon-Free Sandwich Complex [(P
5
)
2
Ti]
2−
Authors: Urne˙žius Eugenijus , Brennessel William W. , Cramer Christopher J. , Ellis John E. , Schleyer Paul von Ragué
Published: July 2002

Reactions of highly reduced titanium complexes with white phosphorus, P
4
, at or below 25°C yielded brown to deep red-brown salts of the first entirely inorganic metallocene, [(η
5
-P
5
)
2
Ti]
2−
(
1
). Like ferrocene and other carbon-based metallocenes, the structure of
1
has parallel and planar five-membered rings symmetrically positioned about the central metal atom. Despite its electron-deficient (16 electron) and formally zerovalent titanium character, salts of
1
are highly stable toward heat and air, both in solution and in the solid state. Computational studies show that the pentaphosphacyclopentadienyl unit, P
5
, functions as an unusually effective acceptor ligand, and this results in substantial stabilization of
1
.
Formation of a Matter-Wave Bright Soliton
Authors: Khaykovich L. , Schreck F. , Ferrari G. , Bourdel T. , Cubizolles J. , Carr L. D. , Castin Y. , Salomon C.
Published: July 2002

We report the production of matter-wave solitons in an ultracold lithium-7 gas. The effective interaction between atoms in a Bose-Einstein condensate is tuned with a Feshbach resonance from repulsive to attractive before release in a one-dimensional optical waveguide. Propagation of the soliton without dispersion over a macroscopic distance of 1.1 millimeter is observed. A simple theoretical model explains the stability region of the soliton. These matter-wave solitons open possibilities for future applications in coherent atom optics, atom interferometry, and atom transport.
Structure of the p53 Tumor Suppressor Bound to the Ankyrin and SH3 Domains of 53BP2
Authors: Gorina Svetlana , Pavletich Nikola P.
Published: July 2002

Mutations in the p53 tumor suppressor are among the most frequently observed genetic alterations in human cancer and map to the 200-amino acid core domain of the protein. The core domain contains the sequence-specific DNA binding activity and the in vitro 53BP2 protein binding activity of p53. The crystal structure of the p53 core domain bound to the 53BP2 protein, which contains an SH3 (Src homology 3) domain and four ankyrin repeats, revealed that (i) the SH3 domain binds the L3 loop of p53 in a manner distinct from that of previously characterized SH3-polyproline peptide complexes, and (ii) an ankyrin repeat, which forms an L-shaped structure consisting of a β hairpin and two α helices, binds the L2 loop of p53. The structure of the complex shows that the 53BP2 binding site on the p53 core domain consists of evolutionarily conserved regions that are frequently mutated in cancer and that it overlaps the site of DNA binding. The six most frequently observed p53 mutations disrupt 53BP2 binding in vitro. The structure provides evidence that the 53BP2-p53 complex forms in vivo and may have a critical role in the p53 pathway of tumor suppression.
The Coulomb Blockade in Coupled Quantum Dots
Authors: Livermore C. , Crouch C. H. , Westervelt R. M. , Campman K. L. , Gossard A. C.
Published: July 2002

Individual quantum dots are often referred to as “artificial atoms.” Two tunnel-coupled quantum dots can be considered an “artificial molecule.” Low-temperature measurements were made on a series double quantum dot with adjustable interdot tunnel conductance that was fabricated in a gallium arsenide-aluminum gallium arsenide heterostructure. The Coulomb blockade was used to determine the ground-state charge configuration within the “molecule” as a function of the total charge on the double dot and the interdot polarization induced by electrostatic gates. As the tunnel conductance between the two dots is increased from near zero to 2
e
2
/
h
(where
e
is the electron charge and
h
is Planck's constant), the measured conductance peaks of the double dot exhibit pronounced changes in agreement with many-body theory.
Could the Internet Balkanize Science?
Authors: Van Alstyne Marshall , Brynjolfsson Erik
Published: July 2002

The information explosion carried by the Internet represents a powerful technology, but at the same time, it carries risk. In their Perspective, Van Alstyne and Brynjolfsson argue that the ability to form global collaborations on the basis of scientific specialty could potentially diminish local interdisciplinary contact, which the authors dub “the balkanization of cyberspace.” Such balkanization is not inevitable, however, and depends on how scientists choose to use information technology. [Available as an Enhanced Perspective on
Science
Online with additional hypertext links and Internet resources]
Ethics in Science: Is Data-Hoarding Slowing the Assault on Pathogens?
Authors: Marshall Eliot
Published: July 2002

The secretive policies of many biotechs and pharmaceutical firms have rekindled a smoldering debate over who should control DNA sequence data, and how quickly it should be shared.
A Tunable Diode Based on an Inorganic Semiconductor|Conjugated Polymer Interface
Authors: Lonergan Mark C.
Published: July 2002

Although in principle semiconductor-metal (Schottky) diodes should be tunable by changing the work function of the metal, such flexibility cannot be achieved in a single device and in practice is often limited by interfacial states that cause Fermi-level pinning. A tunable diode is reported based on a hybrid inorganic-organic,
n
-indium phosphide|poly- (pyrrole)|nonaqueous electrolyte architecture. By electrochemically manipulating the work function of the conjugated polymer poly(pyrrole), the turn-on voltage (more precisely, the forward bias potential required to pass a particular current) of the diode can be continuously and actively tuned by more than 0.6 volt. The work highlights a distinguishing feature of conjugated polymers relative to more traditional semiconductor materials, namely, the ability of dopant ions to permeate conjugated polymers, thereby enabling electrochemical manipulation.
Formation of Molecular Chlorine from the Photolysis of Ozone and Aqueous Sea-Salt Particles
Authors: Oum K. W. , Lakin M. J. , DeHaan D. O. , Brauers T. , Finlayson-Pitts B. J.
Published: July 2002

Halogen atoms from the reactions of sea-salt particles may play a significant role in the marine boundary layer. Reactions of sodium chloride, the major component of sea-salt particles, with nitrogen oxides generate chlorine atom precursors. However, recent studies suggest there is an additional source of chlorine in the marine troposphere. This study shows that molecular chlorine is generated from the photolysis of ozone in the presence of sea-salt particles above their deliquescence point; this process may also occur in the ocean surface layer. Given the global distribution of ozone, this process may provide a global source of chlorine.
Local Orbital Forcing of Antarctic Climate Change During the Last Interglacial
Authors: Kim Seong-Joong , Crowley Thomas J. , Stössel Achim
Published: July 2002

During the last interglacial, Antarctic climate changed before that of the Northern Hemisphere. Large local changes in precession forcing could have produced this pattern if there were a rectified response in sea ice cover. Results from a coupled sea ice–ocean general circulation model supported this hypothesis when it was tested for three intervals around the last interglacial. Such a mechanism may play an important role in contributing to phase offsets between Northern and Southern Hemisphere climate change for other time intervals.