bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantized conductance in an atom-size bridge made from magnetic materials
Authors: Morigaki Y. , Nakanishi H. , Kasai H. , Okiji A.
Published: July 2002

With the aid of the Hubbard model, and considering orbital degeneracies, we investigate how the magnetic properties of atom-size bridges affect the quantum transport of electrons through them. Within the Hartree–Fock approximation, we calculate the corresponding total energy of the electron system of the atom-size bridge as it is stretched in length. Our numerical results show that stretching the atom-size bridge induces changes in its magnetic state, from paramagnetic to ferromagnetic, and also vice versa. Furthermore, the corresponding conductance variation of the atom-size bridge is demonstrated.
Correspondence of potentials of mean force in proteins and in liquids
Authors: Shan Yibing , Zhou Huan-Xiang
Published: July 2002

The concept of potential of mean force (PMF) is now widely used in predicting protein structures. Proteins notably differ from liquids by their inhomogeneity and chain connectivity. Does meaningful correspondence exist between PMFs in proteins and PMFs in liquids? This question was addressed in this article. We constructed “proteins” each with 90 residues selected from a system of 500 hard spheres. The residues were of two types, N and P. They interact among themselves (with energies ENN, EPP, ENP) and the 410 “solvent” spheres (with energies ENS and EPS). Out of the 500 hard spheres, we first identified all chains consisting of 90 residues that have appropriate distances between nearest neighbors. The conformation of a protein was selected as the one having the lowest total energy among the 3.7 million chains. A corresponding liquid system was constructed without imposing distance constraints among solute spheres. The PMFs obtained from the proteins and the liquid system show remarkable similarities. For eleven sets of the energy parameters, the first minima of the PMFs in the proteins agree with their counterparts in the liquid state to within a constant.
Impact strength of cryogenic deuterium pellets for injection into tokamak plasmas
Authors: Lorenz A. , Lang P. T. , Lang R. S.
Published: July 2002

The technical implementation of the potentially very efficient inboard pellet refueling scheme in tokamaks remains so far restricted to low velocities (v=200–300 m/s) due to the fragile nature of cryogenic D2. One specific problem is practically unavoidable pellet impacts in components of the pellet guiding system: first, in a funneling adaptor installed to cope with the angular scatter of pellets from acceleration devices such as centrifuges and, second, inside variably bent guiding tubes required to access the high field side of the torus. In order to inject pellets at speeds close to those available from the ASDEX Upgrade centrifuge (v⩽1200 m/s), knowledge of critical guiding system parameters such as impact angle and speed is needed. In this study design requirements for an optimized guiding scheme are derived by examining grazing incidence pellet impacts on one single and two subsequent flat, rigid plates. The survival of the pellets was found to be determined by the normal impact velocity component. After undergoing two successive collisions under the same angle a critical impact speed of v⊥=40 m/s was detected, permitting incidence angles of up to 2° at v>1000 m/s. Studying impacts under 1°–6° reflection angles from 0.1° to 0.8° were measured, suggesting a considerable ductility of the D2 ice. This was confirmed in measurements of the plane pellet compression where irreversible deformation was found to set in well below the critical impact speed. Therefore the optimized guiding track geometry is proposed on the basis of an elliptic loop to keep the pellets as close as possible to the track and to guide the pellets through a large number of elastic small angle collisions. This makes also use of the lubricating effect of the vapor film form upon collision between the sliding pellet and the chute.
Imaging soft samples in liquid with tuning fork based shear force microscopy
Authors: Rensen W. H. J. , van Hulst N. F. , Kämmer S. B.
Published: July 2002

We present a study of the dynamic behavior of tuning forks and the application of tuning fork based shear force microscopy on soft samples in liquid. A shift in resonance frequency and a recovery of the tip vibration amplitude have been observed upon immersion into liquid. Conservation of the vibration mode is confirmed by both direct stroboscopic observation and by detection of the tip vibration amplitude of the tuning fork. Thanks to the partial recovery of the Q factor upon complete immersion into liquid, it is possible to obtain high-resolution images on soft samples in liquid. This opens a new domain of applications for tuning fork based near-field scanning optical microscopes.
Geometry selects highly designable structures
Authors: Shahrezaei V. , Ejtehadi M. R.
Published: July 2002

By enumerating all sequences of length 20, we study the designability of structures in a two-dimensional hydrophobic-polar (HP) lattice model in a wide range of intermonomer interaction parameters. We find that although the histogram of designability depends on interaction parameters, the set of highly designable structures is invariant. So in the HP lattice model the high designability should be a purely geometrical feature. Our results suggest two geometrical properties for highly designable structures; they have the maximum number of contacts and a unique neighborhood vector representation. Also we show that the contribution of perfectly stable sequences in the designability of structures plays a major role in making them highly designable.
Calculation of the compressibilities of hard-sphere fluids from the radial free space distribution function and its application to sound velocity
Authors: Yoon Byoung Jip , Ohr Young Gie
Published: July 2002

The relation between the compressibility and the radial free space distribution function (RFSDF) has been derived. This method for calculating the compressibility employing RFSDF is useful because the compressibility of any system is obtainable in a single Monte Carlo simulation without obtaining the equation of state. The compressibilities from the newly derived relation using RFSDF of hard-sphere fluids have been compared with those obtained from the equation of state and they are in good agreement. This method has been used to calculate the velocity of sound for hard spheres confined in parallel hard walls with the results showing that the velocity of sound in parallel direction to the wall is faster than that in normal direction.
GaN thin films by growth on Ga-rich GaN buffer layers
Authors: Kim Yihwan , Subramanya Sudhir G. , Siegle Henrik , Krüger Joachim , Perlin Piotr , Weber Eicke R. , Ruvimov Sergei , Liliental-Weber Zuzanna
Published: July 2002

The influence of the composition of the low-temperature GaN buffer layer on the structural, electrical, and optical properties of the subsequently grown GaN epilayer by molecular-beam epitaxy is investigated. It is found that decreasing the N/Ga flux ratio during the buffer layer growth yields excess Ga in the buffer layer. GaN epilayers grown on the buffer layer with excess Ga showed increased electron Hall mobility and reduced threading dislocation density. The tilt and twist disorders of the epilayer were also significantly reduced. The compressive stress in the epilayers at room temperature increased as the N/Ga flux ratio during the buffer layer growth was reduced. The improved properties of the epilayers are explained by the hypothesis that excess Ga in the buffer layer facilitates stress relaxation at the epilayer growth temperature.
Atomic-scale defect control on hydrogen-terminated silicon surface at wafer scale
Authors: Sakaue Hiroyuki , Fujiwara Shinya , Shingubara Shoso , Takahagi Takayuki
Published: July 2002

We have developed a wet preparation method for an atomically defect-free Si wafer surface. In the conventional atomically smooth treatment using ammonium fluoride aqueous solution, dissolved oxygen has been revealed to form defects on the Si (111) surface. We have been able to create an extremely atomically smooth hydrogen-terminated surface with a good periodic step/terrace structure. Moreover, the ordered surface structure was confirmed to be fabricated all over the wafer surface. The atomic-scale defect-free hydrogen-terminated wafer surface with a periodic step/terrace structure is expected to be highly useful as a well-defined substrate for nanostructure fabrication and high-quality film deposition, and to be useful in many other research fields.
Combined Monte Carlo and molecular dynamics simulation of hydrated dipalmitoyl–phosphatidylcholine–cholesterol lipid bilayers
Authors: Chiu S. W. , Jakobsson Eric , Scott H. Larry
Published: July 2002

We have applied a hybrid equilibration and sampling procedure for the atomic-level simulation of a hydrated lipid bilayer to systems consisting of dipalmitoyl phosphatidylcholine (DPPC) and cholesterol. The procedure is applied to a bilayer of 64 molecules of DPPC, 64 molecules of cholesterol, and 4116 waters (1:1 ratio), and to a bilayer of 108 molecules of DPPC, 54 molecules of cholesterol, and 5301 waters (2:1 ratio). After equilibration three separate continuous molecular dynamics runs, separated by 10 000 configurational bias Monte Carlo steps, were carried out for each system. Properties of the systems were calculated and averaged over the three separate runs. A striking result from the simulations is the overall similarity of lipid structure in the 1:1 and the 2:1 systems. The area per DPPC, lipid chain order parameter profiles, and atom distribution are all very similar for the two systems, in agreement with available experimental data. The calculated dipole potential profiles reveal differences in the electrostatic environment in the two systems. Based on simulation data we suggest that there is a critical lipid:cholesterol ratio above 2:1 but probably below 4:1 at which cholesterol forces DPPC chains into gel-like conformational order.
Residual dipolar couplings between quadrupolar nuclei in solid state nuclear magnetic resonance at arbitrary fields
Authors: Wi Sungsool , Frydman Veronica , Frydman Lucio
Published: July 2002

Second-order dipolar effects arise when a nucleus S is in close proximity to a quadrupolar spin I. These couplings originate from cross correlations between quadrupolar and dipolar interactions, and have the notable characteristic of not being susceptible to averaging by magic-angle-spinning. Therefore they can originate noticeable splittings in high resolution solid state nuclear magnetic resonance (NMR) spectra, as has been observed repeatedly for S=1/2. With the advent of high resolution half-integer quadrupole spectroscopy, such effects have now also been noticed in higher (S=3/2,5/2,…) spin systems. Within the last year these couplings have been reported for a number of complexes and analyzed in the high-field limit, when I’s Larmor frequency largely exceeds its quadrupolar coupling. The present study discusses the generalization of these analyses to arbitrary quadrupolar/Zeeman ratios. The predictions of the essentially numerical treatment that results compare well with previously derived high-field analytical models, as well as with experimental solid state NMR spectra observed in a borane compound possessing a B11–As75 spin pair. An alternative analytical variant that can account for these effects in the low-field limit is also derived on the basis of average Hamiltonian theory; its results agree well with the predictions obtained from general numerical calculations of one-dimensional S spectra, but present peculiarities in the bi-dimensional NMR line shapes whose origins are briefly discussed.
Length scale of dynamic heterogeneity in supercooled glycerol near Tg
Authors: Reinsberg S. A. , Qiu X. H. , Wilhelm M. , Spiess H. W. , Ediger M. D.
Published: July 2002

This letter presents the first direct measurement of the length scale of dynamic heterogeneity in a low molecular weight glass former without the perturbing effect of probe molecules or confinement. Using a multidimensional C13 solid-state exchange NMR experiment, 1 nm heterogeneities were found in glycerol for temperatures ranging from 199 K to 207 K (Tg=189 K). This small size and weak temperature dependence allow some distinctions to be made among different models of the glass transition. It is shown that the dynamics are not influenced by a low concentration (0.1 wt. %) relaxation agent Cu(NO3)2.
Yellow and green luminescence in a freestanding GaN template
Authors: Reshchikov M. A. , Morkoç H. , Park S. S. , Lee K. Y.
Published: July 2002

We have studied a broad photoluminescence band in high-mobility freestanding 200-μm-thick GaN template prepared by hydride vapor-phase epitaxy. Variable-excitation intensity and energy experiments showed two defect-related bands: a yellow luminescence (YL) band at about 2.15 eV and a green luminescence (GL) band at about 2.43 eV. In contrast to epitaxial GaN samples prepared by both vapor-phase and molecular-beam epitaxy, the YL in the sample studied is weak and can be easily saturated. However, the GL is dominant. We attribute the GL to isolated defects involving gallium vacancies and the YL to the same defect, but bound to dislocations, or possibly to structural surface defects.
Small-energy asymptotics of the scattering matrix for the matrix Schrödinger equation on the line
Authors: Aktosun Tuncay , Klaus Martin , van der Mee Cornelis
Published: July 2002

The one-dimensional matrix Schrödinger equation is considered when the matrix potential is self-adjoint with entries that are integrable and have finite first moments. The small-energy asymptotics of the scattering coefficients are derived, and the continuity of the scattering coefficients at zero energy is established. When the entries of the potential have also finite second moments, some more detailed asymptotic expansions are presented.
Molecular-dynamics simulations of cluster–surface collisions: Emission of large fragments
Authors: Tomsic Anna , Andersson Patrik U. , Markovic Nikola , Piskorz Witold , Svanberg Marcus , Pettersson Jan B. C.
Published: July 2002

Large-scale classical molecular-dynamics simulations of (H2O)n (n=1032,4094) collisions with graphite have been carried out. The clusters have an initial internal temperature of 180 K and collide with an incident velocity in the normal direction between 200 and 1000 m/s. The 1032-clusters are trapped on the surface and completely disintegrate by evaporation. The 4094-clusters are found to partly survive the surface impact provided that the surface is sufficiently hot. These clusters are trapped on the surface for up to 50 ps before leaving the surface under strong evaporation of small fragments. The time spent on the surface is too short for full equilibration to occur, which limits the fragmentation of the clusters. The size of the emitted fragment is roughly 30% of the incident cluster size. The cluster emission mechanism is found to be very sensitive to the rate of the surface-induced heating and thus to the surface temperature. The incident cluster velocity is less critical for the outcome of the collision process but influences the time spent on the surface. The trends seen in the simulations agree well with recent experimental data for collisions of large water clusters with graphite [Chem. Phys. Lett. 329, 200 (2000)].
Ultrahigh resistivity aluminum nitride grown on mercury cadmium telluride
Authors: Butcher K. S. A. , Tansley T. L.
Published: July 2002

Aluminum nitride insulating layers have been grown at room temperature with a film resistivity of 3.3×1016 Ω cm on mercury cadmium telluride substrates. Insulator breakdown fields of 640 MV/m were reached. Capacitance–voltage measurements with Al/AlN/Hg0.76Cd0.24Te metalinsulator-semiconductor devices demonstrate band bending at the semiconductor surface indicating that damage to the substrate was minimized during film growth. A fixed interface charge density (given here as total charge per unit area divided by the electron charge=Qss/q) of +2×1011 cm−2 and a slow interface state density of 4×1010 cm−2 were measured. The procedures for achieving these high quality insulating layers are reported. Frequency dependent dielectric constant and dielectric loss tangent measurements, carried out at room temperature and 100 K, are also presented.
High-quality, faceted cubic boron nitride films grown by chemical vapor deposition
Authors: Zhang W. J. , Jiang X. , Matsumoto S.
Published: July 2002

Thick cubic boron nitride (cBN) films showing clear crystal facets were achieved by chemical vapor deposition. The films show the highest crystallinity of cBN films ever achieved from gas phase. Clear evidence for the growth via a chemical route is obtained. A growth mechanism is suggested, in which fluorine preferentially etches hBN and stabilizes the cBN surface. Ion bombardment of proper energy activates the cBN surface bonded with fluorine so as to enhance the bonding probability of nitrogen-containing species on the F-stabilized B (111) surface.
Quadrupolar and dipolar contributions to x-ray magnetic circular dichroism at the Tb L3,2 edges: Experiment versus theory
Authors: Wende H. , Li Z. , Scherz A. , Ceballos G. , Baberschke K. , Ankudinov A. , Rehr J. J. , Wilhelm F. , Rogalev A. , Schlagel D. L. , Lograsso T. A.
Published: July 2002

We investigate the x-ray magnetic circular dichroism (XMCD) at the L3,2 edges using a single crystal of Tb as a prototype system for a one-element magnet in order to ascertain the multipolar nature of the features in the dichroic spectra. The high resolution of the experimental data allows for a clear identification of the dipolar (E1: 2p→5d) and quadrupolar (E2: 2p→4f) transitions. On the basis of ab initio calculations we developed a simple procedure to extract the quadrupolar part by subtracting the derivative of the spin-averaged absorption spectra from the experimental XMCD data. The deconvolution has to be carried out before applying sum rules to determine 4f and 5d moments.
Room-temperature intense 320 nm band ultraviolet emission from quaternary InAlGaN-based multiple-quantum wells
Authors: Hirayama Hideki , Enomoto Yasushi , Kinoshita Atsuhiro , Hirata Akira , Aoyagi Yoshinobu
Published: July 2002

We demonstrated room-temperature (RT) intense ultraviolet (UV) emission with wavelength in the range of 300–340 nm from Inx1Aly1Ga1−x1−y1N/Inx2Aly2Ga1−x2−y2N multiple-quantum wells (MQWs) fabricated on SiC by metalorganic vapor phase epitaxy. We found that the UV emission is considerably enhanced upon introducing approximately 5% of In into AlGaN. Maximally efficient emission was obtained at 318 nm from the fabricated In0.05Al0.34Ga0.61N/In0.02Al0.60Ga0.38N three-layer MQW when the QW thickness was approximately 1.4 nm. The intensity of 320 nm band emission from the InAlGaN-based MQWs was as strong as that of 410 nm band emission from InGaN-based QWs at RT. We observed emission fluctuations of submicron size in cathode luminescence images of Inx1Aly1Ga1−x1−y1N/Inx2Aly2Ga1−x2−y2N single QW which might be due to In segregation effect. The temperature dependence of photoluminescence emission for InAlGaN-based QWs was greatly improved in comparison with that of GaN- or AlGaN-based QWs.
Structural and optical properties of epitaxial and bulk ZnO
Authors: Zeuner A. , Alves H. , Hofmann D. M. , Meyer B. K. , Heuken M. , Bläsing J. , Krost A.
Published: July 2002

In this letter, we compare the properties of bulk and epitaxial ZnO. The ZnO thin films were grown on GaN templates and on ZnO single crystals by vapor phase deposition using Zn and NO2 as precursors. We use high-resolution x-ray diffraction to resolve the structural properties. The rocking curves of the bulk crystal are extremely broad caused by a mosaic structure of the substrate. The homoepitaxial ZnO film mimics the properties of the ZnO substrate whereas ZnO films on GaN templates showed superior rocking curve half width as small as 230 arcsec. The optical properties are investigated by temperature-dependent photoluminescence. Different donor and acceptor bound excitons can be distinguished for a half width of the recombination lines less than 1 meV. Free exciton emission is already detectable at liquid-He temperatures proofing the high quality of the epitaxial films.
Elastic interaction of a wedge crack with a screw dislocation
Authors: Ohr S. M. , Chang S.-J. , Thomson R.
Published: July 2002

The elastic field of a semi-infinite wedge crack and its interaction with a screw dislocation under mode III loading conditions are examined. The stress field around the wedge crack is expressed in terms of a wedge stress intensity factor. The rate of falloff of the stress field with distance from the crack tip is found to be a function of the wedge angle. The relationship between the stress intensity factors for the wedge and sharp cracks is derived and used to estimate the magnitude of stress relaxation occurring at the crack tip due to crack blunting. The crack extension force for the wedge crack is calculated and is shown to be zero. The modifications in the stress field and the local wedge stress intensity factor due to the presence of the dislocation are calculated. From the image stress on the dislocation, the condition for the emission of dislocations from the wedge crack is obtained in terms of the critical wedge stress intensity factor for dislocation generation. It is found that the emission of dislocations from the crack tip is more difficult when the crack is blunting.
Energy balance in a nonequilibrium weakly ionized nitrogen discharge
Authors: Boeuf J. P. , Kunhardt E. E.
Published: July 2002

The time evolution of the energy balance in a nitrogen discharge during its early phases of development is simulated using a self-consistent calculation of the electron distribution function (EDF), the vibrational population (VP), and densities of some electronically excited states. The contribution of the V–T (vibration-to-translation), and V–V (vibration-to-vibration) energy exchanges is quantitatively analyzed. The contribution to the energy balance by self-quenching reactions of the N2(A3Σ+u) state is also analyzed. It is shown that the population of this state in a discharge and its contribution to the energy balance may be significant.
Hydrogenated amorphous silicon studied by scanning tunneling microscopy
Authors: Wiesendanger R. , Rosenthaler L. , Hidber H. R. , Güntherodt H.-J. , McKinnon A. W. , Spear W. E.
Published: July 2002

We have used scanning tunneling microscopy under ultrahigh vacuum conditions in order to study the local electronic properties, as well as the topographical and chemical structure of hydrogenated amorphous silicon. Local characteristics were measured and images of the topography, as well as simultaneously recorded images of the local tunneling barrier height, were obtained with subnanometer resolution. The experimental results demonstrate that chemical inhomogeneities on a subnanometer scale, which may be caused by the presence of hydrogen in these samples, can be detected in the local tunneling barrier height images.
Raman scattering by epitaxial GaAs on a Si substrate
Authors: Roughani B. , Kallergi M. , Aubel J. , Sundaram S.
Published: July 2002

The strain distribution and the lattice disorder of GaAs on a Si substrate were studied by Raman scattering technique as a function of distance from the GaAs/Si interface. Epitaxial layers of GaAs grown by the laser-assisted metalorganic chemical vapor deposition technique on a Si substrate were subjected to slow chemical etching in successive steps, and Raman spectra were observed after each etch procedure. The asymmetry of the longitudinal optical (LO) phonon peak and the ratio of the integrated intensity of LO phonon to that of the transverse optical phonon Raman peaks increase for successive chemical etching, and the LO-phonon peak shifts to lower energies. Our Raman results indicate changes in lattice stacking, disorder, and strain in the epilayer as a function of distance from the GaAs/Si interface.
Laser-induced forward transfer of high-T
c YBaCuO and BiSrCaCuO superconducting thin films
Authors: Fogarassy E. , Fuchs C. , Kerherve F. , Hauchecorne G. , Perriere J.
Published: July 2002

For the first time, the deposition of YBaCuO and BiSrCaCuO thin films has been performed by the single-laser pulse-induced forward transfer technique. In addition, the BiSrCaCuO films were successfully converted into the superconducting phase, with an onset critical temperature of about 90 K and a zero resistance at 80 K, by a subsequent thermal anneal in oxygen atmosphere in the 850–900 °C temperature range.
60-Hz ac losses in YBa2 Cu3 O7 cylinders
Authors: Orehotsky J. , Garber M. , Xu Youwen , Wang Y. L. , Suenaga M.
Published: July 2002

60-Hz ac losses at 4.2 and 77 K for sintered YBa2 Cu3 O7 cylinders having a large difference in the grain size were measured as a function of applied magnetic field. The results are discussed in terms of the intra- and transgranular critical current densities in the specimens.
Zone-center magnons in dilute magnetic semiconductors
Authors: Huber D. L.
Published: July 2002

A calculation of the frequency of a zone-center magnon in a dilute magnetic semiconductor is outlined. It is based on the moments of the imaginary part of the dynamic spin susceptibility. Results are obtained at low temperatures for Cd1−xMnxTe in the zero- and high-magnetic-field limits. Good agreement with experiment is found assuming that (1) the dominant anisotropy mechanism is the Dzyaloshinski–Moriya interaction and (2) nearest-neighbor Mn spins are not colinear in the zero-field ground state.
Charging effects of MgO under electron bombardment and nonohmic behavior of the induced specimen current
Authors: Cazaux J. , Kim K. H. , Jbara O. , Salace G.
Published: July 2002

A MgO single crystal partly coated with a thin layer of Ag was irradiated with incident electrons while the time evolution of the surface potential and specimen current were recorded. A zero surface potential, Vs, is measured when the uncoated part of the specimen is irradiated, while the potential is very negative on the coated part for primary beam energies in the 2–5 keV range. The results invalidate the usual approach which uses a theoretical total yield curve for evaluating the sign of the surface potential as well as the use of an ohm law Vs = RIs for correlating the surface potential Vs and the specimen current Is. Explanations of the observed phenomena are suggested and their correlation with the secondary electron emission of insulators is indicated. One surprising result is the possibility of injecting (nonohmic) current densities of 1 mA/mm2 or greater into insulators having electrical resistivities of 1015 Ω cm.
Ion irradiation damage in Er-doped silica probed by the Er3+ luminescence lifetime at 1.535 μm
Authors: Polman A. , Poate J. M.
Published: July 2002

The effect of MeV ion irradiation damage on the luminescence lifetime of erbium-doped silica glass films has been studied. The 10-μm-thick films were first implanted with 3.5 MeV Er at a fluence of 5×1015 cm−2. When optically pumped at 488 nm, the films show a clear photoluminescence spectrum centered around 1.535 μm, corresponding to the 4I13/2→4I15/2 transition of Er3+(4f11), with a luminescence lifetime of 5.5 ms. After thermal annealing at 900 °C, the lifetime increases to 14.1 ms. Radiation damage was then introduced in the annealed films using 1 MeV He, 3.5 MeV C, 5.5 MeV Si, or 8.5 MeV Ge ions. The lifetime is decreased by irradiation with fluences as low as 1011 ions/cm2 and continues to decrease with fluence until saturation occurs above ≊1014 ions/cm2. The saturation lifetime is ion-mass dependent and ranges from 6.6 to 8.5 ms. The lifetime changes are explained in terms of nonradiative energy transfer processes caused by irradiation-induced defects in the silica. A model for lifetime changes as a function of ion fluence is derived, assuming an inverse relation between the nonradiative lifetime and the defect density. Fits to the data show that the defect generation rate is a sublinear function of the ion fluence. The ion damage effects are governed by the electronic component of the energy loss along the ion trajectories.
Defects in epitaxial Si-doped GaInP
Authors: Krynicki J. , Zaidi M. A. , Zazoui M. , Bourgoin J. C. , DiForte-Poisson M. , Brylinski C. , Delage S. L. , Blanck H.
Published: July 2002

We have characterized by capacitance-voltage and deep level transient spectroscopy measurements the only defect detected in Si-doped GaInP layers. This defect exhibits an ionization energy of 0.435 eV but is located only at ∼20 meV below the bottom of the conduction band. All its characteristics, i.e., energy level, apparent capture barrier, ionization energy, can be understood if the defect is a donor associated DX center. Its cross section for electron and hole capture have been measured. The effect of an electric field on the ionization energy confirms that the defect is indeed shallow and a donor.
Effects of a stress field on boron ion implantation damage in silicon
Authors: Mis J. D. , Mader S. R. , Beshers D. N.
Published: July 2002

The effect of stress fields on the nucleation of defects from boron ion implantation into silicon has been examined. Two different processing techniques were used to create the stress field: formation of an oxide-filled trench and formation of windows etched into nitride films. Interstitial dislocation loops created by the boron implantation were found to be preferentially oriented with respect to the stress field. A correlation between the force resulting from the stress field and the orientation of the dislocation loops was found.
Dissolution kinetics of D defects in Czochralski silicon
Authors: Wijaranakula W.
Published: July 2002

The dissolution kinetics of D defects in p- and n-type Czochralski silicon during high temperature annealing were investigated. The results indicate that D-defect dissolution is independent from both the density of the D defects in as-received wafers and the type of doping impurities. For annealing up to 1.08×104 s, no D-defect dissolution was observed in samples annealed at 1000 °C. Based upon the present analysis, it is concluded that D-defect dissolution occurs when the vacancy concentration retained in as-grown silicon is below the thermal equilibrium concentration of vacancies at the annealing temperature. The results suggest that the driving force for D-defect dissolution is vacancy undersaturation in the silicon matrix and not the silicon interstitial saturation.
Comparison between deep level defects in GaAs induced by gamma, 1 MeV electron, and neutron irradiation
Authors: Lai S. T. , Alexiev D. , Nener B. D.
Published: July 2002

The deep level transient spectroscopy technique has been employed to follow closely the effect of 1–300 Mrad 60Co γ irradiation on the deep electron traps in undoped vapor-phase-epitaxy n-type GaAs. The 1 Mrad γ-irradiated Schottky device was identical to the as-grown or control device, with only two electron traps EL2 (Ec−0.820 eV) and EL3 (Ec−0.408 eV) detected. At a γ dose of 5 Mrad, two additional electron traps EL6 (Ec−0.336 eV) and E2 (Ec−0.128 eV) were observed. As the γ doses were increased to ≥10 Mrad, a third electron trap E1 (Ec−0.033 eV) was observed, and the single exponential EL2 capacitance transient became a double exponential, indicating two deep levels lying at Ec−0.820 eV (EL2/EL2-A) and Ec−0.843 eV (EL2-B). The trap concentration of EL2-A remained unchanged up to a γ dose of 50 Mrad before starting to increase slowly as the γ dose was increased to ≥100 Mrad. In contrast, the EL2-B trap concentration was found to increase by 32 times, reaching 2.6×1014 cm−3 at 300 Mrad from a low 8.0×1012 cm−3 at 10 Mrad when it was first observed, whereas for the 1 MeV electron irradiation with low electron fluence of 1014 e cm−2, the EL6, E2, E1, and the double exponential EL2 were detected at the same time. There was no sign of EL2-B, EL3, EL6, E2, or E1, but an additional broad U band was observed after irradiation with 1 MeV neutrons. The results of the γ and neutron irradiation suggest that the presence of the double exponential EL2 transients is not related to either EL6, E2, E1, or the U band, and is unlikely to be due AsGa→VGa+Asi but is probably caused by the AsGa complex defects involving an irradiation defect. The defect concentration of trap E1 increased strongly from 5.4×1013 cm−3 at 10 Mrad to 9.3×1014 cm−3 at 100 Mrad, and E2 increased from 2.1×1013 cm−3 at 5 Mrad to 6.7×1014 cm−3 at 100 Mrad.
Zero field damping capacity in (TbxDy1−x)Fey
Authors: Teter J. P. , Hathaway K. B. , Clark A. E.
Published: July 2002

Materials with a large capacity to dissipate mechanical energy are of interest for passive vibration damping. We have measured the zero-field damping capacity of the giant magnetostrictive terbium–dysprosium–iron compounds by quasistatic stress–strain hysteresis loops. The magnetization and strain of the samples were measured for increasing and decreasing compressive stresses, up to 50 MPa. The zero-field, room temperature damping capacity (ΔW/W) was calculated from the stress–strain loops and is plotted against maximum stress. The damping capacity maximum for the alloy with the nominal Terfenol-D composition occurs at 4.1 MPa with the value 1.75 which corresponds to Q−1=0.28. The damping is almost independent of stress in a multi-phase sample, (Tb0.6Dy0.4)Fe1.4, which contains both Laves phase and elemental rare earth.
The active dopant concentration in ion implanted indium tin oxide thin films
Authors: Vink T. J. , Overwijk M. H. F. , Walrave W.
Published: July 2002

The effect of oxygen ion implantation on the electrical and optical properties of Sn-doped In2O3 (ITO) thin films, sputter deposited from a planar magnetron source on glass substrates, is described. The films were characterized as a function of the implanted dose (3×1013–1×1016 O+ cm−2) by Hall effect, resistivity, and optical transmission measurements. The dependencies observed are explained in terms of the deactivation of the Sn dopant and the removal of oxygen vacancies. In this way an estimate of the amount of electrically active Sn contributing to the carrier density in as-deposited films was obtained. Furthermore, the accompanying changes in the band gap with decreasing free-carrier density could be explained quantitatively in terms of the Burstein–Moss effect.
Measurement of stiffness coefficients of anisotropic materials from pointlike generation and detection of acoustic waves
Authors: Audoin B. , Bescond C. , Deschamps M.
Published: July 2002

A scanned point source-point receiver technique, based on laser generation and detection of acoustic waves, is used to measure the stiffness coefficients of anisotropic materials. The striking effects that anisotropy gives rise to are analyzed and, when possible, advantage is taken of them. The processing developed for recovering the coefficients is presented and applied starting with simulated or experimental signals. A silicon crystal, for which acoustic wave focusing induced by anisotropy is critically sensitive, is first studied. To provide an accurate interpretation of these waves, the two-dimensional problem considering a line source is discussed, before analyzing the point source generation. Secondly, a manufactured composite material is characterized by means of this noncontact technique. By scanning a symmetry plane, four coefficients of the stiffness tensor are then recovered with good reliability.
On the radial distribution and nonambipolarity of charged particle fluxes in a nonmagnetized planar inductively coupled plasma
Authors: Mümken G. , Kortshagen U.
Published: July 2002

The radial distribution of electron and ion fluxes to conducting and nonconducting walls in a planar inductively coupled plasma has been studied experimentally and theoretically. Measurements of electron and ion currents have been performed using electrostatic probe arrays. Radially resolved measurements of ion impact energies have been performed using an ion energy analyzer array. For conducting walls it is shown by calculations and measurements that electron and ion currents are not in balance locally but that diffusion is nonambipolar. The ion impact energies measured on a conducting surface show a significant radial variation in accord with our theoretical model. For nonconducting surfaces the ambipolar fluxes of electrons and ions result in the formation of a surface charge potential profile across the surface. Voltages of the order of several volts between the center and the periphery of the surface are measured.
On the theory of measurement of anisotropic electrical resistivity by flux transformer method
Authors: Levin G. A.
Published: July 2002

A method is developed for computing resistivity tensor from measured current and voltage in the flux transformer method. It is based on the solution of two dimensional Laplace’s equation. A new feature here is the separation of a slowly converging series that generally describes the nonuniform potential distribution inside a rectangular block into rapidly and slowly converging parts. The slowly converging sub series can be calculated exactly, while the rapidly converging one lends itself easily to numerical evaluation. The effects of the finite sizes of current and voltage contacts are also taken into account.
A model of quantum confined state modified by surface potential in porous silicon
Authors: Xue Fang-shi , Bao Xi-mao , Yan Feng
Published: July 2002

By analyzing various experimental photoluminescence (PL) and photoluminescence excitation (PLE) spectra in porous silicon (PS), we have found that the PL spectra in PS are characterized by the transitions among the quantum confined states in nanoscale Si which are influenced by the surface chemical bonds. Since the size of silicon filament is comparable to that of surface potential, its electron states are modified by the surface potential. We use a two-dimensional state-dependent surface potential to model the overlapping of surface chemical bonds, from which we obtain a new set of quantum confined states modified by the surface potential. By using these new quantum states the effective luminescence, experimental PL, and PLE spectra in porous Si are well explained.
Ferromagnetic resonance in a crystal of La0.7Sr0.3MnO3
Authors: Lofland S. E. , Bhagat S. M. , Kwon C. , Tyagi S. D. , Mukovskii Y. M. , Karabashev S. G. , Balbashov A. M.
Published: July 2002

A single crystal of La0.7Sr0.3MnO3 was grown by the floating zone method. The ferromagnetic resonance linewidth at 8.5 GHz has a minimum of 50 Oe at 340 K and increases with lowering temperature. This is indicative of some residual inhomogeneity. The anisotropy of the resonance field was studied as a function of angle in the sample plane, and we find a small uniaxial anisotropy, Han≅230 Oe at 77 K.
Deep and shallow electronic states at ultrathin InAs insertions in GaAs investigated by capacitance spectroscopy
Authors: Krispin P. , Lazzari J.-L. , Kostial H.
Published: July 2002

Single, ultrathin InAs insertions in GaAs are investigated by deep-level transient Fourier spectroscopy and capacitance–voltage measurements near the transition from layer-by-layer to three-dimensional growth. The formation of a broad band of deep levels between 0.60 and 0.80 eV below the GaAs conduction band edge is shown to be related to the incorporation of the strained InAs layer. The defect density can be as high as 5×1010 cm−2. In addition, distinct interfacial levels, the formation of which is correlated to the GaAs growth conditions, are found at the position of the ultrathin InAs sheet. Due to their short-range potential, these interfacial deep-level defects are suitable to probe the local properties of the ultrathin InAs insertion. It is experimentally verified that the small InAs islands which are formed at the onset of three-dimensional growth give rise to laterally confined quantum states in the InAs insertion.
Transient photocurrent response of three-color detectors based on amorphous silicon
Authors: Stannowski B. , Stiebig H. , Knipp D. , Wagner H.
Published: July 2002

Color detectors based on multilayers of amorphous–silicon alloys facilitate the detection of the three fundamental components of visible light in one single pixel of a sensor array. In order to achieve sensitivity for the blue, green, and red components of light, three different bias voltages are applied to the device. By switching them sequentially the detector is read out. n-i-p-i-i-n structures with a controlled band gap and mobility–lifetime product exhibit excellent stationary properties, namely: good color separation and have dynamic behaviors above 95 dB. Besides the stationary behavior the transient response of a color detector is a further optimization criterion. The experimentally found transient photocurrent response after switching on monochromatic light at different applied bias voltages showed reasonable delay times in the range of tens of milliseconds before reaching steady state. Numerical simulations have been carried out which reproduce this characteristic behavior and facilitate a study of time dependent processes within the device, such as charge transport and storage in localized states. The delay times can be explained by the recharging of electrical defect states in the amorphous material. Consequently, the electrical potential within the device changes, which remarkably affects the carrier transport. Based on these results optimization criteria for the transient behavior of the color detectors are discussed.
Real time spectroscopic ellipsometry studies of the nucleation and growth of p-type microcrystalline silicon films on amorphous silicon using B2H6, B(CH3)3 and BF3 dopant source gases
Authors: Koh Joohyun , Fujiwara H. , Koval R. J. , Wronski C. R. , Collins R. W.
Published: July 2002

Real time spectroscopic ellipsometry (RTSE) has been applied to study the nucleation, coalescence, and growth processes for ∼100–200 Å thick microcrystalline silicon (μc-Si:H) p-layers prepared by radio frequency (rf) plasma-enhanced chemical vapor deposition at 200 °C on amorphous silicon (a-Si:H) i-layers in the substrate/(n-i-p) device configuration. Analysis of the RTSE data provides the bulk p-layer dielectric function (2.5–4.3 eV), whose amplitude and shape characterize the void and crystalline Si contents in the p-layer. Among the parameters varied in this study of the deposition processes include the underlying a-Si:H i-layer surface treatment, the p-layer H2-dilution flow ratio, the p-layer dopant source gas and flow ratio, and the p-layer rf plasma power flux. Here we emphasize the differences among p-layer deposition processes using diborane, B2H6, trimethyl boron, B(CH3)3, and boron trifluoride, BF3, dopant source gases. We find that it is easiest to nucleate μc-Si:H p-layers immediately on the i-layer without any surface pretreatment when B2H6 is used as the source gas. In contrast, when B(CH3)3 or BF3 is used, a H2-plasma treatment of the i-layer is necessary for immediate nucleation of Si microcrystals; without pretreatment, the p-layer nucleates and grows as an amorphous film. For H2-plasma-treated i-layers, p-layer microcrystal nucleation at low plasma power is controlled by the catalytic effects of B-containing radicals at the i-layer surface, irrespective of the dopant source, whereas nucleation at higher plasma power is controlled by the bombardment of the i-layer by Si-containing ions. Under high power plasma conditions using BF3, dense single-phase μc-Si:H p-layers can be obtained over a wide range of the dopant gas flow ratio. In contrast, for B2H6 and B(CH3)3, such properties are obtained only over narrow flow ratio ranges owing to the relative ease of dissociation of these gases in the plasma.
Response characteristics of high-performance photorefractive mesogenic composites
Authors: Ono Hiroshi , Hanazawa Akihiro , Kawamura Tomomi , Norisada Hideki , Kawatsuki Nobuhiro
Published: July 2002

This article presents high-speed photorefractive high-liquid crystal (H-LC) and low-molar-mass liquid crystal (L-LC) mixtures (HL-LCMs) which exhibit the fastest response time achieved so far (15 ms), while maintaining a large gain coefficient of 270 cm−1, high resolution, and the required low operating voltage. The response time strongly depended on the viscosity of the HL-LCMs, which was varied by controlling the molecular weight of the H-LC. These superior characteristics were achieved by decreasing the molecular weight of the H-LC.
Plume reduction in segmented electrode Hall thruster
Authors: Raitses Y. , Dorf L. A. , Litvak A. A. , Fisch N. J.
Published: July 2002

A segmented electrode, which is placed at the thruster exit, is shown to affect thruster operation in several ways, whether the electrode produces low emission current or no emission current, although there appear to be advantages to the more emissive segmented electrode. Measured by plume divergence, the performance of Hall thruster operation, even with only one power supply, can approach or surpass that of nonsegmented operation over a range of parameter regimes. In particular, the low gas flow rate can exhibit low plume divergence. This allows flexibility in operation of segmented electrode thrusters in variable thrust regimes.
Variational transition state theory with least-action tunneling calculations for the kinetic isotope effects in the Cl+H2 reaction: Tests of extended-LEPS, information-theoretic, and diatomics-in-molecules potential energy surfaces
Authors: Tucker Susan C. , Truhlar Donald G. , Garrett Bruce C. , Isaacson Alan D.
Published: July 2002

We apply conventional and variational transition state theory with least-action-ground-state and other transmission coefficients to calculate the rate constants and kinetic isotope effects for the reaction Cl+H2→HCl+H. First we consider collinear reactions and compare the calculated results to accurate quantal results for a given potential energy surface. This tests the dynamical methods and shows that they are reliable enough for testing potential energy surfaces. We then make calculations for the three-dimensional reactions employing 11 potential energy surfaces that have been proposed in previous work. Seven of the surfaces are extended LEPS surfaces, as proposed by Persky, Klein, and Stern; Truhlar, Magnuson, and Garrett; and Valencich and co-workers; one is an information-theoretic-bond-order-plus-anti-Morse-bend surface (called AL/AB) proposed by Agmon, Levine, Truhlar, Magnuson, and Garrett; and the final three surfaces are a diatomics-in-molecules-plus-three-center-terms surface proposed by Baer and Last and two diatomics-in-molecules surfaces proposed by Isaacson and Muckerman. Three of the surfaces (the final surface of Persky, Klein, and Stern; the first surface of Truhlar, Magnuson, and Garrett; and the AL/AB surface—all of which have relatively symmetric saddle points) are shown to be more reasonable than the others for predicting the rate constants and the H2/D2 and the HD/DH kinetic isotope effects. The calculations also indicate that the room temperature rate constants are dominated by quantum mechanical tunneling.