bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-varying phenomena in the photoelectric properties of porous silicon
Authors: Frello T. , Veje E.
Published: July 2002

We have undertaken a systematic study of porous silicon with the use of photoconductivity and photoluminescence. During this, we have observed at least three kinds of time-varying photoconductivity, of which two reduce the conductivity and one increases the conductivity during illumination. In addition, we have observed persistent photoconductivity. The time developments of the photoconductivity as well as the persistent photoconductivity depend in intricate ways on parameters such as the wavelength and intensity of the illuminating light, the potential drop across the sample, the surface treatment, and the dark current value. The time scale of these time-varying effects ranges from a few minutes to several hours. The results are discussed in terms of the photoelectric properties of the supporting silicon wafer, diffusion of hydrogen, and photoinduced desorption of hydrogen.
On the origin of periodic inclusions in metals frozen onto a moving substrate
Authors: Biriukov A. S. , Bogatyrjov V. A. , Khitun A. G.
Published: July 2002

Metal coating application onto optical fibers by the freezing technique is accompanied by the emergence of cavities empty of metal, which are situated at regular intervals along the fiber. This article attempts to explain the nature of this phenomenon, which is shown to be inherent in the freezing technique. The regular cavities are due to the instability of the melt flow during the phase transition process at the point of contact of the cool fiber and the melt. Resonance of one of the eigen vibrations of the entrance meniscus with chaotic perturbations of the melt flow accounts for the periodicity of the cavities. The results of the theoretical analysis are confirmed by the experimental data. The conclusions of this analysis allow one to explain the presence of a similar phenomenon in the “spinning” process used for the fabrication of amorphous-alloy thin ribbons.
Preparation of Cu(In,Ga)2Se3.5 thin films by radio frequency sputtering from stoichiometric Cu(In,Ga)Se2 and Na2Se mixture target
Authors: Tanaka Tooru , Demizu Yasutaka , Yoshida Akira , Yamaguchi Toshiyuki
Published: July 2002

Defect chalcopyrite thin films of Cu(In,Ga)2Se3.5 were prepared by rf sputtering from stoichiometric CuInxGa1−xSe2 (x=0.6) and Na mixture target. The composition of the thin films fabricated in the ratio of [Na]/[Cu(In,Ga)Se2] above 5% was changed from the stoichiometric composition of Cu(In,Ga)Se2 to Cu-poor one, and identified as Cu:(In+Ga):Se=1:2:3.5. From the results of x-ray diffraction, the lattice parameters of these thin films were slightly smaller than that of Cu(In,Ga)Se2 and, besides the peaks appearing for chalcopyrite structure Cu(In,Ga)Se2, the additional peak was observed. The optical band gap is increased from 1.24 to 1.36 eV with increasing the [Na]/[Cu(In,Ga)Se2] ratio from 0% to 10% in the target. These films showed n- or p-type conduction.
Aligned carbon–hydrogen complexes in GaAs formed by the decomposition of trimethylgallium during metalorganic vapor phase epitaxy and atomic layer epitaxy
Authors: Davidson B. R. , Newman R. C. , Fushimi H. , Wada K. , Yokoyama H. , Inoue N.
Published: July 2002

Highly carbon doped (001) oriented GaAs layers were grown by metalorganic vapor phase epitaxy using the precursor trimethylgallium with either arsine or trimethylarsenic. Postgrowth infrared measurements using polarized light demonstrate that proposed grown-in planar carbon–hydrogen [H–(CAs)2] complexes, giving vibrational absorption at 2688 cm−1, have the two CAs atoms aligned along the [110] direction. A previously unreported absorption peak at 576 cm−1 exhibits polarization dependent absorption orthogonal to that of the 2688 cm−1 line. Investigation of a sample containing the [D–(CAs)2] complex indicates that the 576 cm−1 line would have to be a transverse mode of the unpaired CAs atom. Polarization difference spectra show the presence of other aligned carbon related complexes giving absorption close to the line (582 cm−1) due to isolated CAs atoms.
Observation of domains in obliquely evaporated Co–CoO films by spin-polarized scanning electron microscopy
Authors: Kohashi Teruo , Matsuyama Hideo , Koike Kazuyuki , Takayama Takanobu
Published: July 2002

Magnetic domains in the remanent magnetization state of obliquely evaporated recording media Co–CoO films were studied by spin-polarized scanning electron microscopy. Inverse domains densely distributed in the media were found to have a closure-domainlike structure at the surface to reduce the magnetostatic energy. The size of the inverse domains depends on the direction of the external magnetic fields that were applied when the remanent states were formed. Using these results, the relationship between the inverse domains and the carrier-to-noise ratio for this kind of recording media is discussed.
Structural stability of hydrogenated (100) surface of cubic boron nitride in comparison with diamond
Authors: Komatsu Shojiro , Yarbrough Walter , Moriyoshi Yusuke
Published: July 2002

In view of (1×1):2H dihydride/(2×1):H monohydride reconstruction, structural stability of (100) surfaces of both cBN and diamond was comparatively investigated by semiempirical molecular orbital methods using isoelectronic clusters of B52N42H80−2n(10−), N52B42H80−2n(10+), and C94H80−2n, to model (100)B and (100)N of cBN, and diamond surface, respectively, where n=0, 1, 2, or 3. The n denotes the number of monohydride dimers formed. These clusters were nanometer-sized pyramidal crystallites bound by four of {111} faces and one (100). The (100)N of cBN was found unique because of the great stability as (1×1):2H dihydride phase, which retains the bulk structure truncated at the surface without reconstruction and is expected to be chemically inert. This passivation seems to be related to the difficulty in chemical vapor deposition of high quality cBN. The (100)B of cBN was predicted to stabilize as (2×1):H monohydride phase as much as hydrogenated (100) of diamond does.
Passivation effect of silicon nitride against copper diffusion
Authors: Miyazaki Hiroshi , Kojima Hisao , Hinode Kenji
Published: July 2002

The use of Cu in ultralarge scale integrated (ULSI) conductors has resulted in the need to prevent Cu diffusion. We evaluated the passivation effect of plasma-enhanced chemical-vapor-deposited silicon nitride (PECVD-SiN) using secondary ion mass spectrometry and atomic absorption spectrometry. From these measurements, it was found that a large amount of Cu diffused through PECVD-SiN films during the heat treatments of the metallization process, probably due to the rapid diffusion paths along the microdefects of PECVD-SiN films. However, Cu contamination was barely detected in the current–voltage measurements and bias-temperature stressing tests of Cu/PECVD-SiN/SiO2/Si capacitors because the leakage current through SiN films slightly increased as a result of Cu diffusion. This result is attributed to the electric-field relaxation caused by a large number of electrons trapped in the PECVD-SiN films, of which the negative charge compensates the positive charge of Cu ions. Although the degradation of electrical characteristics is not explicitly observed in simulation using Cu/PECVD-SiN/SiO2/Si capacitors, Cu atoms reach Si devices in the actual process. Therefore, the passivation effect of PECVD-SiN films is insufficient to allow application to ULSI devices.
Structural investigation of self-aligned silicidation on separation by implantation oxygen
Authors: Deng F. , Ring K. , Guan Z. F. , Lau S. S. , Dubbelday W. B. , Wang N. , Fung K. K.
Published: July 2002

Self-aligned silicidation is a well-known process to reduce the source, drain, and gate parasitic resistances of submicron metal-oxide-semiconductor devices. This process is particularly useful for devices built on very thin Si layer (∼1000 Å or less) on insulators. Since the amount of Si available for silicidation is limited by the thickness of the Si layer, once the Si in the source and drain region is fully consumed during silicidation, excessive silicide formation could lead to void formation near the silicide/silicon interface beneath the oxide edge. In this article, we study the effects of different metals (Ti, Ni, Co, and Co/Ti bilayer) with varying thickness on the formation of voids. A change in the moving species during lateral silicide formation was found to be the likely cause for the voids, even if the metals are the moving species during silicidation in the thin film case.
Parametric resonance in superconducting micron-scale waveguides
Authors: Fomin N. V. , Shalaev O. L. , Shantsev D. V.
Published: July 2002

A parametric resonance due to temperature oscillations in superconducting micron-scale waveguides is considered. Oscillations of superconductor temperature are assumed to be induced by the irradiation of the waveguide with a laser beam. The laser power and parameters of the waveguide providing a possibility of parametric excitation have been calculated. It is shown that for a waveguide made of a YBa2Cu3O7 microstrip with resonant frequency of 10 GHz a laser with a power of about 70 W/cm2 is needed to excite oscillations. The effect can be used for the creation of high-sensitivity tuneable filters and optoelectric transformers on superconducting microstrips in the GHz range.
Observation of selective thermal desorption of electron stimulated SiO2 with a combined scanning reflection electron microscope/scanning tunneling microscope
Authors: Maruno S. , Fujita S. , Watanabe H. , Ichikawa M.
Published: July 2002

Selective thermal desorption (STD) of SiO2 film of less than 1 nm thick induced by a focused electron beam has been investigated with a combined apparatus of scanning reflection electron microscope and scanning tunneling microscope. It has been found that clean Si open windows formed in the SiO2 film by STD have atomically flat surface morphology and uniform linewidth. The surface morphology depends on electron dose, oxide film thickness, and desorption temperature. The fabrication process of Si nanowires using the STD technique has been also investigated.
Interfacial reactions between nickel thin films and GaN
Authors: Venugopalan H. S. , Mohney S. E. , Luther B. P. , Wolter S. D. , Redwing J. M.
Published: July 2002

Thin Ni films on GaN were annealed at temperatures between 400 and 900 °C in N2, Ar, and forming gas and were analyzed using glancing angle x-ray diffraction and Auger depth profiling. The first indication of an interfacial reaction was found after an anneal at 600 °C for 1 h, after which Ga was observed to be dissolved in the face-centered cubic Ni film. The extent of dissolution increased with continued annealing. After annealing at 750 °C for 1 hr in either N2 or Ar, greater intermixing occurred. The reaction product was either Ni3Ga or face-centered cubic Ni with dissolved Ga. Annealing at 900 °C resulted in the formation of the B2 phase NiGa. It was clear from Auger depth profiles that the reacted film contained significantly more Ga than N and that N2 gas was released to the annealing environment, even when the samples were annealed in N2 gas at 1 atm. Thus, a trend of increasing Ga content in the reacted films was observed with increasing temperature. The observed reactions are consistent with the thermodynamics of the Ni–Ga–N system.
Hot electron transport in n-GaAs field emitter at nitrogen temperature
Authors: Semykina Elena A. , Roze Ksenia S.
Published: July 2002

Results of a Monte Carlo study of electron processes in n-GaAs emitter are presented. The simulation of high-field electron transport through the surface region of semiconductor is performed by coupling the Boltzmann to the Poisson solution accounting for the nonparabolic electron spectrum, all relevant scattering processes (phonons, ionized impurities, and impact ionization), and the nonlocal properties of electron transport. It is shown that at nitrogen temperature hot electrons of L valley give the main contribution to the emission current. The variation of the emission process at 78 K with the applied electric field is presented.
Double-peaked decay of transient photocurrents in GaAs metal–semiconductor–metal photodetectors
Authors: Hurd C. M. , McKinnon W. R.
Published: July 2002

A double peak is sometimes seen on a picosecond time scale in the decay of the transient photoconduction of GaAs metal–semiconductor–metal structures. This is known from experiments on micron-scale structures of both conventional and backgated design. A previous numerical simulation, using a drift-diffusion calculation and an idealized one-dimensional model of the conventional structure, shows that the double-peaked behavior can arise from the effect of the photogenerated carriers on the internal fields. Here we extend this to a two-dimensional calculation to investigate the photoconduction decay in both the conventional and backgated designs. We calculate the instantaneous distribution of charge and field in the structures during and after a pulse of illumination, and we show for each design how the interplay of carrier and displacement currents can produce intrinsically a nonmonotonic decay with features qualitatively like those seen experimentally.
Channel optical waveguides formed by deuterium passivation in GaAs and InP
Authors: Kumar Mukesh , Boyd Joseph T. , Jackson Howard E. , Zavada John M. , Jenkinson Howard A. , Wilson Robert G. , Theys B. , Chevallier J.
Published: July 2002

Channel optical waveguides have been formed on both GaAs and InP substrates utilizing deuterium passivation of the surface to provide planar confinement and etching to provide lateral confinement. Design criteria were established for obtaining single mode channel waveguides for the present case of small index changes and thick surface layers associated with deuterium passivation. Planar and channel waveguide operations were demonstrated and channel waveguide propagation losses have been measured. For GaAs channel waveguides, optical loss was measured as a function of channel waveguide width, ranging from 3 to 9 μm, with a minimum loss found for a width of 6.0 μm. Channel waveguide losses as low as 12.7 dB/cm for GaAs and 6.0 dB/cm for InP have been measured at λ=1.3 μm. For InP this loss value is close to the limiting value imposed by free carrier absorption in the semiconductor region below the passivated region. Since the waveguide loss due to free carriers can be reduced by increasing waveguide confinement, we anticipate that lower loss optical channel waveguides could be formed by this technique.
Generalized analysis of the use of induction loop probes in azimuthally symmetric inductively coupled plasma sources
Authors: Haas F. A. , Braithwaite N. St. J.
Published: July 2002

Using B-dot probes Meyer et al. [J. Appl. Phys. 79, 1298 (1996)] have described a method for calculating the azimuthal current, electric field, plasma permittivity, and electron density in a planar inductively coupled plasma source. They assume the phases associated with the time rate of change of the radial and axial field components to be the same. Furthermore, they restrict their analysis to the situation where the phase is independent of radius. We demonstrate that the phases for the two field directions are different. The subsequent modified forms for the plasma properties considered by Meyer et al. are presented. We also show that their procedure can be extended to cover phases which are an arbitrary function of position, and for completeness, give forms for the generalized plasma properties.
Optical studies of carrier kinetics in a type II multiple quantum well hetero-n-i-p-i structure
Authors: Yang Zhenyin Jean , Garmire Elsa M. , Doctor Daniel
Published: July 2002

In this paper, carrier kinetics in a type II multiple quantum well hetero-n-doped-intrinsic–p-doped-intrinsic are studied through optical measurements under continuous illumination. Both field measurement by photoreflectance and time measurement confirmed our hypothesis that the recombination time is a function not only of photocarrier density, but also of photocarrier distribution. A simple model is used for recombination time, assuming exponential dependence on the barrier height and nearest distance between recombining carriers. Quantum efficiency has also been determined.
The chemisorption of H2C[Si(CH3)3]2 and Si6(CH3)12 on Si(100) surfaces
Authors: Sutherland D. G. J. , Terminello L. J. , Carlisle J. A. , Jiménez I. , Himpsel F. J. , Baines K. M. , Shuh D. K. , Tong W. M.
Published: July 2002

The chemisorption of bis(trimethylsilyl)methane (BTM, CH2[Si(CH3)3]2) and dodecamethylcyclohexasilane (DCS, Si6(CH3)12) on clean Si(100) surfaces has been studied by C 1s core-level and valence-band photoemission spectroscopy. Our model for the deposition of carbon by BTM involves decomposition into a –CH2Si(CH3)3 surface moiety for room-temperature adsorption, which further decomposes upon annealing to 550 °C to form a surface terminated primarily by CHx units. DCS deposits almost three times as much C on the surface as BTM. The data are consistent with DCS undergoing a ring opening and bonding to the surface as polydimethylsilane chains. Annealing both adsorbates to 950 °C causes a large decrease in the C 1s signal due to the fact that Si segregates to the surface at temperatures above 900 °C. The valence-band photoemission of Si(100) dosed with DCS at 950 °C is in good agreement with that of β-SiC, whereas the analogous BTM spectrum deviates significantly.
Optical properties of self-assembled arrays of InP quantum wires confined in nanotubes of chrysotile asbestos
Authors: Romanov S. G. , Sotomayor Torres C. M. , Yates H. M. , Pemble M. E. , Butko V. , Tretijakov V.
Published: July 2002

Three-dimensional arrays of structurally confined InP wire-like nanostructures were grown in channels (nanotubes) of a chrysotile asbestos matrix by metalorganic chemical vapor deposition. The formation of the InP compound was confirmed by absorption spectroscopy, X-ray diffraction and Raman scattering. It is shown that the density of states around the band edge increases with the InP loading of the matrix. Photoluminescence spectra of the asbestos filled in with InP consist mainly of two bands: a high energy band which is interpreted to be associated with charge transfer from InP to defect states of the asbestos and a low energy band which is associated with energy relaxation in the InP deposit itself. We show that the optical properties of this material are dominated by the size and dimensionality of the pore system of the matrix for heavy loading and by the semiconductor-to-matrix interface for light loading of the matrix with InP.
Energy transfer in dye impregnated porous silicon
Authors: Létant S. , Vial J. C.
Published: July 2002

Two kinds of porous silicon layers (fresh and oxidized) were impregnated with laser dyes solutions before drying. An original technique of diffuse reflectance allowed us to give an estimation of the concentration of the dye molecules in the pores, which appears to be higher for oxidized samples than for fresh ones. Selective excitation of the red photoluminescence of the silicon crystallites with a pulsed ultraviolet laser was performed and it showed a rapid luminescence of the laser dye. This emission is more important for fresh samples than for oxidized ones, although the dye concentration is lower in fresh samples. This indicates that the luminescence of the dye molecules does not come from direct excitation of the laser but that it is provided by an energy transfer from the porous silicon crystallites. In as far as the coupling between the crystallite and the dye molecule theoretically decreases when the distance between them increases, a lowering of the transfer on oxidized samples for whom the distance is larger is expected and is experimentally observed.
Modeling of pyrolytic laser direct writing: Noncoherent structures and instabilities
Authors: Arnold N. , Kargl P. B. , Bäuerle D.
Published: July 2002

Three-dimensional simulations of pyrolytic laser direct writing from gas-phase precursors are presented. They are based on a fast method for the calculation of temperature distributions induced by an energy beam in deposits of arbitrary shape. Analytical approximations, fast Fourier transform, and the multigrid technique are combined in the algorithm. Temperature dependences of the absorptivities and heat conductivities of the deposit and the substrate have been taken into account. Self-consistent modeling of the growth process allows one to explain oscillations in the height and width of lines caused by the feedback between the shape of the deposit, the temperature distribution, and the growth rate. For the deposition of W from an admixture of WCl6+H2 and a-SiO2 substrates, the oscillations originate from a sharp increase in the absorptivity of the deposit with temperature. With the deposition of Si from SiH4, or C from CH4, C2H2, and C2H4, onto a-SiO2, the oscillations are related to the large ratio of height/width of the deposit and the increase in temperature on its upper surface. This increase also explains the transition from line-type to fiber-type growth. The hysteresis of this transition with respect to laser power and scanning velocity is explained as well. The same algorithm can be used in the modeling of pyrolytic etching and e-beam microprocessing when the feedback between the temperature distributions and changes in the processing geometry is important.
Atomic diffusion induced by stress relaxation in InGaAs/GaAs epitaxial layers
Authors: Roura P. , Vilà A. , Bosch J. , López M. , Cornet A. , Morante J. R. , Westwood D. I.
Published: July 2002

The origin of the microscopic inhomogeneities in InxGa1−xAs layers grown on GaAs by molecular beam epitaxy is analyzed through the optical absorption spectra near the band gap. It is seen that, for relaxed thick layers of about 2.8 μm, composition inhomogeneities are responsible for the band edge smoothing into the whole compositional range (0.05<x<0.8). On the other hand, in thin enough layers strain inhomogeneities are dominant. This evolution in line with layer thickness is due to the atomic diffusion at the surface during growth, induced by the strain inhomogeneities that arise from stress relaxation. In consequence, the strain variations present in the layer are converted into composition variations during growth. This process is energetically favorable as it diminishes elastic energy. An additional support to this hypothesis is given by a clear proportionality between the magnitude of the composition variations and the mean strain.
Unstable behavior of Ga atoms in ZnSe epitaxial layers
Authors: Hino T. , Haga T. , Abe Y. , Miyajima T. , Okuyama H. , Akimoto K.
Published: July 2002

Using ion-channeling methods, the thermal stability of crystallinity for heavily Ga-doped ZnSe epitaxial layers grown on GaAs (100) substrates was investigated. Some distinctive features were found: The crystallinity of as-grown heavily Ga-doped ZnSe is similar to that of undoped ZnSe. After thermal annealing at 450 °C, the crystallinity of Ga-doped ZnSe is degraded, although that of undoped ZnSe is not significantly changed. It is expected that doped Ga atoms are displaced from the regular lattice sites after annealing. The degree of this degradation depends on the crystallinity of the as-grown Ga-doped ZnSe itself. These results indicate the degradation of the crystallinity for Ga-doped ZnSe epilayers is mainly caused by the interaction between the doped Ga atoms and grown-in point defects in the epitaxial layers.
Tunable optical nonlinearities in Cd1−xZnxTe ternary alloys
Authors: Ohlmann D. , Mazilu M. , Levy R. , Hönerlage B.
Published: July 2002

Bulk Cd1−xZnxTe ternary compounds are studied through nonlinear optical pump and probe experiments for different zinc concentrations x. These measurements, similar to excitation spectra, enable us to determine the frequency dependence of the optical nonlinearity. It is attributed to the presence of impurity levels in the material. The two-photon absorption through these states gives rise to a high induced absorption.
Interface form birefringence in native oxided microcavities
Authors: Valadares E. C. , Cury L. A. , Matinaga F. M. , Moreira M. V. B.
Published: July 2002

A novel form birefringence effect associated with periodic corrugations in semiconductor/oxide interfaces of large-index-step high contrast Bragg mirrors of vertical microcavities is discussed in this communication. A simple model is proposed in which the corrugations have a square-wave profile with alternating refraction index whose period is much smaller than the cavity mode wavelength. This new effect has potential use in polarization controlled emission of vertical cavity surface emitting lasers.
Deep level transient measurements of DX centers in GaAlAs up to room temperature
Authors: Dózsa L. , Van Tuyen Vo , Hubik P. , Terziev Nikolay , Mares J. J. , Kristofik J.
Published: July 2002

DX centers were investigated in Si doped Ga0.4Al0.6As by capacitance-voltage (C-V) characterization, by deep level transient spectroscopy (DLTS), and by fast defect transient (FDT) measurements. Since the last method is capable of measuring transients in micro- and nanosecond ranges, it allowed us to measure transients of DX centers up to room temperature. The investigated samples were laser structures where only the cladding layer is doped by Si, so the DX centers are localized within a few Debye lengths in the vicinity of the depleted layer edge. The capture and emission activation energies determined by capacitance DLTS and by FDT measurements are in agreement with the values reported in the literature, in contrast to our previous FDT measurements in bulk GaAlAs samples. The results suggest that the DLTS signal is dominated by transients in the vicinity of the depleted layer edge even in bulk samples. The differences between FDT measurements in bulk and laser structure samples are explained by the recently proposed Auger capture model of the DX shallow–deep transition.
Unipolar-current dipoles and other optimum reduced-symmetry multipoles
Authors: Crow James T.
Published: July 2002

Two-dimensional dipole (uniform) fields produced by parallel currents are generally considered to have two symmetry planes, with a current direction reversal in reflection across one plane. Similarly, quadrupoles have four symmetry planes, and so on for higher multipoles. Expansion of the fields of filamentary currents shows that there are lower symmetry multipoles with highly uniform field characteristics. These solutions include configurations in which there is no current reversal across reflection planes, producing unipolar-current dipoles, quadrupoles, and other multipoles. In all cases, the solutions can yield systems that have the best uniformity of field characteristics (minimum undesired multipole content over the largest area) that can be achieved with the number of wires used, although the unipolar systems are less efficient and produce a lower field magnitude for a given current. These optimum solutions are not limited to circular configurations, or to systems with all wire current magnitudes identical. The configurations are easily extended by summation or integration to wire bundles or finite radius conductors.
Three-dimensional self-consistent simulation of interface and dopant disorders in delta-doped grid-gate quantum-dot devices
Authors: Thean V. Y. , Nagaraja S. , Leburton J. P.
Published: July 2002

We study the influence of interface roughness and dopant disorder on the electronic and charging properties of a δ-doped Al1−xGaxAs/GaAs quantum-dot grid-gate device by computer simulation. The analysis is performed for a device unit cell at T=4.2 K by self-consistent adiabatic one-dimensional (1D) Schrödinger and 3D Poisson’s simulation, within the local density approximation. Random boundary conditions that accounts for cell-to-cell influence of disorder is considered as well. The random effect of interface roughness produces very short-ranged potential distortions over distances of the order of a monolayer, resulting in a maximum deviation of 20% in the charging energy. Sensitivity analysis of the device to both random fluctuations in the number and positioning of the dopant ions shows that dopant number variations from device to device can result in fluctuations in the charging energy of as much as 50% per dopant. Isolated acceptor point charges in the quantum well change the modes of electron confinement, giving rise to fluctuations in the charging energy of the order of 25%.
Spatial distribution of Cu sputter ejected by very low energy ion bombardment
Authors: Doughty C. , Gorbatkin S. M. , Berry L. A.
Published: July 2002

Filling of submicron width trenches with Cu from a highly ionized plasma is sensitive to both the directionality of the depositing flux and reflection and resputtering at the film surface. The spatial distributions of Cu atoms sputtered by He+, Ar+, and Xe+ ions incident at 30°, 45° and 60° and over an ion energy range of ∼55–600 eV have been investigated. In all cases, the distribution is forward directed and cannot be described by a cosine distribution about the surface normal. Decreasing energy, increasing angles of incidence, and increasing ion mass yield more forward directed distributions. For ∼55 eV Ar+ incident at 60°, ∼85% of the resputtered flux is in the forward direction. From these distributions, in a 50% ionized physical vapor deposition system the depositing flux due to forward directed resputtering is estimated to be of the same order of magnitude as the neutral flux.
Coupled wave analysis of anisotropic conical diffraction in doped (K0.5Na0.5)0.2(Sr0.61Ba0.39)0.9Nb2O6 crystals
Authors: Jiang Yongyuan , Li Yan , Sun Xiudong , Zhou Zhongxiang , Xu Kebin
Published: July 2002

Three anisotropic diffraction cones in doped (K0.5Na0.5)0.2(Sr0.61Ba0.39)0.9Nb2O6 crystals with two extraordinary polarized incident beams can be observed for the experimental configuration where the c axis is parallel or perpendicular to the incident plane. These cones are attributed to anisotropic conical self-diffraction and cross-diffraction process, respectively. Photorefractive multiwave interaction caused by the incident beams and forward scattered beams satisfying the phase matching condition is proposed to explain the experimental results. The coupled wave equations and its numerical solution including anisotropic self-diffraction and cross diffraction are also presented. Theoretical analysis and experimental results show that anisotropic conical cross-diffracted light arising from the contribution of both incident beams.
Initial growth of chemical-vapor-deposited SiO2
Authors: Ishikawa M. , Egashira Y. , Komiyama H.
Published: July 2002

Amorphous SiO2 was deposited on three single-crystalline substrates: alumina, CaF2, and hydrogen-terminated silicon, by chemical vapor deposition (CVD) using tetraethoxysilane (TEOS) and ozone (O3). The deposited layers were then examined by using atomic force microscopy (AFM). For deposition times of 3, 8, 15, and 180 s, the film thickness ranged from less than 2.4 to 150 nm, respectively. Comparison of the surface roughness with the film thickness, as determined independently by ellipsometry, indicated that the thinnest film formed on the silicon surface (2.4 nm) was continuous. During film formation, for all three substrates, the surface roughness increased initially, reached a maximum, and then decreased. Surprisingly, the surface became smooth (within the resolution of 0.2 nm of our AFM). For the silicon substrate, this smoothing occurred between 3 and 8 s, which corresponds to 2.4 and 6.4 nm of deposition, respectively. Furthermore, just before the smoothing started, the surface of the deposited film contained protuberances, 1.6 nm high and 16 nm×16 nm in area. This indicates that smooth film of a few tens angstroms in thickness can form on the silicon. The smoothing of the protuberances occurred between 3 and 8 s for the CaF2 substrate, and between 15 to 180 s for the alumina substrate. The initial formation of a rough surface followed by smoothing is likely to be inherent in TEOS/O3 CVD systems, irrespective of substrate type. Surface-tension-induced flows of the surface, which has liquidlike properties, is a plausible mechanism.
Enhanced energy loss of short pulses of electrons in plasma
Authors: Fulton R. D. , Goldstein J. C. , Jones M. E. , Kinross-Wright J. M. , Kong S. H. , Nguyen D. C.
Published: July 2002

The enhancement of energy loss of short pulses of electrons in plasma has been experimentally observed. An enhancement of up to 3.5×104 over single-particle losses was observed when a 15 ps electron bunch was injected into a preformed target plasma with an electron density of approximately 1013 cm−3. This matches the theoretical prediction that the energy loss should be enhanced when the temporal duration of the electron bunch is approximately equal to π/ωpe, where ωpe is the plasma frequency of the target plasma. 2D numerical simulations are in good agreement with the observations.
Studies of flux pinning by proton-induced fission tracks in multifilamentary tapes of (Bi,Pb)2Sr2Ca2Cu3O10/Ag superconductors
Authors: Budhani R. C. , Willis J. O. , Suenaga M. , Maley M. P. , Coulter J. Y. , Safar H. , Ullmann J. L. , Haldar P.
Published: July 2002

The effects of proton-induced linear defects on the dissipationless flow of current in multifilamentary Ag-sheathed tapes of (Bi,Pb)2Sr2Ca2Cu3O10 superconductors in the temperature range of 55–77 K are reported. Randomly oriented columnar defects of estimated areal density 5×1010/cm2 produced by 800 MeV proton-induced fission fragments provide substantial enhancement of critical currents of these tapes. Furthermore, these defects are more isotropic flux pinning centers than high-energy heavy ion irradiation-induced columnar defects. However, there is a tradeoff between enhanced pinning and the deleterious effects of radiation damage at higher proton fluences. It is shown that some of the global damage caused by the products of fission can be repaired by a suitable heat treatment of the samples.
Experimental evidence of Josephson noise in superconductor-insulator-superconductor quasiparticle mixers
Authors: Leridon B. , Febvre P. , George S. , Feautrier P. , McGrath W. R.
Published: July 2002

Josephson currents have been observed to drain instabilities and decrease the sensitivity of a heterodyne superconductor-insulator-superconductor (SIS) receiver operated at submillimeter wavelengths. A static magnetic field is usually applied on the SIS junction to reduce the magnitude of its critical Josephson currents and enable quasiparticle mixing. We measured the SIS mixer output power at the intermediate frequency (IF) versus the dc bias voltage of an array of two 1 μm2 Nb/Al–Al2O3/Nb junctions. We show evidence of additional, magnetic-field-sensitive noise, peaked at the Shapiro steps voltages, which is due to high-frequency Josephson currents converted to the IF by quasiparticle mixing.
Fokker–Planck analysis of the Langevin–Lorentz equation: Application to ligand-receptor binding under electromagnetic exposure
Authors: Moggia Elsa , Chiabrera Alessandro , Bianco Bruno
Published: July 2002

The statistical properties of the solution of the Langevin–Lorentz equation are analyzed by means of the Fokker–Planck approach. The equation describes the dynamics of an ion that is attracted by a central field and is interacting with a time-varying magnetic field and with the thermal bath. If the endogenous force is assumed to be elastic, then a closed-form expression for the probability density of the process can be obtained, in the case of constant magnetic exposure and, for the time-varying case, at least asymptotically. In the general case, a numerical integration of the resulting set of differential equations with periodically time-varying coefficients has been implemented. A framework for studying the possible effects of low-frequency, low-intensity electromagnetic fields on biological systems has been developed on the basis of the equation. The model assumes that an exogenous electromagnetic field may affect the binding of a messenger attracted by the endogenous force field of its receptor protein. The results are applicable to the analysis of experiments, e.g., exposing a Petri dish, containing a biological sample, to a periodically time-varying magnetic field generated by a pair of Helmholtz coils, most widely used in the scientific literature. The proposed model provides a theoretical mean for evaluating the biological effectiveness of low-frequency, low-intensity electromagnetic exposure.
Thermal expansion of GaAs:Te and AlGaAs:Te at low temperatures
Authors: Leszczynski M. , Pluzhnikov V. B. , Czopnik A. , Bak-Misiuk J. , Slupinski T.
Published: July 2002

We studied the influence of free electrons on the lattice parameters, thermal expansion, and negative thermal expansion of GaAs:Te and AlGaAs:Te. The samples were examined at temperatures in the range of 10–295 K. Dylatometry and high-resolution x-ray diffraction were used as experimental techniques. Additionally, for the doped layer of Al0.32Ga0.68As, the free-electron concentration was changed in situ by illumination and emptying the metastable DX centers. These results were compared with those for GaAs bulk crystals examined using dylatometry. It was confirmed that n-type doping increases thermal expansion at temperatures higher than about 120 K. At lower temperatures, a phenomenon of the negative thermal expansion becomes much more pronounced for the doped GaAs sample.
An analytical model for Type I magnetic contrast enhancement with sample tilting
Authors: Chim W. K.
Published: July 2002

In this article, an analytical model for Type I magnetic contrast enhancement in the scanning electron microscope, achieved by sample tilting, is presented. The model accounts for the energy filtering of the secondary electrons emitted from the sample. The results showed that the optimum sample tilt angle for maximum contrast is a function of the field-distance integral. For small field-distance integrals less than 10−7 Tm, corresponding to a weak magnetic field situation, the optimum sample tilt is close to 0°. For large field-distance integrals, the optimum sample tilt angle increases and approaches 45° for a field-distance integral of 8×10−6 Tm.
Formation and relaxation of poled order in dye doped polystyrene probed by isothermal and nonisothermal current measurements
Authors: Fedosov Sergei , Giacometti José A. , Leal Ferreira G. F. , Costa Mauro M.
Published: July 2002

Experimental results are presented on isothermal and nonisothermal electric current measurements obtained with a polystyrene and disperse red 1 guest–host system containing different percentages of the dye. The superposition principle holds for isothermal measurements and polarization currents seem to be almost independent of temperature in the range from 30 to 70 °C, below the glass transition temperature Tg. Thermally stimulated depolarization (TSD) current measurements show that the polarization induced in the samples depends strongly on poling conditions. Samples poled at temperatures above Tg have a lower polarization than those poled just below Tg. If the poling temperature Tp is far below Tg, the TSD current peak appears at much higher temperatures than Tp. The TSD curves differ somewhat from corresponding curves obtained during linear heating under applied voltage.
Anderson localization and Monte Carlo simulation of vertical transport in disordered finite superlattices
Authors: Roberts R. G. , Hagston W. E. , Chen P. , Nicholls J. E. , O’Neill M.
Published: July 2002

The effects on the carrier wave functions of introducing monolayer disorder at the heterojunction interfaces of an undoped superlattice are elucidated. In particular the rate and extent of the resulting localization is quantified by means of an entity C, which is the joint probability of finding two carriers in the same region of space. Evaluating the latter for all states in the miniband provides clear evidence for the existence of a mobility edge in finite two-dimensional structures. Using the entity C as the basis for a semiclassical evaluation of carrier transport, it is shown that the experimentally observed activated transport can be accounted for in terms of the excitation of carriers from the localized states at the bottom of the miniband into the more extended midband miniband states. Comparison of the theoretical results with the experimental results given [Chen et al., J. Cryst. Growth 159, 1066 (1996)], enables conclusions to be drawn concerning the mechanism of activated carrier transport in doped superlattice systems.
Near-threshold photoablation characteristics of polyimide and poly(ethylene terephthalate)
Authors: Krajnovich Douglas J.
Published: July 2002

Photoablation of polyimide (Dupont Kapton™) and poly(ethylene terephthalate) (PET) (Dupont Mylar™) were studied in vacuum using 248 nm, 16 ns excimer laser pulses. A sensitive electron beam ionizer/quadrupole mass spectrometer detector was used to measure mass, translational energy, and angular distributions of the neutral photoproducts at fluences very close to the threshold (average material removal rates <100 Å/pulse). The experiments were performed by combining results from many discrete sample spots, in order to minimize the changes in yield, energy, and surface topography caused by cumulative pulsing (“radiation hardening”). For both polyimide and PET, the dominant neutral photoproducts are fragments of the monomer, although there is a weak tail of intensity extending up to and beyond 1000 amu. The mean translational energy increases with increasing mass, due to the collisional “seeded beam” effect. The mean translational energy of the majority species (i.e., those below 200 amu) is around 1.6–4.9 eV for polyimide at 30 mJ/cm2, and 0.9–1.5 eV for PET at 21 mJ/cm2. The translational energy distributions of the heavier species are close to Boltzmann while those of the lighter species show small but reproducible deviations from Boltzmann form. The angular distribution of the CN product from polyimide is sharply peaked about the surface normal even in this near-threshold regime.
Ge
x
Si
1−x
infrared detectors II. Carrier escape probability and detector performance
Authors: Strong R. , Greve D. W. , Pellegrini P. , Weeks M.
Published: July 2002

Ge x Si 1−x / Si heterojunction internal photoemission (HIP) detectors with thresholds in the medium-wave infrared and long-wave infrared (LWIR) regions were fabricated and characterized. Measurements of the photoresponse are fit well by a theory which takes into account the scattering of excited carriers. The probability of escape of an excited hole is calculated and compared with that observed in another detector, the multiple quantum well structure. It is shown that HIP detectors can achieve background-limited performance in the LWIR region when operated at 40 K.
Photoinduced transient spectroscopy of deep levels in GaAs/Ga1−xAlxAs multiple quantum wells
Authors: Arikan M. Ç. , Cenk S. , Balkan N.
Published: July 2002

The capture and emission dynamics of deep levels in GaAs/Ga1−xAlxAs multiple quantum well structures are investigated by using the photoinduced transient spectroscopy technique. In nominally undoped samples three trapping levels with activation energies in the range between 0.4 and 0.8 eV are observed. These are compared with the observations based on other conventional techniques. Large capture cross sections associated with the trapping centers implies that the presence of these can be detrimental for the high speed operation of optoelectronic devices based on GaAs/Ga1−xAlxAs quantum well structures.
High-quality Nb3Sn thin films on sapphire prepared by tin vapor diffusion
Authors: Perpeet M. , Hein M. A. , Müller G. , Piel H. , Pouryamout J. , Diete W.
Published: July 2002

Stoichiometric Nb3Sn films on sapphire were prepared with a tin vapor diffusion process from sputtered Nb precursors. The superconductive properties of the precursor and the converted films were measured at 1 kHz and 87 GHz. The nanocrystalline Nb films yielded Tc=7.8–9.3 K, ΔTc=0.05–0.30 K, Jc (4.2 K)=0.8–2.8 MA/cm2, and ρ(Tc)=1–35 μΩ cm, indicating a high sensitivity to granularity and impurities. In contrast, the 0.5–3.0 μm thick and large-grained Nb3Sn films showed reproducibly Tc=18.0 K, ΔTc=0.1 K, Jc (4.2 K)=5–6.5 MA/cm2, and ρ(Tc)=7.7–9.1 μΩ cm. A reduced energy gap Δ/kTc=1.8–2.2 and a penetration depth λ0 (T=0 K)=65–80 nm were deduced from the surface impedance measurements. The residual surface resistance dropped below the sensitivity limit of 0.3 mΩ.
Optical electromodulation of surface-intrinsic-doped structures
Authors: Urı́as Jesús , Balderas Raúl
Published: July 2002

The modulation of the electric field within the intrinsic layer of surface-intrinsic-doped structures by means of a chopped pump beam and in the presence of a constant probe beam is calculated by considering the modulation of the surface voltage by the photoactivated majority carrier flow. The analysis is focused as to determine the dependency on the probe and pump photocurrents and on the chopping frequency of the observed rise and fall transient time constants of the time resolved photoreflectance signal. The non linear features of the carrier dynamics are worked out in detail. A rescaling of the time constants by the power of the probe beam is predicted.
Effect of capture and escape phenomena in Monte Carlo technique on the simulation of the nonlinear characteristics in high electron mobility transistors
Authors: Abou-Khalil Michel , Schreurs D. , Nauwelaers B. , Van Rossum M. , Maciejko R. , Wu Ke
Published: July 2002

The transport properties of electrons in a heterostructure high electron mobility transistor are analyzed by considering the capture and escape phenomena as scattering events in Monte Carlo simulation. Through the variation of the equivalent charge in time, the drain current of an AlInAs/InGaAs/InP quantum-well device with planar doping is deduced for given drain and gate voltages. We studied the energy distribution of carriers in the device and we compared the results to those obtained by other quantum mechanical treatment and the classical treatment of the confinement. We demonstrated that by using our capture procedure for the quantum mechanical treatment, carriers have higher energy values than in the case of classical treatment and therefore the current values are different by an amount of 30%–40%.
The directionality of quantum confinement on strain-induced quantum-wire lasers
Authors: Chou S. T. , Wohlert D. E. , Cheng K. Y. , Hsieh K. C.
Published: July 2002

Optical properties of Ga0.47−xIn0.53+xAs (x∼0.14) multiple-quantum-wire (MQWR) lasers prepared by the strain-induced lateral-layer ordering process were studied. A typical ratio of threshold current density, ∼10, was observed from the GaInAs MQWR lasers with contact stripes aligned to the [110] ([110] MQWR laser) and the [1̄10] ([1̄10] MQWR laser) directions. The threshold current density of the [110] MQWR laser is ∼30% lower than that of a Ga0.47In0.53As multiple-quantum-well (MQW) reference laser. The 77 K lasing wavelengths were 1.46, 1.57, and 1.69 μm for the MQW laser, the [1̄10] and [110] MQWR lasers, respectively. This strong anisotropy of threshold current densities and lasing wavelengths is the first direct evidence of the directionality of two-dimensional quantum confinement in the MQWR structure.
Nanoelectronics using conductance quantization
Authors: Cosby Ronald M. , Humm Dustin R. , Joe Yong S.
Published: July 2002

Using a simple adiabatic transport model, we show that basic electronic functions may theoretically be achieved by manipulating a single propagating mode in a system with quantized levels of conduction. For a quantum point contact with a centered and tunable depletion island formed in a two-dimensional electron gas, a sinusoidal island bias results in a pulsed source–drain conductance. For single, multiple parallel, and multiple independent configurations of this device, the functions of a digital clock, digital frequency doubler, and hexadecimal counter are described.