bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth and interfacial chemistry of insulating (100) barium fluoride on gallium arsenide
Authors: Stumborg M. F. , Santiago F. , Chu T. K. , Price J. L. , Leibowitz J. R.
Published: July 2002

The epitaxial growth of insulating BaF2 films on (100) and (111)-oriented GaAs substrates has been investigated. (100)-oriented BaF2 was successfully deposited on (100) GaAs at temperatures as low as room temperature, in contrast to a previous report. This was accomplished by first establishing a chemically reacted template layer a few monolayers thick at the BaF2/GaAs interface. These films consistently exhibited epitaxial reflection high-energy electron diffraction patterns with three-dimensional growth modes for a wide range of incident BaF2 flux rates. The epitaxial quality of the (100) films was, however, temperature dependent. A film deposited on a (111) wafer at 600 °C was (111) oriented and showed two-dimensional growth. X-ray photoelectron spectroscopy studies of the interface chemistry indicate the existence of a Ba state other than that of the BaF2, authenticating the relevance of the template layer. The (100) BaF2 films are insulating, with a breakdown field of ∼1×106 V/cm.
Origin of perfect selectivity of Si epitaxial growth through disilane molecular-beam epitaxy using synchrotron radiation
Authors: Utsumi Yuichi , Akazawa Housei
Published: July 2002

It has been reported that selective Si epitaxial growth between Si and SiO2 substrates can be maintained irrespective of the growth period by synchrotron radiation (SR) during disilane molecular-beam epitaxy. The effects of SR on the initial stage of the poly-Si growth on SiO2 are investigated as they relate closely to the appearance of perfect selectivity. It is observed that SR changes the surface composition of SiO2 and directly affects the incubation period of Si nucleation ending in poly-Si film formation on SiO2. A temperature above 700 °C is necessary for perfect selectivity since the SiO2 surface becomes Si rich below 700 °C due to radiation. The incubation period dependence on the pause/irradiation period in intermittent SR suggests that a photothermal reaction completely removes Si nuclei from SiO2 and brings about perfect selectivity.
Structure and giant magnetoresistance of granular Ag-Co and Ag-Ni alloys grown epitaxially on MgO{100}
Authors: Kubinski D. J. , Holloway H.
Published: July 2002

We describe the structural and magnetoresistive properties of Ag-Co and Ag-Ni granular alloys that were grown epitaxially on MgO{100} substrates. With substrate temperatures in the range 300–400 °C and up to 25 vol% of the ferromagnetic constituent (Co or Ni) x-ray analyses showed that the Ag matrices were elastically strained (by stretching at the interface with the substrate), but had relaxed lattice constants that were the same as that of pure Ag. This excludes the possibility of significant amounts of dissolved Co or Ni. The Co and Ni precipitates gave lattice spacings that were consistent with somewhat smaller strains. Growth at 200 °C appears to yield only partial precipitation of the Co or Ni, as judged by a 20% reduction in saturation magnetization from the values obtained with growth at 300–400 °C. Replacement of Co by Ni in otherwise similar alloys causes an increase in the size of the ferromagnetic precipitates. This leads to a decrease in surface area that combines with a decrease in saturation magnetization to give substantially smaller magnetoresistance in Ag-Ni than in Ag-Co. With both alloy systems, decrease of the substrate temperature leads to decreased precipitate size whose influence is seen in the magnitude of the magnetoresistance, the temperature dependence of the hysteresis, and the rate of approach to saturation with applied magnetic field.
Evaluation of dispersion in a misaligned grating pair pulse compressor
Authors: Zhang Zhigang , Yagi Takashi
Published: July 2002

A theoretical model is introduced for analysis of the dispersions in a grating pair under a small angular misalignment. It is shown that the misalignment causes dispersion errors particularly for higher orders.
Control of emittance growth due to mismatches in accelerators that use stellarator transport
Authors: Krall J. , Slinker S. , Lampe M. , Joyce G.
Published: July 2002

In the spiral line induction accelerator, an intense electron beam is transported along an open-ended beam pipe that makes multiple passes through the accelerating cavities. In the straight sections of the beam line, solenoidal focusing is used; in the bends, an l=2 stellarator field is used. At the solenoid/stellarator transition, where the beam equilibrium changes, a mismatch can occur, exciting oscillations of the beam envelope. Numerical simulation is used to show that the frequency, damping rate, and emittance growth associated with these oscillations are sensitive to nonlinear space-charge forces that depend significantly on the radial profile of the beam. Comparisons between simulation and experimental results illustrate this sensitivity. It is shown that mismatch oscillations can be avoided by using a single thick quadrupole lens at the solenoid/stellarator transition. Simulation and experimental results show excellent agreement.
Photoreflectance study on the two-dimensional electron gas of InAlAs/InGaAs heterojunction bipolar transistor grown by molecular beam epitaxy
Authors: Chen Y. H. , Jan G. J.
Published: July 2002

InAlAs/InGaAs heterojunction bipolar transistor with a 300 Å spacer inserted between emitter and base grown by molecular beam epitaxy was characterized by using photoreflectance spectroscopy. The energy features observed above the InGaAs fundamental band gap are attributed to the quantum confined subband transition of two-dimensional electron gas which was confined in the spacer channel. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the two-dimensional electron gas concentration. The behavior of two-dimensional electron gas at temperatures between 10 and 300 K was also characterized and the sheet-density in proportion to temperature was observed. Furthermore, using the temperature dependence of effective mass measured by cyclotron resonance combined with photoreflectance to analyze built-in electric field is also reported.
Properties of amorphous Si:H films grown by high-vacuum codeposition of Si and H atoms
Authors: Lui K. M. , Chik K. P.
Published: July 2002

Amorphous Si:H films were prepared by allowing silicon and hydrogen atoms to react at the substrate surface under high vacuum. The films were characterized by measuring their electrical and optical properties and electron-spin-resonance (ESR) spectra. Films deposited at a substrate temperature of 250 °C have a typical room-temperature dark conductivity of 8.6×10−11 (Ω cm)−1, a photo-to-dark conductivity ratio of 2.3×104 under 100 mW/cm2 white-light illumination, an optical gap of 1.7 eV, and a Fermi level at 0.87 eV above the valence-band edge. These film properties are stable under illumination and thermally up to at least 510 °C. Infrared data showed that these films are free from polyhydrides. The residual ESR centers have a density around 1017 cm−3 and have a g value of 2.0043. These centers are not neutral dangling bonds but are better assigned as T+2-T−3 pairs.
The diffusion process of metastable carriers in bismuth
Authors: de Sande J. C. G. , Sánchez Balmaseda M. , Guerra Pérez J. M.
Published: July 2002

The diffusion process of electrons and holes which have been pumped into a metastable band in bismuth films by a 1.064-μm laser pulse is considered in the calculation of the induced thermal gradient. The fit between the temporal evolution of this calculated thermal gradient and that of the thermoelectric response of films to the laser excitation allows us to estimate an upper limit of the ambipolar diffusion coefficient of metastable carriers.
Propagation of laser-generated surface acoustic waves visualized by shake-off of fine particles
Authors: Kolomenskii Al. A. , Maznev A. A.
Published: July 2002

Shake-off of micron-sized alumina particles by nanosecond laser-generated surface acoustic wave (SAW) pulses is used to visualize SAW beam propagation on surfaces of semiconductor crystals. Various phenomena in SAW propagation such as reflection from a sample edge, anisotropic diffraction, and beam steering are demonstrated.
A double S-type instability in semiconductor heterostructures
Authors: Gružinskis V. , Starikov E. , Shiktorov P. , Reggiani L. , Varani L.
Published: July 2002

A double S-type instability in an original semiconductor n+/n-GaAs/i-AlGaAs/i-GaAs/p+ heterostructure is investigated theoretically by a Monte Carlo simulation. The mechanism responsible for the second instability is the hot-electron current which, besides itself being a source of an S-type instability, drives a hot-hole current whose sharp switching-on provokes a sudden increase of the heterostructure conductance. An oscillatory behavior of the voltage drop between the heterostructure contacts is predicted. The maximum frequency of the generation so obtained is found to increase from 100 to 500 GHz when the temperature is lowered from 300 to 4 K.
Characterization of photodeposited selenium planar structures by scanning force microscopy
Authors: Peled A. , Baranauskas V. , Rodrigues C. , Art-Weisman D. , Grantman L. , Friesem A. A.
Published: July 2002

This article describes the results of a surface morphology study of photodeposited thin film devices of Selenium by scanning force microscopy (SFM). First, the structures of the photodeposited films were investigated at device level dimensions of the order of visible wavelength. Specifically, ultrathin sinusoidal holographic gratings with spatial periods in the range 480–514 nm were visually identified from SFM nanograph images. Second, grain level structural investigation was performed using image processing techniques such as filtering and one- and two-dimensional Fourier transforms analysis. The variation of the surface grain structure was sampled across the Gaussian profiles of the laser photodeposited patterns. It was found that the random amorphous clustering at the perimeter of the deposited structures becomes progressively grainy towards the center, creating protrusions above the surface with trigonal Selenium (t-Se) crystalline features. Third, performing image enhancement analysis at high magnification—the nanometer level structure was investigated for amorphous Selenium (a-Se) and the laser thermally induced structural transformations of the a-Se films. It was found that the atomic solid-state structure of a-Se films, previously deduced only by indirect methods, consists mainly of a random mixture of Sex branched chains containing also a small concentration of imperfect ring structures characteristic of the α- and β-monoclinic phases. The triclinic crystalline phase (t-Se) was identified in the center of the laser overheated regions of the film Gaussian profile. The results enable us to conclude about the debate in the literature regarding the crystalline and amorphous structure of Selenium thin films.
Sputtered SiNx film for self-aligned Si-Zn diffusion into GaAs and AlGaAs
Authors: Zou W. X. , Boudreau R. , Han H. T. , Bowen T. , Shi Song Stone , Mui D. S. L. , Merz J. L.
Published: July 2002

A new technology for self-aligned Si-Zn diffusion into GaAs and AlGaAs is described. In this technology, closed-tube Si diffusion is obtained from a sputtered SiNx film, and Zn diffusion self-aligned to the Si diffusion window is obtained by reusing the SiNx film as the mask. The key to a successful self-aligned Si-Zn diffusion is that the SiNx film is controlled to have a proper refractive index profile.
High coercivity and high wear resistance γ-Fe2O3 thin films
Authors: Ishii Osamu , Senda Masakatsu
Published: July 2002

High coercivity and high wear resistance γ-Fe2O3 thin films are described, which were prepared by bias sputtering followed by heat treatments. The coercivity, squareness ratio, and wear resistance of the films were increased by applying a negative bias voltage of about −60 V to the substrate. The improvements in magnetic and mechanical properties are attributed to the enhanced ion irradiation which increased both the internal strain of the films and the strength with which the films adhered to the substrates. When the bias voltage was too low (−180 V), many cracks appeared in the film and this resulted in low coercivity and poor wear resistance.
Magnetoresistance and phase decomposition in Cr-Fe bulk alloys
Authors: Okano R. , Hono K. , Takanashi K. , Fujimori H. , Sakurai T.
Published: July 2002

We report magnetoresistance in Cr-Fe bulk alloys and its variations associated with the microstructure change due to the phase decomposition. Large negative magnetoresistance (MR) is observed in the as-quenched Cr-Fe bulk alloy at liquid helium temperature, but not at room temperature. This is attributed to the spin cluster glass in the homogeneous solid solution. A unique feature of the MR in these homogenous alloys is that it does not saturate even in the presence of very high magnetic fields. However, such MR behavior changes when ferromagnetic particles precipitate by annealing. A lower degree of compositional fluctuation of Fe progresses by annealing the alloy at 773 K for 400 h and then a higher degree of decomposition occurs after a prolonged aging for 2600 h. In the annealed specimens, MR is observed even at room temperature. The MR change becomes more prominent at a lower field region, since ferromagnetic particles precipitate from the solid solution. The degree of the phase decomposition of the Cr-Fe solid solution is quantitatively evaluated by the atom probe analysis, and the origin of the MR dependence on the annealing conditions is discussed.
The influence of boron doping on the structure and thermal decomposition of ultrathin C/B:H films
Authors: Schenk A. , Winter B. , Lutterloh C. , Biener J. , Schubert U. A. , Küppers J.
Published: July 2002

Few monolayers thick hydrogenated carbon films doped with boron (C/B:H) were prepared and investigated in an ultrahigh-vacuum environment by high-resolution electron energy loss, Auger electron, electron energy loss, and thermal desorption/decomposition spectroscopies with specific emphasis on their chemical erosion behavior as compared to their undoped C:H counterparts. Films of thicknesses ranging from 1 to about 10 monolayers, with a maximum B/C ratio of 0.5, were grown by ion-beam deposition at room temperature on a carrier consisting of a Pt(100) single-crystal surface covered with a graphite monolayer. The process gas used was a mixture of ethane and trimethylboron of varied compositions. While at zero boron concentration the films exhibit a graphiticlike structure with about equal amounts of carbon atoms in the sp2 and sp3 hybridization state, with increasing boron concentration the film structure becomes increasingly sp3 dominated. This is evidenced by decreasing HREELS loss intensities of the vibrational modes related to graphitic hydrogenated carbon, i.e., C=C, sp2 CH stretches, and aromatic CH deformations, but enhanced C–C and sp3 CHn stretch mode intensities. No BH vibrational modes have been observed at any doping level. In accordance with these observations, the C-272 eV Auger peak line shape underwent a change characteristic for a sp3-dominated network upon B doping. The π-plasmon energy was found to shift toward lower energies at C/B:H films which also is in line with a decrease of the carbon sp2 concentration, giving further support for a change to a less graphitic structure. The observed enhanced capacity for hydrogen in the films was found to correlate in a linear fashion with the increase of the fraction of carbon atoms in the sp3 configuration. The relative hydrogen content of the films, H/C, starting at 0.4 at zero boron content, was observed to increase, saturating at 0.75 for boron concentrations greater than 10%.

This in turn coincides with a substantial growth of the film hydrogen capacity, as judged from the amount of H2 desorbing from the films between 500 and 1100 K upon thermal decomposition of the films. Although hydrogen originating from sp3 CH groups increased significantly, the amount of chemically eroded species, monitored by CnHm production in the thermal decomposition spectra, was unaffected by boron doping. However, the desorption maxima for either species, hydrogen and hydrocarbons, shift to lower temperatures at boron doped films. As reasons for the effect of B doping on the chemical constitution of C/B:H films and the resulting chemical erosion behavior, the capability of B to block the formation of aromatic structures is proposed.
GaP1−xNx alloys formed by ion implantation
Authors: Yang Xizhen , Lin Zhenjin , Li Zhigang , Wu Liu , Mao Chenjie
Published: July 2002

GaP1−xNx alloys (x=0.02–0.07 and x≂0.99) have been formed by ion implantation of Ga+ and N+ into GaP. X-ray diffraction measurements proved that both a wurtzite structure and a zincblende structure of the crystals have been formed within the implanted layers. They are similar to but not the same as GaN and GaP, respectively. An emission band at an energy higher than the energy gap of GaP appeared on the photoluminescence (PL) spectra of the implanted layers and consists of several components. Formation of the alloy and origins of the PL components are discussed.
Interface traps in InP/InAlGaAs p-n junctions by metal organic chemical vapor deposition
Authors: Huang Z. C. , Wie C. R. , Chen J. C. , Davis G.
Published: July 2002

We have studied the deep levels in the lattice-matched InP/(InAlGa)As heterojunction system. Five p-n junction samples were grown by metalorganic chemical vapor deposition with varied Al composition of 0%, 16%, 36%, 45%, and 55%. A vacancy-related deep electron trap was found at the InP/(InAlGa)As interface. This trap has less concentration in the sample with higher Al composition, and is not observed when Al composition is greater than 55%. Its activation energy within the band gap increases with the increasing Al composition. The trap can be eliminated by rapid thermal annealing at 700 °C. Current-voltage characteristics show that this trap acts as recombination centers under forward bias condition.
Superlattice parameters for optimum absorption in InAs/InxGa1−xSb superlattice infrared detectors
Authors: Heller Eric R. , Fisher Kent , Szmulowicz Frank , Madarasz Frank L.
Published: July 2002

The linear absorption coefficient of InAs/InxGa1−xSb superlattices is optimized with respect to layer widths, indium content, substrate type and substrate orientation, interface type, and choice of buffer layers based on a model envelope-function approach (EFA) involving the solution of a 6×6 EFA Hamiltonian (heavy, light, and conduction bands) for wave functions and subband energies. Free-standing superlattices as well as superlattices matched to a number of substrates are considered. In general, increasing the indium mole content from 0 to 0.4 doubles the magnitude of absorption. Changing the substrate orientation from [001] to [111] significantly increases absorption in all cases studied due to the increased heavy-hole mass and the larger InAs-conduction-band–InGaSb-valence-band offset in the [111] direction. The use of an In0.4Ga0.6Sb substrate leads to higher absorption because all the beneficial effects of strain are placed in the InAs layer, which is more sensitive to strain than is the InGaSb layer. The larger valence–conduction-band offset for InSb than for GaAs interfaces also leads to higher absorption. The model results agree best with available data when a 100 meV InAs-conduction-band–GaSb-valence-band offset is used. Specific superlattice parameters that optimize absorption for free-standing superlattices on GaSb at three cutoff wavelengths are proposed.
Effects of walk-off and group velocity difference on the optical parametric generation in KTiOPO4 crystals
Authors: Nishikawa Tadashi , Uesugi Naoshi
Published: July 2002

The effects of walk-off and group velocity difference on the optical parametric generation in KTiOPO4 crystals are numerically revealed. These calculations treat the transverse-mode profile and the temporal pulse shape of the interaction pulses simultaneously. Combining calculations with experimental results, the values of a second-order nonlinear coefficient and two-photon absorption coefficient at 0.6 μm are obtained as d24=4.2 pm/V and 0.35 pm/W, respectively. Due to the back-conversion, the maximum conversion efficiency with a 5-mm-length crystal is limited to about 10%. This is because the beam diameter and the pulse duration of the signal and idler fields become small in the propagation and interaction with the pump field cannot take place at the entire portion of the pump field. The calculation results also indicate that 280 fs output pulses are obtained from a 1 ps pump pulse by the pulse narrowing effect. It is also clarified that the back-conversion causes depression and splitting of the transverse mode and walk-off plays an important role in the transformation of the transverse-mode profile. The depression and splitting can be suppressed by using a short-length crystal or decreasing pump intensity.
In situ gettering of edge-defined film-fed growth silicon in a CO ambient
Authors: Balster S. G. , Schroder D. K. , Bailey J. , Kalejs J. P.
Published: July 2002

Edge-defined film-fed growth (EFG) Si grown with carbon monoxide (CO) added to the Ar ambient during crystal growth yields solar cells with higher efficiencies than when grown without CO. This increase in cell efficiency is not fully understood. Surface photovoltage, deep-level transient spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to determine the minority carrier diffusion lengths, impurity distributions, and defect structures in uncontaminated, Cr contaminated, and V contaminated EFG material grown with and without CO added to the Ar ambient. We conclude that ‘‘SiC-like’’ complexes in the near-surface region of the CO ambient material act as gettering sites during crystal growth, and that this gettering action results in lower bulk impurity levels and higher solar cell efficiencies.
Composition dependence of microwave properties of Y-Ba-Cu-O thin films grown by metal-organic chemical-vapor deposition
Authors: Waffenschmidt E. , Sjamsudin G. , Musolf J. , Arndt F. , He X. , Heuken M. , Heime K.
Published: July 2002

Thin films of Y-Ba-Cu-O of different composition were grown on MgO and LaAlO3 substrates by metal-organic chemical-vapor deposition. Using a microwave cavity resonator, their microwave surface resistance at 24.5 GHz was measured at a temperature of 77.5 K. It varies from less than 3 mΩ to more than 50 mΩ depending on the composition of the films. The lowest surface resistances could be obtained with samples having a composition close to the ideal stoichiometry 1:2:3 but with a slight excess of copper and yttrium.
Precise determination of the periodicity for Mo/Si and W/C metallic multilayers by electron and x-ray diffraction
Authors: Jiang S. S. , Zou J. , Cockayne D. J. H. , Hu A. , Sikorski A.
Published: July 2002

Mo/Si and W/C multilayer films were fabricated by magnetron sputtering. Cross-section transmission electron microscopy shows that these multilayers have a well-formed layer structure. The modulation wavelength of the multilayer can be precisely determined from the spacing of the satellites in the low-angle regions of both electron and x-ray-diffraction patterns. Experimental results show that the refraction corrections to the Bragg condition for electron and x-ray diffractions are in opposite directions. The physical significance of the difference between the two corrections is discussed.
Lateral junctions of molecular beam epitaxial grown Si-doped GaAs and AlGaAs on patterned substrates
Authors: Takamori Takeshi , Kamijoh Takeshi
Published: July 2002

We investigated laterally aligned pn junctions formed by a one-step molecular beam epitaxial growth of Si-doped GaAs and AlGaAs on patterned substrates. Current-voltage (I-V) and cross-sectional electron beam induced current measurements suggested that the junctions’ configuration was rather nn−pn with a highly compensated n− region than simple npn configuration. Temperature dependence of the I-V curves was also examined and the breakdown mechanism was found to be avalanche multiplication. Transmission electron microscope measurement revealed that the critical angle of crystal plane which determines the conduction type of Si-doped GaAs is exactly 25°, i.e., the (311)A plane.
Time evolution of grating decay during photorefractive fixing processes in LiNbO3
Authors: Müller R. , Arizmendi L. , Carrascosa M. , Cabrera J. M.
Published: July 2002

Decays of the diffraction efficiency of gratings written in Fe:LiNbO3 have been studied in connection with photorefractive fixing. Measurements have been carried at five different temperatures within the range of interest in photorefractive fixing, 144–168 °C. The decay curves are nonexponential and can be fitted to a sum of three simple exponential components, whose effective activation energies and preexponential factors have been determined. The dependence of the last exponential component on the grating period Λ has been measured and a 1/Λ2 law has been found. The decay constants have also been found to depend on the time the crystal is kept at the setting temperature before writing the grating. The difficulties for establishing a model taking into account the occurrence of several sites for protons in LiNbO3 are discussed.
Surface resistance and residual losses of Ag-doped YBa2Cu3O7−δ thin films on sapphire
Authors: Pinto R. , Apte P. R. , Hegde M. S. , Kumar Dhananjay
Published: July 2002

High-quality Ag-doped YBa2Cu3O7−δ thin films have been grown by laser ablation on R-plane 〈11̄02〉 sapphire without any buffer layer. Thin films have been found to be highly c-axis oriented with Tc=90 K, transition width ΔT≤1 K, and transport Jc=1.2×106 A cm−2 at 77 K in self-field conditions. The microwave surface resistance of these films measured on patterned microstrip resonators has been found to be 530 μΩ at 10 GHz at 77 K which is the lowest reported on unbuffered sapphire. Improved in-plane epitaxy and reduced reaction rate between the substrate and the film caused due to Ag in the film are believed to be responsible for this greatly improved microwave surface resistance.
Electron emission from deep states and evaluation of the density of states in a-Si:H
Authors: Yan Baojie , Adriaenssens Guy J.
Published: July 2002

Electron emission from deep states in the intrinsic layer of a-Si:H p-i-n structures was investigated by post-transit time-of-flight photocurrents in space-charge-limited current mode. The emission time tE from the deep levels is thermally activated for temperatures above 250 K. The frequency pre-factor ν0 (attempt-to-escape frequency) varies between 1011 Hz and 1013 Hz, with the corresponding activation energies Ea in the range of 0.42 to 0.52 eV for our samples. A Meyer–Neldel relation between ν0 and Ea is observed. Below 250 K, tE becomes less temperature sensitive. The density of states in the upper part of the gap is evaluated from the current in the post-extraction regime. At low temperature, the resolved distribution of gap states becomes inconsistent with higher-temperature data, similar to what is observed by standard post-transit photocurrent analysis. Possible mechanisms for the observed behavior of both the tE and the density of states are discussed: re-trapping, hopping-assisted release from deep states, the shift of transport energy with temperature and the energy dependence of the attempt-to-escape frequency. The shift of transport energy with temperature can partially explain the temperature dependence of the measured ν0 and the density of states.
Analysis of intergrowth structure and growth model of (Bi2Sr2CuOy)m(CaCuO2)m′ superlattices synthesized by alternate deposition
Authors: Nakamura Keikichi , Hatano Takeshi
Published: July 2002

It is shown that the stacking sequence of any phase of the Bi-Sr-Ca-Cu-O layered compounds synthesized by alternate (layer-by-layer) deposition can be treated as a (Bi2Sr2CuO6)m(CaCuO2)m′ superlattice. For m=1 and m′=n−1, the superlattice is just identical to a pure phase belonging to the Bi2Sr2Can−1CunOy [=22(n−1)n] series. In the alternate impingement of fluxes necessary to form two BiO layers, two SrCu0.5O1.5 layers (or one SrCuO2 and one SrO layer) and x layers of CaCuO2 in one repetition, two different CaCuO2 supply sequences result in the formation of a superlattice with the same composition and superlattice period but with different ordering. For example, when the CaCuO2 supply x is periodic as -(-2-2-3)-(-2-2-3)-, ordered stacking with modulation wavelength of two 2223 plus one 2234 layers, which is equivalent to that of three 2201 plus seven 0011 layers, is formed. Contrary to this, when x is constant throughout the deposition as -(2.33-2.33-2.33)-, the resultant film is a statistical mixture of three different stacks of 334, 343, 433, and these different stacks are distributed statistically parallel to the substrate plane, where 3 and 4 in the 334 expression means n in a unit 22(n−1)n layer. The calculated intensity based on the above stacking model agrees well with the observed x-ray-diffraction patterns. A very simple and convenient indexing is proposed to characterize these films synthesized by two different CaCuO2 supplies. Tc of the intergrowth films is also discussed based on the proposed structure model.
Sputtering of boron-doped graphite USB15—Investigation of the origin of low chemical erosion
Authors: Schwörer R. , Roth J.
Published: July 2002

The changes in surface composition of USB15—a boron doped graphite containing 15 wt.% of boron—during bombardment with D ions were determined by in situ Auger electron spectroscopy at temperatures from 300 up to 1000 K. For energies above 100 eV no strong increase of the boron surface concentration could be observed even around 800 K, i.e., at the maximum for chemical erosion of pure graphite. Chemical factor analysis of the carbon Auger peak in this energy-regime results in a much larger carbidic fraction of carbon atoms than suspected from the boron content of 15%. Thus, boron influences much more carbon atoms in their chemical reactivity with deuterium ions than is expected for the stoichiometric B4C precipitates. For ion energies below 100 eV a strong increase of boron surface concentration with decreasing ion energy at room temperature was observed. The chemical erosion of carbon in this energy regime is not suppressed by boron doping and indicates a different, surface related release process of hydrocarbon molecules.
Low ion energy electron cyclotron resonance etching of InP using a Cl2/N2 mixture
Authors: Miyakuni S. , Hattori R. , Sato K , Takano H. , Ishihara O.
Published: July 2002

The low-energy (about 30 eV) ion etching of InP with an excellent vertical sidewall profile, smooth surface, and a high etch rate (more than 1000 Å/min) has been demonstrated in an electron cyclotron resonance plasma using a Cl2/N2 mixture. Surface analysis by x-ray photoelectron spectroscopy and plasma diagnostics by optical emission spectroscopy suggest that the etching mechanism is dominated by the reduction of Cl neutral radical density by N2 dilution and the formation of InN and P3N5 due to the reaction between the atomic nitrogen in the Cl2/N2 plasma and the InP substrate. The success of this technique appears to be based on a new InP dry etching concept which controls the volatility of PClx (x=1–5) products, thus balancing the desorption rate of InClx (x=1–3) products, in contrast to the conventional reactive ion etching method which enhances the desorption of nonvolatile InClx products by high-energy ion bombardment.
Space charge generation in ZnS:Mn alternating-current thin-film electroluminescent devices
Authors: Shih S. , Keir P. D. , Wager J. F. , Viljanen J.
Published: July 2002

The electrical characteristics of ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices grown by atomic layer epitaxy are assessed as a function of the thickness of the phosphor layer using capacitance-voltage (C-V) and internal charge-phosphor field (Q-Fp) analysis. Deviations from the ideal in the measured C-V and Q-Fp characteristics are ascribed to the generation of space charge in the phosphor layer during ACTFEL device operation. C-V overshoot deviations are correlated to space charge generation via a simulation employing a single sheet of charge model which assumes that the centroid of the generated space charge is located at a discrete sheet within the phosphor. Space charge generation in these atomic layer epitaxy (ALE) ZnS:Mn ACTFEL devices is ascribed to impact ionization of the zinc vacancy portion of chlorine-zinc vacancy self-activated defect complexes. A thermodynamic argument is provided which suggests that zinc vacancies are created via self-compensation of ZnS when the ZnS is unintentionally doped with chlorine. It is contended that space charge generation could even be desirable in ALE ZnS:Mn ACTFEL devices because it leads to better aging stability and improved performance.
A sensitive technique for detecting low concentrations of deep level traps: Current-source deep level transient spectroscopy
Authors: Rancour David P.
Published: July 2002

A defect characterization method is presented that promises to provide improved defect detection sensitivity. The new technique involves the use of current-source biasing of a pn diode. Under current-source bias the junction voltage is transient, whereas standard deep level transient spectroscopy typically involves voltage-source bias with either capacitance or current transients. Computer simulations predict voltage transients of 5 mV from samples that, under identical trap capture and emission conditions, yield capacitance and current transients of only 0.2 fF and 0.7 pA. The new approach can tolerate large leakage currents, and the voltage transient amplitude is independent of device area.
Effect of intragrain current on low-field magnetic-flux distributions of zero-field-cooled polycrystalline YBa2Cu3O7−δ
Authors: Noguchi Yuji , Ohara Satoshi , Huybrechts Ben , Takata Masasuke
Published: July 2002

Magnetic-flux distributions B(r) below the lower critical field of the grains in a zero-field-cooled disk and a ring of polycrystalline YBa2Cu3O7−δ are measured at 77 K with a Hall probe. The measured distribution in the disk had a steep gradient on the sample surface, and the distribution in the ring, over the applied magnetic-field range of 1.5–4 mT, was ‘‘W’’ shaped. These results can only be explained consistently by considering the intergrain current, the intragrain currents, and their interaction. It is shown that the magnetic-flux density measured by a Hall probe with an active area much larger than the grain size is not the intergranular magnetic-flux density Binter but μBinter, where μ is effective magnetic permeability (intergranular volume fraction), and dB(r)/dr gives μ times the intergrain (transport) critical current density Jct and not Jct as is assumed by many authors.
Influence of the interface bond type on the far-infrared reflectivity of InAs/GaSb superlattices
Authors: Gadaleta C. , Scamarcio G. , Fuchs F. , Schmitz J.
Published: July 2002

We have investigated the far-infrared reflectivity spectra of two series of InAs/GaSb superlattices grown with GaAs- or InSb-like interfaces. Significant differences in the spectra induced by the interface bond type can be observed. The theoretical simulation of the experimental data allowed us to accurately determine the energy of the interface TO phonons and the actual thickness of the region in which they are localized. The inactivity of the InAs TO phonon vibration at the interface can be explained by modeling a nonhomogeneous strain accommodation across the heterointerfaces.
Excitation spectra of photoinduced absorption in a-Si:H
Authors: Malinovsky I. , Hajiev F. , Uğur S. , Uğur H.
Published: July 2002

Photoinduced absorption (PA) of hydrogenated amorphous silicon (a-Si:H) was studied by excitation spectra analysis. The nonlinear PA response taken at different excitation photon energies was found to follow the corresponding absorption coefficient for intrinsic and p-doped samples in the 80–300 K temperature range. Significant contribution of the electronic PA in the observed response was demonstrated. Thermal effects are discussed. A simple model of trapping during the pump with consequent recombination from the localized state was found to be in good agreement with experimental observations.
Optical transmissions in metal/insulator (Fe/MgF2) multilayered thin films
Authors: Nishikawa M. , Kita Eiji , Tasaki Akira
Published: July 2002

Optical transmissions were studied for Fe/MgF2 multilayered thin films whose total Fe thickness is kept at 1000 Å. The optical transmission at 700 nm was 26%, and 67% at 2000 nm for films with 15 Å Fe layer thickness. These transmissions were almost proportional to the inverse of the Fe layer thickness. The Maxwell-Garnett model can be a possible mechanism for the granularlike multilayer, for example, the (15 Å/30 Å) 70 film; however, transmissions were also enhanced in films with continuous Fe layers.
General shock wave equation of state for solids
Authors: Boogerd P. , Verbeek H. J. , Stuivinga M. , van der Steen A. C. , van der Put P. J. , Schoonman J.
Published: July 2002

A theoretical shock wave equation of state has been derived based on the Morse potential function, known from classical spectroscopy. Using this potential function and an interstitial-electron model, a linear relation between the shock wave velocity and the mass displacement velocity can be found, analogous to the common empirical relation, but with parameters derived from easily accessible nonshock-wave data. The equation is applicable to a wide range of inorganic materials, as will be demonstrated in two forthcoming articles on metals and ceramics. Examples are shown for silver and copper. Calculations are within 5% average error between experimental data and the model.
Deep levels in indium selenide single crystals doped with iodine
Authors: Micocci G. , Tepore A. , Rella R. , Siciliano P.
Published: July 2002

Electron trapping levels in indium selenide single crystals doped with iodine have been investigated by deep-level-transient spectroscopy measurements. Two traps located at about 0.60 and 0.21 eV have been detected below the conduction band and the corresponding thermal capture cross sections have been evaluated. The first trap is present both in undoped and doped InSe crystals, whereas the second trap appears in doped samples.
An analysis of phase shifts in photoreflectance spectra of strained Si/Si1−xGex structures for x<0.24
Authors: Hosea T. J. C. , Hall D. J. , Carline R. T.
Published: July 2002

Photoreflectance spectra have recently been obtained for a range of single strained Si1−xGex epilayers for 0.12<x<0.24, which are buried under a Si cap. Spectra measured at different positions on a sample wafer showed pronounced changes in the SiGe line shape. Here these changes are shown to be due to phase shifts arising from changing optical interference effects caused by variations in the Si cap thickness. The phase shifts are determined accurately using a Kramers–Kronig analysis and are interpreted in terms of a multiple-reflection treatment incorporating a calculation of the Seraphin coefficients. This allows the Si cap thiakness changes to be determined and compared to the0results of speatroscopic ellipsometry measurements.
Picosecond time-resolved luminescence of tetracene thin films
Authors: Wappelt A. , Bergmann A. , Napiwotzki A. , Eichler H. J. , Jüpner H. J. , Kummrow A. , Lau A. , Woggon S.
Published: July 2002

The energy relaxation of excitons in the organic semiconductor tetracene (film thickness 160 nm) is studied by photoluminescence from 15 to 300 K using time-correlated single-photon counting. The well-known relaxation of triplet excitons with time constant ∼7 ns is accompanied by a faster relaxation process with time constants ranging from 260 ps to 1.2 ns, depending on temperature and wavelength.
On the spatial confinement in energy beam microprocessing
Authors: Arnold Nikita
Published: July 2002

The influence of nonlinearities in the absorptivity, heat conductivity, and chemical kinetics on the spatial resolution of patterns thermally produced by a focused energy beam is studied theoretically. It is shown that a decrease in substrate temperature and proper choice of processing temperatures can significantly improve the resolution. Example calculations are presented for Si.
A solution of the doping problem for Ga delta-doping layers in Si
Authors: Zagwijn P. M. , van der Veen J. F. , Vlieg E. , Reader A. H. , Gravesteijn D. J.
Published: July 2002

We have studied the incorporation of Ga in silicon during the fabrication of delta-doping layers. The delta-function doping profiles were grown by molecular beam deposition following a solid phase epitaxial growth method. Medium-energy ion scattering, secondary ion mass spectrometry, and Rutherford backscattering spectrometry were used to determine the structure and composition of the grown films. The interface velocity of the crystallization front and the diffusion coefficient of the impurity atoms in the Si matrix, both relevant parameters of the growth process, were measured. Optimum growth conditions were found that yield Ga doping profiles of less than 1.0 nm (full width at half maximum), with more than 95% of the buried dopant atoms on lattice sites. For these optimum growth conditions, a model is derived explaining the observed incorporation of the Ga atoms.
Organic molecular beam deposition of highly nonlinear optical 4′-nitrobenzylidene-3-acetamino-4-methoxy-aniline
Authors: Schlesser R. , Dietrich T. , Sitar Z. , Gitmans F. , Kündig A. , Eng L. , Münch B. , Günter P.
Published: July 2002

Thin films of the nonlinear optical material 4′-nitrobenzylidene-3-acetamino-4-methoxy-aniline have been prepared using the organic molecular beam deposition technique. High quality homoepitaxial layers have been grown at substrate temperatures of 80 °C and moderate growth rates of 0.1–0.5 Å/s. The samples have been characterized by optical polarization and interference microscopy as well as atomic force microscopy. Growth experiments on inorganic substrates, including silicon and glass, have been performed in a substrate temperature range of −190 to 100 °C and led to amorphous films at low temperatures and polycrystalline films at temperatures above 50 °C.
Solution of the multivalley Boltzmann transport equations in Si and GaAs based on the time scales of hydrodynamic equations
Authors: Cheng Ming-C. , Chennupati Rambabu , Wen Ying
Published: July 2002

The previously developed hydrokinetic transport theory is used to arrive at a multivalley transport model for the electron distribution function evolving at the energy relaxation scale. The hydrokinetic distribution described by hydrodynamic parameters, including the density, mean energy, and average velocity, is introduced to approximate the kinetic distribution. The developed multivalley hydrokinetic model, together with the Monte Carlo method, is applied to study nonequilibrium energy and momentum distribution functions of electrons in n-type Si 〈100〉 and GaAs. It is shown that the hydrokinetic concept can be used to characterize extreme nonequilibrium phenomena of the distribution and transport parameters in terms of the relaxation scales of hydrodynamic parameters. The study suggests that evolution of the distribution is strongly influenced by energy relaxation. It is also found that in ultrafast transient situations the influence of velocity relaxation on the distribution function is more pronounced if the ratio τε /τm is larger, where τε and τm are energy and momentum relaxation times, respectively. In general, similar influences of energy and momentum dependences also show in the relaxation times. In Si at room temperature, the ratio is near or below 10 at low or medium field, and the distribution, which is subjected to a rapid change in field, weakly depends on the velocity relaxation. In the Γ valley of GaAs, although the ratio is not larger than that in Si, effects of velocity relaxation are considerably stronger due to much more pronounced velocity overshoot. The hydrokinetic distribution at the energy relaxation scale therefore provides a good description for electrons in Si in extreme nonequilibrium situations, but not in GaAs during the strong overshoot/undershoot interval. In the L valleys the ratio is much larger than 10 at low or medium fields. Consequently, The L-valley distribution function subjected to a drastically increasing field from a low value is also strongly influenced by velocity relaxation even though no overshoot is observed.
Properties and chemical stability of hot-rolled Ag(7 at. % Cu)-sheathed Bi2Sr2Ca0.64Cu1.64Ox powder-in-tube tapes
Authors: Guo J. , Lewis J. A. , Goretta K. C. , Schwartz J.
Published: July 2002

Bi2Sr2Ca0.64Cu1.64Ox (nominally Bi2212) powders were fabricated into powder-in-tube Ag- and Ag(7 at. % Cu)-sheathed tapes by cold and hot rolling to investigate the effects of sheath composition and rolling conditions on their microstructural development and superconducting properties. Bi2212 tapes with Ag(Cu) sheaths exhibited improved grain alignment and interfacial uniformity, as well as enhanced formation of the Bi-free phase (≊Sr7.5Ca6.5Cu14Ox), relative to the Ag-sheathed specimens. The hot-rolled Ag(Cu)-sheathed tapes displayed superior critical current densities (Jc), where magnetization Jcm=1.5×106 (H∥c) and 4.6×105 A/cm2 (H⊥c) at T=5 K, H=1 T. Correspondingly, these specimens had transport critical current densities (Jct) of 6.7×104 A/cm2 (H∥c) and 5.4×104 A/cm2 (H⊥c) at T=4.2 K, H=0 T and 2.2×104 A/cm2 (H∥c) and 3.0×104 A/cm2 (H⊥c) at T=4.2 K, H=14 T. The chemical stability of the Ag(Cu) sheath regions during the partial melting process was also studied. Rapid oxidation of copper produced Cu2O precipitates in the sheath at 885 °C, and subsequently a Cu2O-free zone developed near the core/sheath interface. A theoretical analysis of Cu2O precipitate formation and decomposition during thermal processing is presented.
Multisource excitations in a stratified biphase structure
Authors: Cao Wenwu , Qi Wenkang
Published: July 2002

A stratified biphase structure can have many mechanical resonance modes due to the existence of several length scales in the system. The resonance effect is greatly enhanced if there are periodic synchronized multiple driving sources in the structure. For example, a single beam or a linear array composite transducers used in medical ultrasonic imaging. Such resonance behavior can be studied using an extended transfer matrix technique which we name: multisource T-matrix technique. Using this technique we have studied the effects of randomization in a 2-2 composite. It is found that for dispersing the pitch resonance the randomization of ceramic spacing is more effective for low ceramic content, while randomization of ceramic width is more effective for high ceramic content.
Explanation of spurious features in tungsten deposition using an atomic momentum model
Authors: Smy T. , Salahuddin M. , Dew S. K. , Brett M. J.
Published: July 2002

Adequate uniformity of thin film coverage is necessary in very large scale integration. Recently, cross-sectional scanning electron microscope micrographs of thin films deposited by sputter systems producing highly directional flux have exhibited unusual growth features. A model based on the momenta of the incident sputter flux and resultant asymmetric diffusion has been developed to explain the effects and has been incorporated into simulation of ballistic deposition, a Monte Carlo simulation package. Experimental growth of tungsten films deposited over VLSI topography is presented illustrating the unique features of the deposited films which necessitated the use of the momentum model. The model reflects a general effect for thin film growth in sputtering systems that is more apparent for directional deposition, for example, tungsten deposition at low pressure or when using a collimated system. Experimental results of tungsten films deposited over high aspect ratio contact cuts are carefully compared to simulations incorporating the atomic momentum model.