bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser photodissociation of chlorine and methyl chloride on low-temperature silicon substrates
Authors: Kawasaki Masahiro , Sato Hiroyasu , Nishi Nobuyuki
Published: July 2002

Laser irradiation at 193 or 351 nm of a multilayer of Cl2 or CH3Cl on an Si wafer at 100 K leads to both photodissociation of these molecules and formation of photoetching products. The kinetic energy distributions of photofragments (Cl, CH3) and etching products (SiCl, SiCl2) were measured. The kinetic energy distribution of Cl or CH3 is bimodal for the case of thin deposition of parent molecules on substrates, in which the two-component analysis of Maxwell–Boltzmann represents experimental results, assuming average kinetic energies of ∼1 and 10 kcal/mol. The etching products have kinetic energies of 1 kcal/mol. For the thick-deposition case, the kinetic energy distribution of Cl or CH3 fragments is composed of a single component with an average kinetic energy of 10 kcal/mol.
Ion spectra of vacuum arc plasma with compound and alloy cathodes
Authors: Sasaki Jun , Brown Ian G.
Published: July 2002

We have carried out an experimental investigation of the charge state distribution of ions produced in the vacuum arc plasma for the case when the cathode is a compound or alloy. The plasma was generated in a metal vapor vacuum arc ion source, and the charge state spectra were measured using a time-of-flight method. We have compared these spectra to the spectra obtained from cathodes of the constituent elements. The cathode materials used and reported on here were the following groups: SiC/Si/C [i.e., we have compared the spectra obtained using a cathode of silicon carbide with the spectra obtained using (a) a silicon cathode and (b) a carbon cathode], TiC/TiN/TiO2/Ti/C, WC/W/C, (UC-ZrC)/UN/U/Zr/C, brass/Cu/Zn, and stainless steel/Fe/Cr/Ni. The arc current employed throughout was approximately 100 A. We find that the charge state distributions change depending on the elemental composition of the alloy or compound of which the cathode is fabricated.
The complexing of silicon impurities with point defects in plastically deformed and annealed GaAs
Authors: Ono Haruhiko , Newman Ronald C.
Published: July 2002

Si-doped GaAs crystals grown by the horizontal Bridgman method were plastically deformed at 450 °C and then annealed at various temperatures up to 800 °C. Localized vibrational mode (LVM) absorption due to Si-related defects was investigated using infrared Fourier-transform spectroscopy with the samples at liquid He temperature. Two LVM lines near 367 cm−1 due to a complex labeled Si-Y increased in strength in deformed samples which were subsequently annealed around 650–700 °C for 1 h. A new result was the observation of an LVM line from Si-Y superposed on the absorption from SiAs. Thus three separate lines occur at 366.8, 367.5, and 397.8 cm−1. It is proposed that the Si-Y center is a SiGa−VGa complex.
Theory of two-wave mixing gain enhancement in photorefractive InP:Fe: A new mechanism of resonance
Authors: Picoli G. , Gravey P. , Ozkul C. , Vieux V.
Published: July 2002

We present a new model (including both temperature and electron-hole effects) of two-beam coupling in photorefractive semiconductors under an external dc field E0. This model predicts that the exponential gain Γ can exhibit an intensity-dependent resonant behavior, yielding a π/2 phase shift of the space-charge field with respect to the incident fringe pattern. This optimum intensity strongly depends on crystal temperature but it is practically independent of the grating period. As an illustration this model is applied to InP:Fe. In this case the resonance occurs when the hole photoionization rate and the electron thermal emission rate are equal. Values of Γ as high as 20 cm−1 at 1.06 μm, for a fringe spacing of 15 μm and E0 =10 kV/cm, are predicted. The comparison between theory and experimental data requires taking into account the variation of the pump intensity throughout the sample (due to optical absorption), which reduces the volume where the resonance condition is fulfilled; in this way a satisfactory fit of the experimental gain versus intensity curve has been obtained. The characteristics of this new resonance mechanism suggest several gain enhancement techniques which are briefly discussed. In particular, an increase of Γ by using an additional incoherent backward propagating beam is predicted and experimentally demonstrated.
Theory of the acoustical properties of symmetric multilayer structures
Authors: Martin B. G.
Published: July 2002

A theoretical investigation has been made of the acoustical properties of symmetrical multilayer structures. Acoustic symmetric multilayers (ASMs) exhibit behavior analogous to that of optical symmetric multilayers. For a three-layer ASM, a range of effective acoustic impedances is possible merely by varying the thickness of the middle layer for a given frequency. This has the advantage that one can design an antireflection coating for virtually any substrate impedance. Some illustrative examples are given.
Magnetization reversal in CoCr perpendicular thin films
Authors: Zhu Jian-Gang , Bertram H. Neal
Published: July 2002

Computer simulation has been utilized to study magnetization reversal processes in CoCr perpendicular films. The model is based on the columnar structure of the film. Each column is considered to be a single crystal with perpendicular uniaxial crystalline anisotropy. The model assumes that each column is always uniformly magnetized even during its magnetization reversal. The gyromagnetic equation of motion with phenomenological Landau–Lifshitz damping is utilized to describe the magnetization rotation of this coupled system. The study focuses on the collective magnetization reversal modes of the particles due to magnetostatic interactions and intergranular exchange coupling. Low nucleation fields occur which are characterized by a planar chain nucleation mode. This yields coercivities equal to or less than the crystalline anisotropy field 2K/M. It is argued that, due to this collective process, the uniform rotation reversal mechanism for the individual particles in the film is energetically more favorable than nonuniform reversal, such as curling. Intercolumn exchange coupling significantly reduces the coercivity and an increase of the exchange coupling strength changes the magnetization behavior from ‘‘particulate’’ to ‘‘continuous.’’
Separation of magnetic and topographic effects in force microscopy
Authors: Schönenberger C. , Alvarado S. F. , Lambert S. E. , Sanders I. L.
Published: July 2002

Several techniques are presented which allow magnetic force microscopy to be performed while simultaneously mapping the surface topographic features of a magnetic sample. The separation of magnetic and topographic features measured simultaneously with a scanning force microscope is made possible by an instrument based on a differential interferometer that can detect cantilever deflections of 0.005 nm at a frequency as low as 1 Hz. Two different applications are presented.
Monte Carlo determination of femtosecond dynamics of hot-carrier relaxation and scattering processes in bulk GaAs
Authors: Zhou Xing , Hsiang Thomas Y.
Published: July 2002

We present results of ensemble Monte Carlo simulations of the initial femtosecond dynamics of hot-carrier relaxation and scattering processes in bulk GaAs. Contributions of each operative scattering mechanism to the primary stage of carrier relaxation are investigated by turning them on or off separately. We find that when scattering to both L and X valleys is energetically possible, the initial relaxation process, which occurs on a time scale of ∼50 fs, is dominated by the intervalley scattering out of the Γ valley, but cannot be described by a single, averaged scattering time. When the carriers are excited between the X and L valleys, thermalization occurs on a time scale of ∼140 fs, which is mainly due to Γ-L intervalley scattering, and then, optical-phonon and e-e scatterings start to contribute in the relaxation. At low temperatures, however, optical-phonon scattering plays only a secondary role.
A numerical simulation of rf glow discharge containing an electronegative gas composition
Authors: Oh Yong-Ho , Choi Nak-Heon , Choi Duk-In
Published: July 2002

A one-dimensional continuum model of glow discharge, which contains three charged particle species of electrons, positive and negative ions, is numerically solved. The model consists of three continuity equations of each particle species and electron temperature equation with Poisson’s equation. The charged particle formation mechanisms of the ionization and the attachment which depend on the electron energy are included in the model. A numerical algorithm based on the finite difference method on nonuniform staggered grids is developed and the results are obtained on a relatively small workstation computer. General behaviors of particle movements are similar to the previous works, but the reaction rates reveal somewhat different features.
Lateral restoring force on a magnet levitated above a superconductor
Authors: Davis L. C.
Published: July 2002

The lateral restoring force on a magnet levitated above a superconductor is calculated as a function of displacement from its original position at rest using Bean’s critical-state model to describe flux pinning. The force is linear for small displacements and saturates at large displacements. In the absence of edge effects the force always attracts the magnet to its original position. Thus it is a restoring force that contributes to the stability of the levitated magnet. In the case of a thick superconductor slab, the origin of the force is a magnetic dipole layer consisting of positive and negative supercurrents induced on the trailing side of the magnet. The qualitative behavior is consistent with experiments reported to date. Effects due to the finite thickness of the superconductor slab and the granular nature of high-Tc materials are also considered.
Selective ions adsorption and nonlocal anchoring energy in nematic liquid crystals
Authors: Barbero G. , Durand G.
Published: July 2002

Recent measurements have demonstrated the nonlocal character of the angular anchoring energy of a nematic liquid-crystal orientation close to a solid substrate. Assuming a sign selective adsorption for ions on such a substrate, we calculate the surface charge and field. It exists a macroscopic cell thickness below which the surface can adsorb most of the volume charges. In this regime, because of the coupling of the surface field with the nematic director, the surface anchoring energy depends on the thickness, and can even become zero. This model explains the observed thickness dependence of the anchoring energy and the ‘‘spontaneous’’ Freederickcsz transition.
Low-energy electron attachment to BCl3
Authors: Petrović Z. Lj. , Wang W. C. , Suto M. , Han J. C. , Lee L. C.
Published: July 2002

The rate constants of low-energy electron attachment to BCl3 diluted in N2 are measured as a function of E/N at 1–11 Td, corresponding to mean electron energies at 0.4–1.0 eV. The negative ions produced by hollow-cathode discharges of either pure BCl3 or mixtures of BCl3 in N2 are mass analyzed to identify the products of electron attachment to BCl3. Only Cl− ion is found in the discharge media, although BCl−3 is observed at the applied voltage significantly lower than the breakdown voltage. The electron attachment processes of BCl3 are discussed.
The interface morphology of diamond film grown on Si
Authors: Kweon Dae-Weon , Lee Jai-Young
Published: July 2002

The interfaces between diamond films and silicon substrate have been investigated to obtain a better understanding of the characteristics of diamond-substrate interface. In order to investigate the interface morphology, the backside surface of diamond film and the substrate surface have been observed after the deposited diamond film is separated from the silicon substrate. The vacant space between particles is found on the backside surface of the separated film and the central part of each particle caves in. Also, many of the hillocks are present on the Si substrate surface after the film is removed. From the observations, it is found that the interfaces between the diamond film and the silicon substrate is very rough. It is suggested that such interface morphology is attributed to the etching of the silicon substrate which takes place at the early stage of the synthesis. For the energy dispersive spectroscopy experiment of the backside surface of the separated film, more silicon is detected at the central part of each particle. This result implies that strong Si—C bonds are localized at the center of each particle, which would result in the poor adhesion.
Growth processes in the initial stage of Ge films on (811)Si surfaces by GeH4 source molecular beam epitaxy
Authors: Koide Y. , Zaima S. , Itoh K. , Ohshima N. , Yasuda Y.
Published: July 2002

The growth process in the initial stage of growth of Ge films on (811)Si substrate surfaces by GeH4 source molecular-beam epitaxy has been studied by in situ reflection high-energy electron diffraction observation. It has been found that a strained film by the monolayer overgrowth mode is formed initially with an epitaxial relationship of the parallel orientation, and that plate-shaped Ge islands with (811) facets are grown early in the growth, but that the predominant facet changes to {311} and the (100) planes with further growth. These growth processes are similar to those of Ge films on (100)Si surfaces reported previously. It is concluded that Ge islands with {811} facets are energetically stable in the initial stage of the island growth on the (811)Si surfaces as well as on the (100)Si surfaces.
Hydrogenation of Si- and Be-doped InGaP
Authors: Dallesasse J. M. , Szafranek I. , Baillargeon J. N. , El-Zein N. , Holonyak N. , Stillman G. E. , Cheng K. Y.
Published: July 2002

Data are presented on the hydrogenation of Be-doped (p-type) and Si-doped (n-type) In1−xGaxP epitaxial layers grown lattice matched to GaAs (x ∼ 0.5). Low-temperature (1.7 K) photoluminescence, electrochemical carrier concentration profiling, and scanning electron microscopy are used to study the effects of hydrogenation on carrier recombination, carrier concentration, and surface morphology. Hydrogenation is found to passivate Si donors and Be acceptors and to improve photoluminescence efficiency, but causes mild surface damage. The carrier concentration following hydrogenation is found to be lowest in acceptor-doped material.
Recrystallization of amorphous silicon films deposited by low-pressure chemical vapor deposition from Si2H6 gas
Authors: Nakazawa Kenji
Published: July 2002

This paper investigated the recrystallization of low-pressure chemical vapor deposition amorphous silicon (a-Si) films deposited using Si2H6 gas at various substrate temperatures. The grain size of recrystallized films formed from Si2H6 is larger than that formed from SiH4. The maximum grain size is obtained at the substrate temperature of 460 °C, where the nucleation rate is minimum due to the maximum structural disorder of the Si network. The structural disorder is increased not only by lowering the substrate temperature but also by increasing the deposition rate. The field effect mobility of thin-film transistors (TFTs) using the recrystallized films reaches 120 cm2 V−1 s−1, even though the highest temperature during the TFT fabrication process is only 600 °C.
A comparison of the amorphization induced in AlxGa1−xAs and GaAs by heavy-ion irradiation
Authors: Jenčič I. , Bench M. W. , Robertson I. M. , Kirk M. A.
Published: July 2002

The response of AlxGa1−xAs/GaAs samples to bombardment with heavy ions (50 keV Kr+, 50 keV and 1.5 MeV Xe+) was studied as a function of ion dose at temperatures of 30 and 300 K using transmission electron microscopy. Samples with x=0.2 and 0.85 were used. Under all irradiation temperature and ion combinations, the AlGaAs was more resistant to amorphization than GaAs. The resistance increased with increasing Al content and decreased with decreasing irradiation temperature. This difference in the response may be attributed to differences either in the mechanisms by which a region is rendered amorphous (i.e., by direct impact amorphization or by the buildup of point defects) or to differences in the inherent stability (recrystallization temperature and rate) of individual amorphous zones in AlGaAs and GaAs.
Measurements of magnetoresistance in magneto-optical recording media
Authors: Hajjar Roger A. , Mansuripur M. , Shieh H.-P. D.
Published: July 2002

The magnetoresistance effect in compositionally modulated transition-metal/transition-metal films (Co/Pt and Co/Pd) and in amorphous binary rare-earth–transition-metal alloy films (TbFe) has been investigated. Results of measurements on several samples at room temperature as a function of the strength of the applied magnetic field are reported. For each sample three different configurations are investigated in which the direction of the applied field is (i) perpendicular to the plane of film, (ii) in the plane of the film and parallel to the direction of the electric current, (iii) in the plane of the film and perpendicular to the direction of the current. During these measurements the extraordinary Hall effect and the magneto-optic Kerr effect have also been monitored. This additional information, together with the magnetization measurement results obtained from a vibrating sample magnetometer, are used to analyze the data and to explain some of their interesting features.
Fabrication and structure of epitaxial terbium silicide on Si(111)
Authors: Kaatz F. H. , Spiegel J. Van der , Graham W. R.
Published: July 2002

The epitaxial growth of terbium silicide by codeposition on Si(111) is demonstrated. Terbium and silicon are evaporated onto substrates held at room temperature and subsequently annealed up to 850 °C, where the pressure during evaporation and annealing is maintained below 1×10−9 Torr. Low-energy electron diffraction shows a sharp (3)1/2×(3)1/2 pattern of the hexagonal silicide after an 850 °C anneal. The morphology in these films in much improved over that of metal reacted layers with pinholes of <0.15 μm in diameter. Rutherford backscattering analysis indicates single-crystal growth with a channeling minimum yield of 9% for 150-Å-thick silicide films annealed to 800–850 °C. Plan view electron microscopy shows the evidence of faults along 〈2̄20〉 directions, but no indication of a superstructure in the silicide layer. The microstructure of the thick film is shown to be strongly affected by the formation of a template layer prior to thick film deposition.
Observation of a magnetized low-pressure rf plasma for thin-film preparation
Authors: Okuno Yoshihiro , Yagura Shinya , Fujita Hiroharu
Published: July 2002

A magnetized low-pressure rf plasma used for thin-film preparation has been observed with probe measurements. The sheath structure near the rf electrode was also obtained by measuring the potential profile in the sheath with an emissive probe. The results revealed that the magnetic field increases the electron density but has no effect on the electron temperature. Further the magnetic field increases the magnitude of the self-bias potential of a rf electrode, and it decreases the effective sheath thickness. The application of a magnetic field to the conventional rf processing plasma is confirmed to be useful for control of a low-pressure rf plasma.
Computer simulation of magnetization reversal: The effect of shape and uniaxial crystalline anisotropy
Authors: Spratt Geoffrey , Uesaka Yasutaro , Nakatani Yoshinobu , Hayashi Nobuo
Published: July 2002

The Landau–Lifshitz–Gilbert equation has been used to compare the hysteresis loops and magnetization-reversal mechanisms of a cubic particle, with uniaxial crystalline anisotropy, and a 2:1-aspect-ratio elongated particle. The switching fields of these particles were 700 Oe. The cubic particle exhibited a square hysteresis loop, but the elongated particle had a nonsquare hysteresis loop due to the tilting of spins in the surface layers. Magnetization reversal for the elongated particle was initiated at the surface layers. For the cubic particle the reversal was initiated at an edge of the particle and spread through to the central elements. Differences between the hysteresis loops and reversal mechanisms, for the different particles, are attributed to the different nature of the magnetic anisotropies.
Photoreflectance studies of GaAs containing a Si-δ-doping layer
Authors: Zhou Weimin , Perry Clive H. , Ma Ling , Lee Kyu-Seok , Worlock John M. , Zrenner Artur , Koch F. , Ploog K.
Published: July 2002

The electronic structure of several n-type GaAs samples containing ‘‘δ-doping’’ layers of Si have been studied using photoreflectance (PR) spectroscopy. Well-defined oscillatory features due to electronic transitions well above the band gap are observed at 300 K and identified as Franz–Keldysh oscillations. The energy spacing and the intensity of the oscillations decrease with decreasing temperature as a consequence, we believe, of changes in the electric field due to the surface charges. Self-consistent energy-band calculations support the interpretation that the oscillatory structure is due to Franz–Keldysh effects. The imposition of magnetic fields up to 15 even at room temperature has a pronounced influence on the PR spectrum. Parallel fields suppress the oscillatory structure but cause a large increase in the PR peaks near the GaAs energy gap. At 4.2 K Landau-like spectral features are observed for fields applied perpendicular to the doping layer.
Homogeneous nucleation of diamond powder by CO2 -laser-driven gas-phase reactions
Authors: Buerki Peter R. , Leutwyler Samuel
Published: July 2002

Diamond powders with grain diameters up to 0.3 μm were obtained by CO2 -laser-induced decomposition of C2H4 at low pressures and temperatures. C2H4 or mixtures of C2H4, H2, and SiH4 were irradiated in a gas-flow reactor with the 10P14 line (10.532 μm) of a grating-tunable CO2 laser with 50-W cw output power. Solid products were produced in a yellow-to-orange colored flame (500–550 °C) and collected in filters. The product contained polyaromatic species, high molecular polymers, graphite, amorphous carbon, and spherical diamond particles. Several diamond particle populations, with mean diameters of 6–120 nm, were observed by transmission electron microscopy. Both diamond modifications, cubic and hexagonal, were identified by electron diffraction. Diamond formation is dependent on the residence time in the reaction flame, but relatively independent of the reactant gas compositions. Hydrogen-free pyrolysis of pure ethylene produced diamond of high purity (no diffraction rings of graphite detectable).
Raman scattering from boron nitride coatings at high temperatures
Authors: Exarhos Gregory J. , Schaaf Joel W.
Published: July 2002

The temperature dependence from ambient to 2325 K of the high frequency E2g mode intrinsic to the hexagonal phase of boron nitride has been determined using a pulsed-excitation gated-detection technique which effectively discriminates against sample blackbody emission. A monotonic decrease of 80 cm−1 in mode frequency accompanied by a marked increase in linewidth was observed over this temperature range. Simultaneous acquisition of both Stokes and anti-Stokes Raman scattering has been demonstrated and provides another method by which sample temperatures may be verified. Variations in measured linewidth and mode frequency are described in terms of an anharmonic oscillator model from which the intralamellar thermal expansion coefficient is estimated.
Highly thermally stable electro-optic response in poled guest-host polyimide systems cured at 360 °C
Authors: Wu J. W. , Binkley E. S. , Kenney J. T. , Lytel R. , Garito A. F.
Published: July 2002

In a major development of the new poling process for electro-optic polyimide guest-host systems [Wu et al., Appl. Phys. Lett. 58, 225 (1991)], a thermally stable EO response at 200 °C for 80 h is demonstrated for poled polyimide films cured at 250 °C for 1 h plus an additional curing at 360 °C for 0.5 h. Thermal stability at 300 °C for 2 h is demonstrated for the same system cured at 360 °C for 0.25 h plus an additional curing at 300 °C for 0.33 h. High-temperature densification curing (360 °C) is found to be essential in realizing a thermally stable EO response of poled guest-host polyimides at high temperatures.
Electron-energy distribution function measurements in capacitively coupled rf discharges
Authors: Dilecce G. , Capitelli M. , De Benedictis S.
Published: July 2002

Measurements by Langmuir probes have been performed in a capacitively coupled, parallel- plate rf discharge of He and Ar with pressures ranging from 0.1 to 2 Torr and discharge power from 10 to 50 W at 27 MHz. The first part of this paper is devoted to a discussion of the preparation of the electrostatic probe technique, for which a rf discharge is well known to be a very hostile environment, and of its intrinsic limitations, in order to assess the reliability of the measurements. Results showing a marked non-Maxwellian character of the electron-energy distribution function are then presented and discussed in the light of the most recent theories about the sustainment of this kind of gas discharge.
A dc high-current low-energy electron beam gun
Authors: Ohgo Tadashi , Hara Tamio , Hamagaki Manabu , Ishii Kazushige , Otsuka Masamoto
Published: July 2002

A dc high-current and low-energy electron beam gun has been developed. The maximum electron beam current is 63 A for an accelerating energy of 140 eV. The maximum ratio of the beam current to the discharge current is 78%. The ion current, which is extracted from the helium plasma produced by the electron beam, is 12.3 A. This value corresponds to the current density of about 7 A/cm2, which is the highest value among the ion sources in steady operation. The electron density of the plasma is estimated from the ion saturation current to be about 4.6×1013 cm−3, which corresponds to the degree of ionization of 21%. This gun will open the way for many electron beam applications.
SiF2 as a primary desorption product of Si etching by F atoms: Interpretation of laser-induced fluorescence spectra; rate constant of the gas phase SiF2+F reaction
Authors: Vanhaelemeersch S. , Van Hoeymissen J. , Vermeylen D. , Peeters J.
Published: July 2002

Using a laser-induced fluorescence technique, SiF2 radicals are detected as primary gas-phase products of the chemical etching of silicon by fluorine atoms. The bending frequencies for the lower (X̃ 1A1) and the excited (Ã 1B1) state of the SiF2 molecule are determined from the observed excitation and fluorescence spectra and are in close agreement with literature. The v″2 intensity distribution of the fluorescence spectra corroborates the assignment of the upper-state vibrational levels. In a second series of experiments, the rate constant for the SiF2+F reaction at room temperature was measured: k = (5 ± 1) × 10−13 cm3 molecule−1 s−1 at pressures of 1–5 Torr helium. The absence of pressure dependence is rationalized in terms of radiative deactivation of the SiF*3 product.
Inhibition of atomic hydrogen etching of Si(111) by boron doping
Authors: Chen P. J. , Colaianni M. L. , Yates J. T.
Published: July 2002

Subsurface boron doping reconstructs the Si(111) surface and alters the electronic character of the surface Si atoms. The interaction of atomic hydrogen with the boron-modified Si(111)-(√3×√3)-R30° surface was studied using temperature programmed desorption (TPD), high-resolution electron energy-loss spectroscopy (HREELS), and low-energy electron diffraction. In comparison to the Si(111)-(7×7) surface, we observe a significantly reduced hydrogen saturation coverage, measured by TPD and HREELS, and the absence of silane production. The ordered (1/3 ML) subsurface boron atoms passivate the surface Si atoms and reduce their reactivity with atomic hydrogen. This leads to a surface condition causing suppression of silicon etching by atomic hydrogen, compared to the unmodified Si(111)-(7×7) surface.
Enhanced volume production of negative ions in the post discharge of a multicusp hydrogen discharge
Authors: Hopkins M. B. , Bacal M. , Graham W. G.
Published: July 2002

In this paper we demonstrate a new concept in the production of negative hydrogen ions in a low-pressure multicusp discharge. The discharge voltage is modulated to produce a non-Maxwellian, hot-electron plasma during the current pulse, followed by a cool Maxwellian electron plasma in the post discharge. This procedure, of separating in time the required hot and cold electron plasmas required for volume H− production, is called a temporal filter. The time evolution of the electron energy distribution function is measured using the time-resolved second derivative of a Langmuir probe characteristic. Time-resolved measurements of the negative ion density are made using laser photodetachment. The measurements show that the negative ion density in the center of the source, at a gas pressure of 0.07 Pa, increases by a factor of 2 when the discharge is switched off. At this low pressure the average H− beam current extracted from the source, when operated with a discharge current of 1 A in the pulse modulated mode exceeds the H− beam current from a 5 A continuously operated source. The increase in efficiency of the pulsed source is explained in terms of a two-step H− production mechanism.
Electron paramagnetic resonance study of thermal donors in Fe-doped InP
Authors: Bardeleben H. J. von , Stievenard D. , Kainosho K. , Oda O.
Published: July 2002

The effect of thermal annealing of semi-insulating InP wafers in the 660–820 °C temperature range under SiNx capping condition is studied by electron paramagnetic resonance (EPR) spectroscopy. The annealing leads to the formation of electrically active, deep thermal donors with total defect concentrations up to 1016 cm−3. The thermal donors are of intrinsic origin. By transient EPR spectroscopy the activation energies for electron emission of the dominant thermal donors were determined to be 0.40 and 0.14 eV, respectively.
Sputtering pressure effect on microstructure of surface and interface, and on coercivity of Co/Pt multilayers
Authors: He Ping , McGahan William A. , Nafis S. , Woollam John A. , Shan Z. S. , Liou S. H. , Sequeda F. , McDaniel T. , Do H.
Published: July 2002

Thin Co/Pt multilayers were prepared on Si and glass substrates by sputtering with Ar pressures ranging from 2.5 to 15 mTorr. The bilayer structure of the samples was Co(3 Å)/Pt(15 Å)×17, and all samples had the easy axis of magnetization perpendicular to the sample surface as determined with a SQUID magnetometer. All samples retained the layered structure, as revealed by low-angle x-ray diffraction. In addition, diffraction peaks due to the formation of Co-Pt compounds (presumably at the interfaces between Co and Pt) were identified. The coercivity of samples changed from about 400 Oe for films deposited at low Ar sputtering pressure (2.5 mTorr) to as high as 2300 Oe for films deposited at high Ar pressure (15 mTorr). Ellipsometry and atomic force microscopy were used to study surface roughness and microstructure of samples prepared at different sputtering pressures.
Microwave properties of YBa2Cu3O7−δ high-transition-temperature superconducting thin films measured by the power transmission method
Authors: Miranda F. A. , Gordon W. L. , Bhasin K. B. , Heinen V. O. , Warner J. D.
Published: July 2002

The microwave response of YBa2Cu3O7−δ superconducting thin films has been studied by performing power transmission measurements. The measurements were made at frequencies within the 26.5–40.0-GHz frequency range, and at temperatures from 20 to 300 K. The films were deposited on LaAlO3, MgO, yttria-stabilized zirconia, and LaGaO3 substrates by laser ablation and dc off-axis magnetron sputtering. From these measurements the complex conductivity σ* = σ1 − jσ2, the magnetic penetration depth λ, and the surface resistance Rs of the films have been determined. It was observed that both σ1 and σ2 increased when cooling the films below their transition temperature. This behavior disagreed with that expected from the two-fluid model. In addition, it was observed that the temperature behavior of σ1 deviates from the predictions of the Bardeen–Cooper–Schrieffer theory. Values of λ have been obtained that are in good agreement with the best reported values for high quality c-axis-oriented thin films and single crystals (∼140 nm). The anisotropy of λ was determined by measuring this parameter in c- and a-axis-oriented films. An intrinsic penetration depth value (λ∼90±30 nm) has been estimated from the film thickness dependence of λ. Values of Rs for the YBa2Cu3O7−δ films were calculated and found to be comparable or lower than that of copper at temperatures below 80 K. These Rs values were consistent with those found on the same films using resonant-cavity techniques.
Detection of ferroelectric domain reversal in KTiOPO4 waveguides
Authors: Laurell F. , Roelofs M. G. , Bindloss W. , Hsiung H. , Suna A. , Bierlein J. D.
Published: July 2002

Segmented optical waveguides in KTiOPO4 are fabricated by ion exchange of Rb/Ba for K using molten nitrate salts. The waveguides have been utilized to generate efficient second-harmonic generation (SHG) of blue light. Four techniques are described which demonstrate that the ion exchange can produce domain reversal in these waveguides, and quasi-phase matching is utilized to obtain blue light. The experiments involve surface SHG, electrostatic toning, selective etching, and piezoelectric measurements.
The design, synthesis, and evaluation of chromophores for second-harmonic generation in a polymer waveguide
Authors: Burland D. M. , Miller R. D. , Reiser O. , Twieg R. J. , Walsh C. A.
Published: July 2002

In order to develop nonlinear optical chromophores for frequency doubling applications, it is important to understand the magnitudes of the relevant chromophore properties that will be required. In this paper the use of organic polymers with attached optically nonlinear chromophores to form waveguide second-harmonic-generation devices will be discussed. In particular, the use of such a frequency doubling device in optical storage applications will be considered. To this end limits on the quantity μβ are obtained where μ is the dipole moment and β the hyperpolarizability. Experimental values for electron donor/acceptor-substituted benzenes, stilbenes, and tolanes are compared to these requirements. None of the chromophores treated here have both sufficiently high optical nonlinearity and sufficiently low optical absorption at a doubled optical frequency of 400 nm to be practical in the specific application described.
Growth of Ge on a Te adsorbed Si(001) surface
Authors: Higuchi S. , Nakanishi Y.
Published: July 2002

The growth process in the initial deposition stage of Ge films on an Si(001) surface where tellurium is adsorbed was investigated. The growth mode of these films was found to change to layered growth on a Te/Si(001) surface, i.e., Te atoms were observed on the surface of the Ge films as a result of successive site exchanges between the Te and Ge atoms. Thus, Te is believed to act as a surfactant to grow Ge in a layer-by-layer mode on a Si substrate. The growth process was observed in situ by reflection high-energy electron diffraction and low-energy electron diffraction, with the microstructure of Ge films being examined in detail using a transmission electron microscope.
The chemical interaction between high-concentration, mixed-ion-implanted group-III and -V impurities in silicon
Authors: Shiryaev S. Yu. , Nylandsted Larsen A. , Deicher M.
Published: July 2002

High-concentration, mixed group-III (In, Ga)/group-V (P,As) -implanted silicon single crystals have been studied by combining Rutherford backscattering spectroscopy/channeling spectrometry, Hall/resistivity measurements, transmission electron microscopy, and perturbed γγ angular correlation. A variety of processes and physical properties of the implanted layers are found to be dramatically changed in the mixed compared to the single implants. These include solid-phase recrystallization, redistribution of group-III and group-V atoms, phase nucleation and separation, lattice location of the impurity atoms, and electrical activation of the group-V atoms. The cause of these changes on a microscopic level is shown to be a strong chemical interaction between the donor and acceptor atoms, resulting in the formation of AIIIBV chemical compounds. For the As/In and P/In in Si systems, the formation of moleculelike AIIIBV complexes with donor and acceptor atoms sitting in substitutional silicon lattice sites is directly revealed by perturbed angular correlation measurements. A remarkable difference in the high-temperature annealing behavior is found between In- and Ga-doped, mixed implants: The precipitates found after high-temperature annealing in the In-doped samples are concluded to be mainly metallic particles whereas those found in the Ga-doped samples are AIIIBV compound precipitates coherent to the host lattice. The thermal stability of the AIIIBV compounds in silicon is suggested to be dependent on both the strength of the chemical bonds between the donor and acceptor atoms and the lattice mismatch between silicon and the crystalline phase of the AIIIBV compound.
Modeling and measurements of the negative ion flux from amplitude modulated rf discharges
Authors: Overzet Lawrence J. , Lin Yun , Luo Laizhong
Published: July 2002

The time dependent as well as amplitude modulation frequency dependent ion flux from parallel plate radio frequency discharges have been measured by mass spectrometry and modeled using the fluid equations. The negative ion flux to the grounded electrode becomes large after the rf excitation is turned off and has complex temporal features which depend upon the ion, gas mixture, applied dc bias voltage and other externally controlled parameters. Some of these features can be explained with the assistance of the fluid equation model. In short, negative ions can be formed during the active glow as well as after the glow excitation has been turned off. The resulting negative ion flux to the grounded electrode during the afterglow is determined in part by when the negative ion is formed. For example, if the amplitude modulation frequency at which the flux of a negative ion goes to zero is larger than 100 kHz, it indicates that ion is formed in the afterglow. (Of course, it may exist during the active glow as well.) Conversely, ions with signal only at small modulation frequencies are unlikely to have been produced by attachment in the afterglow. The experimental results are from CF4 and Cl2-He discharges. One finding was that the temporal and modulation frequency dependencies of F− and SiF5− ions from CF4 discharges indicate that SiF5− exists only in a thin layer above the grounded electrode.
dc voltage holding experiments of vacuum gap for high-energy ion sources
Authors: Watanabe Kazuhiro , Mizuno Makoto , Ohara Yoshihiro , Tanaka Masanobu , Kobayashi Kazuo , Takahashi Eiki , Uede Taisei
Published: July 2002

dc voltage holding characteristics were investigated to obtain a data base for designing high-energy and high-power ion sources of neutral beam injectors. We confirmed that the voltage holding characteristics almost obey the clump theory in the experimental gap length of up to 50 mm. The magnetic field in the gap lowered the breakdown voltage in a gas discharge region higher than a pressure of 10−3 Torr. The breakdown voltage of 30% was reduced by seeding cesium on the electrode with one order higher density than that of actual ion source at the pressure region of lower than several mTorr.
Effect of transition-metal overlayers on the perpendicular magnetism of MBE-grown, ultra-thin Co films
Authors: Engel Brad N. , Wiedmann Michael H. , Van Leeuwen Robert A. , Falco Charles M.
Published: July 2002

We have used in situ polar Kerr ellipticity measurements to study the perpendicular magnetic behavior of MBE-grown Pd/Co/TM sandwich structures, where TM is the nonmagnetic transition metal overlayer Pd, Cu, or Ag. These structures are epitaxially deposited on thick Pd (111) buffer layers grown on Co-seeded GaAs (110) substrates. Hysteresis curves were measured in situ for systematically varied Co and TM layer thicknesses 2 Å≤tCo≤10 Å and 0 Å≤tTM≤200 Å. We observed perpendicular loops with a coercive field of Hc≤200 Oe for the uncovered Co films for tCo≤6 Å, becoming in-plane above this thickness. However, subsequent deposition of just one atomic layer (≊2 Å) of any of the TM over the Co resulted in strongly perpendicular, square hysteresis curves with Hc≥700 Oe for all films in the Co thickness range studied. Deposition of TM overlayers causes nonmonotonic behavior in Hc as a function of coverage. We find a peak in Hc at a TM coverage of tTM∼1.5 Å for all materials, with a subsequent monotonic increase above tTM≊4 Å to a limiting value by tTM≊100 Å. The peak is most pronounced for Cu, where Hc changes by more than 200%.
Deposition and surface characterization of high quality single crystal GaN layers
Authors: Asif Khan M. , Kuznia J. N. , Olson D. T. , Kaplan R.
Published: July 2002

Recently, using low pressure metalorganic chemical vapor deposition we grew extremely high quality single crystal GaN films over basal plane sapphire substrates. Optimization of the buffer layers and the total film thickness played a key role in determining the electrical, optical, crystalline and surface qualities. In this communication we present the surface characterization results for these high quality GaN layers. Reflection high energy electron diffraction, low energy electron diffraction (LEED), and electron energy loss spectroscopy data are presented for clean GaN surfaces. The cleaning procedure was developed using Auger electron spectroscopy analysis. These electron diffraction data indicate the grown surface to be excellent single crystal GaN with a wurtzite structure. To the best of our knowledge ours is the first reported LEED data for single crystal GaN.
Anisotropic etching versus interaction of atomic steps: Scanning tunneling microscopy observations on HF/NH4F-treated Si(111)
Authors: Pietsch G. J. , Köhler U. , Henzler M.
Published: July 2002

After ex situ etching with various solutions of hydrofluoric acid (HF) and ammonium fluoride (NH4F) Si(111) samples are transferred into ultrahigh vacuum with an ultrafast load-lock and characterized by scanning tunneling microscopy (STM): Concentrated HF selectively removes any surface oxide and, thus chemically prepares the initially burried, isotropically rough Si/SiO2 interface while highly buffered HF (i.e., NH4F) attacks bulk silicon anisotropically. After a rapid homogenization of the chemical surface termination (HF: various hydrides, fluorine, ...) towards a perfect, unreconstructed monohydride phase, Si(111)-(1×1):H, NH4F etching leads to a time-dependent transformation of isotropic roughness into a pattern of triangular etch defects with monohydride steps perpendicular to <2̄11≳ due to a preferential removal of lower-coordinated atomic defect sites. A predominant atomic step structure due to sample miscut (vicinal surfaces with azimuth ≠<2̄11≳) can oppose the anisotropic NH4F etching: At low step density (small polar angle of miscut) a meandering of atomic steps with straight monohydride portions is observed while at high step density strong step-step interaction counterbalances anisotropic removal and forces an etching by a homogeneous flow of (nonmonohydride) steps along the macroscopic misorientation. Local findings obtained with STM are compared to macroscopically averaged results from a simultaneous quantitative analysis of low-energy electron diffraction profiles.
Mössbauer study of the amorphization process induced in α-Fe by boron implantation
Authors: Jagielski Jacek , Kopcewicz Michal/ , Thomé Lionel
Published: July 2002

Amorphization of iron by boron implantation was studied by conversion electron Mössbauer spectroscopy and Rutherford backscattering techniques. The results indicate that the amorphization process depends on the thickness of the sample. After implantation at RT, the amorphous fraction amounting to 0.85 was found for 150 nm thick iron film whereas in the bulk sample this fraction reached only about 0.65. The residual crystalline α-Fe phase persists in the sample amorphized by B implantation in a wide concentration range corresponding to nominal compositions from Fe90B10 to Fe65B35 in clear distinction from melt-spun and sputtered samples.
Thermal characteristics of alexandrite fluorescence decay at high temperatures, induced by a visible laser diode emission
Authors: Zhang Zhiyi , Grattan K. T. V. , Palmer A. W.
Published: July 2002

Resulting from an experimental study to develop a fiber-optic temperature sensor based on the fluorescence decay of Cr3+-doped crystalline alexandrite, a simple configurational coordinate model is proposed to describe the kinetics of alexandrite fluorescence over the temperature region from 20 to 700 °C, to link experimental and theoretical work. This model fits well the experimental fluorescence lifetime data over the above region and successfully explains the observed phenomenon that a sharp reduction in the fluorescence lifetime occurs beyond ∼400 °C. The model was also used to study the temperature dependence of the intensity of the fluorescence emission induced by light from a visible laser diode, and as a result it is suggested that a phonon-assisted absorption is involved in a specific absorption process, where the energy of a single incident photon is below that of the primary absorption band 4T2.
Computational complexity and the observation of physical signals
Authors: Saloma Caesar
Published: July 2002

The effects of computational complexity on the characteristics of a physical signal that is reconstructed from its representation of sampled data are analyzed. It is found that a more complex algorithm does not only require longer time to implement, but also yields an erroneous reconstruction. The reconstruction suffers from contrast degradation, phase shifts, and attenuation of details relative to the true signal. These unwanted effects are caused by the existence of spurious frequencies in the computed spectrum due to rounding-off errors. The amplitude distribution of the spurious frequencies across the spectral bandwidth strongly depends on the number of data points handled and on the complexity of the particular reconstruction algorithm employed. Since the floating point representation of numbers in a computer is always finite, an upper limit exists in the maximum number of additions or multiplications required to compute a quantity reliably without errors.
Optical and electrical characterization of an AlGaAs/GaAs heterostructure
Authors: Gray M. L. , Pollak F. H.
Published: July 2002

Photoreflectance (PR) spectroscopy and Hall-effect measurements have been used for the analysis of a molecular beam epitaxially grown AlGaAs/GaAs heterostructure. The photoreflectance spectrum provided valuable information regarding the quality of the undoped GaAs, the aluminum composition of the AlGaAs layers, impurity diffusion, and the quantum well widths. Successive layer removal aided with the identification of some photoreflectance features and provided insight into the electrical transport properties of the heterostructure. Quantum well widths obtained from PR lineshape fits are compared with layer thicknesses measured from cross-sectional transmission electron micrographs.
Dynamic properties of a superconducting quantum interference device containing arrays of Josephson junctions
Authors: Darula M. , Seidel P. , Busse F. , Beňačka Š.
Published: July 2002

The dynamic properties of a superconducting quantum interference device (SQUID) containing arrays of Josephson junctions in a superconducting loop, and in particular the case of a four-junction SQUID, is analyzed theoretically via computer simulations. It is shown that phase locking of Josephson junctions determines the dynamic behavior of the SQUID. In the case of a stable phase-lock state hysteretic I-V curves as well as unusual voltage-flux dependencies appear. The influence of a small spread in the Josephson junction parameters upon the stability of the phase-lock state is investigated in parameter space.