bluuebunny/crossref_metadata_2025 huggingface dataset:
Charge state dependence of the diffusivity of interstitial Fe in silicon detected by Mössbauer spectroscopy
Authors: Gunnlaugsson H. P. , Weyer G. , Dietrich M. ,   ISOLDE collaboration, Fanciulli M. , Bharuth-Ram K. , Sielemann R.
Published: July 2002

Interstitial Fe57m atoms excited in the 14.4 keV Mössbauer state have been created in silicon at 400–800 K as a result of the recoil imparted on these daughter atoms in the β− decay of ion-implanted, substitutional Mn57. Diffusional jumps of the interstitial Fe57m cause a line broadening in their Mössbauer spectra, which is directly proportional to their diffusivity. Thus, the charge-state-dependent diffusivity has been determined in differently doped material.
Anisotropy in breakdown field of 4H–SiC
Authors: Nakamura Shun-ichi , Kumagai Hironori , Kimoto Tsunenobu , Matsunami Hiroyuki
Published: July 2002

The breakdown fields along the 〈112̄0〉 and 〈033̄8〉 directions in 4H–SiC have been measured. For the measurements, epitaxial p+n diodes with mesa structures were fabricated on the (112̄0) and (033̄8) faces, and they showed good rectification properties and avalanche breakdown. The breakdown fields along these directions calculated from the breakdown voltage were found to be about three quarters of that along the 〈0001〉 direction in 4H–SiC. The cause of the anisotropy in breakdown field is discussed.
Real-time observations of interface formation for barium strontium titanate films on silicon
Authors: Mueller A. H. , Suvorova N. A. , Irene E. A. , Auciello O. , Schultz J. A.
Published: July 2002

Ba .5 Sr .5 TiO 3 (BST) film growth by ion sputtering on bare and thermally oxidized silicon was observed in real time using in-situ spectroscopic ellipsometry and time of flight ion scattering and recoil spectrometry techniques. At the outset of BST film deposition on silicon, an approximately 30 Å film with intermediate static dielectric constant (K∼12) and refractive index (n∼2.6 at photon energies of 1.5–3.25 eV) interface layer formed on bare silicon. The interface layer growth rate was greatly reduced on an oxidized silicon substrate. The results have profound implications on the static dielectric constant of BST.
Pattern dynamics and heat transport in rotating Rayleigh–Bénard convection
Authors: Zhong Fang , Ecke Robert
Published: July 2002

Pattern dynamics and heat transport in moderate aspect ratio (Γ=10) rotating Rayleigh–Bénard convection are discussed. Patterns are obtained from shadowgraph visualization of the temperature field. Using 2-D Fourier transform analysis of the patterns, it is demonstrated that organized patterns with one dominant roll orientation produce greater thermal heat transport than do more disordered patterns generated by the Küppers–Lortz instability and consisting of combinations of different roll orientations.
Comment on ‘‘The elastic stiffness coefficients of nickel-iron single-crystal alloys at room temperature’’
Authors: Ledbetter H. M.
Published: July 2002

We respond to the recent claim that in face-centered-cubic Fe-Ni alloys the two cubic elastic-shear coefficients, C44 and (C11−C12)/2, vary linearly with composition. Both theory and measurement support a nonlinear variation.
Simple, nondestructive silicon membrane thickness measurement using a scanning electron microscope
Authors: Raley Norman F. , Van Duzer Theodore
Published: July 2002

We present a simple, fast, and nondestructive method using an electron beam for measuring the thickness of a silicon membrane supported by adjacent silicon bulk material. The method involves determination of the incident electron-beam voltage at which primary electron current transmitted through a membrane is close to zero. This voltage is determined indirectly by observation of secondary electron contrast as a function of beam voltage; consequently, the technique is readily implemented in a conventional scanning electron microscope equipped to allow beam voltage variation. The dependence of membrane thickness on null beam voltage is in reasonable agreement with reported results based on more complicated measurements of primary electron transmission, when compared at the same value of null transmission coefficient. For null voltages from about 2 to 20 kV, silicon membrane thicknesses have been measured using this technique in the range from 0.1 to 5.0 μm with ±10% accuracy. Application to measurement of silicon dioxide–silicon membranes is discussed.
Effect of polycrystalline sublayer films on the magnetic and structural properties of CoCr films
Authors: Howard J. K. , Ahlert R. , Lim G.
Published: July 2002

The magnetic and structural properties of Co-18 at. % Cr films were evaluated for high density longitudinal recording applications. The films exhibited in-plane magnetization, high coercivity (≲1000 Oe), and hysteresis loop squareness of 0.7–0.89 when the CoCr films were sputtered on sublayer films of Cr or Cr-20 at. % Co. The coercivity was correlated with the thicknesses of the CoCr film and the sublayer, the deposition conditions, and the percentage of CoCr grains with c-axis preferred orientation in the film plane.
Characterization of silver-related deep levels in silicon
Authors: Baber N. , Grimmeiss H. G. , Kleverman M. , Omling P. , Iqbal M. Zafar
Published: July 2002

Results from a comprehensive study of silver-related deep levels in silicon are presented. Thermal emission rates and capture cross sections (along with their temperature dependence) for the two dominating Ag-related levels have been measured using deep level transient spectroscopy. For both levels the measured capture cross sections are temperature independent in the investigated temperature range. The activation energies, thus, equal the enthalpies of the corresponding thermal emission processes. Using the same samples photoionization cross sections for electron and hole emission from one of the levels and hole emission from the other have been determined in absolute values using junction photocapacitance measurements. Enthalpies obtained from thermal measurements concur with optical threshold energies to place the two Ag-related levels at EC−0.54 eV and EV+0.34 eV in the band gap of Si.
Excimer-laser-induced etching of ceramic PbTi1−xZrxO3
Authors: Eyett M. , Bäuerle D. , Wersing W. , Thomann H.
Published: July 2002

The etching of ceramic PbTi1−xZrxO3 (PZT) by XeCl excimer laser radiation has been investigated. In air, the threshold fluence for etching was about 2 J/cm2. At fluences of 10 J/cm2, etch rates of 0.1 μm/pulse were observed. The geometry of etched structures can readily be defined by choosing suitable experimental conditions, suggesting potential applications of this process to the production of devices.
Electron impact excitation cross sections for chlorine
Authors: Ganas P. S.
Published: July 2002

An analytic atomic independent particle model potential adjusted to experimental single-particle energy levels is used to generate wave functions for the valence and excited states of chlorine. Using these wave functions in conjunction with the Born approximation and LS coupling, we calculate generalized oscillator strengths and integrated cross sections up to 5 keV for various electron impact excitations from the 1s2 2s2 2p6 3s2 3p5(2P3/2) ground state.
Ultrahigh vacuum i
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i
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u transmission electron microscopy observations of molecular-beam epitaxially grown InSb(111)
Authors: Yata M. , Toda A. , Nagatsuyu H. , Hariu T. , Nakada T. , Tsukui K. , Osaka T.
Published: July 2002

Ultrahigh vacuum in situ transmission electron microscopy has been used to investigate homoepitaxial growth processes and a 2×2 surface reconstruction of InSb(111) by molecular-beam epitaxy. When the incident fluxes (1:1) of Sb4 and In1 are impinged onto the substrate, the respective molecules form homoepitaxially grown InSb films, leaving an excess molecule Sb* which does not contribute to the formation of the InSb films. As a result, there exists a critical temperature (Th) for the condensation of Sb* molecules. Below Th, the surface concentration of Sb* (nSb*) becomes higher than a critical concentration for the condensation (ncSb*), so that two phases (InSb+Sb) grow in a polycrystalline state. However, above Th as nSb* becomes lower than ncSb*, InSb films grow with the 2×2 reconstructed surfaces. This critical temperature is defined as a homoepitaxial temperature. Quantitative interpretations of Th are discussed.
Scanning tunneling microscope investigation of (100) and (001) faces of YBa2Cu3O7−δ
Authors: Niedermann Ph. , Scheel H. J. , Sadowski W.
Published: July 2002

Thin as-grown side faces, and edge and corner regions of crystals of the tetragonal precursor phase of the high-temperature superconductor YBa2Cu3O7−δ were studied by scanning tunneling microscopy under scanning electron microscope control of tip positioning. From observed changes in slope of the (100) surface, in particular near the (100)/(001) edges, it was concluded that the (100) and (001) faces have different growth mechanisms.
Gettering in silicon
Authors: Kang J. S. , Schroder D. K.
Published: July 2002

A series of gettering experiments have been carried out for a better understanding of gettering mechanism(s) in silicon. We find that oxidation and oxynitridation, which are known to inject silicon interstitials, do not getter metallic impurities such as Au, Cu, Fe, and Ni while phosphorus (P) diffusion does produce effective gettering of these metals. We also find from P diffusion, Ar ion implantation, and Ni film gettering performed as a function of temperature, there exists an optimum gettering temperature. From a comprehensive discussion of the existing models, we conclude that neither the enhanced metal solubility nor the silicon interstitial model explains our experimental results. Furthermore, it is shown that generation of dislocations is not a prerequisite for effective gettering. A model, based on the segregation of impurities at high temperatures and on the release/diffusion of metallic impurities at lower temperatures, is proposed to explain all of our results. A general form of the segregation coefficient has been developed using an extended concept of solid solubility.
On the saturation effect in the picosecond near ultraviolet laser ablation of polyimide
Authors: Chuang Mei-Chen , Tam Andrew C.
Published: July 2002

The saturation effect in the ablation rate is observed in the laser ablation of polyimide at a wavelength of 355 nm and duration of 30 ps. The fluence dependence of the reflectivity, emission intensity, and photoacoustic signal have been measured to provide some understanding of the mechanism for the saturation. A mechanism invoking the blocking of the trailing part of the light pulse by both the plume and the excited polymer generated by the leading part of the pulse explains all the observation satisfactorily. However, the possibility of blocking by only the excited polymer followed by nonstatistical disposal of energy in the ejection process cannot be ruled out.
Narrow-band microwave generation from an oscillating virtual cathode in a resonant cavity
Authors: Fazio M. V. , Hoeberling R. F. , Kinross-Wright J.
Published: July 2002

An experimental approach using a high Q, resonant cavity surrounding an oscillating virtual cathode has achieved frequency stabilization, and repeatable narrow-band operation of the virtual cathode microwave source. A cylindrical cavity resonator is used with the microwave power being extracted radially through circumferential slot apertures into dominant-mode L-band waveguide. The electron-beam/cavity interaction produces strong feedback between the induced cavity field and the oscillating virtual cathode, forcing it to lock to the resonant frequency of a cavity mode over a large variation in electron beam current. The 3-dB frequency bandwidth observed during single 100-ns pulses is less than 1%. The 3-dB bandwidth appears to be limited by the finite temporal width of the microwave pulse.
Tungsten-carbon multilayer composition and the effects of annealing: A glancing angle extended x-ray absorption fine structure study
Authors: Lamble G. M. , Heald S. M. , Sayers D. E. , Ziegler E. , Viccaro P. J.
Published: July 2002

Results are presented from a glancing angle extended x-ray absorption fine structure study to investigate the structure of W/C multilayers and the effects of moderate annealing. There is evidence of considerable interlayer mixing prior to any heat treatment. It is found that after annealing to 350 °C for several hours, drastic changes can occur in the structural environment of the W atoms, with minimal changes in the reflectivity of the sample. There is extensive formation of W2C depending on the W thickness and the W/C ratio. Identification of this carbide as the product of annealing is unambiguous. Above a critical thickness, W begins to grow in its bcc form within the W layer, as it is prepared. This bcc layer is unaffected by the annealing process. The presence of carbon is shown to stabilize the amorphous state of W and inhibits any formation of W2C when the W thickness and W/C ratio are small.
Characterization of sub-10-μm 30-ps flash duration point sources for x radiography
Authors: Failor Bruce H. , Gabl Edward F. , Johnson R. R. , Shepard Chester
Published: July 2002

Flash x radiography can provide important information about high-density, high-opacity plasmas. Experiments at KMS investigated the characteristics of point x-ray sources produced when coated fibers, wires, and mounted particles were irradiated with high intensity (I>1016 W/cm2) green laser light. The target materials Bi, CsI, Au, and Ag were evaluated in the areas of (1) the spatial and temporal resolutions that can be achieved, (2) the effect of source size and laser pulse duration on x-ray signal strength, and (3) the practical considerations associated with using the technique to actually obtain radiographs of accelerated polyvinyl alcohol shells.
A model of dc glow discharges with abnormal cathode fall
Authors: Schoenbach Karl H. , Chen Hao , Schaefer G.
Published: July 2002

A model for an abnormal glow discharge, including a self-consistent analysis of the cathode fall, was developed. It combines microscopic particle simulation by means of Monte Carlo methods with a fluid model of the gas discharge. The model allows calculations of the steady-state electrical field distribution, the charged-particle densities, and the current densities along the axis of the discharge. The model was used to simulate a glow discharge in 80% He and 20% SF6 at a pressure of 8 Torr with a current density of 1 A/cm2. The computed discharge voltage agrees well with measured values. The computer code can easily be modified to describe the charged-particle densities and energies not only in the cathode fall region, but in any plasma boundary layer.
The substitution of cobalt for nickel in stoichiometric nickel-zinc ferrite
Authors: Hahn H. T.
Published: July 2002

In the past, cobalt (II) ions have been added to nickel-zinc ferrites to capitalize on the anisotropic character of Co+2 in stabilizing domain walls and reducing magnetic losses. This paper emphasizes increasing losses. The effect of Co+2 on the real permeability (μ′) and magnetic loss (μ″) when cobalt was sequentially substituted for nickel in stoichiometric nickel-zinc ferrite was measured at 50–1500 MHz. The compositions examined were included in the formulation CoxNi0.5−xZn0.5Fe2O4. in which the cobalt atom fraction x took values from x=0.032 to 0.50. Air density of the measured toroids decreased from 4.52 g/cm3 at x=0.032 to 4.28 g/cm3 at x=0.50. A maximum for μ″ was found in the region x=0.03–0.04, depending upon the frequency. This increase was marked at the lower frequencies, but it essentially disappeared by 800 MHz. The magnetic loss spectrum for cobalt-zinc ferrite was described by a single relaxation dispersion with a relaxation time of 6.22×10−10 s. As nickel replaced cobalt, the relaxation time increased to 14.9×10−10 s at x=0.032. An earlier analysis of nickel-zinc ferrite (x=0) yielded a relaxation time of 11.9×10−10 s, confirming the approximate coincidence of maxima in both μ″ and relaxation time. The fact that μ″ decreases at x≳0.04 (as cobalt replaces nickel) does not mean that the reflected attenuation must decrease. Sample thickness also influences the result. For shorted samples 0.250 in. thick, the replacement of nickel by cobalt increased attenuation despite decreasing values of μ″, and shifted the frequency of maximum attenuation to higher values.
Defect structure and phase transitions in epitaxial metastable cubic Ti0.5Al0.5N alloys grown on MgO(001) by ultra-high-vacuum magnetron sputter deposition
Authors: Adibi F. , Petrov I. , Hultman L. , Wahlström U. , Shimizu T. , McIntyre D. , Greene J. E. , Sundgren J.-E.
Published: July 2002

Ti0.5Al0.5N alloy films, typically 1.5 μm thick, were grown on MgO(001) at temperatures Ts between 400 and 850 °C by ultra-high-vacuum reactive magnetron sputtering in pure N2. Films grown at Ts between ≂480 and 560 °C were single crystals in which the lattice misfit strain was partially relieved by glide of 〈001〉 misfit dislocations, with Burgers vector =a0/2〈011〉, on {011̄} planes. Cross-sectional transmission electron microscopy investigation showed no evidence of residual extended defects in the films until thicknesses of ≂150 nm at which point threading dislocations, oriented along the [001] growth direction, were observed. Surface-initiated spinodal decomposition, resulting in the formation of compositionally modulated NaCl-structure platelets along [001] with width ≂1 nm, occurred over a narrow growth temperature range between 540 and 560 °C as a precursor to bulk phase separation of wurtzite-structure AlN at Ts≥560 °C. The alloy was continuously depleted of AlN at higher growth temperatures until the equilibrium two-phase structure, cubic TiN and wurtzite AlN, was obtained at Ts≥750 °C.
Measurement of nematic liquid crystal splay and bend elastic constants with obliquely incident light
Authors: Ong Hiap Liew
Published: July 2002

With the use of an obliquely incident light propagating in the plane containing the liquid crystal director, a simple optical method is presented to study the planar oriented liquid crystal cell properties. This method is very useful for the measurement of the liquid crystal splay and bend elastic constants. Simple analytic formulas are obtained for the optical responses in the parallel and homeotropic oriented nematic liquid crystal cells. The results are applied to the magnetic field-, electric field-, and optical field-induced Freedericksz transitions.
Ferro and paramagnetic resonance studies of natural volcanic glasses: Teotihuacan obsidians
Authors: Chávez-Rivas F. , Zamorano-Ulloa R. , Galland D. , Regnard J. R. , Chappert J.
Published: July 2002

Magnetic resonance spectra of iron and manganese in black obsidians from Teotihuacan (Central Mexico) were investigated at 9 GHz (X band) and 35 GHz (Q band). The specimens were found to be magnetically very heterogeneous. The X band spectrum exhibited, at g=4.28, the classical paramagnetic signal of isolated high spin Fe(III) ions. At g=2.0 a ferromagnetic resonance occured, associated with magnetite-like particles previously characterized by Mössbauer spectroscopy. At Q band there are two signals, a superparamagnetic signal at g=2.00 and a weak hyperfine sextet, superimposed on the g≂2.00 signal, attributed to isolated Mn(II) free ions. At X band a strong grain-size dependence of the overall line shape was also observed. The temperature, frequency, and grain-size dependence of the spectral lines of these natural glasses indicate, in addition to isolated high spin Fe(III) and Mn(II) ions, the presence of superparamagnetic clusters of magnetite and a range order, in these obsidians, larger than the common short-range order of the glassy state.
Nematic director orientation in a liquid-crystal-dispersed polymer: A deuterium nuclear-magnetic-resonance approach
Authors: Stannarius R. , Crawford G. P. , Chien L. C. , Doane J. W.
Published: July 2002

The nuclear-magnetic-resonance (NMR) technique is employed to liquid-crystal-dispersed-polymer (LCDP) systems. Deuterium NMR is used to determine director distributions in LCDP cells in the presence of a magnetic field. The samples consist of a nematic liquid crystal mixed with small percentages of a monomer that is polymerized under different conditions after dissolving it in the liquid crystal. NMR spectra of the deuterated liquid-crystal molecules give information on orientation and order in such systems. The orientation of the polymer skeleton formed during the polymerization is found to be stable. It determines the orientation of the nematic director even in the presence of a strong external magnetic field. Simple models for the director field are presented.
Lateral coupling between a silicon-oxinitride waveguide and an amorphous Si photodiode
Authors: Kapser K. , Deimel P. P.
Published: July 2002

We have measured the absorption coefficient at λ=850 nm for an amorphous Si (a-Si:H) p-i-n photodiode laterally coupled to a silicon-oxinitride single mode planar waveguide both for TE and TM modes. The experimental results are compared to calculations considering a five-layer system with a complex index of refraction for the absorbing layer. Theoretical and experimental results agree well considering the inaccuracies in the determination of the layer thicknesses.
Interaction of an edge dislocation with an interfacial crack
Authors: Zhang Tong-Yi , Li J. C. M.
Published: July 2002

The stress and displacement fields of an edge dislocation near a semi-infinite or a finite interfacial crack are formulated by using the complex potential theory of Muskhelishvili’s elasticity treatment of plane strain problems. The image forces exerted on the dislocation have an oscillatory character (with respect to the dislocation position) if the dislocation is originated elsewhere and moves to the vicinity of a finite interfacial crack. There is no such oscillation of image forces if the edge dislocation is emitted from the finite interfacial crack or if the crack is semi-infinite. The stress intensity factors produced by the edge dislocation also have an oscillatory character for both semi-infinite and finite interfacial cracks. They also depend on whether the dislocation is emitted from the crack or comes from elsewhere.
Well-width dependence of the excitonic lifetime in strained III-V quantum wells
Authors: Amand Th. , Marie X. , Dareys B. , Barrau J. , Brousseau M. , Dunstan D. J. , Emery J. Y. , Goldstein L.
Published: July 2002

The dependence of the excitonic lifetime on the well width in InGaAs/InP (unstrained), InGaAs/GaAs, and InGaAsP/InP strained quantum well heterostructures is investigated by time-resolved photoluminescence spectroscopy. We show that the main contribution to the lifetime variation comes from the change of the exciton binding energy. In the strained InGaAs/GaAs wells, the widest wells have a shorter lifetime than expected. We attribute this to the onset of plastic relaxation.
Photoluminescence characterization of GaAs quantum well laser structure with AlAs/GaAs superlattices waveguide
Authors: Dotor M. L. , Recio M. , Golmayo D. , Briones F.
Published: July 2002

Dependence on the excitation power and temperature of the photoluminescence emission of GaAs quantum well laser structures using GaAs/AlAs superlattices in the waveguide is reported. The effects related to a quality reduction due to the presence of a thick ternary AlGaAs cladding layer in the bottom of the laser structure were elucidated by comparing to the photoluminescence of a similar waveguide structure, except for the AlGaAs bottom layer. The excitation power dependence shows the strong excitonic origin of the light emission in the temperature range 4–300 K in both structures. Carrier transport mechanisms through the superlattices is analyzed from the evolution of the photoluminescence of the quantum well and the superlattice confining layers; a structure dependent transparency temperature is defined, at which transport changes from tunneling assisted to extended minibands regime. The value of this parameter depends on the localized states in the superlattice minibands, caused by interface roughness.
Analytical model for the out-diffusion of an exponentially decaying impurity profile. Application to nitrogen in silicon
Authors: Willems G. J. , Maes H. E.
Published: July 2002

Analytical solutions of Fick’s one-dimensional diffusion equation for a semi-infinite medium with an exponentially decaying initial impurity concentration profile and different boundary conditions are presented. The properties of the solution for a constant surface concentration are discussed more extensively. The theoretical results are applied to the diffusion of nitrogen incorporated in silicon by laser melting. The diffusion coefficient of nitrogen in silicon near its melt temperature is estimated to be of the order of 1×10−6 cm2/s. This confirms the recently reported high diffusion coefficient values for nitrogen in silicon.
Characteristics of implantation-induced damage in GaSb
Authors: Callec R. , Poudoulec A.
Published: July 2002

The production and annealing of radiation damage in GaSb Ne-implanted at high energy are studied by transmission electron microscopy and Rutherford backscattering spectrometry in combination with the channeling technique. The anomalous swelling phenomenon of implanted GaSb, previously reported, is found to be related to the formation of voids and microtwins in the crystalline implanted layer. These defects appear when the introduced damage exceeds a critical amount and lead to the formation of a porous polycrystalline GaSb layer, whereas in most other III-V materials an amorphous layer is formed. Provided the swelling is avoided, there is a good recovery of the lattice upon rapid thermal annealing at 600 °C. Otherwise, a heavily perturbed layer remains, containing voids, dislocations and, at sufficiently high doses, polycrystalline GaSb. From previously published results, it is inferred that the InSb behavior towards radiation damage is similar to that of GaSb.
Correlations between ultrathin film microstructure and magnetic properties for room temperature epitaxial films of fcc Fe/Cu(100)
Authors: Heim K. R. , Healy S. D. , Yang Z. J. , Drucker J. S. , Hembree G. G. , Scheinfein M. R.
Published: July 2002

Correlation studies between thin film nanostructure and macroscopic magnetic properties in ultrathin fcc Fe films grown epitaxially on room temperature Cu(100) substrates were performed in situ using an ultrahigh vacuum scanning transmission electron microscope and the surface magneto-optic Kerr effect. Nanometer lateral spatial resolution secondary electron microscopy revealed no gross morphological changes in the 2–10 monolayer thickness range. The use of broad-beam Auger electron spectroscopy as an indicator of Cu surface cleanliness is shown to have insufficient sensitivity to detect surface contamination as evidenced by corresponding secondary electron micrographs. Cu(100) surfaces with both (nearly) perfect and imperfect surface structure, and identical Fe coverages, possess nearly identical polar and longitudinal Kerr hysteresis loops. Analysis of reflection high-energy electron diffraction patterns confirms that Fe films grown on room temperature Cu(100) remain fcc with the same in-plane lattice constant as the Cu template, for thicknesses up to 10 ML.
1.54 μm wavelength emission of highly Er-doped CaF2 layers grown by molecular-beam epitaxy
Authors: Daran E. , Legros R. , Muñoz-Yagüe A. , Fontaine C. , Bausá L. E.
Published: July 2002

CaF2:Er layers have been grown by molecular-beam epitaxy on (100)-oriented CaF2 substrates; the Er concentration ranges from 1% to 50% (mole fraction). The 1.54 μm emission observed under excitation around 800 nm was studied by photoluminescence. Up to 35% Er concentration the integrated emission increases monotonously, quenching appearing for higher doping levels. Photoluminescence results are discussed within the framework of previous studies of Er3+ emission in the near-infrared range (830–860 nm) in order to gain insight into the Er centers involved in the 1.54 μm emission.
Acoustic transient generation by holmium-laser-induced cavitation bubbles
Authors: Asshauer T. , Rink K. , Delacrétaz G.
Published: July 2002

The acoustic effects of free-running 2.12 μm Cr:Tm:Ho:YAG laser pulses delivered in water are studied. Laser pulses of 10 to 1200 mJ energy and 230 μs duration (full width at half-maximum) are used. Delivery fiber diameters of 200–600 μm are investigated. Combined fast flash video imaging and needle probe hydrophone pressure sensing are used. The experimental results show that the laser-induced water vapor bubbles can generate strong acoustic transients at the bubble collapse several hundreds of μs after the start of the laser pulse. Pressures of up to 3600 bar are measured. Above a laser fluence threshold of 40 J/cm2 the pressure amplitude increases sharply, reaching a maximum value between 100 and 200 J/cm2. At higher fluences up to more than 1000 J/cm2, the pressure amplitude is found to decrease again. A two-phase mechanism is proposed to describe the complex bubble dynamics generated by the free-running pulses: The isotropic expansion of an initially superheated water volume is followed by a continuous ablation phase. The results suggest a mechanism of possible unwanted acoustic damage during Holmium laser medical applications in a liquid environment.
Ion channeling studies of epitaxial Fe and Co silicides on Si
Authors: Schwarz C. , Onda N. , Goncalves-Conto S. , Sirringhaus H. , von Känel H. , Pixley R. E.
Published: July 2002

High quality epitaxial Co and Fe silicides have been grown by molecular beam epitaxy on Si(111) and Si(001) substrates with film thicknesses ranging between 25 and 8400 Å. We used Rutherford backscattering spectrometry channeling techniques to measure the lattice distortion as a function of film thickness. The critical thickness hc corresponding to the film thickness at which strain relieving dislocations begin to appear was determined for CoSi2 on Si(111) and Si(001) as well as for Si on CoSi2(111). For CoSi2 on Si(001), a larger critical thickness was obtained than on Si(111), where hc is ∼45 Å. Epitaxial Si on CoSi2(111) was found to be under a compressive strain up to thicknesses of about 350 Å depending on substrate misorientation. Strain measurements were also performed on epitaxially stabilized Co and Fe monosilicides with the CsCl structure. Channeling measurements on thick epitaxial films of bcc-Fe, Fe3Si, FeSi, and Fe0.5Si were used to determine the crystalline quality. Excellent channeling minimum yields of 4.0% were found for bcc-Fe/Si(111). The results are compared with structural information obtained from x-ray diffraction and Brillouin scattering spectroscopy.
Spatial and temporal evolution of the electric field in a longitudinally electric-discharge-pumped gas laser: Application to a large-bore copper vapor laser
Authors: Blau Pinhas
Published: July 2002

Calculations of the electric field in the excitation pulse of a 8-cm-diam copper-vapor laser are presented. The electric field evolution is governed by a diffusion equation. Both radial and axial diffusions are considered. The boundary conditions are determined by the geometry of the tube, which is coaxially shielded by a metal sleeve, considering the tube inductance and parasitic capacitance. The diffusion coefficients are determined by the electrical conductivity of the plasma that is calculated based on measured plasma parameters. These calculations are essential both for modeling the excitation process and for estimation of the overall tube impedance in such lasers. Inadequate penetration of the electric field into the plasma because of high pre-pulse conductivity turns out to be a main cause of power and pulse repetition rate limitations of this laser. The optimization of the tube geometry in order to minimize the field penetration problem is discussed.
Piezoresistivity modeling of grain-boundary junctions in positive temperature coefficient of resistivity BaTiO3
Authors: Alles Aldo B. , Murphy Michael W. , Symanski Jesse J. , Tremper Christine L. , Schulze Walter A.
Published: July 2002

A phenomenological method of modeling positive temperature coefficient of resistivity junctions under different thermal and elastic boundary conditions as well as different interfacial trap distributions is detailed. Devonshire’s thermodynamic approach and a modified Heywang–Jonker model are employed. Model calculations are applied to isothermal empirical results obtained in samples under uniaxial compressive loads as well as four-point bending loads of trilaminate sensors. Some explanation of discrepancies found in the literature in sign and in magnitude of piezoresistive coefficients under uniaxial loads is offered. Trilaminate sensor behavior under four-point bending loads followed satisfactorily the general trends predicted by the model.
Determination of conduction band tail and Fermi energy of heavily Si-doped GaAs by room-temperature photoluminescence
Authors: Lee Nam-Young , Lee Kyu-Jang , Lee Chul , Kim Jae-Eun , Park Hae Yong , Kwak Dong-Hwa , Lee Hee-Chul , Lim H.
Published: July 2002

A line-shape analysis of room temperature photoluminescence (PL) spectra was carried out on Si-doped GaAs samples grown by molecular beam epitaxy. The electron concentration n of the samples ranges from 1.0×1017 to 4.2×1018 cm−3. It was found that the conduction band tail ηc and the Fermi energy εf measured from the conduction band minimum can be expressed as ηc=2.0×10−8n1/3(eV) and εf=−0.074+1.03×10−7n1/3(eV), respectively. The PL peak energy, at which the electron concentration per unit energy in the conduction band is maximum, can also be expressed as 1.426+2.4×10−14n2/3(eV).
Measurement of the refractive indices of a ferroelectric liquid crystal
Authors: Yang Fuzi , Sambles J. R. , Bradberry G. W.
Published: July 2002

Optical excitation of half-leaky guided modes has been used to determine the refractive indices of a ferroelectric liquid crystal (Merck-SCE13) in a homeotropically aligned state. The uniform homeotropic alignment is realized, with no surface treatment, by the application of an in-plane dc electric field. This applied field, of the order of 5×105 V m−1 is sufficient to fully unwind the S*c helix giving a uniformly tilted homeotropic monodomain for optical characterization. Analytic and numerical modeling results indicate that, for a slab with its optic axis tilted in a plane orthogonal to the plane of incidence, two distinct critical angles appear in the half-leaky guided mode response. These independently relate simply to the ordinary and extraordinary refractive indices of the S*c material. By fitting theoretical angle dependent reflectivities to those recorded experimentally the two refractive indices have been obtained for a range of temperatures in both the S*c and SA phases.
In situ measurements of changes in the structure and in the excess charge-carrier kinetics at the silicon surface during hydrogen and helium plasma exposure
Authors: Neitzert H. C. , Layadi N. , Roca i Cabarrocas P. , Vanderhaghen R. , Kunst M.
Published: July 2002

The damage induced by hydrogen and helium plasmas at the surface of crystalline silicon has been monitored in situ by time-resolved microwave conductivity and by spectroscopic ellipsometry measurements. Both plasma treatments increase the decay rate of the optically generated excess charge carriers and decrease the amplitude of the microwave reflection transients. While for the helium plasma a high density of electronic defects is created immediately after plasma ignition, a continuously increasing number of recombination centers is observed in the case of the hydrogen plasma exposure. In support of the transient microwave measurements, the analysis of the spectroscopic ellipsometry measurements reveals the creation of a damaged surface layer, which in the case of the helium plasma exposure has a high and in the case of the hydrogen plasma a low fraction of amorphous silicon. This can be explained by the different nature of the processes involved in the interaction of hydrogen (chemical) and helium (physical) plasmas with the silicon surface. After a constant plasma exposure time the damaged surface layer is thicker in the case of the hydrogen plasma exposure. Moreover, the helium plasma treatment produces a more defective overlayer as deduced from the faster decay of the transient microwave signals.
Ga-concentration dependence of magnetocrystalline anisotropy in Gd2Fe17−xGax compounds
Authors: Cheng Zhao-hua , Shen Bao-gen , Liang Bing , Zhang Jun-xian , Wang Fang-wei , Zhang Shao-ying , Zhao Jian-gao , Zhan Wen-shan
Published: July 2002

The structure and magnetic properties of arc-melted Gd2Fe17−xGax compounds were studied by means of x-ray diffraction and magnetization measurements. X-ray diffraction patterns demonstrate that all samples have a rhombohedral Th2Zn17-type structure and the substitution of Ga leads to an approximately linear increase in the unit cell volumes. The Curie temperature is found to first increase and then decrease with increasing Ga concentration. It is noteworthy that the substitution of Ga has a significant effect on the magnetic anisotropy of iron sublattice. The easy magnetization direction of Gd2Fe17−xGax compounds changes from basal plane to c axis with increasing Ga concentration. The sample with x=7 exhibits a uniaxial anisotropy.
On the formation and loss of S2 molecules in a reactive ion etching reactor operating with SF6
Authors: St-Onge L. , Sadeghi N. , Booth J. P. , Margot J. , Barbeau C.
Published: July 2002

Laser-induced-fluorescence (LIF) spectroscopy was used to study, with spatial and temporal resolution, the processes by which diatomic sulfur S2 is formed and lost in SF6 plasmas. We present results concerning the relative S2 number density in steady-state or pulsed discharges in a reactive ion etching (RIE) reactor operated at different SF6 gas pressures and RF powers, in the presence or absence of a silicon wafer. It is found that S2 is formed mainly on surfaces under conditions when the F-atom density is high, but that volume formation can also occur when the F-atom population is depleted (namely, when Si is present). It is also shown that loss of S2 is mainly due to diffusion out of the inter-electrode space to the main reactor volume, excluding electron-impact dissociation processes. It is apparent that, in a RIE reactor, the only electron process pertinent to the balance of S2 density is the formation of its precursors (probably S atoms and SF molecules) by fragmentation of the SF6 gas. The remaining reactions controlling the density of S2 are neutral-neutral interactions in the volume and on surfaces.
Excitonic emissions from hexagonal GaN epitaxial layers
Authors: Chichibu S. , Azuhata T. , Sota T. , Nakamura S.
Published: July 2002

Excitonic photoluminescence (PL) peaks from hexagonal GaN epilayers were investigated making a connection with the analysis of the photoreflectance spectra. Free exciton emissions associated with transitions from the conduction (Γ7c) band to the A (Γ9v) and B (Γ7uv) valence bands are dominant above 100 K. Values of the full widths at half maximum of them were smaller than the thermal energy kBT up to room temperature, which suggests the dominance of excitons in the PL spectra.
Dominant mechanism for limiting the maximum operating temperature of InP-based multiple-quantum-well lasers
Authors: Seki Shunji , Oohasi Hiromi , Sugiura Hideo , Hirono Takuo , Yokoyama Kiyoyuki
Published: July 2002

We study the basic mechanism for limiting the maximum operating temperature (Tm) of InP-based multiple-quantum-well (MQW) lasers emitting at a wavelength of 1.3 μm. It is shown that laser operation is characterized in terms of the temperature dependence of threshold gain (gth) by introducing a critical temperature (Tc): gth exhibits a linear relationship with temperature below Tc, while it superlinearly increases with increasing temperature above Tc. This rapid increase in gth leads to a marked increase in threshold current and a significant reduction in differential quantum efficiency above Tc. We indicate that Tc exhibits a direct correlation with Tm: the higher Tc, the higher Tm. In the temperature range above Tc, laser operation moves into a loss-multiplication regime, where a considerable portion of the injected carriers brings about a significant increase in internal loss rather than gain due to their pileup in the separate confinement heterostructure layers. We demonstrate that an anomalous increase in internal loss, which occurs at the final stage in this loss-multiplication regime, determines Tm of 1.3 μm InP-based MQW lasers.
Influence of interdiffusion processes on optical and structural properties of pseudomorphic In0.35Ga0.65As/GaAs multiple quantum well structures
Authors: Bürkner S. , Baeumler M. , Wagner J. , Larkins E. C. , Rothemund W. , Ralston J. D.
Published: July 2002

Interdiffusion has been investigated in molecular-beam epitaxially (MBE)-grown, highly strained In0.35Ga0.65As/GaAs multiple quantum well (MQW) structures. Thermal intermixing and impurity-free interdiffusion (IFID) was induced via rapid thermal annealing (RTA) at temperatures between 700 and 950 °C using GaAs proximity caps and electron-beam evaporated SiO2 cap layers, respectively. Both reduced photoluminescence (PL) linewidths and increased PL intensities were observed following interdiffusion-induced band-gap shifts ranging from 6 to 220 meV. PL microscopy (PLM) investigations were utilized to study the onset of strain relaxation due to dislocation generation. Two types of line defects were found in the proximity-cap annealed samples, depending on the annealing temperature and the number of QWs: misfit dislocations with the dislocation lines parallel to 〈110〉 directions and 〈100〉-oriented line defects. No dislocations were observed in the SiO2-cap annealed samples over the entire temperature range investigated here. Resonant Raman scattering measurements of the 1LO/2LO phonon intensity ratio were used for a semiquantitative assessment of the total defect densities, including point defects (PDs). Whereas increasing PD densities and the formation of line defects were observed in the proximity-capped samples as the annealing temperature was increased, no deterioration of the structural quality due to an increased PD density was observed in the case of the SiO2-cap annealed samples.
Nuclear magnetic resonance study of R2Fe17 (R=Y, Sm, and Gd) hydrides
Authors: Shen N. X. , Zhang Y. D. , Budnick J. I. , Hines W. A. , Binninger U.
Published: July 2002

In order to study the location of hydrogen atoms and the effects of their insertion into R2Fe17, spin–echo NMR experiments have been carried out on the hexagonal Y2Fe17Hx (x=0, 3.0, 4.7), rhombohedral Sm2Fe17Hx (x=0, 1.7, 5.7), and mixed-phase Gd2Fe17Hx (x=0, 2.3, 5.8) compounds. 1H and 89Y spectra obtained from Y2Fe17Hx clearly demonstrate that the hydrogenation process is reversible upon vacuum annealing. For both Y2Fe17Hx and Sm2Fe17Hx, the 1H spectra show two broad peaks; the peaks are tentatively assigned to H atoms in the tetrahedral and octahedral interstitial sites, and a hydrogen filling scheme is proposed. For Gd2Fe17Hx, a single broad 1H peak near 70 MHz is observed; application of an external magnetic field indicates that the hyperfine field has the same direction as the net magnetization.
Electroluminescence from polyurethanes with emissive and charge-transporting chromophores and their blends
Authors: Kim Dong Uk , Tsutsui Tetsuo
Published: July 2002

Polyurethanes with a diphenylamino-substituted 1,4-bisstyrylbenzene unit for an emissive and charge-transport chromophore were prepared. Single-layer electroluminescent (EL) diodes were made of spin-coated polyurethane films. An indium-tin-oxide (ITO) anode and a magnesium-silver alloy cathode were used. The EL diode made of the polyurethane exhibited the maximum luminance of 35 cd/m2 at 570 mA/cm2 at applied voltage of 26 V. When cyano groups were attached to the 1,4-bisstyrylbenzene unit, the maximum luminance of 1000 cd/m2 for 870 mA/cm2 at voltage of 26 V was attained. The external quantum efficiency of the latter device was calculated to be 0.035%. The emission peaks of these two devices were 505 and 590 nm. Two polyurethanes were found to be mixed homogeneously and EL diodes were fabricated using blend films.
Fluorescence and photoelectrochemical behavior of chlorophyll a adsorbed on a nanocrystalline SnO2 film
Authors: Bedja Idriss , Kamat Prashant V. , Hotchandani Surat
Published: July 2002

Fluorescence and photoelectrochemical studies of chlorophyll a (Chl a) adsorbed on nanocrystalline SnO2 film were carried out. The results of fluorescence and incident photon to current conversion efficiency (IPCE) as a function of applied bias suggest that the fluorescence quenching and the photocarrier generation are interrelated. Fluorescence quenching has thus been utilized to determine the photogeneration efficiency, η(e), of charges in a SnO2/Chl a based photoelectrochemical cell. A value of 0.75 was obtained for η(e) for unbiased cells. With an IPCE of 13%, η(e) of 75%, and a light harvesting efficiency of 70%, the charge collection efficiency of ∼23% was evaluated. These results suggest that the losses due to the charge recombination are a major factor that limit the efficiency of the cells.
Bistable reflective cholesteric liquid crystal display
Authors: Lu Min-Hua
Published: July 2002

A new type bistable reflective cholesteric liquid crystal display is reported. The texture and the relaxation process are very different from other types of the reflective cholesteric displays reported previously. The contrast ratio of the display is about 2.5 times higher than regular cholesteric displays. The response time of the display is 10 ms, which is much faster than the 230 ms response of regular cholesteric displays made with the same liquid crystal materials. A model to explain the unique textures of the display is presented.
Limiting electrical response of conductive and dielectric systems, stretched-exponential behavior, and discrimination between fitting models
Authors: Macdonald J. Ross
Published: July 2002

Given a fitting model, such as the Kohlrausch–Williams–Watts (KWW)/stretched-exponential response, three plausible approaches to fitting small-signal frequency or time-response data are described and compared. Fitting can be carried out with either of two conductive-system formalisms or with a dielectric-system one. Methods are discussed and illustrated for deciding which of the three approaches is most pertinent for a given data set. Limiting low- and high-frequency log–log slopes for each of the four immittance levels are presented for several common models; cutoff effects are considered; and an anomaly in the approach to a single-relaxation-time Debye response for one of the conductive-system approaches is identified and explained. It is found that the temporal response function for the most appropriate conductive-system dispersion (CSD) approach, designated the CSD1, one long used in approximate form for frequency-response data analysis, does not lead to stretched-exponential transient behavior when a KWW response model is considered. Frequency-domain fitting methods and approaches are illustrated and discriminated using 321 and 380 K Na2O–3SiO2 data sets. The CSD1 approach using a KWW model is found to be most appropriate for fitting these data exceedingly closely with a complex nonlinear least-squares procedure available in the free computer program LEVM. Detailed examination and simulation of the approximate, long-used CSD1 modulus fitting formalism shows the unfortunate results of its failure to include separately the effects of the always present high-frequency-limiting dielectric constant, εD∞. The stretched-exponential exponent, β, associated with this fitting approach has always been misidentified in the past, and even after its reinterpretation, the result is likely to be sufficiently approximate that most physical conclusions derived from such fitting will need reevaluation.