bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical evaluation of electron-beam-excited vacuum-ultraviolet F2 lasers
Authors: Kim Yong-Pyung , Obara Minoru , Suzuki Tokio
Published: July 2002

A theoretical kinetic model for an electron-beam-excited F2 laser (157 nm) was successfully developed to evaluate the performance characteristics in terms of electron-beam excitation rate, pumping pulse width, and total operating pressure. As a result, it is made clear that a high excitation rate (above 0.2 MW/cm3 atm) is essential to obtain efficient laser operation. An intrinsic laser efficiency of 4.3% is obtainable when a 6.5-atm mixture of He/F2=1000/1 pumped at an excitation rate of 0.5 MW/cm3 atm, giving a laser output of over 4 J/l. It is also found that a higher laser output is obtainable with increasing total operating pressure, while the intrinsic laser efficiency slowly decreases.
Relative carrier densities and trap effects on the properties of CdS/CdTe
Authors: Fortmann Charles M. , Fahrenbruch Alan L. , Bube Richard H.
Published: July 2002

It is shown that the open-circuit voltage of CdS/CdTe heterojunction solar cells is a maximum when the carrier densities in the CdS and CdTe are approximately equal, because of the effects of a level lying 0.45 eV below the conduction band in CdTe on the junction transport. Cells investigated include those prepared by vacuum evaporation of CdS on thin-film CdTe, deposited by hot-wall vacuum evaporation on graphite substrates and on single-crystal p-CdTe:P substrates, and on single-crystal p-CdTe:P substrates. The existence of a level in the CdTe lying 0.45 eV below the conduction band is identified by measurements of the extrinsic photoresponse of CdS/CdTe junctions, and a model using this level defining junction transport is consistent with the dark and light current versus voltage relationships observed. This model involves two recombination paths: (1) depletion-layer recombination in the CdTe through the 0.45-eV level, which dominates when the electron density in the CdS in much larger than the hole density in the CdTe, and (2) tunneling of electrons from the CdS conduction band to the 0.45-eV level in the CdTe, followed by recombination, which dominates in other cases.
A phenomenological model for surface deposition kinetics during plasma and sputter deposition of amorphous hydrogenated silicon
Authors: Kushner Mark J.
Published: July 2002

The surface processes during the plasma-enhanced chemical vapor deposition and reactive sputter deposition of amorphous hydrogenated silicon (a-Si:H) are investigated by use of a phenomenological model. The model consists of an accounting, in rate equation form, of adsorption of radicals from the plasma onto the surface, surface diffusion, incorporation into the lattice, interconnection of bonds in the lattice, and burial of species on the surface, thereby constituting film growth. By accounting for the coordination partners of Si atoms in the film, the atomic fraction of hydrogen in the film is computed for the lattice and for hydrogen in polymeric or isolated configurations. Results from the model are discussed while parametrizing the probability for hydrogen elimination during incorporation and the probability for saturation of dangling bonds by gas phase species. We find that the mode of hydrogen elimination during incorporation distinguishes films grown dominantly from SiH2 or SiH3 radicals. Characteristics of films grown by sputter deposition are investigated as a function of the composition of the radical flux. We find that films grown from hydrogen-rich fluxes are composed dominantly of dihydride (Si HH) configurations, whereas hydrogen-lean mixtures are composed of dominantly hydride ( 3/4 Si–H) configurations.
Dynamics of a z pinch for focusing high-energy charged particles
Authors: Dothan F. , Riege H. , Boggasch E. , Frank K.
Published: July 2002

The dynamics of a linear z pinch is studied. Through the pinched plasma column of length 270 mm and diameter up to 40 mm flows a current of maximum 400 kA; the resulting azimuthal magnetic field, spatially and temporally stable for up to 0.5 μs, shall be used as a focusing lens for a charged particle beam. The discharge current and voltage were measured, as well as the magnetic field strength at different radii. The current density as a function of radius and time was derived, and also the velocity and width of the imploding current layer. Current loops (‘‘inductons’’), excited near the maximum contraction moment, were found; they lead to a favorable amplification of the pinch current and of the magnetic field in the column. By choosing aluminum oxide as the wall material and hydrogen as the filling gas the formation of harmful wall currents can be avoided.
Monte Carlo calculation of velocity-field characteristics in GaInAs/InP and GaInAs/AlInAs single-well heterostructures
Authors: Yoon K. S. , Stringfellow G. B. , Huber R. J.
Published: July 2002

Two-dimensional scattering rates are formulated using a two-subband triangular well approximation for GaInAs/InP and GaInAs/AlInAs single-well heterostructures, and two-dimensional electron gas (2-DEG) velocity characteristics are calculated using one-particle Monte Carlo simulations. Although the alloy scattering in GaInAs limits the low field mobility at low temperatures, a significant increase in the velocity characteristics is obtained for the 2-DEG at both 77 and 300 K. This result clearly illustrates that GaInAs is a promising material for high speed devices at 300 K and even at 77 K.
X-ray lithography with a Ag-Se/Ge-Se inorganic resist using synchrotron radiation
Authors: Saito Kunio , Utsugi Yasushi , Yoshikawa Akira
Published: July 2002

A Ag-Se/Ge-Se inorganic resist is applied in x-ray lithography using synchrotron radiation (SR). Usable sensitivity of 0.3–3 times that of a polymethylmethacrylate (PMMA) resist and high contrast (γ∼3.5) are obtained through SR exposures. By utilizing the fringes of masked SR flux caused by Fresnel diffraction, 500-Å-wide fine lines are formed by using x rays of 2–10 Å in wavelength. It is determined that a high-density resist, such as a Ag-Se/Ge-Se inorganic resist, is favorable for forming micropatterns using x rays because of the short ranges of the electrons generated by the x rays in the resist.
Collisional cross sections of c-C4F8 and transport coefficients of c-C4F8 and N2-c-C4F8 mixtures
Authors: Novak J. P. , Fréchette M. F.
Published: July 2002

An effective set of collisional cross sections of c-C4F8 was determined by fitting the transport coefficients, i.e., drift velocity, diffusion, ionization, and attachment coefficients, to available experimental data. The best fit was obtained for the following values: a total momentum transfer cross section of 8×10−15 cm2 for electron energies of z≤4 eV dropping to 1.75×10−15 cm2 for z>12 eV, a maximum vibrational cross section of 8×10−15 cm2 at 0.4 eV, and an excitation cross section of about 4×10−16 cm2 between 9 and 20 eV. Ionization and attachment cross sections based on measurements by M. V. Kurepa [Transactions of the 3rd Czechoslovak Conference on Electronics and Vacuum Physics (Academia, Prague, 1967), p. 107] were used; the ionization cross section was multiplied by a correction factor of 0.59, while the attachment cross sections were readjusted according to measurements by C. Lifshitz and R. Grajower [Int. J. Mass Spectrom. Ion Phys. 10, 25 (1972/73)] and I. Sauers, L. G. Christophorou, and J. G. Carter [J. Chem. Phys. 71, 3016 (1979)]. Calculations of the transport coefficients of N2-c-C4F8 mixtures were also performed.
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1P NMR relaxation study of spin dynamics in layered transition-metal compounds MPX3
Authors: Ziolo J. , Torre S. , Rigamonti A. , Borsa F.
Published: July 2002

Measurements of 31P NMR relaxation times in layered antiferromagnets FePSe3, FePS3, and NiPS3 have been carried out around the transition temperature TN to the ordered state, in the presence of different external magnetic fields. The relaxation rates have been related to the fluctuations of the hyperfine magnetic field at the P site due to the six nearest-neighbors magnetic ions. From the temperature behavior of the relaxation rates, information on the correlation properties of the spin fluctuations and on their dependence from the external field has been extracted.
Magnetic and transmission electron microscopy studies of the formation of cobalt silicide thin films
Authors: Madar R. , d’Anterroches C. , Arnaud d’Avitaya F. , Boursier D. , Thomas O. , Senateur J. P.
Published: July 2002

The cobalt reactivity on monocrystalline silicon has been studied for annealing temperatures lower than 500 °C. The techniques used were thermomagnetism measurements and transmission electron microscopy. The results demonstate that the reaction starts during deposition. During annealing the three types of formation kinetics were shown to exist as a function of temperature (20–220 °C, 220–370 °C, 370–420 °C). At the latter temperature the cobalt is completely transformed into CoSi and Co2Si.
Defect generation in silicon dioxide from soft x-ray synchrotron radiation
Authors: Williams C. K. , Reisman A. , Bhattacharya P. , Ng W.
Published: July 2002

Generation of fixed positive charge, neutral electron traps, and fixed negative charge in SiO2 due to exposure to monochromatic x-ray radiation in the photon energy range from 300 to 1000 eV in a synchrotron source is reported upon. At a constant exposure level of 2×107 rads, the number of defects generated is approximately independent of x-ray photon energy. The generated defects, independent of x-ray radiation energy, show normal post-metal annealing characteristics.
Polar Kerr-effect observation of perpendicular surface anisotropy for ultrathin ferromagnetic films: fcc Fe/Cu(100)
Authors: Liu C. , Moog E. R. , Bader S. D.
Published: July 2002

The theoretically predicted prependicular surface anisotropy of ultrathin ferromagnetic Fe films has been confirmed using the magneto-optical Kerr effect. Polar and longitudinal Kerr-effect measurements have been performed in situ on the same fcc Fe/Cu(100) films to study the dependence of the magnetic properties on film thickness and growth temperature conditions. Auxiliary LEED and Auger studies are used to characterize the structure and growth of the films. For films 1.2–5.9 monolayers (ML) thick grown at ∼100 K, for instance, square hysteresis-loop behavior in the polar Kerr effect confirmed the dominance of the perpendicular surface anisotropy. Examples are presented to document how the polar and longitudinal Kerr-effect signals evolve for films thicker than 6 ML due to the easy axis reorienting into the film plane. To differentiate between intrinsic temperature effects and those associated with interfacial compositional changes, the films were thermally cycled and their Kerr signals monitored. Films of thickness around 4 ML retained their characteristic polar Kerr-effect square loops despite thermal cycling between ∼100 and ∼400 K, while those <2 ML thick changed irreversibly. These studies confirm that intermixing can be limited to the monolayer range by low-temperature growth conditions, but the LEED results show that the degree of structural ordering of the films is reduced, as expected.
Influence of the substrate orientation on Ge incorporation in molecular-beam epitaxial GaAs
Authors: Lee B. , Bose S. S. , Kim M. H. , Stillman G. E. , Wang W. I.
Published: July 2002

The incorporation of Ge in GaAs layers grown simultaneously on (100), (311)A, and (311)B GaAs substrates by molecular-beam epitaxy has been studied using Hall-effect measurements, photothermal ionization spectroscopy, and photoluminescence. The quantitative analysis shows that Ge is more amphoteric than Si, but the orientation dependence of the amphoteric behavior of Ge is similar to Si. The amphoteric site preference of Ge is more selective for growth on the (311) polar orientation than on the (100) nonpolar orientation. Like Si, Ge incorporates predominantly as a donor in (311)B growth, while it incorporates predominantly as an acceptor in (311)A growth.
Mass and dose dependence of ion-implantation-induced intermixing of GaAs/GaAlAs quantum-well structures
Authors: Leier H. , Forchel A. , Hörcher G. , Hommel J. , Bayer S. , Rothfritz H. , Weimann G. , Schlapp W.
Published: July 2002

The influence of ion mass and dose on the intermixing of GaAs/GaAlAs quantum-well structures using photoluminescence (PL) and secondary-ion-mass spectroscopy (SIMS) techniques has been studied. Ga, Zn, Ar, Mg, Ne, and He ions are implanted in a single-quantum-well (SQW) structure at different doses. After annealing, the amount of intermixing between Al and Ga is extracted from the PL peak energy shift of the near-band-gap emission of the SQW. The measured Al diffusion length values ΔAl for different ion species agree with a simple model which assumes that the implantation damage in conjunction with low T (T<600 °C) defect diffusion is responsible for the mixing. We observe a similar dose dependence for ΔAl for all the above ions. For high implantation doses we have studied the mixing by Ar ions after implantation and annealing with SIMS. The SIMS data indicate that at high doses collisional mixing is the dominant mechanism for the disordering. Drastic mixing effects are obtained in the above quantum-well structures after high-temperature annealing of samples implanted with electrically active impurities (S,Si).
Improvement of oxide quality by rapid thermal annealing
Authors: Wendt H. , Spitzer A. , Bensch W. , Sichart K. V.
Published: July 2002

Rapid thermal processing as a post-oxidation annealing (POA) process is of considerable influence on the quality of thin thermal oxides: The number of defect-related oxide failures is reduced, together with a slight reduction of the intrinsic oxide quality, but to a degree that is of no importance as regards reliability aspects. According to our investigations, it is important that POA is performed after poly-Si deposition and doping, whereas POA after poly-Si patterning is found to be an undesirable process. Infrared spectroscopy indicates stress relaxation caused by the POA treatment, but no change of the SiO2/Si-interface roughness could be observed by high-resolution electron microscopy. This relaxation process was accompanied by the creation of electron traps. Constant-current investigations indicated an enhanced electron trapping in the oxides after POA. This electron trapping increased with increasing POA temperature. The reduced defect-related oxide failures after POA are attributed to a retarded runaway of the injection current by negative space charges.
Optical properties of Ga1−xInxP1−ySby alloys grown by organometallic vapor phase epitaxy
Authors: Jaw D. H. , Jou M. J. , Fang Z. M. , Stringfellow G. B.
Published: July 2002

The new family of GaInPSb quaternary alloys grown by organometallic vapor phase epitaxy was characterized by photoluminescence, transmission, photoluminescence excitation, and Raman spectroscopies. Transmission and photoluminescence excitation spectroscopic techniques were applied to determine the energy band gap of the alloys. It was found that the values of band gap obtained for the GaInPSb alloys were close to the theoretical values deduced from the data obtained for the end ternary alloys. The photoluminescence peak energies for all the samples were found to be about 80 to 150 meV below the band gap deduced from the transmission measurements. The large difference between the photoluminescence peak energy and band gap, together with other evidence, suggests that the photoluminescence is from recombination involving a deep center. The long wavelength lattice dynamics of the alloys were studied by Raman spectroscopy in the wave-number range from 150 to 400 cm−1. The long wavelength optical phonons displayed a ‘‘three- and four-mode’’ behavior.
An electron diffraction study of amorphous hydrogenated germanium-carbon thin films
Authors: White S. B. , McKenzie D. R.
Published: July 2002

The structure of a-Ge1−xCx:H films produced by glow-discharge decomposition of germane and methane gases has been studied by electron diffraction. The reduced density function of five films for different values of x shows evidence that germanium and carbon form a bond in the amorphous solid state. The structure does not correspond to the tetrahedrally bonded network of the amorphous silicon-carbon analogue but consists of molecular fragments and has a low stability.
Proton irradiation of silicon: Complete electrical characterization of the induced recombination centers
Authors: Hüppi Marcel W.
Published: July 2002

The control of minority-carrier lifetimes in n-type silicon by means of low-dose, high-energy proton irradiation at room temperature has been investigated. The irradiation-induced defects, their electrical properties, their annealing behavior, and their relevance for carrier lifetimes have been studied by deep level transient spectroscopy (DLTS) and the open-circuit voltage decay technique. Emphasis was placed on the complete electrical characterization of the dominant recombination centers induced by the proton bombardment of silicon. The energy level position within the forbidden band, the capture cross sections, and emission rates for majority and minority carriers, as well as the defect concentration were determined for each center. By means of a special recombination selective DLTS measurement mode (double-double DLTS) it was even possible to localize in the spectra the deep centers which are relevant to minority-carrier lifetimes. The results are quantitatively discussed in the framework of Shockley–Read–Hall recombination statistics with respect to the reduction of carrier lifetimes for high- and low-injection conditions in semiconductor devices. The results have been corroborated qualitatively by the correlation of the minority-carrier lifetimes in proton-irradiated devices with the concentrations of the induced recombination centers which were influenced by different temperature treatments.
Dynamic approach for finding effective elastic and piezoelectric constants of superlattices
Authors: Akçakaya E. , Farnell G. W. , Adler E. L.
Published: July 2002

A dynamic approach is presented for determining a set of equivalent elastic, piezoelectric, and permittivity constants for a superlattice consisting of a periodic multilayer of anisotropic materials. The objective is to predict the acoustic properties of such a periodic structure in the long-wavelength (low-frequency) limit. It is shown that the acoustic wave propagation properties for propagation in any plane defined by an arbitrary direction and the superlattice axis are contained in a 6×6 system matrix requiring only a subset of at most 29 material constants (from a total of 46) for its construction. The computation of an equivalent system matrix for a superlattice is demonstrated and the manner of obtaining the equivalent density, six permittivities, 21 stiffness, and 18 piezoelectric constants is given.
Temperature dependence of a 2.94-μm YAG:Er laser and population of the laser levels
Authors: Spring R. , Lüthy W.
Published: July 2002

The temperature dependence of the laser threshold and the slope efficiency of a YAG:Er laser operated at 2.94 μm have been measured. With a reduction of the temperature from 55 to 4 °C the output of the YAG:Er laser can be increased by a factor of 2.8. From the above measurements the relative population Nu of the 4I11/2 and Nl of the 4I13/2 levels are determined, leading to further insight into the mechanism of this laser system.
Characterization and analysis of the CO2 laser-induced ablation of lithium target via laser-induced fluorescence and absorption spectroscopy of emitted atoms
Authors: Harnafi M. , Dubreuil B.
Published: July 2002

Pulsed CO2 laser-induced ablation of solid lithium is studied in the low-energy-density regime where no plasma forms on the surface. Li atoms emitted from the surface are characterized using laser-induced fluorescence and absorption spectroscopy. Atom densities measured as a function of time for different distances from the surface are well described by a full-range Maxwellian in a center-of-mass coordinate system. For 0.9 J/cm2 incident energy density (a fraction being absorbed), the beam velocity and the characteristic temperature are 3×105 cm/s and 8500–10 000 K, respectively. Under these conditions, the number of ablated atoms is about 5×1011 per laser shot. The determined effective beam temperature is much higher than the boiling point of pure lithium. This could be explained considering that a film of oxide with greater vaporization temperature is always present on the surface even in relatively good vacuum conditions.
The permeability tensor of composite consisting of magnetic particles
Authors: How H. , Vittoria C.
Published: July 2002

The demagnetizing energy of spheroidal magnetic particles dispersed in cubic lattices has been calculated. The demagnetizing energy contains contributions from the demagnetizing self-energy and magnetostatic coupling energy between particles. The total demagnetizing energy can be conveniently expressed in terms of three demagnetizing factors whose sum equals unity. For low volume loading of particles the self-energy contribution is the dominant one, which is insensitive to particle coordinations and can be approximated by that of an isolated particle. However, when particle loading increases appreciably, significant differences arise for different particle lattices (sc, bcc, and fcc). One advantage of our formulation is that the permeability tensor of the composite can then be calculated for either a magnetic nonsaturated or saturated state of the particle. In contrast to conductivity calculations at the percolation limit there is no divergence of the permeability.
A theory on the x-ray sensitivity of a silicon surface-barrier detector including a thermal charge-diffusion effect
Authors: Cho T. , Hirata M. , Takahashi E. , Teraji T. , Yamaguchi N. , Matsuda K. , Takeuchi A. , Kohagura J. , Ogura K. , Kondoh T. , Osawa A. , Yatsu K. , Tamano T. , Miyoshi S.
Published: July 2002

An analytical method based on a new theoretical model for the x-ray energy responses of silicon surface-barrier (SSB) detectors has been proposed. This method may address a recent confusing issue in the x-ray detection characteristics of SSB semiconductor detectors; that is, the x-ray responses of SSB detectors as well as p-i-n diodes used in underbiased operations were recently found to be contrary to the commonly held belief that the x-ray sensitivity of an SSB detector is determined by the thickness of the depletion layer. The model presented includes a signal contribution from thermally diffusing charge that is created in the field-free substrate region within a diffusion length from the depletion layer along with a signal contribution from charge created in the depletion layer. This model predicts a large signal contribution from the charge-diffusion effect on the SSB responses to high-energy x rays. Formulas and calculated results supporting SSB calibration data have been represented. These analytical methods might be developed to apply the analyses and predictions of energy responses of various types of silicon detectors including p-i-n diodes as well as charge-coupled devices.
Study of optical dispersion parameters of WO3 polycrystalline thin films
Authors: Davazoglou D. , Donnadieu A.
Published: July 2002

The optical properties of WO3 polycrystalline thin films prepared by oxidation of ‘‘reflective’’ tungsten films in three different conditions of temperature and ambient have been studied by standard reflection-transmission measurements. The resulting dispersion parameters E0 and Ed [S. H. Wemple and M. DiDomenico, Phys. Rev. B 3, 1338 (1971)] have been analyzed using the effective-medium approximation. The experimental values of E0 and Ed can be reproduced if the films are considered to be composed of a crystalline phase, disordered phase, and voids, the concentration of each phase depending on the annealing mode and the thickness. The arguments proposed have been supported by x-ray diffraction, scanning electron microscope, and density measurements. Films prepared at 600 °C in air exhibit a high concentration of crystalline phase (≳85%) while the void content remains at low levels and unaffected by thickness variations (<10%). In films obtained by annealing at 500 °C in air or in a stream of 90% argon-10% oxygen, the void content increases while the concentration of disordered phase decreases with thickness.
High-resolution simultaneous photothermal measurements of thermal parameters at a phase transition with the photopyroelectric technique
Authors: Marinelli M. , Zammit U. , Mercuri F. , Pizzoferrato R.
Published: July 2002

Photothermal techniques allow, under particular experimental conditions, the simultaneous determination of specific heat, thermal conductivity, and thermal diffusivity. A comparison has been made between measurements performed with the gas-microphone (P) configuration and the photopyroelectric (PPE) one at a liquid-crystal second-order phase transition. An increase in the sensitivity with respect to thermal parameter variation has been observed in the case of PPE.
Molecular-orbital analysis of magneto-optical Bi-O-Fe hybrid excited states
Authors: Dionne Gerald F. , Allen Gary A.
Published: July 2002

Molecular-orbital estimates of the excited-state spin-orbit coupling multiplet splitting are presented to support the theory that enhanced magneto-optical effects in Y3−xBixFe5O12 originate from covalent interactions between Bi3+ and Fe3+ ions. A self-consistent approximation is applied to a two-level bonding-antibonding hybrid formed from the excited 4P term of Fe3+ and the excited 3P of Bi3+. For term energy values chosen from spectroscopic data, overlap integrals s<0.05 are sufficient to account for bonding-state depressions that match those reported for the tetrahedral and octahedral site-enhanced transitions at 2.6 and 3.15 eV. The hybrid eigenfunctions of the upper states can contain more than 30% of the Bi3+ 3P orbital term. Multiplet splittings that are enhanced by the large Bi3+ spin-orbit coupling constant (λ≊2 eV) are found to be on the order of 0.5 eV with a full complement of Bi surrounding Fe in the garnet lattice, in agreement with estimates based on experiment.
Measurement of tunneling current through Al/polyimide Langmuir–Blodgett film/Al structure
Authors: Isono Yasuo , Nakano Hiroshi
Published: July 2002

The dc conduction properties of Al/polyimide Langmuir–Blodgett (LB) film/Al structure are reported. The current-voltage characteristics and the temperature dependence of the current are measured and compared with expectation from a tunneling theory, which takes natural oxide of aluminum into consideration. It is concluded that the conduction in the structure with five- or seven-layered LB film is predominantly by direct tunneling of electrons.
Investigation and reduction of excess low-frequency noise in rf superconducting quantum interference devices
Authors: Mück M. , Heiden C. , Clarke J.
Published: July 2002

A detailed study has been made of the low-frequency excess noise of rf superconducting quantum interference devices (SQUIDs), fabricated from thin niobium films and operated at 4.2 K, with rf bias frequencies of 0.15, 1.7, and 3 GHz. When the SQUIDs were operated in an open-loop configuration in the absence of low-frequency flux modulation, the demodulated rf voltage exhibited a substantial level 1/f noise, which was essentially independent of the rf bias frequency. As the rf bias frequency was increased, the crossover frequency at which the 1/f noise power was equal to the white noise power moved to higher frequencies, because of the reduction in white noise. On the other hand, when the SQUID was flux modulated at 50 kHz and operated in a flux locked loop, no 1/f noise was observed at frequencies above 0.5 Hz. A detailed description of how the combination of rf bias and flux modulation removes 1/f noise due to critical current fluctuations is given. Thus, the results demonstrate that the 1/f noise observed in these SQUIDs is generated by critical current fluctuations, rather than by the hopping of flux vortices in the niobium films.
Nonradiative recombination and saturation of traps in multiple intrinsic quantum wells
Authors: Ding Y. J. , Veliadis J. V. D. , Khurgin J. B.
Published: July 2002

We have investigated the dependence of the continuous-wave photoluminescence on the laser intensity, which is affected by the trap saturation and the dominant nonradiative recombination in multiple intrinsic quantum wells. If the laser intensity is relatively low, the photoluminescence is proportional to the 1.5 power of the laser intensity. As the laser intensity increases, however, and above a critical intensity, the photoluminescence increases at a rate that is higher than square-law and finally approaches this dependence. This is the result of trap saturation. In order to examine the trap saturation in a more sensitive scale, we have calculated the dependence of the effective nonradiative decay time on the laser intensity. In addition, the laser-intensity dependences of the trapping efficiency and of the ratio of the electron and hole nonradiative decay times, have been determined for different ratios of the trapping and nonradiative recombination rates. The laser-intensity dependences of the electron and hole nonradiative decay times have also been determined.
Diamond homoepitaxial growth on (111) substrate investigated by scanning tunneling microscope
Authors: Tsuno Takashi , Tomikawa Tadashi , Shikata Shin-ichi , Fujimori Naoji
Published: July 2002

Epitaxial films were grown on diamond(111) substrates by microwave-plasma-assisted chemical vapor deposition, and their surfaces were studied by scanning tunneling microscope (STM). STM images with atomic order resolution were obtained, and showed 1×1 periodicity, which was also observed by low-energy electron diffraction. 〈112〉 single bilayer steps, 〈112̄〉 single bilayer steps, and 〈112〉 double bilayer steps were observed, and the majority of the steps were found to be of the first type. Steps were found to be very straight in the atomic scale, and there were suggestions of the incorporation of carbon atoms at the kinks. Even a small island with a nanometer order size showed a triangular shape surrounded by three 〈112〉 single bilayer steps.
A comprehensive model for Coulomb scattering in inversion layers
Authors: Gámiz F. , López-Villanueva J. A. , Jiménez-Tejada J. A. , Melchor I. , Palma A.
Published: July 2002

A comprehensive model for Coulomb scattering in inversion layers is presented. This model simultaneously takes into account the effects of: (i) the screening of charged centers by mobile carriers, (ii) the distribution of charged centers inside the structure, (iii) the actual electron distribution in the inversion layer, (iv) the charged-center correlation, and (v) the effect of image charges. A Monte Carlo calculation to obtain the effective mobility of electrons in an n-Si(100) inversion layer by using the model proposed for Coulomb scattering has been developed. The importance of correctly taking into account the effects above to study Coulomb scattering in inversion layers is pointed out.
Critical state models for flux-pinning-induced magnetostriction in type-II superconductors
Authors: Ikuta H. , Kishio K. , Kitazawa K.
Published: July 2002

Models and calculated results are presented for the flux-pinning-induced magnetostriction in type-II superconductors. Three models are considered for the critical state; the original Bean model, the Kim model, and the exponential model in which the critical current density is assumed to depend exponentially on the flux density. The analytic expressions of the length change under the magnetic field are derived for specimens having infinite slab geometry and under the assumption that the reversible part of the total magnetization is negligibly small. The results are used for computer simulations and the hysteresis loops are calculated over a wide range of the relevant parameters. Since the pinning-induced magnetostriction reveals the pinning force directly, the hysteresis loops for each model have different features which are discussed in detail. It is also shown that the pinning force of the sample can be directly inferred from the width of the magnetostriction hysteresis loop. Finally, the experimental results on high-temperature superconductors are discussed in comparison with the calculated results.
Magnetostriction of melt-spun Dy-Fe-B alloys
Authors: Lim S. H. , Noh T. H. , Kang I. K. , Kim S. R. , Lee S. R.
Published: July 2002

The magnetostriction of melt-spun ribbons of Dyx(Fe1−yBy)1−x (x=0.2, 0.25, 0.3; 0≤y≤0.2) alloys is systematically investigated as a function of the wheel speed during melt quenching. As the wheel speed increases from 10 to 50 m/s, the magnetic softness improves with the wheel speed rather continuously for the alloys with the Dy content x=0.2 and 0.25 but it exhibits a maximum at the wheel speed of 30 or 40 m/s for the alloys with the highest B content (x=0.3). The softness also improves with the B content for a fixed wheel speed. Homogeneous and ultrafine grain structure is observed for the first time even in the as-spun state when the ribbons of the alloy Dy0.3(Fe0.8B0.2)0.7 are fabricated at the wheel speed of 30 m/s. The ribbon having the ultrafine grain structure exhibits good magnetic softness together with a high strain.
Vortex structures in YBa2Cu3O7 (invited)
Authors: Keimer B. , Lynn J. W. , Erwin R. W. , Dogan F. , Shih W. Y. , Aksay I. A.
Published: July 2002

Extensive small angle neutron scattering experiments have been conducted on the vortex system in YBa2Cu3O7 in a magnetic field range of 0.5 T≤H≤5 T, and with various orientations of the magnetic field with respect to the crystallographic axes. For H parallel to the c axis, the vortex lattice is oblique with two nearly equal lattice constants and an angle of 73° between primitive vectors. One principal axis of the vortex lattice coincides with the (110) direction of the crystal lattice. It is shown that this structure cannot be explained in the framework of a purely electrodynamic (London) model, and that it is intimately related to the in-plane anisotropy of the superconducting coherence length. When the field is inclined with respect to the c axis, the uniaxial anisotropy due to the layered crystal structure of YBa2Cu3O7 becomes relevant. The interplay between the square in-plane anisotropy and the uniaxial anisotropy leads to both a continous structural transition and a reorientation of the vortex lattice as a function of inclination angle. For the largest inclination angles, the vortex lattice decomposes into independent chains.
Limitations of the uniform effective field approximation due to doping of ferroelectric thin-film capacitors
Authors: Chai Francis K. , Brews J. R. , Schrimpf R. D. , Birnie D. P.
Published: July 2002

The electric-field distribution in a ferroelectric capacitor often is treated as a uniform effective field for circuit-level modeling. By solving Poisson’s equation and treating the ferroelectric capacitor as a back-to-back Schottky-barrier system, the nonuniform electric-field distribution is calculated inside a ferroelectric thin film, assuming that the thin-film capacitor is completely depleted and has a constant doping concentration. It is found that the departure of the local field from the uniform effective field increases with an increase in the doping concentration of the film. Within this model, the uniform field approach to extraction of microscopic ferroelectric parameters is inaccurate for doping levels great enough that the surface field exceeds the coercive field even at zero bias. Based on this criterion, the critical doping concentration for parameter extraction using the uniform field approximation is found to be about 5×1017 cm−3. That is, according to the assumed model, for the high doping concentrations reported for typical lead-zirconate-titanate thin films (1018–1019 cm−3), the extraction of microscopic film properties based on a uniform electric field approximation is inaccurate.
Effects of tandem and colliding shock waves on the initiation of triaminotrinitrobenzene
Authors: Tarver Craig M. , Urtiew Paul A. , Tao William C.
Published: July 2002

The shock initiation of the insensitive high-explosive LX-17, which contains 92.5% triaminotrinitrobenzene and 7.5% Kel-F binder, was studied under simulated accident conditions in which two shock waves interact producing locally high pressures and temperatures. Two experimental geometries were studied using embedded manganin pressure gauges to measure the increases in pressure due to exothermic reaction at various locations as functions of time. These pressure histories were compared to ignition and growth reactive flow model calculations to determine whether a second shock compression of reacting LX-17 caused unusually rapid reaction rates and thus more extreme hazards. One experiment used a tandem flyer plate of aluminum and steel separated by a gap to shock the LX-17 charge, allow it to rarify, and then reshock the damaged charge to even higher pressures. These experiments revealed no significant enhancement of the LX-17 reaction rates under this shock, rarefaction, and reshock loading. The second experiment used a grooved flyer plate to produce a subcritical shock wave in LX-17, which then diverged and collided, producing a Mach stem interaction at the charge axis. The threshold conditions under which the Mach stem grew to detonation were measured. The standard LX-17 ignition and growth model yielded excellent agreement with the pressure gauge records in the Mach stem interaction region. The formation of Mach stem interactions by nearly simultaneous multiple high-velocity impacts was identified as a serious shock initiation hazard for heterogeneous solid explosives.
The temperature dependence of threshold current of chemical beam epitaxy grown InGaAs–InP lasers
Authors: Rees P. , Blood P. , Vanhommerig M. J. H. , Davies G. J. , Skevington P. J.
Published: July 2002

The temperature dependence of threshold current between 130 and 310 K of 1.65 μm In0.53Ga0.47As–InP bulk lasers grown by chemical beam epitaxy has been measured. Comparison with a calculation of the spontaneous recombination current at threshold allows one to determine the proportion of current loss over this temperature range. It is found that the loss can be described using an Auger recombination component of the form Rauger=C′n3 exp(−Ea/kBT) where n is the carrier population density in the undoped active region. The activation energy Ea is found to be 39±5 meV which is in excellent agreement with the theoretical value for the conduction to heavy hole band/split-off to heavy hole band Auger process. The values obtained for the Auger coefficient C over the temperature range are in close agreement with published values obtained by time resolved photoluminescence.
A study of neodymium-chelate-doped optical polymer waveguides
Authors: Lin Sihan , Feuerstein Robert J. , Mickelson Alan R.
Published: July 2002

The problem of doping optical quality polymers with chelated Nd3+ has been studied. A number of material systems were evaluated. The best results in terms of optical quality at a high Nd3+ concentration were achieved by doping a fluorinated polyimide with fluorinated neodymium chelate. Slab and channel waveguides were fabricated in this material system. Optical absorption and luminescence studies were carried out.
Electron spectroscopic studies of colossal magnetoresistance material La1−xCaxMnO3
Authors: Park J.-H. , Chen C. T. , Cheong S.-W. , Bao W. , Meigs G. , Chakarian V. , Idzerda Y. U.
Published: July 2002

High resolution photoemission measurements have been performed for the ferromagnetic transition of La0.67Ca0.33MnO3 and for the order–disorder transition of La0.4Ca0.6MnO3. The band gap collapsed below the Curie temperature and the density of states at the Fermi level increases with cooling. Upon cooling through the order–disorder transition temperatures, TCO, the band gap increases by ∼50 meV. This change in the gap is consistent with the change in the activation energy above and below TCO estimated from conductivity data. We have also performed soft x-ray magnetic circular dichroism measurements at Mn L2,3 edges for ferromagnetic La0.67Ca0.33MnO3 and La0.8Ca0.2MnO3.
Numerical micromagnetics in hard magnetic and multilayer systems (invited)
Authors: Schrefl T. , Fidler J.
Published: July 2002

Micromagnetic finite element calculations using a magnetic vector potential to treat long-range dipolar interactions describe the interactive magnetization processes which determine the magnetic properties of fine-grained magnetic materials. Micromagnetic models give a quantitative treatment of the correlation between the microstructure and the magnetic properties of melt-spun Nd2Fe14B magnets and of Co/Pt multilayers. In nanocrystalline permanent magnets, intergrain exchange interactions cause nonuniform magnetic states which increase the remanence and act as nucleation sites. Remanence increases with decreasing grain size. Remanence enhancement of about 15% with respect to noninteracting particles can be achieved for an average grain size D≤20 nm. On the other hand, the nucleation field reduces to about 20% of the anisotropy field. Once a reversed domain has been nucleated, strong internal stray fields cause the expansion of the domain over several grains. The nucleation field of Co/Pt multilayers increases with improving texture. Spatial fluctuations of the magnetocrystalline anisotropy energy create barriers for domain wall motion. The pinning field of reversed domains increases with increasing misalignment and with increasing grain size. The quality of texture and the grain size significantly influence the jaggedness of domains in Co/Pt multilayers. Large grains and strong deviations of the easy directions from the film normal deteriorate the smoothness of domains and increase the transition width.
Permeability of microstrip thin films of various materials
Authors: Yamakawa Kiyoshi , Ise Kazuyuki , Honda Naoki , Ouchi Kazuhiro , Iwasaki Shun-ichi
Published: July 2002

Soft magnetic bilayered microstrip films have been studied for various materials such as Ni–Fe–Mo–Cu, Co–Zr–Mo–Ni, Fe–Si–N, and Ni–Fe in terms of their magnetization dynamics. Effects of the strip width and the interlayer thickness on permeability were investigated. An optimum interlayer thickness was found for obtaining the maximum permeability for those films except Fe–Si–N, but the optimum value differs in magnetic film material. A scanning Kerr effect microscope observation reveals that permeability variations with strip width related to changes of the dynamic domain configurations of the strips. All magnetic film materials except Fe–Si–N show a high permeability of more than 1000 at 2 μm strip width at each optimum interlayer thickness. When the film width is reduced to 1 μm, however, the permeability decreases significantly regardless of the interlayer thickness and kinds of the film materials. For such narrow strip film, a higher permeability is obtained for higher anisotropy field. It is concluded that the shape anisotropy of the micropatterned films must be controlled for achieving a higher permeability.
Magnetization and finite-size effects in Gd/W multilayers
Authors: Jiang J. Samuel , Chien C. L.
Published: July 2002

We have studied the magnetization and finite-size effects of thin Gd layers in sputter-deposited Gd/W multilayers. The interfacial Gd atoms lose about 9% of their moment, as the result of being in proximity with W. Annealing the multilayers at high temperatures improves the crystalline quality and sharpens the susceptibility peaks. The Curie temperature TC decreases with decreasing dGd according to the finite-size scaling relation [TC(∞)−TC(d)]/TC(∞)=(d/d0)−λ. The shift exponent λ has been found to be 1.5±0.1 and the constant d0 to be 13 Å.
Stoichiometry-dependent deep levels in n-type InP prepared by annealing under controlled phosphorus vapor pressure
Authors: Nishizawa Jun-ichi , Oyama Yutaka , Suto Ken , Kim Kiyoon
Published: July 2002

The photocapacitance method is applied to investigate the stoichiometry-dependent deep levels in intentionally-undoped n-type InP crystals prepared by 4 h annealing at 700 °C under controlled phosphorus vapor pressure. The present photocapacitance measurements reveal three dominant deep levels at 0.63 and 1.1 eV below the conduction band and 0.74 eV above the valence band, respectively. The phosphorus vapor pressure dependence of these level densities is also shown. The excitation photocapacitance results show the precise optical transition mechanism of these deep levels. In conjunction with the phosphorus vapor pressure dependence of the photocapacitance results, the defect formation mechanism is also discussed.
Electroluminescence, photoluminescence, and photocurrent studies of Si/SiGe p-i-n heterostructures
Authors: Förster M. , Mantz U. , Ramminger S. , Thonke K. , Sauer R. , Kibbel H. , Schäffler F. , Herzog H.-J.
Published: July 2002

Comparative electroluminescence and photoluminescence measurements were performed on Si/Si0.7Ge0.3 p-i-n single quantum well structures, and on one p-i-n and one undoped multiple quantum well structure in a wide temperature range. The samples were grown pseudomorphically by molecular beam epitaxy, and mesa diodes for electroluminescence and photocurrent measurements were fabricated. In electroluminescence, optical emission comes primarily from the SiGe quantum wells whereas no emission from Si is observed except for high temperatures of ≊200 K and up. All p-i-n structures exhibit maximum emission intensities in a temperature range between 80 K and 220 K, depending on the quantum well width. This temperature characteristic is very different from undoped quantum well samples. A model is discussed that accounts satisfactorily for all observed temperature dependent data. As an essential feature, the model includes Auger recombination in addition to radiative recombination in the n+ and p+ sides of the junctions and in the SiGe quantum well due to the high electron or hole densities in these regions. Photocurrent spectra due to single quantum wells are measured showing the SiGe absorption threshold in addition to the Si threshold. Quantitative fits to these spectra yield threshold energies for SiGe and Si consistent with the electroluminescence spectra. The question of how photogenerated excess holes that are bound in a quantum well can escape the well at 4.2 K to yield the measured photocurrents is discussed.
Nanosecond laser induced single atom deposition with nanometer spatial resolution using a STM
Authors: Ukraintsev V. A. , Yates J. T.
Published: July 2002

Nanosecond laser pulses, with 2.33 eV photon energy and ∼0.6 MW/cm2 radiation flux, have been used to initiate a transient increase of tunneling current between a W tip and a Si sample surface in an ultrahigh vacuum scanning tunneling microscope (STM) apparatus. As the laser power is increased to ∼2.5 MW/cm2, single atom transfer from the tip to a silicon surface occurs. For both polarities, the laser induced tunneling current is linear with laser pulse energy up to ∼0.6 MW/cm2. A transient tunneling current up to 15 μA has been observed. The similarity of the laser induced transient tunneling for both polarities, and hence its independence on material, suggest that the same mechanism is operative in both directions of tunneling. Both ballistic electron tunneling and band bending effects have been considered in the analysis of the electron transfer. It is proposed, however, that pulse laser heating of the tip causes this transient increase of the tunneling current due to a transient thermal expansion, reducing the tip-sample tunneling distance. The increase in tunneling current may lead to additional Nottingham heating of the tip apex. At a laser flux of 2.5 MW/cm2, single atom transfer between the W tip and the silicon surface occurs. The number of atoms transferred can be controlled by the laser flux, and the transfer process is virtually independent of the tip-sample bias polarity. Since a maximum tip temperature of 650 K is estimated during the pulse, W atom transfer must occur under the influence of strong W–Si chemical interaction. The speed of the pulse laser atom transfer (8 ns) exceeds by orders of magnitude the transfer speed that could be achieved by pulsing the STM piezodrive.