bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation of superconducting properties and oxygen concentration in Y1Ba2Cu3O7−δ films via Raman spectroscopy
Authors: Liu Y. , Pangilinan G. , Sooryakumar R. , Sumner Michael J. , Lemberger Thomas R.
Published: July 2002

We report Raman scattering and resistance measurements on a YBa2Cu3O7−δ film in which δ is varied by repeated annealing in oxygen or argon. As oxygen is removed, the resistance rises, the transition moves to lower temperature and broadens, and ‘‘forbidden’’ Raman modes are observed. Group theory, the Raman activity of another oxygen rich (δ∼0) film of different orientation, and comparison with results on single crystals allow us to assign each Raman phonon. We hence demonstrate that several ‘‘forbidden’’ Raman peaks are associated with depletion of oxygen chain sites. Our results establish that the oxygen concentration δ, and hence the superconducting properties, can be varied reversibly by reannealing in oxygen or argon with the Raman spectra providing a useful microscopic probe of the lattice.
Calorimetric absorption and transmission spectroscopy for determination of quantum efficiencies and characterization of ultrathin layers and nonradiative centers
Authors: Juhl A. , Bimberg D.
Published: July 2002

A novel absorption technique, calorimetric absorption spectroscopy (CAS), is described and modeled in detail. The technique is more sensitive than any other low-temperature absorption technique reported hitherto and it is quantitative. Quantum efficiencies can be determined by combining CAS with calorimetric transmission spectroscopy. The method is based on integral detection of phonons emitted during nonradiative recombination processes. A low-temperature carbon resistor acts as phonon detector. The sensitivity of CAS is so large that a simple combination of a tungsten lamp and a spectrometer can be used as the illumination source. Depending on the excitation density, αd products down to 10−8 can be detected. These features make it possible to apply CAS for the characterization of the interface structure of single quantum wells with thicknesses of less than 1 nm as well as for the study of the fine structure of deep traps in semiconductors. Examples for all these applications are given.
Monte Carlo electron backscattering in ZnS and PbS films
Authors: Antolak A. J. , Williamson W.
Published: July 2002

The condensed history Monte Carlo transport code sandyl is used to calculate the electron backscattering coefficients and energy distributions for thin ZnS and PbS films. The backscattering coefficients from normally incident 20-, 30-, and 40-keV electrons on various thickness films are compared with experimental data and an empirically derived fitting formula. The agreement between our calculations and the experimental data where available is within 10%. However, for some of the thinnest films, is a somewhat larger discrepancy between the empirical and either the Monte Carlo or experimental results.
The influence of annealing on the density and mechanical properties of amorphous Se100−xSbx
Authors: Elshafie A. , El-Zaidia M. M. , Abdel-Aal A. , Abou-Ghazala M.
Published: July 2002

The density and microhardness of the system Se100−xSbx (where x=5, 10, and 15 at. %) were determined at room temperature using the stepwise method. The values of the density and microhardness increase with time of annealing, temperature of annealing, and Sb content. The energy required for the crystallization processes was calculated from the density as 31.82, 39.8, and 49.7 K cal/mol for the samples Se95Sb5, Se90Sb10, and Se85Sb15, respectively. On the other hand, these energies are given by 43.76, 49.72, and 54.7 K cal/mol, using the microhardness data. The dimensional nature of the growth process has been looked for through the determination of the n exponent of the Avrami rate equation. The value of the ‘‘n’’ exponent of the Avrami rate equation indicates that the crystals grow in a one-dimensional way.
X-ray topographs of silicon crystals with superposed oxide film. A theoretical study by means of simulations
Authors: Epelboin Y.
Published: July 2002

Stresses induced in a silicon wafer by mask edges or recessed oxides may be studied by the means of x-ray topographs. The contrast of such topographs has been previously studied using the Blech and Meieran theory [J. Appl. Phys. 38, 2913 (1967)] for the stresses and Kato’s theory [Acta Crystallogr. 16, 282 (1963)] for the diffraction. Simulations using Takagi’s equations [S. Takagi, Acta Crystallogr. 15, 1131 (1962)] show that this model is not correct: the magnitude of stresses is too large and should lead to additional fringes in the image. This could not be predicted using Kato’s theory where the interaction between the wavefields and the most deformed areas of the crystal is underestimated. It suggests that a relaxation occurs along the surface to decrease the value of the stresses.
Reverse field injection for the gradient B drift transport of a high-current electron beam
Authors: Lee J. R. , Faucett D. L. , Halbleib J. A. , Hedemann M. A. , Stygar W. A.
Published: July 2002

Reverse field injection was used to inject a 0.9-MeV electron beam produced in a ring diode into a drift tube. The annular beam then was transported distances of up to 183 cm by gradient B drift in an applied magnetic field. The electron beam struck a tantalum target at the end of the drift tube, producing bremsstrahlung x-rays. An electron beam current of 1.1 MA was inferred from measurements of x-ray production in the target. This implies an injection efficiency of 69%.
Concentration profiles of composing ions in radio frequency sputtered Sr (Zr0.2Ti0.8)O3 films
Authors: Matsuoka Tomizo , Kuwata Jun , Fujita Yosuke , Abe Atsushi
Published: July 2002

Sr(Zr0.2 Ti0.8 )O3 dielectric thin film for electroluminescent devices was prepared by in-line-type rf magnetron sputtering method using oxide target. Novel phenomenon of periodic concentration fluctuations of the composing ions was observed in SIMS in-depth profiles. Partial concentration anomaly with Sr deficiency was also observed in the first-to-grow region of the film. The periodic concentration fluctuations were confirmed to be ascribed to periodic appearance of Sr(Zr1−x Tix )O3 with x larger than 0.8.
Band offsets in GaAs/amorphous Ge and GaP/amorphous Ge heterojunctions measured by internal photoemission
Authors: Coluzza C. , Lama F. , Frova A. , Perfetti P. , Quaresima C. , Capozi M.
Published: July 2002

Internal photoemission has been used to determine the valence- and conduction-band discontinuities (ΔEv and ΔEc) in GaAs/amorphous Ge and GaP/amorphous Ge heterojunctions. The method is straightforward, accurate, and assumes a minimum of unknown parameters. We have found ΔEv =(0.43±0.05) eV and ΔEc =(0.34±0.05) eV for GaAs/a-Ge and ΔEv =(0.88±0.05) eV and ΔEc =(0.68±0.05) eV for GaP/a-Ge.
Properties of CdS/ZnCdTe heterojunctions
Authors: Peters Michael G. , Fahrenbruch Alan L. , Bube Richard H.
Published: July 2002

A series of CdS/ZnxCd1−xTe heterojunctions have been prepared by evaporation of CdS onto single-crystal p-type ZnxCd1−xTe substrates with x=0, 0.1, and 0.3. The junction properties have been evaluated as a function of x, the substrate surface preparation, and the substrate temperature during CdS deposition. The junctions with the best quality correspond to those formed on stoichiometric ZnxCd1−xTe surfaces. Even with the optimum surface preparation, an increase in x results in an increase in the reverse saturation current and a decrease in the open-circuit voltage. Current versus voltage measurements as a function of temperature indicate that the junction transport can be described by thermally assisted tunneling. Illumination increases the junction transport current, an effect that persists for a measurable time after cessation of illumination. Both results can be described in terms of the effects of interface states, controlling tunneling/recombination and the width of the depletion layer in the ZnxCd1−xTe.
Calculation of optical absorption associated with indirect transitions in silicon n-i-p-i structures
Authors: de Sterke C. Martijn
Published: July 2002

Indirect transitions in silicon n-i-p-i structures are investigated theoretically. A method is used in which these structures are characterized by their internal electric fields, which change the absorption in a Franz–Keldysh-like manner. Our calculations, in which the influence of excitons is included, are based upon the effective-mass approximation. The conclusions are that the absorption only increases in an energy region for which the absorption is very small in the first place. It is further shown that excitonic effects are very important in this geometry, in spite of the strong internal electric fields, or, equivalently, in spite of the large spatial separation of the carriers.
Characteristics of GaAs/AlGaAs heterostructures grown by liquid-phase epitaxy on molecular-beam-coated GaAs on Si
Authors: van der Ziel J. P. , Logan R. A. , Chand N.
Published: July 2002

Liquid-phase epitaxy has been used to grow four-layer GaAs/AlGaAs double heterostructures on Si substrates that had been precoated with GaAs by molecular-beam epitaxy and coating the remaining exposed parts of the Si wafer with SiO2. Direct growth of GaAs on Si by liquid-phase epitaxy is not feasible because of the dissolution of the Si substrate by the Ga melt. As with the other growth techniques, photoluminescence from the GaAs cap layer and electroluminescence from the 0.2-μm-thick GaAs active layer indicate the luminescence peaks are shifted to lower energy as a result of the tensile strain in the plane of the layers that is caused by the different thermal contraction of the GaAS and Si layers as the wafer is cooled from the growth temperature. Optically pumped stimulated emission is observed from the cap layer at 80 K. The efficiency of the electroluminescence from the active layer was low, indicating the p-n junction was displaced from the active layer due to the accidental excess doping resulting from a slight dissolution of the Si substrate in the Ga melt.
Preparation and electrical properties of bromine intercalated vapor-grown carbon fibers
Authors: Endo Morinobu , Yamanashi Hidenori , Doll G. L. , Dresselhaus M. S.
Published: July 2002

Vapor-grown carbon fibers possessing a high degree of preferential orientation have been prepared using either a catalyzed substrate or a fluidized catalytic method, yielding fibers with diameters of about 10 and 1 μm, respectively. After heat treatment at 2600–2960 °C, the fibers were used as a host material for bromine intercalation to yield highly conductive filaments. Intercalation was carried out at room temperature by direct immersion into liquid bromine. Through bromination, the room-temperature electrical conductivity of these fibers measured under ambient conditions increased by a factor of about 6 relative to their pristine state. Typical values for the resistivity of the brominated fibers are ∼10 μΩ cm. Of particular importance is the stability of these brominated fibers under ambient conditions; the fibers showed less than a 3% increase in resistivity after 1 month of exposure to air. The fibers also displayed a high degree of thermal stability up to 500 °C, which was established by measuring the change in resistivity and the weight loss with increasing temperature. Whereas the thick fibers prepared by the substrate method showed a high degree of staging structure, the thin fibers prepared by the fluidized seeding method did not show significant staging fidelity, based on x-ray, Raman, and TEM observations. Nevertheless, the increase in conductivity observed for the thin fibers is almost as large as for the thick fibers. The fiber preparation and intercalation method used in this work, as well as the chemical and thermal stability that has been achieved, offer attractive possibilities for practical applications of these materials in light weight, highly conducting composites.
Limit cycle oscillation in negative differential resistance devices
Authors: Hellman E. S. , Lear K. L. , Harris J. S.
Published: July 2002

We present experimental current-voltage curves for GaAs/AlGaAs resonant tunneling diodes which show complicated multiple step structures when biased into negative differential resistance. We show that these results can be explained as limit cycle oscillations in the nonlinear dynamical system consisting of a negative differential resistance device loaded with a resonant circuit. Two circuit models, a resistor-capacitor-inductor load, and the dispersionless transmission-line load, are discussed. The limit cycles in the second model exhibit a variety of behaviors characteristic of nonlinear systems, such as bifurcations, period doubling, and Devil’s Staircases, resulting in good qualitative agreement with experiment.
Neutron-irradiation effect on the absorption edge of NaBrO3 single crystals
Authors: Abass A. K. , Jassim S. A. , Jeiad K. M.
Published: July 2002

Single crystals of sodium bromate are prepared from an aqueous solution of pure material. The effect of neutron irradiation on the absorption edge and indirect band-gap energy, as well as phonon-assisted indirect transitions, are studied in the low-absorption region.
The preionized metal vapor jet of a room-temperature metal vapor laser
Authors: Harvey E. C. , Tobin R. C.
Published: July 2002

In the room-temperature metal vapor laser studied, a rapid flow of noble gas entrains sputtered metal atoms, metal ions, and gas ions into the laser discharge volume in the form of a narrow jet. A short rise-time discharge pumps the laser transition and is observed to concentrate onto the preionized metal vapor jet. Radial and axial measurements of the electron temperature and ion number density in the preionized jet have been made using an instantaneous triple probe, and show that the ions are concentrated within the jet and extend up to about 20 cm downstream from the sputtering cell. Electrons are carried along the jet by an axial ambipolar field, and are most energetic at the boundary of the jet where they are heated through three-body recombination. Under certain conditions, flowing gas through the sputtering glow discharge causes the plasma to oscillate with a period of between 2 and 20 μs. These oscillations are due to plasma instabilities associated with the anode glow, and have been eliminated by a suitable arrangement of the discharge electrodes. In so doing the anode glow has been extinguished, the slope resistance of the sputtering discharge reduced, and the performance of the sputtering cell improved.
Laser-pulse vaporization of uranium dioxide and other refractory materials
Authors: Olander D. R. , Yagnik S. K. , Tsai C. H.
Published: July 2002

The nature of the vapor produced by pulse surface heating of UO2 and several other materials was investigated using normal-mode laser pulses with peak power densities between 104 and 106 W/cm2. Maximum surface temperatures measured by optical pyrometry ranged from 1900 to 4200 K. At temperatures below ∼2400 K for UO2, the vapor pulse followed the Hertz–Langmiur vacuum vaporization theory. More energetic transients produced gasdynamic blowoffs accompanied by significant supersaturation in the expanding plume. Neutron activation analysis of the collected vapor indicated that the total quantity of liquid UO2 evaporated in the pulse followed the Hertz–Langmuir formula. However, mass spectrometric analysis of the emitted vapor showed large deviations in the quantity and waveform shape of the monomer species. These and other features of the blowoff were satisfactorily explained in terms of nucleation and growth of polymeric species. Although UO2 clusters were undetectable by the mass spectrometer, polymers of MgO and ZrO2 were observed.
Further improvement of the high-efficiency, high-energy hydrogen-fluoride chain chemical lasers initiated by an intense electron beam
Authors: Kannari Fumihiko , Ashidate Shu-ichi , Obara Minoru
Published: July 2002

In order to further improve the output performances, the operational characteristics of the hydrogen fluoride (HF) chain chemical lasers initiated by an intense electron beam were theoretically analyzed with a new computer code including ionic reactions. Both theoretical HF laser output energy and pulsewidth were in good agreement with the experiment. Furthermore, we made an optimization of the laser gas mixtures and the electron-beam-current density in order to obtain a high efficiency within the limit of our experimental conditions. In consequence of the experiment based on the theoretical results, a maximum output energy of 5.0 kJ (268 J/ℓ ) was obtained from the low total pressure mixture (282 Torr) of F2/H2/SF6/O2=139/58/45/42 (Torr) with a high electrical efficiency of 284% and a chemical efficiency of 20.3%. The corresponding total efficiency reached 18.9%. Moreover, aiming at the achievement of the high chemical efficiency, a maximum chemical efficiency of 29.6% was attained with a 317-Torr mixture of F2/H2/SF6/O2=167/30/70/50 (Torr). The maximum output energy was 4.9 kJ (259 J/ℓ ), corresponding to a high total efficiency of 26.4% and an electrical efficiency of 246%.
Characterization of GaAs crystals with different degrees of compensation: Electronic Raman scattering of photoneutralized acceptors
Authors: Wagner J. , Seelewind H.
Published: July 2002

Electronic Raman scattering (ERS) with below band-gap excitation at 1.064 μm has been used to assess residual shallow acceptors in undoped GaAs crystals with different degrees of compensation. Continuous wave excitation at a power density of ∼1–5 kW/cm2 photoneutralizes those shallow acceptors quantitatively which are compensated by the midgap EL2 donor level. Therefore, the concentration of EL2 compensated acceptors is measured by ERS. Pulsed excitation at a power density of ∼20 MW/cm2 leads furthermore to a partial neutralization of residual acceptors compensated by donors shallower than the EL2 level. This allows a semiquantitative analysis of those donors. Pulsed excitation enables also to assess residual shallow acceptors by ERS in n-type semiconducting GaAs.
The inversion of surface potential measurements to determine crack size and shape
Authors: Connolly M. P. , Michael D. H. , Collins R.
Published: July 2002

The ac field measurement technique (acfm) for sizing surface breaking cracks in metals exploits the fact that the high-frequency alternating current tends to flow in a thin skin on the metal and crack surfaces. The method seeks to interpret measured perturbations in the surface voltages produced by a crack so as to deduce its size and its shape and for this purpose a suitable mathematical model of the field is required. In the past, solutions of direct problems in which the crack shape is assumed to be known have been used for this purpose. This paper addresses the inverse problem in which the crack shape is not specified. The problem is formulated as a two-dimensional Laplace problem and a boundary element method is used for its iterative solution. The algorithm for predicting crack profile and size has been tested against synthetic input data obtained from known solutions of direct problems and also against surface voltage data obtained on real surface-breaking cracks. Good agreement has been found in all cases.
A photoluminescence comparison of CdTe thin films grown by molecular-beam epitaxy, metalorganic chemical vapor deposition, and sputtering in ultrahigh vacuum
Authors: Feng Z. C. , Bevan M. J. , Krishnaswamy S. V. , Choyke W. J.
Published: July 2002

High perfection CdTe thin films have been grown on (001) InSb and CdTe substrates by molecular-beam epitaxy, metalorganic chemical vapor deposition (MOCVD), and sputtering in ultrahigh vacuum techniques. The quality of the as-grown CdTe films are characterized by 2-K photoluminescence. The spectra show strong and sharp exciton transitions and weak 1.40–1.50-eV defect-related bands. Radiative defect densities of lower than 0.002 are realized. High-resolution spectroscopy shows that the full width at half maximum of the principal bound exciton lines is about 0.1 meV. Such small ρ values and narrow photoluminescence lines have not been previously reported. The largest luminescence efficiency is observed for MOCVD-CdTe films grown on CdTe substrates. A variety of impurities appear to be responsible for the observed radiative transitions in these three kinds of CdTe films. We attempt to assign the observed impurity related lines by a comparison with ‘‘known’’ impurities in bulk CdTe spectra given in the literature.
Production and characterization of CdCl2 intercalated graphite fibers
Authors: Gooden Clarence E. , Pham Chuong , Naud Steve , Gaier James R.
Published: July 2002

Graphite fibers were intercalated using CdCl2 as a Cl source and Cl2 as carrier gas. Fibers were synthesized with a resistivity-density figure of merit comparable to that of copper. These fibers can withstand practical electrical current densities as high as 54 000 A/cm2. Their thermal coefficient of resistivity is 0.0016 °C−1, which is approximately 41% of those for copper and aluminum.
High-quality epitaxial growth of ZnSe on (100) ZnSe by atmospheric pressure metalorganic vapor phase epitaxy
Authors: Yodo Tokuo , Koyama Tadashi , Yamashita Ken
Published: July 2002

Single crystalline layers of undoped ZnSe have been grown on ZnSe substrates at 250 °C by atmospheric pressure metal organic vapor-phase epitaxy using dimethylzinc and hydrogen selenide (H2 Se). The substrates are ZnSe wafers with (100) orientation cut from bulk crystals grown by iodine vapor transport. A homoepitaxial layer (homoepilayer) of ZnSe with excellent crystallographic properties, as revealed by double crystal x-ray diffraction, was obtained. The full width at half maximum (FWHM) of the (400) diffraction pattern was 38 arcsec, which was superior to the crystallographic property (the FWHM was 250 arcsec) of the heteroepitaxial layer (heteroepilayer; a GaAs substrate was used). However, the intensity of deep emissions near 560 nm (PL at 77 K) is several times stronger than that of the heteroepilayer. The origin of deep emissions is not supposed to be due to iodine or copper diffused from the ZnSe substrate but to complexes between Zn vacancies and impurities in the layer.
A defect relaxation model for bias instabilities in metal-oxide-semiconductor capacitors
Authors: Zvanut M. E. , Feigl F. J. , Zook J. D.
Published: July 2002

We have studied electron trapping and detrapping resulting from bias stress applied to a metal-sputtered oxide-native oxide-semiconductor capacitor. The trapping process is described as band-to-trap tunneling. Based on the assumption of a trap with a delta function spatial distribution, a model was developed that predicts a trap energy distribution and defect relaxation energy. Application of this model to experimental data reveals a value for the relaxation energy of approximately 1 eV. We suggest that this model may apply to hysteretic instabilities observed in p-channel transistors and dual dielectric memory devices.
Piezoelectric plate resonances due to first Lamb symmetrical mode
Authors: Alippi A. , Craciun F. , Molinari E.
Published: July 2002

The resonances of the first Lamb s0 mode in parallelepiped piezoelectric plates excited by surface electrodes have been measured. Below the first thickness mode, the resonances are found to reproduce the dispersion of s0 in an infinite plate even for plates of small length. Moreover, a significant difference is observed between the behavior of the fundamental and its harmonics, reflecting the different effect of metallization in the two cases.
Performance of protective polymeric coatings for nonlinear optical materials
Authors: Marshall K. L. , Schmid A. W. , Smith D. J. , Bevin A. A. , Guardalben M. J. , Jacobs S. D.
Published: July 2002

KDP, KD*P, and LiNbO3, three nonlinear optical materials that have been difficult to coat, are treated with polymeric surface layers. These layers hermetically seal the hygroscopic crystal surfaces. Their optical properties, thermal compatibility, high-power laser damage behavior, abrasive resistance, and suitability for overcoating with traditional, dielectric antireflection multilayers are reported.
Interfacial microstructure of tungsten silicide Schottky contacts to n-type GaAs
Authors: Shih Yih-Cheng , Callegari A. , Murakami M. , Wilkie E. L. , Hovel H. J. , Parks C. C. , Childs K. D.
Published: July 2002

To investigate the effects of microstructure of the Schottky characteristics of WSix contacts to n-type GaAs, cross-sectional transmission electron microscopy, x-ray diffraction, and secondary-ion mass spectrometry have been used to study the interfacial and bulk film microstructures. The barrier heights and ideality factors of WSi0.1 and WSi0.6 contacts were obtained by forward current-voltage and capacitance-voltage measurements. These Schottky characteristics were found to be unrelated to the bulk film microstructure, but closely related to the interfacial microstructure at the WSix/GaAs interfaces. Both the WSi0.1/GaAs and WSi0.6/GaAs interface morphologies were observed to be stable and remain smooth during annealing at 800 °C for 10 min, while a rough interface with W protrusions and Ga and As out-diffusion was observed in two-layer W/WSi0.6 contacts. The stability of the WSix interfacial microstructure is suggested to depend on both the chemical stability of the WSix films with GaAs and the intervening oxides between WSix and GaAs. Nontrivial amounts of W and Si were observed to diffuse from the WSi0.1 film into the GaAs substrate during annealing at 800 °C for 10 min. Although these in-diffused impurities in the GaAs substrate do not seem to affect the Schottky characteristics after the 800 °C annealing, they could be a potential problem in long-term stability. Of the three WSix film compositions, the single-layer WSi0.6 films were found to have the least W and Si in-diffusion and thus the best thermal stability.
Electronic conduction of tin oxide thin films prepared by chemical vapor deposition
Authors: Kojima Masahiko , Kato Hisao , Imai Atsuo , Yoshida Akira
Published: July 2002

SnO2 thin films having low resistivity were prepared by chemical vapor deposition. Conductivity, its dependence on temperature, and Hall effect of films were measured. Examination of the surface morphology by scanning electron microscope showed that the SnO2 thin films are made up of many grains. Experimental results were discussed by the grain-boundary model. The carrier transport mechanism across a grain boundary is the tunnel effect rather than the thermionic emission.
Electrical and ellipsometric characterization of the removal of silicon surface damage and contamination resulting from ion beam and plasma processing
Authors: Vitkavage S. C. , Irene E. A.
Published: July 2002

The silicon surface damage resulting from low-energy Ar ion beam and plasma bombardment under a variety of bombardment conditions has been measured and compared. Techniques used to characterize the damage and its removal included x-ray photoelectron emission spectroscopy and reflection ellipsometry to determine the type of incorporated impurities and the damage layer depth, respectively; and high-frequency and quasistatic capacitance-voltage measurements of interface trapped and fixed oxide charge to determine the electrical nature of the damage. While the ellipsometrically measured damage could be removed using various described treatments, the electrical damage persisted. Both electrical and optical differences resulting from lattice damage versus impurity-related damage associated with the different dry etching environments could be discerned.
Nucleation and growth kinetics of the tetragonal to orthorhombic transition in YBa2Cu3O7−∂
Authors: Shi D. , Zhang K. , Capone II D. W.
Published: July 2002

The nucleation and growth geometry of the tetragonal to orthorhombic transformation in YBa2Cu3Ox is determined to be grain-boundary nucleation after saturation. In this geometry the orthorhombic phase rapidly nucleates along the grain boundaries, then propagates into the grains as a planar front governed by the much slower volume diffusion of oxygen. This conclusion is reached by fitting the volume fraction of the superconducting, and hence orthorhombic, phase as a function of annealing time at 400 °C to the Avrami relation. The volume fraction of the superconducting phase is obtained using magnetization versus field data.
Modeling of x-ray generation during the breakdown phase of surface discharges
Authors: Beverly III R. E.
Published: July 2002

High electric fields associated with the breakdown process in surface discharges generate a flux of nonequilibrium, runaway electrons. These electrons impact the dielectric substrate and anode producing bremsstrahlung radiation which can be used to photopreionize gas laser media. A computational model is given which calculates the electron and photon distribution functions and integrated x-ray dose. For a nanosecond-duration surface discharge in helium gas at a pressure of 104 Pa, the computed total radiated energy (∼10−5 J) is in approximate agreement with published experimental data. The integrated x-ray dose is larger in a helium gas atmosphere than in nitrogen and predicted doses for both gases decrease with increasing pressure as observed experimentally. The efficacy for CO2 laser preionization is examined in terms of the x-ray dose required to ensure discharge stability.
A reactive flow model for a monomolecular high explosive
Authors: Nutt Gerald L.
Published: July 2002

The thermodynamics of a reacting fluid element is discussed in a context appropriate to a hydrodynamic computer code. This is followed by a treatment of reaction initiation and reaction growth with reference to global reaction rates measured by reactive flow Lagrange analysis. The reactant and product gases are represented in the equation of state of the fluid element as two unmixed phases. The parameters in a common phenomenological representation of the global reaction rate are fit to the measured rate, and fine tuned by iterative code calculations of measured flow variables. We discuss changes in the form of the rate equation which would improve the agreement with experiment.
Covariant transformations of wave equations for initial-boundary-value problems with moving boundary conditions
Authors: Wilhelm H. E.
Published: July 2002

It is shown that the wave equation ψxx−ψyy=0 for the field ψ(x,y) in the domain R(xy) can be transformed into a wave equation Ψξξ−Ψηη=0 for the field Ψ(ξ,η) in the domain S(ξη). The transformation is accomplished through a complex function F(x,y)=ξ(x,y) +iη(x,y), which is not analytic. For the transformation to exist, the real transformation functions ξ=ξ(x,y) and η=η(x,y) have to satisfy wave equations in the domain R(xy) and the first-order partial equations ξx=±ηy and ξy=±ηx [‘‘±’’ distinguishes transformations of the first (+) and second (−) kinds]. Thus, the hyperbolic transformation theory is different from the conformal mapping theory, where the real transformation functions satisfy the Laplace equation and the Cauchy–Riemann conditions. As applications, the linear Lorentz transformation and nonlinear mappings of time-varying regions into fixed domains are discussed as solutions of the indicated partial differential equations. Furthermore, an initial-boundary-value problem for the electromagnetic wave equation with moving boundary condition is solved analytically (compression of microwaves in an imploding resonator cavity).
Edge dislocation inside a lamellar inhomogeneity
Authors: Stagni L. , Lizzio R.
Published: July 2002

The elastic field induced by an edge dislocation located inside a lamellar inhomogeneity is evaluated, the resulting formulas applying as well to any internal stress source within a lamella in generalized plane stress or plane strain, and the Peach–Koehler force on the dislocation derived. The features of the interaction are determined by the mismatch in both the shear moduli and the Poisson’s ratios of lamella and matrix. The differences from the case of an edge dislocation in either a free or a clamped strip turn out to be mainly of a quantitative nature and to become relevant when the lamella is up to one order of magnitude harder or softer than the matrix.
ArF laser excited in a capacitively coupled discharge tube
Authors: de la Rosa J. , Eichler H. J. , Herweg H.
Published: July 2002

Lasing in ArF was obtained with a longitudinal capacitively coupled high-voltage discharge. A laser energy of 15 μJ at 193-nm wavelength has been obtained by discharging a two-stage Marx generator with 60 pF capacitance and a voltage of 80 kV per stage into a discharge tube with a length of 35 cm and 2 mm inner diameter.
Ion beam rehydrogenation and post-hydrogenation of a-Si:H
Authors: Tsuo Y. S. , Deng X. J. , Smith E. B. , Xu Y. , Deb S. K.
Published: July 2002

We have studied the rehydrogenation and post-hydrogenation of a-Si:H using a Kaufman ion beam source. The achievement of an air-mass-one (AM1), photo-to-dark conductivity ratio of 5.6×105 with a rehydrogenated a-Si:H sample was reported earlier [Y. S. Tsuo, E. B. Smith, and S. K. Deb, Appl. Phys. Lett. 51, 1436 (1987)]. In this communication we report recent results of the rehydrogenation study and new results of a study of the post-hydrogenation of amorphous silicon deposited by glow discharge at 480 °C. AM1 photo-to-dark conductivity ratios as high as 9.5×106 (with a photoconductivity of 8.6×10−6 Ω cm−1) and 1.1×105 (with a photoconductivity of 6.3×10−6 Ω cm−1) have been obtained with a rehydrogenated sample and a post-hydrogenated sample, respectively. We also report the results of the hydrogen depth profile and photostability measurements of these samples.
Influence of cathode potential drop on some electron attachment rate measurements
Authors: Letardi T. , Zheng C. E.
Published: July 2002

The HC1 attachment rate constants have been measured by observing the electron drift current decay after termination of an x-ray ionizing pulse. In order to discuss the influence of the cathode potential drop on the results, a simplified cathode-drop model for non-self-sustaining discharge is described.
Pulsed electromagnetic inductive plasma-enhanced chemical-vapor deposition of amorphous carbon films
Authors: Ebihara Kenji , Kanazawa Seiji , Yamagata Yukihiko , Maeda Sadao
Published: July 2002

A pulsed electromagnetic inductive methane discharge process was developed to form amorphous carbon thin films. In order to estimate the methane plasma state in the pulsed plasma process, the time-resolved excitation temperature was measured by means of relative spectral intensity method. At the high electromagnetic compression phase the pulsed plasma has an excitation temperature of the same order (20 000–50 000 K) as in the conventional rf glow discharges. The deposited thin films are transparent in the IR and adhere well to room-temperature substrates. The optical energy gap and the electrical conductivity of the amorphous carbon films are investigated and compared with the amorphous carbon films prepared with rf glow plasma chemical vapor deposition. The optical gap is observed to decrease from 1.26 to 1.14 eV as the deposition temperature and the charging voltage increase. It is shown that dynamic pulsed plasma flows affect the phase transition from a diamondlike structure to a graphitic structure.
Organometallic vapor-phase epitaxial growth and characterization of the metastable alloy InP1−xSbx
Authors: Jou M. J. , Cherng Y. T. , Stringfellow G. B.
Published: July 2002

The III-V metastable alloy InP1−x Sbx has been grown for the first time with compositions well inside the miscibility gap. Despite the large miscibility gap at the growth temperatures of 480–600 °C, epilayers with compositions covering the entire range from x=0 to 1.0 have been grown successfully by organometallic vapor-phase epitaxy at atmospheric pressure using the reactants trimethylindium, trimethylantimony, and phosphine. The 10-K energy band gap as a function of composition was determined from photoluminescence measurements combined with x-ray diffraction and electron microprobe analysis. The bowing parameter for the band-gap energy of the InP1−x Sbx was estimated to be 1.9±0.1 eV. The lattice dynamics have been studied using Raman spectroscopy in the frequency range from 150 to 400 cm−1. Long wavelength optical phonons display a ‘‘two-mode’’ behavior throughout the entire composition range. The InP-like longitudinal-optical and transverse-optical modes shift to lower frequency with increasing Sb concentration.
n-type amorphous (or microcrystalline) silicon/p-type crystalline silicon heterojunction electrodes for efficient and stable solar-to-chemical conversion
Authors: Ueda Keiichi , Nakato Yoshihiro , Sakai Yuichi , Matsumura Michio , Tsubomura Hiroshi
Published: July 2002

Silicon (Si) electrodes having a n-p heterojunction for hydrogen photoevolution were prepared by depositing n-type amorphous or microcrystalline Si layers on p-type single-crystal Si wafers by the plasma chemical-vapor-deposition method. Electrodes of this type, each coated with a thin platinum layer (about 1 nm) as a reaction catalyst, generated photocurrents much higher than those for Pt-coated n+ -p homojunction single-crystal Si electrodes, together with photovoltages nearly the same as those for the latter electrodes, clearly indicating the ‘‘window’’ effect of the n-type amorphous or microcrystalline Si layer. A photoelectrochemical cell equipped with a Si electrode of the present type and a Pt-plate counterelectrode, separated with a cation exchange membrane, photodecomposed hydrogen iodide into hydrogen and iodine with a solar-to-chemical conversion efficiency of 10.8% (air mass 1, 100 mW/cm2 ), the highest of the efficiencies so far reported for the direct solar-to-chemical conversion without any externally applied voltage.
Gate current of modulation-doped field-effect transistors
Authors: Ruden P. P. , Han C. J. , Shur M.
Published: July 2002

We present a new room-temperature model for modulation-doped field-effect transistors and heterojunction insulated gate field-effect transistors that accounts for effects induced by the gate current at large gate voltages. For large gate currents the quasi-Fermi levels in the GaAs channel and the AlGaAs barrier layers are different because of the limited electron exchange imposed by the heterojunction band offset. As a consequence, the channel electron concentration for large gate voltages exceeds the value expected for the n-AlGaAs/GaAs material system in thermal equilibrium. The channel electron capacitance can have two peaks as a function of gate voltage leading to similar structure in the gate voltage dependence of the transconductance. We find good agreement between our calculated and measured gate currents as a function of the gate voltage.
Type conversion near the p-Si substrate surface by growing GaAs on Si substrates
Authors: Nishioka Takashi , Itoh Yoshio , Yamamoto Akio , Yamagichi Masafumi
Published: July 2002

Type conversion was observed near the p-type Si substrate surface by growing GaAs on the Si. Electron-beam induced currents and the spectral response of GaAs/Si solar cells indicated n-p junction formation in the Si near the interface. It was found that the type conversion was attributed not to As diffusion in the Si during the growth process, but to a positive interface charge in the GaAs/Si system. The interface charge density was estimated to be ≳8×1012 cm−2. Secondary-ion mass spectrometry depth profiles showed a certain amount of C accumulation at the interface, which had little effect on the type conversion.
Microhardness measurements on dopant modified superconducting YBa2Cu3O7 ceramics
Authors: Ling H. C. , Yan M. F.
Published: July 2002

We have studied the microhardness of undoped and dopant modified YBa2Cu3O7 superconducting ceramics as a function of load and dopant concentration using the Vickers indentation technique. In the undoped composition, the hardness value increased monotonically with decreasing load. We concluded that the higher hardness values, measured at lower loads, represent the hardness of single-crystalline grains. The higher loads [∼100–200 gf (1–2 N)] usually resulted in complex crack patterns and extensive crack propagation along the grain boundaries. With Zn or Li as the substitutional dopant in the Cu site, the hardness increased linearly with the dopant concentration up to 10% of dopant to Cu ratio at a load of 25 gf (0.25 N). Our data can be modeled by a solute solution strengthening mechanism.
Nonradiating electromagnetic oscillations of a thin metal film with rough surfaces
Authors: Paulick Thomas C.
Published: July 2002

A simple model is developed for the surface–plasmon polariton on a rough surface. Only plane-surface Maxwell boundary conditions are used, with the roughness entering through a single complex-valued parameter. The calculated dispersion relation for silver in air agrees well with published data over a useful frequency range. The model is extended to describe the corresponding nonradiating modes of a thin film with two rough surfaces. The resulting dispersion relation for silver films again corresponds closely to the available data. The complex roughness parameter can be calculated from published perturbation theories. The transmission of an ordinarily opaque metal film can be very large under kinematical conditions in which the internal excitation couples to radiation at both rough surfaces.
Time-dependent theory of optical-beam-induced current imaging of defects in semiconductors
Authors: Pester P. D. , Wilson T.
Published: July 2002

We calculate the beam-induced currents in a semi-infinite Schottky barrier due to arbitrary spatial and temporal excitation. We specialize it to the case where the spatial generation represents optical excitation and the temporal term represents a step function in light intensity. It is shown how, by collecting images at short times after carrier injection before the diffusion process dominates, the resolution of the optical-beam-induced contrast imaging system may be increased.
Multiple-scattering theoretic approach to the thermal expansion of inhomogeneous materials
Authors: Ballabh T. K. , Middya T. R. , Basu A. N.
Published: July 2002

Expression for the effective thermal expansion coefficient (TEC) of statistically homogeneous and isotropic, inhomogeneous material has been derived within the framework of the multiple-scattering theory. Then, from the general expression, effective TEC is obtained in the single-grain scattering approximation, for this type of materials, consisting of piecewise homogeneous phases (grains). For simplicity, only spherical grains are considered. Previous exact results for two-phase composites and polycrystals with crystallites having a preferred axis (e.g., tetragonal, trigonal, and hexagonal) are shown to follow from this approach in the single-grain scattering approximation. Apart from this, it is shown that the effective TEC for general multiphase composites and polycrystals can be obtained in a self-consistent way. Finally, the self-consistent solutions thus obtained have been employed to calculate the effective TEC of polycrystals belonging to different symmetry classes, and the results are found to compare favorably with observations.
Diffusion of silicon in Pd2Si during silicide formation
Authors: Comrie C. M. , Egan J. M.
Published: July 2002

Inert and radioactive markers have been used to study the mechanism of diffusion during Pd2Si formation. With the aid of Ti as an inert marker it has been shown that silicon is the dominant diffusing species during polycrystalline Pd2Si formation. When radioactive silicon is used as a marker it is found that the radioactive silicon is uniformly distributed throughout the Pd2Si after silicide formation. The self-diffusion of silicon in Pd2Si was investigated and found to be much lower than that necessary to produce a uniform radioactive silicon distribution, had silicon diffused by a grain boundary or pure interstitial mechanism. It is therefore proposed that silicon diffuses by a vacancy mechanism during silicide formation.
Frequency dependence of response of the D
X center in AlGaAs and its influence on the determination of the band discontinuity of GaAs/AlGaAs heterojunctions
Authors: Subramanian S.
Published: July 2002

An analysis to simulate the apparent carrier profiles of n-N heterojunctions obtained from capacitance-voltage measurements is presented. The analysis takes into account the partial ionization and the frequency dependence of the response of deep donors, if any, present in the N material. In the case of GaAs/AlGaAs heterojunctions, the latter effect has not been included in the previous analyses but appears to be important since the reciprocal time constant of the Si-related DX centers in AlGaAs at room temperature seems to be comparable to the frequency of the test signal (1 MHz) normally used for capacitance measurements.
Surface effect on spin waves in semi-infinite ferrimagnets
Authors: Lin D. L. , Zheng Hang
Published: July 2002

The surface effect on the spin-wave spectrum in two-sublattice ferrimagnets is investigated by means of the retarded Green’s function equation of motion method. The two-sublattice Heisenberg model with nearest neighbor interactions is assumed in our discussion. We find that for a NaCl structure, the (001) free surface spin-wave spectrum splits up into acoustic and optical branches as usual. For the case of a CsCl structure, the (001) surface spin wave depends solely upon the nature of the magnetic ions in the surface layer. It has only the optical branch when the surface layer belongs to one sublattice, and has only the acoustic branch when the surface layer belongs to the other. Other properties are also discussed.
Gettering mechanisms in silicon
Authors: Polignano M. L. , Cerofolini G. F. , Bender H. , Claeys C.
Published: July 2002

Various procedures for heavy metal gettering in silicon p-n junctions have been compared in order to test the effectiveness of dopants and extended defects as getter sites; the role of silicon interstitials in the gettering process has also been studied. It has been found that only phosphorus and boron atoms are effective getter sites, while arsenic and antimony are not; such gettering effectiveness is independent of the presence of extended defects in the heavily doped regions. During a moderate temperature annealing (segregation annealing) dissolved metal impurities diffuse from the space-charge region of the devices and segregate at getter sites. Extended defects generated by oxygen precipitation and stacking fault backside damage have some ability to capture metal impurities, but electrical tests show that they do not provide a satisfactory gettering technique. The role of self-interstitials consists of increasing the dissolution rate of metal precipitates with subcritical radii, so that the segregation of metal impurities into getter sites is made easier; however, a complete gettering process requires three steps, that is: (1) the creation of effective getter sites; (2) the dissolution of metal precipitates; (3) the segregation of dissolved metal impurities at getter sites. Some of these steps may be activated simultaneously.
Analysis of planar channeling effects on the threshold voltage uniformity of GaAs metal-semiconductor field-effect transistors using stereographic projection
Authors: Mikami Hitoshi , Uchitomi Naotaka , Toyoda Nobuyuki
Published: July 2002

Planar channeling effects on the threshold voltage uniformity of GaAs metal-semiconductor field-effect transistors within 3-in. GaAs wafers have been investigated using the stereographic projection method. This method is very useful for a quantitative understanding of the angular relationship between the direction of an incident ion beam and wafer orientation during implantation. The ratio N(X1)/N(X2) of the peak carrier concentration at a depth of X1 to the carrier concentration of a depth of X2=2X1, obtained from the carrier depth profiles, was employed as a suitable parameter which effectively reflects the variation in the carrier profiles arising from the planar channeling effects. The conclusion, that the most uniform implants are attained at an azimuthal angle of 26.5° when tilting the wafer 10°, was analytically derived from the stereographic projection method and was experimentally confirmed.