bluuebunny/crossref_metadata_2025 huggingface dataset:
Double-resonance overtone photofragment spectroscopy of trans-HONO. II. State- and time-resolved dissociation and OH-product state distributions
Authors: Reiche F. , Abel B. , Beck R. D. , Rizzo T. R.
Published: July 2002

Linewidths, unimolecular dissociation rates and product state distributions (PSDs) have been measured for single rovibratational states of the ν1=5–7 levels of gas-phase trans-nitrous acid (HONO) by double-resonance overtone photofragment spectroscopy (DROPS). The linewidth measurements, together with the unimolecular dissociation rates in 5ν1, suggest that the intramolecular dynamics are not statistical but rather depend sensitively upon specific intramolecular couplings and the vibrational character of the initial state. Comparison with calculated rate constants from statistical unimolecular rate theory reveals that intramolecular vibrational energy redistribution (IVR) is the rate determining step in the dissociation of HONO subsequent to vibrational overtone excitation. Despite this, we find the measured product state distributions to be close to the predictions of statistical theory. We explain these observations in terms of a simple tier model incorporating hierarchical IVR. The experimental findings underscore the importance of the preparation technique, and hence the nature of the initially excited state, in determining the subsequent intramolecular dynamics.
Magnetotransport and microstructure of annealed magnetic tunnel junctions
Authors: Schmalhorst J. , Brückl H. , Reiss G. , Gieres G. , Wecker J.
Published: July 2002

We investigated the structural, the magnetic, the magnetotransport, and the tunneling properties of CoFe/Al2O3/Ni81Fe19 junctions with an artificial ferrimagnet as a pinning layer for different Al thickness and oxidation time after isochronal annealing up to 500 °C. The main purpose of these experiments is to find relations between the structural changes upon annealing and the modifications of the physical magnetotransport and barrier properties, which could also be important for the further application of such tunneling elements in spinelectronics. The tunneling magnetoresistance (TMR) shows a strong increase up to 37% after annealing at 300 °C accompanied by an improvement of the dielectric stability and the voltage dependence of the TMR. At higher temperature, the TMR starts to decrease. The dielectric stability remains good up to annealing temperatures of 500 °C, indicating an excellent thermal stability of the Al2O3 barrier. All results can be related to thermally induced structural changes of the microstructure which have been determined by Auger depth profiling and complementary methods.
Compositional intermixing at CdS/Cu(In,Ga)Se2 rough interface studied by x-ray fluorescence
Authors: Kim S. , Soo Y. L. , Kioseoglou G. , Kao Y. H. , Ramanathan K. , Deb S. K.
Published: July 2002

The effects of compositional intermixing and high interfacial roughness in a series of CdS/Cu(In,Ga)Se2 heterojunctions have been investigated using the technique of angular dependence of x-ray fluorescence. In the present case, the average interfacial roughness actually exceeds the nominal thickness of CdS film. A method of data analysis has been worked out to account for the large roughness and this technique allows a possibility of nondestructive determination of the concentration profile of both CdS and Cu(In,Ga)Se2 as well as the effective roughness parameters in the system.
Nanostructures of the turbostratic BN transition layer in cubic BN thin films deposited by low-pressure inductively coupled plasma-enhanced chemical vapor deposition
Authors: Yang H. S. , Iwamoto C. , Yoshida T.
Published: July 2002

Nanostructures of the turbo-stratic boron nitride (tBN) transition layer in cubic BN (cBN) thin films deposited on silicon substrates by low-pressure inductively coupled plasma-enhanced chemical vapor deposition have been studied by means of cross-sectional high-resolution transmission electron microscopy. The variation of tBN (0002) basal plane orientation at different growth stages, from random to texture and then random again, was observed. Quantitative analysis of tBN orientation revealed that cBN nucleation occurred when approximately 64% of the tBN c axis oriented parallel to the substrate surface in the angle ±10° and the distribution function drawn in a polar diagram was approximated by the function: cos6 (θ). At the interface between tBN and cBN, both hexagonal and rhombohedral configurations were observed, and both were found to act as structural precursors and/or deposition sites for cubic boron nitride nucleation. In addition, the compressed tBN configuration accompanied by d spacing contraction and distortion was directly observed.
Bias-dependent photoluminescence in CdTe photovoltaics
Authors: Shvydka Diana , Karpov V. G. , Compaan A. D.
Published: July 2002

We show that external bias significantly affects the photoluminescence (PL) in CdTe photovoltaics. The main observations are: (1) reverse bias suppresses PL, (2) PL increases with moderate forward bias and tends to saturate when it is above the open-circuit voltage, and (3) PL in the region of saturation is extremely sensitive to device degradation. We attribute the observed phenomena to the competition between the field-induced separation of electrons and holes and their nonradiative recombination. We have developed a model that describes bias-dependent PL more quantitatively and forms a basis for using it as an indicator of device degradation.
Photodissociation of ethylbenzene at 248 nm
Authors: Huang Cheng-Liang , Jiang Jyh-Chiang , Lin Sheng Hsien , Lee Yuan T. , Ni Chi-Kung
Published: July 2002

Photodissociation of jet-cooled ethylbenzene at 248 nm was studied using VUV photoionization/multimass ion imaging techniques. The photofragment translational energy distribution obtained at 248 nm showed that after the excitation 75% of the ethylbenzene molecules dissociate from electronic excited state, and the rest 25% of the molecules dissociate through a hot molecule mechanism. This is the first experimental evidence which proves that the dissociation of alkyl-substituted benzenes can occur not only from hot molecule mechanism in this UV region.
Thin film alloy mixtures for high speed phase change optical storage: A study on (Ge1Sb2Te4)1−x(Sn1Bi2Te4)x
Authors: Lee Tae-Yon , Kim Ki-Bum , Cheong Byung-ki , Lee Taek Sung , Park Sung Jin , Lee Kyeong Seok , Kim Won Mok , Kim Soon Gwang
Published: July 2002

An approach is proposed to develop recording materials for high speed phase change optical data storage. It utilizes a thin film alloy mixture between a stoichiometric GeSbTe alloy and an additive ternary telluride alloy. Selection rules for an additive alloy are suggested. For a test, (Ge1Sb2Te4)1−x(Sn1Bi2Te4)x thin films are deposited by co-sputtering and their structural and thermal properties are studied. Ge1Sb2Te4 and Sn1Bi2Te4 are found to form a completely soluble pseudo-binary system, whose crystalline lattice parameters obey Vegard’s rule over the entire range of x (0<x<1). Furthermore, the alloy mixtures display an increasing tendency for crystallization with Sn1Bi2Te4 content. Dynamic tests of disk samples are made to show the effectiveness of the approach for high speed erasure.
One-flux theory of a nonabsorbing barrier
Authors: Gildenblat Gennady
Published: July 2002

Equations of the one-flux method are integrated to obtain the scattering matrix for a nonabsorbing barrier with an arbitrary potential profile. Earlier solutions for a field-free and uniform-field region are recovered as special cases of a general result. An important expression representing the results of the Monte-Carlo simulations for the “downstream” backscattering coefficient in a strong field is derived directly from the one-flux method. The generalized Bethe criterion is obtained formally within a general context.
Radial electric fields and global electrostatic microinstabilities in tokamaks and stellarators
Authors: Villard L. , Bottino A. , Sauter O. , Vaclavik J.
Published: July 2002

The effects of applied radial electric fields on the stability of ion temperature gradient (ITG) modes and trapped particle modes are investigated with a full radius gyrokinetic formulation in both axisymmetric and helically symmetric configurations. The validity of a simplified stabilization criterion often applied to experimental analysis (the shearing rate larger than the linear growth rate) is examined for various profiles of E×B flows using an expression for the shearing rate derived for arbitrary quasisymmetric geometries [T. S. Hahm, Phys. Plasmas 4, 4074 (1997)]. The results show that the criterion holds for profiles with finite shearing rate and for toroidal, helical, and slab-like ITG modes. However, it is not always applicable for trapped particle instabilities: a case is shown for which E×B flows are destabilizing. For profiles with zero shearing rate, another stabilizing mechanism is found for toroidal and helical ITG modes but not for slab-like modes.
Atomic level characterization of ultrathin flat cobalt disilicide film with three crystalline domains
Authors: Ohtomo Takumi , Kawasaki Tadahiro , Takai Yoshizo
Published: July 2002

The crystal structure and interface structure of ultrathin cobalt disilicide films, prepared on a Si(001) substrate at 400 °C, have been examined using transmission electron microscopy (TEM). Analysis of the electron diffraction data reveals that there are three general types of epitaxy between the silicon substrates and the cobalt disilicide films: (1) CoSi2(110)//Si(001), CoSi2[001]//Si[110]; (2) CoSi2(110)//Si(001), CoSi2[1̄10]//Si[110]; and (3) CoSi2(001)//Si(001), CoSi2[110]//Si[110]. Using cross-sectional high-resolution TEM for all three types of epitaxy showed that (1) the cobalt disilicide films form an ultrathin flat layer and (2) the interface between the substrate and the film is abrupt with an atomic level for all three types of epitaxy. Dark-field observations also revealed unique localized distributions of three different types of crystalline domains. The domains of CoSi2(110) had elliptic and those of CoSi2(001) had round shapes that were several hundred nanometers in diameter. Each domain consisted of small crystalline epitaxial particles approximately 5–30 nm in diameter as well as an amorphous phase consisting of a cobalt disilicide crystal structure characterized by short-range order.
Light-induced effects on transport in hydrogenated amorphous silicon-sulfur alloys at different temperatures
Authors: Yoon Jong-Hwan , Kim Kyoseon , Park Jin Yong
Published: July 2002

The effect of light illumination on the electronic transport in hydrogenated amorphous silicon-sulfur alloys (a-SiSx:H) has been investigated at various temperatures. The light-induced effects were monitored using photoconductivity and dark conductivity measurements, including subgap defect density by the constant photocurrent method. The light-induced changes in the dark conductivity, as illumination temperature increases, reveal a transition from normal to inverse Staebler–Wronski (S–W) behavior at a critical temperature (Tc) that decreases with increasing sulfur concentration. On the other hand, the changes in the photoconductivity reveal normal S–W behavior at all the observed temperatures, but those are significantly reduced with increasing temperature and sulfur concentration. Correlation between subgap defect density and photoconductivity indicates that the defects created at temperatures higher than Tc act as inefficient recombination centers as compared with those created at temperatures lower than Tc. Two competing reaction processes, controlled by hydrogen diffusion, for light-induced defect generation are proposed to account for the experimental results.
Bias dependence of magnetoresistance in Fe–Al2O3 granular thin films
Authors: Boff M. A. S. , Geshev J. , Schmidt J. E. , Flores W. H. , Antunes A. B. , Gusmão M. A. , Teixeira S. R.
Published: July 2002

This paper reports on the magnetotransport behavior of Fe–Al2O3 granular thin films when the injected dc current is varied. The electric resistance as a function of temperature, magnetoresistance, and the current vs applied bias potential measurements were used to characterize the samples. It was found that the transport mechanism which best describes the electronic properties of these samples is variable range hopping. Non-Ohmic behavior was observed and is claimed as responsible for the great modification of the electronic characteristics of the system as a function of the applied bias potential. Inversion of the tunneling magnetoresistance is observed for applied bias potential greater than 3 V. Such inverted magnetoresistance comes from the activation of low resistivity tunneling paths that are promoted by increasing the bias potential. An expression is proposed to describe the magnetoresistance behavior.
Ammonia-molecular-beam epitaxial growth and optical properties of GaN/AlGaN quantum wells
Authors: Tang H. , Webb J. B. , Sikora P. , Raymond S. , Bardwell J. A.
Published: July 2002

GaN/AlGaN quantum wells on GaN templates have been grown on (0001) sapphire substrates using the ammonia-molecular-beam epitaxy technique. The GaN template layers were of the type used previously for growing high-mobility, heterostructure field-effect transistor structures. The photoluminescence properties of the quantum wells showed strong quantum-confined Stark effect in good agreement with theoretical calculations, as well as evidence of carrier localization due to in-plane well width fluctuation. At low temperature, the quantum well emissions were dominated by two or more localized exciton levels. Carrier thermalization to the lower localized level was observed as the temperature was raised. Evidence of interwell transfer of photocarriers was observed, with the holes appearing to transfer in the direction of the internal field within the AlGaN barrier.
Role of cyclic process in the initial stage of diamond deposition during bias enhanced nucleation
Authors: Hayashi Y. , Matsushita Y. , Soga T. , Umeno M. , Jimbo T.
Published: July 2002

The initial nucleation of diamond film on a Si substrate, deposited by a three-step growth process together with a cycle bias enhanced nucleation (BEN) growth/H2 plasma etching technique in a microwave plasma enhanced chemical vapor deposition system has been investigated by atomic force microscopy, Raman spectroscopy and x-ray photoelectron spectroscopy. The surface morphology and the uniformity of {100}-oriented textured grains at the BEN step were found to be greatly increased by applying the cyclic process with some optimal H2 plasma etching time during the BEN stage.
Nonequilibrium optical phonon generation by steady-state electron transport in quantum-cascade lasers
Authors: Spagnolo Vincenzo , Scamarcio Gaetano , Troccoli Mariano , Capasso Federico , Gmachl Claire , Sergent A. Michael , Hutchinson Albert L. , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

Observation of the nonequilibrium optical phonons population associated with electron transport in quantum-cascade lasers is reported. The phonon occupation number was measured in the range 75–280 K by using a combination of microprobe photoluminescence and Stokes/anti-Stokes Raman spectroscopy. The excess phonon population is observed to decrease as the lattice temperature increases. From the nonequilibrium phonon population, we extracted interface phonon lifetimes of 5 ps at 75 K and 2 ps at 280 K.
Stress as a governing parameter to control the crystallization of amorphous silicon films by thermal annealing
Authors: Hazra Sukti , Sakata Isao , Yamanaka Mitsuyuki , Suzuki Eiichi
Published: July 2002

We have fabricated nanocrystalline dots by thermal annealing (TA) of thermal chemical-vapor-deposited amorphous silicon (a-Si:H) films. In S. Hazra, I. Sakata, M. Yamanaka, and E. Suzuki, Appl. Phys. Lett. 80, 1159 (2002), we observed that ultrathin a-Si:H films (2–10 nm) are stressed because of the presence of deformed crystallites or paracrystallites. With the increase of thickness, volume fraction of paracrystallites decreased in the films and stress in the films gradually reduced. Therefore, by changing the thickness, we can control the stress in the a-Si:H films and thereby the dimensions (in the range of 5 to 10 nm) as well as volume fractions of nanocrystallites formed by TA. On the other hand, it has been found that relaxed a-Si:H networks form polycrystalline films by TA.
Rectifying characteristic in all-perovskite oxide film p-n junction with room temperature ferromagnetism
Authors: Zhang Jun , Tanaka Hidekazu , Kawai Tomoji
Published: July 2002

We fabricated an all-perovskite oxide p-n junction comprised of hole-doped (p-) manganite La0.9Ba0.1MnO3 and electron-doped (n-) titanate Sr0.99La0.01TiO3 films. The junction showed good rectifying properties at both room temperature and low temperature in a simple structure without inserting an insulating layer. By optimizing junction fabrication conditions, a thin La0.9Ba0.1MnO3 layer in the junction exhibited room temperature ferromagnetism and metallic conduction, which may be modulated by carrier injection from the n-type layer under an electric field. These results indicate that this p-n junction may be developed into functional, strongly correlated electronic devices able to work at room temperature.
Current-induced dendritic magnetic instability in superconducting MgB2 films
Authors: Bobyl A. V. , Shantsev D. V. , Johansen T. H. , Kang W. N. , Kim H. J. , Choi E. M. , Lee S. I.
Published: July 2002

Magneto-optical imaging reveals that in superconducting films of MgB2 a pulse of transport current creates avalanche-like flux dynamics where highly branching dendritic patterns are formed. The instability is triggered when the current exceeds a threshold value, and the superconductor, shaped as a long strip, is initially in the critical state. The instability exists up to 19 K, which is a much wider temperature range than in previous experiments, where dendrites were formed by a slowly varying magnetic field. The instability is believed to be of thermomagnetic origin indicating that thermal stabilization may become crucial in applications of MgB2.
Reaction steps of silicidation in ZrO2/SiO2/Si layered structure
Authors: Muraoka Kouichi
Published: July 2002

Reaction steps of silicidation in ZrO2/SiO2/Si layered structure have been investigated in terms of ultrahigh vacuum annealing. Comparison of 2- and 20-nm ZrO2 films at 920 °C revealed that the trigger of silicidation is the contact of ZrO2, SiO, and Si accompanying disappearance of interfacial SiO2 layer due to SiO desorption. In the contact position, a small amount of SiO gas can easily change ZrO2 to ZrSi2. Moreover, this reaction model is also applicable to the silicidation of gate polycrystalline-Si (poly-Si)/ZrO2 interface.
Green luminescence of Er3+ in stoichiometric KYb(WO4)2 single crystals
Authors: Mateos X. , Güell F. , Pujol M. C. , Bursukova M. A. , Solé R. , Gavaldà Jna. , Aguiló M. , Dı́az F. , Massons J.
Published: July 2002

We grew good-optical-quality KYb(WO4)2 single crystals doped with erbium ions by the top seeded solution growth slow cooling method. Optical absorption of erbium was performed at room temperature (RT) and at 6 K. Green photoluminescence of erbium was achieved at RT and 6 K after selective excitation of ytterbium ions at 940 nm (10 638 cm−1). The splitting of all found excited energy levels and the ground energy level of erbium in KYb(WO4)2 is presented derived from the accurate absorption and emission measurements, respectively. The lifetime of the Stokes and the anti-Stokes green emissions of erbium were measured after excitation at 488 nm (20 490 cm−1) and 940 nm(10 638 cm−1), respectively. We propose applying the up-conversion model to the observed green emission.
Channeling and percolation in two-dimensional chaotic dynamics
Authors: Chaĭkovsky D. K. , Zaslavsky G. M.
Published: July 2002

The Hamiltonian dynamics of a particle moving in a nearly periodic two-dimensional (2-D) potential of square symmetry is analyzed. The particle undergoes two types of unbounded stochastic or random walks in such a system: a quasi-1-D motion (a ‘‘stochastic channeling’’) and a 2-D motion which results from a sort of stochastic percolation. A scenario for the onset of this stochastic percolation is analyzed. The threshold energy for percolation is found as a function of the perturbation parameter. Each type of random walk has the property of intermittency. The particle transport is anomalous in certain energy intervals.
Dynamics of a nonlinear parametrically excited partial differential equation
Authors: Newman W. I. , Rand R. H. , Newman A. L.
Published: July 2002

We investigate a parametrically excited nonlinear Mathieu equation with damping and limited spatial dependence, using both perturbation theory and numerical integration. The perturbation results predict that, for parameters which lie near the 2:1 resonance tongue of instability corresponding to a single mode of shape cos nx, the resonant mode achieves a stable periodic motion, while all the other modes are predicted to decay to zero. By numerically integrating the p.d.e. as well as a 3-mode o.d.e. truncation, the predictions of perturbation theory are shown to represent an oversimplified picture of the dynamics. In particular it is shown that steady states exist which involve many modes. The dependence of steady state behavior on parameter values and initial conditions is investigated numerically.
Computing Dirac’s atomic hydrogen wave functions of the continuum, using summation of mathematically divergent series
Authors: Belinfante F. J.
Published: July 2002

Accurate computation of the positive and negative energy wave functions for the continuous parts of the atomic hydrogen spectrum, at large distances from the proton, provides an example of the problems in which sums of mathematically divergent series have to be computed. Here it is shown how such summations can be performed and what checks on the accuracy of the methods are used. Similar methods should be applicable in many other cases.
Dual channel formation in a laser-triggered spark gap
Authors: Kushner M. J. , Kimura W. D. , Ford D. H. , Byron S. R.
Published: July 2002

During self-break in spark-gap switches, multiple streamers can form in close proximity to one another. The rate of expansion of these streamers is sufficiently fast that they can interact during the current pulse. To help understand how these closely spaced, expanding spark columns interact, a laser-triggered spark gap has been studied in which two parallel columns (separation 1.3 mm) are simultaneously preionized, resulting in a pair of nearly identical, axisymmetric spark columns. The spark gap (electrode separation 1.2 cm) switches a 100 ns, 40–60 kV, 12-20 kA, 1.5 Ω waterline. Interferograms of the expanding arc channels are obtained with a laser interferometer having a time and spatial resolution of 5 ns and 10 μm, respectively. Voltage and current were measured with an internal capacitive-voltage divider and a current viewing resistor. The interferograms show that for initially identical axisymmetric columns, the individual channels do not merge into a single larger axisymmetric spark column. Instead, regions of high gas density remain inside the combined column long into the recovery period. The columns also do not remain axisymmetric as they grow, indicating a long-range interaction between the channels. The voltage drop and resistance of the dual channel spark gaps changes by less than 15% from that of a single spark channel. A scaling model is presented to explain the resistance measurements and to predict the change in resistance for multichannel spark gaps.
Determination of the dynamic response of AD-85 alumina with in-material Manganin gauges
Authors: Rosenberg Z. , Yeshurun Y.
Published: July 2002

Plate impact experiments were performed on commercial alumina disks (AD-85, manufactured by Coors), with in-material Manganin gauges, in order to determine the dynamic response of this material in the 0–100 kb range. The measured stress signals showed that the transmitted waves, at the high stress range, are composed of an initial elastic jump, to about 60 kb, followed by a dispersive rise to the final stress level. This behavior was found recently for pure alumina but not for AD-85. We used the gauges to determine the Hugoniot curve of AD-85 in the 0–100 kb range and its spall strength in this region.
Atom movements of gold in lead-tin solders
Authors: Hu Chao-Kun , Huntington H. B.
Published: July 2002

Measurements have been made of the diffusivity and electromigration of 195Au in lead-tin solders in the composition range 0–12 at. % tin. A marked decrease in the diffusivity is attributed to tin trapping of gold with a binding energy of about 0.21±0.03 eV. The addition of the tin has no perceptible effect on the electromigration of the gold. A curious minimum in the Dx/D vs concentration x appears in the concentration range, 1–5 at. % tin, for specimens that have been prepared by a slow quench from the melt. This effect disappears when the specimens are heated above 180 °C. The origin of this behavior is still a topic of open speculation.
Thermoelectric and thermal properties of GaAlAs Peltier-cooled laser diodes
Authors: Hava S. , Sequeira H. B. , Hunsperger R. G.
Published: July 2002

Analyses of heat spreading, temperature distribution, and resultant cooling effects in a monolithically Peltier-cooled laser (MPCL) structure are presented. The analyses were obtained by using Laplace’s equation and were made under steady-state conditions, assuming constant thermal conductivity. In this MPCL structure a metal surface layer surrounds a heat-generating p-n laser junction. It is shown that by depositing relatively thick metallic cooling plates a 15% temperature reduction and 25% thermal spreading can be achieved. This heat spreading due to the passive cooling is added to the cooling obtained when the Peltier cooler is operated. Experimental measurements of the effect of Peltier cooling reveal a 6.8 °C reduction in junction temperature corresponding to a wavelength shift of as much as 20 Å.
Time-Fourier transform by a focusing array of phased surface acoustic wave transducers
Authors: Wilcox Jaroslava Z. , Brooks Robert E.
Published: July 2002

The generation of Fourier transforms of a time varying signal by frequency-dependent beam steering of focused surface acoustic waves (SAWs) is described. This transform is accomplished by using a focusing array of phased SAW interdigital transducers (IDT) to generate, focus, and angularly disperse the acoustic waves according to the input signal frequencies. The spectrum is retrieved at the output by an array of independent SAW IDTs placed along the focal arc. Substrate anisotropy affects SAW diffraction, but need not present a problem when properly accounted for. This paper discusses the rationale for the design of the input transducer and its effect on the acoustic diffraction pattern, frequency response, and sidelobe suppression on an anisotropic substrate.
Transport theory of the double heterojunction bipolar transistor based on current balancing concept
Authors: Lee Si-Chen , Lin Hao-Hsiung
Published: July 2002

The effect of the two-interface conduction-band spikes of a double heterojunction bipolar transistor on the current transport was theoretically studied. The model was based on the current-balancing concept at both the emitter-base and base-collector heterojunctions. The analytical expressions for the current-voltage characteristics and the offset voltage are derived and relate to material parameters. It is found that the back-and-forth bouncing of carriers between two potential spikes and the associated recombination loss represented by an important parameter ΔE0 play an important role in determining the transistor characteristics. The theory also reveals that in order to eliminate the offset voltage of a double heterojunction bipolar transistor, the emitter and the collector must have the same area, the electron injection efficiency at the base-collector junction must be made close to unity, and the potential spike energy ΔE1 must be smaller than ΔE0.
A study of deep levels by transient spectroscopy on p-type liquid-phase-epitaxial GaxIn1−xAsyP1−y grown on semi-insulating InP
Authors: Pelloie J. L. , Guillot G. , Nouailhat A. , Antolini A. G.
Published: July 2002

p-type layers of InP, Ga0.28In0.72As0.6P0.4 (λ=1.3 μm) and Ga0.47In0.53As (λ=1.65 μm) grown by liquid -phase epitaxy on semi-insulating InP:Fe substrates have been investigated by deep level transient spectroscopy. In InP, we have found near the surface a majority-carrier trap which is located at 0.22 eV above the top of the valence band. A very similar trap (capture cross section and energy) appears with about the same concentration in the quaternary layer but is not present in the ternary layer. Thus, we suggest that this trap could be related to the phosphorus sublattice (e.g., a complex, a vacancy, or a substitutional impurity), caused by a phosphorus depletion produced at the end of the epilayer growth. Other traps have been found in quaternary and ternary layers but are not identified.
Optical superlattices for modulation and deflection of light
Authors: Russell P. St. J.
Published: July 2002

A novel kind of doubly resonant light scattering involving the Floquet–Bloch waves of a fine-period stratified structure is discussed. These waves can be Bragg scattered by a weak coarse-period superlattice provided a certain Bragg-like resonant condition is satisfied. Included among the unique properties of this kind of scattering are wavelength selective wide-angle beam deflection by acoustic waves in the 10 MHz–1 GHz range, the ability to enhance or impair the efficiency of Bragg reflection by a conventional fine-period grating, and, in one particular case, very wide-band modulation of light. Coupled-wave equations describing this interaction are solved for a variety of different geometries, and some potential applications proposed.
Generalization of the model for the J-V characteristics of dc sputtering discharges
Authors: Maniv S.
Published: July 2002

A model for the J-V-P characteristics for the dc diode sputtering discharges had already been developed. Here we analyze this model and show that the J-V law is independent of the specific form of the electric field distribution and of the distribution functions of the velocities with which the ions reach the target. The distribution functions have an influence on the J-V coefficient and on the pressure dependence only. We also show two equivalent possible ways for developing the model: one based on the average velocity of the ions and the other on the average velocity of the electrons. A generalized model which is valid for both discharges: diodes and magnetrons is described herein. Its′ J-V law is a superposition of the terms V3/2 and V2. Experimental results are given to support the model.
Resistance and mobility changes in InGaAs produced by light ion bombardment
Authors: Tell B. , Brown-Goebeler K. F. , Bridges T. J. , Burkhardt E. G.
Published: July 2002

We have measured sheet resistance and mobility changes for a series of In0.53Ga0.47As layers as a result of hydrogen, boron, and beryllium implantation. We find that boron and beryllium implantation can produce a two order-of-magnitude increase in sheet resistance due mainly to a decrease in mobility accompanied by a smaller decrease in the sheet carrier concentration. Hydrogen implantation results in a decrease in sheet resistance due to an increase in electron concentration accompanied by only a small mobility decrease. The increase in sheet resistance due to boron and beryllium implants is not large enough to have obvious application for device isolation.
Origin of the low-frequency internal friction background of gold
Authors: Baur J. , Benoit W.
Published: July 2002

The internal friction (IF) background of gold is studied in the kHz frequency range. Systematic measurements of IF as a function of frequency, strain amplitude, and temperature show that the IF is due to the superposition of two contributions: the thermoelastic effect and a dislocation effect. The thermoelastic effect is responsible for the IF background observed when the strain amplitude tends to zero. It is the only contribution to the IF background which is strain amplitude independent. On the contrary, the dislocation effect contributes only to the strain amplitude-dependent IF background. This effect is proportional to the strain amplitude. In particular, it is zero when the strain amplitude tends to zero. Furthermore, the dislocation contribution is frequency independent. The experimental results show that the dislocation effect cannot be explained by a viscous damping of dislocation motion, but must be related to an hysteretic and athermal motion of dislocations.
Grain boundary electronic states in some simple ZnO varistors
Authors: Gambino J. P. , Kingery W. D. , Pike G. E. , Philipp H. R. , Levinson L. M.
Published: July 2002

The spatial density and energy distribution of grain boundary electronic states were measured in Bi-Co-ZnO and Pr-Co-Al-ZnO varistors. A large density of trap states was observed 0.6 to 0.7 eV below the conduction-band edge in both compositions. The physical origin of these states is extrinsic and may be attributed to Bi or Pr ions in the boundaries.
Effect of a substrate on the phase transformations of amorphous TiSi2 thin films
Authors: Thompson R. D. , Takai H. , Psaras P. A. , Tu K. N.
Published: July 2002

The crystallization of an amorphous TiSi2 evaporated film has been studied on both poly-Si and SiO2 substrates. A metastable form of TiSi2 (base-centered orthorhombic; a=3.62 A, b=13.76 A, and c=3.605 A) [P. G. Cotter, J. H. Kohn, and R. A. Potter, J. Am. Ceram. Soc. 39, 11 (1956)] is formed first at a temperature of approximately 350 °C on both substrates. This phase consumes the entire amorphous layer before undergoing a polymorphic transformation to face-centered orthorhombic TiSi2 (a=8.24 A, b=4.78 A, and c=8.54 A) [F. Laves and H. J. Wallbaum, Z. Kristallogr. 101, 78 (1979)] at 600 and 800 °C on poly-Si and SiO2, respectively. These transformations were investigated using in situ resistivity, x-ray diffraction, and transmission electron microscopy. The room-temperature resistivities observed were 96 and 20 μΩ cm for the base-centered and face-centered TiSi2, respectively. The enhanced polymorphic transformation on poly-Si over SiO2 is explained by a lowering of surface energy barrier to nucleation.
RKKY interactions across yttrium layers in Gd-Y superlattices
Authors: Yafet Y.
Published: July 2002

A simple calculation of the RKKY coupling between two Gd arrays in a Gd-Y superlattice is presented. The calculation is valid for Gd arrays consisting of a single atomic layer. The Fourier transform J(q) of the exchange interaction between two Gd layers separated by Y is taken to be that of Gd layers embedded in an Y matrix. Plausible arguments are given in support of this procedure. The coupling is calculated by making use only of the measured J(q) in bulk Gd and of the calculated generalized susceptibilities of Gd and Y. The coupling is found to be of fairly long range and to be consistent in sign with recent neutron scattering measurements of long-range order in Gd-Y superlattices. Numerical calculations on a model, one-dimensional superlattice with two free-electron metals of different electron densities have not provided a useful guide for calculating RKKY interactions in superlattices and further theoretical work is necessary.
The Faraday rotation of bismuth- and thulium-substituted yttrium iron garnet
Authors: Mitchell T. B. , Wigen P. E.
Published: July 2002

The substitution of bismuth in the dodecahedral site of rare-earth garnet films is known to enhance the Faraday rotation by orders of magnitude. The effect of the bismuth ion on the iron sublattice that produces this enhancement is not known, and experimental evidence suggests that the rare-earth ion is not a significant contributor to the Faraday rotation. As the Faraday rotation probes the orbital angular momentum states of the garnet film, Tm3+-substituted yttrium iron garnet (YIG) films were investigated because of the large orbital contribution to the angular momentum and the crystal-field effect of the Tm ion. The temperature dependence of the Faraday rotation in Tm:YIG did not show any significant variations other than the bismuth enhancement. The major contribution to the Faraday rotation comes from the tetrahedral symmetry site, which is evaluated from the fit of the experimental data to a simple sublattice model. The temperature dependence of the Faraday rotation is presented between 4 and 300 K at wavelengths of 546, 578, and 633 nm. The liquid-phase epitaxial films have a composition of Y2.4−xTmxBi0.6Fe5−yGayO12, where x=1.2, y=0; x=1.9, y=1.3; and x=2.4, y=1.3. The Faraday rotation passes through a compensation point in the Ga-doped samples, and the Faraday rotation is shown to be independent of the Tm concentration.
Spectroscopic studies of plasma during cw laser gas heating in flowing argon
Authors: Mazumder J. , Rockstroh T. J. , Krier H.
Published: July 2002

cw laser gas heating in flowing argon was studied using emission spectroscopy. The local electron temperature was calculated from the 415.8-nm Ari line spectra ratio to the adjacent continuum emission. The two-dimensional temperature fields were utilized in the calculation of laser power absorption, beam refraction, radiative losses, and net thermal energy retained by the gas. The experiments were performed over a range of laser powers from 2.5 to 6.4 kW and flow velocities between 15 and 120 cm/s. The plasmas were determined to absorb as much as 78% of the incident laser energy at high powers and low flowrates. The calculations indicate that thermal energy retained by the gas is in excess of 35% of the incident energy at intermediate power levels and high flowrates. The results compare favorably with independent calorimetric and thermometric measurements as well as a numerical model. The primary conclusion is that laser propulsion appears feasible in that the earlier dynamic concerns have been shown to be nonexistent or circumventable.
Self-consistent domain theory in soft-ferromagnetic media. III. Composite domain structures in thin-film objects
Authors: van den Berg H. A. M. , van den Brandt A. H. J.
Published: July 2002

The self-consistent domain theory, based on micromagnetic principles, is further developed in order to incorporate all possible solenoidal two-dimensional magnetization distributions in plane-parallel thin-film objects with arbitrary lateral shape. A decomposition of the object into a number of disjunct plane-parallel subregions that completely cover the object’s area is put forward. In each subregion, a solenoidal M distribution is defined with the M vector parallel to the subregion’s boundary, so that the M distributions in adjacent subregions properly link either via a continuous transition, or via a 180° wall at the intermediate boundary. Two types of subregions are distinguished; namely, the simple connected regions and the so-called parallel regions, being a special type of multiple connected region. In the first category, the basic structures as defined in the preceding paper on this subject are present. The parallel regions are closed ringlike configurations that are built of simpler units—the parallel segments. A parallel segment is a region bounded by two orthogonal trajectories of the same set of straight lines, while two of these straight lines close the segment at either end. No points of intersection of members of this family of lines are found inside the segment. In a specific parallel region, the distance between the orthogonal trajectories is the same for all segments. Adjacent segments in a parallel region are separated by a domain wall which is the locus of centers inside the cross section of the segments of circles that touch at corresponding orthogonal edges of both of the segments involved. A systematic procedure is developed for constructing the parallel subregions, and it is shown that, with this, all possible two-dimensional solenoidal M distributions can be recovered.
Temperature runaway of microwave irradiated materials
Authors: Roussy Georges , Bennani Abderrahim , Thiebaut Jean-Marie
Published: July 2002

A numerical procedure for calculating the steady-state temperature distribution generated by microwave absorption is described. The procedure can be applied to the case of material for which the permittivity is nonlinearly temperature dependent and can be used to simulate the conditions when temperature runaway occurs. The criteria for temperature runaway are explicitly given for a cylindrical sample which is irradiated with a uniform electric field and is simultaneously cooled by convection. The results illustrate the method of predicting runaway and avoiding it by changing the experimental conditions as a function of the characteristics of the irradiated materials.
Effects of substrate misorientation and background impurities on electron transport in molecular-beam-epitaxial-grown GaAs/AlGaAs modulation-doped quantum-well structures
Authors: Radulescu D. C. , Wicks G. W. , Schaff W. J. , Calawa A. R. , Eastman L. F.
Published: July 2002

Single GaAs quantum wells, clad with Al0.3Ga0.7As, and modulation doped with silicon introduced in the Al0.3Ga0.7As after the quantum wells are grown have been grown by molecular-beam epitaxy on GaAs substrates tilted a few degrees from the nominal (001) plane towards either of the (111) planes. The low-field two-dimensional electron gas mobility is observed to be a function of the tilt angle (0°, 2°, 4°, 6.5°) and of the direction of tilt [towards (111)A or (111)B]. The two-dimensional electron gas mobilities in quantum-well structures grown on substrates tilted towards (111)A are larger than those in structures grown on nominally flat (001) substrates. The improvement in two-dimensional electron gas transport is attributed to an improvement in the quality of the inverted interface (i.e., GaAs grown on AlGaAs). Quantum wells grown on substrates tilted toward (111)A also exhibit larger two-dimensional electron gas mobilities than quantum wells grown on substrates tilted toward (111)B for a given angle of tilt. For quantum-well structures where interface scattering from the inverted interface is significant, the two-dimensional electron gas mobility is observed to be anisotropic and larger in the [110] direction in comparison to the [1̄10] direction. The anisotropy in electron transport in the GaAs quantum well is observed to be larger for structures where the substrate tilt is towards (111)B in comparison to (111)A. For quantum wells grown on substrates tilted toward (111)A the anisotropy in two-dimensional electron gas mobility gets progressively larger as the tilt angle gets smaller. Larger molecular-beam epitaxy machine background impurity concentrations are observed to significantly increase the magnitude of the anisotropy in two-dimensional electron gas mobility suggesting that impurities and/or defects introduced during MBE growth are the origin of the anisotropic transport.
Kr+ laser-induced chemical vapor deposition of W
Authors: Zhang G. Q. , Szörényi T. , Bäuerle D.
Published: July 2002

Kr+ laser-induced pyrolytic direct writing of W stripes by H2 reduction of WF6 has been investigated. The reproducibility of the process and the morphology and electrical properties of deposits depend heavily on the partial pressures of both WF6 and H2; the best results have been obtained with p(WF6)=5 mbar and 100 mbar≤p(H2)≤800 mbar. For a laser focus of 2w0=7 μm and laser powers between 30 and 200 mW, the widths of stripes varied between 1.5 and 15 μm with corresponding thicknesses between 0.1 to 3 μm. The width of stripes is independent of the scanning speed within the range 20 μm/s≤Vs ≤400 μm/s. The electrical resistivities of these stripes were about a factor of 1.3–2.3 larger than the bulk value.
Effects on H− production in a multicusp ion source by mixtures of H2 with H2O, NH3, CH4, N2H4, and SF6
Authors: Orient O. J. , Chutjian A. , Leung K. N.
Published: July 2002

Effects of H− production in a multicusp ion source are measured by separately mixing with hydrogen small amounts (0.33%–10%) of water, ammonia, methane, and hydrazine—molecules which produce large amounts of H− via dissociative attachment (DA) resonances at higher electron energies. The mixing was done in a separate reservoir, with careful measurement of individual pressures. Experimental enhancements of 1.4 and less were observed, whereas calculated enhancements, using accurate DA cross sections for ground-state H2, should have produced factors of 1.5, 3.0, 1.3, and 2.4 enhancements for water, ammonia, methane, and hydrazine, respectively, at a mean electron energy of 1.0 eV in the extraction region. The difference is accounted for by including, in the enhancement calculation, vibrationally and rotationally excited H2 molecules, with v″=5–11, and J″=0–5, and the large DA cross sections for the excited H2(v″,J″). The relative populations of H2(v″,J″) thus obtained are found to be substantially smaller than those predicted by theoretical calculations. The effect on H− current was also studied by mixing small amounts of SF6 with H2. A 1.5% mixture was found to reduce the H− output by one half.
Study of irradiation-induced defects in germanium
Authors: Nagesh V. , Farmer J. W.
Published: July 2002

Radiation-induced defects in germanium were studied using deep-level transient spectroscopy and stress transient spectroscopy. Four electron traps have been identified: the planar four vacancy at Ec −0.09 eV, the divacancy at Ec −0.17 eV, the vacancy-oxygen pair at Ec −0.27 eV, and the donor-vacancy pair level at Ec −0.35 eV.
Initial evaporation rates from GaAs during rapid thermal processing
Authors: Haynes T. E. , Chu W. K. , Aselage T. L. , Picraux S. T.
Published: July 2002

The initial rates of evaporation of Ga and As from the surface of single-crystal GaAs during rapid thermal processing (RTP) have been determined in the temperature range from 600 to 750 °C. Absolute vaporization rates for capless annealing of GaAs were measured by collecting the evaporated species on a copper film, which condenses 50%–100% of the evaporated material. Initial evaporation rates during RTP are shown to be in agreement with the maximum predicted rates based on the Hertz–Knudsen equation. In a second set of experiments, the preferential loss of arsenic for GaAs-GaAs proximity annealing was measured using ion channeling. All experimental results are interpreted in terms of a model which we develop for capless and proximity annealing based on the kinetic theory of gases.
Electrical activity of the first- and second-order twins and grain boundaries in silicon
Authors: Bary A. , Nouet G.
Published: July 2002

The structural and electrical properties of twins in polycrystalline silicon materials have been investigated by electron-beam-induced current and transmission electron microscopy. The electrical activity of the first- and second-order twins is related to segregation effects and to their structure, as described by geometrical models.