bluuebunny/crossref_metadata_2025 huggingface dataset:
Conformation-selective photoionization of covalently-linked diaryl compounds: Excimer-mediated one-color two-photon ionization in seeded beams of 1,3-diphenylpropane
Authors: Lee Jae Kwang , Judge Richard H. , Boo Bong Hyun , Lim Edward C.
Published: July 2002

Hole-burning spectroscopy and ionization threshold measurement were combined with fluorescence excitation (FE) and resonant two-photon ionization (R2PI) spectroscopies to probe conformation dependence of photoionization for jet-cooled 1,3-diphenylpropane (DPP). The S1←S0 excitation energy dependence of excimer fluorescence and that of one-color R2PI indicate formation of an intramolecular excimer and an efficient photoionization of the excimer that occur at higher energies. The energy threshold for the enhanced photoionization, which concurs with that for the excimer formation, is considerably lower for the trans–gauche (tg) conformer than for the trans–trans (tt) or the gauche–gauche (gg) conformer. These results demonstrate the occurrence of conformation selective excimer-mediated photoionization in DPP. The important precursor role of the singlet excimer in the photoionization is attributed to the conformational similarity between the intramolecular singlet excimer and the most stable ground-state cation of DPP, which leads to a very favorable Franck–Condon factor for the photoionization of the excimer. Even for excitation energies far below the energy threshold for excimer formation, the efficiency of one-color R2PI is substantially greater for tg than for tt or gg for photoionization with a high intensity laser. Comparison of the ionization threshold measured with high photon flux with that obtained with low photon flux suggests that the greater ionization efficiency of the tg conformer at high photon flux may be related to direct photoionization of the S1 tg to the ionic state of DPP with T-shaped arrangement of the phenyl moieties.
Drastic coercivity relaxation in amorphous Fe74Al5P11C6B4 and its dependence on the preparation method
Authors: Schlorke-de Boer N. , Schäfer R. , Eckert J. , Schultz L.
Published: July 2002

Amorphous Fe74−xAl5P11C6B4Gax (x=0 or 2) powders with a wide supercooled liquid region were prepared by high energy ball milling of rapidly quenched ribbons and subsequently hot pressed in the viscous state to receive bulk glasses. Almost complete coercivity relaxation of the ball milled powders was achieved by annealing. The coercivity Hc of as-milled ribbons drops drastically from more than 2200 Am−1 after 1 h of milling to 14.6 Am−1 after optimum annealing. The coercivity of this powder and that of the as-quenched ribbons (7 Am−1) differs only by the different contributions of surface irregularities Hcsurf to the total coercivity Hctot. Drastic coercivity relaxation was also achieved by consolidating the as-milled powders. A minimum coercivity Hc of 30 Am−1 was found for the bulk sample after compaction of 1 h ball milled powders. It is of the same order of magnitude as Hc of cast amorphous rods. The milling-induced coercivity increase as well as the coercivity relaxation are discussed in terms of the change of magnetic anisotropy and the creation and elimination of stress and shear bands in the amorphous phase.
Unraveling the highly overlapping Ã 2B2–B̃ 2A1 photoelectron bands of Cl2O: Nonadiabatic effects due to conical intersection
Authors: Mahapatra Susanta
Published: July 2002

The highly overlapping Ã 2B2–B̃ 2A1 photoelectron bands of Cl2O molecule are studied theoretically with the aid of ab initio quantum dynamical methods. The theoretical results are compared with the high-resolution 58.4 nm He I recording of Motte-Tollet et al. [Chem. Phys. 284, 452 (1998)]. The theoretical findings reveal the existence of a conical intersection in the Ã 2B2–B̃ 2A1 electronic manifold of the radical cation (Cl2O+) and the highly overlapping photoelectron bands originate from the associated nonadiabatic interactions. A diabatic vibronic Hamiltonian for the Ã 2B2–B̃ 2A1 interacting manifold of Cl2O+ is constructed in terms of the dimensionless normal coordinates of the neutral Cl2O employing a linear vibronic coupling scheme. The coupling parameters of the Hamiltonian are derived from ab initio electronic structure results. The photoelectron bands are then calculated with this Hamiltonian by solving the eigenvalue equation using a quantum dynamical method and the Lanczos algorithm. The photoelectron bands reveal a prominent progression of the bend vibrational mode and a weak progression of the symmetric stretch vibrational mode of the cation. The vibronic fine structures of the photoelectron bands are carefully examined and an adiabatic ionization energy value of 12.171 eV is estimated for the B̃ 2A1 band. The impact of nonadiabatic coupling between the Ã 2B2 and B̃ 2A1 electronic states of Cl2O+ on the photoelectron dynamics is also explicitly discussed.
Microstructure and magnesium diffusion in YBa2Cu3O7−δ films on bicrystal MgO substrates
Authors: Hao Z. , Wu Y. , Enomoto Y. , Tanabe K. , Koshizuka N.
Published: July 2002

Various kinds of microstructures exist in the c-axis oriented bicrystal YBa2Cu3O7−δ films grown by liquid phase epitaxy (LPE), which have an important effect on the properties of the bicrystal junctions. In our work, distinct configurations of the bicrystal boundary near the bicrystal substrate have been observed directly. The cross-sectional view of different kinds of bicrystal films provides a platform for the study of the mechanism of single crystal growth. Furthermore, magnesium diffusion, introduced by the MgO substrates, has also been studied by a transmission electron microscope equipped with an energy dispersive spectrometer. Grain boundaries in seed layer, thereafter the grain boundaries in the LPE layer, act as an important channel for magnesium diffusion.
Digitally graded active region for optically pumped intersubband lasers and nonlinear wavelength convertors
Authors: Tomić Stanko , Indjin Dragan , Ikonić Zoran , Milanović Vitomir
Published: July 2002

A strategy is proposed for the realization of quantum-well structures optimized for devices based on intersubband optical transitions. It relies on the recently established techniques for the design of optimal smooth quantum-well profile by using the inverse spectral theory, followed by the design of an (almost) equivalent digitally graded structure, comprising just two different alloy compositions. Digital grading greatly simplifies the structure growth while essentially fully retaining the properties achieved in smoothly graded optimized structures. Example designs are presented for an optically pumped quantum-well laser and for a quantum well intended for the second-harmonic generation.
The liquid–solid transition in a micellar solution of a diblock copolymer in water
Authors: Castelletto V. , Caillet C. , Fundin J. , Hamley I. W. , Yang Z. , Kelarakis A.
Published: July 2002

The structure of a diblock copolymer solution in the vicinity of the transition between micellar liquid and solid phases was investigated using small-angle x-ray scattering (SAXS). An amphiphilic poly(oxyethylene)–poly(oxybutylene) diblock was studied in water. Static and dynamic light scattering techniques were used to provide an independent measure of micelle dimensions and aggregation number. Dynamic shear rheometry and mobility measurements were used to locate phase transitions. A micellar liquid phase was identified at low concentration and a cubic micellar phase at higher concentration, the transition between the two occurring at higher temperature as the concentration increased. The cubic micellar phase behaves rheologically as a solid and SAXS confirmed a face-centered cubic structure. Intermediate between these two phases, a viscoelastic soft solid was observed, with a finite yield stress but with a much lower dynamic modulus than the crystalline solid. Several distinct suggestions have been put forward for the structure of the solution in this region. In a poly(oxyethylene)–poly(oxypropylene)–poly(oxyethylene) Pluronic triblock, small-angle neutron scattering and rheology provided evidence for a percolation transition between micellar liquid and solid phases [L. Lobry et al., Phys. Rev. E. 60, 7076 (1999)], indicating a fractal structure of micelles aggregated due to attractive interactions. Alternatively, a defective solid phase has been proposed. We analyzed the structure of solutions of our diblock copolymer via detailed model fits to the SAXS data for concentrations spanning the liquid–solid transition. The micellar form factor was modeled as a homogeneous micellar core with attached Gaussian chains; and the intermicellar structure factor could be described using the hard sphere model. Thus there is no evidence for percolation induced by effective attractive interactions between micelles in our system. In contrast SAXS data indicates there is a coexistence region between hard sphere fluid and solid crystal phases, in which small grains of close-packed crystal coexist with fluid. It is apparent that block copolymer micelles acts as model colloidal systems in which it is possible to investigate the influence of attractive and repulsive interactions between spherical particles by varying the copolymer composition.
AlGaInN-based ultraviolet light-emitting diodes grown on Si(111)
Authors: Kipshidze G. , Kuryatkov V. , Borisov B. , Holtz M. , Nikishin S. , Temkin H.
Published: July 2002

Ultraviolet light-emitting diodes grown on Si(111) by gas-source molecular-beam epitaxy with ammonia are described. The layers are composed of superlattices of AlGaN/GaN and AlN/AlGaInN. The layers are doped n and p type with Si and Mg, respectively. Hole concentration of 4×1017 cm−3, with a mobility of 8 cm2/Vs, is measured in Al0.4Ga0.6N/GaN. We demonstrate effective n- and p-type doping of structures based on AlN/AlGaInN. Light-emitting diodes based on these structures show light emission between 290 and 334 nm.
Scanning tunneling potentiometry search for mesoscopic phase separation in La0.7Sr0.3MnO3
Authors: Grévin B. , Maggio-Aprile I. , Bentzen A. , Kuffer O. , Joumard I. , Fischer O/.
Published: July 2002

Optimally doped epitaxial La0.7Sr0.3MnO3/SrTiO3 thin films have been probed by scanning tunneling potentiometry in a search for possible mesoscopic phase separation. The experiments were performed in ultrahigh vacuum after an in situ deposition, which prevents local inhomogeneities from forming due to surface contamination. Resulting reproducible two-dimensional images reveal clear topographic features and continuous potentiometric variations. These results support the idea that mesoscopic phase separation is not a generic feature shared by all optimally doped manganites.
Spectrally modified chirped pulse generation of sustained shock waves
Authors: McGrane S. D. , Moore D. S. , Funk D. J. , Rabie R. L.
Published: July 2002

A method is described for generating shock waves with 10–20 ps risetime followed by >200 ps constant pressure, using spectrally modified (clipped) chirped laser pulses. The degree of spectral clipping alters the chirped pulse temporal intensity profile and thereby the time-dependent pressure (tunable via pulse energy) generated in bare and nitrocellulose-coated Al thin films. The method is implementable in common chirped amplified lasers, and allows synchronous probing with a <200 fs pulse.
Symbol dynamic maps of spatial-temporal chaotic vibrations in a string of impact oscillators
Authors: Moon F. C. , Holmes W. , Khoury P.
Published: July 2002

Spatially complex, temporally chaotic dynamics of N-coupled impact oscillators connected by a string are studied experimentally using a discrete measure of the motion for each of the masses. For N=8, a binary assignment of symbols, corresponding to whether or not the masses impact an amplitude constraint, is used to code the spatial pattern as a binary number and to store its change in time in a computer. A spatial pattern return map is then used to observe the change in spatial patterns with time. Bifurcations in spatial impact patterns are observed in this experiment. An entropy measure is also used to characterize the dynamics. Numerical simulation shows behavior similar to the experimental system.
KAM theorems for the product of two involutions of different types
Authors: Quispel G. R. W. , Sevryuk M. B.
Published: July 2002

The properties of quasiperiodic motions in Hamiltonian, volume preserving, and reversible systems are summarized. KAM theorems concerning lower-dimensional invariant tori are announced for G-reversible mappings A such that the fixed point manifolds Fix(G) and Fix(AG) of the reversing involutions G and AG are of different dimensions. The case where the manifold Fix(G) itself consists of several connected components of different dimensions is also briefly discussed.
Unbiased reconstruction of the dynamics underlying a noisy chaotic time series
Authors: Jaeger Lars , Kantz Holger
Published: July 2002

Refined methods for the construction of a deterministic dynamical system which can consistently reproduce observed aperiodic data are discussed. The determination of the dynamics underlying a noisy chaotic time series suffers strongly from two systematic errors: One is a consequence of the so-called ‘‘error-in-variables problem.’’ Standard least-squares fits implicitly assume that the independent variables are noise free and that the dependent variable is noisy. We show that due to the violation of this assumption one receives considerably wrong results for moderate noise levels. A straightforward modification of the cost function solves this problem. The second problem consists in a mutual inconsistency between the images of a point under the model dynamics and the corresponding observed values. For an improved fit we therefore introduce a multistep prediction error which exploits the information stored in the time series in a better way. The performance is demonstrated by several examples, including experimental data.
Inverting chaos: Extracting system parameters from experimental data
Authors: Baker G. L. , Gollub J. P. , Blackburn J. A.
Published: July 2002

Given a set of experimental or numerical chaotic data and a set of model differential equations with several parameters, is it possible to determine the numerical values for these parameters using a least-squares approach, and thereby to test the model against the data? We explore this question (a) with simulated data from model equations for the Rossler, Lorenz, and pendulum attractors, and (b) with experimental data produced by a physical chaotic pendulum. For the systems considered in this paper, the least-squares approach provides values of model parameters that agree well with values obtained in other ways, even in the presence of modest amounts of added noise. For experimental data, the ‘‘fitted’’ and experimental attractors are found to have the same correlation dimension and the same positive Lyapunov exponent.
A technique for modeling interference patterns from two-beam interferometers
Authors: Atkinson J. Dudley , Cleary David D.
Published: July 2002

We present a technique for modeling the interference pattern produced by two-beam interferometers such as the Michelson interferometer. This technique performs a ray-trace analysis of the two paths of the interferometer and then constructs the resulting pattern of Fizeau fringes. The program is written for the IBM PC/486 and the Windows operating environment. It was found that a position-dependent phase correction must be made for interferometers that employ diffraction gratings. An algorithm is presented for including the effects of longitudinal coherence and the artifact of moiré patterns are addressed.
Supermodes of multiple-stripe quantum-well heterostructure laser diodes operated (cw, 300 K) in an external-grating cavity
Authors: Epler J. E. , Holonyak N. , Burnham R. D. , Paoli T. L. , Streifer W.
Published: July 2002

The far-field supermode patterns of a phase-locked multiple-stripe quantum-well heterostructure (QWH) laser diode are described as a function of injection current and emission wavelength, the latter controlled by an external grating. The external-grating cavity is used to isolate single or multiple supermodes of the multiple-stripe QWH laser (Pout>170 mW cw, λ∼7400 Å). The progression of supermode patterns consists of a discrete set of mode configurations for each longitudinal mode of the spectrum. The progression is cyclic with a ∼2.8-Å period which corresponds to the longitudinal mode spacing of the diode. Under high gain conditions, i.e., near the center of the recombination-radiation spectrum or at higher current levels, continuous tunability is observed with gradual transitions between supermode eigenstates. As the gain is reduced (low current), the number of supermodes observed decreases until only the in-phase pattern, i.e., each emitter at the same phase, remains above threshold. The far-field patterns range from a double-lobe pattern with a 10° peak separation (5 μm between emitter phase reversals) to a narrow (<2° full angle half power) single-lobe in-phase pattern. The experimental data are compared to the results of coupled-mode analysis.
Magnetic and crystallographic properties of Nd15B7(Fe1−xMx)78
Authors: Yang Ying-chang , Ho Wen-Wang , Chen Hai-ying , Wang Jin , Lan Jian
Published: July 2002

The Nd15B7Fe78-based four-component systems in which Fe is substituted by Co, Mn, and Cr have been studied by x-ray diffraction and magnetic measurements. The lattice constants are found to decrease with raising the concentration of Co and increase with raising the concentration of Mn and Cr. The Curie temperature TC increases significantly with the content of Co, whereas the substitution of Fe with Mn results in a sharp decrease of the Curie temperature. The saturation magnetization and magnetocrystalline anisotropy were measured with a field up to 70 kOe at 1.5 and 300 K. The variation of magnetization as a function of the concentration x of Co shows a maximum around x=0.2. The easy magnetization axis of Nd15B7(FeCo)78 changes with temperature.
Iron-based rare-earth magnets (invited)
Authors: Narasimhan K. S. V. L.
Published: July 2002

Rare-earth iron alloys offer promise for the realization of the highest-energy product attainable. We have investigated rare-earth iron alloys by melt spinning and identified compositions that are beneficial for magnet fabrication. Compositions of the type R3Fe16B and R3Fe20B provide high values of coercive force. Processing of the compositions into magnets resulted in energy product of up to 45 MG Oe with a potential for 47.5 MG Oe. The B-H loop characteristics indicate that these magnets can be used up to 150 °C.
Calculated gain and spontaneous spectra of multi-quantum-well lasers
Authors: Ohtoshi T. , Uomi K. , Chinone N. , Kajimura T. , Murayama Y.
Published: July 2002

Optical gain and spontaneous spectra of multi-quantum-well (MQW) lasers are investigated theoretically. A band-to-band radiative recombination rate is calculated using an energy-dependent density of states and k-selection rule. It is found that the gain and spontaneous spectra are significantly dependent upon degrees of two-dimensionality of subbands created in MQW structure.
Ion implantation of Si and Se donors in In0.53Ga0.47As
Authors: Penna Thereza , Tell B. , Liao A. S. H. , Bridges T. J. , Burkhardt G.
Published: July 2002

Ion implantation of Si and Se donors in In0.53Ga0.47As is reported. Both room-temperature and elevated temperature (200 °C) implants are performed. Rapid thermal annealing as well as conventional furnace anneals have been used. Both species yield relatively shallow profiles with peak electron concentrations ∼1×1019 cm−3 and with sheet resistance less than 20 Ω/sq. Our results indicate that elevated temperature implants are effective in reducing implant damage and are important for heavy ions like Se+ to achieve high activation and mobility.
Photoconductivity and electronic doping effects in hydrogenated amorphous silicon
Authors: McMahon T. J. , Madan A.
Published: July 2002

The concept of electronic doping is used to explain unexpectedly large values of the diode quality factor (exceeding two) and supralinearity which is sometimes observed in amorphous silicon p-i-n-type diodes and materials, respectively. This suggests the presence of an extra set of defect states in lightly boron-doped films with a capture rate for electrons which is much larger than that of the inherent defect states. We also report that for high-quality undoped intrinsic layers, the photoconductivity versus intensity behavior exhibits sublinear power dependence which increases with intensity in distinct contrast to the previously reported results. We provide a self-consistent model which is able to explain the above observations.
Magnetic and magneto-optic properties of Tb-FeCo-Al films
Authors: Aratani K. , Kobayashi T. , Tsunashima S. , Uchiyama S.
Published: July 2002

The influence of an addition of Al on the corrosion resistance of amorphous 100-nm-thick Tb-Fe coevaporated films has been studied. From measurements of the change of the optical transmittance, it is seen that the growth of pin holes is reduced considerably. However, the addition of Al also results in a decrease of the magnetic and magneto-optic properties of the Tb-Fe films such as the Curie temperature, the Kerr rotation, the perpendicular anisotropy, and the coercivity. These problems, however, are recovered easily by replacing part of the Fe by Co. This replacement also contributes to the improvement of the corrosion resistance. It has been shown that Tb-Fe-Co-Al films have a much better corrosion resistance than Tb-Fe or Tb-Fe-Co films, while having the same good magnetic and magneto-optic properties.
Superconductivity and normal-state electronic properties of URu3 and UCo: New behavior for U-compound superconductors
Authors: Chen J. W. , Hake R. R. , Lambert S. E. , Maple M. B. , Rossel C. , Torikachvili M. S. , Yang K. N.
Published: July 2002

The compound URu3 has been found to exhibit superconductivity below 0.15 K. Polycrystalline specimens of URu3 and two other U-compound superconductors, UPt3 and UCo, have been investigated by means of ac magnetic susceptibility measurements in zero and finite magnetic fields. Electrical resistivity, dc magnetic susceptibility, and specific-heat measurements on URu3 and UCo reveal non-heavy-fermion behavior for both compounds and evidence for relatively large exchange enhancement of the Pauli paramagnetic susceptibility for UCo.
Test of the electronic structure of Fe(100) by absorbed current spectroscopy
Authors: Kisker E. , Kirby R. E. , Garwin E. L. , King F. K. , Tamura E. , Feder R.
Published: July 2002

The absorbed electron current for a clean Fe(100) surface as a function of energy rises steplike at the vacuum-energy cutoff with an absorption close to 1. The smooth decrease of absorbed current at higher electron energy due to secondary electron emission is superimposed by a considerable amount of fine structure, the amplitude of which decreases with increasing energy. These features are found in good agreement with the results of a calculation of the elastic part of the electron reflection coefficient. Further, they are compared with the ferromagnetic electronic bulk band structure calculated above the vacuum energy. From the comparison with the experimental data, the energy dependence of the real and imaginary parts of the inner potential is determined.
Double heterostructure lasers with facets formed by a hybrid wet and reactive-ion-etching technique
Authors: Salzman J. , Venkatesan T. , Margalit S. , Yariv A.
Published: July 2002

Double heterostructure lasers were fabricated in which one of the laser facets was produced by a hybrid wet and reactive-ion-etching technique. This technique is suitable for GaAs/GaAlAs heterostructure lasers and utilizes the selectivity of the plasma in preferentially etching GaAs over GaAlAs. Lasers fabricated by this technique are compatible with optoelectronic integration and have threshold currents and quantum efficiency comparable to lasers with both mirrors formed by cleaving. The technique enables the use of relatively higher pressures of noncorrosive gases in the etch plasma resulting in smoother mirror surfaces and further eliminates the nonreproducibility inherent in the etching of GaAlAs layers.
A proposal for p-type ZnS1-xSex–ZnTe superlattices
Authors: Fujiyasu Hiroshi , Mochizuki Koji
Published: July 2002

Energy band structures ZnSe–ZnTe and ZnS0.5Se0.5 –ZnTe superlattices are calculated using the Kronig-Penney model taking into account strain effects due to the lattice mismatches between the materials. The ZnSe(30 Å)–ZnTe(5 Å) and the ZnS0.5Se0.5 (30 Å)–ZnTe(5 Å) superlattices have energy gaps of 2.5 and 2.7 eV, respectively. The ZnS0.5Se0.5 (30 Å)–p-type ZnTe(5 Å) superlattice is considered to be a p-type material of 1017 cm−3 and to be available for a hole injection layer to an n-type ZnSe layer.
Radiation-induced traps in low-energy proton-irradiated GaAs solar cells
Authors: Roux Michel , André Philippe
Published: July 2002

Trapping centers produced by 250-keV, 400-keV, 700-keV, and 1.5-MeV proton irradiations at room temperature in AlGaAs-GaAs solar cells have been studied. The traps detected by deep-level transient spectroscopy are the same as those produced by electron irradiations, only their introduction rates are different. An introduction rate was determined for each trapping center at the various proton energies. The large concentration of the E4 trap suggests it is associated with a cluster-type defect.
Applicability of the microwave cavity perturbation method for conductivity measurements on carbon fibers
Authors: Azzeer Abdallah M. , Silber Leo M. , Spain Ian L. , Patton Carl E. , Goldberg H. A.
Published: July 2002

A microwave cavity perturbation technique has been used to measure the resistance per unit length of carbon fibers. These materials and other test materials used in these measurements such as copper, evenohm, and tungsten, have much higher conductivities than polymeric materials which have been investigated previously with this technique. The 1500–5000-Ω resistance of the carbon fibers lay in the region near the limit of applicability of the perturbation technique, thereby affording a means of investigating this limit. The condition Rs/Zc>2 where Rs is the sample resistance and Zc is the characteristic impedance of the cavity, appears to be a useful condition for estimating the limit of applicability.
Quantitative analysis of carbon in liquid-encapsulated Czochralski GaAs
Authors: Homma Yoshikazu , Ishii Yoshikazu , Kobayashi Takashi , Osaka Jiro
Published: July 2002

The quantitative analysis of carbon in liquid-encapsulated Czochralski (LEC) GaAs crystals has been investigated using secondary ion mass spectrometry (SIMS). Localized vibrational mode (LVM) infrared absorption measurements are calibrated by SIMS results. A LVM absorption of 1 cm−2 arisen from carbon was found to be produced by a carbon concentration of 9.5±2.9×1015 atoms cm−3 at room temperature. This carbon LVM optical cross section is 2.7 times larger than the value which had previously been reported. Carbon concentrations in LEC GaAs crystals are estimated to be 1–3×1015 cm−3 or lower.
Raman study of GaAs-InxAl1−x As strained-layer superlattices
Authors: Nakayama M. , Kubota K. , Kanata T. , Kato H. , Chika S. , Sano N.
Published: July 2002

Raman spectroscopy has been used to study the lattice-mismatch strains in GaAs-InxAl1−xAs strained-layer superlattices grown by molecular beam epitaxy with the layer thicknesses of 10–200 Å and In content x of 0.11, 0.20, and 0.35. The strain-induced shifts of the longitudinal optic phonon modes indicate that the GaAs and InxAl1−xAs layers have the tensile and compressive strains, respectively, along the interfaces. The strain calculated from the observed frequency shift agrees with the lattice-mismatch strain given by the elastic theory.
Generation and propagation of defects into molecular beam epitaxially grown GaAs from an underlying GaAs substrate
Authors: Shinohara Masanori , Ito Tomonori , Imamura Yoshihiro
Published: July 2002

Dislocation and growth striation hereditability from liquid-encapsulated Czochralski (LEC) grown GaAs substrates to molecular beam epitaxially (MBE) grown epilayers is studied for the different types of epilayer structures including homo-, hetero-, and superlattice. Also the correlation of surface defects in the epilayers with dislocations in the substrates is studied. It is found that growth striation in the substrate is not inherited into the epilayer, whereas dislocation propagation strongly depends on the epilayer structure. Dislocation distributions in GaAs epilayers for homo- (GaAs/GaAs substrate) and hetero- (GaAs/Al0.3Ga0.7As or AlAs/GaAs substrate) structures inherit the dislocation distribution maps for their substrates. On the other hand, dislocation propagation can be effectively prevented by introducing an AlAs/GaAs superlattice spacer between the GaAs epilayer and the substrate. As for surface defects, micron-sized hillocks emerge when the superlattice structure is fabricated. Their morphological configurations are very similar to those of previously reported oval defects in MBE-grown GaAs layers. However, these newly found surface defects are apt to be generated by the shutter operation and this close correlation with the dislocations in the substrate is quite different from the usual oval defects.
Spectroscopic study of cylindrically converging shock waves
Authors: Matsuo Hideo , Ebihara Kenji , Ohya Yuji , Sanematsu Hiroaki
Published: July 2002

Cylindrically converging shock waves have been produced by detonating 20-cm-diam explosive shells in atmospheric air. Shadowgraphs were taken to monitor the production of waves. A spectroscopic measurement of temperature was made using a monochromator combined with a photomultiplier. The spectral radiant energy was compared with the blackbody function (Planck’s law). The dependence of temperature upon the initiation energy (the chemical energy of explosives) was also investigated. It was shown that temperature was approximately proportional to the square root of the initiation energy. The attained temperatures ranged from 13 000–34 000 K depending on the initiation energy. These are 3500–9500 K lower than those estimated from shock propagation speeds. The implosion pulse duration (the half-value width) of 2.0–2.5 μs was observed. A possibility of the further increase of temperature as well as some problems was suggested.
Spreading of electromagnetic pulses
Authors: Wu Tai Tsun , Lehmann Harry
Published: July 2002

It is shown that for any finite-energy solution of the free-space Maxwell equations without sources, the energy contained in an infinite slab of finite thickness approaches zero as time tends to infinity.
Molecular beam epitaxial growth and photoluminescence of near-ideal GaAs-AlxGa1−xAs single quantum wells
Authors: Juang F.-Y. , Nashimoto Yasunobu , Bhattacharya Pallab K.
Published: July 2002

Single quantum wells with ∼120-Å GaAs wells and Al0.3Ga0.7As or GaAs-Al0.3Ga0.7As superlattice barriers were grown by molecular beam epitaxy under conditions known to produce very high-purity material. Low-temperature photoluminescence measurements indicate that the dominant recombination transitions are associated with free and bound excitons involving both light and heavy holes. A forbidden transition, possibly E21h, is also observed. The transition associated with electron-heavy-hole free excitons is most intense and has a linewidth of 0.3 meV at 2 K. The linewidths observed for these samples, grown with As4 species at 630 °C, are the smallest for 120-Å single quantum wells and are close to theoretically calculated limits.
Growth of diamond at room temperature by an ion-beam sputter deposition under hydrogen-ion bombardment
Authors: Kitabatake Makoto , Wasa Kiyotaka
Published: July 2002

The structure and crystal phase of carbon films prepared by an ion-beam sputter deposition were studied. The properties of the deposited carbon films were influenced by a hydrogen-ion bombardment during the film growth. It is noted that the hydrogen-ion bombardment activates the growth of diamond in the deposited carbon film. Diamond particles of 0.1∼1 μm in diameter, which showed well-defined morphology of diamond, were successfully grown at room temperature on nondiamond substrates.
Marker experiments in growth studies of Ni2Si, Pd2Si, and CrSi2 formed both by thermal annealing and by ion mixing
Authors: Hung L. S. , Mayer J. W. , Pai C. S. , Lau S. S.
Published: July 2002

Inert markers (evaporated tungsten and silver) were used in growth studies of silicides formed both by thermal annealing and by ion mixing in the Ni/Si, Pd/Si, and Cr/Si systems. The markers were initially imbedded inside silicides and backscattering spectrometry was used to determine the marker displacement after different processing conditions. The results obtained in thermal annealing are quite consistent with that found in previous investigations. Ni is the dominant diffusing species in Ni2Si, while Si is the diffusing species in CrSi2. In Pd2Si, both Pd and Si are moving species with Pd the faster of the two. In contrast, in growth of silicides by ion irradiation Si is the faster diffusing species in all three systems.
Characterization of silicon implanted GaAs buffer layers grown by metalorganic chemical vapor deposition
Authors: Kuech T. F. , Potemski R. , Chappell T. I.
Published: July 2002

The uniform and reproducible activation of silicon, ion-implanted directly into GaAs substrates, is often difficult to achieve. Epitaxial GaAs buffer layers have been used as an alternative implantation media which offers improved electrical characteristics. The characteristics of Si implanted GaAs buffer layers grown by the metalorganic chemical vapor deposition technique are presented here. The influence of the gas phase stoichiometry, a key determinant in the electrical properties of the layer, on the characteristics of the Si implanted and capless annealed layers was studied over the implantation dose range of 3×1012 to 1×1014 cm−2 at an implant energy of 150 keV. The electrical activation, mobility, deep-level concentration, and impurity distribution both prior to and after the implant and anneal were determined through optical and electrical characterization techniques. Undoped GaAs grown by this technique provides reproducible high levels of implant activation. Implantation into thick buffer layers allows the study of the implant and anneal processes free from the complicating influence of the GaAs substrate.
Thermodynamics of ferrimagnetic Ising chains
Authors: Georges R. , Curely J. , Drillon Marc
Published: July 2002

The exact solutions of the so-called ferrimagnetic Ising chain made up of two sublattices (S0,S1) are derived from a transfer matrix method. The short-range ferrimagnetic order occurs when considering different spins and/or different Landé factors on both sublattices. Most of the physical features of interest are shown to be involved in the S0=S1=1 system including an alternation of Landé factors (g0,g1) and local anisotropies (K0,K1). Thus, in the limit K0→∞ stabilizing a Kramers doublet, Sz=±1, on even sites, the system behaves like the ferrimagnetic chain (S0=1/2, S1=1). The susceptibility and magnetization curves are discussed in various situations as a function of the significant parameters.
Simple case of mixed symmetries in the measurement of uniaxial magnetic anisotropies
Authors: Pastor G. , Torres M.
Published: July 2002

An ideal experiment for a uniaxial magnetic sample measured in a torque magnetometer is analyzed by means of an angular transformation over the measurement plane. By using this transformation the torque curves are developed in Fourier’s series. This development permits the finding of an analytical solution for the angular variation of the magnetization with respect to the field. Furthermore, an uniaxial anisotropy measured in a finite field is expressed as an infinite superposition of n-axial anisotropies whose constants are functions of the field. From this the general problem of a composite anisotropy in a heterogeneous magnetic object constituted of uniaxial uncoupled regions can be studied.
The growth and characterization of CdTe epitaxial layers on CdTe and InSb by metalorganic chemical vapor deposition
Authors: Wang C. H. , Cheng K. Y. , Yang S. J. , Hwang F. C.
Published: July 2002

Epitaxial layers of CdTe were grown on CdTe and InSb substrates by metalorganic chemical vapor deposition using dimethylcadmium and dimethyltelluride as alkyl sources. Specular CdTe layers were grown on InSb at temperatures between 350 and 375 °C with serious out-diffusion of In from substrates. Dimethyltelluride is the controlling species of this growth system. Typical growth rates were 4 to 7 μm/h. Low-temperature photoluminescent measurements revealed the superior quality of epitaxial layers grown on CdTe substrates. The bound-exciton emission at 1.590 eV and the band-edge emission at 1.548 eV are the dominant peaks. Homostructure CdTe epitaxial layers grown between 330 and 410 °C possess the best surface morphology. Hole concentrations in the 1013-cm−3 range and a carrier mobility over 100 cm2/V sec were observed in these layers at 77 K. The highest hole mobility is 555 cm2/V sec in the sample grown at 400 °C. Layers grown outside this range show n-type conductivity with deteriorated electron mobilities and photoluminescent spectra.
Electron spin resonance study of interface states induced by electron injection in metal-oxide-semiconductor devices
Authors: Mikawa R. E. , Lenahan P. M.
Published: July 2002

We find that electrons emitted from silicon into the oxide of metal-oxide-silicon devices generate amphoteric trivalent silicon (Pb center) defects at the Si/SiO2 interface. The Pb centers are generated in numbers approximately equal to that of the electron injection induced interface states. The effects of electron injection are similar to those of ionizing radiation to the extent that in both cases Pb centers are generated at the Si/SiO2 interface. However, the effects are not identical; ionizing radiation creates another trivalent silicon defect, termed E′, in the oxide. We are unable to observe any E′ generation in oxides subjected to electron injection. There appears to be a strong correlation between the number of trapped electrons and the electron injection induced Pb center interface states; this observation suggests that the trapping of electrons in the bulk of the oxides is in some way related to the creation of the Pb center interface state defects. We find that dry oxides subjected to a deuterium/nitrogen anneal exhibit less electron trapping than otherwise identical oxides which have been subjected to a hydrogen/nitrogen anneal. This observation is consistent with the idea that a hydrogen bond breaking event may be involved in electron capture.
Infrared absorption and photoluminescence of defect levels in the 204- to 255-meV range in p-type GaAs
Authors: Fischer David W. , Yu P. W.
Published: July 2002

Infrared absorption and photoluminescence measurements have been performed on a series of p-type GaAs materials, all of which exhibited the so-called 78-meV double acceptor. Additional details concerning the negative charge state of the acceptor (204-meV level) were obtained. A new infrared absorption spectrum which appears to be due to a level at about 255 meV was found and might be related to the 1.284-eV photoluminescence line. Direct evidence for the existence of the boron antisite defect in as-grown material is shown for the first time but it is not the source of the double-acceptor spectra.
Atomic migration and surface evaporation of Hg in Hg1−x CdxTe crystals observed by 40-MeV O5+ ion backscattering method
Authors: Takita K. , Ippōshi T. , Murakami K. , Masuda K. , Kudo H. , Seki S.
Published: July 2002

Dissociation and evaporation of Hg from the near-surface region of Hg1−xCdxTe crystals was investigated, up to several microns, by means of 40-MeV O5+ backscattering. Samples with x=0.18 and 0.32 were heat treated in a vacuum for 60 min at various temperatures up to 320 and 340 °C, respectively. The change of the backscattering spectra with the temperature of heat treatment indicated that the Hg atoms outdiffused partly from the near-surface region of the crystals at higher temperatures than 280 and 320 °C for x=0.18 and 0.32, respectively. This behavior of outdiffusion is in contrast to the case of HgTe crystal.
A nonempirical model of the gas-cell atomic frequency standard
Authors: Camparo J. C. , Frueholz R. P.
Published: July 2002

In this paper a signal model of the rubidium (Rb) atomic frequency standard is developed. The model combines an analysis of the atomic physics required to describe processes occurring within the Rb absorption cell with a feedback analysis of the clock’s servocontrol circuitry, and thus allows clock performance in terms of Allan variance to be predicted from a number of electronic and physical parameters. All previous models of the Rb clock have been limited to an analysis of the clock’s short-term performance, Allan variance averaging times less than 10 000 s. However, by explicitly including the effects of discharge lamp intensity fluctuations, which are transformed into output frequency variations via the light shift effect, clock performance can be predicted for averaging times greater than 10 000 s. Furthermore, the present model is the first which incorporates the influence of an optically thick Rb vapor, along with the diffusion of optically pumped atoms to the walls of the absorption cell, into the calculation of clock performance. As part of the model’s validation, the calculations are compared with the results from a recent Rb clock long-term performance experiment. Agreement between measured and predicted Allan variances for both short and long averaging time periods is excellent.
A highly efficient, compact chemical oxygen–iodine laser
Authors: Yoshimoto Hideaki , Yamakoshi Hideo , Shibukawa Yukio , Uchiyama Taro
Published: July 2002

The dependence of the laser output power on the flow velocity at a cavity for a compact chemical oxygen–iodine laser is reported. The flow velocity is changed by using two vacuum pumps (3000 and 15 000 l/min). It becomes experimentally clear that the laser output power is proportional to the flow velocity and the extractable O2(1Δ) concentration. Output power in excess of 100 W has been extracted efficiently. The maximum overall reaction efficiency of 16.8% with the chlorine flow rate of 413 mmol/min is obtained.
Magneto-optical properties of substituted cobalt ferrites: CoFe2−xMexO4 (Me=Rh3+, Mn3+, Ti4++Co2+)
Authors: Martens J. W. D.
Published: July 2002

The Faraday rotation has been calculated at 1.55-eV photon energy (800 nm wavelength) for various substitutions in CoFe2O4 from the optical constants and the Kerr rotation and ellipticity spectra of polycrystalline bulk samples. The Faraday rotation consists of contributions both from the Co2+→Fe3+ charge transfer transition on octahedral sites as well as from the Co2+ 4A2→4T1 (P) crystal-field transition on tetrahedral sites, both centered around 2-eV photon energy. For the various substitutions the contribution of the crystal field transition to the Faraday rotation at 1.55 eV is determined by the amount of Co2+ on tetrahedral sites as well as the shift in energy of this transition upon substitution. A significant contribution of the 4A2→4T1 (P) transition to the Faraday rotation at 1.55 eV is reached in Rh3+ substituted CoFe2O4.
Correspondence between coherently strained multilayers and a single coherently strained layer on lattice mismatched substrate
Authors: People R.
Published: July 2002

The equilibrium in-plane lattice parameter for lattice mismatched multilayers, as first derived by Matthews and Blakeslee [J. Cryst. Growth 32, 265 (1976)], has been used to obtain an expression for the misfit between the multilayers as a whole and a lattice mismatched substrate. The resulting expression immediately suggests a correspondence between the multilayers as a whole and a single coherently strained layer of equal thickness but having a misfit equal to the misfit between the constituent layers spatially averaged over a single repeat distance. The present results are obtained without reference to the energy density of a misfit dislocation. In particular, the present correspondence allows one to predict the critical thickness of coherently strained multilayers if single-layer critical thicknesses are known.
Band structure and density of states changes in heavily doped silicon
Authors: Bennett Herbert S.
Published: July 2002

The Klauder self-energy method is used to calculate the effects of the one-body interactions among the dopant ions and the carriers in heavily doped silicon at 300 K. The many-body interactions of exchange and correlation are estimated by interpreting optical absorption measurements and by calculations based on degenerate theory. When densities exceed 5×1019 cm−3, one-body and many-body terms become of the same order of magnitude and should be included in calculations of band structure changes and of properties such as carrier transport which depend on the density of states.
Oxygen in silicon: A positron annihilation investigation
Authors: Dannefaer S. , Kerr D.
Published: July 2002

Positron lifetime and Doppler broadening investigations of oxygen in silicon (Cz silicon) have been performed. It was found that positrons may be trapped by defects yielding a positron lifetime of only 100 ps and a momentum distribution of the annihilation quanta which is wider than that for defect-free silicon. This unequivocally shows that clusters of oxygen constitute positron traps. The trapping process was found to be thermally activated in the 25–300 K range. Isochronal and isothermal annealing showed that the clusters alternate during growth between having an interstitial character and a vacancy character. At 450 and 500 °C, growth of oxygen clusters takes place with an activation energy of 1.5 eV, while at 650 and 714 °C, these clusters disappear with an activation energy of 1.1 eV. At 714 and 790 °C, growth of new clusters takes place with an activation energy of 2.6 eV.
Properties of the metal–polymer interface observed with space-charge mapping techniques
Authors: Kurtz S. R. , Anderson R. A.
Published: July 2002

A space-charge mapping technique was used to measure the space charge and internal electric fields in metal–poly(ethylene terephthalate)–metal samples subjected to electric field stressing. From these results the role of electronic injection and transport in the observed electrical properties of capacitor structures could be inferred. Below breakdown fields, there was no evidence that the dark current can be attributed to electronic processes. As breakdown fields were approached, a decrease in the injected space charge trapped adjacent to the electrode was observed. This behavior was attributed to the onset of tunneling into high-energy electronic states in the polymer. This process may produce large current densities leading to dielectric breakdown.
High-intensity laser irradiation of metallic surfaces covered by periodic structures
Authors: Ursu I. , Mihăilescu I. N. , Prokhorov A. M. , Tokarev V. N. , Konov V. I.
Published: July 2002

An analysis is performed, on the role of the periodic structures, either preexisting or induced by laser irradiation, in the process of interaction of high-intensity laser radiation with metallic surfaces. The significance of the new results is discussed in connection with previous experimental and theoretical results reported in literature.