bluuebunny/crossref_metadata_2025 huggingface dataset:
Compositional dependence of the Auger coefficient for InGaAsP lattice matched to InP
Authors: Bardyszewski W. , Yevick D.
Published: July 2002

We present a detailed discussion of previous theoretical and experimental results relating to the pure-collision and phonon-assisted Auger coefficients of semiconductor laser materials. Subsequently, we calculate the dependence of the Auger coefficient of In1−xGaxAsyP1−y lattice matched to InP (x≊0.45y) on the compositional parameter y. Our analysis, which incorporates improved Kane model wave function overlap integrals and spectral density functions, yields results which are in good agreement with recent experiments.
Equation-of-state parameters for poly(vinyl acetate)
Authors: Cho Bongkee
Published: July 2002

The temperature and pressure dependence of the thermal pressure coefficient and internal pressure for poly(vinyl acetate) are discussed on the basis of the equation of state, previously reported. The principle results are as follows: (1) The thermal pressure coefficient of the liquid polymer increases with increasing pressure at constant temperature. (2) The internal pressure of the glassy polymer decreases with increasing pressure at constant temperature. (3) The thermal pressure coefficient of the glassy polymer and the internal pressure of the liquid polymer can be approximated to be independent of pressure. (4) The zero-pressure thermal expansivity can be approximated to be independent of the glass-forming pressure. Also, the relationship between the Gruneisen constant and sonic wave velocity is derived and is found to be identical to the Broadhurst–Mopsik equation.
Characterization of the electro-optic effect in styrylpyridinium cyanine dye thin-film crystals by an ac modulation method
Authors: Yoshimura Tetsuzo
Published: July 2002

The electro-optic effect in styrylpyridinium cyanine dye (SPCD) thin-film crystals is characterized by a newly developed ac modulation method that is effective in characterizing thin-film materials of small area. SPCD thin-film crystals 3–10 μm thick were grown from a methanol solution of SPCD. The crystal shows strong dichroism and anisotropy of refractive index, indicating that molecular dipole moments align along a definite direction (z axis). When an electric field is applied along the z axis, SPCD thin-film crystals show a large figure of merit of electro-optic phase retardation of 6.5×10−10 m/V, which is 5 times as large as in LiNbO3 crystal, 2 times that in 2-methyl-4-nitroaniline (MNA) crystal, and is the largest ever reported in organic solids. The electro-optic coefficient r33 of SPCD crystals is estimated to be approximately 4.3×10−10 m/V, which is 6 times larger than that of an MNA crystal. This value is consistent with that expected from second-harmonic generation measurements.
An exactly solvable model for calculating critical misfit and thickness in epitaxial superlattices: Layers of equal elastic constants and thicknesses
Authors: van der Merwe Jan H. , Jesser W. A.
Published: July 2002

A parabolic interaction potential has been used to develop a model for calculating the misfit dislocation (MD) energy in the case of a superlattice of alternating layers of materials with equal elastic constants and thicknesses. The model, which is believed to be a good one for small misfits and to have some merit for covalent bonded materials, is exactly solvable for the critical thickness above which it is energetically favorable to lose coherency by the introduction of MDs into the interfaces. It was found, for a given misfit f, that the critical thickness for epitaxial superlattices free from their substrate is somewhat more than 4 times that for a single epilayer on a thick substrate. Furthermore, the critical thickness varies almost inversely with misfit to the power 1.22 when Poisson’s ratio is 1/3. It was also shown that the critical misfit fc obtained by equating maximal misfit strain and MD energies is a significant overestimate of fc. The results for a superlattice are compared with those of a thin layer on a thick substrate.
Modeling for rf discharge characteristics
Authors: Maniv S.
Published: July 2002

A radio-frequency discharge characteristics model for the dc self-bias current density Jsb versus the dc self-bias voltage Vsb and the gas pressure p is developed. The model considers positive ions and electrons only and is consistent with a previously developed model for dc sputtering discharge characteristics. Two different concepts for the cathode fall distance are established: Li, for the ions, which is time independent, and Le, for the electrons, which is a time-averaged length. A relationship between the cathode fall distance L and the discharge efficiency n is developed. The area ratio power law for the voltage distribution between the electrodes is theoretically analyzed and the resulting quadratic power law is experimentally confirmed. The predicted pressure dependence of Vsb is rather complicated, and its mathematical expression has been left with an adjustable exponential parameter to be determined by the best fit to experimental data.
Tunneling process in AlAs/GaAs double quantum wells studied by photoluminescence
Authors: Tada Tetsuya , Yamaguchi Atsushi , Ninomiya Toshiyuki , Uchiki Hisao , Kobayashi Takayoshi , Yao Takafumi
Published: July 2002

The tunneling process of photogenerated carriers in AlAs/GaAs double quantum well structures in which two wells have different widths (60 and 80 Å) was studied by steady-state and time-resolved photoluminescence spectra. The tunneling process was found to play an important role for a barrier thickness narrower than about 40 Å. The tunneling rate was determined as 2×1010 s−1 for a 30-Å barrier. The quantum-mechanical penetration depth of the wave function into an AlAs barrier was estimated as 6 Å from the barrier width dependence of luminescence intensity ratio between the two wells. The tunneling rate and penetration depth are consistent with a simple envelope function approximation with no Γ-X mixing.
Atomic steps in thin GaInAs/lnP quantum-well structures grown by organometallic vapor phase epitaxy
Authors: Wang T. Y. , Fry K. L. , Persson A. , Reihlen E. H. , Stringfellow G. B.
Published: July 2002

InP/GaInAs/InP quantum-well structures have been grown using atmospheric pressure organometallic vapor phase epitaxy (AP-OMVPE). The effects of growth parameters such as V/III ratio and substrate orientation have been studied. For thin wells the 10-K photoluminescence spectra show clearly resolved doublets or in some cases triplets. The energy separation of the peaks increases with decreasing well width down to a width of approximately 12 Å. For thinner wells the energy separation decreases with decreasing well width. The doublet is interpreted as being due to the photoluminescence from two wells differing in thickness by a single monolayer. A simple calculation for a finite quantum well describes the general features of the energy splitting versus well width. For the thin wells, where the individual photoluminescence peaks are resolved, the half-widths are extremely narrow. The value of <15 meV for the thinnest well is much better than observed previously for low pressure or AP-OMVPE and matches the results obtained by chemical beam epitaxy.
Equation of state for liquids
Authors: Cho Bongkee
Published: July 2002

A simple equation of state for polymer liquids and solids, previously reported, is modified for simple liquids. Values of the parameters in the equation of state are given for 18 liquids. Comparison of the predictions of this equation with experimental data is made for all of the liquids.
Impurity scattering with semiclassical screening in multiband quasi-one-dimensional systems
Authors: Weng Yilin , Leburton J. P.
Published: July 2002

We present a calculation of screened impurity scattering in multiband GaAs one-dimensional systems. The influence of background impurities is extended to a domain outside the wire within a phenomenological model. Screening effects are evaluated in the semiclassical Thomas–Fermi approximation. A feature of our model is the consideration of level degeneracy which enhances carrier occupation and screening in higher subbands. For confinement conditions corresponding to subband separation equal to the thermal energy, we found that this effect equalizes the different scattering rates in each subband.
Photoreflectance measurements of unintentional impurity concentrations in undoped GaAs
Authors: Sydor Michael , Angelo James , Mitchel William , Haas T. W. , Yen Ming-Yuan
Published: July 2002

Modulated photoreflectance is used to measure the unintentional impurity concentrations in undoped epitaxial GaAs. A photoreflectance signal above the band gap spreads with the unintentional impurity concentrations and shows well-defined Franz–Keldysh peaks whose separation provide a good measure of the current carrier concentrations. In samples less than 3 μm thick, a photoreflectance signal at the band edge contains a substrate-epilayer interface effect which precludes the analysis of the data by using the customary third derivative functional fits for low electric fields.
Experimental studies of misfit dependence of critical layer thickness in pseudomorphic InGaAs single-strained quantum-well structures
Authors: Weng Shang-Lin
Published: July 2002

The dependence of critical layer thickness on a misfit in pseudomorphic GaAs/InGaAs/GaAs single quantum-well structures grown by molecular-beam epitaxy is studied using a phase-contrast optical microscope and photoluminescence spectroscopy. The results show that the critical layer thickness as a function of misfit follows more closely with Matthews and Blakeslee’s mechanical equilibrium model [J. Cryst. Growth 27, 118 (1974)] than People and Bean’s energy balance model [Appl. Phys. Lett. 49, 229 (1986)]. However, the observed critical layer thickness is slightly less than that predicted by the mechanical equilibrium model for single quantum-well structures.
Critical layer thickness and strain relaxation measurements in GaSb(001)/AlSb structures
Authors: Gossmann H.-J. , Davidson B. A. , Gualtieri G. J. , Schwartz G. P. , Macrander A. T. , Slusky S. E. , Grabow M. H. , Sunder W. A.
Published: July 2002

Measurements of critical layer thicknesses and strain relaxation have been made for AlSb single layers and GaSb/AlSb superlattices on GaSb(001) using ion scattering/particle-induced x-ray techniques, x-ray diffraction, and Raman scattering. The ion beam techniques and x-ray diffraction indicate a critical thickness on the order of 100–170 Å. Raman scattering from superlattice samples using 5145 Å excitation shows the most rapid strain relaxation versus layer thickness and indicates a lower apparent critical thickness on the order of 50–100 Å. The influence of the finite experimental resolution is addressed. The experimental critical thickness and the strain relief are compared to theoretical models. The equilibrium critical thickness of these models is consistent with the experimental value, indicating that metastable strain retention is not a dominant process in the GaSb(100)/AlSb system under the present growth conditions.
On the effect of ionization dead spaces on avalanche multiplication and noise for uniform electric fields
Authors: Marsland J. S.
Published: July 2002

The effect of the impact ionization dead space on avalanche multiplication and noise has been assessed for uniform p-i-n avalanche photodiodes. This has required the development of a new numerical technique. The well-established McIntyre theory of avalanche noise [IEEE Trans. Electron Devices ED-13, 164 (1966)], which neglects the ionization dead-space effect, has been shown to overestimate the excess noise factor. The implications of the dead-space effect, for ionization coefficient determination and the interpretation of measured excess noise factors, are discussed.
Photoluminescence study of impurity states in aluminum antimonide
Authors: Hofmann G. , Lin C. T. , Schönherr E. , Weber J.
Published: July 2002

We have characterized Czochralski-grown undoped AlSb single crystals by low-temperature photoluminescence and photoluminescence excitation spectroscopy. An excitonic band-gap energy Egx =1.667±0.001 eV of AlSb was determined. In all samples we detect a dominant donor and acceptor. The donor binding energy ED =151±3 meV and acceptor binding energy EA =37±3 meV are deduced from donor-acceptor pair luminescence and the excited states of the acceptor from selective pair excitation. We tentatively identify the donor as sulfur and the acceptor as carbon.
p- and h-type adaptive mesh generation
Authors: Raizer A. , Hoole S. R. H. , Meunier G. , Coulomb J.-L.
Published: July 2002

Adaptive mesh generation involves the improvement of finite elements on the basis of error estimates by either refining the mesh (h-type refinement) or improving the trial function (p-type refinement). In this paper, both methods are employed in conjunction, thereby reaping the advantages of both approaches. The strategy is tested with two different error criteria, one based on nodal perturbations and the other on discontinuities of the magnetic field or the flux density between neighboring elements. Forces on permanent magnets are computed to demonstrate the method.
Transport critical current of aligned polycrystalline Y1Ba2Cu3O7−δ and evidence for a nonweak-linked component of intergranular current conduction
Authors: Ekin J. W. , Hart H. R. , Gaddipati A. R.
Published: July 2002

A study of grain alignment and its effect on the dc transport critical current in fine-grained bulk Y1Ba2Cu3O7−δ is reported in magnetic fields from 10−4 T to 26 T. Two features distinguish the critical current density Jc of aligned bulk Y1Ba2Cu3O7−δ from unaligned material. First, the effective critical field where the intergranular Jc approaches zero is about four times higher (30 T) for aligned samples with field parallel to the a, b planes, than it is for polycrystalline unaligned samples (7 T). Second, the nearly field independent plateau value of Jc between 10 mT and 1 T is one to two orders of magnitude higher than typical plateau values of Jc in unaligned bulk-sintered Y1Ba2Cu3O7−δ, for field parallel to the a, b planes. A low-field (<10 mT) weak-link decrease in Jc with magnetic field is still observed, but it is much smaller than for unaligned material. These data clearly demonstrate that alignment alone significantly reduces the weak-link problem in fine-grained polycrystalline samples with low-aspect-ratio (4:1) grains (unlike melt-grown samples where there has been some ambiguity as to the relative importance of alignment versus large grain growth). Furthermore, the results provide strong evidence that there are two parallel components of intergranular current conduction, one consisting of weak-linked material, the other behaving like intrinsic intragranular material that is not weak-linked. A comparison with unaligned Y1Ba2Cu3O7−δ indicates that the volume fraction of such nonweak-linked material is significantly enhanced by grain alignment, but still only 0.01%–0.1% of the grain boundary area. Field-cooled and force-free Jc data are also presented, along with detailed measurements of the shapes of the voltage-current characteristics.
C-V analysis of the Schottky barrier in semi-insulating semiconductors
Authors: Dubecký Frantis̆ek , Olejníková Boz̆ena
Published: July 2002

We analyze the screening length and the base series admittance effect on the measurement of the barrier capacitance and voltage in semi-insulating (SI) semiconductors. Specific experimental conditions are summarized which enable the utilization of C-V analysis in SI materials. It is shown that under such experimental conditions the formula of Sah and Reddi for low-frequency C-V dependence can be applied for the calculation of the concentration of residual shallow impurities. These considerations are applied for the low-frequency C-V analysis of the Schottky barrier in SI GaAs:Cr formed by Au contact and the concentration of residual shallow donors is determined. Some limitations of the presented C-V analysis are discussed.
Transport properties of silver-doped Y-Ba-Cu-O thin films
Authors: Song L. W. , Shao H. M. , Kao Y. H. , Narumi E. , Yang F. , Shaw D. T.
Published: July 2002

Transport properties of dilute silver-doped Y-Ba-Cu-O thin films prepared by a laser ablation technique have been investigated. The Hall coefficient and critical current density were measured as a function of temperature and in magnetic field up to 1.0 T. Silver doping is shown to result in significant changes in the temperature- and magnetic-field dependence of the transport properties of these films. The Hall coefficient in silver-doped films shows a clear decrease from that in silver-free films, consistent with an increase in the hole concentration caused by the silver impurities. This result lends a direct support to the observation that dilute silver dopants behave as acceptors but without appreciable influence on the transition temperature. The magnetic-field dependence of the normalized critical current density Jc(H)/Jc(0) is enhanced by silver doping, mainly due to a decrease in the effective weak-link area at the grain boundaries.
Thermally stimulated current studies of sillénite-type oxide crystals: Retrapping kinetics and identity of trapped carriers
Authors: Takamori Takeshi
Published: July 2002

Thermally stimulated currents (TSC) of Bi12SiO20 and Bi12GeO20 crystals were measured after uv excitation at two different temperatures. This double excitation technique was demonstrated to be a simple but effective means of studying the detrapping and retrappping kinetics. In particular, it leads us to suggest that most, if not all, of the trap levels detected by TSC may be hole traps, and calls into question the general belief that these traps are electron traps. In addition, our previous contention regarding the stepwise trapping kinetics among the major traps observed in the TSC spectra was confirmed by this technique. It was found that some of the minor but deeper trap levels at higher temperatures are also related to shallower traps by such stepwise kinetics.
Generalization of the vector Preisach hysteresis model
Authors: Friedman G.
Published: July 2002

A generalization of the vector Preisach hysteresis model is proposed. This new vector hysteresis model represents a larger class of hysteresis nonlinearities than either the Stoner-Wohlfarth model or the previously developed vector Preisach model. To demonstrate this fact, it will be shown that the Stoner-Wohlfarth model and earlier vector Preisach models are particular cases of the generalized vector Preisach model and can be obtained by specific choices of parameters in the new model.
Frequency pulling by hyperfine σ transitions in cesium beam atomic frequency standards
Authors: Cutler L. S. , Flory C. A. , Giffard R. P. , De Marchi A.
Published: July 2002

It is demonstrated that frequency pulling by Δm=±1 hyperfine σ transitions, here called Ramsey pulling, is a real calculable effect in atomic beam frequency standards. An analytic expression for the effects of Ramsey pulling is derived using perturbative techniques for the driving σ transitions, while treating the primary π transitions exactly. It is shown that these Ramsey pulling effects are intrinsically different from Rabi pulling in origin, manifestation, and elimination. These predictions are compared to measurements performed on a cesium beam atomic frequency standard, giving good quantitative verification of the theory and a clear demonstration of the existence of these effects.
Magneto-optic images of flux distribution and creep in YBa2Cu3O7
Authors: Muller H. , Suenaga M. , Yokoyama Y.
Published: July 2002

A magneto-optic imaging technique was used to examine the flux distributions at 50, 60, and 70 K in a single-crystal platelet of YBa2Cu3O7. The superconducting sample was covered with a layer of iron garnet film, and a polarization microscope was used to examine the magnetic domains in this imaging film. Since the spacing of these domains was proportional to the local magnetic field in the underlying superconductor, it was possible to determine field profiles directly from the optical density of the image. Field profiles indicated that the distributions of remnant flux in the sample were well described by the critical state model. It was also possible, through the use of simple linear models of initial flux penetration, to measure the flux trapped in the sample as a function of time. The flux creep data derived from analysis of the digitized images showed a logarithmic decrease of magnetization with time, in good agreement with SQUID magnetometer measurements performed on the sample. There was no evidence of an axial critical state.
Extended two-dimensional analysis of electrical breakdown between a wire and a plane
Authors: Donovan K. G. , Cohen I. M.
Published: July 2002

The formation of a discharge between a wire and a plane has been simulated numerically through the development of large electron and positive ion number densities. When the number densities grow sufficiently large and equal a new region within the computational model is formed, within which the governing equations of the quasi-neutral region are solved for the common ion and electron number density and electric potential. Results for the temporal development of the quasi-neutral region, radial profiles of charged particle number density, electric potential, and electric field strength from discharge initiation to program termination are described. The quasi-neutral region is shown to develop first off of the discharge axis, above the wire-to-plane gap. Near the quasi-neutral region the electric field strength and electric potential are highly distorted and within the quasi-neutral region are greatly diminished.
High-purity ion beam production at high current densities with a liquid-helium-cooled series-field-coil extraction ion diode
Authors: Hanson D. L. , Porter J. L. , Williams R. R.
Published: July 2002

Experiments are described in which a high-purity, high-power (0.15 TW, 1 MeV) proton beam is generated from an ion source consisting of H2 gas frozen onto a liquid-helium-cooled copper anode at 4.2 K in a series-field-coil extraction diode on the 0.7 TW HydraMITE-II accelerator. Peak anode proton current densities of 2 kA/cm2 were measured. This current density is a factor of 100 higher than those obtained in previous liquid-helium-cooled cryogenic diode experiments on small accelerators and is in the range required for high-power ion beam applications. Thomson parabola, Faraday cup, and carbon activation measurements indicate an ion beam proton fraction close to 100% for the cryogenic source, compared to 50–70% for the standard hydrocarbon anode tested. The cryogenic proton source is believed to consist of no more than a few monolayers of molecular hydrogen. The hydrogen-coated cryogenic anode shows a faster initial anode turn-on than other materials. However, source-limited emission from the thin hydrogen layer results in a somewhat longer current risetime, reduced ion diode efficiency, lower proton current enhancement over the Child–Langmuir limit, and a proton spectrum of lower average energy than for the hydrocarbon anode. Techniques to overcome these limitations are discussed. Cryogenic ion sources consisting of frozen N2, CH4, and Ne have also been studied. In each case, high intensity beams consisting predominantly of components of the refrigerated gas were produced.
Thermodynamic stability of the Mie–Grüneisen equation of state, and its relevance to hydrocode computations
Authors: Segletes Steven B.
Published: July 2002

The role of the equation of state (EOS) is reviewed as an analytical vehicle to express material pressure as a function of density and temperature (or internal energy in the case of adiabatic transition). Starting from the fundamental laws of thermodynamics, three criteria are developed to measure the stability of equations of state, and are applied specifically to the Mie–Grüneisen EOS, with Hugoniot reference, to investigate the stability characteristics thereof. This EOS is singled out for study because of its relative importance in the computational modeling of shock transition. Results indicate numerous possibilities of instability under various circumstances. Various remedies are proposed to correct the observed instabilities and compared with existing models.
A quantitative treatment for deep level transient spectroscopy under minority-carrier injection
Authors: Fourches N.
Published: July 2002

We introduce a theoretical treatment of deep level transient spectroscopy (DLTS) under minority-carrier injection condition. The occupancy function for a hole trap lying in the n-type region of a pn junction is calculated versus the injected minority-carrier current density for a semiconductor with a low residual doping concentration. This calculation is then used to determine the DLTS signal corresponding to hole emission in the valence band. The change of the DLTS signal magnitude with the injected current enables an estimation of the ionization energy of the trap and the diffusion length for minority carriers. Experimental results are reported for the case of neutron-induced traps in high-purity germanium as illustration.
Phosphorus diffusion into silicon from a spin-on source using rapid thermal processing
Authors: Hartiti B. , Slaoui A. , Muller J. C. , Stuck R. , Siffert P.
Published: July 2002

Diffusion of phosphorus into silicon from a doped spin-on glass source using rapid thermal processing is described. The structural and electrical characteristics of the resulting shallow junctions including atomic and carrier concentration profiles, sheet resistance, as well as the effects on bulk carrier transport properties were studied and compared to those resulting from the use of conventional furnace heating. The results show that sheet resistance as low as 15 Ω/⧠ and surface carrier concentration higher than 1 × 1020 cm−3 are obtained in the annealed samples. Furthermore, a gettering effect is observed as the minority-carrier diffusion length measured by the surface photovoltage technique is improved after processing.
An analytical model for longitudinally pumped continuous-wave laser
Authors: Hwang In Heon , Meador Willard E.
Published: July 2002

An analytical model for a longitudinally end-pumped laser is developed using the circulating photon flux approach, and the threshold pump intensity and slope efficiency are expressed with the laser parameters. The thermal population in the lower laser level is included in the model. The threshold pump intensity decreases as the absorption cross section increases. The model shows that the threshold pump intensity increases as the length of the laser rod increases over the optimum value. The slope efficiency of the system depends on the laser rod length, the absorption cross section, and the reflectance of the output mirror. When the model is applied to a GaAlAs diode laser pumped Nd:YAG laser, the achievable slope efficiency reaches up to 76%, which is the ultimate quantum efficiency, at the output mirror reflectance lower than 50% with a laser crystal of loss coefficient 0.1%/cm. The experimental results of 946 nm Nd:YAG laser by Fan and Byer, Opt. Lett. 12, 809 (1987) are compared with the present model, and shows a good agreement with calculations.
Structural and electrical defects in amorphous silicon probed by positrons and electrons
Authors: Roorda S. , Hakvoort R. A. , van Veen A. , Stolk P. A. , Saris F. W.
Published: July 2002

The structure of pure amorphous Si, prepared by ion implantation, has been investigated by variable-energy positron annihilation spectroscopy (PAS) and lifetime measurements of optically generated free carriers. In general, PAS measurements are thought to be sensitive to vacancy-type defects while the carrier lifetime depends on the density of band-gap states (e.g., dangling bonds). The PAS measurements indicate that the density of positron-trapping defects can be reduced by thermal annealing at 500 °C. Concurrent with the removal of structural defects the density of band gap states is reduced as indicated by an increased photocarrier lifetime by a factor of 10. Some material has been implanted with H+ and annealed at a low temperature (150 °C). The hydrogen is expected to passivate electrical defects associated with strained and dangling bonds and indeed the photocarrier lifetime is increased in this material. Moreover, the PAS measurements cannot distinguish this material from 500 °C annealed amorphous Si, indicating that (some of) the electrical defects are associated with positron-trapping, and therefore possibly vacancy-type, structural defects. Finally, both methods have been used to detect small amounts of ion irradiation damage in annealed amorphous Si.
On the characteristics of magnetic order of Nd2Fe14B-type compounds
Authors: Zhong Xue-Fu , Ching W. Y.
Published: July 2002

Based on the angle-dependent free-energy formalism, a general analysis of the characteristics of magnetic order in Nd2Fe14B-type compounds is given. The focus is on the symmetry in magnetic order and its relation to the crystal structure. The spin-reorientation and the first-order magnetization process are discussed from the viewpoint of phase transition with the emphasis on the change in the symmetry of magnetic order.
Electric forming and telegraph noise in tunnel junctions
Authors: Kohlstedt H. , Gundlach K.-H. , Kuriki S.
Published: July 2002

The normal state resistance of Pb/Bi/In-oxide-Pb/Bi and Pb/Bi/In-oxide-Ag can be changed by applying sufficiently high voltage across the oxide barrier. To explain this effect we discuss an electrical dipole, a donor, a metal ion, and an oxygen migration model. The oxygen migration model best accounts for the experimental observations. Our investigations include telegraph noise measurements. A significant change in the low-frequency noise spectrum before and after electrical forming is found. It is possible to create and destroy two-level signals by applying an electric field on the order of 2.5×108 V/m.
Remote n-type modulation doping of InAs quantum wells by ‘‘deep acceptors’’ in AlSb
Authors: Shen Jun , Dow John D. , Ren Shang Yuan , Tehrani Saied , Goronkin Herb
Published: July 2002

Due to the fact that impurities normally change their doping characters when they undergo shallow to deep transitions or deep-to-false-valence transitions, a single defect, such as a cation on an Sb site, can explain all of the following facts for nonintentionally doped AlxGa1−xSb alloys and InAs/AlxGa1−xSb superlattices and quantum-well structures: (i) Bulk GaSb is p type; (ii) bulk AlSb is semi-insulating; (iii) InAs/AlSb superlattices with InAs quantum wells thicker than a critical thickness dc(x=1.0) are n type, where the InAs shallow–deep critical thickness function dc(x) is around ≂100–≂150 Å for 0.5<x≤1.0 for InAs/AlxGa1−xSb superlattices; (iv) InAs/AlSb superlattices with InAs quantum wells thinner than dc(x=1.0) are semi-insulating. In addition, the theory predicts that Al0.5Ga0.5Sb and AlSb will be semi-insulating when nonintentionally doped, but can be converted to p type by the application of hydrostatic pressure P: P≳90 kbar and P≳150 kbar, respectively. These changes of doping character, which lie outside the conventional effective-mass theory, occur often in type-II band-alignment systems, such as InAs/AlxGa1−xSb.
Microstructure and texture evolution of Cr thin films with thickness
Authors: Tang Li , Thomas Gareth
Published: July 2002

Microstructure and texture of Cr thin films sputter deposited on NiP-coated AlMg substrates were studied using transmission electron microscopy (TEM). The sample tilt method in TEM has proven to be very effective in determining the fiber axis of weakly textured films. It was found that both the grain size and texture of the Cr films are strongly dependent on their thickness and deposition conditions. For Cr films deposited on 300 °C preheated substrates, the grain size increases monotonically with the film thickness. The texture changes from no texture (∼100 Å) to a [001] fiber texture (200–2000 Å), and then to a [011] fiber texture (∼5000 Å). A 2000-Å-thick film deposited on a room-temperature substrate shows a strong [011] texture and a smaller grain size than that of a film with the same thickness but deposited on a preheated substrate. Correlations between the in-plane coercivity of Co-based alloy thin films for longitudinal magnetic recording and the grain size and texture of Cr underlayers are also discussed.
Raman scattering study of lead zirconate titanate thin films prepared on silicon substrates by radio frequency magnetron sputtering
Authors: Taguchi I. , Pignolet A. , Wang L. , Proctor M. , Lévy F. , Schmid P. E.
Published: July 2002

Raman scattering is used to characterize thin films of PbTi1−xZrxO3. The films have been prepared on platinum-coated (100) silicon by radio-frequency (rf)-magnetron sputtering without substrate heating followed by a post-deposition annealing at 600–650 °C. As the concentration of Zr is increased, the Raman peaks broaden and their intensities decrease more rapidly compared with bulk ceramic or powder samples, while the background intensity increases. The observations show that the crystal structure of the films is locally deformed and significantly disordered. With increasing x, the soft E(TO) mode shifts to lower frequencies. An additional Raman peak appears besides the phonon peaks expected in a perfect crystal. The soft mode strongly couples with the extra mode, similarly with the case of ceramic samples. The results are related to features of the thin films such as a frustrated phase transition, i.e., smaller differences between the lattice parameters a and c in comparison with powder data, and an electrical behavior showing a diffuse ferroelectric-to-paraelectric transition with a broad peak.
Estimation of strength distribution of Nb3Sn in multifilamentary composite wire from change in superconducting current due to preloading
Authors: Ochiai Shojiro , Osamura Kozo , Watanabe Kazuo
Published: July 2002

Bronze-processed multifilamentary Nb3Sn superconducting composite wires were loaded and then unloaded at room temperature, resulting in a large change in the critical current (Ic) and the upper critical magnetic field (Hc2) at 4.2 K: the Ic and Hc2 increased, reaching maximum and then decreasing with an increasing applied stress level. The changes in Ic and Hc2 below the loading stress to cause breakage of the Nb3Sn were described well from the viewpoint of the change in residual strain in Nb3Sn. When the applied stress was high enough to cause breakage of the Nb3Sn, the Ic was reduced seriously. From the reduction in Ic in such a case, the strength distribution of Nb3Sn was estimated by using the Weibull distribution function. The result indicated that the shape parameter for the Weibull distribution was 12 and the average strength was 1.02 GPa in the present samples. From the calculation of the residual strain at 4.2 K in combination with the estimated strength distribution of Nb3Sn, the change in Ic at 4.2 K as a function of applied stress at room temperature could be described well.
Sound velocity in model superlattices with graded profile
Authors: Mon K. K.
Published: July 2002

We study by means of numerical solution to the wave equation the long-wavelength sound velocities in model superlattices with a periodic graded profile. The deviations of the sound velocity squares from the uniform limits are observed to be linearly proportional to the squares of the elastic constant modulation amplitude for the small amplitude regime.
Triple-crystal x-ray diffraction analysis of reactive ion etched gallium arsenide
Authors: Wang V. S. , Matyi R. J. , Nordheden K. J.
Published: July 2002

The effect of BCl3 reactive ion etching on the structural perfection of GaAs has been studied with diffuse x-ray scattering measurements conducted by high-resolution triple-crystal x-ray diffraction. While using a symmetric 004 diffraction geometry revealed no discernible differences between etched and unetched samples, using the more surface-sensitive and highly asymmetric 113 reflection revealed that the reactive ion etched samples etched displayed less diffusely scattered intensity than unetched samples, indicating a higher level of structural perfection. Increasing the reaction ion etch bias voltage was found to result in decreased diffuse scattering initially, until an apparent threshold voltage was reached, after which no further structural improvement was observed. Furthermore, we have shown that this reduction in process-induced surface structural damage is not due merely to the removal of residual chemical-mechanical polishing damage.
Long wavelength (λ∼1.5 μm) native-oxide-defined InAlAs-InP-InGaAsP quantum well heterostructure laser diodes
Authors: Caracci S. J. , Krames M. R. , Holonyak N. , Ludowise M. J. , Fischer-Colbrie A.
Published: July 2002

Native oxidation (‘‘wet’’ oxidation via H2O vapor+N2) of InAlAs is employed to fabricate long wavelength (λ∼1.5 μm) InAlAs-InP-InGaAsP quantum well heterostructure laser diodes. Data are presented on gain-guided native-oxide-defined stripe-geometry lasers (40 μm×500 μm) with threshold currents of 200 mA (1 kA/cm2) emitting with multiple longitudinal modes centered at λ∼1.5 μm. The threshold currents, approximated as Ith=I0 exp(T/T0), exhibit a characteristic temperature of T0∼49 K and an operating temperature as high as T=70 °C. Maximum continuous output powers of 140 mW/facet (uncoated facets) and a differential quantum efficiency of 38% are achieved.
‘‘Exchange’’ spin waves in nonuniform yttrium iron garnet films
Authors: Temiryazev A. G. , Tikhomirova M. P. , Zil’berman P. E.
Published: July 2002

Spin wave resonance spectra have been observed in yttrium iron garnet films. The spectra exhibit as many as 120 strongly excited modes with large deviation from quadratic spacing, which are interpreted in terms of a thickness variation of the effective magnetization 4πM(x) depending both on the magnetization and the anisotropy field. Profiles of 4πM(x) are calculated from the spectra measured both in parallel and in perpendicular bias magnetic fields. It has been shown that the films tested have monotonic variations of the effective magnetization. The difference 4πΔM between upper and lower 4πM(x) values is as large as 330 G. It is shown that exchange-dominated spin waves (ESW) with the maximum wave number of 2.5×105 cm−1 have been excited under experiment conditions. Microwave pulse delay due to ESW propagation across the film thickness has been observed. Delay versus frequency characteristics strongly depend on the profile M(x) and the direction of the applied magnetic field. Profiles M(x) have been calculated to provide both constant and linear delay-versus-frequency characteristics.
Raman scattering study of [hhk]-GaAs/(Si or CaF2) strained heterostructures
Authors: Puech P. , Landa G. , Carles R. , Pizani P. S. , Daran E. , Fontaine C.
Published: July 2002

Raman spectroscopy is used to measure the frequency shift, symmetry, and activity of long-wavelength optical phonons in several GaAs strained epilayers. The results are compared with theoretical evaluations using the elastic compliances, phonon deformation potentials, and Raman tensors. The effect of growth direction ([001], [111], and [112]) and the substrate nature (Si or CaF2) is analyzed. The importance of nonstandard growth directions, [111] or [112], on residual stress and piezoelectric effect is discussed.
Impurity optical absorption of Cd1−xHgxGa2Se4:Co2+ single crystals
Authors: Lee Yang-Lae , Kim Chang-Dae , Kim Wha-Tek
Published: July 2002

Structural and optical properties of Cd1−xHgxGa2Se4 and Cd1−xHgxGa2Se4:Co2+ single crystals grown by the chemical transport reaction method have been investigated. It has been found that these single crystals have a defect chalcopyrite structure and a direct optical energy gap. It has been seen that the optical energy gap decreases as a composition x increases and as a cobalt impurity is introduced. The temperature dependence of the optical energy gap is well presented by the Varshni equation. Impurity optical absorption peaks due to a cobalt atom were observed in the optical-absorption measurements of Cd1−xHgxGa2Se4:Co2+ single crystals. It has been analyzed that the impurity optical-absorption peaks are attributed to the electronic transition between the split energy levels of a Co2+ ion occupying a cubic Td symmetry site of the host lattice.
Specific heat behavior for pseudobinary compounds ErxDy1−xSb
Authors: Long Y. , Hashiguchi T. , Hashimoto T. , Okamura M. , Sori N.
Published: July 2002

To search the new magnetic regenerator materials in the temperature range near 4 K, specific heat behavior of pseudobinary compounds ErxDy1−xSb(x=0, 0.2, 0.6, 0.8, and 1) was studied at temperatures from 2 to 40 K. The ordering temperature shifts from 3.5 to 9.5 K with increasing the concentration of the Dy+3 ion. The compounds of 1≳x≳0 show the gradual magnetic phase transition. The change of entropy associated with the magnetic phase transition is close to R ln 2 for all compounds. The change of magnetic entropy up to 40 K increases rapidly with decreasing the concentration of the Dy+3 ion, which implies that more excited states exist. The compounds of lower concentration of the Dy+3 ion have properties well suited for magnetic regenerative material around the temperature range of 4 K.
Enlarged quantum well in a semiconductor superlattice studied by depth resolved grazing incidence x-ray diffraction
Authors: Pietsch U. , Metzger T. H. , Seifert W.
Published: July 2002

A lattice matched {InP/GaInAs}30/InP[001] superlattice containing an enlarged quantum well (EQW) was investigated by means of grazing incidence x-ray diffraction (GID) using synchrotron radiation. The in-plane (220) rocking curve was measured choosing a grazing angle for the incident beam with respect to the surface, αi. At the angular position of maximum intensity we recorded the intensity distribution of the reflected beam normal to the surface (rod scan) using a position sensitive detector. The rod contains information about the density variation towards the surface normal. Instead of a single superlattice Bragg peak we found a double peak which can be explained by the phase shift of the partial x-ray waves scattered at the two superlattices sandwitching the EQW. For fixed αithe intensity ratio of the two peaks is a measure of the EQW thickness. An additional advantage of the GID technique is that this ratio can be modified by changing the penetration depth of the probing x-ray beam into the sample. This is performed by keeping αi smaller or larger than the critical angle for total external reflection. The EQW thickness and its position below the surface is determined by simulation of the recorded rod scans using the kinematic approach of the GID. Both quantities are obtained with monolayer accuracy.
Properties of AlN films grown at 350 K by gas-phase excimer laser photolysis
Authors: Radhakrishnan Gouri
Published: July 2002

Thin films of aluminum nitride have been deposited at 350 K on Si(100), fused quartz, and KBr substrates using gas-phase excimer laser photolysis of trimethylamine alane and ammonia at 193 nm. Depth profiles of these laser-grown films using secondary-ion-mass spectrometry indicate that no AlN film is produced without photolytic processes. The films are amorphous, and are smooth and featureless as established by scanning electron microscopy and atomic force microscopy. Optical-absorption spectra of these films have been measured and a band gap of 5.8 eV has been calculated from these spectra. Ellipsometric measurements have been used to determine a refractive index of 1.9–2.0 for these films. The infrared spectrum of the films displays a characteristic absorption due to TO phonons in AlN. Electrical measurements reveal that the films have excellent dielectric properties. A breakdown electric field of 108 V m−1 and a low-frequency dielectric constant of 6.0–6.9 have been established from I–V and C–V measurements, respectively.
Thermal stability of (Zn,Cd)(Se,S) heterostructures grown on GaAs
Authors: Bacher G. , Tönnies D. , Eisert D. , Forchel A. , Möller M. O. , Korn M. , Jobst B. , Hommel D. , Landwehr G. , Söllner J. , Heuken M.
Published: July 2002

We have analyzed the thermal stability of ZnSe-based single quantum well structures grown on a GaAs substrate by applying a rapid thermal annealing process. The photoluminescence intensity of the quantum well was used as a monitor for the thermal changes induced by the annealing process. X-ray diffractometry yields information about the crystal quality and the strain condition before and after the thermal treatment. As a main result, we found that the thermal stability of the quantum well photoluminescence signal critically depends on the thickness of the II–VI buffer layer, i.e., the distance between the active layer and the GaAs-II–VI heterointerface. For a buffer layer thickness of about 38 nm, the quantum well signal is totally quenched after a 1 min annealing step at 500 °C, while clear luminescence signals can be observed in samples with a 1 μm buffer even for a 750 °C process. Additionally, by comparing CdZnSe/ZnSe and ZnSe/ZnSSe quantum wells, we found that the Cd–Zn interdiffusion seems to be more efficient than the S–Se interdiffusion.
Site occupancy and lattice changes on nitrogenation in Nd3Fe29−xTixNy
Authors: Hu Z. , Yelon W. B. , Kalogirou O. , Psycharis V.
Published: July 2002

The structure of Nd3Fe29−xTixNy has been determined by neutron diffraction and compared with the unnitrided parent. Nitrogen is found in the two large octahedral interstitial sites with four Fe and two rare-earth neighbors, leading to a concentration of y=4. The lattice expands by 6.4% (ΔV/V) but the expansion is anisotropic in the axes. Although most of the Fe–Fe bond lengths expand, a few show significant contraction, most notably the Fe3–Fe3 dumbbell, which has an extremely short 2.18 Å length.
Fabrication and properties of intrinsic Josephson junctions in thin films of Tl-Ba-Ca-Cu-O
Authors: Veith M. , Eick T. , Brodkorb W. , Manzel M. , Bruchlos H. , Köhler T. , Schmidt H.-G. , Steinbeiss E. , Fuchs H.-J. , Schlenga K. , Hechtfischer G. , Müller P.
Published: July 2002

We have observed intrinsic Josephson effects for current flow perpendicular to the CuO2 planes in HTSC thin films. Tl-Ba-Ca-Cu-O thin films were deposited on step edges in LaAlO3 substrates. Due to the special preparation process, microbridges across such steps act as stacks of intrinsic Josephson junctions with current flow perpendicular to the CuO2 planes (STEP STACK junctions). Up to 130 individual junctions could be observed exhibiting high IcRN products up to 26 mV per junction. We observed ac-Josephson effects as Shapiro steps and in microwave emission experiments. At low temperatures sharp emission peaks were detected at frequencies of 11.3 and 24.2 GHz. Broad emission peaks from the whole stack were observed at temperatures close to Tc.
Kinetics of low pressure rf discharges with dust particles
Authors: Wang De-Zhen , Dong J. Q.
Published: July 2002

The kinetics of dusty plasma in low pressure rf discharges is studied by solving self-consistently the time dependent electron Boltzmann and dust charging equations. The electron temperature, the excitation and ionization rate coefficients, and the dust particle charge are shown to be modulated in the periodic state. It is demonstrated that the presence of dust particles enhances the modulated amplitude, reduces the high energy content of the electron energy distribution, the corresponding effective electron temperature, and the electron excitation and ionization rate coefficients.