bluuebunny/crossref_metadata_2025 huggingface dataset:
Atom diffusion and impurity-induced layer disordering in quantum well III-V semiconductor heterostructures
Authors: Deppe D. G. , Holonyak N.
Published: July 2002

The process of impurity-induced layer disordering (IILD) or layer intermixing, in AlxGa1−xAs-GaAs quantum well heterostructures (QWHs) and superlattices (SLs), and in related III-V quantum well heterostructures, has developed extensively and is reviewed. A large variety of experimental data on IILD are discussed and provide newer information and further perspective on crystal self-diffusion, impurity diffusion, and also the important defect mechanisms that control diffusion in AlxGa1−xAs-GaAs, and in related III-V semiconductors. Based on the behavior of Column III vacancies and Column III interstitials, models for the crystal self-diffusion and impurity diffusion that describe IILD are reviewed and discussed. Because impurity-induced layer disordering has proved to be an important method for III-V quantum well heterostructure device fabrication, we also review the application of IILD to several different laser diode structures, as well as to passive waveguides. We mention that it may be possible to realize even more advanced device structures using IILD, for example, quantum well wires or quantum well boxes. These will require an even greater understanding of the mechanisms (crystal processes) that control IILD, as well as require more refined methods of pattern definition, masking procedures, and crystal processing.
A model for hysteresis in magnetostriction
Authors: Sablik M. J. , Jiles D. C.
Published: July 2002

The domain wall pinning model used previously by the authors to explain magnetic hysteresis and stress effects on magnetic hysteresis is used in conjunction with the Callen and Callen expression for magnetostriction λ to qualitatively explain magnetostriction hysteresis both with respect to magnetic intensity H and flux density B. The Callen and Callen form for the magnetostriction is used because it depends functionally on effective field He rather than M, and this produces hysteresis in λ vs B whereas λ=λ(M) does not. To our knowledge, this is the first time that magnetic hysteresis and magnetostriction hysteresis have been modeled simultaneously.
Phase formation in the Pd-InP system
Authors: Caron-Popowich R. , Washburn J. , Sands T. , Kaplan A. S.
Published: July 2002

The interaction of 40-nm Pd layers on chemically cleaned (100) InP substrates has been examined. The samples were isochronally annealed in an Ar-5%H2 ambient at temperatures from 175 to 650 °C. Transmission electron microscopy, x-ray diffraction, and Auger electron spectroscopy were used to study the phases that formed. The reaction began upon deposition. With subsequent annealing at 175 °C, an amorphous ternary phase of approximate composition Pd4.8InP0.7 was formed. For samples annealed at 215 and 250 °C, the tetragonal ternary phase, Pd5InP (a=0.3928 nm, c=0.6917 nm), was found. After higher-temperature annealing (45–650 °C), the simple cubic phase PdIn (a=0.326 nm), was observed.
The temperature dependence of contact resistivity of the Ge/Pd and the Si/Pd nonalloyed contact scheme on n-GaAs
Authors: Yu L. S. , Wang L. C. , Marshall E. D. , Lau S. S. , Kuech T. F.
Published: July 2002

The temperature dependence of the contact resistance of the Ge/Pd and Si/Pd metalization scheme on n-GaAs was investigated. These two contact systems are based on solid-phase reactions, thus leading to nonspiking ohmic contacts to n-GaAs. The experimental results show that the ohmic behavior is likely due to both a highly doped surface n+ region and/or a small barrier at the interface. The origin of this small barrier and nonlinear current-voltage characteristics for certain samples are also discussed.
High coercivity in non-rare-earth containing alloys
Authors: Gao C. , Wan H. , Hadjipanayis G. C.
Published: July 2002

Large coercive fields have been obtained in melt-spun Co-Zr(Hf)-B based alloys. The highest coercivities have been observed in Co76 Zr18 B3 Si3 and Co78 Hf16 B3 Si3 samples with values 6.7 and 6.5 kOe, respectively. The saturation magnetization for both samples is estimated to be around 60 emu/g. Thermomagnetic data showed the presence of two phases with the major phase having a Curie temperature at around 450 °C. TEM and SEM studies showed a microstructure consisting of a mixture of two phases with the grain size in the range of 1000–5000 Å. The origin of the high coercivity is believed to be due to the fine grain microstructure leading to a single-domain particle-type behavior.
Iron magnetic moments in iron/silica gel nanocomposites
Authors: Shull R. D. , Ritter J. J. , Shapiro A. J. , Swartzendruber L. J. , Bennett L. H.
Published: July 2002

Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe have been prepared by low-temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X-ray diffraction, electron microscopy, Mössbauer effect, and magnetization measurements have been used to show that these bulk materials are paramagnetic composites at room temperature and remain in that state to 10 K. In this condition the Fe is present in nanometer-sized regions and exists in ionic form (both Fe3+ and Fe2+ ). It possesses a large magnetic moment which decreases linearly from 3.9 μB/ Fe atom to 2.8 μB /Fe atom as the Fe content increased from 11% to 40%. For this composition increase, a negative Curie-Weiss temperature was found which increased in magnitude linearly from −13 to −46 K. It is suggested that many of the iron atoms in the as-cured nanocomposites interact antiferromagnetically, and that the magnitude of the effect increases with the Fe concentration. After treatment in hydrogen, the state of the Fe changes.
Hg adsorption on optically thin Au films
Authors: Butler M. A. , Ricco A. J. , Baughman R. J.
Published: July 2002

Monitoring the reflectivity of optically thin Au films on the ends of multimode optical fibers during exposure to Hg vapor provides a new means of determining the nature and extent of chemisorption and reaction of Hg with Au. Upon Hg vapor exposure, a freshly deposited Au film shows an initial increase in reflectivity, followed after some time by a substantial reflectivity decrease. The initial reflectivity increase is a result of the chemisorption of Hg on the Au, augmenting the film’s optical thickness. The subsequent reflectivity decrease, which is observed only for saturated or near-saturated Hg vapor concentrations (p/psat >0.1), is a consequence of amalgamation of the Au by the Hg. The amalgamation process, which begins at grain boundaries when multilayers of Hg exist on the surface of the Au, results in the formation of large voids in the Au film. Energy-dispersive x-ray fluorescence spectroscopy shows the resulting amalgam to have the approximate composition Au2 Hg3. Reflectivity measurements have been made as functions of Hg partial pressure and of Au film thickness; models are presented which describe both behaviors. Detection of Hg vapor in the parts per 109 concentration range has been demonstrated, suggesting that this effect could be used to monitor Hg vapor concentrations at the Occupational Safety and Health Administration exposure limits.
Implant activation and redistribution in AlxGa1−xAs
Authors: Pearton S. J. , Hobson W. S. , Von Neida A. E. , Haegel N. M. , Jones K. S. , Morris N. , Sealy B. J.
Published: July 2002

The electrical activation characteristics of implanted Be, Mg, Si, and S in AlxGa1−xAs (x=0–1) were investigated as a function of ion dose for rapid annealing in the range 600–950 °C. The apparent activation energy for electrical activity of these species increases with increasing AlAs mole fraction—for Be, the activation energy is 0.35 eV for GaAs, and 0.49 eV for Al0.54Ga0.46As. There is no evidence for pairing of Be and O in AlGaAs, in contrast to the situation for GaAs. Self-compensation is the predominant limiting mechanism for Si activation in AlGaAs as determined by the relative photoluminescence intensities of the SiGa–to–SiAs -related transitions. No significant redistribution of implanted Si is observed for any AlAs mole fraction for rapid annealing (5 s) up to 900 °C, whereas S shows motion into the AlGaAs and no tendency to outdiffuse. By contrast, both Be and Mg display loss of the dopant to the surface and little redistribution toward the bulk. Minimal damage is observed by transmission electron microscopy in as-implanted AlGaAs for Be or Si doses below the amorphization threshold. Upon annealing at the conditions for optimum activation, a high density of small dislocation loops is observed near the end of the ion range.
Thermodynamic properties and crystallization kinetics of glass-forming undercooled liquid Au-Pb-Sb alloys
Authors: Fecht H. J. , Perepezko J. H. , Lee M. C. , Johnson W. L.
Published: July 2002

The heat capacities of liquid and crystalline Au-Pb-Sb alloys in the glass-forming composition range were measured with droplet emulsion and bulk samples. Based on the measured Cp data, the entropy, enthalpy, and Gibbs free-energy differences between the eutectic solid mixture and undercooled liquid were determined as a function of temperature over ∼60% of the undercooling range below the liquidus temperature and compared with theoretical predictions. The results indicate an isentropic temperature at 313 (±5) K, which agrees well with experimental data for the glass transition. The thermodynamic evaluation was applied further to develop a kinetics analysis of the nucleation undercooling response during cooling. Use of different approximations for the Gibbs free energy leads to a variation of the prefactor terms of six orders of magnitude for classical nucleation theory and, consequently, large variation in calculated transformation diagrams which is more pronounced with increasing undercooling. Extrapolations into the glass-forming temperature range and the effects of viscosity, transient nucleation, and estimated Kauzmann temperatures on the crystallization kinetics at high undercooling have been evaluated. This analysis reveals the importance of using measured values of thermophysical properties, even if they represent a limited temperature range at modest undercooling, rather than model approximations in order to obtain reliable evaluations of crystallization kinetics at high undercooling in the glass-forming temperature range.
Growth by molecular-beam epitaxy and characterization of (InAs)m(GaAs)m short period superlattices on InP substrates
Authors: Toyoshima Hideo , Anan Takayoshi , Nishi Kenichi , Ichihashi Toshinari , Okamoto Akihiko
Published: July 2002

(InAs)m(GaAs)m short period superlattices (SPSs) have been grown by molecular-beam epitaxy on InP substrates with their layer index m value systematically changed from 1 to 3. Their structural and electrical property dependencies on the layer index m value have been examined. During the first growth stage for the SPSs, with layer index m values of 2 and 3, two-dimensional reflection high-electron energy diffraction growth patterns were observed. The intended periodic structures without misfit dislocation generation were confirmed by x-ray diffraction and transmission electron microscopy (TEM) measurements. However, the obtained electrical properties were still poor, indicating the existence of a large amount of disorder in the SPSs. On the other hand, though the thickness of consisting binary compounds was as thin as one monolayer, a high-quality (InAs)1(GaAs)1 SPS was obtained. The highly ordered monolayer arrangement for InAs and GaAs was first observed by a TEM lattice image as well as x-ray diffraction measurement. The obtained electron Hall mobilities for the modulation-doped N-AlInAs/(InAs)1(GaAs)1 SPS structure were as high as 11 000 cm2/V s at room temperature and 74 000 cm2/V s at 25 K.
Dislocation distribution near the crack tip of I and II modes in bulk aluminum single crystal
Authors: Xu Y. B. , Ha K. F. , Wang Z. G. , Wang X. H. , Li J.
Published: July 2002

Dislocation distribution near the crack tip of I and II modes in bulk aluminum single crystal has been decorated by using the etching pit technique and then was observed by scanning electron microscopy. It has been shown that two kinds of distribution of dislocations, i.e., shielding and antishielding dislocations are found to exist near the crack tip. In addition to these, the dislocation-free zone is still observed to be between the crack tip and the plastic zone. According to Rice and Thomson [Philos. Mag. 29, 73 (1974)], the dislocation mode of the elastic-plastic crack is extended to include the antishielding dislocations as part of a crack-tip equilibrium configuration, which may be described as follows: (μb/2π) (∫−e−a+∫−c−s +∫sc+∫ae)[ f(x′)/(x−x′)]dx′+σa=0, when ‖x‖<C, or σf, when c<‖x‖<s and e<‖x‖<a. The factors affecting the dislocation distribution are discussed in detail.
Coercivity enhancement of melt-spun Nd-Fe-B ribbons using low-level Cu additions
Authors: Herbst J. F. , Fuerst C. D. , Mishra R. K. , Murphy C. B. , Van Wingerden D. J.
Published: July 2002

We have explored the impact of low-level Cu additions on the magnetic properties of melt-spun Nd-Fe-B ribbons. The addition of 0.5 at. % Cu enhances the intrinsic coercivity as much as 30% over that of Cu-free materials. Transmission electron microscopy analyses demonstrate that Cu segregates preferentially to the intergranular regions.
Absolute evaluation of combined hybridization-induced and RKKY-induced two-ion interaction in correlated electron systems
Authors: Sheng Q. G. , Cooper Bernard R.
Published: July 2002

Cerium and light actinide materials have unusual properties, including suppressed crystal-field splitting and highly anisotropic ordered magnetism, that can be understood on the basis of the Coulomb interaction between partially delocalized f electrons and the non-f-band electrons. In setting the absolute scale of interaction energy for materially predictive theory, it is crucial to take account of all consequences of the Coulomb interaction which contribute to the two-ion interaction. These include both the hybridization-induced effect (i.e., band-f mixing effects treated by a one-electron potential) and the RKKY-type effect (arising from the band-f exchange interaction) and the cross effect. The RKKY interaction, when treated fully, provides both isotropic and anisotropic contributions to the two-ion interaction. We found the anisotropic part is similar in its angular dependence and range dependence to the hybridization-induced effects. Therefore, the qualitative anisotropic nature of observable magnetic effects is quite similar to that caused by hybridization-induced effects by themselves, but the scale of energy is changed. Our results for CeBi and CeSb are in agreement with both the qualitative nature of the magnetic ordering and crystal-field dressing and with the absolute scale of energy as shown by the Néel temperature.
Charge injection asymmetry: A new route to strong optical nonlinearity in poled polymers
Authors: Yitzchaik Shlomo , Berkovic Garry , Krongauz Valeri
Published: July 2002

Amorphous polymer films containing hyperpolarizable dye molecules are poled by an electric field applied parallel to the plane of the thin film. The resultant second harmonic generation is much stronger than predicted by the standard model of dipolar interactions, and furthermore indicates asymmetry both parallel and perpendicular to the poling field. These results are attributed to a new, efficient mechanism for the production of optical nonlinearity in poled films in which asymmetric charge injection into the polymer and its trapping by dye species sets up a charge gradient perpendicular to the direction of the external poling field.
Properties of titanium nitride thin films deposited by rapid-thermal-low-pressure-metalorganic-chemical-vapor-deposition technique using tetrakis (dimethylamido) titanium precursor
Authors: Katz A. , Feingold A. , Pearton S. J. , Nakahara S. , Ellington M. , Chakrabarti U. K. , Geva M. , Lane E.
Published: July 2002

Titanium nitride (TiNx) thin films were deposited onto InP by means of the rapid-thermal-low-pressure-chemical-vapor-deposition (RT-LPMOCVD) technique, using the tetrakis (dimethylamido) titanium (Ti(NMe2)4 or DMATi) complex as the precursor. Depositions were successfully carried out at temperatures below 550 °C, pressure range of 5–20 Torr and duration of 50 to 90 s, to give layer thicknesses up to 200 nm and growth rates in the range of 0.8 to 4.5 nm/s. These films had a stoichiometric structure and contained nitrogen and titanium in a ratio close to unity, but also contained a significant amount of carbon and oxygen. The elements were spread uniformly through the films, the nitrogen was Ti bounded, and the carbon was partially titanium bonded and organic bonded as well. The film resistivity was in the range of 400–800 μΩ cm−2; the stress was always compressive, in the range of − 0.5 × 109 to − 2 × 1010 dyne cm−2, and the film had a good morphology. These layers performed as an ohmic contact while deposited onto p-In0.53Ga0.47As material, (Zn-doped 1.2 × 1018 cm−3), provided an excellent step coverage for high aspect ratio via holes and were deposited selectively onto the InP and based materials when using SiO2 mask. This represents the first report of TiNx films deposited in a commercial RT-LPMOCVD reactor using the DMATi precursor.
Diphenyl-diacetylene liquid crystals for electro-optic application
Authors: Wu Shin-Tson , Meng Hsiang-Hui B. , Dalton Larry R.
Published: July 2002

Three diphenyl-diacetylene nematic liquid-crystal homologs were synthesized and two mixtures formulated. Dielectric constant, threshold voltage, splay elastic constant, birefringence, order parameter, and rotational viscosity of these liquid crystals were evaluated at elevated temperatures. These highly conjugated liquid-crystal mixtures are found to exhibit a large birefringence, relatively low viscosity, wide nematic range, and modest dielectric anisotropy. Potential applications of these liquid crystals in high definition displays, infrared spatial light modulators, and high-speed light switches are foreseeable.
Dimensionality effects on the multicritical phase diagrams of the Blume–Emery–Griffiths model with repulsive biquadratic coupling: Mean-field and renormalization-group studies
Authors: Hoston William , Berker A. Nihat
Published: July 2002

Six new phase diagrams, including a novel multicritical topology and two new ordered phases, are obtained by the global mean-field theory of the spin-1 Ising model with only nearest-neighbor interactions, for negative biquadratic couplings. Renormalization-group studies indicate a threshold spatial dimension, above and below which different sequences of phase diagrams occur.
Laser drilling with focused Gaussian beams
Authors: Olson Radley W. , Swope William C.
Published: July 2002

A computational model for drilling holes with focused Gaussian laser beams is presented and compared with experimental results. Unlike earlier models, the beam divergence near the focus is taken into account and is shown to affect the hole profile strongly. The model correctly mimics experimental observations, which include the effect of moving the beam waist away from the sample surface and the dependence of the hole profile on beam divergence. It also demonstrates the optical conditions needed to drill cylindrical or conical holes.
The magnetocaloric effect: The role of magnetic anisotropy
Authors: Bennett L. H. , McMichael R. D. , Shull R. D. , Swartzendruber L. J. , Watson R. E.
Published: July 2002

In a ferromagnet, the magnetic entropy change induced by the application of a magnetic field is greatest in the temperature regime near the Curie point. In the paramagnetic temperature regime, the magnitude of the magnetocaloric effect is expected to rise monotonically with the size of the individual moments that make up the material. The magnetic properties of such materials are relevant to devices employing magnetic refrigeration. One aspect of the problem, namely the impact of uniaxial magnetic anisotropy on the magnetocaloric effect, is explored. The results of Monte Carlo simulations for classical Heisenberg ferromagnets on a fcc lattice with anisotropy are presented.
Frequency dependence of the transverse biased permeability in thin permalloy films
Authors: Doyle W. D. , He X. , Tang P. , Jagielinski T. , Smith N.
Published: July 2002

The frequency dependence of the transverse biased permeability μ′t(f ) has been measured up to 250 MHz in thin (0.044 μm) and thick (2.1 μm) unpatterned sputtered permalloy films with uniaxial anisotropy. In these films, the initial permeability along the hard axis can be described by the classical eddy current damping model assuming uniform rotation. For both films, μ′t at low frequency agrees reasonably well with Hoffman’s ripple model assuming appropriate values for the structure factor. In the thin film, μ′t(f ) is independent of frequency within experimental error up to at least 250 MHz. However, in the thick film μ′t(f ) decreases more rapidly with frequency than can be explained by classical eddy currents, when the transverse bias field HB is not very large compared to the net anisotropy field. The value of HB at which μ′t(f ) reaches a maximum amplitude is also found to increase monotonically with increasing frequency, also inconsistent with simple eddy current predictions. It is suggested that the formation of ripple walls leads to anomalous eddy current losses similar to that observed in bulk materials.
Atomic scale friction of a diamond tip on diamond (100) and (111) surfaces
Authors: Germann Geoffrey J. , Cohen Sidney R. , Neubauer Gabi , McClelland Gary M. , Seki Hajime , Coulman D.
Published: July 2002

The friction of a clean diamond tip on diamond (111) and (100) surfaces is studied using an ultrahigh vacuum force microscope that simultaneously measures forces parallel and perpendicular to the surface. The 30 nm radius diamond tip is fabricated by chemical vapor deposition. The attractive normal force curve between the tip and surface agrees well with calculated dispersion interactions. The frictional force exhibits periodic features, which on the (100) surface are tentatively associated with a 2×1 reconstructed surface convoluted over an asymmetric tip shape. The (111) surface shows features that cannot be simply related to the surface structure. As the tip is scanned back and forth along a line, the same features are observed in each direction, but offset, suggesting the presence of a conservative force independent of the direction of motion as well as a nonconservative force. The friction is approximately ≂3×10−9 N independent of loads up to 1×10−7 N.
Free-standing silicon microstructures fabricated by laser chemical processing
Authors: Westberg Helena , Boman Mats , Johansson Stefan , Schweitz Jan-Åke
Published: July 2002

Laser-assisted chemical-vapor deposition (LCVD) is used for growth in ‘‘free space’’ of microscale fibers and helical structures of silicon. The LCVD technique is also used for fabrication of a tungsten coil on a cylindrical silicon substrate, i.e., a microsolenoid is realized. The microstructure of the silicon deposits is investigated by transmission electron microscopy, and their mechanical strength is evaluated by micromechanical testing in situ in a scanning electron microscope. The resistivity of the tungsten coil is measured, and the magnetic properties of the microsolenoid are investigated by means of superconducting quantum interference device equipment.
Fundamentals of epitaxial silicon film thickness measurements using emission and reflection Fourier transform infrared spectroscopy
Authors: Zhou Zhen-Hong , Yang Isabel , Yu Fuzhong , Reif Rafael
Published: July 2002

A theoretical analysis of the principles and results of film thickness measurements using a reflection and an emission Fourier transform infrared spectrometer (FT-IR) is presented. Epitaxial film thickness measurements by emission FT-IR (E/FT-IR) have been recently demonstrated and the study reported here was conducted to understand further the underlying physical principles. The theoretical basis for spatial Fourier transformation using the Michelson interferometer is introduced. Moreover, a transfer function for a linear system model of the Michelson interferometer is derived. The transfer function and the model are subsequently used to calculate interferograms, which agree well with our experimentally measured interferograms. The difference in the interferograms obtained by IR reflection and IR emission is explained theoretically. The limitation of the FT-IR techniques is discussed. Furthermore, a comparison of the reflection FT-IR and emission FT-IR for epitaxial film thickness measurement will be given. Finally, an application of E/FT-IR for real-time in situ monitoring of epitaxial silicon film thickness is demonstrated.
Structual and magnetic properties of RMo1.5Fe10.5Nx
Authors: Yang Ying-Chang , Pan Qi , Zhang Xiao-Dong , Zhang Ming-Hou , Yang Chang-Li , Li Yan , Ge Sen-Lin , Zhang Bao-Feng
Published: July 2002

A systematic study of the structural and magnetic properties of the RMo1.5Fe10.5Nx series, where R is Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, or Er, has been carried out using x-ray diffraction, magnetic measurements, and the Mössbauer effect. The 1-12 phase with Ce and Pr has been successfully stabilized. PrMo1.5Fe10.5Nx possesses excellent intrinsic magnetic properties, favorable for permanent magnet applications. The 1-12 phase, RMo1.5Fe10.5Nx, was also stabilized. Compared to RMo2Fe10Nx, this compound shows a higher Curie temperature and a larger saturation magnetization due to the lower Mo content.
Polyimide ablation using intense laser beams
Authors: Soběhart J. R.
Published: July 2002

The ablation of polyimide by intense UV laser beams is modeled with a photothermal process that considers the shielding of the laser beam by the decomposed polymer. The heating of the decomposed material may drive the polymer degradation process for fluences above several J/cm2.
Coulomb energy of traps in semiconductor space-charge regions
Authors: Schulz M.
Published: July 2002

Large Coulomb barriers exceeding ΔE≊250 meV are estimated for capture and emission rates of trap centers in semiconductor space-charge regions. Depending on the charge state of the trap, the capture rate or both the capture and emission rates are activated or deactivated, respectively. The Coulomb energy raises the equilibrium energy state of a trap center that is repulsively charged when occupied. Quantitative agreement of the calculated Coulomb energy is obtained with trapping rates for single individual interface traps in metal-oxide-semiconductor (MOS) structures measured by random telegraph signals. The Coulomb barrier is reduced in MOS capacitors by partial screening due to mobile charge carriers in the inversion channel. The Coulomb energy can be externally controlled in MOS structures by the gate bias voltage.
Hydrogen Balmer alpha line shapes for hydrogen-argon mixtures in a low-pressure rf discharge
Authors: Djurović S. , Roberts J. R.
Published: July 2002

The spectral and spatial profiles of atomic hydrogen emission (the Balmer lines Hα and Hβ) from a low-pressure rf (13.56 MHz) discharge in H2+Ar mixtures have been studied. The plasma emission was observed in a direction normal to the applied electric field. The Hα profiles exhibit central narrow components and wide components which are due to Doppler broadening. Comparisons of the Hα profiles in a pure hydrogen plasma with those in H2+Ar mixtures show that collisions of molecular hydrogen ions and hydrogen atoms with argon atoms play a significant role in the production of the Hα profile shapes.
Efficiency improvements of silicon solar cells by the impurity photovoltaic effect
Authors: Keevers M. J. , Green M. A.
Published: July 2002

A theoretical investigation of the impurity photovoltaic (IPV) effect for improving silicon solar-cell efficiency is presented. The approach is better than previous analyses because of the improved treatment of generation and recombination via impurities, and because it includes the effects of optical competition and light trapping. The approach is applied to the nonmidgap, deep-level impurity indium as the IPV effect impurity incorporated into an idealized silicon solar cell. The analysis is based on experimentally determined parameters for indium. Improvements of cell current, subgap spectral response, and energy conversion efficiency are quantified. The analysis reveals the importance of light trapping and proper selection of indium and dopant concentrations. The impurity photovoltaic effect is predicted to improve solar-cell efficiency.
Pressure-induced phase transition and pressure dependence of crystal structure in low (α) and Ca/Al-doped cristobalite
Authors: Parise J. B. , Yeganeh-Haeri A. , Weidner D. J. , Jorgensen J. D. , Saltzberg M. A.
Published: July 2002

The phase stability and atomic-level compression mechanisms for both SiO2 cristobalite, and for cristobalite partially stabilized by Ca/Al doping (Cax/2 Si2−xAlxO4), have been investigated. A phase transition to a lower symmetry phase, observed with in situ high-pressure energy-dispersive x-ray diffraction, occurs at about 1.2 GPa. Structure models of the low-pressure phase were obtained by Rietveld analysis of neutron powder-diffraction data from powdered samples contained in a gas pressure apparatus. These data were collected at pressures up to 0.6 GPa and at 298 and 60 K. The results suggest collapse of the corner-connected framework from rotations of the rigid SiO4 tetrahedra at high pressures and low temperatures as the dominant mechanism for the densification of both materials. Compared to pure SiO2 cristobalite at the same pressure and temperature, the Ca/Al-doped material has a larger unit-cell volume. It also has a larger Si-O-Si bending angle and a more expanded framework as evidenced by the smaller rotations of the rigid SiO4 tetrahedra. The rate of change of these parameters as a function of pressure and temperature is the same for both pure and Ca/Al-doped cristobalite. These observations are consistent with Ca occupying positions within the cavities formed by the (Si, Al)-O framework and bracing it against collapse.
Uniformly layered mixtures of the Bi2Sr2Can−1CunOn+4 phases formed in artificially layered films—structural analysis and superconducting properties
Authors: Hatano Takeshi , Nakamura Keikichi , Narita Hideo , Sato Jun-ichi , Ikeda Shozo , Ishii Akira
Published: July 2002

Uniformly layered mixtures of the succeeding members of Bi2Sr2Can−1 CunO2n+4 series were found in the films synthesized by alternating sputter deposition of BiO, SrCu0.5O1.5, and CaCuO2 layers. The uniformly layered intergrowth was caused by the deviation of the CaCuO2 supply amount in each sequence, which is constant throughout the synthesis, from the stoichiometric amount. It is presented that the x-ray diffraction peaks from the films with uniform intergrowth can be indexed by treating them as the satellites of superlattices. In this treatment, the number of CuO2 layers, which varies randomly but uniformly along the c axis, is expressed on average by noninteger n. The superconducting transition temperature (Tc) and concentration of holes in the CuO2 layers in those films decreased with increasing n (2.88≤n≤6) monotonically. From the continuity of the decrease in Tc with n, we conclude that the hole concentration is averaged between the two phases in the intergrowth by the diffusion of the holes.
Intermetallic phase formation during annealing of Al/Ni multilayers
Authors: Edelstein A. S. , Everett R. K. , Richardson G. Y. , Qadri S. B. , Altman E. I. , Foley J. C. , Perepezko J. H.
Published: July 2002

The phase evolution during annealing of Al/Ni multilayer samples prepared by ion-beam sputtering with composition modulation wavelengths Λ between 10 and 400 nm was determined using x-ray diffraction and differential scanning calorimeter measurements. Samples with average compositions of Al0.40Ni0.60 and Al0.75Ni0.25 were investigated. For the Al0.40Ni0.60 samples the following results were obtained. A measure of the degree of periodicity and the sharpness of the interfaces in a sample with Λ=80 nm was the large number (over 20) of peaks observed in small-angle x-ray scattering measurements. A sample with Λ=10 nm was transformed by heat treatment directly to the AlNi phase. In the Λ=80 nm sample, the first phase formed after annealing was the metastable η phase. The η phase was identified as Al9Ni2. In the 400 nm wavelength sample, both the metastable η phase and the stable Al3Ni formed after the first exothermic reaction. For the Al0.75Ni0.25 samples two results were obtained. A Λ=11.4 nm sample transformed directly on annealing into Al3Ni. The η phase was the first phase formed on annealing a Λ=100 nm sample. The difference in the component diffusivities and the concentration gradient play an important role in controlling phase formation and evolution.
X-ray magnetic circular dichroism in the near and extended absorption edge structure (invited)
Authors: Schütz G. , Fischer P. , Attenkofer K. , Knülle M. , Ahlers D. , Stähler S. , Detlefs C. , Ebert H. , de Groot F. M. F.
Published: July 2002

Magnetic circular dichroism (MCD) studies at K and L edges using hard and soft x rays are presented. The relation of this universal phenomenon in the x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) regions to local magnetic structures in the case of 3d and 4f elements is illustrated. The validity of atomic and band-structure pictures to describe the MCD in the XANES range are outlined and the applicability of sum rules and two-step vector coupling models to deduce spin and orbital moments of the absorbing atom as well as spin-density profiles from the experimental data are critically discussed. The correlation of the dichroic contribution in the EXAFS to the magnetic moments and spin densities of the neighboring atoms are addressed by systematic studies which provide new insights into the exchange phenomena of the interaction of spin-polarized electrons with ferromagnetic media. The potential but also the limitations of this new spectroscopy is demonstrated.
Probing porous media with first and second sound. II. Acoustic properties of water-saturated porous media
Authors: Johnson David Linton , Plona Thomas J. , Kojima Haruo
Published: July 2002

The ultrasonic properties (reflection/transmission and bulk attenuation/speed) of porous and permeable media saturated with a Newtonian fluid, namely water, are considered. The frequency dependence of the transmission amplitudes of pulses is measured through a slab of thickness d1, repeated for another slab of thickness d2 for a given material. With these two measurements on two different thicknesses, it is possible in principle to separate bulk losses from reflection/transmission losses for compressional waves in these materials. The bulk properties are calculated from the Biot theory for which all of the input parameters have been measured separately; the attenuations are particularly sensitive to the values of Λ, determined from second-sound attenuation measurements reported in the companion article. There is excellent quantitative agreement between the theoretical and experimental values in the cases considered; there are no adjustable parameters involved. The reflection and transmission coefficients are reported for some of the multiply reflected pulses and their amplitudes are compared with those calculated from the Deresiewicz–Skalak and Rosenbaum boundary conditions appropriate to either the open-pore or sealed-pore surfaces, as the case may be. Again, there is excellent quantitative agreement between theory and experiment. Compared with the open-pore boundary conditions, it is noted that there is a large reduction, both theoretically and experimentally, in the efficiency with which the slow compressional wave is generated when the sealed-pore boundary conditions apply, but this efficiency is not reduced to zero.
The synthesis, structure, and characterization of α″-Fe16N2 (invited)
Authors: Jack K. H.
Published: July 2002

Controversy about the magnetic properties of α″-Fe16N2 thin films makes it desirable to examine bulk material. In the original preparation and crystal structure determination of α″ in 1951, N-austenite (γ) prepared by nitriding α-iron powder at 700–750 °C was quenched to give N-martensite (α′) which then gave α+α″ on long tempering at 120 °C. The final product was a mixture of either α+α″ or α+α″+γ with less than 50%α″. Recent repetitions of this preparative method in three different laboratories have given mixtures with varying amounts of α″, again never greater than 50%, and with magnetic moments of α″—assessed from measurements made on the mixtures—that show considerable variation. Studies of α′ tempering by XRD, and of α″ precipitation from supersaturated N ferrite (α) by high-resolution TEM, both show that slow ordering of N atoms to produce α″ occurs only after localized regions of α′ and α reach the Fe8N composition by a clustering process. Thus α′-Fe8N can be obtained with a tetragonality c/a equal to that of α″, but without the complete N ordering that is characteristic of Fe16N2. This might explain some of the variability in magnetic properties. The inhibiting effect of oxygen and other impurities on the nitriding of iron is emphasized, the existence of the α″-carbonitride Fe16(C,N)2 is noted, and possible methods for 100% production of bulk α″ are suggested.
Recrystallization of a planar amorphous-crystalline interface in silicon by low energy recoils: A molecular dynamics study
Authors: Caturla Maria Jose , Diaz de la Rubia Tomas , Gilmer George H.
Published: July 2002

We discuss the motion of an amorphous-crystalline interface in silicon induced by low energy recoils. We employ molecular dynamics simulation with the Stillinger–Weber interatomic potential for silicon. The temperature of the substrate in these simulations was 250 K. Our results show that when 15 or 20 eV recoils are initiated from the amorphous side of the interface, the crystal regrows by solid-phase epitaxy. On the other hand, no interface motion was detected for 15 eV recoils launched from the crystalline side, and damage accumulation resulted when the recoil energy was set to 20 eV. The efficiency of recrystallization for this process is 0.67, for both 20 and 15 eV recoils. That is, approximately two silicon atoms transform from the amorphous to the crystalline phase per every three incident recoils. The calculated threshold energy required to produce a stable defect in silicon was found to be substantially lower in an amorphous matrix than in a crystalline lattice.
Thermal diffusivity/conductivity in SiAlON ceramics
Authors: Liu Dean-Mo , Chen Cheng-Jen , Lee Ray R.-R.
Published: July 2002

Thermal diffusivity and conductivity of ceramic materials in a system of Si3N4-SiO2-Al2O3-AlN composed of α and β SiAlON (which is well known as duophase SiAlON ceramics) are investigated. Single-phase β-SiAlON ceramic has a thermal conductivity of 12.44 W/m K. The thermal conductivities of SiAlON ceramics decrease linearly with increasing content of α phase and can be best described by the following equation: K=12.46–0.043 f, where f is the weight percentage of α-SiAlON content and K is the thermal conductivity of SiAlON ceramics. The thermal conductivity of single-phase α-SiAlON was then estimated to be ∼8.16 W/m K, which is in excellent agreement with the literature reported values, ∼8.4 W/m K. This estimation is also close to the value, 8.22 W/m K, obtained by using Bruggeman’s variable dispersion theory. The temperature dependence of the thermal conductivity of duophase SiAlON ceramics varies with α and β phase contents in the materials.
Growth and ferroelectricity of epitaxial-like BaTiO3 films on single-crystal MgO, SrTiO3, and silicon substrates synthesized by pulsed laser deposition
Authors: Lin W. J. , Tseng T. Y. , Lu H. B. , Tu S. L. , Yang S. J. , Lin I. N.
Published: July 2002

The epitaxial-like BaTiO3 (BTO) films with good ferroelectricity are obtained both on (001)SrTiO3 (STO) single crystal and on CeO2 buffered silicon substrate by pulsed laser deposition. The deposition parameters need to be stringently controlled in order to grow BTO films with good crystallinity. The BTO films grown on YBa2Cu3O7−x (YBCO)/CeO2/STO substrates are epitaxial, as confirmed by rocking curve, φ scan, and wide-angle x-ray-diffraction techniques. The alignment of a and b axes of BTO films on YBCO/CeO2/Si substrate is, however, not as perfect as BTO film on YBCO/STO substrate. The BTO/YBCO/CeO2/Si films are only (00l) textured. The ferroelectric property measurement, using the YBCO layer as the base electrode material, shows that the remanent polarization Pr and coercive field Ec of the BTO/YBCO/CeO2/Si films (Pr=3.6 μC/cm2, Ec=11.1 kV/cm) are, however, as good as those of the BTO/YBCO/STO films (Pr=4.0 μC/cm2, Ec=12.5 kV/cm).
Oxygen gettering and oxide degradation during annealing of Si/SiO2/Si structures
Authors: Devine R. A. B. , Warren W. L. , Xu J. B. , Wilson I. H. , Paillet P. , Leray J.-L.
Published: July 2002

The microscopic nature of the degradation of oxide layers in Si/SiO2/Si structures induced by annealing in the temperature range 1200–1320 °C in inert or weakly oxidizing atmospheres has been studied. Electron-spin-resonance measurements have been performed on unannealed and annealed samples subsequently subjected to γ or X radiation or hole injection. Two oxygen-vacancy-related defect centers were observed, the monovacancy Eγ′ center and the multiple vacancy Eδ′—both were observed in substantially larger numbers in annealed oxides as compared to unannealed oxides. Etchback profiling of the paramagnetic defect distributions shows that they are distributed nonuniformly throughout the annealed oxides with the highest densities close to the two Si/SiO2 interfaces. Electrical measurements of fixed oxide charge induced by X irradiation indicate that annealing results in the creation of both positive and negative charge traps. The numbers of positive trapped charges and their radiation dose dependence are inconsistent with their origin being identified simply with the paramagnetic oxygen-vacancy centers. Infrared measurements of the O interstitial content of the float-zone Si substrates of annealed and unannealed samples reveal that the interstitial concentration increases as a function of anneal temperature/time. Atomic force microscopy measurements reveal that the SiO2/Si substrate interfaces are roughened during high-temperature annealing. The data are interpreted in terms of a model in which oxygen is gettered from the oxide film into the over- and underlying Si. The O are incorporated into the Si as interstitials and it is their solubility limit at the anneal temperature which drives the gettering process. The oxygen-vacancy defect profiles near to both Si/SiO2 interfaces are not well predicted by the gettering model suggesting that other interface-related defect creation processes may be active.
Chemical transformations of the polyimide Kapton brought about by ultraviolet laser radiation
Authors: Srinivasan R. , Hall R. R. , Loehle W. D. , Wilson W. D. , Allbee D. C.
Published: July 2002

By the use of ultraviolet laser pulses of microsecond and millisecond duration it is shown that the chemistry of the transformations of Kapton by UV laser radiation is strongly dependent on the intensity (power/unit area) of the laser beam. With these long pulses, the polymer was not ablated. The decomposition resulted in 51% of the polymer weight being converted to gaseous products consisting mostly of CO (67%), HCN (15%), C2H2 (12%), and some (<5%) CO2. The major solid product that remained was ‘‘glassy’’ carbon which was identified from its Raman spectrum. This material can be viewed as the product of the secondary addition reactions of the residue that is left after the loss of the gaseous products listed above. With 20 ms pulses, the evolution of the gaseous products increased linearly with intensity and the product composition was constant within the experimental uncertainty over a 12-fold range of intensity up to 50 kW/cm2 (≡1 kJ/cm2). These results show that pulses of duration much greater than ns do not lead to ablation even at fluences that are 104 greater than the threshold for ablation using nanosecond pulses. It is therefore more appropriate to view the ablation of this polymer by UV laser pulses of nanosecond duration as being due to the scaling of an intensity threshold rather than a fluence threshold as has become the practice.
Identification of passive layer in ferroelectric thin films from their switching parameters
Authors: Tagantsev A. K. , Landivar M. , Colla E. , Setter N.
Published: July 2002

The switching of the sandwich structure ‘‘ferroelectric+thin dielectric layer’’ is theoretically studied. The behavior of the remanent polarization, Pr, coercive field, Ec, maximal polarization on the loop, Ps, and the slope of the hysteresis loop at Ec as a function of both, the relative thickness of the layer (the thickness of the dielectric divided by the thickness of the ferroelectric) and the amplitude of the measuring field, is analyzed. It is shown that the effect of the layer on Pr and the slope at Ec is qualitatively different from that on Ec and Ps. As the result of this analysis the set of characteristic features of the sandwich with the variable relative thickness of the dielectric layer have been have formulated. These characteristic features can be used for the identification of the presence of such a dielectric layer using the data on the switching parameters of the system. It is shown that the increase of the coercive field with increasing relative thickness of the dielectric layer, which was considered as an important sign of the presence of the dielectric layer, is not a real property of the sandwich structure but rather an artifact of the approximations which were used in the analysis performed by the previous workers. In fact, Ec is a decreasing function of the relative thickness of the dielectric layer.
Charge-carrier kinetics in semiconductors by microwave conductivity measurements
Authors: Swiatkowski C. , Sanders A. , Buhre K.-D. , Kunst M.
Published: July 2002

Theory and measurements of the conductivity and the (transient) photoconductivity in the microwave frequency range are presented. The theory is tested on noninvasive measurements of semiconductors with known properties, i.e., Si wafers, in a simple apparatus. Quantitative agreement between theory and experiment is found without the use of adjustable parameters. A contactless and accurate determination of the conductivity of Si wafers in a restricted conductivity range is proposed. The quantitative evaluation of photoconductivity measurements makes a detailed discussion of nonuniform photoconductivity possible. The requirements for reliable measurements of nonhomogeneous charge carrier kinetics are discussed.