bluuebunny/crossref_metadata_2025 huggingface dataset:
Projects of the Consortium for Materials Development in Space
Authors: Lundquist Charles A.
Published: March 2011

The Consortium for Materials Development in Space amalgamates private industries, the federal government, and universities for a common goal, commercial developments in space. The Consortium embraces research and development projects that benefit from unique attributes of space and that also rely on innovative applications of physical chemistry, material transport and their interactions. Three projects employ vapor transport of material from a solid source at one end of a sealed container to a growing crystal at the other end. The fourth and fifth projects have desired surface properties of materials as their objectives. In one of these, surfaces are electrodeposited, often with inert solid particles codeposited in the surface layer. The other project investigating surface properties makes use of the atomic oxygen environment outside a spacecraft. A surface exposed toward the direction of orbital motion is impacted by a beam predominantly of 5 electron volt oxygen atoms. This can produce unusual chemical reactions and surface morphology. The sixth project uses demixing of immiscible polymers in low-g to better understand the role of phase segregation in the properties of polymer blends and to accomplish the purification of materials by partitioning between two liquid phases.
The Use of Molten Salts as Physical Models for the Study of Solidification in Metals and Semiconductors
Authors: Koziol Jurek K. , Sadoway Donald R.
Published: March 2011

Product morphology in metal casting and semiconductor crystal growth is strongly influenced by the nature of the solid-liquid interface during solidification. Since in situ observation of solification in these technologically important systems is not possible, investigators have resorted to the use of physical models, such as water and organic liquids, which while they are transparent to visible light, suffer from the fact that their Prandtl numbers are too high. Molten alkali halides are better physical models in this respect, and are transparent to visible light. The purpose of the present study is to examine solidification in the LiCl-KCl system to determine if phenomena such as solute rejection can be observed by laser schlieren imaging.
Gravity and Conduction Driven Melting in a Sphere
Authors: Bahrami P A. , Wang T. G.
Published: March 2011

The fundamental processes of melting, the well known Stefan and Neumann problems, have been of great interest from a theoretical point of view as well as for their wide applications. The yet unmolten part of the material undergoing phase change within spherical containments is generally presumed to remain stationary, an unlikely occurrence in practice. The differing densities of the liquid and the solid may readily cause a force imbalance on the solid in gravitational and perhaps microgravitational environments, thereby moving the solid away from the center. In the present work, an approach related to the theories of lubrication and film condensation was employed and an approximate closed-form solution of melting within spheres was obtained. It was shown that a group of dimensionless parameters containing, Prandtl, Archimedes and Stefan numbers describes the melting process. Fundamental heat transfer experiments were also performed on the melting of a phase-change medium in a spherical shell. Free expansion of the medium into a void space within the sphere was permitted. A step function temperature jump on the outer shell wall was imposed and the timewise evolution of the melting process and the position of the solid-liquid interface was photographically recorded. Numerical integration of the interface position data yielded information about the melted mass and the energy of melting that support the theory.
Charged Drop Dynamics Experiment Using an Electrostatic-Acoustic Hybrid System
Authors: Rhim W. K. , Chung S. K. , Trinh E. H. , Elleman D. D.
Published: March 2011

In this paper, we describe the design and the performance of an electrostatic-acoustic hybrid system and its application to a charge drop rotation experiment. This system can levitate a charged drop electrostatically and induce drop rotation or oscillation by imposing an acoustic torque or an oscillating acoustic pressure. Using this system, the equilibrium shapes and stability of a rotating charged drop were experimentally investigated. A 3 mm size water drop was rotated as a rigid body and its gyrostatic equilibrium shapes were observed. Families of axisymmetric shapes, two-lobed shapes, and eventual fissioning have been observed. With the assumption of “effective surface tension” in which the surface charge simply modified the surface tension of neutral liquid, the results agree exceptionally well with the Brown and Scriven's prediction for uncharged drops.
Influence of Thermal Gravitational Convection on Solidi Fication Processes
Authors: Tensi Hans M. , Schmidt Joachim J.
Published: March 2011

The aim of these experiments in the Gradient Furnace with Quenching device (GFQ) during the German D-1 Mission was to study the influence of thermal convection on

– the diffusion in the melt ahead of the solidification front in an AlCu-alloy

– the stability of a smooth solidification front in an Al-Cualloy and

– the morphology of the solidification front in an AlSi-alloy

during the directional solidification of the binary alloys AlCu with 0.3 wght.-% Cu and AlSi with 7.0 wght.-% Si. Altogether five samples had been successfully processed during the D1-Mission. After getting the complete values from the process data (above all the velocity of the solidification front and the temperature gradient) reference experiments were made in the same GFQ under lg-conditions.

Additional experiments under lg with a transparent fluid were done to obtain information about the thermal convection in a cylindrical cell while measuring temperature distribution at the same time.

A first result having the same crystallization condition is the more than twice thicker diffusion boundary layer in AlCu 0,3-specimen than in the lg-reference specimen. The diffusion coefficient and the dendritic morphology are to be determined. The evaluation of all specimens (flight- and lg-samples) is not finished in the moment.
Synthesis and Characterization of TiO2-Nb2O5Borosilicate Glasses for Nonlinear Optical Waveguides
Authors: Vogel E. M. , Fribérg S. R. , Jackel L. , Smith P. W.
Published: March 2011

Nonlinear optical glasses with fast responding nonlinearities and very low absorption are promising materials for use in ultra-fast (picosecond) all-optical switches. We have synthesized a TiO2Nb2O5. B2O3Na2O SiO2glass with the intention of optimizing both the third order optical nonlinearity and suitability for fabrication of optical waveguides. The nonlinear index coefficient n2for this glass composition was found to be 4X 10-19m2/W, a factor of two greater than the largest value previously reported for commercially available TiO2-silicate glasses. Optical waveguides with a maximum index change of Δn = 0.12 were made using silver-sodium ion exchange. These waveguides may become building blocks for future all-optical logic elements and circuits.
Synthesis, Ultrapurification and Analysis of LaF3 for Fluoride Optical Fibers
Authors: Gabbe D. R. , Folweiler R. C. , Pink F. X.
Published: March 2011

Lanthanum chloride was synthesized by reacting La2O3 with CCl4. Purification carried out by zone refining resulted in reduction of Nd and Eu concentrations by at least a factor of 103. Impurity segregation coefficients were calculated from the concentration profiles in the charge measured by neutron activation analysis. Values of 0.85, 0.5, and 0.15 were found for Ce, Nd and Eu while a value of 0.65 was obtained for Pr from a plot of measured distribution coefficient versus ionic radius. These results show the feasibility of removing rare earth impurities to the ppb level from LaCl3. After purification, LaC13 is converted to LaF3
Optical Fiber Coating Evaluations for Composite Embedment Applications
Authors: Martin David A.
Published: March 2011

An emerging growth market for integrated optic devices and data transmission is in non-metallic composite aircraft structures. There is a regional need to identify optical fiber coatings which are suitable for embedment in composite structures. This paper assesses fibers with various coatings for the following: 1) critical structural test performance, 2) reproducibility, and 3) low attenuation when subjected to representative manufacturing and service conditions. A brief discussion of follow-on system verification tests, and an overview of desirable fiber and coating developments to meet future application requirements are also included.
Control of Radiation Induced Defects of Pure Silica Fiber Prepared by VAD Process
Authors: Muta K. , Kashiwazaki A. , Kohketsu M. , Kawazoe H.
Published: March 2011

The γ-induced centers in SiO2 glass such as the Si-E' center and OHC were found to be effected by the atmosphere present during melting. In the case of the VAD process, a porous preform rod is sintered to fabricate a transparent preform rod. The generation of defect species could be easily controlled by controlling the atmosphere of the sintering process. Only small amounts of OHC are generated in fibers sintered in Cl2 or H2 atmospheres, and most of the OHC is recovered at room temperature. This indicates the possibility of making fibers which are highly resistant to irradiation. The correlation between the γ-induced loss increase and the formation of these defects was examined.
Plastics and Polymers for Optical Transmission
Authors: Blyler L. L. , Cogan K. A. , Ferrara J. A.
Published: March 2011

The current stktus of worldwide research in the use of polymers as active lightwave transmission media is examined. Applications include plastic optical fibers, plastic optical components, polymeric optical fiber sensors and non-linear optical polymers.
The Use of an Organosilsesquioxane for the Coating/Cladding of Silica Fibers
Authors: Bagley B. G. , Kurkjian C. R. , Quinn W. E.
Published: March 2011

The properties necessary for a material to be useful as a fiber cladding (appropriate Δn) and/or coating (provide mechanical protection) were evaluated for an organosilsesquioxane (Owens-Illinois Resin GR 650). The samples were 120 μm silica fibers coated with 12 μm of the cured organosilicon polymer. We observe, from an index profile of the clad fiber, that Δn = 0.04. The tensile strength measured in air (65% RH, 25 °C) was 765 ksi and the dynamic fatigue constant determined under the same ambient conditions was 29, both of which are typical of a mechanically protected fiber. From aging experiments in 90 °C distilled water we observe that the onset for a marked loss of strength with time is ∼1 day for an epoxy-acrylate coated fiber and slightly more than a week for the organosilicon polymer coated fiber, a notable improvement.
Influence of Formation Parameters on the Mechanical Properties of Wet Gels
Authors: Pardenek S. A. , Fleming J. W. , Klein L. C.
Published: March 2011

The mechanical behavior of wet silica gels formed by various sol-gel processes has been studied. Samples were analyzed with a beam-bending apparatus specifically designed for accurate measurements of very low strength solids. Breaking strength, viscosity and relaxation measurements were made on gelled bodies. Specimens were studied in an undried gel state, molded in shapes suitable for the particular experiments. Tests were made in several ambient conditions including immersion in a liquid bath. The mechanical properties of gels are dependent upon the gel composition, gel age and other formation parameters such as pH, temperature and pressure. An understanding of these properties is particularly important in determining the nature of subsequent processing steps. Models for the structures of the various gels are proposed which explain variation in mechanical behavior.
Advanced Ti-Implanted Optical Waveguides in LiNbO3
Authors: Buchal Ch. , Ashley P. R. , Thomas D. K. , Appleton B. R.
Published: March 2011

LiNbO3 is the best substrate for modulators and switches for integrated optics. Efficient low loss waveguides for light in LiNbO3 are formed by introducing Ti-ions into its lattice, thus increasing locally the ordinary and the extraordinary indices of refraction. We are the first to use the very versatile technique of ion-implantation to administer Ti into LiNbO3. This implantation process offers the possibility to introduce significantly more Ti into a well-defined volume than conventional diffusion techniques. During this process first an amorphous non-equilibrium phase is generated, which has to be kept at low temperatures in order to prevent segregation. Subsequent thermal treatment leads to solid phase epitaxy and restores the desired stable crystalline state. We have used this technique to fabricate excellent planar waveguides, channel waveguides and Mach-Zehnder modulators.
Semimagnetic Lead Salt Alloys
Authors: Bauer Günther
Published: March 2011

The magnetic and electronic properties of pseudobinary alloys of the lead compounds (PbTe,PbSe)with Manganese, Europium and Gadolinium are reviewed. These materials have a direct minimum gap at the L-point of the Brillouin zone. The addition of Mn, Eu, or Gd to the diamagnetic hosts causes a rather weak antiferromagnetic exchange interaction. For sufficiently high carrier concentrations (10 2 0 -102 1 cm−3) a ferromagnetic interaction occurs due to RKKY interaction. The spin-spin exchange interaction between the Bloch states and the localized 3d or 4f states is characterized by rather small exchange integrals and the limited applicability of the mean field approximation for the description of energy band parameters like g-factors. Interband transitions in PbMnTe and PbMnS lend support to a zeromagnetic field splitting (“spin-splitting”)of band states which increases linearly with Mn-content up to several meV at temperatures below 4K.
Electronic Theory of Mn - Alloyed Diluted Magnetic Semiconductors
Authors: Ehrenreich H. , Hass K. C. , Larson B. E. , Johnson N. F.
Published: March 2011

Recent calculations of the electronic structure and magnetic interactions in Mn - alloyed II-VI diluted magnetic semiconductors (DMS) are summarized. Detailed band structure results are obtained using an empirical tight-binding, coherent potential approximation approach with input from experiment and local spin density band calculations. The dominant magnetic interactions in these systems result from hybridization between spin-split Mn d states and sp valence bands. Superexchange between Mn moments is well described by a simple three-level model which yields accurate Mn - Mn exchange constants for a variety of II-VI DMS as well as the rocksalt insulators MnO and α-MnS.
Magnetization of Random and Nonrandom (Cd,Mn)Te Alloys
Authors: Heiman D. , Isaacs E. D. , Becla P. , Foner S.
Published: March 2011

Pulsed-field magnetization measurements up to 40 T by Faraday rotation were made for random alloy Cd1−xMnx Te with 0.1 < x < 0.5. At liquid helium temperature the magnetization can be separated into the sum of two components: a paramagnetic Brillouin-like part that saturates by B=15 T, and a part linear in B. For large x, the linear part dominates and becomes independent of x. Mean-field calculations suggest that an enhanced magnetization can be obtained with new ordered-alloy crystal structures which have reduced antiferromagnetic cancellations.
Surface and Bulk Photoconductivity of Cd1−xMnxTe
Authors: Neff H. , Lay K. Y. , Park K. , Bachmann K. J.
Published: March 2011

Double beam photoconductivity experiments are reported for the system Cd1−xMnxTe. The technique allows a separation of surface and bulk contributions, respectively. Bulk effects dominate for manganese rich material and reveal a sharp peak at the band gap energy while surface conductivity reveals a step function type spectral behavior. The growth of a native oxide on the surface causes an increase in the surface recombination velocity and a change from surface to bulk conduction. An oxide related trap state was discovered that is located at approximately 400 meV above the valence band edge.
Influence of Electric Fields on Exciton Luminescence in ZnSe/(Zn,Mn)Se Superlattices
Authors: Fu Qiang , Nurmikko A. V. , Kolodziejski L. A. , Gunshor R. L.
Published: March 2011

Application of moderate external electric fields to ZnSe/(Zn,Mn)Se superlattices is shown to yield readily measurable spectral shifts of the exciton ground state resonance in the quantum well limit. This shows that confinement effects increase the classical field ionization threshold. At high applied fields and low temperatures, externally injected hot electron excited luminescence from internal excitations of the Mn-ion is observed.
Optical Properties of (Pb,Eu)Te Thin Films and Superlattices
Authors: Goltsos W. C. , Nurmikko A. V. , Partin D. L.
Published: March 2011

Photoluminescence, transmission, and reflectance measurements have yielded information about the states defining an optical gap in thin films and superlattices based on the (Pb,Eu)Te system, including the limit of high Eu concentration. Magneto-optical measurements show the presence of finite spin exchange processes at low Eu-concentrations.
Atomic Layer Epitaxy of Diluted Magnetic Semiconductors
Authors: Pessa M. , Lilja J. , Jylhä O. , Ishiko M. , Asonen H.
Published: March 2011

At the time being, there is evidently a sharp increase of interest in growth of low-dimensional thin-film structures of semiconducting compounds by the Atomic Layer Epitaxy (ALE) method. Many of the wide-gap II-VI compounds can be grown from elemental source materials by an ALE mode related to Molecular Beam Epitaxy (MBE). This paper concentrates on recent ALE work involving ZnSe- and CdTe- derived diluted magnetic semiconductors (DMS) where a fraction of Zn or Cd is substituted by Mn atoms. It shows that Zn1-xMnxSe/ZnSe and Cd1-xMnxTe/CdTe DMS structures with abrupt interfaces and high structural perfection can be produced at low growth temperatures by the ALE method.
From Concentrated to Dilute Magnetic Semiconductor: Old Concepts and New Ideas
Authors: Molnar S. Von
Published: March 2011

Magnetic semiconductors have enjoyed a renaissance with the discovery by Komarov et al. in 1977 of very large enhancements of magneto-optical properties in Mn doped CdTe. Although these magnetic semiconductors are dilute, that is to say they are solid solutions of II-VI or IV-VI semiconductors in which the cations have been replaced (randomly) by divalent transition metal or rare earth ions, their physical properties may be understood, in general, with concepts developed for the concentrated systems, for example the europium chalogenides. It is the purpose of this paper to review some of these ideas and point out new developments which have broadened our understanding, particularly as regards magnetic polarons, the highly correlated ferromagnetic region surrounding trapped carriers in an otherwise paramagnetic (P.M.) or antiferromagnetic (A.F.) semiconducting hosts.
Low-Temperature Magnetic Spectroscopy of Dilute Magnetic Systems
Authors: Awschalom D. D. , Warnock J.
Published: March 2011

Optically induced magnetization has been observed in a 10 micron diameter sample of Cd.8Mn.2Te. A newly developed integrated SQUID magnetic spectrometer was used to study both the magnitude and the picosecond dynamics of the magnetic response as function of the energy and polarization of the optical excitation. The dramatic energy dependence of the response provides an understanding of the way in which the overall sample magnetization changes upon illumination, and the subsequent relaxation through the spin lattice interaction.
Spin-Texture in Acceptor-Bound Magnetic Polarons
Authors: Isaacs Eric D. , Wolff Peter A.
Published: March 2011

We outline a theory of the acceptor-bound magnetic polaron in cubic diluted magnetic semiconductors that exhibits a nonuniform magnetization. Calculations show that the manganese spins have an overall z-alignment, but are canted in an azimuthally symmetric way from that direction. This is new phenomenon we call BMP spin-texture. Spin-texture results from valence band degeneracy, and the important spin-orbit effects associated with it. The possibility of studying BMP spin-texture via neutron scattering is discussed.
Investigation of Metallic and Metallic Glass Hollow Spheres for Fusion Target Application
Authors: Lee Mark C. , Kendall James M. , Wang Taylor G. , Johnson William L. , Keck W. M.
Published: March 2011

High quality metallic and metallic-glass microballoons (MMB and MGMB) are of considerable interest for fusion target applications on account of the intrinsic properties of these materials such as high density, high strength and high atomic number. We report the first successfully formed submillimeter and millimeter spherical shells of tin and of a gold-lead-antimony alloy by means of the hollow-jet instability technique developed by one of us (JMK). Examination of tin specimens by means of SEM has revealed that surface quality varied from poor to excellent. Whereas this metal has been employed only as a convenient and inexpensive material, the gold alloy is important because it is hard, has high atomic number, and may be solidified into the amorphous state through the provision of a modest cooling rate. We have produced AuPbSb spherules up to 1.5 mm in diameter using LN2 or chilled methanol as a coolant, and have found that these amorphous samples possess a superb surface smoothness compatible with fusion target requirements. Hollow spheres currently made of this alloy have an average O.D. of 2000 μm.
The Calculation of Transport Phenomena in Electromagnetically Levitated Metal Droplets
Authors: El-Kaddah N. , Szekely J.
Published: March 2011

A mathematical representation has been developed for the electromagnetic force field, the fluid flow field, the temperature field (and for transport controlled kinetics) in a levitation melted metal droplet. The technique of mutual inductances was employed for the calculation of the electromagnetic force field, while the turbulent Navier-Stokes equations and the turbulent convective transport equations were used to represent the fluid flow field, the temperature field and the concentration field. The governing differential equations, written in spherical coordinates, were solved numerically.

The computed results were found to be in good agreement with measurements reported in the literature, regarding the lifting force and the average temperature of the specimen.
Effect of Rayleigh Accelerations Applied to an Initially Moving Fluid
Authors: Dressler Robert F. , Robertson S. J. , Spradley L. W.
Published: March 2011

Although the instability of an initially motionless fluid subjected to a Rayleigh-type acceleration is well known, this type of excitation applied to an initially moving fluid has not been studied previously. We have therefore performed numerical analyses for two-dimensional unsteady thermal convection in circular cylinders under variable low-g conditions associated with space flight. When an acceleration vector is applied parallel to the thermal gradient for a fluid at rest, no convection results for the stable direction, and an instability leads to Rayleigh convection for the opposite direction. However, when the acceleration has a component which is orthogonal to the gradient, convection always results at any Rayleigh number; this is the usual situation during space flight. We therefore study the resultant effect on convection when both types of acceleration are applied, concurrently or sequentially, and when the resultant vector varies in direction with time. Our results indicate that for space flight conditions, the Rayleigh accelerations impose significant, but not dominating, alterations in the established convection even when the Rayleigh number is less than critical.
Theoretical Studies in Isoelectric Focusing
Authors: Mosher R. A. , Palusinski O. A. , Bier M.
Published: March 2011

Our overall objective is the development of a process for large scale purification of biologicals by isoelectric focusing (IEF). Thus, it was important to gain a better understanding of the underlying physical phenomena through mathematical modeling and computer simulation of the process. A mathematical model of the steady state in IEF was developed for an L-component system of ampholytes or monovalent buffers. It is based on the fundamental equations describing the components' dissociation equilibria, mass transport due to diffusion and electromigration, electroneutrality, and the conservation of charge. The model consists of ordinary differential equations coupled to a system of algebraic equations. The validity and usefulness of these simulationshas been confirmed in the formulation of actual buffer mixtures for experimental IEF. This model was recently extended to include the transient evolution to a steady state. Furthermore the new formulation applies to other electrophoretic processes, notably isotachophoresis and zone electrophoresis. As a result, we now have the first general model which predicts the behavior of a given chemical system in a variety of electrophoretic processes.
Atomic Fluorescence Study of High Temperature Aerodynamic Levitation
Authors: Nordine Paul C. , Schiffman Robert A. , Sethi D. S.
Published: March 2011

Ultraviolet laser induced atomic fluorescence has been used to characterize supersonic jet aerodynamic levitation experiments. The levitated specimen was a 0.4 cm sapphire sphere that was separately heated at temperatures up to 2327K by an infrared laser. The supersonic jet expansion and thermal gradients in the specimen wake were studied by measuring spatial variations in the concentration of atomic Hg added to the levitating argon gas stream. Further applications of atomic fluorescence in containerless experiments, such as ideal gas fluorescence thermometry and containerless process control are discussed.
A Study of the Coalescence Process Inside the Miscibility Gap in Zn-Bi Alloys
Authors: Bergman Å. , Fredriksson H.
Published: March 2011

The coalescence process has been studied in four different hyper-monotectic Zn-Bi alloys with 4–10wt % Bi . The alloys have been studied by isothermal treatments at a temperature just above the monotectic temperature and at different holding times. The alloys have been studied both at microgravity and at normal gravity. The change of the particle distribution as a function of composition and holding time has been investigated. The changing of the particle distribution has been compared with the theory of particle collision. The validity of the collision theory is investigated with respect to the theory of liquid phase sintering.
Levitation, Coating and Transport of Particulate Materials
Authors: Hendricks Charles D.
Published: March 2011

Several processes in various fields require uniformly thick coatings and layers on small particles. The particles may be used as carriers of catalytic materials (platinum or other coatings), as laser fusion targets (various polymer or metallic coatings), or for biological or other tracer or interactive processes. We have devisea both molecular beam and electro-dynamic techniques for levitation of the particles during coating and electrodynamic methods of controlling and transporting the particles between coating steps and to final use locations. Both molecular beam and electrodynamic techniques are describea and several advantages and limitations of each will be discussed. A short movie of an operating electrodynamic levitation and transport apparatus will be shown.
The Effect of Solute on the homogeneous Crystal Nucleation Frequency in Metallic Melts
Authors: Thompson C.V. , Spaepen F.
Published: March 2011

We have made a complete calculation that extends the classical theory for crystal nucleation in pure melts to binary alloys. Using a regular solution model, we have developed approximate expressions for the free energy change upon crystallization as a function of solute concentration. They are used, together with model-based estimates of the interfacial tension, to calculate the nucleation frequency. The predictions of the theory for the maximum attainable undercooling are compared with existing experimental results for non-glass forming alloys. The theory is also applied to several easy glass-forming alloys (Pd-Si, Au-Si, Fe-B) for qualitative comparison with the present experimental experience on the ease of glass formation, and for assessment of the potential for formation of the glass in bulk.
Finite Elenent Analysis of The Effect of a Non-Planar Solid-Liquid Interface on The Lateral Solute Segregation During Unidirectional Solidification.
Authors: Carlson F. M. , Chin L-Y , Fripp A. L. , Crouch R. K.
Published: March 2011

The effect of solid-liquid interface shape on lateral solute segregation during steady-state unidirectional solidification of a binary mixture is calculated under the assumption of no convection in the liquid. A finite element technique is employed to compute the concentration field in the liquid and the lateral segregation in the solid with a curved boundary between the liquid and solid phases. The computational model is constructed assuming knowledge of the solid-liquid interface shape; no attempt is made to relate this shape to the thermal field. The influence of interface curvature on the lateral compositional variation is investigated over a range of system parameters including diffusivity, growth speed, distribution coefficient, and geometric factors of the system. In the limiting case of a slightly non-planar interface, numerical results from the finite element technique are in good agreement with the analytical solutions of Coriell and Sekerka obtained by using linear theory. For the general case of highly non-planar interface shapes, the linear theory fails and the concentration field in the liquid as well as the lateral solute segregation in the solid can be calculated by using the finite element method.
Low Gravity Solidification Structures in the TIN-15WT %Lead and TIN-3WT%Bismuth Alloys
Authors: Johnston Mary H. , Parr Richard A.
Published: March 2011

The tin-15wt%lead and tin-3wt%bismuth alloys have been solidified in the low-gravity environment provided by the Space Processing Applications Rocket (SPAR), on the KC–135 airplane, and at high “g” levels in a centrifuge furnace. In each case the resultant cast structure was significantly different from that obtained in ground based experiments. Earlier low-gravity studies with the metal-model system NH4Cl-H2O presaged these results. This paper presents and discusses the influence of changes in the gravity force on the grain structure of these materials.
Review of The European Microgravity Activities
Authors: Malmejac Yves , Huth Ulrich
Published: March 2011

A few months ago, european activities in the field of microgravity research appeared as being an addition of single programmatic elements without enough logics, coordination and scientific background. Under the active pressure and guidance of many european scientists who became more and more aware of the basic interest of materials research in the microgravity environment, and due to the efforts of the ESA executive, the situation could evolve in a significant way, succeeding now in getting the official recognition of the necessity and content of a coherent european microgravity programme by both the ESA Council and the community of classical Space scientists. This presentation will deal with the scientific motivations of this new-born Programme through both the initial status and the most rational ambitions for future. The poirts of emphasis of the necessary multidisciplinary cooperative exertion together with the possible involvement of these scientists who could not derive benefit from any of the existing national programmes will then be stressed. At last the resulting priority objectives will be briefly reviewed before discussing the needed coordination with other european and non-european microgravity activities.
Structural Studies of Hydrogenated Amorphous Carbon Infrared Coatings
Authors: Robinson C. J. , Samant M. G. , Stohr J. , Speriosu V. S. , Guarnieri C. R. , Cuomo J. J.
Published: March 2011

Hydrogenated amorphous carbon thin films are well known for their mechanical hardness and optical properties which make them useful for applications in infrared device coatings. In this work films have been prepared by plasma. decomposition of methane using an RF diode reactor operating under conditions of high self bias potential (Vb = 1 KeV). The resulting ion bombardment during film growth leads to the formation of hard, insulating carbon coatings which have a band gap of ≃1.1 eV and are transparent in the IR. Nuclear reaction analysis has been used to quantify the atomic concentration of hydrogen incorporated in the films and extended x-ray absorption fine structure (EXAFS) has been used to determine local site geometry. Only first and second nearest neighbor bond lengths are observed with no evidence of further long range order or microcrystallinity. A model for atomic structure is proposed which includes both sp2 and sp3 bond configurations and direct comparisons are made with data obtained from sputtered carbon films, graphite and diamond.
Materials Requirements for IR Detectors and Imagers
Authors: Kinch M. A.
Published: March 2011

The requirements of infrared systems have increased significantly over the years, from the simple linear, low resolution, photoconductive array to the present-day, large area, high-density, photodiode (MIS and metalurgical) arrays, with on-focal-plane signal processing of considerable complexity. The success that has been achieved in meeting the performance goals appropriate for these systems has been due, to a large degree, to significant advances in the relevant materials technologies. The technologies of importance over the last twenty years are briefly reviewed, and the current state of the art, with its dominance by intrinsic alloy materials, is addressed in detail. The limitations of current bulk and epitaxial intrinsic materials technologies are considerable, both from a performance and a producibility point of view, when compared to the quality and quantity of material required by future infrared systems. These limitations are considered together with possible ways to overcome them.
Subsurface Micro-Lattice Strain Mapping
Authors: Ananthanarayanan T. S. , Rosemeier R. G. , Mayo W. E. , Becla P.
Published: March 2011

SYNOPSIS

Defect morphology and distribution up to depths of 20um have been shown to be critical to device performance in micro-electronic applications. A unique and novel x-ray diffraction method called DARC (Digital Automated Rocking Curve) topography has been effectively utilized to map crystalline micro-lattice strains in various substrates and epitaxial films. The spatial resolution of this technique is in the the order of 100um and the analysis time for a 2cm2 area is about 10 secs. DARC topography incorporates state-ofthe- art 1-dimensional and 2-dimensional X-ray detectors to modify a conventional Double Crystal Diffractometer to obtain color x-ray rocking curve topographs.

This technique, being non-destructive and non-intrusive in nature, is an invaluable tool in materials’ quality control for IR detector fabrication. The DARC topographs clearly delineate areas of microplastic strain inhomogeniety. Materials analyzed using this technique include HgMnTe, HgCdTe, BaF2, PbSe, PbS both substrates and epitaxial films. By varying the incident x-ray beam wavelength the depth of penetration can be adjusted from a 1–2 micron up to 15–20um. This can easily be achieved in a synchrotron.
Characterization of Ultra High Purity Silicon Epitaxy Using Photoluminscence Spectroscopy
Authors: Huffman J. E. , Thewalt M. L. W. , Steele A. G.
Published: March 2011

High purity epitaxial silicon samples, grown on indium doped and on ultrahigh resistivity silicon substrates, were analyzed for impurity content using photoluminescence spectroscopy (PL) and spreading resistance analysis (SRA). Calibrated SRA indicated typical net carrier concentrations of < 3×1012cm-3 in the epitaxial layers, and about 7×1011 cm-3 in the substrates. Impurities were identified by collecting highly resolved, very clean no-phonon and TO-phonon replica PL spectra at liquid helium temperatures. Spectra were taken on the substrate material alone and on substrates with epitaxy. Ga, As, A1, B and P contamination was evident in the epitaxy. Correlation of SRA and PL results on samples with various levels of contamination at the epitaxy substrate interface identified Al as the main
interfacial impurity.
On the Auger Recombination Process in P-Type Lpe HgCdTe
Authors: Chen J. S. , Bajaj J. , Tennant W. E. , Lo D. S. , Brown M. , Bostrup G.
Published: March 2011

Minority carrier lifetime measurements at 77K were carried out in ptype liquid phase epitaxial (LPE) Hg 1-xCdx Te/CdTe (x = 0.22) using the photoconductive decay technique. Lifetimes of 20 to 7000 ns were obtained in samples with hole concentrations, p, in the range 1014 to 1016 cm-3. The hole concentrations were determineg by analyzing the Hall data using a double-layer model. It was found that the minority carrier lifetime is inversely proportional to p01.86. This result demonstrates that the Auger mechanism may be the dominant recombination process in p-type LPE Hg0.78 Cd0.22Te/CdTe. The temperature dependence of minority carrier lifetime was also measured between 10 and 200K for several samples.
Characterization of HgCdTe Epilayers and HgTe-CdTe Superlattice Structures Grown by Molecular Beam Epitaxy
Authors: Myers T. H. , Yanka R. W. , Karins J. P. , Harris K. A. , Cook J. W. , Schetzina J. F.
Published: March 2011

The results of an in-depth study of HgCdTe epilayers and HgTe-CdTe superlattice structures are summarized. In particular, both spectral and transient photoconductance measurements have been made on samples which were also characterized by x-ray diffraction, Hall, IR transmission, IR photoluminescence, and transmission electron microscopy measurements. Selected HgCdTe samples grown at General Electric during this investigation exhibit some of the best structural and electrical properties reported to date for MBE-grown HgCdTe. Sharp photoconductance spectra have been obtained for HgTe-CdTe superlattice structures grown at North Carolina State University. This study is part of an on-going collaborative effort between General Electric's Electronics Laboratory and North Carolina State University.
A Study of the MBE HgTe Growth Process
Authors: Koestner Roland J. , Schaake H. F.
Published: March 2011

The MBE growth of HgTe on CdTe is examined over a hundred-fold range in Hg/Te2 flux ratio and over four separate substrate surface orientations [(111)Te, (111)Te-4 degrees, (112)Te and (001)]. The 77K Hall mobility of the (112)Te and (001) oriented HgTe layers approaches the best bulk values reported to date, although our (111)Te and (111)Te-4 deg oriented HgTe films yield much lower values. The growth process is shown to be very far from thermodynamic equilibrium at our optimal substrate temperature and calculated equivalent Hg beam pressure. Important clues to help understand the kinetics governing the HgTe growth process are uncovered by studying the defects that form under Hg- or Te-rich conditions with cross-sectional transmission electron microscopy (XTEM). Since multilayered structures are an important application for MBE growth of Hg-based semiconductors, we have also examined the interfacial roughness present in HgTe-CdTe superlattices (SL) as a function of growth orientation.
InSb: A Key Material for IR Detector Applications
Authors: Jost S. R. , Meikleham V. F. , Myers T. H.
Published: March 2011

InSb has served as an important mid-wave IR (λ=3−5μm) detector material for several decades. In this presentation, we will briefly review General Electric's InSb Charge Injection Device technology. Emphasis will be placed on device performance as a function of material parameters. A new InSb materials technology utilizing liquid phase epitaxy will be described. This epitaxial growth technology improves InSb material parameters and increases minority carrier lifetimes by more than two orders of magnitude to near the Auger limit. Comparisons will be made between available bulk material parameters and that of the epitaxial material.
Dual Ion Beam Sputter Deposition of Cdte, Hgte and Hgcdte Films
Authors: Krishnaswamy S. V. , Rieger J. H. , Doyle N. J. , Francombe M. H.
Published: March 2011

Experiments have been performed to assess the feasibility of using ionbeam sputter deposition for the growth of CdTe, HgTe and HgCdTe films. Some simple cryogenically cooled dual-target configurations have been employed in an investigation of epitaxial growth on CdTe substrates. Good-quality epitaxy was achieved for CdTe at temperatures down to 140°C, and for HgTe and HgCdTe at temperatures extending to below 50° C.Based upon compositional and phase analyses, and upon IR absorption measurements, we conclude that, using an excess Hg flux, stoichiometric transfer of the HgCdTe target composition to the substrate is approximately obtained. However, some departure from stoichiometry is produced at higher substrate temperatures (> 150”C) due to thermal re-evaporation of Hg, and under high sputtered Hg fluxes due to selective re-sputtering of HgTe. The good structural quality and excellent compositional uniformity of the films indicate that ion-beam sputter deposition may be suitable for low-temperature processing of IR detector structures.
Issues in the Growth of Bulk Crystals of Infrared Materials
Authors: Bachmann K. J. , Goslowsky H.
Published: March 2011

Selected issues in the growth of bulk single crystals for applications in infrared optoelectronics are reviewed including an overview over materials choices, bulk III-V crystal growth, and the growth of II-VI, IV-VI and I-III-VI2 compounds and alloys. The most important issues are the control of purity, perfection, stoichiometry, and uniformity during crystal growth and the control of the surface properties in wafer fabrication. Specific examples are given to illustrate problems related to these issues and to discuss approaches to their solution.
Real Time Observation of Periodic Step Arrays During Silicon Vapour Phase Homoepitaxy
Authors: Robbins D. J. , Young I. M. , Pidduck A. J. , Pickering C. , Glasper J. L. , Gasson D. B.
Published: March 2011

Light scattering has been used to characterise the periodic surface topography which develops on (001) Si epitaxial layers growing by a step flow mechanism. The periodicity provides a measure of the average kink density along the misorientation steps which develop a sawtooth form during growth. The diffracting properties of the layers have been determined after growth by angle-resolved scattering measurements. In-situ light scattering at fixed angle has been used to fol low the build-up of the periodic step arrays. The periodic structure is a metastable state of the surface maintained by the supersaturation during growth, and it decays on switching off the silane source gas or when the temperature is reduced below the point at which surface diffusion will support the sawtooth step shape.
Microscopic Model for the Epitaxy of CaF2 ON Si(111)
Authors: Himpsel F. J. , Karlsson U. O. , Morar J. F. , Rieger D. , Yarmoff J. A.
Published: March 2011

The epitaxy of CaF2 on Si(111) is characterized on an atomic scale using various photoelectron spectroscopy techniques. We find both F and Ca bonding to Si with Ca forming most of the interface bonds. The bonding orbital is found at 1.5 eV below the valence band maximum of Si. It consists of the Si3p-like dangling bond orbital and the 4s electron of Ca1+. Ca has changed its oxidation state at the interface from Ca2+ to Ca1+, which is demonstrated by the multiplet structure of the Ca 2p absorption edge. The electronic properties of the CaF2 interface layer are altered dramatically relative to bulk material, with the band gap shrinking from 12 eV to about 2 eV. Such strong effects raise prospects for creating new materials in the vicinity of an interface.
A Photoemission Study of the Epitaxial Growth of Si on Gap(110)
Authors: Niles David W. , Tang Ming , Höchst Hartmut
Published: March 2011

We have used angular resolved ultraviolet photoemission spectroscopy to study the epitaxial growth of Si on GaP(110). Surface state emission obscures the top of the valence band (TVB). The Fermi level for the clean GaP(110) surface is 1.20±0.05eV above the TVB. 1ML (monolayer) of Si pins the Fermi level position at 1.40±0.05eV above the TVB. Further deposition of Si leads to a valence band discontinuity ΔEv=1.07 ±0.10eV.
Angle-Resolved X-Ray Photoelectron Spectroscopy for the Characterization of GaAs(001) and Inp(001) Surfaces
Authors: Alnot Patrick , Olivier J. , Wyczisk F.
Published: March 2011

Electron scattering and diffraction in X-ray photoemission spectroscopy (XPS) have been used to characterize GaAs(001) and InP(001) chemically etched surfaces. 6a(3d),As(3d), In(4d) and P(2p) photoelectrons have been observed as a function of polar angles for the two [1–10] and [110] azimuths For kinetic energy range of these photoelectrons the experimental results have been correctly predicted by the single-scattering cluster model with spherical-wave corrections.

The problems of quantitative measurements in XPS have been discussed in relation with the diffraction phenomena.
Structure Analysis of the Al-Inp (100) Interface
Authors: d'Anterroches C. , Houzay F. , Bensoussan M.
Published: March 2011

High Resolution Electron Microscopy (HREM) images of the Al/InP interface were obtained from as-deposited films. The high purity Al films were deposited onto a clean (100) InP surface in a Molecular Beam Epitaxy (MBE) chamber. The in situ Reflection High Energy Electron Diffraction (RHEED) and Ultra-Violet Photoemission Spectroscopy (UPS) analyses showed a transformation of the InP surface during the Al deposition. The UPS data are interpreted as a strong exchange reaction between incoming Al and substrate In atoms. The In atoms are released and recover metallic behavior while Al atoms are involved in covalent bonds. At high coverages the RHEED analysis shows an epitaxial relationship in between Al and InP such as (110)Al//(100)InP with the two variants: [001]Al//[011]InP and [001]Al//[0–11]InP. The HREM images show that the interface AI/InP is perturbed and an intermediate layer is found. This layer appears to have the same crystal structure as InP indicated by extension of atomic planes from InP to the layer. However, the observed intensity, which corresponds to the mean potential of the forming atoms, is lighter than that of InP. Hence,out of these HREM and UPS results it is derived that an AlP or AlxIn1−xP compound is located at the Al/InP interface.
A Theoretical Study of Condensation Processes in OMCVD of GaAs
Authors: Shastry S. K.
Published: March 2011

Kinetics of epitaxial growth of GaAs from trimethylgallium (TMG) and arsine in organometallic chemical vapor deposition (OMCVD) have been suggested in the past to occur according to the Langmuir-Hinshelwood (L-H) or Langmuir-Rideal (L-R) mechanism [1–3], where competitive chemisorption of the Ga- and As-containing species is assumed. In contrast, formation of sp3 bonds on the GaAs growth front suggests that the Ga-containing species are less likely to chemisorb onto Assites, while the As-containing species are less likely to chemisorb onto Ga-sites. In addition, an analysis of probable chemical reactions and the unlikely event of homogeneous dissociation of hydrogen molecules indicate that the chemisorption of hydrogen must be included in the growth kinetics. Since H-As and H-Ga bonds have similar characteristics, such hydrogen chemisorption probably occurs on all sites. Thus, a mix of selective (Ga, As) and competitive (H2) chemisorption processes is likely to be present in practice. Furthermore, the presence of chemisorbed hydrogen will alter the surface As bonds, which, in the absence of hydrogen, are known to dehybridize and dimerize[4]. These basic issues have not been addressed in existing OMCVD growth models. Therefore, an analysis of the adsorption and growth processes, in the epitaxy of (100)GaAs is presented in this paper, with particular attention to the above issues.