bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical Characterization of Defects in GaAs Grown on Si by MBE
Authors: Chand N. , Fischer R. , Sergent A. M. , Lang D. V. , Cho A. Y.
Published: March 2011

We show that MBE-grown GaAs on Si exhibits only a modest increase in the concentrations of the well-known electron traps typical of MBE-GaAs with no evidence for any new electron deep levels in the upper half of the bandgap in spite of the dislocations and other defects in the material. As shown by Au-GaAs Schottky contacts, the defects are unnoticeable when the device is forward biased but become very active in reverse biased condition, causing large leakage current and low breakdown voltage (although the device is still acceptable for many applications, especially FET's). The defects become more active after hydrogenation and more inactive after a post-growth rapid thermal annealing (RTA). Performance of devices made on thermally annealed GaAs on Si is comparable to those of GaAs on GaAs. Also, following the application of a large- current the device behavior improves, indicating a self-annealing action as a result of internal heating. The reverse current in the as-grown material shows a very weak temperature dependence, indicating its origin is not thermionic emission or carrier generation. It is speculated that a large part of the leakage current in the as-grown GaAs on Si is due to the defect assisted tunneling. After RTA, the average spacing between defect clusters increases, thus reducing the tunneling probability and tremendously improving the device characteristics.
Photoluminescence Study of GaAs/AlGaAs Quantum Wells Grown On Si Substrates
Authors: Jagannath C. , Zemon S. , Norris P. , Elman B.S. , Shastry S.K.
Published: March 2011

Photoluminescence and photoluminescence excitation spectroscopies are utilized to study excitons in GaAs/AlGaAs quantum wells (QW's) fabricated using MBE on MOCVD grown GaAs/Si. The experimental results are understood in terms of the biaxial tension of approximately 3 kbar present in the plane of growth for both the QW's and the GaAs buffer. An important consequence of the biaxial tension is that for QW's with well widths larger than ≈15 nm the light- and heavy- hole sub-bands cross each other in energy. This results in the light-hole exciton energy being lower than that of the heavy-hole exciton, opposite to the case of QW's grown on GaAs substrates.
Optical Characterization Of GaAs/Si: Spectral Features in the Excitonic Region
Authors: Zemon S. , Shastry S. K. , Jagannath C. , Norris P. , Lambert G.
Published: March 2011

Using 4.2 K selective photoluminescence (PL) excitation and PL excitation spectroscopy, the n = 2 excited state of the light-hole exciton is observed for the first time in GaAs/Si. The excited state is about 3 meV above the ground state, similar to exciton results for GaAs/GaAs. A spectral width of 1.8 meV is observed for the ground state transition, the narrowest yet reported.
GexSi1−x/Si Heterostructures: Physics and Device Applications
Authors: Bean John C.
Published: March 2011

Work on GexSi1−x strained layer epitaxy is reviewed including: The limits of single layer and superlattice growth, and comparison with theory; Physical properties including bandgap and heterostructure band alignment; Work on non-random alloys; Device applications including MODFET's, PIN and APD photodetectors.
Theoretical Analysis of a Si/SiGe Hemt Structure
Authors: Smith C , Jones M E
Published: March 2011

An analysis has been made of a Si/SiGe HEMT structure, grown on a (001) oriented substrate, by adapting an existing model for III–V HEMT's. Strain plays a critical role in the behaviour of these SiGe structures by separating the degenerate conduction band minimum into 2- and 4- fold symmetric minima. This effect has been incorporated into a ID solution of Poisson's equation in the device structure. The effects of strain, Schottky barrier height, layer width and doping density on both enhancement and depletion mode HEMT properties are all considered. In addition initial estimates of the effect of strain on electron mobility are presented.
Optical Properties of Ordered Ge-Si Atomic-Layer Superlattices
Authors: Pearsall T. P. , Bevk J. , Bean J. C. , Bonar J. M. , Mannaerts J. P.
Published: March 2011

A systematic study of the band-to-band optical transitions in commensurate strainedlayer superlattices of Ge and Si grown on (001) Si substrates show the presence of new electronic energy bands. Our measurements suggest that superlattice structure on the same scale as the unit cell results in a band structure significantly different from that of a random alloy.
New Ordering-Induced Optical Transitions in Strained Sige Superlattices
Authors: Froyen Sverre , Wood D. M. , Zunger Alex
Published: March 2011

First principles total energy and electronic structure calculations for SinGen superlattices grown epitaxially on a Si (001) substrate reveal a nearly direct band gap despite the pronounced indirectness of the Si0.5Ge0.5alloy. While the new direct superlattice transitions have their origin in folded indirect transitions of the random alloy, their transition matrix elements are considerably enhanced by superlattice atomic relaxation and the superlattice ordering potential. For a superlattice grown on a substrate with an lager lattice contant than we predict a nearly direct band gap.
The Nucleation and Growth of GaAs on Si
Authors: Harris J. S. , Koch S. M. , Rosner S. J.
Published: March 2011

Substantial progress has been realized in both understanding the nucleation and growth of GaAs/Si and demonstration of device application of this technology. In this paper, we review the recent progress in the role of the Si surface, initial nucleation, the GaAs/Si interface, GaAs thick layer growth and defect generation and control in the GaAs layer. This last area is the remaining area where substantial progress is still required. Several new approaches for defect control are discussed.
Stress Distribution and Critical Thicknesses of Thin Epitaxial Films
Authors: Sharan S. , Jagannadham K. , Narayan J.
Published: March 2011

We have calculated stress distribution and critical thickness for the growth of strained epilayers having various values of mismatch between the epilayer and the substrate. The present analysis has been carried out assuming that the nucleation of a misift dislocation is controlled by the activation energy. Further, a misfit dislocation is nucleated when the areal strain energy density of the coherent film exceeds the activation energy associated with the misfit dislocation configuration. The latter term has been deternmined using the discrete dislocation method in conjunction with a surface dislocation analysis. The surface dislocation configuration is obtained by minimizing the total energy associated with the two-phase medium. It is verified that the free surface boundary conditions and the continuity of stresses acrosss the interface are satisfied by the surface dislocation array. These energy calculations are expected to be more accurate than those performed by previous workers.
Epitaxy Of GaAs on Si: MBE and OMCVD
Authors: Lee Jhang W. , Salerno Jack P. , Gale Ron P. , Fan John C.C.
Published: March 2011

Growth technique dependent factors are compared for molecular beam epitaxy (MBE) and organometallic chemical vapor deposition (OMCVD) that are related to the GaAs on Si epitaxy. The comparison, both for growth processes and as-grown material characteristics, indicates that material qualities of these layers provided by the two growth techniques are comparable in many aspects, but differ in morphological texture, residual stress, and occasionally Schottky barrier height. These issues are discussed further with our recent OMCVD results in order to ensure the OMCVD advantages for GaAs on Si wafer engineering, which are referred as an easy scale-up and a large throughput.
Heteroepitaxial Si-, Ge-, and Gaas-On-Insulator Structures on Si Substrates by Use of Fluoride Insulators
Authors: Asano T. , Ishiwara H. , Furukawa S.
Published: March 2011

Heteroepitaxial growth of alkaline earth fluoride films on Si substrates and Si, Ge, and GaAs films on the fluoride/Si structures, is reviewed. Growth of single crystalline fluoride films on Si is first discussed. Then the usefulness of novel heteroepitaxial technologies, the predeposition method and the electron beam irradiation method, is demonstrated in the growth of Si and Ge films on CaF2/Si structures. Finally fundamental growth characteristics of GaAs films on CaF2/Si structures and annealing effects on the crystallinity of the GaAs films are described.
The Effect of Rapid Thermal Annealing on Heteroepitaxial Structures
Authors: Phillips Julia M. , Batstone J. L. , Pfeiffer L.
Published: March 2011

We present evidence on the types of structural changes caused by the rapid thermal annealing of two types of heteroepitaxial layers: CaF2/CoSi2/Si(111) and Si(100) on A12O3 (1102). We find that grains in a film can be merged into a single crystal and that the microtwin density can be dramatically lowered. We also find a number of changes in the structure of the heteroepitaxial interfaces.
Properties Of Epitaxial Cdte On Si(lll) With a (Ca,Ba)F2 Buffer Layer
Authors: Zogg H. , Blunier S.
Published: March 2011

Epitaxial CdTe has been grown onto Si(lll) wafers by MBE with the aid of a composition graded (Ca,Ba)F2 buffer layer to surmount the large misfit of 19%. Untwinned CdTe layers with smooth surfaces, narrow X-ray lines and strong photoluminescence with a narrow near band edge peak were obtained. The results indicate a comparable structural quality to well known CdTe layers on sapphire, InSb or GaAs used as buffers to grow (Hg, Cd)Te for IR-device applications. In addition, the CdTe layers are near strain free despite a large thermal expansion mismatch. This is most probably due to dislocations which are able to move along the fluoride/Si interface even after growth and down to near room temperature.
Silicon Quality Versus Thickness For Novel Silicon On Boron Phosphide Soi Process
Authors: Robertson Perry J. , Dumin David J. , Carver Gary P. , Novotny Donald B. , Freytag Michael
Published: March 2011

Results are reported for a single temperature Si-BP-Si process which utilizes 0.2 and 0.4 μm of high resistivity BP covered by a single 1–5 μm silicon epitaxial layer on which PMOS devices were fabricated. Transistor and capacitor measurements were used to characterize the quality of the silicon films. Impurity concentrations on 1.0 and 2.0 μm silicon layers were as high as 3×1017cm−3 due to auto doping from the BP layer. MOS transistors manufactured on these layers did not work properly. Device characteristics improved on thicker silicon layers. The impurity concentration was as low as 1015 cm−3 on the 5.0 μm silicon layers. Surface state densities were found to be as low as 5×1011 eV−1cm−2 with subthreshold currents as low as 10−11 amperes. On all wafers the subthreshold slope varied from 90 to 100 mV/decith little variation due to silicon thickness. Linear and saturation mobilities were process limited with values on the 5.0 μm layers as high as 250 and 160 cm2/V-s respectively. Characteristics of devices fabricated on 5 μm silicon layers were comparable to those of devices fabricated on bulk silicon processed at the same time indicating high quality silicon growth.
Formation of Si-On-Insulator Structure By Lateral Solid Phase Epitaxy
Authors: Miyao. M. , Moniwa K , Kusukawa K , Warabisako T. , Murakami E. , Sinke W.
Published: March 2011

Formation of a Si-on-insulator structure by a solid phase process was investigated. Examination by µ-RHEED method revealed that oriented crystal growth propagated from the seeding area by a solid phase epitaxy (SPE) in the low temperature range around 550 °C. In addition . a new local doping method was developed. This realized a relatively large lateral-SPE area on insulating regions (14 µm from the seeding area). Crystal quality and electrical properties of SPE layers were precisely examined utilizing µ-Raman spectroscopy and MOSFET fabrication. A small stress field and high electron mobility comparable to that of bulk Si were obtained.
Lattice Defects in β-Sic Grown Epitaxially On Silicon Substrates
Authors: Pirouz P. , Chorey C. M. , Cheng& T. T. , Powell J. A.
Published: March 2011

Defects generated in β-SiC grown on a (001) silicon substrate by chemical vapor deposition (CVD) are charaterized and their mechanism of formation discussed. It is argued that nucleation plays a primary role in this heteroepitaxial system where the lattice mismatch is so large.
Mocvd Growth Of Single Crystal Znsxsel−x On Si
Authors: Sekula Patricia A. , Vernon Stanley M. , Keavney Chris J.
Published: March 2011

Single crystal ZnSxSel−x films have been grown for the first time on (111) Si substrates by low-pressure MOCVD. The epitaxial films clearly show a passivating effect on silicon solar cells, and act as antireflective coatings. Attempts at incorporating Al as an n-type dopant were unsuccessful as the films remain semi-insulating.
The Physics of Metal Base Transistors
Authors: Rosencher E. , Arnaud D'Avitaya F. , Badoz P.A. , d'Anterroches C. , Glastre G. , Vincent G. , Pfister J.C.
Published: March 2011

Epitaxial Si/CoSi2/Si structures can be grown under ultra-high vacuum conditions. The metallic CoSi2 films can be extremely thin typically between 1 nm and 20 nm. The electrical properties of these heterostructures are presented, mainly the transport of electrons in the metallic films parallel to the interfaces and the transfer of electrons through the metal ilm. The influence of pinholes in the CoSi2 layers will be discussed.
Characterization of Ultrathin Cosi2 on Si(111) Layers.
Authors: Batstone J.L. , Phillips Julia M. , Gibson J.M.
Published: March 2011

Ultrathin epitaxial CoSi2 films on Si(111) have been grown in ultrahigh vacuum by room temperature deposition of Co on Si(111) followed by a high temperature anneal at ~600°C. Characterization of the thin films with transmission electron microscopy has revealed pseudomorphic growth up to thicknesses ~30Å. Pinholes present in the pseudomorphic thin films are thought to prevent the trapping of dislocations within the film. A clear transition to films containing a regular network of misfit dislocations occurs at ~40Å. Evidence for the growth of CoSi2 via intermediate metal-rich silicide phases is observed.
Growth of Single Crystal type A and type B CoxNi1-xSi2 Layers on Si(111)
Authors: Tung R. T. , Hellman F. , Gibson J. M. , Boone T.
Published: March 2011

Single crystal type A and type B CoxNi1-xSi2 layers have been grown on Si(111) by deposition of Co-Ni alloy in UHV and annealing. The dependence of CoxNi1-xSi2 layer orientation on the thickness of deposited metal is similar to that observed for pure nickel reaction. Silicon deposition along with the Co-Ni alloy allows the growth of type B oriented CoxNi1-xSi2. Auger, LEED, RBS and TEM analyses suggest that the composition of the ternary silicide layers is inhomogeneous. The advantages and disadvantages of this material in device application will be briefly discussed.
Formation of Epitaxial NiSi2 and CoSi2 On Laterally Confined (111)Si
Authors: Chang C.S. , Nieh C.W. , Chen L.J.
Published: March 2011

Epitaxial NiSi2 of single orientation was grown on laterally confined (111)Si. Striking oxide opening size effects on the growth of NiSi2 epitaxy were observed. The formation temperature of NiSi2 on (111)Si was found to be as low as 550°C inside oxide openings 1.8 μm or smaller in size. Epitaxial NiSi2 of single orientation which is identical to that of (111)Si substrate was formed inside oxide openings of or smaller than 1.8, 1, and 0.8 μm in size in samples annealed at 550-750, 800, and 850-900°C, respectively. Preliminary results on the epitaxial growth of CoSi2 are also reported.
Formation Of A Metastable Nisi Phase On <111> Si Induced By Pulsed Laser Irradiation
Authors: Priolo F. , Baeri P. , rimaldi M.G. , Rimini E.
Published: March 2011

Thermally grown NiSi layers on <111> Si substrates were irradiated by 35 nsec Nd glass laser pulses in the energy density range 0.3−2.0 J/cm2. Time resolved reflectivity measurements were performed during the irradiation to detect surface melting. The samples were analyzed by 2.0 MeV He+ Rutherford Backscattering Spectrometry in combination with channeling effect. The measured threshold for surface melting was 0.5 J/cm2. Irradiation at energy densities higher than 1.3 J/cm2 changed the silicides layer composition because of the mixing with the underlying silicon. In the intermediate energy density range (0.7−1.1 J/cm ) slight changes in composition were observed, a strong alignement of NiSi molecules along the <111> substrate direction was however observed. The measured Xmin was about 30%. It seems then that an epitaxial NiSi phase can be grown by pulsed laser irradiation with a suitable choice of the incident energy density. Work is in progress to identify this new NiSi phase by TEM. However this ordered phase is a metastable one since after anunealing at 250°C, 30min the channeling yield reduction disappeared without any appreciable change in composition.
Characterisation Of Quantum Wells By Auger Analysis On Chemical Bevels
Authors: Olivier Jean , Etienne P. , Razeghi M. , Alnot P.
Published: March 2011

In recent papers, we have described a depth profiling method using chemical beveling and Auger line scan measurement, quick and easy to perform, with a sufficient resolution to check the abruptness of chemical interfaces obtained in epitaxial growth of III-V compounds.

The variation law of chemical species concentrations at the interfaces is tighly related to the mecanisms of initial growth. We present here a theoretical analysis applied to chemically beveled interfaces with sharply graded concentrations.

We show on two examples of LP-MOCVD quantum wells (InP/GaInAs/InP and GaInP/GaAs/GaInP) the good agreement between experimental and theoretical curves, assuming exponentially varying concentrations. Finally, we discuss the correlations with the dynamics of the first steps of the LP-MOCVD growth.
Optical Reflectance and Rheed Transients During Mbe Growth on (001) GaAs
Authors: Aspnes D. E. , Harbison J. P. , Studna A. A. , Florez L. T.
Published: March 2011

We compare the dynamic responses of reflection high energy electron diffraction (RHEED) and optical reflectance-difference (RD) signals caused by abrupt changes in incident fluences of Ga and As during MBE growth of GaAs on (001) GaAs. Our results reveal that RHEED and RD originate mainly, although not completely, from structural order and chemical bonding, respectively, and thus provide complementary information for real-time studies of MBE growth. We also measure the wavelength dependence of the RD spectrum and show that it can be described approximately from 2 to 4 eV by a single Lorentzian absorption line centered at 2.4 eV.
Stresses in Multilayered Thin Films on a Thick Substrate
Authors: Suhir E.
Published: March 2011

The analysis contains an engineering method for the prediction of thermally induced stresses in single- and multilayered heteroepitaxial structures on a thick substrate. The examined stresses include 1) normal stresses acting in the film layers themselves and responsible for their ultimate and fatigue strength, and 2) interfacial stresses responsible for film blistering and peeling. The developed formulas are simple, visible, easy-to-use, and clearly indicate how material and structural characteristics affect the magnitude and the distribution of stresses and deflections. Some recommendations for smaller stresses in film structures are presented. The obtained results can be utilized as a guidance for an optimal physical design of multilayered heteroepitaxial structures used in Microelectronics.
Effect of Free Surfaces and Interfaces on Dopant Distribution Profiles
Authors: Jagannadham K. , Narayan J.
Published: March 2011

The drift force exerted by a free surface, a coherent or a noncoherent interface on dopant atoms has been analyzed. We have considered a coherent or a noncoherent interface present in an epilayer of finite thickness. The difference in the shear modulus of the matrix and of the second phase is responsible for the configurational energy of a dopant atom in a coherent two-phase medium. On the other hand, the hydrostatic component of stress associated with a misfit dislocation in a noncoherent interface gives rise to a first-order size interaction. The drift forces are responsible for dopant diffusion towards a free surface or an interface. The diffusion equation including the drift term is solved using an eigen function expansion method with appropriate boundary conditions. The concentration profiles obtained from the analysis of the diffusion equation are in qualitative agreement with those obtained experimentally. The attractive drift force gives rise to a peak in the dopant concentration at the interface followed by a minimum near the interface. These calculations and the observed concentration profiles enable us to evaluate the interaction energy term of the dopants with free surfaces and interfaces and the dilatation associated with the point defects.
The Structure of GaAs/Si(211) Heteroepitaxial Layers
Authors: Liliental-Weber Zuzanna , Weber E.R. , Washburn J. , Liu T.Y. , Kroemer H.
Published: March 2011

Gallium arsenide films grown on (211)Si by molecular-beam epitaxy have been investigated using transmission electron microscopy. The main defects observed in the alloy were of misfit dislocations, stacking faults, and microtwin lamellas. Silicon surface preparation was found to play an important role on the density of defects formed at the Si/GaAs interface.

Two different types of strained-layer superlattices, InGaAs/InGaP and InGaAs/GaAs, were applied either directly to the Si substrate, to a graded layer (GaP-InGaP), or to a GaAs buffer layer to stop the defect propagation into the GaAs films. Applying InGaAs/GaAs instead of InGaAs/InGaP was found to be more effective in blocking defect propagation. In all cases of strained-layer superlattices investigated, dislocation propagation was stopped primarily at the top interface between the superlattice package and GaAs. Graded layers and unstrained AlGaAs/GaAs superlattices did not significantly block dislocations propagating from the interface with Si. Growing of a 50 nm GaAs buffer layer at 505°C followed by 10 strained-layer superlattices of InGaAs/GaAs (5 nm each) resulted in the lowest dislocation density in the GaAs layer (∼;5×l07/cm2) among the structures investigated. This value is comparable to the recently reported density of dislocations in the GaAs layers grown on (100)Si substrates [8]. Applying three sets of the same strained layersdecreased the density of dislocations an additional ∼2/3 times.
Physical Properties of Ternary Re-Fe Base Alloys and their Relationship to Permanent Magnet Applications
Authors: Buschow K. H. J.
Published: March 2011

Several ternary compounds of tetragonal structure and with the composition R2Fe14B have found an application as starting materials for permanent magnets. Several physical properties of R2Fe14B compounds will be discussed in relation to the modification in properties that can be obtained by substituting other elements for R, Fe or B. Also investigated is the extent to which it will be possible to find alternative materials differing in structure and composition from R2Fe14B. For this purpose the results obtained on 3d-rich ternaries based on well-known structure types will be reviewed. Included in this survey will be novel ternary intermetallics made by crystallization from the amorphous state. These compounds, though stable enough for applications, are actually metastable. They are not found in the corresponding equilibrium diagrams.
Magnetic Anisotropy and Spin Reorientation in Nd2-xPrxCo14B
Authors: Pourarian F. , Sankar S. G. , Pedziwiatr A. T. , Boltich E. B. , Wallace W. E.
Published: March 2011

Nd2-xPrxCo14B intermetallics crystallize in tetragonal form isostructural to Nd2Fe14B. Structural and magnetic properties of the polycrystalline materials have been determined. Room temperature magnetic measurements indicate that these intermetallics exhibit uniaxial anisotropy throughout the entire composition range. Substitution of praseodymium in Nd2-xPrxCo14B significantly enhances the anisotropy field both at 78 K and at room temperature. Magnetization versus temperature plots show spin reorientation TSR2 at 546 K for Nd2Co14B and at 668 K for Pr2Co14B. TSR2 increases linearly with composition x. The 3d sublattice anisotropy which seeks the tetragonal plane appears to dominate above these temperatures. In addition to the above, low temperature magnetic measurements demonstrate the spin reorientation in Nd2Co14B and Nd1.9Pr0.1Co14B. Further additions of praseodymium strengthen the axial anisotropy at temperatures down to 4.2 K. Effect of competing crystal field interactions and the consequences of praseodymium substitution that result in considerable increase in anisotropy field in these quaternaries are discussed.
High Field Torque Magnetometry Measurements on Y1.8Er0.2Fe14B
Authors: Williams C. M. , Koon N. C. , Das B. N.
Published: March 2011

The magnetocrystalline anisotropy energy has been determined for single crystal Y1.8 Er0.2Fe14B in the (001) and (100) planes between 5 K and 300 K using torque magnetometry techniques. The results are compared with a model based on crystal field theory. Excellent agreement was obtained between the model and experiment. Both experiment and model showed a first order spin reorientation between the [001] and an angle 70 degrees from the [001] in the (100) plane.
Misch-Metal and/or Aluminum Substitutions in ND-FE-B Permanent Magnets
Authors: Ma B. M. , Willzian C. J.
Published: March 2011

The effects of misch-metal–and/or Al substitutions in the Nd-Fe-B alloy system have been investigated. Selected compositions were processed into magnets using the conventional powder metallurgy technique. As expected, misch-metal substitutions act to degrade the intrinsic magnetic characteristics and thus degrade the properties of the sintered magnet. It was observed that at a level of 25% of the total rare earth being comprised of misch-metal, energy products of the order of 25 MGOe were still able to be achieved. Processing of such material is difficult because higher sintering temperatures are required to fully densify the magnet, and the grain growth that occurs at these temperatures dramatically reduces the coercivity. The effects of misch-metal substitutions are quantified in this discussion.

Al was found to be beneficial for improving the coercivity in both the Nd-Fe-B and misch-metal-Fe-B alloy systems. Unfortunately Al substitutions reduce both the remanence and the Curie temperature. If the Al content is kept to 1 wt.% or less, magnets exhibiting reasonable properties can be made. Al doping as opposed to doping with expensive heavy rare earth elements such as Th or Dy is a less costly method of improving Hei, in the Nd-Fe-B magnets.
Magnetic Hardening Mechanism in Sintered R-Fe-B Permanent Magnets
Authors: Sagawa M. , Hirosawa S.
Published: March 2011

The state-of-the-art description of the magnetic hardening mechanism in the sintered Nd-Fe-B permanent magnet is given. Recent experimental results concerning the coercivity-anisotropy (Hc -HA) correlation in B-rich Pr-Fe-B and Nd-Fe-B sintered magnets and the inhluence of the surface conditions of the sintered Nd-Fe-B magnets on the coercivity are reported. These results are interpreted in terms of the μ0Hc versus cμ0 HA -NIS plot, where I is the spontaneous magnetization of R2 Fe14 (R=Pr or Nd) and N the effective demagnetization field coefficient.
Microstructural Studies of High Coercivity Nd15Fe77B8 Sintered Magnets
Authors: Ghandehari M. H. , Fidler J.
Published: March 2011

Microstructures of Nd15−xDyxFe77B8 prepared by alloying with Dy, and by using Dy2O3 as a sinl'ken adidive, have been determined using electron microprobe and transmission electron microscopy. The results have shown a higher Dy concentration near the grain boundaries of the 2–14–1 phase for magnets doped with Dy2O 3, as compared to the Dy-alloyed magnets. A two-step post sintering heat treatment was also studied for the two systems. The resultant concentration gradient of Dy in the 2–14–1 phase of the oxide-doped magnets is explained by the reaction of Dy2O3 with the Nd-rich grain boundary phase and its slow diffusion into thg 4–14–1 phase. Increased Dy concentration near the grain boundary is more effective in improving the coercivity, as domain reversal nucleation originates at or near this region.
Aluminum Substitutions in Nd-Fe-B Sintered Magnets
Authors: Chen J. K. , Thomas G.
Published: March 2011

A microstructural and microanalytical study of aluminum substituted Nd-Fe-B sintered permanent magnets were carried out to determine the effect, if any, of aluminum on structure, composition, and magnetic properties, particularly on the observed increase in coercivity. It was found that Al enrichment occurred in the Nd-rich phase at the grain boundaries. The possible role(s) of this enrichment on the observed coercivity increase are discussed.
Effect of Hydrogen on Magnetic Properties of Quaternary Pr2Fe14−xCoxB Compounds
Authors: Pourarian F. , Jiang S. Y. , Wallace W. E.
Published: March 2011

The effect of absorbed hydrogen on the structural and magnetic behavior of Pr2Fe14−xCoxB has been investigated. Introduction of hydrogen in the host material reduces the average magnetic moment of the 3d element by ˜ 4 to 7%. Anisotropy fields for all systems studied are significantly reduced upon hydrogenation. Evidence of spin reorientation at temperatures ranging between ˜ 295 K and 496 K for some of the hydrides is observed. Pr2Co14B and its hydride exhibit uniaxial anisotropy at room temperature and at 77 K.

The role of hydrogen in expanding the lattices of these intermetallics and modifying their magnetic properties is discussed.
Neutron Diffraction Studies of Magnetic Materials
Authors: James William J.
Published: March 2011

The ability of neutron diffraction in determining the nature and extent of magnetic ordering is illustrated for the intermetallic compounds, Y6(Fe,Mn)23, and ErFe3. Substitution with other 3d transition metals influences the Fe-Fe exchange forces such as to alter, sometimes considerably, the magnetic properties, e.g., local site magnetic anisotropies in Er(Fe,Ni)3 and thermal expansion anomalies in the R2 (Fe,Co)14B compounds. When the 3d atoms are near neighbors in the periodic chart, their nuclear scattering lengths for neutrons are sufficiently different to permit the detection of preferential occupation of the several nonequivalent crystallographic 3d metal sites, i.e., atomic ordering, present in the R6M23, and R2Fe14B structures.
Synthesis of R2 Fe14 B Single Crystals
Authors: Das B. N. , Koon N. C.
Published: March 2011

Single crystals of R2 Fe14 B weighing 5 to 15 g were grown from a liquid melt by tri-arc and levitation Czochralski methods. The arc method was used for the growth of smaller size crystals from 5 to 7 g, and the levitation method was used for the growth of larger sizes, from 10 to 15 g. Crystals of (R1)2 –x(R2)xFe14 B could also be grown with isomorphous replacement of rare earth atoms. The starting alloy composition for the crystal growth process was chosen based on solidification microstructure. In this paper we discuss the solidification microstructure, isomorphous replacement, seeding and their interactions with the crystal growth processes and crystal quality.
Microstruoture-Property Relationships in Magenqubnch Magnets
Authors: Rishra Raja K.
Published: March 2011

Transmission electron microscopy has been used to characterize the microstructure of Nd-Fe-B magnets produced by melt-spinning and subsequent hot-pressing/die-upsetting. For a material of starting composition Nd.135Fe.815B.05, the basic microstructure od melt-spun, hot-pressed and dieupset magnets consists of two phases. In the optimally processed melt-spun ribbons and hot-pressed samples, small and randomly oriented Nd2Fe14B grains are surrounded by a thin noncrystalline Nd-rlch phase. The die-upset material consists of closely stacked flat Nd2Fe14B grains surrounded by a second phase of approximate composition Nd7Fe3. No Nd11Fe4B4 phase is observed in these materials, but it can form if the chemical composition and/or processing parameters are varied. In all these materials, Lorentz microscopy reveals that magnetic domain walls are pinned by the second phase. The differences in the hard magnetic properties of the three kinds of MAGNEQUENCH magnets closely correlate with the differences in the distribution of Nd2Fe14B crystallites and the pinning sites.
Optimization of Liquid Dynamic Compaction for Fe-Nd-B Magnet Alloys
Authors: Veistinen Mauri K. , Hara Y. , Lavernia E. J. , O'Handley R. C. , Grant N. J.
Published: March 2011

Permanent magnets from the Fe77Nd15B8 alloy were produced using liquid dynamic compaction (LDC). Process parameters such as gas and metal mass flow rates were carefully controlled to refine the microstructure. Accordingly, grain sizes between 0.1 and 5 microns were achieved in the LDC compact. In contrast to our earlier LDC results high intrinsic coercivity in excess of 7500 Oe was obtained for thick (> 7 nm) as-LDC'd samples with lower neodymium content.
Optical Properties of Dopant Induced States in La2−xSrxCuO4-δ Compounds
Authors: Etemad S. , Aspnes D. E. , Barboux P. , Hull G. W. , Kelly M. K. , Tarascon J. M. , Thompson R. , Herr S. L. , Kamaras K. , Porter C. D. , Tanner D. B.
Published: March 2011

Optical data over a broad energy range (∼ 0.01 eV to ∼ 6 eV) for a series of La2−xSrxCuO4-δ compounds are obtained in parallel with the Meissner effect and the superconductivity transition temperature, Tc. Two noteworthy trends in the optical spectra are observed as the Sr concentration is increased in small steps from x = 0 to x = 0.3. First is the appearance of a low frequency band in the reflectivity, R, whose strength follows closely the Meissner effect and Tc measured on the same set of samples. The position of the onset of this band is pinned at ∼0.9 eV for all values of x. The origin of this band in R is identified as an absorption process due to an electronic transition with a large oscillator strength. Second is the appearance and disappearance of an intense vibrational mode whose strength also tracks “superconductivity”. This sharp structure in the far infrared is a characteristic vibrational mode associated with the dopant induced electronic state.
Observation of Hexagonally Ordered Flux Quanta in YBa2Cu3O7
Authors: Gammel P. L. , Bishop D. J. , Dolan G. J. , Kwo J. R. , Murray C. A. , Schneemeyer L. F. , Waszczak J. V.
Published: March 2011

The high resolution Bitter Pattern technique has been used to reveal the structure of the array of flux lines which is present when single crystal samples of the high Tc superconductor YBa2Cu3O7 are placed in a magnetic field. At 4.2K with the magnetic field parallel to the c axis, the patterns formed are strongly reminiscent of the analogous structures in ordinary type II superconductors. The structures typically consist of flux spots with short range hexagonal correlations. We interpret the patterns to consist of singly quantized vortices. Very uniform patterns are observed in thin, flat samples cooled in a constant field. Sampling the spot density yields a vortex density consistent with a flux quantum of hc/2e.

Experiments at 77K, the only other temperature studied, showed no spatially varying magnetic structure. Recent mechanical measurements which are sensitive to the bulk modulus and dissipation within the vortex lattice show evidence for a mobility transition near 75K, which would be consistent with this observation. We believe this stems from materials properties rather than some more fundamental property of the new superconductors.
Phase Equilibria Studies Near the Superconducting YBa2Cu3Ox Composition by a Solid-State Ionic Technique
Authors: Ahn Byung Tae , Gür Turgut M. , Huggins Robert A. , Beyers Robert , Engler Edward M.
Published: March 2011

This paper reports the findings of a phase-equilibrium study of the Y-Ba-Cu-O quaternary system in which the oxygen activity was monitored and controlled by a solid-state ionic technique. A stabilized zirconia electrolyte was used to titrate oxygen in or out of the sample and to monitor the oxygen activity inside a sealed sample chamber.

Compositions at or near YBa2Cu3Ox were studied at elevated temperatures in a wide range of oxygen pressures. All compositions indicated the presence of an invariant polyphase reaction- e.g., at 1 × 10-3 atm of oxygen pressure at 850 °C-- below which the superconducting phase becomes thermodynamically unstable and undergoes a decomposition reaction. Complementary x-ray diffraction analysis of samples prepared under well-defined oxygen pressures identified the phases present and, along with the oxygen coulometric titration data, allowed the construction of a portion of the relevant quarternary phase diagram.
X-Ray Absorption Studies of La2−xSrxCuO4 and Y1−xPrxBa2Cu3O7-δ
Authors: Alp E. E. , Shenoy G. K. , Soderholm L. , Goodman G. L. , Hinks D. G. , Veal B. W. , Montano P. A. , Ellis D. E.
Published: March 2011

The question of valence of Cu and rare-earth atoms in the newly discovered oxide superconductors with high transition temperatures is crucial to the understanding of their electronic structure. We have measured the X-Ray Absorption Near Edge Structure (XANES) of Cu K transition and Pr Ljjj transition to obtain information on the valence of Cu, and Pr in La2−xSrxCuO4 and Y1−xPrxBa2Cu3O7-δ. We will present the experimental results, compare them with calculations of absorption cross-section for model atomic clusters, and discuss the valence of Cu as a function of oxygen concentration.
Developments of Tape and Wire Fabrication Processes for High Tc Ba2YCu3O7-y Oxide Superconductors
Authors: Togano K. , Kumakura H. , Shimizu H.
Published: March 2011

The application of superconducting material to a magnet requires the winding tape or wire conductor with sufficient flexibility into a solenoid. Tape conductors of Ba2YCu3O7-y were successfully prepared by a new technique of surface reaction. Cu tape or Cu-electroplated Ag tape was coated with a fine mixture of Ba2CO3, and Y2O3 and then heat treated at about 900°C to form Ba2YCu3O7-y oxide layer with high density on both surfaces of the tape. Zero resistance state has been achieved at the temperature above liquid nitrogen temperature for Ag-substrate tape. The critical current density is about 40A/cm at 77K and zero magnetic field. Although, at present, the superconducting properties of these tapes are somewhat inferior to those of conventional powder metallurgy processed conductors, the surface reaction process has the potential feasibility of processing Ba2YCu3O7-y into a long-length ductile tape desirable for practical application.
The Deposition, Fabrication and Characteristics of High Critical Temperature Devices
Authors: Morris G. W. , Blamire M. G. , Somekh R. E. , Tomlinson E. J. , Evetts J. E.
Published: March 2011

In the first section of this paper the sputter deposition of thin films of YBa2Cu3O7−x is considered. In the second section low and high temperature heat treatments are discussed in the light of the thermodynamics of the material and in the third the development, fabrication and current-voltage (IV) characteristics of a variety of device structures are described, including the hysterctic characteristics of certain YBa2Cu3O7−x / YBa2Cu3O7−x devices that reveal Josephson supcrcurrcnt tunnelling.
Field Dependence of Meissner Effect in a Single Crystal of YBa2Cu3Oxin the Ultra-Low Magnetic Field Regime
Authors: Krusin-Elbaum L. , Malozemoff A. P. , Yeshurun Y.
Published: March 2011

We report a strong field-dependence (from 0.1% to 71%) of flux expulsion (Meissner effect) in a single crystal of YBa2Cu3Oxin magnetic fields from 36 kOe down to 50 mOe. The data is discussed in the context of flux pinning forces and glassy behavior.
Application of Gel Technology in Preparation of High-Tc Perovskite Superconductors
Authors: Liu R. S. , Lin G. C. , Sung H. M. , Chen Y. C. , Lin O. C. C.
Published: March 2011

La-Ba-Cu-O and Y-Ba-Cu-O superconducting systems have been successfully prepared by gel techniques with high degree of chemical homogeneity. The precursor gel was synthesized from mixture of the corresponding metallic nitrates and di-carboxylic acids. The sintered oxides prepared from the different dicarboxylic acids were all Tc = 90K perovskite superconductors. However differences in electrical and magnetic properties were also observed. Effects due to the different acids elucidated by magnetic susceptibility measurements and X-ray diffraction analysis will be discussed.
YBa2Cu3O7-δ Superconductors Produced by Hot-Pressing of Freeze-Dried, Carbonate- and Oxalate-Coprecipitated Powders
Authors: Wang C. T. , Lin L. S. , Lin J. H. , Su J. Y. , Yang S. J. , Hsu S. E.
Published: March 2011

Reactive YBa2Cu3O7-δ powders have been produced via freeze-drying, carbonate- and oxalate-coprecipitating methods. In the coprecipitating methods, sodium carbonate and sodium oxalate were used as precipitants. These powders were characterized by TGA, XRD, and SEM. The morphologies of the YBa2Cu3O7-δ powders produced from these methods are different from each other. The influence of hot-press process on the bulk density, micro-structure development and superconducting properties of YBa2Cu3O7-δ samples was also studied.
Theories of High Tc Superconductors
Authors: Bardeen John
Published: March 2011

Many theories have been given to account for the high transition temperatures in the oxide superconductors. While most are based on pairing, they differ as to the origin of the attractive interaction that gives rise to the pairs and whether the pairing is weak or strong. If weak(λ < ∼ 0.5), the energy range of the pairing interaction must be wider than is consistent with one mediated solely by phonons. With measurements on single crystals becoming available, giving data covering a wide range of frequencies and temperatures, it is possible to narrow down the possible options. The data show that anisotropie 3D models are required. Observed isotope shifts indicate that phonons must play a role, but additional mechanisms are likely necessary to account for the high T's. Thermal, magnetic and transport data are consistent with anisotropie Ginzburg-Landau theory near T, with expected departures at low temperatures.
Top Seeded Solution Growth of Lanthanum Copper Oxide
Authors: Picone P. J. , Jenssen H. P. , Gabbe D. R. , Tuller H. L.
Published: March 2011

Single crystals of pure and Sr doped La2CuO4 have been grown with a top seeded solution growth technique. Crystals grown from a lithium borate flux incorporate impurities detrimental to superconductive properties while those grown from excess copper oxide are free from this difficulty.
Formation and Processing of High Tc Superconductor films with Pulsed Excimer Laser
Authors: Kawaiw Tomoji , Kanaiw Masaki , Kawai Maki
Published: March 2011

High Tc YBa2 Cu3 Oy and GdBa2 Cu3 Oy thin films can be made by a laser sputtering method using LnBa2 Cu3 Oy (Ln=Y and Gd) ceramics as targets and cubic zirconia as a substrate in a high vacuum system. The superconcucting onset temperature and the zero resistance temperature of the film are 95 and 80K, respectively. The surfaces of these films were subjected to an irradiation with a focused pulsed excimer laser. Scanning of the beam led to the etching of the surface to form a groove. Photochemical processes on these superconductor films were studied using dynamic mass-spectroscopic method.
Epitaxial Films of High Tc Oxide Superconductors Y1Ba2Cu3O7 Grown on SrTiO3 by Molecular Beam Epitaxy
Authors: Kwo J. , Hsieh T. C. , Hong M. , Fleming R. M. , Liou S. H. , Davidson B. A.
Published: March 2011

Orientation ordered high-Tc oxide films of Y1Ba2Cu3O7 were prepared by molecular beam epitaxy on SrTiO3 substrates. A combination of effusion cells and e-beam evaporators was used with the addition of an oxygen jet. The superconducting phase is formed by further O2 anneal at temperatures ≥800° C. For SrTiO3 (100), the Y1Ba2Cu3O7 films grow in the orientation of either the c-axis or the a-axis perpendicular to the film plane. Films with predominant “c⊥” orientation exhibit a high degree of crystalline order with a rocking curve 0.3° wide. The Jc is about two times higher than those with the “a⊥” orientation of a comparable Tc (R=0). For a typical film of 9000Å thickness, a Tc (R=O) of 87K, and a Jc of 7 × 104 A/cm2 at 77K were reproducibly achieved. For SrTiO3 (110), X-ray showed twinning of (110) and (103) orientations in Y1Ba2Cu3O7, thus precluding the observation of anisotropy expected for the in-plane axes of [001] and [110].
Photoemission Study of the Electronic Structure of Copper Oxide Superconductors
Authors: Shen Z.-X. , Allen J. W. , Yeh J. J. , Kang J.-S. , Ellis W. , Spicer W. E. , Lindau I. , Sun J. Z. , Geballe T. H. , Maple M. B. , Dalichaouch Y. D. , Torikachvei M. S.
Published: March 2011

We report the photoemission spectra from a high quality thin film sample of Y1Ba2Cu3C7-δ at room temperature and bulk samples of Y1Ba2Cu3C7-δ at different temperatures. The experimental data are interpreted within the configuration interaction model of Anderson Hamiltonian. The experimental values for Δ, U and T are determined. We also give the reasons why we think PES data reflect the intrinsic property of the superconductors. Furthermore, we emphasize the importance of the presence of a 3d8-like satellite which signals the strong correlation among 3d electrons.
Electronic Structure and Antiferromagnetism in La2CuO4
Authors: Sterne P. A. , Wang C. S. , Stocks G. M. , Temmerman W. M.
Published: March 2011

Self-consistent spin-polarized energy bands have been calculated for tetragonal and orthorhombic La2CuO4. No stable antiferromagnetic order is found in either the stoichiometric compound or oxygen deficient La2CuO3.75. Constraining the occupancy of Brillouin zone boundary states leads to a small antiferromagnetic moment in both systems which is rapidly quenched by Ba doping.
Noise Power Spectrum of Copper Oxide Superconductors in the Normal State
Authors: Testa Joseph A. , Song Yi , Chen Xiao-Dong , Golben John P. , McMichael Robert D. , Lee Sung-Ik , Patton Bruce R. , Gaines James R.
Published: March 2011

The discovery of superconductivity above liquid nitrogen temperature in copper oxide compounds has resulted in much research activity aimed at understanding the nature of these materials, as well as at using them in practical devices from sensors to levitated trains. Despite this intense interest, there have been relatively few studies of fluctuation phenomena in them. One fluctuation-induced property of interest is the electrical noise power spectrum. Although excess noise above the thermal background in metals is usually observable only in thin films, we report here measurements of significant noise power in bulk composite samples of the nominal 90K superconductors Y1Ba2Cu3O7-δ (YBCO) and Er1Ba2Cu3O7-δ (EBCO) from 77K to 290K. The “excess” noise spectrum (the total spectrum minus the frequency-independent amplifier and thermal noise contributions) is 1/f in shape in the normal state; no 1/f noise is observed while superconducting. The excess noise magnitude is found to be too large to originate from the expected number of charge carriers, and to have an unusual temperature dependence. We discuss two possible explanations for these observations.
Magnetization of a Superconducting Glass
Authors: Tuominen M. , Goldman A. M. , MeCartney M. L.
Published: March 2011

The time-dependent magnetization of single crystal samples of the high temperature superconductor YBa2Cu3O7 has been studied as a function of temperature and magnetic field. Long relaxation times have been found to persist up to the transition temperature, implying that the transition temperature is the upper bound on glass-like behavior.