bluuebunny/crossref_metadata_2025 huggingface dataset:
A Dislocation Model for Flow at Intermediate Temperatures in Hard-Oriented NiAl
Authors: Mills M. J. , Daw M. S. , Angelo J.E. , Miracle D. B.
Published: April 2011

Recent studies of single crystals and bicrystals indicate that dislocations of the type a<011> are important, and may actually control, deformation at intermediate temperatures (above the brittle-to-ductile transition temperature) in hard-oriented NiAl. In the present work, the fine structure of a<011> dislocations has been examined using both high resolution and diffraction-contrast transmission electron microscopy. Evidence has been found for the decomposition of a<011> dislocations into two a<001> dislocations. The initial driving force for the decomposition is due to core effects, as revealed by molecular statics and dynamics Embedded Atom Method calculations. Additional decomposition occurs by a combination of climb and glide. A continuum-based dislocation model is introduced which incorporates these relevant microstructural features.
The Effects of Interstitial Content and Annealing on the Flow and Fracture behavior of Polycrystalline ÿ-NiAl
Authors: Weaver M. L. , Levit V. , Kaufman M. J. , Noebe R. D.
Published: April 2011

The strain aging behavior of three polycrystalline NiAl alloys has been investigated at temperatures between 300 and 1200 K. Yield stress plateaus, yield stress transients upon a tenfold increase in strain rate, work hardening peaks, and dips in the strain rate sensitivity (SRS) have been observed between 700 and 800 K. These observations are indicative of dynamic strain aging (DSA) and are discussed in terms of conventional strain aging theories.
A Preliminary Research on the Mechanical Properties of TiAl+Ti5Si3 Dual Phase Alloys
Authors: Zhang Lanting , Qiu Guohua , Wu Jiansheng
Published: April 2011

A sufficient Si addition to TiAl matrix has led to TiAl + Ti5Si3 dual phase alloys, showing coupled-growth microstructure. Compression tests at R.T. as well as high temperatures indicated that the yield stress increased with increasing Ti5Si3 volume fraction, and decreased at higher temperature. The reinforcement from Ti5Si3 phase was obvious, while high Si and Al contents resulted in low ductility. The fracture surfaces were quasi-cleavage. Further research should concern with the adjustment of the shape and amount of the second phase.
Fracture Toughness and Microstructure of Nial-Based Composite with TiC Reinforcement
Authors: Xing Z. P. , Guo J. T. , Dai J. Y. , Yu L. G. , Hu Z. Q. , An G. Y. , Chen Q. Y.
Published: April 2011

Fracture toughness of NiAl–based composite reinforced by TiC particulate was measured using single–edge notched three–point bend specimens. The composite was fabricated by HPES (hot pressing aided exothermic synthesis) technique. It is shown that the ambient toughness of stoichiometric NiAl was increased from 5.9 MPa m1/2 to 8.9 MPa m1/2 by incorporating 20 vol.% TiC particulates. From fracture surface analysis along with the high–resolution electron microscopy (HREM) observation, toughening mechanisms of the NiAl–TiC composite were also discussed.
A New Approach to Study Vacancy Defects in High-Temperature Intermetallic Compounds
Authors: Collins Gary S. , Sinha Praveen
Published: April 2011

Perturbed angular correlations of gamma rays (PAC) is being applied to study defects in ordered intermetallic alloys. Vacancies on both Pd and In sublattices in the B2 system Pdln were detected after quenching through quadrupole interactions induced at nearby 111In probe atoms. Fractions of probe atoms having each type of neighboring defect were observed to increase monotonically with quenching temperature over the range 8251500 K. For compositions close to 50.15 at.% Pd, nearly equal site fractions were observed for Pd and In vacancies, indicating that the Schottky vacancy-pair defect is the thermal defect at high temperature. The formation enthalpy of the Schottky defect was determined to be 1.3(2) eV through analysis of quenching data from in the range 825–1200 K. Above 1200 K, however, the vacancy concentration was observed to saturate at a value of 1.4(2) atomic percent, perhaps due to breakdown of the law of mass action for high defect concentrations.
Effect of Rotation Relationship Between γ Grains on Compatibility of Deformation of Ti-A1 Alloys
Authors: Morris M. A. , Luster J
Published: April 2011

The specific rotation relationships between adjacent γ grains have been measured in an alloy of composition Ti-47.5Al-2.5Cr consisting of lamellar and equiaxed distributions of γ grains in which the α2 phase was distributed as long lamellae or smaller globules respectively. Detailed analyses of active slip systems have been carried out by TEM observations of deformed samples and from these, the effect that specific rotations between γ grains have on the compatibility of deformation across adjacent grains has been discussed. Theoretical calculations of the geometric compatibility factor characterising the best slip transfer across adjacent grains have been used and compared to the experimental results. It has been possible to deduce the role played by the type of orientation relationship between grains in producing active deformation systems that allow the maximum compatibility of deformation.
Strength, Ductility, and Fracture Mode of Ternary FeAl Alloys
Authors: Schneibel J. H. , George E. P. , Specht E. D. , Horton J. A.
Published: April 2011

Iron aluminides with the composition Fe-45Al-5X-0.2B-0.1Zr (at. %), where X stands for the first row transition metals Ti, V, Cr, Mn, Fe, Co, Ni, Cu, were examined at room temperature with respect to their strength, ductility, environmental sensitivity, and fracture mode. The extruded materials were annealed at 1273 K to produce similar grain sizes and subsequently at 673 K to reduce the amount of quenched in vacancies. All alloys were essentially single phase. Their solid solution strengthening was found to correlate with the atomic size misfit derived from the lattice parameters. The “binary” alloy Fe-45Al-0.2B-0.1Zr exhibited predominantly transgranular fracture. Ternary alloying additons with atomic numbers less than that of Fe tended to enhance intergranular fracture, whereas those with atomic numbers higher than that of Fe favored substantial amounts of transgranular fracture. Tensile testing in a partial pressure of dry oxygen increased the ductilities of the ternary alloys only slightly, whereas the ductility of the binary alloy increased from about 8 to about 19%. The ductilities in air correlated inversely with the yield strength. However, those alloys exhibiting substantial amounts of transgranular fracture always showed higher ductilities than those fracturing intergranularly. We interpret our fracture results in terms of yield strengths and ternary element site occupations.
Development of Tial-Based Automotive Engine Valves
Authors: Dowling William E. , Donlon William T. , Allison John E.
Published: April 2011

The application of TiAl-based alloys as an exhaust valve material would allow automotive engines to operate at higher temperatures with increased efficiencies. Development of these materials at Ford initially concentrated on the Ti-48A1-1V (at%) system. This included; 1) room and elevated temperature fatigue, 2) creep and 3) tensile ductility optimization. Results from this test program in conjunction with other available data, previous ceramic experience and limited engine testing led to the conclusion that the major roadblock to implementation was not optimizing mechanical properties, but rather low cost and reliable valve manufacturing technology. When a cost effective manufacturing technology is developed, then the emphasis will shift to ensuring the product durability. Hence, the focus of the current program is the development of valve manufacturing technologies, in particular valve blank fabrication. Currently, casting appears to be the lowest cost alternative for valve blank fabrication. This paper reviews the technology development process as it pertains to TiAl-based valves.
Properties of Reactively Synthesized Titanium Aluminides
Authors: Alman D. E. , Hawk J. A. , Wilson R. D.
Published: April 2011

Reactive synthesis directly forms compounds from their elemental constituents. In this U.S. Bureau of Mines study the microstructure, hot-hardness and tensile properties of alloys and composites based on TiAl and Ti2AlNb were studied. Ti2AlNb sheets were processed from elemental Ti, Al and Nb foils. The elemental foils were consumed during processing, leaving a microstructure consisting of three phases: O (orthorhombic Ti2AlNb), B2 (ordered cubic Ti), and α2 (Ti3Al). The composite sheet structures were heat-treated to produce a microstructure consisting of O lathes in a B2 matrix. A significant advantage of reactive synthesis is the ease of formation of complex shapes prior to synthesis of the aluminide. TiAl-based alloys and in situ TiAl/boride and TiAl/Ti5Si3 composites were produced from elemental mixtures of Ti and Al with either additions of B or Si powders. The hot-hardness and tensile properties of these alloys and composites are reported. The Kirkendall porosity associated with reactive sintering of TiAl can be eliminated by simultaneously reactive sintering TiAl and Ti5Si3.
Strength and Ductility of Ni3Al Alloyed with Boron and Substitutional Elements
Authors: Ishikawa K. , Aoki K. , Masumoto T.
Published: April 2011

The effect of simultaneous alloying of boron (B) and the substitutional elements M on mechanical properties of Ni3Al was investigated by the tensile test at room temperature. The yield strength of Ni3Al+B increases by alloying with M except for Fe and Ga. In particular, it increases by alloying with Hf, Nb, W, Ta, Pd and Si. The fracture strength of Ni3Al+B increases by alloying with Pd, Ga, Si and Hf, but decreases with the other elements. Elongation of Ni3Al+B increases by alloying with Ga, Fe and Pd, but decreases with other elements. Hf and Pd is the effective element for the increase of the yield strength and the fracture strength of Ni3Al+B, respectively. Alloying with Hf leads to the increases of the yield strength and the fracture strength of Ni3Al+B, but to the lowering of elongation. On the other hand, alloying with Pd improves all mechanical properties, i.e. the yield strength, the fracture strength and elongation. On the contrary, alloying with Ti, V and Co leads to the lowering of mechanical properties of Ni3Al+B. The reason why ductility of Ni3Al+B is reduced by alloying with some elements M is discussed.
Ductile Phase Toughened Ceramics by Partial Reduction Reactions in the Ni-AI-O System: Mechanical Properties and Effect of Dopants
Authors: Üstündağ E. , Ret P. , Shapiro Y. , Subramanian R. , Dieckmann R. , Sassa S. L.
Published: April 2011

Partial reduction reactions in the Ni-Al-O system, starting with the spinel compound NiAl2O4, are used to form metal-ceramic microstructures in situ. Two different morphologies of nearly pure Ni particles, equiaxed and rod-like, form within a ceramic matrix depending on the choice of processing parameters. Electron microscopy studies were performed for microstructural characterization, phase identification and chemical analysis. The fracture toughness of the Ni-Al2O3 mixture was significantly improved with respect to that of the original spinel phase. It is shown that cracking at the original spinel grain boundaries, likely due to the large volume changes associated with the reduction reaction, can be avoided by the addition of small amounts of ZrO2. It is seen that ZrO2 also acts as a nucleation site for the precipitating metal and hence allows morphology control in microstructures obtained by partial reduction reactions.
Analytical Modeling in Support of the Development of Fiber Reinforced Ceramic Composite Materials for Re-Heater Burners
Authors: Kibler J.J. , DiPietro S.G.
Published: April 2011

Development of Continuous Fiber reinforced Ceramic Composite (CFCC) materials is a process of identifying components which will benefit from CFCC properties, and defining appropriate composite constructions which will provide materials which will meet the structural and thermal requirements of the application. Materials Sciences Corporation (MSC) has been providing analytical support to Textron Specialty Materials in the development of re-heated tubes for metal reheating furnaces. As part of this support, a study has been made of the sensitivity of composite properties to fiber orientation as well as a number of matrix properties which control the stress-strain behavior of the composite.
An Experimental and Theoretical Investigation of Stick-Slip, Steady-State and Roughness Dominated Sliding in Fiber-Reinforced Composites
Authors: Mackin Thomas J.
Published: April 2011

The mechanical properties of fiber reinforced composites depends strongly upon the properties of the fiber/matrix interface. Enhanced fracture resistance and strain to failure are synonymous with debonding and sliding of the reinforcement phase. Thus, the two key properties of the composite are the interfacial toughness and the post-debond sliding stress. After debonding a variety of interfacial sliding phenomena are noted, including: stick-slip, steady-state, and roughness dominated sliding. The interfacial properties, including the coefficient of friction, the radial clamping pressure, asperity amplitude, the elastic properties of the constituents, and the compliance of the test machine, each play a role in the operative sliding phenomenon. Experiments have been conducted to explore each of these phenomena. In addition, models have been developed that rationalize all of the observed behavior.
Fabrication of Advanced C/C Composites by a Hybrid Process of CVI/Liquid Precursor Impregnation
Authors: Ishizaki Masato , Shioda Muneyoshi , Miyahara Kaoru , Sasa Tadashi
Published: April 2011

Fabrication process of advanced C/C composites was developed by combining the CVI and liquid impregnation-pyrolysis processes. Time-controlled temperature-gradient (TC-TG-) CVI process was developed to accelerate the carbon matrix deposition in the C-fiber preform within relatively short process-time. The temperature gradient in the preform was changed according to a controlled program. The present TC-TG-CVI process gave twice as much weight gain as that of the conventional steady TG-CVI process. The CVI process, however, showed limitation to fill up the large pores between the bundles. Liquid impregnation-pyrolysis was, therefore, carried out after the TC- TG-CVI process to improve the infiltration of these pores. Polycarbosilane (PCS) with the yield of about 50 wt% was used as the liquid precursor to form a SiC-rich matrix to improve the oxidation-resistance. SiC matrix was observed in the pores between bundles after pyrolysis of PCS. The weight gain after pyrolysis was found to be nearly saturated after 10 cycles of the impregnation-pyrolysis process.
Characterization of Pseudo-Porous SiC/C Coatings on Nextel™440 and Nicalon™ Fibers
Authors: Khasgiwale N.R. , Butler E.P. , Tsakalakos L. , Hensley D.A. , Cannon W.R. , Danforth S.C. , Gonczy S.T.
Published: April 2011

Pseudo-porous SiC/C coatings were deposited on Nextel™440 and Nicalon™ fibers by CVD. The morphology and chemistry of the coatings was evaluated, both before and after oxidation, using Scanning Electron Microscopy (SEM), X-Ray Diffraction Analysis (XRD), X-Ray Photoelectron Spectroscopy (XPS) and Auger spectroscopy. Coated fibers were subjected to two different oxidation treatments to assess coating stability: a) oxidation at 600°C for 20 hours, and b) oxidation at 1000°C for 20 hours. Pseudo-porous SiC/C on Nicalon™ fibers appear to be more oxidation resistant than the same coatings on Nextel™440 fibers.
Neutron Diffraction Study of Highly Radioactive U3Si2 Reactor Fuel
Authors: Richardson J. W. , Birtcher R. C. , Mueller M. H.
Published: April 2011

Neutron diffraction data were collected from a highly radioactive, low-enrichment uraniumU3Si2/A1 fuel mini-plate that had been irradiated to 42% uranium burnup. The experiment was performed using the General Purpose Powder Diffractometer at IPNS. Most prominent in the diffraction pattern were the Bragg reflections from crystalline Al (fuel plate) and V (sample container). Rietveld refinement revealed a broad oscillatory signal from the U3Si2 typical of amorphous material. This “amorphous” scattering pattern has peaks at the same positions as the amorphous scattering components observed following room temperature neutron irradiation of U3Si2 powder. The irradiated U3Si2 is amorphous with a remote possibility that it consists of recrystallized nanocrystalline particles subject to the requirements that the crystallites: (i) are no larger than 100-200 Å in size, and (ii) possess a crystalline lattice which is distorted relative to the unirradiated material.
Defect Configurations in Neutron-Irradiated U3Si2
Authors: Chan S.-K. , Birtcher R. C. , Mueller M. H. , Richardson J. W.
Published: April 2011

Investigations of the nature of defects in neutron-irradiated intermetallic U3Si2 as a function of dose have been carried out. Defect categories of vacancies and interstitials, their clusters and dislocation loops have been considered. The effects of the displacement field from each on the neutron scattering intensity profiles are presented. The theoretical predictions are compared with experimentally determined intensity profiles from time-of-flight data using the General Purpose Powder Diffractometer at the Intense Pulse Neutron Source of Argonne National Laboratory. The results are illustrated for a particular peak at various doses and the physical implications discussed.
Altering the Oxidation Resistance of Iron Via ion Beam Alloying
Authors: Murzin Ivan H. , Potter Donald I.
Published: April 2011

Fe-Cr, Fe-Y and Fe-Cr-Y surface alloys were produced by direct ion implantation, ion beam mixing, and combinations of implantation and vapor deposition. The influence of these treatments on the oxidation behavior of iron was investigated in 1 atm. of oxygen at 520°C. The oxidation rates were less in all the ion beam alloyed iron samples than in untreated iron. The oxidation follows parabolic kinetics in most cases, with the rate constants, Kp, in the range (3-8)×10−6 mg2cm−4 sec−l versus 2.2×10−5 mg2 cm−4 sec−1 for untreated iron. Yttrium fluences between 5×1014 and 5×lO15 cm−2 did not alter the microstructures of iron significantly. However, fluences of 1×1016, 3×1016, 5x1016 and 1x1017 cm−2 caused the crystalline structure of iron to be replaced by an amorphous phase. The presence of this phase was demonstrated with selected area channeling patterns and transmission electron microscopy.
Density Changes in Amorphous Pd80Si20 During Low Temperature ion Irradiation
Authors: Schumacher G. , Birtcher R.C. , Rehn L.E.
Published: April 2011

Density changes in amorphous Pd80Si20 during ion irradiation below 100K were detected by in situ HVEM measurements of the changes in specimen length as a function of ion fluence. A decrease in mass density as a function of the ion fluence was observed. The saturation value of the change in mass density was determined to be approximately -1.2 %.
Radiation Effects in Ceramic Insulators
Authors: Pells G P
Published: April 2011

Radiation damage mechanisms in oxide ceramics are briefly discussed for irradiation by electrons, ions and neutrons. The effects of electron, ion and neutron irradiation on the evolution of the damage microstructure of MgO, A12O3 and MgAl2O4 are followed in detail. Particular attention is paid to differences in the microstructure produced by ions of varying mass and energy. The effects of damage microstructure on other properties are indicated.
Study of Neutron-Irradiated Quartz Upon Heat Treatment: A Relation Between Tunneling States and Microtwins?
Authors: Keppens V. , Laermans C.
Published: April 2011

This paper deals with the study of the tunneling states (TS) in neutron-irradiated quartz. Ultrasonic attenuation measurements are carried out for a dose of 1.2 x 1019 n/cm2, before and after heat treatment at 700°C and 840°C. The data give evidence for significant changes in the distributions of the tunneling parameters upon heat treatment. These modifications provide interesting information about the nature of the tunneling states and will be discussed in view of the possible relation between the TS and the twin domains: the influence of heat treatment on the distribution of the tunneling states can be related to changes in the twin formation upon annealing.
Temperature Effects on ion Irradiation Damage in MgAl2O4 Spinel Single Crystals
Authors: Yu Ning , Sickafus Kurt E. , Nastasi Michael
Published: April 2011

Single crystalline samples of magnesium aluminate spinel (MgAl2O4), <100> oriented, were irradiated at 100 K and 670 K with 370-400 keV Xe ions to doses of (1-2)x1016 Xe/cm2. The microstructures of irradiated samples were subsequently examined by cross-sectional transmission electron microscope. A uniform layer of amorphous phase was observed on the surface of spinel irradiated at 100 K. At the end of the damage range underlying the amorphous layer, a disordered transition layer resided on the undamaged substrate. Both high resolution electron microscopy and microdiffraction revealed that the transition layer retained single crystallinity with epitaxial relationship to the underlying substrate. However, the intensity of <220> reflections in the transition layer was significantly weaker than that of the undamaged spinel. No evidence of amorphization was found in the spinel sample irradiated at 670 K to a dose of 2x1016 Xe/cm2. The <220> reflections exhibit only limited diminution in the heavily damaged region. The observation of reduced intensity of <220> reflections or absent reflections suggests that spinel experiences a structural transition from its original cubic phase (a=0.808 nm) to a new cubic phase (a=0.404 nm). A transition sequence from the original phase to a metastable phase and then to an amorphous phase has been observed. The temperature dependence of metastable and amorphous phase formation has revealed that the accumulation efficiency of cation disorder decreases with increasing irradiation temperature due to the enhancement of interstitial-vacancy recombination.
Dissolution of Ordered Precipitates under ion Irradiation
Authors: Camus Eric , Bourdeau Françise , Abromeit Christian , Wanderka Nelja , Wollenberger Heinrich
Published: April 2011

The stability of the ordered γʹ precipitates under 300-keV Ni+ irradiation was investigated between room temperature and 623 K. The two competing mechanisms of destabilization by cascade producing irradiation, i. e. disordering and dissolution of the γʹ precipitates in Nimonic PE16 alloy, has been studied separately by electron microscopy and field-ion microscopy with atom probe. At high temperatures, the precipitates are stable. At intermediate temperatures, the precipitates dissolve by ballistic mixing into the matrix, but the interface is restored by the radiation-enhanced atomic jumps. The order in the precipitates remains stable. At low temperatures, the precipitates are dissolved by atomic mixing. The dissolution proceeds in a diffusional manner with a diffusion coefficient normalized by the displacement rate D/K = 0.75 nm2dpa−1. The precipitates become disordered by a fluence of 0.1 dpa, whereas precipitate dissolution needs much higher fluences.
Flowing Afterglow Synthesis of Polymer Films with Nonlinear Optical Properties
Authors: Haaland Peter , Jiang Hao
Published: April 2011

Extensively conjugated polymer films with excellent morphology, homogeneity, robustness, and nonlinear optical coefficients have been synthesized in the flowing afterglow of a lowpressure argon plasma. Polymers of benzene, thiophene, furan, and napthalene are among the systems studied. The polymers, which grow by accretion of positive ions through a low-voltage sheath, have bulk and surface textures which scale systematically with film thickness. The hopes and perils of employing these new materials as active elements in gradient-index reflectors and optical switches will be described, as will the link between morphology and deposition mechanism.
New Self-Focusing Materials: Synthesis, Structures and Nonlinear Optical Properties of Butterfly Shaped Clusters
Authors: Shi S. , Xin X. Q.
Published: April 2011

Compounds WCu2OS3(PPh3)4 (I) and MoCu2OS3(PPh3)3 (II) were synthesized by reactions of (NH4)2MO2S2 (M = W, Mo), Cu2S and PPh3 in solid state for nonlinear optical studies. A wide transparent window (λ = 400 - 1000 nm) was observed for both clusters, which makes them attractive for nonlinear optical (NLO) applications. NLO properties of the clusters were studied with a 7-ns pulsed laser at 532 nm. Cluster I exhibits mainly optical self-focusing (n2 = 8 × 10-18 m2 W-1, as measured with an 1.2 × 10-4 M acetonitrile solution) with negligibly small nonlinear absorption. Cluster II exhibits both optical self-focusing (n2 = 5 × 10-17 m2 W-1, as measured with a 7.4 × 10-5 M acetonitrile solution) and nonlinear absorption (α2 = 2.6 × 10-10 m W-1). The third-order NLO susceptibilities (χ(3)) of the two clusters at the above-mentioned concentrations are 2 × 10-11 esu for I and 1.2 × 10-10 esu for II respectively. These nonlinear optical properties of the clusters were compared with those of cubic cage shaped and nest shaped clusters to reveal a qualitative structure/NLO property correlation.
The Design of Candidate Optical Limiters
Authors: Pachter Ruth , Crane Robert L. , Adams W. Wade
Published: April 2011

Series of molecules and polymeric systems are being evaluated as optical limiters for the design of a high second molecular hyperpolarizability γ. For example, results for a series of tolanes are discussed, as well as those of semi-flexible, stiff-chain polymers, including substituted biphenyl (X, Y = H, CH3, NO2, or a halogen atom) and benzo-bisthiazole moieties. The non-linear optical response was calculated using a finite field approach and compared for various derivatives.
Modeling of Spiropyran: Calculation Of Absorption Spectra
Authors: Young J. A. , Farmer B. L. , Adams W. W.
Published: April 2011

Semiempirical computational chemistry techniques were used to investigate the absorption spectra of 1′-isopropyl-3′,3′-dimethyl-indoline-spiro-6-nitro-8-methoxy-benzopyran. The spectra of both the closed (spiropyran) and open (merocyanine) forms were calculated using the INDO/CI method. Spectroscopic calculations showed the effects, both electronic and steric, that substituents such as hydrogen, methoxy and thiophene have on the absorption spectrum. Knowledge of the absorption spectra as a function of the choice and placement of substituents and molecular geometry will potentially allow optical properties to be optimized for specific applications.
Low Threshold ZnSe1−x Tex Optical Limiters
Authors: Ji W. , Tan H. S. , Feng Z. C. , Becla P.
Published: April 2011

We report an experimental investigation of optical limiting with bulk ZnSe1−xTex crystals at wavelengths just below the band edge, using nanosecond laser pulses. The limiting threshold as low as a few mJ/cm2 has been demonstrated. The generation of free charge carriers by singlephoton absorption, and subsequent free-carrier nonlinearities (nonlinear absorption and selfdefocusing) are responsible for the limiting behavior.
Effects of SiH2Cl2 Addition on the Growth Mechanism of Si Films Prepared by Photochemical Vapor Deposition
Authors: Oshima T. , Yamaguchi K. , Abe K. , Yamada A. , Konagai M. , Takahashi K.
Published: April 2011

Effects of SiH2Cl2 and H2 addition to SiH4 on photochemical vapor deposition (photo-CVD) of amorphous Si (a-Si) and epitaxial Si films were investigated, and roles of Cl and H in the growth mechanism were discussed. The surface morphology and the defect density of the obtained a-Si films were evaluated by atomic force microscope (AFM), and constant photocurrent method (CPM), respectively. It was found that the H2 dilution of SiH4 is effective to prepare the a-Si films with smooth surfaces and low defect density. Furthermore, nearly flat surface with the root mean square (RMS) of the roughness of 0.35nm was obtained by the SiH2Cl2 addition to H2-diluted SiH4. In the Si epitaxy, the SiH2Cl2 addition is also efficient to improve the film quality. It is suggested that the growing surface is terminated by both Cl and H, and the surface is more stable than that terminated by only H. Therefore, the diffusion on the growing surface of the film precursors such as SiH3 is enhanced, resulting in the improvement of the surface morphology and the film quality.
Microcrvstalline and Mixed Phase Si:H : Preparation. Potential and Properties for Devices
Authors: Dalai Vikram L. , Kaushal Sanjeev , Ping E. X. , Xu Jun , Rnox Ralph , Han K.
Published: April 2011

We discuss the preparation, properties and device potential of microcrystalline Si:H (μc-Si:H) material deposited using plasma-CVD techniques. We have been able to deposit high quality μc-Si:H using both glow discharge and ECR plasma CVD techniques. In both cases, the critical parameter was the presence of a high flux of H, and high power. The quality of the material was measured using sub-gap photo-conductivity, in both films and devices. We find that the absorption coefficient in μc-Si:H films follows the c-Si curve very closely, with little excess absorption. We have also fabricated p-i-n devices in these materials. We find that the properties of the devices depend critically upon the p-i interface and on the deposition technique used to make the devices. We use quantum efficiency techniques to study the devices in detail. We find that often, a good device characteristic corresponds to a case where the undoped layer is a mixed phase (amorphous-microcrystalline) and not just microcrystalline. In contrast, using just the microcrystalline phase for i layers in devices leads to severe interface problems, which lead to inflexion points in I (V) curves. Using the mixed phase for i layer, we have made solar cells with 0.46 V open circuit voltage, and a fill factor of 0.58. In contrast, using a microcrystalline phase can lead to devices with high voltages (0.6), but poor fill factors. We also discuss the device potential of microcrystalline devices, and we find that it may be possible to fabricate devices with 8% solar conversion efficiency in this material once all the interface problems are solved. Such devices may have applications as low-gap cells in an a-Si:H/μc-Si:H cell structure.
Formation of a-Si:H Film by p-CVD Method with SiH4-He Mixture and its Opto-Electronic Properties
Authors: Nakamura Youichi , Yamaguchi Tsuyoshi , Okagawa Atsushi
Published: April 2011

Formation of hydrogenated amorphous silicon (a-Si:H) was carried out by rf glow discharge method in the reactive gas systems of silane. By preliminary experiments on a-Si:H for helium dilution, a relatively high deposition rate up to 2–3 μm/hr was obtained. In order to investigate the opto-electronic properties on a-Si:H films due to different dilution of helium or hydrogen, measurements on optical band gap Eop, electric conductivity and FT-IR were done. Optical emission spectra were also observed. Optical band gap value Eop on a-Si:H film for SiH4 (10%) -He (90%) was nearly constant about 1.73 eV, while that for SiH4 (10%) -H2 (90%) was increased from 1.75 eV to 1.78 eV with increase of residence time of silane molecule and it was relatively high. From the experimental results of FT-IR, ratio of bonding mode of SiH to (SiH+SiH2) for hydrogen dilution was about 90% and higher than that for helium dilution, while hydrogen concentration in the film for SiH4 (10%) -H2 (90%) was less than 10% and that for SiH4 (10%) -He (90%) was 20–30%.
Diffusion, Drift, and Recombination of Holes In a-Si:H
Authors: Schwarz R. , Wang F. , Grebner S. , Gu Q. , Schiff E. A.
Published: April 2011

We compare measurements in a-Si:H of ambipolar diffusion length Lamb (from steady-state photocarrier gratings (SSPG)) and hole drift Χ (t) (from time-of-flight (TOF)). Using the response time tR from small-signal photocurrent decay measurements, we find that the equation L2amb = 2 (kT/e) Χ (tR) /E is consistent with the measurements, where E is the electric field inducing hole drift in TOF. Several samples under different temperature and light intensity levels have been studied. This equation has several implications. Under the usual SSPG illumination conditions, electron-hole recombination occurs while holes are still occupying valence bandtail states; hence SSPG is not sensitive to hole capture by deep levels. Furthermore, the experiments show that the Einstein relation is valid for holes in a-Si:H. We are unaware of prior direct tests of this relation in an amorphous semiconductor.
Flexible a-Si Solar Cells with Plastic Film Substrate
Authors: Fujikake S. , Tabuchi K. , Yoshida T. , Ichikawa Y. , Sakai H.
Published: April 2011

An amorphous silicon solar cell deposited on plastic film substrate has been studied. The solar cell has a new monolithic series-connected structure called SCAF, and will be fabricated by a newly developed fabrication process based on “Stepping Roll” film deposition system.

The process was compared with the conventional glass solar cell process. Preliminary results for small and large area SCAF solar cells were presented and discussed.
Relation of the Electroluminescence Measurements to A-Si.H Solar Cell Parameters
Authors: Wang Keda , Han Daxing
Published: April 2011

We have measured the intensity of electroluminescence (EL) and its energy spectrum in a-Si:H solar cells having an initial energy conversion efficiency from 5.75 to 9.8 %, and open-circuit voltages (Voc) between 0.799 and 0.952 V. We found that (a) at room temperature, EL efficiency is proportional to the initial solar cell energy conversion efficiency; (b) the defect energy distribution in the i-layer can be detected by EL energy spectrum at room temperature; and (c) Voc is simply related to the quasi-Fermi level splitting obtainable in the i-layer and that the buffer layer serves to increase this splitting.
Design and Growth of Band-GAP Graded a-SiGe:H Solar Cells
Authors: Vasanth K. , Payne A. , Crone B. , Sherman S. , Jakubowski M. , Wagner S.
Published: April 2011

The i-layers of the middle and bottom cells in stable triple-junction amorphous silicon solar cells are composed of a-SiGe:H alloys which are graded in composition to enhance performance. We compare modeling and experimental results for three i-layer band gap grading schemes to determine the optimal profile. We find a good correlation between model trends and measured device parameters for all grading schemes. This is encouraging for the use of the model in predictive device design. We find that the highest white and red light performance do not necessarily have the same cell parameter set. Modeling and experiment indicate that thin cells without band gap profile and with suitably designed p/i and n/i buffer layers, have the best red light performance.
Analysis of Non-Uniform Creation of Light-Induced Defects in Schottky Barrier Solar Cell Structures
Authors: Liu Hongyue , Malone C. T. , Fortmaiiii C. M. , Wronski C. R.
Published: April 2011

Hydrogenated amorphous silicon (a-Si:H) TCO/n+/i/Ni Schottky barrier solar cells were degraded with illuminations of white and red light through both sides of the structure. Because the forward dark I-V's are sensitive to the distribution and any spatial variation of defects in the i-layer, these measurements were used to characterize the degraded cell structures. These characteristics were analyzed using a charged defect distribution of gap states consisting of D+, D°, and D states derived from corresponding film analysis. It was found that the non-uniformities of light induced defects created in the i-layer can be represented by two regions of different defect densities. The relative densities depend on the direction of illumination and their ratio even with white light is about a factor of four. This non-uniformity is expected to be less for p-i-n cells which have higher built-in potential and hence electric fields.
Thin Film Transistors with Layered a-Si:H Structure
Authors: Kuo Yue
Published: April 2011

Thin film transistors TFTs with the layered a-Si:H structure are presented and discussed. Compared with the conventional single layer a-Si:H TFT, transistor characteristics of this new structure can be superior or inferior, depending on the deposition condition and number of the bulk (non-interface) a-Si:H layers. The mechanism influencing transistor characteristics is discussed. Changes of these TFTs' characteristics are not significant, e.g., the mobility varies within 20%. More data are being collected to verify the statistical significance of this kind of TFT.
Very low off Current Behavior in high Performance a-Si:H TFT
Authors: Yoon Jung-Kee , Choi Woong-Sik , Jang Yong-Ho , Seo Seong-Mo
Published: April 2011

The off currents in high performance a-Si:H TFT's were measured below 10−12 A. The Ioff-Vgs behavior is similar to that of a poly-Si TFT, in which the current increases rapidly with negative gate bias under a large drain bias while it stays almost constant under a small drain voltage. It seems the thermally-assisted tunneling via localized states occurs near the drain region as in the case of poly-Si TFT, although the current level is quite different. The off current change after negative bias stress at room temperature can be explained by the defect creation, but defects can be reduced by negative stress at 180 °C according to the defect pool model.
New Bias-Controlled Three-Color Detectors using Stacked a-SiC:H/a-Si:H Heterostructures
Authors: Topič Marko , Smole Franc , Groznik Aleš , Furlan Jože
Published: April 2011

A novel family of three-terminal three-color (blue, green, red) detectors based on stacked a-SiC:H/a-Si:H heterostructures is presented: TCO/PIN/TCO/PINIP/TCO/metal and TCO/PINIP/TCO/PIN/TCO/metal structure. The analysis of stacked photodetectors and the optimization of their geometrical dimensions is performed using the adopted ASPIN simulation program. Both structures are mutually compared with regard to calculated current-voltage characteristics and spectral responsivity. They both exhibit linear photocurrent/generation-rate relationship for all three colors at peak wavelengths 430, 530, 630 nm, applying ±1 V or more. This linearity allows that all three colors can be detected with high rejection ratio using simple system electronics.
Metastable Defect Formation by Hydrogen Relocation and Rebonding
Authors: Li Qiming , Biswas R.
Published: April 2011

Molecular dynamics with the tight-binding approach are utilized to examine the fundamental process of dangling bond creation via the rebonding of H from Si-H bonds to weak Si-Si bonds. The defect formation energy is found to strongly correlate with the bond-length of the weak Si-Si bond, indicating that the distribution of weak Si-Si bonds controls the total defect density. Rate equations for thermally generated and light-induced defects are developed and utilized to calculate the equilibrium and saturated defect density. The results agree well with experimental data.
Dependence of Sheet Hole Concentrations on Growth Kinetics of a Carbon Doped Electron Beam Heated Graphite Source in GaAs and Al0.3GaD0.7As Films Grown by Solid Source MBE
Authors: Sato D. L. , Szalkowski F. J. , Lee H. P.
Published: April 2011

A series of carbon delta- and uniformly- doped GaAs and Al0.3Ga0.7As films have been grown by solid source Molecular Beam Epitaxy using an electron-beam heated graphite rod as the dopant source. In contrast to heated filament carbon sources, this electron-beam source produces atomic carbon C1, rather than C3 as the predominant species. The purpose of these experiments was to compare measured sheet carrier concentration of delta-doped GaAs and Al0.3Ga0.7As films under fixed growth conditions and with similar carbon beam fluxes using this electron-beam source. The films were characterized by Hall-effect measurements. The Al0.3Ga0.7As films have consistently higher hole sheet carrier concentrations than GaAs films with similar carbon fluxes. Delta-doped Al0.3Ga0.7 As sheet carrier concentrations of up to 2.2×1013 cm−2 were measured, and thin uniformly-doped Al0.3Ga0.7 As carrier densities were measured up to 5.7×1020 cm−3. The uniformly doped Al0.3Ga0.7As films appear to have higher atomic carbon activation than this delta-doped Al0.3Ga0.7As films. These results can be possibly explained in terms of several concurrent processes involving C-C pairing, stronger Al-C bonding, and dopant surface segregation taking place on the growth surface of the delta-doped layer. Photoluminescence intensity measurements of uniformly-doped GaAs shows degradation at hole concentrations greater then 1018 cm−3 and bandgap contraction at hole densities greater than mid 1018 cm−3.
Photodiffractive Characterization of Growth-Defects in GaAs
Authors: Jarasiunas K. , Sudzius M. , Kaniava A. , Vaitkus J.
Published: April 2011

The transient grating technique has been developed over the years and several reviews have been written concerning the subject [1,2]. The present paper will focus on its advantages for studies of main native defect in semiconducting GaAs crystals, namely, of EL2, which plays a crucial role in semi-insulating properties and optical absorption below band-gap, photorefractivity, metastability, etc. We will consider the role of EL2 in optical nonlinearities and its contribution to carrier transport in subnanosecond time domain. Nonlinear character of TG technique allows to get deeper insight into spatial distribution of growth defects (EL2 and dislocations) in the wafers and perform their nondestructive monitoring. Different experimental techniques, as DFWM in nano- and picosecond time domain, light selfdiffraction, and set of differently grown GaAs crystals enabled us to show applicability of this technique for basic research and nondestructive determination of parameters, defect distribution, reveal fast transients of optical nonlinearities. The photorefractive nonlinearity at 300K is shown being dependent on dislocation density due to local strain fields around charged dislocations. Moreover, transient quenching of EL2 by short laser pulses and enhancement of low-temperature photorefractive nonlinearity is demonstrated.
Crystallographic Defect Related Degradation in High Density Memory Devices
Authors: Kim S.S. , Wijaranakula W.
Published: April 2011

Crystallographic defect related degradation in high density memory devices was investigated. The results indicate that the refresh time degradation and bit failure mechanism are directly related to the crystal originated defects generated during Czochralski crystal growth. Defects associated with vacancy aggregations are found to have a detrimental effect on the overall performance of memory devices. Other defects, such as oxide polyhedral precipitates, contribute to a high number of cumulative fail bits, particularly in the bottom section of the crystal.
Analysis and Control of the Performance-Limiting Defects in HEM-Grown Silicon for Solar Cells
Authors: Khattak Chandra P. , Schmid Frederick , Cudzinovic Michael , Symko Martha , Sopori Bhushan L.
Published: April 2011

Multicrystalline silicon ingots of 55 cm × 55 cm cross section, 100 kg have been grown by the Heat Exchanger Method (HEM). Controlled growth features have been used to produce large grain size, vertically oriented grain boundaries, large areas of twins with low defect density and rejection of impurities to the top of the ingot. Ambient control has reduced C, N, and O concentration and minimized precipitates with no detectable metallic impurities. High performance solar cells have ben fabricated, and further improvements can be achieved by minimizing dislocation tangles and impurities in localized regions.
Resolving Degradation Mechanism in Carbon and Beryllium Doped HBT’s Using Pulsed Mode Current Stress
Authors: Chou Y. C. , Li G.P. , Wu C.S. , Chu Peter , Pao C.K. , Cisco T.C.
Published: April 2011

The reliability of high performance carbon and beryllium-doped heterojunction bipolar transistors (HBT’s) is investigated using a pulsed mode current stress. After the current stress, the collector current reduction (measured at a fixed Vbe) and the inverted collector current ratio change (measured at two different reverse Vbe biases) are used as measures of HBT degradation due to dopant acceptors migration. For carbon and beryllium doped HBT’s, degradation was found to be only significant beyond a threshold current density (Jth), showing that the carbon doped devices have a higher Jth than that of the Be-doped HBT's. Beyond the Jth, the device degradation is insensitive to the stress current density but depends on the total stress charge. As a result, the device degradation mechanisms of electric field assisted dopant and carrier recombination enhanced dopant migration can be distinguished under pulsed mode current stress.
Improved Performance of MESFET Devices with L.T.GaAs Buffer Layers
Authors: Lagadas M. , Hatzopoulos Z. , Karadima F. , Konstantinidis G. , Kornilios N. , Papavasiliou C. , Christou A.
Published: April 2011

We have investigated the electrical properties of n-GaAs epilayers grown by MBE on the top of L.T.GaAs .The Hall mobility and the channel depth of the epilayer decreases as the growth temperature of the buffer decreases. The degradation of the electrical properties is attributed to the outdiffusion of precipitates and point defects from the L.T. buffer layer. The use of intermediate layers (GaAs grown at 400°C-600°C and GaAs/AlxGa1-xAs (x=0.5, 1) superlattices ) between L.T. buffer and n-GaAs active layers in MESFET devices improves the Hall mobility of the channel, the gm and fT performance of the devices.This improvement has been attributed to the AlGaAs layer which hinders the defects outdiffusion.
Scanning Tunneling Microscopy Studies of GaAs1-xPx Single Crystals
Authors: Liu X. , Weber E. R. , Ogletree D. F. , Salmeron M. , Slupinski T.
Published: April 2011

We report cross-sectional scanning tunneling microscopy studies of GaAsP single crystals grown by the Liquid Encapsulated Czochralski technique. We show that the two group-V elements can be clearly distinguished, which is attributed to the difference in energies of surface dangling bond states of As and P. Our atomic scale imaging results show alloy composition in agreement with spectroscopic studies. They also provide valuable information about atomic scale alloy fluctuations and clustering effects.
Atomic Structure of the Interfaces Between Silicon Directly Bonded Wafers
Authors: Benamara M , Rocher A , Laporte A , Sarrabayrouse G , Lescouzères L , PeyreLavigne A , Fnaiech M , Claverie A
Published: April 2011

The so-called Direct Wafer Bonding (DWB) technique opens new possibilities for the electronic industry but still suffers from the poor knowledge we have of the microstructure of these interfaces and hence of their electrical activity. In this work, we have extensively used Transmission Electron Microscopy techniques in plan-view and cross-section to identify the structure of the interfaces found between two bonded silicon wafers. The general structure of these interfaces is that of a perfect grain boundary and evidently depends on the misorientation between the two bonded wafers. A twist component in the range 0>θ>13˚ creates a square network of pure screw dislocation whereas an unavoidable tilt component (<0.5˚) is compensated by a periodic array of 60˚ dislocation lines perpendicular to the tilt direction. Therefore, the regularity of these networks can be disrupted by the presence of steps (of up to several nanometers) in the interface plane. Silicon oxide precipitates are seen heterogeneously distributed on the interface with preferential nucleation sites on the dislocations.
Bistability of Defects in Semiconductors
Authors: Mizuta Masashi
Published: April 2011

The DX center in AlGaAs alloy is reviewed as an example of bistability of defects in semiconductors. Also presented is some technological importance of the defect bistability.