bluuebunny/crossref_metadata_2025 huggingface dataset:
Dependence of Hardness on Modulation Amplitude in Compositionally Modulated Cu-Ni Thin Films
Authors: Oberle R.R. , Cammarata R.C.
Published: April 2011

The hardness dependence on the composition modulation amplitude of electrodeposited Cu-Ni multilayered thin films was investigated. The composition modulation amplitude was reduced by annealing the films, resulting in interdiffusion between the Cu and Ni layers. The hardness enhancement ΔH over that for a completely intermixed film was measured as a function of composition modulation amplitude A. It was found that for films with a sine wave composition profile, the hardness enhancement ΔH varied as A1.1±0.1, in good agreement with the model given by Krzanowski [1].
Effect of Copper Film Thickness on Stress and Strain in Grains of Different Orientation
Authors: Vinci R. P. , Zielinski E. M. , Bravman J. C.
Published: April 2011

Models describing the effect of grain orientation on dislocation glide in a thin film are reviewed; it is predicted that differences in stress-temperature behavior should exist between grains of different orientation in a polycrystalline copper film. Direct x-ray evaluation of strains in {111} and {100| grains within primarily {111} textured copper films shows that these stresses are film thickness or grain size dependent. The {111} oriented grains behave as expected, with room temperature flow stress increasing linearly with 1/film thickness. The flow stress of grains of {100} orientation, however, is approximately constant with film thickness and is not well represented by the models. It is proposed that simplifying assumptions about orientation dependent yield may only be appropriate for the majority grain orientation population in a textured film.
Stress That Counteracts Electromigration: Threshold Versus Kinetic Approach
Authors: Glickman E. , Osipov N. , Ivanov A.
Published: April 2011

The paper analyzes electromigration (EM) conditions and material properties that determine the maximum EM induced stress, σa, and stress gradient, ∇σ, which counteract EM flow in interconnects.

The first systematic data on the drift velocity vs. stripe length, L, current density, j, and temperature are presented for Al lines. In contrast to the conventional approach to the Blech problem with σa taken to be a material constant (“yield strength”), the observations suggest that σa increases with j. The stress adjustment is shown to result from the imperative coupling of the net flux of material directed to the downwind end of the stripe with the flux of plastic flow (creep) responsible for stress relaxation. The effect of parameters of the constitutive equation assumed to describe the plastic flow kinetics, namely that of strain rate exponent, threshold stress, and creep, effective viscosity, on the stress cya is considered. To account for the creep viscosity, η, obtained unpassivated aluminum stripes from EM experiments, a model for the attachment-controlled Coble creep is suggested.
Stress behavior of CVD-PSG Films Depending on Deposition Methods and Hillock Suppression
Authors: Kim Eungsoo , Kang Suk-Hee , Lim Soon-Kwon
Published: April 2011

In multilevel metallization processes, stress induced void and hillock formations cause serious problems with respect to the reliability of metal line, such as pitting corrosion of metal during the pad etching step, mask misalignment due to surface roughness, and open or short failure of metal line. It is generally known that these types of metal-film related problems are due to stress relaxation process in the metal line stress-induced by thermal expansion mismatch between metal and over/underlayer oxide film or compressive stress of overlayered passivation dielectric film.

In this study, the stress behavior of phosphosilicate glass (PSG) film on the Al-l%Si film, deposited by different CVD methods, is investigated. Here we found that the degree of PSG film stress is related with the suppression of hillock growth, void formation, and cracking of passivation layer, depending on the heat treatment and heat treatment. Using stress measurement, surface profilometer, and SEM, the relation between passivated PSG film stress on metal line and the metal line failures is analyzed. In addition, an effective method of hillock suppression is suggested, which is the use of PECVD-PSG film on the metal line before heat treatment of 400°C or that of any type of PSG film before 350°C.
Morphology of Damage in Al Films Tested Under Electromigration Conditions Using the Drift Velocity Method
Authors: Kononenko Oleg V , Matveev V.N.
Published: April 2011

Pure Al films deposited by the partially ionized beam technique at bias Iavels of 0, 3 and 6 kV onto TiN sublayers were patterned for measurement for direct drift velocities. Then films were annealed in a vacuum 10−4 Pa for 1 hour. Electromigration tests were performed in an air atmosphere at a temperature of 280°C. The current density during tests was 6·105 A/cm2. Examinations in the SEM showed that the morphology of the damage on cathode and anode ends of the Al stripes depends on the bias during deposition. In the case of 0 kV bias, voids nucleate on the back and side edges of the cathode end and propagate toward the anode end. Hillock and whisker formation was observed on the anode end. The height and diameter of the whiskers were about 50 and 1–2 μm, respectively. In the case of 3 and 6 kV bias the drifting edge was smooth. Only hillocks (no whiskers) grew on the anode edge. The morphological differences of these films are discussed on the basis of self-ion bombardment effects.
Measuring the Mechanical Properties of Thin Metal Films by Means of Bulge Testing of Micromachined Windows
Authors: Paviot V. M. , Vlassak J. J. , Nix W. D.
Published: April 2011

Free-standing films of gold and aluminum have been fabricated using standard micro-machining techniques. LPCVD silicon nitride films are deposited onto (100) silicon wafers. Square and rectangular silicon nitride membranes are made by anisotropic etching of the silicon substrates. Then, metal films are deposited onto the silicon nitride membranes by means of evaporation. Finally, the sacrificial silicon nitride film is etched away by means of reactive plasma etching, resulting in well-defined, square and rectangular metal membranes.

Bulge testing of square windows allows one to determine the biaxial modulus of the film as well as the residual stress in it. Testing rectangular windows yields the plane-strain elastic modulus and the residual stress. Since deformation in rectangular membranes approaches plane-strain deformation, this geometry is ideal for studying the plastic properties of the metal films. Stress-strain curves can be readily determined from the load-deflection curves of rectangular membranes. The gold films have a biaxial modulus of 161±3 GPa and a plane-strain modulus of 105±5 GPa, slightly lower than the literature values for a (111) textured film. The yield stress of these films is approximately 231±17 MPa at 10−4% plastic strain. The elastic moduli of the aluminum films are 105±3 GPa and 76.4±0.7 GPa, respectively; the yield stress of these films is 187±30MPa.
Blister Test Analysis Methods
Authors: Hohlfelder Robert J , Vlassak Joost J. , Nix William D. , Luo Huihong , Chidsey Christopher E.D.
Published: April 2011

We present a general mathematical model of the blister test describing the relation between adhesion energy and other test parameters, and discuss the assumptions made therein. We describe the effects of excessive compliance in the blister test apparatus and discuss the design of our apparatus. Preliminary results from adhesion tests of polymer films which use self-assembled monolayers as adhesion promoters are presented. Some features of the data can be explained by a simple model incorporating a rate-dependent adhesion energy.
Ferroelectric Domain Inversion in LiNbO3 and its Application to High-Precision Piezoelectric Actuators
Authors: Nakamura Kiyoshi , Yamada Ken
Published: April 2011

Heat treatment of bare single-domain LiNbO3 plates near the Curie temperature induces a local polarization reversal, thereby yielding a ferroelectric inversion layer. For a sufficiently long heat treatment the inversion layer thickness becomes equal to one half of the plate thickness. Similar domain inversion is also induced by heat treatment of protonexchanged LiTaO3. These domain inversion phenomena and a model for explaining the mechanism are reviewed.

The inversion layer has piezoelectric constants opposite in sign to those of the uninveiled domain. Therefore, a LiNbO3 plate with such an inversion layer can be used as a piezoelectric actuator similar to the bimorph, because bending motion or torsional motion can be piezoelectrically produced, depending on the plate orientation. The measured characteristics of these actuators are presented and demonstrated to exhibit excellent linearity and no hysteresis.
Deposition Parameters for Sputter-Deposited Thin Film TiNi
Authors: Jardine A.Peter
Published: April 2011

Sputter deposition of TiNi has been problematic due to the extreme reactivity of titanium. There are several advantages in depositing TiNi with other metals, such as Ti and Cu, as the Shape Memory Effect properties can be changed selectively, including coring of the thin film. Simultaneous co-deposition of cored material using sputter deposition becomes technically difficult as this implies that the sample to target distance increases. In this paper, we discuss a relationship developed so that a basic base vacuum pressure can be found with target-sample distance and correlated with the observed phases for both Ti and TiNi samples. Practical limitations imposed by the vacuum system on both devices and device quality are discussed
Composite Piezoelectric Sensors and Actuators
Authors: Newnham R. E. , Fernandez J. F. , Markowski K. A. , Fielding J. T. , Dogan A. , Wallis J.
Published: April 2011

Composite materials have found a number of structural applications, but their use in the electronics industry has been relatively limited. As the function of electroceramic composites are better understood, we can expect this picture to change. In this paper some of the piezoelectric composite sensor and actuator studies carried out in our laboratory during the past decade will be reviewed. The ideas that provide a basic understanding of functional composites have previously been discussed [1]. This paper describes recent advances in the processing and properties of composites possessing 0-3, 1-3, and 2-2 connectivity made of polymers, metals and ferroelectric ceramics. The introduction of open spaces in the previous connectivity patterns allows the development of new designs of several piezoelectric composites with connectivity patterns of 0(0)-3, 1(0)-3, 2(0)-2-2, and 2-0-2. As in most electronic systems that are developed with improved properties a push toward lower cost and smaller sizes of the piezoelectric ceramics, together with improved reliability and performance occurs. The piezocomposites with open spaces in their structures clearly demonstrate the growth of the functional ceramics into the field of the smart ceramics as the materials for the beginning of the next century.
Structure and Thermal Stability in Titanium-Nickel Thin Films Sputtered at Elevated-Temperature on Inorganic and Polymeric Substrates
Authors: Hou Ll , Pence T. J. , Grummon David S.
Published: April 2011

Deposition of crystalline titanium-nickel shape-memory alloys on polymeric substrates is complicated by the elevated temperatures that must be withstood either during deposition or during post-deposition crystallization anneals. In this paper we report results on thin films of titanium-nickel, with Ni-rich compositions, which were prepared by dc magnetron sputtering onto quartz substrates, and onto thin polyimide membranes, at various elevated substrate temperatures between 623K and 703K. All of the as-deposited films possessed ordered bcc microstructures and had nano-scale grain sizes. Films deposited at the higher end of the temperature range displayed familiar two-step transformation behavior in electrical resistivity experiments, with transformation temperatures that were stable with respect to annealing for 22-hrs at 700K. Classic shape-memory was observed for a bilayer consisting of an 7.6 μm thick Kapton® polyimide sheet onto which 3 μm of near-equiatomic TiNi had been sputtered at 703K. By applying a pattern-etch technique to the metallized polymer we were able to fabricate a prototype electrically-excitable thin-film actuating element, potentially applicable to microelectromechanical and biomechanical systems.
Piezoelectric Fiber Coatings and Micro-Tubes
Authors: Fox G.R. , Damjanovic D. , Danai P.A. , Setter N. , Limberger H.G. , Ky N.H.
Published: April 2011

Axially symmetric coatings of ZnO and PZT have been deposited onto fiber substrates by reactive dc magnetron sputtering. Multi-layer Cr/Au/ZnO/Cr/Au coating structures on optical fibers have been used to make integrated phase modulator devices. An analysis of the structural properties and piezoelectric response of the phase modulator devices will be presented. Microtubes of ZnO and Pt/ZnO/Pt multi-layers have been prepared by coating polyester fibers that act as a fugitive phase. After burning away the polyester fiber, up to 3 cm long micro-tubes with a 23 μm inside diameter and 3 to 9 μm wall thickness were obtained.
Ferroelectric Behavior and Microstructure of Calcium-Modified Lead Titanate Ceramics
Authors: Zhuang Zhiqiang , Kulesha G.A. , Du H. , Gallois B.
Published: April 2011

Calcium-doped lead titanate ceramics exhibiting piezoelectric anisotropy were fabricated for applications in ultrasonic transducers, infrared detectors and surface acoustic wave devices. Transmission electron microscopy and X-ray diffraction techniques were used to characterize the development of the piezoelectric anisotropy and the dependence of the piezoelectric anisotropy on microstructure. Electron diffraction patterns indicated that most of the ferroelectric domains in samples with [CaA] up to 30 mole% were 90° domains. The size of the ferroelectric domains was not related to piezoelectric anisotropy. Ferroelectric domains could not be detected above this concentration. Measurements of the temperature dependence of dielectric properties at different frequencies did not show any ferroelectric relaxor behavior.
Residual Stress Distribution in Feal Weld Overlay on Steel
Authors: Wang X.-L. , Spooner S. , Hubbard C. R. , Maziasz P. J. , Goodwin G. M. , Feng Z. , Zacharia T.
Published: April 2011

Neutron diffraction was used to measure the residual stress distribution in an FeAl weld overlay on steel. It was found that the residual stresses accumulated during welding were essentially removed by the post-weld heat treatment that was applied to the specimen; most residual stresses in the specimen developed during cooling following the post-weld heat treatment. The experimental data were compared with a plasto-elastic finite element analysis. While some disagreement exists in absolute strain values, there is satisfactory agreement in strain spatial distribution between the experimental data and the finite element analysis.
Initial Survey on Fracture Toughness of Commerical Nd2Fe14B
Authors: Horton J.A. , Schneibel J.H.
Published: April 2011

Nd2Fe14B is currently the best (highest energy product) hard magnet available. Like most magnetic materials, it is rather brittle. The fracture toughness of specimens of commercially available Nd2Fe14B magnets were measured using a chevron notch three point bend test. Values ranged from nearly 2 to 5.5 MPa√m. Consistent differences were found between the five different types of materials tested. In this initial survey, insufficient tests were done for good statistics to be obtained. However, it appears that this test will allow differences in fracture toughness between alloys with different compositions and fabricated by different production methods to be measured and tracked as improvements are made.
Mechanical Properties and Structure of Cubic (Al,Cr)3 Ti Alloys
Authors: Nic J. P. , Mikkola D. E.
Published: April 2011

Single phase LI2 (Al,Cr)3Ti alloys show no anomalous strengthening at intermediate temperatures, but the strength rises rapidly at low temperatures. It is likely that a transition from SISF to APB superdislocation dissociation occurs with increasing temperature based on the more rapid decrease in APB energy compared to SISF energy because of entropy considerations. High strain hardening rates are suggested to arise in large part from the formation of large numbers of dipoles. Calculations of interaction forces illustrate the complexity of superdislocation interactions and provide a reasonable mechanism for initiating cross slip.
Understanding The Nb-Ti-A1 System: First Principles Calculations
Authors: Mehl Michael J.
Published: April 2011

The discovery of ductile cubic phases in the Nb-Ti-Al system has led to increased study of these high-temperature intermetallics. I have performed first-principles calculations for ordered crystal structures in this system, paying particular attention to the Nb7Ti7Al2 structure. Somewhat surprisingly, the electronic density of states, lattice constant, and bulk modulus are nearly independent of the ordering of these materials, even though the changes in the total energy are significant.
Abrasive Wear Of Intermetallics
Authors: Hawk J. A. , Alman D. E. , Wilson R. D.
Published: April 2011

The U.S. Bureau of Mines is investigating the wear behavior of a variety of advanced materials. Among the many materials under evaluation are intermetallic alloys based on the compounds: Fe3Al, Ti3Al, TiAl, Al3Ti, NiAl and MoSi2. The high hardness, high modulus, low density, and superior environmental stability of these compounds make them attractive for wear materials. This paper reports on the abrasive wear of alloys and composites based on the above compounds. The abrasive wear behavior of these alloys and composites are compared to other engineering materials used in wear applications.
Estimation of Defect Structure and Site Preference of Additional Elements in B2-Type Nial, Coal and Feal at Offstoichiometry
Authors: Hosoda Hideki , Inoue Kanryu , Mishima Yoshinao
Published: April 2011

Defect structures induced by deviation of composition from stoichiometry and site preference of additional elements are predicted for B2-type aluminides (NiAl, CoAl and FeAI) consisting of 8A-group elements by comparing formation energies of particular atomic configurations under the pseudo-ground state. Interaction energies between atoms and vacancies are estimated using a modification of Doyama’s relationship. The results show that vacancy-type defects are formed in NiAl and CoAl only when the total composition of Al and additional elements occupying the Al-site is more than the stoichiometry, whilst with less than the stoichiometry, substitutional defects are formed. In FeAI, only substitutional defects are formed in either case of deviation. Similar to Al, Si occupies only the Al-site, although elements in 1A- and 8A-groups of the periodic table mostly occupy only the Ni-, Co- and Fe-site. The other elements occupy either Al-site or the site of 8A-group elements, or both, depending on the composition.
Solution of the Atomic Structure of the Σ=5 (310) [001] Grain Boundary in Nial by Hrem and Atomistic Simulations
Authors: Fonda R. W. , Yan M. , Luzzi D. E.
Published: April 2011

The atomic structure of the Σ = 5 (310) [001] grain boundary in NiAl has been determined by a synergistic approach combining high resolution electron microscopy (HREM) and atomistic structure calculations. A bicrystal of controlled orientation was produced by diffusion bonding and imaged with the electron beam parallel to the [001] tilt axis. The results showed that the material remains chemically ordered up to the boundary plane. Atomistic structure calculations employed N-body empirical potentials developed for the NiAl phase to examine the changes in interfacial energy due to the incorporation of various point defects at the grain boundary. A self-consistent model structure was determined which was of lowest energy and produced calculated images which matched experimental images of the boundary. Monte Carlo simulations confirm the stability of this structure at finite temperatures.
The Observations on Faulted Dipoles in Deformed Tial—Based Alloys
Authors: Gao Y. , Zhu J. , Cai Q. G.
Published: April 2011

The deformation structure of polycrystalline TiAl-based alloys after uniaxial compression at temperature range from 77K to 1073K has been examined using transmission electron microscopy. It was observed that a large number of faulted dipoles are commonly present in deformation structure of the alloys compressed at low temperature 77K and room temperature. The nature of the faulted dipoles has been determined to be intrinsic stacking fault lying on {111} plane, bounded by 1/6<112] partial dislocations. A possible mechanism for the formation of the faulted dipoles was suggested. The results of the statistic observation shows that faulted dipoles in deformed Ti-48A1 and Ti-(47–48) Al-X (X = V,Cr,Mn) alloys are less than those in single phase Ti-52A1 alloy, and the number of the faulted dipoles decreases with increasing deformation temperature. The effect of the faulted dipoles on plastic deformation of the alloy was discussed.
Temperature-Dependent Structure of a<101> Superdislocations in Ni3Al
Authors: Chrzan D. C. , Foiles S. M. , Daw M. S. , Mills M. J.
Published: April 2011

The dissociated structure of the a〈101〉 superdislocations in Ni3Al is predicted as a function of temperature. The temperature dependence of the relevant fault Helmholtz free energies and elastic constants are calculated within the quasiharmonic approximation using the embedded atom method. The results of these calculations are then incorporated into anisotropic elasticity theory-based calculations of the dissociation distances. The cross-slip activation enthalpy is estimated and found to decrease by 24% at 600 K when compared with its 0 K value. The calculations point to the need to perform experiments to address this temperature dependence.
The Effects of Notches on the Deformation of Ductile Intermetallics
Authors: Schulson Erland M. , Xu Yixin
Published: April 2011

The notch-sensitivity of the ductile L12 aluminides Ni-rich Ni3A1 doped with boron and Zr3A1 is attributed to the triaxial tensile state of stress induced by a notch.
Study of the Effects of Al, Ta, W Additions on the Microstructure and Properties of Mosi2 Base Alloys
Authors: Stergiou A. , Tsakiropoulos P.
Published: April 2011

Solidification microstructures and phase selection in ingots and melt-spun ribbons of MoSi2, MoSi2-W-Ta and MoSi2-Al alloys were studied. Rapid quenching from the melt refined the grain size of all alloys by two orders of magnitude compared to the ingots. Zone A microstructures were formed in the wheel side of all alloy ribbons. Addition of W and Ta and of Al suppressed the formation of Mo5Si3 in the ingots and ribbons. Tungsten exhibited complete solid solubility in MoSi2. In the ribbons the solubility of Ta in MoSi2 was extended to 2 at% in Zone A, and in the ingots and zone B of the ribbons hexagonal (C40) TaSi2 and tetragonal (C11b) MoSi2 structures were formed. A fine lamellar microstructure between the Cllb and C40 phases was formed at 1350°C after 48h. Addition of Al changed dramatically the solidification microstructure which consisted of two phases C40 Mo(Al0.5Si0.5)2 and C54 MoAl1.3Si0.7. In the ingots the room temperature hardness decreased with addition of W and Ta and of Al. Addition of Al increased the thermal stability of the MoSi2-Al alloy.
Incipient Melting and Oxidation of Grain Boundary in a Two-Phase Ni3Al Alloy Containing Zr
Authors: Li Huaxin , Chaki T.K.
Published: April 2011

Incipient melting and oxidation of grain boundaries at elevated temperatures have been investigated in the investment cast billet of a two-phase (γ + γ′) nickel aluminide alloy (Ni74.48Al16.98Cr8.02Zr0.51B0.10), designated as IC-218. Due to enrichment of Zr, which can form eutectic alloys with Ni, the inter-dendritic regions and grain boundaries melted incipiently at 1150°C. During quenching the molten layers often opened up producing cracks at the boundaries. Annealing at 1200°C in air for 570 min produced elongated ZrO2 precipitates at the grain boundaries. There was Zr depletion inside the grains in the vicinity of the precipitates. Such precipitates were scarcely observed in the specimens annealed in air at 1100°C for 23 h. Thus, the molten Ni-Zr alloy at the boundaries promoted internal oxidation.
Nondestructive Characterization of Woven Fabric Ceramic Composites
Authors: Hsu D. K. , Liaw P. K. , Yu N. , Saini V. , Miriyala N. , Snead L. L. , Lowden R. A. , Mchargue C. J.
Published: April 2011

Woven fabric ceramic composites fabricated by the chemical vapor infiltration method are susceptible to high void content and inhomogeneity. The condition of such materials may be characterized nondestructively with ultrasonic methods. In this work, longitudinal and shear waves were used in the quantitative determination of elastic constants of NicalonTM/SiC composites as a function of volume percent of porosity. Elastic stiffness constants were obtained for both the inplane and out-of-plane directions with respect to fiber fabric. The effect of porosity on the modulus of woven fabric composites was also modelled and compared to the measured results. Scan images based on the amplitude and time-of-flight of radio frequency (RF) ultrasonic pulses were used for evaluating the material homogeneity for the purpose of optimizing the manufacturing process and for correlation with the mechanical testing results.
Burner Rig Thermal Fatigue Failure of SiC Continuous Fiber/Si3N4 Ceramic Composites
Authors: Ertürk T. , Park K. , Sung C.
Published: April 2011

The burner rig thermal fatigue properties of SiC continuous fiber/Si3N4 ceramic composites were examined under impinged jet fuel flame, a constant applied tensile stress and thermal cycling in the temperature range 500-1350 °C. The SCS-9 SiC fiber/Si3N4 composites failed within the flame impinged zone, whereas the SCS-6 fiber/Si3N4 composites failed outside the flame impinged zone due to the high thermal stresses resulting from high-temperature gradients. Analytical transmission electron microscopy was used to investigate the microstructure and chemistry of the fiber, matrix and fiber/matrix interface in the failed SCS-9 SiC fiber/Si3N4 composites. The partial degradation of columnar structure of the fiber was interpreted as the dominant mechanism of burner rig thermal fatigue failure of SCS-9 SiC fiber/Si3N4 composites.
Molecular Dynamics Simulations of High Energy Cascades in Iron
Authors: Stoller Roger E.
Published: April 2011

A series of high-energy, up to 20 keV, displacement cascades in iron have been investigated for times up to 200 ps at 100 K using the method of molecular dynamics simulation. Thesimulations were carried out using the MOLDY code and a modified version of the many-bodyinteratomic potential developed by Finnis and Sinclair. The paper focuses on those results obtained at the highest energies, 10 and 20 keV. The results indicate that the fraction of the Frenkel pairs surviving in-cascade recombination remains fairly high in iron and that the fraction of the surviving point defects that cluster is lower than in materials such as copper. In particular, vacancy clustering appears to be inhibited in iron. Some of the interstitial clusters were observed to exhibit an unexpectedly complex, three-dimensional morphology. The observations are discussed in terms of their relevance to microstructural evolution and mechanical property changes in irradiated iron-based alloys.
A Phenomenological Model for the Effect of Nanocrystalline Microstructure on Irradiation-Induced Amorphization In U3Si
Authors: Rest J.
Published: April 2011

A rate theory model is formulated wherein amorphous clusters are formed by a damage event. These clusters are considered centers of expansion (CEs), or excess-free-volume zones. Simultaneously, centers of compression (CCs) are created in the material. The CCs are local regions of increased density that travel through the material as an elastic (e.g., acoustic) shock wave. The CEs can be annihilated upon contact with a sufficient number of CCs, to form either a crystallized region indistinguishable from the host material, or a region with a slight disorientation (recrystallized grain). Recrystallized grains grow by the accumulation of additional CCs.

Preirradiation of U3Si above the critical temperature for amorphization results in the formation of nanometer-size grains. In addition, subsequent reirradiation of these samples in the same ion flux at temperatures below the critical temperature shows that the material has developed a resistance to radiation-induced amorphization (i.e., a higher dose is needed to amorphize preirradiated samples than those that have not been preirradiated). In the model, it is assumed that grain boundaries act as effective sinks for defects, and that enhanced defect annihilation is responsible for retarding amorphization below the critical temperature by, for example, preventing a buildup of vacancies adjacent to the grain boundaries. The calculations have been validated against data from ion-irradiation experiments with U3Si. For appropriate values of the activation energy of thermal crystallization, the model predicts the evolution of a two phase microstructure consisting of nanocrystalline grains and amorphous clusters.
Neutron Diffraction Study of Radiation Damage in U3Si at 30°C and 350°C
Authors: Birtcher R. C. , Richardson J. W. , Mueller M. H.
Published: April 2011

We compare damage evolution in U3Si produced by neutron irradiation at 30°C and 350°C and measured by neutron diffraction. Initial studies found that increasing neutron irradiation dose at 30°C results in monotonic expansion of the a-axis and contraction of the c-axis that transforms the crystal structure from tetragonal to cubic [1]. Additional irradiation results in amorphization. Neutron irradiation at 350°C results in little change to the a-axis and expansion of the caxis.The complete alteration in lattice dilatation during irradiation is interpreted as due to modification of surviving defect configurations at the higher temperature. The high temperature lattice dilations can be explained by defect loop formation in the a-b plane. Confinement of lattice strain to the c-axis during irradiation at 350°C may be the mechanism that prevents the total lattice dilatation from exceeding the critical level required for amorphization.
Effect of Irradiation Spectrum on the Microstructure of Ion-Irradiated A120O3
Authors: Zinkle S. J.
Published: April 2011

Polycrystalline samples of alpha-alumina have been irradiated with various ions ranging from 3.6 MeV Fe+ to 1 MeV H+ ions at 650°C. Cross-section transmission electron microscopy was used to investigate the depth-dependent microstructure of the irradiated specimens. The microstructure following irradiation was observed to be dependent on the irradiation spectrum. In particular, defect cluster nucleation was effectively suppressed in specimens irradiated with light ions such as I MeV H+ ions. On the other hand, light ion irradiation tended to accelerate the growth rate of dislocation loops. The microstructural observations are discussed in terms of ionization enhanced diffusion processes.
Quantitative Hrem of Electron-Irradiated Quartz
Authors: Qin L.C. , Hobbs L.W.
Published: April 2011

The progress of heterogeneous amorphization of electron-irradiated quartz was studied in situ by high-resolution electron microscopy (HREM). The transformation front appears surprisingly sharp when viewed close to edge-on, but the apparent morphological evolution when aperiodic and periodic material overlap in projection is one in which gradual changes in image regularity are observed. Electron-microscopical contrast obtained in HREM irradiations was interpreted quantitatively, through the use of image contrast simulations. When the amorphized portion occupies less than about two-thirds of the total thickness, the HREM images continue to appear crystalline. Only when the amorphized thickness fraction exceeds about two-thirds does the resultant image contrast show mainly non-crystalline features.
Evidence for Electronic Energy Loss Processes Stimulating Solid Phase Epitaxial Regrowth of Spatially Isolated Amorphous Regions in Semiconductor Systems
Authors: Jencic I. , Bench M. W. , Robertson I. M. , Kirk M. A.
Published: April 2011

Solid phase epitaxial regrowth of spatially isolated amorphous regions in Si, Ge and GaP has been stimulated by using an electron beam with energies in the range of 50 to 300 keV. In all materials, the rate at which the amorphous zones disappear decreases as the energy of the electron beam increases from 50 keV reaching a minimum below the threshold displacement voltage before it again increases with increasing electron energy. The experimental results are interpreted in terms of creation and motion of defects (dangling bonds, charged defects) along the amorphouscrystalline interface.
Preparation of Thin Film Phthalocyanine Glasses for Optical Limiting
Authors: George R. D. , Snow A. W. , Shirk J. S. , Flom S. R. , Pong R. G. S.
Published: April 2011

To explore the use of phthalocyanine materials as a solid-state component in an optical limiter, the issues of morphology, film processing and optical limiter activity/stability are investigated. Metal-free and lead phthalocyanine films with glassy morphologies are obtained by chemically functionalizing the ring periphery with an amine-epoxy substitution. Uniform films were obtained by a shearing technique. Comparative optical measurements are made on the amine-epoxy lead phthalocyanine compound as a concentrated solution and as a glassy thin film. Both the solution and film are reverse saturable absorbers at 590 nm. The concentrated solution is a very effective optical limiter. The glassy thin film is susceptible to damage (cavitation caused by the incident beam) and performs comparably only at low incident energies. A liquid matrix may have the benefit of self-healing such damage.
Optical Bandwidth Determinants for Excited State Absorptive Limiters
Authors: Miles Perry A.
Published: April 2011

The performance of optical limiters depends on the ratio of the absorptive cross section for the transition from an excited state, σe, to that for transition from the ground state, σg. Both these factors depend on wavelength. It is appropriate to define two bandwidths, one for high energy pulse suppression, depending on σe, and the other for linear transmission, depending on σg. These bandwidths are affected differently by the use of molecular mixtures.
Local and Long-Range Hydrogen Motion in a-Si:H
Authors: Hari P. , Taylor P. C. , Street R. A.
Published: April 2011

Nuclear magnetic resonance (NMR) measurements of 1H dipolar echoes in a-Si:H indicate that the dipolar spin-lattice relaxation time, T1D, can be used as a measure of local hydrogen motion. In general, the microscopic hydrogen motion is orders of magnitude faster, and the microscopic activation energies are much smaller, than those measured macroscopically using secondary ion mass spectroscopy (SIMS). Although no detailed model exists to explain these apparently divergent results, these discrepancies can be understood phenomenologically by assuming a very broad distribution of rates. In this case the measured activation energies can be shown to scale roughly as the logarithm of the time over which the measurement is made.
Photoelectric-Yield Studies of c-Si/a-Si:H Interfaces
Authors: Sebastiani M. , Di Gaspare L. , Bittencourt C. , Evangelisti F.
Published: April 2011

We report the first yield spectroscopy study on well characterized c-Si/a-Si:H heterojunctions grown in situ under UHV conditions. We find that this spectroscopy, when operated in the constant final state mode, allows a direct and precise determination of the valence-band discontinuity at the interface. A value of δEv = 0.44 ± 0.02 eV was found for the discontinuity.
Bonding of Hydrogen and Deuterium in Silicon Nitride Films Prepared by Remote Plasma Enhanced Chemical Vapor Deposition
Authors: Stevens G. , Santos-Filho P. , Habermehl S. , Lucovsky G.
Published: April 2011

We have deposited Si-nitride thin films by remote plasma-enhanced chemical-vapor deposition using different combinations of hydrogen and deuterium source gases. In one set of experiments, NH3 and SiH4 were injected downstream from a He plasma and the ratio of NH3 to SiH4 was adjusted so that deposited films contained IR-detectable bonded-H in SiN-H arrangements, but not in Si-H arrangements. Similar results were obtained using the same ND3 to SiD4 flow ratio; these films contained only SiN-D groups. However, films prepared from ND3 and SiH4 displayed both SiN-D and SiN-H groups in essentially equal concentrations establishing that H and D atoms bonded to N are derived from both source gases SiH (D) 4 and NH (D) 3, and further that inter-mixing of H and/or D atoms occurs at the growth surface. This reaction pathway is supported by additional studies in which films were grown from SD4 and ND3 with either i) He or ii) He/H2 mixtures being plasma excited. The films grown from the deuterated source gases without H2, displayed only SiN-D bands, whereas the films grown using the He/H2 mixture displayed both SiN-H and SiN-D bands. The total concentration of N-H and N-D bonds in the films grown from the He/H2 excitation was the same as the concentration of N-D, supporting the surface reaction model. In-situ mass spectrometry provides additional insights in the film deposition reactions.
Alternative Mechanism for Defect Generation in a-Si:H
Authors: Redfield David
Published: April 2011

Recombination-driven mechanisms for defect formation do not account well for several experimental facts, and indeed conflict with some facts. To overcome these problems, an alternative mechanism is proposed in which latent defect centers are converted to metastable defects (dangling bonds) without a recombination event. The transitions are driven simply by electron capture at the center with no energy barrier. A new configuration-coordinate diagram expressing this mechanism includes essential configuration-induced changes in the energies of band edges, and accounts for several other previously problematic observations. This carrier-induced mechanism (CIM) is consistent with defect formation in forward bias and defect inhibition in reverse bias, light-enhanced annealing, as well as the usual light-induced and beam-induced formation. For low orħħωthe process starts by ionization of an existing defect.
Origin and Incorporation Mechanism for Oxygen Contaminants in a-Si:H and μc-Si:H Films Prepared by the Very High Frequency (70 MHz) Glow Discharge Technique
Authors: Kroll U. , Meier J. , Keppner H. , Littlewood S. D. , Kelly I. E. , Giannoulès P. , Shah A.
Published: April 2011

The effect of plasma power, reactor outgassing rate and feed gas purity on the oxygen content of amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) layers, prepared by the VHF (70 MHz) glow discharge technique has been investigated. The oxygen concentration in the films was measured by Secondary Ion Mass Spectrometry. The gas purity could be optionally enhanced by the application of a getter-based gas purifier mounted in the gas pipe just before the reactor. The purifier reduces the oxygen contaminants in the feed gas to levels in the ppb range. It was found that oxygen incorporation was enhanced at lower deposition rates. The deposition rate dependence of the oxygen incorporation is in excellent agreement with the proposed incorporation mechanism. Furthermore, the reactor outgassing rate and the purity of the feed gas itself were identified as the main contamination sources for the oxygen contaminants in the deposited a-Si:H films as well as in μc-Si:H films : e.g. at outgassing rates in the range of 10−6 mbar 1 s−1, at least around half of the oxygen detected in both a-Si:H and in the μc-Si:H layers originates from the feed gas if the latter is unpurified. By combining reduced outgassing rates and the use of purified feed gas, a-Si:H and μc-Si:H films with an oxygen concentration of 5×1017cm−3 and 2×1018cm−3, respectively, were deposited. These values both represent the lowest concentrations of oxygen contaminants reported to date for these kinds of materials.
Correlation Between Phototransport and Network Order in a-Si:H
Authors: Morell G. , Katiyar R. S. , Weisz S. Z. , Jia H. , Shinar J. , Balberg I.
Published: April 2011

Hydrogenated amorphous silicon (a-Si:H) films prepared by the glow discharge (GD) technique show superior optoelectronic properties over those prepared by rf sputtering (RFS). To find out whether this is associated to structural differences in the amorphous network, we have carried out a comprehensive comparison of the Raman spectra of the two types of films grown at different substrate temperatures. The use of two properly chosen excitation radiations allowed the observation of the Raman spectra from the near surface versus that from the bulk of the films. The results show that the short-range order in the bulk of GD films is close to that of the ideal tetrahedral network, having an rms bond angle deviation (Δθ) of ≈9°. In contrast, the smallest value of Δθfound in the RFS films was ≈15°. There is also a short-range order inhomogeneity in both sets of materials that can be reduced significantly by selecting the appropriate substrate temperature. The intermediate-range disorder is relatively small and uniform in GD films, while large differences exist in this parameter between the surface and bulk of RFS films. In general, the results indicate that the short-range order and the inhomogeneity in intermediate-range order present in the RFS films cannot be improved to equal those of GD materials by annealing at temperatures low enough that no substantial hydrogen effusion occurs. These structural differences are argued to be the reasons for the superior phototransport properties of GD over RFS materials and are interpreted in terms of the differences between the two deposition processes.
Numerical Modelling of Time-of-Flight in SCLC-Mode
Authors: Brüggemann Rudolf , Bauer Gottfried H.
Published: April 2011

The application of time-of-flight (TOF) in the space charge limited current (SCLC) mode to a-Si:H pin-diodes has led to the discovery of new features in the current transients. An alternative to analytic models, our numerical modelling describes the specific nature of amorphous semiconductors by taking into account the interaction of free carriers with tail states and the contribution of trapped carriers to the space charge. Typical features of experimental SCLC-TOF-currents such as a cusp in the transient can be reproduced. We confirm the concept of ‘effective thickness’ for the determination of the transit time. At high injection intensity only a small fraction of the charge can be collected. Bimolecular recombination via tail states and surface recombination is identified as a loss mechanism that competes with recombination via dangling bonds. The post-transit behaviour of electron-TOF is dominated by holes emitted from the valence band (vb) -tail. The density of states distribution from post-transit-spectroscopy (PTS) shows features of the vb-tail. Finally, hole SCLC-TOF shows a different behaviour from electron SCLC-TOF.
Defect Properties of Cathode Deposited Glow Discharge Amorphous Silicon Germanium Alloys
Authors: Zhong Fan , Chen Chih-Chiang , David Cohen J. , Wickboldt Paul , Paul William
Published: April 2011

We have characterized the deep defect densities and their energy distributions for a series of a-Si1-xGex:H alloys with large Ge content (0.57< × < 1.00) prepared by the cathode deposited glow discharge method. Our results indicate markedly superior properties for these samples. A small Urbach tail slope (about 45meV) was found for all samples in this alloy range. The defect densities were either obtained directly from the drive-level capacitance profiling or deduced from the sub-band-gap optical spectra. Both are substantially lower than the trend line determined from previous studies of a-Si1-xGex:H samples produced by conventional glow discharge and by Photo-CVD methods. However, the relation between the total defect densities and the optical spectra in the cathodic samples obeys the same defect formation model that has been used to successfully predict the defect densities in other types of a-Si1-xGex:H material.
Wide GAP a-SiC:H Alloys for Novel Photovoltaic-Electrochromic Window Coatings
Authors: Bullock John N. , Xu Yueqin , Benson David , Branz Howard M.
Published: April 2011

Self-powered “smart” windows utilize an electro-optic transmittance modulator based on electrochromic (EC) thin films that exhibit reversible and controlled changes in optical properties with an applied voltage between 0.7 and 2.0 V. Existing window designs require an external electrical connection, which may be economically unfeasible. This problem is solved by the tandem photovoltaic-electrochromic device, in which a wide-gap amorphous silicon-based alloy photovoltaic device is deposited together with an electrochromic optical transmittance modulator in a monolithic device on a single substrate. In this paper, we discuss our proposed monolithic photovoltaic-electrochromic device.

We also present studies of transparent, wide-gap (1.8 to 2.2 eV) amorphous silicon-carbon thin films and p-i-n devices designed for use in the photovoltaic-electrochromic device. These photovoltaic cells can operate at low current (<1 mA/cm2) because a total injected charge of only 60 μC/cm2 will darken the EC layer to a visible transmission of 5%, but they need a high open-circuit voltage (>1.0 V) and high transparency (≈70%). We present our progress toward these design targetxxss.
Deposition of Polysilicon Films by Hot-Wire CVD at Low Temperatures for Photovoltaic Applications
Authors: Puigdollers J. , Bertomeu J. , Cifre J. , Andreu J. , Delgado J. C.
Published: April 2011

Polysilicon (poly-Si) thin films have been obtained using hot-wire chemical vapor deposition (HWCVD) from silane-hydrogen mixtures. The films were prepared at low substrate temperatures (down to 200°C) and at very high deposition rates (up to 40 Å/s). They showed good crystalline properties and no amorphous phases were detected. The films can also be efficiently doped by adding diborane or phosphine to gas phase. In this paper, an overview of the properties of the poly-Si films, intrinsic and p and n-doped, deposited at our laboratory by HWCVD is presented and discussed. The properties of the material and the features of the deposition technique which are interesting for their application in photovoltaics are emphasized.
Application of the Defect Pool Model in Modelling of a-Si:H Solar Cells
Authors: Zeman Miroslav , Tao Guoqiao , Trijssenaar Marcel , Willemen Joost , Metselaar Wim , Schropp Ruud
Published: April 2011

The increasing complexity of hydrogenated amorphous silicon (a-Si:H) based solar cells requires continuous extending and testing of the computer models which are used for their simulation. The calculation of the defect states density in the bandgap of a-Si:H based on the defect pool model (DPM) has been rally implemented in the ASA (Amorphous Semiconductor Analysis) computer program developed at Delft University of Technology. We used the technique of inverse modelling to verify the DPM and to calibrate it by fitting the dark and illuminated J-V characteristics of a single-junction a-Si:H solar cell fabricated at Utrecht University. The DPM was also used in simulating the absorption coefficient of a-Si:H layers. Using the DPM for the defect states distribution, a good agreement between simulated and measured data was obtained. The values of the material parameters needed for obtaining the best fits are more realistic than using a conventional model for the defect states distribution.