bluuebunny/crossref_metadata_2025 huggingface dataset:
Fracture Toughness, Fracture Planes and BDT in Stoichiometric Feal and Nial Single Crystals
Authors: Specht Petra , Brede Markus , Neumann Peter
Published: April 2011

Fracture toughness data of stoichiometric FeAl and NiAl single crystals were measured in four-point-bending tests and the crystallographic orientations of the resultant fracture surfaces were determined.

For FeAl single crystals unexpectedly high fracture toughness values were measured even at low temperatures (17 MPa [001] at 77K). However, these crystals failed in a brittle manner for temperatures up to 300K. The crystallographic orientation of the fracture surfaces were strongly dependent on the environment due to hydrogen embrittlement.

The NiAl specimens were precracked and the fracture toughness was measured in the weakest crystal orientation. The fracture surfaces were similar to those obtained in previous investigations. Although the room temperature (RT) KIC-value was the highest for pure weak-oriented NiAl (7.5 MPa [001]) the BDT temperature was found in the same region where it occurs for the hard orientation (673K to 693K). The onset of the BDT will be discussed.
Rapid Solidification Joining of Intermetallics using Capacitor Discharge Welding
Authors: Wilson R. D. , Alman D. E. , Hawk J. A.
Published: April 2011

Capacitor Discharge Welding (CDW) is a rapid solidification joining process capable of cooling rates greater than 106 K/s. The Bureau of Mines is investigating the CDW process as a method of joining TiAl, Fe3A1 and MoSi2. Experimental results show that the fusion zone of the CDW welds is less than 0.1 mm wide, is uniform in composition, and has a cellular solidification microstructure. This paper compares the CDW microstructure of several intermetallics to the microstructures obtained from the gas tungsten arc welding.
Influence of Ni5Zr Particles on Fatigue Property of Ni3Al Alloy
Authors: Li Gang , Guo Jian-Ting , Wang Zhong-Guang
Published: April 2011

In this investigation, the influence of second phase particles on high cycle fatigue behavior of Ni3Al alloy is studied. A single phase Ni3Al-B alloy and a Ni3Al-B/Zr alloy with a few second phase particles (Ni5Zr) at the grain boundaries are selected for investigation. High cycle fatigue tests at room temperature with R (minimum stress/maximum stress) 0.1 are conducted in air and at 30 Hz. The results show that the second phase particles are detrimental to high cycle fatigue resistance. It may be explained in terms of the second phase particles promoting fatigue crack initiation. The characteristics of fracture surfaces are examined by Scanning Electron Microscopy (SEM).
Fracture Toughness of Multiphase Intermetallics
Authors: Chan Kwai S.
Published: April 2011

An overview of recent advances in ductile-phase toughening of multiphase intermetallics is presented. The fracture characteristics of aluminides, silicides, chromides, and their composites are reviewed to elucidate relationships between microstructure, toughening mechanism, and fracture resistance. The roles of crack-tip blunting, trapping, bridging, and interface delamination in instigating fracture resistance are considered. Prospects for improving fracture toughness are discussed in conjunction with relevant microstructural variables such as the volume fraction, size, morphology, constraint, and work-of-fracture of the ductile phase.
High Temperature Creep of Alpha-2 Ti-34mo1%Al Polycrystals
Authors: Oikawa Hiroshi , Fukuda Toshihiko , Ohtsuka Makoto
Published: April 2011

Constant-stress compressive creep tests were carried out on an Al-rich a2 single-phase material, which had equiaxed-grains of 60μim in grain size, at 1050∼1250 K under 100∼500MPa. The type of the primary creep stage and the microstructures developed during creep depend greatly on the creep condition. The minimum creep-rate, however, can be represented by one set of parameters over the whole range of experimental condition. The stress exponent is 5.0±0.2 and the (modulus-compensated) activation energy is 360 ± 10kJ/mol. The Dorn-type plot of the minimum creep rate reveals that the normalized creep strength of fine-grained Ti-34mol%Al is not greatly different from that of disordered solid-solution hardened alloys.
The Strain Rate Sensitivity of Polycrystalline and Single Crystal NiAl
Authors: Lee Tsang-Sheau , Hon Wei-Pirn , Yang Sy-Cherng , Hsu Shu-En , Chin Stephen , Anton Donald L.
Published: April 2011

The compressive and tensile strain rate sensitivities of polycrystalline and single crystal NiAl have beem evaluated at 877°C which is well above the static DBTT. Samples were prepared to specific grain sizes by hot press consolidation of appropriate powder sizes and minimizing post-consolidation thermal exposures. NiAl single crystals were grown in the near [100] orientation using a modified Bridgeman technique. The use of high oxygen content powders to fabricate polycrystalline NiAl with very fine grain sizes resulted in increased strengths but did not debit ductility. The yield and tensile strength of polycrystalline NiAl as a function of grain size generally follows a Hall-Petch type relationship. A tensile strain rate effect was found at strain rates of 1 to 10−4 sec−1. The strain rate sensitivity exponent, m, in the equation σ = Cεm is in the range 0.10 to 0.13. The highest strain rate (1 sec−1) resulted in significantly higher strengths and little or no observed tensile ductility. Cleavage-type transgranular fractures were found on samples tested at the highest strain rate (1 sec−1) and ductile fractures were found on samples tested at the lower strain rates (10−1 and 10−4 sec−1). Compression testing at strain rates of 10−5 to 10−3 sec−1 also indicated an effect of strain rate on strength.
The Temperature and Strain Rate Dependence of the Flow Stress of Single Crystal Nial Deformed Along <110>
Authors: Maloy Stuart A , Gray George T
Published: April 2011

Single crystal NiAl and Ni-49.75Al-0.25Fe have been deformed along <110> at temperatures of 77, 298 and 773K and strain rates of 0.001/s, 0.1/s and 2000/s. The flow stress of <110> NiAl is rate and temperature sensitive. A significant decrease in the work hardening rate is observed after deformation at 77K and a strain rate of 2000/s. Coarse {110} slip traces are observed after deformation at a strain rate of 2000/s at 77K, while no slip traces were observed after deformation under all other conditions. TEM observations reveal distinct {110} slip bands after deformation at 77K and a strain rate of 2000/s.
Thermally Activated Processes and Dislocation Mobilities in Two-Phase γ-Titanium Aluminides
Authors: Appel Fritz , Sparka Ulf , Wagner Richard
Published: April 2011

The processes controlling the dislocation mobility in two-phase γ-titanium aluminides have been investigated over a wide temperature range by determining the activation volumes and activation energies of thermally activated dislocation glide processes. The deformation tests are supplemented by electron microscope observations. Accordingly, at room temperature the mobility of ordinary dislocations is determined by a combination of localized pinning and lattice friction. Additional glide resistance arises from dislocation dipoles and debris defects, which are trailed and terminated at jogs in 1/2 <110] screw dislocations. Dislocation climb processes start above 900 K and seem to initiate the transition from brittle to ductile material behaviour.
What Determines the Peak-Temperature in Binary Ni3Al?
Authors: Karnthaler C. Rentenberger , Karnthaler H.P.
Published: April 2011

In Ni3Al alloys the yield stress increases with increasing temperature up to a maximum stress, reached at the temperature TP. Below TP glide occurs on primary octahedral planes whereas above TP glide on primary cube planes is dominating. Therefore this change of glide planes was considered to determine TP.

TEM investigations were carried out to study the mechanisms that control TP in Ni3Al specimens with [001] compression axis. In this case no shear stresses are acting on any of the cube glide systems. The TEM results show a pronounced change of the dislocation structure on the {111} glide planes below and above the peak. The breakdown of the barriers (locked screw dipoles) starts when the diffusion is high enough to facilitate their annihilation. The dynamic recovery of the locked screws occurs by multiple cross-slip via octahedral planes. It is proposed that, in this and other ordered alloys, the value of TP is diffusion controlled.
Activation Volumes in the Yield Strength Anomaly Domain of Ni3(Al,Ta).
Authors: Spätig P. , Bonneville J. , Martin J.-L.
Published: April 2011

Ni3(Al,Ta) single crystals have been deformed in compression in the temperature range of the flow stress anomaly (293–780K). The strain-rate sensitivity (SRS) of the flow stress has been characterised by using a technique of repeated stress relaxations that allows for the measurement of the true (or effective) activation volume (Veff). When measured at the conventional critical resolved shear stress (CRSS), Veff exhibits as a function of temperature a sharp discontinuity close to 470K. When the temperature is held constant (420K), the discontinuity of Veff occurs along the stress-strain curve at approximately 3% strain; the stress for both discontinuities is approximately the same. These results suggest a change in the rate controlling mechanism that is dependent on stress as much or more than temperature.
The Yield Stress Anomaly in Fe-Al Alloys: The Role of the Local Climb Locking Mechanism
Authors: Morris David G. , Peguiron David
Published: April 2011

A detailed examination of the variation of yield stress with temperature in an Fe3Al alloy shows a maximum at a temperature of about 500°C, slightly below the critical temperature for loss of DO3 order. At this temperature the dislocations present in the material change from being <111= superdislocations separated by APB to being single dislocations with Burgers vector <100=. At slightly lower temperatures the superdislocations become pinned by a local climb process involving point defect transfer between the partial dislocations.

Analysis of the forces between the dislocations which induce the local climb locking process allows an estimation of the role which will be played by variations in composition of the Fe-Al alloy considered, changes in deformation rate and orientation of the applied stress.

Examination of data available in the literature shows that each of the three aspects discussed, namely the influence of variations in ordered state, in Al content over the range 25% to 50%, or additions of alloying elements such as Si, straining at very fast or at slow rates, and stressing along different crystallographic axes, is completely consistent with the model proposed.
Near-Transus Processing (NTP) of Powder Metallurgy Tial-Based Alloys
Authors: Fuchs G. E.
Published: April 2011

Alloys based on the intermetallic compound γ-TiAl are rapidly being developed for elevated temperature structural applications. In order to produce an alloy, by either I/M or P/M techniques, with useful engineering properties, it is necessary to control the microstructure by heat treatment and/or hot working. By combining consolidation with the heat treatment, unique microstructures can be produced in powder metallurgy (P/M) materials. The effect of processing on the microstructures and tensile and creep properties are discussed.
A Preliminary Investigation of the Cr3Si-Mo Pseudo-Binary Phase Diagram
Authors: Dickerson R. M. , Raj S. V. , Locci I.E.
Published: April 2011

An investigation was undertaken to study the phase relations in Cr3Si alloyed with Mo varying from 10 to 83.5 wt. % of the material. Specimens were prepared from arc-melted buttons that were subsequently heat treated at 1673 K for 200 h and air quenched to room temperature to preserve the high temperature microstructures. Alloys containing more than 20 wt. % Mo were primarily two-phase materials of M3Si and M5Si3, where M is (Cr,Mo). Three alloys contained less than 5 % of a third phase, which also had the M5Si3 crystal structure. Differential thermal analysis (DTA) was performed on several specimens at temperatures up to 2073 K in order to determine a solidus curve for the M3Si phase. Since only one DTA peak was observed in each alloy, the M5Si3 phase must melt above 2073 K, the maximum temperature examined. A preliminary pseudo-binary phase diagram for (Cr,Mo)3Si and a portion of the 1673 K isothermal section of the Cr-Mo-Si ternary phase diagram are presented.
Prediction of the Fiber-Matrix Interface Failure due to Longitudinal Tensile Loading Using Finite Element Methods
Authors: Mahfuz Hassan , Mian A.K.M. Ahsan , Vaidya Uday K. , Brown Timothy , Jeelani Shaik
Published: April 2011

A 3D-unit cell for 0/90 laminated composites has been developed to predict the composite behavior under longitudinal tensile loading condition. 3D contact element has been used to model the fiber matrix interface. Two interface conditions, namely, infinitely strong and weakly bonded, are considered in the analysis. Both large displacement and plastic strain behavior for the matrix are considered to account for the geometric and material non-linearities. Investigations were carried out at three temperatures to compare the composite response obtained from mechanical tests at those temperatures. Stress-strain behavior and the local stress distributions at the fiber as well as at the matrix are presented, and their effects on the failure of the interface are discussed in the paper. The material under investigation was SiCf/Si3N4.
Evolution of Internal Stresses in Composites during Creep
Authors: Lara-Curzio Edgar , Ferber M. K.
Published: April 2011

The redistribution of internal stresses in a composite with linear viscoelastic constituents was calculated when the composite is subjected to a constant stress at a temperature where the phases would exhibit time-dependent deformation. It was found for the case of an elastic fiber embedded in a matrix that behaves as a Burgers material under distortion and elastically under dilation, that the normal interfacial stress and the axial stress in the matrix undergo complete relaxation at long times. The implications of these findings are discussed in relation to the behavior of ceramic matrix composites.
Finite Element Analysis of the Fiber Twist Test
Authors: Gaudette F. , Ertürk T. , Robertson S.
Published: April 2011

Interface and torsional shear stresses in the fiber twist test (FTT) were computed using the ABAQUS finite element program. Interface stress singularities were compared with an elasticity solution for the torsion of a fiber embedded in an elastic half space [1]. Single fiber composite systems having perfectly bonded interfaces and fiber/matrix shear modulus ratios of Gf /Gm = 1 – 3 were considered. The decay rates and depths of the interface shear stress σrθ and the torsional shear stress σZθ in the fiber and matrix were evaluated for each Gf/Gm ratio.
Alpha-Si3N4 Precipitation in C/C-SiC Composites from inherent Fiber Impurity Sources
Authors: Pleger Roland , Braue Wolfgang
Published: April 2011

The formation of crystalline α-Si3N4 filaments grown inside the pore channels of HT-carbon fibers from a 2D C/C-SiC composite is investigated by transmission electron microscopy. Precipitation of α-Si3N4 is promoted by a low carbonization heat treatment of the C/C material prior to liquid silicon infiltration and results from the interaction of a highly reactive nitrogen-rich vapor phase released from the fiber and silicon vapor diffusing ahead of the SiC reaction front into the porous microtexture of the fiber.
Temperature and Strain-Rate Effects on Deformation Mechanisms in Irradiated Stainless Steel
Authors: Brimhall J. L. , Cole J. I. , Vetrano J. S. , Bruemmer S. M.
Published: April 2011

Analysis of the deformation microstructures in ion-irradiated stainless steel shows twinning to be the predominant deformation mode at room temperature. Dislocation channelling also occurs under slow strain rate conditions. Stresses required for twinning were calculated by the model of Venables and are compatible with observed yield stresses in neutron-irradiated material if loops are the principal twin source. Computation of the expected radiation hardening from the defect structure, based on a simple model, is consistent with yield strengths measured on neutron-irradiated steels. Lower yield stresses and greater thermal energy at 288°C lessen the probability of twinning and dislocation channeling becomes the primary deformation mode at the higher temperature. However, preliminary results show that some twinning does occur in the irradiated stainless steel even at the higher temperature when higher strain rates are used.
Effect of Prior-Deformation on the Stability of the Intermetallic Precipitate in Zircaloy-2
Authors: Huang Peter Y , Adamson R.B.
Published: April 2011

Both stress-relieved (SR) and recrystallized (RX) samples irradiated near 300°C tofluences between 1 and 9 x 1021 n/;cm2 (E>IMeV) were examined using analytical scanning transmission electron microscopy (STEM). The aim was to extend our knowledge of irradiation effects on microstructure by examining the effect of prior-cold-work on the dissolution of intermetallic precipitates in Zircaloy-2. Resulting from prior mechanical deformation and fast neutron irradiation, SR samples contain a high density of -component, mixed  line dislocations and “black dot” damage. On the other hand, RX samples contain mostly “black dot” damage. Pure -dislocations are detected in the high fluence samples in RX materials. For identical irradiation conditions, different degrees of amorphization and dissolution are observed in RX and SR samples. Also, preferential diffusion of solute is observed to occur along -dislocations. These results are discussed in terms of possible interactions between irradiation produced defects, precipitates and solutes.
The Effect of Displacement Cascades on Small Helium Bubbles in Aluminum and Gold
Authors: Donnelly S E. , Birtcher R.C. , Templier C. , Valizadeh R. , Vishnyakov V.
Published: April 2011

The evolution of individual helium bubbles in thin foils of gold and aluminum irradiated with 400 keV Ar+ and 200 keV Xe+ has been followed with in-situ transmission electron microscopy for a comparison between the effects of dilute (Al) and dense (Au) collision cascades. Bubble shrinkage in Al has been attributed to direct displacement of the gas out of the bubbles. Effects in Au, include the disappearance and Brownian motion of bubbles under irradiation, and are consistent with thermal spike processes seen in molecular dynamics simulations.
Tem Investigation of Hydrogen Implanted and Laser Annealed Silicon
Authors: Zheng P. , Saint-Jacques R.G. , Terreault B. , Veilleux G.
Published: April 2011

In order to explain the relatively easy laser-induced desorption of hydrogen implanted in silicon, and particularly the lower temperature needed for desorption at higher implantation energy, the microstructural modifications produced by laser pulses were studied by means of transmission electron microscopy. The structural damage, such as defect clusters and hydrogen gas bubbles was observed. In the case of low dose implantation (H/Si ≤ 15&), most of the bubbles were produced during laser annealing rather than during implantation. This bubble formation in the course of desorption explains the higher temperature needed. When blisters are already present on the as-implanted surface, desorption starts at a lower temperature.
Control of Point Defects in Semiconductors
Authors: Loginov Yuri Y. , Brown Paul D. , Humphreys Colin J.
Published: April 2011

The prospects for localised point defect control within II-VI compound semiconductors are considered with reference to doping, thermal annealing, electron and ion beam irradiation and localised strain. Interstitials and vacancies generated within electron beam irradiated II-VI compounds interact and coalesce in a TEM foil to form voids decorated by metallic precipitates and the processes of CdTe chemical decomposition are examined. Ion beam and electron beam irradiated material show clear differences in behaviour following annealing of doped and un-doped CdTe. Controlled directional climb of dislocations within electron beam irradiated ZnSe is demonstrated. In view of the differential diffusion of interstitials and vacancies, the prospect of localised semiconductor type conversion is considered.
Thin Film Thermochromic Materials for Non-Linear Optical Devices.
Authors: Lewis Keith L , Pitt A M , Wyatt-Davies T , Milward J R
Published: April 2011

A number of materials are available which exhibit thermochromic properties. These include the superionic materials (eg silver sulphide), phase transition materials (eg vanadium dioxide) and mixed valence materials (eg SmS). Such materials can be exploited as optical thin films and when incorporated into multilayer structures exhibit non-linear properties, particularly at wavelengths in the infra-red (1–12μm). The response of such devices is dependant on the degree of temperature rise produced in the active material, which in turn is dependant on the optical properties of the material and the heat sinking characteristics of the substrate. A summary is presented highlighting techniques for film deposition, multilayer device design and temporal response characteristics measured under conditions of pulsed irradiation.
Photorefractive Optical Limiters
Authors: Salamo Gregory J. , Duree Galen C. , Morin Matthew , Sharp Edward J. , Wood Gary L. , Neurgaonkar Ratnakar R.
Published: April 2011

We consider the application of photorefractive beam fanning as an optical limiter. Results indicate that by focusing the incident laser light into the photorefractive crystal and by applying electric fields that limiting is possible at the 10μJ range with an optical density or O. D. of 3. These results are independent of the incident optical pulse width.
New Self-Defocusing Materials: Synthesis, Structures and Nonlinear Optical Properties of Nest Shaped Clusters
Authors: Shi S. , Xin X. Q. , Ji W. , Xie W.
Published: April 2011

Mixed metal clusters (n-Bu4N)2[MoCu3OS3X3] (X = SCN, Cl and Br) were synthesized by solid state reactions. X-ray single crystal diffraction data show that the anionic clusters assume nest shaped structures. The clusters exhibit large self-defocusing effect as demonstrated by their Z-scan traces. The effective third-order polarizability of |γ| = 4.8 × 10−29 esu was determined for (n-Bu4N)2[MoCu3OS3(SCN)3] by a degenerate four-wave mixing technique with 7-ns laser pulses of 532 nm wavelength. An irradiance-dependent transmission measurement was conducted to assess the contribution of nonlinear absorption to the γ-value. These results in combination with the self-defocusing property of the cluster yielded a negative value for the real part of γ, Reγ = − 4.8 × 10−29 esu. These numerical values are in good agreement with those obtained in a Z-scan experiment. Even though the clusters absorb laser light at 532 nm significantly, the results of temporal profile analysis on transmitted pulses show that the solvent thermal effect makes little contribution to the observed nonlinear optical phenomenon.
A New Role for Bboron in Advanced Materials
Authors: Kaszynski Piotr , Lipiak Dariusz
Published: April 2011

Application of highly polar dative bonds of heteroatoms to boron atoms for the preparation of a new class of liquid crystals is discussed. Semiempirical calculations show that compounds incorporating polar bonds of tetra- and higher-coordinated boron centers are characterized by large coaxial dipole moments and relatively large first hyperpolarizabilities β. It is expected that these new liquid crystalline materials will be useful for electrooptical applications.
New Optical Limiting Materials: Synthesis, Structures and Nonlinear Absorption of Cubic Cage Shaped Clusters
Authors: Shi S. , Ji W. , Xin X. Q.
Published: April 2011

Mixed metal clusters (n-Bu4N)3[WCu3Br4S4] (I), (n-Bu4N)3[Wag3Br4S4] (II), (n-Bu4N)3[MoAg3BrCl3S4] (III) and (n-Bu4N)3[MoAg3BrI3S4] (IV) were synthesized by solid state reactions. The cluster anions assume cubic cage shaped structures and possess strong optical limiting capability. At very low fluences the molecules respond linearly to the incident light obeying Beer's law. As light fluence rises their molecular absorptivities increase rapidly exhibiting limiting effect with threshold and saturation fluence of 1.3 J/cm2 and 0.7 J/cm2 for compound I, 0.7 J/cm2 and 0.5 J/cm2 for compound II, 0.6 J/cm2 and 0.3 J/cm2 for compound III, and 0.5 J/cm2 and 0.3 J/cm2 for compound IV respectively. The threshold and saturation values of compound IV are about 3 and 2 times better than those of C60 measured under identical conditions. The optical limiting capability of the clusters is derived from excited state absorption, a process that turns on within nanoseconds and off over milliseconds.
On Fourier's Phase and Gradient Index Optical Design
Authors: Haaland Peter D.
Published: April 2011

Gradient index or rugate optical filters are continuous generalizations of the familiar quarterwave stack which find widespread applications in sensor hardening. In the design of a filter or mirror there are infinitely many refractive index profiles n(x) whose wavelength-dependent reflectance R(λ) is specified as a design objective. The relationship between R(λ) and n(x) is nearly that of a Fourier-transform pair. Exploiting this relationship, specifically the physically indeterminate phase of the Fourier transforms, permits constrained optimization of rugate designs. In this contribution we outline the approach of optimal phase modulation and indicate by examples its application to problems in optical limiting.
Study of the Microstructure of Amorphous Germanium Alloys Using AFM and SAXS
Authors: Moffitt Paul R. , Naseem Hameed A. , Ang Simon S. , Brown William D.
Published: April 2011

Many researchers have found the photoresponse of germanium-based films to be directly related to the homogeneity of the film's microstructure, with more homogeneous films having better photoresponse. Transmission electron microscopy (TEM) gives the resolution needed for studying microstructure, but is typically limited to imaging films with a thickness of less than 200 nm. Surface measurement by atomic force microscopy (AFM) gives a three-dimensional representation of the film surface and can be used for measuring any thickness film. A more complete idea of the microstructure can be gained by combining AFM with small-angle x-ray scattering (SAXS) measurements. SAXS measurements yield the size of internal voids or less dense regions and indicate whether the inhomogeneities have a preferred orientation, such as might be associated with columnar growth. Films of amorphous germanium and germanium carbon alloy were grown by the PECVD method from SiH4, GeF4, H2 and CH4 source gases. The films were then characterized using AFM, SAXS, and FTIR. The effect that various growth conditions had on the microstructure and photoresponse will be reported.
Nonlinear Optical Defect Absorption in Hydrogenated Amorphous Silicon
Authors: Rjeger W. , Lanz M. , Graeff C. F. O. , Nebel C. E. , Stutzmann M.
Published: April 2011

By using a Nd: YAG-pumped optical parametric oscillator (OPO) as excitation light source (8 ns pulse width), the dynamic range of photothermal deflection spectroseopy (PDS) is increased by a factor up to 1000. This enables the study of nonlinear defect absorption in intrinsic, phosphorus and boron doped a-Si:H. To probe nonlinear absorption of defects, the intensity of the fundamental emission of the Nd:YAG (1064 nm) has been varied over three orders of magnitude. For intensities greater than 1024 photons/ (cm2s), a significant increase of a by a factor 1.4 in p-, 1.2 in i-, and 1.15 in n-a-Si:H is detected. From electron spin resonance (ESR) and transient photoconductivity experiments on intrinsic a-Si:H, a bleaching of the defect density by electron excitation into the conduction band is measured. A hole-like signature in the ESR spectra indicates that electrons from the valence band are excited into the defect band as well. The data are interpreted based on a model for IR-induced nonlinear optical effects.
Stress and the Goal of Improved Amorphous Silicon Stability
Authors: Fortmann C. M.
Published: April 2011

The stability of amorphous silicon-based solar cells is related to the i-layer carrier mobilities as these determine the carrier densities under illumination. Materials with high mobilities quickly diffuse the carriers to high recombination regions such as the contacts and interface regions. The quick out diffusion of carriers lowers the i-layer carrier densities and therefore the driving force for defect generation. Increased compressive stress appears to be correlated to increased electron mobility and is related to the hydrogen content. Solar cell configurations also play a role in the i-layer carrier density. For example, factors that increase the V∝ tend to decrease the stability because the associated decrease in interface recombination results in increased i-layer carrier densities. Factors that reduce the blue response such as increased interface recombination tend to increase the stability as these results in reduced i-layer carrier densities.
Measurement of Two Deep H Bonding Levels in Device Quality Glow Discharge a-SI:H
Authors: Mahan A. H. , Johnson E. J. , Crandall R. S. , Branz H. M.
Published: April 2011

We present the results of H effusion studies on device-quality glow discharge deposited hydrogenated amorphous silicon (a-Si:H) films. We measure the decrease in the amount of Si-H infrared absorption as pieces of an a-Si:H sample are annealed isothermally at temperatures from 425°C to 500°C, until more than 95% of the initial H is evolved. We use the rate equation for loss of H due to annealing to calculate the activation energy for H effusion. For anneals below 500°C we observe two distinct rate processes, a fast process corresponding to release of ∼ 70% of the total H (∼10 at. %) contained in the sample, and a slower process corresponding to effusion of the remainder. The fast loss process yields an activation energy of ∼1.4 eV; this is the energy level widely observed in diffusion coefficient measurements. A similar analysis for the slow loss process yields an activation energy of ∼2.1 eV and a diffusion constant prefactor higher than that for the fast loss process. We suggest that this slow component represents the first determination of the energy depth of the “isolated” H component commonly observed in nuclear magnetic resonance experiments.
Amorphous Silicon Three Color Detector
Authors: Stiebig H. , Giehl J. , Knipp D. , Rieve P. , Böhm M.
Published: April 2011

Band gap and defect engineered amorphous silicon based nipin photo diodes with bias controlled spectral response have been fabricated successfully. The devices exhibit good linearity over a wide illumination range and linearly independent spectral response curves which are required to generate a standard RGB-signal. In the bias range from -1.5 V to 1.5 V a dynamic range exceeding 90 dB for two color sensors and 80 dB for three color sensors has been observed. The general operation principle of the multispectral photo diode is discussed using a numerical simulation program. The model describes the defect state distribution of dangling bonds according to the defect-pool model and uses coherent wave propagation in the device to calculate the profile of photo generated carriers. Additionally, an analytical model has been developed to be included into standard circuit simulation programs like SPICE (Simulation Program with Integrated Circuit Emphasis). The analytical model uses linear field approximations in both i-layers of the device.
Effect of Plasma Damage on Interface State Density Between a-Si:H and Insulating Films
Authors: Umezu Ikurou , Kuwamura Takahiro , Maeda Keiji
Published: April 2011

The interface state density between a-Si:H and insulating film is very important in characteristics of a-Si:H thin film transistors. In this study, the interface state density was measured by photothermal deflection spectroscopy (PDS). Layered structures of a-SiN, 1.7:H on a-Si:H and a-SiO2.0 on a-Si:H were deposited by P-CVD method. The a-SiN1.7:H layer was grown from a gas mixture of SiH4 and NH3 and the a-SiO2.0 layer was grown from a gas mixture of SiH4 and N2O. While the interface state density of a-SiN1.7:H on a-Si:H structure was smaller than the free surface state density of a-Si:H, that of a-SiO2.0 on a-Si:H structure was larger than the free surface state density of a-Si:H. The difference in the surface state density between these specimens is discussed in terms of plasma damage of a-Si:H surface by the source gases during deposition of insulating layer. Surface of the a-Si:H was treated by plasma of NH3 or N2o gas which is dominant constituent of source gases of insulating layer. Although the surface state density of the N2o plasma-treated sample increases, that of NH3 plasma treated sample does not increase. The shape and intensity of the spectra of N2o plasma-treated sample is similar to that of the a-Sio2.0 on a-Si:H structure. These results indicate that the interface defect between a-Sio2.0and a-Si:H layer was induced by plasma damage of the a-Si:H surface.
Deposition of Amorphous Hydrogenated Silicon Films by VUV Laser CVD: Influence of Substrate Temperature
Authors: Karstens H. , Hess P.
Published: April 2011

Amorphous hydrogenated silicon (a-Si:H) films were deposited from disilane at substrate temperatures between 180 and 390 °C using a F2-laser (157 nm) in a parallel configuration. Material properties such as hydrogen content, SiH and SiH2 group concentration, photo-and dark conductivity, band-gap energy and the Urbach parameter were determined as a function of the deposition temperature. The material with the best optical and electronical properties was found for a substrate temperature of 260 °C. Using argon as the buffer gas instead of helium results in films of poor quality.
Low Energy Ion Implantation of as During Si-MBE
Authors: Collart E. J. H. , Gravesteijn D. J. , Lathouwers E. G. C. , Kersten W. J.
Published: April 2011

As-doped Si layers were grown using Molecular Beam Epitaxy (MBE) together with simultaneous Low Energy Ion Implantation (LEII). The influence of growth conditions such as Si-substrate temperature, ion energy and ion dose was investigated using structural and electrical characterization techniques. Below the As solid solubility limit, well defined and 100 % electrically active As-doped layers were grown. Above solid solubility segregation occured, with broadened profiles and less than 100 % activation.
The Ultrafast Carrier Dynamics in Semiconductors: The Role of Defects
Authors: Fauchet P. M. , Wicks G. W. , Kostoulas Y. , Lobad A. I. , Ucer K. B.
Published: April 2011

The presence of point defects is expected to influence the properties of free carrier in semiconductors. We have used the techniques of ultrafast laser spectroscopy to characterize the dynamics of photoinjected carriers in several III–V semiconductors grown at low temperature. The initial scattering time and the lifetime of the carriers become very short at low growth temperatures. Results obtained with low-temperature grown III–Vs are compared to those obtained with III–Vs grown at normal temperatures and amorphous silicon.
The Formation of Defects Degrading Gate Oxide Integrity During CZ-Si Crystal Growth
Authors: Tsumori Y. , Nakai K. , Iwasaki T. , Haga H. , Kojima K. , Nakashizu T.
Published: April 2011

The formation of grown-in defects degrading the gate oxide integrity (GOI) has been studied. The growth-halting experiments were carried out to investigate the temperature ranges at which the formation of the defects was promoted or suppressed. GOI is improved in the crystal regions slowly cooled above 1330°C and between 1060°C and 1100°C. It is degraded in the crystal regions held below 1060°C. In the peripheral of the crystals, those temperature ranges are about 30°C lower. The defects are formed and grown below 1060°C in the center part of the crystal. The defect density is decreased with cooling time between 1060°C and 1100°C. These phenomena are considered to be closely related with reactions of intrinsic point defects, that is, the pair annihilation or the aggregation. The temperatures at which the pair annihilation and the aggregation of the point defects occur are dependent upon the supersaturation of the point defects.
External Gettering Comparison and Structural Characterization of Single and Polycrystalline Silicon
Authors: Hieslmair Henry , McHugo Scott , Weber Eicke
Published: April 2011

Various silicon samples, both single and polycrystalline, were intentionally contaminated and externally gettered using phosphorus, aluminum and co- phosphorus/aluminum gettering. Gettering efficiencies were quantified via diffusion length improvements. Structural characterization was used to correlate defects with low gettering efficiencies. External gettering was found to be particularly effective at recovering diffusion length in large grain polycrystalline silicon and solar grade single crystal silicon despite Fe contamination and high defect densities. One of two explanations is possible, 1) the structural defects are initially undecorated and are completely gettered after Fe contamination, or 2) metal decoration on as-grown structural defects are structurally and/or chemically different from intentional Fe decoration.
Theory of Defects in Wide-Band-Gap Semiconductors
Authors: Van de Walle Chris G , Neugebauer Jörg
Published: April 2011

We discuss the application of state-of-the-art first-principles calculations to the problem of defects, impurities, and doping levels in semiconductors. Since doping problems are of particular relevance in wide-band-gap materials, we focus here on studies of ZnSe and GaN. For ZnSe, we discuss our latest insights in the influence of compensation and dopant solubility on the experimentally observed limitation of the free carrier concentration in p-type ZnSe. For GaN, we focus on the role of native defects in doping or compensation of the material, with particular emphasis on the n-type conductivity of as-grown GaN.
Photoluminescence Studies of GaN and AlGaN Layers Under Hydrostatic Pressure
Authors: Wetzel C. , Walukiewicz W. , Haller E. E. , Amano H. , Akasaki I.
Published: April 2011

Wide bandgap GaN very often shows a high electron concentration. Although several impurities such as O and Si have been identified, the concentration is not high enough to account for the number of free carriers. As a consequence native defects namely the nitrogen vacancies are widely considered to be present at high densities. Several calculations predict different energy levels of this strongly localized defect. We present photoluminescence experiments of wurtzite GaN and AlGaN layers under large hydrostatic pressure to search for localized defects within the questionable energy range of 3 .0 to 3 .8 eV above the valence band edge.
Local Vibrational Mode (LVM) Spectroscopy of Semiconductors
Authors: Haller Eugene E.
Published: April 2011

Impurities and defects with masses smaller than the masses of the host semiconductor crystal atoms typically exhibit vibrational frequencies well above the phonon frequency spectrum. These vibrational modes produce sharp spectral absorption features in the infrared. Because of their strong spatial localization these modes are not affected by neighboring impurities and/or defects with concentrations up to 1019 cm−3. This insensitivity is especially advantageous when the free carrier concentration must be reduced through the introduction of electron irradiation defects or when highly doped thin layers must be investigated. LVM spectroscopy with perturbations such as polarization of the probe light, uniaxial and hydrostatic stress, and isotope substitution has been highly successful in identifying the structure and composition of a large number of defect complexes. Hydrogen, in particular, forming a wide variety of complexes in elemental and compound semiconductors has been extensively studied with LVM spectroscopy. For example, it has been shown recently that nitrogen acceptors are hydrogen passivated in MOCVD grown ZnSe. Carbon and oxygen have been investigated in all major semiconductors with LVM spectroscopy. The extreme simplification of the spectrum of bond centered oxygen through isotope enrichment of several Ge crystals has been demonstrated. Additional recent investigations of importance to the currently much studied semiconductors will be reviewed.
Effect of RF Power on the Characteristic Parameters of W/n-GaAs Schottky Contacts Fabricated by RF Sputtering
Authors: Singh A. , Aroca G. , Velásquez L.
Published: April 2011

W/n-GaAs/In Schottky contacts of area 1.75 mm2 were fabricated by deposition of W on (100) n-GaAs by rf Sputtering using rf power values in the range 200–400 Watt. The I-V and high frequency C-V measurements at 300 K, in the 200 Watt W/n-GaAs Schottky contact indicated that W formed a good rectifying contact to n-GaAs, with a rectification ratio (r) of 270, ideality factor (n) of 1.39, reverse saturation current (Io) of 1.2×10−6 A and the C-V barrier height (φbo) of 1.6 V. However, n and Io increased, whereas r and φbo decreased for the W/n-GaAs Schottky contacts prepared using higher rf power. For the 300 Watt W/n-GaAs contact, the values of 70, 1.70, 6.3}10−6 A, and 1.2V for r, n, Io and φbo, respectively, were estimated. The low frequency forward bias capacitance (or surface defect density) was an order of magnitude higher in the 300 Watt contact than in the 200 Watt contact. This fact suggested that the degradation in the quality of W/n-GaAs Schottky contacts fabricated by using high rf power was caused by high density surface defects created during sputter deposition of W on n-GaAs.
A Systematic Study of the Structural and Luminescence Properties of P-Type Porous Silicon
Authors: Yoon H. , Goorsky M. S.
Published: April 2011

The structural and luminescence properties of (001) p-type porous silicon samples (p∼0.1-0.2 Ω•cm) fabricated electrochemically under various conditions were investigated using high resolution double and triple axis diffraction and photoluminescence spectroscopy. We show the sensitivity of the structure of the porous silicon to the current density in the range of 10-50 mA/cm2, HF acid concentration in the range of 15% - 30%, and the evolution of the structure with time. We have found a systematic dependence of the amount of strain in the porous silicon layer (PSL) on the current density. The effect of the HF concentration is such that at 25% and 30% HF, PSLs are formed which are crystalline and strained, but at a lower HF concentration (15%), strained layers are not formed. The perpendicular strain in the layer increases linearly with storage time but the in-plane lattice constant of the porous silicon remains matched to the substrate. Further, we utilized x-ray reciprocal space maps to observe that, with storage time, there is an increase in the diffuse scattering from the PSL due to an increase in the range of tilts in the layer. Room temperature photoluminescence emission was observed for all 15% and 25% HF samples, but not for all 30% HF samples. Higher peak luminescence energy was obtained with lower HF concentration. Finally, we note the relationship between the strain in the PSL and the luminescence properties.
Chemistry of Point Defect in Silicon and its Applications in Semiconductor Technology
Authors: Pizzini S. , Binetti S. , Acciarri M.
Published: April 2011

The chemistry of the interactions between point defects and impurities is discussed by considering first the general thermodynamic and kinetic aspects of these reactions, deserving major attention to the identity of of the stable chemical species eventually formed and to the boundary conditions for diffusion controlled and reaction controlled interaction processes.

The second part of the paper is instead dedicated to the analysis of the chemistry of carbon, oxygen, hydrogen and point defects in silicon, which is a system of major technological interest.

We postulate that at low enough temperatures, when homogeneous nucleation processes are slow, spinodal decomposition assists oxygen aggregation phenomena. We postulate, also, on the basis of the existing knowledge, that carbon and hydrogen favour alternative reaction paths for oxygen in the due of clustering processes and discuss the hydrogen-enhanced oxygen diffusivity in the frame of a conventional trapping model.
Low Temperature Anneal of the Divacancy in P-Type Silicon
Authors: Trauwaert M.-A. , Vanhellemont J. , Maes H. E. , Van Bavel A.-M. , Langouche G. , Stesmans A. , Clauws P.
Published: April 2011

Results are reported of a Deep Level Transient Spectroscopy (DLTS) study of the conversion of the divacancy, with energy level at Ev+0.19eV, to a level at Ev+0.24eV after anneal at temperatures below its dissociation temperature (300°C). In literature both levels have been associated with the donor level of the divacancy.

Diodes processed on p-type Float Zone (FZ) and Czochralski (Cz) silicon wafers with boron concentration between 0.2 and 3E15 cm−3 are irradiated with 2 MeV electrons. Before and after anneal (200°C and 250°C) DLTS spectra are recorded to get a full electrical characterisation of the induced defects.

The observed conversion is proposed to be a gradual transformation of the divacancy to a divacancy-oxygen complex.
Superlattice Nanowires
Authors: Attenborough K. , Hart R. , Schwarzacher W. , Ansermet J-PH. , Blondel A. , Doudin B. , Meier J.P.
Published: April 2011

CoNiCu/Cu superlattice nanowires have been grown by electrodeposition in nuclear tracketched nanoporous membranes. Transmission electron microscopy (TEM) images show a good layer structure and allow an estimate of the current efficiency. Current perpendicular to plane (CPP) giant magnetoresistance of up to 22%, at ambient temperature, has been measured but appears to be limited by defects, giving rise to ferromagnetic interlayer coupling, at low nonmagnetic layer thicknesses. Magnetic properties of the superlattice nanowires are influenced by in-plane anisotropy and magnetostatic coupling.
Reactions of Defined Oxidized Carbon Fiber Surfaces with Model Compounds and Polyurethane Elastomers
Authors: Pittman Charles U. , Gardner Steven D. , He Guoren , Wang Lichang , Wu Zhihong , Singamsetty C. , Wu Biahua , Booth Glyn
Published: April 2011

Ex-PAN carbon fiber surfaces, oxidized to varying degrees by HNO3, have been characterized by NaOH, dye and HCl uptake, ion scattering spectroscopy (ISS), and angleresolved X-ray photoelectron spectroscopy (ARXPS). Subsequent treatments with tetraethylenepentamine to introduce amino groups or epichlorohydrin to introduce epoxy groups have been thoroughly characterized. The efficiency of using these surface functions to bond polymers onto fiber surfaces has been investigated using model anhydrides and isocyanates (for amines) and diols and diamines (for epoxides). The fraction of surface-bound groups which react drops with an increase in molecular size of the species being grafted. Crosslinked elastomeric polyurethane layers with designed modulus values and thicknesses have been bonded to these fibers. Composites have been prepared (epoxy matrices). The impact strengths and interlaminar shear strengths (ILSS) were studied as a function of the interphase modulus and thickness. Impact strengths increased (even at 500–1500 Å thicknesses). ILSS values depend on interphase modulus.