bluuebunny/crossref_metadata_2025 huggingface dataset:
Using Dx Centers to Write Erasable Metallic Patterns in AlGaAs
Authors: Thio Tineke , Linke R.A. , Devlin G.E. , Bennett J.W. , Chadi J.D. , Macdonald R.L. , Mizuta M.
Published: April 2011

DX centers are semiconductor dopants which form deep states due to a large lattice relaxation. At low temperature, the DX centers exhibit persistent photoconductivity. When exposed to light in a spatial pattern, the photocarriers are confined to the illuminated regions by Coulomb interaction with the localised DX centers. The resulting spatial modulation of the free carrier density gives rise to a modulation of both the electrical conductivity and the dielectric constant. We demonstrate both effects by measurements of the conductance anisotropy and optical diffraction of samples exposed to excitation in a striped pattern. Erasure is achieved by thermal annealing. The constrast ratio of the conductivity modulation is greater than 108; in our experiment it is limited to ∼100 by light scattering. We estimate that 100nm resolution is feasible. Optical diffraction efficiencies up to 40% have been demonstrated in a stripe-illuminated thick sample. The persistence of the written patterns at low temperature is potentially useful in high-density data storage applications and the fabrication of erasable submicron devices.
First-Principles Studies of γ-Tial Based Ternary Alloys
Authors: Wolf W. , Podloucky R. , Rogl P.
Published: April 2011

Self-consistent total energy calculations were performed to investigate the effects of ternary additions of 3d, 4d and 5d transition metal elements (V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, Pt) to Ll0 ordered γ-TiAl. In general, we studied ordered solids of 4 atoms per unit cell. In the case of Mn additions larger supercells with up to 32 atoms were also treated. Minimizing the total energy with respect to volume and c/a ratio leads to the determination of lattice distortions due to alloying by the ternary elements. These distortions prove to be strongly dependent on whether the alloying element substitutes Al or Ti atoms. Furthermore, the site preference can be derived by comparing the formation energies of Tin-1XAln and TinXAln-i, where X is the substituting element. As a result the change of the geometry of L10 TiAl due to alloying can be estimated and may thereby give insight into the conditions at semicoherent γ/γ′ and α2/γ interfaces, which are associated to be responsible for ductility improvements of γ-TiAl based alloys.
Effects of Hydrogen on Fatigue Crack Growth of Iron Aluminides
Authors: Castagna A. , Stoloff N.S
Published: April 2011

Three Fe-Al alloys, FAP-Y, FA-129, and Fe-35a%Al, containing 16, 28, and 35a%Al, respectively, have been subjected to fatigue crack growth testing in moist air, in oxygen, and in gaseous hydrogen. In each case hydrogen and air were embrittling. Crack growth rates increased significantly as frequency decreased. Fatigue crack growth results have been compared with those for other structural iron-base alloys. Surprisingly, FAP-Y displays the highest crack growth rate of any alloy examined, except at very low levels of stress intensity range. The mechanisms for embrittlement by hydrogen and by moisture in air are discussed.
Anomalous Oxidation of Intermetallics
Authors: Berztiss D. A. , Pettit F. S. , Meier G. H.
Published: April 2011

MoSi2, (γ-NiAl and TiAl with Cr additions are of interest for high temperature applications in oxidizing environments, where an oxide layer such as SiO2 or Al2O3 should form to protect the base material. At elevated temperatures (600-1700°C), a protective SiO2 layer forms on MoSi2, while near 500°C pesting and/or accelerated oxidation could disintegrate the material to powder as Mo and Si oxidize to form a complex, thick, non-protective oxide layer. Use of γ -TiAl is limited by poor oxidation resistance, whereby layered mixed oxides of TiO2 and Al2O3 form. With the addition of Cr from 4 to 34 at%, results are varied: protective Al2O3 formation, mixed oxide formation as with TiAl or more rapid oxidation than TiAl. NiAl is currently used as a diffusion coating on Ni-based superalloys and is being considered for use as a structural material itself because of its excellent oxidation resistance, i.e. forming α-alumina above 1000°C. Recent work indicates that pure NiAl oxidized under low oxygen partial pressures in a contained atmosphere develops nonprotective oxide scales similar to accelerated oxidation of MoSi2. This study explores the parameters defining protective behavior of these intermetallics and attempts to describe and explain anomalies at low temperatures and pressures.
Core Structure And Mobility Of a <101] Dislocations In L10 TiAl
Authors: Rao S. , Woodward C. , Simmons J. , Dimiduk D.
Published: April 2011

An empirical atomistic potential, fit to the structural and elastic properties of L10 TiAl within the embedded atom method (EAM), is used to simulate the mobility of two possible planar forms of a<101] dislocations in a model L10 compound. The two configurations examined were: the planar SISF-APB-CSF coupled (P core) and the decomposed 1/2<110]-SISF-SESF coupled (D core). Six different line orientations are considered for the P core: 0° (screw), 30°, 60°, 90° (edge), 120° and 150°. The ‘ideal’ friction stress at 0°K of a<101] dislocations in the P form is found to be a function of line orientation, with the close packed line directions, <101] (screw) and <110] (60°), having friction stresses ranging from 0.001–0.002μ. Previously calculated results on the friction stress of a/2<110] dislocations, using an identical potential are consistently higher than the friction stress of a<101] dislocations. Simulations of the interaction of glide strains with the D core for the 60° (line directions < 110]) and 120° (line directions <011]) orientations show that the Shockley partial trailing the SESF in the D core is strongly pinned. The dislocation moves by extension of SESF when glide stresses are applied with SESF as the trailing fault.
The High-Temperature Oxidation of Nb-40Ti-15Al and the Effect of Cr Alloying and Silicide Diffusion Coatings
Authors: Cockeram B. V. , Schmutzler H. J. , Shyue J. , Hoshino K. , Meng S. , Wheeler R. , Fraser H. L.
Published: April 2011

The high-temperature oxidation resistance of Nb-40Ti-15Al-(0 or 5)Cr alloys has been studied. The kinetics and activation energy for the isothermal oxidation of Nb-40Ti-15Al are similar to pure titanium. Detailed TEM analysis of the oxidized Nb-40Ti-15Al alloy reveals that oxygen dissolution has produced a case layer by the decomposition of the matrix B2 phase into oxygen saturated alpha-Ti and an A15 phase. Internal oxidation of the A15 phase to form AlNbO4occurred in the case layer near the scale/metal interface. The phase separated case layer evolved into a segregated oxide layer containing a co-continuous TiO2phase. These morphologies of the oxide scale and case layer correlate well with the kinetic data, and suggest that oxygen dissolution in α-Ti and diffusion through TiO2control the high-temperature oxidation rates. The Cr additions reduced the thickness of the case layer and slowed the oxidation kinetics, but did not change the mechanism. In addition, excellent cyclic oxidation resistant was provided by the silicide diffusion coatings.
A Statistical Approach for Some Stress Anomalies
Authors: Louchet François , Viguier Bernard
Published: April 2011

The yield anomaly of TiAl is modelled in terms of a mechanism of mobile dislocation exhaustion. Several consequences on mechanical properties are derived, that are shown to be applicable to a wide range of anomalies, irrespective of the specific dislocation mechanisms.
Application of Norbury Rule to Thermal Conductivity in Intermetallic Compounds
Authors: Terada Yoshihiro , Mohri Tetsuo , Suzuki Tomoo
Published: April 2011

Thermal conductivity data at room temperature, which are measured by laser-flash method, are presented in B2 aluminides and titanides, and in nickel based L12 compounds. The thermal conductivity λ is changed in the following order of the compounds. λ(NiAl) >λ(CoAl)>λ(FeAl), λ(NiTi)<λ(CoTi)<λ(FeTi), λ(Ni3Al)>λ(Ni3Si) and λ(Ni3Ga)>λ(Ni3Ge). According to Norbury rule originally proposed for the concentration dependence of electrical resistivity, the increasing rate is greater in the solid solution, where the position of solute elements is more remote in horizontal distance from a host metal in the periodic table. It is found that this rule holds for the thermal conductivity measured for the intermetallic compounds with the combination of a series of guest constituents and a fixed host constituent both in the B2 and Ll2 intermetallic compounds.
The Mechanical Properties of Fe-36.5Al and its Cr or ti Containing Alloys at Elevated Temperature
Authors: Li Dingqiang , Liu Yi , Shan Aidang , Lin Dongliang
Published: April 2011

The mechanical properties of B2 structural FeAl alloys, prepared by hot rolling, at elevated temperatures have been measured by tensile tests. The alloys of Fe-36.5at.%A1, Fe-36.5at.%A1-5at.%Cr and Fe-36.5at.%Al-2at.%Ti were taken for tensile tests at a temperature range from room temperature to 1000°C. The fracture surfaces of these alloys were observed by SEM. The results showed that elongations of these alloys increased with increasing temperature when the testing temperatures were above 600°C. All the maximum elongations of these alloys appeared at 1000°C and those of Fe-36.5A1, Fe-36.5Al-5Cr, and Fe-36.5Al-2Ti alloys were 120%, 183% and 208% respectively. Fracture surfaces showed that failure of these alloys was by a combination of intergranular fracture and transgranular cleavage below 700°C. but showed a ductile fracture above 700°C. The ductility and strength of ternary alloys were higher than that of binary FeAl alloy at elevated temperatures, especially at high temperature. The <111> dislocations and helices have been observed in Fe-36.5A1 alloy by TEM. The large elongation of FeAl alloy at high temperature resulted from <111> dislocations slipping and <111> helices climbing.
<111> Slip in Nial Single Crystals Between 4.2K and 100K
Authors: Sun Y. Q. , Taylor G. , Darolia R. , Hazzledine P. M.
Published: April 2011

Slip in <111> directions is a deformation mode found in many intermetallic compounds ordered into the B2 (CsCl) structure, but in NiAl it is active only under the limiting conditions of uniaxial loading along <100> and low testing temperatures. This paper investigates the deformation characteristics of <100> oriented NiAl tested between 4.2K and 300K. Compression tests have shown that below about 100K, deformation takes place by slip in <111> directions without kinking, whereas between 100K and 300K kinking in the form of shearing on {100} becomes the dominant mode of plastic deformation, occurring usually near one or both ends of the compression samples. The yield stress has been found to exhibit a normal decrease with temperature, but at a rate that is smaller than most bcc metals. The sensitivity of the flow stress to strain-rate changes is also characteristically small, giving activation volumes around b3, b being the Burgers vector of a<111> superdislocation. Both slip line observations and transmission electron microscopy have shown slip mainly on {110} near 80K, but with increasing activity on {112} and cross-slip between {110} and {112} with the lowering of temperature to 4.2K.
Site Preference of Ternary Additions in Ni3A1
Authors: Sluiter M. , Takahashi M. , Kawazoe Y.
Published: April 2011

The site preference of a large number of alloying elements in Ni3AI (γ′) was studied in a systematic way by using electronic structure calculations based on the local density approximation. Alloying elements in this intermetallic may occupy exclusively the Ni or Al sublattices, or may exhibit no particular site preference. By performing the calculations both for spin-polarized and non-spin-polarized cases, it was found that magnetism strongly affects the computed site substitution behavior of Mn, Fe, and Co in Ni3Al. In the case of Fe, for example, a preference for the Ni sublattice is computed when magnetic effects are ignored, but when magnetic effects are accounted for it is found that Fe has no significant site preference, in much better agreement with a large number of experimental determinations. Some trends regarding the site preference across the periodic table are shown and discussed.
Quantitative Evaluation of Crack Bridging in Fully Lamellar Ti-48A1-2Cr-2Nb
Authors: Wissuchek D. J. , Lucas G. E. , Evans A. G.
Published: April 2011

The toughening effect of ligaments in a fully lamellar Ti-48Al-2Cr-2Nb alloy has been determined by evaluation of J integrals of the crack tip and crack wake regions from surface strain maps. The crack tip fracture toughness KtiP was found to vary with lamellae orientation and ranged from Ktip = 13 MPa√m for inter-lamellar fracture to Ktip = 19.5 MPa√m for translamellar fracture The bridging contribution to fracture toughness was found to arise from a bimodal distribution of ligament sizes. Fracture surface reconstructions showed a distribution of large hinges that initiated at grain or subcolony boundaries and smaller ligaments associated with delaminations within a grain. The stress-displacement functions for these two ligament populations have been determined from a large scale bridging analysis of the resistance curves along with fractographic information. The small ligamerts appear to account for the initial rapid rise in crack growth resistance; and the large hinges, for the slow continued rise in toughness at large crack extensions.
Experimental Tem and Image Simulation of  Dislocations in Ti3A1
Authors: Wiezorek JÖrg M.K. , Court S. A. , Humphreys C.J.
Published: April 2011

The fine structure and the character of a dislocations on prism planes have been determined for room-temperature deformed polycrystalline Ti3Al using a combination of experimental and computational techniques of transmission electron microscopy (TEM). Within the resolution limits of the weak-beam technique the fine structure of prism plane a dislocations in Ti3Al is found to be planar and to comprise only a single ribbon of antiphase boundary and no experimentally observable stacking faults.
Fatigue Crack Growth Characteristics and Fracture Toughness of γ-Tial Base Alloy Sheet Material
Authors: Behr R. , Wanner A. , Clemens H. , Glatz W.
Published: April 2011

Room temperature tension-tension fatigue crack growth experiments were perfomed on single edge notch specimens of Ti-48Al-2Cr sheet materials produced on industrial scale with different microstructures ranging from near gamma to fully lamellar. Crack extension was monitored using a travelling optical microscope and a DC potential method. Crack paths and fracture surfaces were investigated by SEM. Fracture toughness data were determined by monotonic loading of previously fatigued samples.
Microstructures in Rapidly Solidified Gamma Titanium-Aluminum Alloys with Erbium Additions
Authors: Kral M. V. , Bassler B. T. , Hofmeister W. H. , Wittig J. E.
Published: April 2011

Erbium additions of 0.25, 0.5, 1.0 and 2.0 atomic percent were incorporated into base alloys of 40 at.% titanium - 60 at.% aluminum by arc melting. Samples of 0.30g were electromagnetically levitated and melted and then rapidly solidified by double anvil splat quenching with liquid temperatures ranging from the liquidus temperature to near the maximum undercooling temperature for each alloy. Microstructures of TiAl with 0.25 and 0.5 at.% Er showed the presence of small erbium rich particles within γ grains as well as antiphase domain boundaries, while TiAl with 1.0 and 2.0 at.% Er showed no evidence of these features. These observations were correlated with solidification velocity measurements on levitated samples. Evidence for disordered primary solidification and a solid state disorder/order reaction is presented.
Characterization of the High Temperature Dislocation Substructure of Mosi2
Authors: Campbell J. P. , Chang H. , Gibala R.
Published: April 2011

The dislocation substructure of polycrystalline MoSi2 deformed in compression at temperatures ranging from 900°C to 1300°C has been investigated. Slip is found to occur primarily by <100> Burgers vectors. A quantitative characterization of the <100> dislocation substructure is developed for several deformation temperatures, including the slip systems present and the relative occurrence of each. Orientation distributions showing the screw/edge character of the <100> dislocations are generated at each deformation temperature. Variations in these distributions with temperature are noted, and the implications of these variations to the deformation behavior of MoSi2 are discussed. Notable observations include the onset of dislocation climb between 900°C and 1100°C, a strong preference for dislocations of mixed character at 900°C, and the complete absence of pure screw dislocations from 900°C to 1300°C.
Toughened Microstructures for Ductile Phase Reinforced Molybdenum Disilicide
Authors: Packard S. M. , Ghosh A.K.
Published: April 2011

Various morphologies of ductile Nb refractory metal reinforcement are incorporated into a MoSi2 matrix using powder metallurgy, including single-ply laminates, continuous metal ribbons and sections of 2-dimensional wire mesh. Hot forging techniques are used to redistribute the reinforcement and change the dimensions and the aspect ratio of the reinforcing metal ligaments. Work-of-rupture measurements are conducted on bend test specimens and precracked tensile specimens of the composite so that the toughness contribution from the various ductile metal morphologies can be assessed according to its effectiveness. Accompanying microstructural examination of crack bridging interaction with the reinforcement is conducted.
The Effect of Grain Refinement on the Room-Temperature Ductility of As-Cast Fe3Al-Based Alloys
Authors: Viswanathan S. , Andleigh V. K. , McKamey C. G.
Published: April 2011

Fe3Al-based alloys exhibit poor room temperature ductility in the as-cast condition. In this study, the effect of grain refinement of the as-cast alloy on room-temperature ductility was investigated. Small melts of Fe-28 at. % Al-5 at. % Cr were inoculated with various alloying additions and cast into a 50- × 30- × 30-mm graphite mold. The resulting ingots were examined metallographically for evidence of grain refinement, and three-point bend tests were conducted on samples to assess the effect on room-temperature ductility. Ductility was assumed to correlate with the strain corresponding to the maximum stress obtained in the bend test. The results showed that titanium was extremely effective in grain refinement, although it severely embrittled the alloy in contents exceeding 1%. Boron additions strengthened the alloy significantly, while carbon additions reduced both the strength and ductility. The best ductility was found in an alloy containing titanium, boron, and carbon.
Silicon Nitride Ceramic Matrix for CMCs from Silane Derived Si Powders and Preceramic Polymers
Authors: Rhine Wendell E. , Lightfoot Annamarie , Seyferth Dietmar , Haggerty John S.
Published: April 2011

Si powders synthesized by the laser pyrolysis of silane can be nitrided at 1200–1250°C, producing an improved reaction bonded silicon nitride. This RBSN is a potential matrix for composites since it can be formed at temperatures which do not degrade the strengths of commercially available amorphous ceramic fibers. However, the density of the RBSN matrix has been limited to about 75% using the silane-derived Si powder. Combining Si powders and preceramic polymers offers an approach for increasing the density and mechanical properties of the reaction-formed Si3N4matrix. The incorporation of polysilazanes inhibited the nitridation at 1250°C, but samples could be effectively nitrided at 1400°C. These higher nitriding temperatures are compatible with SCS-6 and other lowoxygen, crystalline SiC fibers which can be used as reinforcements for ceramic composites.
Interface Characterisation Using an SEM-Based Micro-Indentor
Authors: Lewis M.H. , Daniel A.M. , Cain M.G.
Published: April 2011

The design and performance of an SEM-based microindentor, for interfacial property measurements in CMCs, is described. It enables high resolution imaging and simultaneous load/displacement monitoring with capacity and resolution of 20N ± lmN (load) and 100μm ± 10nm (displacement). Its application to measurement of interface debond and shear stresses for a wide range of fibres and monofilaments is described.
Growth of Diamond Film By CVD on Near Net Shape Fabricated β-SiC/TiC Composites Synthesized Using SHS
Authors: Raghunathan R. , Chowdhury R. , Jagannadham K. , Narayan J.
Published: April 2011

β-SiC/TiC composites were synthesized using the process of self propagating-high temperature combustion synthesis (SHS). The heat released during the exothermic reaction between Si and C powders (with a δH of -14kcal/mole) and that Ti and C powders (with a δH of -44.1 kcal/mole) is sufficient to cause the melting of the powders into which the carbon diffuses and from which β-SiC and TiC precipitate out of the supersaturated solution. The composite was characterized using X-ray diffraction techniques, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. An attempt was made to understand the mechanism of formation of the composite. We have proposed a mechanism to understand the formation of the SHS compounds based on dissolution, diffusion and precipitation from the supersaturated solution. There is no evidence for the presence of TiSi2 and an attempt was made to explain this observation based on free energy considerations. Diamond film was then grown on the pellet by hot filament CVD technique using methane and hydrogen gas as the reactants. The deposition was conducted for a period of four hours. A continuous film of diamond was found to grow on β-SiC/TiC composite using this technique. The diamond film was characterized by using Raman spectroscopy and SEM. The diamond film showed both (001) and (11) facets with average grain size of 5 μm.
Displacement Reactions in the Ni-Al-O System Resulting in Periodic Layer Structures
Authors: Song Daniel W. , Subramanian Ramesh , Dieckmann RÜdiger
Published: April 2011

Displacement reactions may play an important role in in situ processing technologies for the production of metal-ceramic composites. To better understand such reactions displacement reactions between NiO and Al were studied at high temperatures. Different reaction layers with periodic structures were observed involving Al2O3, Al3Ni, Al3Ni2, Ni and Al. The experimental observations are presented and discussed with regard to the reaction mechanism.
Computer Simulation of Displacement Cascade Damage in Metals
Authors: Gao F. , Wooding S.J. , Calder A.F. , Bacon D.J.
Published: April 2011

MD simulations of displacement cascades in a variety of metals of different crystal structure and an ordered alloy are discussed. Frenkel-pair production at the end of the cascade process is well below the NRT theoretical value in all cases and a new empirical relationship between Frenkel-pair number and damage energy is assessed. In contrast with this, antisite production efficiency in ordered alloys increases with increasing energy, as does the atomic mixing of the different atomic species. The degree of clustering of interstitials in cascades is materialdependent. These results are discussed in relation to the highly disordered zone formed in the thermal spike.
A Spectral Effect on Phase Evolution in Neutron-Irradiated Aluminum?
Authors: Farrell K.
Published: April 2011

It is proposed that the spatial distribution of transmutation-produced silicon precipitates formed in aluminum by capture of thermal neutrons is systematically modified by the fast neutron portion of the spectrum, and the tensile strength is altered accordingly. The concept is used to rationalize some previously erratic tensile data.
Microstructural Characterization of Iron Implanted Sapphire Nanocomposites
Authors: Ren Shelly X. , Mchargue Carl J. , Allard L. F. , Chen Y. , Hunn J. D. , Lucas B. N. , Williams R. K.
Published: April 2011

Nanocomposites of iron in sapphire (α-A12 O3) prepared by ion implantation have been studied as a model to investigate the potential of such materials for applications in high technology areas. The implantation was performed with 160 keV ions at several doses; the nanocomposites were then annealed at selected temperatures between 700 and 1400°C in an Ar-4&H2 atmosphere for 1 hour. Rutherford backscattering spectroscopy and high resolution transmission electron microscopy (TEM) were used to characterize the structure of these nanocomposites. Measurements showed that damage depth extended to about 300 nm and the embedded iron extended to about 200 nm. This region became amorphous when the fluence reaches 2x1017 Fe/cm2 at this energy. At this dose, oriented precipitates with diameters of 2 to 3 nm were identified by TEM techniques as α-Fe which had the following orientation relationship with the sapphire matrix: <111>Fe||<310>Sapphire and (011)Fe||{006}sapphire. Thermal annealing could be used to restore the crystallinity to the damaged near-surface region, to form the metallic colloids, and also to coarsen the precipitates. The optical density and luminescence spectra were also measured. The predominant defects were oxygen vacancies with two electrons (F center) at the known absorption peak of 200 nm.
Defects Induced by Protons and γ-Rays in Semi-Insulating Gaas Detectors
Authors: Castaldini A. , Cavallini A. , Del Papa C. , Fuochi G. , Alietti M. , Canali C. , Nava F. , Paccagnellam A. , Lanzieriv C.
Published: April 2011

Semi-insulating gallium arsenide has been irradiated by protons and by gamma-rays with different doses. The irradiation-induced deep level defects have been investigated by current transient spectroscopy to find their energy, capture cross sections and generation rate.Two electron traps at Ec+0.14eV(E13) and Ec-0.70eV(E4) and a hole trap at Ec-0.14eV(H2) in addition to the levels existing before the irradiation have been detected in the irradiated samples. These findings have been related to the performance of gallium arsenide charge particle detectors.
Effect of Contacts on Capacitance Transient Measurements in N-Type Hydrogenated Amorphous Silicon
Authors: Jackson W. B. , Johnson N. M. , Walker J.
Published: April 2011

Measurement of the dependence of emission capacitance transients on filling pulse duration has been extended to devices with Ohmic back contacts. Capacitance transients on devices possessing identical bulk 20-ppm P doped a-Si:H but either Ohmic contacts or blocking contacts were compared. The devices with blocking contacts completely reproduced in quantitative detail the previously observed anomalous dependence of capacitance transients on filling pulse duration. The diodes with Ohmic contacts showed no evidence of the anomalous filling pulse effect even for light-degraded, resistive samples. Current injection measurements show that blocking contacts delay the charge injection into the device by about 10–100 msec.
Nanostructure of a-Si:H and Related Materials by Small-Angle X-Ray Scattering
Authors: Williamson D. L.
Published: April 2011

The use of small-angle x-ray scattering to examine nanostructural features of a-Si:H and the related alloys a-SiGe:H and a-SiC:H will be reviewed. A wide range of H, Ge, and C compositions has been investigated. The films examined came from several film- and device-making groups and represent current state-of-the-art solar cell material or attempts to develop improved and more stable material. A detailed comparison of the three classes of materials reveals dramatic differences in nanostructure. The diffuse component of the small-angle scattering, not recognized or discussed in previous small-angle scattering experiments on these materials by other groups, is shown to contain potentially valuable information on the atomic-scale structure.
Carrier Recombination in A-SI:H P-I-N Devices Studied by PL and EL Spectroscopies
Authors: Yeh C. N. , Han D. X. , Wang K. D. , McNeil L. E.
Published: April 2011

Both photoluminescence (PL) and electroluminescence (EL) energy spectra were studied in the temperature range of 80 to 300 K on a-Si:H p-i-n structures. The EL spectrum depends on several parameters such as the i-layer thickness and the sample structure with or without a buffer layer (b-layer), while the PL spectrum shows no difference when those parameters are varied. Comparing PL and EL in 0.5 μm p-i-n devices with and without a buffer layer, we found that (a) at 80 K, the main-band peak energy is 1.3 eV for PL and 1.2 eV for EL; (b) the PL spectral line shape does not change with the insertion of a buffer layer, but the EL spectra show more enhanced main-band luminescence with the buffer layer; (c) the temperature dependence of the PL intensity shows a slope of 26 K which is similar to that of a-Si:H films, but the EL efficiency shows a weaker temperature dependence that varies with the diode structure.
Effect of Light Soaking on Hot Wire Deposited a-Si:H Films
Authors: Kwon Daewon , David Cohen J. , Nelson Brent P. , Iwaniczko Eugene
Published: April 2011

We present the results of studies on the defect properties and the effect of light soaking for various hot wire deposited (HW) films. We employ junction capacitance measurements together with the transient photocapacitance spectroscopy to measure the deep defect densities in as-grown state (state A) and in light soaked state (state B). Good agreement is found between the defect densities measured from both measurements. The HW film with a hydrogen content of 10 – 12 at.% shows physical characteristics and defect densities similar to conventional PECVD films. The HW films with hydrogen content, CH, in the range 2 – 9 at.% show a smaller defect density in state B than the defect density of the film with higher CH. However, the film with a hydrogen level of less than 1 at.% exhibits markedly inferior physical properties.
Light-Induced Metastable Changes in a-SiSx:H
Authors: Wang S. L. , Taylor P. C.
Published: April 2011

Metastable light-induced increases in the dark conductivities of a-SiSx:H alloys are explained as photo-activation of hydrogen-passivated sulfur donor sites. For a sulfur concentration (sulfur-to-silicon ratio) of 5.6 × 103 the excess dark conductivity as a function of illumination temperature is thermally activated with an activation energy of 0.72 eV. When the sulfur concentration is 3.3 × 102, the temperature dependence is very weak. This dramatic difference in the temperature dependence of the creation of increased dark conductivity is explained by a lowering of the annealing temperature for the metastable changes as the sulfur concentration increases. We discuss the influence of this new metastability on the possibility of obtaining more stable films.
An Expanding Thermal Plasma for Deposition of a-Si:H
Authors: Severens R. J. , Brussaard G. J. H. , Verhoeven H. J. M. , Van de Sanden M. C. M. , Schram D. C.
Published: April 2011

A remote argon/hydrogen plasma is used to deposit amorphous hydrogenated silicon. The plasma is generated in a DC thermal arc (typical operating conditions 0.5 bar, 5 kW) and expands into a low pressure chamber (20 Pa) thus creating a plasma jet with a typical flow velocity of 103 m/s. Pure silane is injected into the jet immediately after the nozzle, in a typical flow mixture of Ar:H2:SiH4=55:10:6 scc/s. The electron temperature in the jet is low (typ. 0.3 eV) : silane radicals are thought to be produced mainly by hydrogen abstraction, but also by a sequence of dissociative charge exchange and consecutive dissociative recombination. In-situ ellipsometry yields refractive indices of 3.6–4.2 at 632.8 nm and growth rates of 10–20 nm/s. FTIR analysis yields a hydrogen content of 9–25 at.% and refractive indices of 2.7–3.3 in the infrared. The SiH density decreases with increasing hydrogen content, whereas the SiH2 density increases. Above 11 at.%, the majority of hydrogen is bonded in the SiH2 configuration. The optical bandgap remains constant at approximately 1.72 eV. The photoconductivity is of the order 101–6 (Ωcm) 1–6 and the photoresponse 106.
Pulsed-Light-Induced Metastable Defect Creation in Hydrogenated Amorphous Silicon
Authors: Yoon Jong-Hwan , Kim H. L.
Published: April 2011

We report the results of a study of metastable defect creation by pulsed light soaking in undoped hydrogenated amorphous silicon (a-Si:H). An illumination time dependence of the defect density, a saturated defect density, and light-induced annealing under pulsed laser light have been studied. Measurements show approximately a t1/2 time-dependence of the defect creation, which is independent of light intensity. It is observed that the saturation value of the defect density is about one order of magnitude higher than by cw illumination in device quality films. It has been suggested that these results would be due to the difference in the light-induced defect annealing rate between cw and pulsed lights, in which it is found that the light-induced annealing rate by pulsed light is lower than by cw light.
Study of Electron Transport in a-Si:H p-i-n Diodes: Use of the Transient Space-Charge-Limited-Current Technique
Authors: Adriaenssens G. J. , Yan B. , Eliat A.
Published: April 2011

A full and detailed transient space-charge-limited current (T-SCLC) study of a-Si:H p-i-n diodes has been carried out in the time range from 108s to 10s. In the short-time regime, general features of T-SCLC such as the current cusp and the carrier extraction period were observed, and related transport parameters were deduced. Electron emission from deep states was studied by measuring the current transients well beyond the extraction time. The emission time is thermally activated at temperatures higher than 250K and levels off at lower temperatures. The high temperature behaviour places the upper edge of the deep states at 0.42–0.52eV below the conduction band edge, and the attempt-to-escape frequency in the range of 1011-1013Hz. An observed shift of emission time with light intensity is attributed to defect relaxation.
Transient Infrared Stimulated Photoconductivity in a-SI:H at Low Temperatures
Authors: Heck S. , Stradins P. , Fritzsche H.
Published: April 2011

Dual beam photoconductivity with bandgap primary light and hv = 0.4- 0.6eV infrared light steps was measured with Ims time resolution in hydrogenated amorphous silicon (a-Si:H) at 4.2K. The results can be described by assuming that the photocurrent transients are due to energy-loss hopping of photocarriers and that the infrared light promotes recombination by reexciting photocarriers thereby enhancing the probability of tunneling recombination.
Analytical approach for transient photoconductivity in undoped a-Si:H
Authors: Goerlitzer M. , Pipoz P. , Beck H. , Wyrsch N. , Shah A. V.
Published: April 2011

Transient photoconductive response of undoped a-Si:H has been studied; the changes were analysed between two slightly different steady-state illumination conditions, at room temperature. A theoretical model is developed to describe transient photoconductivity; it yields good agreement with the measured curves for a whole range of light intensities. Numerical evaluations allows one to extract the recombination time of electrons. Comparison with steady-state photoconductivity yields a band mobility of free electrons between 0.1 and 6 cm2V−1s−1, depending upon sample quality.
Improved Ambipolar Diffusion Length in a-Si1-xGex:H Alloys for Multi-Junction Solar Cells
Authors: Fölsch J. , Finger F. , Kulessa T. , Siebke F. , Beyer W. , Wagner H.
Published: April 2011

To prepare hydrogenated amorphous silicon-germanium alloys as low gap material for multi-junction solar cells in plasma enhanced chemical vapour deposition, the well established concept of strong dilution of the process gases with hydrogen has been used. Two different regimes of alloying were found: for low Ge content (x < 0.40) we observe material with low defect density, small Urbach energy and high values of the ambipolar diffusion length. In the regime of high Ge content (x > 0.40) the defect densities and Urbach energies are high and the values of the ambipolar diffusion length low. The transition is accompanied by the appearance of a low-temperature peak in hydrogen effusion experiments indicating a void rich film structure. Material from just above and below the transition zone is used in pin solar cells leading to a much enhanced red response compared with a-Si:H cells. The differences seen in the material quality are mirrored in the solar cell properties. By carefully adjusting the active layer thickness material with low diffusion length shows also reasonable solar cell performance.
Utilization of Amorphous Silicon Carbide (a-Si:C:H) as a Resistive Layer in Gas Microstrip Detectors
Authors: Hong W. S. , Cho H. S. , Perez-Mendez V. , Gong W. G.
Published: April 2011

Thin semiconducting films of hydrogenated amorphous silicon (a-Si:H) and its carbon alloy (a-Si:C:H) were applied to gas microstrip detectors in order to control gain instabilities due to charges on the substrate. Thin (∼100 nm) layers of a-Si:H or p-doped a-Si:C:H were placed either over or under the electrodes using the plasma enhanced chemical vapor deposition (PECVD) technique to provide the substrate with a suitable surface conductivity. By changing the carbon content and boron doping density, the sheet resistance of the a-Si:C:H coating could be successfully controlled in the range of 1012 ∼ 1017 μ/□, and the light sensitivity, which causes the resistivity to vary with ambient light conditions, was minimized. An avalanche gain of 5000 and energy resolution of 20% FWHM were achieved and the gain remained constant over a week of operation. A-Si:C:H film is an attractive alternative to ion-implanted or semiconducting glass due to the wide range of resistivities possible and the feasibility of making deposits over a large area at low cost.
Brightness Degradation Controlled by Current Induced Metastable Defect Creation in a-SiC:H Based Light Emitting Diodes
Authors: Rizzoli R. , Summonte C. , Galloni R. , Ruth M. , Desalvo A. , Zignanf F. , Rava P. , Demichelis F. , Pirri C. F. , Tresso E. , Crovini G. , Giorgis F. , Madan A.
Published: April 2011

A study of the electroluminescence degradation of a-SiC:H based light emitting devices (LED) is presented for the first time. The best initial peak brightness obtained is 4.2 cd/m2. All LEDs reported in this paper emit a red light which, when operated under continuous bias in a not fully darkened room, is visible for several minutes, depending on degradation rate. The time dependence of LED degradation, which is reversible upon annealing, can be explained if self-annealing is taken into account. There is evidence of an improved LED performance for lower temperature operation. Pulsed operation, with respect to dc operation, produces a markedly lower defect production rate, associated to a higher brightness after degradation. The possibility of some optimization of the operation parameters (peak current, duty cycle) is discussed.
Phase separation of Si1-xGex alloys
Authors: Kim Eunja , Lee Young Hee
Published: April 2011

We generate liquid and amorphous Si1_xGex alloys for various Ge compositions using ab initio molecular dynamics approach. The electronic bonding characters and structural properties are discussed in terms of radial distribution function, bond angle distribution, and order parameters. Although the order parameters suggest approximately random alloy for all compositions, the snapshots reveal clearly phase separation. We will discuss how the phase can be separated in SiGe alloy system.
Nucleation & Growth of Defects in SOI Materials
Authors: Silvestre G. C. M. , Moore R. A. , Kennedy B. J.
Published: April 2011

To produce Silicon-On-Insulator (SOI) materials with thin Si overlayer, sacrificial oxidation is often used. This creates defects which have adverse effects on device performance. It has been observed that Stacking Faults (SFs) in thin Separation-by-IMplantation-of-OXygen (SIMOX) or Bonded-and-Etched-back-SOI (BESOI) films of less than 600 Å, do not shrink as expected during neutral Ar anneals. Shrinkage of SFs in standard bulk substrates with different capping layers has been investigated to promote the understanding of the Si/Si02 interface effects on Si interstitial incorporation during anneals. The activation energy for growth and shrinkage of SOI samples thicker than 800 A was found to be the same as bulk Si: 2.3 eV (growth) and 4.6 eV (shrinkage). Bulk silicon implanted with low doses of oxygen, permitted investigation of the nucleation sites of SFs in SIMOX where oxygen precipitates are believed to act as nuclei for SFs. A five step etch procedure was modified to reveal the defects in very thin SOI and an automatic defect counting system developed at T.C.D. permitted fast and reliable measurements of size and density of the defects. It appears that the two Frank partial dislocations that bound SFs, are pinned at the two Si/Si02 interfaces for both SIMOX and BESOI films thinner than 500 Å. In thicker SOI, the mechanisms for growth and shrinkage of SFs are the same as for bulk silicon.
Low-Temperature, Solid-Phase Epitaxial Growth of Amorphized, Non-Stoichiometric GaAs
Authors: Belay K. B. , Llewellyn D. L. , Ridgway M. C.
Published: April 2011

Non-stoichiometric GaAs layers with semi-insulating properties can be produced by low-temperature molecular beam epitaxy or ion implantation. The latter is the subject of the present report wherein the solid-phase epitaxial growth of amorphized, non-stoichiometric GaAs layers has been investigated with time-resolved reflectivity, Rutherford backscattering spectrometry and transmission electron microscopy. GaAs substrates were implanted with Ga and/or As ions and annealed in air at a temperature of 260°C. The recrystallized material was composed of a thin, crystalline layer bordered by a thick, twinned layer. Non-stoichiometry results in a roughening of the amorphous/crystalline interface and the transformation from planar to non-planar regrowth. The onset of the transformation and the rate thereof can increase with an increase in non-stoichiometry. Non-stoichiometry can be achieved on a macroscopic scale via Ga or As implants or on a microscopic scale via Ga and As implants. The influence of the latter is greatest at low doses whilst the former dominates at high doses.
Lateral Diffusion and Capture of Iron in P-Type Silicon
Authors: Beaman Kevin L. , Agarwal Aditya , Koveshnikov Sergei V. , Rozgonyi George A.
Published: April 2011

The lateral motion of iron impurities was observed and studied in ptype iron contaminated silicon. The lateral diffusion was induced by and then measured using Schottky diodes with a special interdigitated fingers design. Capture of the impurities was done by diffusing to laterally placed dislocation loops formed by a self aligned ion implantation. Lateral changes in Fe concentration were determined using capacitance-voltage and deep level transient spectroscopy.
Phosphorus and Aluminum Gettering of Gold in Silicon: Simulation and Optimization Considerations
Authors: Gafiteanu R. , Gösele U. , Tan T. Y.
Published: April 2011

Using the diffusion-segregation equation, modeling and simulations of gettering Au (a substi-tutional-interstitial species in Si) away from the Si bulk have been performed. Three external gettering schemes have been considered: wafer frontside P indiffusion gettering, wafer backside Al deposition gettering, and a combination of the two processes. Under the same processing conditions, it has been shown that P indiffusion gettering is faster than Al gettering, but P gettering has a lower gettering capacity and is less stable than Al gettering for longer gettering times. The combined P and Al gettering process is as fast as P gettering in reaching an optimum gettered state, and possesses the capacity and stability of the Al gettering process.
Gettering of Interstitial Iron in P/P+ Epitaxial Silicon Wafers
Authors: Wijaranakula W. , Gao X. , Curtis K. , Haddad H.
Published: April 2011

Gettering of interstitial iron in p/p+ epitaxial silicon wafers during an integrated circuit device simulation annealing was examined. The results from the deep level transient spectroscopy and optical microscope examination suggest no correlation between the interstitial iron concentration in the epitaxial layer and the bulk microdefect density. An electron microscopic analysis revealed that gettering of interstitial iron took place at an oxide polyhedral precipitate located at the center of a bulk stacking fault. By comparing the results of iron implanted to non-implanted samples, it is concluded that iron gettering occurs via a phase transformation of Si02 to an α FeSi2 iron disilicide. In this study, it is suggested that gettering of iron atoms is fundamentally different from that of other transition metals where the silicide formation occurs predominantly along the Frank partial dislocations of the bulk stacking fault.
Mechanism of Iron Gettering by Polycrystalline Silicon Film in P-Type Czochralski (CZ) Silicon
Authors: Mishra Kamal K.
Published: April 2011

Iron gettering in p-type CZ silicon at 600C has been investigated using deep level transient spectroscopy(DLTS), and the surface photovoltage(SPV) method. A quantitative analysis of the gettering kinetics of iron using the polycrystalline silicon film is presented for the first time. Depth profiles of iron concentration indicated a sharp gradient in the Fe concentration near the polycrystalline silicon/substrate interface. Concurrent decrease in the minority carrier diffusion length was also observed in the same region. The majority of iron gettering from the bulk silicon was found to be associated with the enhancement of the internal gettering.

The presence of small oxygen precipitates/nuclei generated by prolonged heat treatment at 600C was found to prevent regeneration of FeB pairs at the room temperature. On the other hand, large precipitates formed at lOOOC do not influence the diffusion or the recombination of Fei with B to form FeB pairs.
Hydrogen-Related Electron Traps in GaAs and their Relation to Crystal Stoichiometry
Authors: Shinagawa Tatsuyuki , Okumura Tsugunori
Published: April 2011

Deep-level formation upon plasma hydrogenation has been studied with n-GaAs grown by various methods. Four electron traps (EH0-EH3) were generated in As-rich n-GaAs crystals. No electron traps were observed in the LPE layer before and after hydrogenation. The hydrogen as well as excess arsenic defects are responsible for the formation of these deep levels. Two of the generated levels in our study, EH0/EH2, exhibit metastability and are identical to the M3/M4 levels reported by Buchwald et al. It can be speculated that both diffused hydrogen and already existing As antisite defects are responsible for the generation of the metastable defects.
Non-Destructive Method for Simultaneous Mapping of Diffusion Length and Surface Recombination Velocity
Authors: Ostendorf Hans-Christoph , Endrös Arthur L.
Published: April 2011

Bulk and surface recombination are the main material parameters that determine the performance of crystalline silicon solar cells. We present a new method for the nondestructive, simultaneous mapping of the diffusion length and the surface recombination velocity of a silicon wafer. The method uses the hardware of the electrolytical metal tracer (ELYMAT). The separation between bulk and surface recombination is achieved by illuminating the sample with laser beams of two different colors. By solving the diffusion equation for both laser penetration depths the diffusion length and the surface recombination velocity can be calculated from the measured diffusion currents. First experiments are presented which show the basic feasibility of the method.