bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanoscale Phase Separation Induced by Mechanical Alloying in the Iron-Erbium-Nitrogen System
Authors: Fu Z. , Fecht H. J. , Johnson W. L.
Published: March 2011

Metastable phases, including nanocrystalline and amorphous structures, can be prepared by high energy cyclic deformation processes. In the present study, we compare the behavior of a stable congruent melting compound (Fe2Er Laves phase) with a mixture of pure elemental Fe and Er powders subjected to high energy ball milling. X-ray diffraction and transmission electron microscopy reveal similar results in both cases. In the early stages, a nanocrystalline fcc phase with lattice parameter a = 0.484 nm and a grain size of 6 nm is formed together with a bcc Fe-rich phase. Extended milling results in a nanoscale phase separation into Fe-rich and Er-rich crystallites with average grain sizes of 1.8-4 nm. Based on a lattice parameter analysis, the fcc phase was initially thought to be a metastable FeEr3 phase. Further studies revealed nitrogen gas in the milling vial had reacted with the powder during ball milling to produce the cubic ErN phase (“NaCl” structure with a lattice parameter of 0.4836 nm). Our experiments demonstrate that the steel vials for ball milling do not remain hermetically sealed during the milling process and a nitride phase can be formed easily if a catalyst for the dissociation of nitrogen molecules (such as Fe) exists in the system.
Grain Boundary Intermetallic Phases in Alloy 718
Authors: Burke M. G. , Miller M. K.
Published: March 2011

A nickel base superalloy, Alloy 718, has been studied by analytical electron microscopy in order to trace the development of the complex microstructure which is produced during a typical multistage thermal treatment. The distribution of δ γ″, γ′ and Laves phases was found to be strongly dependent on aging treatment.
Atomistic Modeling of Extended Defects in Metalic Alloys: Dislocations and Grain Boundaries in Ll2 Compounds
Authors: Vitek V. , Ackland G. J. , Cserti J.
Published: March 2011

Extended defects, such as dislocations and grain boundaries, control a wide variety of material properties and their atomic structure is often a governing factor. A necessary precursor for modeling of these structures is a suitable description of atomic interactions. We present here empirical many-body potentials for alloys which represent a very simple scheme for the evaluation of total energies and yet reflect correctly the basic physical features of the alloy systems modeled. As examples of atomistic studies we show results of calculations of the core structures of screw dislocations in Ll2 compounds. The same potentials have also been used to calculate structures of grain boundaries in these compounds. The deformation and fracture behavior of Ll2 alloys is then discussed in the light of grain boundary and dislocation core studies.
Deformation and Fracture Behavior of Tial
Authors: Fu C. L. , Yoo M. H.
Published: March 2011

First-principles total-energy calculations of the elastic constants, shear fault energies, and cleavage energies of TiAl are presented. We find a large elastic shear anisotropy along the [011] direction, and high APB energies due to the strong cohesion between Ti and Al layers. Shear faults of SISF, SESF, and twin boundary are predicted to be prevalent due to their low energies. The anomalous temperature dependence of flow stress is explained by the cross-slip pinning and fault dragging mechanisms. The intrinsic brittleness of TiAl is discussed in terms of the low mobility of 1/2[110] dislocations.
Ground State Properties And Magnetism In Substitutionally Disordered Fe1-xCrx Alloys
Authors: Shelton W. A. , Pinski F. J. , Johnson D. D. , Nicholson D. M. , Stocks G. M.
Published: March 2011

We have performed calculations of the electronic structure of the random substitutional bcc Fe1-xCrx alloys, using the spin-polarized, self-consistent Korringa, Kohn and Rostoker coherent potential approximation (KKR-CPA) method. This is a first principles method based on a local spin density approximation for electron exchange and correlation energy. For the iron-rich alloys, we find that the average moment decreases linearly with Cr concentration, although the individual moments show a different concentration dependence and the Cr moment is anti-parallel to the Fe moment. This system is similar to Fe1-xVx system, although some details are different.
Design of High Elastic Modulus Alloy Using First Principles Electronic Theory
Authors: Masuda-Jindo K. , Terakura K.
Published: March 2011

The first principles ASW (augmented spherical wave) method is used to predict the elastic properties of Al-base and Ni-base fcc alloys. It is shown that the elastic modulus increase or decrease of the fcc alloys is closely correlated with the change in the lattice constants and that the results for various solute atoms can be summarized in the simple curves, depending on the species of the solute atoms. We demonstrate that the theoretical calculations for the binary alloys can be used for the design of the (multi-component) fcc alloys with desired elastic properties and lattice constants.
Microstructures of Ni3 Al Rapidly Solidified by Hammer-Anvil Technique
Authors: Mishima Yoshinao , Sasaki Shirou , Suzuki Tomoo
Published: March 2011

The Hammer-Anvil technique is employed for the rapid solidification of Ni3Al in the present work. The effect of off-stoichiometry on the change in the microstructure in the quenched alloy is examined for compositions within the L12 phase field at equilibrium. It is shown that non-equilibrium phases of various morphologies appear with fine grains of Ll2 phase, which have never been reported for the compound rapidly solidified by the roll techniques. With increasing aluminum concentration and with increase in cooling rate, the latter being judged by the difference in the thickness of the quenched foil, the fraction of the non-equilibrium phases appearing is found to increase giving more complex microstructure.
Designing Precipitation-Strengthened Iron-Aluminides for High-Temperature Applications
Authors: Maziasz P.J. , Mckamey C.G. , Hubbard C.R.
Published: March 2011

A combination of minor solutes was added to an Fe3AI-type base alloy (Fe-28%AI6%Cr-0.05%B) to produce carbide precipitates. These solutes reacted to produce coarse Ti-rich MC and Cr- and Mo-rich M6C precipitates during fabrication at 650–1000°C. X-ray diffraction of hot-worked material revealed a significant amount of the higher temperature B2 phase is retained at room temperature, indicating that the D0s.↔,B2 phase transformation was retarded. During subsequent heat-treatments, the coarse carbides partially dissolved during annealing at 1100–1200°C, and no grain coarsening was observed at temperatures up to 1200°C. Specimens annealed for lh at 1250°C and quenched exhibited fine carbide precipitates both along some of the dislocations and at grain boundaries. The annealed and quenched specimens also contained many fine dislocation loops which coarsened and evolved into networks during further aging at 750°C for 1-10h. Tensile testing at 600°C showed improvements in strength and ductility of annealed and aged material relative to the hot-worked condition. Correlation of microstructural behavior with tensile properties changes supports the idea that microstructural control can be used to develop better high-temperature strength in the B2 phase. However, more data is needed on the combinations of alloying and heattreating which produce fine, stable matrix carbide dispersions in iron-aluminides.
The Stability of the T2 (AL6CuLi3) Phase During Examination in the Tem
Authors: Ramani A. S. , Tosten M. H. , Bartges C. W. , Michel D. J. , Reed J. R. , Ryba E. R. , Howell P. R.
Published: March 2011

Transmission electron microscopy (TEM) has been employed to examine the stability of the (presumed) icosahedral T2 (A16 Cu Li3) phase. The T2 phase was found to be unstable either when irradiated by the electron beam or during in-situ heating. In addition, certain specimen preparation techniques (e.g., ion-beam thinning) also led to the decomposition of the T2 phase. When the T2 particles were formed during conventional aging of aluminum-rich Al-Li-Cu based alloys, the transformation products were invariably microcrystalline. Individual microcrystals have been identified as the aluminum rich ∝-solid solution which, in certain instances, contained the δʹ (Al3Li) phase. TB (A17.5 Cu4Li) and T1 (Al2CuLi) particles were also found. When the T2 phase was prepared by casting alloys of the proposed stoichiometry of T2, then the transformation products were more complex, although certain reaction products have been identified as the a solid solution, TB and T1.
A Comparative First Principles Study of Phase Stability in Ni-Ti and Ni-Al Alloys Around Equiatomic Composition
Authors: Turchi P.E.A. , Sluiter M. , Pinski F.J. , Johnson D.D.
Published: March 2011

Electronic structure and phase stability properties of Ni-Ti and Ni-Al around equiatomic composition are investigated with a first principles approach. The study is based upon the generalized purturbation method applied to the Korringa-Kohn-Rostoker multiple scattering formulation of the coherent potential approximation. Within this framework, effective cluster interactions which build up the configuration-dependent part of the internal energy are calculated. The strength of ordering tendencies in both bcc-based substitutional alloys is compared and contrasted in terms of hybridisation effects, in relation with experimental evidences.
Theoretical Investigation of Phase Stability in Ti-V and Ti-Cr Substitutional Alloys
Authors: Sluiter M. , Turchi P.E.A.
Published: March 2011

The phase stability of Ti-V and Ti-Cr alloys is examined within the context of an electronic structure description. Energies of mixing, effective pair interactions, and phase diagrams are computed and compared to available experimental data. A metastable distorted B2 type of structure is predicted in near equiatomic Ti-Cr.
Percolation Structure Observed in Evaporated Nd-Fe-B Films
Authors: Shang C.H. , Liu B.X. , Li H.D.
Published: March 2011

Thin alloy films were prepared by evaporating a standard magnetic Nd2Fe14B ingot in a high vacuum e-gun system at roam temperature. The as-deposited films of 50 nm thick were characterized by transmission electron microscopy (TEM) and in situ energy dispersive spectroscopy (EDS). Bright field examination showed that a novel morphology with holes of varied sizes emerged in the films. The hole-like white areas enriched with Nd, and the dark network with Fe. The electron micrograph was digitized by an VAX image processor. An infinite cluster was proved to be a two-dimensional percolating structure at the percolation threshold. The fractal dimension and the percolation threshold were calculated to be 1.90±0.04 and 0.60±0.04, respectively, which are in good consonance with the theoretical predictions. The as-deposited samples were aged at roam temperature for about two months. Enhanced diffusion and solid state reaction induced another percolating networks camposed essentially by Fe7Nd and Nd-riched solid solution.
Microstructure and Stabiulty of TiB2 and Cu Multilayers
Authors: Basu S. N. , Hubbard K. M. , Hirvonen J-P. , Mitchell T. E. , Nastasi M.
Published: March 2011

The Interfacial stability of a high strength TiB2/Cu multilayer structure was examined by subjecting the layers to ion irradiation by 400 keV Ne++ ions up to a maximum dose of 12×1015 ions/cm2. Even at the highest dose, with a maximum dpa value of 4.92, the TiB2/Cu interface did not show any mixing. This stability of the multilayers has been explained by examining the maximum thermodynamic driving force for Interfacial reactions in this system.
MBE Growth of Artificially-Layered Magnetic Metal Structures on Semiconductors
Authors: Farrow R.F.C. , Lee C.H.
Published: March 2011

In this review, recent developments in MBE growth of artificially-layered magnetic metal structures on semiconductor substrates are considered. It is seen that the ability to seed specific orientations of such structures and to probe their structural and chemical properties with a variety of in situ probes are key advantages of MBE over more conventional deposition techniques.
Tracer Diffusion in Amorphous NI-ZR Alloys
Authors: Wu H.-M. , Hahn H. , Averback R.S.
Published: March 2011

Tracer impurity diffusion coefficients for Co and Hf were measured in a series of amorphous NixZr(1−x) alloys in the range 0.30 < × < 0.70 and at temperatures between 250°C and 350° C. The diffusion coefficients for the Co atoms are between 2 and 4 orders of magnitude greater than those for Hf depending on alloy composition and temperature. Diffusion coefficients for Hf are independent of alloy composition, whereas those for Co decrease strongly with increasing Ni content. The activation enthalpies of diffusion and the pre-exponential factors were both found to be significantly smaller for the Hf than the Co tracer impurity atoms. These results are discussed in terms of possible diffusion mechanisms in amorphous metallic alloys.
Atomistic Structure and Composition of a Ag/Ni Interphase Boundary
Authors: Gumbsch P. , Daw M. S. , Foiles S. M. , Fischmeister H. F.
Published: March 2011

Using the embedded atom method we atomistically model the compensation of the misfit induced stresses in a “cube on cube” oriented Ag/Ni bicrystal with (011) interface plane, in which zones of maximum misfit (misfit dislocations) run along the [100] and the [011] directions.

The ideal interface corresponds to an abrupt transition between Ag and Ni. The interfacial enthalpy is found to be lowered by the introduction of vacancies on the Ni side (equivalently, vacancies are trapped on the Ni side of the boundary). Pursuing this perspective, we find that the interfacial enthalpy is lowered considerably by the removal of a complete line of Ni atoms along the [011] direction from the Ni side of the boundary. The minimum energy configuration consists of a Ni layer whose atomic density is reduced by 16% sandwiched between the ordinary Ni and Ag lattices.
Diffusional Asymmetry in Amorphous Alloys:Implications for Interfacial Reactions
Authors: Greer A.L. , Dyrbye K. , Andersen L.-U. Aaen , Somekh R. E. , Bøtiger J. , Janting J.
Published: March 2011

Earlyllate transition metal systems such as Ni-Zr and Co-Zr exhibit solid state amorphization (SSA) in which the amorphous phase is formed by reaction between the crystalline elements. The rate of the amorphization is governed by the diffusion of the faster species, Ni or Co. Here results are presented on the homogenization of compositionally modulated thin films which show that the Zr diffusion is up to 106 times slower. The difference in diffusivities is correlated with atomic size. The consequences of the marked diffusional asymmetry are considered, particularly for the interpretation of results on the indiffusion of Co into amorphous Co-Zr. It is proposed that for amorphous alloys such as Ni-Zr and Co-Zr, changes in composition by rapid diffusion of Ni or Co can yield structures which are not in internal equilibrium. This would affect, for example, the validity of the common tangent construction as applied to predict the limiting compositions of the amorphous phase in contact with the elemental layers during SSA.
Interfacial Composition and Structure in Pt/Ni and Pt/Nb Multilayer Films
Authors: Bain J.A. , Clemens B.M. , Brennan S.
Published: March 2011

The interfacial structure of Pt/Nb and Pt/Ni sputtered multilayer films was studied using x-ray diffraction in symmetric, asymmetric, and grazing incidence modes. The grazing incidence and asymmetric diffraction were used to distinguish alloying effects on the lattice spacing from strain in the films. This strain was shown to be consistent with semi-coherent interfaces in the Pt/Ni but not in the Pt/Nb in which another strain generating mechanism dominates.
Determination of the Strain Source in Mo/Ni Multilayers
Authors: Chyung L.J. , Clemens B.M. , Brennan S.
Published: March 2011

Structural characterization and strain measurements were conducted on Mo/Ni multilayers, with bilayer periods between 10 Å and 200 Å, utilizing symmetric, asymmetrin, and grazing incidence x-ray diffraction techniques. The structural difference between the 15 Å and 20 Å modulation wavelength samples, evident in the symmetric high angle diffraction spectra, is attributed to the absence of crystalline registry in the early stages of sputter deposition, yielding crystalline order only when the bilayer deposition thickness exceeded 15 Å. The dominant mechanism for the observed modulation wavelength dependent strain behavior, both in the growth direction and in the plane of the film, is consistent with a coherency strain model. The orientation relationship predicted for the Mo/Ni system, based on their atomic radii ratio is the Nishiyama-Wasserman relationship, (110)BCC//(111)FCCand[001]BCC//[101]FCC. This relationship would provide the source for the observed coherency strain.
Al-Ge Phase Separation During Vapor Deposition
Authors: Adams C. D. , Atzmon M. , Cheng Y.-T. , Srolovitz D. J.
Published: March 2011

The phase- and grain microstructure and surface morphology of Al-Ge films formed by electron beam co-deposition have been studied as a function of the deposition temperature. X-ray diffraction, scanning- and transmission electron microscopy with x-ray fluorescence analysis were used to characterize the films. At room temperature and above, crystalline, phase separated films are obtained. The phase dimensions and surface roughness were observed to increase with the deposition temperature and the dependence yields activation energies for atomic migration consistent with a surface diffusion mechanism. The surface roughness is observed to reflect the microstructure under the surface.
Formation of Amorphous Interlayers by Solid-State Diffusion in Refractory Metal/Silicon Systems
Authors: Cheng J.Y. , Wang M.H. , Chen L.J.
Published: March 2011

Formation and growth of amorphous interlayers (a-interlayers) in nine refractroy metal and silicon systems by solid-state diffusion have been investigated by conventional and high resolution transmission electron microscopy. The amorphous interlayers were found to form in samples annealed at 350–650 °C. The growth was found to follow a linear growth law initially then slow down until a critical thicknees was reached. The interface structures were examined. The correlations among difference in atomic size between metal and Si atoms, growth rate and activation energy of the linear growth, critical and maximum a-interlayer thickness, the largest heat of formation energy for crystalline silicides, the calculated free energy difference in forming amorphous phase as well as atomic mobility in refractroy metal/silicon systems are discussed.
Laser Assisted Deposition of Thin BiSrCaCuO Films
Authors: Cheenne A. , Perriere J. , Kerherve F. , Hauchecorne G. , Fogarassy E. , Fuchs C.
Published: March 2011

A study of the pulsed laser evaporation process of thin superconducting films using both Nd:YAG and ArF eximer lasers is presented. Nuclear reaction analysis, Rutherford backscattering spectrometry (RBS) and scanning electron microscopy were used to characterize composition, thickness and surface morphology of the deposited films. The influence on the properties of the films, of the various parameters of the method such as photon wavelength, laser power density and geometrical conditions are presented and discussed.
Spectroscopic Evidence for Cluster Emission from Laser Ablated Surface in Group IV Elements
Authors: Kasuya A. , Nishina Y.
Published: March 2011

Transient process of laser ablation has been investigated by space/ time-resolved optical spectrometry. An excimer laser excitation on crystal surfaces of group IV elements produces highly excited microclusters of atoms which continue to absorb laser energy and begin to decompose in time scale of nanosecond. The spectral characteristics indicate that they are highly charged and decompose dominantly through ejections of neutral or singly charged monomers of constituent element rather than fragmentation into smaller clusters like a fission process.
Foreign Gas Effect upon Excimer Laser Ablation of Polymer
Authors: Shimo N. , Uchida T. , Masuhara H.
Published: March 2011

We have examined effect of foreign gas and its pressure upon etch rate, surface morphology, and surface chemical composition. Etch rate is suppressed by adding inert gases such as argon and helium. On the contrary, oxygen enhances the rate by increasing its pressure, indicating that oxygen molecules react with ablated polymer surface. From the results of XPS analyses, active surface of ablated polymer reacts also with ammonia and hexamethyldisilane. It is considered that laser ablation has a high potentiality of simultaneous microfabrication and photochemical surface modification of polymer.
Pulsed Laser Deposition of Magneto-Optic Material
Authors: Erington K. B. , Mcgahan W. , Ianno N. J. , Woollam J. A.
Published: March 2011

A wide range of materials can be deposited by Pulsed Laser Deposition. We will discuss the deposition of materials that can be employed in magneto-optic systems. These materials include dysprosium, cobalt, and aluminum nitride. We will present the results of our work involving the fabrication of multilayer structures of the previously mentioned material. These structures and materials have been characterized by a scanning electron microscope (SEM) and X-ray diffractometry
Excitation Spectroscopy of Photoluminescence of a-Si:H
Authors: Gu S. Q. , Taylor P. C.
Published: March 2011

Photoluminescence excitation (PLE) spectra at 77 K have been measured over the range 1.201.75 eV using the Ti sapphire cw tunable laser as the excitation source. Two undoped a-Si:H samples on rough substrates have been investigated. The first sample has a very high dangling bond (DB) density; the second one has low DB density. The PLE spectrum of photoluminescence (PL) at 0.8 eV for the first sample follows the shape of the absorption spectrum measured by photothermal deflection spectroscopy (PDS) at room temperature. This behavior can be understood within the context of the existing models as due to recombination through defects which produces PL centered around 0.8 eV. However the PLE spectrum of PL at 0.8 eV for the second sample drops very rapidly with decreasing energy for energies less than about 1.3 eV. This behavior, which differs dramatically from that of the absorption spectrum, is consistent with earlier results and suggests that the PL measured at 0.8 eV for the second sample may be largely due to a contribution of the tail of the PL band which peaks near 1.3 eV. The PLE spectra for PL at 1.0 eV and 1.1 eV for the second sample approach the PLE spectrum previously obtained using the integrated PL intensities.
Effects of Interfaces on the a-Si:H Schottky Barrier Characteristics
Authors: Li Y. M. , Malone C. , Kumar S. , Wronski C. R. , Nguyen H. V. , Collins R. W.
Published: March 2011

A detailed study of hydrogenated amorphous silicon (a-Si:H) surfaces during etching and subsequent low temperature (T≤200°C) oxidation was carried out using in-situ and spectroscopic ellipsometry (SE). The microstructural information from SE is correlated with the properties of the corresponding evaporated Pd Schottky barrier structures. Oxide layers thinner than ∼ 10 Å have little effect on the diode characteristics and electron surface recombination velocities. This is consistent with a porous structure for these oxides. Evidence is found from growth kinetics that higher density, compact oxide growth occurs for greater thicknesses. These oxides have a large effect on electron transport and surface recombination velocities consistent with the formation of metal-insulator-semiconductor structures.
Optical Characterization of Interface, Surface and Bulk Defects in A-Si:H
Authors: Persans P. D. , Arnzen D. , Possin G. E
Published: March 2011

We present a new analysis approach which combines photoconductivity and photother-mal deflection spectroscopies in a-Si:H thin films of various thicknesses to profile electronic defects.
High Quality Interfaces in a-Si:H/a-Sic:H Superlattices
Authors: Bernhard N. , Kirsch M. , Eigenschenk R. , Bollu M. , Wetzel C. , Müller F. , Schwarz R.
Published: March 2011

Amorphous multilayers with low interface defect density were deposited in an ultrahigh-vacuum RF-glow discharge reactor by computer-controlled fast gas switching. An extensive film characterization was done including determination of carbon content by PES (proton enhanced scattering), grazing-incidence X-ray diffraction, measurement of ambipolar diffusion length, FTIR spectroscopy and hydrogen effusion. From PDS (photothermal deflection spectroscopy) a defect density of below 1011 cm−2 per interface is found.
Study of Band Tails in a-SiGe:H Alloys from Time of Flight and Thermally Stimulated Current Experiments
Authors: Longeaud C. , Kleider J. P. , Mencaraglia D. , Mohammed-Brahim T. , Rahal A.
Published: March 2011

Thermally stimulated current (TSC) and Time of flight (TOF) experiments have been performed on the same set of Pt/a-SiGe:H Schottky diodes in order to compare the band tail characteristics obtained from the two methods. For the silicon rich alloys, good agreement is found in the values of the band tail width. However, the values of the attempt-to-escape frequencies obtained from TSC are typically orders of magnitude smaller than those obtained from TOF. To analyse the origin of this discrepancy we propose another kind of TSC experiment denoted Pulsed TSC: short voltage pulses are applied to the sample while it is heated and the resulting current is monitored. Preliminary results are presented and discussed .
Studies on Carrier Transport Through a-Si:H/a-Si1−xCxH Quantum Well Structures Imbedded in the i Layer of a p-i-n Structure
Authors: Jiang Y. L. , Hwang H. L. , Feng M. S.
Published: March 2011

a-Si:H/a-Si1−XCX:H single and double barrier structures imbedded at different locations of the i layer of a p-i-n structure were used to study the carrier transport properties of amorphous QW. The shapes of the observed I-V curves of both structures are similar. The bound states of the well layer of double barrier structures do not play a role in the carrier tunneling process. The resonant tunneling time is larger than the inelastic scattering time by 5 to 12 orders of magnitude. When the tunneling process of amorphous QW is in the completely incoherent tunneling regime, the peak-to-valley ratio of NDR and the density of states in the well go to zero. From the analysis of I-V characteristics of both structures under different monochromatic light illumination, the observed current dips are due to the collection of photogenerated carriers in two regions of the i layer separated by the imbedded barrier structure.
Photoinduced Noise in Amorphous Semiconductor Films
Authors: Demichelis F. , Pirri C.F. , Tagliaferro A.
Published: March 2011

Electrical noise measurements have been performed on amorphous semiconductor films under irradiation with monochromatic light at different wavelengths. The Power Spectrum Densities (PSDs) of highly photoconductive undoped a-Si:H and a-SiC:H (deposited by glow discharge) in the frequency range .1 – 20 kHz are obtained at room temperature. The analysis is performed upon metal/semiconductor/ITO Schottky barriers. The PSDs vs frequency follow l/fn law and show a change of magnitude when the wavelength of the incident light is varied around the value corresponding to the energy gap.
Proximity Recovery Layers to Speed up the Recovery of Stressed Amorphous Silicon Thin-Film Transistors
Authors: Hack M. , Jackson W. B. , Lujan R.
Published: March 2011

We have developed a means to speed up the recovery of both the threshold voltage shift of hydrogenated amorphous silicon (a-Si:H) transistors and the Vx shift of high voltage a-Si devices. This is accomplished by placing a lightly doped compensated layer adjacent to the active layer in these transistors. This proximity recovery layer does not alter the initial characteristics of a-Si:H transistors and is completely process compatible with standard fabrication procedures.
Thermal-CVD Produced Amorphous-Silicon Thin-Film Transistors-Ambipolar Characteristics
Authors: Kanoh Hiroshi , Sugiura Osamu , Breddels Paul A. , Matsumura Masakiyo
Published: March 2011

Amorphous-silicon thin-film transistors (a-Si TFTs) have been fabricated by using an a-Si layer deposited by low-temperature thermal-CVD method using higher silanes. The TFT with thermally grown SiO2 gate was operated in both the n-channel mode and the p-channel mode. The maximum field-effect mobility and typical on/off current ratio were 1.5 cm2/Vs and 107 for the n-channel operation, and 0.2 cm2/Vs and 106 for the p-channel operation, respectively. Free electron mobility in the conduction band and free hole mobility in the valence band have been estimated, using the temperature dependence of the field-effect mobilities. It was found that the hole mobility is as high as the electron mobility.
Threshold Voltage Shifts in Amorphous Silicon Thin Film Transistors under Bias Stress
Authors: Ogawa Tetsu , Hotta Sadayoshi , Takezawa Horoyoshi
Published: March 2011

Through the time and temperature dependence measurements on threshold voltage shifts (Δ VT) in amorphous silicon thin film transistors, it has been found that two separate instability mechanisms exist; within short stress time ranges Δ Vτ increases as log t and this behavior corresponds to charge trapping in SiN. On the other hand, in long stress time ranges Δ VT increases as t t/4 and can be explained by time-dependent creation of trap in a-Si.
Measurements of 1/f Noise in A-Si:H Pin Diodes and Thin-Film-Transistors
Authors: Cho Gyuseong , Drewery J. S. , Fujieda I. , Jing T. , Kaplan S. N. , Perez-Mendez V. , Qureshi S. , Wildermuth D. , Street R. A.
Published: March 2011

We measured the equivalent noise charge of a-Si:H pin diodes (5 ∼ 45 μm i-layer) with a pulse shaping time of 2.5 μ.sec under reverse biases up to 30 V/μm and analyzed it as a four component noise source. The frequency spectra of 1/f noise in the soft-breakdown region and of the Nyquist noise from contact resistance of diodes were measured. Using the conversion equations for a CR-RC shaper, we identified the contact resistance noise and the 1/f noise as the main noise sources in the low bias and high bias regions respectively. The 1/f noise of a-Si:H TFTs with channel length of 15 μm was measured to be the dominant component up to ∼100kHz for both saturation and linear regions.
Amorphous Silicon Sensor Arrays for Radiation Imaging
Authors: Street R. A. , Nelson S. , Antonuk L. , Perez Mendez V.
Published: March 2011

Large two-dimensional a-Si:H imaging arrays have applications in imaging of X-ray, gamma-ray and charged particle fluxes. Radiation imaging using a-Si:H p i n sensors together with a phosphor to transform the radiation into visible light is described, and some examples of images are shown. These arrays are very efficient in imaging large area diffuse sources, and the a-Si:H sensors have little radiation damage. The main limitation to the sensitivity of the array is the leakage current of the sensors, which is a source of noise and limits the integration time of the arrays. The contributions to the leakage current from bulk thermal generation and contact injection are discussed
Tunneling Assisted Photocurrent Multiplication in a-Si Based p-i/Sinx/i-n Structure Junction
Authors: Yoshimi M. , Hattori K. , Okamoto H. , Hamakawa Y.
Published: March 2011

Photocurrent multiplication has been observed in a-Si based p−i/SiNx/i−n junction cells under reverse biased high electric field. An apparent external quantum efficiency exceeds 20. A systematic investigation on electric and optoelectronic properties has been made to clarify the mechanism of photocarrier multiplication. The results indicate the possibility of inter-band tunneling via localized states in the a-SiN layer, which is induced by field-redistribution due to the built-up of trapped charges at the a-SiN/a-Si interface.
Nmr and Esr Studies of the Structural Defects of Amorphous and Microcrystalline Sic Prepared by the Polymer Route
Authors: Ting S.-J. , Chu C.-J. , Mackenzie J.D
Published: March 2011

Amorphous SiC was prepared by firing the polycarbosilane to various temperatures ranging from 500°C to 1200°C. However the l3c CPMAS NMR and ESR studies show that the carbon rich SiC has two major structural defects, C=C bonds and C-dangling bonds. Both defects concentrations vary with different sample firing conditions. Hydrogen has dramatic effect on decreasing the defect concentrations when used in the sample firing.
Direct a-Si:H Photo-CVD Deposition with the Excimer Spectra of a Dielectric-Barrier Discharge Lamp
Authors: Kessler F. , Mohring H.-D. , Bauer G.H.
Published: March 2011

A vacuum UV-photo-CVD reactor for direct decomposition of Si2H6, SiH4, GeH4, and B2H6 for a-Si:H, a-Ge:H, and a-SiGe:H deposition has been operated with a large area dielectric-barrier discharge lamp emitting excimer radiation of Xe (7.3eV), Kr (8.6eV), or Ar (9.8eV).

It is shown that the deposition process (Si2H6 or GeH4 at 0.5 mbar) is governed by the diffusion of long-lifetime radicals.

High quality undoped and p-type doped a-Si:H has been deposited from Si2H6 and B2H6 with the Xe lamp spectrum (σph/σ≈106 at Eg=1.9eV) and film growth rates of up to 5 nm/min have been achieved.
Deposition of a-Ge:H in a Remote Plasma System
Authors: Heintze M. , Nebel C. E. , Bauer G. H.
Published: March 2011

The remote plasma deposition process was studied by optical emission spectroscopy using Ne and N2 to detect He-metastables. a- Ge:H was prepared and its optoelectronic and structural properties were characterized. AM 1.5 photoconductivities around 10−6(Ωcm)−1 were obtained in intrinsic material with the Fermi level position lying near midgap. However, even in the best film the defect density is higher than 1017cm−3.
Hydrogen Incorporation in Amorphous Silicon Prepared at High Deposition Rates by the VHF-GD Technique
Authors: Finger F. , Viret V. , Shah A. , Tang X.-M. , Weber J. , Beyer W.
Published: March 2011

The influence of deposition temperature on hydrogen incorporation in a-Si:H prepared by VHF glow discharge at 70 MHz is investigated using hydrogen evolution, elastic recoil detection analysis and infrared spectroscopy.The films were further characterized by dark- and photoconductivity and by photothermal deflection spectroscopy. While the electronic film properties deteriorate in the usual manner with decreasing substrate temperature it is found that the total hydrogen content CH and the degree of microstructure that can be directly correlated to CH increase only moderately. It is concluded that a higher flux of low energy ions in the VHF plasma plays a key role in this context, possibly by increasing the surface mobility of the H atoms and thereby preventing the build-in of a large amount of hydrogen at low substrate temperatures.
Energy Resolved Mass Spectrometry of the a-Si:D Film Growth Species During DC Magnetron Sputtering
Authors: Myers A. M. , Ruzic D. N. , Maley N. , Doyle J. R. , Abelson J. R.
Published: March 2011

Double modulated beam mass spectrometry was used to obtain the first measurements of the identity, relative flux, and energy of film precursors in reactive dc magnetron sputtering. In these experiments, a 2″ diameter silicon target was sputtered in an argon plus deuterium plasma at an argon pressure of 1.5 mTorr. This system produces high quality a-Si:D. Energetic neutral Si and SiD were observed, while energetic neutral SiD2 and SiD3 were absent. The arrival rate of D in the form of SiD increased and then saturated at 19 percent of the total flux as the deuterium pressure was varied from 0 to 5 mTorr. The sputtered Si energy agreed qualitatively with the standard Thompson theory. Energetic ions located at mass 2 and 4 were measured with energies much greater than the sputtered Si. Thermal silane was also detected, raising the possibility that thermal silane radicals in addition to energetic Si and SiD contribute to film growth.
Annealing of Defects During the Deposition of Amorphous Silicon
Authors: Hata N. , i Cabarrocas P. Roca , Wyrsch N. , Wagner S. , Favre M.
Published: March 2011

The thickness dependence of the sub-gap optical absorption in plasma-deposited hydrogenated amorphous silicon is carefully studied by photo-thermal deflection spectroscopy. The deep-level defect concentration decays from the top surface into the bulk where it approaches the thermal equilibrium defect density. This defect profile is interpreted in terms of the annealing, during growth, of growth-induced surface defects. It is also shown that the defect profile is compatible with the known growth-temperature dependence of the average defect density in amorphous silicon.
Characterization of a-Si:H,Cl and a-Sic:H Films Prepared by Electron Cyclotron Resonance Plasma
Authors: Gangopadhyay S. , Trost T. , Kristiansen M. , Young C. , Zheng P. , Palsule C. , Pleil M.
Published: March 2011

Microwave excited electron cyclotron resonance (ECR) plasmas have been used to fabricate a-Si:H,Cl and a-SiC:H films. By varying the absorbed microwave power, highly photoconductive a-Si:H,Cl films were obtained. The a-SiC:H films do not have appreciable photoconductivity, but they show a bright, broad continuous wave (cw) photoluminescence (PL) in the visible region under near-uv excitation. We have measured the photoluminescence decay in the picosecond range and have resolved the spectra into four distinct components. The four luminescence components may correspond to various recombination mechanisms of photogenerated electron-hole pairs.
A New General Method for the Determination of Optical Properties of Amorphous Silicon Films
Authors: Amato G. , Boarino L. , Fizzotti F. , Manfredotti C.
Published: March 2011

We propose to apply a new method to model the optical response of amorphous silicon thin films. This method presents the advantage of having a good physical insight. On the other hand, although the model has been originally tested on different materials like a-Si, a-Ge and a-GaAs, we show that it is also sensitive to small differences like those that can exist between intrinsic and doped a-Si:H.
Optimization of Textured Tco Substrates for a-Si Solar Cells
Authors: Komakine Mitsuhiko , Nishimura Hiromichi , Gotoh Yoshio , Sato Kazuo , Kondo Kei
Published: March 2011

Textured TCO (Sn02:F) substrates were fabricated using atmospheric pressure CVD (APCVD). Various kinds of TCO developed were classified into three types (A, B, C) in terms of shape of texture, electrical and optical properties. TCO substrates of type A with low carrier concentration and pyramidal texture fulfill the conflicting properties of low absorption and low resistivity after hydrogen plasma treatment. By using type A TCO substrates, the highest conversion efficiencies of 12 % for a small area of 1cm×1cm as a-Si single junction solar cell were certified by several related NEDO research organizations.
Light-Induced Electron Spin Resonance in Gate-Quality Nitrogen-Rich Amorphous Silicon Nitride: Photo-Production and Photo-Bleaching
Authors: Tober E. D. , Crowder M. S. , Janicki J.
Published: March 2011

Electron spin resonance spectroscopy is used to monitor the light-induced, paramagnetic (neutral) defect density in gate-quality nitrogen-rich hydrogenated amorphous silicon nitride (a-SiN1.6:H) thin films. A new phenomenon has been observed in which the light-induced electron spin resonance (LESR) signal can be reduced (photo-bleached) by re-illuminating with monochromatic light. The extent to which the LESR signal is bleached depends strongly upon the incident photon energy used for re-illumination. Photo-bleaching as well as the photo-production of the LESR signal follow a stretched exponential dependence upon illumination time.
Annealing of Irreversible Defects in Hydrogenated and Unhydrogenated Amorphous Silicon Thin Films
Authors: Parsons G. N. , Wang C. , Lucovsky G.
Published: March 2011

We have prepared unhydrogenated and hydrogenated ([H] = 14 at.%) amorphous silicon thin films using magnetron sputtering with substrate temperature, TS = 40°C. After deposition, the films were annealed at temperatures between 150 and 200°C and conductivity was measured as a function of anneal time. We find that for that for both materials, the conductivity changes non-exponentially with annealing time. The characteristic time constant for annealing at 175°C is approximately the same in unhydrogenated films (τ≈100min) as found in films containing 14 atomic % hydrogen (τ≈200min).
Defect Density in Doped a-Si:H Films
Authors: Pierz K. , Mell H. , Fuhs W.
Published: March 2011

We compare the experimentally observed correlation between the density of charged dangling–bond defects and the Fermi–level position in n– and p–type a–Si:H films with the predictions of two hydrogen–related models for the thermal equilibrium state at the glass–transition temperature of the hydrogen glass in a–Si:H. Reasonable agreement is obtained with a defect reaction in which the transfer of hydrogen from Si–H bonds to weak bonds is limited to distances comparable to one atomic spacing. The results for p–type (boron–doped) films suggest that the formation of positively charged dangling–bond defects is not only determined by electronic energy differences but also by a relaxational energy gain of approximately 0.2eV.
Forces and Geometry Optimization in First-Principles Atomic Cluster Calculations
Authors: Jackson Koblar , Pederson Mark R. , Erwin Steven C.
Published: March 2011

We make use of the existing formalism for calculating atomic forces within the local density approximation (LDA) to determine forces in all-electron, local orbital electronic structure calculations. The forces are calculated as the proper total energy derivatives, including the necessary basis-set corrections. Our technique evaluates the LDA potential exactly on a variationally determined integration mesh which allows all integrals relevant to the electronic structure problem to be computed to any desired accuracy. We demonstrate the high accuracy of forces calculated using our method with an application to the molybdenum dimer. Several issues concerning the accuracy of the forces are discussed, including self-consistency effects, and the effects of integration error. We discuss the use of the forces in dynamical routines for geometry optimizations.
Theory of the Quasiparticle Effective Masses in Semiconductors based on an Electron Self Energy Approach
Authors: Zhu Xuejun , Hybertsen Mark S. , Louie Steven G.
Published: March 2011

A conceptually complete formalism for the quasiparticle effective masses in semiconductors is proposed. Our approach is based. on a generalized form of the theory, including the effects of the nonlocal, energy dependent electron self: energy operator Σ, which accounts for the electron-electron interaction. This introduces two -important effects on the expression of the effective mass: an explicit energy renormalization and an extra contribution to the matrix element that enters the usual . Our preliminary numerical results for prototypical GaAs show promising improvements over the results from the local density approximation for the calculated electron effective mass compared to experimental data.
On the Possibility of Passivation of Si(100) by Adsorption of Group-VI Atoms
Authors: Kaxiras Efthimios
Published: March 2011

The possibility of passivating the Si(100) surface by adsorption of Group-VI atoms (S and Se) is investigated through first-principles calculations. The structure of the ideal (1×1) configuration with the Si surface dangling bonds saturated by full monolayer coverage is examined in detail. The Group-VI adsorbates form covalent bonds to the substrate with bond-lengths very close to the sums of the covalent radii. The bond-angles are larger than in bulk configurations of the Group-VI elements. The ideal (1×1) configuration gives rise to a surface electronic state with large dispersion spanning the entire band-gap of Si. Deviations from this configuration by in-phase or out-of-phase tilting of the adsorbate atoms result in energy costs which can give qualitative information on the relative strength of adsorbate-adsorbate and adsorbate-substrate interactions.
The Atomic and Electronic Structures of Grain Boundaries in Silicon-Carbide and Silicon
Authors: Kohyama M. , Kose S. , Kinoshita M. , Yamamoto R.
Published: March 2011

The atomic and electronic structure of the {122} Σ=9 grain boundary in cubic SiC has been calculated for the first time using the self-consistent tight-binding (SCTB) method. An atomic model consisting of zigzag arrangement of 5-membered and 7-membered rings similar to that in the same boundary in Si or Ge has been constructed from a HREM image, although Si-Si and C-C wrong bonds are repeated alternately at the interface in this model. We have also performed calculations of the same boundary in Si using the SCTB method for comparison, and have obtained the results similar to those previously obtained by other theoretical schemes. The calculated boundary energy in SiC has shown that the present atomic model can exist stably as compared with the two surfaces, and the calculated boundary electronic structure in SiC has no deep states in the gap as well as that in Si. However, it has been found that the the increase in the electrostatic energy caused by the wrong bonds is a large part of the present boundary energy in SiC differently from that in Si, and it has been shown that the wrong bonds introduce the wrong-bond localised states at the band edges and within the valence band.
The Relationship between the Microscopic Properties of Semiconducting Grain Boundaries and their Orientation
Authors: Tarnow Eugen , Arias Tomas , Bristowe P. D. , Dallot P. , Francis G. P. , Joannopoulos J. D. , Payne M. C.
Published: March 2011

The microscopic structure of a tilt and a twist boundary in germanium are explored using a state-of-the-art total energy calculation. The structure of the tilt boundary (Σ = 5 (310)) is found to be simple as it exhibits a well defined minimum energy structure, consistent with previous experimental and theoretical results on this and other tilt boundaries. The structure of the twist boundary (Σ = 5 (100)), however, is found to be very complex. The boundary bonds are distorted and weak, and their weakness makes the twist boundary exhibit a wealth of local energy minima. The different types of energy minima for the twist boundaries are identified and studied in some detail. We summarize the consequence of the orientation of the two crystal grains upon the microscopic structure of the boundaries, and speculate about future applications.
Computer Simulation of Grain Boundary Structures in Ni3 Al
Authors: Lin Dongliang , Lin T. L. , Chen Da
Published: March 2011

The embedded atom type potentials and static relaxation method combined with steepest gradient computational technique have been used to simulate the grain boundary cohesive energies, the distribution of electron density and stress field in the grain boundary region, and other related problems of [100], [110] and [111] symmetric tilt grain boundaries in Ni3 Al with different grain boundary geometrical index and composition. Their relations with the segregation of boron, behaviors of the grain boundary, and especially the stoichiometrical effect of B induced ductility have also been studied and discussed.
The Nature of Crack Tip Fields in Atomic Scale Models of Aluminum
Authors: Hoagland R. G. , Daw M. S. , Foiles S. M. , Baskes M. I.
Published: March 2011

The stresses, displacement gradients, and Eshelby's F and M integrals are obtained for two crack orientations in an EAM atomic model of aluminum. For a sharp crack, the stresses are shown to agree quite well with the linear elastic prediction, and F is essentially path independent and also in good agreement with the linear elastic prediction. When dislocation emission and blunting ensues, the path independence of F disappears. In addition, for circular contours with origin at the crack tip, the M-integral is linear in contour radius with slope equal to twice the surface energy and zero intercept for a sharp crack, but acquires a nonzero intercept as blunting occurs. The shift in intercept is related to the movement of singularities away from the origin.