bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Neutron Irradiation and Post-Irradiation Annealing on the Radiant Output Power of Infrared Light Emitting Diodes
Authors: Hajghassem H. S. , Brown W. D. , Yeargan J. R. , Williams J. G.
Published: March 2011

This paper presents results of a study of the degradation of commercially available GaAs and AlGaAs light emitting diodes subjected to neutron bombardment at a TRIGA reactor. Devices were characterized using current-voltage and light output measurements prior to and following a sequence of neutron irradiations and after high temperature annealing. A model is derived which can be used to determine the lifetime damage constant product, τoK, if the light output measurements as a function of IMeV equivalent neutron fluence are made at a fixed operating current. For current levels smaller than approximately 1 ma, τoK and operating current is logarithmic with τoK decreasing as current increases. Annealing at temperature up to 275°C recovers some of the neutroninduced damage but does not affect the validity of the model.
Reliability of Ohmic Contacts for AlGaAs/GaAs HBTs
Authors: Jackson G. S. , Tong E. , Saledas P. , Kazior T. E. , Sprague R. , Brooks R. C. , Hsieh K. C.
Published: March 2011

The reliability of ohmic contacts to thin, heavily doped layers of GaAs is investigated. Pd/Ge/Au contacts to n-type GaAs display excellent electrical stability over extended periods of thermal stress. The contact resistance stays below 0.50Ω-mim during a 2500h, 280°C bake. Reactive ion beam assisted evaporation of Ti with N forms TiN which is introduced as a barrier layer in Pt/TiN/Ti/Au contacts to a thin p+ layer. The TiN layer allows greater process latitude in the sintering process and improves long term stability of the ohmic contact. The microstructure of the p-type contacts is examined with TEM and Auger profiling at different instances of the 2500h, 280°C bake and compared to the contact resistance measurements.
Pd-Ge-Au Ohmic Contacts to GaAs: Reliability and Failure Analysis
Authors: Kazior T. E. , Hieslmair H. , Brooks R. C.
Published: March 2011

We report on experiments that were performed to evaluate the temperature stability and long term reliability of non-alloyed Pd-Ge-Au ohmic contacts on N-type GaAs. Low resistance contacts (≈1×10−6 Ωcm2) were obtained for samples that were sintered in a conventional furnace or flash sintered in a graphite susceptor. Elevated temperature storage (≈4000 hours at 280°C) showed improved contact stability when compared to Ni-AuGe-Ni-Au control samples. Gateless MESFETs subjected to bias temperature stress measurements (Ids ≈300–350mA/mm, 2000–4000 hours at 200°C) showed no significant change in device current. This result is in contrast to devices with Ni-AuGe-Ni-Au ohmic contacts which exhibited a 6–27% decrease in current under the same test conditions. Failure analysis reveals significant electromigration and Au diffusion in the drain fingers of devices with Ni-AuGe-Ni-Au contacts. In contrast, devices with Pd-Ge-Au contacts show no elect romigration or Au diffusion in the GaAs.
Making Reliability Estimates when Zero Failures are Seen in Laboratory Aging
Authors: Nash Franklin R.
Published: March 2011

To estimate the probability of failure for a deployed population, when the laboratory aging of a sample certified population produces zero failures, an appropriate model is required. The widely used exponential (constant hazard rate) model, and a declining-hazard-rate Weibull model are contrasted. On balance the use of the Weibull model seems preferred because: (i) It is consistent with laboratory aging studies on many types of electronic components; (ii) It is less sensitive to the choices for parameters (e.g., thermal activation energy) whose values are not known accurately; and, (iii) It may yield more conservative (i.e., larger) values for the relevant hazard rate than the exponential model. The unavoidable uncertainties connected with the use of either model are discussed. An example is given of a plausible, but in fact mistaken, use of the exponential model that leads to disastrously incorrect reliability predictions.
Electroluminescence from Gunn Domains in GaAs MESFETS as a Means of Defect Detection
Authors: Zappe Hans P. , Jantz Wolfgang
Published: March 2011

The emission of visible light from the high-field region near the drain of GaAs/AlGaAs MESFETs has been used to study the quality of fabricated transistors. Inhomogeneities or bright spots in the emission have proven to indicate the presence of defects in the gate/drain spacing or surface contaminants, such as re-deposited gate material. Processing abnormalities of 0.5 μm diameter are easily seen. The time-variation of emission from the brightest spots is characteristic of the formation of microplasmas the location of which will often predict the site of destructive device breakdown.
Degradation of Hydrogen-Passivated p-Type Layers in GaAs by Minority Carrier Injection and Reverse Bias Annealing
Authors: Tavendale A. J. , Pearton S. J. , Williams A. A. , Alexiev D.
Published: March 2011

We detail experiments showing that acceptor passivation by atomic hydrogen in p-type GaAs is unstable to either illumination, forward bias annealing or reverse bias annealing. The long-term stability of operation of devices employing hydrogen in or near the active region of FETs or quantum-well lasers is therefore questionable. The systematics of acceptor reactivation during minority carrier injection or reverse bias annealing are presented.
Ordering Energy of B2 Alloys Calculated in the Frozen Potential and Harris Approximations
Authors: Shelton W. A. , Nicholson D. M. , Stocks G. M. , Pinski F. J. , Johnson D. D. , Sterne P. , Temmerman W. M.
Published: March 2011

It has been established that the coherent potential approximation can successfully describe the energy of random alloys [1]. It has also served as the basis of generalized perturbation method [2] and concentration wave [3] xscalculations of the energy of short range ordered alloys. The multisublattice coherent potential, (MCPA) is the natural extension of the CPA with which to address long range order (LRO). Using the recently developed multisublattice coherent potential approximation Korringa Kohn Rostoker [4], (MCPA-KKR) code the elgenvalue sum can be calculated as a function of LRO. This allows the evaluation of the ordering energy by either of two approximations. The frozen potential approximation (FPA) [5] assumes that the muffintin single site potentials do not change as the long range order is varied; the Harris Approximation, (HA) [6], as applied in this work, assumes that the single site charge densities do not change as the long range order is changed. These two methods of calculating the ordering energy will be compared with each other and to experiment for several systems including CuZn, NiAl, and NiAl with zinc additions.
Thermodynamic Stability of Ordered Intermetallic Compound Phases
Authors: Chang Y. Austin , Neumann Joachim P. , Chen Shuang-Tin
Published: March 2011

The Wagner-Schottky model was used to describe the thermodynamic behavior of ordered intermetallic compound phases. To demonstrate the utility of the approach, the models developed for triple-defect B2 (and B32) and anti-structure L10phases were used to describe the thermodynamic properties of β-AlLi and γ-TiAl respectively. Since any potential engineering materials to be developed on the basis of intermetallics will be multi-component systems, the methodology was extended to describe the thermodynamic properties of ternary intermetallics. The ternary Ti-Mo-Al system was used as an example for discussion. It is believed that the general topologies concerning the phase equilibria of Ti-M-AI with M being V, Nb, Ta, Mo and W are similar. The relative stabilities of the competing phases, i.e. BCC and HCP, in the mid-composition region of Ti-M-AI were discussed.
Kinetics of Ordering in Fe3Al: Experiment
Authors: Fultz Brent , Gao Zheng-Qiang , Anthony Lawrence
Published: March 2011

Alloys near the Fe3Al stoichiometry were prepared far from thermodynamic equilibrium by rapid solidification methods. These highly disordered materials were then annealed at low temperatures to develop B2 and D03 order. Both shortrange and long-range order (SRO and LRO) were measured by Mössbauer spectrometry and by x-ray diffractometry, respectively. The SRO was found to evolve in two stages: an initial quick growth of the SRO alone, followed by a slower growth of SRO along with LRO. As expected, at lower temperatures the absolute rates of growth of order were suppressed, but more interestingly, the relative rates of growth of B2 and D03 long-range order parameters were different at different temperatures of annealing. When the B2 order parameter is graphed against the D03 order parameter, at lower temperatures the “kinetic path” through this graph shows that B2 order evolves relatively more rapidly than D03 order.
Morphological and Structural Characterization of the Spinodal Decomposition of FE-CR Alloys.
Authors: Cerezo A. , Hetherington M.G. , Hyde J.M. , Miller M.K.
Published: March 2011

The position sensitive atom-probe (POSAP) is capable of extremely high resolution (subnanometer) microanalysis. Its 3-dimensional capabilities are unique and offer a new opportunity to study the topology and structure of materials. This paper is an initial overview of the new types of parameters and analysis that this technique makes possible.
The Measurement of Composition During the Early Stages of the Spinodal Decomposition of Fe-Cr Alloys
Authors: Hetherington M.G. , Hyde J.M. , Miller M.K.
Published: March 2011

The properties of many advanced alloys are derived from extremely fine-scale microstructures. This poses interesting questions about the measurement of composition on this scale. The phase separation of model Fe-Cr alloys has been been studied with the atom-probe. Statistical techniques have been used to estimate the composition and compare the results with the predictions of linear and non-linear theories of spinodal decomposition and the distributions obtained from Monte-Carlo calculations.
Dislocation Core Structures in Nial.
Authors: Pasianot R. , Farkas D.
Published: March 2011

Since the early studies on bcc materials, computer simulation of dislocation core structures has been recognized as a valid means of understanding yield behavior with temperature. Many of today's high temperature ordered alloys do have yield anomalies and computer simulation of core structures is a basic ingredient in the corrent theories for the anomalous yield behavior in Ll2alloys. In the case of B2 type alloys there are very few of these studies. The present work deals with computer simulation of core structures in B2 NiAl. An embedded atom type potential is used to simulate interactions and several slip systems are considered, particularly those having [100] and [111] slip vcctors that are the experimentally observed.
The Electronic Structure And Phase Stabilities Of Metallic Alloys
Authors: Györffy B.L. , Barbieri A. , Jolnsoni D.D. , Nicholson D.M. , Pinski F.J. , Shelton W.A. , Stocks G.M.
Published: March 2011

Condensed matter consists of positively charged atomic nuclei and the electron glue which holds them together. Although it is attractive to attempt to oilodl it by nentral atoms interacting via. relatively weak pairwise ‘effective interactions’ in genbreak down the predictive and inter;retative power of the analysis. Hence, there is no substitute to forging a parameter-free and quantitative theory of the complex degenerate Fermi liquid ‘glue’ to complement phenomenological calculations. Such theory is the general aim of ’first-principles’ calculations of the electronic structure. This talk will concern past present and future first-principles calculations relevant it the state of compositional order in metallic alloys. Special attention will be paid to phaase insta bilities induced by Fermi Surface effects.
Microstructure and Compressive Properties of Fe-Modified L12-TYPE Al3Ti
Authors: Pak H.R. , Wayman C.M. , Favrow L.H. , Cooper C.V. , Pak J.S.L.
Published: March 2011

The microstructures and mechanical properties of an Fe-modified L12 alloy containing 7.5 at.% Fe have been investigated. This alloy has been determined to be essentially single phase following a homogenization heat treatment (HHT) at 1100°C for 100h, with a very small volume fraction of precipitates having been observed to form along dislocations. Conversely, in the case of the as-cast (AC) condition, the alloy has been determined to contain band-like precipitates, which have also formed along dislocations. In addition, a high density of very thin plate-like precipitates have formed parallel to {001} planes of the L12 matrix. Although these plate-like precipitates appear to exhibit lattice tetragonality, their crystal structure cannot be explained by assuming D022 and D023 structures. Five different <110>-type dislocations havfe been activated within a small region of a matrix grain during deformation at 1100°C, some of which cross slipped from {111} to {001} planes. Specimens in both AC and HHT conditions were deformed in compression to approximately 0.5% without fracture at both 22 and 1 100°C, the yield stress for the HIT condition having been determined to be 192 MPa at 22°C and 98 MPa at 1100°C.
Mechanical Properties, Fracture Behavior, and Grainboundary Chemistry of B-Doped Nial
Authors: George E. P. , Liu C. T. , Liao J. J.
Published: March 2011

This paper summarizes the results of our work aimed at overcoming the intrinsic grainboundary weakness of NiAI by microalloying with boron. In previous work we have shown that 300 wppm boron is very effective in suppressing intergranular fracture in NiAI [1]. It does this by segregating strongly to the grain boundaries and strengthening them. Despite this dramatic effect on the fracture mode, however, boron is unable to improve ductility because it is a potent solid solution strengthener, more than doubling the yield strength relative to that of undoped NiA1. The present work attempts to decrease this deleterious hardening effect by lowering the bulk concentration of boron in NiA1. Our results show that if the boron concentration in the bulk is lowered to 30 wppm, the yield strength of boron-doped NiA1 is only about 30% higher than that of undoped NiAI. In addition, there is enough boron at the grain boundaries of this alloy to suppress intergranular fracture. Under these conditions, boron-doped NiAI has a tensile ductility of 2%, which is essentially identical to that of undoped NiA1. This result, namely that the strengthening of grain boundaries by boron does not by itself improve ductility, indicates that although grain boundaries might well be the weakest links in NiAI, cleavage planes are not much stronger. In other words, even though boron additions serve to strengthen the grain boundaries and suppress intergranular fracture, ductility is not improved, because the next brittle fracture mode, namely transgranular cleavage, takes over before significant plastic deformation can occur.
A First Principles Examination of Phase Stability in Fcc-Based Ni-V Substitutional Alloys
Authors: Mikalopas J. , Turchi P.E.A. , Sluiter M. , Sterne P.A.
Published: March 2011

The phase stability of fcc-based Ni-V substitutional alloys is investigated using linear muffin-tin orbitals total energy (LMTO) calculations. The method of Connolly and Williams (CWM) is used to extract many body interactions from the ground state energies of selected ordered configurations. These interactions are used in conjunction with the cluster variation method (CVM) to calculate the alloy phase diagram. The dependence of the interactions on the choice of configurations used to calculate them is examined.
Room Temperature Oxidation of Silicon Catalyzed by Cu3Si
Authors: Harper J.M.E. , Charai A. , Stolt L. , d'Heurle F.M. , Fryer P.M.
Published: March 2011

We demonstrate remarkably rapid oxidation of (100) silicon at room temperature catalyzed by the presence of Cu3Si. Thermal oxidation of Si is normally carried out at temperatures above 700°C. Oxidation of many metal silicides occurs more rapidly than that of Si, but under controlled conditions results in a surface layer of SiO2. In contrast, the oxidation process described here produces a thick layer of SiO2 underneath
the copper-rich surface layer. The SiO2 layer grows spontaneously to over one micrometer in thickness in several weeks in air at room temperature. Analysis by Rutherford backscattering, Auger electron spectroscopy, cross-sectional transmission electron microscopy and scanning electron microscopy reveals the presence of Cu3Si at the buried SiO2/Si interface, epitaxially related to the underlying Si substrate. Catalytic action by this silicide phase appears responsible for the unusual oxidation process.
Crystal Nucleation in Amorphous Si Thin Films During Ion Irradiation
Authors: Im James S. , Atwater Harry A.
Published: March 2011

The nucleation and transformation kinetics of the amorphous-to-crystal transition in Si films under 1.5 MeV Xe+ irradiation have been investigated by means of in situ transmission electron microscopy in the temperature range T = 480–580°C. After an incubation period during which negligible nucleation occurs, a constant nucleation rate was observed in steady state, suggesting homogeneous nucleation. A significant enhancement in nucleation rate during high energy ion irradiation (6 orders of magnitude) was observed as compared with thermal crystallization, with an apparent activation energy of Qn = 3.9 ± 0.75 eV. Independent analyses of the temperature dependence of the incubation time, the crystal growth rate, and nucleation rate suggest that interface rearrangement kinetics and not the thermodynamic barrier to crystallization, are affected by ion irradiation.
Structure and Phase Transformations in Thermoelastic Ni(1−x)TiCux) Thin Films Prepared by D.C. Magnetron Sputtering.
Authors: Chang L. , Hu-Simpsono C. , Grummon D. S. , Pratt W. , loloee R.
Published: March 2011

The intermetallic compound NiTi and its copper containing variants, Ni(1−x)TiCu(x), are capableof stress induced displacive tansformations which give rise to superelasticity and shape-memory effects. Thin films of these alloys, which may find use as sensors, microactuators, or as protective surface microalloys, are of increasing interest. In this study, thin films in the Ni(1−x)TiCUx) system, ranging in thickness from 250 nm to 5 μm, were prepared with a triode-type D.C. magnetron sputtering apparatus using a ternary alloy target. Both homogeneous films and periodic multilayer structures (with interspersed Ti-rich layers) were fabricated. of particular interest were shifts in composition with respect to the target, and the structural and phase transformation characteristics of the sputtered films. These phenomena were examined using transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray microanalysis. Transformation temperatures and enthalpies were determined by differential scanning calorimetry and confirmed by 4-point D.C. resistivity measurements. The as-sputtered films were amorphousbut crystallized during annealing at 923K. Electron diffraction and XPS data suggested the presence of oxygen. Single layer film compositions were shifted to slightly lower Ti fractions which depressed the onset of the transformation sequence by ˜35K and degraded phase stability during annealing. It was possible, however, to produce periodic multilayer films which showed evidence of thermoelastic phase transformations during DSC, resistivity, and in-situ TEM experiments. These films were stable with respect to vacuum annealing at 923 K for 6 h.
A Phase Diagram Approach for Predicting Reactions in Al/TiW(-Nitride) Thin-Film Systems
Authors: Bhansali A.S. , Raaijmakers I.J.M.M. , Sinclair R. , Morgan A.E. , Burrow B.J. , Arst M.
Published: March 2011

A quaternary phase diagram for Ti–W–N–Al has been calculated from existing thermodynamic data, and is used to predict the Al/TiW-nitride reaction. The predicted reaction products-TiAl3, WAl5, WAl12, and AlN—were observed by XRD and TEM in annealed Al/TiW(-nitride) thin films. The sheet resistance of Al/TiW films increased by an order of magnitude at 550°C, whereas the increase in the case of the Al/TiW-nitride films was not even two-fold. The formation of an interfacial AlN layer was observed in the Al/TiW-nitride metallization. This AlN layer limits the interaction between Al and TiW-nitride, thus providing good thermal stability.
Composition and Structure Characterization of WNx Films Produced by RF Reactive Sputtering
Authors: Lin Dongliang , Yan Bewda , Yu Weili
Published: March 2011

WNx film is one of the most promising materials for self-aligned GaAs NASFET because of its low electrical resistivity and high Schottky Barrier Height at the WNx /GaAs contact.

In this paper, the effect of the sputtering conditions and the annealing environment on the chemical composition and structure of the WNx films deposited on Si and GaAs by RF reactive sputtering are studied.

The results show that with the increase of the partial pressure ratio of nitrogen gas or decrease of the working pressure, deposition rate of WNx film decreases, whereas the atomic percentage of N in the deposited film increases before approaching saturation. The WNx films formed at high working pressure (≈5 × 10−2 torr) consist of W, WN or W2N phases depending on the nitrogen partial pressure ratio. Whereas the films formed at low working pressure (≈ 3 × 10−3 torr) are usually amorphous. Annealing in a flowing N2 gas causes the crystallization of the amorphous films, which mainly consist of W+W2N. There is no change for the crystalline films. However, annealing in H2 gas causes severe loss of nitrogen of the film, the film becoming single W phase eventually.
Laser Deposition of Molybdenum Oxide Thin Films From Organometallic Precursors
Authors: Olson Kurt A. , Schrader Glenn L.
Published: March 2011

Thin films of crystalline molybdenum oxides were formed on silicon surfaces by laser deposition. An ArF excimer laser was used to photodissociate molybdenum hexacarbonyl in an oxygen-containing ambient. Normal incidence of the laser led to the formation of very thin (< 1000 Å) films of crystalline MO4O11. The films were characterized by color, SEM, Raman spectroscopy, and X-ray diffraction.
Correlating CVD Process Parameters and Film Properties
Authors: Chow D. K.
Published: March 2011

This paper addresses an approach of predicting the effects of deposition process parameters on the film properties. The film resistivity is shown to correlate with other properties and testing conditions in explicit group form, and so is the rate of deposition with process parameters. Through a physical property relationship the two correlations merge to permit the prediction. Justifications for the correlations derive from both experimental and theoretical work already reported. Discussions on relative sensitivities of the variables toward another will follow.
The Epitaxial Growth of Copper on the (111) Surface of Silicon
Authors: Walker F. J. , Conner J. R. , Mckee R. A.
Published: March 2011

The progression of the epitaxy of Cu(111) on Si(111) has been studied using surface sensitive techniques and the as-grown film studied using high resolution transmission electron microscopy (HRTEM). A molecular beam epitaxy (MBE) system has been used to deposit films on 3-inch-diameter silicon substrates. Across this large area a graded thickness of 0 to 10 nm of copper was deposited and scanned using Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). For films grown at 373 K, silicon alloys with the copper up to a thickness of 5.0 rim. LEED indicates an ordered R30°-√3×√3 Cu(111) surface which results from the alloying of copper and silicon. HRTEM images from the interface region show the alloy to be 2.3 nm thick, crystalline, and epitaxial to the Si(111) surface. There are indications that the interface alloy may be a metastable copper silicide. This alloy is important in the mechanism for the growth of Cu(111) on Si(111).
Atom Probe and Hrem Characterization of Transition Metal Multilayers.
Authors: Cerezo A. , Hetherington M.G. , Petford-Long A.J.K.
Published: March 2011

The direct measurement of the composition variations in transition metal multilayers has been carried out using the atom-probe and the position sensitive atom-probe (POSAP). The atom-probe and POSAP do not have sufficient resolution to determine the crystal structure and this complementary information has been obtained using high resolution electron microscopy.
Kinetics and Thermodynamics of Amorphous Silicide Formation in Metal/Amorphous-Silicon Multilayer Thin Films
Authors: Thompson C. V. , Clevenger L. A. , DeAvillez R. , Ma E. , Miura H.
Published: March 2011

Formation of intermetallic phases upon heating of films composed of alternating layers of metal and amorphous silicon has been studied using power-compensated differential scanning calorimetry, crosssectional transmission electron microscopy, and thin film x-ray diffractrometry. Results for Ni/amorphous-Si (Ni/a-Si), Ti/a-Si, V/a- Si, and Co/a-Si are reviewed. In the first three cases, an amorphous silicide is the first phase to form. Further heating leads to thickening of the amorphous silicide and eventually to formation and growth of layers of crystalline silicides. In the case of Co/a-Si multilayer films, a crystalline silicide (CoSi) appears to be the first phase to form. In these systems calorimetric measurements suggest that there are barriers to nucleation of the crystalline phases, even though the energy reduction that would accompany their formation from pure components is large. It is suggested that interdiffusion may precede the formation of new phases at the original metal/a-Si interfaces, resulting in a significant decrease in the driving force for nucleation of the crystalline phases.
Structure and Thermodynamics of Amorphous Ti-Si Produced by Solid-State Interdiffusion
Authors: Holloway Karen , Moine Pierre , Delage Jacques , Bormann Rüdiger , Capuano Linda , Sinclair Robert
Published: March 2011

The thermodynamic and structural aspects of amorphous titanium silicide formation by solid-state reaction have been addressed. The radial distribution function of an amorphous Ti60Si40 reaction product has been determined by electron diffraction. It is an amorphous metallic alloy, with a coordination number of about 11.4, and a local arrangement of atoms which is similar to that in a crystalline silicide, Ti5Si4. The free energy of the amorphous alloy as a function of composition has been determined from the heats of crystallization of co-deposited amorphous Ti-Si alloys, and the heat of formation of the amorphization product as measured by differential scanning calorimetry (DSC). The latter was measured to be 35 ± 5 kJ/mole. Using these data, free energy of the amorphous Ti-Si phase has been calculated by applying the CALPHAD method. These results are discussed in light of the mechanism and energetics of the amorphization reaction.
Kinetic and Thermodynamic Aspects of Phase Evolution in Ti/a-Si Multilayer Films
Authors: Ma E. , Clevenger L.A. , Thompson C.V. , Tu K.N.
Published: March 2011

The growth of an amorphous Ti-Si phase and subsequent formation of crystalline silicides during solid-state reactions in Ti/a-Si multilayer films have been studied using power-compensated differential scanning calorimetry, cross-sectional transmission electron microscopy, and thin-film x-ray diffraction. By analyzing calorimetric data we have determined the activation energies for the formation of the various silicides (amorphous Ti-silicide, TiSi, C49-TiSi2, Ti5Si3) as well as their heats of formation. An amorphous silicide is the first phase to form during heating and we have measured the composition profile of this amorphous layer using scanning transimission electron microscopy. Metastable phase equilibria in the Ti-Si system are discussed in light of the thermodynamic and compositional information obtained in our experiments.
Formation and Growth of Amorphous Phases by Solid-State Reactions Between Co Thin-Films and III-V Compound Semiconductors
Authors: Shiau F.Y. , Chang Y.A.
Published: March 2011

Solid-state amorphizaiton reactions (SSAR's) between Co thin-films and GaAs and InP have been investigated by transmission electron microscopy and Auger electron spectroscopy. Upon annealing at 260–300 °C, an amorphous phase was observed in the Co/GaAs system. Annealing at higher temperatures or for longer times led to the crystallization of this phase into a supersaturated CoAs structure. In the Co/InP system, annealing at 200°C for 1 hour led to the simultaneous formation of Co2P and an amorphous Co-In phase. At higher temperatures the amorphous phase crystallized into CoIn2 structure. For both systems, the amorphization reactions are attributed to the rapid diffusion of Co into the III-V substrates. The factors governing SSAR in ternary systems were discussed, with emphasis on the role of kinetics as opposed. to thermodynamic considerations.
Chemical Reactivity of Metal Particles Produced by Laser Ablation into Liquids
Authors: Parker W. L. , Boudjouk Philip
Published: March 2011

Spectroscopic evidence of the chemical differences between metal particles produced by laser ablation of metals into reactive fluid solvents and metal particles produced by less energetic means is presented. Specifically. 1) ablation of copper into fluid methanol produces metal particles in which some fraction of the metal is present In the cuprous oxidation state and 2) ablation of nickel into tetrahydrofuran results In metal particles less reactive to the solvent than those produced by ultrasound promoted chemical reduction.
Computer Simulation of Pulsed Laser Ablation for YBaCuO Superconducting Films
Authors: Nakamiya Toshiyuki , Ebihara Kenji , John P. K. , Tong B. Y.
Published: March 2011

The dynamics of melting and ablation of high Tc YBa2Cu3O7-x superconducting thin films flashed by a pulsed KrF excimer laser(λ=248nm) or a pulsed Nd-YAG laser (λ =1.06μ m) were studied numerically. The fundamental model during a pulsed laser irradiation was a one-dimensional heat conduction equation. The finite element method was applied to solve the equation including the temperature dependence of the thermal conductivity of YBaCuO thin films. In addition, the microstructure of YBa2Cu3O7-x bulk(l.5mm thick) flashed by a pulsed XeCl excimer laser (λ =308nm) was investigated by scanning electron microscopy (SEM) in order to estimate the threshold incident laser energy density for surface melting and ablation. The good agreements between the numerical calculations and the experimental results were obtained.
Pulsed Laser Deposition of High Tc Superconducting thin Films: Present and Future
Authors: Wu X. D. , Venkatesan T. , Inam A. , Xi X. X. , Li Q. , McLean W. L. , Chang C. C. , Hwang D. M. , Ramesh R. , Nazar L. , Wilkens B. , Schwarz S. A. , Ravi R. T. , Martinez J. A. , England P. , Tarascon J. M. , Muenchausen R. E. , Foltyn S. , Estler R. C. , Dye R. C. , Garcia A. R. , Nogar N. S.
Published: March 2011

Pulsed laser deposition (PLD) has been widely used for deposition of high Tc superconducting thin films, and is recognized as one of the best physical vapor techniques for the preparation of these films. The most important advantage of this technique is stoichiometric deposition; films can be made with the same composition as the target. Utilizing PLD, not only thin films but also multilayers and superlattices of high Tc superconductors have been fabricated. In this paper, the performance of the technique will be reviewed, and speculations regarding the future would be made.
Heteroepitaxial Metal Oxides on Silicon by Laser Ablation
Authors: Fenner D. B. , Fork D. K. , Connell G. A. N. , Boyce J. B. , Ponce F. A. , Tramontana J. C. , Viano A. M. , Geballe T. H.
Published: March 2011

Thin epitaxial films of cubic - fluorite structured PrO2 and YSZ (yttria- stabilized zirconia) were grown on single crystal silicon substrates using the laser ablation - deposition technique. X-ray diffraction theta two - theta, omega rocking and phi scans indicate a high degree of epitaxial orientation of the films to the Si lattice. The highest quality of epitaxy was obtained with the PrO2 [111] oriented normal to Si(111) surfaces and the cubic YSZ [100] normal to Si(100) surfaces. For both PrO2 and YSZ, high epitaxial quality required the removal of the Si native oxide prior to deposition and careful control of the deposition environment. It was further found that the YSZ films on Si(100) were an excellent surface for subsequent laser ablation of YBCO films by the usual in situ process. The resistivity of this YBCO was ≈ 250 micro-ohm-cm at 300 K, extrapolated to the resistivity -temperature origin, showed a sharp transition to zero resistance at ≈ 85 K and was nearly identical to high quality YBCO films deposited on (bulk) YSZ substrates.
Spectroscopic and Ion Probe Characterization of the Transport Process Following Laser Ablation of Yba2Cu3Ox
Authors: Geohegan David B. , Mashburn Douglas N.
Published: March 2011

Spatial and temporal measurements of the optical absorption, optical emission and ion probe response in the ablation plume formed following pulsed 248 nm irradiation of Y1Ba2Cu3Ox are reported over laser energy densities from near threshold into the film growth regime. Time of flight absorbance-velocity profiles in vacuum indicate the formation and acceleration of a plasma front, with ions leading neutrals on the edge of the expanding plume. Ion probe screening measurements show that the laser plume is a well-shielded plasma with Debye lengths <10 μm at film deposition distances. Velocity distributions and estimates of ground state Ba+, Ba, Y+, and Y densities indicate that the populations of the ions outnumber those of the neutrals at high energy densities in vacuum. Measurements of the slowing of the plasma front and attenuation of the total charge reaching the substrate are reported for laser ablation in background pressures of oxygen. Absorption by ground state YO and BaO in the region close to the pellet indicates oxide densities ˜5 × 1013 cm−3 close to the pellet.
Influence of Laser Irradiation and Ambient Gas in Preparation of PZT Films by Laser Ablation
Authors: Morimoto Akiharu , Otsubo Shigeru , Shimizu Tatsuo , Minamikawa Toshiharu , Yonezawa Yasuto , Kidoh Hideo , Ogawa Toshio
Published: March 2011

Pb(Zr0.52Ti0. 48)O3 (PZT) films were prepared on r-plane sapphire substrates by the laser ablation method utilizing ArF excimer laser in O2 or N2O environment. The composition of the films deposited in O2 environment was found to be fairly close to the composition of the target material for a wide range of substrate temperatures, 400 – 750 °c. Increasing the laser fluence (the laser power density) for the ablation enhances the formation of the perovskite structure rather than the pyrochlore one. Use of N2O ambient gas instead of O2 gas enhances the formation of the perovskite structure of PZT films. Furthermore, it was found that a laser irradiation on the growing film surface during deposition enhances the formation of the perovskite structure.
Picosecond UV Excimer Laser Ablation of LiNbO3
Authors: Beuermann Th. , Brinkmann H. J. , Damm T. , Stuke M.
Published: March 2011

Ultrashort UV excimer laser ablation of materials shows several advantages over conventional UV excimer laser ablation. We describe new experimental results on lithium niobate, LiNbO3, using 308 nm pulses of 1 ps duration. The fluence threshold for material removal is reduced by one order of magnitude when compared with 16 ns pulses at 308 nm. In addition the etch rate is up to three times higher for picosecond pulses and it is independent of the diameter of the ablation area. We discuss a simple model describing the etching and ablation behaviour of materials upon interaction with UV excimer laser pulses of different pulse duration.
Congruent Evaporation and Epitaxy in Thin Film Laser Ablation Deposition (Lad) of Rare Earth Transition Metal Elements and Compounds
Authors: Gavigan J. P. , Givord D. , Lienard A. , Mcgrath O. F. K. , Rebouillat J. P. , Souche Y.
Published: March 2011

Laser ablation deposition (LAD) is a versatile thin film preparation technique which has been slowly developing for a number of years, and is currently receiving a lot of attention as demand increasingly exploits its advantages over other established techniques. Apart from its simplicity, one of its main advantages is the possibility of congruently evaporating any solid compound target, be it metal or insulator, due to the extremely high energy and instantaneous power densities attainable with pulsed lasers (up to 50 Jcm−2 and 1012 Wcm−2 for picosecond pulses). In this paper, we report on tests for both congruent evaporation in LAD of a number of rare earth - transition metal intermetallic compounds including Nd2 Fe1 3 B, Yzn0.7, Yni3, Y2 Fe15 and Yni5 for different preparation conditions (using a Nd:YAG laser λ = 1064, 532, 355 nm, τ = 35 ps and 20 ns) and on the epitaxial growth of Yni5 and W on monocrystalline sapphire substrates. Optical and electron microscopy were used to examine film morphology while congruent evaporation was confirmed using x-ray microprobe analysis. In-situ RHEED revealed good epitaxy of the films deposited on sapphire, with the hexagonal diffraction patterns obtained for YNis being identical to those of an YNi 5 reference single crystal.
Nanometer-Scale Pulsed Laser Modification of the Basal Plane of Graphite Observed with Stm
Authors: Schildbach M. A. , Tench R. J. , Balooch M. , Siekhaus W. J.
Published: March 2011

Scanning tunneling microscopy has been used to document changes in the nanometer-scale morphology of the basal plane of highly oriented pyrolytic graphite after exposure to 7 ns, 1064 nm laser.pulses in air. Surface modification was visible at fluences far below those that produce melting. Damage appears first on step edges and consists of exfoliation of graphite layers and recession of steps through removal of mono- or multilayer patches.
Probing the Densities of Gap States in Intrinsic a-Si:H Using Space Charge Limited Currents of Electrons and Holes
Authors: Dawson Robin M. , Smith J. H. , Wronski C. R.
Published: March 2011

Space charge limited currents of holes in intrinsic hydrogenated amorphous silicon (a-Si:H) have been obtained using novel p+-intrinsic-p+ (p-i-p) structures. The presence of these hole space charge limited currents is verified from their temperature dependence and their dependence on a wide range of intrinsic layer thickness. The carrier transport and space charge limited currents in the p-i-p structures are compared with those of n-i-n structures and the results are discussed in terms of a self consistent density of states in the gap.
Density of States of Amorphous Germanium Thin Films Deposited by the Pecvd of H2-Diluted Germane
Authors: Godet C. , Chu V. , Equer B. , Bouizem Y. , Chahed L. , El Zawawi I. , Theye M. L. , Basrour S. , Bruyere J. C. , Stoquert J.P.
Published: March 2011

The disorder in a-Ge:H thin films produced by the plasma-enhanced chemical vapor deposition (PECVD) technique is strongly reduced when the GeH4 gas is diluted at 1% in H2 and the radiofrequency power density is increased to 0.1 W.cm−2. This improvement is attributed to a better surface passivation by the hydrogen atoms during the growth. However, the poor transport properties indicate a still high defect density. The midgap defect absorption and the Urbach energy, obtained from the photothermal deflection spectra calibrated with optical data, both decrease as a function of the film thickness. The optical defect density is calibrated with EPR spin measurements. For a-Ge:H films thicker than 2 μm, obtained at a deposition temperature Ts ranging from 150 to 250°C, the Urbach tail parameter E° is lower than 50 meV and not sensitive to Ts ; the dangling bond density is around 4.107 cm−3, which is higher by a factor of 100 than in a-Si:H. Preliminary transport measurements indicate that the Fermi level density of states is larger than 1018 cm−3.eV−1.
Determination of a-Si:H Film Quality Throughfst and Sclc Techniques
Authors: Martins R. , Vieira M. , Fortunato E. , Ferreira I. , Soares F. , GuimarĀes L.
Published: March 2011

The ambipolar diffusion length (L*) and the effective lifetime (τ*) in undoped a-Si:H films have been measured by the Flying Spot Technique (FST). This technique consists in measuring the decay range of charges in Schottky barriers and the potential induced by a laser beam with a constant motion rate. The data show that the FST measurements are quite suitable to determine separately the effective diffusion length and the effective lifetime of the carriers, taking into account the surface effect. The results also indicate that the photosensitivity is not the best parameter to characterize film quality. Indeed, films with high photosensitivity can present a poor lifetime and diffusion length that make them unsuitable to be used in optoelectronic devices. The results obtained by FST have been correlated with the Density of States (DOS) inferred by the Space Charge Limited Current (SCLC) measurements performed in the temperature range of 300–430 K and by the Constant Photocurrent Method (CPM) in forward biased Schottky diodes at room temperature. The dependence of the DOS on temperature shows that the SCLC technique is clearly influenced by the thermally activated carriers.
The Influence of Interfaces on the Gap State Distribution of Undoped a-Si:H
Authors: Schumm G. , Bauer G. H.
Published: March 2011

Modulated primary photocurrent (MPC) studies on pin structures show spatial variations of the gap state distribution across the i-layer that can be correlated with Fermi level shifts by band bending towards interfaces. These results as well as reverse bias annealing effects are explained in terms of the defect pool model. It is demonstrated that MPC measurements are basically identical to TOF measurements with clear advantages in the post-transit time regime.
Strong Thickness Dependence of Photoelectronic Properties in Hydrogenated Amorphous Silicon
Authors: Yang L. , Balberg I. , Catalano A. , Bennett M.
Published: March 2011

Many of the photoelectronic properties of a-Si:H, including photoconductivity and ambipolar diffusion length, were observed to deteriorate rapidly with decreasing film thickness. Calculations show that the thickness dependence can be attributed mainly to the effect of surface recombination. In fact, any surface recombination velocity above 100 cm/s will result in an equally good fit to the experimental data. We conclude that the intrinsic photoelectronic properties of bulk a-Si:H and other related materials can be measured only on thick samples (≥1μm) and by using uniformly absorbing light (at long wavelengths).
Defect Thermodynamics, Inhomogeneity, and the Density of Gap States in Hydrogenated Amorphous Silicon
Authors: Branz Howard M. , Silver Marvin
Published: March 2011

A new hydrogenated amorphous silicon (a-Si:H) density of states (d.O.s.) in+cluding the transition levels of both neutral (T3o) and charged (T3+ and T3−) dangling-bond defects is proposed. We derive closed-form and numerical solutions for the d.o.s. from a thermodynamic equilibrium theory of defect concentrations in which material inhomogeneity is assumed to give rise to ∼1020 cm−3 of electrostatic potential fluctuations. The connection between thermodynamic transition level energy and defect formation energy implicit in this and other “defect pool” models is included explicitly in the calculation. We calculate the d.o.s. for a range of parameters and for different values of Fermi energy. We apply the calculated d.o.s. to explain and unify various experimental results in a-Si:H. In particular, we reconcile recent depletion-width-modulated ESR data with the near-perfect Curie law T-dependence of the dangling-bond spin density observed by several groups. It is seen +that the depletion results in roughly equal numbers of T3−T3–>° and T3°–>T3+ transitions despite the positive value of effective correlation energy. We also discuss possible sources of the short-to-medium range potential fluctuations in amorphous silicon.
Saturation of Metastable-Defect Density in a-Si:H
Authors: Redfield David , Bube Richard H.
Published: March 2011

The existence of saturation (or steady state) in the density of light-induced defects in amorphous Si:H is shown to have major importance for the interpretation of the nature and origin of these defects. First, a number of characteristics of the steady-state and transient responses to light and temperature are described and contrasted. These lead to the conclusion that the saturation value is the only useful criterion of the number of defects in these materials. We then describe a new atomic model for defects, unifying both dopant-induced and light-induced defects. This model invokes foreign atoms in defects, and saturation reflects the limitation imposed by the numbers of such atoms. Many other observed properties of defects are explained by this model.
Electronic States in the Gap of a-Si from Bond Angle Variations
Authors: Davidson B. N. , Lucovsky G.
Published: March 2011

We investigate the formation of defect states in the gap of a-Si arising from deviations from the ideal tetrahedral bond angles. The local density of states for Si atoms in disordered environments is calculated using tight-binding parameters for the cluster-Bethe lattice method. The Hamiltonian for a-Si with bond angle distortions is taken as an average over many configurations associated with a random choice of bond angles, weighted by Gaussian distributions with standard deviations between 2°.and 10°. Bond angle deviations in this range generate a density of defect states at the valence band edge that: 1) increases as the average bond angle deviation increases; and 2) is significantly larger than the density of band tail states generated at the conduction band edge. We obtain a shift of the absorption edge from the joint density of states (DOS) as a function of bond angle deviations. In addition, a calculation of the DOS for a distorted tetrahedral cluster embedded in an idealized Bethe lattice yields a threshold bond angle distortion of ±20° for the appearance of a discrete state in the gap near the valence band edge.
The First Fifty Billion Nanoseconds of Electron Photocarrier Evolution in Undoped a-Si:H
Authors: Antoniadis Homer , Schiff E.A.
Published: March 2011

The drift of electrons generated by a 3 ns illumination impulse was studied in several undoped a-Si:H specimens by using transient photocharge measurements from 10 ns to 50 sec. The transients show four stages of evolution: band transport, deep-trapping and subsequent re-emission, and recombination. From the temperature dependence of the emission time we estimated the mean energy of the trap distribution and the attempt-to-escape frequency associated with this trap to be 0.6 eV below the conduction band and 5×1012 sec−1 respectively; this trap has been identified in previous work with the D-center found in electron spin resonance. In addition the data resolve the two orders of magnitute discrepancy between mobility-lifetime estimates from earlier charge-collection experiments and from steady-state photoconductivity.
A General Analysis of Steady State Photocarrier Grating Technique for the Determination of Ambipolar Diffusion Length
Authors: Li Yuan-Min
Published: March 2011

A general photoconductivity formula is derived for the case of a semiconductor steady state photocarrier grating (SSPG)1. It is shown that, under the condition of weak applied electric field, the ambipolar diffusion length can be determined by the SSPG technique1without the lifetime-regime restriction2,3 if the lifetime of photocarriers is known. The general formula presented here is reduced to the simple lifetime-regime formula1–3 under the condition of fast dielectric relaxation.