bluuebunny/crossref_metadata_2025_split huggingface dataset:
Improving the Electrical Conductivity of Composites Comprised of Short Conducting Fibers in a Nonconducting Matrix: The Addition of a Nonconducting Particulate Filler
Authors: Chen Pu-Woei , Chung D. D. L. 
Published: April 2011

ABSTRACTThe addition of a second discontinuous filler (silica fume) that is essentially nonconducting to a composite with a comparably non-conducting matrix (cement) and a conducting discontinuous filler (carbon fibers) was found to increase the electrical conductivity of the composite when the conducting filler volume fraction was less than 3.2%. The maximum conducting filler volume fraction for the second filler to be effective was only 0.5% when the second filler was sand, which was much coarser than silica fume. The improved conductivity due to the presence of the second filler is due to the improved dispersion of the conducting filler. The silica fume addition did not affect the percolation threshold, but the sand addition increased the threshold.
Mechanical Behavior of Eutectic Sn-Ag and Sn-Zn Solders
Authors: Mavoori Hareesh , Vaynman Semyon , Chin Jason , Moran Brian , Keer Leon M. , Fine Morris E. 
Published: April 2011

ABSTRACTEutectic Sn-Ag and Sn-Zn solders are currently being investigated as the basis for replacement of Sn-Pb solders. Some mechanical properties of these solder systems - fatigue, tensile, stress relaxation and creep are presented here along with some damage feature observations for eutectic Sn-Ag solder. Semi-empirical modeling, constitutive modeling and numerical simulation based on damage models have been carried out for lifetime prediction. Preliminary agreement of this numerical simulation with experimental results appears encouraging, but the models need further refinement.
Metal - Containing Ferroelectric Liquid Crystals
Authors: Espinet P. , Etxebarría J. , Marcos M. , Pérez-Jubindo M. A. , Ros M. B. , Serrano J. L. 
Published: April 2011

AbstractFerroelectric metallomesogens are a new type of ferroelectric liquid crystal which have unique properties arising from the presence of metal atoms. Here we report an overview of results based on the wide range of structural changes, either in the metallic centre or in the organic part, that these compounds offer.Ferroelectric salicylaldiminate derivatives, imine ortho-palladated dinuclear complexes and ortho-metallated imine-β-diketone mononuclear compounds give rise to chiral smectic C ordered materials with macroscopic polarizations up to 200 nC.cm-2 while their response times are in the range of ms.Depending on the metal and the structure of the metallomesogens, additional properties such as paramagnetic behavior as well as non-linear optical reponses are described for these new materials. Thus, imine-β-diketone compounds of Pd(II) and Pt(II) are the first metallomesogens ever reported which show second harmonic generation. deff values in the range of 0.03 to 0.06 pm.V-1 at 10 °C below the transition to the SmC* phase have been measured for these materials under phase matching conditions.
Exciton Confinement in Ultrathin Crystalline Organic Films Grown by Organic Molecular Beam Deposition
Authors: Forrest S. R. , Haskal E. I. , Burrows P. E. 
Published: April 2011

AbstractWe use the low temperature fluorescence spectra of organic multiple quantum well samples consisting of the archetype materials, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA); 3,4,7,8 naphthalenetetracarboxylic dianhydride (NTCDA), and 3,4,9,10 perylenetetracarboxylic-bis-benzimidazole (PTCBI), to study the effects of quantum confinement on the lowest energy excited and ground electronic states of these molecular crystals. Both the Franck-Condon and the dominant ground state vibronic mode energies are observed to undergo significant shifts with decreasing PTCDA layer thicknesses (ranging from 500Å to 10Å) in PTCDA/NTCDA MQWs, while no such effects are observed for PTCBI/NTCDA MQWs. These results are interpreted in the context of confinement of spatially extended excitons in ultra thin PTCDA layers, whereas the considerably smaller radius PTCBI excitons are not affected over the range of layer thicknesses investigated. These results unambiguously rule out previous hypotheses suggesting that binding of small radius excitons to interfaces results in the blue shifts previously observed in the absorption spectra of PTCDA-based MQWs.
Low Resistivity Aluminum Nitride: Carbon (AIN:C) Films Grown by Metal Organic Chemical Vapor Deposition
Authors: Wongchotiqul K. , Chen N. , Zhang D. P. , Tang X. , Spencer M. G. 
Published: April 2011

ABSTRACTLow resistivity single crystal aluminum nitride-carbon (AIN:C) films were grown by metal organic chemical vapor deposition (MOCVD). The growth system used ammonia (NH3), trimethylaluminum (TMA), hydrogen (H2), and propane (C3H8) precursors. Films produced with high partial pressure of propane during growth exhibited high conductivity. Van der Paw measurements indicated that the resistivity of the as grown films changed dramatically from 108 ohm-cm for unintentionally doped samples to less than .2 ohm-cm for partial pressures of propane greater than 0.5×10−3 torr. Reflection electron diffraction (RHEED) measurements performed "in situ" just after film growth indicated that the material is single crystal up to a propane partial pressure of 2.5×10−3 torr. P-n junctions of n-type 6H-SiC and p-type AIN:C were fabricated, blue emission (centered at 490nm) was observed from the heterojunction under forward bias.
Low Temperature Epitaxial Growth of AlN & GaN Thin Films by the Method of Ion Beam Assisted Deposition
Authors: Kim Ig-Hyeon , Jeon Chan-Wook , Kim Seon-Hyo 
Published: April 2011

ABSTRACTThe epitaxial layers of AIN and GaN were grown on Si and Sapphire substrate at a relatively low temperature of around 500 °C using the process of reactive ion beam assisted deposition. The optimum ion beam energy for epitaxial growth of AIN and GaN films was found to be about 50 eV. Characterization of the epitaxial layers was carried out by GID (Grazing-Incidence x-ray Diffraction) and high resolution TEM observation. The orientational relations between epitaxial layer and substrate were determined through these analysis. Very thin amorphous layers were observed at the interfaces of bom AIN and GaN films grown on Si(111) substrate, whereas the films grown on Sapphire substrate has no amorphous layer. The amorphous layer may act as a buffer layer enabling the growth of the epitaxial layers of AIN and GaN by relaxing the misfit strain in the early growing stage.
High-Precision Characterization of III-Nitride Semiconductor Alloys with Secondary Ion Mass Spectrometry (SIMS)
Authors: Erickson J W. , Gao Y. , Wilson R. G. 
Published: April 2011

ABSTRACTSamples of representative AlxGayIn1−x-yN compositions have been studied with secondary ion mass spectrometry (SIMS). First, ionized species of common interest (H, B, C, O, Mg, Si, and Cd) were implanted into the Ill-nitride samples to provide calibrated standards. Depth profiles and conversion factors for quantification of dopants were then obtained using O2+ or Cs+bombardment and positive or negative SIMS to measure B+ and Mg+; H−, B−, C−, O−, and Si−; and CdCs+. In addition calibration curves for quantification of stoichiometry were prepared using MCs+ ions (NCs+, AlCs−, GaCs+, InCs+) for which the ion yields are relatively independent of the matrix composition; and using atomic, dimer, and trimer ions (Al, Ga, In, Al2, Ga2, In2, Al3, Ga3) which are very sensitive to matrix composition. The empirical calibration curves show small non-linearities. Dopant concentrations can be quantified with great sensitivity (detection limits usually below 1 ppm), accuracy (usually better than 10%), and precision (better than 25%). Matrix stoichiometry can be quantified with an accuracy of about 1–3%.
A Microscopic Evaluation of the Surface Structure of OMVPE Deposited α-GaN Epilayers
Authors: Rohrer G. S. , Payne J. , Qian W. , Skowronski M. , Doverspike K. , Rowland L. B. , Gaskill D. K. 
Published: April 2011

ABSTRACTThe surface structure of organometallic vapor phase epitaxy (OMVPE) grown α-GaN films was investigated using optical and scanning force microscopy (SFM). Optical microscopy shows that the surface is decorated with several different types of faceted features that have lateral dimensions of 10 to 75 μm and occur with a density of approximately 104/cm2. SFM images show that on the flat regions of the surface, single diatomic layer steps, 2.6 Å high, are straight, evenly spaced (at 500 to 1500 Å intervals), and oriented along á101ñ directions. The SFM images also show that the regular step patterns are often interrupted by faceted growth hillocks, 0.8 to 5 μm in diameter and 120 to 400 Å high, that occur with a density of 106/cm2. An open-core screw dislocation with a Burgers vector of 5.2 Å occurs at the center of each hillock and is a source for spiral steps. Other dislocations are also observed to intersect the flat regions of the surface and create a step, but these have smaller Burgers vectors, do not form spirals, and do not have open cores. Based on these observations, we conclude that thick OMVPE GaN films grow by a combination of the layer-by-layer and spiral growth mechanisms.
Defect Studies of GaN under Large Hydrostatic Pressure
Authors: Wetzel C. , Fischer S. , Walukiewicz W. , Ager III J. , Haller E.E. , Grzegory I. , Porowski S. , Suski T. 
Published: April 2011

ABSTRACTGaN plays a key role in the exploration of the properties of group-Ill nitrides. As grown GaN often shows a high electron concentration, e.g. 1019 cm−3, of as yet unidentified origin. Applying large hydrostatic pressure we studied the behavior of these donors and a frequently observed strong luminescence band at 3.42 eV. We find a drop of the electron concentration to 3×1017 cm−3 at 27 GPa and derive a binding energy of 126 meV for the neutral singlet donor level at this pressure. Such a pressure behavior of a donor is consistent with the model of strongly localized defects. Within the framework of a bandstructure calculation we predict the neutral level of this donor at 0.40 ± 0.10 eV above the conduction band edge at ambient pressure.
Shallow and Deep Level Defects in GaN
Authors: Götz W. , Johnson N.M. , Bour D.P. , Chen C. , Liu H. , Kuo C. , Imler W. 
Published: April 2011

ABSTRACTShallow and deep electronic defects in MOCVD-grown GaN were characterized by variable temperature Hall effect measurements, deep level transient spectroscopy (DLTS) and photoemission capacitance transient spectroscopy (O-DLTS). Unintentionally and Si-doped, n-type and Mg-doped, p-type GaN films were studied. Si introduces a shallow donor level into the band gap of GaN at ∼Ec - 0.02 eV and was found to be the dominant donor impurity in our unintentionally doped material. Mg is the shallowest acceptor in GaN identified to date with an electronic level at ∼Ev + 0.2 eV. With DLTS deep levels were detected in n-type and p-type GaN and with O-DLTS we demonstrate several deep levels with optical threshold energies for electron photoemission in the range between 0.87 and 1.59 eV in n-type GaN.
Picosecond Raman Studies of Electron-Phonon Interactions in the Wide Bandgap Semiconductor GaN
Authors: Tsen K.T. , Ferry D.K. , Botchkarev A. , Sverdlov B. , Salvador A. , Morkoc H. 
Published: April 2011

ABSTRACTPicosecond Raman spectroscopy has been employed to study electron-phonon interactions in the wide bandgap semiconductor GaN. An ultraviolet picosecond laser with photon energy ћω= 4.36eV was used to excite electron-hole pairs in an undoped bulk GaN. The relaxation of these high energy electrons and holes were used to interrogate electron-phonon interactions. We have found that electrons thermalize toward the bottom of the conduction band by emitting primarily longitudinal optical phonons. Our work demonstrates that the Fröhlich interaction is much stronger than the deformation potential interaction in wurtzite GaN.
Photoluminescence Related to the 2-Dimensional Electron Gas in Modulation Doped GaN/AlGaN Structures
Authors: Bergman J.P. , Lundström T. , Monemar B. , Amano H. , Akasaki I. 
Published: April 2011

ABSTRACTWe report low temperature photoluminescence (PL) spectra related to a two-dimensional electron gas confined at a GaN/AlGaN heterointerface. The recombination between electrons confined in the bottom of the interface potential and photoexcited holes causes a broad PL emission about 50 meV below the bulk GaN exciton emission. A second emission, attributed to the recombination of electrons in the first excited level at the interface, is also observed close to the excitonic band gap in GaN. The data agrees with a self consistent calculation of the energy levels and the electron concentration at the interface. Similar PL data from a modulation doped AlGaN/GaN quantum well exhibit three PL emissions related to the 2D electron gas.
Gallium Nitride Epitaxy on Silicon: Importance of Substrate Preparation
Authors: Martin G. A. , Sverdlov B. N. , Botchkarev A. , Morkoç H. , Smith D. J. , Tsen S.-C. Y. , Thompson W. H. , Nayfeh M. H. 
Published: April 2011

ABSTRACTHexagonal GaN films grown on non-isomorphic substrates are usually characterized by numerous threading defects which are essentially boundaries between wurtzite GaN domains where the stacking sequences do not align. One origin of these defects is irregularities on the substrate surface such as surface steps. Using Si <111> substrates and a substrate preparation procedure that makes wide atomically flat terraces, we demonstrate that reduction of these irregularities greatly improves the crystalline and luminescent quality of GaN films grown by plasma-enhanced molecular beam epitaxy. X-ray rocking curve width decreases from over 1 degree to less than 20 minutes, while PL halfwidth decreases from over 15 meV to less than 10 meV.
Hydrogen in GaN
Authors: Johnson N. M. , Götz W. , Neugebauer J. , Van de walle C. G. 
Published: April 2011

ABSTRACTHydrogen is implicated in the low doping efficiency of acceptors (e.g., Mg) in MOCVD-grown GaN by the following observations: the well-established role of hydrogen in the same phenomenon in other semiconductors; re-passivation of thermally activated Mg in GaN upon exposure to NH3 or H at temperatures > 400°C; correlation between Mg and H concentrations; and computational studies on the diffusivities of charged H in GaN and on the configuration and stability of the Mg-H complex in GaN. Strong experimental evidence for the Mg-H complex comes from variable-temperature Hall effect measurements which reveal that after hydrogenation of activated GaN:Mg the hole mobility increases as the hole concentration decreases, which is consistent with removal of ionized scattering centers and not with the introduction of separate compensating defects. However, experimental evidence is also accumulating to suggest that the Mg activation process involves the generation or transformation of electrically active defects.
Growth of GaN on (100)Si Using a New C-H and N-H Free Single-Source Precursor
Authors: Kouvetakis John , McMurran Jeffrey , Beach David B. , Smith David J. 
Published: April 2011

ABSTRACTWe have demonstrated growth of crystalline GaN on Si substrates by using, for the first time, a novel inorganic precursor Cl2GaN3 and ultra-high-vacuum chemical vapor deposition techniques. Cross-sectional electron microscopy of the highly conformal films showed columnar growth of wurtzite GaN while Auger and RBS oxygen- and carbon-resonance spectroscopies showed that the films were pure and highly homogeneous. In addition to the high growth rates of 70–500 Å per minute, the low deposition temperature of 550–700 °C, and the nearly perfect GaN stoichiometry that we obtain, another notable advantage of our method is that it provides a carbon-free growth environment which is compatible with p-doping processes.
Thermal Stability of Ohmic Contacts to n-InxGa1−xN
Authors: Duibha A. , Pearton S. J. , Abernathy C. R. , Lee J. W. , Holloway P. H. , Ren F. 
Published: April 2011

ABSTRACTThe microstructural properties and interdiffusion reactions of Au/Ge/Ni, Ti/Pt/Au, WSix and AuBe contacts on GaN and In0.5Ga0.5N have been examined using Scanning Electron Microscopy and Auger Electron Spectroscopy. The WSix contacts possess excellent thermal stability and retained good structural properties at annealing temperatures as high as 800°C on GaN. The electrical characteristics of WSix contacts on In0.5Ga0.5N had a specific contact resistivity of 1.48×10−5Ωcm2 and an excellent surface morphology following annealing at 700°C. The increase in contact resistance observed at higher temperatures was attributed to intermixing of metal and semiconductor. In contrast the Ti/Pt/Au and Au/Ge/Ni contacts were stable only to ≤ 500°C. AuBe contacts had the poorest thermal stability, with substantial reaction with GaN occurring even at 400°C. The WSix contact appears to be an excellent choice for high temperature GaN electronics applications.
Optically Pumped GaN-AlGaN Double-Heterostructure Lasers Grown by ECR-GSMBE and HVPE
Authors: Maki P. A. , Molnar R. J. , Aggarwal R. L. , Liau Z-L. , Melngailis I. 
Published: April 2011

ABSTRACTWe have observed laser emission with well-defined cavity modes in optically pumped GaN-Al0.1Ga0.9N double-heterostructure (DH) lasers. The laser structures were grown using an electron-cyclotron-resonance nitrogen-discharge source and gas-source molecular beam epitaxy (ECR-GSMBE) on thick (≥10 µm) GaN buffers grown by hydride vapor-phase epitaxy (HVPE) on c-plane sapphire. Transversely pumped cavities using a 337.1 nra nitrogen laser pump source exhibit a threshold pump fluence ranging from 0.15 to 0.3 mJ/cm2 at 77 K, a linear light output above threshold, a lasing wavelength of 358 nm, and an estimated differential quantum efficiency of 1%. The room-temperature threshold is about 1.7 times higher. Longitudinal mode structure has been resolved in a shorter-cavity (23 µn) device at 77 K. The measured mode spacing of 0.56 nm corresponds to a group index of 5.0. Far-field measurements in a plane perpendicular to the plane of the heterostructure indicate a double-lobed pattern for a 1000 Ǻ thick GaN active region, and a single lobe with a FWHM of 60° for a 4000 Ǻ active region. The thick HVPE GaN buffer layer provides for a lattice-matched growth and results in improved nucleation in MBE, as indicated by a high-quality reflection-electron-diffraction pattern of the as-loaded wafers. The surface morphology of the MBE layers on the HVPE buffer shows improved optical smoothness as compared to layers grown directly on sapphire using a low-temperature, MBE-grown GaN buffer. Laser facets were formed either by saw cutting or cleaving of the GaN buffer and epilayer along crystal planes. Details of the material development and laser performance are described.
Thin Films of Ferroelectrics Made by Pulsed Laser Deposition for Optoelectronic Applications
Authors: Chrisey D.B. , Knauss L.A. , Horwitz J.S. , Auyeung R.C.Y. , Knies D. , Grabowski K.S. , Donovan E.P. , Mccallion K. , Johnstone W. 
Published: April 2011

ABSTRACTWe have deposited ferroelectric thin films by pulsed laser deposition to be used as an inline optoelectronic modulator utilizing evanescent field coupling to a side-polished optical fiber. In order to satisfy waveguiding conditions, amorphous quartz substrates (n= 1.447) were necessary. Ferroelectric films of (Pb0.91La0.90)(Zr0.65,Ti0.35)O3, Pb0.80La0.20TiO3 (PLZT), and Sr0.5Ba0.5TiO3 were deposited on quartz substrates and thin Bi4Ti3O12 buffer layers were explored as a chemical barrier and to improve crystallographic texturing. The morphology of thick films (˜3 µm) of PLZT was not acceptable for optical applications, whereas the Sr0.5Ba0.5TiO3 films were very smooth, but had some cracking. The Sr0.5Ba0.5TiO3 films were optically determined to be uniform and of sufficient quality for use in fabricating in-line fiber optic modulators.
Application of Laser Microetching in Formation of Air-Bearing Surface for Magnetic Head Sliders
Authors: LU Y.F. , YE K.D. 
Published: April 2011

ABSTRACTLaser-induced etching of polycrystalline Al2O3TiC material by a tightly-focused CW Ar ion laser has been investigated in a KOH solution with different concentrations. It is found that the KOH concentration can strongly affect the etching quality where low KOH concentration can result in rough and irregular patterns. The etching effect is also related to laser power and scanning speed. Laser-induced etching of polycrystalline AI2O3TiC in a KOH solution is found to be a photothermal reaction in which a threshold laser power exists. With an appropriate set of etching parameters, well defined grooves can be obtained with clean side walls and with an etching rate up to several hundred micrometers per second. It is also found that the grains in the polycrystalline Al2O3TiC material play an important role in the etching dynamics and etching quality. This etching process is believed to be applicable to the formation of a slider surface of magnetic heads in the future.
Laser Dry Cleaning of Zro2 Particles from Air Bearing Surface of Magnetic Head Sliders
Authors: Lu Y.F. , Song W.D. , Hong M.H. , Chong T.C. , Low T.S. 
Published: April 2011

ABSTRACTLaser dry cleaning of ZrO2 particles from air bearing surface (ABS) of magnetic head sliders has been investigated. The experimental results show that the cleaning threshold of removing ZrO2 particles from ABS is about 100 mJ/cm2. For laser fluence larger than this threshold, cleaning efficiency increases with increasing laser fluence and pulse number, but does not depend on repetition rate up to 30 Hz. The mechanisms of this laser dry cleaning are laser-induced ABS vibration, particle thermal expansion and particle vibration, which produce forces strong enough to detach ZrO2 particles from ABS. With increasing laser fluence and pulse number, the average acceleration and vibration number of particles induced by laser increase respectively, so that it is easier to remove particles which corresponds to higher cleaning efficiency. For fixed laser fluence and pulse number, changing the repetition rate does not change the average acceleration or the vibration number of particles, therefore laser cleaning efficiency is almost the same for different repetition rates.
Amide to Active Amine Conversion of Nylon by Deep UV Irradiation
Authors: Kelley Michael J. , Cohen Jeffrey D. , Erkenbrecher Carl , Haynie Sharon L. , Kobsa Henry , Roe A. Nicholas , Scholla Michael H. 
Published: April 2011

ABSTRACTUltraviolet light having sufficiently short wavelength can drive photochemical processes at polymer surfaces. We find that irradiation at 193 nm, but not at 248 nm, results in conversion of amide groups at the nylon surface to amines, still bound in the polymer chain. These amines show anti-microbial activity [1].
Mechanical Properties of High Strength Aluminum Alloys Formed by Pulsed Laser Deposition
Authors: Knapp J. A. , Follstaedt D. M. 
Published: April 2011

ABSTRACTVery high-strength alloys of Al(O) have been formed using a pulsed laser deposition (PLD) system to deposit from alternating targets of Al and Al2O3. Ion beam analysis and transmission electron microscopy show that the deposited material is uniform in composition with up to 33 at.% O and has a highly refined microstructure consisting of a fine, uniform dispersion of ∼1 nm diameter γ-Al2O3 precipitates. Ultra-low-load indentation testing combined with finite-element modeling is used to determine the mechanical properties of the layers. Yield stresses as high as 5.1 GPa have been measured in these materials, greatly exceeding the strengths of aerospace Al alloys (∼0.5 GPa) and even high strength steels. The key to the properties of these materials is the dispersion of small, hard precipitates spaced only a few Burgers vectors apart; dislocations are apparently unable to cut through and must bow around them.