bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Low Temperature-Grown GaAs on Lateral Thermal Oxidation of Al0.98Ga0.02As
Authors: Ferrer J. C. , Liliental-Weber Z. , Reese H. , Chiu Y.J. , Hu E.
Published: April 2011

The lateral thermal oxidation process of Al0.98Ga0.02As layers has been studied by transmission electron microscopy. Growing a low-temperature GaAs layer below the Al0.98Ga0.02As has been shown to result in better quality of the oxide/GaAs interfaces compared to reference samples. While the later have As precipitation above and below the oxide layer and roughness and voids at the oxide/GaAs interface, the structures with low-temperature have less As precipitation and develop interfaces without voids. These results are explained in terms of the diffusion of the As toward the low temperature layer. The effect of the addition of a Si02 cap layer is also discussed.
Fabrication of Bi2Sr2CaCu2O8 Superconductor Thick Films on Cu Substrates
Authors: Han Sang-Chul , Sung Tae-Hyun , Han Young-Hee , Lee Jun-Seong , Kim Sang-Joon
Published: April 2011

Well oriented Bi2Sr2CaCu2O8(Bi2212) superconductor thick films were formed successfully on Cu tapes by liquid reaction between Cu-free precursors and Cu tapes. Cu-free Bi-Sr-Ca-O powder mixtures were screen-printed on Cu tapes and heat-treated at 850-870°C for several minutes in air. Cu-free precursors were composed of BixSrCaOy(x= 1.2-2). In order to obtain the optimum heat-treatment condition, we studied on effects of the precursor composition, heattreatment temperature and time, the screen-printing thickness, and the heat-treatment atmosphere on the superconducting properties of Bi2212 films. Microstructures and phases of films were analyzed by XRD and optical microscopy. The electric properties of superconducting films were examined by the four probe method. At heat-treatment temperature, the specimens were in a partially molten state by liquid reaction between CuO in the oxidized copper tape and the precursors. The non-superconducting phases in the molten state are mixtures of Bi-free phase and Cu-free phases.
Growth Morphology and Electronic Structure of Ultra-Thin TaOx Films on Ag(100)
Authors: Howard M. M. , Ventrice C. A. , Geisler H. , Hite D. A. , Sprunger P. T.
Published: April 2011

A study of the growth morphology and electronic structure of TaOx films on the Ag(100) substrate has been performed to determine the properties of ultra-thin TaOx films without the influence of a mixed interfacial oxide (i.e., a disordered SiO2/TaOx interface for growth on Si). The TaO, films were grown by thermal evaporation of Ta in an oxygen atmosphere of 1 × 101−6 Torr. Growth on a Ag(100) surface held at room temperature results in an amorphous TaOx overlayer, as determined by low energy electron diffraction. The onset of ordering of these films occurs for a post-anneal at ∼500°C. A diffraction pattern that corresponds to a multi-domain overlayer structure is observed for anneals at ∼550°C. Deposition of Ta without oxygen results in the formation of Ta islands. These results indicate that there is a very weak adsorbatesubstrate interaction. Photoemission measurements of the TaOx films show the formation of a band gap with the valence band maximum residing at 3.5 eV below the Fermi level. Core level shifts of ∼3.5 eV are observed for the Ta with no indication of metallic Ta at the surface.
Matrix Assisted Pulsed Laser Evaporation Direct Write (MAPLE DW): A New Method to Rapidly Prototype Active and Passive Electronic Circuit Elements
Authors: Fitz-Gerald J.M. , Chrisey D.B. , Piqu A. , Auyeung R.C.Y. , Mohdi R. , Young H.D. , Wu H.D. , Lakeou S. , Chung R.
Published: April 2011

We demonstrate a novel laser-based approach to perform rapid prototyping of active and passive circuit elements called MAPLE DW. This technique is similar in its implementation to laser induced forward transfer (LIFT), but different in terms of the fundamental transfer mechanism and materials used. In MAPLE DW, a focused pulsed laser beam interacts with a composite material on a laser transparent support transferring the composite material to the acceptor substrate. This process enables the formation of adherent and uniform coatings at room temperature and atmospheric pressure with minimal post-deposition modification required, i.e., ≤400°C thermal processing. The firing of the laser and the work piece (substrate) motion is computer automated and synchronized using software designs from an electromagnetic modeling program validating that this technique is fully CAD/CAM compatible. The final properties of the deposited materials depend on the deposition conditions and the materials used, but when optimized, the properties are competitive with other thick film techniques such as screen-printing. Specific electrical results for conductors are < 5X the resistivity of bulk Ag, for BaTiO3/TiO2composite capacitors the k can be tuned between 4 and 100 and losses are < 1-4%, and for polymer thick film resistors the compositions cover 4 orders of magnitude in sheet resistivity. The surface profiles and fracture cross-section micrographs of the materials and devices deposited show that they are very uniform, densely packed and have minimum resolutions of -10 jtm. A discussion of how these results were obtained, the materials used, and methods to improve them will be given
Direct Focused Ion Beam Writing of Printheads for Pattern Transfer Utilizing Microcontact Printing
Authors: Longo David M. , Hull Robert
Published: April 2011

We describe how focused ion beam (FIB) direct write technology may be combined with microcontact printing (μCP) and other pattern transfer techniques to enable nanoscale fabrication of complex patterns over both curved and planar surfaces. Nanoscale printheads are fabricated by direct sputtering or deposition (Pt or SiO2) in the FIB. These printheads are capable of transferring 100 nm features over fields of view up to 1 mm2, onto planar and curved surfaces. We are also investigating the concept of “programmable” printheads by fabrication of individually addressable printhead arrays, coupled with selective desorption of a relevant transfer medium (e.g. hexadecanethiol self-assembled monolayers) by heating or by application of an electric field
Direct Write Techniques for Fabricating Unique Antennas
Authors: Taylor Robert M. , Church Kenneth H. , Culver James , Eason Steve
Published: April 2011

The current fabrication methods used to produce many antennas are limited by variances in the precision and skill levels of individual laborers. These variances slow production and often create inconsistent results. As radio-frequency transmitter and receiver design moves towards higher operating frequencies, the physical dimensions of the supporting antennas decrease. Smaller sizes add new complexities to the fabrication of these antennas. Several designs that may be considered high-performance antennas are difficult to reproduce; many times, they cannot be fabricated at all due to the sophisticated patterning and precision necessary for successful function. Direct-write technologies provide the tools necessary to fabricate unique patterns in two and three dimensions. A demonstration of a directly written antenna, constructed from a silver-based thick-film paste pen-deposited onto cylindrical alumina substrates, is presented for review
Lateral Dye Distribution With Ink-Jet Dye Doping of Polymer Organic Light Emitting Diodes
Authors: Madigan Conor F. , Hebner Thomas R. , Sturm J. C. , Register Richard A. , Troian Sandra
Published: April 2011

In this work we investigate the lateral dye distribution resulting from the dye doping of a thin polymer film by ink-jet printing (UIP) for the integration of color organic light emitting diodes (OLED's). The dye is found to segregate into distinct outer rings following rapid droplet evaporation, while slower evaporation rates are found to significantly reduce (or eliminate) this effect. The dye segregation phenomena are found to depend critically on the mechanisms of droplet evaporation. Good dye uniformity was obtained using a low vapor pressure solvent, and integrated, 250 micron red, green, and blue polymer organic light emitting diodes (OLED's) were fabricated with this technique. These devices had good color uniformity over most of the device area and similar electrical properties to comparable spin-coated devices without IJP
Rapid Prototyping of Patterned Multifunctional Nanostructures
Authors: Fan Hongyou , López Gabriel P. , Jeffrey Brinker C.
Published: April 2011

The ability to engineer ordered arrays of objects on multiple length scales has potential for applications such as microelectronics, sensors, wave guides, and photonic lattices with tunable band gaps. Since the invention of surfactant templated mesoporous sieves in 1992, great progress has been made in controlling different mesophases in the form of powders, particles, fibers, and films. To date, although there have been several reports of patterned mesostructures, materials prepared have been limited to metal oxides with no specific functionality. For many of the envisioned applications of hierarchical materials in micro-systems, sensors, waveguides, photonics, and electronics, it is necessary to define both form and function on several length scales. In addition, the patterning strategies utilized so far require hours or even days for completion. Such slowprocesses are inherently difficult to implement in commercial environments. We present a series of new methods of producing patterns within seconds. Combining sol-gel chemistry, Evaporation-Induced Self-Assembly (EISA), and rapid prototyping techniques like pen lithography, ink-jet printing, and dip-coating on micro-contact printed substrates, we form hierarchically organized silica structures that exhibit order and function on multiple scales: on the molecular scale, functional organic moieties are positioned on pore surfaces, on the mesoscale, mono-sized pores are organized into 1-, 2-, or 3-dimensional networks, providing size-selective accessibility from the gas or liquid phase, and on the macroscale, 2-dimensional arrays and fluidic or photonic systems may be defined. These rapid patterning techniques establish for the first time a link between computer-aided design and rapid processing of self-assembled nanostructures
CFD Modeling of Laser Guided Deposition for Direct Write Fabrication
Authors: Sheu J. C. , Giridharan M. G. , Lowry S.
Published: April 2011

A computational model for simulating laser-guided particle deposition process is described. This model solves for the transport of gas and particle phases in a fully coupled manner. The optical forces on the particles are evaluated from Mie theory based on local laser intensity. Simulations were performed for different operating conditions of laser power, ambient pressure, and substrate traverse speed. Simulations revealed potential problems such as particle deflected away from substrate due to ambient air current disturbances, substrate overheating, and optical fiber clogging. Possible solutions for these problems are discussed
Role of Powder Production Route in Direct Write Applications
Authors: Ranade M. B. , Gonen Z. Serpil , Eichhorn Bryan W.
Published: April 2011

Various production techniques are available for many materials for the direct write (DW) applications. Single metals and alloys are prepared from vapor and spray reactors as well as by precipitation techniques. Single and multicomponent oxides such as ferrites are also possible from these techniques in the nanometer to micrometer size ranges. Examples with the use of these techniques for production of metals and ferrites for DW are described with examination of crystallinity and morphology.
Offset Printing of Liquid Microstructures for High Resolution Lithography
Authors: Miller Scott M. , Darhuber Anton A. , Troian Sandra M. , Wagner Sigurd
Published: April 2011

We have investigated the direct printing of polymer solutions from a chemically patterned stamp onto a hydrophilic target substrate as a new high-throughput alternative to optical lithography. The patterns on the stamp, which are typically in the micron size range, define regions of alternating wettability. They are produced by patterning a hydrophobic self-assembled monolayer previously deposited onto a hydrophilic surface, typically a glass slide or silicon wafer with a natural oxide coating. Polar liquids or aqueous polymeric solutions are then deposited only onto the hydrophilic surface patterns by dip-coating the stamp in a liquid reservoir. The deposited film thickness depends critically on the speed of withdrawal and the feature size and shape. For vertically oriented hydrophilic stripes dipped in a reservoir containing a polar liquid, we have developed a theoretical model whose prediction for the maximum deposited film thickness agrees exceptionally well with experimental measurements. After deposition, the wetted stamp is pressed against a target substrate by means of a motion controlled press. In this way we have so far printed 5µm wide polyethylene oxide lines onto a silicon wafer.
Sol–Gel-Derived 0–3 Composite Materials for Direct Write Electronics Applications
Authors: Coleman Steven M. , Parkhill Robert L. , Taylor Robert M. , Knobbe Edward T.
Published: April 2011

The use of sol–gel-derived 0–3 composite ceramics for low-temperature direct-write electronics applications was investigated. The 0–3 composite paste materials were prepared using selected metal alkoxides and commercial low- and high-κ' dielectric powders. The composite pastes were deposited onto alumina and polyimide substrates using conventional screening and micro-dispensing techniques. The deposited films were oven-dried at or below 200°C and thermally densified using a CO2 laser. The 0–3 composites exhibited good adhesion and structural density. Electrical characterization of the laser-processed dielectrics revealed κ' values as high as 295 and tan δ as low as 0.02 on polyimide substrates.
Direct Write Metallizations for Ag and Al
Authors: Curtis C. J. , Miedaner A. , Rivkin T. , Alleman J. , Schulz D. L. , Ginley D. S.
Published: April 2011

We have employed inks containing nanometer-sized particles of Ag and Al (nano-Ag and nano-Al, respectively) as precursor inks for the formation of contacts to n- and p-type Si, respectively. The particles as formed by the electroexplosion process were dispersed in toluene, applied to Si and annealed above the respective eutectic temperatures. In the case of nano-Ag, this directly yields an ohmic contact. However, the nano-Al was found to be coated with an oxide layer that impairs the formation of an ohmic contact. A chelating chemical etch involving treatment with hexafluoroacetylacetone was developed to remove this oxide coat. This treated nano-Al produced a good ohmic contact. Smooth, pure Ag films have also been deposited by spray printing organometallic inks prepared from Ag(hfa)(SEt2) and Ag(hfa)(COD). These films are deposited in one step onto heated glass and Si substrates at one atmosphere pressure. The films show resistivities of ∼2 µΩ·cm. These inks appear to be amenable to ink-jet printing of Ag lines and as a low temperature glue for the Ag nanoparticles for thicker metallizations.
Electrostatic Printing, A Versatile Manufacturing Process for the Electronics Industries
Authors: Detig Robert H.
Published: April 2011

Functional materials configured as liquid toners are printed on a variety of substrates for various manufacturing processes. The materials include metal toners, resistor toners, high k dielectric toners, phosphors and glass. The substrates printed upon include glass, bare and coated metal, polymeric films and even paper. A fixed configuration electrostatic printing plate is used in most manufacturing applications though traditional photo receptor plates can be used if electronic addressability is desired.

Applications of electrostatic printing for electronic packaging products (printed wiring boards and flex circuits) and of passive electronic components themselves will be shown. Results with a pure silver toner printed on both glass and paper will be reported. Examples of passive electronic components like resistors, capacitors, and even inductors that have been electrostatically printed with liquid toners will be shown. Possible applications of toners to the manufacture of flat panel displays will be discussed.
Microstructures of Laser Deposited 304L Austenitic Stainless Steel
Authors: Brooks John A. , Headley Thomas J. , Robino Charles V.
Published: April 2011

Laser deposits fabricated from two different compositions of 304L stainless steel powder were characterized to determine the nature of the solidification and solid state transformations. One of the goals of this work was to determine to what extent novel microstructures consisting of single-phase austenite could be achieved with the thermal conditions of the LENS process. Although ferrite-free deposits were not obtained, structures with very low ferrite content were achieved. It appeared that, with slight changes in alloy composition, this goal could be met via two different solidification and transformation mechanisms.
Gas Phase Solid Freeform Fabrication of Saldvi of SiC Cermets
Authors: Crocker J.E. , Shaw L. , Marcus H.L.
Published: April 2011

In this work, the solid freeform fabrication of cermets was explored. Using a laser-based approach, SiC was deposited by chemical vapor deposition from tetramethylsilane gas into powder layers of Cu, Mo, or Ni. The resulting structures were examined to observe the extent of reaction between the metal powders and the vapor deposited SiC. Silicide formation was observed, most readily with the Ni powder. The thermal expansion of the metals compared to that of the vapor deposited SiC affected the interfacial stresses generated in the cermets during fabrication.
Characterization of Laser Deposited Niobium and Molybdenum Silicides
Authors: Brice C. A. , Schwendner K. I. , Amancherla S. , Fraser H. L. , Zhang X. D.
Published: April 2011

Recent advances in laser deposition technology have made the production of advanced composite materials more technically feasible. By utilizing the unique characteristics of the laser deposition process, materials can be made that are difficult to produce by conventional methods. Structural silicide materials hold promise for high temperature applications, unfortunately, their low toughness has prevented their practical use [1-4]. The introduction of a second phase can greatly increase their low temperature mechanical properties. Laser direct deposition can be used to deposit these materials near net shape and produce an in-situ alloy that has the desired structure for improving properties. In-situ alloying was accomplished by using an Optomec LENS™ (Laser Engineered Net Shaping) machine to deposit elemental niobium-silicon and molybdenum-silicon-boron powder blends. The resultant microstructures showed a homogeneous structure with a primary silicide phase surrounded by a continuous eutectic phase. These deposits were characterized using scanning and transmission electron microscopy, x-ray diffraction, and by energy dispersive spectroscopy.
Rapid Electroforming Tooling
Authors: Yang Bo , Leu Ming C.
Published: April 2011

A method of rapid electroforming tooling for the production of metal tools including molds, dies, and electrical discharge machining electrodes is developed. Geometry and material of the solid freeform fabricated part, properties of the electroformed metal, and process parameters are significant factors that cause inaccuracy in the manufactured tools. Thermomechanical modeling and numerical simulation are used to simulate thermal stresses induced during the burnout process that removes the rapid prototyping part from the electroform. The analysis demonstrates that appropriate design of the rapid prototyping part geometry and selection of electroform thickness not only reduce thermal stress, thus improving tooling accuracy, but also minimize manufacturing time and cost.
Understanding the Microstructure and Properties of Components Fabricated by Laser Engineered Net Shaping (LENS)
Authors: Griffith M. L. , Ensz M. T. , Puskar J. D. , Robino C. V. , Brooks J. A. , Philliber J. A. , Smugeresky J. E. , Hofmeister W. H.
Published: April 2011

Laser Engineered Net Shaping (LENS) is a novel manufacturing process for fabricating metal parts directly from Computer Aided Design (CAD) solid models. The process is similar to rapid prototyping technologies in its approach to fabricate a solid component by layer additive methods. However, the LENS technology is unique in that fully dense metal components with material properties similar to wrought materials can be fabricated. The LENS process has the potential to dramatically reduce the time and cost required realizing functional metal parts. In addition, the process can fabricate complex internal features not possible using existing manufacturing processes. The real promise of the technology is the potential to manipulate the material fabrication and properties through precision deposition of the material, which includes thermal behavior control, layered or graded deposition of multi-materials, and process parameter selection.
Effect of Grain Size Distribution on Tensile Properties of Electrodeposited Nanocrystalline Nickel
Authors: Ebrahimi Fereshteh , Ahmed Zunayed , Morgan Kristin L.
Published: April 2011

We have produced dense and ductile nanocrystalline nickel with various grain size distributions using electrodeposition techniques. The strength of the nickel deposits fell within the scatter band of the general Hall-Petch curve for nickel. However, large variations in yield strength, strain hardening rate and tensile elongation were associated with a relatively small change in the average grain size. The scatter in the elongation data has been attributed to the formation of nodules and the presence of voids. The variations in strength and strain hardening rate have been shown to be associated with the changes in the grain size distribution. A model based on confined dislocation motion and composite behavior has been developed for predicting the stress-strain behavior of the nanocrystalline nickel.
Femtosecond Ultrasonics for the Characterization of Layered Micro- and Nano Structures
Authors: Vollmann Jacqueline , Profunser Dieter , Dual Jürg
Published: April 2011

The precise measurement of thicknesses, mechanical properties, and film-substrate bonding qualities of sub-micron metallic films, multi-layers, and microstructures in a nondestructive way is of growing importance for the semiconductor industry as well as for MEMS manufacturers.

Short laser pulses having durations of less than 100 fs are an ideal – not to say the only –way in order to provide sub-surface visualization in nanometer scale perpendicular to the film surface.

In this investigation, an enhanced pump-probe set up is presented. A dual frequency modulation technique is implemented in order to eliminate optical and electrical cross talk between the excitational pump pulse and the measuring probe pulse.

Up to the 5th acoustic echo in an aluminum film having a thickness of 50 nm on a sapphire substrate has been measured. From the time of flight and from the amplitude decay between the echoes the material properties of the film and of the substrate have been derived.

Further directions of the on going research project are presented.
Positron Lifetime Measurements in Nanostructured Ni-Al Samples
Authors: Petegem S. Van , Segers D. , Dauwe C. , Torre F. Dalla , Swygenhoven H. Van , Yandouzi M. , Schryvers D. , Tendeloo G. Van , Kuriplach J. , Hou M. , Zhurkin E.E.
Published: April 2011

Positron lifetime spectroscopy is an effective tool to study various types of defects in materials including nanostructured ones. The size of free volumes associated with defects can be estimated using the lifetime components found in the measured spectra. Positron lifetime experiments are performed on nanocrystalline Ni-Al samples synthesized by the inert-gas condensation technique. The samples are further characterized by means of X-ray diffraction, electron diffraction and microscopy techniques as well as density measurements. In the lifetime spectra we observe three lifetime components corresponding to different annihilation sites in the samples. These lifetimes are compared with the results of simulations of positron lifetimes in modeled Ni-Al samples obtained using molecular dynamics and Monte Carlo calculations. Finally, we present positron lifetime results for nanocrystalline Ni3Al samples which were produced or annealed at different temperatures.
Superelastic Deformation of Adaptive Nano-Composites
Authors: Roytburd Alexander L. , Slutsker Julia
Published: April 2011

An adaptive composite containing a shape memory alloy as an active component is the subject of the paper. The elastically heterogeneous nano-composite containing ultra-thin layers of a passive material and thin layers of an active material is investigated in this paper. The superelastic modulus of the composite depends on the fraction of the active layer and can be negative if relative thickness of the active layer exceeds some critical value depending on the ratio between the elastic moduli of the layers and the characteristics of transformation. The negative superelastic modulus corresponds to the thermodynamic instability of superelastic deformation and results in stress-strain hysteresis. The relation between the elastic properties of the composite's components and relative thickness of the layers, which enable to stabilize the superelastic deformation, is obtained.
Size-dependent melting of matrix-embedded Pb-nanocrystals
Authors: Ehrhardt H. , Weissmüller J. , Wilde G.
Published: April 2011

We report calorimetric data for the size-dependence of the melting temperature as well as the enthalpy and entropy of melting for nanoscale Pb particles in an Al matrix, prepared by high energy ball-milling. The results are discussed with respect to various models for the melting of small confined systems. We can rule out models based on a temperature-independent Gibbs excess free energy of the particle-matrix interface, and a model based on an increased meansquare displacement of the interfacial layer. The best agreement to the data is provided by modeling the interface as an inert layer of finite thickness, which does not participate in the phase transition.
Stacking Faults Created by Mechanical Milling in Nanostructured WC-Co Composite Powder
Authors: Zhimin Yang , Changhui Mao , Jun Du , Daniel Michel , Yannick Champion , Serge Hagège , Martin Hÿtch
Published: April 2011

Nanostructured WC-Co powders obtained by mechanical milling were investigated by combination of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques. HRTEM image analysis shows that in the as-milled nanostructured powder, many WC grains contain stacking faults lying on the plane{10.0}. Analysis of phase images showed that these defects were nearly periodically ordered along the [10.0] direction. Based on these observations, a structural model is proposed for the WC grains with ordered stacking faults, which is in fact equivalent to a superstructure of WC with space group Amm2. When this model is introduced together with the normal WC structure (space group P6m2) into the Rietveld refinement, a much better agreement between the calculated and experimental XRD profiles is obtained. This study allowed obtaining the lattice parameters, grain size, microstrain and other structural information on the as-milled powders.
Structure and Electrical Properties of an Assembly of Au Nanoclusters
Authors: Muralidharan G. , Maya L. , Thundat T.
Published: April 2011

Conduction through an assembly of nanosized clusters coupled by tunneling barriers is of significant interest both for understanding the fundamental physics involved and for potential applications. In this study, we describe a technique for preparing relatively large (dimensions of a few 100 µm to a few mm in size) monolayer films consisting of 3 nm diameter Au clusters coated with mercaptododecanoic acid, using low molecular weight-polymers as coupling agents. Electrical measurements of the assembly show non-linear characteristics. Below a certain threshold voltage, the current does not vary with an increase in voltage. Above this threshold voltage, current increases with voltage and can be described by a power-law relationship with an exponent close to unity. These characteristics of the I-V curve are discussed with specific reference to theoretical studies on conduction through an array of capacitance-coupled metallic islands and previous experimental results in similar systems.
Convenient Molecular Approach of Size and Shape Controlled ZnSe and ZnTe Nanocrystals
Authors: Jun Young-wook , Park Jong-Il , Cheon Jinwoo
Published: April 2011

Our study describes a convenient one-step synthesis of ZnSe and ZnTe nanocrystals (NC) whose sizes and shapes are precisely tuned by varing the growth temperature or stabilizing surfactants. We utilized molecular precursors, bis(phenylselenolate or phenyltellurolato)zinc -N,N,N',N'-tetramethylethylenediamine (TMEDA), which effectively produce 0-dimensional sphere or 1-dimensional nanorods of ZnSe or ZnTe, respectively. Nanocrystals are highly monodispersed and luminescent; the emission wavelength varies over a wide range depending on the particle size. This study constitutes a nice demonstration of direct size and shape controlled synthesis of semiconductor nanocrystals and this method can be extended to the synthesis of nanocrystals of other materials.
Nanoparticle Assembly via Hydrogen-Bonding: IRS, TEM and AFM Characterizations
Authors: Han Li , Maye Mathew M. , Zhong Chuan-Jian
Published: April 2011

This paper reports results of the characterizations of nanoparticle assembly formed via spontaneous core-shell and shell-shell reactivities at thiolate-capped gold nanoparticles. Gold nanoparticles of two different core sizes and thiols with carboxylic acid terminals are exploited as a model system. The reactivities involve covalent Au-thiolate bonding and non-covalent hydrogen-bonding with anisotropic linking character. We employed infrared reflection spectroscopy (IRS), atomic force microscopy (AFM) and transmission electron microscopy (TEM) for the characterizations. While IRS provides structural assessment, TEM and AFM imaging measurements probe the morphological properties.
4-Point Resistance Measurements of Individual Bi Nanowires
Authors: Cronin Stephen B. , Lin Yu-Ming , Gai Pratibha L. , Rabin Oded , Black Marcie R. , Dresselhaus Gene , Dresselhaus Mildred S.
Published: April 2011

We have synthesized single crystal bismuth nanowires by pressure injecting molten Bi into anodic alumina templates. By varying the template fabrication conditions, nanowires with diameters ranging from 10 to 200nm and lengths of ~50[.proportional]m can be produced. We present a scheme for measuring the resistance of a single Bi nanowire using a 4-point measurement technique. The nanowires are found to have a 7nm thick oxide layer which causes very high contact resistance when electrodes are patterned on top of the nanowires. The oxide is found to be resilient to acid etching, but can be successfully reduced in high temperature hydrogen and ammonia environments. The reformation time of the oxide in air is found to be less than 1 minute. Focused ion beam milling is attempted as an alternate solution to oxide removal.
DNA-Directed Assembly of Anisotropic Nanoparticles on Lithographically Defined Surfaces and in Solution
Authors: Reiss Brian D. , Mbindyo Jeremiah N. K. , Martin Benjamin R. , Nicewarner Sheila R. , Mallouk Thomas E. , Natan Michael J. , Keating Christine D.
Published: April 2011

Anisotropic, noble metal nanoparticles have been synthesized using a template synthesis strategy. In short, metallic salts are reduced in the nanometer scale pores of either an alumina or polycarbonate membrane. The particles can then been released from the template to form suspensions of anisotropic nanoparticles. These nanoparticles have been modified with deoxyribonucleic acid (DNA) oligomers of varying length using several different attachment chemistries. The thermodynamics and kinetics of modifying these particles with DNA has been explored. DNA has also been used to assemble the particles on planar Au surfaces as well as lithographically defined Au pads on Si wafers. In addition to surface assembly, DNA has been used to assemble the nanowires into simple, yet deterministic structures in solution.
The Effect of Surface Finish on the Dislocation Density in Sublimation grown SiC Layers
Authors: Sanchez E. K. , Liu J. , Vetter W. M. , Dudley M. , Bertke R. , Mitchel W. C. , Skowronski M.
Published: April 2011

The effect of the seed surface finish on the dislocation density of sublimation grown silicon carbide was investigated. Growth on seeds that were polished down to 1 μm diamond paste resulted in the nucleation of threading screw dislocations in a density of 106 cm−2 and threading edge dislocations in densities of 107 cm−2. Following the mechanical polish of the seeds with a hydrogen etch or chemo-mechanical polish prior to growth resulted in the screw dislocation density decreasing by four orders of magnitude and the threading edge dislocation density dropping two orders of magnitude. Using the dislocations density and the hydrogen etch rate, the depth of damage in mechanically damaged seeds was determined to be between 400 and 1000 Å.
Defect Evolution in 4H-SiC Sublimation Epi-Layers Grown on LPE Buffers with Reduced Micropipe Density
Authors: Yakimova R. , Syväjärvi M. , Jacobsson H. , Kakanakova-Georgieva A. , Tuomi T. , Rendakova S. , Dmitriev V. , Janzén E.
Published: April 2011

The objective of this work is to study defect occurrence and appearance in thick sublimation epitaxial layers grown on top of LPE layers with different thickness and substrates with reduced micropipe density. Data from growth on C-terminated surfaces are also presented. The results were analyzed with the aid of optical microscopy, SEM images, HRXRD and synchrotron white beam X-ray topography. A pronounced effect of the LPE (buffer) layer surface roughness on formation of dislocations and micropipes is observed in the sublimation epitaxy layers. While with increasing the thickness of the buffer layer the efficiency of the micropipe reduction increases, the structure quality of the top sublimation epilayer degrades. LPE layers with a thickness of 0.1 μm appear to be the best compromise.
Investigation of Lo-Hi-Lo and Delta-Doped Silicon Carbide Structures
Authors: Konstantinov A. , Karlsson S. , Adås C. , Harris C. , Linnarsson M.
Published: April 2011

Feasibility of lo-hi-lo and delta-doped structures for evaluation of high-field silicon carbide material properties is investigated. Delta-doped structures are grown using the hot-wall CVD technique. Aluminum and nitrogen doping profiles are demonstrated with FWHM below 10 nm. Nitrogen doping transients are found to be slower than those for aluminum. The growth interrupt technique has been developed for achieving narrow nitrogen doping peaks. Lo-hi-lo structures were fabricated using the implantation-and-regrowth technique. Structures with confined avalanche multiplication are demonstrated using the lo-hi-lo and delta doping techniques. The effect of substrate imperfections on early avalanche breakdown is investigated using confined avalanche multiplication devices.
Electroluminescence from 4H-SiC Schottky Diodes
Authors: Carlsson F. H. C. , ul-Wahab Q. , Bergman J. P. , Janzén E.
Published: April 2011

We have observed electroluminescence from 4H-SiC Ni-Schottky diodes on 1015cm−3 nitrogen doped n-type epilayers. A high barrier Schottky contact will form an inversion layer close to it. This creates minority carriers that can be injected into the epi and recombine to emit light. The spectral composition and its temperature dependence have been investigated from liquid He temperatures to room temperature. Band edge luminescence, Al related luminescence and DI bound exciton have been observed. To study the electroluminescence from Schottky diodes provides an easy and additional technique for defect characterization of epitaxial layers.
The Growth and Characterization of Large Diameter Silicon Carbide Substrates
Authors: Gupta A. , Yoganathan M. , Burton J. , Byrd N. , Dimondi A. , Edwards S. , Giordana A. , Glesener J. , Harding J. , Long F. , Mitchel W. , Saxler A. , Sostak R. , Whitlock J. , Thomas R. N. , Anderson T.
Published: April 2011

Affordable, high quality SiC wafers are very desirable for a variety of new technologies including GaN based lighting, RF, and high-power electronics based on wide band gap materials. At Litton Airtron we have a major effort in the growth and characterization of SiC. We will present data on 35, 50 and 75-mm diameter crystals. We are growing both n-type, semiinsulating 4H, 6H, and 15R material. A variety of characterization techniques are being used at Litton Airtron to determine wafer quality. These include Raman microscopy, digital wafer photography, and crossed polarizer images. Raman spectroscopy is an excellent probe of polytype and carrier concentration for n-type materials; in addition it can be done at room temperature and is sufficiently fast that it can be used in an industrial environment. The use of digital photography allows for the collection of images that can be quantitatively analyzed and archived.
Ordinary and Extra-Ordinary Dielectric Function of 4h- and 6H-SiC in the 0.7 to 9.0 eV Photon Energy Range
Authors: Lindquist O. P. A , Järrendahl K. , Arwin H. , Peters S. , Zettler J.-T. , Cobet C. , Esser N. , Aspnes D. E. , Henry A. , Edwards N. V.
Published: April 2011

We report ordinary <ε┴> and extra-ordinary <ε║> dielectric function data of 4H- and 6H-SiC from 0.7 to 9.0 eV. These data, which were obtained by with spectroscopic ellipsometry, are in good qualitative agreement with trends recently reported in ab initio calculations.
Sublimation Growth of 6H-SiC Boule on Various a-Plane Substrates
Authors: Nishino S. , Nishiguchi T. , Masuda Y. , Sasaki M. , Ohshima S.
Published: April 2011

Sublimation growth of 6H-SiC was performed on {1100} and {1120} substrates. The difference between the growth on {1100} plane and {1120} plane was observed. {1100} facet was almost flat and there were grooves oriented toward <1120> direction. The step bunching was observed on {1100} plane 5° off-axis. A lot of pits were introduced on {1120} plane of the crystal grown both on {1100} and {1120} substrates. Step flow growth toward <1120> direction created the pits on {1120} plane. It was important to grow crystal by layer by layer growth on {1120} plane. By changing the growth mode from step flow growth to layer by layer growth, pit on the {1120} plane may be reduced as same as CVD growth on {1120} plane. Growth temperature and C/Si ratio should be optimized to keep layer by layer growth.
Titanium Tungsten (TiW) for Ohmic contacts to n-and p-type 4H-SiC
Authors: Lee S.-K. , Zetterling C.-M. , Östling M.
Published: April 2011

In the present work, we investigated sputtered titanium tungsten (TiW) contacts for Ohmic contacts to both n- and p-type 4H-SiC with long-term stability under high temperature (500°C).. Epitaxial layers with a doping concentration of 1.3×1019 and 6×1018 cm−3 were used. After high temperature annealing (>950°C) sputtered TiW contacts showed Ohmic behavior with good uniform distribution of the specific contact resistance. We obtained an average specific contact resistance (pc) of 4×10−5 Ωcm2 and 1.2 ∼ 1.7×10−4 Ωcm2 for p- and n-type, respectively from linear TLM measurement. We also found some variation of the specific contact resistance and the sheet resistance from our TLM measurement for p-type contacts. We will discuss this behavior with the measurement of SIMS. Long-term stability with a top-cap layer is also discussed
A Single-Band Constant-Confining-Potential Model for Self-Assembled InAs/GaAs Quantum Dots
Authors: Califano M. , Harrison P.
Published: April 2011

A simple and versatile numerical method for electronic structure calculations in InAs pyramidal dots is presented, and its predictions compared with both theoretical and experimental data. The calculated ground state energy eigenvalues agree well with those of more sophisticated treatments which take into account band mixing and the microscopic effects of the strain distribution. The number of electron bound states predicted is in excellent agreement with very recent calculations for strained quantum dots performed in the framework of the 8-band k · p theory. Very good agreement is obtained with both the number and the energy of the peaks in several experimental photoluminescence spectra. Furthermore our ca agreement with that deduced from capacitance and photoluminescence measurements.
Quantum Dot Cell Coupled to a Single Mode Quantum Cavity
Authors: Rojas F. , Cota E. , Ulloa S. E.
Published: April 2011

We study the properties of the charge distribution (polarization) of a 4-quantum dot square cell with two electrons, induced or modified by the coupling to a single quantized mode field. The effect of the relevant parameters (tunneling between dots, the amplitude of the electron-cavity interaction, cavity frequency) is investigated in the properties of polarization of the two-electron fundamental states. We further investigate the effects of asymmetry in the electron-cavity interaction in each dot and evaluate the modification to the dynamic evolution of the polarization. We find that there is a range of values of the asymmetry for which the polarization is improved. This suggests that an optimization of the coupling of the quantum dot cell with its environment may yield more reliable and stable operation.
The Enhancement of Band Edge Emission from ZnS/Zn(OH)2 Quantum Dots
Authors: El-Khair Hatim Mohamed , Xu Ling , Huang Xinfan , Li Minghai , Gu Xiaofeng , Chen Kunji
Published: April 2011

Wurtzite structure monodisperse ZnS quantum dots (QDs) of 1 to 5 nm diameter, synthesized by colloidal chemical method, were confirmed by transmission electron microscopy (TEM) images and electron diffraction (ED) patterns. Enhanced blue shifted band edge emission from Zn(OH)2 capped ZnS QDs with decreasing size has been observed, which indicates the role of inorganic surface passivation and hence supports the quantum size effect. Detectable far-red shifted emission from bare ZnS QDs has been observed when QDs precursors and stabilizer dispersed in solvents with different polarities. This emission is attributed to the surface trap states of different energies.
Observation of CdSe Colloidal Nano-Dot Films by Scanning Probe Microscopy
Authors: Tanaka Ichiro , Kawasaki Eri , Ohtsuki O. , Saita S. , Kamiya I.
Published: April 2011

We observed the surface topography of CdSe colloidal nano-dot film by cyclic contact mode atomic force microscopy. The observed structure changes with cantilever oscillation amplitude, and non-uniform images with long-range corrugations are obtained with relatively large oscillation amplitude while fine structures are revealed with smaller oscillation amplitude. When the amplitude is larger and the surface is weakly ‘tapped’, the topography of the soft organic matrix of the film dominates, and when the tapping force is increased, the hard CdSe dots begin to reveal.
Phase Diagram and Electrostatic Levitation Undercooling Studies of Polytetrahedral Phases in Ti-Fe-Si-O and Ti-Zr-Ni Alloys
Authors: Croat T.K. , Davis J.P. , Gangopadhyay A.K. , Kelton K.F. , Lee G.W. , Simmons J. , Hyers Robert W. , Robinson Michael B. , Rathz Thomas J.
Published: April 2011

New information on the phase diagrams of Ti-Fe-Si-O and Ti-Zr-Ni alloys near the quasicrystal and rational approximant compositions is presented. α(TiFeSiO), the 1/1 rational approximant, is shown to form in a peritectic mode from the liquid, indicating the possibility to produce single-crystal samples. Very long duration annealing studies demonstrate unambiguously that the TiZrNi i-phase and 1/1 approximant form at low temperatures by a solid-state transformation; their phase fields do not extend to the liquidus temperatures. The first undercooling measurements of electrostatically-levitated droplets of the Ti-Zr-Ni alloys are presented. These nucleation studies provide new information on the structural relations between polytetrahedral phases and the undercooled liquid, and on the phase transformation processes. The reduced undercooling for the polytetrahedral phases in these alloys is less than for crystal phases of a similar composition, demonstrating a low interfacial energy between the polytetrahedral phase and the liquid.
Classification of Icosahedral Quasicrystals and their Approximants by the Electronic Conduction Mechanisms
Authors: Takeuchi Tsunehiro , Banno Eiichi , Onogi Tomohide , Mizunol Takayuki , Sato Takuya , Vincent Fournée , Mizutani Uichiro
Published: April 2011

In this paper, two independent factors, electronic structure at the Fermi energy and electron scattering, both of which determine the electrical resistivity, are clearly separated by using a plot of ρ4K versus RRR (RRR = ρ4K/ρ300K) for icosahedral quasicrystals and their approximants. Each contribution of the electronic structure and the electron scattering on the electrical resistivity was systematically revealed, and the origin for the high resistivities observed in the quasicrystals and approximants is well understood by taking each effect into account. We found that the quasicrystals and approximants are classified into three groups in terms of the electron scattering mechanism, which dominates the temperature dependence of the resistivity. The temperature dependence of the electrical resistivity in the first group is well understood in terms of the Boltzmann transport mechanism, and those in the second and the third groups are in terms of the weak localization and the Anderson localization, respectively.
Electron Transport Properties and Thermoelectric Effect of Al-Pd-Mn Single Quasicrystals Doped with M=Au or Re by ion Implantation and its Comparison with Quaternary Quasicrystalline Samples.
Authors: Fournée V. , Mizutani U. , Takeuchi T. , Saitoh K. , Ikeyama M. , Guo J.Q. , Tsai A.P.
Published: April 2011

Ion implantation experiments were carried out on a single grain Al-Pd-Mn icosahedral quasicrystal. The influence of Re or Au dopant on the thermopower and the electrical resistivity has been measured. Comparison with ternary and quaternary alloys produced by usual metallurgical techniques is made. The results indicate that the efficiency of this quasicrystalline alloy as a thermoelectric material can be improved by adjusting composition and structural quality, as well as by the addition of a small amount of Re, whereas Au implantation is inefficient in that respect.
Decagonal Model Based on Periodic Structures in the Al-Pd-Mn System
Authors: Boudard Michel
Published: April 2011

The structure of periodic phases, in the Al-Pd-Mn system, showing the importance of chemical ordering is reviewed as well as their description in term of pentagonal chains of transition metals. These features are discussed in relation with structural models for AlPdMn decagonal quasicrystals.
Processing and Mechanical Properties of Quasicrystal-Reinforced Al-Alloys
Authors: Schurack F. , Eckert J. , Schultz L.
Published: April 2011

We investigated the suitability of powder metallurgy as well as die casting for the fabrication of light-weight Al-based composites with quasicrystalline particles embedded in an aluminum matrix. Al-Mn-Ce and Al-Cu-Fe quasicrystalline powders were synthesized by milling of elemental powder mixtures or arc-melted prealloys using a planetary ball mill. The mixture of the quasicrystalline phase and the Al-matrix phase with an appropriate ratio was realized by an initial aluminum excess or by blending of quasicrystalline powder with pure aluminum. The powders were consolidated by hot extrusion. Bulk samples of Ø3mm diameter and 50mm length were also directly prepared by squeezing the melt into a copper mold. CCRT-compression tests revealed a yield strength of about 400 MPa, an ultimate strength of 565 MPa and a ductility of up to 19 % fracture strain as optimum mechanical properties.