bluuebunny/crossref_metadata_2025 huggingface dataset:
Cathodoluminescence of Lateral Epitaxial Overgrowth GaN: Dependencies on Excitation Conditions
Authors: Cargill G. S. , Campo Eva , Yue Lanping , Ramer J. , Schurman M. , Ferguson I. T.
Published: April 2011

We have found that near-band-edge cathodoluminescence (CL) emission decreases with time for some nominally undoped GaN samples. The rate of intensity decrease depends on the incident beam current. It also depends on the size of the area being scanned, which is determined by the magnification used, although the electron beam voltage and current are held constant and similar regions of GaN are being examined. Faster decrease with time occurs with higher beam currents and higher magnifications. The reduced luminescence efficiency persists over at least 24 hour beam-off periods. The dependence of CL intensity on beam current and on scanned area may be a result of different levels of local charging, and structural modifications caused by this charging, for different beam current and magnification settings.
Photoconductivity Recombination Kinetics in GaN films
Authors: Misra M. , Moustakas T.D.
Published: April 2011

Photoconductive detectors were fabricated on autodoped n-GaN films, with resistivity varying from 10 Ohm-cm to 107Ohm-cm, by molecular beam epitaxy. The mobility-lifetime product, determined from the measurement of photoconductive gain, was found to decrease monotonically from 10−2 cm2/V to 10−7 cm2/V as the dark resistivity was increased. This variation in the ντ products is attributed to changes in photocarrier lifetimes. In order to understand the recombination mechanisms responsible for this photoconductive behavior, the dependence of photoconductivity on excitation intensity (δμσ fλ) was investigated. The exponent λ was found to vary from 0.5 to 1.0, as the dark resistivity of the films increased. These results indicate the presence of exponential band tails extending from the conduction band edge. Furthermore, the dependence of photoconductivity on dark resistivity indicates that the photoconductive response is governed primarily by the location of the dark fermi level. A model accounting for these observations is presented.
Formation and Characterization of Oxides on GaN surfaces
Authors: Mistele D. , Rotter T. , Fedler F. , Klausing H. , Semchinova O.K. , Stemmer J. , Aderhold J. , Graul J.
Published: April 2011

We characterized oxides formed directly on n-GaN surfaces. The methods used for oxide layer formation were both photoanodic oxidation and thermal oxidation. The photoanodic oxidation took place in aqueous solutions of potassium hydroxide with pH values lower than 13. Homogenous oxide films were obtained in the voltage range from -0.6 V to 0.4 V vs the saturated calomel electrode (SCE). The characterization of the oxide layers was performed primarily by Auger electron spectroscopy (AES). First the surface chemistry was determined, proving that Ga-oxide is formed with an attributed stoichiometry of Ga2O3. Secondly, depth profiling shows the oxide thickness to be dependent on the photoanodic voltage and oxidation time. Complementary X-ray diffraction (XRD) studies suggest an amorphous state of the formed layers. Annealing GaN in O2-atmospheres above 900°C also lead to surfaces fully covered with gallium oxide. We found that N-polar surfaces oxidize faster than Ga-polar surfaces, which is in agreement to the theoretical work of Zywietz et al [1]. Furthermore, we report on the electrical properties of the anodized oxide layers by analyzing MOS structures.
Monte Carlo Based Calculation of Transport Parameters for Wide Band Gap Device Simulation
Authors: Bellotti E. , Farahmand M. , Nilsson H.-E , Brennan K. F. , Ruden P. P.
Published: April 2011

We present Monte Carlo based calculations of transport parameters useful in the simulation of III-nitride and SiC based devices. The calculations are performed using a full band ensemble Monte Carlo model that includes numerical formulations of the phonon scattering rates and impact ionization transition rates. Calculations are made for the wurtzite and zincblende phases of GaN, the wurtzite phase of InN, and the 3C (cubic) and 4H phases of SiC. The basic transport parameters determined are saturation drift velocity, and the ionization coefficients as a function of applied electric field. Results from the various materials are finally compared.
Design of AlGaN/GaN Heterojunction Bipolar Transistor Structures
Authors: Zhang Yumin , Cai Cheng , Paul P.
Published: April 2011

The potential of III-nitride materials for the fabrication of bipolar transistors is investigated theoretically. Several different pseudomorphic AlGaN/GaN n-p-n heterojunction bipolar transistor structures are examined through calculations of their band profiles and majority carrier distributions in equilibrium. Spontaneous and piezoelectric polarization charges are utilized to create large hole sheet carrier densities in the base layer, thus minimizing the base spreading resistance. At the same time, a large accelerating field in the base can help reduce the base transit times of the electrons and, hence, increase the current gains of these devices. The effect of strain due to substrate mismatch is also investigated.
Thermal Modeling of III-nitride Heterostructure Field Effect Transistors
Authors: Li T. , Ruden P.P. , Albrecht J.D. , Ancona M.G. , Anholt R.
Published: April 2011

The output characteristics of AlGaN/GaN semiconductor heterostructure field effect transistors are strongly dependent on the ambient temperature of the channel region due to relatively large variations in the electron drift velocity with temperature. A model to simulate the heat flow in AlGaN/GaN HFETs on sapphire and SiC substrates is presented. The non- linearity of the problem arising from the temperature dependencies of the III-nitride and substrate thermal conductivities is examined through the technique of self-consistent boundary conditions. It is found that the use of the linearizing Kirchhoff transformation is a good approximation for these systems. Alternative approaches of heatsinking the devices from the top of the wafer are explored also.
Ohmic Contact Formation Mechanism of Pd-based Contact to p-GaN
Authors: Kim Dae-Woo , Bae Joon Cheol , Kim Woo Jin , Baik Hong Koo , Kim Chong Cook , Je Jung Ho , Hong Chang Hee
Published: April 2011

We have investigated surface treatment effect on the interfacial reaction of Pd/p-GaN interface and also room temperature ohmic contact formation mechanism of Pd-based ohmic contact. In order to examine room temperature ohmic behavior, various metal contact systems were deposited and current-voltage measurements were carried out. In spite of large theoretical Schottky barrier height between Pd and p-GaN, Pd-based contact showed perfect ohmic characteristic even before annealing. According to the results of synchrotron X-ray radiation, the closed-packed atomic planes (111) of the Pd film were quite well ordered in surface normal direction as well as in the in-plane direction. The effective Schottky barrier height of Au/Pd/Mg/Pd/p-GaN was 0.47eV, which was estimated by Norde method. This discrepancy between theoretical barrier height and the measured one might be due to the epitaxial growth of Pd contact metal and so the room-temperature ohmic characteristic of Pd-based ohmic contact was related strongly to the in-plane epitaxial quality of metal on p-GaN.
Reactive Ion Etching of CVD Diamond in CF4/O2, O2 and O2/Ar Plasmas
Authors: Leech Patrick W. , Reeves Geoffrey K. , Holland Anthony S.
Published: April 2011

The reactive ion etching of diamond in O2, CF4/O2, CHF3/O2, O2/Ar) discharges has been examined as a function of bias voltage, flow rate and composition of the gas mixtures. Etching in O2 and O2/Ar plasmas (with flow ratio of O2/Ar >25% ) was characterised by a high etch rate (∼35 nm/min) and an increase in surface roughness with rising bias voltage. The CF4/O2 plasmas also produced a high etch rate (∼50 nm/min) but with only minor dependence of roughness on bias voltage. In comparison, the O2/Ar (with O2/Ar flow ratio <25%) and CHF3/O2 plasmas resulted in a low etch rate (7-10 nm/min). The high and low rate regimes were identified as ion- enhanced chemical etching and physical sputtering respectively. Etching in the O2/Ar plasmas has been attributed to a combination of the two processes dependent on the O2 content.
A Study of Pt/AlN/6H-SiC MIS Structures for Device Applications
Authors: Thompson Margarita P. , Auner Gregory W. , Huang Changhe , Hilfiker James N.
Published: April 2011

AlN films with thicknesses from 53 to 79 nm were deposited on 6H-SiC substrates via Plasma Source Molecular Beam Epitaxy (PSMBE). The influence of deposition temperature on the growth mode and film roughness was assessed. The optical constants of the films in the range 0.73-8.75 eV were determined using spectroscopic ellipsometry. Pt/AlN/6H-SiC MIS structures were created and current-voltage (I-V) and capacitance-voltage (C-V) measurements were performed at room temperature and at 250°C. Most of the MIS structures showed rectifying I-V characteristics regardless of growth temperature. A 120-nm-thick AlN film was deposited at 500°C. MIS structures created on this film showed a very low leakage current densities of 6×10−8 A/cm2. The dielectric constant of the film was estimated at approximately 9. The relation between film structure and electrical properties of the films is discussed.
Device Processing for GaN High Power Electronics
Authors: Pearton S.J. , Cao X.A. , Cho H. , Lee K.P. , Monier C. , Ren F. , Dang G. , Zhang A.P. , Johnson W. , LaRoche J.R. , Gila B.P. , Abernathy C.R. , Shul R.J. , Baca A.G. , Han J. , Chyi J.-I. , Hove J.M. Van
Published: April 2011

Recent advances in developing process modules for GaN power devices are reviewed. These processes include damage removal in dry etched n- and p-GaN, implant doping and isolation, novel gate dielectrics, improved Schottky and ohmic contacts and deep via etching of SiC for hybrid GaN/SiC structures.
Characterization of n-type layer by S+ ion implantation in 4H-SiC
Authors: Tanaka Yasunori , Kobayashi Naoto , Okumura Hajime , Yoshida Sadafumi , Hasegawa Masataka , Ogura Masahiko , Tanoue Hisao
Published: April 2011

We investigated the optical, electrical and structural properties of the layer which was implanted with sulfur ion(S+) in 4H-SiC. By using the high temperature ion implantation technique more less residual defects were observed compared with the room temperature ion implantation by Rutherford backscattering spectrometry and channeling(RBS-channeling). After annealing at 1700°C there was no significant difference between the implanted sample and virgin sample in crystallinity within the detection limit of RBS-channeling. From the result of low temperature photoluminescence(LTPL) we could see the photoluminescences, so-called D1 and D2center, originating in the defects formed by ion implantation and post-annealing(∼1700°C) processes and confirmed that their intensities decreased with the increasing of the total dose of S+. The result of Hall effect measurement suggested that the conduction type of S+-implanted layer is n-type and their activation energies were 275meV and 410meV by the fitting of neutrality equation assuming the two activation energies for the hexagonal and cubic lattice sites in 4H-SiC.
Effects of Electrode Spacing on Reactive Ion Etching of 4H-SiC
Authors: Bonds Janna R. , Carter Geoff E. , Casady Jeffrey B. , Scofield James D.
Published: April 2011

4H-SiC was selectively etched in a Reactive Ion Etch (RIE) system using a nickel mask. The power, pressure, and electrode spacing were varied within a RF generated SF6:O2 (1:2) plasma. Peak etch rates of up to 2600 Aring;/min. were obtained at a pressure of 350 mT, power of 90 W (2 W/cm2), and electrode spacing of 3.180 cm. Etches were all residue-free, although power levels above 60 W (1.36 W/cm2) resulted in the SiC surface being roughened, which limited smooth surface etch capability to 2000 Aring;/min. When comparing electrode spacing from 3.180 cm to 1.270 cm, the 3.180 cm spacing was found to have the highest etch rate at pressures ranging from 250 mT to 500 mT.
Thin Na0.5K0.5NbO3 Films for Varactor Applications
Authors: Cho C.-R. , Koh J.-H. , Grishin A. , Abadei S. , Petrov P. , Gevorgian S.
Published: April 2011

Single phase Na0.5K0.5NbO3 (NKN) thin films have been pulsed laser deposited on SiO2/Si(C-01) wafers and LaAlO3(001) and MgO(001) single crystals. Radio frequency (up to 1 MHz) and microwave (up to 50 GHz) dielectric spectroscopy studies have been carried out to characterize thin NKN films for electrically tunable microwave device applications. Films on single crystal oxide substrates showed tunabilities as high as 30-40 % at 40 V bias and dissipation factor of 0.01-0.02 at 1 MHz. The films on Si substrates showed low dielectric losses of < 0.01, and low leakage currents. Dielectric properties of ferroelectric films on Si substrates at low frequencies are greatly influenced by the depletion capacitance and the resistance inserted by semiconductor substrate. Microwave frequency measurements for NKN film on Si wafers yield more than 10 % tunability at 50 GHz and loss tan σ <0.1 at 10 GHz.
Simulation of a Simplified Design for a Nanoscale Metal-Oxide Field Effect Transistor
Authors: Newns D. M. , Donath W. M. , Pattnaik P.C.
Published: April 2011

We describe simulations on a simplified design for a metal-oxide nanoscale Field Effect Transistor (FET). The device features an oxide channel with a high dielectric constant ferroelectric as the gate insulator. In the present model, the gate and source/drain electrodes are unconventionally placed on opposite sides of the channel. Simulations are quantum mechanical and are based on a simplified transport model. Results on a 10 nm. channel device show adequate conductance and ON/OFF ratio, while simulation of a ring oscillator yields an estimated device switching time of 300 fs..
The Effects of Mg Doping on the Materials and Dielectric Properties of Ba1-xSrxTiO3 Thin Films
Authors: Cole M.W. , Joshi P.C. , Pfeffer R.L , Hubbard C.W. , Ngo E. , Ervin M.H. , Wood M.C.
Published: April 2011

We have investigated the dielectric, insulating, structural, microstructural, interfacial, and surface morphological properties of Ba0.60Sr0.40TiO3 thin films Mg doped from 0 to 20 mol%. A strong correlation was observed between the films structural, dielectric and insulating properties as a function of Mg doping. Non textured polycrystalline films with a dense microstructure and abrupt film--Pt electrode interface were obtained after annealing at 750°C for 30 min. Single phase solid solution films were achieved at Mg doping levels up to 5 mol%, while multiphased films were obtained for Mg doping levels of 20 mol%. Decreases in the films dielectric constant, dielectric loss, tunability and leakage current characteristics were paralleled by a reduction in grain size as a function of increasing Mg dopant concentration. Our results suggest that Mg doping serves to limit grain growth and is thereby responsible for lowering the dielectric constant from 450 to 205. It is suggested that Mg behaves as an acceptor-type and is responsible for the doped films low dielectric loss and good leakage current characteristics. Performance-property trade-offs advocates the 5 mol% Mg doped Ba0.60Sr0.40TiO3 film to be an excellent choice for tunable microwave device applications.
Fabrication of MgIn2O4 Thin Films With Low Resistivity on MgO (100) Surface by PLD Method
Authors: Noshiro R. , Ueda K. , Hosono H. , Kawazoe H.
Published: April 2011

Thin films of MgIn2O4 spinel, which is a recently discovered TCO material, were deposited on MgO (100) surface by PLD. The thin films were prepared under low oxygen partial pressure to enhance formation of oxygen vacancies, from which carrier electrons were generated. X-ray analyses and AFM observations suggest epitaxial growth of the grains with diameter of 100∼200nm. The grains showed strong orientations both along the normal of the thin film and in plane. Epitaxial growth of the spinel was also confirmed by high-resolution transmission electron microscopic observations. The lattice image of the interface region suggests formation of structural imperfections such as dislocations, grain boundaries and amorphous phase in significant fraction. Strong optical absorption due to electron carriers was detected in near infrared region. Very large Burnstein-Moss shift was observed in ultraviolet region, and the optical band gap was estimated to be 4.3eV. DC conductivity observed was 4.5×103Scm−1, which is the highest value reported for the material so far. Concentration and Hall mobility of carrier electrons were found to be 2.1×1021cm−3 and 14cm2V−1s−1, respectively.
Development and Implementation of New Volatile Cd and Zn Precursors for the Growth of Transparent Conducting Oxide Thin Films Via Mocvd
Authors: Babcock J.R. , Wang A. , Edleman N.L. , Benson D.D. , Metz A.W. , Metz M.V. , Marks T.J.
Published: April 2011

For the growth of thin zinc group metal oxide films [i.e. cadmium oxide (CdO), cadmium stannate (Cd2SnO4), and zinc oxide (ZnO)] via metal-organic chemical vapor deposition (MOCVD), volatile Cd and Zn precursor families are needed. Starting with Cd, β-ketoiminates of varying substitution were prepared to elucidate structure-property relationships. The nature of the ligand substituents strongly influences the melting point (liquid precursors are desired). Unlike conventional Cd β-diketonates, these complexes are monomeric as determined by x-ray crystallography. Despite these advantageous characteristics, attempts to grow CdO thin films in a cold-wall MOCVD reactor using two such derivatives were not successful. This class of Cd complex appears to decompose thermally with time-- a likely cause of the poor performance. Therefore, a new series of more thermally stable Cd precursors was sought. Using the chelating diamine N,N,N1,N1-tetramethylethylenediamine (TMEDA), monomeric β-diketonates were prepared. The molecular structure of Cd(hfa)2(TMEDA) (hfa = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) confirms the monomeric structural assignment. This series of Cd complexes is appreciably more volatile and sublime more cleanly than the aforementioned β-ketoiminates, as determined by vacuum thermogravimetric analysis (TGA). In addition to this advantageous characteristic, these complexes are easily prepared under ambient laboratory conditions from commercially available starting materials in a single step. Following the protocol established for Cd, a volatile series of Zn precursors was also prepared. For the Zn series, the melting point was effectively tuned through variation of both the β-diketonate and diamine ligands. The use of the Cd and Zn β-diketonate precursors in the successful growth of CdO and ZnO thin films, respectively, by MOCVD is also presented.
Evaluation of LSCO Electrodes for Sensor Protection Devices
Authors: Schwartz R. W. , Sebastian M. T. , Raymond M. V.
Published: April 2011

We have evaluated lanthanum strontium cobalt oxide (La0.5OSr0.50COOx; LSCO 50/50) as a candidate “transparent” electrode for use in an electrostatic shutter-based infrared sensor protection device. The device requires that the electrode be transparent (80% transmission) and have moderate sheet resistance (300 – 500 Δ/sq.). To meet these needs, the effects of post-deposition annealing on the resistivity and optical absorption characteristics of sputter deposited LSCO thin films were studied. The as-deposited films were characterized by an absorption coefficient of ∼ 12,500 cm1−1 and resistivities of ∼ 0.08 to 0.5 Δ-cm. With annealing at 800°C, the resistivity decreased to 350 νΔ-cm, while the absorption coefficient increased to ∼ 155,000 cm1−1. By using a post-deposition annealing step at 800°C and controlling film thickness, it appears that a standard LSCO 50/50 material may possess the requisite conductivity and optical transmission properties for this sensor protection device.
Preparation and Characterization of LiCoO2 and LiMg0.05Co0.95O2 Thin Films on Porous Ni/NiO Cathodes for MCFC by Complex Sol-Gel Process (CSGP)
Authors: Lada W. , Deptula A. , Sartowska B. , Olczak T. , Cimielewski A.G. , Carewska M. , Scaccia S. , Simonetti E. , Giorgi L. , Moreno A.
Published: April 2011

The major disadvantage of Ni/NiO cathodes for a Molten Carbonate Fuel Cells (MCFC) application is dissolution of NiO in K/Li electrolyte that significantly decreases the cell lifetime. Thin films of LiCoO2 or LiMg0.05Co0.9502 were prepared on a cathode body in order to protect them against dissolution. For preparation of starting sols the Complex Sol-Gel Process (CSGP) has been applied. These sols have been prepared by adding of LiOH to aq. acetates solution of Co 2+(Mg2+) with ascorbic acid and then by alkalizing them with aqueous ammonia to pH=8. The cathode plates of various dimensions (to several hundreds cm2) have been dipped in these sols and withdrawn at rate a 1.7 cm/s. Commercially sintered Ni plates were always initially oxidized by heating at various temperatures. Their microstructure and mechanical properties as a function of temperature were observed. Heat treatment should be carried out under the dead load of the ceramic plates in order to avoid their waving. The best non-folded plates were obtained by treating them for lh at 550°C. The covered substrates were calcined for lh at 650°C, using low heating ratel°C/min. The presence of LiCoO2 in a deposited coating has been proved by EDS patterns. The resultant film thicknesses were measured by scanning electron microscopy (SEM) on the fractured cross-sections; they ranged from 0.5 to 2νm and depended on sol concentration and viscosity. A 350 hundred hours test in molten carbonates, proved that the cathode bodies covered with LiCoO2 are completely prevented from dissolution of Ni in a molten K/Li electrolyte. Dissolution of LiCoO2 coating was not observed as well. After treatment in a molten electrolyte SEM observations did not show any changes in microstructures and morphology of the covered cathodes.
Thermal Spray Techniques for Fabrication of Meso-Electronics and Sensors
Authors: Sampath S. , Herman H. , Patel A. , Gambino R. , Greenlaw R. , Tormey E.
Published: April 2011

Thermal Spray is a directed spray process, in which material, generally in molten form, is accelerated to high velocities, impinging upon a substrate, where a dense and strongly adhered deposit is rapidly built. In the case of ceramic deposits, it is necessary to bring the particles to well above the melting point, which is achieved by either a combustion flame or a thermal plasma arc. The deposit microstructure and, thus, properties, aside from being dependent on the spray material, rely on the processing parameters, which are numerous and complex. In recent years, through concerted, integrated efforts of the Center for Thermal Spray Research at the State University of New York at Stony Brook and others, significant fundamental understanding of the process has been achieved, allowing for an enhanced control of the process.
Numerical Simulation of Laser Induced Substrate Heating for Direct Write of Mesoscopic Integrated Conformal Electronics (MICE)
Authors: Lowry S. , Sheu J. C. , Stewart Robert , Parkhill Robert
Published: April 2011

A numerical tool is developed to simulate the optical and thermal interactions of selected lasers with precursors and substrates in support of the emerging technology for the direct write of Mesoscopic Integrated Conformal Electronics (MICE). The code couples the Discrete Ordinate Method (DOM) radiation model with the multi-physics computation fluid dynamics code CFD-ACE to predict the conductive and radiative heat transport in the process

This paper provides a brief overview of the numerical model. Selected simulations are presented including comparison with empirical data. The capabilities, limitations, and potential applications of the model with respect to MICE are discussed. Future model enhancements are proposed
Calculation of Hamaker Constants in Nonaqueous Fluid Media
Authors: Bell Nelson , Dimos Duane
Published: April 2011

Calculations of the Hamaker constants representing the van der Waals interactions between conductor, resistor and dielectric materials are performed using Lifshitz theory. The calculation of the parameters for the Ninham-Parsegian relationship for several non-aqueous liquids has been derived based on literature dielectric data. Discussion of the role of van der Waals forces in the dispersion of particles is given for understanding paste formulation. Experimental measurements of viscosity are presented to show the role of dispersant truncation of attractive van der Waals forces
Rapid Prototyping of Patterned Multifunctional Nanostructures
Authors: Fan Hongyou , Laópez Gabriel P. , Brinker C. Jeffrey
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 slow processes 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.
Process-Property-Performance Relationship for Fused Deposition of Ceramics (FDC) Feedstock Materials
Authors: Venkataraman N. , Rangarajan S. , Harper B. , Matthewson M.J. , Safari A. , Danforth S.C.
Published: April 2011

Fused deposition of ceramics (FDC) is an extrusion based layered manufacturing process. It uses a high solids loaded (>50 vol. % ceramic or metal powder) thermoplastic binder in filament form as the feedstock material. The filament acts as both the piston driving the extrusion process and the feedstock material being deposited in the X-Y direction onto a Z-stage platform. The primary mode of failure of the filament during the FDC process is via buckling. Earlier work has shown that the filament compressive modulus and the feedstock viscosity control the buckling behavior of the filament material in FDC. A study was conducted to investigate the effect of particle/polymer interface on the viscosity and compressive modulus of the PZT filled ECG9 system. The relative viscosity of the untreated and the surface treated particle filled systems increases with solids loading. It is found that both of the surface treated materials (stearic acid and titanate coupling agent) exhibit a lower relative viscosity than the untreated material. A rheological model (Krieger-Dougherty model) was used to investigate the possible reasons for the decrease in relative viscosity due to the surface treatment of particles. The investigaton showed that the coupling agent acted as a dispersant (increasing value of ϕm) and thereby decreased the viscosity. The stearic acid may act as a dispersant (increasing value of ϕm) and as a lubricant (decreasing value of KE). The compressive modulus of all the different systems studied showed an increase with solids loading. A generalized Nielsen model was used to describe the relative modulus vs. solids loading behavior for all the different systems studied. Also, the measured compressive modulus of the filled system was found to be insensitive to the modifications in the particle surface treatment.
Refrigerative Stereolithography Using Sol-Gel Transformable Photopolymer Resin and Direct Masking
Authors: Murakami T. , Kamimura A. , Nakajima N.
Published: April 2011

The authors have been studying a new fabrication method termed “refrigerative stereolithography” which uses a gel resin layer instead of a liquid layer as in conventional stereolithography. This paper proposes the “direct masking method” in which a masking pattern is drawn to block direct light exposure to the surface of the gel resin layer. With such masks, we can avoid surplus growth only in the regions where it is unnecessary to improve height direction accuracy and resolution. Also, we can solidify the required section shape selectively using the masks and a lamp instead of laser scan patterns. The effectiveness of refrigerative stereolithography with the direct masking method is discussed and confirmed by some experimental results.
An Overview of Plasticity in Nanoscale Composites
Authors: Embury J.D. , Sinclair C.W.
Published: April 2011

In the past two decades there has been great activity in the area of nanoscale composites. This has included enormous effort in the areas of epitaxial structures for microelectronics applications, organometallic systems, coatings [1], layered metallic structures and drawn in-situ composites. A great deal of progress has been made in the development of controlled fabrication methods including sputtering, electrodeposition and crystallization of amorphous structures. Also, attention has been given to the integration of ultrafine scale structures into the design of many engineering applications from high field magnets operating at cryogenic temperatures [2] to future gas turbines [3].
Formation of Nano-Sized Crystals During Plastic Deformation in Amorphous Alloys
Authors: Ogura A. , Sato M. , Tarumi R. , Shimojo M. , Takashima K. , Higo Y.
Published: April 2011

Microstructural change in deformation bands was observed in a Ni-P amorphous alloy using transmission electron microscopy (TEM). Bending strain was applied to a micro-sized cantilever beam specimen. A TEM specimen was prepared from the micro-sized specimen with keeping the information on the relationship between the deformed direction and the specimen orientation. TEM observation in deformation bands revealed that plastic deformation induces the precipitation of nanocrystalline Ni particles. An orientation relationship between deformation direction and crystal orientation was obtained, that is, a (111) plane of all crystalline particles are parallel to the side surface of the specimen.
Cyclic Deformation and Fatigue Properties of Ultrafine Grain Size Materials: Current Status and Some Criteria for Improvement of the Fatigue Resistance
Authors: Mughrabi Haël , Höppel Heinz Werner
Published: April 2011

In this review, some general conclusions based on studies performed to date on fatigued materials of ultrafine grain (UFG) size produced by equal channel angular pressing (ECAP) are drawn, and open issues of current interest are defined. Important aspects addressed include the apparent discrepancy between improved fatigue strengths in Wöhler (S-N) plots as opposed to inferior fatigue strengths in Manson-Coffin plots, the clarification of the microstructural mechanisms of severe cyclic softening in conjunction with dynamic (local) grain/subgrain coarsening and damaging large-scale catastrophic shear banding. The important roles of the cyclic slip mode, the friction stress, the crystal structure and the temperature of cyclic deformation with respect to stable cyclic deformation behaviour are emphasized. Based on such considerations, criteria are formulated that must be observed, when designing ECAP-processed UFG-materials of superior fatigue strength.
Simulation of Positron Characteristics in Nanocryartalline Materials
Authors: Kuriplach Jan , Petegem Steven Van , segers Danny , Dauwe Charles , Hou Marc , Zhrukin Eugenij E. , Swygenhoven Helena Van , Morales Alvaro L.
Published: April 2011

The theoretical calculations of the position response to different types of defects can be very helpful in order to interpret properly positron measurements. In this contribution a new computational technique to determine position properties in nanocrystalline materials is presented. In such calculations we employ the realistic models of n-materials obtained using molecular dynamics. The new technique is based on the so-called atomic superposition method where atomic densities are superimposed in a selected region of the model (virtual) sample to approxiamte the electron density of the system. We study the virtual samples of n-Cu, n-NiAl, and n-Ni3Al, for which we calculate position lifetime and position binding energies corresponding to defects located in selected regions of the samples. The regions of interest for position calculations comprise grain boundaries inculding triple points, nano-voids, and bulk-like regions.
Atomistic Studies of Plasticity in Nanophase Metals
Authors: Swygenhoven H. Van , Derlet P. , Caro A. , Farkas D. , Caturla M. , Rubia T. Díaz de la
Published: April 2011

Molecular dynamics computer simulation of nanocrystalline Ni and Cu with mean grain sizes ranging from 5 to 20 nm show that grain boundaries in nanocrystalline metals have structures similar to most grain boundaries found in conventional polycrystalline materials. Moreover, the excess enthalpy density in grain boundaries and triple junctions appears to be independent of grain in both, computer generated and experimental measured samples. Simulations of deformation under constant uniaxial stress demonstrate a change in deformation mechanism as function of grain size: at the smallest grain sizes all deformation is accommodated in the grain boundaries, at higher grain sizes, intragrain deformation is observed
Calculating Surface Energies of Lead Magnesium Niobate Using Density Functional Theory
Authors: Kavarnos George , Richards Roger
Published: April 2011

Computations involving density functional theory have been performed on lead magnesium niobate (PMN) single crystal models in an effort to calculate their surface energies, which are believed to play a role in brittle fracture mechanisms. To establish credibility of this approach, test calculations were performed on MgO and SiC single crystal models. The surface energy of MgO was determined to be 1.2 J/m2, which is in close agreement with the experimental value. Similarly, the value for SiC, 8.03 J/m2, supported a level of confidence with this methodology. Surface energies were calculated for several simple perovskites and several PMN models. The calculated values suggest that changes in the A-site ion of PMN do not result in any significant changes in the surface energies.
Atomic Scale Modelling of Supported and Assembled Nanoparticles
Authors: Zhurkin E. , Hou M. , Swygenhoven H.Van , Pauwels B. , Yandouzi M. , Schryvers D. , Tendeloo G.Van , Lievens P. , Verschoren G. , Kuriplach J. , Peteghem S.Van , Segers D. , Dauwe C.
Published: April 2011

The properties of elemental and bimetallic free, supported and assembled nanoclusters are modeled at the atomic scale and the models are discussed on the basis of experimental observations. This way, the memory of some free cluster properties in nanostructured materials may be evaluated.

The combination of molecular statics with High Resolution Transmission Electron Microscopy (HRTEM) allows to predict fine detail of the lattice relaxation of a truncated octahedral gold cluster deposited on MgO. Metropolis Monte Carlo (MC) predicts that a lattice mismatch may contribute to disordering in deposited Cu3Au nanoclusters. In both Cu-Au and Ni-Al free clusters, offset of equilibrium stoichiometry may result in segregation of Au or Al at the cluster surface. An ordered stoichiometric core is surrounded by a disordered mantle where the excess species resides. Different modeling methods predict different nanometer scale textures.

Therefore, cluster assembled Ni3Al alloys formed by condensation and pressing are modeled in two different ways. Both make use of a combination of Molecular Dynamics and MC. Whatever the model nanostructure, the segregation properties of free clusters remain in the nanostructured material. This segregation is one possible cause that can inhibit the formation of a metastable martensitic phase as observed in bulk Ni-Al alloys.

The occurrence of vacancy clusters and voids is hardly identified by HRTEM. On the other hand, their distribution and sizes are sensitive to the nanostructure modeling. Therefore, a new characterization method is developed, which combines positron lifetime spectroscopy with the calculation of positron lifetimes from selected areas of the model samples.
Site-Specific Attachment of Gold Nanoparticles to DNA Templates
Authors: Stevenson Karen A. , Muralidharan Govindarajan , Maya Leon , Wells Jack C. , Barhen Jacob
Published: April 2011

DNA was used as a scaffold for the binding of gold nanoparticles using a standard chemical technique. A DNA template was designed with amino-modified thymines located every 3.7 nm, which would allow the attachment of the carboxylic acid functionalized gold nanoparticles. The gold particles were covalently bound to the amino groups on the DNA using standard 1-ethyl-3- (3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) chemistry in the presence of a competitor to block excess gold binding sites. The products were analyzed by transmission electron microscopy (TEM) and atomic force microscopy (AFM).
Crystallographic Description for Nanoparticle Asemblies – Application to Cadmium Selenide Clusters
Authors: Vasiliev A L , Aindow M , Lee J , Papadimitrakopoulos F , Jain F
Published: April 2011

The stacking sequence of several layers of self-assembled semiconducting CdSe nanoparticles has been investigated using transmission electron microscopy. FCC, HCP, saddle sites occupation and ring structures were found coexisting in the supercrystals formed by CdSe nanoparticles.
Direct Synthesis of Silicon Nanowires, Silica Nanospheres, Wire-Like Nanosphere Agglomerates, and Silica-Based Nanotubes and Nanofiber Arrays
Authors: Gole J. L. , Stout J. D. , Dai Z. R. , Wang Z. L.
Published: April 2011

For several decades, the vapor-liquid-solid (VLS) process,1,2 where gold particles act as a mediating solvent on a silicon substrate, forming a molten alloy, has been applied to the generation of silicon whiskers. The diameter of the whisker is established by the diameter of the liquid alloy droplet at its tip. The VLS reaction generally leads to the growth of silicon whiskers epitaxially in the <111> direction on single crystal silicon <111> substrates.1-3 Recently, Lieber,4 Lee,5 Yu,6 and coworkers have extrapolated on the ideas entailed in the VLS technique to develop laser ablation of metal containing silicon targets, obtaining bulk quantities of silicon nanowires. More recently, Lee et al.5,7 have shown that oxides play a dominant role in the nucleation and growth of semiconductor nanowires be it by laser ablation, thermal evaporation, or chemical vapor deposition. Lee et al.5 have suggested a new growth mechanism, referred to as oxide assisted nanowire growth, which represents a new approach to nanowire synthesis. Our initial approach8-10 to this problem has involved the application of the techniques of high temperature synthesis to modify the approach of Lee et al. and generate virtually defect free SiO2 sheathed crystalline silicon nanowires and silica (SiO2) nanospheres which can be agglomerated to wire-like configurations impregnated with crystalline silicon nanoclusters. Further controlled condensation can extend this agglomeration to produce nanotubes and nanofiber arrays.
Nanoparticles of Polypyrrole and Their Effect on Mediating Li Ion Transport from Liquid Electrolyte to Cathode
Authors: Pang Yong , Chen Ningpine , Hong Liang
Published: April 2011

Absctract

Nano-scale particles of polypyrrole (PPy) have been synthesized by polymerizing the respect monomer in the presence of its substituted homologue with oxyethylene (POE) oligomers as the substitute groups. The polymerization was carried out by introducing an aqueous solution of oxidant into the tetrahydrofurane (THF) solution of the monomers. The particles obtained possess brush-like architectures in the dispersing medium, and their sizes in dry state fall into the range of 10 to 40 nm according to TEM analysis. When dispersed by a low content (< 0.1 wt %) in an aprotic organic solvent, the nanoparticles synthesized have been foundto affect the conduction of lithium ion. The effect is interpreted as a significant reduction in the electrical resistance at the electrode/electrolyte interface according to gain-phase impedance measurement.This has been attributed to the mediating role of the graft POE chains on the colloidal particles.
Purpose-Built Anisotropic Metal Oxide Nanomaterials
Authors: Vayssieres Lionel , Guo Jinghua , Nordgren Joseph
Published: April 2011

Large arrays of perpendicularly oriented anisotropic nanoparticles of ferric oxyhydroxide (Akaganeite, β-FeOOH) and oxide (Hematite, α-Fe2O3) of typically 3-5 nm in diameter, self-assembled as bundles of about 50 nm in diameter and of up to 1 μm in length have been successfully grown onto polycrystalline substrates without template and/or surfactant by heteronucleation from an aqueous solution of ferric salts and their optical and electronic properties investigated.
Epitaxial Ferroelectric BaTiO3 Thin Films for Microphotonic Applications
Authors: Niu F. , Teren A.R. , Hoerman B.H. , Wessels B.W.
Published: April 2011

Epitaxial ferroelectric BaTiO3 thin films have been developed as a material for microphotonics. Efforts have been directed toward developing these materials for thin film electro-optic modulators. Films were deposited by metalorganic chemical vapor deposition (MOCVD) on both MgO and silicon substrates. The electro-optic properties of the thin films were measured. For BaTiO3 thin films grown on (100) MgO substrates, the effective electro-optic coefficient, reff depended on the magnitude and direction of the electric field. Coefficients as high as 260 pm/V have been measured. Investigation of BaTiO3 films on silicon has been undertaken. Epitaxial BaTiO3 thin films were deposited by MOCVD on (100) MgO layers grown on silicon (100) substrates by metal-organic molecular beam epitaxy (MOMBE). The MgO serves as the low index optical cladding layer as well as an insulating layer. X-ray diffraction and transmission electron microscopy (TEM) indicated that BaTiO3 was epitaxial with an orientational relation given by BaTiO3 (100)//Si (100) and BaTiO3[011]//Si [011]. Polarization measurements indicated that the BaTiO3 epitaxial films on Si were in the ferroelectric state.
Enhanced Emission from a Light-Emitting Diode Modified by a Photonic Crystal
Authors: Erchak Alexei A. , Ripin Daniel J. , Fan Shanhui , Joannopoulos John D. , Ippen Erich P. , Petrich Gale S. , Kolodziejski Leslie A.
Published: April 2011

Enhanced light output is observed from a light-emitting diode (LED) structure containing a two-dimensional (2D) photonic crystal. The capture of emitted light into planar waveguide modes reduces the extraction efficiency of LEDs. Here, 2D photonic crystals are utilized to: 1) enhance the extraction of light in the vertical direction from an LED and 2) enhance optical pumping by directly coupling light from a pump laser into the LED structure. Spatially and spectrally-resolved photoluminescence (PL) is used to characterize the enhancements. A 100-fold enhancement in extraction for a single wavelength into the vertical direction is obtained without the photonic crystal penetrating the active material.
Investigation of silicon carbide physical vapor transport growth on the C-terminated face of 6H seeds
Authors: Schulz D. , Doerschel J. , Irmscher K. , Rost H.-J. , Siche D. , Wollweber J.
Published: April 2011

Sublimation growth of 6H-SiC has been studied with respect to surface morphology, growth temperature, supersaturation and growth rate. Growth was performed on the C-terminated face of 6H seeds mainly and for comparison also the Si-terminated face was used. Step bunching is observed dependent on different parameters and is strongly influenced by the seed orientation. The growth rate of 4H on the C-face is found to be higher than the rate of 6H grown on the Siface.
A New Sublimation Etching for Reproducible Growth of Epitaxial Layers of SiC on SiC Substrate in a CVD Reactor
Authors: Wang Rongjun , Bhat Ishwara , Chow Paul
Published: April 2011

We have developed a new and simple method to etch SiC at relatively low temperature with very high rates. This method is particularly useful for cleaning the susceptor after growth. By employing this etching method to clean the susceptor prior to every growth run, we have greatly improved the reproducibility of SiC epitaxial growth and increased the lifetime of the susceptor. The method can also be used to etch SiC wafers at a very fast rate as an alternative to mechanical polishing. An etch rate of over 100μm/hr was obtained at 1600°C using this new method. The surfaces of the etched wafers were examined by atomic force microscope (AFM). Results of epitaxial growth using new etching method are presented.
Epitaxial Growth of SiC on Non-Typical Orientations and MOS Interfaces
Authors: Matsunami Hiroyuki , Kimoto Tsunenobu , Yano Hiroshi
Published: April 2011

High-quality 4H-SiC has been epitaxially grown on (1120) substrates by chemical vapor deposition. The physical properties of epilayers and MOS interfaces on both (1120) and off-axis (0001) substrates are elucidated. An unintentionally doped 4H-SiC epilayer on (1120) shows a donor concentration of 1×1014 cm−3 with a total trap concentration as low as 3.8×1012 cm−3. Inversion-type planar MOSFETs fabricated on 4H-SiC (1120) exhibit a high channel mobility of 96 cm2/Vs. The channel mobility decreases according to the T−2.2 dependence above 200K, indicating reduced Coulomb scattering and/or electron trapping. The superior MOS interface on (1120) originates from the much lower interface state density near the conduction band edge.
Characterization of Light Emission from 4H and 6H SiC MOSFETs
Authors: Macfarlane P. J. , Stahlbush R. E.
Published: April 2011

While SiC devices are an attractive alternative to Si in high power applications, interface trap densities measured in SiC-based MOSFETs are significantly larger than in Si-based ones. Here, we study SiC MOSFETs using both spatial images and spectral analysis of light emission due to electron-hole recombination. The light emission is produced by alternately driving the channel between accumulation and inversion using what is essentially a charge-pumping set-up. Emission is due to interface trap and bulk electron-hole recombination. The spatial imaging studies suggest that recombination occurs at both interface traps and bulk defects. Spectral studies of the emission indicate the presence of a narrow band centered at 425 nm and a broad band extending from approximately 500 to 800nm. The former we suggest is due to bulk recombination and the latter to interface trap recombination. The spectral studies of the 500 to 800 nm band are timed to separate light emitted during the inversion-to-accumulation transition from that emitted during the accumulation-to-inversion transition and visa versa. Comparisons of the emission spectra collected during these specific periods are consistent with a larger Dit in the upper half of the bandgap than the lower half in both 4H and 6H devices.