bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct Patterning of Hydrogenated Amorphous Silicon by Near Field Scanning Optical Microscopy
Authors: Hollingsworth Russell E. , Bradford William C. , Herndon Mary K. , Beach Joseph D. , Collins Reuben T.
Published: April 2011

Practical methods for directly patterning hydrogenated amorphous silicon (a-Si:H) films have been developed. Direct patterning involves selectively oxidizing the hydrogen passivated aSi:H surface, with the oxide then serving as an etch mask for subsequent hydrogen plasma removal of the unoxidized regions. Photo induced oxidation has been extensively studied using both far field projected patterns and near field scanning optical microscopy (NSOM) for direct write patterning. Examination of the threshold dose for pattern generation for excitation wavelengths from 248 to 633nm provides indirect evidence for involvement of electron-hole recombination in optically induced oxidation. Optical exposure of a-Si:H in vacuum demonstrated that oxygen must be present in the ambient atmosphere during exposure for successful pattern generation. This suggests that oxidation of the surface may not involve removal of hydrogen, but rather breaking of Si-Si backbonds and insertion of oxygen. An additional mechanism for oxide generation was observed whereby pattern generation resulted from simple proximity of an NSOM probe within ∼30nm from the sample surface. The probe dither amplitude was found to greatly affect the line width and height of patterns generated without light. Line widths of approximately 100nm, comparable to the probe diameter, were obtained.
Pattern Writing by Implantation in a Large-Scale PSII System With Planar Inductively Coupled Plasma Source
Authors: Wu Lingling , Gao Hongjun , Manos Dennis M.
Published: April 2011

A large-scale plasma source immersion ion implantation (PSII) system with planar coil RFI plasma source has been used to study an inkless, deposition-free, mask-based surface conversion patterning as an alternative to direct writing techniques on large-area substrates by implantation. The apparatus has a 0.61 m ID and 0.51 m tall chamber, with a base pressure in the 10−8 Torr range, making it one of the largest PSII presently available. The system uses a 0.43 m ID planar rf antenna to produce dense plasma capable of large-area, uniform materials treatment. Metallic and semiconductor samples have been implanted through masks to produce small geometric patterns of interest for device manufacturing. Si gratings were also implanted to study application to smaller features. Samples are characterized by AES, TEM and variable-angle spectroscopic ellipsometry. Composition depth profiles obtained by AES and VASE are compared. Measured lateral and depth profiles are compared to the mask features to assess lateral diffusion, pattern transfer fidelity, and wall-effects. The paper also presents the results of MAGIC calculations of the flux and angle of ion trajectories through the boundary layer predicting the magnitude of flux as a function of 3-D location on objects in the expanding sheath.
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.
Processing of Organic/Inorganic Composites by Stereolithography
Authors: Lee J. H. , Prud'homme R. K. , Aksay I. A.
Published: April 2011

Ceramic StereoLithography (CSL) is used to fabricate complex shaped ceramic powder compacts by laser photocuring a concentrated ceramic dispersion in photocuring solutions layer-by-layer. The main processing parameters in CSL such as layer thickness, resolution, hatch spacing, and overcure depend on knowledge of the light propagation in a concentrated dispersion. In studies dealing with the processing of ceramic-filled organics, we investigated the depth of curing for model resin systems as a function of photoinitiator concentration. An optimal photoinitiator concentration that maximized the gel cure depth was observed The study showed that photoinitiator plays a significant role in controlling the quality and performance of the formed gel network, with special regard to thickness of cured layers. This has potential application to fields as diverse as industrially cured coatings and dental fillings, and more generally, 3-dimensional fabrication techniques.
Freeform Fabrication of Ceramics by Hot-Melt Ink-Jet Printing
Authors: Derby B. , Reis N. , Seerden K.A.M. , Grant P.S. , Evans J.R.G.
Published: April 2011

Ink-jet printing is a versatile freeform fabrication technique with a high spatial resolution. By suspending ceramic particles in low melting point organic materials and printing above the melting point, rapid cooling on impact after printing results in rapid layer growth. Current results from a collaborative programme studying the hot wax ink-jet printing of structural ceramic components will be reported. The influence of key fluid properties on the ink-jet deposition process are discussed.
Conformal Cooling Versus Conventional Cooling: An Injection Molding Case Study With P-20 and 3DP™-Processed Tooling
Authors: Schmidt Wayde R. , White Ronald D. , Bird Connie E. , Bak Joseph V.
Published: April 2011

A series of designed experiments was performed in an attempt to evaluate and quantify the historically “anecdotal” benefits of conformal cooling for injection molding tooling. The study considered different generic part geometries, gating schemes, mold materials, plastic resins and cooling approaches. This paper provides an overview of the mold design approach, cooling simulation, tool fabrication via the 3DP™ process, as well as part molding and inspection results.
Microstructural Evolution in cryomilled Inconel 625
Authors: He Jianhong , Lavernia Enrique J.
Published: April 2011

Nanocrystalline Inconel 625 alloy, with a uniform distribution of grains, was synthesized using cryogenic mechanical milling. Microstructures of the powder, cryomilled for different times, were investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that both the average powder particle size and average grain size approached constant values as cryomilling time increased to 8 hours. The TEM observations indicated that grains in the cryomilled powder were deformed into elongated grains with a high density of deformation faults, and then fracturedviacyclic impact loading in random directions. The fractured fragments from the elongated coarse grains formed nanoscale grains. The occurrence of the elongated grains, from development to disappearance during intermediate stages of milling, suggested that repeated strain fatigue and fracture caused by the cyclic impact loading in random directions, and cold welding were responsible for the formation of a nanocrystalline structure.
Mechanical Properties of Nanocrystalline Ni in Relation to its Microstructure
Authors: Torre F. Dalla , Swygenhoven H. Van , Victoria M. , Schaeublin R. , Wagner W.
Published: April 2011

Mechanical properties of nanocrystalline Ni made by Inert Gas Condensation and Electrodeposition are presented in relation to their microstructure. Significant plasticity is only observed at elevated temperatures for both types of nanocrystalline Ni. However, a higher temperature is needed in the Inert gas condensated material. Careful analysis of the microstructure by means of X-ray diffraction and conventional electron microscopy reveal initial differences in as-prepared samples. The change in microstructure during deformation at elevated temperatures and during heat treatment without external load is investigated and information about the deformation mechanisms is reported.
The Effect of Deposition Parameters on Tensile Properties of Pulse-Plated Nanocrystalline Nickel
Authors: Morgan K. L. , Ahmed Z. , Ebrahimi F.
Published: April 2011

The microstructure and tensile properties were investigated for pulse-plated nanocrystalline nickel electrodeposits prepared from an additive-free sulfamate-based solution. Square-wave cathodic current densities were varied from 25 to 75 mA/cm2. The samples deposited at 25 mA/cm2showed the smallest grain size (d ∼ 12 nm), and the grain size was found to increase with increasing current densities. The grain size of the electrodeposits ranged from approximately 27 to 12 nm. Tensile results indicated that the yield strength of the specimens increased with decreasing grain size. Scanning electron microscopy revealed that all electrodeposits fractured through a ductile mechanism.
Extension of High Cycle Fatigue Life by the Formation of Nano-Sized Martensite Particles at Intersections of Dislocations in an Austenitic Stainless Steel
Authors: Inamura T. , Shimojo M. , Takashima K. , Higo Y.
Published: April 2011

A new fatigue strengthening method which is applicable for both the micro-sized materials and ordinary-sized materials have been proposed. Dislocations are pinned at their intersections by the formation of nano-crystals after cold work in this strengthening method.

Nano-sized α'-martensite particles, the diameter of which was approximately 5 nm, were formed by a cryogenic treatment at a certain temperature above(martensite burst starting temperature) in a commercially available 316-type austenitic stainless steel. These nano-particles are considered to be formed at intersections of dislocations. Fatigue life tests using ordinary-sized specimens revealed that high cycle fatigue life of the 316-type austenitic stainless steel was extended by the cryogenic treatment and this is considered to be due to the pinning of dislocations by the formation of nano-sized α'-martensite particles.
Novel Tungsten Carbide Nanocrystalline Composites by Pulsed Laser Deposition
Authors: Venkatesan Ravi K. , Kvit A. , Wei Q. , Narayan J.
Published: April 2011

We have developed a novel processing technique to fabricate “artifact free” tungsten carbide (WC) nanocomposites. In this method, pulsed laser deposition of WC in conjunction with a few monolayers of nickel aluminide (NiAl) is used to control the grain size of nanocrystalline composites. The grain size of WC was controlled by the thickness of tungsten carbide and the substrate temperature. The role of NiAl is to ensure the nucleation of tungsten carbide islands, and it is also insoluble in WC. Using this approach, we have fabricated nanocomposites of grain sizes ranging from 6 nm to 35 nm. The hardness of the composite increases with the decrease in grain size, following approximately Hall-Petch relationship. The role of NiAl in grain boundary deformation is of particular interest in strengthening the nanocrystalline composites. The potential of this technique to go to even lower grain sizes is discussed.
Processing and Properties of Ceramic Nanocomposites Produced from Polymer Precursor Pyrolysis, High Pressure Sintering and Spark Plasma Sintering
Authors: Wan Julin , Gasch Matt J. , Kuntz Joshua D. , Mishra Rajiv , Mukherjee Amiya K.
Published: April 2011

Silicon nitride/silicon carbide nanocomposites and alumina-based nanocomposites were investigated in an effort to produce materials with high structural integrity and service properties. Bulk nano-nano composites of silicon nitride and silicon carbide were processed by crystallization of amorphous Si-C-N ceramics that were consolidated in-situ during pyrolysis of a polymer precursor. This material was developed for the purpose of examining the creep behavior of covalent ceramics when there is no oxide glassy phase at grain boundaries. Si3N4/SiC micro-nano composites were sintered by spark plasma sintering (SPS), aiming at better microstructural control and improved creep resistance. Composites of alumina with diamond, silicon carbide and metal (Nb) were developed by high pressure sintering and SPS. These composites maintain microstructures with a nanometric alumina matrix and are targeted for studying the toughening mechanisms and superplastic deformation mechanisms.
Structural Disorder in the Anion Lattice of Nanocrystalline Zirconia and Hafnia Particles
Authors: Vollath Dieter , Forker Manfred , Hagelstein Michael , Szabó D. Vinga
Published: April 2011

Nanoparticles of ZrO2 and HfO2 with sizes around 5 nm are – depending on the production route - found in the cubic or tetragonal or monoclinc phase. To shed some light into this difference, an analysis of crystal structure, distances, and symmetry around the cations was performed on alumina coated nanoparticles prepared by the microwave plasma process. Within the range of accuracy, electron-diffraction and -microscopy revealed a possibly cubic cation lattice for both materials. In the case of ZrO2, the evaluation of the amplitude of the modified radial distribution function derived from extended x-ray absorption fine structure (EXAFS) data lead to a structure with a narrow distribution of oxygen ions in the first shell of neighbors. This is expected for the cubic structure. Comparing the imaginary part with model calculations, a distinct similarity with the cubic structure is obvious. In the second shell of neighbors, consisting of metal ions, one radial distribution peak is expected and found experimentally. Perturbed angular correlation measurements indicate a pronounced disorder of the positions of the first neighbors of the metal ions. Together with the EXAFS observation of a narrow radial distribution of the first neighbor oxygen ions, these measurements provide evidence for a randomc distribution of Zr-O bond angles in the vicinity of their crystallographic positions. Above 500°C a partially reversible transformation to almost perfect tetragonal symmetry is observed. Therefore, one has to conclude that the cubic structure with disordered anion lattice is more stable than the tetragonal one at room temperature. Structural relationships like this are unknown in coarse-grained material.
Simulation of Surface Morphology and Defect Structure in Copper Nanoparticles
Authors: Kogure Yoshiaki , Doyama Masao
Published: April 2011

Molecular dynamics simulation for the morphology and the defect structure in nanoparticles has been performed. The nanoparticles are consisted of 1300 – 5000 atoms and the EAM potential developed by the present authors is adopted to calculate the interactions between atoms. The atom consisting the surfaces or defects are selected thorough the potential energy of individual atoms and structure is investigated by calculating the local crystalline order. A relation between the cooling rate and the particle morphology is also investigated.
Achieving Superplasticity and Superplastic Forming through Severe Plastic Deformation
Authors: Furukawa Minoru , Horita Zenji , Langdon Terence G.
Published: April 2011

The application of severe plastic deformation to metals provides a convenient procedure for achieving nanometer and submicrometer microstructures. Several different processing methods are available but Equal-Channel Angular Pressing (ECAP) is especially attractive because it provides an opportunity for preparing relatively large bulk samples. This paper describes the use of ECAP in preparing materials with ultrafine grain sizes and the subsequent properties of these materials at elevated temperatures. It is demonstrated that, provided precipitates are present to retain these small grain sizes at the high temperatures where diffusion is reasonably rapid, it is possible to achieve remarkably high superplastic elongations in the as-pressed materials and there is a potential for making use of this processing procedure to develop a superplastic forming capability at very rapid strain rates.
High Spin Mn Molecular Clusters: Spin State Effects On the Outer Core-Level Multiplet Structures
Authors: Nelson A. J. , Reynolds J. G. , Christou George
Published: April 2011

Oxo-bridged manganese polynuclear complexes have applications in a variety of technologies, such as single-molecule nanomagnets, catalysis and photosynthetic redox chemistry. The reason that these types of compounds are capable of such important and varied technologies is thought to be because they possess ground states with large spin values. However, the electronic, structural and magnetochemical relationships are not well understood and need to be thoroughly investigated to adequately explain why Mn is such an integral part of so many useful processes. X-ray photoemission spectroscopy was used to study the Mn 3p, 3s and valence band electronic behavior as a function of Mn cluster structural properties, where the cluster size and nuclearity are systematically varied. Results show a chemical shift of the Mn 3p3/2,1/2 spin-orbit pair related to the cluster size and nuclearity. Also, the Mn 3s 7S and 5S final state multiplet components shift since they involve the binding energy of a ligand valence electron. In addition, the branching ratio of the 7S:5S states is related to the 3s–3d electron correlation. Specifically, in the 7S state, the remaining 3s electron is well correlated with 3d electrons of parallel spin, while in the 5S state the two spins are antiparallel. Changes in this electron correlation are clearly observed in the 7S:5S branching ratio as a function of cluster size and ligand electronegativity.
Transport Properties of Bi-related Nanowire Systems
Authors: Lin Y. M. , Cronin S. B. , Ying J. Y. , Heremans J. , Dresselhaus M. S.
Published: April 2011

We present here an electrical transport property study of Te-doped Bi nanowires, and Bi1−xSbx alloy nanowires embedded in a dielectric matrix. The crystal structure of the nanowires were characterized by X-ray diffraction measurements, indicating that the nanowires possess the same lattice structure as bulk Bi in the presence of a small amount of Te or Sb atoms. The resistance measurements of 40-nm Te-doped Bi nanowires were performed over a wide range of temperature (2 K≤ T ≤ 300 K), and the results are consistent with theoretical predictions. The 1D-to-3D localization transition and the boundary scattering effect are both observed in magneto-resistance measurements of Bi1−xSbx alloy nanowires at low temperatures (T < 4 K).
Studies of the Dielectric Constant of Thin Film Bismuth Nanowire Samples Using Optical Reectometry
Authors: Blacka M. R. , Lin Y.-M. , Cronin S. B. , Rabin O. , adi M. P , Dresselhaus M. S.
Published: April 2011

Arrays of 10 to 120 nm diameter single crystalline bismuth nanowires havebeen formed inside amorphous alumina templates. ince bismuth has a small e ective mass compared to other materials, signi cant quantum mechanical con nement is expected to occur in wires with diameter less than 50nm. he subbands formed b yquantum con nement cause in teresting modi cations to the dielectric function of bismuth. his study measures the dielectric function of bismuth nanowires in an energy range where the e ects of quantum con nement are predicted (0.05 to 0.5e). Using F ourier transforminfrared re ectometry, the dielectric constant as a function of energy is obtained for the alumina/bismuth composite system. E ective medium theory is used to subtract the e ect of the alumina template from the measurement of the composite material, thus yielding the dielectric function of bismuth nanowires. A strong absorption peak is observed at ∼1000cm−1 in the frequency dependent dielectric function in the photon energy range measured. he dependence of the frequency and intensity of this oscillator on incident light polarization and wire diameter are reviewed. n addition, the dependence of the optical absorption on antimony and tellurium doping of the nanowires are reported.
Optical Response of Gold Nanoparticles in Dielectric Materials
Authors: Ueda Akira , Bryant Elana M. , Maxwell Carlton B. , Blake Charline M. , Mu Richard R. , Wu Marvin H. , Zavalin Andrey I. , Meldrum Alkiviathes , Henderson Don O.
Published: April 2011

The fundamental studies of metallic nanoparticles embedded in various host materials have been made. The host-guest interaction causes the shapes of embedded nanoparticles, and the surface plasmon resonances of the metallic nanoparticles are affected by the host materials. The control of the surface plasmon resonance condition is a challenging question. We will discuss the interface effect of the systems where gold nanoparticles were fabricated between materials of MgO and SiO2.
New Crucible Design for SiC Single Crystal Growth by Sublimation
Authors: Nishizawa Shin-ichi , Yamaguchi Hirotaka , Kato Tomohisa , Khan M. Nasir , Arai Kazuo , Oyanagi Naoki , Kitou Yasuo , Bahng Wook
Published: April 2011

SiC bulk single crystal growth by sublimation was investigated. A new crucible design, double-walled crucible, was proposed, and its effect was confirmed numerically and experimentally. On the point of heat transfer in a growth cavity, double-walled crucible is better than conventional crucible. With a double-walled crucible, temperature of seed and source surfaces could be kept constant with better uniformity than that with a conventional crucible. It was deduced that a crystal growth rate could be kept constant with flat surface. Furthermore, in case of a double walled crucible, crystal enlarged rapidly with less inclusion. As the results, a double-walled crucible is useful to grow high quality SiC single crystal by sublimation.
Growth of thick 4H-SiC epilayers in a vertical radiant-heating reactor
Authors: Tsuchida H. , Kamata I. , Jikimoto T. , Izumi K.
Published: April 2011

Growth of very thick 4H-SiC epilayers up to 215 μm has been demonstrated in a vertical radiant-heating reactor. Surface roughness is maintained as small as ˜0.2 nm even for epilayers over 150 μm in thickness, and a regular step structure without macro step bunching is observed from the very thick epilayers indicating a stable step-flow growth. Photoluminescence and secondary ion mass spectroscopy (SIMS) were performed for a 150 μm-thick epilayer. The Photoluminescence showed strong free excitons and comparatively small nitrogen bound excitons, while aluminum-, boron- and titanium-related lines were almost negligible. The SIMS analysis found no impurities exceeding 1 × 1014 cm−3. The influence of growth parameters on thickness uniformity will also be shown.
Hollow defect elimination during solution growth of SiC
Authors: Epelbaum B. M. , Hofmann D. , Müller M. , Winnacker A.
Published: April 2011

Using pure and alloyed silicon melt saturated with carbon we investigated systematically hollow defect elimination during SiC solution growth over a wide temperature range from 1500°C to 2100°C. In the process of solution growth all hollow defects present in a substrate demonstrate an evident tendency to act as growth centers and after an adequate period of treatment they were overgrown. Growth morphologies observed in the vicinity of hollow defects are rather different. A new visualization method is proposed, which reveals crystalline defects associated with hollow cores and allows to detect the quantity and the distribution of HD in a whole SiC wafer. Classification of hollow defects based on these observations is presented and the corresponding closing mechanisms are discussed.
Electronic Properties of Nitrogen Delta-Doped Silicon Carbide Layers
Authors: Yokogawa Toshiya , Takahashi Kunimasa , Uenoyama Takeshi , Kusumoto Osamu , Uchida Masao , Kitabatake Makoto
Published: April 2011

Nitrogen delta-doped silicon carbide (SiC) layers were grown by a new pulse doping method in a chemical vapor deposition. Doping distribution with high peak concentration (1.×1018 cm−3) and narrow distribution width (12 nm) was fabricated in the nitrogen delta-doped structure of SiC. Mobility enhancement due to spatial separation of electrons and their ionized parent donors was observed for the delta-doped structure. Metal-semiconductor field-effect transistors with a nitrogen delta-doped channel and a recess gate structure were fabricated. The devices had large source-drain breakdown voltages, high drain current capability and easy control of the threshold voltage with a good pinch-off characteristics.
Structural Characterization Of Sic Epitaxial Layers Grown On Porous Sic Substrates
Authors: Saddow S. E. , Melnychuk G. , Mynbaeva M. , Nikitina I. , Vetter W. M. , Jin L. , Dudley M. , Shamsuzzoha M. , Dmitriev V. , Wood C. E. C.
Published: April 2011

A layer of porous SiC was fabricated by surface anodization of commercial 4H and 6H-SiC (0001)Si face off-axis wafers. A 8.5 μm 4H–SiC epilayer was grown on porous SiC (PSC) substrates using atmospheric pressure CVD. TEM investigation on cross-sectional specimens of the CVD epitaxial layers revealed that the presence of pores in the substrate does not lead to the formation of any micropipe in the epitaxial layer. The investigation also failed to detect a more than usual dislocation density on the basal plane of the epitaxial layer. Based upon the results of various analytical techniques applied to the CVD deposit we propose that the density of screw dislocations in the epitaxial layer is less than 5–104 cm−3. It should be noted that the density of similar types of dislocations in the initial substrate as determined by the TEM was ∼106 cm−3, so this preliminary investigation indicates that the epitaxial layer grown on PSC may have a reduction in dislocation density of more than an order of magnitude over those grown on conventional SiC substrates that are not porous. Synchrotron white beam x-ray topography (SWBXT) was performed on these layers. Comparison between the dislocation density on the porous and standard epitaxial layers proved to be very similar using this technique.
Traps at the SiC/SiO2-Interface
Authors: Pensl Gerhard , Bassler Michael , Ciobanu Florin , Afanas'ev Valeri , Yano Hiroshi , Kimoto Tsunenobu , Matsunami Hiroyuki
Published: April 2011

The density of interface states Dit at SiC/SiO2 interfaces of different SiC polytypes (4H-, 6H- and 15R-SiC) is monitored and the origin of these states is discussed. The hydrogenation behavior of interface states in the temperature range from 250°C to 1000°C is studied by C-V and G-V investigations. The strong increase of Dit close to the 4H-SiC conduction band is attributed to defects located in the oxide (so-called “Near Interface Traps”).
Micromachining Techniques for Advanced SiC MEMS
Authors: Mehregany Mehran , Zorman Christian A.
Published: April 2011

This paper reviews the development of a multilayer, micromolding-based surface micromachining process for SiC microelectromechanical systems (MEMS). The micromolding process uses polysilicon and SiO2 thin films that are deposited onto polysilicon and SiO2 sacrificial layers, patterned into micromolds by reactive ion etching, filled with polycrystalline SiC (poly-SiC), planarized by mechanical polishing, and eventually dissolved and released in selective wet chemical etchants. In addition, a SiC lift-off technique that exploits the microstructural differences between SiC films deposited on Si, SiO2 and Si3N4 surfaces has been developed. The micromolding and lift-off techniques are being used as the basic patterning processes for a four-layer, poly-SiC surface micromachining process that we call the MUSiC (Multi-User SiC) process.
Point and planar defect formation in SiC during PVT growth
Authors: Khlebnikov Yuri I. , Drachev Roman V. , Rhodes Curtis A. , Cherednichenko Dmitry I. , Khlebnikov Igor I. , Sudarshan Tangali S.
Published: April 2011

The spontaneous nucleation of “negative” crystals from the solute of vacancies in SiC does not appear to be dominant due to the low super-saturation of vacancies. However, clustering of the vacancies is possible due to the energy gain in the system caused by coalescence of any two vacancies. The major reasons for point and planar defect formation in SiC are the liquid phase of free silicon and non-stoichiometry of the vapor.
Epitaxial Growth of SiC on AlN/ Sapphire Using Hexamethyldisilane by MOVPE
Authors: Teker Kasif , Lee Ki Hoon , Jacob Chacko , Nishino Shigehiro , Pirouz Pirouz
Published: April 2011

High quality SiC and AlN films allow the fabrication of metal/AlN/SiC MIS structures and SiC/AlN heterostructures that require a low lattice mismatch and excellent thermal stability. Epitaxial SiC on AlN/sapphire was grown using hexamethyldisilane (HMDS) by MOVPE. 2HAlN is epitaxially grown on sapphire by MOCVD, and subsequently SiC is deposited on it. The growth of high quality SiC was achieved in a one step process without any nucleation step using dilute hydrogen in argon (12% H2 + Ar) as the carrier gas, which is less explosive than pure H2. The effect of growth temperature and thickness of AlN on the SiC crystal quality and the surface smoothness were studied. All films were analyzed using reflection high energy electron diffraction (RHEED), Nomarski differential interference contrast microscopy (NDIC), X-ray diffraction (XRD), and atomic force microscopy (AFM). Optimum temperature for SiC growth was between 1300°C and 1350°C. At these temperatures, the grown films show strong epitaxial relationship with AlN and very smooth surfaces (RMS ∼ 0.1- 0.75 nm). At temperatures below 1300°C, the film becomes polycrystalline. At 1400°C, the films show highly textured features, observed by XRD. In the RHEED, however, weak rings appear superimposed on the spot pattern, which implies the grown films are polycrystalline but highly textured. In order to evaluate the effect of underlying AlN thickness on the SiC film, layers with various thicknesses (50, 200, 400 nm) have been used at 1350°C. The SiC film on a 50 nm thick AlN layer shows a very smooth surface (RMS ∼ 0.1 nm) compared to the SiC film on a 400 nm (RMS ∼ 0.7 nm) AlN layer. This seems to be caused by the increasing roughness of the underlying AlN, as it becomes thicker. However, all the films show highly epitaxial growth features, which implies that 50 nm is sufficient to relieve the mismatch strain of the underlying AlN/sapphire.
Structural and electrical characterizations of oxynitride films on solid phase epitaxially grown silicon carbide
Authors: Bera L. K. , Choi W. K. , McNeill D. , Ray S. K. , Chatterjee S. , Maiti C. K.
Published: April 2011

We have investigated the structural and electrical properties of as-prepared and rapid thermal oxynitride films on C+ implanted solid phase epitaxially grown SiC. The oxynitride was grown using N2O. The C concentration of the samples was estimated to be 1, 2 and 5 at. %. From the infrared spectra, samples with 1 and 2 at. % carbon showed that the carbon was substitutionally incorporated into the silicon. No precipitation of SiC was detected. However, for the 5 at. % C sample, some precipitation was observed as indicated by a broad peak at ∼800 cm−1. The oxynitride films showed the Si-O-Si stretching mode at ∼1100 cm−1. The shoulder at 980–1067 cm−1 was due to the O-Si-N bond. The peak at 830 cm−1 was due to the Si-N and Si-C bonds and C-O complex vibrational mode was observed at 663 cm−1. Electrical characterization of the oxynitride films was carried out using the MOS capacitor structure. The interface state density was found to range between 5.7×1011 to 3.35×1012 cm−2eV−1 and increased with an increase in the C concentration. The electrical breakdown field was found to be in the range of 5–7 MV cm−1 and reduced with an increase in C concentration. The charge-to-breakdown value was measured and decreased with an increase in C concentration.
One of Many Sources of Defect Generation in SiC
Authors: Khlebnikov Igor I. , Sudarshan Tangali S. , Khlebnikov Yuri I. , Wood Colin
Published: April 2011

Silicon carbide is a unique material for the study of process of defect generation and crystallization. In this paper, for the first time, we report the observation of the entrapment of whiskers and dendrites (tree-like defects) within the volume of the growing monocrystalline SiC. The encapsulation of the tree-like defects in the volume of the grown crystal leads to solid state polytype transformation. According to Oswald rule, the most probable transformation sequence is as follows: 2H→3C→xR→4H, 6H until a stable phase is established for the given conditions of crystal growth. We observe that the entrapments of tree-like defects are the source of SiC defects such as micropipes and planar defects. It is very likely that the above process is also the source of dislocations. Practically, every branch of the tree-like structure generates the above mentioned defects (micropipes, planar defects, etc.). Our investigation (by EDAX) shows that the chemical composition of the tree-like defect is the same as that of bulk SiC. In this paper, we will present the mechanism of the entrapment of the tree-like defects in the bulk crystal.
Dislocation Content of Etch Pits in Hexagonal Silicon Carbide
Authors: Khlebnikov Igor I. , Lari Mohsen B. , Khlebnikov Yuri I. , Bondokov Robert T. , Ayyagari Ramakrishna , Muzykov Peter , Sudarshan Tangali S. , Anderson T. , Whitlock J. B.
Published: April 2011

6H- and 4H- SiC crystals grown on the Si-face were chemically etched on the as-grown (virgin) surface and the C-face (sliced side). The etching of both the surfaces revealed a strong relationship between a variety of etch pits and the morphological features of the grown boule surface. Several types of etched patterns were revealed. On the Si face, we observed small, medium, and large hexagonal shaped pits and a linear array of small etch pits. However, the C face contained only small pits and a linear array of small pits.

We observed individual or group of dislocations that were connected from the Si face to the opposite C face of the wafer. Also, etch pit lines oriented along specific crystallographic directions were seen.

Our experimental observations have provided a physical basis to explain the generation of defects in SiC. An analysis of our observations show that a correlation exists between the distribution of different size etch pits and the condition of the crystal growth process.
Multi-Axial Channeling Study of Disorder in Gold Implanted 6H-SiC
Authors: Jiang W. , Weber W. J. , Thevuthasan S. , Shutthanandan V.
Published: April 2011

Single-crystal wafers of 6H-SiC were irradiated at 300 K with 2 MeV Au2+ ions over fluences ranging from 0.029 to 0.8 ions/nm2. The accumulated disorder on both the Si and C sublattices in the irradiated specimens has been studied in situ using 0.94 MeV D+ channeling along <0001>, <1102> and <1011> axes. At low doses, results show that some of the Si and C defects are well aligned with the <0001> axis with more C defects shielded by the <0001> atomic rows; a higher level of C disorder is observed, which is consistent with a smaller threshold displacement energy on the C sublattice. There is only a moderate recovery of disorder, produced at and below 0.058 Au2+/nm2, during the thermal annealing at 570 K; similar behavior is observed in the higher-dose samples annealed between 720 and 870 K. The results suggest the presence of defect clusters and amorphous domains formed during the Au2+ irradiation. Reordering processes at 570 K in the weakly damaged 6H-SiC (0.12 Au2+/nm2, 300 K) appear to occur closely along the <1011> direction.
Defect Structures in Neutron Irradiated 6H-SiC Studied by X-Ray Diffraction Line Profile Analysis
Authors: Seitz C. , Magerl A. , Hock R. , Heissenstein H. , Helbig R.
Published: April 2011

We have investigated by x-ray diffraction defect structures in 6H-SiC after neutron irradiation with different fluences and followed by different annealing procedures. An interpretation along a model of Klimanek [1, 4–6] shows, that higher fluences lead to a stronger than linear reduction of the correlation length, whereas higher annealing temperatures correlate with a better recovery of the correlation length. In addition defects of 1st kind created by irradiation are reduced by annealing. We find that annealing changes the character of the defects and it accentuates a defect structure already present in the original samples.
Electron Irradiation of 4H SiC by TEM: An Optical Study
Authors: Sridhara S. G. , Persson P. O. Å. , Carlsson F. H. C. , Bergman J. P. , Janzén E. , Evans G. , Steeds J. W.
Published: April 2011

An intrinsic defect spectrum, commonly observed after ion-implantation, electron, proton or neutron irradiation and even after SIMS measurements is investigated using photoluminescence techniques. The spectrum is associated with carbon related isoelec-tronic centers having a pseudodonor like behaviour. Vacancy-interstitial pair complexes are tentatively suggested as the defect centers responsible for this intrinsic spectrum.
Simulations of Quantum Confinement in Self-Assembled InAs/GaAs Island Quantum Dot Arrays
Authors: Johnson H.T. , Nguyen V. , Bower A. F.
Published: April 2011

Quantum confinement and photoluminescence properties of self-assembled InAs quantum dot arrays on GaAs substrates are studied theoretically using a coupled morphology/electronic structure finite element approach. Simulations of island evolution due to strain energy driven diffusive mass transport are first used to generate realistic island arrays. The resulting finite element meshes are input to a continuum, single charge-carrier model in which band structure properties are prescribed using a k•p Hamiltonian formulation. The results of the electronic properties analysis are spectra of electron and hole energies and corresponding wave functions. The electron and hole states have characteristics unique to arrays, the most prominent of which are the energy separations due to size and shape variations among dots in the array. This feature is shown to lead directly to inhomogeneous broadening in the photoemission spectra. Other phenomena observed here include the onset of a discrete density of states at the point of island formation during growth; the presence of states with wave functions coupling multiple dots in the array; and the nature of wetting layer states in the system.
InAs Quantum Dots Grown on an AlGaAsSb Strain-Relief Buffer
Authors: Gray A.L. , Dawson L. R. , Lin Y. , Stintz A. , Xin Y.-C. , Garza A.A. , Lester L. F.
Published: April 2011

An In(Ga)As-based self-assembled quantum dot laser test structure grown on strain-relief Al0.5Ga0.5As1-ySby strain-relief buffer layers (0≤y ≤ 0.24) on a GaAs substrate is investigated in an effort to increase dot size and therefore extend the emission wavelength over conventional InAs quantum dots on GaAs platforms. Cross-section transmission electron microscopy, and high-resolution x-ray diffraction are used to monitor the dislocation filtering process and morphology in the buffer layers. Results show that the buffer layers act as an efficient dislocation filter by drastically reducing threading dislocations, thus providing a relaxed, low dislocation, compositionally modulated Al0.5Ga0.5Sb0.24As0.76 substrate for large (500Å height x 300Å width) defect -free InAs quantum dots. Photoluminescence shows a ground-state emission of the InAs quantum dots at 1.45 μm.
Formation of Quasicrystalline Phases in the Equilibrium Immiscible Co-Cu and Fe-Cu Systems by Ion Mixing/Solid-State Reaction
Authors: Li Z.F. , Lai W.S. , Yang G.W. , Zhang Q. , Liu B.X.
Published: April 2011

Anomalous structural evolution was induced in the equilibrium immiscible Co-Cu and Fe-Cu systems by 100 or 200 keV xenon ion irradiation at 77 K or room temperature. In the Co15Cu85 and Fe70Cu30 multilayered films, nanosized quasicrystals were formed in an amorphous matrix, through a two-step transition of crystal-to-amorphous-to-quasicrystal. The obtained quasicrystals are Co-Cu dodecagonal and Fe-Cu icosahedral phases with twelve-fold and five-fold rotational symmetries, respectively. The real compositions of the amorphous matrix were determined to be close to Co10Cu90 and Fe70Cu30, while those for quasicrystals are nearly Co20Cu80 and Fe50Cu50, respectively. Moreover, the same dodecagonal and icosahedral phases were also obtained in the specifically designed Co50Cu50 and Fe50Cu50 multilayered samples upon thermal annealing at 500°C and 850°C, respectively, confirming the existence of these new metastable states in the respective systems. Besides, our molecular dynamics study showed that either Co or Fe could be mixed with Cu at an atomic scale in forming some metastable alloy phases. The amorphous-to-quasicrystal transition was discussed in terms of the similarity in the atomic configuration between the quasicrystal and amorphous short-range orders.
The Ten-Fold Surface of The Decagonal Al72Ni11Co17 Quasicrystal Studied by Leed, Spa-Leed, Aes and Stm.
Authors: Cox Erik J. , Ledieu Julian , Mcgrath RÓn'n , Diehl Renee D. , Jenks Cynthia J. , Fisher Ian
Published: April 2011

The ten-fold surface of the decagonal Al72Ni11Co17 (d-Al-Ni-Co) quasicrystal has been investigated using low energy electron diffraction (LEED), spot profile analysis LEED (SPA- LEED), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). This was done as a function of both annealing temperature and annealing time. The long-range order of the surface, as indicated by LEED, increases both as a function of annealing time and temperature. STM shows the surface to be rough and cluster-like at low annealing temperatures (≤725 K), whilst annealing to temperatures in excess of 725 K results in the formation of terraces. These terraces are small (≤ 100 Å width) at lower annealing temperatures and increase in size (100 Å ≤ x ≤ 500 Å) as the annealing temperature is increased (≥ 850 K). They are characterised by the presence of three-fold protrusions which align preferentially. STM images show single height steps as expected due to the periodicity of d-Al-Ni-Co in the z direction. To date it has not been possible to obtain atomic resolution, although this work is continuing.
Origin of Magnetism in Al-Pd-Mn and Al-Mn Quasicrystals and Approximants
Authors: Hippert Françoise , Simonet Virginie , Audier Marc , Calvayrac Yvonne , Bellissent Robert , Laissardière Guy Trambly de , Mayou Didier
Published: April 2011

Localized moments are observed on a small fraction of the Mn atoms in Al-Pd-Mn and Al-Mn quasicrystals while a large proportion of magnetic moments is found in liquids in equilibrium with these quasicrystalline phases. As magnetic sites cannot be identified in quasicrystals because of the very small number of Mn atoms involved, we have studied the case of approximant phases in both Al-Mn and Al-Pd-Mn systems. Most of the approximants are non- magnetic but magnetic moments are present on a fraction of the Mn sites in the μ-Al4Mn phase and in the Mn-rich TAlPdMn phase. From both experimental and theoretical investigations, the magnetic sites are identified in these phases. The moment formation can be explained by analysing the local environment of the Mn atoms and indirect Mn-Mn interactions mediated by conduction electrons on relatively large distances. A clear picture of the origin of magnetism in quasicrysals, approximants and liquids in the Al-Mn and Al-Pd-Mn systems can therefore be obtained.
Modeling Thermopower in AlPdMn Based Quasicrystalline Systems
Authors: Pope A. L. , Zawilski B. M. , Gagnon R. , Tritt T. M. , Strom-Olsen J. , Schneidmiller R. , Kolis J.
Published: April 2011

We report room temperature thermopower values and the temperature dependence for several AlPdMn based quasicrystals. In an effort to further understand the complexities of electrical transport in quasicrystalline systems, thermopower data for icosahedral Al71Pd21Mn8-XReX will be presented and discussed. A relation of room temperature thermopower to the curvature of the thermopower is demonstrated. We propose an empirical fit to the thermopower data, utilizing three free variables. The physical significance of the fit parameters is discussed. These results are discussed in brief concerning the relation to the application of quasicrystals for use as thermoelectric materials.
Quasicrystalline Coatings Through Laser Processing: A Study on Process Optimisation and Microstructure Evolution
Authors: Chattopadhyay K. , Biswas K. , Bysakh S. , Phanikumar G. , Weisheit A. , Galun R. , Mordike B.
Published: April 2011

Composite coatings containing quasicrystalline (QC) phases in Al-Cu-Fe alloys were prepared by laser cladding using a mixture of the elemental powders. Two substrates, namely pure aluminum and an Al-Si alloy were used. The clad layers were remelted at different scanning velocities to alter the growth conditions of different phases. The process parameters were optimized to produce quasicrystalline phases. The evolution of the microstructure in the coating layer was characterized by detailed microstructural investigation. The results indicate presence of quasicrystals in the aluminum substrate. However, only approximant phase could be observed in the substrate of Al-Si alloys. It is shown that there is a significant transport of Si atoms from the substrate to the clad layer during the cladding and remelting process. The hardness profiles of coatings on aluminum substrate indicate a very high hardness. The coating on Al-Si alloy, on the other hand, is ductile and soft. The fracture toughness of the hard coating on aluminum was obtained by nano-indentation technique. The K1C value was found to be 1.33 MPa m1/2 which is typical of brittle materials.
Where are the Atoms in Quasicrystals? Experimental and Theoretical Studies of Ternary and Quaternary Approximants
Authors: Miller Gordon J. , Lee Chi-Shen , Tsai Wen-Jiuan
Published: April 2011

Problems concerning complete structural characterization of quasicrystals involve locating the atomic positions as well as determining the distribution of elements at each site. Quasicrystalline approximants provide models for potential building units of quasicrystals, but a clear determination of the elementaldecorations in such approximants also remains incomplete. We report experimental and theoretical studies of new, quaternary Bergman phases in the Li-Mg-Zn-Al system and a new, quasicrystalline approximant Li10Mg6Zn31Al3(A16M34-type). A theoretical model using averaged Mulliken populations provides a means to track the segregation of elements (and vacancies) onto different sites as a function of valence electron concentration. As the Li content decreases, vacancies begin to occur at a specific site in the Bergman structure. The new approximant demonstrates how truncated tetrahedra can play an important role in forming clusters with possible fivefold symmetry in quasicrystalline structures.
Fundamental Cluster and Hydrogen Sites in Ti-Zr-Ni Quasicrystals
Authors: Majzoub E.H. , Hennig R.G. , Kelton K.F. , Gibbons P.C. , Misture S.T.
Published: April 2011

A Rietveld refinement confirmed by ab initio calculations [1] has shown that the structure of the related W-phase [2] 1/1 approximant consists of Bergman clusters connected by “glue” sites. To investigate if the quasicrystal structure contains the same cluster we construct two constrained icosahedral glass models using the Bergman or Mackay cluster, respectively. A comparison with neutron diffraction data yields better agreement if the Bergman cluster is used, suggesting that this is a frequently occurring element in the quasicrystalline structure. Since absorbed hydrogen is known to locate preferentially in tetrahedral interstitial sites in many metals, the tetrahedral sites in the constrained glass were filled with hydrogen. The calculated powder neutron diffraction spectra are similar to the experimental data. It was not possible, however, to differentiate between possible tetrahedral sites where the hydrogen atoms sit. Hydrogen site energies from ab initio calculations indicate preferences that are consistent with fillings of non-Ni bearing tetrahedral sites.