bluuebunny/crossref_metadata_2025_split huggingface dataset:
Anomalous Field Enhancement in Planar Semiconducting Cold Cathodes from Spontaneous Ordering in the Accumulation Region
Authors: Bilbro Griff L. , Nemanich Robert J. 
Published: April 2011

ABSTRACTWide band gap semiconductors exhibit a low electron affinity and may prove suitable for cold cathode applications. We introduce a simple closed-form analytic approximation for the stability of electrons in the electron accumulation layer of planar Low Electron Affinity (LEA) semiconducting cathodes. This analysis extends our previous results, which used Runge-Kutta numerical integration of the linearized equations of motion for the electric potential and quasi Fermi level. The model shows conditions in which the electrons in the accumulation layer form a two dimensional array of regions of higher and lower electron density. This instability could lead to field enhancement without surface roughness and could account for observed electron emission at low applied fields.
Dispersions and fibers of carbon nanotubes
Authors: Vigolo Brigitte , Pénicaud Alain , Coulon Claude , Sauder Cédric , Pailler René , Journet Catherine , Bernier Patrick , Poulin Philippe 
Published: April 2011

AbstractWe study the phase behavior of single walled carbon nanotubes in aqueous solutions of surfactant molecules or amphiphilic polymers. Homogeneous dispersions can be obtained by using sodium dodecyl sulfate (SDS) in a well-defined concentration range. In contrast, polyvinyl alcohol (PVA) is not efficient at stabilizing the tubes. Carbon nanotubes stick with each other when PVA is added to homogeneous dispersions initially stabilized by SDS. This behavior is the basis of a simple method that we developed to assemble single walled carbon nanotubes into indefinitely long ribbons and fibers. The processing consists of dispersing the nanotubes in SDS solutions, re-condensing the nanotubes in the flow of a PVA solution to form a nanotube mesh, and then collating this mesh to a nanotube fiber. Flow induced alignment may lead to a preferential orientation of the nanotubes in the mesh that has the form of a ribbon. Unlike classical carbon fibers, the nanotube fibers can be strongly bent without breaking. Their obtained elastic modulus is 10 times higher than the modulus of high-quality bucky paper.
Vibrational modes of graphitic fragments and the nucleation of carbon nanotubes
Authors: Volpe Manuela , Cleri Fabrizio , D'Agostino Gregorio , Rosato Vittorio 
Published: April 2011

AbstractWe studied the nucleation mechanism of carbon nanotubes based on the hypothesis that the starting nanotube seed can be nucleated by rolling a small fragment of a graphite sheet (graphene) under thermal fluctuations. The energy barriers for rolling a graphene along different crystallographic directions are calculated from a tight-binding model,. We then estimate the relative weight of the large-amplitude fluctuations corresponding to low-frequency vibrational modes of graphene sheets of increasing size. Direct molecular dynamics simulation of the high- temperature fluctuation of a pair of parallel graphenes demonstrates that a nanotube closed at one end can spontaneously form. We discuss the combined effects due to: (a) the decrease of the energy barriers against rolling with increasing nanotube radius, and (b) the increase of random fluctuations with increasing size of the graphene sheet. The superposition of such effects may lead to a preferential range of nanotube diameters which could nucleate more abundantly than others.
Laser Synthesis of Single-wall Carbon Nanotubes Utilizing High Temperature Induction Heating.
Authors: Gennett T. , Dillon A.C. , Alleman J.L. , Jones K.M. , Parilla P. A. , Heben M.J. 
Published: April 2011

AbstractAn inductive heating system was used to control the temperature of the target during laser synthesis of SWNTs. The position of the target relative to the heating coil, the type of catalyst metals and the synthesis temperatures were all varied in attempts to gain more control over the types of carbon single wall nanotubes grown. Raman spectroscopy results suggest that narrower diameter distributions are produced when the gas-phase species are rapidly condensed. TEM analysis of the raw soot shows that many tube ends are present and that the tubes are short due to incomplete growth. This investigation shows that it is possible to gain further control over SWNT diameter distributions through fine control of the reaction environment made possible with inductive heating.
Structure of Self-Assembled Fe and FePt Nanoparticle Arrays
Authors: Yamamuro S. , Farrell D. , Humfeld K. D. , Majetich S. A. 
Published: April 2011

AbstractArrays were self-assembled by evaporating suspensions of 4 nm FePt or 8 nm Fe nanoparticles. The monolayers had a hexagonal close packed (hcp) structure, but the multilayer structure varied. To identify the multilayer structures, transmission electron microscopy (TEM) images were compared with phase contrast image simulations. The results showed that Fe could be grown as both hcp and face-centered cubic (fcc), or fcc-like, structures. The results of image analysis of the FePt arrays were consistent with fcc structures.
Development of Induced Crystallization as a Pattern Transfer Mechanism for Nanofabrication
Authors: Chraska T. , Cabral M.J. , Mesarovic S. , Longo D.M. , Stach E.A. , Bean J.C. , Hull R. 
Published: April 2011

AbstractA novel pattern transfer technique, based upon direct contact between nanofabricated printheads and amorphous films/substrates is being developed. Transfer of features from printhead to substrate is via an induced crystallization mechanism, with a goal of controlling crystal nucleation at dimensions comparable to the printhead features. To realize such a printing mechanism, appropriate mediums/materials that can be crystallized through contact with the printhead elements must be identified. From a range of candidate materials we focus on Ge and Indium Tin Oxide (ITO) films grown by molecular beam epitaxy or sputtering. The crystallization may be induced directly by heating (thermally), pressure (mechanically), or electrical conduction through the contact points. Crystallization of many amorphous materials is known to be self sustaining (so called “explosive crystallization”), raising the need to identify mechanisms for abruptly stopping crystallization if feature dimensions are to be constrained. A set of amorphous or nanocrystalline samples have been studied during crystallization by a heated printhead fabricated with a Focused Ion Beam (FIB). A simplified continuum model suggests the feasibility of achieving lateral confinement of the induced crystallization. In order to characterize fully the transitional and final substrate states, real-time observation of nanoscale crystallization in the transmission electron microscope (TEM) is in progress. A special TEM goniometer is being developed which incorporates a piezoelectrically driven tip and a heating capability to enable observation of tip-substrate contact at elevated temperatures that should give invaluable insight into crystallization mechanisms.
Electrodeposition of Three-Dimensionally Periodic Metal Meshes and Spheres
Authors: Xu Lianbin , Zhou Weilie L. , Baughman Ray H. , Zakhidov Anvar A. , Wiley John B. 
Published: April 2011

AbstractElectrochemical methods have been used to produce three-dimensionally periodic metal meshes and spheres. Nickel is initially deposited into porous opal sheets. The opals themselves consist of close-packed silica spheres, which serve as a template for the growth of the nickel arrays within the void space between SiO2 spheres. Dissolution of the SiO2 spheres results in open, three-dimensionally periodic nickel mesh structures. The metal meshes can then be oxidized in air to produce nonconducting NiO meshes. This results in an inverse template that can be used for the growth of three-dimensionally periodic metal sphere arrays. Details on the preparation and characterization of these materials are presented.
Novel Colloidal Assembly Methods for the Preparation of Core-Shell Composite Materials
Authors: Fleming Michael S. , Mandal Tarun K. , Walt David R. 
Published: April 2011

AbstractColloidal assembly is a process by which particles ranging in size from nanometers to micrometers are organized into structures by mixing two or more particle types. Assembly is controlled by either specific or non-specific interactions between particles. Examples include chemical bonding, biological interactions, electrostatic interactions, capillary action and physical adsorption. The assembly process is performed such that smaller particles assemble around larger ones. In this paper, we report on colloidal assembly of polymer nanoparticles (50-200 nm diameter) onto silica particles (3-5 μm diameter) using specific chemical interactions (i.e. aminealdehyde). Annealing the assembled composites at temperatures above the glass transition (Tg) of the polymer nanospheres allows polymer to flow and uniformly coat the microsphere surfaces. Polystyrene and poly(methyl methacrylate) nanospheres were used to produce such materials. Shell composites were created by mixing both nanosphere types prior to assembly/annealing. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the materials presented herein.
Non-lithographic Nanocolumn Fabrication with Application to Field Emitters
Authors: Colgan M. J. , Vick D. , Brett M. J. 
Published: April 2011

AbstractA non-lithographic method of fabricating high-density arrays of nanometer-scale vertical columns was investigated. The use of oblique deposition techniques allows the fabrication of isolated vertical columns in a single-step evaporation process without the need for either pre- or post-deposition lithographic processing. Extreme oblique incidence deposition with computer controlled substrate motion was utilized to fabricate columns with diameters near 100 nm and densities exceeding 109 columns/cm2. The desired column geometry may be engineered through choice of deposition angle and substrate spin rate. In one potential application of these microstructures, arrays of vertical columns were fabricated from silicon and carbon and tested for field emission characteristics. Further studies were made on the use of ion milling to modify the tips of the nanocolumns in order to improve the field emission properties.
A FIB Micro-Sampling Technique and a Site-Specific TEM Specimen Preparation Method for Precision Materials Characterization
Authors: Yaguchi Toshie , Urao Ryoichi , Kamino Takeo , Ohnishi Tsuyoshi , Hashimoto Takahito , Umemura Kaoru , Tomimatsu Satoshi 
Published: April 2011

AbstractA technique to cut out small pieces of samples directly from chips or wafer samples in a focused ion beam (FIB) system has been developed. A deep trench is FIB milled to cut out a small, wedge-shaped portion of the sample from the area of interest A micromanipulator with tungsten (W) probe is employed for lifting the micro-sample. The lifted micro-sample is then mounted on a carrier to prepare electron transparent thin foil specimens for transmission electron microscope (TEM) observation. We have also developed a method for site-specific TEM specimen preparation. In this method, FIB system and TEM/scanning transmission electron microscope (STEM) equipped with secondary electron (SE) detector are employed. An FIB–TEM/STEM compatible specimen holder has also been developed so that a specimen can be milled in the FIB system and observed in a TEM/STEM without remounting the specimen. STEM and scanning electron microscopy (SEM) images are used for locating a specific site on a specimen. SEM image observation at an accelerating voltage of 200kV enabled us to observe not only surface structures but also inner structures near the surface of a cross section with depth of field of around 1 micrometer. The STEM image allows the observation of inner structures of 3-5 micrometer thick specimens. Milling of a specimen by FIB and observation of the milled sample by SEM and STEM are alternately carried out until an electron transparent thin foil specimen is obtained. The position accuracy of the method in TEM specimen preparation is approximately 100nm.
Investigations of Silicon Nano-crystal Floating Gate Memories
Authors: Kumar Arvind , Welser Jeffrey J. , Tiwari Sandip , Rana Farhan , Chan Kevin K. 
Published: April 2011

AbstractAs memory continues to be scaled to ever smaller dimensions, the floating-gate memory transistor, which offers a single-element storage cell, becomes more attractive. Typically, this structure has been reserved for nonvolatile applications, where the comparatively high voltages, slow write speeds, and limited cyclability could be tolerated. However, if the floating-gate, which is usually a continuous film of polysilicon, is replaced with a discontinuous film of small floating islands (nano-crystals), a new set of tradeoffs in these performance factors becomes possible, opening the door to broader applications. If these islands are further reduced to the point where Coulomb charging or energy quantization effects become relevant, it is possible to control the charge on the islands at a single (or few) electron level, which offers very low power operation and may enable new functionality. This paper will discuss design and fabrication of these memories, experimental results on fabricated devices, and modeling of what could ultimately be achieved, as well as what limitations will ultimately be reached.
Comparative SEM and Cathodoluminescence Microanalysis of Porous GaP Structures
Authors: Stevens-Kalceff M.A. , Langa S. , Tiginyanu I.M. , Carstensen J. , Christophersen M. , Föll H. 
Published: April 2011

AbstractElectron microscopy and cathodoluminescence (CL) microanalysis were used for a comparative study of porous layers fabricated by electrochemical etching of n-GaP substrates in a sulfuric acid solution. Both the CL and morphology of porous layers were found to depend upon the anodic current density. At high current density (100 mA/cm2) anodization leads to the formation of so-called current-line oriented pores and an increase in the CL intensity. We observed self-induced voltage oscillations giving rise to a synchronous modulation of the diameter of pores and CL intensity. When the current density decreased to values as low as 1 mA/cm2 the pores began to grow along <111> crystallographic directions and the CL intensity was observed to be lower than that of bulk GaP.
Raman Spectroscopy of Si Nanocrystals in Nanocrystalline Si Superlattices: Size, Shape and Crystallographic Orientation
Authors: Grom G. F. , Fauchet P. M. , Tsybeskov L. , McCaffrey J. P. , Labbé H. J. , Lockwood D. J. , White B. E. 
Published: April 2011

AbstractWe use Raman spectroscopy to study the size, shape and crystallographic orientation of silicon nanocrystals formed by solid phase crystallization of amorphous Si/SiO2 superlattices (SLs) grown by radio-frequency sputtering. The first and second Raman peaks broadening, their relative positions and intensities indicate the presence of nanoscale Si objects with a degree of disorder (grain boundaries) and strain (Si/SiO2 interfaces). Shapes of Si nanocrystals sandwiched between SiO2 layers strongly influence the Si/SiO2 interface roughness, which is inferred from the intensities of folded acoustic phonon scattering. The averaged crystallographic orientation of Si nanocrystals is determined by polarized Raman analysis. The laterally elongated nanocrystals exhibit <111> preferred crystallographic orientation along the SL axis due to orientation-dependent crystallization rates. These results demonstrate that control over Si nanocrystals structural parameters has been achieved and that solid phase crystallization of nanometer-thick amorphous Si films remains one of the most promising techniques for Si-based nanoelectronic device fabrication.
Growth of Self-Seeded Aluminum Nitride by Sublimation-Recondensation and Substrate Preparation
Authors: Rojo J. Carlos , Slack Glen A. , Morgan Kenneth , Schowalter Leo J. , Dudley Michael 
Published: April 2011

ABSTRACTBulk aluminum nitride boules have been grown at driving rates of 0.9mm/h by the self-seeded sublimation-recondensation technique. Up to 15mm diameter substrates cut from those boules present large single crystal grains that have been analyzed using different techniques. X-ray double crystal diffraction shows a full-width-at-half-maximum of around 100 arcsec and X-ray topography reveals extensive areas with a density of dislocations less than 104 cm−2. These substrates have been prepared by chemical mechanical polishing techniques to obtain a surface roughness of 1.4-1.6nm.
Fabrication and Characterization of InxAlyGa1−x−yN Ultraviolet Detectors
Authors: Oder Tom N. , Li Jing , Lin Jingyu , Jiang Hongxing 
Published: April 2011

ABSTRACTUltraviolet (UV) photoconductive detectors based on InxAlyGa1−x−yN quaternary alloy lattice-matched to GaN have been fabricated and characterized. The detectors consisted of about 0.1 μm thick InxAlyGa1−x−yN quaternary alloy of different In (x) and Al (y) compositions grown by metalorganic chemical vapor deposition (MOCVD). The films were characterized by different techniques including x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), atomic force microscopy (AFM), Hall-effect and time-resolved photoluminescence measurements. The characteristics of these UV detectors including the cut-off wavelength, photoresponsivity and device speed have been measured. The cut-off wavelength of the InxAlyGa1−x−yN detectors was found to vary to the deep UV range with varying In and Al compositions. The most important and intriguing result is that the responsivity of the InxAlyGa1−xyN quaternary alloy exceeded that of AlGaN alloy of comparable cut-off wavelength by a factor of five. This makes the nitride quaternary alloy very important material for solar blind UV detector applications in the deep UV range where Al rich AlGaN alloys have problems with low quantum efficiency and cracks due in part to lattice mismatch with GaN.
A Comparative Study of the Influence of the Local Density Approximation and the Generalized Gradient Approximation on the Calculated Properties of the III-Nitride (110) Surfaces
Authors: Alves H. W. Leite , Alves J. L. A. , Nogueira R. A. , Leite J. R. 
Published: April 2011

ABSTRACTWe present a systematic theoretical study of several III-nitride (110) surfaces based on accurate parameter-free, self-consistent total energy and force calculations using the density functional theory, the local density approximation (LDA), as parametrized by Perdew and Zunger, and the generalized gradient approximation (GGA), as proposed by Perdew, Burke, and Ernzerhof, for the exchange-correlation term; we use the Full Potential Linear Augmented Plane Wave (FPLAPW) approach (WIEN-97 code) associated with the slab supercell model to simulate the (110) surface. We studied BN, AlN, GaN, InN and compared the theoretical results as related to the use of the LDA and the GGA. We conclude that although the results for both approximations are similar, differences in structural parameters may be as large as 10%.
Optical Characterization of AlGaN/GaN MQW's
Authors: Rocha Ricardo A. , Monteiro Teresa , Pereira Estela , Alves Eduardo 
Published: April 2011

ABSTRACTAlGaN/GaN multi-quantum wells (MQW's) were optically studied in this work. Photoluminescence spectra revealed a quantum Stark effect in the samples. Calculated builtin electric fields were found to be significantly less than the values expected from theoretical models. An approach concerning the existence of free charges responsible for screening of the electric field is made, where we find carrier densities very similar to values described in other works.
Shallow-impurity-related photoluminescence in homoepitaxial GaN
Authors: Kirilyuk V. , Zielinski M. , Christianen P.C.M. , Zauner A.R.A. , Weyhera J.L. , Hageman P.R. , Larsen P.K. 
Published: April 2011

ABSTRACTPhotoluminescence (PL) spectra of GaN epitaxial layers grown on vicinal (0001) GaN substrates (i.e. N-polar) demonstrate high optical quality showing free exciton transitions in addition to narrow bound-exciton peaks (line width 1-2 meV). In contrast, N-polar films deposited on exactly oriented (0001) substrates exhibit a broad PL emission resulting from high free carrier concentrations. As derived from Secondary Ion Mass Spectroscopy (SIMS), exact (0001) epilayers reveal very high concentrations of oxygen (1.5×1019 atom/cm3), which are about one order of magnitude higher then in the layers grown on a 4°-off substrate. It is therefore concluded that oxygen is responsible for the high free carrier concentration and could be a dominant shallow donor in the homoepitaxial N-polar films. It is also shown that incorporation of oxygen strongly depends on the polarity of the films (Ga or N). Concentrations of other impurities, such as Si, C and H are found to be similar for homoepitaxial films of both polarities being consistent with the results of exciton-related PL.
Development of an AlN Deep UV Detector for Space Application
Authors: Zhong Feng , Huang Changhe , Danylyuk Yuri V. , Auner Gregory W. 
Published: April 2011

ABSTRACTAlN is a very promising deep UV sensing material suitable for space application such as exoatomospheric solar blind detectors due to its large band gap (6.2 eV), excellent radiation and thermal stability. In this paper, a deep UV detector based on AlN photoconductor was successfully grown and characterized. High quality AlN thin films on Sapphire have been confirmed by in-stu RHEED and X-ray diffraction measurements. The film has an optical Eg of 5.96 eV. The detector has extremely low dark current. Different electrodes were investigated as contact materials. Al and Pt are more suitable electrode contacts for AlN photoconductor compared to Ti. The DC responsivity of the detector starts to rise for photon energies of about 255 nm (4.9eV) and reaches its maximum 7×10-5 A/W at 195 nm (6.5eV).
High Density Plasma Etching Damage Effects on Contacts to n-GaN
Authors: Singh Rajwinder , Eddy C. R. , Moustakas T.D. , Ng H.M. 
Published: April 2011

ABSTRACTThe effects of inductively-coupled plasma etching on the quality of ohmic contacts to etched n- GaN are reported. A high density chlorine plasma and a range of rf bias power levels (incident ion energies) are employed to etch n-GaN films with varying doping levels. Resulting plasma damage degrades contact ohmicity in all cases examined. At moderate levels of rf bias power the extent of this degradation for nitride layers with lower doping levels (mid-1017 cm−3) is similar to the degradation for nitride layers with higher doping levels. With increased rf bias, the degradation becomes more severe in films with higher carrier concentration. Annealing at 700 °C rapidly improves the contact quality. Studies of cumulative annealing time, up to 160 seconds, show that the improvement in contact quality takes place within the first twenty seconds of annealing.
Propagation and Scattering of Exciton-Polaritons in Nitride-Based Multiple Quantum Wells
Authors: Malpuech Guillaume , Zamfirescu Marian , Kavokin Alexey , Carlo Aldo Di 
Published: April 2011

ABSTRACTWe show theoretically that disorder-induced scattering of light between « bright » and « dark » exciton-polariton modes governs the coherent optical response of GaN/AlGaN quantum wells (QWs) on a femtosecond scale. The incident light essentially excites the “bright” modes, they emit the most part of light radiatively during first few hundreds of femtoseconds. The rest of the coherent signal, i.e. the temporal behaviour on a scale from 1 to few picoseconds, comes from the “dark” modes, populated due to the elastic scattering processes in the system, and mixed so that they form two bulk-like polariton branches. The pico-second scale optical response of GaN/AlGaN QWs is essentially incoherent because of a very efficient scattering of exciton-polaritons by a structural disorder.