bluuebunny/crossref_metadata_2025 huggingface dataset:
Micro-Structuring of Silicon by Pulsed-Laser Ablation Under Reactive Atmospheres
Authors: Jesse S. , Pedraza A. J. , Fowlkes J. D. , Budai J. D. , Lowndes D. H.
Published: April 2011

Micro-holes hundreds of μm-deep below the initial surface surrounded by 40- μm-tall micro-cones protruding over the initial surface form in silicon as a response to multiple UV pulsed-laser irradiation in an SF6 atmosphere. The micro-holes and micro-cones are arranged in a characteristic ensemble displaying a self-organized pattern. Similarly, deep-holes and micro-column ensembles form when the irradiation is performed in an oxygen-rich atmosphere. The formation mechanism of microcolumns and microcones in these reactive atmospheres has been studied using scanning electron microscopy, profilometry and x-ray diffraction. The growth of cones under SF6 atmospheres was studied in-situ using an ICCD gated camera coupled to a long distance microscope. The images obtained with the ICCD camera show that the fluorescent plume is correlated with the amount of ablated material. There is also a strong correlation between the plume and the growth of the cones. The cones grow in the region where the laser-generated plume is intense. When the flux of silicon-rich material ceases because the holes are very deep the balance between redeposition and ablation is tilted toward the latter and the cones begin to recede. Laser ablation is not only a function of the nominal fluence but also of the surface roughness and microstructure. These results strongly support the growth mechanism whereby laser-ablated silicon-rich molecules and clusters are transported to the tip and side of the cones by the laser- generated plasma.
Electroreflectance Study of Porous Silicon made from Substrates with Different Resistivities
Authors: Toyama Toshihiko , Nakai Yasuharu , Okamoto Hiroaki
Published: April 2011

Employing electroreflectance (ER) spectroscopy, we have studied optical transitions in porous Si (PSi) with a thickness of 100±50 nm made from crystalline Si (c-Si) substrates with different resistivities, 4–10 ωcm (p-), 0.1–1 ωcm (p), and < 0.018 ωcm (p+). The ER features observed at 1.1–2.8 eV in p+ PSi are analyzed with a simple effective mass approximation (EMA) model for confined electron-hole (e-h) pairs in a spherical quantum dot as we have previously done for those observed at 1.2–3.1 eV in p- PSi. From the ER analysis with the EMA model, the effective crystal size is estimated without destruction, and the kinetic energy and the Coulomb attraction energy of the confined e-h pairs are also deduced.

Furthermore, the ER features corresponding to the optical transitions at E1(E0') critical point (CP) are observed at 2.8–3.3 eV in p+ PSi. With an increase in the crystal size, the transition energy of E1(E0') CP in p+ PSi is decreased, while that in p- and p PSi is unchanged from that of 3.4 eV found in c-Si including the p+c-Si. From the Raman results, strain- and disorder-induced spectral changes are found to be negligible, so that the high doping induced effect is the most acceptable mechanism for the red-shift in E1(E0') transitions of Si nanocrystals with the crystal size of 2–3 nm.
Carrier Transport and Lateral Conductivity in Nanocrystalline Silicon Layers
Authors: Kim H. B. , Montes L. , Krishnan R. , Fauchet P. M. , Tsybeskov L.
Published: April 2011

We have studied carrier transport and lateral electrical properties of nanocrystalline Si layers containing size controlled Si nanocrystals. Using results from direct current (dc) and alternating current (ac) conductivity measurements, the charging of Si nanocrystals and Coulomb blockade effect are discussed.
Self-Formation of Au Microwires on Au-Covered Si Electrode Surface by Electrochemical Etching in Dilute Hydrofluoric Acid Solution
Authors: Kimura Yasuo , Nemoto Jun , Kusakabe Atsushi , Kondo Yusuke , Niwano Michio
Published: April 2011

We have investigated the method of fabricating microstructures on a Si surface that is covered with a patterned gold (Au) mask, by electrochemical etching (anodization) in dilute hydrofluoric acid (HF) solution. We found that at electrode potentials below approximately 0.5 V, the Si surface is preferentially etched on the fringe of the Au mask, where a number of pores are formed. At higher electrode potentials, Au microwires with about 1 µm in width form along the fringe of the Au mask overlayer. We suggest that electromigration of Au towards the fringe of the Au mask induces self-assembling of Au atoms to form microwires. The observed self-formation of metal microwires would be beneficial to the fabrication of metal micro- or nano-structures on Si.
Development of Silicon-Based UV-Photodetector Prototypes using Photoluminescent Nanocrystalline Silicon Overlayers
Authors: Navarro Carlos , Fonseca Luis F. , Nery Guillermo , Resto O. , Weisz S. Z.
Published: April 2011

The maximum photoresponse of a normal silicon photodetector, that uses a p-n junction as the active zone, is obtained when the incident radiation wavelength is around 750nm. This response diminishes significantly when the incident radiation is near or in the UV region. Meanwhile, nanocrystalline silicon (nc-Si) films with high transparency above 650nm and high absorbance in the UV can be prepared. By quantum confinement effects, a fraction of this absorbed UV energy is re-emitted as visible photons that can be used by the junction. We study the enhancement of the UV-photoresponse of two silicon detector prototypes with a silicon p-n junction active zone and with a photoluminescent nc-Si overlayer. One prototype is made with a porous silicon/n-type silicon/p-type silicon/p++-silicon/metal configuration and the other with an Eu-doped Si-SiO2 overlayer instead of the porous silicon one. The comparison between both prototypes and the control is presented and discussed stressing on the enhancement effect introduced by the photoluminescent overlayers, stability and reproducibility.
Quantum Effects on the Dielectric Function of Porous Silicon
Authors: Cruz M. , Díaz S. F. , Wang C. , Rubo Y. G. , Tagüeña-Martínez J.
Published: April 2011

In this work, the imaginary part of the dielectric function of porous silicon is studied by means of both the tight-binding and the effective medium approaches, in the latter exact result is obtained for the case of 50% porosity. Within the tight-binding approximation, the dielectric function is calculated by using the interconnected and chessboard-like supercell models for the Si skeleton. These microscopic models give quantitatively similar results, which are by a factor of three larger than those from the effective medium theory.
Size Reduction of Silicon Nanopillars by Photo-Electrochemical Etching
Authors: Juhasz Robert , Linnros Jan , Kleimann Pascal
Published: April 2011

Silicon nanopillars, formed by electron beam lithography, were electrochemically etched to provide controlled size reduction. The smallest dimensions achieved were pillars of 15 nm in diameter, restricted mainly by the scanning electron microscope used for characterization. The etch rate was mainly determined by the photogeneration of carriers, by the HF concentration and by the applied voltage bias. The applied bias also controlled the resulting shape of the pillars such that a high bias resulted in etching of the pillar top whereas a negative bias caused etching only at the pillar base. For 0 V, a relatively conform etching of the pillar was observed. We discuss these phenomena in terms of electropolishing or pore formation effects on a local scale.
Electronic Structure of GaN Quantum Dots with an Adjacent Threading Dislocation
Authors: Andreev A. D. , Downes J.R. , O'Reilly E. P.
Published: April 2011

We present a theoretical analysis of the electronic structure of GaN quantum dots (QD) with an adjacent threading dislocation. The QD carrier spectra and wavefunctions are calculated using a plane-wave expansion method within an 8-band k.p model. The method is very efficient, because the strain and built-in electric fields can be included through their discrete Fourier transforms. The QD structures considered have been analysed experimentally by other groups. The GaN QDs are truncated hexagonal pyramids on a wetting layer with an edge dislocation adjacent to each dot. The built-in piezoelectric potential strongly influences the localisation of the carrier wavefunctions. This potential pushes the electrons to the top of the dot, the holes to the bottom and, additionally, causes strong lateral confinement of the carriers. The effect of the dislocation strain field at the dot edge on the carrier states in each GaN/AlN QD is shown to be insignificant. Results are presented for the confined state energies and optical matrix elements for a range of different sized dots with and without dislocations. The size of the dot influences the energies and overlaps, but the presence of the dislocation has minimal effect. The dependence of the ground state optical transition energy on the size of the dot is in good agreement with experimental data.
Metal-Semiconductor Contacts and CPW MMIC Issues for AlGaN/GaN FETs
Authors: Jacobs Bart , Kramer Mark , Straaten Bram van , Kwaspen Thieu , Karouta Fouad , Hek Peter de , Dijk Raymond van , Vliet Frank van
Published: April 2011

In this report two different aspects in the development of AlGaN/GaN power amplifiers will be discussed. In the first part of this paper we report on the optimization of the Ti/Al/Ni/Au metallization scheme on a doped AlGaN/GaN FET structure. By a systematic investigation we were able to reduce the contact resistance to 0.2 ωmm (7.3×10×7ωcm2). The Al/Ti thickness ratio for this contact was 6, which according to the Al-Ti binary phase diagram, does not result in excess Ti which should react with nitrogen in the AlGaN layer to render the surface heavily doped. Preliminary results on Schottky contacts indicate an improvement in the reverse leakage current if a RIE oxygen plasma in combination with a NH4OH dip is performed prior to metallization.

Coplanar waveguides on AlN are discussed in the second part of this paper. These transmission lines can be used in AlGaN/GaN power amplifiers if no via-hole technology is available or if a hybrid solution is pursued. The signal line should have a large metal cross- sectional area (> 5 × 50 [.proportional]m2) in order to carry enough current in the output stage of an amplifier. It is shown that CPWs with large dimensions show non-quasi TEM behavior related to propagation of parallel plate modes.
Influence of Er depth profiles on luminescence properties of Er-doped GaN
Authors: Uekusa Shin-ichiro , Hirano Tomoaki
Published: April 2011

Erbium (Er) ions were implanted into gallium nitride (GaN) and the temperature-dependent photoluminescence (PL) and PL lifetime were characterized. Thermal quenching of the Er3+ luminescence was restrained by using GaN as a host material and PL was observed from Er3+ at room temperature. We prepared a flat depth profile of the density distribution of Er by ion-implanted into GaN and monitored the thermal quenching process due to the residual defects after implantation and annealing. We also monitored the auger effect that is believed to be the main cause of the thermal quenching process and concluded that, in the temperature range 15 K to 100 K, the thermal quenching process is dominated by nonradiative recombination from the first excited state (4I13/2) to the ground state (4I15/2) of Er3+.
Optical and Electrical Properties of Al1−xInxN Films Grown on Sapphire (0001) by Plasma Source Molecular Beam Epitaxy
Authors: Danylyuk Y. V. , Lukitsch M. J. , Huang C. , Auner G. W. , Naik R. , Naik V. M.
Published: April 2011

A series of epitaxial InxAl1−xN alloy films (thickness ~ 150 nm) with 0 ≤ × ≤ 1.0 were grown by Plasma Source Molecular Beam Epitaxy (PSMBE) on sapphire (0001) at a low substrate temperature of 375 °C. X-ray diffraction (XRD) measurements confirm ac-axis oriented epitaxial growth of alloy films without any alloy segregation. However, the degree of crystalline mosaicity, compositional fluctuation and surface roughness, all increase with increasing x. The direct energy band gap of alloy films were determined using optical (UV-VIS) transmission and reflection measurements. The observed bowing of the direct gap versus x plot, when compared to the theoretical prediction, is less pronounced than seen in earlier studies reported in literature. Electrical resistivity and Hall effect measurements show n-type electrical conductivity in these alloys with carrier concentrations ~1019-1020cm−3for x > 0.5.
Visible Emission from Thin-Film Phosphors of Amorphous AlN:Cu, Mn, and Cr
Authors: Caldwell M. L. , Martin A. L. , Spalding C. M. , Patten P. G. Van , Kordesch M. E. , Richardson H. H.
Published: April 2011

Luminescence studies of amorphous AlN doped with Cu, Mn, or Cr were performed at 300 K. Thin films of Cu, Mn, and Cr doped amorphous AlN, ∼200 nm thick, were grown on p-doped silicon (111) substrates using RF magnetron sputtering in a nitrogen atmosphere. Cathodoluminescence (CL) showed that pure Cu doped amorphous AlN has strong emission in the blue (∼420 nm) and Mn and Cr doped films luminesce in the red (∼690 nm). Cr+3 emission is more intense than Mn+4 because chromium does not suffer from incomplete charge compensation in the III-V semiconductor. Luminescence studies of crystalline and amorphous AlN:Mn thin films showed a red shift in the emission peak by almost 100 nm and is believed to be caused by the different crystal field of the amorphous host compared to the crystalline host material. Secondary ion mass spectrometry (SIMS) depth profiling was conducted to confirm the presence of the Cu and Cr in the films and to show the amount of dopant in relation to the Si substrate.
A High-Temperature STM-Study of Iron Silicide Formation on High Index Silicon Surfaces
Authors: Kneppe Martin , Kot Evgueni , Dorna Volker , Kahler Ulrich
Published: April 2011

The nucleation and initial growth of iron silicide on Si(113), Si(5 5 12) and Si(114) is studied using high temperature STM. During MBE and gas source deposition (precursor Fe(CO)5 STM-“movies” at temperatures up to 600°C directly show the kinetics of silicide formation on the surface. A complete 2D-wetting layer is formed on all vicinal surfaces for temperatures below 510°C. A chain-like silicide structure with a mixture of different periodicities (4×n), with n ranging from 2-6, is found. Above one monolayer growth proceeds with anisotropic 3D-silicide-islands. The wetting layer was found to be only metastable at lower growth temperature. At temperatures above 510°C the 2D-silicide layer does not wet the substrate anymore. In the equilibrium growth mode, 3D-islands directly grow on the bare Si-substrate. The anisotropy of the 3D-islands is extremly temperature dependent resulting in an aspect ratio up to 50 at 600°C. Especially on Si(114) a one-dimensionally nanostructured silicide layer can be grown in this way. A lateral spreading of the 3D-islands forming an interconnected silicide layer is achieved on Si(113) by a stochiometric codeposition of Fe and Si.
Relationship Between Graded Layer Structures and Defects in Silicon-Germanium Virtual Substrates
Authors: Mizushima Kazuki , Shiono Ichiro , Yamaguchi Kenji , Muraki Naoki
Published: April 2011

Silicon-germanium virtual substrates have been synthesized by low-pressure chemical vapor deposition. We obtained threading dislocation densities ranging from 105 to 106 cm−2, surface roughness ranging from 1.5 to 4 nm, and also cross-hatch pattern densities, depending on the grading rate and top layer germanium composition. For the typical sample, which has a linear-graded structure with a grading rate of 20%/[µm, and germanium composition of 30 % at the top layer, we obtained dislocation densities of about 106 cm−2 and root mean squared surface roughness of about 3 nm. The obtained dislocation densities are equivalent with the virtual substrates synthesized by ultra-high vacuum system. On the other hand the surface roughness is superior to the typical reported values. In this study three kinds of structures, i.e. linear-graded, stepwise, and graded-step structures, were considered. We found the defects are effectively reduced by introduction of an optimum number of steps in the graded layer.
Electronic Structure of Ag/Cu/Ag/Cu(111) Double Nanofilm Structures Studied by Angle-Resolved Photoemission Spectroscopy
Authors: Tanaka Akinori , Takahashi Kazutoshi , Sasaki Hiroyuki , Suzuki Shoji , Sato Shigeru
Published: April 2011

We have carried out an angle-resolved photoemission study of double Ag nanofilm structures, composed of an outer Ag nanofilm / Cu barrier layer / inner Ag nanofilm / Cu(111) substrate. From these results, we discuss the electronic coupling between the quantized electronic states in the double Ag nanofilm structures with various configurations.
Simulations of molecular beam epitaxy growth of GaAs
Authors: Zhang Z. , Orr B. G.
Published: April 2011

Numerical simulations have been performed for generic III-V MBE growth. The key aspects of the simulation include two deposited species one volatile and the second with high surface mobility. Simulations reproduce the experimentally observed adatom concentrations for GaAs and show that smooth surfaces are produced for films deposited with a substrate temperature in a crossover regime between kinetically limited and entropically roughened growth.
Ultra-Thin Zirconium Silicate Filmsc With Good Physical And Electrical Properties For Gate Dielectric Applications
Authors: Dharmarajan Easwar , Qi Wen-Jie , Nieh Renee , Kang Laegu , Onishi Katsunori , Lee Jack C.
Published: April 2011

The need for alternative gate dielectrics to replace conventional SiO2 is increasing to facilitate further CMOS scaling. One of the most promising materials for use as an alternative gate dielectric is Zr silicate due to its thermodynamic stability on Si and its good interface quality with Si. In this study, ultra-thin Zr silicate films (45 – 60 Å thick) with different Zr compositions have been deposited on Si using magnetron reactive co-sputtering. The Zr composition was kept below the stoichiometric value of about 16% to prevent precipitation of ZrO2and to have Si rich films for better interface quality. Films were rapid thermal annealed in N2 ambient up to 9000C and Pt was used as the gate electrode. Electrical characterization of these films was done using HP 4156 and HP 4194 parameter analyzers. Based on these studies, we demonstrate Zr silicate films with equivalent oxide thickness (EOT) of less than 14 Å with gate leakage significantly lower thanSiO2 of similar thickness and hysteresis of < 20mV ( in a sweep from –3 to 3 V). The films exhibit good thermal stability on Si even after 900 0C annealing as shown by a minimal increase in EOT with annealing. TEM and XPS analyses show high quality Zr silicate films that remain stable and amorphous even at 900 0C.
Characteristics of Laser-Ablated Plasma and Properties Of Diamond-Like Carbon Film In Pulsed Laser Deposition
Authors: Yamagata Yukihiko , Ohshima Tamiko , Ikegami Tomoaki , Thareja Raj K. , Ebihara Kenji , Sharma Ajay , Mayo Robert M. , Narayan Jagdish
Published: April 2011

Characteristics of laser-ablated carbon plasma and properties of diamond-like carbon film in KrF pulsed laser deposition were investigated using laser-induced fluorescence (LIF) and optical emission spectroscopy. Two-dimensional LIF images of C2 (Swan band, d3Φg – a3Φu) and C3 (Comet Head System, A1Φu –X1Σg+) molecules were detected as a function of laser energy density by narrow band pass filters and an intensified CCD camera. C2 LIF intensity is found to be weaker in the central part of the plume than that at the periphery at incident energy greater than 6 J/cm2. It is conjectured that C2 molecules are dissociated by collision with energetic species in the central part of the ablation plume, and degrade the diamond-like property of deposited films.
Growth of AlN and TiN Structures by Plasma-Enhanced Pulsed Laser Deposition
Authors: Poindexter Edward , Xin Yan , Durbin Steven M.
Published: April 2011

Nitride materials are of interest for a wide variety of applications, including wear-resistant coatings, insulating layers, high-temperature semiconductor devices, and short-wavelength emitters and detectors. TiN and AlN appear to be particularly amenable to crystalline thin film deposition, with stoichiometric material easily obtained even without the use of active nitrogen species. This paper describes the growth of crystalline AlN and TiN thin films on silicon and sapphire substrates using a KrF excimer laser (λ = 248 nm) to ablate elemental metallic targets, and an inductively-coupled RF plasma source to supply active nitrogen species. Growth was monitored in-situ using reflection high-energy electron diffraction (RHEED), and films were characterised using fourier-transform infrared spectroscopy (FTIR) and electron microscopy techniques. Optimised growth conditions led to single-crystal growth of TiN on both substrates, but only polycrystalline AlN was formed directly. Use of a TiN buffer layer on (0001) sapphire led to the successful growth of a single-crystal AlN layer as confirmed by RHEED and high-resolution transmission electron microscopy (HRTEM).
The Heteroepitaxial Nucleation and Growth of Metal Oxides by Llvsztu Oxidation
Authors: Bharadwaj Mridula D. , Zhou Gnang-wen , Yang Judith C.
Published: April 2011

Here we report our investigations on the initial stages of Cu(OO1) oxidation in dry and moist atmosphere using in situ ultra high vacuum (UHV) transmission electron microscopy (TEM), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Cu20 islands were observed to grow 3-dimensionally into the Cu film as seen through the above mentioned techniques. Further, we discuss our interpretation of the experimental observations that presence of water vapor in the oxidizing atmosphere retards the rate of Cu oxidation and Cu20 shows surprising reduction when exposed to water vapor.
Modeling Gas Separation Membranes
Authors: Malanoski Anthony P. , Swol Frank van
Published: April 2011

Recent advances in the development and application of self-assembly templating techniques have opened up the possibility of tailoring membranes for specific separation problems. A new self-assembly processing route to generate inorganic membrane films has made it feasible to finely control both the three-dimensional (3D) porosity and the chemical nature of the adsorbing structures. Chemical sites can be added to a porous membrane either after the inorganic scaffolding has been put in place or, alternatively, chemical sites can be co-assembled in a one-step process. To provide guidance to the optimized use of these ‘designer’ membranes we have developed a substantial modeling program that focuses on permeation through porous materials. The key issues that need to be modeled concern 1) the equilibrium adsorption behavior in a variety of 3D porous structures, ranging from straight pore channels to fractal structures, 2) the transport (i.e. diffusion) behavior in these structures. Enriching the problem is the presence of reactive groups that may be present on the surface. An important part of the design of actual membranes is to optimize these reactive sites with respect to their strength as characterized by the equilibrium constant, and the positioning of these sites on the adsorbing surface. What makes the technological problem challenging is that the industrial application requires both high flux and high selectivity. What makes the modeling challenging is the smallness of the length scale (molecular) that characterizes the surface reaction and the confinement in the pores. This precludes the use of traditional continuum engineering methods. However, we must also capture the 3D connectivity of the porous structure which is characterized by a larger than molecular length scale. We will discuss how we have used lattice models and both Monte Carlo and 3D density functional theory methods to tackle these modeling challenges.
Probing Dynamics of water Molecules in Mesoscopic Disordered media by NMR Dispersion and 3D Simulations in Reconstructed Confined Geometries
Authors: Levitz P. , Korb J.P. , Quynh A. Van , Bryant R.G.
Published: April 2011

Disordered mesoporous materials with pore sizes ranging from 2 nm to some 10 nm develop large specific surface areas. These matrices can be easily filled with polar fluids And the interfacial region between the solid matrix and the pore network strongly influences the molecular dynamics of the entrapped fluid. A promising way to probe such a coupling on a large time-scale is to look at the dispersion of the nuclear spin-lattice relaxation rate of the polar liquid using field cycling NMR relaxometry technique. We have performed such an experiment on a fully hydrated porous Vycor glass, free of electron paramagnetic impurities. The proton nuclear magnetic relaxation rate (1/T1) exhibits a logarithmic dependence on Larmor frequency over the range from 0.01 to 30 MHz. A cross-over is observed below 0.1 MHz. In order to understand the relationship between geometric disorder, interfacial confinement, and nuclear magnetic relaxationdispersion (NMRD), we first compute an off-lattice reconstruction of the Vycor glass This model agrees with available experimental data (specific surface, porosity, chord length distributions, small angle scattering and tortuosity). A Brownian dynamics simulation is performed to analyze long time molecular self-diffusion and NMRD data. These later are well reproduced and appear to be connected with the translation diffusion of water near the SiO2 interface. The logarithmic character of the NMRD is specifically related to the interfacial geometry of the Vycor glass. Several other multiconnected interfacial structures such as periodic minimal surfaces do not exhibit such an evolution. Therefore, NMRD appears to be selectively sensitive to the interfacial geometry of mesoscopic disordered materials (MDM).
Probing Confining Geometries with Molecular Diffusion: A Revisited Analysis of NMR-PGSE Experiments
Authors: Rodts Stéphane , Levitz Pierre
Published: April 2011

We present an interpretation of the self diffusion propagator for a molecular fluid confined in a porous medium. Breaking with conventional routes of interpretation, we focus in reciprocal space on the time dependence of the propagator at a fixed wave vector q. New theoretical results are reported, as well as NMR measurements on a water-saturated packing of glass beads and on a stack of plastic platelets with rough surfaces. It is shown that at least three time regimes may be distinguished, characteristic of new ways in which the propagator is affected by the geometry of the system: a short-time exponential regime of almost unrestricted diffusion, a pseudo-exponential regime probing the transport process across the material at the length scale λ=2π/q, and an algebraic regime at long times with exponent -d/2, where d is the dimensionality of connected parts of the pore space.
Static Friction between Elastic Solids due to Random Asperities
Authors: Sokoloff J. B.
Published: April 2011

Several workers have argued that should be negligible static friction per unit area in the macroscopic solid limit for interfaces between both incommensurate and disordered atomically flat weakly interacting solids. In contrast, the present work argues that in the low load limit, when one considers disorder on the multiasperity scale, the asperties act independently to a good approximation. This can account for the virtual universal occurence of static friction.
Cayley Tree Random Walk Dynamics
Authors: Katsoulis Dimitrios , Argyrakis Panos , Pimenov Alexander , Vitukhnovsky Lexei
Published: April 2011

We investigate diffusion on newly synthesized molecules with dendrimer structures. We model these structures with geometrical Cayley trees. We focus on diffusion properties, such as the excursion distance, the mean square displacement of the diffusing particles, and the area probed, as given by the walk parameter SN, the number of the distinct sites visited, on different coordination number, z, and different generation order g of a dendrimer structure. We simulate the trapping kinetics curves for randomly distributed traps on these structures, and compare the finite and the infinite system cases, and also with the cases of regular dimensionality lattices. For small dendrimer structures, SN approaches the overall number of the dendrimer nodes, while for large trees it grows linearly with time. The average displacement R also grows linearly with time. We find that the random walk on Cayley trees, due to the nature ot these structures, is indeed a type of a “biased” walk. Finally we find that the finite-size effects are particularly important in these structures.
Spatial Fluctuations and the Flipping of the Genetic Switch in a Cellular System
Authors: Metzler Ralf
Published: April 2011

– It has been realised that noise plays an important rôle in cellular processes where fluctuation induced number fluctuations of certain messenger molecules become non–negligible, due to the small total number of these molecules within one cell. In the following, it is argued that spatial fluctuations of such molecules and their impact on genetic switches should be considered as well.
Transport Properties in Ionic Media
Authors: Rollet A.-L. , Jardat M. , Dufrêche J.-F. , Turq P. , Canet D.
Published: April 2011

Transportcoe[ ]cients in charged media exhibit strong variations, according to the conditions of displacement of the particles. Electrical transport, characterized by the simultaneous displacement of positive and negative charges in opposite directions obeys Ohm's law, but its variation with concentration (non-ideality), depends on several types of interactions, whose time of establishment varies from picosecond to nanosecond. Several diffusion processes can occur: mutual diffusion, where ions move simultaneously in the same direction, keeping local elec- troneutrality, and self diffusion where individual ionic particles move separately. The variation of diffusion coefficients with concentration dependson non-ideality factors analogous to those occuring in conductance, and their experimental evidence is facilitated b y the availability of experimental tec hniquesowing different characteristic times of observation. This phenomenon is particularly noticeable for self-diffsuion coefficients, where the dynamical processes can be observed from the picosecond range (neutron quasi-elastic scattering), to millisecond (NMR) and to hour scale (radiactive tracers). The results are especially enhanced for porous charged media like ion exchanging membranes (nafions).

Those results are be explained here theoretically in the framework of contin uous solv en t model theories (brownian dynamics) and experimentally in the study of self-diffusion in nafion membranes.
Grain Orientation of Aluminum Titanate Ceramics during Formation Reaction
Authors: Ohya Yutaka , Nakagawa Zenbe-e , Hamano Kenya , Kawamoto Hiroshi , Kitaoka Satoshi
Published: April 2011

A microstructural change during the formation reaction of aluminum titanate from a mixture of rutile and corundum powders has been studied. The characterization was carried out using a polarization microscope, a scanning electron microscope and a micro-focus X-ray diffractometer. The formation of aluminum titanate was controlled by a nucleation step. The formation reaction proceeded to form spherically oriented regions of aluminum titanate grains among the matrix of rutile and corundum. At the end of the reaction, the specimen was entirely filled with the oriented region of consisting several hundred micrometers. The oriented region was composed of primary aluminum titanate grains of several micrometers and pores. Large cracks due to a thermal expansion anisotropy were formed at the boundaries of the orientated regions. The formation of the oriented region was caused by a small change in free energy, increasing elastic energy, and the endothermic nature of the reaction.
Interference Effect between Electron and Ion Flows in Semiconducting Fe3−δO4
Authors: Hong Jeong-Oh , Yoo Han-Ill
Published: April 2011

The effective valence, zFe–αFe*, of mobile Fe-ions (Fe2+, Fe3+) in semiconducting Fe3O4was determined at elevated temperatures via the electrotransport experiment in association with the literature data on the cation diffusivity and total electrical conductivity. It has been found that the value for zFe–αFe* varies systematically from below 2 up to 3 with oxygen partial pressure at a fixed temperature. The effective valence is determined not only by the mobility difference of Fe2+and Fe3+ions (zFe), but also by the cross effect between the cations and electrons upon their transfer (αFe*). A value of zFe–αFe* between 2 and 3 may be attributed to the mobility difference between Fe2+and Fe3+ions even in the absence of the cross effect, but the values of zFe–αFe* < 2 clearly indicate that the cross effect is in play in Fe3O4.
Transport Properties And Crystal Chemistry Of Ba-Sr-Bi Oxides
Authors: Gökçen Oya A. , Meen James K. , Jacobson Allan J. , Elthon Don
Published: April 2011

Rhombohedral (R) phases of the binary systems AE-Bi-O (AE = Ca, Sr, Ba) have low temperature (β2) and high temperature (β1) polymorphs and the β1 polymorph is a good oxygen ion conductor. In a previous study, we showed that in the ternary system Ba-Sr-Bi-O, conductivities of polycrystalline R samples are unaffected by Ba:Sr, whereas temperature ranges of the β2↔β1 polymorphic transition are. Recently, we proved that the conductivity of both polycrystalline and single-crystal R samples is sensitive to spatial direction. For single crystals, the conductivity is highest perpendicular to the c-axis. Along the conduction plane, the conductivities are significantly higher than those of polycrystalline R samples; perpendicular to that plane they are significantly lower. The BaBiO3 perovskite (P) phase, predominantly an electronic conductor, has conductivities of 10 to 102 S/cm at 350-800°C in air. Conductivity measurements of the R-P assemblages with different R:P showed that percolation threshold is around 35 volume % P.
Effect of Polymerization of Precursor Solutions on Crystallization and Morphology of Ce0.9Gd0.1O1.95 Powders
Authors: Wang Shuqiang , Awano Masanobu , Maeda Kunihiro
Published: April 2011

Synthesis of Ce0.9Gd0.1O1.95 (CGO) powders from polymeric precursor solutions formed by mixing nitrates and ethylene glycol at 60-85 was investigated with emphasis on the effect of the polymerization of the precursor solution on the crystallization and morphology of the derived intermediate and the resultant oxide powders. It was revealed by FTIR that the molecular structures of the polymeric precursor solution change from aldehyde or ketone groups to carboxylic acid and carboxylate groups with increasing heating time. TG-DTA analyses demonstrated the temperature shifting and the disappearance of the exothermic reactions of the derived powders with different heating times of the polymeric precursor solutions. Furthermore, it was identified by XRD, SEM and TEM that the derived powders can be changed from well crystallized organic formates of Ce1-xGdx(HCOO)3 with dendritic growth to loose agglomerated cubic CGO powders with grain sizes below 10 nm.
NO Decomposition Property of Lanthanum Manganite Porous Electrodes
Authors: Matsuda Kazuyuki , Kanai Takao , Awano Masanobu , Maeda Kunihiro
Published: April 2011

The NOx decomposition activity of electrochemical cells composed of YSZ solid electrolyte and La1−xCaxMnO3 porous electrodes was examined. La1−xCaxMnO3 powders were prepared by solid-state reaction using corresponding metal oxides and carbonates. The powders were dispersed in polyethylene glycol and screen-printed on both sides of the electrolyte pellet. The electrochemical decomposition of NO was carried out in the temperature range from 973 to 1173 K by passing a mixed gas of 1000 ppm NO and 2% O2 in He through the cell. When the DC voltage was applied to the cell, NO was directly decomposed to N2 and O2 in the oxidizing atmosphere. The cells composed of the La0.8Ca0.2MnO3 electrodes showed a higher NO decomposition ratio and current efficiency than that of other La1−xCaxMnO3 electrodes.
Technology transfer challenges in the manufacturing of a-Si tandem solar cells
Authors: Carlson D. E. , Ganguly G. , Lin G. , Gleaton M. , Bennett M. , Arya R. R.
Published: April 2011

BP Solar started commercial production of amorphous silicon tandem solar cells in Toano, Virginia in 1997. The scale-up process has involved overcoming technology challenges in several areas. It was necessary to develop high speed, multiple-beam laser scribing systems that could pattern photovoltaic modules over an area of 0.8 square meters with close dimensional tolerances. In addition, the deposition rate of amorphous silicon was doubled while production was being ramped up, and the deposition system was modified to meet capacity requirements. The utilization of the germane feedstock gas was increased by about 25% in order to reduce material costs. In addition, the dimensional tolerances of the deposition system geometry were determined and controlled in order to assure uniform deposition of the amorphous silicon alloys. Effects of contaminants (such as pump oils and residual dopants) on device performance were quantified. Sources of debris that could cause shunts and shorts in the devices were identified and minimized. Current research efforts are focused on further increases in the amorphous silicon deposition rate, improvements in device performance and the development of in-situ diagnostic tools to monitor and control the manufacturing process.
Diffusion and Solubility of Hydrogen in Amorphous and Microcrystalline Si:H Films
Authors: Beyer Wolfhard
Published: April 2011

Hydrogen diffusion and solubility effects in hydrogenated amorphous and microcrystalline silicon films are reviewed. Various diffusion-related effects have been observed which need to be considered in models of hydrogen diffusion. Hydrogen solubility is found to affect hydrogen incorporation and hydrogen transport.
Laplace-transform Transient Photocurrent Spectroscopy as a Probe of Metastable Defect Distributions in Hydrogenated Amorphous Silicon
Authors: Gueorguieva Mariana J. , Main Charlie , Reynolds Steve
Published: April 2011

Three Laplace transform methods for recovering the density of electronic states from transient photocurrent data are evaluated through a study of light-induced defect creation in PECVD a-Si:H films. A mathematically approximate method is shown to be sufficient to resolve the deep defects, whose density is estimated to increase by a factor of five from the annealed state after 3 hours' exposure to simulated AM1 illumination. An exact method, and a method employing Tikhonov regularisation, are found to give very similar results, provided the current-time data are smoothed beforehand in the former case. The increased resolution available is, however, unnecessary here, and these methods are shown to be more suited to the study of discrete levels or narrow distributions.
Electrical Stability of a-Si:H TFTs Fabricated at 150°C
Authors: Gleskova H. , Wagner S.
Published: April 2011

We investigated the electrical stability of amorphous silicon thin-film transistors (a-Si:H TFTs) fabricated on 51 μm thick polyimide foil at 150°C. Gate dc-voltage bias stressing caused an increase in threshold voltage and subthreshold slope, and a minor decrease in mobility. Annealing in H2+N2 substantially improved the stability of the TFTs. In annealed TFTs the threshold voltage shift exhibited a power law dependence on time with the exponent γ depending on gate bias Vg. For Vg = 20 V, γ = 0.45, while for Vg = 80 V, γ = 0.27. The threshold voltage shift also exhibited a power law dependence on Vg with the exponent β depending slightly on stress duration. β = 2.1 for t = 100 sec and 1.7 for t = 5000 sec. These values fall into the range experimentally observed for a-Si:H TFTs fabricated at the standard temperatures of 250-350°C.
Generation- Recombination Noise in Amorphous Semiconductors
Authors: Main Charlie , Reynolds Steve , Badran Rashad I.
Published: April 2011

We examine different approaches to the analysis of noise in amorphous hydrogenated silicon associated with trapping and generation – recombination processes, which appear to predict very different noise spectra. In one approach the broad noise spectrum observed is assumed to be composed of a distribution of Lorentzian noise spectra, each associated with traps at a given energy depth, with appropriate weighting according to the energy distribution of characteristic time constants. This latter weighting is taken to mirror the energy distribution of states in the gap. This represents a linear superposition of the (weighted) contribution from individual trapping levels, each with its own characteristic time constant. This approach thus assumes that each trap level is an independent source of fluctuation in free carrier number, unaffected by the presence of other traps in the material. At first sight this assertion seems plausible, since in the multi-trapping situation envisaged, cross-correlation effects must be very small. However, the presence of several groups of traps, or, in the limit, a continuum, results in a distribution of characteristic time constants, which is not a simple linear superposition of the time constants for each level. Thus the assertion that a flat density of states, or a region which is flat, such as the top of a broadened level, results in a region of 1/f slope in the noise spectrum, may not be valid. We present an alternative model in which the distribution of time constants is appropriately incorporated, and compare the predictions of this model with the ‘superposition’ approach, using computed noise spectra.
Silicon Thin Film Deposition on Nano-Structured ZnO Substrates
Authors: Könenkamp R. , Chu V. , Conde J. , Dloczik L.
Published: April 2011

Amorphous and micro-crystalline Si films have been deposited by the hot-wire technique on deeply structured columnar ZnO films. It is found that nearly perfect conformal deposition occurs for amorphous films, similar to what was previously found for rf-plasma deposited films. Micro-crystalline films deposited by the hot wire technique are less conformal since deposition occurs preferentially on protruding features of the substrate, while films deposited by rf-plasma CVD exhibit high conformality.
Electromechanical properties of amorphous and microcrystalline silicon micromachined structures
Authors: Gaspar J. , Boucinha M. , Chu V. , Conde J.P.
Published: April 2011

Electrostatic actuation of bridge and cantilever structures is studied. The structures are composed of bilayers of amorphous or microcrystalline silicon and aluminum fabricated using surface micromachining on glass substrates. The structures are actuated by applying the sum of a DC and a low-frequencyAC voltage. The resulting AC deflection is detected optically. The dependence of this deflection upon theapplied voltages and the bridge length is studied and compared with the predictions of an electromechanical model. The deflection amplitude can be electrostatically controlled and detected with a precision estimated to be better than a nanometer.
Films and Devices Deposited by Hwcvd at Ultra High Deposition Rates
Authors: Mahan A. H. , Xu Y. , Iwaniczko E. , Williamson D. L. , Beyer W. , Perkins J. D. , Vanecek M. , Gedvilas L. M. , Nelson B. P.
Published: April 2011

The structure of a-Si:H, deposited at rates in excess of 100Å/s by the hot wire chemical vapor deposition (HWCVD) technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). As the film deposition rate (Rd) is increased, we find that the XRD, Raman and the H evolution peak curves are invariant with Rd, and exhibit structure consistent with state-of-the-art, compact a-Si:H films deposited at low Rd. The only exception is the SAXS signal, which increases by a factor of ∼100 over that for our best low Rdfilms. We relate changes in the film electronic structure (Urbach edge) to the increase in the SAXS signals. We also note the invariance of the saturated defect density versus Rd, and discuss possible reasons why the increase in the SAXS does not play a role in the Staebler-Wronski Effect for this type of material. Finally, device results are presented.
Properties of Nano-crystalline n-type Silicon Films Produced by Hot Wire Plasma Assisted Technique
Authors: Ferreira I. , Fernandes F. Braz , Vilarinho P. , Fortunato E. , Martins R.
Published: April 2011

In this work, we present the properties of n-type silicon films obtained by hot wire plasma assisted technique produced at different rf power and gas flow rate. The films were produced at a filament temperature of 2000°C and the rf power was varied from 0W to 200W while gas flow rate was varied from 15 to 100sccm keeping rf power at 50W. In this flow rate range, the growth rate of the films varied from 5Å/s to 250Å/s and the corresponding electrical room dark conductivity varied from 10−2 to 10(Ωcm)-1. On the other hand, we observed that the electrical conductivity increased from 2 to 6(Ωcm)-1, and the Hall mobility from 0.1 to 2cm2/V.s as rf power change from 0W to 200W. The infrared, EDS and XPS analyses revealed the existence of oxygen incorporation, which is not related to post-deposition oxidation. The X-ray diffraction and [.proportional]Raman data show the presence of Si crystals in the films structure and the SEM micrographs reveal a granular surface morphology with grain sizes lower than 60nm.
High-rate deposition of a-SiNx:H films for photovoltaic applications
Authors: Kessels W.M.M. , Assche F.J.H. van , Hong J. , Moschner J.D. , Lauinger T. , Schram D.C. , Sanden M.C.M. van de
Published: April 2011

The feasibility of the new ‘Expanding Thermal Plasma’ technique for the deposition of a-SiNx:H at high deposition rates (typically ∼20Å/s) has been investigated. The structural film properties of the a-SiNx:H are reported for various process conditions and the application of the material as antireflection coating on (multi)crystalline silicon solar cells is studied. Furthermore, the performance of the material for surface and bulk passivation is investigated.
Nanostructured silicon films produced by PECVD
Authors: Martins R. , Águas H. , Silva V. , Ferreira I. , Cabrita A. , Fortunato E.
Published: April 2011

This paper presents the process conditions that lead to the production of nanostructured silicon films grown by plasma enhanced chemical vapour deposition close to the so-called gamma regime (powder formation), highly dense and with low density of bulk states. Thus, the powder management is one important issue to be addressed in this paper. As a general rule we observed that high quality films (low density of states and high μτ products) are obtained when films are grown under low ion bombardment at high hydrogen dilution and deposition pressure conditions, to allow the proper surface passivation and surface activation.
Growth Monitoring of Cu-Poor Prepared CuInS2 Thin Films
Authors: Luck Ilka V. , Alvarez-Garcia Jacobo , Calvo-Barrio Lorenzo , Werner Axel , Perez-Rodriguez Alejandro , Morante J. R. , Bräunig Dieter
Published: April 2011

The CuInS2 thin film formation from a Cu/In precursor stack in the presence of elemental sulfur using a rapid thermal process under Cu-poor conditions has been studied. The process has been aborted at appropriate stages and the corresponding samples were investigated by XRD, Raman spectroscopy and SEM. The sulfurisation starts from elemental Cu and CuIn2. Elemental In and the binary phases Cu11In9 and Cu7In3 appear as intermediate phases. At the end of the sulfurisation the sample contains the ternary phases CuInS2 and CuIn5S8. CuS and β-In2S3 are detected by Raman spectroscopy at the sample surface and at distinct stages of the sulfurisation only. A difference in CuInS2 crystal quality is observed between the surface and the bottom of the samples.