bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of Experimental and Theoretical Melting Behavior of DNA
Authors: Marx Kenneth A. , Bizzaro J. W. , Assil Iman , Blake R. D.
Published: April 2011

High resolution melting curves of total nuclearDictyostelium discoideumDNA (A×3 strain) are compared to theoretical melting calculated from GENBANK sequences(1.74 % of total) by the statistical thermodynamics program MELTSIM, parameterized for long DNA sequences(1,2). The lower and upper limits of simulated melting agree quantitatively with the experimental melting of total DNA. Calculated melting of coding, intron and flanking regions indicate that intron and flanking DNAs are extremely (A+T)-rich and account for the earliest melting DNA. There is no temperature overlap of these regions with coding DNA. A theoretical denaturation map of DNA containing the ribosomal DNA genes showed excellent agreement with subtransition positions of these genes in experimental curves. Agreement between these calculated and experimental melting data demonstrates our ability to accurately simulate DNA melting in complex eukaryotic genomes. This has important consequences for the understanding of sequence dependent energetic properties of nucleic acids and their potential use as biomaterials.
Pattern Evolution of Granular Media Rotated in a Drum Mixer
Authors: Hill K. M. , Kakalios J. , Caprihan A.
Published: April 2011

A homogeneous mixturea of different types of granular material will often segregate when rotated in a drum mixer. In the traditional axial segregation effect, the mixture of two different sizes of granular media will appear from the surface to separate into bands of relatively pure single concentrations along the axis of rotation. The initial pattern is not stable, but evolves in time with continued rotation through metastable states of fewer and fewer bands. We describe two experimental methods for measuring this evolution which provide a more complete picture of the dynamics involved in the pattern progression. The use of a CCD camera in conjunction with digital analysis techniques provides a consistent and precise measure of the state of the surface as a function of time. Nuclear Magnetic Resonance Imaging (MRI) techniques are used to noninvasively measure the segregation beneath the surface. These methods indicate that the underlying mechanisms of the pattern evolution originate in the bulk of the material beneath the avalanching surface.
Random Walks for Magnetic Decay in Porous Media
Authors: Cai Weicheng , Halsey Thomas C. , Hardenbergs John H. , Leibig Michael
Published: April 2011

We propose a Monte Carlo method to simulate the magnetization decay for NMR in porous media. In this method, the diffusive spins in the fluid phase are modeled by 3-d random walkers, whose absorption at the solid surface simulates surface-mediated decay. Our method is 20 times faster than direct relaxation of the diffusion equation when a variable-step size implementation is used. We demonstrate our method by computing the surface relaxivity ρ for a Fontainebleau sandstone, whose structure was determined by X-ray tomography.
Monte Carlo Simulations of Phase Transitions in a Two-Dimensional Random-Bond Potts Model
Authors: Paredes R. , Valbuena J.
Published: April 2011

Motivated by recent experiments on phase behavior of systems confined in porous media, we have studied the effect of quenched bond randomness on the nature of the phase transition in the two dimensional Potts model. To model the effects of the porous matrix we chose the couplings of the q state Potts Hamiltonian from the distribution P(Jij) = pδ(Jij – J) + (1 – p)δ(Jij). For a range of p values, away from the percolation threshold, the transition temperature follows the mean field prediction Tc(p) = Tc(1)p. Furthermore, we observed that the strong first order transition, that appears in the pure case for q = 10, changes two a second order transition. It is also clear from our simulations that the second order transition of the q = 3 pure case changes to a second order transition of a different universality class. A finite size scaling analysis suggests that in both cases the critical exponents, in the presence of disorder, fall into the universality class of the two dimensional pure Ising model. This result agrees with theoretical calculations recently published [1].
Pattern Formation and Unusual A + B → 0 Reaction Kinetics between Charged Reactants in Low Dimensions
Authors: Kotomin E. A. , Kuzovkov V. N. , von Niessen W.
Published: April 2011

The effect of non-equilibrium charge screening in the kinetics of the one-dimensional, diffusion-controlled A + B → 0 reaction between charged reactants in solids and liquids is studied. Incorrectness of static, Debye-Hückel theory is shown. Our microscopic formalism is based on the Kirkwood superposition approximation for three-particle densities and the self-consistent treatment of the electrostatic interactions defined by the nonuniform spatial distribution of similar and dissimilar reactants treated in terms of the relevant joint correlation functions. Special attention is paid to the pattern formation due to a reaction-induced non-Poissonian fluctuation spectrum of reactant densities. This reflects a formation of loose domains containing similar reactants only. The effect of asymmetry in reactant mobilities (DA - 0, DB > 0) contrasting the traditional symmetric case, i.e. equal diffusion coefficients, (DA = DB) is studied. In the asymmetric case concentration decay is predicted to be accelerated, n(t) ∞ t−α, α = 1/3 as compared to the well-established critical exponent for fluctuation-controlled kinetics in the symmetric case, α - 1/4 and/or the prediction of the standard chemical kinetics, α = 1/2. Results for the present microscopic theory are compared with the mesoscopic theory.
Stretched, Twisted, Supercoiled and Braided DNA
Authors: Marko John F.
Published: April 2011

The DNA double helix is a semi-flexible polymer with twist rigidity. Its bending elasticity gives rise to entropie polymer elasticity, which can be precisely studied in single-molecule experiments. DNA's twist rigidity causes it to wrap around itself, or ‘supercoil’, when it is sufficiently twisted; thermal fluctuations destabilize supercoiling for DNAs twisted fewer than once per twist persistence length. Twisted DNAs under tension, braided DNAs, and the internal dynamics of supercoiled DNAs are discussed. The interplay between braiding and supercoiling free energy is argued to be important for the decatenation of duplicated DNAs in prokaryote cells.
Avalanche Segregation of Binary Mixtures of Granular Media
Authors: Koeppe J. , Enz M. , Kakalios J.
Published: April 2011

When an initially homogeneous binary mixture of granular media such as fine and coarse sand is poured near the closed edge of a “quasi-two-dimensional” Hele-Shaw cell consisting of two vertical transparent plates held a narrow distance apart, the mixture spontaneously forms alternating segregated layers. Using digital images taken with a CCD camera and Fourier transformed to obtain the structure function of the banding pattern, the wavelength selection mechanism and degree of segregation are studied as the plate separation of the Hele-Shaw cell is systematically increased. For a given flow rate, the degree of segregation, as reflected in the amplitude of the peak in the structure function, decreases as the plate separation increases. For wider plate spacings the peak in the structure function shifts to lower wavelengths. These results are compared to numerical simulations of the avalanche segregation using a stochastic sandpile model.
Twist-Stretch Elasticity of DNA
Authors: Kamien Randall D. , Lubensky T. C. , Nelson Philip , O'Hern Corey S.
Published: April 2011

The symmetries of the DNA double helix require a new term in its linear response to stress: the coupling between twist and stretch. Recent experiments with torsionally-constrained single molecules give the first direct measurement of this important material parameter. We extract its value from a recent experiment of Stricket al.and find rough agreement with an independent experimental estimate recently given by Marko. We also present a very simple microscopic theory predicting a value comparable to the one observed.
Sponge Phase Under Shear
Authors: Mahjoub H. F. , Bourgaux C. , McGrath K. M. , Tassin J.F. , Kleman M.
Published: April 2011

The L3 phase under shear transforms to a lamellar phase L*α above some critical shear rate. We study the sequence of successive states (rheothinning, flow birefringence, transition region, lamellar) with simultaneous rheooptical methods, in situ X-ray scattering, and light microscopy observations. The transition region is biphasic, and the two phases display epitaxial relationship immediately after shearing is ceased.
Initial Distribution Effects on Diffusion-Limited Reactions in Constrained Geometries
Authors: Lindenberg Katja , Romero A. H. , Sancho J. M.
Published: April 2011

We present a study of the effects of the initial distribution on the kinetic evolution of irreversible binary reactions in low dimensions. We focus on the role of initial density fluctuations and, in particular, on the role of the long wavelength components of the initial fluctuations, in the creation of the macroscopic patterns that lead to the well-known kinetic anomalies in this system. The frequently studied random initial distribution is but one of a variety of possible distributions leading to interesting anomalous behavior. Our discussion includes initial distributions that suppress and ones that enhance the initial long wavelength components.
Dynamics of Spontaneous Spreading with Evaporation for thin Solvent Films
Authors: Dussaud Anne D. , Troian Sandra M.
Published: April 2011

We have investigated the spreading behavior of solvent droplets on a bulk water support using solvents with different vapor pressures and spreading coefficients. Instead of seeding the surface with tracer particles, as is usually done to track moving fronts, we employ laser shadowgraphy to visualize the entire surface of the spreading film including the leading edge. For non-volatile systems it has previously been shown that the leading edge advances in time as t3/4. We find that volatile systems with positive initial spreading coefficients exhibit two spreading fronts, both of which demonstrate power law growth but with exponents closer to 1/2. Surprisingly, differences in the liquid vapor pressure or the spreading coefficient seem only to effect the speed of advance but not the value of the exponent. We are presently investigating the behavior of the subsurface flow to determine the mechanism leading to the smaller spreading exponent.
Molecular dynamics simulations of sliding friction of Langmuir-Blodgett monolayers
Authors: Koike Asako , Yoneya Makoto , Ito Yutaka
Published: April 2011

Molecular dynamics simulations have been performed to study friction in Langmuir-Blodgett monolayers of perfluorocarboxylic acid, semifluoro acid and hydrocarboxylic acid on SiO2. The frictional coefficient of perfluorocarboxylic acid is about three times as large as that of hydrocarboxylic acid, while the frictional coefficient of semifluorocarboxylic acid is about two times as large as that of hydrocarboxylic acid. The qualitative aspects of these simulation results are consistent with known experimental results. In order to interpret the difference in the frictional coefficient, a series of simulations have been carried out by changing molecular potential parameters. The simulation results suggest that the 1,4-van der Waals interaction is the main cause of the larger frictional force for perfluorocarboxylic acid than that for hydrocarboxylic acid. Further frictional coefficients of semifluorocarboxylic acid are found to change by the fluorination ratio. The results also show that frictional force is roughly proportional to the excess r.m.s. fluctuation of the potential energy under shear from the equilibrium. The relation between the frictional force and the energy needed for molecular deformation under shear conditions is also discussed.
STM Investigations of Confined Transition Metal Chalcogenide Colloidal Particles
Authors: Kivuitu F. , Kelty S. P.
Published: April 2011

The chemical reactivity of catalytic surfaces often arises from a unique crystal or electronic structure confined to the first few atomic layers. Scanning Tunneling Microscopy (STM) is particularly well suited to studying the spatially confined structural properties of such systems. In this paper, I report recent advances in the characterization of colloidal layered transition metal chalcogenides using STM. These particles mimic the surface (layer edge) properties of bulk catalytic materials. It is found that the materials adopt distinct equilibrium chemical and electronic structures as compared to the bulk. The possible significance of these novel structures in regard to the bulk catalytic functionality of the parent material will be briefly discussed.
Irreversible Adsorption of Polymer Melts
Authors: Sawhney U. , Durning C.J. , O'Shaughnessy B. , Smith G.S. , Majewski J.
Published: April 2011

We studied the equilibrium architecture of polymer layers strongly adsorbed from the melt. Immobilized layers of poly-(methyl methacrylate) (PMMA) were produced by the following method: 1) The polymer was spin-coated onto silanol bearing surfaces of single crystal and fused quartz, and annealed at melt conditions, 2) The annealed layer was quenched to room temperature (below the glass transition temperature) in order to “freeze in” the melt structure near the substrate, 3) Unbound material was leached away in good solvent (benzene) to leave a residual, strongly-adsorbed layer. The architecture of this layer was studied by neutron reflection. Data on dried adsorbed layers indicates a dense PMMA film whose thickness gradually increases with annealing time in the melt from a minimal value. Evidently, annealing gradually relaxes a rather flat non-equilibrium structure produced by spin-coating. The thicknesses, h, in a series of dry layers annealed long enough to achieve equilibrium conditions in the melt scale as h ∼ N1/2. Data on swollen layers suggest a dilute, extended layer, but the preliminary results cannot give a definitive confirmation of the brush structure predicted by Guiselin.11
Experimental Investigations of the Kinetics of a Catalytic Trapping Reaction in Confined Spaces
Authors: Feldman Mark S. , Lin Anna L. , Kopelman Raoul
Published: April 2011

We investigate the anomalous kinetics in one dimension of a diffusion limited catalytic trapping reaction, A + T → T, by measuring the oxidation of glucose. The reaction is carried out in a thin capillary tube, and the depletion of oxygen in the vicinity of the reaction front is monitored by the fluorescence of a Ru(II) dye. Theoretical results and simulations have predicted an asymptotic t1/2 dependence for the rate coefficient. We observe a depedence on t0.56, with what appears to be an asymptotic behavior approaching t1/2.
Random walks, trapping and reactions in quasi-one dimensional lattices
Authors: Argyrakis Panos , Lin Anna L. , Kopelman Raoul
Published: April 2011

We performed Monte Carlo simulations of diffusion processes on baguette-like lattices, which have very small width and height, but long length, in effect providing quasi-one dimensional systems. This is done by investigating random walk properties of single particles, and also the well known model bimolecular reactions A + A and A + B. We monitor the number of distinct sites visited, as a function of time, and also the survival probability in the presence of static traps. For the reaction systems we monitor, as usual, the decay of the particle density. The expected one-dimensional behavior is recovered, in the long time limit, for all cases studied. Our interest here is in the crossover time, from 3- (or 2-)dimensionaI behavior (early time) to one-dimensional (long time). We find that this crossover time scales with respect to the baguette's short dimension. However, this scaling deviates significantly from a mean square displacement law, and it is specific to both tube dimensionality and reaction type (e.g. A + A or A + B). Specifically, instead of an expected power of 2, the exponents range between 1 and 4. The densities of the A + B reactions at the dimensional crossover are compared to the densities at the segregation crossover in regular lattices at all three dimesnions. As expected, the time evolution of the A + A reaction parallels the behavior of the average number of distinct sites visited.
Massively Parallel Molecular Dynamics Simulation of Gas Permeation across Molecular Sieving Porous Membranes
Authors: Pohl Phillip I. , Heffelfinger Grant S.
Published: April 2011

In this work we simulate the diffusion of gases in a microporous solid models using a newly developed dual control volume grand canonical molecular dynamics technique. This allows spatial variation of chemical potential and hence an accurate simulation of steady-state pressure driven diffusion. The molecular sieving nature of microporous zeolites are discussed and compared with that for amorphous silica from sol-gel methods. Massively parallel supercomputers allow a quick and insightful study of these microporous structures.
Notes on the Interpretation of Nanorheology Experiments
Authors: Dhinojwala Ali , Cai Lenore , Granick Steve
Published: April 2011

We discuss the resolution of shear force measurement in determining the effective shear viscosity of molecularly-thin films. Recent findings for the shear of confined OMCTS (octamethylcyclotetrasiloxane) are presented.
Surface Rotational Dynamics of Liquids Confined in Nanopores
Authors: Korb J.-P. , Malier L. , Cros F. , Xu Shu , Jonas J.
Published: April 2011

2H NMR relaxation times of selectively deuterated polar molecules confined to a set of calibrated nanoporous silica glasses are reported. These experiments, combined with the consideration of different time scales in the theory of surface relaxation, show how confinement effects can provide detailed information on the rotational dynamics of temporarily adsorbed liquid layers in presence of biphasic fast exchange.
Fluid and Protein Motion in Monomolecular and Bilayer Films Overlying Sublayers of Finite Depth
Authors: Stone H.A.
Published: April 2011

Motion of a monolayer, bilayer or other surface film is affected by the presence of a surrounding fluid phase. Several recent experimental and theoretical studies have examined this viscous coupling with particular attention given to situations where the subphase provides the dominant resistance to motion. This research is summarized and the translation of a membrane-bound particle over a sublayer of finite depth is considered.
Electron Spin Resonance of Hydrogenated Amorphous Silicon Alloyed with Sulfur
Authors: Yan Baojie , Taylor P. C.
Published: April 2011

Electron spin resonance (ESR) and light-induced electron spin resonance (LESR) measurements were performed on sulfur-doped hydrogenated amorphous silicon (a-SiS:H). At low S doping levels (H2S/SiH4 ≤ 10−3 in gas phase), the ESR and LESR line shapes are similar to those observed in undoped a-Si:H. The dark spin density generally increases with S doping and reaches 5×1016 cm−3 at H2S/SiH4 ≈ 10−3. On the other hand, at high S concentration, the dark spin density increases significantly with S concentration. The ESR and LESR line shapes become identical and asymmetric, a fact that implies the ESR and LESR signals result from the same kind of ESR center. Since S doping is very inefficient, the ESR signals probably are due to defects instead of trapped carriers in band tails. The asymmetry of the ESR and LESR line shapes at high sulfur concentration may result from either more than one type of defect or an asymmetry in the g tensor of a single defect. The photo-excitation of H-passivated, S-related defects could also contribute to the LESR.
Hydrogen Populations in PECVD a-Si:H,D
Authors: Cull T. S. , Kernan M. J. , Chan P. H. , Fedders P. A. , Leopold D. J. , Norberg R. E. , Wickboldt P. , Paul W.
Published: April 2011

Proton NMR and deuteron NMR (DMR) have been used to measure hydrogen populations in a series of PECVD a-Si:H,D films. The sharp DMR doublet from Si-D is fitted and subtracted out. The residual spectra then show specific signatures for molecular D2 and HD. The fitting procedures yield quantitative measures of Si-bonded and molecular species. A particular comparison is made between a pair of films prepared as the powered and unpowered electrodes in the same plasma deposition. Both silicon-bonded and molecular populations axe significantly different in the two films and correlate with photoresponse products ημT and with IR as well as other materials characterizations.
Determination of the Density of States in Semiconductors from Transient Photoconductivity
Authors: Main C.
Published: April 2011

This paper discusses the techniques we have developed to analyse the photo-response of amorphous semiconductors to transient optical excitation to determine the energy distribution of gap states (DOS). We highlight the difficulties arising from a direct ‘single point’ approach using the instantaneous transient photocurrent i(t) when there is significant structure in the DOS or in the presence of recombination. Frequency domain methods which involve measurement of the amplitude I(ω) and phase φ(ω)of the steady ac photocurrent in response to ac excitation are shown to overcome these problems, but include other, experimental shortcomings. The paper describes a solution which combines the best features of time and frequency domain methods. The method involves numerical Fourier transformation of the time sampled impulse response i(tk) into a complex ac response (ω n), followed by analysis for gap-state distribution. Essentially, the method succeeds since it folds in information from the whole of the measured response for each energy considered. Other advantageous features include applicability to semiconductors exhibiting dispersive or non-dispersive transport, and to pre- and post- recombination regimes of the transient photocurrent, without modification. The features of this analytical method are demonstrated with computer simulated and experimental transient photocurrent data for a-Si:H.
Spin Dependent Photoinduced Absorption in a-Si:H
Authors: Schultz N. , Vardeny Z. V. , Taylor P. C.
Published: April 2011

We have studied photoexcitation dynamics in undoped a-Si:H from 80 K to 300 K by the techniques of photoluminescence (PL), photoinduced absorption (PA) and their respective versions of optically detected magnetic resonance, namely PLDMR and PADMR. Both PL and PA spectra are composed of low and high energy bands, respectively. Using their respective temperature dependences we correlate the low energy PA band (∼ 0.4 eV) to the high energy PL band (∼1.3 eV) and the high energy PA band (∼ 1 eV) to the low energy PL band (∼ 0.8 eV). We also found that the PADMR spectrum is composed of three main contributions: a narrow resonance at g ∼ 2 (FWHM = 15 G), a broad resonance at g ∼ 2 (160 G) and a narrow asymmetric resonance at g ∼ 4 (20 G). By measuring their photon energy spectrum we found that the g ∼ 4 and the broad g ∼ 2 resonances are correlated with each other and therefore are due to trapped electron-hole pairs in the triplet spin configuration.
Fluctuating Defect Density Probed with Noise Spectroscopy in Hydrogenated Amorphous Silicon
Authors: Verleg P.A.W.E. , Uca O. , Dijkhuis J. I.
Published: April 2011

Resistance fluctuations have been studied in hydrogenated amorphous silicon in the temperature range between 300 K and 450 K. The primary noise source has a power spectrum of approximately 1/f and is ascribed to hydrogen motion. Hopping of weakly bound hydrogen is thermally activated at such low temperatures with an average activation energy of 0.85 eV. The attempt rate amounts to 7 · 1012 s−1.
Experimental Determination of the Distribution of Tail States of Hydrogenated Amorphous Silicon: A Transient Photocurrent Analysis
Authors: Webb D. P. , Chan F. Y. M. , Zou X. C. , Chan Y. C. , Lam Y. W. , Lin S. H. , O'Leary S. K. , Lim P. K.
Published: April 2011

Recent experimental developments have cast doubt on the validity of the common assumption that the distribution of tail states of hydrogenated amorphous silicon exhibits a single exponential functional form. We employ transient photocurrent decay measurements to determine this distribution of tail states. In our approach, however, we determine the distribution of tail states directly from the experimental data, without assuming, a priori, a specific functional form. It is found that these experimental results are consistent with other more recent experimental determinations of the distribution of tail states, suggesting the possibility of deviations from a single exponential distribution of tail states in hydrogenated amorphous silicon.
Ambipolar Phototransport (μτe = μτh) Observed as an Intrinsic Property of a-SiGe:H
Authors: Wickboldt Paul , Pang Dawen , Paul William , Chen Joseph H. , Chen Chih-Chiang , David Cohen J.
Published: April 2011

A study is presented of a series of high quality PECVD a-Si0.33Ge0.67 films, produced by cathodic deposition, in which small concentrations of PH3, B2H6 or air impurities were added during deposition. The quantum efficiency-mobility-lifetime product (ημτ) increases, and the ambipolar diffusion length (Lamb) decreases monotonically with dopant concentration for both PH3 and B2H6. This result is strong evidence that for these films neither photocarrier is dominant (μτe= μτh) at zero doping. This result is very different from what has been typically observed by other researchers, that the electron is the dominant photocarrier for undoped a-SiGe:H.

Drive level capacitance (DLC) measurements of these alloys show an unusual behavior of being temperature-independent, and the dark conductivity activation energy is maximum for zero doping. It is proposed that all of these unusual properties are due to the unusually low impurity concentration of these films, and that these properties are, in fact, the intrinsic properties for a-SiGe:H alloys. To verify this, films were prepared with a calibrated and controlled air leak introduced during deposition. As the air leak was increased, the film properties changed to typical behavior. Even for air concentrations as low as 2 ppm (gas phase), the transport measurements showed changes consistent with a shift in the Fermi level toward the conduction band.
Electronic transport and structure of microcrystalline silicon deposited by the VHF-GD technique
Authors: Goerlitzer M. , Beck N. , Torres P. , Kroll U. , Keppner H. , Meier J. , Koehler J. , Wyrsch N. , Shah A. V.
Published: April 2011

Electronic transport parallel and perpendicular to growth direction has been studied in a series of microcrystalline silicon samples obtained by various dilutions of silane in hydrogen. It is clearly shown that the transport properties (dark conductivity, drift mobility, ambipolar diffusion length and photoconductivity) under dark and under illumination conditions are enhanced as the dilution is increased. Furthermore, these films exhibit no degradation upon light-soaking. X-Ray diffraction patterns of the samples confirm that there is a correlation between the amount of crystalline fraction in the samples and the transport properties, as well as a preferential orientation along the growth direction. A similar correlation is found with the shift of the Si-H stretching mode peak of the infrared spectra (IR). Because transport properties have been measured by different techniques (dark conductivity, ambipolar length and photoconductivity in the direction perpendicular to growth direction, drift mobility in the direction parallel to growth direction), no statement can be made about a possible anisotropy in transport, as it would be expected from the columnar shape of the crystallites.
Direct Nucleation of Crystalline SiGe on Substrates by Reactive Thermal CVD with Si2H6 and GeF4
Authors: Yoshizawa Fumio , Shiota Kunihiro , Inoue Daisuke , Hanna Jun-ichi
Published: April 2011

Polycrystalline SiGe (poly-SiGe) film growth by reactive thermal CVD with a gaseous mixture of Si2H6 and GeF4 was investigated on various substrates such as Al,Cr, Pt, Si, ITO, ZnO and thermally grown SiO2.

In Ge-rich film growth, SEM observation in the early stage of the film growth revealed that direct nucleation of crystallites took place on the substrates. The nucleation was governed by two different mechanisms: one was a heterogeneous nucleation on the surface and the other was a homogeneous nucleation in the gas phase. In the former case, the selective nucleation was observed at temperatures lower than 400°C on metal substrates and Si, where the activation of adsorbed GeF4 on the surface played a major role for the nuclei formation, leading to the selective film growth.

On the other hand, the direct nucleation did not always take place in Si-rich film growth irrespective of the substrates and depended on the growth rate. In a growth rate of 3.6nm/min, the high crystallinity of poly-Si0.95Ge0.05in a 220nm-thick film was achieved at 450°C due to the no initial deposition of amorphous tissue on SiO2 substrates.
Transport and Recombination Channels in Undoped Microcrystalline Silicon Studied by ESR and EDMR
Authors: Will D. , Lerner C. , Fuhs W. , Lips K.
Published: April 2011

We present a detailed study of ESR and spin-dependent transport (EDMR) on μc-Si. We identify to different types of defects at g=2.0055(±3) and g=2.0044(±5) and study their influence on transport and recombination by stepwise annealing the samples. We find that transport is not controlled by defects if ND<1018 cm−3. For ND>1018cm−3 a dramatic decrease of the conductivity is found and we identify a hopping contribution in transport. To explain our ESR and EDMR results we propose a simple model where most defects are distributed at the surface of the columns and transport is along percolation paths. We also observe minor metastable changes of the defect density which are assigned to adsorption of atmospheric oxygen.
Correlation Between Surface Morphology and Lattice Orientation of Microcrystalline Silicon
Authors: Kondo M. , Nishimiya T. , Saitoh K. , Ohe T. , Matsuda A.
Published: April 2011

The surface morphology and preferential orientation of μc-Si:H has been studied on c-Si substrate in relation to the Si-H bonding mode. Epitaxy-like growth is observed on Si (001) substrate at a moderate temperature and under low ion bombardment, while the surface is significantly roughened. With increasing or decreasing the temperature or increasing the ion bombardment, the lattice orientation is randomized and an amorphous component is increased, resulting in the reduction of the surface roughness. The growth mode is intimately related to the Si-H bonding configuration. The epitaxy-like growth is collapsed accompanying the disappearance of the surface Si-H 2,3 mode. The resemblance of these phenomena to the low temperature MBE is discussed.
Real-Time ESR Observation During Film Growth of a-Si:H
Authors: Yamasaki S. , Umeda T. , Isoya J. , Tanaka K.
Published: April 2011

In-situ electron-spin-resonance (ESR) measurements of film growth of hydrogenated amorphous silicon (a-Si:H) using a remote hydrogen plasma technique have been performed. The Si dangling-bond signal in a-Si:H during and after deposition has been detected, in addition to the gas-phase ESR signals both of atomic hydrogen and photo-excited SiHx molecules. Dynamic changes of the Si dangling-bond signal intensity were observed when the deposition started and stopped, which has suggested the existence of a subsurface region with higher spin density than that in the bulk region.
Structural Properties of a-Ge1-xCx:H Alloys Prepared by the RF Sputtering Technique
Authors: Marques F. C. , Vilcarromero J. , Freire F. L.
Published: April 2011

Structural and mechanical properties of hydrogenated amorphous germanium carbon (a-Ge1-xCx:H) alloys are presented. The films were prepared by the rf-co-sputtering technique using a graphite/germanium composed target. The carbon and germanium relative concentrations were determined by RBS, and the total hydrogen concentration by ERDA measurements. An increase in the optical gap was measured for low carbon content (0 < × < 0.15). For higher values of x the optical gap is almost constant. Infrared transmission absorption spectra show several absorption bands related to Ge-C stretching, C-Hn (n = 1,2,3) and Ge-H stretching and bending modes. The mechanical internal stress was strongly affected by the incorporation of carbon. The trends of the optical gap, refractive index, infrared absorption and mechanical stress as a function of the carbon content suggest that the high carbon concentration alloys have polymeric and/or graphite-like contribution in their structure.
Defect Formation Mechanism During PECVD of a-Si:H
Authors: Maeda Keiji , Umezu Ikurou
Published: April 2011

Defect formation mechanism in a-Si:H during PECVD at substrate temperature below 250°C is considered to be breaking of weak bonds in the Urbach tail. To break weak bonds, an extra energy is necessary. This energy is supplied by the reaction energy of SiH3 precursor at the growing surface incorporating SiH2 into the network. The defect density is experimentally shown to be proportional to a product of the energy supply frequency, i.e., SiH2 density, and the weak bond density which is obtained by the Urbach energy. By analysis using the configurational coordinate diagram the energy level of the broken weak bond is determined to be 0.2 eV above the valence band mobility edge. There is similarity of the defect formation mechanism during deposition to that of the Staebler-Wronski effect.
High-Rate Deposition of a-Si:H Films in 55 kHz Glow Discharge: Growth Mechanisms and Film Structure
Authors: Budaguan B. G. , Aivazov A. A. , Sazonov A. YU , Popov A. A. , Berdnikov A. E.
Published: April 2011

The 55 kHz GD technique allowed the high-rate deposition of a-Si:H films (up to 30 A/s) with good electronic properties in spite of inhomogeneous structure. In this work, we investigated the growth mechanisms and the correlation between plasma parameters and film structure. The electrical parameters of discharge, the properties of plasma and of the films were investigated with using of wide range of methods (optical emission spectroscopy, mass-spectrometry, IR-spectroscopy, atomic force microscopy).

The films were deposited in an industrial reactor of diode type with both non-grounded electrodes used as substrate holders. The deposition mechanism in this case includes the alternation of growth phases when the negatively charged particles reach the anode (at a moment) during the first half-period of an oscillation of electric field, and the improvement of growing surface under the influence of the bombardment by positive ions during the second half-period. It is shown that the increase of the power increases the ionic flux leading to the increase of the size of structural inhomogeneities. The pressure influences the energy of positive ions, and, therefore, the hydrogen content and its bonding form on the growing surface.
Ion Bombardment and Disorder in Amorphous Silicon
Authors: Sidhu L. S. , Gaspari F. , Zukotynski S.
Published: April 2011

The effect of ion bombardment during growth on the structural and optical properties of amorphous silicon are presented. Two series of films were deposited under electrically grounded and positively biased substrate conditions. The biased samples displayed lower growth rates and increased hydrogen content relative to grounded counterparts. The film structure was examined using Raman spectroscopy. The transverse optic like phonon band position was used as a parameter to characterize network order. Biased samples displayed an increased order of the amorphous network relative to grounded samples. Furthermore, biased samples exhibited a larger optical gap. These results are correlated and attributed to reduced ion bombardment effects.
Amorphous Silicon-Carbon Alloys and Amorphous Carbon from Direct Methane and Ethylene Activation by ECR
Authors: Conde J. P. , Chu V. , Giorgis F. , Pirrt C. F. , Arekat S.
Published: April 2011

Hydrogenated amorphous silicon-carbon alloys are prepared using electron-cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition. Hydrogen is introduced into the source resonance cavity as an excitation gas. Silane is introduced in the main chamber in the vicinity of the plasma stream, whereas the carbon source gases, methane or ethylene, are introduced either with the silane or with the hydrogen as excitation gases. The effect of the type of carbon-source gas, excitation gas mixture and silane-to-carbon source gas flow ratio on the deposition rate, bandgap, subgap density of states, spin density and hydrogen evolution are studied.
Microcrystalline Silicon Germanium: An Attractive Bottom-Cell Material for Thin-Film Silicon-Based Tandem-Solar-Cells
Authors: Ganguly Gautam , Ikeda Tom , Kajiwara Kei , Matsuda Akihisa
Published: April 2011

We have prepared hydrogenated macrocrystalline silicon germanium by plasma enhanced CVD of a mixture of silane and germane gas diluted with hydrogen. The growth conditions have been systematically controlled to obtain large (∼400Å) crystallites of silicon-germanium as observed using Raman scattering and x-ray diffraction. The dangling bond (germanium) density has been reduced to <5×1016 cm−3 at low substrate temperatures (∼150°C). The optical absorption spectra of the 50% Ge containing material is red-shifted compared to microcrystalline silicon, consistent with a reduction of the indirect optical gap to 0.9eV. Schottky type cells fabricated using Au on an n+ crystalline silicon substrate confirm that the long wavelength response is remarkably enhanced in this material.
Improved Amorphous Silicon Alloy Solar Cells for Module Fabrication
Authors: Banerjee A. , Yang J. , Guha S.
Published: April 2011

An initial conversion efficiency of 13.5% has been obtained on a triple-junction triple-bandgap device fabricated in a large-area deposition reactor capable of producing one-square-foot modules. The intrinsic layer of the top cell is a wide bandgap amorphous silicon alloy. The middle and bottom cells employ high quality amorphous silicon-germanium alloy. The high efficiency of the triple-junction cell is attributed to the relative reduction of the optical loss in the top tunnel junction and the improvement in the quality of the middle and bottom component cells. Triple-junction devices with initial efficiency of 13.3% have shown saturation at 11.6% after light soaking. Modules of aperture area 909cm2 have been fabricated using an assembly process similar to the one being currently used in our manufacturing line. The module design consists of onelarge-area, high-current monolithic multijunction device. The status of the small-area devices andmodules is described
Space-Charge-Limited Currents in n-i-n Devices Incorporating Glow-Discharge and Hot-Wire Deposited a-Si:H
Authors: Molenbroek Edith C. , Van Der Werf C. H. M. , Feenstra K. F. , Rubinelli F. , Schropp R. E. I.
Published: April 2011

Space-charge-limited currents have been examined in a wide variety of n-i-n devices. If the devices were completely symmetric, the current-voltage characteristics would be identical for positive and negative bias, but in several devices differences between the two polarities were observed. In order to understand in which part of the device these differences originate, the influence of the contacts and interfaces on the JV characteristics were examined by using different metal top contacts, different n-layers and different i-layers. Ag and Al top contacts gave minor differences between the polarities, whereas with Cr contacts no differences were observed. Incorporation of a defect layer in the i-layer results in asymmetric JV curves. We have observed a small asymmetry in an experimental device, and a large asymmetry using AMPS modeling. N-i-n devices appear to be a sensitive probe for interface defects.
Photocapacitance and Hole Drift Mobility Measurements in Hydrogenated Amorphous Silicon (a-Si:H)
Authors: Nurdjaja Indra , Schiff E. A.
Published: April 2011

We present measurements of the photocapacitance in hydrogenated amorphous silicon (a-Si:H) Schottky barrier diodes under reverse bias. A calculation relating photocapacitance to hole drift mobility measurements is also presented; the calculation incorporates the prominent dispersion effect for holes in a-Si:H usually attributed to valence bandtail trapping. The calculation accounts quantitatively for the magnitude and voltage-dependence of the photocapacitance.
Al/ZnO/a-SiGe:H: A System Protected by the ZnO Buffer from Metal-Induced Crystallization
Authors: Edelman F. , Brener R. , Cytermann C. , Weil R. , Beneking C. , Beyer W. , Skorupa W. , Yankov R. , Werner P.
Published: April 2011

Metal-Induced-Crystallization (MIC) by the contact of amorphous semiconductors with metals is one of the degradation factors in solar cells. This study has been made on the barrier properties of a ZnO layer between undoped a-SiGe:H and Al metallization films in the structure (001)Si/SiO2/a-SiGe:H/ZnO/Al. Plasma assisted CVD deposition was used to produce a-Si1.xGex:H (x=0 to 1) undoped films over thermally oxidized Si-wafers. There were covered with 500Å and 1000Å thick transparent conductive layers of ZnO. Al and then 1000Å thick films of Al. A set of Al-implanted a-Si, a-Ge, and a-Sio.5Geo.5 films on Si/SiO2 substrates was also prepared to study MIC in an amorphous system with dispersed Al. The structures were annealed in vacuum in the temperature range of 200°C to 400°C for lh. X-ray diffraction studies demonstrated the a-SiGe:H stability against crystallization under ZnO protection up to 400°C. Secondary Ion Mass Spectroscopy didn't reveal any noticeable redistribution of Al inside Al-implanted a-Si:H and a-Si0.5Ge0.5:H samples after annealing at 400°C for lh, but strong Al diffusion was seen in the a-Ge:H layer. Nevertheless, no MIC was observed in any of the Al-implanted a-materials.
Impact of Hydrogen Dilution on the Properties and Light Induced Changes of A-SI:H Based Materials and Solar Cells
Authors: Lee Yeeheng , Jiao Lihong , Koh Joohyun , Fujiwara Hiroyuki , Lu Zhou , Collins R. W. , Wronski C. R.
Published: April 2011

Studies have been carried out on a-Si:H materials and corresponding solar cells fabricated with and without hydrogen dilution of silane by rf PECVD. The effect of hydrogen dilution on the growth kinetics and microstructures and their dependence on the substrate temperature have been studied. Hydrogen diluted a-Si:H materials and solar cells exhibit improved properties and higher stability to light induced changes. Distinct differences are found in the electron mobility lifetime (μτ) products and subgap absorption over a wide range of generation rates. Striking differences are also found in the kinetics of light induced degradation in both the materials and their corresponding solar cells. Direct correlations are presented between the degradation kinetics of p(a-SiC:H)/i(a-Si:H)/n(μc-Si) solar cells and those of thin film materials constituting the i-layers.
Energy Dependence of Defects in a-Si:H Solar Cells During Degradation and Annealing Processes
Authors: Caputo Domenico , Lemmi Francesco , Palma Fabrizio
Published: April 2011

In this work we report on the effect of current-induced degradation and annealing on p-i-n amorphous silicon solar cells. Current-voltage curves and capacitance measurements under forward bias have been used to monitor the current-induced changes as a function of time. We found that the recovery rate increases with the annealing current, while the stabilized value of efficiency decreases. Comparison of short circuit current and capacitance evolution suggests that defect kinetics in the electronic gap occurs in a different way during degradation and annealing. This behavior can be modeled assuming a faster annealing of defects closest to the extended band and a slower annealing of mid-gap defects.
The Effect of the Amorphous Silicon Alpha-Gamma Transition on Thin Film Transistor Performance
Authors: French I D , Deane S C , Murley D T , Hewett J , Gale I G , Powell M J
Published: April 2011

We have investigated the effect of depositing amorphous silicon (a-Si:H) under α and γ (or dusty) plasma conditions on hydrogen bonding and TFT performance. By infrared measurements three deposition regimes could be identified due to their distinctive absorption curves in the bending mode range of 800 to 900 cm'1. These corresponded to a films deposited below 300°C, γ films deposited below 300°C, and films deposited in either plasma condition at temperatures of 300°C and above. There was a correlation between the a-Si:H material regimes and TFT performance. When the deposition temperature was below 300°C the a-TFTs had a higher field effect mobility than γ-TFTs, but lower stability. For deposition temperatures of 300°C and above the films had more similar properties regardless of whether they were deposited under a or γ conditions. TFT mobilities were the same, but TFTs containing a-Si:H deposited under γ conditions were still more stable. These results show that the mobility and stability of TFTs are optimised for different growth conditions, and that the overall best conditions for TFT manufacture depends upon the specific application.
The Impact of Field Plate Resistivity on the Performance of a Novel High Voltage Thin Film Transistor Incorporating a Semi-Insulating Layer
Authors: Chen Y. , Clough F. J. , Sankara Narayanan E. M. , Xu Y. Z. , Eccleston W. , Milne W. I.
Published: April 2011

The Semi-Insulating field plated High Voltage TFT (SI HVTFT) is a new poly-Si HVTFT structure with a much improved blocking capability and enhanced on-state performance [1][2]. The unique feature of the SI HVTFT is the semi-insulating amorphous oxygen-doped Si (SIAOS) field plate which connects the gate to the drain and reshapes the potential distribution in the offset region. The leakage current, flowing through the SIAOS field plate during device operation, determines device performance. In this paper, both experimental and 2-D simulation results for devices with different field plate conductivities are used to investigate the impact of the field plate properties on the performance of the SI HVTFT structure.
Microstructures of GaN Films Grown on a LiGaO2 New Substrate by Metalorganic Chemical Vapor Deposition
Authors: Li Jing-Hong , Kryliouk Olga M. , Holloway Paul H. , Anderson Timothy J. , Jones Kevin S.
Published: April 2011

Microstructures of GaN films grown on the LiGaO2 by metalorganic chemical vapor deposition (MOCVD) have been characterized by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). TEM and HRTEM results show that high quality single-crystal wurtzite GaN films have been deposited on the LiGaO2 and that the GaN film and the LiGaO2 have the following orientation relationship: (2110)(0002)GaN ̂ (002)LiGaO2 ^ 5–8°. A higher density of threading dislocations and stacking faults have been observed near the GáN/LiGaO2 interface, even though the lattice mismatch of GaN to LiGaO2 is only ∼1%. Threading dislocations with burgers vector b=<0001> and b=a/3<1120> are predominant in the GaN films. Also the GaN films contain some columnar inversion domain boundaries (IDBs). Both TEM and HRTEM results reveal that there is an unexpected amorphous or nano-crystalline inter-layer between the GaN and the LiGaO2 with a thickness of 50–100 nm.