bluuebunny/crossref_metadata_2025 huggingface dataset:
Compositionally Dependent Band Offsets In Aln/AlxGa1-xN Heterojunctions Measured By Using X-Ray Photoelectron Spectroscopy
Authors: Beach R.A. , Piquette E.C. , Grant R.W. , McGill T.C.
Published: April 2011

Although GaN has been extensively studied for applications in both light emitting and high power devices, the AlN/GaN valence band offset remains an area of contention. Values quoted in the literature range from 0.8eV (Martin)[1] to 1.36eV (Waldrop)[2]. This paper details an investigation of the AIN/AlxGa1-xN band offset as a function of alloy composition. We find an AlN/AlxGa1-xN valence band offset that is nearly linear with Al content and an end point offset for AlN/GaN of 1.36 ± 0.1 eV. Samples were grown using radio frequency plasma assisted molecular beam epitaxy and characterized with x-ray photoelectron spectroscopy(XPS). Core-level and valence-band XPS data for AIN (0001) and AlxGa1-xN (0001) samples were analyzed to determine core-level to valence band maximum (VBM) energy differences. In addition, oxygen contamination effects were tracked in an effort to improve accuracy. Energy separations of core levels were obtained from AlN/AlxGa1-xN(0001) heterojunctions. From this and the core-level to valence band maximum separations of the bulk materials, valence band offsets were calculated.
Photoemission Study of The Electronic Structure of Wurtzite Gan(0001) Surfaces
Authors: Smith Kevin E. , Dhesi Sarnjeet S. , Stagarescu Cristian B. , Downes James , Doppalapudi D. , Moustakas Theodore D.
Published: April 2011

The surface electronic structure of wurtzite GaN (0001) (1 × 1) has been investigated using angle-resolved photoemission spectroscopy. Surfaces were cleaned by repeated cycles of N2 ion bombardment and annealing in ultra-high vacuum. A well-defined surface state below the top of the valence band is clearly observed. This state is sensitive to the adsorption of both activated H2 and O2, and exists in a projected bulk band gap, below the valence band maximum. The state shows no dispersion perpendicular or parallel to the surface. The symmetry of this surface state is even with respect to the mirror planes of the surface and polarization measurements indicate that it is of spz character, consistent with a dangling bond state.
An Investigation of the Electron Escape Time within a Biased AlGaN/GaN Quantum Well
Authors: Lefebvre Kevin R. , Anwar A. F. M.
Published: April 2011

In this paper, the electron escape time from an Al.25Ga.75N/GaN/Al.25Ga.75N quantum well system is calculated as a function of electric field. The calculation includes the piezoelectric field induced by the material system, the continuous density of states within a quantum well as a result of the applied electric field, the change in the group velocity and the proper partitioning of between the tunneling and thermionic emission currents. The calculation of the escape time is achieved by solving Schroedinger equation, through the logarithmic derivative of the wavefunctions, and Poisson's equation self-consistently.
Acceptor Binding Energies in GaN and AIN
Authors: Mireles Francisco , Ulloa Sergio E.
Published: April 2011

We present binding energy calculations for Mg, Zn, and C substitutional shallow acceptors in GaN and AIN for both, wurtzite (WZ) and zincblende (ZB) crystal phases. The calculations are performed within the effective mass theory through the 6 × 6 Rashba-Sheka-Pikus and the Luttinger- Kohn matrix Hamiltonians for WZ and ZB bulk crystals, respectively. An analytic representation for the pseudopotential is used to introduce the nature of the impurity atom. The energy shift due to polaron effects is also considered in this approach. The estimated ionization energies are in good agreement with those reported experimentally and those reported theoretically employing other methods. We find that the binding energies for ZB GaN acceptors are shallower than the corresponding impurities in the WZ crystalline phase. The binding energy dependence upon the crystal field splitting in the WZ compounds is analyzed.
Uniaxial Stress Effects On Valence Band Structures Of GaN
Authors: Yamaguchi A. A. , Mochizuki Y. , Sasaoka C. , Kimura A. , Nido M. , Usui A.
Published: April 2011

Valence band modification by uniaxial stress in GaN is investigated by reflectance spectroscopy. It is observed that the energy separation between the A and B valence bands increases with the applied uniaxial stress in the c-plane. Changes of the wavefunctions by the stress are also investigated by the polarization characteristics of the reflectance spectra. The experimental results are analyzed on the basis of k•p theory, and deformation potential D5 is experimentally determined as -3.3 eV. It is indicated that the uniaxial strain effect could be utilized for improving GaN-based laser performance.
Theory of Ga, N and H terminated GaN (0001)/(0001) surfaces
Authors: Eisner J. , Haugk M. , Gutierrez R. , Frauenheim Th.
Published: April 2011

We present a theoretical study of atomic structures, electrical properties and formation energies for a variety of possible reconstructions with 1×1 and 2×2 periodicity of the GaN(0001) and (0001) surfaces. We find that during MBE growth in the (0001) direction 2×2 structures become stable under N rich growth conditions while Ga rich environment should yield structures with 1×1 periodicity. Considering MBE growth on (0001) surfaces, among the investigated structures only those with 1×1 periodicity are predicted to be stable. During MOCVD growth, where H terminated surfaces may occur, only structures with lx1 periodicity are found to be stable for both growth directions.
Selective Area Etching of GaN and AlGaN by Thermally Chemical Reaction in Hydrogen Ambient
Authors: Hiramatsu K. , Matsushima H. , Hanai H. , Sawaki N.
Published: April 2011

The selective area etching of wurtzite GaN (0001) and AlGaN (0001) with SiO2 masks is investigated for different temperatures of 800 to 1 100°C and different ambient gases of H2, N2 and Ar including NH3. The etching rate of GaN increases with increasing annealing temperature under H2 ambient. This etching is attributed to the chemical reaction between Ga-N and H2. On the other hand, the etching of GaN does not occur in the N2 or Ar ambient gas. The NH3 gas in H2 ambient suppresses the chemical reaction, while the NH3 gas in N2 enhances it. The surface etching of Al0.1Ga0.9N is not observed even in H2 ambient.
Silicon Carbide And Gallium Nitride Rf Power Devices
Authors: Weitzel C. E. , Moore K. E.
Published: April 2011

Impressive RF power performance has been demonstrated by three radically different wide bandgap semiconductor power devices, SiC MESFET's, SiC SIT's, and AlGaN HFET's. AlGaN HFET's have achieved the highest fmax 97 GHz. 4H-SiC MESFET's have achieved the highest power densities, 3.3 W/mm at 850 MHz (CW) and at 10 GHz (pulsed). 4H-SiC SIT's have achieved the highest output power, 450 W (pulsed) at 600 MHz and 38 W (pulsed) at 3 GHz. Moreover a one kilowatt, 600 MHz SiC power module containing four multi-cell SIT's with a total source periphery of 94.5 cm has been demonstrated.
Growth Control And Characterization Of Wide Band Gap Silicon-Carbon Films
Authors: Kerdiles S. , Rizk R. , Pérez-Rodríguez A. , Garrido B. , González-Varona O. , Calvo-Barrio L. , Morante J.R.
Published: April 2011

Silicon-carbon films have been grown by reactive hydrogen magnetron sputtering at a substrate temperature of 730°C, with different values of carbon-to-silicon sputtered area ratio, rc. The layers were investigated by infrared spectroscopy, Raman scattering, x-ray photoelectron spectroscopy and optical absorption. For rc below 30%, almost only Si nanocrystallites were formed with a few fraction of amorphous SiC, whereas for rc exceeding 30%, a drastic change was noticed, leading to the achievement of SiC crystals in the layers. These latter were found of near-stoichiometric composition with an atomic ratio C/Si ˜1.04. The results suggest that the excess C is of graphitic-like configuration being likely located in the intergrain regions, in addition to some silicon-oxygen bonds. These features are accompanied by an abrupt widening of the band gap in the transition region that is consistent with the formation of SiC nanocrystals. The large value measured for the band gap (≥3 eV) is thought to be due to more than one origin, such as size effect of SiC, Si-O bonds and possible presence of different SiC polytypes.
A Novel Technique For Rtp Annealing Of Compound Semiconductors
Authors: Fu M. , Sarvepalli V. , Singh R. K. , Abernathy C. R. , Cao X. , Pearton S. J. , Sekhar J. A.
Published: April 2011

We introduce for the first time a novel rapid thermal processing (RTP) unit called ZapperTM, which has recently been developed by MHI Inc. and the University of Florida for high temperature thermal processing of semiconductors. This ZapperTM unit is capable of reaching much higher temperatures (>1500 °C) than conventional tungsten-halogen lamp RTP equipment and achieving high ramp-up and ramp-down rates. We have conducted implant activation annealing studies of Si+-implanted GaN thin films (with and without an AIN encapsulation layer) using the ZapperTM unit at temperatures up to 1500 °C. The electrical property measurements of such annealed samples have led to the conclusion that high annealing temperatures and AIN encapsulation are needed for the optimum activation efficiency of Si+ implants in GaN. It has clearly been demonstrated that the ZapperTM unit has tremendous potential for RTP annealing of semiconductor materials, especially for wide bandgap compound semiconductors that require very high processing temperatures.
Electronic Properties of Wide Bandgap Materials
Authors: Yater J.E. , Shih A. , Abrams R.
Published: April 2011

Secondary electron emission spectroscopy is used to investigate the generation and transport of impact-ionized electrons in wide bandgap material. Secondary electron yield and energy distribution measurements from C(100) and CVD diamond samples are analyzed to obtain information about the internal gain and electron energy distribution following impact ionization, as well as the effects of the transport process on the internal electron distribution. By studying the emission from surfaces having a negative electron affinity (NEA), the total transmitted intensity and the full energy spectrum of the internal electrons are revealed in the measurements. Energy spectra measured from the diamond samples contain a low-energy peak whose energy position and width are independent of incident beam energy. This suggests that the peak represents the electron distribution produced by impact-ionization events. A large percentage of the total emitted electrons lie within this peak, indicating that the impact-ionization process is very efficient at generating low-energy electrons. Very high yields are measured from both samples, establishing the presence of high internal gain and efficient electron transport in the material. From the linear slope of the yield curves, the escape depth of the low-energy electrons is deduced to be much larger than ˜ 0.1 μm in both diamond samples.
Metalorganic Chemical Vapour Deposition of CuInSe2 From Copper and Indium Diselenocarbamates for Solar Cell Devices
Authors: McAleese John , O'Brien Paul , Otway David J.
Published: April 2011

Thin film(s) of chalcopyrite CulnSe2 have been grown by low-pressure metal-organic chemical vapour deposition (LP-MOCVD) using the precursors In(Se2CNMenHexyl)3 and precursors Cu(Se2CNMenHexyl)2. The precursors were prepared from carbon diselenide. Films were grown on glass between 400 – 450 °C, and characterized by X-ray diffraction, optical spectroscopy (UV/Vis), EDAX and scanning electron microscopy.
Surface Morphology of LPE SiGe Layers Grown on (100) Si Substrates
Authors: Sembian A. M. , Silier I. , Davies K. , Gutjahr A. , Lyutovich K. , Konuma M. , Banhart F.
Published: April 2011

We have investigated the surface morphology of thick SiGe layers grown on Si(100) substrates. SiGe layers containing different Ge concentrations (from 0 to 16 at.%) and having thickness of about 15μm are prepared by liquid phase epitaxy (LPE) method using various growth conditions. The wavelength of undulation of SiGe layers is found to be increasing when we adopt low cooling rates during LPE process. The roughness of the layer does not show any significant change with cooling rate.
Mocvd as a Technique for the Growth of CdTe/CdS Solar Cells
Authors: Berrigan R. A. , Irvine S. J. C. , Stafford A. , Ellis D. , Colehamilton D. J.
Published: April 2011

The MOCVD growth of cadmium sulphide on indium tin oxide (ITO) coated glass is investigated in this work. Optimum transmission characteristics and grain size are observed for films grown at 290° C at VI/MI ratios of 1–1.5. It is postulated that the nucleation delay observed during in situ growth monitoring via laser interferometry s a critical step in acieving these results. The nucleation delay is noted to vary with temperature and also VI/II ratio, allowing the rationalisation of the resulting film quality in terms of this observation. Structural analysis indicates polycrystalline CdS of hexagonal phase of (100) preferred orientation. Results of preliminary doping studies indicate that CdS can be n-type doped during growth with nbutylchloride.
Preparation And Photovoltaic Properties Of Tin Sulfide And Tin Oxysulfide Thin Films By Spray Pyrolysis Technique
Authors: Kosugi Tsuyoshi , Murakami Kenji , Kaneko Shoji
Published: April 2011

Tin sulfide and a related novel compound, tin oxysulfide, thin films were deposited from the aqueous solution containing tin (11) chloride and thiourea by a spray pyrolysis technique. The constituent atoms of the thin films have merits of nontoxicity and availability. Physical and photovoltaic properties of both films were also investigated. It has, therefore, been considered that these two compounds become a promising absorbing layer for solar cell on account photovoltaic ability with high absorption coefficient and appropriate bandgap energy.
Development of High Efficiency Thin Film Polycrystalline Silicon Solar Cells using Vest Process
Authors: Ishihara T. , Arimoto S. , Morikawa H. , Nishimoto Y. , Kawama Y. , Takami A. , Hamamoto S. , Naomoto H. , Namba K.
Published: April 2011

Thin film Si solar cell has been developed using Via-hole Etching for the Separation of Thin films(VEST) process. The process is based on SOI technology of zone-melting recrystallization (ZMR) followed by chemical vapor deposition (CVD), separation of thin film, and screen printing. Key points for achieving high efficiency are (1)quality of Si films, (2)back surface emitter (BSE), (3)front surface emitter etch-back process, (4)back surface field (BSF) layer thickness and its resistivity, and (5)defect passivation by hydrogen implantation. As a result of experiments, we have achieved 16% efficiency(Voc:0.589V, Jsc:35.6mA/cm2, F.E:0.763) with a cell size of 95.8cm2 and the thickness of 77μm. It is the highest efficiency ever reported for large area thin film Si solar cells.
Deposition of Thick Silicon Layers on Glass Substrate for Photovoltaic Application
Authors: Sarret M. , Mohammed-Brahim T. , Baudry G. , Briand D. , Bonnaud O.
Published: April 2011

A new silicon deposition reactor fabricated in our laboratory is presented in this paper. It works at Sub-Atmospherical total pressure, so it is a SAPCVD reactor. The deposition rates and electronic transport properties are compared with a conventional LPCVD reactor.
Polycrystalline Si Films Fabricated by Low Temperature Selective Nucleation and Solid Phase Epitaxy Process
Authors: Chen Claudine M. , Atwater Harry A.
Published: April 2011

With a selective nucleation and solid phase epitaxy (SNSPE) process, grain sizes of 10 μm have been achieved to date at 620°C in 100 nrm thick silicon films on amorphous SiO2, with potential for greater grain sizes. Selective nucleation occurs via a thin film reaction between a patterned array of 20 rnm thick indium islands which act as heterogeneous nucleation sites on the amorphous silicon starting material. Crystal growth proceeds by lateral solid phase epitaxy from the nucleation sites, during the incubation time for random nucleation. The largest achievable grain size by SNSPE is thus approximately the product of the incubation time and the solid phase epitaxy rate. Electronic dopants, such as B, P, and Al, are found to enhance the solid phase epitaxy rate and affect the nucleation rate.
Low-Temperature Sputter-Deposition of Poly-Crystal Silicon Thin-Films
Authors: Yeh Wen-Chang , Matsumura Masakiyo
Published: April 2011

Poly-Si films were deposited at temperatures as low as 325°C by a newly proposed method. In the proposed method, both Ni-enhanced crystallization effects and vapor-phase deposition were utilized to reduce the deposition temperature. The X-ray diffraction measurement shows that the films were (110)-oriented at temperatures among 325°C to 419°C. When the partial pressure of hydrogen gas in the deposition chamber exceeds 0.1%, crystallization was disrupted. The characteristics of TFTs and pn junction diodes showed its potential applicability to AMLCDs and sollar cells.
Tailored Si-Layers on Silicon Oxide Obtained by Thermal CVD
Authors: Beaucarne G. , Poortmans J. , Caymax M. , Nijs J. , Mertens R.
Published: April 2011

In this paper, a method to obtain by CVD Si layers on silicon oxide with the desired grain size is described, involving nucleation control through the growth parameters. Nucleation experiments are carried out with a hydrogen - dichlorosilane - HCl ambient at high temperature. The nucleus density is observed to drop to low values at a threshold HCl-flow. A qualitative explanation using concepts from atomistic nucleation theory is proposed. The effect of addition of diborane to the gas flow is investigated and appears to be small or non-existent . Finally, preliminary results of a thin-film crystalline silicon solar cell process applied on such layers are given to illustrate the potential of such layers.
Mbe Growth And Characterization Of Er/O And Er/F Doped Si Light Emitting Structures
Authors: Ni W.-X. , Du C.-X. , Joelsson K. B , Hansson G. V. , Pozina G. , Buyanova I. A. , Chen W. M. , Monemar B.
Published: April 2011

By using a low temperature growth process, Er doping in Si during MBE using Er2O3 or ErF3 as dopant sources has achieved a level of 5 × 1019 cm−3 without precipitation and generation of other extended defects. Luminescence properties of these Er-doped MBE Si structures have been extensively studied using both photon and hot electron impact excitation at a wide range of temperatures (2–300 K). It has been found that by incorporating C into Er/O doped layers, the room temperature EL emission with a FWHM value of 14 meV was ten times more intense than that with lower C doping. Post thermal annealing gave a strong effect on Er/F doped layers, leading to a 7-fold increase of the highest peak intensity while the peak line width reduced to 0.12 meV, which is very important for laser applications.
Resonant Excitation Spectroscopy of Light-Emitting Silicon Nanostructures
Authors: Kanemitsu Yoshihiko , Okamoto Shinji
Published: April 2011

We have studied optical properties and electronic structures of zero-dimensional (OD) Si nanocrystals and two-dimensional (2D) Si quantum wells by means of resonant excitation spectroscopy. Resonantly excited luminescence spectra of 0D Si nanocrystals are sensitive to the surface structure. The size dependence of the photoluminescence (PL) properties and resonantly excited PL spectra of SiO2-capped Si nanocrystals indicate that excitons are localized at the interface between the c-Si and SiO2 surface layer. The TO-phonon related structure in resonantly excited luminescence is clearly observed in H-passivated Si nanocrystals. Hpassivated Si nanocrystals show their crystalline nature, while the oxidized Si nanocrystals show their disordered nature. Luminescence properties of Si/SiO2 nanocrystals are similar to those of Si/SiO2 quantum wells. The electronic structures and luminescence properties of Si nanostructures are discussed.
Optical Activation of Erbium Doped Porous Silicon by Hydrogen Plasma Treatment
Authors: Dejima T. , Saito R. , Yugou S. , Isshiki H. , Kimura T.
Published: April 2011

Er3+;-doped porous silicon (Er:PS) shows strong room temperature emissions at ˜ 1.54μm. However, its spectrum is usually much broader than that of Er-doped crystalline silicon (fullwidth at half maximum - FWHM - is ˜ 10 nm). It is probably because Er ions are located in amorphous phases. We report in this paper that strong and very sharp Er3+ 1.54μm emissions are obtained, when Er:PS samples are treated in a hydrogen plasma. Porous silicon layers are formed by anodic etching and then doped with Er3+ ions in an ErCl3/ethanol solution by an electrochemical method, and then treated in a hydrogen plasma at ˜ 1000°C from 0.5 min to 90 min for the optical activation. Several sharp peaks are observed at 20K, of which the strongest peak is located at 1.538 μm with an FWHM less than 1 nm. This value is comparable to that obtained from Er3+-doped crystalline silicon formed by means of molecular beam epitaxy (MBE) or ion implantation. Comparisons are made among hydrogen plasma, argon plasma, H2 flow and vacuum for the post-dope annealing atmosphere. Fourier-transform infrared (FT-IR) absorption and secondary ions mass spectrometry (SIMS) measurements are also carried out. We conclude that preferential etching of amorphous surface layers, and termination of dangling bonds of silicon nanocrystallites with hydrogen atoms and formation of Er-H complexes may be responsible for the strong and sharp Er3+-related luminescence.
Infrared Reflection Spectroscopy Of As-Anodized And Passivated 6H And 4H Porous Silicon Carbide
Authors: Spanier Jonathan E. , Herman Irving P.
Published: April 2011

We present a study of the infrared reflectance of porous silicon carbide (PSC) formed by the electrochemical dissolution of silicon carbide substrates of both 6H and 4H polytypes. The formation of porous silicon carbide from a 4H-SiC substrate is reported for the first time, as is the reflectance from n-PSC, both as-anodized and passivated. The passivation of PSC has been accomplished using a short thermal oxidation. Fourier transform infrared (FTIR) reflectance spectroscopy is employed ex situ after different stages of the thermal oxidation process. The characteristics of the reststrahlen band normally observed in bulk SiC are altered by anodization; further changes in the reflectance spectra occur following oxidation for different short periods of time. An effective medium theory model which includes air, SiC and SiO2 as component materials is shown to characterize the observed changes in the reflectance spectra after different stages of PSC oxidation.
Ion-Beam Synthesized Semiconducting β-FeSi2 Controlled By Annealing Procedures And Phase-Transitions
Authors: Maeda Y. , Fujita T. , Akita T. , Umezawa K. , Miyake K.
Published: April 2011

The ion beam synthesis (IBS) of β-FeSi2 was examined by Rutherford backscattering spectroscopy (RBS) and x-ray diffractometry (XRD), and the structural characterization was carried out by Raman spectroscopy and scanning electron microscopy (SEM). We found that the IBS of β- FeSi2 is controlled by two different processes depending on the annealing temperature (Ta) and Fe surface concentration (Cs); (I) precipitation of β-FeSi2 on the surface in Cs˜30 at% and Ta⩾700° C and (II) phase transition from γ -FeSi2 to β-FeSi2 in Cs<∼20 at%and Ta⩾600°C. The precipitation process(1) created large sized (-10 μm) polycrystalline grains of β-FeSi2. The good crystalline β-FeSi2 obtained above 800°C showed a clear reflectance maximum at 0.88 eV due to the optical transition at the direct band-gap of 0.84 eV observed in the characteristic plot ((ahv)2 vs. hv) of the optical absorption.
Integrated Optic Devices in Silicon and Silicon-on-Insulator Materials
Authors: Schüppert B. , Petermann K.
Published: April 2011

This paper reviews various integrated optics techniques and devices in the silicon material system. Different approaches for waveguides, passive and active components in silicon and the performance of such devices will be discussed.
First-Principles Study Of Photoluminescence From Silicon/Silicon-Oxide Interfaces
Authors: Kageshima H. , Shiraishi K.
Published: April 2011

Experimentally reported interfacial luminescence from silicon nano structures are studied by using a first-principles calculation of a Si(100) quantum slab covered with silicon oxide. When Si-OH bonds were introduced at the silicon/silicon-oxide interface, wave functions at the valence band top and near conduction band bottom localized vertically and laterally near the Si-OH bonds. Such strong localization is the result of the cooperation of the coupling of non-bonding 2p lone pair orbitals on interfacial 0 atoms and the strong dipole of OH. Furthermore, the localization is significant only in silicon nano structures. This localization of the wave functions can be the source for creating localized excitons, which can dramatically enhance the intensity of photoluminescence. Therefore, interfacial Si-OH bonds are a theoretically convincing possible source of reported interfacial luminescence.
Photoelastic Waveguides In Sige/Si Heterostructures And Bulk Si
Authors: Lea E. , Weiss B. L. , Rho H. , Jackson H. E.
Published: April 2011

Photoelastic waveguides in bulk Si and SiGe/Si heterostructures have been modelled and characterised. The calculated transverse strain profiles of photoelastic waveguide structures in SiGe/Si heterostructures and bulk silicon are in good agreement with those obtained by microRaman experiments. The waveguide characteristics are also found to be in good agreement with those obtained from the strain modelling and demonstrate that low loss waveguides can be fabricated using these structures
Quasi-Two-Dimensional Smectic States of DNA Molecules Intercalated between Lipid Membranes in Multi-Lamellar Phases
Authors: Golubović Leonardo , Moldovan Dorel , Golubović Mirjana
Published: April 2011

We study fluctuations of DNA-cationic lipid complexes in their lamellar membrane phases with DNA intercalated between lipid membranes. We theoretically elucidate this novel state of matter by characterizing it as the very first realization of adecoupled(unregistered) phase of strongly fluctuating 2-d smectic manifolds weakly interacting across membranes. Due to couplings between adjacent 2-d smectic Lx, × Lyplanes, the experimentally observed ordinary 2-d smectic behavior [Salditt et al., Phys. Rev. Lett. 79, 2582 (1997)] of DNA in-plane undulations, with, must cross over, at the longest scales, to a novel fluctuation behavior, with  ˜ (logLy)2˜ (logLx.)2.
Self-Assembly of Tyrocidines in Nanotubular Structures
Authors: Thies M. , Paradies H. H.
Published: April 2011

Hollow tubular structures of tyrocidines, a cyclic decapeptide with two amino acids residues in the D-configuration and eight in the L-configuration, were formed in acid aqueous solutions in the presence of ethanol (20°C). Discrete nanotubes can be produced under controlled conditions, e.g. the amount of water added, or in the presence of protons from ethanol-water solutions (45/55 %v/v). These cyclic oligopeptides crystallize into tubular structures hundreds of nanometer long with an internal diameter of 9.0 Å and outer diameter of 35 Å. Support for the proposed tubular structure is provided by electronmicroscopy, SAXS and Fourier-transform infrared spectroscopy. These structures are open-ended, having a uniform shape and constant internal diameter.
Phase-Selective Synthesis of Calcium Carbonate Polymorphs
Authors: Doxsee Kenneth M. , Saulsbery Robin L.
Published: April 2011

Through the use of solubilizing agents, simple salt metathesis reactions may be translated from aqueous solution to the milieu of organic solvents. When applied to the crystallization of calcium carbonate through the reaction of calcium chloride with sodium bicarbonate in “complexation-mediated” crystallization leads to the selective formation of a metastable phase, vaterite, rather than to calcite, the phase obtained from aqueous solution. Impacts on the formation of the third anhydrous phase of calcium carbonate, aragonite, are also noted. These studies suggest an additional mechanism whereby Nature may control the morphology and phase of the calcium carbonate biominerals.
Spatio-Temporal Evolution Model for Competitive Ligand Binding
Authors: Condat C.A. , Delsanto P. P. , Iordache D. , Perego G. , Ruffino E.
Published: April 2011

Ligand binding is an essential step in many biological and biochemical processes, notably protein activation. In many cases, the binding rates are determined by competition: blockers competing with ligands or decoys competing with receptors. Often, the diffusivities of the various species also play a key role. The usual modelling of these processes, however, either involves well-mixed systems, for which spatial variations are neglected, or steady-state situations for which all time evolution has ceased. The model presented here can instead account for the spatial variations that originate as a consequence of inhomogeneous initial conditions and localized binding sites. It describes their evolution in time due to a combination of mechanisms of diffusion, competition and reaction. The model, which is formulated in terms of a coupled system of equations for the probability densities of the various populations, is easy to adapt to different specific conditions and is particularly well suited for numerical calculations, e.g., on a parallel computer. Since it yields concrete predictions, it can be readily used to predict the concentrations and distributions of the various species that are needed to obtain prescribed responses.
Potential Dependent Endothelial Cell Adhesion, Growth and Cytoskeletal Rearrangements
Authors: Zhou Tiean , Braunhut Susan J. , Medeiros Diane , Marx Kenneth A.
Published: April 2011

Normal endothelial cells (ECs), lining the blood vessels, are influenced by their interaction with the underlying potentially piezoelectric extracellular matrix (ECM). That this interaction may affect the EC metabolic state and functions in vivo prompted us to study the subsequent response of cultured ECs on indium-tin oxide (ITO) glass electrodes subjected to 1 hr of constant DC surface potential ranging from -0.3 to +0.6 V (vs. Ag/AgCl). We measured, relative to controls, cellular viability, growth rate and changes in actin microfilament organization in ECs over a subsequent 6 days in culture. The growth rate of ECs was stimulated by negative potential and inhibited by positive potential. Differences could be detected as early as three days post-potential. We also observed a potential dependent cellular shape change and actin microfilament rearrangement at positive potentials within four days of treatment. ECs changed in average cell surface area and assumed a polygonal cell shape in response to treatment. Using NBD-phalloidin stain for actin and fluorescence microscopy, microfilaments were observed to re-distribute to the periphery of the cell at positive potential, indicative of cellular stress.
Model for Dynamic Shear Modulus of Semiflexible Polymer Solutions
Authors: Gittes F. , Schnurr B. , Schmidt C. F. , Olmsted P. D. , Mackintosh F. C.
Published: April 2011

We discuss a dynamical model for the frequency-dependent shear modulus of an entangled solution of semifexible polymers, based on longitudinal fluctuations in filaments between entanglement points or crosslinks. The goal is to explain non-Rouse, power-law scaling of the bulk shear modulus that is found via microscopic rheology of highly entangled F-actin solutions. This generalizes a previous model for the static modulus. Hydrodynamic effects, and the validity of a local drag approximation below the scale of the mesh size, are discussed. We test aspects of our model via a molecular dynamics simulation, and also present for comparison experimental results from microrheology on F-actin.
Quantum Electrodynamics Calculation of Long - Range Interaction of Biopolymers in Intracellular Dispersion Medium
Authors: Pinchuk A. O. , Vysotskii V. I.
Published: April 2011

The general Lifshitz theory of van der Waals interaction was used to formulate and compute energies between the nucleotides situated on the opposite ends of a broken DNA helix. Our calculation show that infrared and ultraviolet resonances in the dielectric functions of DNA and the intracellular liquid account for less then 10 percent of the forces of interaction, at a range of 5-15 angstroms, between nucleotides. Thus the fundamental contribution to the interaction is presented by the group of resonances with frequencies of X - ray range. It was shown that during the interaction between thymine - guanine, adenine - guanine and cytosine - guanine there exists a potential barrier which prevents DNA selfrepairing after a mutanous, over a distance of about 7 - 20 angstroms, at the room temperature and with reference to the viscosity factor for pure water. All the remaining pairs of nucleotides have no such barrier. In addition the barrier vanishes and DNA undergoes complete selfrepairing with the decreace in viscosity of intracellular medium.
High Resolution Two-Dimensional Detection of Ncd Motility with Optical Tweezers
Authors: Allersma M. W. , Gittes F. , deCastro M. J. , Stewart R. J. , Schmidt C. F.
Published: April 2011

The ATP-dependent motility of the kinesin-related non claret disjunctional (ncd) mechanoenzyme was observed in anin vitrobead motility assay using optical tweezers in combination with a new two-dimensional displacement detection method. The detection technique is based on observing the far-field interference pattern formed in the back focal plane (BFP) of the microscope condenser by the illuminating laser focus and the light scattered from the trapped dielectric bead. The ability to observe the two-dimensional motion, with high temporal and spatial resolution, and in a manner largely independent of position in the microscope field-of-view, is the particular advantage of this detection method. In the assay, a fusion protein (GST-N195) of truncated ncd and glutathione-Stransferase was adsorbed to silica beads and the axial and lateral motions of the beads along the microtubule surface were observed. The average axial velocity of the ncd coated beads was 230 ± 30 nm/s (± std. dev.). Spectral analysis of bead motion showed an increase in viscous drag near the surface. Furthermore, we also found that any elastic constraints of the moving motors are much smaller than the constraints due to binding in the presence of the non-hydrolyzable nucleotide adenylylimido-diphosphate (AMP-PNP).
Optimal Design of Stagnation-Flow MOVPE Reactors with Axisymmetric Multi-Aperture Inlets
Authors: Gupta V. , Theodoropoulos C. , Peck J. D. , Mountziaris T. J.
Published: April 2011

An approach for optimal design of vertical stagnation flow Metalorganic Vapor Phase Epitaxy (MOVPE) reactors that minimizes parasitic pre-reactions between the film precursors is presented. The use of axisymmetric multi-aperture inlets (e.g. tube-in-tube or concentric-ring inlets) enables the separation of incompatible precursors, while preserving the axial symmetry of the reactor. A careful selection of the inlet velocity of each stream and the distance between the inlet and the susceptor (reactor height) can lead to complete mixing just above the substrate, while keeping the contact time between the precursors in the gas phase low enough to suppress pre-reactions. This idea has been used by our group for growing high quality ZnSe films on GaAs substrates from (CH3)2Zn:N(C2H5)3 and H2Se diluted in H2 in a stagnation flow MOVPE reactor with an axisymmetric split inlet. A transport model describing the MOVPE of ZnSe, for conditions at which the growth rate is limited by the precursors' arrival rate at the surface, has been developed. A parametric study was performed aiming at identifying operating conditions in industrial-scale reactors that maximize film thickness uniformity while minimizing precursor contact time. Operation atGr/Re2<100 eliminated flow recirculations in the region above the substrate. Such recirculations may lead to formation of particulates by trapping reactants. Optimal conditions correspond to equal velocities of the inlet streams, satisfying the above criterion, and to the minimum possible reactor height leading to uniform film thickness across the substrate.
Towards the Optimization of AMT Barrel Reactors for Silicon Epitaxy
Authors: Masi M. , Radaelli G. , Roda N. , Raimondi P. , Carra S. , Vaccari G. , Crippa D.
Published: April 2011

In this work, the epitaxial silicon deposition by SiHCl3-hydrogen mixtures in a AMT 7700 barrel reactor was analyzed through a detailed model where a three dimensional flow dynamic was solved by reduction to a two dimensional geometry under cylindrical coordinates. The model was used to investigate the role of the reactor geometry and of the process parameters on the axial deposition uniformity. A comparison with industrial experimental data was also performed.
The Roles of “3d/2d” and “3d/3d” Topography Simulators in Virtual Wafer Fabs
Authors: Cale T. S. , Merchant T. P. , Borucki L. J.
Published: April 2011

After discussing topography simulation, we summarize two approaches used to move surfaces in topography simulators used in virtual semiconductor wafer fabs; “front tracking” and “level set”. Front tracking is presented for two dimensional (2d) surfaces, and a number of examples are shown to demonstrate the approach. The level set approach is presented for three dimensional (3d) surfaces, and examples are shown. Though either approach could be used in both 2d and 3d topography simulators, this is by and large the current usage. Transport and reaction submodels needed for physically based process simulations will continue to be developed using experiments performed on structures that are inherently 2d, combined with three dimensional 3d transport simulations; i.e., “3d/2d” simulations. Three dimensional device structures will be generated using “3d/3d” topography simulations, using robust codes. Plasma enhanced deposition of silicon dioxide from TEOS is used as an example of how 3d/2d and 3d/3d simulations are used.
Atomistic Modeling of the Ion Implantation Step Within a 2D Process Simulator
Authors: Schmidt Bruno , Posselt Matthias , Strecker Norbert , Feudel Thomas
Published: April 2011

2D calculations of as-implanted dopant profiles are performed using a new module of the process simulator DIOS which is based on the binary collision code Crystal-TRIM. Examples derived from MOS and trench technology are considered. Good lateral and depth resolution is achieved within an acceptable computing time by combining a trajectory splitting algorithm with a method for the lateral duplication of ion trajectories.
Theoretical Modeling and Improved Thermoelectric Properties in (111) and (001) Oriented PbTe/Pb1-xEuxTe MQWs
Authors: Koga T. , Cronin S. B. , Harman T. C. , Sun X. , Dresselhaus M. S.
Published: April 2011

Theoretical modeling ofZTof (111) and (001) oriented PbTe/Pb1-xEuxTe multiple-quantum-wells (MQWs) is carried out assuming parabolic energy bands and the constant relaxation time approximation. The model calculation for the (111) oriented MQWs is compared with the recently obtained experimental results for the Seebeck coefficient and the Hall carrier concentration. The thermoelectric properties for (001) oriented PbTe MQWs are expected to have even better thermoelectric properties than the (111) oriented PbTe MQWs.
3D Atomistic Simulations of Submicron Device Fabrication
Authors: Buneat Marius M. , Dunham Scott T.
Published: April 2011

We use the Lattice Monte Carlo method to simulate the coupled motion of vacancies, interstitials and dopants during the annealing of a 50nm channel length NMOS structure. The initial defect locations are taken from Monte Carlo ion implantation simulations. The resulting defects diffuse, recombine, pair and cluster, with rates of these atomic processes calculated based on the local environment. Dopant redistribution occurs via both displacement by vacancies as well as the formation and diffusion of mobile boron and arsenic interstitials. We use these simulations to demonstrate the potential of atomistic simulations for deep submicron devices and to explore the influence of atomistic processes on macroscopic behavior.
First-Principles Study of Point-Defect Production in Si and SiC
Authors: Windl W. , Lenosky T. J. , Kress J. D. , Voter A. F.
Published: April 2011

We have calculated the displacement-threshold energy Ed for point-defect production in Si and SiC using empirical potentials, tight-binding, and first-principles methods. We show that—depending on the knock-on direction—64-atom simulation cells can be sufficient to allow a nearly finite-size-effect-free calculation, thus making the use of first-principles methods possible. We use molecular dynamics (MD) techniques and propose the use of a sudden approximation which agrees reasonably well with the MD results for selected directions and which allows estimates of Ed without employing an MD simulation and the use of computationally more demanding first-principles methods. We compare our results for Ed with the available experimental values. Furthermore, we have examined the temperature dependence of Ed for C in SiC and found it to be negligible.
Magnetism and Spin Tunneling in Nanostructures
Authors: Bratkovsky Alexander
Published: April 2011

In the present paper different tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.

The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.
Tight-Binding Linear Muffin-Tin Orbital Implementation of the Difference Equation Green's Function Approach for 2D-Periodic Systems
Authors: Schilfgaarde Mark Van , Lambrecht Walter R. L.
Published: April 2011

The central ideas and the advantages of the difference equation Green's function approach for layered systems with 2D-periodicity are presented. The tight-binding linear muffin-tin orbital and other practical aspects of the implementation are discussed. Preliminary test results are presented.
Three-Body Correlation in the Diluted Generalized Hubbard Model
Authors: Navarro O. , Avignon M.
Published: April 2011

A real-space method has been used to solve the generalized Hubbard Hamiltonian for a system with few electrons. The method is based on mapping the correlated many-body problem onto an equivalent tight-binding one in a higher dimensional space. For a linear chain, we have obtained an exact solution of the problem of three non-parallel electrons. The three-body correlation are studied by examining the binding energy in the ground state, for different values of the hopping parameters and of the on-site (U) and nearest-neighbor (V) interactions.
Self-Consistent Tight-Binding Approximation Including Polarisable Ions
Authors: Finnis M. W. , Paxton A. T. , Methfesselt M. , van Schilfgaarde M
Published: April 2011

Until recently, tight-binding has been applied to either covalent or metallic solid state systems, or charge transfer treated in a simple point charge framework. We present a self-consistent tight-binding model which, for the first time, includes electrostatic ion polar-isabihty and crystal field splitting. The tight-binding eigenvectors are used to construct multipole moments of the ionic charges which are used to obtain angular momentum components of the electrostatic potential in structure constant expansions. Our first test of the model is to study the phase stability in zirconia; in particular the instability of the fluorite phase to a spontaneous symmetry breaking, and its interpretation in terms of band effects and ion polarisability. This new formalism opens up the way to apply the tight-binding approximation to problems in whichpolarisationof atomic charges is important, for example oxides and other ceramic materials and surfaces of metals.
Tight-Binding Calculations of Complex Defects in Semiconductors: Comparison with AB Initio Results
Authors: Kohyama M. , Arai N. , Takeda S.
Published: April 2011

Complex defects in Si and SiC such as coincidence tilt boundaries, planar defects and self-interstitial clusters were dealt with by using the transferable tight-binding method for Si and the self-consistent tight-binding method for SiC. These results have been compared with ab initio calculations of similar configurations. Essential features of the tight-binding results have been supported by the ab initio results. Especially, the agreement on stable atomic configurations is good, although there exits a tendency that energy increases are somewhat overestimated by the tight-binding methods. Serious faults have been found for the electronic structure by the tight-binding method for SiC.
Development of Simple SPD Tight-Binding Models for Transition Metals
Authors: Le Bacq O. , Willaime F. , Pasturel A.
Published: April 2011

A simple spd tight-binding scheme for atomistic simulations in transition metals is developed using an orthogonal basis set in the two-center approximation. The purpose of the present approach is to cope with the limitations of the canonical d-band model for elements at the beginning or at the end of the transition metal series while keeping a reduced number of parameters, and simple decay-functions. The parameters for the hopping integrals are fitted to FP-LMTO calculations of the volume dependence of band energies and band structures for several selected structures. Constant values are taken for the on-site energies, and the Born-Mayer pair potential is used for the repulsive term. Two approaches are compared for the total energy: the band model and the bond model. First applications are presented in the case of zirconium, where the difference between these models on phase stability results is particularly drastic. The bond model reproduces the stability of the hep phase and displays a good agreement with experiments for the elastic constants.
Optical Properties of Materials using the Empirical Tight-Binding Method
Authors: Lew Yan Voon L. C.
Published: April 2011

Procedures for calculating the optical properties of periodic systems using the empirical tight-binding method are compared. Results of the linear and nonlinear susceptibilities are presented using a recently developed exact formalism [Lew Yan Voon and Ram-Mohan. Phys. Rev. B47, 15500 (1993)].