bluuebunny/crossref_metadata_2025_split huggingface dataset:
Preparation of 30×30 mm2 Free-Standing GaN Wafer by Mechanical Liftoff and Optical Properties in The Backside of The Free GaN by Cathodoluminescence
Authors: Kim Hwa-Mok , Oh Jae-Eung , Kang Tae-Won 
Published: April 2011

ABSTRACTIn this study, free-standing GaN was produced from 350- to 400-μm-thick GaN films grown on sapphire by using hydride vapor phase epitaxy. The thick films were separated from the substrate by using the mechanical polishing method with a diamond slurry. After liftoff, the bow is slight or absent in the resulting free-standing GaN. Double-crystal X-ray diffraction, hall measurements, and cathodoluminescence were used for characterizing the free-standing GaN wafer. To investigate spatially the backside of the free-standing GaN substrate, we controlled the electron beam energy from 5 keV to 30 keV. As the beam energy increased, dark regions, i.e., nonradiative regions, become smaller than bright regions. We think this means that nonradiative centers, i.e., threading dislocations, merge during film growth.
Persistent Photoconductivity in a High Mobility two Dimensional Electron Gas in an AlGaN/GaN Heterostructure
Authors: Elhamri S. , Saxler A. , Cull D. , Mitchel W. C. , Elsass C.R. , Smorchkova I.P. , Heying B. , Poblenz C. , Fini P. , Keller S. , Petroff P.M. , DenBaars S.P. , Mishra U.K. , Speck J.S. 
Published: April 2011

ABSTRACTTemperature-variable Hall and Shubnikov- de Haas effects have been used to study persistent photoconductivity in an AlGaN/GaN heterojunction. At liquid helium temperatures, the mobility in this structure was close to 55000 cm2/Vs. A blue GaN-based light emitting diode was used to illuminate the sample. This illumination resulted in a persistent photocurrent, which allowed us to vary the carrier density and study the dependence of the mobility on the carrier concentration. Exposing the sample to this light resulted in an increase in the carrier density. For small increases in the density, the mobility also increased. However, unlike in previous reports by other authors, extended illumination resulted in an increase in the density and a decrease in the mobility. The initial increase in the mobility is attributed to increased screening due to the increase in the carrier density, while the decrease in the mobility may be attributed to alloy scattering.
Time-Resolved Photoluminescence Measurements of In0.15Ga0.85N/In0.015Ga0.985N Quantum Wells with Si-doped Barriers
Authors: Ryu Mee-Yi , Yu Young Jun , Yu Phil Won , Shin Eun-joo , Oh Eunsoon , Sone Chul Soo , Nam Ok Hyun , Park Yong Jo 
Published: April 2011

ABSTRACTA systematic study of photoluminescence (PL) and time-resolved PL spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si doping concentration in the barriers has been carried out. As the Si doping concentration increases, the PL emission intensity was increased and the PL peak energy was blueshifted. The energy separation between the spontaneous emission (SPE) and stimulated emission (SE) peaks decrease with increasing Si doping concentration. We also observed that the slow decay time τ2 in the QWs decreases with increasing Si doping concentration, from ∼ 130 ns for [Si] = 2 × 1018 cm−3 to ∼ 30 ns for [Si] = 1 × 1019 cm−3. The PL emission peak shifts to lower energies with delay time after a pulsed excitation and this shift decreases with increasing Si doping concentration. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the SPE and SE peaks with increasing Si doping concentration are attributed to the screening of piezoelectric field by carriers originated from Si doped barriers.
Island decay on the anisotropic Ag(110) surface
Authors: Morgenstern Karina , Lægsgaard Erik , Besenbacher Flemming 
Published: April 2011

AbstractWe have investigated the decay of two-dimensional islands on the anisotropic Ag(110) surface using variable-temperature scanning tunneling microscopy. Contrary to predictions from traditional Ostwald ripening theory, a quasi-one-dimensional decay mode is observed at low temperatures (175-220 K). A surprisingly sharp transition to the quasi-two-dimensional decay mode is observed around 220 K. This transition is accompanied by a fast equilibration of the island shape. These findings have tentatively been rationalized within a simple model to identify the underlying rate limiting atomistic processes.
Ordering in AlGaN: Thermodynamical and X-Ray Analysis
Authors: Ruffenach-Clur Sandra , Briot Olivier , Moret Matthieu , Aulombard Roger-Louis 
Published: April 2011

AbstractUp to date, different groups have reported ordering phenomena in nitride alloys like InGaN or AlGaN. These evidences of ordering during and after growth lead us to study the atomic arrangement of Al and Ga on site III in the AlGaN alloy using a thermodynamical approch. We have defined the enthalpy and the entropy of our system using the Keating model and the cluster variation method limited to the first coordination shell. This study gives us a good way to understand ordering, and moreover to see why at three special compositions (25%, 50% and 75%), the alloys have good chances to present a better crystalline quality than in other cases.We have grown AlGaN samples increasing the Al composition by 10% steps, in the full range of composition from GaN to AlN. All the growth parameters were kept constant except for the alloy composition. X-Ray diffraction measurements performed on these samples indicate that FWHM increases linearly with Al composition, except for three compositions, where according to our thermodynamical study, we observe an improved crystalline quality.
Surfactant-Mediated Growth of MnSi1.7 on Si(001)
Authors: Teichert S. , Hortenbach H. , Sarkar D. K. , Beddies G. , Hinneberg H.-J. 
Published: April 2011

AbstractThe formation of the silicide MnSi1.7 by reactive deposition of Mn onto Si(001) has been studied using Sb as a surfactant. The growth was performed under UHV conditions by simultaneous or consecutive exposure of the Si substrates, held at high temperatures (550°C, 600 °C), to a flux of Sb and Mn atoms. The presence of Sb during the growth strongly increases the island density and changes the crystalline orientation of the MnSi1.7 grains. The morphology and the orientation of the resulting silicide are the same both for the deposition of Mn on a Sb terminated Si(001) surface and for the co-deposition of Mn and Sb on Si(001). A residual Sb coverage close to one monolayer (ML) at the sample surface has been determined for both of the preparation modes at Tsub = 550 °C. The transition from the growth mode without Sb to the surfactant-controlled growth has been studied for Tsub = 600 °C. It has been found that the silicide morphology and orientation strongly depend on the thickness of Sb pre-coverage, which was increased from 0 to about 0.7 ML (1 ML = 6.78·1014 atoms cm-2).
Dynamics of InAs Quantum Dots Formation on AlAs and GaAs
Authors: Yakimov M. , Tokranov V. , Oktyabrsky S. 
Published: April 2011

AbstractWe have studied the formation of InAs quantum dots (QDs) grown by molecular beam epitaxy on top of GaAs and 2 ML-thick AlAs layers in the temperature range from 350 to 500°C. In-situ reflection high energy electron diffraction (RHEED) patterns were recorded in real time during the growth and analyzed to characterize the 2D-to-3D transition on the surface, including QD formation, and ripening process. The kinetics of QD formation was studied using the InAs growth rates ranging from 0.01 to 1 ML/s and different ratios of As2/In fluxes. RHEED patterns and ex-situ atomic force microscopy images were analyzed to reveal the development of sizes and shapes of the single-layer and stacked QD ensembles. The critical InAs coverage for QD formation was shown to be consistently higher for dots grown on the AlAs overlayer than for those grown on GaAs surface. Self-assembly of multilayer QD stacks revealed the reduction of the critical thickness for dots formed in the upper layers.
In Situ Investigation of TixAly Film Growth As A Function of Substrate Temperature and Film Stoichiometry
Authors: Lackner St. , Abermann R. 
Published: April 2011

AbstractThe growth of TixAly-alloy films was investigated by measuring the film stress during as well as after the film deposition under UHV-conditions with a cantilever beam technique. The alloy films were deposited on alumina substrates from two separate evaporation sources. The substrate temperature was varied from room temperature to 500°C and the alloy stoichiometry from Ti25Al75 to Ti75Al25.At a substrate temperature of 200°C the dependence of the growth stress of the alloy film on stoichiometry is pronounced and varies from tensile stress in the case of titanium rich alloy-films to compressive stress in aluminium rich films. The stress changes after deposition at 200°C are negligible for titanium rich alloys but in the case of aluminium rich alloys small tensile stress changes are measured.At a substrate temperature of 500°C the film stress is compressive and the dependence on stoichiometry is rather small. The stress vs. thickness curves of Ti25Al75-films, however, show unexpected oscillations during growth while at lower aluminum content stable stress curves are measured. The stress changes after deposition are large in each case, interpreted to indicate film recrystallisation and/or continued alloy formation.In a second series of experiments we have measured the internal stress changes in Ti25Al75-alloy films during annealing. It can be shown that the changes in the film stress can be correlated with changes in the film microstructure seen in the electron microscope (TEM).
Oblique Stacking of Three-Dimensional Dome Islands in Ge/Si Multilayers
Authors: Sutter P. , Sutter E. , Vescan L. 
Published: April 2011

AbstractThe organization of Ge ‘dome’ islands in Ge/Si multilayers has been investigated by cross-sectional transmission electron microscopy. Ge ‘domes’ are found to spontaneously arrange in oblique stacks, replicating at a well-defined angle from one bilayer to the next. The formation of oblique island stacks is governed by a complex interplay of surface strain - generated by the already buried islands - and surface curvature - caused by the inherent tendency of large ‘domes’ to carve out material from the surrounding planar substrate.
Nanoassembled Model Catalysts: Changing Efficiency and Selectivity Atom-by-Atom
Authors: Heiz U. , Abbet S. , Häkkinen H. , Landman U. 
Published: April 2011

AbstractCluster-assembled materials open fascinating new routes for tuning physical and chemical properties by changing cluster size and often these materials behave completely differently than their bulk analogues. By depositing gas phase cluster cations on various surfaces, cluster model catalysts are fabricated, which exhibit remarkable catalytic activity. While inert as bulk material, gold cluster catalysts oxidize carbon monoxide with Au8 to be the smallest active size. In contrast to the high selectivity of Pd(111) surfaces for the cyclotrimerization of acetylene, small supported Pdn clusters reveal a strongly size-dependent selectivity for the polymerization of acetylene and catalyze the cyclotrimerization as well as the hydrogenation of the formed intermediate C4H4.
Anomalous Diffusion in Active Intracellular Transport
Authors: Caspi Avi , Granek Rony , Elbaum Michael 
Published: April 2011

AbstractThe dynamic movements of tracer particles have been used to characterize their local environment in dilute networks of microtubules, and within living cells. In the former case, 300 nm diameter beads are fixed to individual microtubules, so that the movements of the bead reveal undulatory modes of the polymer. The mean square displacement shows a scaling of t3/4 in keeping with mode analysis arguments. Inside a cell, beads show a more complicated behavior that reflects internal dynamics of the cytoskeleton and associated motors.When placed near the cell edge, 3 micron diameter beads coated by proteins that mediate membrane adhesion are engulfed underneath the membrane and drawn toward the center by a contracting flow of actin. On reaching the region surrounding the nucleus, the beads continue to move but lose directionality, so that they wander within a restricted space. Measurement of the mean square displacement now shows a scaling of t1 up to times of ~1 sec. At longer times the scaling varies between t and t1/2 in the various runs. The data do not fit a crossover between ballistic (t2)and diffusive (t1) behavior. The movement is clearly driven by non-thermal interactions, as it cannot be stopped by an optical trap. Treatment of the cell to depolymerize microtubules restores ordinary diffusion, while actin depolymerization has no effect, indicating that microtubule-based motor proteins are responsible for the motion. Immunofluorescence microscopy shows that the mesh size of the microtubules is smaller than the bead diameter.We propose that the observations are related, and that the non-trivial scaling in the polymer system leads to time-dependent friction in a network, which in turn leads to a generalized Einstein relation operative in the intracellular environment. This results, in the driven system, in sub-ballistic motion at short times and sub-diffusive motion at longer times.
Molecular Dynamics and Relaxation Methods in the Stability Calculations for the Study of Distortions of Confined Nematic Liquid Crystals
Authors: Calles A. , Valladares R.M. , Castro J.J. 
Published: April 2011

AbstractWe present a calculation using a relaxation method for the study of the orientational ordering of a nematic liquid crystal near the surfaces confining the system. We comment on the advantage of using this method as compared with molecular dynamics simulation. The system is simulated by a lattice model with a superposition of isotropic and anisotropic intermolecular interaction of the Maier-Saupe and induced dipole-induced dipole type force for the bulk nematic phase. For the nematic confining surface we consider a Rapini-Papoular interaction. We present simulations for negative dielectric anisotropy.
Ordered Morphologies of Confined Diblock Copolymers
Authors: Tsori Yoav , Andelman David 
Published: April 2011

AbstractWe investigate the ordered morphologies occurring in thin-films diblock copolymer. For temperatures above the order-disorder transition and for an arbitrary two-dimensional surface pattern, we use a Ginzburg-Landau expansion of the free energy to obtain a linear response description of the copolymer melt. The ordering in the directions perpendicular and parallel to the surface are coupled. Three dimensional structures existing when a melt is confined between two surfaces are examined. Below the order-disorder transition we find tilted lamellar phases in the presence of striped surface fields.
Spectral Properties of Various Cerium Doped Garnet Phosphors for Application in White GaN-based LEDs
Authors: Wu Jennifer L. , Denbaars Steven P. , Srdanov Vojislav , Weinberg Henry 
Published: April 2011

ABSTRACTRecently, a renewed research interest has emerged for the development of visible light, down-converting phosphors for application in white light emitting diodes (LEDs). In such devices, a blue GaN LED can act as the primary light source, exciting photoluminescence in a phosphor with subsequent broad-band emission occurring at lower energies. It was recently reported that use of a combinatorial approach to synthesize and screen potential inorganic phosphors for such an application could aid in identifying improved phosphors for blue to yellow down conversion. Using solution chemistry techniques developed by Symyx Technologies, solid state thin-film arrays (libraries) based on the garnet structure (A1-xBx)3(C1-yDy)5O12:Ce3+, where x and y = 0 to 1.0 and A, B = Y, Gd, or Lu; C., D = Al or Ga, were synthesized. X-ray diffraction was used to select library samples of the crystalline garnet phase. Libraries of these various garnets were then characterized, and their spectral properties compared to traditionally prepared bulk powder phosphors of similar composition. Emission and excitation trends show that as larger cations are substituted for Y (A = Y), emission and excitation are red-shifted and as larger cations are substituted for Al (C = Al), emission and excitation are blue-shifted. If smaller cations are substituted for Y and Al, an opposite trend is observed.
Origin of Ferroelectricity in Aurivillius Compounds
Authors: Y. Donají , Reaney Suárez Ian M. , Lee William E. 
Published: April 2011

ABSTRACTThe structures and microstructures of a range of Aurivillius phases have been investigated using transmission electron microscopy. Superlattice reflections arising from rotations of octahedra around the c-axis have been identified and their intensity at room temperature has been shown to diminish as the tolerance factor of the perovskite blocks increases. The paraelectric to ferroelectric phase transition temperature (Tc) has been monitored using permittivity measurements as a function of temperature and Tc has also been shown to decrease as tolerance factor increases. It is proposed that the onset of octahedral tilting and Tc are related in Aurivillius phases.
Investigation of Lead Borosilicate Glass Structure With 207Pb and 11B Solid-State NMR
Authors: Gibson James M. , Vogt Frederick G. , Barnes Amy S. , Mueller Karl T. 
Published: April 2011

ABSTRACTA series of three lead borosilicate glasses were synthesized and analyzed for structural information with both 11B and 207Pb solid-state nuclear magnetic resonance (NMR) spectroscopic methods. Results showed that increasing lead content caused lead to take a more active role in the network as a former and that the populations in these sites can be approximately quantified. 207Pb phase-adjusted-spinning sidebands (PASS), 11B magic-angle spinning (MAS), and 11B multiple-quantum MAS (MQMAS) experiments were used to determine structural parameters for the two nuclei. The 207Pb PASS experiment showed that at higher lead content, more covalent bonding was present. This principle was demonstrated in both an overall shift of the spectral resonances and a quantitative change in site ratios. The 11B MAS experiment showed that the ratio of BO3 to BO4 units was dependent on the amount of lead and boron, consistent with previous studies. Preliminary 11B MQMAS experiments failed to detect any BO3– units, previously hypothesized to exist in this system.
Crystal Chemistry of Colloids Obtained by Hydrolysis of Fe(III) in the Presence of SiO4 Ligands
Authors: Doelsch Emmanuel , Masion Armand , Rose Jérôme , Stone William E.E. , Bottero Jean Yves , Bertsch Paul M. 
Published: April 2011

ABSTRACTThe crystal chemistry of a number of Fe-Si systems (Si/Fe 0-4, pH 3-10) was investigated by combining local scale spectroscopic methods (EXAFS, FTIR and NMR) and at the semi local scale (SAXS). The Fe clusters within the precipitates have two growth regimes depending on the Si/Fe ratio: the growth is three and two dimensional for Si/Fe fl and Si/Fe ≤ 1 respectively. The presence of Fe-O-Si bonds within the precipitated phases has been demonstrated. Their formation and relative proportion was found to be very dependent on the pH and Si concentration The size of silica domains within the precipitates was shown to increase with increasing Si/Fe and/or decreasing pH. The high fractal dimension (Df) of the aggregates is attributed to the presence of the SiO4 ligands, but the evolution of Df linearly depends on the polymerization state of iron.
Slater Transition in the Pyrochlore Cd2Os2O7
Authors: Mandrus D. , Thompson J. R. , Woods L. M. 
Published: April 2011

ABSTRACTCd2Os2O7 crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. Here we present resistivity, heat capacity, and magnetization results on Cd2Os2O7. Both single crystal and polycrystalline material were examined. We also present LAPW electronic structure calculations on Cd2Os2O7. We interpret the results in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition--unlike a Mott transition--is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below TMIT.
Metal Cyanide Networks Formed at an Air-Water Interface: Structure and Magnetic Properties
Authors: Culp Jeffrey T. , Morgan A. Nicole , Meisel Mark W. , Talham Daniel R. 
Published: April 2011

ABSTRACTExtended two-dimensional coordinate covalent networks have been formed at an air-water interface by condensing a Langmuir monolayer of an amphiphillic pentacyanoferrate(III) complex and aqueous nickel(II) ions. The cyanide-bridged arrays were transferred to solid supports by the Langmuir-Blodgett (LB) technique. The resulting thin films were structurally characterized by UV-Vis and FT-IR spectroscopies, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and SQUID magnetometry. Magnetic measurements on the thin films transferred to Mylar show evidence for ferromagnetic exchange interactions between the S = 1/2 Fe(III) and S = 1 Ni(II) centers.
Decoration of silicon carbide nanotubes by CoFe2O4 Spinel nanoparticles.
Authors: Estournès Claude , Pham-Huu Cuong , Keller Nicolas , Ledoux Marc J. 
Published: April 2011

ABSTRACTSilicon carbide nanotubes have been prepared and decorated with CoFe2O4 spinel. These nanocomposites were prepared by incipient wetness impregnation of the dried SiC support with a stoichiometric aqueous solution of the cobalt and iron nitrates. After drying, subsequent annealing treatments were performed under air in the temperature range 873 to 1073 K. X-ray diffraction and high resolution transmission electron microscopy reveal the presence of spinel nanoparticles at the surface of the tubes. Magnetic properties of the nanocomposites show in the magnetic field explored (up to 5 T) that saturation magnetization is not observed neither at room temperature nor at 5Kelvin. The nanometric character and the nature of the spinel particles are also confirmed by the coercive field (1.6 T) observed at low temperature and the curie temperature.
Synthesis of Large ZSM-5 Crystals under High Pressure
Authors: Wang Xiqu , Jacobson Allan J. 
Published: April 2011

ABSTRACTThe synthesis temperature for silicalite-I (MFI) can be raised to 300°C by applying high pressure to stabilize the structure-directing organic template. The elevated temperature and pressure favor theformation of crystals with improved quality. Prismatic silicalite-I crystals with a uniform size of about 0.7× 0.2 × 0.2 mm have been obtained by heating a gel prepared from TMA-silicate solution, TPABr andsodium hexafluorosilicate at 250°C under a pressure of 80 MPa. The influence of synthesis conditions on the crystal sizes has been studied by systematically changing temperature, pressure and gel compositions. Underthe specific conditions of 250°C and 80 MPa, a strong correlation was found between the crystal size andthe F/Si mole ratio of the starting gel, which enables the preparation of uniform crystals of silicalite-I with preset dimensions.