bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optical Properties of MBE Grown Cubic AlGaN Epilayers and AlGaN/GaN Quantum Well Structures
Authors: As D.J. , Frey T. , Bartels M. , Khartchenko A. , Schikora D. , Lischka K. , Goldhahn R. , Shokhovets S. 
Published: April 2011

ABSTRACTCubic AlyGa1−yN/GaN heterostructures on GaAs(001) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, spectroscopic ellipsometry and cathodoluminescence were used to characterize the structural and optical properties of the alloy epilayers. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic AlyGa1−yN films. Both SE as well as room temperature CL of the AlyGa1−yN epilayer show a linear increase of the band gap with increasing Al-content. A pseudomorphically strained cubic 10 × (2.4 nm GaN/ 4.8 nm Al0.12Ga0.88N) multi-quantum well (MQW) structure has been realized. Cathodoluminescence clearly demonstrates strong radiative recombination due to quantized states in the GaN well layer at a photon energy of 3.323 eV.
Comparative study of HVPE- and MOCVD-grown nitride structures for UV lasing application
Authors: Lam J. B. , Gainer G. H. , Bidnyk S. , Elgawadi Amal , Park G. H. , Krasinski J. , Song J. J. , Tsvetkov D. V. , Dmitriev V. A. 
Published: April 2011

ABSTRACTWe studied and compared the emission properties of optically excited (Al)GaN structures grown by two different techniques: hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD). We successfully achieved stimulated emission (SE) in an HVPE-grown GaN epilayer and a GaN/AlGaN double heterostructure at 10 K and room temperature. We found that the SE threshold and photoluminescence efficiency of the HVPE-grown samples are similar to those of high-quality MOCVD-grown structures. Photoluminescence measurements from 10 to 300 K show that the HVPE GaN has a high density of non-radiative recombination channels, especially those activated below 100 K. This study represents the first demonstration of SE in HVPE-grown (Al)GaN heterostructures.
Luminescence properties of amorphous AlN thin film phosphors incorporated with mixtures of Tb, Cu or Cu, Cr
Authors: Martin Andrea L. , Caldwell Meghan L. , Kordesch Martin E. , Spalding Chance M. , Patten Paul G. Van , Richardson Hugh H. 
Published: April 2011

ABSTRACTAmorphous thin films of AlN doped with Cu (blue luminescence), Tb (green luminescence) and Cr (red luminescence) were deposited on p-doped Si (111) substrates using RF magnetron sputtering in a nitrogen atmosphere and made luminescent active by heat treatment at 1000°C. Single layered amorphous AlN films deposited with Tb and Cu showed cathodoluminescence from only the Tb3+ ions. Presumably, energy is transferred from Cu luminescence centers to Tb centers in close proximity. In contrast to this, double layered amorphous AlN films doped with Cu (∼200 nm) on top of Tb (∼50 nm) emits from both ions. This behavior is observed in a double layered amorphous AlN film doped with Cu (∼200 nm) on top of a Cr (∼200 nm) doped film. Secondary-ion mass spectrometry depth profiling revealed that the incorporated metals moved from one layer into the other during the heat treatment step necessary for luminescence activation.
Planar Force-constant Method for Lattice Dynamics of Cubic InN
Authors: Alves H. W. Leite , Alves J. L. A. , Scolfaro L. M. R. , Leite J. R. 
Published: April 2011

ABSTRACTUsing the density-functional theory within the Full Potential Linear Augmented Plane-Wave (FP-LAPW) method, we have calculated, in this work, the equation of state, the effective charges and the phonon dispersion along [100] and [111] directions for the cubic Indium Nitride(c-InN), including hydrostatic strains dependence. A good agreement with the micro-Raman scattering experiment is obtained for the phonon modes at ã. Our results show that the apparent divergence between the known experimental results is a consequence of hydrostatic effects on the sample due to differences of the used Raman methods.
Role of Localized Quantum Well Excitons in InGaN Quantum Well Structure Correlated with Microstructural Analysis
Authors: Chichibu S. F. , Sota T. , Nakamura S. 
Published: April 2011

ABSTRACTInxGa1−xN multiple-quantum-well laser diode structure, which lased at 405 nm, was shown to have atomically-flat interfaces between each layer. Nanometer-probe compositional analysis showed that InN mole fraction, x, in the wells and barriers are approximately 6 % and 2 %, respectively, which agreed with the result obtained from high-resolution x-ray diffraction measurement. The Stokes-like shift (SS) at 300 K was 49 meV, being approximately 65 % of the luminescence linewidth. The localization depth, E0, of qunatum-well (QW) excitons was estimated to be 35 meV at 300 K though the compositional fluctuation in the well was as small as 1 % or less (detection limit) within adjacent 20-30 nm lateral length scale. Since the well thickness fluctuation is insufficient to reproduce SS or E0, effective bandgap inhomogeneity is attributed to be due to large bandgap bowing in InGaN. The spontaneous emission was thus assigned as being due to the recombination of QW excitons weakly localized in exponential tail-type potential minima in the QW. The size of localization is smaller than the quantum-disk [M. Sugawara, Phys. Rev. B 51, 10743 (1995)]-size. Such small bandgap inhomogeneity can be leveled by injecting high density carriers under lasing conditions, which can explain the general experimental finding that the quantum efficiency decreases with increasing carrier density in InGaN QW devices due to free carrier trapping into threading dislocations.
Pattern Formation via a Two-Step Faceting Transition on Vicinal Si(111) Surfaces
Authors: Men F.K. , Liu Feng , Wang P.J. , Chen C.H. , Cheng D.L. , Lin J.L. , Himpsel F.J. 
Published: April 2011

AbstractWe demonstrate a self-organized pattern formation on vicinal Si(111) surfaces that are miscut toward the [211] direction. All the patterns, consisting of a periodic array of alternating (7×7) reconstructed terraces and step-bunched facets, have the same periodicity and facet structure, independent of the miscut angle; while the width of the facets increases linearly with miscut angle. We attribute such unique pattern formation to a surface faceting transition that involves two transition steps: the first step forms a stress-domain structure defining the universal periodicity; the second step forms the low-energy facets controlling the facet width.
Surface Self-Diffusion Instability in Electric Fields
Authors: Ojovan Michael I. , Poluectov Pavel P. 
Published: April 2011

AbstractThe plane form is the equilibrium one for surfaces of condensed matter. Deviations can be caused usually by crystal structure. Herein we will describe an effect of surface instability due to self- diffusion processes of atoms and molecules in the near surface electric field.Self-diffusion processes (as it was shown by Mullins) cause relaxation of any deviation (protuberance) from the plane form due to the increased concentration of surface atoms and its consequent smoothing. This process we studied for the case when there is an electric field near the surface. The near surface electric field can be due to either the location of material in an external (homogeneous or inhomogeneous) electrical field or self-charges on the surface. There is an increasing of electric field intensity near protuberances both in external and self-formed electrical fields: the higher is the curvature of surface the stronger is the intensity of the near surface electrical field. Consequently two competing processes occur during surface molecules mass transfer: both the self- diffusion smoothing of surface molecule concentration and drawing of molecules in the strong electric field regions. Depending on the initial shape of the protuberance either relaxation or instability occurs. There is a critical wavelength λ0=RskBT/2Uz, which shows that shorter wavelength deviations decrease their amplitudes and longer wavelength deviations grow in amplitude by time. Here Rs is characteristic of the material, T is temperature, and Uz is the interaction energy of surface molecules with the electric field. Since there are random variations of any surface from the plane form, being placed in an electric field these surfaces will be unstable depending on the intensity of electric field and properties of material.
Electro-resistive Memory Effect in Colossal Magnetoresistive Films and Performance Enhancement by Post-annealing
Authors: Liu Shangqing , Wu Naijuan , Ignatiev Alex , Tavizon Gustavo , Papagianni Christina 
Published: April 2011

AbstractColossal magnetoresistive thin films have shown a large electric-pulse-induced resistivity change effect in zero magnetic field and at room temperature. The resistance of such films can be both decreased and increased through multiple nonvolatile intermediate levels by short electrical pulses. The effect provides a potential to develop a novel nonvolatile memory with high density, fast speed, and low power-consumption. An example of this effect has been seen for Pr0.7Ca0.3MnO3 films within which the thermal behavior of the film revealed a method for signal enhancement through annealing. An increase of 700% of the resistance ratio has been demonstrated for a film annealed at 170oC for 30 min. The effect is also observed to be active at room temperature but inefficient at low temperatures, which is interestingly contrary to the behavior of the colossal magnetoresistance effect and provides a clue to understanding the effect.
Unusual Magnetic Properties of Fe/Ni Bilayers on Cu(100)
Authors: Schirmer B. , Liu X. , Wuttig M. 
Published: April 2011

AbstractUltrathin iron films grown on Cu(100) have been found to exhibit a rich variety of structural and magnetic phases. In the present work, Fe/Ni bilayers have been prepared by molecular beam epitaxy to explore novel magnetic phenomena introduced by the ferromagnetic (FM) Ni underlayer. Unusual properties have been observed by measuring the temperature dependent magnetic properties. For 5.3 ML Fe on 7 ML Ni, a temperature dependent exchange coupling in the Fe film has been observed between the FM surface layer and FM interface layer.
β-FeSi2 thin-films grown by a pulsed laser deposition
Authors: Uekusa Shin-ichiro , Watanabe Yasuharu , Aida Yasuhiro , Miura Noboru 
Published: April 2011

AbstractOrthorhombic β-FeSi2 thin-films were prepared on Si(100) and Si(111) substrates by a pulsed laser deposition method. When the substrate temperature was 500°C, β-FeSi2 thin-films were grown on Si(100) and Si(111) substrates. The thin-films grown on Si(100) and Si(111) substrates were polycrystalline and monocrystalline structures, respectively. The values of band-gap energy calculated from transmittance measurements were 0.71-0.72 eV. From Raman scattering measurements, it was found that the distortion due to the lattice mismatch between a β-FeSi2 thin-film and a Si substrate originates in the β-FeSi2/n-Si interface. Moreover, the fine crystals of β-FeSi2 existed in an amorphous thin-film which was grown on Si(111) substrate at room temperature (RT).From van der Pauw measurements, conduction type, carrier concentration and Hall mobility were p-type, 1018-1021 cm−3 and 200-500 cm2/Vsec, respectively. The p-n diode characteristics of these heterostructure diodes were investigated by I-V and C-V measurements. The results indicate that the carrier distribution agrees with an ideal one-sided slope junction.
Mechanical, Adhesive and Thermodynamic Properties of Hollow Nanoparticles
Authors: warz U. S. Sch , Safran S. A. , Komura S. 
Published: April 2011

AbstractWhen sheets of layeredmaterial like C., WS2 or BN are restricted to [ ]nite sizes, they generally form single- and multi-walled hollow nanoparticles in order to avoiddangling bonds. Using contin uum approaches to model elastic deformation and van der Waals in teractions of spherical nanoparticles, we predict the variation of mechanical stability, adhesive properties and phase behavior with radius R and thickness h. We find that mechanical stability is limited by forces in the nN range and pressures in the GPa range. Adhesion energies scale linearly with R, but depend only weakly on h. Deformation due to van der Waals adhesion occurs for single-walled particles for radii of few nm, but is quickly suppressed for increasing thickness. As R is increased, the gas-liquid coexistence disappears from the phase diagram for particle radii in the range of 1-3 nm (depending on wall thickness) since the in teraction rangedecreases like 1/R.
Li-insertion Behavior in NanocrystallineTiO2-(MoO3)z Core-Shell Materials
Authors: Moore Gregory J. , Guyomard Dominique , Elder Scott H. 
Published: April 2011

ABSTRACTA fundamental study of the Li insertion behavior of a series of materials consisting of a TiO2 core having MoO3 on the surface has been carried out in order to determine the influence of the shell. These TiO2-(MoO3)z materials, where (z) denotes the fraction of coverage from a partial to a double layer, range in diameter from 40-100 Å. Calculations have been done on their theoretical lithium capacity using a maximum of Li0.5TiO2 for the core, and Li1.5MoO3 at the TiO2/MoO3 interface, and they have been compared to that found experimentally. The reversible Li-insertion capacity was shown to increase from 0.34 per Ti for the pure TiO2 sample, to 0.91 Li per transition metal when the MoO3 coverage increased to one monolayer. There was only one plateau observed in the electrochemical scans for the samples showing that they function as a single-phase material making them interesting for electrodes. The redox voltage of the TiO2/Li0.5TiO2 biphasic transformation increased 60 mV from the pure TiO2 to the sample containing one monolayer of MoO3. This effect was interpreted as due to a change in TiO2 surface charge coming from an inductive effect of Ti-O-Mo bonds.
Vanadium Oxide Frameworks Modified with Transition Metals
Authors: Zavalij Peter Y. , Whittingham M. Stanley 
Published: April 2011

ABSTRACTIt is well known that vanadium oxides readily form open structures that defines the potential use of vanadium compounds as cathode materials for lithium batteries. In order to stabilize open frameworks during electrochemical cycling and improve their performance, vanadium oxide can be modified with other transition metals. This work presents the structural analysis of vanadium oxide frameworks modified with Mn and Zn as well as reporting on several interesting new structures, such as TMA4[Zn4V21O58] and NaMn(VO3)4·2H2O.
Itinerant Vibrons and High-Temperature Superconductivity*
Authors: Goodenough John B. 
Published: April 2011

ABSTRACTThe La2−xSrxCuO4 phase diagram is interpreted within the framework of a transition from localized to itinerant electronic behavior. In the underdoped region 0 < x < 0.1, holes in the x2 – y2 band are not small polarons; each occupies a mobile correlation bag of 5 to 6 copper centers at temperatures T > TF, a spinodal phase segregation into the parent antiferromagnetic phase and a polaron liquid is accomplished below TF by cooperative oxygen displacements. In the overdoped compositions > x > 0.25, holes are excluded from strong-correlation fluctuations within a Fermi liquid. In the intermediate range 0.1 < x < 0.25, the polaron liquid formed below room temperature changes character with increasing x and decreasing T. In the polaron liquid, mobile two-hole bags of four copper centers order with decreasing temperature into alternate CuO-Cu rows of a superconductive CuO2 sheet at a critical composition xc ≍ 1/6. It is argued that hybridization of itinerant electrons with optical-mode phonons propagating along the Cu-O-Cu rows produces heavy electrons responsible for high-temperature superconductivity.
(Rare Earth/Li) Titanates and Niobates as Ionic Conductors
Authors: Morata-Orrantia A. , García-Martín S. , Morán E. , Amador U. , Alario-Franco M. A. 
Published: April 2011

ABSTRACTThe lithium ion conducting properties of materials of composition La0.58Li0.26TiO3, Nd0.58Li0.26TiO3, La0.67Li0.25Ti0.75Al0.25O3 and La0.29Li0.12NbO3 have been compared in relation with their microstructure. All the oxides have powder X-ray diffraction patterns characteristic of a perovskite-related structure with lattice parameters a∼√2ap, b∼√2ap, c∼2ap (p refers to cubic perovskite). However, some important differences are observed in their microstructure by SAED and HRTEM. Ordering between vacancies, Li+ and La3+ or Nd3+ and twinning of the NbO6 or TiO6 octahedra tilting system are shown in La0.29Li0.12NbO3 and Nd0.58Li0.26TiO3, which are the materials having a lower ionic conductivity. The La0.58Li0.26TiO3 and La0.67Li0.25Ti0.75Al0.25O3 oxides do not show ordering between cations.
Ferroelectric Tunability Studies in Doped BaTiO3 and Ba1−xSrxTiO3
Authors: Li Dong , Subramanian M. A. 
Published: April 2011

ABSTRACTAcceptor and Donor codoped BaTiO3 and Ba1−xSrxTiO3 are prepared. For Ba1−xLaxTi1−xFexO3,BaTiO3 remains as tetragonal phase up to about 5mol% LaFeO3. For x ≥0.06, the structure changes to cubic at room temperature. The phase change shifts the Curie temperature to lower value and increases the tunability at room temperature. Doping of other acceptor (Al, Cr) and donor (Sm, Gd, Dy) ions has the same effect although with varying levels of tuning. BaTiO3: 4%LaFeO3 has the highest tunability among the studied systems, which is even higher than Ba0.6Sr0.4TiO3. Co-doping of (La, Fe) and (La, Al) in Ba1−xSrxTiO3 also lowers the Curie temperature and increases the tunability of high Ba content samples at cryogenic temperature.
Electronic and Optical Studies of Coexisting 5- and 6-Atom Rings in Tetrahedral a-C
Authors: Valladares R.M. , Calles A.G. , Nelis M.A. Mc , Valladares Ariel A. 
Published: April 2011

ABSTRACTWe simulate both the amorphous cluster a-C57H52 that contains 5-atom planar rings and 6-atom boat-type rings and the crystalline cluster c-C59H60 that contains only chair-type 6-atom rings, as in the tetrahedral crystal (diamond structure). We carry out ab initio calculations using the DMol code and report the total energy, the electronic (density of states) structure and optical properties of the two types of clusters, with and without relaxation of the structures, in order to see the effect of the types of atom rings found in the amorphous structures compared to those found in the crystal structure.
The Synthesis and Characterization of a Novel Gallium Diphosphonate
Authors: Attfield Martin P. , Harvey Howard G. 
Published: April 2011

ABSTRACTThe gallium diphosphonate, Ga2[O3PC2H4PO3](H2O)2F2·H2O, has been prepared under solvothermal conditions from a HF/ pyridine solvent system. The structure of the material was determined by Rietveld refinement using the isostructural aluminium compound as the starting model for refinement. The structure consists of chains of corner sharing GaO4F2 octahedra with bridging F atoms linking the octahedra. The chains of octahedra are linked together in two directions perpendicular to the chain direction by the ethylenediphosphonate groups. The resulting structure contains a one-dimensional channel system in which extra-framework water molecules reside. Thermal gravimetric analysis of the material shows that 40 % of the extra- framework water can be reversibly absorbed. Thermodiffraction studies show that the material retains its structural integrity until all the extra-framework water is lost and then undergoes a considerable loss in crystallinity as the framework water molecules are lost. The crystallinity of the material then progressively decreases as the temperature is increased.
Microcrystalline Germanium Photodetectors
Authors: Krause M. , Stiebig H. , Carius R. , Wagner H. 
Published: April 2011

ABSTRACTFor sensor applications in the detection of near infrared light we have prepared μc-Ge:H by plasma enhanced chemical vapor deposition (PECVD) with a mixture of germane and hydrogen, investigatedits structural and electronic properties and incorporated it into thin pin diodes. In order to ensure microcrystalline growth we had to use high hydrogen dilution. However, only the material prepared with a ratio of germane to hydrogen of 0.2% shows high crystallinity. The optical absorption is remarkably different from c-Ge and exhibits no indication of a direct gap at 0.8eV. When this material is implemented as part of a 110nm thin absorber, a short circuit current of 20mA/cm2 and a quantum efficiency of 15% at a wavelength of 1.1μm are achieved. Higher germane concentrations in hydrogen lead to poor electronic properties due to an increase of the amorphous phase and the short circuit current of the devices deteriorates. As for crystalline germanium photodiodes cooling of the devices is used to overcome the restrictionoriginating from the high free carrier concentration.
Hydrogen Diffusion Duringc Amorphous Silicon Growth and Its Consequences for the Transition to Nanocrystalline Growth
Authors: Crandall Richard S. , Thiesen Jack 
Published: April 2011

ABSTRACTUsing a closed-form, time-dependent solution to the partial differential equation representing hydrogen diffusion in the frame of a growing silicon film we obtain considerable insight into processes controlled by mobile H in silicon. We apply this insight to quantitatively explain the limiting thickness for Si homoepitaxy and explain by analogy the incubation thickness before the transition from amorphous to nanocrystalline silicon growth.
p-type Window Layers for pin Solar Cells Entirely Fabricated by Hot-Wire CVD
Authors: Weber Urban , Koob Markus , Mukherjee Chandrachur , Chandrashekhar D. , Dusane Rajiv O. 
Published: April 2011

ABSTRACTWe investigate a-SiC:H p-layer deposition for a-Si:H-based solar cells by Hot-Wire CVD using alternatively methane, ethane, and acetylene. Carbon incorporation in the film results from gas-phase reactions and not from direct dissociation at the hot filament for all hydrocarbon gases. Ethane can be dissociated more easily than methane allowing less extreme deposition conditions. With all types of materials the requirements of high dark conductivity and high band gap for the use as window layers in solar cells can be fulfilled. Highest conductivity is observed with ethane indicating a better network structure, which is supported by the IR signatures. A larger band gap (>2 eV) can be obtained at a similar conductivity with the use of acetylene. We compare these results with the utilization of [.proportional]c-Si:H p-layers. All types of p-layers are incorporated into pin solar cells. Methane- and ethane-based a-SiC:H-p-layers yield similar Voc and FF (∼850 mV and 72%). Acetylene-p-layer-based solar cells yield higher current and higher Voc (890 mV) but lower fill factor (∼67%). Microcrystalline p-layers improve Voc and FF up to 900 mV and 72%, respectively, however higher absorption leads to lower short circuit current and prevents an increase of initial efficiency beyond 8%. Using ethane for p-layer deposition, a significant improvement of the stability of all-Hot-Wire CVD pin solar cells is achieved.
Low-Temperature (630°C) Epitaxy of Silicon on Seeded Glass by Ion-Assisted Deposition
Authors: Aberle Armin G. , Harder Nils-Peter , Oelting Stefan 
Published: April 2011

ABSTRACTThin polycrystalline silicon (poly-Si) films on glass are of great interest for electronic and photovoltaic applications. In this paper we report the successful realisation of thick (5 microns), large-grained (5 microns), continuous and smooth poly-Si films on glass at a temperature below 700°C. This significant technological milestone was achieved by using ion-assisted deposition (IAD) of silicon at 630°C for thickening a thin poly-Si seeding layer made on the glass at about 500°C by aluminium-induced crystallisation of amorphous silicon. Transmission electron micro-graphs reveal an epitaxial relation between about 50% of the grains of the IAD-grown layer and the seeding layer, while the remainder grew with a different orientation than the seed grains. The grain size of the IAD-grown poly-Si film is determined by the grain size of the underlying polySi seeding layer, a clear experimental proof of the “seeding layer concept”.