bluuebunny/crossref_metadata_2025_split huggingface dataset:
Design of a 364 nm Electrically Pumped Multi-Quantum Well Continuous Wave Nitride Vertical Cavity Surface Emitting Laser
Authors: Luke Shelia C. , Motayed Abhishek , Christou Aris 
Published: April 2011

ABSTRACTShort-wavelength vertical cavity surface emitting lasers (VCSELS) are typically important for high-density optical memory and optical imaging systems. An AlGaN/GaN multiquantum well (MQW) VCSEL was designed for operation at wavelength of 364 nm. The design process consisted of careful selection of materials and optimization of the parameters for distributed Bragg reflector (DBR) stacks, active region, and calculation of the threshold current density, external quantum efficiency, and threshold carrier concentration. InGaN/AlGaN material system was selected for constructing DBR stacks in order to minimize power dissipation and to achieve high reflectivity. A reflectivity of 99% was calculated for 33 pairs in the bottom DBR stack and 97% was calculated for 30 pairs in the top DBR stack. An external quantum efficiency of 85% has been achieved through parametric optimization. Results were comparable to that of the similar structures based on the review of recent literatures.
Optical Properties of Al0.70Ga0.30As:Er,Yb
Authors: Uekusa Shin-ichiro , Tanaka Isao , Arai Tomoyuki. 
Published: April 2011

ABSTRACTErbium (Er) ions were co-implanted with ytterbium (Yb) into Al0.70Ga0.30As substrates and we realized an increase in the intensity of Er intra-4f-shell luminescence. The photoluminescence (PL) intensity of Er-related dominant peak (1538.2nm) was enhanced by co-implanted Yb. The thermal quenching was improved. PL intensity of Yb-related emission was decreased. We studied the transfer energy and the optical sensitization of Yb ions co-implanted with Er ions in Al0.70Ga0.30As. Energy transfers from 2F5/2 (the first excited state) → 2F7/2 (the ground state) of Yb3+ to 4I13/2 (the first excited state) → 4I15/2 (the ground state) of Er3+ were observed by PL excitation (PLE) and selectively excited PL (SPL).
Rotating Anisotropies Without Superparamagnetic Grains in Exchange Bias Systems
Authors: Schulthess T. C. 
Published: April 2011

ABSTRACTWe introduce a random field model with anisotropic interfacial exchange for ferromagnet / antiferromagnet bilayers and calculate average coupling energies and coupling directions for different grain or domain sizes. The model is shown to reconcile Malozemoff's random field Ising model for exchange bias with the spinflop coupling mechanism. Furthermore, we find that for small and mid size domains, the average coupling direction strongly varies between different interfacial configurations. We show how this behavior can lead to rotating anisotropies in systems with very strong magnetocrystalline anisotropies, where the use of superparamagnetic grains or domains in the antiferrofmagnet is not justified.
Atomic Structure, Band Offsets and Hydrogen in High K oxide:Silicon interfaces
Authors: Robertson J , Peacock P W 
Published: April 2011

AbstractThe bonding, electronic structure and valence band offsets are calculated for various atomic models of interfaces between Si and high dielectric constant insulators ZrO2 and SrTiO3. A non-polar face of the oxide does not necessarily give a semiconducting interface, because of the need to fill Si dangling bond states on the Si side. This can be achieved by stoichiometry changes. Band offsets of semiconducting interfaces are generally the same as those found from bulk charge neutrality levels, indicating no dipole layers. Dipole layers are present at some configurations, where the offset is then changed by up to 1 eV. The states of hydrogen in the oxides are also considered.
Wide-gap P-type Conductive Properties in Layered Oxychalcogenides
Authors: Ueda Kazushige , Takafuji Kouhei , Hiramatsu Hidenori , Ohta Hiromichi , Hirano Masahiro , Hosono Hideo , Kawazoe Hiroshi 
Published: April 2011

ABSTRACTSeveral Cu(I)-containing layered oxysulfides were selected as candidates for wide-gap p-type semiconductors by extending a concept of a materials design for transparent p-type conducting oxides. The electrical and optical properties of the selected oxysulfides were investigated, and their electronic structures were analyzed by energy band calculations. LaCuOS, Sr2Cu2ZnO2S2 and Sr2CuGaO3S were found to be wide-gap p-type semiconductors, and LaCuOS showed the largest energy gap (Eg=3.1eV) among these layered oxysulfides. It was also found that LaCuOS shows band edge emission under uv excitation at room temperature, which is consistent with the results of the energy band calculations that LaCuOS has a direct-allowed-type energy gap at Γ point. In further materials research, analogous layered oxychalcogenides such as LaCuOSe and LnCuOS (Ln=Pr, Nd) were found to show similar optical and electrical properties to those of LaCuOS. Therefore, it is considered that the layered crystal structure and the electronic structure are responsible for the wide-gap p-type conductive properties in these materials.
Quantitative Characterization Of Morphological Evolution In Q = 2 Potts Model Aluminum Thin Films
Authors: Alsem D. H. , Stach E. A. , de Hosson J. Th. M. 
Published: April 2011

ABSTRACTIn this research, we have focused on the morphological evolution of a model metal film / silicon substrate system. When aluminum (Al) is physical vapor deposited on (100) oriented single crystal silicon (Si) at 280 °C it grows heteroepitaxially. Crystallographically, the resulting films are a Potts model system with degeneracy two; their topology simulates a polycrystalline grain structure, with the two types of grains enveloping each other in a convoluted, maze-like arrangement. The morphological evolution of the films during annealing was determined via a combination of transmission electron microscopy (TEM) and atomic force microscopy (AFM). Films with thickness' of 100 to 500 nm were annealed in-situ in the TEM from 250 °C to 550 °C, and characterized using a (220) two-beam condition from one of the grain orientations. This yields predominantly white on black images, which are amenable to quantitative image processing techniques. As anticipated, the grain size increased linearly during the first annealing cycle. Also so-called sub-grain boundaries were observed. These low angle boundaries originate during film deposition when arrays of dislocations are introduced in the areas where coalescing islands of the same orientation meet. After cooling and reheating the sub-grain boundaries disappear. This happens because upon cooling, misfit dislocations were introduced into the films to relieve the thermal misfit strain, resulting in a dense array of dislocations at the film / substrate interface. These thermal misfit dislocations interact strongly with pre-existing sub-grain dislocations; this, in effect, heals the microstructure.
Room temperature grain growth in sputtered Cu films
Authors: Deduytsche D. , Detavernier C. , Debaerdemaeker J. , Van Meirhaeghe R.L. , Dauwe C. , Kuan T.S. 
Published: April 2011

ABSTRACTThe results are presented of a study on grain growth in magnetron sputter deposited Cu films. Similarly to the well-known case of electroplated Cu, we observed significant changes in the microstructure of sputter deposited films during storage at room temperature, as evidenced by a decrease in sheet resistance (up to 40% decrease in less than 6 hours). Direct evidence for grain growth was observed by FIB and SEM and by a strong decrease in the XRD peak width. Moreover, a decrease in the total defect content of the film was observed as a function of storage time from positron annihilation experiments, probably reflecting the decreasing number of grain boundaries in the film. It is well known that the microstructure of an as-deposited film is strongly dependent on the deposition parameters. This relationship is summarized in the Thornton structure-growth zone diagram [7]. We have studied the kinetics of room temperature grain growth as a function of Ar pressure and substrate temperature for Cu films with a thickness between 50 nm and 1 micrometer. It is found that spontaneous grain growth during storage at room temperature occurs mainly for a zone T microstructure (at low Ar pressure and a substrate temperature below 40°C), while it does not occur for zone I (high Ar pressure, low substrate temperature) or zone II (substrate temperature > 100°C) microstructures.
Ge/Si(100) Island and Wetting Layer Composition
Authors: Zhang Yangting , Drucker Jeff 
Published: April 2011

ABSTRATCTEtching water soluble Ge-oxides was used to investigate Si interdiffusion into epitaxial Ge / Si(100) samples. The Ge coverage, θGe, was measured using Rutherford backscattering spectrometry (RBS) before and after water etching of samples grown at substrate temperatures between 400 °C and 650 °C. θGewas correlated with sample morphology determined using atomic force microscopy (AFM). The local Ge concentration was qualitatively assessed using energy dispersive x-ray (EDX) analysis. For samples grown at T=400 °C, water completely dissolves the islands and no Ge is detected by RBS. For samples grown at T=600 and 650 °C, AFM detects no change in the surface morphology and RBS indicates that θGedecreases by about 3 monolayers (ML). These results suggest that for growth at T=400 °C, both the islands and wetting layer are relatively pure Ge while for growth at T≥600 °C, the wetting layer is Ge rich compare to the SiGe alloy islands. EDX confirms this conclusion detecting no Ge signal between islands for etched samples grown at T≥600 °C. Our results suggest that for growth at T≥600 °C, Si interdiffusion into islands is through the region underneath the islands instead of from the wetting layer.
STM studies of island nucleation during hyperthermal atom deposition
Authors: Pomeroy Joshua M. , Brock Joel D. 
Published: April 2011

ABSTRACTWe report fundamental changes in island nucleation dynamics as the kinetic energy of the constituent particles used for film grown is increased. A hyperthermal energy ion beam-line with precise control over ion kinetic energy was used to grow copper islands on a Cu(100) substrate. Dramatic increases in island densities were observed with increasing kinetic energy from thermal energies to 150 eV. We find that sputter erosion and the formation of adatom-vacancy pairs contribute to this increase. In addition, variations in flux and temperature suggest that the mean-field scaling exponent is sensitive to atomistic mechanisms activated by the ion beam.
Time-Dependent Capture Numbers with Repulsive Pair Interactions: Cu/Cu(111) and Ge/Si(001)
Authors: Venables J. A. , Brune H. , Drucker J. 
Published: April 2011

ABSTRACTRecent experiments and calculations have shown that weak repulsive interactions between adsorbate atoms may shift nucleation kinetics from the well-known diffusion limit towards the attachment-limited case. The distinctions between diffusion- and attachment-limited kinetics are clarified, and the increased importance of the transient nucleation regime in the latter case is shown to be due to a combination of delayed nucleation and reduced capture. A time-dependent interpolation scheme between attachment- and diffusion-limited capture numbers is proposed, and tested against KMC simulations. Using this scheme to interpret recent STM results on Cu/Cu(111), bounds on the maximum adatom-adatom potential repulsive energy of 12±2 meV are deduced. Time-dependent effects also occur in the growth and ripening of strained Ge islands on Si(001), and the similarities and differences between these two systems are discussed.
Structural properties of SiGe islands: Effect of capping
Authors: Stangl Julian , Hesse Anke , Holý Vaclav , Bauer Günther , Denker Ullrich , Schmidt Oliver G. , Kirfel O. , Gruützmacher D. 
Published: April 2011

ABSTRACTSelf-organized nanostructures are increasingly important for novel optoelectronic devices, as high densities of quantum dots can be deposited directly during heteroepitaxial growth. Due to the elastic relaxation in 3D nanostructures, properties differing from 2D systems can be achieved, e.g., higher Ge contents in SiGe islands than in planar layers with the same total amount of Ge can be realized, enhancing the design freedom for, e.g., detectors or high frequency transistors. However, when such nanostructures are overgrown, which is inevitable for any application, they often undergo significant changes, partly eliminating their distinct advantages. We present an investigation of SiGe islands formed by deposition of pure Ge on Si (001). X-ray scattering and diffraction is employed to obtain the shape, strain and Ge distribution in uncapped as well as Si-capped islands. For the analysis, model calculations of the strain fields are required. The finite element method is used for uncapped islands, for buried nanostructures a novel analytical approach has been developed. For Tgrowth ∼ 650°C, optimized for island formation, the Ge content in the top part of uncapped islands reaches 100%, at an elastic relaxation of about 50%. During capping, the maximum Ge content considerably decreases to about 50%. At the same time, the islands become very flat, with a decrease in height from 13 to 6nm accompanied by an increase in base diameter from about 100 nm to 180 nm. The elastic relaxation decreases to about 2.5%, so that the buried islands have properties not very much different from 2D layers. Lowering the growth temperature for the Si cap, this effect can be avoided, for Tgrowth as low as 460°C, the structure of the uncapped islands can indeed be preserved.
Microstructural characterization of porous thin films
Authors: Djurfors B. , Brett M.J. , Ivey D.G. 
Published: April 2011

ABSTRACTA preliminary TEM investigation of porous films fabricated by glancing angle deposition has shown the importance of surface diffusion and crystal structure in determining the final microstructure of the film. Although the process variables are extremely important in defining the morphology of the film, their effects are limited in terms of final microstructure.
X-ray scattering studies of low-T Ag(001) and Cu(001) homoepitaxy: Vacancy trapping and surface morphology evolution
Authors: Botez Cristian E. , Li Kaile , Lu Erdong D. , Miceli Paul F. , Conrad Edward H. , Stephens Peter W. 
Published: April 2011

ABSTRACTWe have used synchrotron x-ray diffraction to study the low-T homoepitaxial growth on Ag(001) and Cu(001) surfaces. For both systems, we found that a large, temperature-dependent vacancy concentration is incorporated in films grown below 160K. The vacancy trapping occurs concomitantly with substantial changes in the surface morphology, where a non-monotonic temperature dependence of the mean-square surface roughness has been previously observed. For Cu/Cu(001) we also found that the concentration of vacancies incorporated at 110K evolves, upon heating, according to the vacancy annealing behavior well-known from radiation damage studies of bulk copper and it is consistent with the activation energy for vacancy mobility.
Effects of the Amorphous Oxide Intergranular Layer Structure and Bonding on the Fracture Toughness of a High Purity Silicon Nitride
Authors: Ziegler A. , Kisielowski C. , Hoffmann M. J. , Ritchie R. O. 
Published: April 2011

ABSTRACTThe microstructural evolution and structural characteristics and transitions in the thin grain-boundary oxide films in a silicon nitride ceramic, specifically between two adjacent grains and not the triple junctions, are investigated to find their effect on the macroscopic fracture properties. It is found that by heat treating a model Si3N4-2wt% Y2O3ceramic for ∼200 hr at 1400°C in air, the fracture toughness can be increased by ∼100%, coincident with a change in fracture mechanism from transgranular to intergranular. Structural phase transformations occur in the thin grain boundaries during oxidation that are revealed by XRD, EDX, Raman and EELS analyses. They affect the local bonding of atoms. It is concluded that only specific crystal “building blocks”, i.e., tetrahedra, are transformed along the grain boundary and the resulting difference in the atomic structure of the oxide interface is seen directly to alter the macroscopic fracture behavior.
Grain Boundary Segregation in Titanium Dioxide: Evaluation of Relative Driving Forces for Segregation
Authors: Wang Qinglei , Lian Guoda D. , Dickey Elizabeth C. 
Published: April 2011

ABSTRACTSolute segregation to grain boundaries is a fundamental phenomenon in polycrystalline metal-oxide electroceramics that has enormous implications for the macroscopic dielectric behavior of the materials. This paper presents a systematic study of solute segregation in a model dielectric, titanium dioxide. We investigate the relative role of the electrostatic versus strain energy driving forces for segregation by studying yttrium-doped specimens. Through analytical transmission electron microscopy studies, we quantitatively determine the segregation behavior of the material. The measured Gibbsian interfacial excesses are compared to thermodynamic predictions.
Phase Evolution During Crystallization of an Amorphous TiAl Film
Authors: Senkov O.N. , Uchic M.D. 
Published: April 2011

ABSTRACTPhase evolution during crystallization of an amorphous TiAl film produced by physical vapor deposition (PVD) was studied using differential thermal analysis (DTA), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The following sequence of phase transformations was observed during continuous heating from room temperature: amorphous → body centered cubic (β) → hexagonal close-packed (α) → tetragonal (γ) → ordered α2. The β phase was formed as near-spherical particles that were evenly distributed in the amorphous phase, and the size of these particles was approximately 90 nm. Formation of the α phase by decomposition of β and the remaining amorphous phases led to a very fine feathery-like microstructure arranged in colonies of approximately 100 nm in size. The transformation of the metastable α phase into a mixture of the γ and α2 phases led to formation of an equiaxed γ-grain structure with a grain size of about 150 nm after heating to 850°C.
Diffusion and Relaxation in Glasses and their Melts
Authors: Caprion D. , Kluge M. , Schober H. R. 
Published: April 2011

ABSTRACTResults of computer simulations of diffusion and relaxation in the liquid and the glassy state are presented for model systems and Cu33Zr67 and Se. The diffusion constant and the intermediate self scattering function show the typical behavior upon quenching from the liquid to the glass. The diffusion constant in the undercooled liquid can be fitted both with a Vogel-Fulcher or a mode coupling law. However, its pressure derivative, the activation volume, clearly follows the prediction of the mode coupling theory. From the diffusional isotope effect in the liquid we conclude that collectivity increases upon quenching to the glass transition. Checking the relaxations in the glass we find collective hopping of chains of atoms as elementary process. Both in the glassy state and in the undercooled liquid we observe an increase oc ∝√ι ofthe non-Gaussianity of the self correlation function, which measures the dynamic heterogeneity. This can be understood from the collective jump mechanism.
Relaxation Dynamics of Glass-Forming Liquids Studied by Ultrasonic Spectroscopy: Stretched Response of Supercooled Ethylbenzene
Authors: Cutroni M. , Mandanici A. 
Published: April 2011

ABSTRACTGlass forming liquids exhibit their strong or fragile behaviour as a function of temperature, featuring a smaller or greater deviation from a simple Arrhenius law. The size of such deviation on a typical time scale of 10-6 s characterizes the ‘kinetic fragility’ F1/2 of a given material. Ultrasonic experiments in the MHz region are then suitable to show how the relaxational properties are influenced by the ‘fragile’ character of the liquid investigated. On this basis, measurements of acoustic attenuation at fixed frequency (15 MHz) have been performed on glass forming liquid ethylbenzene in the temperature range 100 K-300 K and compared with previous results on simple supercooled liquids derived from benzene. To prevent crystallization each temperature point below 190 K was reached by cooling the sample directly from 300 K and using also different cooling rates. Measurements have given evidence of a mechanical relaxation process: below the melting temperature the acoustic attenuation exhibits a peak and correspondingly the sound velocity increases from liquid-like to solid-like values. A stretched response function is required to reproduce the observed behaviour. The values of the Kohlrausch-Williams-Watts stretching parameter βkww which describe the experimental data are very low, if compared to those obtained for other glass-forming liquids.
Calculated Effect of Alloy Additions on the Saturation Magnetization of Fe0.80B0.20
Authors: Nicholson D. M. C. , Wang Yang , Widom Mike 
Published: April 2011

ABSTRACTThe great number of different local environments in amorphous alloys leads to the evolution of complicated non collinear magnetic structures. Alloy additions can affect the magnetic structure in surprising ways. For example, replacement of a small amount of Fe with Co increases the saturation magnetization even though Co has a much smaller moment than Fe. The calculated behavior of the magnetic structure of (Fe(1-x)Mx) 0.8B0.2 with M=Co, Cr, Zr, and Mn2Zr are presented.
Structure of Shear Bands in Zirconium-Based Metallic Glasses Observed by Transmission Electron Microscopy
Authors: Gu Xiaofeng , Livi Kenneth J. T. , Hufnagel Todd C. 
Published: April 2011

ABSTRACTWe have used transmission electron microscopy (TEM) to investigate the structure of shear bands produced by bending electron-transparent Zr52.5Cu17.9Ni14.6Al10Ti5metallic glass specimens. Shear bands were located by comparing the structure of the specimens before and after deformation. The shear band spacing is influenced by the structure of the specimen; portions of the specimen with a significant population of nanocrystals show a smaller separation between shear bands. Quantitative high resolution TEM analysis based on ratio technique has been used to explore the defect structure in shear bands. High density and void-like defects with size of about 1 nm were found in shear bands formed in both amorphous and nanocrystalline areas. A simple model was proposed to explain the formation of these defects.
Elevated Temperature Fatigue Crack Propagation of a Zr-Ti-Cu-Ni-Be Bulk Metallic Glass
Authors: Hess Peter A. , Dauskardt Reinhold H. 
Published: April 2011

ABSTRACTThe sub-Tg elevated temperature fatigue propagation behavior of a Zr-based bulk metallic glass was examined. Fatigue crack-growth rates as a function of the applied stress intensity range, ΔK, are reported. Increased testing temperature was found to increase ΔKTH. Examination of crack surfaces at near-threshold growth rates revealed the presence of elongated ridges perpendicular to the crack front at higher temperatures; these ridges were absent in specimens tested at lower temperatures. The size of the features was found to increase with temperature and applied ΔK, finally breaking up at higher growth rates. Mechanisms responsible for the ridge formation are described, and modeled in terms of fluid meniscus instabilities that occur at the fatigue crack tip.