Neutron irradiation-infrared based measurement method for interstitial oxygen in heavily boron-doped silicon

A novel method, based on an infrared absorption and neutron irradiation technique, has been developed for the determination of interstitial oxygen in heavily boron-doped silicon. The new procedure utilizes fast neutron irradiated silicon wafer specimens. On fast neutron irradiation, the free carriers of high concentration in silicon can be trapped by the irradiated defects and the resistivity increased. The resulting calibration curve for the measurement of interstitial oxygen in boron-doped silicon has been established on the basis of the annealing behaviour of irradiated boron-doped CZ silicon.

New method for the growth of highly uniform quantum dots

A new method is realized for the growth of self-formed quantum dots. We identify that dislocation-free islands can be formed by the strain from the strained superlattice taken as a whole. Unlike the Stranski-Krastanow (S-K) growth mode, the islands do not form during the growth of the corresponding strained single layers. Highly uniform quantum dots can be self-formed via this mechanism. The low temperature spectra of self-formed InGaAs/GaAs quantum dot superlattices grown on a (001) GaAs substrate have a full width at half maximum of 26-34 meV, indicating a better uniformity of quantum dot size than those grown in the S-K mode. This method can provide great degrees of freedom in designing possible quantum dot devices. 1998 Published by Elsevier Science B.V. All rights reserved.

Native deep level defects in ZnO single crystal grown by CVT method - art. no. 68410I

Hall effect, photoluminescence (PL), infrared absorption, deep level transient spectroscopy (DLTS), and Raman scattering have been used to study property and defects of ZnO single crystal grown by a chemical vapor transport method (CVT). As-grown ZnO is N type with free electron density Of 10(16)-10(17)cm(-3). It has a slight increase after 900 degrees C annealing in oxygen ambient. The DLTS measurement revealed four deep level defects with energy at 0.30eV, 0.50eV, 0.68eV and 0.90eV in the as-grown ZnO sample, respectively. After the high temperature annealing, only the 0.5eV defect survive and has a concentration increase. PL results of the as-grown and annealed ZnO indicate that the well-known green emission disappear after the annealing. The result suggests a correlation between the 0.68eV defect and the green PL peak. Results of P-doped ZnO were also compared with the undoped ZnO sample. The nature of the defects and their influence on the material property have been discussed.

Metal-free growth of Si/SiO2 nanowires by annealing SiOx (x < 2) films deposited by PECVD

A new metal catalysis-free method of fabricating Si or SiO2 nanowires (NWs) compatible with Si CMOS technology was proposed by annealing SiOx (x < 2) films deposited by plasma -enhanced chemical vapor deposition (PECVD). The effects of the Si content (x value) and thickness of SiOx films, the annealing process and flowing gas ambient on the NW growth were studied in detail. The results indicated that the SiOx film of a thickness below 300 rim with x value close to 1 was most favorable for NW growth upon annealing at 1000-1150 degrees C in the flowing gas mixture of N-2 and H-2. NWs of 50-100nm in diameter and tens of micrometers in length were synthesized by this method. The formation mechanism was likely to be related to a new type of oxide assisted growth (OAG) mechanism, with Si nanoclusters in SiOx films after phase separation serving as the nuclei for the growth of NWs in SiOx films > 200nm, and SiO molecules from thin SiO, film decomposition inducing the NW growth in films < 100nm. An effective preliminary method to control NW growth direction was also demonstrated by etching trenches in SiOx films followed by annealing.

Effects of multistage coupling and disk flexibility on mistuned bladed disk dynamics

The effects ofdisk flexibility and multistage coupling on the dynamics of bladed disks with and without blade mistuning are investigated. Both free and forced responses are examined using finite element representations of example single and two-stage rotor models. The reported work demonstrates the importance of proper treatment of interstage (stage-to-stage) boundaries in order to yield adequate capture of disk-blade modal interaction in eigenfrequency veering regions. The modified disk-blade modal interactions resulting from interstage-coupling-induced changes in disk flexibility are found to have a significant impact on (a) tuned responses due to excitations passing through eigenfrequency veering regions, and (b) a design's sensitivity to blade mistuning. Hence, the findings in this paper suggest that multistage analyses may be required when excitations are expected to fall in or near eigenfrequency veering regions or when the sensitivity to blade mistuning is to be accounted for Conversely, the observed sensitivity to disk flexibility also indicates that the severity of unfavorable structural interblade coupling may be reduced significantly by redesigning the disk(s) and stage-to-stage connectivity. The relatively drastic effects of such modifications illustrated in this work indicate that the design modifications required to alleviate veering-related response problems may be less comprehensive than what might have been expected.

practical type checking of functions defined on context-free languages

A type checking method for the functional language LFC is presented. A distinct feature of LFC is that it uses Context-Free (CF) languages as data types to represent compound data structures. This makes LFC a dynamically typed language. To improve efficiency, a practical type checking method is presented, which consists of both static and dynamic type checking. Although the inclusion relation of CF.languages is not decidable,a special subset of the relation is decidable, i.e., the sentential form relation, which can be statically checked.Moreover, most of the expressions in actual LFC programs appear to satisfy this relation according to the statistic data of experiments. So, despite that the static type checking is not complete, it undertakes most of the type checking task. Consequently the run-time efficiency is effectively improved. Another feature of the type checking is that it converts the expressions with implicit structures to structured representation. Structure reconstruction technique is presented.

pattern matching compilation of functions defined in context-free languages

LFC is a functional language based on recursive functions defined in context-free languages. In this paper, a new pattern matching algorithm for LFC is presented, which can represent a sequence of patterns as an integer by an encoding method. It is a rather simple method and produces efficient case-expressions for pattern matching definitions of LFC. The algorithm can also be used for other functional languages, but for nested patterns it may become complicated and further studies are needed.

Free radial oscillation of a sphere in relativistic elasticity theory

The spherically symmetric free radial oscillation in the first post-Newtonian approximation for a homogeneous and isotropic elastic sphere with a constant density is studied. Based on the Xu, Wu, and Soffel formalism, the relation of the oscillation frequency of the sphere with the radius, mass density, and elastic constants of the sphere is derived by using the successive approximation method.

Heteroepitaxial Growth and Heterojunction Characteristics of Voids-Free n-3C-SiC on p-Si(100)

Highly oriented voids-free 3C-SiC heteroepitaxial layers are grown on φ50mm Si (100) substrates by low pressure chemical vapor deposition (LPCVD). The initial stage of carbonization and the surface morphology of carbonization layers of Si(100) are studied using reflection high energy electron diffraction (RHEED) and scanning electron microscopy (SEM). It is shown that the optimized carbonization temperature for the growth of voids-free 3S-SiC on Si (100) substrates is 1100 ℃. The electrical properties of SiC layers are characterized using Van der Pauw method. The I-V, C-V, and the temperature dependence of I-V characteristics in n-3C-SiC-p-Si heterojunctions with AuGeNi and Al electrical pads are investigated. It is shown that the maximum reverse breakdown voltage of the n-3C-SiC-p-Si heterojunction diodes reaches to 220V at room temperature. These results indicate that the SiC/Si heterojunction diode can be used to fabricate the wide bandgap emitter SiC/Si heterojunction bipolar transistors (HBT's).

New method for the growth of highly uniform quantum dots

A new method is realized for the growth of self-formed quantum dots. We identify that dislocation-free islands can be formed by the strain from the strained superlattice taken as a whole. Unlike the Stranski-Krastanow (S-K) growth mode, the islands do not form during the growth of the corresponding strained single layers. Highly uniform quantum dots can be self-formed via this mechanism. The low temperature spectra of self-formed InGaAs/GaAs quantum dot superlattices grown on a (001) GaAs substrate have a full width at half maximum of 26-34 meV, indicating a better uniformity of quantum dot size than those grown in the S-K mode. This method can provide great degrees of freedom in designing possible quantum dot devices. 1998 Published by Elsevier Science B.V. All rights reserved.

A method to determine fracture toughness using cube-corner indentation

For the cube-corner indenter, an approximate linear relationship between the ratio of hardness (H) to reduced modulus (E-r) and the ratio of unloading work (W-u) to total loading work (W-t) is confirmed by finite-element calculations and by experiments. Based on this relationship a convenient method to determine the fracture toughness (K-IC) of brittle materials, especially for those at small scale, using cube-corner indentations is proposed. Finally, the method is calibrated by indentation experiments on a set of brittle materials. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A global shallow water model using high order multi-moment constrained finite volume method and icosahedral grid

A novel accurate numerical model for shallow water equations on sphere have been developed by implementing the high order multi-moment constrained finite volume (MCV) method on the icosahedral geodesic grid. High order reconstructions are conducted cell-wisely by making use of the point values as the unknowns distributed within each triangular cell element. The time evolution equations to update the unknowns are derived from a set of constrained conditions for two types of moments, i.e. the point values on the cell boundary edges and the cell-integrated average. The numerical conservation is rigorously guaranteed. in the present model, all unknowns or computational variables are point values and no numerical quadrature is involved, which particularly benefits the computational accuracy and efficiency in handling the spherical geometry, such as coordinate transformation and curved surface. Numerical formulations of third and fourth order accuracy are presented in detail. The proposed numerical model has been validated by widely used benchmark tests and competitive results are obtained. The present numerical framework provides a promising and practical base for further development of atmospheric and oceanic general circulation models. (C) 2009 Elsevier Inc. All rights reserved.

A finite-element algorithm for modeling variably saturated flows

A general numerical algorithm in the context of finite element scheme is developed to solve Richards’ equation, in which a mass-conservative, modified head based scheme (MHB) is proposed to approximate the governing equation, and mass-lumping techniques are used to keep the numerical simulation stable. The MHB scheme is compared with the modified Picard iteration scheme (MPI) in a ponding infiltration example. Although the MHB scheme is a little inferior to the MPI scheme in respect of mass balance, it is superior in convergence character and simplicity. Fully implicit, explicit and geometric average conductivity methods are performed and compared, the first one is superior in simulation accuracy and can use large time-step size, but the others are superior in iteration efficiency. The algorithm works well over a wide variety of problems, such as infiltration fronts, steady-state and transient water tables, and transient seepage faces, as demonstrated by its performance against published experimental data. The algorithm is presented in sufficient detail to facilitate its implementation.

Influence of drying method on morphology and properties of asymmetric cellulose hollow fiber membrane

Using a dry/wet spinning process, asymmetric cellulose hollow fiber membranes (CHFM) were prepared from a dope composed of cellulose/N-methylmorpholine-N-oxide/water. The formation mechanism for the finger-like macrovoids at the inner portion of as-spun fibers was explained. Naturally drying and three solvent exchange drying methods were tried to investigate their influence on morphology and properties of CHFM. It was found that the ethanol-hexane exchange drying was an appropriate method to minimize morphology change of the as-spun CHFM, whereas the naturally drying caused the greatest shrinkage of the fibers that made the porous membrane become dense. The result, CHFM from ethanol-hexane exchange drying performed the highest gas permeation rate but gas permeation of the naturally dried membrane could not be detectable. The resultant CHFM from the ethanol-hexane exchange drying also showed acceptable, mechanical properties, thus it was proposed to be an appropriate method for gas separation purpose. The experimental results supported the proposed drying mechanism of CHFM. The free water would evaporate or be replaced by a solvent that subsequently would evaporate but the bonded water would remain in the membrane. What dominated the changes of membrane morphology during drying should be. the molecular affinities of cellulose-water, water-solvent and solvent-solvent. (C) 2004 Elsevier B.V. All rights reserved.

A facile and effective method for the distribution of Mo/HZSM-5 catalyst active centers

Post-steaming treatment of Mo/HZSM-5 catalysts results in more molybdenum species migrating into and residing in the HZSM-5 zeolite channels. This is confirmed by XRF and XPS measurements. H-1 MAS NMR and Si-29 MAS NMR also demonstrate that the number of free Bronsted acid sites decreases in the Mo/HZSM-5 catalysts that underwent post-steaming treatment, compared to untreated Mo/HZSM-5 catalysts. As a result, the deactivation rate constant (kd) on the Mo/HZSM-5 catalyst after post-steaming treatment for 0.5 h is much smaller, and the catalyst therefore shows remarkable stability in the probe reaction of methane dehydro-aromatization. The results suggest that a more beneficial bi-functional balance between active Mo species for methane activation and acid sites for the following aromatization is developed over those Mo/HZSM-5 catalysts that have experienced post-steaming treatment for 0.5 h, in comparison with the untreated Mo/HZSM-5 catalysts.