Shape effects on the yield stress and deformation of silicon nanowires: A molecular dynamics simulation

The tension and compression of single-crystalline silicon nanowires (SiNWs) with different cross-sectional shapes are studied systematically using molecular dynamics simulation. The shape effects on the yield stresses are characterized. For the same surface to volume ratio, the circular cross-sectional SiNWs are stronger than the square cross-sectional ones under tensile loading, but reverse happens in compressive loading. With the atoms colored by least-squares atomic local shear strain, the deformation processes reveal that the failure modes of incipient yielding are dependent on the loading directions. The SiNWs under tensile loading slip in {111} surfaces, while the compressive loading leads the SiNWs to slip in the {110} surfaces. The present results are expected to contribute to the design of the silicon devices in nanosystems.

The effects of surface tension on the elastic properties of nano structures

In the absence of external loading, surface tension will induce a residual stress field in the bulk of nano structures. However, in the prediction of mechanical properties of nano structures, the elastic response of the bulk is usually described by classical Hooke’s law, in which the aforementioned residual stress was neglected in the existing literatures. The present paper investigates the influences of surface tension and the residual stress in the bulk induced by the surface tension on the elastic properties of nano structures. We firstly present the surface elasticity in the Lagrangian and the Eulerian descriptions and point out that even in the case of infinitesimal deformations the reference and the current configurations should be discriminated; otherwise the out-plane terms of surface displacement gradient, associated with the surface tension, may sometimes be overlooked in the Eulerian descriptions, particularly for curved and rotated surfaces. Then, the residual stress in the bulk is studied through the non-classical boundary conditions and used to construct the linear elastic constitutive relations for the bulk material. Finally, these relations are adopted to analyze the size-dependent properties of pure bending of Al nanowires. The present results show that surface tension will considerably affect the effective Young’s modulus of Al nanowires, which decrease with either the decrease of nanowires thickness or the increase of the aspect ratio.

A surface energy model and application to mechanical behavior analysis of single crystals at sub-micron scale

Size effects of mechanical behaviors of materials are referred to the variation of the mechanical behavior due to the sample sizes changing from macroscale to micro-/nanoscales. At the micro-/nanoscale, since sample has a relatively high specific surface area (SSA) (ratio of surface area to volume), the surface although it is often neglected at the macroscale, becomes prominent in governing the energy effect, although it is often neglected at the macroscale, becomes prominent in governing the mechanical behavior. In the present research, a continuum model considering the surface energy effect is developed through introducing the surface energy to total potential energy. Simultaneously, a corresponding finite element method is developed. The model is used to analyze the axial equilibrium strain problem for a Cu nanowire at the external loading-free state. As another application of the model, from dimensional analysis, the size effects of uniform compression tests on the microscale cylinder specimens for Ni and Au single crystals are analyzed and compared with experiments in literatures. (C) 2009 Elsevier B.V. All rights reserved.

On The Thermally Induced Cracking Of A Segmented Coating Deposited On The Outer Surface Of A Hollow Cylinder

In this work, the thermally induced cracking behavior of a segmented coating has been investigated. The geometry under consideration is a hollow cylinder with a segmented coating deposited onto its outer surface. The segmentation cracks are modeled as a periodic array of axial edge cracks. The finite element method is utilized to obtain the solution of the multiple crack problem and the Thermal Stress Intensity Factors (TSIFs) are calculated. Based on dimensional analysis, the main parameters affecting TSIFs are identified. It has been found that the TSIF is a monotonically increasing function of segmentation crack spacing. This result confirms that a segmented coating exhibits much higher thermal shock resistance than an intact counterpart, if only the segmentation crack spacing is narrow enough. The dependence of TSIF on some other parameters, such as normalized time, segmentation crack depth, convection severity as well as material constants, has also been discussed. (C) 2008 Elsevier B.V. All rights reserved.

Stress analysis of ZnO film with a GaN buffer layer on sapphire substrate

A 5.35-mu m-thick ZnO film is grown by chemical vapour deposition technique on a sapphire (0001) substrate with a GaN buffer layer. The surface of the ZnO film is smooth and shows many hexagonal features. The full width at half maximum of ZnO (0002) omega-rocking curve is 161 arcsec, corresponding to a high crystal quality of the ZnO film. From the result of x-ray diffraction theta - 2. scanning, the stress status in ZnO film is tensile, which is supported by Raman scattering measurement. The reason of the tensile stress in the ZnO film is analysed in detail. The lattice mismatch and thermal mismatch are excluded and the reason is attributed to the coalescence of grains or islands during the growth of the ZnO film.

Comparison and combination of several stress relief methods for cubic boron nitride films deposited by ion beam assisted deposition

Cubic boron nitride (c-BN) films were prepared by ion beam assisted deposition (IBAD) technique, and the stresses were primary estimated by measuring the frequency shifts in the infrared-absorption peaks of c-BN samples. To test the possible effects of other factors, dependencies of the c-BN transversal optical mode position on film thickness and c-BN content were investigated. Several methods for reducing the stress of c-BN films including annealing, high temperature deposition, two-stage process, and the addition of a small amount of Si were studied, in which the c-BN films with similar thickness and cubic phase content were used to evaluate the effects of the various stress relief methods. It was shown that all the methods can reduce the stress in c-BN films to various extents. Especially, the incorporation of a small amount of Si (2.3 at.%) can result in a remarkable stress relief from 8.4 to similar to 3.6 GPa whereas the c-BN content is nearly unaffected, although a slight degradation of the c-BN crystallinity is observed. The stress can be further reduced down below I GPa by combination of the addition of Si with the two-stage deposition process. (c) 2008 Elsevier B.V. All rights reserved.

Effect of Nitridation on Morphology, Structural Properties and Stress of AIN Films

We investigate effects of nitridation on AIN morphology, structural properties and stress. It is found that 3 min nitridation can prominently improve AIN crystal structure, and slightly smooth the surface morphology. However, 10 min nitridation degrades out-of-plane crystal structure and surface morphology instead. Additionally, 3-min nitridation introduces more tensile stress (1.5 GPa) in AIN films, which can be attributed to the weaker islands 2D coalescent. Nitridation for 10 min can introduce more defects, or even forms polycrystallinity interlayer, which relaxes the stress. Thus, the stress in AIN with 10 min nitridation decreases to -0.2 GPa compressive stress.

Quantum-dot growth simulation on periodic stress of substrate

InAs quantum dots (QDs) are grown on the cleaved edge of an InxGa1-xAs/GaAs supperlattice experimentally and a good linear alignment of these QDs on the surface of an InxGa1-xAs layer has been realized. The modulation effects of periodic strain on the substrate are investigated theoretically using a kinetic Monte Carlo method. Our results show that a good alignment of QDs can be achieved when the strain energy reaches 2% of the atomic binding energy. The simulation results are in excellent qualitative agreement with our experiments. (C) 2005 American Institute of Physics.

Thermal stress analysis for GaInAsP multiple quantum well wafer chemically bonded to Si (100)

An n-InP-based InGaAsP multiple-quantum-well wafer was bonded with p-Si by chemical surface activated bonding at 70 degrees C, and then annealed at 450 degrees C. Different thermal expansion coefficients between InP and Si will induce thermal stresses in the bonded wafer. Planar and cross-sectional distributions of thermal stress in the bonded InP-Si pairs were analyzed by a two-dimensional finite element method. In addition, the normal, peeling, and shear stresses were calculated by an analytic method. Furthermore, x-ray double crystalline diffraction was applied to measure the thermal strain and the strain caused by the mismatching of the crystalline orientation between InP (100) and Si (100). The wavelength redshift of the photoluminescence (PL) spectrum due to thermal strain was investigated via the calculation of the band structure, which is in agreement with the measured PL spectra.

Study on surface morphology of GaN growth by MOCVD on GaN/Si(111) template

The surface morphology of GaN grown by MOCVD on GaN/Si template was studied. Rough morphology and deep pinhole defects on some surface areas of the samples were observed and studied. The formation of rough morphology is possibly related to Ga-Si alloy produced due to poor thermal stability of template at high temperature. The deep pinhole defects generated are deep down to the surface of MBE-grown GaN/Si template. The stress originated from the large thermal expansion coefficient difference between GaN and Si may be related to the formation of the pinhole defects. The surface morphology of the GaN can be improved by optimizing the GaN/Si template and decreasing the growth temperature.

Influence of ion energy and deposition temperature on the surface morphology of carbon films deposited by ion beams

Carbon films were deposited by mass-selected ion beam technique with ion energies 50-200eV at a substrate temperature from room temperature to 80 degreesC,. For the energies used, smooth diamond-like carbon films were deposited at room temperature. When the substrate temperature was 600 degreesC,rough graphitic films were produced. But highly oriented carbon tubes were observed when the energies were larger than 140eV at 800 degreesC. They were perpendicular to the surface and parallel to each other. preferred orientation of graphite basic plane was observed by high-resolution electron microscopy. Shallow ion implantation and stress are responsible for this orientation.

Stress corrosion studies of ion implanted austenitic steel

Stress corrosion studies of 50 Mn18Cr4 austenitic steel implanted with 120 keV N+, 100 keV Cr+, 200 keV and 400 keV Er+ ions were carried out by constant strain method in the nitrate solution. Surface composition and depth profiles of the implanted material were measured by AES sputter etching technique. The results exhibit that nitrogen implantation has no significant affection to the stress corrosion, but the chromium and erbium implantation has prolonged the incubation period of the stress corrosion cracking. (C) 1999 Kluwer Academic Publishers.

Electrical properties of GaN deposited on nitridated sapphire by molecular beam epitaxy using NH3 cracked on the growing surface

We have found that GaN epilayers grown by NH3-source molecular beam epitaxy (MBE) contain hydrogen. Dependent on the hydrogen concentration, GaN on (0001) sapphire can be either under biaxially compressive strain or under biaxially tensile strain. Furthermore, we notice that background electrons in GaN increase with hydrogen incorporation. X-ray photoelectron spectroscopy (XPS) measurements of the N1s region indicate that hydrogen is bound to nitrogen. So, the microdefect Ga...H-N is an effective nitrogen vacancy in GaN, and it may be a donor partly answering for the background electrons. (C) 1999 Elsevier Science B.V. All rights reserved.

Study on surface morphology of GaN growth by MOCVD on GaN/Si(111) template

The surface morphology of GaN grown by MOCVD on GaN/Si template was studied. Rough morphology and deep pinhole defects on some surface areas of the samples were observed and studied. The formation of rough morphology is possibly related to Ga-Si alloy produced due to poor thermal stability of template at high temperature. The deep pinhole defects generated are deep down to the surface of MBE-grown GaN/Si template. The stress originated from the large thermal expansion coefficient difference between GaN and Si may be related to the formation of the pinhole defects. The surface morphology of the GaN can be improved by optimizing the GaN/Si template and decreasing the growth temperature.