Relaxation Of Layers Research Articles

Entangled Dendritic Polymers and Beyond: Rheology of Symmetric Cayley-Tree Polymers and Macromolecular Self-Assemblies

We present linear rheological data on a series of anionically synthesized, model symmetric Cayleytree polybutadienes having 2 or 3 generations with entangled branches. The signature of each layer relaxation is evident, in both the plateau modulus (that scales with the volume fraction of the unrelaxed layers) and the terminal relaxation as a distinct contribution, well-separated in time. Using a robust tube-model analysis of the viscoelastic relaxation of entangled polymers of given architecture, which is based on the concept of hierarchy of motion in branched structures, we describe quantitatively the frequency spectra without adjustable parameters; the needed plateau modulus, entanglement molecular weight, and Rouse time of an entanglement segment are taken from the data. Last, but not least, we show that this approach can be successfully applied to account for the rheology of macromolecular self-assemblies, such as the Cayley-tree-like associations of telechelic, mono-o-functionalized star polymers. The implications of the latter findings in providing pathways for the analysis of complex soft superstructures are evident.

Relaxation-Induced Excess Leakage Current in Recessed Si1-xGex Source/Drain Junctions

This paper presents an investigation of the effect of the relaxation on the electrical performance of recessed Si1-xGex source/drain junctions. It is shown, that the peripheral leakage current density scales exponentially with the total epilayer thickness, which allows the estimation of the characteristic length in good agreement with previously published values. In addition, the area leakage current density shows for relaxed Si0.7Ge0.3 recessed S/D junctions an exponential increase with the position of the epi interface with respect to the junction. For strained Si0.8Ge0.2 recessed S/D junctions, an unusual dependence of the area leakage current density on the etch depth is revealed. At the same time, for 48 nm etch depth, the leakage current components are not monotonically increasing with the Ge content. This could point to a size effect of the active areas, affecting the strain relaxation in the SiGe layers and the tensile strain in the underlying silicon substrate.

Interface properties and electronic structure of ultrathin manganese oxide films on Ag(0 0 1)

Ultrathin manganese monoxide films on silver single crystal substrates are studied as a function of the thickness using X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). It was found that electronic and geometric properties in ultrathin films differ from the bulk. A tetragonal distortion of the oxide films is revealed by polarisation-dependent X-ray absorption measurements: the extent decreases with increasing film thickness. A structural relaxation of epitaxial oxide layers induced by heating also leads to a decrease of the tetragonal interface strain. The electronic structure of ultrathin manganese oxide films in the valence band region is very similar to that in the bulk oxide.

Monitoring of ZnCdSe layer properties by in situ x-ray diffraction during heteroepitaxy on (001)GaAs substrates

The authors demonstrate in situ high-resolution x-ray diffraction applied during heteroepitaxy on (001)GaAs for instant layer characterization. The current thickness, composition, strain, and relaxation dynamics of pseudomorphic layers are precisely determined from qz scans at the (113) reflection measured at a molecular beam epitaxy chamber with a conventional x-ray tube in static geometry. A simple fitting routine enables real-time in situ x-ray diffraction analysis of layers as thin as 20nm. Critical thicknesses for dislocation formation and plastic relaxation of ZnCdSe layers versus Cd content are determined. The strong influence of substrate temperature on heteroepitaxial nucleation process, deposition rate, composition, and strain relaxation dynamics of ZnCdSe on GaAs is also studied.

Some peculiarities of relaxation and structure-forming processes in polymer boundary layers

The relaxation processes are retarded when the polymer interlayer in adhesive joints is diminished to a few microns. This retardation shifts the maximum on the temperature vs strength curve of such joints (under considerable overstresses) toward higher temperatures, and secondly, reduces the effectiveness of “rest” responsible for the partial strength recovery of the specimen in which rupturing crack propagates. In many systems the rate of polymer spherulite linear growth (G) diminishes with decreasing thickness of the polymer film (δ) from 15 to 30 μ. This was observed for isotactical polypropylene (PPr) and polystyrene, polyformaldehyde, cellulose tribenzoate, guttapercha, polypropyleneoxide (PPO), and polyethylenesebacynate (PES) on silica glass; PPr on copper, low-melting metal alloys (in crystalline or liquid state), various crystallo-graphic planes of NaCl, and planes (100) of CaF2 and LiF. The magnitude of G did not depend on δ in the crystallization of PPr, PPO, and PES on polyvinylacetate, polyvinylbutyral, crystalline and liquid low-melting mixtures of inorganic salts. Inhibition of spherulites growth in polymer boundary layers is strongly influenced by the surface energy irrespective of the phase or aggregate state of the substrate. The retarded growth of spherulites is probably due to the reduced density and lower mobility of polymer in the boundary layers.

Swaging process of power steering hose: Its finite element analysis considering the stress relaxation

Abstract Rubber layers in the power steering hose display the stress relaxation after the swaging forming load is released. This paper presents the nonlinear finite element analysis of the swaging process of the power steering hose by considering the stress relaxation in terms of a Prony series expansion. Numerical results illustrating the theoretical work confirm that the stress relaxation of rubber layers cannot be ignored for the reliable numerical simulation of the swaging process.

Influence of Process Flow on the Characteristics of Strained-Si nMOSFETs

We developed a process design that uses epitaxial growth of strained-Si layers after shallow trench isolation (STI) and well implantation to improve the drive current of n-type metal-oxide-semiconductor field effect transistors (nMOSFETs). Due to the significantly reduced thermal budget, we can minimize the degree of partial relaxation in strained-Si layers based on state-of-the-art MOSFET integration technique. Experimental results show that strained-Si devices fabricated by this flow exhibit enhanced saturated drive currents, up to 12% higher than that of strained-Si device with epitaxial growth of strained-Si layers before STI and well implantation.

Magnetization damping in ultrathin polycrystalline Co films: Evidence for nonlocal effects

The magnetic properties and magnetization dynamics of polycrystalline ultrathin Co layers were investigated using a broadband ferromagnetic resonance technique at room temperature. A variable-thickness $(1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\ensuremath{\leqslant}t\ensuremath{\leqslant}10\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ Co layer is sandwiched between $10\text{\ensuremath{-}}\mathrm{nm}$-thick Cu layers ($10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $\mathrm{Cu}\ensuremath{\mid}t\phantom{\rule{0.3em}{0ex}}\mathrm{Co}\ensuremath{\mid}10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ Cu), while materials in contact with the Cu outer interfaces are varied to determine their influence on the magnetization damping. The resonance field and the linewidth were studied for in-plane magnetic fields in field-swept experiments at a fixed frequency, from $4\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}25\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$. The Co layers have a lower magnetization density than the bulk and an interface contribution to the magnetic anisotropy normal to the film plane. The Gilbert damping, as determined from the frequency dependence of the linewidth, increases with decreasing Co layer thickness for films with outer Pt layers. This enhancement is not observed in structures without Pt layers. The result can be understood in terms of a nonlocal contribution to the damping due to spin pumping from Co through the Cu layer and spin relaxation in Pt layers. Pt layers just $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick are found to be sufficient to enhance the damping and thus act as efficient ``spin sinks.'' In structures with Pt outer layers, this nonlocal contribution to the damping becomes predominant when the Co layer is thinner than $4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

Strain relaxation in strained-Si layers on SiGe-on-insulator substrates

Misfit strain relaxation in tensile-strained Si grown at 600 °C on (0 0 1) SiGe-on-insulator substrates is experimentally studied for a tensile strain range between 0.4 and 1.2%. The critical thickness is found to be rather thinner than Houghton's critical thickness for a compressively-mismatched SiGe/Si(0 0 1) system. The thinner critical thickness in tensile-strained Si is attributed to the fact that the stress-relieving misfit dislocations are 90° partial dislocations, instead of 60° misfit dislocations for the compressive SiGe/Si(0 0 1) system. With increasing strained-Si layer thickness thicker than the critical thickness, 60° misfit dislocation is found to be increasingly formed but the strain relaxation is sluggish. For a supercritical thickness ten times thicker than the critical thickness the tensile strain mostly remains. The initial strain relaxation is discussed in terms of the misfit dislocation morphology.

X-ray Diffraction and STM Study of Reactive Surfaces under Electrochemical Control: Cl and I on Cu(100)

The surface structure of Cu(100) modified by chloride and iodide has been studied in an electrochemical environment by means of in-situ scanning tunneling microscopy in combination with in-situ surface X-ray diffraction with a particular focus on adsorbate and potential dependent surface relaxation phenomena. For positive potentials close to the on-set of the copper dissolution reaction, the X-ray data disclose an extraordinarily large Cu-Cl bond length of 2.61 A for the c(2 x 2)-Cl phase. This finding points to a largely ionic character of the Cu-Cl interaction at the Cu(100) surface, with chloride particles likely to retain their full charge upon adsorption. Together with the positive surface charging at these high potentials, this ionic Cu-Cl bond drives the observed 2.2% outward relaxation between the first two copper layers. These results indicate that the bond between the first and the second copper layer is significantly weakened which appears as the crucial prerequisite for the high surface mobility of copper-chloride species under electrochemical annealing conditions at these high potentials. With 2.51 A the Cu-I bond is 4% shorter than the Cu-Cl bond implying that the nature of the Cu-I bond is mainly covalent. Accordingly, we observe a significant inward relaxation of the top Cu layers upon substituting chloride by iodide at the same electrode potential, which suggests that the iodide adsorption involves charge transfer from the halide to the copper substrate.

Strain relaxation in thin SiGe epilayers doped with carbon

The possibility of obtaining a heterosystem consisting of the upper partially strained and lower relaxed sublayers by stepwise in situ doping of SiGe layers by carbon is considered. The properties of the as-grown and annealed samples are studied by means of Raman spectroscopy and atomic force microscopy. The degree of strain relaxation in the as-grown layers was derived from the Raman spectra. The Si0.7−yGe0.3Cy layer was shown to be stable to annealing up to 700°C. Moreover, the strain relaxes during annealing not homogenously over the whole structure but in a layer-by-layer way. The segregation of carbon atoms is observed for as-grown Si1−x−yGexCy layers in the near-substrate regions.

Ab initiostudy of the geometric dependence ofAgPdsurface segregation

The role of surface orientation and surface relaxation on the segregation profile of $\mathrm{AgPd}$ random alloys is investigated. The calculations are performed using the full charge density exact muffin-tin orbitals method in combination with the coherent potential approximation. The equilibrium concentration profiles for the first three close-packed surfaces are established as a function of bulk composition and temperature. In $\mathrm{AgPd}$ alloys the surface relaxation is significant only on the (100) and (110) facets. For these facets our study shows that the top layer relaxation has negligible effect on the segregation profile. The present results obtained for ideal surface geometries are compared to former theoretical and experimental data. Strong $\mathrm{Ag}$ segregation to the top surface layer is found for all surfaces and all bulk concentrations. At the same time, the behavior of the subsurface layer in $\mathrm{Pd}$ rich alloys depends significantly on the surface orientation.

Elastic strain relaxation in discontinuous wetting layers and its impact on lateral ordering of heteroepitaxial dots

We discuss morphological changes of strained $\mathrm{Si}\mathrm{Ge}∕\mathrm{Si}(001)$ dots grown from an indium solution. In the course of a particular depletion scenario initial, lenslike dots transform into truncated pyramids of fourfold symmetry inside circular rims of rising height and steep inner edge. Further dot nucleation performs close to the rim indicating elastically relaxed lattice sites. The observations made are supported by numerical finite element calculations on the strain energy distribution.

Structural, electronic, and magnetic properties of bcc iron surfaces

Trends in atomic multilayer relaxations, surface energy, electronic work function, and magnetic structure of several low-Miller-index surfaces of iron are investigated employing density functional theory total energy calculations. The calculated topmost layer relaxations reproduce well the experimental contractions and their variation with the surface crystallographic orientation, and surface roughness. The multilayer relaxation sequences correlate with the reduced coordination in surface layers and can be explained in terms of a simple electrostatic picture. The surface energies scale almost linearly with the surface roughness. They agree well with the experimental surface tensions and show a small anisotropy in agreement with predictions based on measurements for other metals. The equilibrium shape of a bcc Fe crystal is determined and discussed. The work function anisotropy is calculated and rationalized in terms of changes in the valence charge distribution. Significantly increased local magnetic moments of atoms in the surface region are determined. The correlation between the anisotropy of the surface magnetic moments and atomic coordination in the outermost layers is demonstrated to follow a simple rule.

Misfit strain relaxation in strained-Si layers on silicon-germanium-on-insulator substrates

The critical thickness of tensile-strained-Si layers grown at 600°C on (001) surfaces of silicon-germanium-on-insulator substrates is experimentally studied for a tensile strain range between 0.4% and 1.2% and is found to be rather thinner than Houghton’s critical thickness [J. Appl. Phys. 70, 2136 (1991)] for compressively mismatched SiGe∕Si (001) system. The thinner critical thickness in tensile-strained Si is attributed to the fact that the stress-relieving misfit dislocations are 90° partial dislocations, instead of 60° misfit dislocations for compressive SiGe layers on Si (001) substrates. Although 60° misfit dislocation is found to be increasingly formed with increasing strained-Si layer thickness, the strain relaxation is sluggish and the tensile strain mostly remains for supercritical thickness ten times thicker than the critical thickness.

Electronic structure ofSrVO3(001)surfaces: A local-density approximation plus dynamical mean-field theory calculation

The influence of local Coulomb correlations on the surface electronic structure of ${\mathrm{SrVO}}_{3}$, a strongly-correlated metal in a perovskite structure, is investigated for both the $\mathrm{SrO}$-layer and ${\mathrm{VO}}_{2}$-layer-terminated (001) surfaces. The electronic structure within the local density approximation of a semi-infinite surface is determined using the embedded Green-function approach, and the resultant density of states projected on the V ${t}_{2g}$ orbitals is used as an input to a subsequent many-body calculation within the dynamical mean field theory (DMFT) and the multiorbital quantum Monte Carlo technique. Qualitatively, the present study confirms the conclusion of recent photoemission experiments and tight-binding DMFT calculations which both indicate that the electronic structure at the surface is more strongly correlated than in the bulk. On a quantitative level significant differences are obtained as a function of orbital polarization at the surface, surface layer relaxation, and $\mathrm{SrO}$ vs ${\mathrm{VO}}_{2}$ surface termination.

Nonequilibrium liquid-phase heteroepitaxy of In xGa 1−xP alloys on different substrates

Using the minimum condition of the free energy, we describe nonequilibrium liquid-phase heteroepitaxy of In x Ga 1− x P alloys on the substrates with different lattice parameters. The developed approach is applied to analyze In x Ga 1− x P growth on GaAs(0 0 1) and In 0.07Ga 0.93As/GaAs(0 0 1) substrates from the same supercooled liquid solution. The presented approach has allowed us to determine the degree of relaxation of the epitaxial layers. The composition self-stabilization effect is demonstrated for the relaxed epitaxial layer.

Ab initio study of the Cr2AlC(0001) surface

Using an ab initio total energy method, we have calculated the surface energy and surface stress of Cr2AlC(0001) with the configuration of the top layer as Al[(0001)Al], Cr[(0001)Cr], and C[(0001)C] atoms, respectively. Both the surface energy and surface stress of (0001)Cr are very close to (0001)C. While those of (0001)Al are the lowest, suggesting that this surface configuration is the most stable one. Furthermore, the interplanar relaxation does not have any obvious effect on the surface energy, while the surface stress decreased dramatically under layer relaxation. The Cr-C bond length in the surface layer contracted by >5%, and the bond contraction for Cr-Al is <1.5%, showing that the bond strength in the surface layer is higher than that in the bulk region.

Physico-chemical and electrical properties of rapid thermal oxides on Ge-rich SiGe heterolayers

Rapid thermal oxidation of high-Ge content (Ge-rich) Si 1− x Ge x ( x = 0.85) layers in dry O 2 ambient has been investigated. High-resolution X-ray diffraction (HRXRD) and strain-sensitive two-dimensional reciprocal space mapping X-ray diffractometry (2D-RSM) are employed to investigate strain relaxation and composition of as-grown SiGe alloy layers. Characterizations of ultra thin oxides (∼6–8 nm) have been performed using Fourier transform infrared spectroscopy (FTIR) and high-resolution X-ray photoelectron spectroscopy (HRXPS). Formation of mixed oxide i.e., (SiO 2 + GeO 2) and pile-up of Ge at the oxide/Si 1− x Ge x interface have been observed. Enhancement in Ge segregation and reduction of oxide thickness with increasing oxidation temperature are reported. Interface properties and leakage current behavior of the rapid thermal oxides have been studied by capacitance–voltage (C–V) and current–voltage (J–V) techniques using metal-oxide-semiconductor capacitor (MOSCAP) structures and the results are reported.

Characterization of crystallographic properties and defects via X‐ray microdiffraction in GaN (0001) layers

AbstractIntrinsic stresses due to lattice mismatch, high densities of threading dislocations, and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion, are present in almost all III‐Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional, cantilever (CE) and in pendeo‐epitaxial (PE) films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X‐ray microdiffraction, high resolution monochromatic X‐ray diffraction and SEM analysis have been used to determine the crystallographic properties, misfit dislocations distribution and crystallographic tilts in uncoalesced GaN layers grown by PE and CE. The crystallographic tilt between the GaN(0001) and Si(111) planes was detected in the CE grown samples on Si(111). In contrast there was no tilt between GaN(0001) and SiC(0001) planes in PE grown samples. The wings are tilted upward for both the PE and CE grown uncoalesced GaN layers. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)