P. Dłużewski

Composition fluctuation in InGaN quantum wells made from molecular beam or metalorganic vapor phase epitaxial layers

Using strain analysis on high resolution electron microscopy images and finite element modeling of InGaN quantum wells (QWs), it is shown that the In composition changes inside the layers can be accurately determined. The analyzed samples were nominally grown with 15%–17% In composition by molecular beam or metalorganic vapor phase epitaxy. Inside these QWs, the In composition is not homogeneous. Finite element modeling strongly suggests that the measured strain corresponds most probably to InN clusters whose size depends on the growth method.

High-pressure, high-temperature synthesis of SiC-diamond nanocrystalline ceramics

Dense and entirely nanocrystalline diamond–SiC ceramics were synthesized by the infiltration of liquid Si into the nanodiamond body under high-pressure (77 kbar) and high-temperature (1400–2000 °C) conditions. Based on x-ray diffraction and transmission electron microscopy observations, a model of as-synthesized material is proposed, where individual polycrystalline diamond particles are bonded via SiC–diamond nanolayers. The nanolayers provide a very high hardness of the entire specimen up to 51 GPa. The phenomenon of self-stop Si infiltration was detected in this system. This phenomenon can be explained by the closure of pores near the boundary between molten Si and diamond nanopowder due to the formation of SiC inside the pores.

ZnTe nanowires grown on GaAs(100) substrates by molecular beam epitaxy

ZnTe nanowires with an average diameter of about 30nm and lengths above 1μm were grown on GaAs(100) substrate by molecular beam epitaxy. The growth process was based on the Au-catalyzed vapor-liquid-solid mechanism. A thin gold layer (3–20A thick) annealed in high vacuum prior to the nanowire growth was used as a source of catalytic nanoparticles. The nanowires are inclined about 55° to the (100) substrate surface normal. They have a zinc-blende crystal structure and their growth axis is ⟨111⟩.

Anisotropic Hyperelasticity Based Upon General Strain Measures

This paper presents stress-strain constitutive equations for anisotropic elastic materials. A special attention is given to the logarithmic strain. Assuming a constitutive equation for the specific internal energy the equation governing the Cauchy stress is derived. Mathematical relations presented take a relatively simple form and concern a very wide class of elastic materials. The dependence of third-order elastic constants on the choice of strain measure is shown. The third-order elastic constants measured experimentally in relation to the Green strain are recalculated here for the logarithmic strain.

On geometry and continuum thermodynamics of movement of structural defects

Abstract A thermodynamic theory of the movement of point, line and surface defects is considered at finite deformations. This theory is based on the balance laws for crystal defects. The defects balance laws together with the well-known balance laws for the mass, momentum, moment of momentum, energy and entropy have been utilized to find the driving forces acting on crystal defects. Some of the derived formulae are well-known, e.g. Peach-Koehler formula, nevertheless, many new relations are obtained, e.g. for osmotic forces and for the energy flux due to the movement of crystal defects. The driving force acting on a grain boundary is found as a thermodynamic force needed to balance the jump in energy density across the moving discontinuity surface. Using the relations derived for driving forces the problem of the constitutive modelling of the crystal defect movement is considered. The elastic behaviour of materials with structural defects is determined by a constitutive equation imposed on the free energy density. This equation takes into account the elastic strain, crystal defect densities and temperature. The crystal plasticity is described by vector constitutive equations stated between the defects velocities and the respective driving forces.

Interplay of superconductivity and ferromagnetism in YBa2Cu3O7/ La1?xSrxMnO3 heterostructures

Oxide manganite/cuprate superlattices are an attractive model system to study the proximity effect in oxide ferromagnet/cuprate superlattices. This report describes recent experimental results obtained on manganite/cuprate heterostructures. The experimental results are discussed in terms of the impact of hole charge transfer from cuprate to manganite layers on the proximity between the oxide ferromagnet and oxide superconductor layers.

On the measurement of dislocation core distributions in a GaAs/ZnTe/CdTe heterostructure by high-resolution transmission electron microscopy

Tensorial maps of misfit dislocations at the strained GaAs-ZnTe-CdTe interfacial zone are reconstructed by use of digital processing of high-resolution transmission electron micrographs. Large distortions of the crystal lattice around Lomer dislocations are measured using the geometric phase technique. The integration of the dislocation distribution tensor field over a dislocation core region gives the in-plane components of their Burgers vectors. The accuracy of the method for the dislocation map reconstruction is tested by comparing the theoretical values of the so-called true Burgers vectors with those obtained from the integration of tensorial maps.

Self-organized MnAs quantum dots formed during annealing of GaMnAs under arsenic capping

Formation of MnAs quantum dots in a regular ring-like distribution has been found on molecular beam epitaxy grown (GaMn)As(100) surfaces after low-temperature annealing under As capping. The appearance of the dots depends on the thickness and Mn concentration in the (GaMn)As layer. With 5 at. % substitutional Mn the quantum dots showed up for layers thicker than 100 nm. For thinner layers the surfaces of the annealed samples are smooth and well ordered with 1×2 surface reconstruction, just as for as-grown (GaMn)As. The annealed surfaces are Mn rich, and are well suited for continued epitaxial growth.

Crystal orientation spaces and remarks on the modelling of polycrystal anisotropy

Abstract recently , in many microstructural models of polycrystals the micro-macro transition is based on the averaging of crystal behaviours over all the crystal orientations. In this paper the elements of the theory of crystal orientation spaces are presented from the viewpoint of Riemannian geometry. The Euler angles are used as coordinate systems on these spaces. The angular metric tensor is introduced. It is shown that each turn of a crystal describes a geodesic line in the crystal orientation space. A simple formula to determine the finite distance between two different crystal orientations is given. It appears that the orientation region frequently used to describe the orientation distribution of f.c.c. crystals does not take into account all orientations of the crystals. Therefore a new, more suitable region has been proposed. A continuity equation for the crystal orientation distribution function (CODF) is obtained as a conservation law in this geometry. It is shown that the frequently used form of this equation is not correct.

3D modelling of misfit networks in the interface region of heterostructures

We present a methodology for the stress–strain analysis of a film/substrate interface by combining crystallographic and continuum modelling. Starting from measurements of lattice parameters available from experimental observations, the heterostructure is recast initially in the form of a crystallographic model and finally as a continuum elastic model. The derived method is capable of handling dense arrays of misfit dislocations as well as large areas of the interface between two crystal structures. As an application we consider the misfit dislocation network in the GaN/Al2O3 interface region through determination of strain relaxation and associated residual stresses. Our calculated results are referred back to and found to be in good agreement with the experimental observations of misfit dislocation arrays obtained from high resolution transmission electron microscopy.