Misfit disclination dipoles in nanocrystalline films and coatings

A theoretical model is proposed for describing the special physical micromechanism of misfit stress relaxation in nanocrystalline (NC) films and coatings. According to this model, under certain conditions, grain boundary sliding occurs in NC films and coatings, which is accompanied by the formation of an ensemble of disclination dipoles (rotational defects). These dipoles produce elastic stress fields, which partially compensate misfit stresses in NC films and coatings. Using the proposed model, it is shown that the nucleation of disclination dipoles in a film (coating) can significantly decrease the total energy of the film/substrate composite for the AlN/6H-SiC and GaN/6H-SiC systems over a wide range of structural parameter values.     

Misfit disclinations at crystal/crystal and crystal/glass interfaces

Theoretical concepts have been developed for a new type of misfit defects, misfit disclinations, at crystal/crystal and crystal/glass interfaces. It is shown, in particular, that the formation of misfit disclinations is an efficient physical micromechanism of misfit stress relaxation at crystal/crystal interfaces. A model describing misfit disclinations at crystal/glass interfaces has been constructed. The energy characteristics of phase boundaries with misfit disclination ensembles are estimated. Fiz. Tverd. Tela (St. Petersburg) 41, 1637–1643 (September 1999)     

Effect of screening of the elastic field of the disclination located at the interface of two half-spaces by a dislocation ensemble

The self-consisted dynamics of a dislocation ensemble in the elastic field of the disclination located at the interface of two half-spaces has been considered for two cases, namely, for half-spaces with different densities of mobile dislocations and for a bicrystal where dislocations are absent in one half-space. The elastic energy W of the disclination screened by the dislocation ensemble has been calculated for the rectangular zone centered relative to the disclination. It has been shown that W increases as ～ $\sqrt R $ (R is the transverse size of the zone in the plastically deformed half-space).     

Defects and separation of amorphous nanofilms from crystalline substrates

The conditions of separation of an amorphous nanofilm from a crystalline substrate are theoretically studied in terms of a disclination-dislocation model for a crystal-glass interface. In this model, such an interface is characterized by a high density of disclinations and dislocations. A criterion for the separation of an amorphous nanofilm from a crystalline substrate is obtained. The critical nanofilm thickness above which a film begins to separate is calculated as a function of the characteristics of the disclination-dislocation system and the dilatation misfit.     

Strained germanium films in Ge/InGaAs/GaAs heterostructures: Formation of edge misfit dislocations at the Ge/InGaAs interface

Heterostructures of the “strained Ge film/artificial InGaAs layer/GaAs substrate” type have been grown by molecular beam epitaxy. A specific feature of these structures is that the plastically relaxed (buffer) InGaAs layer has the density of threading dislocations on a level of 10⁵–10⁶ cm⁻². These dislocations penetrate into the strained Ge layer to become sources of both 60° and 90° (edge) misfit dislocations (MDs). Using the transmission electron microscopy, both MD types have been found at the Ge/InGaAs interface. It has been shown that the presence of threading dislocations inherited from the buffer layer in a tensile-strained Ge film favors the formation of edge dislocations at the Ge/InGaAs interface even in the case of small elastic deformations in the strained film. Possible mechanisms of the formation of edge MDs have been considered, including (i) accidental collision of complementary parallel 60° MDs propagating in the mirror-tilted {111} planes, (ii) induced nucleation of a second 60° MD and its interaction with the primary 60° MD, and (iii) interaction of two complementary MDs after a cross-slip of one of them. Calculations have demonstrated that a critical layer thickness (h _c ) for the appearance of edge MDs is considerably smaller than h _c for 60° MDs.     

Diffusion-induced decay of disclinations and solid state amorphization in mechanically alloyed materials

A theoretical model is proposed which treats the diffusion-induced decay of fragment boundary disclinations as being a micromechanism for the solid state amorphization in mechanically alloyed materials. Within the framework of the suggested model the kinetics of amorphous-phase nucleation centres (spread cores of the decayed disclinations) is studied. In doing so, kinetic equations are suggested and solved, which describe the evolution of the radius of the amorphous core of the decayed disclination.     

Competing relaxation mechanisms in a disclinated nanowire: temperature and size effects

This Letter reports a molecular dynamics study of the temperature and size dependence of disclination relaxation in a bicrystalline titanium nanowire. The simulations show that an unstable disclination may relax via crack nucleation and/or structural transformation. The critical disclination strength to nucleate a crack decreases with the nanowire diameter at 0 K, but an inverse relation may exist at higher temperatures. Similar relaxation mechanisms are operative in other hcp materials. The results suggest that grain boundaries in nanostructured materials can be disordered through disclinations which relax via amorphization.     

Arrays of Charged (±2π)-Twist Disclinations in Ferroelectric Liquid Crystals

The notion of an electrostatic charge of (±2)-twist disclinations is used to approximate the evaluation of the electrostatic interaction energy among disclinations forming arrays in finite samples of ferroelectric chiral smectic C liquid crystals. Screening effects of free charges in a material surrounding the disclination are taken into account by introducing a phenomenological depolarisation factor.The electrostatic interaction energy is important in chiral smectic C materials with high values of the spontaneous polarisation when screening effects of free charges are small. Then the electrostatic interaction leads to elimination of disclinations from the sample. When there is a high concentration of free charges in the sample (smaller value of depolarisation factor), the electrostatic interaction energy is of the order of the elastic interaction energy of disclinations what influences the equilibrium of disclination arrays in the sample. Two disclination configurations are considered. In the Brunet-Williams configuration the disclinations of opposite topological charge have also the opposite electrostatic charge so their attraction is augmented. This attraction can be balanced by the helical structure in the central part of the sample when the sample thickness is rather high.On the contrary, in the Glogarová-Pavel configuration the disclinations of opposite topological charge have the electrostatic charge of the same sign. The equilibrium in this configuration is either a balance of elastic attraction and electrostatic repulsion if elastic and Coulomb forces are of the same order or it is governed by the value of the anchoring energy when electrostatic interaction prevails over the elastic one.     

Faceted grain boundaries in polycrystalline films

A theoretical model is proposed for describing a new mechanism of misfit-stress relaxation in polycrystalline films, namely, the formation of faceted grain boundaries whose facets are asymmetric tilt boundaries. The ranges of parameters (the film thickness, misfit parameter, angle between facets) in which the nucleation of faceted grain boundaries is energetically favorable are calculated. The nucleation of faceted grain boundaries is shown to be facilitated as the film thickness increases.     

晶体相场法模拟异质外延过程中界面形态演化与晶向倾侧

采用晶体相场模型研究了异质外延过程中失配应变与应力弛豫对外延层界面形态演化的影响, 并对由衬底倾角引起的外延层晶向倾侧进行了分析.研究结果表明: 在有一定倾角的衬底晶体上进行外延生长时,若衬底和外延层之间失配度较大 (ε>0.08),外延层中弹性畸变能会以失配位错的形式释放, 最终薄膜以稳定的流动台阶形式生长且外延层的晶向倾角与衬底倾角呈近似线性关系. 而当衬底和外延层之间失配度较小(ε<0.04)不足以形成失配位错时, 外延层中弹性畸变能会以表面能的形式释放,最终使薄膜以岛状形态生长. 在高过冷度条件下,衬底倾角和失配度较大时,衬底和外延层之间会形成由大量位错规则排列而成的小角度晶界从而显著改变外延层的生长位向.     

Electric field generated solitons,disclinations and vortical flows in freely suspended nematic 8CB

We present the results of optical studies on the instabilities in a substrate-free nematic 8CB film, subject to an in-plane electric field. The initial director field involves a −1/2 strength disclination loop, separating the central pseudoisotropic zone from the splay-bend (SB) birefringent boundary. Three regimes of sample thickness are distinguishable on the basis of field-induced instabilities. Thick (∼75 μm) films display growth of SB zones (independently of disclination movement), wall-formation, and reversible transition between walls and disclinations. Moderately thick films, a few μm in the central part, exhibit distinctive undulations at the border of advancing SB layers. Submicron thin films, with smectic-like homeotropic central plateau, show spectacular isotropic vortex-pairs at either end of this plateau. Further, the end regions of the birefringent zone exhibit both electro-convective flows and reorientational effects. The latter are associated with the formation of open and closed walls, and loop-wall emission. The final high field instability involves jet-like flows at the two ends of the film.     

Edge misfit dislocation formation at the interface of a nanopore and infinite substrate with surface/interface effects

The model of an edge misfit dislocation at the interface of the hollow nanopore and the infinite substrate with surface/interface stress is investigated. Using the complex variable method, analytical solutions for complex potentials of a film due to an edge misfit dislocation located in the film with surface/interface effect are derived, and the stress fields of the film and the edge misfit dislocation formation energy can be obtained. The critical conditions for edge misfit dislocation formation are given at which the generation of an edge misfit dislocation is energetically favourable. The influence of the ratio of the shear modulus between the film and the infinite substrate, the misfit strain, the radius of the nanopore and the surface/interface stress on the critical thickness of the film is discussed.     

Twist disclination system in a planar sample of chiral smectic C* liquid crystal

In thick samples of Sm C^* liquid crystals the helicoidal structure in the sample centre is connected to the unwound structure near sample surfaces by a regular system of (?2π) and (+2π)-twist disclinations associated in rows near the upper and lower sample surfaces. The geometry and stability of such a disclination system in a high electric field is investigated and the critical electric field causing the disclination annihilation is estimated. The mutual disclination annihilation is the proposed mechanism for the field induced transition from helicoidal Sm C^* structure to unwound Sm C^* structure with the twist along the sample thickness.     

Misfit dislocations in nanocomposites with quantum dots,nanowires and their ensembles

We review theoretical concepts and experimental results on the physics of misfit dislocations in nanocomposite solids with quantum dots (QDs) and nanowires (quantum wires). Special attention is paid to thermodynamic theoretical models of formation of misfit dislocations in QDs and nanowires, including composite core–shell nanowires. The effects of misfit dislocations on the film growth mode during heteroepitaxy and phase transitions in QD systems are analysed. Experimental results and theoretical models of the ordered spatial arrangement of QDs growing on composite substrates with misfit dislocation networks are discussed. The influence of subsurface dislocations in composite substrates on the nucleation of QDs and nanowires on the substrate surface is considered. Models of misfit strain relaxation and dislocation formation in nanofilms on compliant substrates are also reviewed.     

外延生长薄膜中失配位错形成条件的分子动力学模拟研究

运用分子动力学方法对负失配条件下的外延铝簿膜中失配位错的形成进行了模拟研究.所采 用的原子间相互作用势为嵌入原子法(EAM)多体势.模拟结果显示：在500K下长时间静态弛豫 ，表面和内部结构完整的外延膜在9—80原子层厚度范围内(约为其热力学临界厚度的3—40 倍)均不形成失配位错，而在薄膜表面预置一个单原子层厚、三个原子直径大小的凸台或凹 坑时，失配位错则能够在15个原子层厚的外延膜上迅速形成：在动态沉积生长条件下，表面 自然形成凹凸，初始厚度为9个原子层厚的外延膜在沉积生长中迅速形成失配位错.在三种条 件下，所形成的位错均为伯格斯矢量与失配方向平行的全刃位错.分析发现：在压应力作用 下，表面微凸台诱发了其侧薄膜内部原子的挤出，造成位错形核；而表面微凹坑则直接因压 应力作用形成了一个表面半位错环核. 关键词： 外延薄膜 失配位错 分子动力学 铝     

Stability of Disclinations in Nematic Liquid Crystals

In the light of φ-mapping method and topological current theory, the stability of disclinations around a spherical particle in nematic liquid crystals is studied. We consider two different defect structures around a spherical particle: disclination ring and point defect at the north or south pole of the particle. We calculate the free energy of these different defects in the elastic theory. It is pointed out that the total Frank free energy density can be divided into two parts. One is the distorted energy density of director field around the disclinations. The other is the free energy density of disclinations themselves, which is shown to be concentrated at the defect and to be topologically quantized in the unit of (k-k₂₄)π/2. It is shown that in the presence of saddle-splay elasticity a dipole (radial and hyperbolic hedgehog) configuration that accompanies a particle with strong homeotropic anchoring takes the structure of a small disclination ring, not a point defect.     

Defects in smectic C* liquid crystals

We consider the structure and properties of various topological defects that can occur in smectic C* liquid crystals. The polarization field associated with disclinations, the effect of incommensuration on the structure of dispirations, some interesting situations in the interaction between dispiration and disclination and between dispirations themselves have been discussed in detail. The properties of cholesteric type disclinations and a possible model for the core structure of a wedge disclination have also been dealt with. The author felicitates Prof. D S Kothari on his eightieth birthday and dedicates this paper to him on this occasion.     

Role of edge dislocations in plastic relaxation of GeSi/Si(001) heterostructures: Dependence of introduction mechanisms on film thickness

It has been shown that, in the GeSi/Si(001) heterosystem at lattice parameter mismatches of ～2% and more, a small critical thickness of the introduction of dislocations leads to the implementation of the mechanism of induced nucleation of misfit dislocations. This mechanism consists in that the stress field of an already existing 60° dislocation provokes introduction of a secondary 60° dislocation with an opposite-sign screw component. As a result of the interaction of such dislocation pairs, edge misfit dislocations are formed, which do control the plastic relaxation process. This mechanism is most efficient when dislocations are introduced at the GeSi film thickness only slightly exceeding the critical thickness of the introduction of 60° dislocations, and there are threading dislocations. The dominant type of misfit dislocations (60° or edge) in the Ge-on-Si(001) system can be controlled by varying the mismatch parameter in the heteropair.