Generation and evolution of partial misfit dislocations and stacking faults in thin-film heterostructures

An analysis is made of the specific features in the generation and evolution of partial misfit dislocations at the vertices of V-shaped configurations of stacking fault bands, which terminate in the bulk of the growing film at 90° partial Shockley dislocations. The critical thicknesses h _c of an epitaxial film, at which generation of such defect configurations becomes energetically favorable, are calculated. It is shown that at small misfits, the first to be generated are perfect misfit dislocations and at large misfits, partial ones, which are located at the vertices of V-shaped stacking-fault band configurations emerging onto the film surface. Possible further evolution of stacking-fault band configurations with increasing film thickness are studied.     

The role of misfit dislocations in tilt boundary formation in heterosystems with nonsingular orientations

The structural state of Ge _x Si_{1 ? x} films on miscut Si(1 1 13) substrates is studied by X-ray diffraction. Triclinic distortions that arise in the Ge _x Si_{1 ? x} film are analyzed. The coincidence of the tilt axis and the miscut axis is established. Different misfit-dislocation networks that generate tilt boundaries are considered.     

Nano-structuration of CoO film by misfit dislocations

We have studied the nano-patterning of CoO film induced by misfit dislocation network at the interface with the Ag(0 0 1) substrate. Grazing incidence diffraction (GIXD), X-ray photoemission spectroscopy (XPS) and low energy electron diffraction (LEED) have been used to characterize chemistry and structure of the CoO layers. The XPS spectrum of the Co 2p core level permitted to establish the stoichiometric growth of CoO. The structure of the CoO film together with the absorption sites of cobalt and oxygen atoms was determined, thanks to GIXD measurements. Moreover we have followed the evolution of the in-plane lattice constant of the CoO as a function of the film thickness. It turns out that the CoO film growth starts with the same in-plane lattice constant of the Ag(0 0 1) substrate up to 3-4 ML; afterwards the in-plane parameter of CoO steadily increases before reaching a stable value of 2.98 Å at 23 ML. During the relaxation process, at about 8 ML of film thickness, we observe the formation of a buried misfit dislocation network. These dislocations, that have a period of 9.2 nm for a film thickness of 23 ML, induce mosaicity in the CoO film which then appears as a regular distribution of tilted domains.     

Screw misfit dislocations in soft substrates of epitaxial heterostructures

We derive compact analytical formulae for the elastic field induced by an anti-plane mismatch deformation in a heterostructure with different elastic moduli of the constituents. Unlike previous studies, we consider the possibility that the misfit dislocations may appear in the substrate, not in the epilayer. We show that this situation can be realized in heterostructures where the substrate is softer than the epilayer. In order to avoid cumbersome calculations, we consider screw misfit dislocations. The misfit dislocations emerge with zero density away from the interface in the body of the substrate when the epilayer reaches its critical thickness. Thus the epilayer remains free from dislocations if it is grown on a softer substrate. This property, which was recently observed experimentally, may find numerous applications in electronics, where epilayers are widely used as active elements.     

Two groups of misfit dislocations in GaAs on Si

High-resolution cross-sectional and conventional plan-view transmission electron microscope observations have been carried out for molecular beam epitaxially grown GaAs films on vicinal Si (001) before and after annealing as a function of film thicknesses and observation directions between two orthogonal 110 directions. Two groups of misfit dislocations are characterized at the interface regions between GaAs and Si by analyzing whether their extra half planes exist in the film or the substrate side. Group I misfit dislocations due to stress caused by a lattice misfit between GaAs and Si consist of partial dislocations and 60° and 90° complete dislocations in an as-grown state. With an increase in the film thickness, partial dislocations decrease and complete dislocations increase. After annealing, partial dislocations almost completely disappear and 90° perfect dislocations are predominantly observed. Group II misfit dislocations due to thermal-expansion misfit-induced stress are all 60°-type complete dislocations regardless of film thicknesses and annealing treatment.On leave from Central Research Laboratory, Hitachi, Ltd., Tokyo 185, Japan     

Nano-islands on a composite substrate with misfit dislocations

Spatial arrangements of nano-islands (quantum dots) on the free surface of a composite two-layer substrate containing misfit dislocations of edge type are theoretically examined. It is shown that the elastic interaction between misfit dislocations and nano-islands is capable of causing coagulation of nano-islands. The coagulation of nano-islands is shown to be favourable when the upper-layer thickness is smaller than a critical thickness, H₀. An analytical form of H₀ is presented for the partial case with four-to-one correspondence between nano-islands and cells of the misfit dislocation network. Received: 5 December 2000 / Accepted: 29 March 2001 / Published online: 20 June 2001     

面心立方晶体外延膜沉积生长中失配位错的结构与形成过程

用分子动力学方法对5%负失配条件下面心立方晶体铝薄膜的原子沉积外延生长进行了三维模拟.铝原子间的相互作用采用嵌入原子法(EAM)多体势计算.模拟结果再现了失配位错的形成现象.分析表明，失配位错在形成之初即呈现为Shockley扩展位错，即由两个伯格斯矢量为〈211〉/6的部分位错和其间的堆垛层错组成，两个部分位错的间距、即层错宽度为1.8 nm,与理论计算结果一致；外延晶体薄膜沉积生长中，位错对会发生滑移，但其间距保持稳定.进一步观察发现，该扩展位错产生于一种类似于“局部熔融-重结晶”的表层局部无序紊乱- 关键词： 失配位错 外延生长 薄膜 分子动力学 铝     

Nucleation of misfit dislocations and plastic deformation in core/shell nanowires

During fabrication of metal nanowires, an oxide layer (shell) that surrounds the metal (core) may form. Such an oxide-covered nanowire can be viewed as a cylindrical core/shell nanostructure, possessing a crystal lattice mismatch between the core and shell. Experimental evidence has shown that, in response to this mismatch, mechanical stresses induce plastic deformation in the shell and misfit dislocations nucleate at the core/shell interface. As a result, the mechanical, electrical and optoelectronic properties of the nanowire are affected. It is therefore essential to be able to predict the critical conditions at which misfit dislocation nucleation at the nanowire interface takes place and the critical applied load at which the interface begins deforming plastically. Two approaches are explored in order to analyze the stress relaxation processes in these oxide-covered nanowires: (i) energy considerations are carried out within a classical elasticity framework to predict the critical radii (of the core and shell) at which dislocation nucleation takes place at the nanowire interface; (ii) a strain gradient plasticity approach is applied to estimate the flow stress at which the interface will begin deforming plastically (this stress is termed “interfacial-yield” stress). The interfacial-yield stress, predicted by gradient plasticity, depends, among other material parameters, on the radii of the core and shell. Both approaches demonstrate how the geometric parameters of nanowires can be calibrated so as to avoid undesirable plastic deformation; in particular, method (i) can give the radii values that prevent misfit dislocation formation, whereas method (ii) can provide, for particular radii values, the critical stress at which interface deformation initiates.     

采用纳米晶柱阵列衬底抑制失配位错形成的分子动力学模拟研究

运用分子动力学方法对纳米晶柱阵列衬底上铝簿膜的外延生长进行了模拟研究.所采用的原子间相互作用势为嵌入原子法(EAM)多体势.模拟结果表明：采用纳米晶柱阵列衬底可以在不形成失配位错的条件下释放其上生长的外延薄膜晶体中的失配应变,有效地抑制其中失配位错的形成,获得高质量的外延薄膜晶体;这种纳米晶柱阵列的几何设计应满足两个基本条件：1) 晶柱的横截面尺寸应大于对应温度下的晶柱热失稳临界尺寸,以克服纳米结构的热失稳,模拟显示700K下铝的热失稳临界尺寸为19nm;2) 晶柱的高度与间距之比应大于076,以保证 关键词： 失配位错 分子动力学 纳米晶柱 铝     

Interaction of shockley partial dislocations with reacting forest dislocations

Dislocation combinations formed as a result of interaction between a glissileBσ,d Shockley partial dislocation with reacting undissociated forest dislocations are considered. The value of the parameters characterizing the strength <k> of the dislocation combinations, the probability β_r of their formation, and the interaction intensity α_r of the reacting dislocations are determined for an orientation of the [100] deformation axis of an FCC single crystal for all components of the dislocation loop. Tomsk State Architectural-Building Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–8, May, 1997.     

Multi-soliton method for removal of misfit dislocations from functional area of heterostructures

A theoretical model was developed for reduction of the density of misfit dislocations at boundaries of heterojunctions. The model is based on the repulsion of a multi-soliton chain and of a single dislocation, as a result of which the removal of the dislocation from the functional area of heterostructure occurs.     

Less strain energy despite fewer misfit dislocations: the impact of ordering

The average strain state of Ge films grown on Si(111) by surfactant mediated epitaxy has been compared to the ordering of the interfacial misfit dislocation network. Surprisingly, a smaller degree of average lattice relaxation was found in films grown at higher temperature. On the other hand, these films exhibit a better ordered dislocation network. This effect energetically compensates the higher strain at higher growth temperature, leading to the conclusion that, apart from the formation of misfit dislocations, their ordering represents an important channel for lattice-strain energy relaxation.     

AlSb/GaAs(001)失配位错的高分辨电子显微学研究

用200kV六硼化镧光源的高分辨透射电子显微镜观察了AlSb/GaAs(001)外延薄膜的失配位错,结合解卷处理方法把[110]高分辨电子显微像转换为试样的结构投影图,其分辨率接近电子显微镜的信息极限.根据赝弱相位物体近似像衬理论,通过分析AlSb薄膜完整区解卷像的衬度随试样厚度的变化,确定了哑铃原子对中Al和Sb原子的位置.在此基础上构建出失配位错的结构模型,再结合模拟像与实验像的匹配,确定了AlAs型界面以及Lomer和60°两类失配全位错的核心结构.     

Triple-period partial misfit dislocations at the InN/GaN (0001) interface: A new dislocation core structure for III-N materials

The lattice-misfit InN/GaN (0001) interface supports a triangular network of α-core 90° partial misfit dislocations. These misfit dislocations provide excellent strain relief. However, in their unreconstructed form the dislocation contains numerous high-energy N dangling bonds, which must be eliminated by reconstructing the dislocation core. Existing single-period (SP) and double-period (DP) dislocation reconstruction models eliminate these dangling bonds via a like-atom dimerization, such as N-N dimers. However, we show that these N–N dimers are unstable for the III-N materials, so an entirely new reconstruction mechanism is needed. A “triple-period” (TP) structural model is developed which eliminates N dangling bonds via the formation of N vacancies instead of N-N dimers. The model contains no N–N (or III–III) bonds, fully bonds all N atoms to four group-III neighboring atoms, and satisfies the “electron counting rule” by transferring charge from In dangling bonds to Ga dangling bonds.     

Properties of misfit dislocations and pseudodislocations not typical for defects of homogeneous crystals

The structural properties of SiGe/Si heterosystems with a stepped interface obtained by the deflection of a surface from the (001) plane by the rotation by several degrees about the [110] axis are studied. Dislocation-free systems and systems containing misf it dislocations (MDs) are investigated. The model of pseudodislocations describing the mutual rotation of (001) crystallographic planes of the film and substrate is proposed. The pseudodislocations are the elastically strained interface steps, and the magnitude of their effective Burgers vector is determined by the step height and lattice’s mismatch parameter. Long-range normal and shear stress fields in the epitaxial film are considered for the systems containing dislocations. The ambiguity of the value of the MDs Burgers vector is discussed. The mechanism for a small angle boundary formation under MDs introduction into the vicinal interface is proposed. The expression is obtained allowing one to calculate the fractions of MDs glide in the 111 planes inclined to the interface at the maximum and minimum angles based on the parameters determined from X-ray diffraction reflection curves.     

Equilibrium configurations and phase transitions in dipole lattices

Two-dimensional square and hexagonal lattices of magnetic dipoles with the number of rows 1–4 have been studied. Based on the numerical analysis, equilibrium stable domain configurations, including the minimum number of lattice dipoles, have been revealed; the conditions for the creation and destruction of domains have been determined; and their associated changes in the magnetic moment of the lattice and in the energy of the dipole interaction have been found. The conditions for the occurrence of phase transitions that change the configuration of the lattices have been investigated and the conditions for unidirectional propagation of the front of the phase transition have been established. A comparative analysis of different square and hexagonal lattices has been performed in terms of the specific features of the formed domains and the observed orientation phase transitions.     

Stacking faults and partial dislocations in graphene

We investigate two mechanisms of crystallographic slip in graphene, corresponding to glide and shuffle generalized stacking faults (GSF), and compute their γ-curves using Sandia National Laboratories Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). We find evidence of metastable partial dislocations for the glide GSF only. The computed values of the stable and unstable stacking-fault energies are suggestive of a high stability of full dislocations against dissociation and of dislocation dipoles against annihilation.