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     

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     

异质薄膜外延生长中温度对失配位错出现厚度的影响

采用三维分子动力学模拟方法,使用Ercolessi和Adams建立的嵌入原子法（EAM）多体势函数,模拟了二维晶格失配铝膜晶体中失配位错的形成过程,通过体系结构和能量曲线两种方法研究了温度对位错出现厚度的影响。结果显示：温度对于失配位错的形成有影响。同等条件下,随着温度的增大,失配位错的出现厚度变薄。     

Au/Cu(001)异质外延岛演化的分子动力学研究

利用分子动力学弛豫方法模拟了Au/Cu(001)异质外延生长初期Au异质外延岛的形貌演化，分析了Au外延岛演化过程中的局域应力及与基体结合能随表面岛尺寸的变化. 研究结果表明：当异质外延岛小于7×7时，外延岛原子分布呈现赝Cu点阵形貌；当外延岛达到8×8后，外延岛内开始出现失配位错，失配位错数量随外延岛尺寸的增加而增加. 局域压力分析指出，外延岛上原子之间的近邻环境不同导致了所受应力的差异，而外延岛的形变则是由外延岛原子的应力分布所决定. 研究还发现，失配位错的产生导致错位原子与基体原子之间的结合强度减弱，但相对增加了非错位原子与基体原子之间的结合强度. 关键词： 异质外延 表面形貌 局域压力 分子动力学模拟     

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.     

纳米铜薄膜塑性变形中空位型缺陷形核与演化的分子动力学研究

本文采用分子动力学方法模拟了纳米单晶铜薄膜在单向拉伸载荷作用下的塑性变形过程, 重点分析了空位型缺陷的形核过程和演化机理. 在模拟过程中, 采用镶嵌原子势描述原子间的相互作用. 模拟结果表明纳米铜薄膜中塑性变形起源于位错的表面形核, 而空位型缺陷的形核及演化都与晶体内部的位错运动密切相关. 空位型缺陷通常从位错割阶及层错交截处开始形核, 以单空位、层错四面体和不规则空位团等形式存在. 关键词： 纳米薄膜 塑性变形 空位 层错四面体     

HgCdTe外延薄膜临界厚度的理论分析

基于在任意坐标系内应力与应变的关系、晶体弹性理论和位错滑移理论，研究了生长方向分别为［111］和［211］晶向，HgCdTe外延薄膜临界厚度与CdZnTe衬底中Zn组分和HgCdTe外延层中Cd组分的关系. 结果表明，HgCdTe外延薄膜临界厚度依赖于CdZnTe衬底中Zn组分和HgCdTe外延层中Cd组分的变化. 对于厚度为10μm，生长方向为［111］晶向的液相外延HgCdTe薄膜，要确保HgCdTe/CdZnTe无界面失配位错的前提条件，是CdZnTe衬底中Zn组分和HgCdTe外延层中Cd组分的波动必须分别在±0.225‰和±5‰范围内；而对于相同厚度，生长方向为［211］晶向的分子束外延HgCdTe薄膜，CdZnTe衬底中Zn组分和HgCdTe外延层中Cd组分的波动范围分别为±0.2‰和±4‰. 关键词： HgCdTe/CdZnTe 临界厚度 位错滑移理论 失配位错     

α-Fe纯I型裂纹扩展中位错形成过程的三维分子动力学模拟

通过分子动力学方法模拟了三维 α-Fe I型裂纹的单向拉伸实验中的裂纹扩展过程。研究了在不同温度下裂纹扩展时位错的形成过程和断裂机理。计算结果表明,裂纹扩展过程是位错不断发射的过程。 裂纹尖端附近先形成无位错区和层错,当裂纹处应力增加到KI=0.566 MPam1/2时,裂纹尖端附近的某一层原子会逐渐分叉形成两层原子,分层后的原子层继续分离形成位错;当应力KI 达到0.669MPam1/2时第一个位错发射。随着温度的升高,临界应力强度因子逐渐降低,同时位错发射也相应地加快。     

α-Fe纯I型裂纹扩展中位错形成过程的三维分子动力学模拟

通过分子动力学方法模拟了三维 α-Fe I型裂纹的单向拉伸实验中的裂纹扩展过程。研究了在不同温度下裂纹扩展时位错的形成过程和断裂机理。计算结果表明,裂纹扩展过程是位错不断发射的过程。 裂纹尖端附近先形成无位错区和层错,当裂纹处应力增加到KI=0.566 MPam1/2时,裂纹尖端附近的某一层原子会逐渐分叉形成两层原子,分层后的原子层继续分离形成位错;当应力KI 达到0.669MPam1/2时第一个位错发射。随着温度的升高,临界应力强度因子逐渐降低,同时位错发射也相应地加快。     

α-Fe裂纹的分子动力学研究

通过分子动力学方法，模拟了α-Fe裂纹的单轴拉伸实验中的形变过程.研究了不同晶体取向裂纹的形变特点和断裂机理，观察到各种形变现象，如位错形核和发射，位错运动，堆垛层错或孪晶的形成，纳米空洞的形成与连接等.计算结果表明，裂纹扩展是塑性过程和弹性过程相结合的过程，其中塑性过程表现为由裂尖发射的位错导致的原子切变行为，而弹性过程的发生则是由无位错区中的原子断键所导致.同时还研究了α-Fe裂纹的形变特点和断裂机理与温度场和应力场的依赖关系.     

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.     

硅蹼中位错和层错的X射线貌相研究

利用X射线投影貌相术观察和分析了硅蹼中的位错和层错。在生长态硅蹼中,除观察到柏氏矢量为1/2<110>的刃型、螺型与60°全位错以及柏氏矢量为1/6<112>的Shockley刃型半位错外,还观察到平行于硅蹼表面的大面积层错和蹼中的60°,30°Shockley半位错。位错在热处理过程中运动并发生位错反应形成近六角形的位错网络。热处理改变生长态硅蹼中层错的组态和衬度,并由于杂质聚集破坏了Shockley半位错的消象法则。还观察到层错象中的位错。对所观察的结果都分别作了分析和简要的讨论。 关键词：     

Dislocation and stacking fault core fields in fcc metals

Atomistic models were used to determine the properties of dislocation core fields and stacking fault fields in Al and Cu using embedded atom method (EAM) potentials. Long-range, linear elastic displacement fields due to nonlinear behaviour within dislocation cores, the core field, for relevant combinations of Shockley partial dislocations for edge, screw, and mixed (60° and 30°) geometries were obtained. Displacement fields of stacking faults were obtained separately and used to partition the core field of dissociated dislocations into core fields of partial dislocations and a stacking fault expansion field. Core field stresses were derived from which the total force, including the Volterra field plus core field, between dislocations for several dislocation configurations was determined. The Volterra field dominates when the distance between dislocations exceeds about 50b but forces due to core fields are important for smaller separation distances and were found to affect the equilibrium angle of edge dislocation dipoles and to contribute to the force between otherwise non-interacting edge and screw dislocations. Interactions among the components of a dissociated dislocation modify the equilibrium separation for Shockley partials suggesting that methods that determine stacking fault energies using measurements of separation distances should include core fields.     

Analysis of the dislocation structure at the Ge/Si(111) heterointerface

We demonstrate the formation of a modified triangular network of Shockley edge partial misfit dislocations at a plastic-relaxation level of ρ = 0.72 of 3D Ge(111) islands grown on a wetting layer. The network forms due to the offset of one dislocation family by 40% of the interdislocation spacing. We report ultra-high-vacuum scanning tunnel microscopy and high-resolution transmission electron microscopy data and the results of theoretical calculation of the stress fields induced in the film volume by the introduction of misfit dislocations. We establish the Ge-film-thickness range acceptable for observing elastic undulation of the surface by scanning tunneling microscopy.     

A dissociated dislocation in an ultrathin silicon plate

Simplified explicit expressions are presented to describe the elastic displacement field of a periodic family of misfit dislocations running parallel to the two free surfaces of an elastically isotropic plate. In the situation where the period tends to infinity, the use of these expressions proves to be quite valuable for investigating the change of the separation distance, S, between two partial dislocations as a function of the position of one partial and the orientation of the fault plane. For the two 30° Shockley partials of a dissociated screw dislocation in an ultrathin silicon plate, numerical results indicate that S can change drastically. This property is confirmed in anisotropic elasticity for a dislocation located near the free surface of a semi-infinite crystal. The results emphasize that particular attention should be paid to precise measurement of the local thickness and positions of the partials in weak beam or high resolution transmission electron microscopy experiments.     

Dislocation configurations around a nanoindentation in the surface of a fcc metal

We report a scanning tunnelling microscopy investigation of the emission of dislocations around nanoindentations in the form of dislocation arrangements previously called hillocks , consisting of two pairs of Shockley partial dislocations, each encompassing a stacking fault. The spatial arrangement and size distribution of hillocks around the nanoindentation traces are studied. We show that standard dislocation theory for an isotropic continuum can be used to describe the stability of the hillocks, their size and spatial distribution and the broadening of the corresponding extended dislocations near the surface. A model is proposed in which hillocks originate from the split into dislocations partials of primary perfect dislocation loops punched into the crystal by the scanning tunnelling microscope tip. This model implies the operation of a novel dislocation mechanism involving long-range transport of matter across the surface.     

Misfit dislocations in gold/Permalloy multilayers

Several groups have reported the misfit dislocation structures in Au/Ni_0.8Fe_0.2 multilayers where the lattice parameter misfit is very large. To explore the factors controlling such structures, molecular dynamics simulations have been used to simulate the vapour-phase growth of (111)-oriented Au/Ni_0.8Fe_0.2 multilayers. The simulations revealed the formation of misfit dislocations at both the gold-on-Ni_0.8Fe_0.2 and the Ni_0.8Fe_0.2-on-gold interfaces. The dislocation configuration and density were found to be in good agreement with previously reported high-resolution transmission electron microscopy observations. Additional atomic-scale simulations of a model nickel–gold system indicated that dislocations are nucleated as the first nickel layer is deposited on gold. These dislocations have an (a/6)?112? Burgers vector, typical of a Shockley partial dislocation. Each dislocation creates an extra {220} plane in the smaller lattice parameter nickel layer. These misfit-type dislocations effectively relieve misfit strain. The results also indicated that the dislocation structure is insensitive to the energy of the depositing atoms. Manipulation of the deposition processes is therefore unlikely to reduce this component of the defect population.     

Nondiffusion atomic ordering in the low-temperature deposition of copper

A new mechanism of atomic ordering in the low-temperature homoepitaxial deposition of copper onto a close-packed (111) plane has been discovered by means of molecular-dynamics simulation. This nondiffusion mechanism is caused by the collective motion of clusters along the dislocation lines of partial Shockley dislocations. We predict the existence of dislocation-induced coalescence, which is an increase in the mean size of face-centered cubic (fcc) clusters owing to a decrease in the number of hexagonal close-packed (hcp) clusters due to the motion of surface dislocations.     

Shrinking stacking fault through glide of the Shockley partial dislocation in hard-sphere crystal under gravity

Disappearance of a stacking fault in the hard-sphere crystal under gravity, such as reported by Zhuet al. [Nature 387, 883 (1997)], has successfully been demonstrated by Monte Carlo simulations. We previously found that a less ordered (or defective) crystal formed above a bottom ordered crystal under stepwise controlled gravity [Moriet al. J. Chem. Phys. 124, 174507 (2006)]. A defect in the upper defective region has been identified with a stacking fault for the (001) growth. We have looked at the shrinking of a stacking fault mediated by the motion of the Shockley partial dislocation; the Shockley partial dislocation terminating the lower end of the stacking fault glides. In addition, the presence of crystal strain, which cooperates with gravity to reduce stacking faults, has been observed.