力和扩散机制下外延形貌的演化分析

发布时间：2018-02-14 01:21

本文关键词： 相场法外延生长原子岛应力台阶失稳　出处：《天津大学》2014年硕士论文　论文类型：学位论文

【摘要】：纳米材料的宏观性能取决于其微观结构,而外延薄膜材料的制备总的来说是一个表面动力学过程。因此,对表面上原子的扩散、成核和生长,原子岛间的兼并、失稳、粗化,台阶流动及失稳等一系列过程的研究具有非常重要的意义。本文提出一个新的基于扩散界面的相场模型用以描述外延生长过程中岛的形核、生长及熟化过程。该模型同时考虑了生长过程中弹性场、表面能、质量沉积、扩散、解吸和能量势垒等热力学及动力学过程对表面纳米形貌的影响。采用经典的BCF模型来描述生长过程中的扩散形核过程,而采用一个新的包含弹性应变能的自由能函数,通过变分得到一个可以描述多层岛生长动态边界的相场方程。采用有限差分格式对非线性耦合方程组进行求解,并编制相应的计算程序,通过可视化处理再现了外延生长的真实形貌。通过系统的数值预测真实再现了原子岛的形貌,并定量地对表面粗糙度的演化做了详细的刻画。通过修正岛边界PFTZ的活原子扩散系数D实现岛边界上下台阶运动的不对称性,来模拟SE势垒阻碍活原子的下台阶运动,获得了多层区岛的形貌演化,再现了与实验图片一致的外延形貌。通过高度-高度相关函数的引入,计算了结构表面的局部粗糙度指数和粗糙度随覆盖率的变化,与已经获得的实验和理论分析结果一致。采用弹性本构关系进行了应力分析,结果表明,在形貌演化中,伴随着产生复杂的应力场,在岛边界处存在着高应力区。外延生长中的应力演化明显的影响原子的扩散过程,当应力存在时,外延结构变化较无弹性场时变快。另外还利用相场模型模拟了邻晶面上的台阶生长以及退火过程,获得了与实验一致的台阶演化形貌,并对台阶失稳现象进行了机理性的探究。结果发现:台阶的弯曲是在吉布斯-汤姆逊效应和弯曲势垒的相互竞争作用下动态演化,在沉积过程中,KES效应起主导作用,导致台阶发生弯曲失稳和群化失稳;当中断沉积进行退火处理时,吉布斯-汤姆逊效应起主导作用,弯曲的台阶逐渐变得平滑。
[Abstract]:The macroscopical properties of nanomaterials depend on their microstructure, and the preparation of epitaxial thin films is generally a surface dynamic process. Therefore, the diffusion, nucleation and growth of atoms on the surface, the annexation, instability, coarsening of atomic islands, It is of great significance to study a series of processes such as step flow and instability. In this paper, a new phase field model based on diffusion interface is proposed to describe the nucleation of the island in the process of epitaxial growth. The model also takes into account the elastic field, surface energy, mass deposition and diffusion during the growth process. The effects of thermodynamic and kinetic processes, such as desorption and energy barrier, on the surface morphology are studied. A classical BCF model is used to describe the diffusion nucleation process in the growth process, and a new free energy function containing elastic strain energy is used. A phase field equation which can describe the dynamic boundary of multilayer island growth is obtained by variational method. The nonlinear coupled equations are solved by finite difference scheme, and the corresponding calculation program is compiled. The true morphology of the epitaxial growth is reproduced by visual processing, and the morphology of the atomic island is reproduced by the numerical prediction of the system. The evolution of surface roughness is described in detail. By modifying the living atom diffusion coefficient D of the island boundary PFTZ to realize the asymmetry of the upper and lower steps motion of the island boundary, the SE barrier hinders the lower step motion of the living atom. The morphologic evolution of the multilayer islands is obtained, and the epitaxial morphology consistent with the experimental images is reproduced. The variation of the local roughness index and roughness with the coverage of the structure surface is calculated by the introduction of the high-height correlation function. In accordance with the experimental and theoretical results obtained, the stress analysis is carried out by means of the elastic constitutive relation. The results show that the complex stress field is accompanied by the evolution of the morphology. There is a high stress region at the island boundary. The stress evolution in the epitaxial growth obviously affects the diffusion process of atoms, and when the stress exists, The phase field model is used to simulate the step growth and annealing process on the adjacent crystal plane, and the step evolution morphology is obtained, which is consistent with the experimental results. It is found that the buckling of the step evolves dynamically under the interaction of the Gibbs Thomson effect and the bending barrier, and the KES effect plays a leading role in the deposition process. The step buckling and group instability occur, and the Gibbs Thomson effect plays a leading role when the interrupted deposition is annealed, and the curved step becomes smooth gradually.
【学位授予单位】：天津大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：O484.1

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