本文關鍵詞： 相場法 外延生長 原子島 應力 臺階失穩(wěn)　出處：《天津大學》2014年碩士論文　論文類型：學位論文

【摘要】：納米材料的宏觀性能取決于其微觀結構,而外延薄膜材料的制備總的來說是一個表面動力學過程。因此,對表面上原子的擴散、成核和生長,原子島間的兼并、失穩(wěn)、粗化,臺階流動及失穩(wěn)等一系列過程的研究具有非常重要的意義。本文提出一個新的基于擴散界面的相場模型用以描述外延生長過程中島的形核、生長及熟化過程。該模型同時考慮了生長過程中彈性場、表面能、質量沉積、擴散、解吸和能量勢壘等熱力學及動力學過程對表面納米形貌的影響。采用經(jīng)典的BCF模型來描述生長過程中的擴散形核過程,而采用一個新的包含彈性應變能的自由能函數(shù),通過變分得到一個可以描述多層島生長動態(tài)邊界的相場方程。采用有限差分格式對非線性耦合方程組進行求解,并編制相應的計算程序,通過可視化處理再現(xiàn)了外延生長的真實形貌。通過系統(tǒng)的數(shù)值預測真實再現(xiàn)了原子島的形貌,并定量地對表面粗糙度的演化做了詳細的刻畫。通過修正島邊界PFTZ的活原子擴散系數(shù)D實現(xiàn)島邊界上下臺階運動的不對稱性,來模擬SE勢壘阻礙活原子的下臺階運動,獲得了多層區(qū)島的形貌演化,再現(xiàn)了與實驗圖片一致的外延形貌。通過高度-高度相關函數(shù)的引入,計算了結構表面的局部粗糙度指數(shù)和粗糙度隨覆蓋率的變化,與已經(jīng)獲得的實驗和理論分析結果一致。采用彈性本構關系進行了應力分析,結果表明,在形貌演化中,伴隨著產(chǎn)生復雜的應力場,在島邊界處存在著高應力區(qū)。外延生長中的應力演化明顯的影響原子的擴散過程,當應力存在時,外延結構變化較無彈性場時變快。另外還利用相場模型模擬了鄰晶面上的臺階生長以及退火過程,獲得了與實驗一致的臺階演化形貌,并對臺階失穩(wěn)現(xiàn)象進行了機理性的探究。結果發(fā)現(xiàn):臺階的彎曲是在吉布斯-湯姆遜效應和彎曲勢壘的相互競爭作用下動態(tài)演化,在沉積過程中,KES效應起主導作用,導致臺階發(fā)生彎曲失穩(wěn)和群化失穩(wěn);當中斷沉積進行退火處理時,吉布斯-湯姆遜效應起主導作用,彎曲的臺階逐漸變得平滑。
[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.
【學位授予單位】：天津大學
【學位級別】：碩士
【學位授予年份】：2014
【分類號】：O484.1

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本文編號：1509549