摘要
由于类金刚石(DLC)薄膜具有很多优良的物理学、化学性质和广泛的应用前景，对它们的生长机制和合成方法的研究一直是物理学及材料科学研究领域的重要内容。实验工作者已经成功地用脉冲激光烧蚀沉积(PLD)、离子束辅助沉积(IBAD)等方法合成无氢类金刚石(DLC)薄膜。在PLD实验中，已发现飞秒(fs)脉冲沉积条件下较高能量的小碳分子有利于合成高质量的DLC膜。此外，在一定的实验条件下，IBAD可提高合成薄膜的密度、硬度及增强薄膜与表面的结合。然而，在实验的解释和理论分析方面还缺少系统的研究。因为类金刚石薄膜的合成是一个相当复杂的过程，它不仅要受到衬底和沉积源特性的影响，而且还与表面和界面的原子间相互作用及系统热力学和碰撞动力学作用之间的竞争密切相关。对于离子束辅助沉积还涉及离子注入等动力学过程。实验上很难对轰击粒子、沉积分子与表面相互作用的动力学过程进行跟踪观察，许多与原子有关的细节无法获得。因此，在原子水平上研究团簇与表面的相互作用，了解DLC薄膜生长的微观机制，探索实验条件对合成类金刚石薄膜结构的影响，以及改进制备技术和提高薄膜质量有着重要意义。为了研究PLD实验中粒子流特性对薄膜结构的影响。本文基于分子动力学方法并分别选用Brenner、Tersoff和ziegler,Biersack和Littmark(ZBL)势描述原子间的相互作用，首先研究了小团簇C_2和C-(10)在金刚石(001)-(2×1)表面化学吸附的动力学过程。并模拟了C_2和C_(10)合成类金刚石薄膜的结构特征。进而，设计了IBAD的计算机实验模型，模拟了用Ar离子束辅助沉积的方法在硅(001)-(2×1)表面上沉积类金刚石薄膜。定量分析了合成薄膜的结构特征和薄膜沉积的动力学过程，并和相关的实验结果进行了比较。本文的具体研究内容如下：
(1)实验上，脉冲激光的方法合成无氢类金刚石薄膜，通过实验分析了解到，飞秒脉冲激光烧蚀石墨靶材产生的等离子体羽，主要成分是荷能的小团簇。本文，研究了荷能的C_2和环状C_(10)团簇在金刚石(001)-(2×1)表面的化学吸附过程。采用的入射能量为10eV到100eV，主要讨论了团簇大小和入射能量对沉积团簇结构的影响。研究发现仅当C_(10)团簇的入射能量超过某一阈值(5-40eV)时才能发生化学吸附。而当C_2团簇以很低的能量(如0.01eV)入射时就能被吸附在表面上。随着入射能量的增加，吸附在表面的C_2和C-(10)均改变了自由团簇的结构。模拟观察到C_2在吸附过程中打开金刚石再构表面的二聚体(dimer)反应，此过程是类金刚石薄膜生长的主要途径之一。统计计算显示，C_2团簇的吸附概率随着入射能量的增加而提高。以金刚石为衬底，模拟了由荷能C_2和C_(10)合成薄膜的生长过程。通过对薄膜的结构分析，发现由C_2合成的薄膜比C_(10)膜的SP~3键含
量更高一些，特别是当CZ具有较高入射能量和在低温衬底上沉积效果较好。我
们的结果支持了实验的发现。而且，从原子尺度解释了沉积机制。
(2)近年来，实验发现离子束辅助沉积的方法可以制备出高质量的类金刚
石薄膜。本文分别选CZ分子和Ar离子作为沉积源和辅助沉积粒子。模拟Ar离
子辅助沉积在51(001)一(2xl)表面合成DLC膜。Ar的入射能量分别为10，30，
50和100eV。我们重点研究了辅助轰击离子(Ar)和沉积原子(C)的比率(到
达比)和Ar的轰击能量对沉积动力学过程和薄膜结构的影响。通过计算密度分
布、近邻数分布和碳原子注入深度等物理量，定量分析了合成薄膜的四面体结构
特征与Ar轰击能量及到达比的关系，结果和实验观察相一致。支持了IBAD实验
得到的部分结果。为了研究Ar辅助轰击对薄膜生长的动力学的影响，我们用分
子动力学方法设计级联区域，定量的统计了级联区域内的碳原子的反冲动能、原
子迁移等物理量。分析表明，由于Ar离子的轰击引起的能量和动量的传递，大
大地增强了表面沉积C原子的活性及迁移率，特别是表面SP，杂化原子的移位，
增加了合成薄膜的SP3键含量。结果表明，选择合适的Ar能量，并增加束流通
量可制备出密度高、平滑度好且与衬底结合好的薄膜。我们的研究从合成机制上
给出了一些定量解释。
The growth mechanism and synthesis methods of diamond-like-carbon (DLC) films have attracted much attention, due to their unique physical and chemical properties and potential applications. In experiments, DLC films have been synthesized by various methods, such as chemical vapor deposition (CVD) and microwave plasma deposition, etc. It was observed in Pulsed Laser Deposition (PLD) experiments that energetic small carbon molecules are favorable to DLC film growth. Moreover, Ion Beam Assisted Deposition (IBAD) can improve the corresponding film's density, hardness and adhesion to the substrate. The synthesis of DLC films is a complicated process. It is relative to the atomic interaction on both surface and interface and the competition between thermodynamics and collision dynamics of the system. The microscopic process is difficult to observe experimentally. Therefore, it is important to explore the growth mechanism at atomic scale and the appropriate experimental parameters, which favor the growth of DLC fil
ms. In this paper, we first investigated the chemisorption of small molecules, C2 and C10, deposition on the diamond (001)-(2. 1) surface by molecular dynamics (MD) simulation. We simulated the full process of the DLC films assembled by C2 and C10 and analyzed the structure properties of the films. Secondly, we mimicked the film growth on silicon (001)-(2. 1) surfaces via Ar assisted deposition. The results were compared with experimental measurements. In our simulation, the semi-empirical many-body Brenner (#2) potential was used to describe the interaction between C-C atoms, and the Tersoff potential was used to describe the interaction between C-Si, and Si-Si atoms. The Ziegler, Biersack and Littmark (ZBL) pair potential, was used to describe the interactions between Ar-C and Ar-Si atoms. Our main results can be summarized as the following:
(1) Experimentally, hydrogen-free DLC films were assembled by means of PLD, where energetic small-carbon-clusters were deposited on the substrate. In this paper, the chemisorption of energetic C2 and C10 clusters on diamond (001)-(2xl) surface was investigated by molecular dynamics simulation. The influence of cluster size and the impact energy on the structure character of the deposited clusters is mainly addressed. The impact energy was varied from 10 to 100 eV. The chemisorption of C10 was found to occur only when its incident energy is above a threshold value Eth (5-40 eV). While the C2 cluster was easily to adsorb on the surface even at much lower incident energy (0.01 eV). With increasing the impact energy, the structures of the deposited C2 and C10 are different from the free clusters. We also observed the dimmer-opening reaction caused by C2 deposition, which plays an important role in the process of DLC film growth. The adsorption rate is raised with increasing the incident energy of C2- Finally, th
e growth of films synthesized by energetic C2 and C10 clusters were simulated. The statistics indicate the C2 cluster has high probability of adsorption and films assembled of C2 present higher SP3 fraction than that of C1-films, especially at higher impact energy and lower substrate temperature. Our results agree well with the experimental observation.
(2) The growth of DLC films via ion-beam-assisted deposition (IBAD) is simulated using molecular dynamical simulation. The C2-molecules and Ar ions were selected as deposition and assistance projectiles, respectively. The impact energy of Ar ranged from 10, 30, 50, to 100 eV. We focused our examination on the effects pf the assistance/deposition atomic ratio and the incident energy of Ar ions on the growth dynamics and film structure. We analyzed quantitatively the film structure by calculating the density distribution and the coordinate-number-distribution and atomic mixing at the film/substrate interface. In order to study the mechanism of Ar ion assisted deposition, the cascade volume was defined in the simulation. The recoil energy and migration of carbon atoms within the cascade volume were studied quantitativ
引文
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