【作者】 石林；

【导师】 倪军；

【作者基本信息】 清华大学 ， 物理学， 2007， 博士

【摘要】 合金的低维有序结构具有很多特殊物理性质,应用前景十分广阔,已经成为国内外研究的热点。随着外延生长技术的不断发展,特别是分子束外延等技术被广泛地应用,使人们可以制备各种有序薄膜材料。外延生长是一个非平衡的过程,会形成与体结构明显不同的薄膜结构。在外延生长过程中的动力学特征是合金结构形成的主要控制因素之一,故对动力学性质的研究将使人们能够深入地研究合金结构的形成。本文结合基于量子力学的第一原理计算和基于统计力学的主方程方法和动力学蒙特卡罗方法来研究面心立方合金薄膜外延生长的动力学性质,并且分析如何利用这些性质对有序结构进行剪裁。我们发展了主方程计算面心立方多层合金薄膜外延生长的方法,计算了面心立方多层合金薄膜的动力学相图。在计算中除了正常的平衡有序相和无序相外,我们发现了有序结构隔层出现的振荡有序相,它是由于生长过程中不同层动力学对应不同的生长过程导致的。为模拟真实材料的外延生长,我们结合第一原理计算和集团展开方法计算了原子间相互作用能。计算CoPt合金在不同晶格常数下的相互作用能,发现随着水平晶格常数由大变小体系的平衡态结构由L1₀[001]有序转变成L1₀[100]有序。我们发现可以通过调节生长温度或生长速度来调控外延生长的动力学过程从而实现对有序结构的剪裁。我们发现在外延生长的过程中存在动力学各向异性,其会导致随着生长条件的变化合金薄膜的有序取向发生转变。我们具体以CoPt合金薄膜外延生长为例进行了研究。随着温度的升高,在适当的生长速度下体系的有序结构从L1₀[100]有序结构变到L1₀[001]有序结构。而随着生长速度的升高,在适当的生长温度下体系的有序结构逐渐从L1₀[001]有序结构变到L1₀[100]有序结构。以上结果是当平衡态结构为L1₀[001]有序结构时出现的,当平衡态结构为L1₀[100]有序结构时,有序取向的转变会更复杂。通过对有序结构的剪裁,我们提出了制备有序取向超晶格的方法。利用以上的外延生长动力学特性,可以通过周期性的改变生长速度或生长温度使薄膜的有序取向周期性改变从而形成浓度不变但有序取向周期改变的超晶格。更多还原

【Abstract】 The low dimensional ordered structures of alloys have distinct properties, whichattract considerable interest for applications. The epitaxial methods, especially themolecular beam epitaxy, have been extensively used in alloy growth for the purposeof fabricating various ordered film structures. The epitaxial growth is a nonequilibriumgrowth process, which can form some film structures different from the bulk structure.The kinetic feature of growth is one of the controlling factors in the formation of alloystructures. The study on the kinetic feature will help us to understand further the forma-tion of the alloy structures. In this thesis, we study the kinetic features of face-centeredcubic （FCC） alloy film epitaxial growth by the first-principles based on quantum me-chanics and the master equation method （MEM） based on statistics mechanics, and wepropose the kinetic mechanisms to tailor the ordered structures.We develop the MEM to study the kinetics of ordering in FCC multilayer alloysduring the epitaxial growth. The kinetic phase diagrams for the epitaxial multilayer ofan FCC alloy are calculated. We find that there is a kinetic induced oscillatory orderedphase in addition to the equilibrium ordered phase in the kinetic phase diagrams. Thisordering oscillation is caused by the kinetic correlation of different growth processes.In order to simulate the epitaxial growth of real material, we calculate the inter-action energies between atoms, based on the first-principles and the cluster expansionmethod （CEM）. We obtain the interaction energies of CoPt alloy under different latticeconstants. We find that the equilibrium state changes from L1₀ [001] ordered struc-ture to L1₀ [100] ordered structure with the decrease of lattice constant parallel to thesubstrate plane.We propose that the ordered structure can be tailored by adjusting the growth tem-perature or the growth rate to control the growth kinetic processes. In the epitaxialgrowth, we find that there is the kinetic anisotropy. We show there is an ordering-orientation transition of alloy films with the change of growth condition due to thiskinetic anisotropy. As an example, we calculate the epitaxial growth of CoPt alloyfilms. With the increase of growth temperature, the ordered structure of CoPt films changes from the L1₀ [100] ordered structure to the L1₀ [001] ordered structure at ade-quate growth rate. With the increase of growth rate, the ordered structure of CoPt filmschanges from the L1₀ [001] ordered structure to the L1₀ [100] ordered structure at ade-quate growth temperature. These results can be obtained when the equilibrium state isthe L1₀ [001] ordered structure. The ordering-orientation transitions are more complexwhen the equilibrium state is the L1₀ [100] ordered structure.By tailoring of the ordered structure, we propose a method to synthesize theordering-orientation surperlattices. Based on the kinetic feature of alloy epitaxialgrowth, we can change the growth rate or the growth temperature periodically to syn-thesize the ordering-orientation surperlattices, whose odering-orientation is changedperiodically while concentration is kept constant.更多还原