激光诱导信息材料超快相变的热力效应及其作用机理研究

Laser induced phase transition is a technology to achieve the information storing by the ultrafast transition between the crystalline and amorphous states of the phase change materials (PCMs). How to explain the mechanism of the ultrafast transformation has become one of the key problems to be settled in this field. Different from the thermal equilibrium phase transition, the characteristics of laser induced transformation are regarded as the ultrafast heating and cooling by laser irradiations, therefore, the transition between the crystalline and amorphous states is affected by both the temperature and stress.Firstly, the theory of nonequilibrium phase transition for laser processing and the thermo-elastic-plastic constitutive relation for the PCMs are built up respectively. Based on the theoretical model, a temperature/stress coupled numerical model (FEM) is then built up to simulate the variation of the temperature and stress fields during the process of laser induced transformation. Then the microstrcuture and properties of the laser irradiation zone are studied following the new scheme of the experiment. The relation among the temperature, stress and microstructure is further elucidated based on the experimental and simulated results.Finally,a finite element and ab initio molecular dynamics coupled model (FEM/MD(AIMD)) is developed to explore the origin of the structure and properties during the transformation process from the effect of the temperature and stress.The reason for the ultrafast transformation and the reversability of laser induced transition is subsequently explained according to the experimental and simulated results. The factors to affect the ultrafast transformation of GST are analyzed and the mechanism of the ultrafast transformation induced by laser is uncovered. The work based on the project is very important not only for the development of the basic theory of the ultrafast transformation but also for the engineering applications.

光致相变技术是利用激光作用下物质相在晶态/非晶态之间的转变来实现信息快速存储的一种技术，如何阐明激光诱导信息材料超快相变的机理是当前本领域尚未解决而又亟待解决的根本问题。不同于热致相变过程，光致相变的突出特点是超快的加热和冷却速度使得相变过程同时受温度和应力的驱动。本项目首先在理论上发展激光诱导非平衡相变的动力学理论并建立光致相变材料的本构关系，在此基础上，建立温度场/应力场耦合的数值模型（FEM），阐明激光诱导相变过程中温度场、应力场的变化规律。根据设计的实验方案，研究激光诱导相变区组织和性能的分布特征，建立温度-应力-显微组织的关系。最后，构建有限元与第一原理分子动力学耦合模型（FEM/MD(AIMD)），研究温度、应力作用下微结构和性能的起源及变化规律，解释激光诱导相变的超快性和可循环性,揭示激光诱导相变机理。项目的开展，不仅具有重要的理论意义，也具有深远的工程应用背景。

光致相变技术是利用激光作用下物质相在晶态/非晶态之间的转变来实现信息快速存储的一种技术，如何阐明激光诱导信息材料超快相变的机理是当前本领域尚未解决而又亟待解决的根本问题。本项目主要通过实验及模拟的方法研究材料的快速相变过程及微观组织形貌。在单脉冲纳秒激光辐照下，相变材料（Ge2Sb2Te5）结晶的能量密度为24.4-66.6mJ/cm2且晶体的结构为面心立方结构；结晶区域由上表面的柱状晶和晶体内部的等轴晶组成，且柱状晶会随着激光能量密度的提高而增大。随着薄膜内部温度梯度逐渐下降，柱状晶被逐渐的被等轴晶替代。同时对比研究了纳秒脉冲激光诱导相变和等温退火诱导相变的晶化行为，等温退火诱导的相变过程中，由于体加热模式，使得晶粒尺寸随薄膜厚度的增加而显著增加，甚至达到亚微米量级。对于在纳秒激光诱导下Sn掺杂的Ge2Sb2Te5的相变过程，与Ge2Sb2Te5相比，Sn的掺杂会扩大其点阵常数但降低了结晶的温度和激活能，即Sn掺杂的Ge2Sb2Te5更容易结晶。这主要是由于Sn-Te键的键能（359.8kJ/mol）比Ge-Te键的键能（456kJ/mol）低，因此更易于断裂和形成。在单脉冲皮秒激光辐照下，辐照区域分为四个部分：强烧蚀区，弱烧蚀区，融化区，辐照区。通过理论计算，烧蚀和熔化的阈值分别为173.05mJ/cm2和99.19mJ/cm2。此外，通过三维有限元模拟得到了纳秒和皮秒激光辐照相变材料的温度变化曲线为四个阶段：即升温阶段，保温阶段，降温阶段，常温阶段。模拟结果也表明了材料的结晶并不是发生在相变材料的升温阶段，而是在相变材料的降温阶段发生。为了进一步揭示内在相变机理，采用第一性原理计算，对相变材料非晶态的原子结构和电特性进行了分析。并根据分子动力学轨迹和原子结构阐述了非晶化过程的相变机理，发现空位的存在导致Te原子塌陷，形成了以Ge原子为中心的四面体结构，最终形成了存在大量四方环的非晶态结构，证明了空位的重要性；并推测在晶化过程中键长不需要变化，只需要键角的扭曲即可完成相转变。上述研究成果不仅有助于理解超快速相变的内在机理，同时对工程应用也有很强的指导性意义。