Study of crystallization of phase change memory thin films induced by multiple femtosecond laser pulses

自从Ovshinsky发现硫系化合物的外场诱导可逆相变特性以来，相变存储材料的研究就受到了极大的关注。其中最具代表性的两种相变存储材料—GeSbTe和AgInSbTe已成功应用于可擦重写相变光盘和相变随机存储器。然而相较在市场上取得的成功，对相变存储材料的基础物理性质的研究还不够深入，对相变机制的认识还不够全面。近年来，超短脉冲激光诱导相变引起了人们的极大兴趣，这可能是提高材料相变速度（特别是晶化速度）的有效手段。一般认为，晶核的形成和成长需要一定的最短时间，并不适合通过超快激光脉冲来诱导晶化。然而，研究表明，超快激光可以对相变薄膜晶化程度实现精确控制，多脉冲的积累效应是其中的关键因素之一。多脉冲激发过程中的相变薄膜处于不同的中间状态，这为深入探讨薄膜初始状态对后续晶化过程的影响提供了便利条件。本论文以此为着眼点，利用时间分辨的实验和理论方法研究了Ge2Sb2Te5和 Ag8In14Sb55Te23薄膜在多脉冲飞秒激光诱导下的晶化过程，主要研究内容如下： 建立飞秒激光泵浦-探测反射率测试系统，泵浦光源为脉宽130fs，波长800nm的锁模钛蓝宝石飞秒激光器，重复频率在1-1kHz范围内可调，探测光源为波长650nm的半导体连续激光器，通过电子快门控制作用到样品上的脉冲数，整个系统的时间分辨率为2ns。建立飞秒激光与相变薄膜相互作用的双温模型，通过有限元法可数值计算薄膜中不同深度处晶格和电子温度随时间的演化过程。 基于时间分辨泵浦-探测实验系统和双温模型数值计算研究了多脉冲飞秒激光诱导非晶态Ge2Sb2Te5和Ag8In14Sb55Te23薄膜的晶化特性，重点研究了阈值效应对功率密度和脉冲数的依赖关系，通过解析逐个脉冲作用下的晶化动力学过程讨论了其晶化机制的差异。研究了不同初始状态（不同脉冲数目飞秒激光作用后）Ag8In14Sb55Te23薄膜的多脉冲飞秒激光诱导晶化特性。研究结果对深入理解极端非平衡条件下的相变机制，以及光学性能可精密调控相变器件（如多阶存储和显示器件等）的设计和应用具有参考价值。Since the phase-transition-based memory effect of chalcogenides was found by Ovshinsky, phase change memory materials, such as Ge2Sb2Te5 and AgInSbTe, have been extensively investigated. They are working as recording media of rewritable optical disks and phase change random access memories. Despite the increasing technological interest and advances, many fundamental properties of these materials, including their phase transition mechanisms, remain poorly understood. In recent years, researches on the phase transition driven by ultrafast laser pulses have attracted increasing attention due to that the shorter pulse width may be helpful to accelerate the phase transition (especially crystallization) speed. It is generally recognized that there should be a minimum time required for crystalline nucleation to form and grow, so the crystallization process seems not favored by using ultrashort pulses. Recent research showed that a precious control of crystallization degree can be achieved by the repeatedly exposing the same area of a phase change thin film to femtosecond laser pulses. The accumulation effect of multiple excitations is the key of this process. The phase change thin film is at different intermediate states during the multiple excitation process. It is convenient for studying the influence of initial states on the subsequent crystallization process. Based on these considerations, the crystallization process of Ge2Sb2Te5 and Ag8In14Sb55Te23 thin films driven by multiple femtosecond laser pulses have been studied by time-resolved methods in this dissertation. A pump-probe system was built for the real-time reflectivity measurements during laser induced crystallization. The pumping source was a mode-locked Ti:sapphire laser with a wavelength of 800 nm and a pulse duration of ~130 fs. The repetition rate can be tuned in the 1-1KHz range. The number of laser pulses irradiated on the sample can be controlled by an electronic shutter. The probing source was a continuous-wave laser beam from a semiconductor laser with a wavelength of 650 nm. The total time resolution of this system was about 2 ns. A two-temperature model was used to simulate the time-resolved temperature variation of the femtosecond laser-irradiated phase change materials. The crystallization characteristics of Ge2Sb2Te5 and Ag8In14Sb55Te23 thin films driven by multiple femtosecond laser pulses was studied by real-time reflectivity measurements and two-temperature model calculations. A controllable crystallization of the as-deposited amorphous Ge2Sb2Te5 and Ag8In14Sb55Te23 thin films was realized by multiple femtosecond laser-pulse irradiations. The threshold effects of pulse fluence and the number of pulses were analyzed in detail. The difference of crystallization dynamics and mechanisms between Ge2Sb2Te5 and Ag8In14Sb55Te23 thin films was discussed as well. The crystallization process of Ag8In14Sb55Te23 thin films affected by different initial states (as-deposited amorphous state, and irradiated by 4000 and 8000 successive femtosecond laser pulses) was studied and compared. These results would provide further insight into the phase-change mechanism under extra-non-equilibrium conditions and aid the development of multi-level phase-change devices (such as non-binary memory, gray-scale imaging device and arithmetic processor) based on the reflectivity programming