Optical investigation of non-thermal processes in phase change materials

在光盘和随机存取存储器（RAM）芯片上存储信息是当前和未来信息技术的核心。未来的存储设备必须具有更大的容量、更短的访问时间、更小的物理尺寸和更低的功耗。现有的光存储格式（例如DVD-R/W）是基于磁盘涂层内部结构变化的过程。通过聚焦激光脉冲加热光盘表面，硫化物合金被驱动在非晶相和晶相之间。虽然这项技术已经很成熟，但我们对结构相变的理解却非常有限。最近对最常用的材料Ge2 Sb2 Te5 （GST）的研究表明，该材料不一定会在传统意义上熔化，并表明特定电子态的光激发驱动非热相变。电寻址相变RAM （PCRAM或Ovonic存储器）有可能取代现有的基于flash的存储器，后者在扩展到更小尺寸（更高容量）方面面临困难。相比之下，PCRAM扩展良好，并且具有固有的双稳态。PCRAM电池依靠非晶态和晶态之间的可逆转变来“写入”数据。这种转变伴随着电阻的剧烈变化，这种变化很容易读出。当向非晶材料提供写脉冲时，在结构相变或记忆开关发生之前，可以观察到阈值切换到低电导率状态。阈值开关机制的起源仍然存在争议，据我们所知，阈值开关的内在时间尺度从未被测量过。我们建议使用时间分辨飞秒光学测量技术来研究GST和其他硫族合金的相变。利用传统的光泵浦-探针测量方法，我们将确定材料在开关过程中不同时间所处的相位，并研究其与激发光脉冲持续时间和波长的关系。因此，我们将了解定制的光脉冲是否可以诱导光盘更有效的写入和擦除。高度优化的样品材料将由亚琛工业大学、等离子体数据系统有限公司和意法半导体提供。我们将研究合金对光脉冲偏振的响应。光诱导双折射的观察可以更好地理解跃迁的非热性质，并导致光通信技术中的其他应用。频闪时间分辨光学测量也将在原型PCRAM细胞上进行，以了解真实器件中阈值和存储开关过程的动态。超快激光将与快速电脉冲发生器同步，向电池提供设置和重置脉冲，使我们能够在阈值和记忆开关过程中确定硫系合金的瞬时电子状态。我们还将使用短而强的激光脉冲来辅助开关过程。通过改变脉冲的波长，我们将研究特定电子跃迁在键的非热断裂中的重要性。光辅助PCRAM器件的电子开关，据我们所知，以前没有尝试过。测量结果将由多尺度建模来理解、解释和指导。采用Ab-initio密度泛函理论（DFT）计算确定材料的晶体结构、光学性质和电子能带结构。用于预测真实器件（电和光存储器）性能的物理现实宏观模型将通过“弥合”从头算原子尺度模型和现有现象学结晶模型之间的差距来开发。

Storage of information on optical disks and within random access memory (RAM) chips is central to both present and future information technology. Future storage devices must have larger capacity, shorter access times, smaller physical size and use low power. Existing optical storage formats (e.g DVD-R/W) are based upon a process of structural change within the coating on the disk. By heating the surface of the disk with focused laser pulses, a chalcogenide alloy is driven between amorphous and crystalline phases. Although the technology is well established, our understanding of the structural phase transition is surprisingly limited. A recent study of Ge2 Sb2 Te5 (GST), the most commonly used material, revealed that the material does not necessarily melt in a conventional sense, and suggested that optical excitation of specific electronic states drives a non-thermal phase transition. Electrically addressed phase-change RAM (PCRAM or Ovonic memory) has the potential to replace existing FLASH-based memory, which faces difficulties in scaling to smaller sizes (higher capacities). In contrast PCRAM scales well and is inherently bistable. PCRAM cells rely on a reversible transition between the amorphous and crystalline phases to 'write' data. The transition is accompanied by a dramatic change in electrical resistance that may be easily read out. When a write pulse is supplied to the amorphous material, threshold switching to a low conductivity state is observed before the structural phase change, or memory switching , occurs. The origin of the threshold switching mechanism remains controversial and the intrinsic timescales for threshold switching have, as far as we are aware, never been measured.We propose to use time-resolved femtosecond optical measurement techniques to investigate the phase transition in GST and other chalcogenide alloys. Using conventional optical pump-probe measurements we will determine which phase the material occupies at different times during the switching process and investigate its dependence upon the duration and wavelength of the exciting optical pulse. We will hence understand whether a tailored optical pulse may induce more efficient writing and erasure of an optical disk. Highly optimized sample materials will be supplied by RWTH-Aachen, Plasmon Data Systems Ltd, and ST Microelectronics. We will examine the response of the alloy to the polarization of the optical pulse. The observation of optically induced birefringence may provide a better understanding of the non-thermal nature of the transition and lead to additional applications in optical communications technology. Stroboscopic time resolved optical measurements will also be performed upon prototype PCRAM cells to understand the dynamics of the threshold and memory switching processes in real devices. The ultrafast laser will be synchronised to fast electrical pulse generators that deliver set and reset pulses to the cell, allowing us to determine the instantaneous electronic state of the chalcogenide alloy during both the threshold and memory switching processes. We will also use a short but intense laser pulse to assist the switching processes. By varying the wavelength of the pulse we will investigate the importance of specific electronic transitions in the non-thermal breaking of bonds. Optically-assisted electronic switching of PCRAM devices has, to our knowledge, not been previously attempted. The measurements will be understood, interpreted and guided by multi-scale modelling. Ab-initio Density Functional Theory (DFT) calculations will be performed to determine the crystallographic strucutre, optical properties and electronic band stucture of the material. Physically realistic macroscopic models for predicting the performance of real device (electrical and optical memories) will be developed by 'bridging the gap' between ab-initio atomic scale modelling and existing phenomenological crystallisation models.