Optical investigation of non-thermal processes in phase change materials

在光盘上和随机存取存储器（RAM）芯片内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，更小的物理尺寸和使用低功耗。现有的光学存储格式（例如DVD-R/W）基于盘上涂层内的结构变化过程。通过用聚焦激光脉冲加热盘的表面，硫属化物合金在非晶相和晶相之间被驱动。虽然该技术已经很成熟，但我们对结构相变的理解却令人惊讶地有限。最近对Ge 2 Sb 2 Te 5（GST）（最常用的材料）的研究表明，该材料不一定会在传统意义上熔化，并表明特定电子状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，后者在扩展到更小的尺寸（更高的容量）方面面临困难。相比之下，PCRAM可扩展性好，并且本质上是可扩展的。PCRAM单元依赖于非晶相和晶相之间的可逆转变来“写入”数据。这种转变伴随着电阻的急剧变化，可以很容易地读出。当向非晶材料提供写入脉冲时，在结构相变或存储器切换发生之前观察到阈值切换到低导电率状态。阈值开关机制的起源仍然存在争议，阈值开关的内在时间尺度，据我们所知，从来没有被measured.我们建议使用时间分辨飞秒光学测量技术来调查GST和其他硫属合金的相变。使用传统的光学泵浦探测测量，我们将确定在切换过程中的不同时间占据的材料的相位，并研究其依赖于激发光脉冲的持续时间和波长。因此，我们将了解定制的光脉冲是否可以引起光盘的更有效的写入和擦除。高度优化的样品材料将由RWTH-Aachen、Plasmon Data Systems Ltd和ST Microelectronics提供。我们将研究合金对光脉冲偏振的响应。光致双折射的观察可以提供对转变的非热性质的更好理解，并导致光通信技术中的附加应用。频闪时间分辨光学测量也将在原型PCRAM单元上进行，以了解真实的设备中的阈值和存储器切换过程的动态。超快激光器将与快速电脉冲发生器同步，该发生器向电池提供设置和重置脉冲，使我们能够在阈值和存储器切换过程中确定硫属化物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，我们将研究特定的电子跃迁在非热断裂键的重要性。据我们所知，PCRAM器件的光学辅助电子切换以前没有尝试过。将通过多尺度建模来理解、解释和指导测量结果。将进行从头算密度泛函理论（DFT）计算以确定材料的晶体结构、光学性质和电子能带结构。物理上现实的宏观模型预测性能的真实的设备（电和光存储器）将开发的从头算原子尺度建模和现有的唯象结晶模型之间的“弥合差距”。

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.