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Materials World Network - Understanding and exploiting mixed-mode ultra-fast optical-electrical behavior in nanoscale phase change materials

材料世界网络 - 理解和利用纳米级相变材料中的混合模式超快光电行为

基本信息

批准号：
EP/J018783/1
负责人：
C Wright
金额：
$ 46.72万
依托单位：
UNIVERSITY OF EXETER
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2013
资助国家：
英国
起止时间：
2013 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FJ018783%2F1
关键词：
Materials World Network Understanding exploiting

项目摘要

Phase-change materials, such as GeSbTe or AgInSbTe alloys, exhibit some remarkable properties; they can be amorphized in femtoseconds and crystallised in picoseconds, yet can remain stable against spontaneous changes of state for many years. They show hugely contrasting properties between phases, including an electrical conductivity difference of up to five orders of magnitude and a large refractive index change; properties that have led to their application in electrical (phase-change RAM or PCM devices) and optical (DVD and Blu-Ray disks) memories. The origin of such remarkable properties has been a source of much recent research. Kolobov showed that, contrary to conventional expectations, the short-range order in Ge2Sb2Te5 is higher in the amorphous than in the crystal phase. This was explained by an 'umbrella flip' of Ge atoms from primarily tetrahedral to octahedral bonding in the amorphous to crystalline transition, and was put forward as the potential origin of ultra-fast switching. While this simple 'umbrella-flip' model has since turned out not to be a truly realistic model of the phase-transition, and cannot explain the behavior of phase-materials that do not contain germanium, it sparked a world-wide 'quest' for an accurate understanding of the nature of switching processes in this important class of materials. Part of the answer was revealed by the 'discovery' that the crystalline phase of phase-change alloys is also rather unusual, exhibiting strong resonance bonding, with such bonding being suggested as a 'necessary condition' for technologically useful phase-change properties. Most recently a metal-insulator type disorder induced transition in the crystalline phase has also been reported, and it has also been suggested that distortions in the crystalline phase may trigger a collapse of long-range order, generating the amorphous phase without going through the liquid state.The scientific and technological importance of phase-change materials is clearly extremely high. However, many of their remarkable properties remain poorly understood, and the ways in which such properties might be exploited to deliver exciting applications going way beyond simple binary memories is largely 'uncharted territory'. For example we have, very recently, shown that by crystallizing GeSbTe alloys using femtosecond optical pulses we can perform reliable arithmetic processing, so providing a form of 'phase-change processor', Furthermore, we showed that a fundamental advantage of phase-change materials over other common electronics materials is that they have readily accessible and usable electrical and optical responses, and signals can be transferred relatively simply between these two domains. This mixed-mode behavior of phase-change materials provides a (as yet unused) powerful means to understand the fundamental switching properties of these materials. There are also several potentially very important applications of mixed-mode behavior, such as ultra-fast optically-gated switching for example (or, more speculatively, optically-active memristors - or 'memflectors'). However, this mixed-mode behavior of phase-change materials has never before been explored. Our proposal therefore combines a new route to addressing key scientific questions that remain unanswered, along with an exploration of entirely new ways in which to exploit the remarkable properties of phase-change materials; specifically we ask:1. exactly how fast are these phase-change (crystallization and amorphization) processes?2. does amorphization always involve melting in phase-change materials?3. what are the precise dynamics of switching events; are they different in optically-excited and electrically excited cases; do they remain the same on the nanocale?4. what are the key materials drivers for ultra-fast switching? 5. can we scale mixed-mode behavior to the nanoscale?6. can we exploit mixed-mode behavior to provide advanced functionality?

相变材料，如GeSbTe或AgInSbTe合金，表现出一些显著的性质：它们可以在飞秒内非晶化，在皮秒内结晶，但可以在多年的自发状态变化中保持稳定。它们在不同相之间表现出巨大的对比特性，包括高达五个数量级的电导率差异和巨大的折射率变化；这些特性导致它们在电(相变RAM或PCM设备)和光(DVD和蓝光盘)存储器中的应用。这些非凡特性的起源一直是最近许多研究的来源。Kolobov发现，与传统的预期相反，Ge2Sb2Te5中的短程有序度在非晶相中高于晶相中。这可以用非晶态到晶态转变过程中Ge原子从主要的四面体键到八面体键的“伞形翻转”来解释，并被认为是超快开关的潜在来源。虽然这个简单的“伞形翻转”模型后来被证明不是一个真正现实的相变模型，也不能解释不含Ge的相材料的行为，但它引发了世界范围内对这类重要材料中转变过程本质的准确理解。相变合金的晶相也相当不寻常，表现出强烈的共振键，这一发现揭示了部分答案，这种键被认为是技术上有用的相变性能的“必要条件”。最近还报道了一种金属绝缘体类型的无序导致晶相的相变，也有人提出晶相的扭曲可能会引发长程有序的坍塌，在不经过液态的情况下产生非晶相。相变材料的科学和技术重要性显然是非常高的。然而，它们的许多显著特性仍然鲜为人知，而如何利用这些特性来提供令人兴奋的应用程序，远远超出了简单的二进制存储器，在很大程度上是一个未知的领域。例如，我们最近已经证明，通过使用飞秒光脉冲使GeSbTe合金晶化，我们可以执行可靠的算术处理，因此提供了一种“相变处理器”，此外，我们还证明了相变材料与其他常见电子材料相比的一个基本优势是，它们具有易于访问和使用的电和光响应，并且信号可以在这两个域之间相对简单地传输。相变材料的这种混合模式行为为理解这些材料的基本开关特性提供了一个(尚未使用过的)强有力的手段。混合模式行为也有几个潜在的非常重要的应用，例如超快光学选通开关(或者，更推测地说，光学有源记忆阻器--或‘记忆反射器’)。然而，相变材料的这种混合模式行为以前从未被探索过。因此，我们的建议结合了一条新的路线来解决尚未回答的关键科学问题，并探索了一种全新的方法来利用相变材料的显著特性；具体地说，我们问：1.这些相变(晶化和非晶化)过程到底有多快？2.非晶化总是涉及相变材料中的熔化吗？3.开关事件的精确动力学是什么；它们在光激发和电激发的情况下是否不同；它们在纳米级上保持不变吗？4.超快切换的关键材料驱动因素是什么？5.我们能否将混合模式行为扩展到纳米级？6.我们能否利用混合模式行为来提供高级功能？