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

相变材料，如GeSbTe或AgInSbTe合金，表现出一些显著的性能；它们可以在飞秒内非晶化，在皮秒内结晶，但可以在多年内保持稳定，防止状态的自发变化。它们在相位之间表现出巨大的差异，包括高达五个数量级的电导率差异和大的折射率变化；这些特性导致了它们在电气（相变RAM或PCM设备）和光学（DVD和蓝光光盘）存储器中的应用。这些显著特性的起源一直是最近许多研究的来源。Kolobov表明，与传统预期相反，Ge2Sb2Te5在非晶相中的短程有序度高于晶体相。这可以用Ge原子在无定形到晶体转变过程中从主要的四面体键到八面体键的“伞式翻转”来解释，并被认为是超高速开关的潜在起源。虽然这个简单的“伞状翻转”模型后来被证明不是一个真正现实的相变模型，也不能解释不含锗的相材料的行为，但它引发了世界范围内对这类重要材料中转换过程本质的准确理解的“探索”。“发现”揭示了部分答案，即相变合金的结晶相也相当不寻常，表现出强烈的共振键，这种键被认为是技术上有用的相变特性的“必要条件”。最近也报道了一种金属绝缘体型无序诱导的结晶相转变，也有人提出，晶体相的扭曲可能引发长程有序的崩溃，产生非晶相而不经过液态。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知之甚少，而利用这些特性来提供超越简单二进制存储器的令人兴奋的应用的方式在很大程度上是“未知领域”。例如，我们最近已经证明，通过使用飞秒光脉冲结晶GeSbTe合金，我们可以执行可靠的算术处理，因此提供了一种形式的“相变处理器”。此外，我们表明，相变材料比其他常见电子材料的一个基本优势是，它们具有易于获取和可用的电光响应，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种（尚未使用的）强有力的手段。混合模式行为还有几个潜在的非常重要的应用，例如超快速光门控开关（或者更有可能是光有源忆阻器，或“忆阻器”）。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，我们的建议结合了解决尚未解决的关键科学问题的新途径，以及探索利用相变材料显着特性的全新方法；具体来说，我们要求：1。这些相变（结晶和非晶化）过程到底有多快？相变材料的非晶化是否总是涉及熔化？切换事件的精确动态是什么？它们在光激发和电激发的情况下不同吗？它们在纳米电池上保持不变吗？超高速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尺度？我们可以利用混合模式行为来提供高级功能吗？

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?