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，表现出一些显着的性能;它们可以在飞秒内非晶化，在皮秒内结晶，但可以保持稳定，对状态的自发变化多年。它们在相位之间显示出巨大的对比特性，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。这些非凡特性的起源是最近许多研究的来源。Kolobov表明，与传统的预期相反，Ge 2Sb 2 Te 5中的短程有序在非晶相中比在晶相中更高。这被解释为一个“伞翻转”的Ge原子从主要四面体到八面体键合的非晶晶体的过渡，并提出了作为超快速开关的潜在起源。虽然这个简单的“伞翻转”模型后来被证明不是一个真正现实的相变模型，并且不能解释不含锗的相材料的行为，但它引发了全世界对这类重要材料中开关过程性质的准确理解的“追求”。部分答案是由“发现”揭示的，相变合金的结晶相也是相当不寻常的，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”。最近也报道了金属-绝缘体型无序诱导的晶相转变，并且还提出晶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非常高。然而，他们的许多显着的属性仍然知之甚少，并利用这些属性提供令人兴奋的应用程序超越简单的二进制存储器的方式在很大程度上是“未知的领域”。例如，我们最近已经表明，通过使用飞秒光脉冲使GeSbTe合金结晶，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，我们表明相变材料相对于其他常见电子材料的基本优点是它们具有容易获得和可用的电学和光学响应，并且信号可以在这两个域之间相对简单地传递。相变材料的这种混合模式行为提供了一种（尚未使用的）强大的手段来理解这些材料的基本开关特性。混合模式行为也有几个潜在的非常重要的应用，例如超快速光门控开关（或者更推测地，光学有源忆阻器或“memflectors”）。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仍然没有答案的关键科学问题，沿着探索全新的方法来利用相变材料的显着性能;具体来说，我们问：1。这些相变（结晶和非晶化）过程究竟有多快？2.相变材料的非晶化是否总是包括熔化？3.切换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？它们在纳米笼上是否保持相同？4.超快速开关的关键材料驱动因素是什么？5.我们能把混合模式的行为放大到纳米尺度吗6.我们能否利用混合模式行为来提供高级功能？

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?