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

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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?