State-Dependent Phase-Transition Kinetics in Phase-Change Materials

Analogue photonics states are important for optical computing, beam steering and displays. Phase change materials (PCMs) are promising materials for analogue photonics as they can be tuned to multiple optical levels by controlling the transition between their amorphous and crystalline phases. To design PCM-based optical switches with efficient and accurate multi-level control, the state dependent phase-transition kinetics must be understood.<br/>In this work, we show that the degree of disorder in the ‘amorphous state’ can be controlled by thermally activated crystallisation and melting processes, and in turn that these processes depend on the degree of disorder already present in the material. Firstly, we show using a thermo-optical phase-change model that Ge₂Sb₂Te₅ can be switched into states with different degrees of disorder by partial melting the nanocrystalline microstructure. Subsequent rapid quenching freezes-in the partially molten phase into a solid, with optical properties that depend on the energy applied during melting. The theory is supported by experiment results showing how 16 different reflectance levels can be amorphized into Ge₂Sb₂Te₅ using nanosecond-order heat pulses.<br/>Secondly, we show how the level of disorder (amorphousness) in Sb₂Te₃ can be controlled using pulsed laser heating and that the recrystallisation temperature strongly depends on the local atomic configurations in the amorphous structure. Counterintuitively, high energy heat pulses tends to produce amorphous materials with higher activation energy yet shorter minimum crystallisation times. This catalyst-like effect is important because it provides a way we can switch PCM devices at higher rates. Our results for this work set the scene for energy efficient and high speed PCM-enabled analogue photonics.