How Supercooled Liquid Phase-Change Materials Crystallize: Snapshots after Femtosecond Optical Excitation

Abstract

Glass forming materials are employed in information storage technologies making use of the transition between a disordered (amorphous) and an ordered (crystalline) state. With increasing temperature the crystal growth velocity of these phase-change materials becomes so fast that prior studies have not been able to resolve these crystallization dynamics. However, crystallization is the time limiting factor in the write speed of phase-change memory devices. Here, for the first time, we quantify crystal growth velocities up to the melting point by using the relaxation of photo-excited carriers as an ultrafast heating mechanism.During repetitive femtosecond optical excitation, each pulse enables dynamical evolution for tens of picoseconds before the intermediate atomic structure is frozen-in as the sample rapidly cools. We apply this technique to Ag 4 In 3 Sb 67 Te 26 (AIST) and compare the dynamics of as-deposited and application-relevant melt-quenched glass. Both glasses retain their different kinetics even in the supercooled liquid state, thereby revealing differences in their kinetic fragilities. This approach enables the characterization of application-relevant properties of phase-change materials up to the melting temperature, which has not been possible before.Mankind has utilized glasses during the last five thousand years. They can be prepared by cooling a liquid fast and far enough below the glass transition temperature -to a temperature where its viscosity is sufficiently high that the atomic arrangement is kinetically frozen-in 1 . Until recently, research and technology have focused on good glass formers, i.e. materials which can be vitrified by cooling their liquid state at moderate rates. But in recent decades poor glass formers such as metallic glasses and certain chalcogenide glasses have gained interest due to their remarkable property portfolio 2,3 . These materials need to be cooled at rates in excess of around 3*10 9 K/s to bypass crystallization and to quench the atoms in an amorphous arrangement 4 . This so-called glass transition at temperature is commonly observed at a

Femtosecond x-ray diffraction reveals a liquid–liquid phase transition in phase-change materials

“…The FTS crossover was first proposed to explain the behavior of water, which is a fragile liquid down to the temperature of the homogeneous nucleation limit but behaves kinetically strong when heated from the solid amorphous state above (19). Similar observations were made on the phase-change material (PCM) Ag-In-Sb-Te in its liquid and solid amorphous states (9,12,20). However, in PCMs, liquid quenching is difficult due to the rapid onset of crystallization.…”

“…In case of PCMs, similar data are not yet available, but an anomalous breakdown of the Stokes-Einstein relationship (SER) in the equilibrium liquid of GeSb2Te4 was recently observed and was speculated to be caused by a LLPT below Tm (31). We focused our attention on Sb-based PCMs, because Ag4In3Sb67Te26 (AIST) (9,12,20) and Ge9Sb91 (32) have crossovers in the apparent activation energy of viscosity.…”

Crystallization Kinetics of GeSbTe Phase-Change Nanoparticles Resolved by Ultrafast Calorimetry

“…19,21 At that time it was proposed that the Arrhenius behavior correlates to the glassy state, whereas the non-Arrhenius behavior is associated with the supercooled liquid state. In this scenario, this divergence in temperature dependence of viscosity appears at the junction between glass and supercooled liquid.…”

Redefining the Speed Limit of Phase Change Memory Revealed by Time-resolved Steep Threshold-Switching Dynamics of AgInSbTe Devices

“…This unique switching property-portfolio of AIST devices is ideally suited for PC-RAM and also towards universal memory. There is further evidence in the form of the most successful optical memory products such as re-writable digital versatile discs (DVDs) and Blu-Ray discs that use AIST as the key material, which belongs to the second family of PC materials1 owing to its remarkable properties such as high-speed crystal growth velocities21313334 and easily reversible nature with better structural stability12935. Furthermore, recent work36 on the relation between band gap and resistance drift in amorphous AIST material demonstrated a strikingly lower drift of the apparent activation energy compared with GeTe, GeSbTe materials.…”