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

Abstract

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.

Liquid‐liquid phase transitions in glass‐forming systems and their implications for memory technology

“…The structural origin of these L‐L transitions was characterized by employing femtosecond x‐ray diffraction to observe the change in local atomic environment before crystallization occurred on the nanosecond time scale. It was found that the transition is predominantly caused by the onset of a Peierls distortion similar to that described in Figure . The occurrence of an L‐L transition in these materials therefore enables the fabrication of devices combining high speed with excellent data retention.…”

“…The L‐L coexistence line ends with a critical point and is followed by the Widom line depicted as the blue dotted line. This behavior is representative of Si, ST2 water, and likely some PCMs . For the case of higher order L‐L transitions depicted in Figure B, the transition between the low‐density and high‐density phases is continuous and the boundary between the two phases is represented as a broader region with a finite width rather than a coexistence line.…”

“…Likewise, the phase change material AIST (AgInSbTe) crystallizes rapidly before the anomaly in viscosity can be measured. However, recent structural measurements by ultrafast x‐ray probe have permitted to access the supercooled regime and have provided direct structural evidence for an L‐L transition in this system . Conversely, better glass‐formers such as GeSe 3 or As 5 Se 5 allow viscosity measurements across the whole temperature range and permit to reveal the entire F‐S transition.…”

“…On the other end, the high kinetic factor of fragile systems is disadvantageous in the glassy state as it may lead to crystallization of the amorphous bit at ambient temperature and loss of stored data. It was recently reported that this conundrum can be solved in PCMs such as AIST, Ge 15 Sb 85 or Ge 2 Sb 2 Te 5 where a sharp transition from fragile to strong occurs upon cooling, thereby providing the benefit of fast kinetics at high temperature and stabilization of the amorphous phase at low temperature . The structural origin of these L‐L transitions was characterized by employing femtosecond x‐ray diffraction to observe the change in local atomic environment before crystallization occurred on the nanosecond time scale.…”

Memory materials and devices: From concept to application

“…As shown in Figure D, the 10‐year data retention shows a slightly better performance than that of the GST device (~82°C). This stability may have a relationship with suffocated diffusion or even a potential fragile‐to‐strong crossover in kinetics that has been recently proved to exist in many PCMs …”