Phase-change-memory (PCM) technology stores information as nonvolatile changes in the atomic-structural state of the memory material, written by the application of appropriate light pulses in the case of optical memory, as employed in PC photonic computing devices (or by the application of suitable voltage pulses, in the case of electronic phase-change random-access memory (PCRAM) devices). The different structural states of the memory material are typically glassy (amorphous) (“RESET” state) and crystalline (“SET” state), corresponding to the binary memory (logic) states and , respectively. These two structural phases have a sufficiently large contrast in their physical properties, for example, optical reflectivity for photonic memory (or electrical resistance for electrical memory), to enable the memory state ( or ) to be read out optically (or electrically) after writing a bit. The SET (glass→crystal) and RESET (crystal→glass) phase transformations can be performed reversibly and ultra-rapidly (~ ns or less) in the case of archetypal PCM materials such as tellurides, particularly the canonical PCM material, Ge2Sb2Te5 (GST-225). However, in situ experimental studies to investigate the nature and origin of this PC behavior are extremely challenging: the time-scale of the phase transformations is extremely short (~ns or less); the size of the memory element can be very small (10s of nm for PCRAM devices); and the memory element can be buried in/surrounded by other parts of the memory device, for example, the optical waveguide and capping layer in the case of photonic devices (electrodes, heater, and dielectric for PCRAM devices). Thus, computer simulation has become an essential tool for understanding the behavior of PCM materials, especially at the atomistic level.
