摘要：
Phase change random access memory （PCRAM） has been successfully applied in the computerstorage architecture, as storage class memory, to bridge the performance gap between DRAM and Flash-based solid-state drive due to its good scalability, 3D-integration ability, fast operation speed and compatiblewith CMOS technology. Focusing on phase change materials and PCRAM for decades, we have successfullydeveloped 128 Mb embedded PCRAM chips, which can meet the requirements of most embedded systems.3D Xpoint （3D PCRAM）, invented by Intel and Micron, has been regarded as a new breakthrough in thelast 25 years since the application of NAND in 1989, which represents state-of-the-art memory technology.This technology has some remarkable features, such as the confined device structure with 20 nm size, themetal crossbar electrodes to reduce the resistance variations in PCRAM arrays, and the ovonic thresholdswitching selector that can provide a high drive current and a low leakage current. A good understandingof phase change mechanism is of great help to design new phase change materials with fast operation speed,low power consumption and long-lifetime. In this paper, we firstly review the development of PCRAMand different understandings on phase change mechanisms in recent years, and then propose a new viewon the mechanism, which is based on the octahedral structure motifs and vacancies. Octahedral structuremotifs are generally found in both amorphous and crystalline phase change materials. They are consideredto be the basic units during phase transition, which are severely defective in the amorphous phase. Theseconfigurations turn into more ordered ones after minor local rearrangements, the growth of which results inthe crystallization of rocksalt （RS） phase with a large amount of vacancies in the cation sites. Further drivenby thermodynamic driving force, these vacancies move and layer along certain directions; consequently, themetastable RS structure transforms into the stable hexagonal （HEX） structure. Based on our results, we findthat reversible phase transition between amorphous phase and RS phase, without further changing into HEXphase, would greatly decrease the required power consumption. Robust octahedra and plenty of vacanciesin both amorphous and RS phase, respectively avoiding large atomic rearrangement and providing necessaryspace, are crucial to achieve the nanosecond or even sub-nanosecond operation of PCRAM.
