The origin of hexagonal phase and its evolution process in Ge2Sb2Te5 alloy

Ge2Sb2Te5 (GST) is the most important material for phase change random access memory (PCRAM) applications, while the formation of hexagonal (h-) phase results in low switching speed, large energy consumption, and worse endurance performance. Uncovering the formation mechanism of h-phase is beneficial for the further improvement of GST-based PCRAM devices. In this work, through advanced spherical aberration corrected transmission electron microscopy and transmission electron back-scattered diffraction technique, the mechanism of h-phase microstructure evolution is clearly clarified. We find that the vacancy ordering is more likely to appear around the grain boundary in a face-centered-cubic (f-) phase grain, which is the starting point for the generation of h-phase. More specifically, all the atoms in f-phase undergo a gradual shift into h-lattice positions to complete the f-to-h structural transition. By introducing an elemental dopant, for instance, carbon (C), the aggregation of C clusters prefers to distribute in the grain boundary area, which is the essential reason for postponing the generation and expansion of h-phase and greatly improving the thermal stability of C-GST material. In short, clarification of the origin of h-structure incubated from f-phase guides the optimization strategy of GST-based PCRAM devices.