Multistep Crystallization of Ge-Rich GST Unveiled by In Situ synchrotron X-ray diffraction and (scanning) transmission electron microscopy

Among many phase change materials, Ge-rich GeSbTe (GST) alloys are of considerable interest due to their high thermal stability, a specification required for the next generation of embedded digital memories. This stability results from the fact that these alloys do not crystallize congruently but experience phase separation forming Ge and GST-225 nanocrystals upon crystallization. However, the details of the crystallization process remain unclear. Combining in situ X-ray diffraction studies during isothermal annealing and ex situ (scanning) transmission electron microscopy ((S)TEM) observations, the successive phases through which these alloys crystallize are identified. At low temperature (310 degrees C), the homogeneous amorphous material undergoes phase separation during wherein small regions of different Ge contents are formed. After a long incubation time, Pnma GeTe embryos first crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. While the Ge phase progressively builds up through the addition of new small Ge crystals, cubic GeTe forms. At this point, the microstructure ceases to evolve, and Sb is still dispersed and contained within some remaining amorphous matrix surrounding Ge and GeTe crystals. Higher annealing temperatures (typically 400 degrees C) are needed to force Sb to diffuse and get incorporated into the GeTe grains to form cubic Ge2Sb2Te5.