Quantification of fcc-Ge2Sb2Te5 stoichiometry variations

Abstract

Ge-rich Ge-Sb-Te (GGST) alloys are being currently integrated to the complementary metal oxide semiconductor technology for industrial production of new generation of non-volatile memories, able to support the development of raising technologies such as internet-of-things and artificial intelligence. These phase-change random access memories are based on ultra-fast reversible crystallization/amorphization of the Ge2Sb2T5 (GST225) chalcogenide compound exhibiting a large resistivity contrast between the amorphous and crystalline states. GGST film crystallization is accompanied by the phase separation of Ge and an fcc-GST225 phase of unknown composition, which should vary with Ge excess of GGST films. However, device properties and reliability are expected to vary with fcc-GST225 stoichiometry. This work reports atom probe tomography, X-ray diffraction, and scanning transmission electron microscopy investigations of the crystallization of an amorphous GST225 layer containing pseudo-periodic local composition variations. The results show that the compound fcc-GST225 is not stoichiometric, accepting 10 Ge at% variations, 7.5 Sb at% variations, and 6 Te at% variations. Furthermore, the microstructure as well as element distributions in the crystallized fcc-GST225 film suggest that fcc-GST225 nucleation is piloted by local composition variations. Fcc-GST225 nuclei formed in the Te-richest regions of the amorphous layer, corresponding to bulk hetero-nucleation, lowering the nucleation temperature.