In situ study of vacancy disordering in crystalline phase-change materials under electron beam irradiation

Abstract Unconventionally high amount of atomic vacancies up to more than 10% are known to form in Ge-Sb-Te crystals upon rapid crystallization from the amorphous phase. Upon thermal annealing, an ordering process of these atomic vacancies is observed, triggering a structural transition from the recrystallized rocksalt structure to a stable layered trigonal structure and a transition from insulator to metal. In this work, we demonstrate an opposite vacancy disordering process upon extensive electron beam irradiation, which is accompanied by the reverse transition from the stable trigonal phase to the metastable cubic phase. The combined in situ transmission electron microscopy experiments and density functional theory nudged elastic band calculations reveal three transition stages, including (I) the vacancy diffusion in the trigonal phase, (II) the change in atomic stacking, and (III) the disappearance of vacancy-rich planes. The mechanism of vacancy disordering is attributed to kinetic knock-on collision effects of the high-energy electron beam, which prevail over the heating effects.