In Situ Study of Vacancy Disordering in Crystalline Phase-Change Materials Under Electron Beam Irradiation

Abstract

A disorder-driven metal insulator transition has been reported in crystalline Ge-Sb-Te phase-change materials (PCMs). The high concentration and statistical distribution of atomic vacancies were identified as the key factors in shaping the localization properties of electrons and, thus, the electrical transport. Vacancy ordering has been consistently observed in crystalline Ge-Sb-Te thin films upon thermal annealing, triggering a structural transition from a cubic rocksalt structure to a layered hexagonal structure and an insulator to metal transition. 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 hexagonal 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 hexagonal 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.