Phase transitions and chemical segregation in Ge-rich GST based phase change memory cells

In this work, a detailed chemical and crystallographic analysis of basic logic and a few selected intermediate RESET states of Ge-rich Ge-Sb-Te-based (GST) phase-change memories (PCM) is presented, with a particular focus on the understanding of the microscopic mechanisms underlying their electrical characteristics. We demonstrate that Ge-rich GST-based PCM store the information not only in their crystalline or amorphous phase, as in conventional PCM based on Ge₂Sb₂Te₅, but also in their active material stoichiometries: the amorphous phase is characterized by a Ge-rich composition, guaranteeing automotive-compliant retention, while the crystalline phase shows a composite structure, made of cubic GST and rhombohedral Sb grains, the latter mostly found close to the heater. This switching of composition explains the high temperature retention capability of Ge-rich GST cells and their preserved good functionality in the high conductive state. Analysis of intermediate RESET states unveils the microscopic details of these physical and chemical transitions, giving insights on the physical origin of the electrical characteristics of Ge-rich GST-based PCM.