Understanding the multistate SET process in Ge-Sb-Te-based phase-change memory

Multilevel operation is a topic of much current research in the field of phase-change memory materials, representing the most feasible method for increasing memory density beyond the ultimate scaling limits of the cell size. In this work, we present a combined experimental and ab initio molecular dynamics study of the formation of intermediate states during the crystallisation of Ge ₂Sb ₂Te ₅ (GST). A single intermediate resistance level is formed within a narrow voltage window, and simulations suggest this consists of microscopic crystallites embedded in a bulk amorphous phase. These findings are interpreted within the framework of classical nucleation theory, and a mechanism is proposed to explain the formation of the intermediate state. Our findings suggest that it may be difficult to obtain multiple intermediate states reliably during the crystallisation of Ge ₂Sb ₂Te ₅ and shed light on the fundamental limitations of using this method for multilevel programming.