Phonon Instability and Broken Long-Ranged p Bond in Ge-Sb-Te Phase-Change Materials from First Principles

Abstract

Ge-Sb-Te (GST) compounds exhibit substantial electrical and optical contrast between the amorphous and crystalline phases. Despite extensive studies of GST compounds, the underlying mechanism for fast transitions between the amorphous and crystalline phases is yet to be revealed. In this paper, we study the properties of phonons and a long-ranged p-orbital network of hexagonal GST compounds using first-principles calculations. By investigating volume-dependent phonon dispersions, we observe the structural instability at elevated temperature due to the spontaneous softening of a specific in-plane vibrational mode (E-u). We find that the atomic distortion by the E-u mode is associated with weakening of delocalized p bonding inducing large structural and electrical changes. We also discuss how to manipulate the E-u mode to control the device performance. Our finding helps deepen the understanding of the phase-change mechanism and improve the device performance, especially the switching power and operating temperature.