Atomic Layering, Intermixing and Switching Mechanism in Ge-Sb-Te based Chalcogenide Superlattices.

Xiaoming Yu,John Robertson

doi:10.1038/srep37325

Abstract

GeSbTe-based chalcogenide superlattice (CSLs) phase-change memories consist of GeSbTe layer blocks separated by van der Waals bonding gaps. Recent high resolution electron microscopy found two types of disorder in CSLs, a chemical disorder within individual layers, and SbTe bilayer stacking faults connecting one block to an adjacent block which allows individual block heights to vary. The disorder requires a generalization of the previous switching models developed for CSL systems. Density functional calculations are used to describe the stability of various types of intra-layer disorder, how the block heights can vary by means of SbTe-based stacking faults and using a vacancy-mediated kink motion, and also to understand the nature of the switching process in more chemically disordered CSLs.

Highlights

Ge,Sb,Te-based phase change materials are the basis of electronic non-volatile storage-class memories that are a major contender to replace Flash memories[1,2,3,4]
The Z-contrast STEM images of Momand et al.[19] on switchable chalcogenide superlattices (CSL) samples suggest that CSLs are less ordered structures than the simple models of Fig. 1, and the switching process must be more complex
The STEM images showed that the superlattices form Ge,Sb,Te blocks separated by van der Waals bonding gaps, and that the blocks could be thicker than the 9 atomic layers

Summary

Introduction

Ge,Sb,Te-based phase change materials are the basis of electronic non-volatile storage-class memories that are a major contender to replace Flash memories[1,2,3,4]. Yu and Robertson[14] modeled the overall transition and noted that both cases would be a two-step process with a Ge-Te flip along the hexagonal z axis plus a lateral movement within the layers in the x,y plane to regain the lower energy states. The CSL switching process was modeled as a change of stacking sequence in chemically pure layers, without any change in atomic coordination numbers[14,15,16] It differs from the local displacive transition of the crystalline to amorphous phase transition in bulk GeSbTe systems[17,18], where the Ge coordination number changes, and from the thermally driven process of the thicker superlattice devices[5]. The bilayer connections were seen by other groups[14,21], suggesting a general low energy defect of the system

Methods

Discussion

Conclusion