Surface Energy Driven Cubic-to-Hexagonal Grain Growth of Ge2Sb2Te5 Thin Film

Yonghui Zheng,Feng Rao,Yan Cheng,Keyuan Ding,Songlin Feng,Weijun Yin,Rong Huang,Sannian Song,Zhitang Song,Weili Liu,Ruijuan Qi

doi:10.1038/s41598-017-06426-2

Abstract

Phase change memory (PCM) is a promising nonvolatile memory to reform current commercial computing system. Inhibiting face-centered cubic (f-) to hexagonal (h-) phase transition of Ge2Sb2Te5 (GST) thin film is essential for realizing high-density, high-speed, and low-power PCM. Although the atomic configurations of f- and h-lattices of GST alloy and the transition mechanisms have been extensively studied, the real transition process should be more complex than previous explanations, e.g. vacancy-ordering model for f-to-h transition. In this study, dynamic crystallization procedure of GST thin film was directly characterized by in situ heating transmission electron microscopy. We reveal that the equilibrium to h-phase is more like an abnormal grain growth process driven by surface energy anisotropy. More specifically, [0001]-oriented h-grains with the lowest surface energy grow much faster by consuming surrounding small grains, no matter what the crystallographic reconfigurations would be on the frontier grain-growth boundaries. We argue the widely accepted vacancy-ordering mechanism may not be indispensable for the large-scale f-to-h grain growth procedure. The real-time observations in this work contribute to a more comprehensive understanding of the crystallization behavior of GST thin film and can be essential for guiding its optimization to achieve high-performance PCM applications.

Highlights

As a promising candidate for storage-class memory[1] to mitigate the performance gap between dynamic random access memory (DRAM) and NAND Flash memory[2], phase change memory (PCM) bears excellent properties including sub 10 ns switching speed[3, 4], scalability to sub 10 nm dimension[5, 6], more than 1011 cyclability[6], and up to 220 °C 10-year data retention ability[7]
Into {111} planes achieves and forms non-atomic layers in f-GST resembling the Van der Waals interaction gaps in h-GST, the system energy is as small as the equilibrium h-phase[17]
When temperature increases to 210 °C (Fig. 1c) and 270 °C (Fig. 1d), f-grains continuously grow larger as the average grain size reaching ~20 and ~30 nm, respectively

Summary

Introduction

As a promising candidate for storage-class memory[1] to mitigate the performance gap between dynamic random access memory (DRAM) and NAND Flash memory[2], phase change memory (PCM) bears excellent properties including sub 10 ns switching speed[3, 4], scalability to sub 10 nm dimension[5, 6], more than 1011 cyclability[6], and up to 220 °C 10-year data retention ability[7]. We note previous literature[23] mainly concentrated on the phase transformation procedure and electronic structure of GST films upon in situ annealing, while no vacancy ordering process or h-grain growth mechanism was discussed. In contrast to the “normal” case in which grains get larger in a uniform manner, the abnormal growth of h-grains can be characterized by a subset of h-grains (mainly [0001]-oriented) growing bigger at a high rate and at the expense of their multifarious neighboring (small) grains Such swift expansion of the big h-grain was usually named as the “growth-dominated crystallization” for f-to-h transition of GST16, 21, no matter what crystallographic configurations the small grains would have. The present scenarios may offer a more comprehensive perspective to understand the phase transition physics of this key material, and be essential for optimizing GST-based commercialized phase change materials to boost the performances of high density PCM device

Methods

Results

Discussion

Conclusion