npj: 嵌入式相变存储材料—设计与优化

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相变存储硫属化物相变材料利用硫族相变材料非晶相和晶体相之间快速且可逆的相变能力以及两相之间巨大的电阻差异实现快速且稳定的数据存储。目前，英特尔、美光等半导体公司基于锗锑碲合金Ge₂Sb₂Te₅研发的相变存储器3DXpoint已经作为独立式存储类内存进入全球存储器市场。此外，相变存储亦在嵌入式存储应用，如汽车工业、微控制单元、物联网等方面有着广阔的市场前景。面向嵌入式应用，器件稳定性需要经过260℃数分钟的高温退火考验，而传统锗锑碲合金的结晶化温度仅为150℃。近期，意法半导体公司通过调控锗锑碲合金成分可大幅提升结晶化温度，并证实富锗合金相变存储器件可用于汽车微控制芯片的大规模集成，但同时该公司亦指出过量的锗元素极易引发相分离，进而导致器件失效。因此，如何有效提升锗锑碲合金非晶稳定性，但同时避免相分离是该方向亟待解决的问题。

近日，西安交通大学CAID材料创新设计中心张伟教授团队，利用第一性原理分子动力学方法模拟了十余种不同成分比例锗锑碲合金的非晶结构，全面分析了非晶锗锑碲的局部原子构型、化学成键机制以及中程有序结构，结合“原子位置重叠”SOAP方法定量化表征了锗锑碲合金与单质锗的非晶相似度，并类比于合金形成能的概念，计算了非晶锗锑碲的相分离趋势（如图所示）。

结果表明，偏离经典GeTe-Sb₂Te₃二元平衡线，过量锗可大幅提升锗锑碲合金与单质锗的非晶结相似性，从而增强非晶锗锑碲的热稳定性。但锗含量超过55%，接近Ge₄Sb₁Te₂时，非晶锗锑碲的结合机制发生根本性转变，导致相分离形成单质锗与传统锗锑碲合金的趋势大幅提升。因此，作者提出一个合理的合金成分选择范围，如锗锑碲三元图中绿色虚线所示。基于该范围内的锗锑碲合金，可通过少量碳氮掺杂或微缩器件尺寸引入纳米尺寸效应进一步提升富锗合金的非晶热稳定性，从而满足嵌入式相变存储芯片在存储稳定性与循环工作寿命上的需求。

该文近期发表于npj Computational Materials 7: 29 (2021)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

锗锑碲非晶稳定性与相分离趋势研究

Ab initio molecular dynamics and materials design for embedded phase-change memory

Liang Sun, Yu-Xing Zhou, Xu-Dong Wang, Yu-Han Chen, Volker L. Deringer, Riccardo Mazzarello, and Wei Zhang

The Ge2Sb2Te5 alloy has served as the core material in phase-change memories with high switching speed and persistent storage capability at room temperature. However widely used, this composition is not suitable for embedded memories—for example, for automotive applications, which require very high working temperatures above 300 °C. Ge–Sb–Te alloys with higher Ge content, most prominently Ge2Sb1Te2 (‘212’), have been studied as suitable alternatives, but their atomic structures and structure–property relationships have remained widely unexplored. Here, we report comprehensive first-principles simulations that give insight into those emerging materials, located on the compositional tie-line between Ge2Sb1Te2 and elemental Ge, allowing for a direct comparison with the established Ge2Sb2Te5 material. Electronic-structure computations and smooth overlap of atomic positions (SOAP) similarity analyses explain the role of excess Ge content in the amorphous phases. Together with energetic analyses, a compositional threshold is identified for the viability of a homogeneous amorphous phase (‘zero bit’), which is required for memory applications. Based on the acquired knowledge at the atomic scale, we provide a materials design strategy for high-performance embedded phase-change memories with balanced speed and stability, as well as potentially good cycling capability.

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