Sb2Te相变材料的多尺度模拟：非易失性光子应用

硫族化合物相变材料（PCMs）能够在结晶相和非晶相之间迅速、可逆地转换。两相之间原子结构和键合机制的巨大差异导致两相在电学和光学性质方面存在显著不同，可用于数字数据的编码。

近年来，一种基于PCMs的新型非易失性电子存储器已被开发出来并实现商业化，在开关速度、存储容量和可扩展性方面具有优异的器件性能。随着硅波导平台的快速发展，基于PCMs的光学和光子器件由于能够超越衍射极限并应用于高密度集成而再次回归人们的视线。

掺杂的Sb₂Te合金由于其生长驱动的结晶特性，在芯片级光子应用中受到了越来越多的关注。然而，目前尚不清楚Sb₂Te在器件中纳秒级结晶时是否也会形成与成核驱动的Ge-Sb-Te合金类似的亚稳态结晶相。

Fig. 3 Dielectric function.

来自西安交通大学材料创新设计研究中心的张伟教授团队，对Sb₂Te材料进行了多尺度模拟，包括基于密度泛函理论的从头算分子动力学计算以及有限差分时域模拟，以揭示Sb₂Te在超快结晶和后续热退火过程中结构和光学性质的演化。

他们发现，超快结晶后得到的Sb₂Te晶体是无序的A7结构，碲原子随机分布在体内。进一步退火将导致Te的有序化过程，并逐渐转变为完全有序的结构，所有Te原子在Sb₂Te₃中形成完美的层。

Fig. 5 Electronic structure of random-swap models.

快速结晶和退火后之间的结构差异导致了电子结构的不同，从而导致了光学性质随波长的依赖关系发生变化，即在电信波段有显著的影响，而对可见光区域的影响可以忽略不计。

Fig. 6 Relationship between atomic structure and electronic structure.

作者的工作展示了对材料的原子级理解将如何指导器件设计，并有望促进未来对Sb₂Te基合金在各种光子相变应用中的探索。该文近期发布于npj Computational Materials 9: 136 (2023).

Editorial Summary

Chalcogenide phase change materials (PCMs) can rapidly and reversibly switch between crystalline and amorphous phases. The large difference in atomic structure and bonding mechanism between the two phases gives rise to a notable contrast in electrical and optical properties, which can be used to encode digital data. In the recent decade, an emerging type of non-volatile electronic memory utilizing PCMs has been developed and commercialized with superior device performance in switching speed, storage capacity and scalability. Thanks to the fast development of silicon waveguide platforms, PCM-based optical and photonic devices are gaining renewed attention, since they can go beyond the diffraction limit for high-density integration. Doped Sb₂Te alloys are now gaining increasing attention for on-chip photonic applications, due to their growth-driven crystallization features. However, it remains unknown whether Sb₂Te also forms a metastable crystalline phase upon nanoseconds crystallization in devices, similar to the case of nucleation-driven Ge-Sb-Te alloys.

A group led by Prof. Wei Zhang from the Center for Alloy Innovation and Design, Xi’an Jiaotong University, carried out multiscale simulations on Sb₂Te, including density functional theory based ab initio molecular dynamics calculations and finite-difference time-domain simulations, to unveil the structural and optical evolution of Sb₂Te during ultrafast crystallization and subsequent thermal annealing. They found that the atomic structure of crystalline Sb₂Te obtained after ultrafast crystallization is a disordered A7 structure with Te atoms randomly distributed in the bulk. The crystallized structure can be further annealed to induce a tellurium ordering process, and gradually transformed to a fully ordered structure with all the Te atoms forming perfect layers in the Sb₂Te₃blocks. The structural difference between the rapidly crystallized and post annealed structures leads to a difference in electronic structure and thereby wavelength-dependent variations of the optical properties, i.e., it has notable influence in the telecom region, but negligible influence in the visible light region. This work showcases how atomic understanding of materials can guide device design, and is expected to stimulate future exploration of Sb₂Te-based alloys for various photonic phase-change applications. This article was recently published in npj Computational Materials 9: 136 (2023).

原文Abstract及其翻译

Multiscale simulations of growth-dominated Sb₂Te phase-change material for non-volatile photonic applications (应用于非易失性光子器件的生长型Sb₂Te相变材料的多尺度模拟)

Xu-Dong Wang,Wen Zhou, Hangming Zhang, Shehzad Ahmed, Tiankuo Huang, Riccardo Mazzarello, En Ma & Wei Zhang 

Abstract Chalcogenide phase-change materials (PCMs) are widely applied in electronic and photonic applications, such as non-volatile memory and neuro-inspired computing. Doped Sb₂Te alloys are now gaining increasing attention for on-chip photonic applications, due to their growth-driven crystallization features. However, it remains unknown whether Sb₂Te also forms a metastable crystalline phase upon nanoseconds crystallization in devices, similar to the case of nucleation-driven Ge-Sb-Te alloys. Here, we carry out ab initio simulations to understand the changes in optical properties of amorphous Sb₂Te upon crystallization and post annealing. During the continuous transformation process, changes in the dielectric function are highly wavelength-dependent from the visible-light range towards the telecommunication band. Our finite-difference time-domain simulations based on the ab initio input reveal key differences in device output for color display and photonic memory applications upon tellurium ordering. Our work serves as an example of how multiscale simulations of materials can guide practical photonic phase-change applications.

摘要硫族化合物相变材料（PCMs）已被广泛应用于非易失性存储器和类脑计算等电子和光子器件中。掺杂的Sb₂Te合金由于其生长驱动的结晶特性，在芯片级光子应用中受到了越来越多的关注。然而，目前尚不清楚Sb₂Te在器件中纳秒级结晶时是否也会形成与成核驱动的Ge-Sb-Te合金类似的亚稳态结晶相。本文中，我们开展了从头算模拟，以理解非晶态Sb₂Te在结晶和退火后光学性质的变化。在这一连续变化过程中，介电函数的变化从可见光范围到电信波段与波长高度相关。基于从头算输入的有限差分时域模拟，揭示了在碲有序化之后，彩色显示器和光子存储器等器件输出的关键差异。本工作可作为材料多尺度模拟指导实际光子相变应用的实例。