多场耦合下非晶硅薄膜力学特性及结构演化的原位定量研究

As one of the most important amorphous semiconductor materials, amorphous Si film has wide applications in semiconductor and electronics industries, such as the thin film solar cells, thin film transistors, and liquid crystal displays etc. In addition, amorphous Si is a typical model material for studying the covalent bond amorphous solids. Since the mechanical strength and fracture behaviors of amorphous Si film can seriously influence the performances, life span and security of semiconductor and electronics devices, it is crucial to fully understand the mechanical properties and deformation behaviors of amorphous Si film. However, the mechanical properties of amorphous Si film have not been adequately studied, and the existing researches on its deformation mechanism mainly rely on computer simulations due to the relatively high experimental technology threshold and elusive amorphous structures. For lacking experimental supports, the simulation results are not convincing enough. In this project, we abandon the nanoindentation, a traditional mechanical test method for thin film, and use the standard uniaxial tension and compression tests on amorphous Si film samples to avoid the complex stress states and phase transformation. Instead, we intend to make a systemic study on the mechanical properties and structure evolutions of amorphous Si film under mechanical, thermal and electrical multi-fields using the in-situ transmission electron microscopy and quantitative nanomechanics technology together with high angle annual dark field (HAADF) and electron energy loss spectroscopy (EELS) technology. Multi-field effects are much closer to the real service conditions of amorphous Si film devices compared with the single applied force. The in-situ transmission electron microscopy coupled with the quantitative nanomechanics makes the real-time observation of structural changes with applied fields and the simultaneous acquirement of quantitative mechanical data, for example, the stress-strain curve possible. For lack of the long-range ordered structure, the commonly used characterization means for crystal materials are not suitable for amorphous Si film. To characterize the two distinct structures predicated by computer simulations in amorphous Si film, for the first time, we combine the HAADF with EELS in scanning transmission electron microscope (STEM). The HAADF STEM can demonstrate the distinct structures distributions and evolutions in amorphous Si film at atomic resolution directly and clearly. The EELS STEM can quantitatively characterize the atomic number density of each structure. This project is scientifically meaningful for understanding the deformation mechanism and knowing the structure of covalent bond amorphous materials represented by amorphous Si. Meanwhile, it can lay the experimental foundation for the life evaluation and structure optimization of amorphous Si film devices. Finally, we aim to propose the strategic method for controlling the mechanical behaviors of amorphous Si film.

非晶硅薄膜作为最重要的非晶半导体材料之一，在电子工业有着广泛的应用，其本身的力学行为严重影响着半导体器件的性能、寿命及安全性。然而，由于其自身的薄膜特性及长程无序的结构，截至目前，对非晶硅力学性能、微观结构及形变机理的研究多局限于传统的压入法和计算机模拟，由此得出的结论往往由于缺乏直观的实验证据支持而存疑。本项目拟通过原位透射电镜与定量纳米力学测试相结合的方法，以高角环形暗场像和电子能量损失谱为主要的结构表征手段，对非晶硅薄膜在近使役条件（力、热、电及其耦合）下的力学特性、原子尺度微观结构及其演化机制展开系统研究。甄别出影响非晶硅力学行为的关键因素，阐明理论预测的金属性类液体塑性载体是否本征存在于非晶硅薄膜中，并揭示外场对非晶硅结构演化过程的影响规律和内在机制，最终提出调控非晶硅力学行为的具体策略。本项目的顺利开展有望为非晶硅器件的寿命评估及性能优化等奠定实验基础并提供方法论的指导。

非晶硅作为重要的非晶半导体材料，在微机电、微电子等领域有着广泛的应用，其力学行为对半导体器件的结构设计、服役性能、寿命评估等都有着重要影响。因此，系统研究晶硅的力学特性及形变机理对半导体器件性能优化及理解以非晶硅为代表的共价键非晶材料都有着重要意义。本项目选取微纳尺度非晶硅为研究对象，借助原位电镜与定量纳米力学测试相结合的技术，对小尺度非晶硅在不同受力状态（拉伸、压缩和弯曲）及电场、辐照等影响下的力学行为和微观结构演化进行了原位、定量研究，揭示了小尺度非晶硅多个独特的力学行为及其背后物理机制，取得的主要创新性成果如下：. 1）首次发现并报道了非晶硅纳米线室温下的可调控滞弹性行为并阐明了其背后的方向依赖性弹性变形机制（Nano Letters, 2020）。非晶硅纳米线的弹性或滞弹性变形可通过改变纳米线自身的形貌、外加应力的方向及离子辐照（剂量）等进行调控，借助定量电子能量损失谱，揭示了非晶硅中价键交换诱导的反常滞弹性行为机制。非晶硅纳米线这种可调、可控的独特滞弹性行为使其有望成为一种先进的纳米阻尼材料，在半导体和电子工业上发挥重要作用。. 2）“抗压不抗拉”是人们对以硅为代表的共价键硬脆材料的一贯认知，本项目中，以几乎无裂纹、孔洞等微缺陷的亚微米尺度非晶硅为研究对象，揭示了“拉弱压强”只是缺陷掩盖下的表象，“拉强压弱”才是其本征力学行为并从能量角度阐明了非晶硅“越压越易变形”的微观机制，更新了人们长久以来的认知（Nature Materials，2021）同期NM发表专题评论称，“Wang及合作者发现先前预期的拉压对称性在非晶硅中被打破；推动了我们对非晶硅及其他类似非晶材料塑性的认识，帮助我们设计更好的器件”。. 3）在精准认知以硅为代表的本征硬脆半导体力学行为的基础上，开发了一种兼具优异半导体性能及超常规室温塑性的新型无机半导体材料InSe单晶，并在此基础上发展了一种无机塑性半导体材料的快速判据（Science, 2020, 共一），有望为全无机柔性半导体器件的研发带来变革。