HfO2基新型铁电薄膜的自发极化机制和失效行为研究

As representative high-k dielectrics, HfO2 have been widely implemented in microelectronics industry. Recently, doped-HfO2 thin films prepared by a special technique were discovered to exhibit pronounced ferroelectricity. Due to its superior compatibility to silicon-CMOS integration technology, it is expected this novel ferroelectric material will lead to a significant breakthrough in development of integrated ferroelectric-semiconductor devices such as non-volatile ferroelectric memories. This proposal focuses on study of two fundamental material science issues for HfO2-based ferroelectric thin films, i.e. the origin and stabilization mechanisms of spontaneous polarization and the ferroelectric failure properties. Exact atomic geometry of the ferroelectric phase will be clarified by detailed microstructural analyses. Thereafter a first-principle theory study will be taken to reveal the origin of the spontaneous polarization, and its orientation axis and strength information will be given by the calculation. Additionally the stabilization of the ferroelectricity will be investigated with respect to the impacts of dopant, interfacial stress, and size effects. Systematic sample preparation processing experiments and electrical characterization will be performed to investigate the failure behavior of HfO2-based ferroelectric thin films. The polarization switching characteristics will be correlated to interior defects of the thin films and the electrode-ferroelectric interfacial status, by which it is expected to elucidate the physical mechanisms of the failure phenomena and to guide effective approaches to improve the failure properties. The results of all aforementioned research efforts will provide experimental and theoretical supports for performance optimization of this novel ferroelectric material in practical device service.

作为代表性的高-k材料，HfO2已被广泛应用于微电子工业。最近采用特殊工艺制备的掺杂HfO2薄膜被发现具有显著的铁电性质，这一新型铁电材料与硅基CMOS集成电路工艺良好的兼容性对于集成铁电学的发展具有重要意义，预期将带来铁电存储器研究的新突破。本项目以HfO2基铁电薄膜自发极化的形成和稳定机制及其失效行为特性等材料科学基础问题为研究对象，通过晶体结构分析并结合第一性原理计算，明确铁电相的原子结构，揭示自发极化的形成机制并确定其晶轴取向和大小范围，掌握掺杂、界面应力和尺寸条件对铁电性质稳定性的影响规律；基于系统的制备工艺实验和电学性能测量，通过对薄膜内部缺陷和界面状态与极化翻转特性的关系研究，深入分析材料的各类失效行为，探索发生失效的微观物理机制，提出改善失效的有效途径。本项目研究结果将为优化新型铁电薄膜材料的器件服役性能提供实验依据和理论支撑。

2011年HfO2基纳米薄膜的铁电性质被首次公开报道。该新型铁电材料兼备无铅、原子层沉积技术成熟、特别是优秀的Si基CMOS集成工艺技术兼容性等显著优点，具有重大的超高集成密度非易失存储器应用价值。HfO2基铁电薄膜理论研究的核心问题是自发极化的产生和稳定机制，应用研究的关键问题是其失效行为能否满足器件的可靠性要求。为此，本项目开展了TiN电极薄膜和Si掺杂HfO2铁电薄膜制备工艺、HfO2铁电性质起源的第一性原理计算、以及Si掺杂HfO2铁电薄膜的失效行为特性等三个主要内容的研究。采用磁控溅射方法实现了膜厚介于20至65纳米、表面粗糙度小于1 nm、电阻率低于70 微欧.cm的TiN纳米薄膜电极稳定可重复制备。采用原子层沉积技术制备Si掺杂HfO2薄膜，发现Si含量介于3.8至5.6 mol%时样品呈现明显的铁电性质，继续提高Si掺杂量薄膜转变为反铁电态。计算出了HfO2七种晶体结构转变的原子偏移量，结合晶格总能量对比确定了Pca21铁电相源自于四方相P42/nmc或正交相Pbca。首次实验证实超薄纳米铁电薄膜的低场强介电行为特性符合瑞利关系，由此确定薄膜中存在多电畴和畴壁结构，介电弱滞性来源于不可逆的畴壁运动，该结果对于深入认识铁电薄膜材料的电畴结构和尺寸效应具有重要意义。实验证明材料铁电性质的温度稳定性完全满足铁电存储器应用要求，应用非平衡热力学统计模型成功解释了变温电滞回线测试结果，计算出了新畴稳定成核的临界体积。在国际上率先开展了Si掺杂HfO2铁电薄膜的去老化行为实验研究，采用成核限制电畴翻转理论和空间电荷耗尽层理论成功解释了正负矫顽场强去老化的非对称性。实验确定在2.0 MV/cm双极性脉冲循环加载下，Si掺杂HfO2铁电薄膜可以服役至 4 x 10^9 周期不击穿，但是表现出明显的疲劳现象，剩余极化强度衰减约70%；场强大于2.0 MV/cm时，薄膜未表现出疲劳现象就已击穿；明确了如何提高HfO2铁电薄膜的抗击穿和疲劳特性将是在器件实际应用中急需解决的关键问题。本项目迄今共发表学术论文10篇，硕士学位论文7篇。