Shenyang National Laboratory for Materials Science, Institute of Metal Research , Chinese Academy of Sciences , Wenhua Road 72 , 110016 Shenyang , China.
University of Chinese Academy of Sciences , Yuquan Road 19 , 100049 Beijing , China.
ACS Appl Mater Interfaces. 2018 Jul 25;10(29):24627-24637. doi: 10.1021/acsami.8b07206. Epub 2018 Jul 11.
High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO)(SrTaAlO) (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM). PFM measurements reveal that periodic stripe domains are dominant in 50 nm thick PTO films. Besides stripe domains, a/ c domains appear in films with thickness more than 60 nm. A thickness-dependent evolution of piezoresponse amplitude indicates that the 50 nm thick PTO films demonstrate a superior piezoresponse. Electron diffraction and contrast analysis clarify that all these (101)-oriented PTO films contain periodic stripe ferroelectric 90° domains. The domain periods increase with the film thickness following Kittel's law. Aberration-corrected STEM imaging reveals that the stripe ferroelectric 90° domains have an alternate arrangement of wide and narrow c domains with polarization directions along [100] for c domains and [001̅] for c domains, forming a "head-to-tail" polarization configuration. Further strain analysis reveals that stripe domains have uniform strain distributions and distinct lattice rotations around domain walls. It is proposed that the periodic arrangement of high-density stripe 90° domains in 50 nm thick PTO films is the main contributor to the superior piezoresponse behavior. These results are expected to provide useful information to understand the domain structures in (101)-oriented PTO thin films and thus facilitate further modulation of the properties for potential applications.
高折射率铁电薄膜作为（101）取向的薄膜，表现出增强的介电响应、压电响应和奇特的铁电开关行为，这使其成为应用于存储器和电容器的潜在候选材料。然而，（101）取向铁电薄膜中可能的畴模式和畴壁结构仍然难以捉摸，这导致难以理解其特殊性质的起源并进一步对其进行调制。在这项工作中，通过脉冲激光沉积在（101）取向的（LaAlO）（SrTaAlO）（LSAT（101））衬底上生长了一系列厚度为 35、50、60 和 70nm 的 PbTiO（PTO）薄膜，并通过压电力显微镜（PFM）和（扫描）透射电子显微镜（（S）TEM）进行了研究。PFM 测量表明，周期性条纹畴在 50nm 厚的 PTO 薄膜中占主导地位。除了条纹畴之外，在厚度大于 60nm 的薄膜中出现了 a/c 畴。压电力振幅的厚度依赖性演化表明，50nm 厚的 PTO 薄膜表现出优异的压电力响应。电子衍射和对比度分析表明，所有这些（101）取向的 PTO 薄膜都包含周期性条纹铁电 90°畴。畴周期随薄膜厚度按 Kittel 定律增加。具有相位差校正的 STEM 成像显示，条纹铁电 90°畴具有宽窄 c 畴的交替排列，c 畴的极化方向沿[100]，c 畴的极化方向沿[001̅]，形成“首尾相连”的极化构型。进一步的应变分析表明，条纹畴具有均匀的应变分布和畴壁周围的明显晶格旋转。提出 50nm 厚 PTO 薄膜中高密度条纹 90°畴的周期性排列是优异压电力响应行为的主要贡献者。这些结果有望为理解（101）取向 PTO 薄膜中的畴结构提供有用的信息，并有助于进一步调制其性能以应用于潜在领域。
