Surface and grain boundary energy as the key enabler of ferroelectricity in nanoscale hafnia-zirconia: a comparison of model and experiment
Surface and grain boundary energy as the key enabler of ferroelectricity in nanoscale hafnia-zirconia: a comparison of model and experiment
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DOI：
10.1039/c7nr02121f
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发表时间：
2017-07-28
期刊：
影响因子：
6.7
通讯作者：
Hwang, Cheol Seong
中科院分区：
文献类型：
--
作者：
Park, Min Hyuk;Lee, Young Hwan;Hwang, Cheol Seong
纳米级Hafnia-ZrO2的铁电特性被认为具有广泛的应用前景，包括铁电存储器、场效应管和能源相关应用。然而，尽管最近报道了大量的理论和实验结果，但为什么意外的铁电Pca2(1)相能够稳定下来的原因还不是很清楚。从体积自由能的观点来看，铁电正交相在工艺条件下不是稳定相。尽管理论上提出了表面能效应使铁电相稳定的可能性，但这种理论模型还没有与实际实验结果进行系统的比较。在本研究中，对不同薄膜成分和厚度的纳米级HfO_2-ZrO_2固溶体薄膜的多晶性进行了实验观察，并与基于热力学表面能模型的理论预测进行了比较。理论模型可以半定量地解释相演化的实验结果，但两者之间存在着不可忽略的差异。为了理解这些差异，系统地研究了各种因素，如薄膜应力、TiN覆盖层的作用和结晶动力学。这项工作还报告了薄膜的电学性质，即介电、铁电、反铁电相和准同型相的变化，作为薄膜成分和厚度的函数。对相变的深入分析可望为后续研究提供重要指导。
The unexpected ferroelectric properties of nanoscale hafnia-zirconia are considered to be promising for a wealth of applications including ferroelectric memory, field effect transistors, and energy-related applications. However, the reason why the unexpected ferroelectric Pca2(1) phase can be stabilized has not been clearly understood although numerous extensive theoretical and experimental results have been reported recently. The ferroelectric orthorhombic phase is not a stable phase under processing conditions from the viewpoint of bulk free energy. Although the possibility of stabilization of the ferroelectric phase due to the surface energy effect has been theoretically suggested, such a theoretical model has not been systematically compared with actual experimental results. In this study, the experimental observations on polymorphism in nanoscale HfO2-ZrO2 solid solution thin films of a wide range of film compositions and thicknesses are comprehensively related to the theoretical predictions based on a thermodynamic surface energy model. The theoretical model can semi-quantitatively explain the experimental results on the phase-evolution, but there were non-negligible discrepancies between the two results. To understand these discrepancies, various factors such as the film stress, the role of a TiN capping layer, and the kinetics of crystallization are systematically studied. This work also reports on the evolution of electrical properties of the film, i.e. dielectric, ferroelectric, anti-ferroelectric, and morphotropic phase changes, as a function of the film composition and thickness. The in-depth analyses of the phase change are expected to provide an important guideline for subsequent studies.