硅掺杂二氧化铪反铁电纳米薄膜相变及储能特性研究

Lead-free Si-doped HfO2 anti-ferroelectric nano-film is a quite promising material candidate for three-dimensional (3D) nano-architectured electrostatic capacitors desiring high energy storage density, due mainly to its mature atomic layer deposition (ALD) technique, high saturation polarization and high breakdown field. This proposal focuses on study of the phase transition behavior and energy storage properties in this new-type anti-ferroelectric material, which has not been well understood or been investigated up to now. First the silicon content and film thickness will be carefully modified and controlled, and crystallization of the films will be performed under pre-designed annealing conditions. Followed by measurements of the polarization hysteresis and micro-analyses of the crystal structure, it is expected that the composition required to induce and stabilize the anti-ferroelectric (AFE) phase will be confirmed, and the impacts of film thickness and crystallite size will also be elucidated. As a result, effective approaches will be clarified to improve the volume fraction of the AFE phase and to restrain the appearance of the ferroelectric (FE)/paraelectric (PE) phases, whereby the polarization behavior and correlated energy storage density as well as efficiency can be tailored. Thereafter, the thin films will be subjected to combined electro-thermo stressing conditions. The paths and characteristic features of external field induced AFE-FE-PE phase transition will be investigated in detail to understand the intrinsic mechanism of changes in energy storage density and efficiency, and to guide improvement of their temperature stability. In order to optimize the charge-discharge endurance performance, fatigue behavior of the AFE films will be investigated, and the applicability of existing fatigue theory will be examined. The results of aforementioned research efforts will provide solid materials science knowledge supports for practical application of Si-doped HfO2 anti-ferroelectric nano-films in energy storage electrostatic capacitors.

Si掺杂HfO2反铁电薄膜兼备无铅、原子层沉积技术成熟、高饱和极化强度和耐压强度等工艺及性能优势，具有重大的高储能密度3D纳米结构电容器应用价值，但迄今国际上尚未对该新型反铁电材料的相变和储能行为开展深入研究。本项目将精细调制HfO2薄膜Si掺杂量和膜厚、并进行系统的退火晶化实验，通过晶相结构分析和电滞回线测量等手段，首先研究反铁电相区随膜厚和晶粒尺寸的变化规律，掌握抑制铁电和顺电相、提高反铁电相体积分数的组分和制备工艺调控条件，实现对材料极化行为和相应的储能密度及效率性能指标的剪裁；随后开展电场和温度耦合诱导相变特性研究，揭示铁电、反铁电及顺电三相转变的路径和特征、及其与储能密度和效率变化的内在联系，探明提高储能性能热稳定性的调控手段；最后将对材料的疲劳特性进行测试分析，验证现有疲劳理论的适用性，明确改善充放电循环耐久性的有效途径。项目成果将为新材料的储能电容器应用奠定材料科学研究基础。

本项目以原子层沉积（ALD）技术为主要手段制备了Si掺杂HfO2纳米薄膜，通过改变铪源和硅源前驱体的沉积循环次数比实现了Si掺杂浓度从4.5至6.0 mol%的可控调节。HfO2纳米薄膜的相变首先受Si掺杂量控制，随着Si含量增加，薄膜的物相结构发生从单斜、单斜与正交混合、正交、正交与四方混合、最后到四方相的依次转变，相应的介电行为特性也发生由顺电、铁电、反铁电、最后重新回到顺电的依次转变。Si掺杂量为6.0 mol%的HfO2反铁电薄膜具有最高的储能密度和效率，该薄膜的场致相变规律是：在电场作用下发生非极性四方相与极性正交铁电相之间的可逆转变，对应于薄膜呈现反铁电特征的双电滞回线；温度升高，非极性四方相稳定性提高、含量增多，反之温度降低有利于极性正交铁电相稳定、含量增多。构建了L-D热力学理论模型统一解释了Si掺杂HfO2薄膜相变的成分、电场和温度依赖性。反铁电薄膜的电场循环耐久性和抗击穿特性明显优于铁电薄膜，根据薄膜内瞬态局域电场的定性分析和定量计算可以给出合理的解释。创新性地采用化学溶液沉积（CSD）法制备了Y、Ca掺杂HfO2和HfO2-ZrO2固溶体薄膜，确认HfO2薄膜的相变同时受到掺杂量和薄膜厚度的共同作用，当掺杂元素浓度固定时存在由高对称相转变为单斜相的临界厚度，该临界厚度随掺杂元素浓度的降低而下降。通过在衬底施加负偏压，实现了在极宽的工艺窗口范围内高导电性、表面原子级平滑TiN电极薄膜的制备。执行本项目共发表高质量SCI收录论15篇，申请国家发明专利5项，授权2项，毕业博士研究生4人，硕士研究生6人。项目取得了突出的科学规律发现和人才培养效果。