钙钛矿型铁电薄膜高质量外延异质界面的构筑及空间电荷状态研究

The electrical properties of ferroelectric thin film heterostructures are influenced substantially by space-charge behavior, which includes space charge density, carrier concentration, interfacial potential barrier height, optical dielectric constant, built-in electric field, depletion layer thickness and so on. If these parameters can be controlled effectively, the polarization property and other sensing performance will be greatly improved, and the application of 2D ferroelectric materials will also be extended. It is aimed at how to quantify and optimize the extrinsic contribution of these parameters to the electrical properties of epitaxial ferroelectric thin film heterostructures. The method of strain engineering and 90°off-axis radio frequency magnetron sputtering technique will be used to design and prepare high quality epitaxial hetero-interface of perovskite ferroelectric thin films in the project, and achieving the controllable fabrication of the epitaxial hetero-interface with zero stress state and low defect density. After this, the electrical properties of epitaxial ferroelectric thin films will be characterized, meanwhile, the new metal-ferroelectric contact theories will be established, and then the behaviors of space-charge in “clean” interface and its evolution rule in applied field will be studied. Thus, the quantitative relationship between the related parameters of space-charge and electrical properties can be obtained. The tailoring mechanism of interface space-charge in ferroelectric films can be further studied with the implementation of the project. Furthermore, it also will precipitate an insight into epitaxial hetero-interface state and structure-activity relationship.

空间电荷状态会从本质上影响铁电薄膜异质结构的宏观电学性能。通过有效的调控空间电荷特性所包含的多种状态参量，可以极大地提高铁电薄膜的极化特性和其它传感性能，扩展铁电薄膜材料的应用范围。本项目针对如何定量及优化这些状态参量对外延铁电薄膜异质结构宏观电学性能的非本征贡献，运用应变工程的方法设计铁电薄膜外延异质结构，采用90°离轴射频磁控溅射技术，原位制备具有高质量外延异质界面的钙钛矿型铁电薄膜，实现零应力状态，低缺陷密度外延异质界面的可控构筑。在此结构基础上，利用多种测试方法表征外延铁电薄膜的物理性能，同时新建铁电接触理论模型，研究空间电荷在“干净”界面间的状态特征及其在外场下的演变规律，建立空间电荷相关状态参量与宏观电学性能的定量关系。通过本项目的实施，可以深入研究铁电薄膜界面空间电荷的调控机制，同时促使科学界对外延异质界面状态及构效关系的深入理解。

铁电薄膜异质结构的宏观物理特性及电荷输运在很大程度上是来源于界面状态的非本征贡献。而在高质量异质界面的结构基础上，这种非本征贡献和界面电荷状态参量更容易量化。基于高质量异质界面的构筑及电荷状态的研究，本项目研究内容及结果主要有：（1）对BTO薄膜多元弹性异质结构界面的外延性进行了设计与优化，并通过工艺参数的精细调控，在多种BTO薄膜异质结构中获得了近零应力状态、低缺陷密度的高品质外延界面；（2）在(001)-LaAlO3、YAlO3、SrTiO3、(Sr0.5La0.5)(Al0.5Ta0.5)O3单晶基底上外延生长出了BTO铁电薄膜，由于界面应力的松弛，实际的面内错配应变比未松弛的晶格错配应变要低一个数量级；（3）在 (110)、(111)取向的SrTiO3基底上生长的BTO薄膜为外延结构，而在大错配度的MgO基底上均为(110)取向的强织构，电学性能及电荷参量均表现出强烈的各向异性特征；（4）将半导体接触理论加以改性并成功的运用于铁电薄膜异质结构中，详细研究了外延异质界面结构对空间电荷状态（包括空间电荷密度、载流子浓度、界面势垒高度、光学介电常数、内建电场、耗散层和界面层厚度等参量，传导机制）的影响；（5）BTO薄膜异质结构的漏电传导机制均可用改性的肖特基传导模型来解释，有效的空间电荷密度、损耗层厚度在宏观物性的调控过程中起着重要的作用，如，空间电荷密度高于1019 cm-3量级时，电滞回线会发生显著收缩与倾转。这些研究内容从核心上探讨了高质量外延异质界面的空间电荷状态与铁电薄膜宏观物性之间的关系及相应的影响机理，建立了一个先进理论与实验技术紧密结合的铁电薄膜外延异质结构及其电学性能的研究平台，指出一条能实现并有效利用优化了的外延异质界面空间电荷状态来提高铁电薄膜电学性能的可靠途径。