HfO2-基铁电单晶外延薄膜异质结的制备与极化翻转调控的研究

Novel HfO2-based ferroelectric nano-films is accelerating to apply to non-volatile random access memory, capacitor, and transistor and so on, because it owned high dielectric permittivity, low loss, low leakage current, and good CMOS properties. Preparation of epitaxial single crystal heterostructure [YSZ/ITO/Si:HfO2] films will be favor of discovering the ferroelectricity origin of HfO2-based films, and more understand the physical mechanism of the [ITO/Si:HfO2] interface controlled switching of Si:HfO2 ferroelectric polarization. In this project, the heterostructure [YSZ/ITO/Si:HfO2] is researched as an object. We will prepared it using in-situ Rheed assisted laser-molecular beam epitaxial method, and then we will systematically character the crystal structure and electrical properties of Si: HfO2 single crystal films using microstructure and electrical property measurement technology. At the same time we also need character the polar state of the [ITO/Si:HfO2] interface on different orientation YSZ substrate. The main characterized method involves synchrotron X-ray microdiffraction, hard x-ray absorption spectrum, HRTEM, PFM, and STM. All above preparation and characterization is expected to shed light on how did the HfO2-based films produce the ferroelectric properties and how did the [ITO/Si:HfO2] interface tune the Si:HfO2 ferroelectric polarization switching. Briefly, it is important that this project provide a new insight on the research the origin and related physical mechanism about the new simple oxide ferroelectricity.

新型HfO2-基铁电薄膜具有高介电常数、低损耗、小漏电流及与Si基集成电路工艺兼容的优势，近几年被广泛用于非易失性存储器、电容器、晶体管等电子器件的研究。制备外延单晶薄膜异质结[YSZ/ITO/Si:HfO2]对揭示HfO2-基薄膜铁电性起源和[ITO/Si:HfO2]界面极性调控其体相极化翻转的机制有重要物理意义。本项目以上述异质结为研究对象，采用激光分子束外延（L-MBE）方法，辅以高压Rheed的原位监测，外延生长HfO2-基铁电单晶薄膜异质结[YSZ/ITO/Si: HfO2]，利用同步辐射X-ray微区衍射、硬X-ray吸收谱、HRTEM、PFM、STM等微结构表征手段，结合宏观的电性能测量，深入研究HfO2-基单晶薄膜的本征晶体结构以及铁电性的起源，并揭示衬底几何取向控制的[ITO/Si:HfO2]界面极性对铁电极化翻转调控的物理机制，这将十分有利于进一步应用和理论基础研究。

HfO2薄膜具有高的介电常数，是当前Intel芯片使用的经典栅极材料之一。自2011年，J Müller等人在原子层沉积技术制备的HfO2多晶薄膜中发现铁电性能以来，HfO2作为一种新型铁电材料而备受关注。HfO2作为铁电材料优势是其居里温度高，剩余极化值大，且铁电性随薄膜厚度降低而增强（<10 nm）。然而HfO2薄膜材料铁电性起源的物理机制仍不清楚，主要是由于当前原子层沉积技术制备的HfO2-基铁电薄膜是多晶材料，这种多相混合的结构无法揭示其产生铁电的物理机制。因此，围绕HfO2-基薄膜铁电性起源的物理机制和铁电性能调控，本项目开展了如下的研究内容：（1）采用脉冲激光沉积技术成功地在不同取向的YSZ衬底上，以TiN为电极层和结构过渡层生长外延Hf0.5Zr0.5O2（HZO）薄膜。我们发现Hf0.5Zr0.5O2的铁电起源是大的界面应变使Hf0.5Zr0.5O2结构稳定为非中心对称的Pca21相；（2）我们用脉冲激光沉积（PLD）在n型不同取向SrTiO3衬底上生长外延Si掺杂的HfO2薄膜。利用压电力显微镜，可以进行写入和读取操作，并且铁电畴在180°的相变下可逆地被翻转，发现Pca21铁电晶体结构在界面应变和Si掺杂相互作用的影响下表现为氧亚晶格畸变；（3）我们报道了以ITO为电极层实现的半共格外延生长不同取向的Si-HfO2薄膜。在外延压缩应变下，薄膜的铁电极化率高达42μC/cm2，不需要wake-up 效应。结构表征揭示了铁电畴的存在，短轴的四方晶系结构取向垂直于衬底。利用压电力显微镜，可以电读和写的操作，并在180度的相位变化下可逆翻转。另外，铁电极化可以由ITO界面价键失配来影响界面的静电势实现对极化翻转的控制。