Materials World Network: Transport, Switching and Size Effects in the Lead-Free Ferroelectric, BiFeO3

本研究探讨了人工和自发工程的多组分氧化物异质结构中的基本纳米级现象。这项工作涉及加州大学伯克利分校Ramamoorthy Ramesh教授和亚琛工业大学Rainer Waser教授指导下的研究人员之间的广泛合作。铁电薄膜作为各种微电子应用的功能材料引起了人们的极大兴趣。它们具有广泛的物理响应，包括自发极化的存在（在存储器中有用）大压电响应（在MEMS器件中有用）。近年来，在将钛酸锆铅（PZT）和钛酸锶铋（SBT）等铁电体原型薄膜与硅基电子器件集成以实现第一代非易失性存储器（FRAMS）方面取得了相当大的进展。最近对PZT中铅的毒性的关注推动了对替代材料系统的研究。本研究的重点是利用两个机构的联合专业知识，了解新的多铁性铁电体BiFeO3家族的结构性质关系。第一组问题涉及这些薄膜中基本尺寸缩放效应的探索，第二组问题是对影响这些材料功能的纳米尺度动力学现象的全面理解。这项工作旨在通过结合先进的材料加工方法，结合最先进的纳米光刻工具（如聚焦离子束铣削和电子束光刻）来创建纳米结构，并使用各种探针来研究和理解纳米尺度上的动态（即时间依赖性）物理现象，从而影响这一主题。进行的研究包括相关电导率和使用扫描探针显微镜的域映射，动态开关研究，压电表征。BiFeO3同时具有抗铁磁性和铁电性。由于铁电性质以高残余极化和高居里温度的形式表现出来，因此本研究着重于铁电部分。高残余极化约为90C/cm2，再加上不含铅，使得这种材料在铁电非易失性存储器和机电微/纳米系统中的应用非常有吸引力。在BiFeO3进入各种产品之前，还有许多悬而未决的问题。这项工作集中在最重要的问题上，包括将BiFeO3缩放成超薄膜和纳米级横向结构。掺杂可以降低BiFeO3的高矫顽力场，降低泄漏电流。在与伯克利实验室先进光源的密切合作下，将进行新颖的实验，同时研究原子结构和电特性。加州大学伯克利分校和亚琛研究人员之间拟议的合作具有潜在的互惠互利。亚琛贡献其高度先进的设备处理能力，扫描探针表征将在伯克利完成。此外，这两个机构都有很强的计算和理论建模专业知识。

This research explores fundamental nanoscale phenomena in artificially and spontaneously engineered multicomponent oxide heterostructures. The work involves extensive collaboration between researchers at the University of California, Berkeley under Professor Ramamoorthy Ramesh and at RWTH Aachen under the guidance of Professor Rainer Waser. Ferroelectric thin films have attracted considerable interest as functional materials for a variety of microelectronic applications. They possess a wide range of physical responses, including the existence of spontaneous polarization (useful in memories) large piezoelectric responses (useful in MEMS devices). In recent years, considerable progress has been made in the integration of thin films of prototypical ferroelectrics such as lead zirconate titanate (PZT) and strontium bismuth titanate (SBT) with Si based electronics to enable the first generation of nonvolatile memories (FRAMS). The recent concerns with the toxicity of lead in PZT have fueled the search for alternative material systems. This research is focused on understanding the structure property relationships in the new family of multiferroic ferroelectrics, BiFeO3, using the combined expertise in the two institutions. The first set of problems concerns the exploration of fundamental size scaling effects in these thin films and the second is a comprehensive understanding of the nanoscale dynamical phenomena that impact the functionality of these materials. This work seeks to impact this very topic through a combination of advanced materials processing approaches, incorporation of state-of-the-art nanolithography tools (such as focused ion beam milling and e-beam lithography) to create the nanostructures and the use of a variety of probes to study and understand dynamical (i.e., time-dependent) physical phenomena at the nanoscale. Investigations to be carried out include correlated conductivity and domain mapping using scanning probe microscopy, dynamic switching studies, piezoelectric characterization. BiFeO3 simultaneously exhibits anti-ferromagnetic as well as ferroelectric properties. Due to the fact that the ferroelectric properties are pronounced in form of a high remnant polarization and high Curie temperature, this research emphasizes the ferroelectric part. The high remnant polarization on the order of 90C/cm2, together with the absence of lead, makes this material very attractive for applications in ferroelectric non-volatile memories and mechanical-electrical micro/nano systems. Many open questions still remain before BiFeO3 can find its way into various products. This work concentrates on the most important issues, including the scaling of BiFeO3 into ultrathin films and nano-sized lateral structures. Doping is considered as a way to reduce the relative high coercive filed of BiFeO3 and lower the leakage current. Novel experiments in a close collaboration with the Advanced Light Source at the Berkeley Lab will be performed in which the atomic structure as well as the electrical characteristics are investigated at the same time.The proposed collaboration between the University of California, Berkeley and researchers at Aachen is of potential mutual benefit. Aachen contributes its highly advanced device processing capabilities, the scanned probe characterization will be accomplished at Berkeley. In addition, both institutions have very strong computational and theoretical modeling expertise as well.