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

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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.