Collaborative: Reliability of Ferroelectric Thin Films: A Systematic Study of Point Defect Phenomena and Local Electronic Structure Effects

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This proposal describes a coherent, collaborative research project on the connections between the point defect chemistry and electronic structure of ferroelectric thin films and the fatigue and imprint processes that limit their reliability in non-volatile memory (FeRAM) devices. A key objective of the research program is to understand the relative contributions of field-induced electronic charge injection/trapping and charged oxygen vacancy redistribution during fatigue and imprint of state-of-the-art Pb(Zr,Ti)O3 (PZT) films. Fatigue and imprint testing under optical illumination and DLTS measurements will be pursued in order to characterize both optically- and electrically-active carrier traps in the films. Atomic resolution STEM/EELS studies will be performed on both undegraded and fatigued/imprinted specimens in order to look for degradation-induced changes in bonding arrangements and local electronic structure at the electrode interfaces with PZT. Oxygen isotope depth profiling through ferroelectric capacitors subjected to various electrical biasing conditions will be used to characterize oxygen vacancy motion. Ab initio calculations of the local electronic structure at ferroelectric/metal interfaces, the energies of carrier trap states associated with point defects, and defect formation and migration energies will be performed to properly interpret the experimental results.Ferroelectric materials exhibit a spontaneous polarization, which can be used in a variety of different applications in microelectronics and communications. For example, thin film ferroelectric materials are the key enabler for a new generation of non-volatile semiconductor memories which are currently being developed (and, increasingly, brought to market) by major microelectronics firms worldwide. The physics of switching the ferroelectric polarization state in small-dimension, thin film structures is also an important topic of fundamental scientific interest. Both the science and the technology of ferroelectric thin films provide motivation for better-understanding phenomena that interfere with reliable polarization switching in these materials. Such phenomena include ferroelectric fatigue, the loss of switchable polarization after repeated switching by applied voltage pulses, and imprint, a shift in coercive voltage resulting from repeated voltage pulses of one polarity. A host of experimental observations and theoretical models for ferroelectric fatigue and imprint have been reported over the years. However, the detailed mechanisms responsible for these reliability-limiting processes remain uncertain. This research program will investigate the underlying mechanisms of ferroelectric fatigue and imprint in state-of-the art ferroelectric films provided by our collaborators in the semiconductor industry. The research will be directed by three co-principal investigators based at Stanford University and the University of Illinois at Chicago (UIC). The program builds on our complimentary expertise in measurements of charged defect migration and polarization switching characteristics of ferroelectric thin films, atomic resolution imaging and spectroscopy using the electron microscope, and simulations of the electronic properties of solids. It will strengthen existing educational outreach activities to Chicago-area high school students, and will include summer research projects for UIC undergraduates at Stanford. These summer projects will be well-integrated with the research program objectives and will strengthen the collaboration between our two institutions.