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

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This is the University of Illinois at Chicago (UIC) portion of a 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 devices. A key objective of the research program is to understand the relative contributions of field-induced electronic charge injection/carrier trapping and charged oxygen vacancy redistribution during fatigue and imprint of state-of-the-art Pb(Zr,Ti)O3 (PZT) films. We will use atomic resolution STEM and EELS to study the changes in atomic arrangements and local electronic structure that result from ferroelectric fatigue and imprint electrical testing. Examples of such changes might include development of locally-high oxygen non-stoichiometry near electrode interfaces and grain boundaries, and changes in bonding arrangements and the local density of states at these interfaces. Atomic structure determinations will be made using the Z-contrast imaging technique. Simultaneous acquisition of electron energy loss spectra will allow electronic structure information in the spectrum to be correlated with individual atomic columns in PZT thin film specimens. Electrical testing of the PZT capacitors prior to STEM/EELS studies will be performed by our collaborators at Stanford. Quantitative interpretation of EELS features will be facilitated by ab initio calculations (also performed at Stanford) of the local electronic structure at ferroelectric/electrode interfaces and the energies of carrier trap states associated with point defects.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 UIC with complimentary expertise in measurements of charged defect migration and polarization switching of ferroelectric thin films, atomic resolution imaging and spectroscopy using the electron microscope, and simulations of the electronic properties of solids. The UIC portion of the research will focus on direct examination of local bonding and electronic structure changes induced by fatigue and imprint electrical testing of PZT thin films. This research program will also strengthen our existing educational outreach activities to Chicago-area high school students, an effort that has, over the last 4 years, provided research positions for 32 students from groups typically under-represented in engineering and the natural sciences.