Oxygen Permeability of Ferroelectric Thin Film Top Electrodes and Its Effect on Detectable Fatigue Cycling-Induced Oxyge
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We investigated the discrepancy between the significant 18O isotope motion observed after bipolar voltage cycling used to induce ferroelectric fatigue in unannealed Pt/Pb(Zr,Ti)O3/Ir (PZT) capacitors and the lack of any observable oxygen tracer motion in annealed capacitors. We found that while unannealed Pt electrodes are permeable to oxygen, annealed Pt electrodes are oxygen impermeable. Further, when the initial oxygen tracer profile does not vary strongly with depth, the ability to detect oxygen motion during fatigue voltage cycling depends critically on the oxygen permeability of the capacitor’s top electrode. Our results indicate that oxygen exchange between the PZT film and external oxygen sources and sinks during voltage cycling is not necessary for ferroelectric fatigue to be manifest. In addition, studies of the dependence of ferroelectric materials properties on ambient gases should be accompanied by analysis of the permeability of exposed surfaces to the gases of interest.
I. INTRODUCTION
The motion of oxygen vacancies has long been implicated as a leading cause of ferroelectric fatigue,1–9 the loss of switchable polarization with repeated switching. One of the major, and as yet unanswered, questions regarding ferroelectric fatigue of Pb(Zr,Ti)O3 (PZT) thin films is the extent to which films interact with ambient gases during the fatigue cycling process. Some measurements have provided indirect evidence of such interactions,10 while some results have been contradictory. For example, using gas chromatography, Pan et al.11 measured oxygen liberation during fatigue cycling, though Nuffer et al.12 were unable to reproduce these results. Yet, Brazier et al.13 measured variations in the fatigue rate of PZT thin films when fatigued in an ambient of varying oxygen partial pressure. In addition, McCormick et al.14 observed dramatic changes in the ferroelectric properties of PZT thin films when comparing samples tested in either oxygen or argon environments. In a previous paper15 we presented our initial work, using 100 nm electron beam deposited Pt top electrodes, on isotopic oxygen tracer (18O) drift in ferroelectric PZT thin films as a function of fatigue-inducing, bipolar electricfield cycling. In that work, we observed dramatically different results when we compared capacitors that
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0164 J. Mater. Res., Vol. 19, No. 4, Apr 2004
underwent a post-electrode deposition, post-tracer exchange, 30 min, 650 °C anneal in flowing nitrogen (labeled “Exchanged & Annealed,” or E&A) to capacitors that did not undergo this final processing step (labeled “Exchanged,” or E). We compared two types of capacitors in each film: as-processed capacitors that underwent no electrical testing and those that experienced 108 switching cycles. Figure 1 is a reproduction of a graph from Ref. 15 that shows plots of the measured oxygen tracer secondary ion mass spectroscopy (SIMS) depth profiles, in the PZT layer only, of the ratio of the
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