Microscopic modeling of thin-film ferroelectrics: Fatigue

Ferroelectric thin-films are observed to fatigue (reduce their switched charge) as the number of switching cycles increases. A number of qualitative observations have been made. Initially there is a rapid fatigue then the switched charge roughly levels off for a large number of cycles (it decreases only logorithmically with the number of polarization reversals). During this period the time required to switch the sample decreases as the fatigue increases. The switched charge falls off precipitously as the device fails. After failure the conductivity across the sample is large and dendritic growth has been observed in failed samples. A high temperature annealing procedure in which the sample is kept within the ferroelectric phase can restore the samples to near-new condition. A model involving the diffusion and trapping of charged defects is postulated and examined. This model qualitatively explains these phenomena in terms of the dendritic growth of conducting defects near the surfaces of the sample. References to other topics discussed at the Second Symposium on Integrated Ferroelectrics are given in the paper.