Electron microscopy
Retention & Retention Loss in Ferroelectrics
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Retention is described by the ability of an FeRAM to retain the sign and magnitude of the remanent-polarization established by writing. Lack of retention in ferroelectrics is normally due to electret-like effects, and thus retention is lower at higher temperatures. On the other hand, the reliability of ferroelectric storage at zero bias is described by retention losses, where the polarization slightly decrease with time due to ferroelectric relaxation (see Figure 1793). In practice, for ferroelectric applications, e.g. commercial FeRAM, the retention time must be longer than 10 years.

Schematic illustration of retention losses in ferroelectric

Figure 1793. Schematic illustration of retention losses in ferroelectrics (Ec: coercive field; Ps: spontaneous polarization; and Pr: remanent polarization).

A couple of mechanisms were proposed to interpret retention losses in ferroelectrics:
         i) A polarization-independent built-in bias can result in the back-switching of domains. [1] This built-in bias exists at the interface between the ferroelectric layer and electrode, for instance, due to a formed depletion layer at the interfaces.
         ii) A residual internal electric field pointing in the opposite direction to the spontaneous polarization. This residual field can be caused by insufficient screening from a thin surface layer. [2]

[1] A. Gruvermann, M. Tanaka: J. Appl. Phys. 89, 1836 (2001).
[2] A. K. Tagantsev, M. Landivar, E. Colla, N. Setter: J. Appl. Phys. 78, 2623 (1995).