**2.4. Imprint and biasing effects in ferroelectric thin films**

An internal field developed by charged defects is believed to play an important role in the *imprint phenomenon* in ferroelectric thin films. Imprint generally refers to the preference of a certain polarization state, the so called "hard" state, over the other "ease" state in ferroelectric bi-stable states. The preferential polarization may be induced within the ferroelectric by locking-in a particular domain orientation during electrical, thermal or optical (UV light) treatments.[33] The latter two effects are customarily called *thermal imprint* and *optical imprint*.

The polarization imprint of ferroelectric thin film capacitors, which manifests itself macro‐ scopically by a shift of the polarization hysteresis loop along the electric field axis (Fig. 6), is one of the most serious reliability issues for ferroelectric nonvolatile memories applications. An incomplete polarity reversal of the ferroelectric thin film, which also gives rise to an asymmetry in the remanent polarization and an effective variation of the coercive field, can lead to either "read" or "write" failure of the memory cell.[34] The voltage offset of the imprinted loop is often used as a measure of imprint. The sign of the field offset determines the direction of the preferential polarization in the interior of the ferroelectric thin film capacitor.

An asymmetric hysteresis behavior has also been reported for as-grown ferroelectric thin films in an initial state, i.e. without heat treatment, ultraviolet radiation or application of an external electric field.[35] However, unlike the preferred polarization generated from capacitor

**Figure 6.** (a) Shift of polarization of a ferroelectric in the negative state of remanent polarization due to imprint. (b) Thermal imprint – polarization loss due to shift of the P-E hysteresis loop on heating.

structure, an internal bias field develops in these films intrinsically during the fabrication process. The built-in bias does not vary during switching, instead it provokes polarization back switching and causes the *retention loss*, i.e. a gradual decrease of the remanent polarization with time in poled ferroelectric capacitors. Optical and thermal imprint have been shown to improve the retention performance or aging in PZT film capacitors.[36]
