Ferroelectric electrode effects

Ferroelectric thin films considerably gain in interest within the last couple of years due to their potential application in nonvolatile random-accessmemory devices (FeRAM). Among potential candidates, PbZr. n i, (>> (pzt) is one of the most promising materials because of its large remanent polarization and low coercive field. However, pzt is also well known for its poor fatigue behavior on metal electrodes [1,2] and occurrence of size effects [3-5] which are well due to the ferroelectric/electrode interface properties [1-5]. 

Fig. 13.11 Deformed Helix Ferroelectric effect. Scheme of observation of the effect (a) and the picture of distortion of the hehcal stmcture (b) in the zero, positive and negative field. P and A are polarizers and analyser, ITO means indium-tin oxide electrodes, Iq and I are intensities of incoming and outgoing beams. Note that at E = 0 the hehx is harmonic, for 0 < l l < anharmonic and for l l > l J unwound...

Some of the piezoeleetric materials are being used in applieations in the form of thin films of various thieknesses. The physieal situation is somewhat different from bulk materials. For example the electrode layer thickness is of comparable size with the thickness of piezoelectrically active film. If the film is deposited on the substrate the mechanical clamping is significant. Clamping could influence the film effective properties and the domain stracture for ferroelectric substances. Material coefficients are not usually completely available in the hterature because of the specific geometry and they depend on film thickness. Material properties of thin films are different from the bulk properties of the same material. Especially the size effect could result in loss of ferroelectricity below certain film thickness. Lower limit for the ferroelectricity is not generally known for all materials. 

Figure 12.9 RBS spectra of a typical electrode stack in a substrate used for a thin film ferroelectric capacitor device. Results are shown for the effects of annealing the substrate at 800 °C for 10 min on the Pt Ti, and O profiles. ... 

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