Low-Voltage Pb(Zr,Ti)O 3 Film Capacitors: Control of Charge Relaxation at the Interfaces
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387 Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
electrodes are reported to be weak6,7 . In addition, SrBi 2Ta2O 9 (SBT) films, which are the fatigue-free alternative of PZT also show very small size effects 8 . Based on these facts, one can suppose that the polarization fatigue and size effects can be governed by the same physical mechanisms. As we show in the discussion below the most probable common origin for both of these undesirable phenomena is related to the processes of injection and entrapment of the mobile charge carriers. In Ref. I it was proposed that fatigue is caused by domain wall pinning by entrapped mobile carriers (electrons). An alternative scenario suggests that the origin of fatigue is entrapped-carrier-assisted blocking of centers for opposite domain nucleation at the interfaces 9. For both of these scenarios the injection and entrapment properties of the system play a key role. In this context the well-known fact that high fatigue performance is typically accompanied by rather high leakage conduction is understandable since, high leakage is presumably associated with easy relaxation of entrapped charge. However, recently it was shown that high fatigue endurance can be achieved without any increase of leakage conduction compared to the conventional Pt/PZT/Pt systemlO,I". Based on these results, we proposed that polarization fatigue is controlled not by charge transport across the film (i.e., leakage conduction) but by the local injection, entrapment and relaxation of charge in the near by electrode region11. The origin of size effects on ferroelectric film switching has been analyzed in Ref. 7,12 . It was shown that these effects are governed by injection of charge into the interfacial layer. The strong electric field in the interfacial dielectric layer is shown to be the driving force for this injection. The charge injected, and subsequently entrapped, in the interfacial layer impedes polarization switching, which causes an increase of the coercive field. According to the analysis provided in Ref. 12, three types of size effects on ferroelectric switching are possible depending on the relationship between the critical electric field of injection onset in the dielectric interface layer Eth and the maximum polarization Pm: I. Pm< edEth, where Ed is the dielectric constant of the interfacial layer, corresponds to the case of a perfect insulator layer at the interface. No size effects on ferroelctric switching are expected for this case. II. EdEth
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