Coercive voltage

To characterize ferroelectric materials usually the dependence of the polarization on the applied voltage is measured by means of a Sawyer-Tower circuit or by recording the current response to a voltage step. The / (V/)-hys(crcsis curve is used to determine the remanent polarization and coercive voltage, respectively coercive field. These two parameters are of critical importance to the design of external circuits of FeRAMs. 

Vc+ positive coercive voltage, voltage where the polarization crosses the rr-axis by increasing voltage values... 

The failure mechanism within a memory cell is either due to the inability of the programming voltage to switch the ferroelectric material because of an increase of the coercive voltage (write failure) or due to a decrease in the difference of Ps and Pns. This means the two different states of remanent polarization cannot be distinguished by the memory sense amplifier (read failure). This case is shown in Figure 3.19. 

For few micrometer or sub-micron sized electrodes, an afm (atomic force microscope) is the only tool for contacting. Hereby a conductively coated cantilever of the afm is used to contact the top electrode and also to scan the sample surface to retrieve a topographic image to find the top electrode position. But when we look at afm measurements of sub-micron capacitors, we sometimes face hysteresis loops with strangely increased coercive voltages of e.g. 10 Volts, at a film thickness of e.g. 170 nm (Figure 17.7). 

But the film thickness doesn t change, so how can the coercive voltage increase And the same capacitor can show its true coercive voltage of e.g. 2 Volts after a few measurements. So it appears to be a contact problem, though a pure platinum electrode is contacted to a pure Ptlr alloy coated cantilever, so an excellent contact should be expected. To investigate the reason of this poor contact behavior, afm conductivity scans were performed on a pure Pt coated... 

Figure 17.7 Artificially increased coercive voltage created by poor contact quality...

The piezoelectric hysteresis loops have been studied additionally to above dielectric hysteresis. This kind of loop is shown on Fig. 2.17. It has been recorded on PZT nanotube with outer diameter 700 and 90 nm wall thickness with the help of piezoatomic force microscopy (see Refs. [42, 43]). The obtained loop is the direct evidence of ferroelectric properties of the nanotube. Square form of the loop speaks about sharp polarization switching at coercive voltage 2 V. The residual (at zero voltage) piezoelectric coefficient d ff is of the same order as for the thin PZT film. 

However, in a magnitude of root-mean-square noise amplitude, i.e. DC noise, the SUL without the generation of spike noise takes poor level. Figure 7.3 shows a variation in DC noise voltage of the Co-Ni-Fe-B SUL with in-plane coercivity. DC noise was increased with increasing the value of coercivity [18]. Since the coercivity... 

Fig. 7.3 DC noise voltage of Co-Ni-Fe-B films as a function of in-plane coercivity [18]. 2005 IEEE...

As stated before, electro-optic properties of PLZT materials are related to their ferroelectric properties [131]. For PLDZT, the reduced coercive field (EJ and the enhanced optical transmittance (7) induced by the Dy substitution have led to improved electro-optic properties. Because the working voltage of a device is mainly dependent on the half-wave voltage of the materials, the lowered yi/2 of materials should be favorable to the improvement in performances of the devices. 

Therefore, a P-E loop appears on the oscilloscope, when the AC voltage is applied. On the photograph of the measured P-E loop. Polarization (P, and Pr) and coercive field Ec can be easily obtained. Figure 22-8 is a photograph of the loop of a PZT thin film sample fabricated by the sol-gel method. 

Memory characteristics include soft ferroelectric behavior. Compositions exhibiting low coercive forces, less than 1 MV/m, near the morphotropic phase boundary are typical, such as Pb(Zro 65Tio 35)03 doped with less than 8 percent La. Typically a large field is applied to achieve a large remnant polarization. Intermediate polarization states can be obtained by applying voltages. 

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