Pulse train measurement of ferroelectric switching in thin films of vinylidene fluoride/trifluoroethylene copolymer

Ferroelectric switching processes in thin films of a vinylidene fluoride/trifluoroethylene copolymer are investigated by pulse train measurement that involves the application of a train of unipolar on-off electric field pulses and the analysis of charge response. It is found that polarization reversal is considerably delayed as pulse width becomes shorter than the conventional switching time, but is eventually completed even if pulse width is reduced to 0.2 mu s. The charge response in each on-off cycle was resolved into dielectric and ferroelectric components. The former is nonlinear as well as relaxational and reflects changes in dipolar fluctuation and orientation during polarization reversal. The latter is responsible for polarization reversal that progresses forward and backward depending on the on-off period. Net polarization reversal occurs first gradually and then rapidly, corresponding to nucleation and growth, respectively. The switching current responsible for these processes can be reproduced by the sum of power and lognormal functions. The implications of these results are discussed in relation to the switching process that occurs as a result of the rotation of chain molecules.