Ferroelectric switching dynamics in VDF-TrFE copolymer thin films spin coated on Si substrate

Abstract

Simultaneous measurements of the charge Q and the capacitance C were performed for an MFS capacitor with Au-(vinylidene fluoride-trifluoroethylene copolymer)-(n-Si) structure using a double-frequency voltage consisting of a low-frequency high voltage and a high-frequency sinusoidal low voltage. The use of a triangular high voltage yields asymmetrical Q-V and C-V hysteresis loops that support the full ferroelectric polarization reversal occurring in close relation to charge compensation in the n-Si layer. The use of a rectangular high voltage reveals the details of asymmetric switching dynamics. Polarization switching toward the positive side starts with the loss of the depletion layer and progresses rapidly owing to the accumulation of majority carriers to be completed at a time analogous to that for the case of a metal-ferroelectric-metal capacitor. On the other hand, that toward the negative side is markedly impeded by depletion layer formation but is eventually completed via a constant-current process at a time given by the ratio of switched polarization and constant current. On the basis of a linear relationship between the constant current and the depletion layer width, we conclude that the switching dynamics under a negative voltage is controlled by the rate of minority carrier generation in the depletion layer.