Capacitance of thin-film ferroelectrics under different drive signals.

Thin-film ferroelectric capacitance can be obtained by 2 different methods. Capacitance obtained using the derivative of its hysteresis loop is related to large applied signals and can be called the large-signal capacitance. Capacitance measured directly with a small, applied ac signal together with a slow changing dc bias is called the small-signal capacitance. This paper investigated the voltage dependence of the large- and small-signal capacitances. Measurements show that the large-signal C-V curve of thin-film ferroelectrics has much sharper peaks and higher peak values than the small-signal C-V curve. Analyses based on the Landau-Khalatnikov model shows that practical small-signal capacitance is closer to the ideal capacitance. However, its C-V curve has clearance areas around the coercive voltage, and the polarization switching is not reflected in the small-signal capacitance. This causes the peaks of small-signal C-V curves to be lower than that of large-signal C-V curves.