Hysteresis in ultra thin ferroelectric polymer films observed by capacitance versus voltage measurements

Full text: Ferroelectric polymer films with thickness down to 50 nm have been prepared by spin coating a solution of the polymer onto electroded, glass substrates. Switching in the films has been observed both by measuring hysteresis loops and by capacitance (1kHz) versus voltage plots. The 50 nm films have a higher coercive field and a lower maximum polarisation than the those observed in films of the same materials with thickness above one micron. There appears to be a surprisingly small amount of fatigue in these films, when compared with thicker films. As the frequency of capacitance measurement increases to 1 MHz, it is found that the C-V characteristics change in nature. At 1 MHz, the capacitance maximum at the coercive field is not observed, but the capacitance shows a small increase at the highest field values. We conjecture that this increase is due to a field-induced increase in crystallinity in the films. This change is crystallinity is not permanent and disappears when the field is removed. Leakage currents in the thin films show a similar behaviour to that of thick films, both in terms of field dependence and in displaying a relaxation which persists for times in excess of one hour