Retention of thin ferroelectric VDF-TrFE copolymer films evaluated from dielectric non-linearities

Usual retention tests are based on the application of read pulses to a ferroelectric sample while the charge response is recorded. These tests, however, do not allow the continuous recording of retention as after the application of the read pulse the ferroelectric is in a new state defined by the direction of the read pulse. We propose a novel approach which is based on the nondestructive readout of the remanent polarisation by measurement of small signal dielectric nonlinearities. The temporal development of the remanent polarisation is directly accessible from the measured first and second harmonics in the current response to a small sinusoidal voltage signal. The novel technique has been used to investigate the retention of thin VDF-TrFE copolymer films of molar ratio 70/30 with thickness below 200 nm. This technique may also be useful for the nondestructive readout of ferroelectric memory cells.