Electrical properties of very thin VDF-TrFE copolymer films

Very thin VDF-TrFE copolymer films were characterized on the basis of low-field dielectric experiments, high-field ferroelectric hysteresis measurements, and fast ferroelectric switching experiments. The electrical DC-conductivity of thin copolymer films for electric fields either well below or above the minimum poling field (about 50 MV/m) was investigated to study its dependence on the state of polarization. The peak of the dielectric constant around the Curie temperature decreased and was significantly shifted to lower temperatures for copolymer films thinner than 100 nm. These films still show spontaneous polarization. The characteristic time for the reversal of the polarization grows with decreasing film thicknesses. The coercive field rises with decreasing thickness, reaching a saturation value of twice as high as for thick foils. In low fields for similar Al electrodes the conductivity depends on the polarization. Poled films have a lower resistance then unpoled or depolarized samples. For high fields well above the coercive field the conductivity is at least three orders of magnitude higher than for low fields