Finite size effect in thin and ultra-thin ferroelectric polymer films

Ferroelectric polymer thin films have gained a great deal of attention due to their emerging applications in MEMS, memory devices, and pyroelectric sensors. Thin ferroelectric polymer films also provide unique systems to investigate the finite size, dimensional, and interface effects in the ferroelectric materials. Recently, we examined the structure, morphology, and ferroelectric switching behavior in spin cast P(VDF-TrFE) thin and ultra-thin films (from 40 nm to 1 μm) on electroded silicon wafer. It is found that the ferroelectric behavior in these films does not change with thickness and the observed thickness dependent behaviors are caused by interface effects and the change of the crystallinity with film thickness. For example, as the film thickness is reduced, the activation energy for the polarization switching increases. This phenomenon is mainly caused by the interface effect, most likely due to the charge injection process across metal-polymer film interface. However, for films below about 100 nm, there is an additional large increase in the activation energy for the polarization switching and large reduction in switching speed with reduced film thickness, which is attributed to the effect of reduced crystallinity (as revealed by the X-ray and AFM data) at those thicknesses. These results indicate that for the spin cast P(VDF-TrFE) thin and ultra-thin films, there exists a threshold thickness, below which the crystallization process is strongly hindered. Detailed X-ray study on films annealed under different conditions suggests that this threshold thickness in the crystallization process of spun cast polymer thin films is related to the crystallite lamella width along the film thickness direction. By lowing the threshold thickness, better ferroelectric responses were observed in the ultra-thin films.