Thickness effects in ferroelectric thin films

Abstract

Due to the recent progress of thin film formation techniques, thin film ferroelectric devices are now available. The film thickness is an important parameter for these devices, because it strongly influences the ferroelectric properties and hence the performance of these devices when the film thickness is small. The aim of this project is to find out the primary cause responsible for various thickness effects observed from experiments. It has been suggested that the presence of surface layers at the electrode/film interface induces these thickness dependence. Unlike previous works assuming these layers to be dielectric (or non-ferroelectric), we proposed that they are ferroelectric with parameters different from those of the interior region of the film. Our simulation work starts from the four-state Potts model for a two-dimensional array of dipoles, because of the dominance of 90o domains found in many ferroelectric thin films. It implies that the dipole of each cell is restricted only to one of the four states. The coupling coefficient and magnitude of the dipoles at the surface layer are different from those in the interior. The presence of oxygen vacancies has also been considered to be one of the causes for the thickness dependence. The difference in coupling coefficients at the surface layer can be related to the presence of oxygen vacancies. The different magnitude of dipole moment is caused by the vacancy-induced space charge. An additional energy is required to rotate a dipole in the presence of an oxygen vacancy in the same cell. Consequently, the coupling coefficient is enhanced. Hysteresis loops for different thickness have been simulated. The corresponding coercive field and the remanent polarization were obtained. The theoretical results have been compared with experiment (H. Fujisawa, S. Nakashima, K. Kaibara, M. Shimizu, and H. Niu, Jpn. J. Appl. Phys., 38, 5392 (1999)). Moreover, polarization fatigue has also been simulated. This phenomenon is related to the migration of oxygen vacancies from both electrodes after a number of switching cycles. The reduction of remanent polarization and enhancement of coercive field after many switching cycles, as observed from experiments, have been reproduced.