Theoretical and experimental investigation of the polarization switching process of ferroelectric thin films

Polarization switching of ferroelectric thin films is essential to the operation of ferroelectric memories. The switching process involves domain formation and development. To understand the process, domain nucleation close to a domain wall and within an unswitched domain of a ferroelectric thin film as well as heterogeneous nucleation have been analyzed theoretically in terms of the local field, from which a model of ferroelectric polarization switching is developed. The elongation of the switching current and the polarization relaxation are found to be due to the depolarization field. The asymmetricality of the hysteresis loop arises from the heterogeneous nucleation. The maximum switching current vs. the magnitude of external field shows two regimes: large field and small field, which correspond to nuclei-formation-dominant and domain-wall-motion-dominant switching, respectively. In addition, the coercive field is calculated as a function of film thickness.; The implications of the model are explored experimentally using Lithium niobate (LiNbO₃) thin films on p-type (111) oriented silicon substrates deposited by metallo-organic decomposition (MOD) process. The thicknesses of the films are controlled by both dilution of the precursor and the conventional technique of applying varying numbers of layers. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images show that the surface of the films is uniform. The relation between dynamic switching current and the magnitude of external pulse is investigated, and the total switched charges of the negative pulses are much larger than that of the positive pulses. The result is further verified by the hysteresis measurement. This effect is due to the asymmetrical structure of the samples. In addition, polarization suppression and the effects of the depolarization field are observed. The experimental results are compared with the results obtained from our proposed theoretical models.