Multiscale Simulation Of Domain Switching In Ba TiO₃ Ferroelectric Thin Films By Terahertz Electric Field

Ferroelectric memory, with high storage speed, low power consumption and excellent anti-radiation performance, becomes one of the most promising candidate for the next generation memory. Domain switching is the basic for the physical phenomena and processes of the ferroelectric thin film, which determines the characters and applications. In most of ferroelectric, domain switching is based on atomic displacement which is an ultrafast dynamics processes and usually occurs under a picosecond time scale. The resolution time of the existing observation methods of domain switching, such as transmission electron microscopy and piezoresponse force microscopy, is small. These methods are fails to capture the ultrafast dynamics processes of domain switching. Femtosecond laser with ultrashort pulse can not only detect information in ultrahigh time resolution, but also stimulate the terahertz wave source to generate shaped THz electric field for domain writing. Therefore, we put forward a new solution to investigate the ultrafast dynamics processes of ferroelectric domain switching, in which THz electric field by femtosecond laser is used to stimulate ferroelectric domain switching and femtosecond laser is also used to synchronous detection. In this thesis, by using a typical ferroelectric of barium titanate(BTO) as the research object, the ferroelectric domain switching stimulated by terahertz electric field is studied in theory to discuss the feasibility of the above new scheme and to provide reference for studying domain switching in experiment. Meanwhile, in order to keep consistent with the experiment conditions(e.g. the applied area of THz electric fields and the scale of the ferroelectric domain) and to facilitate research the mechanism of domain switching, the simulations are carried out on macroscopic, mesoscopic and microscopic three spatial scales. The main contents and results are as follows:(1) On the macroscopic aspect, a model of domain switching in BTO ferroelectric was built based on soft-phonon model. And the domain switching by THz electric field was investigated. The results show that the parameters of both THz field and materials have significant effect on ferroelectric domain switching. THz field energy density in the range of 0.5 ~ 50 J/cm2 can stimulate domain switching. And the regional change of domain switching is controlled by terahertz beam waist width. With the decrease of the damping coefficient, coupling coefficient of elasticity, and terahertz frequency coefficient, the threshold values of domain switching reduce.(2) On the mesoscopic aspect, the domain switching by THz field in BTO ferroelectric thin films was studied by phase field simulation method and the influences of THz field strength and duration time on domain switching process were investigated. The results show that with the increase of strength and the duration time of THz field, the system is easy to overcome the potential barrier, which is more favorable to domain switching. The domain structures can affect the THz field threshold of domain switching and the switching thresholds of single domain, 90 degree domain and 180 degree domain are 2 MV/cm, 1.9 MV/cm and 0.2 MV/cm, respectively. In addition, the backswitching of ferroelectric domain appears when the THz field is removed.(3) On the microscopic aspect, the single domain switching by THz field in BTO ferroelectric thin films was investigated via molecular dynamics simulation method based on core-shell model. The influence of THz field strength and duration time on the process of domain switching was investigated. The results show that the single domain of BTO thin films is reversed when the strength of electric field with negative direction is greater than 1.0 MV/cm. On the contrary, when the electric field is smaller than 1.0 MV/cm, the domain reverses incompletely. And the duration time of THz electric field can influence the domain reversal process. The domain switching is complete when the duration time is greater than 10 ps. Otherwise the domain switching is incomplete. In addition, the domain backswitching of BTO ferroelectric thin films appears when the terahertz electric field is removed, which occurs under a picosecond time scale.