The Crystallization Behavior And Kinetics Of ABO₃ Type Ferroelectric Thin Films

The ABO₃ type ferroelectric thin films are important in the application of Integrated Ferroelectric Devices. To fabricate the films, researchers often follow the route that firstly deposite amorphous film and then crystallize it by post annealing. In this route, crystallization is a significant method to control the microstructure and the diffusion between various layers, and further more, control the properties of the thin films. As a new crystallization technology, Rapid Thermal Annealing (RTA) was widely used in the crystallization of ferroelectric thin films. However, some researches showed that there exist new crystallization behaviors and characters in this kind of films processed by RTA, and, unfortunately, these new characters can't be correctly explained by classical crystallization theory. In this essay, the crystallization behaviors and kinetics of this kind of films were studied to establish the crystallization kinectics model and reveal the crystallization mechanics of the ABO₃ type ferroelectric thin films.In way of theory, the first principle simulations for the electronic structure of the crystalline and amorphous ferroelectric thin films were carried on, and the potential function of these materilas was built. Basing on the potential function, a series of melting, quenching and crystallization molecular dynamics simulations of these films were carried on to investigate the amorphous-crystalline transition mechanics and rules. From the simulation results and the application of the theory of reaction velocity, we introduced a crystallization kinetics model of the ABO₃ type ferroelectric thin films. In way of experiment, systematic RTA experiments were conducted to explore the effects of annealing time, temperature and heating rate on the microstructure, grain size distribution and surface morphology of the typical ABO₃ type ferroelectric thin films, such as STO and BTO.(1) The building of the potential function of ABO₃ ferroelectric thin films.Adopting DFT theory and planewave method, the first principle simulations of the electronic structure of the crystalline and amorphous ABO₃ type materials were carried on in CASTEP code. The results showed that, there is strong "hybridization" in these materials, and in amorphous, it is not as strong as that in the crystalline. Also it shows that the effective charges are abnormal in both states. From these results, the potential function, which contains both ionic and covalent interaction, was established. In order to fit the parameters of the potential function, and the energy surface of these systems was constructed by energy simulation.(2) The verifying of the potential function and the obtaining of the amorphous configuration.By using of the potential function, the molecular dynamics simulations of melting and quenching process were carried on. The agreement of the simulating lattice constant and melting point of STO with the real value verified the new potential function. Some structure and kinetic characters of amorphous STO, such as coordination number, radial distribution function, bond angle distribution, and diffusion coefficient were also calculated, which shows that the amorphous configuration of STO was successfully reproduced.(3) The simulations of the crystallization of the ABO₃ ferroelectric thin films.By means of molecular dynamics method, the crystallization procedure, undervarious temperatures and heating rates, of ABO₃ ferroelectric thin films on single crystal substrate of same kind of material was simulated. The results show that heating rate strongly affects the temperature distribution of the film. Rapid heating brings dramatically increasing of the temperature difference between the film surface and the film-substrate interface. The nucleation in these films is mainly heterogeneous. The nucleation near the film-substrate interface occurs in lower temperature, and the critical size of the nuclei is 3 times of lattice constant. On the contrary, the nucleation near the film surface only occurs in relatively higher temperature, and the critical size of nuclei is 5 times of lattice constant.(4) Establishing of the crystallization kinetics model for the ABO₃ type ferroelectric thin films.From the results of MD simulations, adopting the theory of reaction velocity, the crystallization model was established, which contains the effect of heationg rate. The model shows that, firstly, the nucleation energy barrier is far higer than that of grain growth, and the crystallization of ABO₃ type ferroelectric thin films is a kinctic procedure controlled by nucleation. Secondly, the nucleation rate increases with the heating rate. At last, when the heating rate is high enough, the heterogenous nucleation near the film surface arouses because of the rapid promoted temperature by fast heating, and then the nucleatin both near surface and film-substrate interface can exist at the same time. On the contrary, when the heating rate is lower, the surface temperature is not high enough for surface nucleation, and there exist only nucleation neare the film-substrate interface which brings lower nucleation rate and larger grain.(5) RTA technology was adopted in the STO and BST ferroelectric thin films crystallization experiments, which showed that, the grain size is determined not by crystallization time, but the originally nucleation temperature. The originally nucleation temperature is controlled by heating rate, and when it is lower, the original temperature is lower, which leads to larger grain. When the heating rate is higher, the original temperature is higher, which results in smaller grain. The model was verified by the agreement between the results of experiments and the model.