Study Of Surface Structure And Interface Strain Effect Of ABO₃-Type Oxide Thin Films By First-principles

ABO₃-type peroviskite oxides have attracted the attention of physicists and material scientists, for the special physical properties and wide application, and which are the hot topics in the electronic studies recently. In the application of the electronic thin films, it is inevitable that the surface and interface affect the electronic systems. To reveal the law of changes and theoretical explanation is necessary. The Density Functional Theory is a new method to study the electronic structures in theoretic way. Furthermore, the method has been developed rapidly and implicated widely, since the results are very close to the experiment reports with high operation speed. The Density Functional Perturbation Theory can be used to reveal the lattice dynamics based the Density Functional Theory. In the present works, the First-Principles are used to study the surface effects and the epitaxial strain of the interface of the dielectric thin films in the atomic and electronic scale.The first, we studied the ideal lanthanum aluminate (001) films surface based on first-principle method. In the atomic scale, the atoms in the surface layer have relaxed, and the oxygen atoms have been exposed in the surface layer, which leads the surface become double electric layer and the surface rumpling. The surface stability has been studied with the grand thermopotential of the different termination structure, which revealed that the LaO-terminated one is more stable than AlO₂ termination. Moreover, the two possible termination structures could not be co-existent, which is coincident with the experiment reports. With the calculation of the electronic structure of the surface before and after relaxation, the electronic distribution has changed, and the numeric results show that the Fermi level have been shifted, which leads to the surface conduction.The second, we employed the first-principle methods on the oxygen-vacancy LaAlO₃(001) surface based on the ideal surface research works. The two possible terminations have been considered. After the geometric relaxation, the oxygen-vacancy leads large value of the surface rumpling in the two terminations. The LaO-terminated surface rumpling value is larger than ideal and AlO₂-terminated oxygen-vacancy ones. From the thermo-stability analysis, the total energy of AlO₂-terminated oxygen-vacancy surface is lower than LaO-terminated one. The AlO₂-terminated one will be formed spontaneously in the high hypoxia condition. From the numeric results, the oxygen-vacancy makes some surface defect states exist in the energy gap near the Fermi level. In the LaO-terminated surface, the defect electronic states characterize as F-centers.We investigated the epitaxial misfit strain effect on the barium titanate and barium zirconate thin films. With the density functional perturbation theory, we studied the properties of lattice dynamics. From the calculation, the numeric analysis shows that there is a linear relation between the thin films' lattice distortion parameter c/a ratio and epitaxial strain. In comparison to barium zirconate thin films, the barium titanate can be distorted easily. With the total energy analysis, the change of total energy of barium titanate is smaller than barium zirconate thin films, which reveal that barium titanate thin films can easily grow on compressive substrate. The feature of phonon modes in the two thin films exhibit different behaviors with the epitaxial misfit strain of the films, and the dielectric tensors' have changed with a different patternbetween barium titanate and barium zirconate thin films.In thefinal part, the 1/1 short period superlattice model has been computed by first-principles methods. The electronic structure and atomic structure have been calculated. In the superlattice, the misft strain is 5.3%, which made the BaTiO₃ layer distort. The in-plane lattice constant was 3.872 A by the studies of 2, 3 and 4 short period LAO/BTO superlattice, which was close to the experiment reports. In the superlattice with 1/2 period, the c/a ratio of BTO layer was 1.015, which was close to the ferroelectric phase bound of BTO.