Theoretical Study Of SrTiO₃ Ferroelectric Thin Films Initial Homoepitaxial Growth

In this thesis, theoretical study of SrTiO₃ ferroelectric thin films initial homoepitaxial growth have been investigated by using a first-principles based upon the Density Functional Theory. All calculations are used the DMol3 package of Materials Studio 4.0.First, it is very important that the microcosmic reaction and particle states of Sr, Ti, O atoms which are evaporated by laser molecular beam epitaxy in the early growth of SrTiO₃ thin films. The precedence reaction processes of Sr, Ti, O atoms and the reaction mechanisms of SrO, TiO₂ and SrTiO₃ molecules have been calculated. We gain corresponding transition states and intermediates and the activation energy. These results indicate that the SrO, TiO₂ and SrTiO₃ molecules are primary particle states in the early growth of SrTiO₃ thin films.When SrO, TiO₂ molecules in a vacuum formed, it will be deposited on the substrate SrTiO₃(100) surface and be adsorbed growth on the surface. SrTiO₃(100) surface has two kinds of termination structure, respectively SrO and TiO₂ layer. We have denoted SrO-SrTiO₃(100) TiO₂-SrTiO₃(100) surface. The surface was optimized by DFT/GGA methods and was stable after the relaxation. On the basis of the above, the adsorption processes of TiO₂ and SrO molecules on the termination of two kinds of SrTiO₃(100) surface have been simulated using ab initio molecular dynamics method respectively. We gain the reaction trajectory and energy of every moment in the process.At the time, the adsorption energys of corresponding stage has been calculated.By comparison the adsorption energy, we concluded that SrO molecule have been adsorbed growth priority than TiO₂ molecule on the TiO₂-SrTiO₃(100) surface, while TiO₂ molecule have been adsorbed growth priority than SrO molecule on the SrO-SrTiO₃(100) surface. In this instance, SrO and TiO₂ layer are growth alternating layered cycle. The results are well in accordance with experimental.At the same time, the bonding process of SrO molecule on the TiO₂-SrTiO₃(100) surface have been investigated by using Mulliken, highest occupied and lowest unoccupied molecular orbital theory and density of states. Consistent with the results of kinetic studies, it indicated that Sr atoms in SrO molecule combine with O atoms in the surface priority. And then O atoms in SrO molecule combine with Ti in the surface, Sr-O bond was broken in SrO molecule.The bonding process of TiO₂ molecule on the SrO-SrTiO₃(100) surface has been investigated by the same method. Ti atoms in TiO₂ molecule combine with O atoms in the surface combine first, and then O atoms in TiO₂ molecule combine with Sr atoms in the surface.We can see that the final configurations of SrO molecule on the TiO₂-SrTiO₃(100) and TiO₂ molecule on the SrO-SrTiO₃(100) surface are similar with structure of SrTiO₃ unit cell. It confirms that the model of SrTiO₃ early growth is migration of unit cell and layer growth.