Theoretical Study Of Electronic Structure For Doped Ferroelectrics

Ferroelectrics have been widely used in the field of microelectronics and optoelectronics since they exhibit many excellent properties such as piezoelectricity, thermoelectricity, electro-optic, photorefractive and nonlinear optical effect. Nowadays, the presence of ferroelectric devices is practically indispensable in everyday's life. Therefore, understanding the physical properties of ferroelectrics is academically significant and practically important.In recent years, different theoretical approaches, including phenomenological Landau theories and atomic-level first-principles calculations, have been used to study the physical properties of ferroelectrics. With continuing advances in density functional theory (DFT) and computer hardware, first-principles studies of electronic structure and structural energetics have become a routine method for ferroelectrics. In parallel with advances in materials synthesis, it has seen a revolution in the atomic-scale theoretical understanding of ferroelectricity in the past decade, especially in perovskite oxides, through first-principles density functional theory investigations. At early stage, people mainly focused on properties in bulk ferroelectrics. First-principles calculations have greatly contributed to the understating of the origin of ferroelectric structures and properties and to mechanisms of ferroelectric instabilities. The central result of a density functional theory calculation is the ground-state energy computed within the Born-Oppenheimer approximation; from this the predicted ground-state crystal structure, phonon dispersion relations, and elastic constants are directly accessible. Recently, first-principles calculations are gradually extended to complex systems, such as doped systems, superlattices and solid solutions, and low demission systems, such as solid surface, nanotube, quantum well and quantum dots. Within this first-principles modeling framework, it can be more clearly identify specific issues and results for investigation and analysis. So, theoretical analysis not only explains observed properties of complex systems of ferroelectrics, but also provides instructions for experiment. In this dissertation, this method has been applied to study the surface structure and doped systems of perovskite-type ferroelectrics. Calculations are mainly carried out using ESPRESSO package and Castep program which is a module of Material Studio. ESPRESSO package is a set of programs for electronic structure calculations within DFT, using a plane-wave basis set and pseudopotentails. Also, Castep program is using a plane-wave basis set and ultra-pseudopotentials.In this following, I summarize the main contents of my dissertation. Investigation of the (001) surface of cubic BaZrO₃ with BaO and ZrO₂ terminations. Surface structure, partial density of states, band structure and surface energy have been obtained. It is found that the largest relaxation appears on the first layer of atoms, and the relaxation of the BaO-terminated surface is larger than that of the ZrO₂-terminated surface. The surface rumpling of the BaO-terminated surface is also larger than that of the ZrO₂-terminated surface. A reduction of the band gap has been found for both kinds of surface terminations, and the reduction is mainly due to the O 2p and Zr 4d electrons at the surface layers. Results of surface energy calculations reveal that the BaZrO₃ surface is likely to be more stable than the PbZrO₃ surface.Study of doped systems of ferroelectrics. First principles calculations have been performed on Fe-doped BaTiO₃ and SrTiO₃. Dopant formation energy, structure distortion, band structure and density of states have been computed. The dopant formation energy is found to be 6.8 eV and 6.5 eV for Fe-doped BaTiO₃ and SrTiO₃ respectively. The distances between Fe impurity and its nearest O atoms and between Fe atom and Ba or Sr atoms become smaller than those of the corresponding undoped bulk systems. The Fe defect energy band is obtained, which mainly originates from Fe 3d electrons.Electronic and structural properties of Nb-doped SrTiO₃ are studied using the first principles density functional theory (DFT) calculations based on a plane-wave basis and pseudopotentials. Dopant formation energy, structure distortion, band structure as well as density of states have been obtained. Dopant formation energy results show that Nb preferentially enters the Ti site in SrTiO₃, which is good agreement with experimental observations. The Fermi level of the Nb-doped SrTiO₃ move into the bottom of conduction band, and the system undergoes an insulator-to-metal transition. Due to the appearance of the carrier impurity from Nb doping, there is a significant distortion on the top valence band.An investigation of electronic and atomic structural properties of Fe-doped CaTiO₃ has been performed using first-principles density functional theory (DFT) calculations based on a plane-wave basis and pseudopotentials. Dopant formation energy, structure distortion, band structure and density of states have been obtained. It is shown from the dopant formation energy that Fe preferentially enters the Ti-site in CaTiO₃ , which is good agreement with experimental data. Also Fe doping leads to a small local structural distortion. The Fe defect energy band appears between conduct band and valence band, but closer to the valence band. This Fe defect energy mainly originates from Fe 3d electrons. The defect band dispersion is weak throughout the Brillouin zone, and the most distinct dispersion lies at the X point.