Theoretical Study Of Ferroelectric Thin Films And Ferroelectric Superlattices

In current ferroelectric physics, there are two important research trends—low dimensional properties and modulation structures. The study of low dimensional properties is stimulated by the motivation of ferroelectric thin film devices. Ferroelectric thin films with the thickness ranging from tens of nanometers to several microns present a series of important properties and effects, such as dielectric, ferroelectric, piezoelectric, pyroelectric and photoelectric characteristics, which makes ferroelectric thin film highly attractive in ferroelectric physics and high-tech devices. In recent years, integrated ferroelectrics, which deal with the combination of ferroelectric thin films and semiconductors, further enlarge the applications of ferroelectric materials in the fields of high performance microelectronics, photoelectronic integrated devices, etc. Ferroelectric thin films have become one of the significant mass foundations of the information technology, and will undoubtedly continue to make invaluable contributions in the future.In the applications of ferroelectric thin films, the interface effects inevitably exist, including the interface between the film and the semiconductor substrate, and the interface between the film and the metal gate, etc. The quality of the interface properties largely influences the performance of these devices. For example, the band offset, which determines the leakage current in devices, can be altered by the chemical environment of the interface between ferroelectric thin films and semiconductor; the interaction between ferroelectric film and metal gate matters very much to the ferroelectric fatigue properties. Therefore, it is necessary to understand these phenomena from the theoretical point of view.The development of modulation structure is due to the increasing demand of ferroelectric thin films. To fabricate films with better properties, great efforts have been made to improve the fabrication technique and exploit new methods. Ferroelectric superlattices offer a good way to improve the ferroelectric film properties. Short-period ferroelectric superlattices have switchable polarization, giant dielectric constant, enhanced ferroelectricity distinct from those of their individual constituent, and the freedom to choose from a multitude of possible materials combinations, layer thickness, and sequences can be utilized to optimize desirable properties.In recent years, with the progress in density functional theory (DFT) and its numerical methods, DFT based first-principles calculation has become a powerful method for condensed matter physics, quantum chemistry and material science, and also become an important method to theoretically investigate ferroelectrics. Based on the basic formula of quantum mechanics and some reasonable approximations, first-principles calculation studies the complex many-particle systems from the level of electronic structure and gives their basic properties.This dissertation is aimed to study the surface and interface properties of ferroelectric thin films and superlattices from first-principles DFT calculations. The atomic and electronic structures of the relaxor ferroelectrics—Pb(Mg_1/3Nb_2/3)O₃ are also studied.In the first chapter, we give a brief introduction of first-principles studies on ferroelectric thin films and superlattices. At the first step, a general review of the ferroelectric materials and the applications of ferroelectric thin films are given. Then, we introduce the basic framework of DFT, two popular exchange-correlation potentials (Local Density Approximation and Generalized Gradient Approximation) and the modification and extension of DFT. We then review first-principles studies on ferroelectric thin films and superlattices. Along with the basic models, we introduce the main method and results of recent theoretical studies on perovskite ferroelectric thin films and superlattices, including the properties of surfaces, interfaces and strain. At the end of this chapter, we briefly describe two simulation codes we have used in this dissertation—CASTEP and PWscf.In chapter 2, the cubic CaTiO₃ and SrZrO₃ (001) surfaces are studied using the DFT method. CaTiO₃ is incipient ferroelectrics, and SrZrO₃ can become ferroelectrics under strain effect in spite of its paraelectric nature. Two types of surfaces with AO and BO₂ terminations, respectively, are considered. Both terminations show surface rumpling, and the rumpling of AO-terminated surface is more prominent than the BO₂-terminated surface. The interlayer spacing between the first and the second layers decreases compared to that of the bulk, whereas the spacing between the second and the third layers increases. As for the fourth layer, the interlayer spacing shows almost no changes. It is found that the band gap of the AO-terminated surface is nearly the same to that of the bulk, while the band gap of the BO₂-terminated surface decreases, which is mainly due to the upward intrusion of the upper valence band states near the M point. Surface energy calculations reveal that the AO-terminated surface is slightly easier to form than the BO₂-terminated surface.In chapter 3, first-principles study of short-period SrZrO₃/SrTiO₃ ferroelectric superlattices is performed. Neither SrZrO₃ nor SrTiO₃ is ferroelectrics. However, clear ferroelectric hysteresis curves have been observed in artificial SrZrO₃/ SrTiO₃ superlattices fabricated by molecular beam epitaxy (MBE). In our theoretical simulations, the electronic structure and polarization properties of SrZrO₃/SrTiO₃ superlattices with stacking periods of 2, 3, 4 and 5 unit cells are studied by mean of DFT and Berry phase methods. Strong ferroelectricity is found in these superlattices, with the polarization of 0.346～0.427 C/m². The ferroelectricity is derived from the large lattice mismatch between SrZrO₃ and SrTiO₃. When they are combined to become superlattices, the interface will endure strong strain effect and causes the lattice distortion, which leads to the relative displacement between the cations and anions. Both SrZrO₃ and SrTiO₃ layers in the superlattices are found to be strongly polarized, and the polarization contribution from SrZrO₃ layers exceeds that from SrTiO₃ layers. A dependence of the superlattice polarization magnitude on the c/a ratio is revealed from our calculations, in consistent with the experiment. In addition, the strained bulk SrZrO₃ and SrTiO₃ are also studied. It is found that when bulk SrZrO₃ and SrTiO₃ are forced to have the same strain as in (SrZrO₃)₁/(SrTiO₃)₁, the SrZrO₃ presents a polarization of 0.601 C/m², whereas that of SrTiO₃ is merely 0.014C/m².In chapter 4, the BaTiO₃/Ge(001) interfaces are studied by DFT method. BaTiO₃ is high-k dielectric material and Ge is of high electron mobility (2 ×) and hole mobility (4 ×) compared to those of Si, which makes the BaTiO₃/Ge system very attractive in high speed circuit devices. Two BaTiO₃/Ge(001) interfaces (I and II) are constructed following the general bonding rules of Peacock and Robertson et al, and their electronic structure and band offset are studied. Both interfaces are semiconducting with no gap states. The band offset largely depends on the chemical environment of the interface. Changing the interface stoichiometry from O-poor to O-rich modifies the band offset up to 1.5 eV. The valence and conduction band offsets of interface I are 1.2 eV and 1.34 eV, respectively, which satisfy the requirement of band offset (> 1eV) and therefore can be the structure in real devices.In chapter 5, we perform DFT calculations on the interfaces between BaTiO₃ and metal (Ag, Au, Pt). Two terminations (BaO- and TiO₂- terminatied) of BaTiO₃ and several adsorption site of the interface metal on BaTiO₃ are considered. It is found that the TiO₂-terminated surface is energetically more favorable for epitaxial growth of metal films than the BaO-terminated surface. By adopting the experimental gap value of BaTiO₃ and calculating the energy difference between the Fermi energy and the valence band edge of BaTiO₃, the Schottky barrier height is obtained. The interface structure largely influences the value of Schottky barrier height. Our results indicate that metal grown on TiO₂-terminated surface presents lower SBH than grown on BaO-terminated surface.In chapter 6, DFT are used to investigate the geometrical and electronic structure of an important relaxor ferroelectrics—Pb(Mg_1/3Nb_2/3)O₃ (PMN). An ordered perovskite structure based supercell is constructed with the B sites Nb-Mg-Nb stacking along the [001] direction. The calculated band gap is a direct one with the value of 1.05 eV. The Mg-0 bond presents no covalency, whereas there is a considerable covalent bonding between Nb and O atoms. The different B-0 bonding characters might be responsible for the relaxor properties of Pb(Mg_1/3Nb_2/3)O₃ crystal.In chapter 7, the conclusions of this dissertation are summarized, and further studies are previewed. Within the framework of DFT, ferroelectric superlattice composed of two paraelectric perovskites (SrZrO₃ and SrTiO₃) is theoretically investigated for the first time. It is found that the ferroelectricity of the SrZrO₃/SrTiO₃ superlattice is mainly due to the lattice distortion caused by the large lattice mismatch between the two components. For the first time, the interface between Ge and perovskite oxide is studied. We have proposed a (001)BaTiO₃/Ge interface, which satisfies the general bonding rule and meets the technological requirement.