Lateral Size Effects On Phase Transition Properties Of Ferroelectric Thin Films

Ferroelectric thin films as one of important functional materials have been paid widely attention due to their excellent characteristics, such as piezoelectricity, ferroelectricity, pyroelectricity and high dielectric constant and so on. At present, the storage density of ferroelectric dynamic random access memory is mainly determined by the capacitor area of each unit. Therefore, further discussing the influence of lateral sizes (or the area) of capacitors on properties of ferroelectric thin films comes to be currently a hot issue. Additionally, impurities, defects, as well as surface and interfacial stress are unavoidable during the preparation of ferroelectric thin films. These factors can lead to local differences of thin-film structures, and form structural transition zones near surface layers and interfaces of thin films. Structural transition zones have a great impact on properties of ferroelectric thin films, and make their properties different from those of corresponding bulk materials. Therefore, based on the transverse Ising model, this dissertation studies the influence of sizes (lateral size and thickness) and lateral structural transition zones of thin films on phase transition, dielectric and pyroelectric properties of ferroelectric thin films with finite sizes by using the Fermi-type Green function.This dissertation establishes the theoretical model of the ferroelectric thin film with finite sizes and structural transition zones, and distribution functions of nearest-neighbor two-pseudo-spin interactions are introduced to reflect structural non-uniformity of ferroelectric thin films, and mean polarizations and transition temperatures of ferroelectric thin films with second-order phase transition are calculated by using the Fermi-type Green function, and the influence of sizes and lateral structural transition zones of thin films on their phase transition properties is discussed detailedly. The results indicate that lateral structural transition zones affect not transition temperatures of thin films, but their mean polarizations. Broadening lateral structural transition zones and weakening nearest-neighbor two-pseudo-spin interactions in these regions are adverse to increase mean polarizations of thin films. Strengthening nearest-neighbor two-pseudo-spin interactions in the center region of thin films can increase not only mean polarizations of thin films but also their transition temperatures when sizes of ferroelectric thin films with second-order phase transition are fixed. Additionally, mean polarizations and transition temperatures of thin films also relate to their lateral sizes and thicknesses. Mean polarizations and transition temperatures both increase with the increase of lateral sizes and thicknesses of ferroelectric thin films with second-order phase transition when nearest-neighbor two-pseudo-spin interactions in the center region of thin films are fixed.For finite size ferroelectric thin films with second-order phase transition, this dissertation further studies their dielectric and pyroelectric properties. The results show that lateral structural transition zones have a great impact on dielectric and pyroelectric properties of ferroelectric thin films with second-order phase transition. It is the effective way to improve susceptibilities and pyroelectric coefficients of ferroelectric thin films with second-orders phase transition by broadening their lateral structural transition zones and weakening nearest-neighbor two-pseudo-spin interactions in these regions. For fixed size ferroelectric thin films with second-order phase transition, strengthening nearest-neighbor two-pseudo-spin interactions in the center region of thin films can not only make dielectric and pyroelectric peaks shift to higher temperature, but also downscale their critical lateral sizes. The susceptibilities and pyroelectric coefficients of thin films with smaller sizes are more affected by the change of their lateral sizes when nearest-neighbor two-pseudo-spin interactions in the center region of thin films are fixed.The influence of thin-film sizes and lateral structural transition zones on phase transition properties of ferroelectric thin films with first-order phase transition is investigated by introducing four-pseudo-spin interactions and using the Fermi-type Green function. The structural change of thin films can be reflected by introducing distribution functions of two-pseudo-spin and four-pseudo-spin interactions. The results reveal that ferroelectric thin films can take place the first-order phase transition mainly due to the contribution of four-pseudo-spin interactions, and critical four-pseudo-spin interactions are weakened with the increase of their lateral sizes and thicknesses. Broadening lateral structural transition zones and weakening four-pseudo-spin interactions in these regions can decrease mean polarizations of thin films, but cannot influence their transition temperatures. Thin-film sizes and four-pseudo-spin interactions in the center region of thin films are the important factors that influence phase transition properties of ferroelectric thin films with first-order phase transition. The mean polarizations and transition temperatures of thin films both increase with the enhancement of lateral sizes and thicknesses when four-pseudo-spin interactions in the center region of thin films are fixed. For ferroelectric thin films with fixed sizes and first-order phase transition, strengthening four-pseudo-spin interactions in the center region of thin films can increase both their mean polarizations and transition temperatures.For finite size ferroelectric thin films with first-order phase transition, this dissertation also further studies their dielectric and pyroelectric properties, and discusses the influence of lateral structural transition zones, thin-film sizes and four-pseudo-spin interactions on susceptibilities and pyroelectric coefficients of thin films detailedly. The results indicate that broadening lateral structural transition zones and weakening four-pseudo-spin interactions in these regions can increase both susceptibilities and pyroelectric coefficients of ferroelectric thin films with first-order phase transition. Furthermore, dielectric and pyroelectric properties of ferroelectric thin films with finite sizes and first-order phase transition also relate to thin-film sizes and four-pseudo-spin interactions in the center region of thin films. Strengthening four-pseudo-spin interactions in the center region of thin films can make dielectric and pyroelectric peaks shift to higher temperature when thin-film sizes are fixed. Dielectric and pyroelectric peaks both shift to higher temperature with the increase of lateral sizes and thicknesses of thin films, and enlarging their thicknesses can downscale their critical lateral sizes when four-pseudo-spin interactions in the center region of thin films are fixed.The researchful results obtained in this dissertation not only enrich theoretical study contents of ferroelectric thin films, but also have a guiding role in exploring the device miniaturization and device-performance improvement.