Preparation and characterization of ferroelectric yttrium manganite and lead calcium titanate thin films

YMnO₃ is a ferroelectric material with a low dielectric constant and non-volatile constituent elements. This makes it a good candidate for second-generation, non-destructive and non-volatile memories, comprising a metal–ferroelectric–silicon field-effect transistor (MFSFET) structure. In this work a considerable effort was devoted to finding a suitable solution to form the hexagonal phase of YMnO₃. Preferentially c -oriented hexagonal-phase YMnO₃ thin films were successfully prepared at 700°C, using the sol–gel method with inorganic precursors. A reasonable ferroelectric C–V window was obtained. For the first time, low-temperature C–V measurement was used to rule out mobile ions as the origin of the C–V window. The XRD [0004] peak was found to shift with shelf time, while simultaneously the C–V window widened. This peak shift is due to the release of stress in the fresh-made sample, resulting from the lattice mismatch between Si and YMnO₃.; (Pb_0.76Ca_0.24)TiO₃ (PCT) is a ferroelectric material with high anisotropy. When prepared in thin-film form it can have a large d₃₃, comparable to bulk material. These properties make it attractive for MEMS and ultrasonic transducer arrays etc. Using the sol–gel method, purely tetragonal PCT thin films have been successfully prepared on platinized Si substrates. The films have a dense structure with a relatively smooth surface. Conventional P–E hysteresis loops were obtained for these films. For the first time PCT thin films were systematically studied with piezoresponse microscopy (AFM-based). A large apparent piezoresponse signal was obtained. Since the response of domains to heat, stress, electric fields, etc., can explain many of the mechanical and electrical properties of ferroelectrics, domain-structure investigations have been carried out on films of different grain sizes. A clear domain structure has been observed for films as thin as 40 nm. This is uncommon even for other well-studied materials like lead zirconate titanate (PZT) and strontium bismuth tantalate (SBT).; Piezoresponse microscopy was also used to investigate the ferroelectric relaxation of PCT thin films. The results were compared with the dielectric relaxation as measured by an LCR meter. It was found that dielectric relaxation is much slower than ferroelectric relaxation. Through a comprehensive survey of retention-loss models and a detailed analysis of experimental data, it is proposed that dielectric relaxation is due to domain-wall depression, while ferroelectric relaxation is due to the depolarization effect. This conclusion is very similar to the macroscopic findings for PZT thin films.; Systematic comparison of continuous-mode and pulse-mode local piezoelectric hysteresis loops was introduced as a new method for investigating domain stability, i.e., switching and back-switching processes. In continuous mode the duration of each voltage step was varied, while in pulse mode the pulse width was varied. It was found that for PCT thin films it is possible to distinguish the back-switching process from the switching process through the activation field. While a smaller activation field extracted from the continuous-mode hysteresis loops is related to domain-switching process, a higher activation field from the pulse-mode hysteresis loops for shorter pulses is related to domain-back-switching process. The physical origin of this activation energy is still unclear, and further work is needed to clarify it. The method can be applied to other ferroelectric materials as well.