title: Charges and Charged Defects in Pb-based Ferro- and Antiferroelectric Ceramics
Creators: name: Huang, Binxiang
Publisher: name: UNSPECIFIED
Year: 2023

description: Ferroelectric (FE) and antiferroelectric (AFE) ceramics are widely used in capacitors for various purposes because of their specific dielectric properties and outstanding insulating nature. Several types of charges and charged point defects are considered to exist in these materials, participating in the mechanisms of charge compensation from different perspectives, such as the compensation within the bulk material which determines the electrical conductivity, the compensation at the domain walls which rationalizes the domain structures, and the compensation at the electrode/dielectric interfaces which screens the depolarization fields and thereby stabilizes the switched FE states. Therefore, identifying these charge carriers and understanding their compensation behavior are fundamental to the design and optimization of these FE and AFE materials. In this work, three composition series based on the well-known Pb-based perovskite AFE system (Pb,La)(Zr,Sn,Ti)O_3 (PLZST) were studied by dc conductivity measurement and X-ray photoelectron spectroscopy (XPS). By combining these two characterization techniques, fundamental correlations in terms of the electronic structures could be explored between the charge compensations and the dielectric properties. These ceramic samples were prepared with traditional solid-state reaction method. Prior to the conductivity and XPS studies, several basic characterizations were performed first, including hysteresis loops, dielectric permittivities, crystalline structures by X-ray diffraction, and microstructures by scanning electron microscopy, in order to get the basic information of the sintered ceramics. High resistivity is a necessary condition for the FE and AFE materials, and it can be described principally by the charge compensation and transport within the bulk. In order to get an overview of the conduction behavior of these samples, dc conductivities were measured under different thermal and electrical conditions. Variations in the conductivity, which are very likely caused by the migration and redistribution of oxygen and/or lead vacancies, have been observed. The comparisons among different compositions reveal that donor- or “net-donor” doping can effectively reduce the conductivity of these Pb-based ceramics. The Arrhenius relations of the donor-doped samples are nearly parallel at 300°C—400°C, indicating very similar transport mechanisms. Besides, a higher conductivity has been noticed in a field-induced FE state compared to that in its AFE state before the AFE-to-FE phase transition. This implies an additional conducting contribution in a FE phase, probably related to the existence of charged domain walls. As it has been realized in the conductivity studies that the low conductivities of these Pb-based dielectrics cannot be well explained without taking the traps for electrons and holes into account, a novel in situ XPS method has been developed to explore such traps within the band gap of a dielectric. Making use of the migration of oxygen species, the sandwich capacitor structure can be operated as a solid electrochemical cell, where the dielectric acts as the electrolyte. Electrochemical reduction and oxidation, which can be characterized by XPS, are expected to take place at the cathodic and anodic interfaces, respectively. By this means, the reduction and oxidation potentials, which indicate the existence of trap levels, are used to identify the upper and lower limits of the Fermi energy of the dielectric, respectively. In this work, an electron trap Pb^{2+/0} has been detected in some of the studied materials, different from the electron trap Ti^{4+/3+} and the hole trap Pb^{2+/3+} that are widely accepted in the literature on Pb(Zr,Ti)O_3. Moreover, the observation of the reduced Pb exhibits a composition dependence, implying that the preferred site for such reduction might be related to the dopants or the states at the bottom of the conduction band. Regarding the charge compensation at the electrode/dielectric interfaces, selected FE and AFE compositions have been studied by another in situ XPS approach, where sufficiently high electric fields can be applied to the sandwich capacitor structure in order to change the polarization state within the bulk ceramic and XPS is measured meanwhile through a thin top electrode. According to the electrode screening theory, an extra electric potential drop occurs at the electrode interface when the bound charges and the free compensating charges are spatially separated by a screening length. This extra potential drop can be characterized by a binding energy shift of the dielectric elements in the XPS spectrum. By detecting such emission shifts, the magnitude of FE polarization, the extent of dipole alignment, and the depth of the charge compensation can be analyzed. In this part, electrode-material-dependent asymmetric screening behavior has been observed for the FE samples. Moreover, no binding energy shift has been seen at the electrode/AFE interface even when the applied field already exceeds the AFE-to-FE transition point. This implies a fundamental discrepancy between the electrode/FE and the electrode/AFE interfaces in terms of the depth where the bound charges of a FE state are compensated. Based on these observations, several screening models that involve charge injection through the electrode interfaces have been suggested, in which the previously mentioned trap levels of electrons and/or holes may play certain roles. The novelty of this work is reflected in both technology and research. New technical strategies of XPS have been developed for extracting useful information from insulating bulk materials, which is usually thought to be difficult because of the charging problem. More importantly, this work can highlight the roles of different charges existing in the dielectric materials and provide some fundamental understanding of the dielectric properties from the viewpoint of electronic structures.