Phenomenological analysis of polarization reversal of ferroelectric thin films has been done on the basis of Landau theory using a lattice model. Polarization hystereses have been obtained in the lattice models in which dead-ferroelectric and built-in field regions are distributed at random. Dependences of saturation polarization and coercive field on remanent polarization are used for comparison between the calculated and experimental data. The polarization fatigue analyzed in the model including bipolar built-in fields agrees best with the experimental data of sol-gel PZT films. So, it is certified that these analyses using polarization lattice model are powerful to clarify the mechanism of polarization reversal dynamics in ferroelectric thin films. These analyses are also expected to be applied to the electrical characterization of ferroelectric capacitors in FeRAM.
CaBi4Ti4O15 thin films were prepared using a complex alkoxide precursor, which were synthesized by controlling chemical reactions of individual alkoxides. The thin films crystallized at relatively low temperatures on Pt-coated Si substrates, and the crystal phase and orientation strongly depended on the crystallinity of the Pt bottom electrodes. On the basis of the results on the crystallization of the thin films on the substrates Si, the polar-axis oriented CaBi4Ti4O15 films were successfully deposited on Pt foils. The bimorph CaBi4Ti4O15/Pt/CaBi4Ti4O15 showed excellent ferro-and piezoelectric properties and would open up possibilities for devices as Pb-free ferroelectric materials.
Ferroelectric random access memory (FRAM) is one of the most promising devices applications of thin film ferroelectric materials. To develop FRAMs with high reliability it is important to understand how the properties of ferroelectric capacitors are impacted by process integration which exposes the capacitor to hydrogen at high temperature. At first we evaluated the hydrogen degradation of Pb(Zr, Ti)O3 (PZT) capacitors by baking in Deuterium (D2) gas. D2 gas baking caused the same type of degradation as observed with H2. We showed that the hydrogen degradation mechanism occurs in three steps: 1) partial reduction of IrO2 top electrode to Ir, 2) dissociation of D2 to D by Ir catalytic action, 3) D diffusion into PZT film. Next, we showed the capability of MOCVD-PZT films for future FRAMs. A large switching charge with low applied voltage and high reliability are obtained with highly (111)-oriented PZT layers. The measured switching charge is sufficient for 0.18 mm FRAM technology.
Relaxor type ferroelectrics are treasure boxes both for basic physics and practical applications. Among them, Ba(Zr, Ti)O3 is one of the candidate materials for elucidating the mechanism of relaxor phenomena. It has homo-valent B-site ions of Zr4+ and Ti4+. Therefore, there is no driving force to push the ions into ordered structure. On the other hand, it has a possibility of co-existence of multi phases (so-called pinching effect). In case of the bulk state, we have found that the relaxor behavior is strongly affected by the annealing conditions. To make clear the intrinsic parameter for the relaxor, we have demonstrated the artificial control of the positioning of the B-site ions by the superlattice technique. The stacking periodicity and the compositional combination of BaTiO3 and BaZrO3 are changed systematically with this method and the sign of the relaxor is detected.
We have shown that piezoresponse force microscope (PFM) is an effective tool for observing 3-dimensional domain structure formed in ferroelectric crystals. The PFM is based on the measurement of the surface displacement induced by inverse piezoelectric effect by using conductive cantilevers. The small tip of the cantilever plays an important role not only in applying electric field as a top electrode but also in detecting piezoelectric displacements just beneath the tip. In the vertical mode, the component of spontaneous polarization (Ps) parallel to the electric field can be mapped, while the in-plane mode provide an image of the component of Ps parallel to the crystal surface. In a PbTiO3 crystal, 90o domain as well as 180o domain with no interface charge was observed. PFM observations of a layered Bi4Ti3O12 suggest that charged 180o domain walls with tail-to-tail and head-to-head configurations strongly interact with oxygen vacancies, which leads to a smaller remanent polarization.
A property design concept involving the substitution of each crystal site in the pseudoperovskite in Bi4Ti3O12 structure, a site-engineered concept, is proposed for Bi4Ti3O12-based thin films. This concept is based on the selective substitution of each crystal site contributing to the ferroelectric properties. By using this concept, a novel material, i.e., (Bi, Nd)4Ti3O12 was developed. A spontaneous polarization (Ps) value was 58 μC/cm2 beyond the single crystal data of Bi4Ti3O12. In addition, taking account of the realizable orientation of the film on (100)Si substrate, the expected remanent polarization (Pr) value of (Bi3.5Nd0.5)Ti3O12 film is almost the same with those of widely-used Pb(Zr0.35Ti0.65)O3 films.
Ferroelectrics has generated considerable interest as promising storage media. In this paper, an investigation of ultrahigh-density ferroelectric data storage based on scanning nonlinear dielectric microscopy (SNDM) was carried out. For the purpose of obtaining fundamental knowledge of high-density ferroelectric data storage, several experiments on nanodomain formation in lithium tantalate (LiTaO3) single crystal were conducted. As a result, a very small inverted domain with a radius of 6 nm was successfully formed in stoichiometric LiTaO3 (SLT), and in addition, a domain dot array with an areal density of 1.5 Tbit/inch2 was formed on congruent LiTaO3 (CLT). Moreover, it was found that fast nanosecond domain switching could be achieved by reducing the sample thickness of CLT plate. Additionally, the first prototype high-density ferroelectric data storage system was developed. Using this system, writing data transfer rates were evaluated and 256×256 bit data were recorded with the areal densities of 258 Gbit/inch2 and 717 Gbit/inch2.
Traditional coloring techniques used in Japanese metal crafts, such as niiro and iroage, are explained. In niiro for copper-gold alloys, a Cu2O layer in which gold particles are dispersed is formed on the alloy surface. This gold alloy is colored through absorption and reflection of light by gold particles in the layer. Besides coloring, this layer functions to prevent corrosion. In iroage for gold-silver alloys, silver dissolves selectively by heating after a chemical is applied on the metal surface. As a gold-rich layer is formed on the gold-silver alloy surface, the gold color is produced.