Uchida Hiroshi

Lead-free ferroelectric Bi4Ti3O12 (BIT) thin films were modified by the cosubstitution of Sr2+ for Bi3+ and of Nb5+ for Ti4+ by spin-coating and decomposition of chemical solutions of metal-alkoxide materials (the nominal compositions of Bi4-xSrxTi3-xNbxO12 (BSTN) are $x=0.5$, 1.0, 1.5). Single-phase thin films with a BIT-type structure were crystallized above 550°C. At room temperature, the ferroelectric and dielectric properties were found to be $P_{\text{r } }=10$ μC/cm2, $E_{\text{c } }=100$ kV/cm, $\varepsilon_{\text{r } }=300$, and $\tan\delta<5$% for $x=0.5$ after annealing at 650°C. The films with $x=1.0$ and 1.5 did not exhibit ferroelectric hysteresis behavior because of the decrease in Curie temperature ($T_{\text{c } }$) below room temperature. The $x$ dependence of $T_{\text{c } }$ was studied by considering the soft mode behavior in Raman scattering spectra and the $T_{\text{c } }$ values were 400, $-25$, and $-220$°C for $x=0.5$, 1.0, and 1.5, respectively. The cosubstitution by Sr2+ and Nb5+ is effective for reducing ferroelectric interaction between electrical dipoles leading to a large shift in $T_{\text{c } }$. Because of its high solubility of Sr2+ and Nb5+ and efficiency for shifting $T_{\text{c } }$, the BSTN system may find a novel application as a dielectric material rather than as a ferroelectric material.

Ti-site ions in bismuth titanate (BIT) -based thin films were substituted by higher-valent ions than Ti⁴⁺ (i.e., V⁵⁺, Nb⁵⁺, Ta⁵⁺, W⁶⁺) in order to compensate the charge valence of oxygen vacancies in BIT crystal. Those films were fabricated using a chemical solution deposition (CSD) technique. Ion substitution using higher-valent ions did not only enhance the remanent polarization (Pr) of BIT-based thin films [e.g., La³⁺ - and Nd³⁺ -substituted BIT (BLT and BNT), etc.] but also lower the leakage current densities, while ion substituting using one of the same valent (Zr⁴⁺) did not improve the ferroelectric properties of BIT-based thin films. Pr values of V⁵⁺- and W⁶⁺-substituted BLT films (13 and 12 μC/cm², respectively) were larger than that of BLT film (8 μC/cm²), while Pr value of Zr4⁺-substituted BLT film (9 μC/cm²) was similar to that of BLT film. V⁵⁺-substitution lowered the leakage current density of BLT film from approximately 10^-7 down to 10^-5 A/cm².

Relationships among in-plane residual stress and some dielectric properties (remanent polarization, coercive field, dielectric permittivity, etc.) were investigated on ferroelectric thin films. Epitaxial PbTiO_3 thin films were deposited on Pt/(100)SrTiO_3 substrate by MOCVD. Residual stresses of the films were determined by a modified sin^2ψ method. The external stress in this study could be varied from +2.0 GPa up to +6.0 GPa. We first observed the change of the dielectric properties together with the tensile stress application in actual experiments; decrease of the remanent polarization from 11 μC・cm^&lt-2> down to 3 μC・cm^<-2> and of the coercive field form 115 kV・cm^<-1> down to 95 kV・cm^<-1>. The Curie temperature of the film was decreased from 450℃ to 437℃ with increasing the residual stress, while the dielectric permittivity at room temperature was measured to be approximately 210, irrespectively of the residual stress. Further, the tensile stress caused deformation of PbTiO_3 crystal lattice, which was described as the decrease of the c/a ratio. We conclude that the residual stress varied the dielectric properties by deforming the crystal lattice.

An analytical technique to determine residual stress in eqitaxial thin film by asymmetric X-ray diffraction (XRD) was studied. The residual stresses of PbTiO_3 films were determied by XRD technique and displace measurement technique. Measurement results by these techniques were compared with each other to consider the respective advantages of each technique. The stresses measured by XRD technique were not consistent with those by displacement measurement technique, because the latter technique involves errors which origined from the shape and the size of the substrate. The stress in a polycrystalline film measured by modified sin^2ψ method was in good agreement with that measured by normal sin^2ψ method. This result suggests that modified sin^2ψ method can be applied to stress measurement not only in epitaxial thin films, but also in polycrystalline thin films. We further discuss the precision of the stress determination technique. Residual stress of epitaxial PbTiO_3 film on (100) SrTiO_3 substrate measured by modified sin^2ψ method was comparable to the theoretical stress estimated from the difference in thermal expansion coefficients between the film and the substrate. Stress values measured by modified sin^2ψ method may thus have the precision required for actual application.

The effect of yttrium oxide (Y₂O₃) addition on the sintering of magnesium silicon nitride (MgSiN₂) powder has been examined. The sintering conditions examined in this paper were firing temperature (1400 and 1500°C), firing time (1 to 7 h), and the amount of Y₂O₃ (1 to 7 mass%). The phases present, microstructures, flexural strength, and thermal conductivity of the resulting sintered compacts were evaluated using X-ray diffractometry, scanning electron microscopy, flexural testing, and atomic force microscopy. When the MgSiN₂ compact without Y₂O₃ addition was fired at 1500°C for 1 h or more, it partly decomposed to form α- and β-silicon nitride (Si₃N₄). The formation of α- and β-Si₃N₄ upon firing at 1500°C for 3 h could be restricted by the addition of Y₂O₃ as a sintering aid; no α- and β-Si₃N₄ were detected with increasing Y₂O₃ amount up to 3 mass%; further increases in the amount of Y₂O₃ up to 7 mass%, however, produced Y2Si₃O3N₄. Maximum relative density (-90%) was attained when a MgSiN₂ compact with 3 mass% of Y₂O₃ addition was fired at 1500°C for 3 h. The flexural strength of MgSiN₂ compacts with 3 to 7 mass% of Y₂O₃ addition fired at 1500°C for 3 h attained 160 to 200 MPa. Thermal conductivities of the sintered MgSiN₂ compacts with 1 to 7 mass% of Y₂O₃ addition were in the approximate range of 15 to 18 W·m^-1·K^-1.

Magnesium silicon nitride (MgSiN_2) was synthesized by the nitridation of powders in the magnesium (Mg) -silicon (Si) system. Although an Mg and Si powder with an Mg/Si ratio of 1.0 ( = stoichiometric ratio of MgSiN_2) was heated up to 1400℃ in nitrogen (N_2) , a single phase of MgSiN_2 could not be obtained, due to evaporation of Mg during heating. On the basis of the information that Mg_2Si formed as an intermediate phase during the synthesis of MgSiN_2, an Mg_2Si powder was heated at a temperature between 600 and 1600℃ for 1h in N_2. Although MgSiN_2 and other phases such as Si and amorphous Mg_3N_2 were present in the temperature range of 900℃ to 1300℃, the powder heated at 1400℃ consisted of only MgSiN_2 phase. The chemical composition of the powder heated at 1400℃ for 1h was almost stoichiometric; the content of the oxygen was as low as 0.76 mol%. The powder consisted of agglomerates with sizes of 〜5μm; these agglomerates were furthermore composed of primary particles with an average size of 0.54 μm. The oxidation resistant temperature of MgSiN_2 was estimated to be 〜830℃. The MgSiN_2 powder had good resistivity to hydration.