内田 寛

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.

本研究ではV5+やW6+などの高価数イオン置換によりチタン酸ビスマス(BIT)基薄膜の強誘電特性を改善することを目的とし,化学溶液法を用いた薄膜組成の制御により置換イオン種および置換量の最適化を検討する.ランタン置換チタン酸ビスマス薄膜Bi3.25La0.75Ti3.00O12 (BLT)の残留分極(Pr)は8μC/cm2であったが, V5+およびW6+を置換したBLT薄膜[Bi3.24La0.75Ti2.97V0.03O12 およびBi3.23La0.75Ti2.97W0.03O12 (V-およびW-BLT)]のPrはそれぞれ13および12μC/cm2まで向上した.一方, Zr4+を置換したBLT薄膜Bi3.25La0.75Ti2.97Zr0.03O12(Zr-BLT)のPrはBLTのものとほぼ同じ値(9 μC/cm2)であり, 同価数イオン置換による特性改善の影響は認められなかった. 更に, BIT基薄膜における高価数イオン置換は残留分極値の向上だけでなく,漏れ電流量の低減にも寄与することが確認された.

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.

窒化ケイ素マグネシウム (MgSiN₂) の焼結に及ぼす酸化イットリウム (Y₂O₃) 添加の影響を検討した.本論文で検討した焼結条件は, 焼成温度 (1400℃および1500℃), 焼成時間 (1～7h), およびY₂O₃添加量 (1～7mass%) であった.MgSiN₂単味成形体を1500℃で1h焼成したところ, MgSiN₂は部分的に熱分解しα一窒化ケイ素 (Si₃N₄) とβ一Si₃N₄が生成した.一方, 焼結助剤としてY₂O₃を3mass%MgSiN₂に添加したところ, 1500℃で3h焼成してもMgSiN₂の熱分解が起こらなかった.Y₂O₃の添加量を3～7mass%まで増加させると, Y2Si₃O₃N₄の生成と生成量の増加が認められた.MgSiN₂成形体を1500℃で3h焼成したところ, Y₂O₃無添加の場合の相対密度は約67%であったが, 3mass%Y₂O₃添加の場合には約90%となった.Y₂O₃の添加量を3～7mass%まで増加させると, MgSiN₂焼結体の曲げ強度は160MPaから200MPaまで増加したが, 熱伝導率は15～18W・m-1・K-1の範囲であった.

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.