Shuichiro Kuwajima

Ferro- and piezoelectric properties of vinylidene fluoride oligomer thin film fabricated on flexible polymer film

The ferro- and piezoelectric properties of vinylidene fluoride oligomer thin films fabricated on flexible polymer substrates were investigated under mechanical bending. Rectangular electric-displacement–electric-field hysteresis curves were observed for a polyimide substrate, and the remanent polarization and coercive electric field were unaffected on one-dimensional bending with a radius of curvature down to 10mm. The piezoelectric charge generated by the bending and restoring process was measured and found to be dependent on the radius of curvature. These results indicate the possibility using the vinylidene fluoride oligomer in the construction of both flexible nonvolatile memories and curvature sensors.

Piezoelectric Properties of Epitaxial NaNbO3 Thin Films Deposited on (001)SrRuO3/Pt/MgO Substrates

Sodium niobate (NaNbO3, NN) thin films were deposited on (001)SrRuO3/Pt/MgO substrates by rf magnetron sputter deposition. The X-ray diffraction (XRD) pattern of the sputtered NN thin films showed epitaxial growth with a (001)-oriented perovskite structure. From the reciprocal space maps, the lattice parameters of the in-plane and out-of-plane directions were a=0.392 nm and c=0.395 nm, respectively. The relative dielectric constant er and the range of the dielectric loss tan δ were about 270 and 0.05–0.13 at 1 kHz, respectively. The P–E hysteresis loop showed clear ferroelectricity with spontaneous polarization Ps of 20 µC/cm2 and coercive electric field Ec of 50 kV/cm. The transverse piezoelectric properties were evaluated from the tip displacement of NN/MgO unimorph cantilevers. The transverse piezoelectric coefficient e31* (=d31/s11E) was -0.9 C/m2, which is almost compatible with that of the bulk NN ceramics. The tip deflection showed typical butterfly curves owing to the polarization reversal, indicating that the sputtered NN thin films had a high piezoelectric performance.

Piezoelectric properties of (K,Na)NbO 3 thin films deposited on (001)SrRuO 3 /Pt/MgO substrates

Thin films of Pb-free single crystal (K_xNa_1-x)NbO₃, KNN, were epitaxially grown on (001)SrRuO₃ / (001)Pt / (001)MgO substrates by rf-magnetron sputtering. In the pseudo-tetragonal system, c-lattice parameters of the sputtered KNN films were around 1% longer than those of the bulk KNN. The sputtered KNN thin films showed bulk like dielectric properties and/or ferroelectricity. Dielectric constant epsiv_r spontaneous polarization P_s, and coercive electric field E_c of KNN (x= 0.1: (K_0.1, Na_0.9)NbO₃) were 300, 20 [muC/cm²] and 50 [kV/cm], respectively. The piezoelectric coefficient e₃₁ of NaNbO₃ (NN) films was e₃₁*= -0.7 [C/m²], which was 60% of e₃₁ of bulk NN ceramics. On the other hand, the piezoelectric properties increased with the doping of K and KNN (x=0.1) showed relatively large e₃₁* of-2.4 [C/m²].

Effect of ferroelectric/metal interface structure on polarization reversal

The ferroelectric behaviors of vinylidene fluoride (VDF) oligomer thin films deposited on various bottom electrode materials (i.e., Au, Pt, Al, Al2O3/Al, and Al2O3/Au) were investigated. The coercive electric fields and full width at half maximum (FWHM) of the switching current peak of VDF oligomer thin films on the Au and Pt bottom electrodes were respectively lower and narrower than those of Al. These results indicate that the native oxide layer Al2O3 on the Al electrode surface works as a linear capacitor on ferroelectric capacitors. However, ferroelectric capacitors using Au and Pt bottom electrodes have the tendency to easily break down with the sudden application of an electric field. Their polarization switching and fatigue lifetime are affected by electrode material and the ferroelectric/metal interface.

Piezoelectric properties of (K, Na)NbO 3 thin films deposited on (001)SrRuO 3 /Pt/MgO substrates

(K_x,Na_1-x)NbO₃ (KNN) thin films were deposited on (001)SrRuO₃/(001)Pt/(001)MgO substrates by RF-magnetron sputtering, and their piezoelectric properties were investigated. The X-ray diffraction measurements indicated that the KNN thin films were epitaxially grown with the c-axis orientation in the perovskite tetragonal system. The lattice constant of the c-axis increased with increasing concentrations of potassium. The KNN thin films showed typical ferroelectric behavior; the relative dielectric constant epsiv_r was 270 ~ 320. The piezoelectric properties were measured from the tip displacement of the KNN/MgO unimorph cantilevers; the transverse piezoelectric coefficient e₃₁* (= d₃₁/s₁₁ ^E) of KNN (x = 0) thin films was calculated to be -0.9 C/m². On the other hand, doping of potassium caused an increase in the piezoelectric properties, and the KNN (x = 0.16) films showed a relatively large transverse piezoelectricity of e₃₁* = -2.4 C/m².

Flexible programmable logic gate using organic ferroelectric multilayer

In order to simplify the design and to further improve the integration, programmable, three-dimensional circuits are required alongside the development of fine process technology. We demonstrate the operation of programmable logic gates using organic ferroelectrics. Three-dimensionally stacked organic ferroelectric capacitors were assembled, which form logic elements with two inputs and one output. By controlling the directions of the electric dipoles, we have demonstrated the logical “and” and “or” operations using our device.

Development of Nonvolatile Memory using Well-Ordered Ferroelectric Linear Molecules

Ferroelectric vinylidene fluoride (VDF) molecular films were fabricated by simple vacuum evaporation method, and the ferroelectric properties and its fatigue were investigated. Formation of ferroelectric phase in VDF oligomer with low molecular weight is favored at low substrate temperature around -150°C. The well-ordered VDF oligomer thin films exhibit a lager value of remanent polarization(130mC/m 2 ) than that of Poly(VDF). Fatigues of polarization reversal can be performed over 10∧5 cycles. The VDF oligomer films can be one of candidates for disposable nonvolatile memory with unique features such as flexible, wide areas and low cost processing.

Switching characteristics in the ferroelectric organic molecular memories

Organic molecular memories are expected to become one of the essential key devices in the next-generation flexible digital instruments, and in this paper a novel non-volatile organic memory will be introduced. It is shown that the polarization switching characteristics in the ferroelectric films of vinylidene fluoride (VDF) oligomer and P(VDF/TrFE) copolymer change with their structural factors such as crystallinity and chain length of the materials, and the switching time becomes shorter and shorter with increasing applied electric field and reaches in the order of nano-second. Moreover, the possibilities of the application of such thin organic ferroelectric films to an infrared image sensor as well as the creation of the ultra high-density molecular memory by utilizing recent scanning probe microscopy (SPM) technique will be discussed.