Hiroshi Maiwa

Electromechanical properties of Nd-doped Bi4Ti3O12 films: A candidate for lead-free thin-film piezoelectrics

Neodymium-doped Bi4Ti3O12 (BNT) films are evaluated for use as lead-free thin-film piezoelectrics in microelectromechanical systems. Bi4Ti3O12, Bi3.25La0.75Ti3O12, and Bi3.25Nd0.75Ti3O12 films were fabricated by chemical solution deposition on Pt/TiOx/SiO2/Si substrates. Nd substitution promoted random orientation with low (00l) diffraction peaks. The 1-μm-thick Bi3.25Nd0.75Ti3O12 film annealed at 750 °C exhibited a remanent polarization of 26 μC/cm2. Typical butterfly field-induced strain loops were obtained in the BNT film capacitors. The electrically induced strain is 8.4×10−4 under the bipolar driving field of 220 kV/cm. These results show that BNT is a promising candidate for use in lead-free thin-film piezoelectrics.

Film Thickness Dependence of Dielectric Properties of BaTiO3 Thin Films Prepared by Sol-Gel Method

Ferroelectric BaTiO3 thin films were prepared on Pt(111)/ SiO2/Si(100) substrates at 650° C by the sol-gel method. Film thickness could be varied by repeating a dip coating/heating cycle. The texture of the BaTiO3 thin films became more dense and homogeneous when the film thickness increased. The crystalline thin films showed microstructure with grains as small as 20–30 nm. The dielectric properties could be measured for films thicker than 0.25 µ m. With increasing film thickness, the dielectric constant and remanent polarization increased and the coercive field decreased. Loss tangent of the thin films was independent of the film thickness. BaTiO3 thin film with a thickness of 0.58 µ m exhibited a dielectric constant of 1000, remanent polarization of 8 µ C/cm2 and coercive field of 30 kV/cm.

Lead-Free Piezoelectric (K,Na)NbO3 Thin Films Derived from Metal Alkoxide Precursors

Lead-free piezoelectric K0.5Na0.5NbO3 thin films for microelectromechanical systems were fabricated via chemical solution process using metal alkoxide. Perovskite K0.5Na0.5NbO3 (KNN) single-phase thin films with good leakage current properties were successfully prepared by optimizing the KxNaxNbO3 (x0.5) composition of the precursor solution. The KNN thin films prepared from the solution with K0.55Na0.55NbO3 composition showed typical ferroelectric P–E hysteresis and field-induced strain loops. The 2Pr and 2Ec values of the K0.55Na0.55NbO3 films were 14 µC/cm2 and 140 kV/cm, respectively. From the slope of the field-induced butterfly loop, the effective d33 was found to be 46 pm/V.

Preparation and Properties of Ru and RuO2 Thin Film Electrodes for Ferroelectric Thin Films

Ru and RuO2 thin films were prepared by RF magnetron sputtering. The crystalline structure and electrical properties of these films were investigated. Reactive sputtering of a ruthenium metal target was carried out in a mixed gas of argon and oxygen at 580° C. Total pressure (P O2 +P Ar) was controlled to maintain a constant pressure of 1 Pa. The RuO2 single phase could be obtained when P O2 was 0.5 Pa and the deposition rate was about 30 nm/min. The resistivity of the 290 nm-thick RuO2 thin films deposited on SiO2(1000 nm)/Si was 54.9 µ Ω cm. However, the resistivity increased with decreasing film thickness. Fatigue resistance of (Pb, La)TiO3 thin films with RuO2 as top and bottom electrodes was improved compared to the films on Pt electrodes.

Dielectric and Electromechanical Properties of Ba(ZrxTi1-x)O3 (x = 0.1 and 0.2) Ceramics Prepared by Spark Plasma Sintering

Ba(ZrxTi1-x)O3 (BZT; x = 0.1 and 0.2) ceramics are prepared by spark plasma sintering (SPS) and conventional sintering. By normal sintering, the density, grain size, and dielectric constant of the ceramics increase with sintering temperature. The temperature dependence of the dielectric constant is enhanced by sharp dielectric anomaly in the ceramics sintered at high temperature. By the application of SPS, ceramics with more than 96% relative densities could be obtained by sintering at 1100 °C for 5 min in air atmosphere. Grain growth is suppressed in the ceramics prepared by SPS. The average grain size obtained was less than 1 µm. The BZT ceramics obtained by SPS and the subsequent annealing at 1000 °C for 12 h exhibit a mild temperature dependence of their dielectric constant, which was maintained at high nominal values at approximately room temperature. The field-induced displacement of the BZT ceramics is less hysteretic and smaller than that of the ceramics sintered by the conventional method.

Preparation and properties of ferroelectric BaTiO3 thin films by sol-gel process

Ferroelectric BaTiO3 thin films have been successfully prepared by sol-gel process using barium and titanium isopropoxides as precursor solutions for dip-coating. Homogeneous BaTiO3 thin films have been obtained on various substrates by preheating gel films at 120°C in a flow of oxygen/water vapor mixed gas, and successive heating at 650°C in oxygen atmosphere. The introduction of water vapor during preheating is very effective for the crystallization of BaTiO3 at relatively low temperatures. These films exhibit good crystallinity and smooth surfaces with grains as small as about 50 nm. BaTiO3 thin films prepared on Pt/SiO2/Si substrates with a thickness of about 0.5 µm show dielectric constants of 800~1000 and loss tangents of 0.07~0.09 at 1 kHz. Ferroelectric hysteresis loops were observed for these thin films which possessed the remanent polarization of 8 µC/cm2 and the coercive field of 25 kV/cm.

Preparation and Properties of BaTiO3 Ceramics by Spark Plasma Sintering

BaTiO3 (BT) ceramics are prepared by spark plasma sintering (SPS) and conventional sintering. In conventional sintering, the density and grain size of the ceramics increase with sintering temperature. In SPS, ceramics with more than 97% relative densities can be obtained by sintering at 900 °C for 5 min in air atmosphere. Compared with conventional sintering, SPS suppresses grain growth in ceramics. The average grain size of the BT ceramics obtained by SPS and subsequent annealing at 1000 °C for 12 h is less than 1 µm. The ceramics exhibit a high relative dielectric constant of 6020. The field-induced displacement of the BT ceramics is larger than that of the ceramics sintered conventionally. The dynamic strain/field at 15 kV/cm was 540 pm/V.

Temperature dependence of the electrical and electromechanical properties of lead zirconate titanate thin films

The temperature dependence of the electrical and electromechanical properties of Pb(Zr,Ti)O3 (PZT) (Zr/Ti=30/70, 52/48, and 70/30) thin films are investigated. The 1-μm-thick PZT thin films were prepared on (111)Pt/Ti/SiO2/Si substrates by chemical solution deposition. The temperature dependence of the dielectric and ferroelectric properties of the films were measured in the temperature range from −250 to 150 °C. The temperature-dependent strain loops from PZT films were measured using scanning probe microscopy in the temperature range from −100 to 150 °C. The field-induced displacement increased with increase of the temperature; however, their temperature dependence was relatively small, compared with that reported for bulk PZT ceramics. The results indicate that a thermally activated extrinsic ferroelastic contribution is not predominant in thin PZT films.

Fatigue and Refreshment of (Pb, La)TiO3 Thin Films by Multiple Cathode Sputtering

The ferroelectric fatigue and refreshment of (Pb, La)TiO3 thin films by multiple cathode rf-magnetron sputtering were investigated. The remanent polarization of the fatigued (Pb, La)TiO3 thin films increased after the application of the DC bias voltage of -20 V for 104 s. By heat treatment of the fatigued films at 400° C for 1 h, the coercive field decreased, and squareness of the loop was improved. These behaviors were discussed from the viewpoint of the behavior of defect dipoles.

Measurement of Piezoelectric Displacements of Pb(Zr, Ti)O3 Thin Films Using a Double-Beam Interferometer

The double-beam interferometric method is applied to measure the field-induced displacement of Pb(Zr, Ti)O3 thin films. The dc electric field dependence of the longitudinal piezoelectric coefficient (d33) response of Pb(Zr, Ti)O3 thin films deposited by metal organic chemical vapor deposition (MOCVD) was measured. Experimental d33 values were compared with coefficients calculated using a phenomenological approach and bulk parameters. Qualitative agreement was obtained between measured and calculated coefficients.