Daichi Togawa

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.

Preparation and Properties of Bi4-xLaxTi3O12 Ferroelectric Thin Films Using Excimer UV Irradiation

Low-temperature processing of Bi4-xLaxTi3O12 (BLT) thin films was investigated by chemical solution deposition using an excimer UV irradiation, and their ferroelectric properties, crystallinity and microstructure were characterized. BLT thin films were prepared on Pt(200 nm)/TiO2(50 nm)/SiO2/Si substrates by a spin-coating technique from alkoxide precursor solutions. The excimer UV irradiation onto as-deposited BLT thin films was highly effective in removing organic species of the gel films, leading to the decrease of the crystallization temperature and an increase of the crystallinity. The UV-processed BLT films started to crystallize at 550°C and showed a high crystallinity and a high (117) preferred orientation for 600°C-annealed films. BLT thin films prepared at 600°C showed a homogeneous and dense microstructure with grain sizes of 200–300 nm. The excimer UV irradiation onto as-crystallized BLT thin films was also effective in improving the ferroelectric properties of the thin films. Bi3.35La0.75Ti3O12 thin films prepared at 600°C using excimer UV irradiation showed a well-saturated P–E hysteresis loop with a Pr of 9.8 µC/cm2 and Ec of 78 kV/cm. Moreover, the Bi3.35La0.75Ti3O12 thin films exhibited good fatigue endurance up to 1010 switching cycles.

Chemical Solution Processing and Properties of (Bi,Nd)4Ti3O12 Ferroelectric Thin Films

Low-temperature processing of (Bi,Nd)4Ti3O12 (BNT) thin films was investigated by a chemical solution deposition method, and their ferroelectric properties, crystallinity and microstructure were characterized. BNT thin films were prepared on Pt/TiOx/SiO2/Si substrates by a spin-coating technique using metal-organic precursor solutions. The BNT precursor films crystallized into the Bi4Ti3O12 (BIT) structure above 600°C. The synthesized BNT films exhibited high crystallinity with low (00l) preferred orientation. BNT thin films prepared at 600°C showed a homogeneous and dense microstructure with a grain size of 50–100 nm. They also showed a well-saturated P–E hysteresis curve with a Pr of 7.0 µC/cm2 and Ec of 83 kV/cm. The Nd-substitution for the Bi site in the BIT structure was found to be effective for promoting the (117) preferred orientation and for improving the ferroelectric properties of the resultant films.

Preparation and Properties of SrBi2Ta2O9 Ferroelectric Thin Films Using Excimer UV Irradiation and Seed Layer.

Low-temperature processing of SrBi2Ta2O9 (SBT) thin films on Pt(200 nm)/Ti(50 nm)/SiO2/Si substrates was investigated by the sol-gel method. The excimer UV irradiation onto as-deposited SBT thin films at 200–300°C in O2 atmosphere and the use of a Sr–Ta–O seed layer were very effective in lowering the crystallization temperature for SBT thin films to 500°C. The SBT thin films prepared by an excimer UV process and subsequent rapid thermal annealing process with the seed layer showed a homogeneous and smooth surface microstructure with fine grains of approximately 50 nm in size. The 600°C-annealed SBT thin films of approximately 200 nm thickness with the seed layer exhibited a Pr of 2.3 µC/cm2 and a Ec of 43 kV/cm.

Excimer UV Processing of (Bi,Nd)4Ti3O12 Ferroelectric Thin Films by Chemical Solution Deposition Method

Ferroelectric (Bi,Nd)4Ti3O12 (BNT) thin films were prepared on Pt/TiOx/SiO2/Si substrates by the chemical solution deposition (CSD) method. Organic species of as-deposited BNT precursor films were completely removed using excimer UV irradiation at 300°C. Synthesized BNT films using excimer UV irradiation showed a random orientation with a strong (117) reflection and a dense microstructure having grain sizes of 100–150 nm and a smooth surface. Excimer UV irradiation was very effective in the improvement of surface morphology of the BNT thin films. BNT thin films crystallized at a low temperature of 550°C through the excimer UV irradiation process showed a well-saturated P-E hysteresis loop with a Pr of 16.1 µC/cm2 and Ec of 102 kV/cm. The ferroelectric properties of BNT thin films were greatly improved by irradiating excimer UV onto as-deposited BNT precursor films.

Preparation and Properties of V-Doped (Bi,Nd)4Ti3O12 Ferroelectric Thin Films by Chemical Solution Deposition Method

Vanadium-doped (Bi,Nd)4Ti3O12 (BNTV) ferroelectric thin films were prepared on Pt/TiO x /SiO2/Si substrates by a chemical solution deposition method. BNTV precursor thin films crystallized as a single phase at 600°C and showed a higher crystallinity than BNT thin films. BNTV thin films consisted of a dense microstructure with a uniform grain size of approximately 160 nm. 600°C-annealed BNTV thin films exhibited a well-saturated P-E hysteresis loop. The values of P r and E c for the BNTV films were 15 μC/cm2 and 111 kV/cm, respectively. The polarization values of non-doped BNT thin films decreased by 30% after the fatigue measurement with 1010 switching cycles, while those of BNTV thin films decreased by 18%. The BNTV thin films were found to show the fatigue and leakage current properties superior to the non-doped BNT thin films.