Naoya Iizawa

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.

Processing and Properties of Rare Earth Ion-Doped Bismuth Titanate Thin Films by Chemical Solution Deposition method

Ferroelectric (Bi,R)4Ti3O12 [R: rare earth] thin films were synthesized by the chemical solution deposition (CSD) method. Heat treatment above 600°C was required for the fabrication of crystalline (Bi,R)4Ti3O12 thin films of Bi-layered perovskite structure on Pt/TiOx/SiO2/Si substrates. The orientation of the films prepared at 750°C depended upon the kind of substituent rare earth ions. Bi4.12Ti3O12 (BIT) and Bi3.35La0.75Ti3O12 (BLT) thin films revealed (00l) preferred orientation. On the other hand, Bi3.35Nd0.75Ti3O12 (BNT), Bi3.35Sm0.75Ti3O12 (BST) and Bi3.35Gd0.75Ti3O12 (BGT) thin films showed random orientation with strong (117) reflection. Synthesized (Bi,R)4Ti3O12 films exhibited the typical ferroelectric behavior. The BNT thin film showed Pr of 21.6 µC/cm2, which was higher than the values of the other rare earth-doped BIT thin films.

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.

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.

Fabrication and Properties of Ge-Doped (Bi,Nd)4Ti3O12 Thin Films by Chemical Solution Deposition

Nd- and Ge-codoped Bi4Ti3O12 thin films have been fabricated by chemical solution deposition. The effects of Ge doping in (Bi,Nd)4Ti3O12 on microstructural and ferroelectric properties were investigated for the development of low-temperature fabrication of ferroelectric Bi4Ti3O12-based thin films. Bi3.35Nd0.75Ti3-xGexO12 [0≤x≤0.5] thin films were found to crystallize into the Bi4Ti3O12 phase above 600°C without forming any second phase. The crystal orientation and surface morphology of the synthesized films strongly depended on the Ge content of Bi3.35Nd0.75Ti3-xGexO12. Among various compositions of Bi3.35Nd0.75Ti3-xGexO12, Bi3.35Nd0.75Ti2.9Ge0.1O12 thin films showed excellent surface morphology and ferroelectricity regardless of the processing temperature. Furthermore, the doping of Ge below x=0.1 was found to be effective in improving the ferroelectric properties of the low-temperature-processed films. The Pr values of Bi3.35Nd0.75Ti3-xGexO12 thin films with the optimum Ge content prepared at 600°C were above 15 µC/cm2 and equal to those of the films crystallized at 700°C.

Synthesis of (Bi,Nd)4(Ti,Ge)3O12 Thin Films by Chemical Solution Deposition

Ferroelectric (Bi,Nd)4Ti3O12-based thin films were synthesized by chemical solution deposition. In this study, Ge substitution for the Ti site in (Bi,Nd)4Ti3O12 was studied for the improvement of microstructural and ferroelectric properties of low-temperature-annealed thin films. Bi3.35Nd0.75Ti3-xGexO12 thin films crystallized into the Bi4Ti3O12 phase above 600°C. The surface morphology of Bi3.35Nd0.75Ti3-xGexO12 thin films was greatly improved by optimizing the amount of Ge substitution compared with that of nonsubstituted Bi3.35Nd0.75Ti3O12 films. In addition, the Bi3.35Nd0.75Ti2.9Ge0.1O12 thin films were found to show excellent ferroelectric properties after crystallization even at 600°C.

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.