SrBi_2Ta_O_9 (SBT) thin films were epitaxially grown on SrTiO_3 single crystals by metal organic chemical vapor deposition (MOCVD) . c-axis-oriented SBT film was deposited on (100) SrTiO_3 substrate above 620℃. On the other hand, (1 16)-oriented SBT film was grown on (110) SrTi03 substrate at 820℃. The remanent polarization and the coercive field of (116)-oriented SBT film were 11.4μC/cm^2 and 80 kV/cm, respectively. On the other hand, the (001) -oriented SBT film showed no ferroelectricity. These ferroelectric annisotropy of SBT agreed with the estimation from the crystal anysotropy of SBT.
Superlattices of transition metal oxides have been constructed by a laser Molecular Beam Epitaxy (MBE) technique combinded with RHEED monitoring system. Not only ferroelectric superlattices but also magnetic superlattices have been constructed on SrTiO_3(100), (110) and (111) surfaces. These surface selections enables us to control the three dimensional positioning of atoms and spins. In the case of LaCrO_3/LaFeO_3 superlattices on (111) surface, ferromagnetism has been realized for the first time. In the case of LaMnO_3/LaFeO_3 superlattices, on the other hand, spin frustration is expected competing with ferromagnetism of self-doped LaMnO_3 and antiferromagnetism of LaFeO_3 on SrTiO_3 (100) surface. Spin glass phase transition (casp shape in magnetization vs. temperature measurements) is observed in the case of LaMnO_3/LaFeO_3 superlattices. It is due to the spin frustration between ferromagnetic couping in LaMnO_3 layer and antiferromagnetic coupling in LaFeO_3 Iayer. This behavior is quite different from that of non-doped LaFeO_3/LaCrO_3 superlattices.
Transparent and highly oriented Ba_2NaNb_5O_<15> (BNN) thin films have been prepared by using metal alkoxides. A homogeneous precursor solution was prepared by the controlled reaction between Nb (OC_2H_5)_65, NaOC_2H_5 and barium metal . The BNN precursor included a molecularlevel mixture of NaNb (OC_2H_5)_6 and Ba[Nb (OC_2H_5)_6]_2 in ethanol . The alkoxy derived powder crystallized to a low-temperature phase, and then transformed to orthorhombic BNN (tungsten bronze) at 600'C. BNN precursor films on substrates crystallized to orthorhombic BNN at 800℃ via the low temperature phase. The tungsten bronze phase of BNN thin films was confirmed by Raman spectroscopy. Highly(002) oriented BNN films of tungsten bronze structure were successfully prepared on MgO(100) and Pt(100)/MgO(100) substrates at 700℃ by using BNN underlayer. Transparent BNN thin films on MgO and fused silica substrates showed the absorption edge at 370 nm. The maximum of dielectric constant was observed at around 540'C . The remanent polarization and coercive field of the BNN film (thickness, 1.0μm) crystallized at 700℃ were 12.3μC/cm^2 and 101 kV/cm, respectively, at 123 K. BNN films on fused silica substrates exhibited second harmonic generation upon irradiation with 1064 nm light.
Thin films of ferroelectric compounds such as Bi-based layer-structured perovskite (SrBi_2Ta_2O_9, CaBi_2Ta_2O_9, and BaBi_2Ta_2O_9) and Sr_2Nb_2O_7 family (Sr_2Nb_2O_7, Sr_2 Ta_2O_7, and Sr_2(Ta, Nb)_2O_7) were synthesized by using structure-controlled precursor solutions. Low temperature crystallization of the Bi-based layer-structured perovskite thin films was attained and the technique would be applied to fabrication of ferroelectric non-volatile memories . The Sr_2Nb_2O_7-family thin films exhibiting the relatively low dielectric constants have been considered to be good candi-dates for the memories with MFMIS (metal-ferroelectric-metal-insulator-semiconductor) structure. However, some crystallographic properties were clarified to imply the difficulty of the application.
In recent years, photocatalysts which show catalytic action on irradiation have attracted much attention from the ecological view points. In this review article , we briefly present (1) the definition and types of the photocatalyst, (2) the mechanism, (3) the applications and then (4) the characteristics of the sol-gel derived photocatalytic films.