Study on Substitution Effect of Bi4Ti3O12 Ferroelectric Thin Films

Nowadays, ferroelectric thin films have attracted considerable attention because of their potential uses in device applications, such as sensors, micro electro-mechanical system (MEMS) and nonvolatile ferroelectric random access memory (NvFRAM) especially (Scott & Paz De Araujo, 1989; Paz De Araujo et al., 1995; Park et al. 1999). Lead zirconate titanate [PbZrxTi1-xO3 (PZT)] ferroelectric thin film is an early material for NvFRAM. PZT and related ferroelectric thin films, which are most widely investigated, usually have high remanent polarization (Pr). However, they are generally suffered from a serious degradation of ferroelectric properties with polarity switching, when they are deposited on platinum electrodes. Bismuth-layered perovskite ferroelectric thin films, with the characteristics of fast switching speed, high fatigue resistance with metal electrodes, and good retention, have attracted much attention. Bismuth titanate [Bi4Ti3O12 (BIT)] is known to be a typical kind of layerstructured ferroelectrics with a general formula (Bi2O2)2+(Am-1BmO3m+1)2-. Its crystal structure is characterized by three layers of TiO6 octahedrons regulary interleaved by (Bi2O2)2+ layers. At room temperature the symmetry of BIT is monoclinic structure with the space group B1a1, while it can be considered as orthorhombic structure with the lattice constant of the c axis (c = 3.2843 nm), which is considerably larger than that of the other two axis (a = 0.5445 nm, b = 0.5411 nm). The BIT has a spontaneous polarization in the a-c plane and exhibits two independently reversible components along the c and a axis (Takenaka & Sanaka, 1980; Ramesh et al., 1990). It shows spontaneous polarization values of 4 and 50 μC/cm2 along the c and a axis respectively. The ferroelectric properties of these bismuth layer-structured thin films are mostly influenced by the orientation of the films (Simoes et al., 2006). The BIT thin film is highly c-axis oriented, thus its spontaneous polarization is much lower than that for a-axis oriented (Fuierer & Li, 2002). For applications in NvFRAM devices, ferroelectric materials should have high remanent polarization, low coercive field (Ec), low fatigue rate and low leakage current density. However, BIT thin film has much lower values of switching polarization and suffers from poor fatigue endurance and high leakage current as a result of the internal defects (Uchida et al., 2002). Numerous works have been made to substitute BIT thin film with proper ions to optimize the ferroelectric properties. In recent years, it was reported that some A-site or B-site substituted BIT showed large remanent polarizations. In the case of A-site substitution in BIT, La-substituted BIT