X-ray photoelectron spectroscopy and high resolution electron microscopy studies of Aurivillius compounds:: Bi4-xLaxTi3O12(x=0, 0.5, 0.75, 1.0, 1.5, and 2.0)

In order to determine the fatigue-free origin of the ferroelectric Bi3.25La0.75Ti3O12, a series of x-ray photoelectron spectroscopy and high resolution electron microscopy studies were performed on the polycrystalline Bi4-xLaxTi3O12 (BLTx, x=0, 0.5, 0.75, 1.0, 1.5, and 2.0) powders. From the XPS study, the surfaces of all La-containing compounds are found to consist of one outermost Bi-rich region and followed by a La-rich region, instead of the chemical stoichiometry in the bulk. An HREM study on Bi3.25La0.75Ti3O12 further confirms that this surface configuration arises from some intergrowth defects with a thickness of 5 nm appearing on the crystal edge, which is also observed in compounds with x=1.0 and 2.0, but not in the poor fatigue-resistant Bi4Ti3O12 (x=0). This La-induced defect locally changes the chemical composition of the crystal surface, which probably possesses a different physical characteristic as compared to the bulk. Consequently, it could can be the physical nature of the interface, when in contact with the metal electrode, Pt, in ferroelectric nonvolatile memory. Since the fatigue phenomenon on a ferroelectric capacitor arises predominately from the pinning of domain walls on the metal-ferroelectric interface, the surface configurations of La-containing Bi4Ti3O12 compounds should be considered as one of the fatigue-free factors as well as the self-regulation of the Bi2O2 layer and the chemical stability of the perovskite slabs. In addition, the band gap of Bi3.25La0.75Ti3O12 is also estimated by UV absorption spectrum to be 3.9+/-0.1 eV. (C) 2002 American Institute of Physics.