Growth of ferroelectric bismuth lanthanum nickel titanate thin films by rf magnetron sputtering

摘要

The epitaxial growth, structural properties, and ferroelectric properties of bismuth lanthanum nickel titanate (Bi_(1-x)La_x) (Ni_(0.5)Ti_(0.5))O_3 (BLNT) thin films deposited on Pt(100)/MgO(100) substrates by rf magnetron sputtering have been investigated using x-ray diffraction, transmission electron microscope, and polarization-electric field hysteresis loop measurements. The ferroelectric BLNT(00l) phase with c-axis orientation and a single-phase tetragonal perovskite structure appeared at x ≥ 0.3. The tetragonality (c/a) increased significantly from 1.004 to 1.028 with increasing La content. The fabricated BLNT films of x ≥ 0.3 indicated the apparent fourfold rotational symmetry observed for a MgO(202) substrate, a bottom Pt(202) electrode, and a BLNT(101) ferroelectric film, based on φ scan measurements. These results imply that the present La-substituted BLNT films are grown heteroepitaxially at x ≥ 0.3. It was confirmed that Bi in the BLNT films is in a trivalent state at A sites in the perovskite crystal, based on x-ray anomalous diffraction measurements and x-ray absorption near edge structure spectra. The sputtering technique using compacted powder targets designed by taking the bond dissociation energy of metal oxides into account provided epitaxial perovskite-structured BLNT thin films on Pt(100)/MgO(100) substrates. It is shown that the c-axis oriented epitaxial BLNT film exhibits a hysteresis loop shape with a P_r value of 12 μC/cm~2 that is comparable to typical high-performance Bi_(3.25)La_(0.75)Ti_3O_(12) (BLT) film.