Large remanent polarization of (Bi,Nd)4Ti3O12 epitaxial thin films grown by metalorganic chemical vapor deposition

Abstract

(104)-oriented ${\mathrm{Bi}}_4{\mathrm{Ti}}_3{\mathrm{O}}_{12},$ La-substituted ${\mathrm{Bi}}_4{\mathrm{Ti}}_3{\mathrm{O}}_{12}\mathrm{[(}{\mathrm{Bi}}_{\text{3.44}}{\mathrm{La}}_{\text{0.56}}){\mathrm{Ti}}_3{\mathrm{O}}_{12}]$ and Nd-substituted ${\mathrm{Bi}}_4{\mathrm{Ti}}_3{\mathrm{O}}_{12}\mathrm{[(}{\mathrm{Bi}}_{\text{3.54}}{\mathrm{Nd}}_{\text{0.46}}){\mathrm{Ti}}_3{\mathrm{O}}_{12}]$ films were epitaxially grown on $\text{(111)}{\mathrm{SrRuO}}_3\text{//(111)}{\mathrm{SrTiO}}_3$ substrates at 700 °C by metalorganic chemical vapor deposition. All deposited films showed strong (104) orientations. The values of the remanent polarization ${\mathrm{(P}}_r)$ and coercive field ${\mathrm{(E}}_c)$ of the (104)-oriented epitaxial $({\mathrm{Bi}}_{\text{3.54}}{\mathrm{Nd}}_{\text{0.46}}){\mathrm{Ti}}_3{\mathrm{O}}_{12}$ thin film were 25 μC/cm² and 135 kV/cm, respectively. This $P_r$ value was larger than that of the (104)-oriented $({\mathrm{Bi}}_{\text{3.44}}{\mathrm{La}}_{\text{0.56}}){\mathrm{Ti}}_3{\mathrm{O}}_{12}$ film: $P_r$ and $E_c$ values of the $({\mathrm{Bi}}_{\text{3.44}}{\mathrm{La}}_{\text{0.56}}){\mathrm{Ti}}_3{\mathrm{O}}_{12}$ were 17 μC/cm² and 145 kV/cm, respectively. These good ferroelectric properties of $({\mathrm{Bi}}_{\text{3.54}}{\mathrm{Nd}}_{\text{0.46}}){\mathrm{Ti}}_3{\mathrm{O}}_{12}$ films can be explained by a large tilting of ${\mathrm{TiO}}_6$ octahedra induced by the substitution of ${\mathrm{Nd}}^{\text{3+}},$ the ionic radius of which is smaller than that of ${\mathrm{La}}^{\text{3+}}.$ Moreover, this $P_r$ value is almost equal to that of commercially used lead zirconate titanate (PZT) films for nonvolatile ferroelectric random access memory (FeRAM) applications. These (104)-oriented epitaxial $({\mathrm{Bi}}_{\text{3.54}}{\mathrm{Nd}}_{\text{0.46}}){\mathrm{Ti}}_3{\mathrm{O}}_{12}$ thin films also showed a fatigue-free character. As a result, lead-free Nd-substituted ${\mathrm{Bi}}_4{\mathrm{Ti}}_3{\mathrm{O}}_{12}$ films with good ferroelectric properties comparable with those of PZT films are useful candidates for FeRAM applications.