The effects of donor Nb$^{5+}$-ion doping on the structural, electrical, and multiferroic properties of an Aurivillius Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$ thin film were investigated. Incorporation of Nb$^{5+}$-ions into the Ti$^{4+}$-sites of the Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$ thin film resulted in a substantial improvement of its electrical and multiferroic properties. From the study of the electrical properties, the Bi$_{7}$Fe$_{3}$Ti$_{2.94}$Nb$_{0.06}$O$_{21+\delta}$ thin film exhibited a low leakage current density of 5.11$\times$10$^{-6}$ A/cm$^{2}$ at 100 kV/cm, which was about one order of magnitude lower than that of the untreated Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$ thin film. The ferroelectric $P-E$ hysteresis loops of the Bi$_{7}$Fe$_{3}$Ti$_{2.94}$Nb$_{0.06}$O$_{21+\delta}$ thin film showed a large remnant polarization (2$P_{r}$) of 20.6 $\mu$C/cm$^{2}$ at 630 kV/cm whereas the 2$P_{r}$ value measured for the untreated Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$ thin film was 3.5 $\mu$C/cm$^{2}$ at 318 kV/cm. Furthermore, a well-saturated magnetic hysteresis loop with an enhanced magnetization was observed for the Bi$_{7}$Fe$_{3}$Ti$_{2.94}$Nb$_{0.06}$O$_{21+\delta}$ thin film at room temperature.

Keywords: Nb-doped Bi$_{7}$Fe$_{3}$Ti$_{3}$O$_{21}$ thin film, Chemical solution deposition, Electrical properties, Multiferroic properties