Improved electric behaviors of the Pt/Bi_1-xLa_xFe _0.92Mn_0.08O₃/n⁺-Si heterostructure for nonvolatile ferroelectric random-access memory

Multiferroic BiFeO₃ (BFO) and lanthanum-substituted Bi _1-xLa_xFe_0.92Mn_0.08O₃ (BLFMx, 0 ≤ x ≤ 0.2) films have been directly deposited on heavily doped Si(100) with an electric resistivity of about 0.001 Ω cm. The La substitution effects on the microstructure and lattice dynamics of the BLFMx films have been investigated by X-ray diffraction (XRD), scanning electron microscopy, far-infrared reflectance and Raman scattering studies. XRD analysis shows that the rhombohedral structure of BFO films reduced toward the orthorhombic or tetragonal structure by Mn and La substitution. It was found that the leakage current density tended to decrease with increasing La composition in a low electric field because the La dopant can suppress the formation of oxygen vacancies by stabilizing the oxygen octahedron and controlling the volatility of Bi atoms. On the other hand, the leakage current density in the high electric field can be suppressed by Mn substitution due to compensation of the charge of Fe²⁺ ions. The well-saturated polarization hysteresis can be obtained in the Pt/BLFMx/n⁺-Si prototype devices. As an example, the electric remanent polarization (2P _r) and coercive field (2E_c) at the electric field of 1600 kV cm^-1 for the BLFM0.15 film are 150 μC cm^-2 and 870 kV cm^-1, respectively. Moreover, its relative dielectric constant at the frequency of 3.5 × 10⁵ Hz is about 125. These results could be crucial for future applications of silicon-based nonvolatile ferroelectric random-access memory.