The physical and electrical characterizations of Cr-doped BiFeO₃ ferroelectric thin films for nonvolatile memory applications

Metal-ferroelectric (Cr-substituted BiFeO₃)-insulator (HfO₂)-semiconductor (MFIS) structures have been fabricated by co-sputtering technique. The crystalline phases of Cr³⁺ substitution at Fe-site for BiFeO₃ (BFO) films were investigated by XRD patterns at the postannealing temperatures of 500 °C, 600 °C, and 700 °C. The microstructure of Cr-substituted BiFeO₃ thin film and interfacial layer between HfO₂ and Si substrate were characterized by TEM. The memory window as functions of insulator film thickness and DC power for Cr was investigated. The maximum memory window is 1.7 V when the HfO₂ thickness increases to 60 nm. The ferroelectric polarization increases with increasing substitution amount due to the reduction in charge injection effect. The result is consistent with the leakage current measured. As the substitution amount increases or postannealing temperature decreases, the surface roughness becomes smooth due to less crystallization of Cr-substituted BiFeO₃.