Nb2O5 Interlayer Engineering for Switchable Polarization and Tunneling Electroresistance in HZO-Based Ferroelectric Tunnel Junctions

Hf1–xZrxO2 (HZO)-based ferroelectric tunnel junctions (FTJs) enable polarization-controlled resistance switching at nanometer-scale thicknesses, offering potential for nonvolatile memory and multilevel conductance operation. In metal–ferroelectric–insulator–ferroelectric–metal (MFIFM) structures, highly insulating interlayers such as Al2O3 can suppress leakage at the expense of switchable polarization and low-resistance-state (LRS) transport. Here, Nb2O5 is explored as an alternative interlayer with a relatively low electron-transport barrier, and Al2O3- and Nb2O5-inserted MFIFM FTJs with identical HZO layer thicknesses and total dielectric-stack thicknesses are directly compared. Replacing Al2O3 with Nb2O5 increases the positive-up–negative-down (PUND)-derived 2Pr from 17.2 to 27.4 μC/cm2, accompanied by a higher fitted orthorhombic-phase contribution and a higher out-of-plane texture index. The Nb2O5-inserted device also exhibits a higher tunneling electroresistance (TER) ratio than its Al2O3-inserted counterpart (60 vs 36 at a read voltage of 1.2 V), accompanied by a larger relative increase in LRS current than in high-resistance-state (HRS) current. The Nb2O5-inserted device further exhibits a higher breakdown voltage than the Al2O3-inserted counterpart, maintains distinguishable polarization states up to approximately 107 switching cycles, and demonstrates repeatable overall multilevel conductance-update behavior over 20 consecutive cycles, despite variations in individual intermediate conductance levels. These results establish interlayer selection as an effective strategy for modulating switchable polarization, resistance-state transport, and electrical reliability in HZO-based MFIFM FTJs.