Role of Ferroelectric Layer Thickness in Resistive Switching and Depolarization Effects in Hf₀.₅Zr₀.₅O₂-Based Structures

Abstract

The development of low-power and high-density ferroelectric memories requires scaling the thickness of the functional layer. However, the thickness could have a significant impact on the performance of memory devices. This work examines the effect of ferroelectric layer thickness on resistive switching and depolarization phenomena in a metal-Hf0.5Zr0.5O2-Si (MFS) structure, which is the functional structure of both ferroelectric field-effect transistors (FeFETs) and ferroelectric tunnel junctions (FTJs). The MFS structures show one stable and another unstable polarization state, with the instability caused by the strong electric field produced by charged donor surface states at the Si interface and poor screening of this polarization. The rate of the unstable state depolarization slows down with increasing thickness from 5 to 10 nm, which is due to the decrease in the depolarization field at the same potential difference in the structure. The resistive effect increases with increasing thickness up to ${R}_{\text {OFF}}$ / ${R}_{\text {ON}}=24$ , which is related to the reduction of undesired conductivity along the grain boundaries of the polycrystalline Hf0.5Zr0.5O2 film. The temporal dynamics of the depolarization of the unstable polarization state and the resulting gradual switching off of the low-resistance state are close to the characteristic times of the weight change of biological synapses, and therefore, such ferroelectric memristors are suitable for emulating their behavior. The results may be useful for the development of building blocks for neuromorphic computing as well as a new generation of FeFET and FTJ-based memories.