Impedance Investigation of MIFM Ferroelectric Tunnel Junction using a Comprehensive Small-Signal Model

The urge to develop efficient and ultra-low power architectures for modern and future technological needs lead to an increasing interest and investigation of neuromorphic and ultra-low power computing. In this respect, ferroelectric technology is found to be a perfect candidate to guide this technological transition. Elucidating the physical mechanisms occurring during ferroelectric-based devices operations is fundamental in order to improve the reliability of emerging architectures. In this work, we investigate metal/insulator/ferroelectric/metal (MIFM) ferroelectric tunnel junctions (FTJs) consisting of a ferroelectric hafnium zirconium oxide (HZO) layer and an alumina (Al2O3) layer by means of C-f and G-f measurements performed at multiple voltages and temperatures. For a trustworthy interpretation of the measurements results, an innovative small signal model is introduced that goes beyond the state of the art by i) separating the role played by the leakage in the two layers; ii) including the impact of the series impedance (that depends on the samples layout); iii) including the frequency dependence of the dielectric permittivity; iv) accounting for the fact that not the whole HZO volume crystallizes in the orthorhombic ferroelectric phase. The model correctly reproduces measurements taken on different devices in different conditions. Results highlight that the typical estimation method for interface trap density may be misleading.