Impedance Spectroscopy of Ferroelectric Capacitors and Ferroelectric Tunnel Junctions

Ferroelectric devices are currently considered as a viable option for ultra-low power computing, thanks to their ability to act as memory units and synaptic weights in brain-inspired architectures. A common methodology to assess their response in different conditions (especially the role of material composition and charge trapping in ferroelectric switching) is impedance spectroscopy. However, test devices may be affected by the parasitic impedance of the metal lines contacting the electrodes of the device, which may alter the measured response and the results interpretation. In this work, we investigate the frequency response at different voltages of ferroelectric tunnel junction (FTJ) having a metal-dielectric-ferroelectric-metal (MDFM) stack, starting from the analysis of single layer capacitors (MFM and MDM). A simple but reliable method, validated by physics-based simulations, is proposed to estimate and remove the parasitic access impedance contribution, revealing the intrinsic device response. The method is used to quantify the intrinsic device-level variability of FTJs and to highlight for the first time the relation between the thickness of the dielectric layer, the phase composition of the ferroelectric, and the magnitude of the peak in the frequency response, usually thought as related to charge trapping only.