Impedance Spectroscopy of Ferroelectric Capacitors and Ferroelectric Tunnel Junctions

Abstract

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 braininspired 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.

Linking the Intrinsic Electrical Response of Ferroelectric Devices to Material Properties by Means of Impedance Spectroscopy

“…In this research, we extend our conference paper [35] and expand upon our previous findings on the validation of the small-signal model for ferroelectric tunnel junctions (FTJs) [33], [34], also used to investigate aging mechanisms [36], and examine the electrical response of FTJs with a metaldielectric-ferroelectric-metal (MDFM) stack at various voltages through multi-voltage capacitancefrequency/conductance-frequency (C-f/G-f) measurements.…”

“…Such physics-based simulations are carried out using Ginestra® simulation platform [37], which includes Schottky and thermionic emission, direct (WKB approximation), trap- assisted tunneling (TAT -including trap-to-trap contribution), and Fowler-Nordheim tunneling, as well as the trapped charge term in the Poisson's equation. We include defects in the HfO2 bulk (defects density 𝛿 = 10 𝑐𝑚 with a normal distribution in space centered at the middle of the stack and a uniform lateral distribution) having thermal (ETH = 2.1±1 eV) and relaxation (EREL = 1.2 eV) energies [44] which are very close to those predicted by hybrid-DFT calculations for oxygen vacancies in the orthorhombic ferroelectric phase of HfO2 (ETH ≈ 1.8 eV, EREL ≈ 0.7 eV) [45]. Consistently with the value extracted from experiments using the small-signal compact model we imposed an equivalent 𝜖 (0V) = 26, in order to effectively include the presence of different HZO phases (e.g., orthorhombic, monoclinic, and tetragonal).…”

The Role of Defects and Interface Degradation on Ferroelectric HZO Capacitors Aging

“…The bottom electrode (BE), shared by all capacitors, is accessible via a common metal pad, and introduces an inevitable parasitic series impedance (Fig. 1a in red) [41] between the grounded tip and the actual capacitor, with consequences on the interpretation of the results [36], [41]. Fabrication details for these devices are reported in [42].…”