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

Abstract

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 Cf 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.

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

“…2 shows the results of PUND and C-f/G-f measurements, highlighting the difference between the pristine and the last cycle (before breakdown (BD)). As expected, leakage increases with stress, as highlighted by the low-frequency Gp/ω [36], [37] (Fig. 2d).…”

“…This is further confirmed by the analysis of the C-f/G-f curves by the smallsignal model (Fig. 3) including HZO physical properties [36], [37] (i.e., voltage dependence of HZO permittivity (𝜖 𝑟𝐹𝐸 ) and of HZO conductance (𝐺 𝐹𝐸 )), a first-order equivalent trap response and equivalent interfacial layers effect [39], [44], [45] (Cit and Git), together with the parasitic series impedance (ZSER = CSER//GSER). It is worth noting that 𝜖 𝑟𝐹𝐸 represent an equivalent contribution of the different ferroelectric and nonferroelectric phases inside the layer, together with the effect of the interfacial layers permittivity.…”

“…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].…”

“…In this work, we elucidate the degradation dynamics in M-HZO-M capacitors (Fig. 1a) by combining ab-initio calculations, physics-based simulations, dedicated experiments and custom data analysis based on a recently introduced small-signal device model [36], [37].…”

“…1b) are conducted to: i) extract the 2𝑃 𝑟 (remnant polarization) evolution throughout device lifetime (from pristine through wake-up and fatigue); ii) determine parasitic series impedance between grounded tips and the capacitor (Fig. 1a in red) and key material properties through impedance spectroscopy [38], analyzed with our small-signal model [36], [37]; iii) evaluate the leakage current. Asymmetries in leakage response and its evolution are reproduced and interpreted by means of physicsbased simulations, including the presence of interfacial parasitic layers at both interfaces (e.g., TiOx and TiONx suggested earlier in similar structures [39]) with different properties.…”