Impedance Spectroscopy on Hafnium Oxide‐Based Memristive Devices

Abstract

Memristive devices for neuromorphic computing have been attracting ever growing attention over the last couple of years. In neuromorphic electronics, memristive devices with multi‐level resistance states are required to accurately reproduce synaptic weights. Here, a memristive device based on a multilayer oxide system (Nb/NbOx/Al2O3/HfO2/Au), which features a filamentary‐free, homogenous interfacial resistive switching mechanism, is investigated. To gain a deeper insight into the switching mechanism, impedance spectroscopy (ImpSpec), X‐ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) are exploited. While this work focuses on the analysis of impedance and current‐voltage characteristics, XPS and TEM investigations can be found in a companion paper (Zahari et al.). In the course of this investigation, potentiodynamic impedance spectroscopy (PD‐ImpSpec) and time resolved impedance spectroscopy (TR‐ImpSpec) in combination with transient analysis are used. Evidence is presented of switching kinetics at voltages above 2.1 V directly related to changes in Schottky barrier resistance. These switching kinetics can in turn be interpreted by the charging and discharging of double positively charged oxygen vacancies ≈ 0.9 eV. The results of the impedance analysis are translated into a more general model for memristive devices to map the physical processes during switching.

Trap‐Assisted Memristive Switching in HfO2‐Based Devices Studied by In Situ Soft and Hard X‐Ray Photoelectron Spectroscopy

“…The worked-out model is in agreement with the companion paper Marquardt et al that exploits ImpSpec to probe the current transport and switching kinetics of the same devices. [74] Based on two different experimental approaches (XPS/HAXPES and ImpSpec), our findings strongly indicate an electronic charging and discharging of traps as the fundamental origin of resistive switching in our devices, instead of ionic drift.…”

“…This is consistent with the results of ImpSpec conducted on the reference devices, performed by Marquardt et al and reported in the companion paper. [74] Here, the kinetics of the set process and of the retention imply charging of traps rather than ion movement. Furthermore, quantitative model parameters describing current transport in HRS and LRS and switching dynamics are given.…”

“…Spectroscopic evidence (by XPS/HAXPES) for such a switching mechanism has not been reported so far. Furthermore, impedance spectroscopy (ImpSpec) [73] is exploited by Marquardt et al in a companion paper [74] to probe memristive switching in the same device. The findings presented in both papers indicate that the memristive switching can be attributed to a Schottky barrier modulation by a charge-trapping mechanism.…”