Valence change detection in memristive oxide based heterostructure cells by hard X-ray photoelectron emission spectroscopy

Abstract

The switching mechanism of valence change resistive memory devices is widely accepted to be an ionic movement of oxygen vacancies resulting in a valence change of the metal cations. However, direct experimental proofs of valence changes in memristive devices are scarce. In this work, we have employed hard X-ray photoelectron emission microscopy (PEEM) to probe local valence changes in Pt/ZrOx/Ta memristive devices. The use of hard X-ray radiation increases the information depth, thus providing chemical information from buried layers. By extracting X-ray photoelectron spectra from different locations in the PEEM images, we show that zirconia in the active device area is reduced compared to a neighbouring region, confirming the valence change in the ZrOx film during electroforming. Furthermore, we succeeded in measuring the Ta 4f spectrum for two different resistance states on the same device. In both states, as well as outside the device region, the Ta electrode is composed of different suboxides without any metallic contribution, hinting to the formation of TaOx during the deposition of the Ta thin film. We observed a reduction of the Ta oxidation state in the low resistance state with respect to the high resistive state. This observation is contradictory to the established model, as the internal redistribution of oxygen between ZrOx and the Ta electrode during switching would lead to an oxidation of the Ta layer in the low resistance state. Instead, we have to conclude that the Ta electrode takes an active part in the switching process in our devices and that oxygen is released and reincorporated in the ZrOx/TaOx bilayer during switching. This is confirmed by the degradation of the high resistance state during endurance measurements under vacuum.

From Photoemission Microscopy to an “All-in-One” Photoemission Experiment

“…The instrument is mainly used for material science studies in information and energy research and a recent example is given in Figure 7. It represents a spectromicroscopic study of the resistive switching in ZrOx [38]. The PEEM image [ Figure 7…”

In‐Memory Binary Vector–Matrix Multiplication Based on Complementary Resistive Switches

“…To confirm the linear relationship between the output voltage and the HD, a single row of lateral CRS cells, each based on two ReRAM cells, was fabricated. [ 24 ] The ReRAM devices are based on a platinum, tantalum oxide, tungsten stack capped by another platinum layer. A detailed description of the fabrication process can be found in Section 6.…”

Chemically addressed switching measurements in graphene electrode memristive devices using in situ XPS

“…Switching mechanisms and valence change in memristive structures and ReRAMs have been and are being studied using a variety of spectroscopy techniques. [13][14][15][16] Verifying the theoretical descriptions, and understanding their modes of operation and ionic dri as well as the effects of device dimensions upon device characteristics, are signicant steps in building on the usability of these devices through expanding application areas and developing a generalised model. The fact that these are nanoscale devices brings about some challenges.…”

“…Electron spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS) have been implemented in order to chemically address memristive characteristics. 13,17,18 XPS has been predominantly used to investigate the composition of memristive devices or to carry out ex situ measurements where devices are excited prior to XPS measurements. Similar to this effort, a recent study reported in situ measurements using X-ray photoelectron emission microscopy (PEEM) to detect valence change in TaO x devices during switching.…”

“…Similar to this effort, a recent study reported in situ measurements using X-ray photoelectron emission microscopy (PEEM) to detect valence change in TaO x devices during switching. 13 Another group, sharing our motivation to better understand the switching mechanisms of memristors and ReRAMs to accelerate technological progress, has employed high-angle annular dark-eld scanning transmission electron microscopy (HAADF-STEM) and twodimensional energy dispersive X-ray spectroscopy (2D EDX). 19 These techniques were utilised to show the differences between a 'pristine' device (which hadn't been switched yet) and undamaged and damaged areas of a device in the low resistive state.…”