115HIGHLIGHTS 2020

Fig. 99: a) Initialisation procedure of the Pt/ L2NO4 \Ti devices annealed in O2, Ar or H2/Ar. Only the read pulses are shown. Illustration of the programming sequence at the bottom of the figure. b) Sketch of the sample showing the biasing conditions. c) I-V characteristics of the three devices after initialisation.

Fig. 98: a) XANES spectra at the Ni K-edge obtained for L2NO4 films with different anneals. Evolution of (b) the c lattice parameter and (c) film resistivity with δ.

in the structure. Figure 98c demonstrates how this δ increase leads to a decrease in the electrical resistivity of the L2NO4 film, ultimately resulting in an improved memory window for the devices.

L2NO4 thin films with three different oxygen stoichiometries were used to build functional memristive devices. The high work function and chemical inertness of Pt, combined with the lower work function and high oxygen affinity of Ti, were used to build asymmetric memristive devices composed of an ohmic Pt/ L2NO4 contact and a chemically-active L2NO4 \Ti contact. As shown by a combination of STEM, electron energy loss spectroscopy maps and vertex component analysis, titanium formed an amorphous TiOx interlayer at the L2NO4 \Ti junction, key for the appearance of the memristive behaviour. The initialisation of the devices results in a resistance increase in all cases (Figure 99a). Remarkably, the film with the highest oxygen content presents the highest (more than 20-fold) resistance increase and the largest opening of the hysteretic current-voltage curve after initialisation (Figure 99b). The results obtained from the various characterisation

on SrTiO3 single crystals before being subjected to a series of post-annealing treatments under oxidising or reducing conditions (in O2, Ar and a highly reducing H2(6%)/Ar gas mixture). The combination of structural (X-ray diffraction, or XRD), chemical (XANES), microscopy (scanning transmission electron microscopy, or STEM) and photoemission electron microscopy results, together with electrical measurements, clearly provide evidence that increasing the oxygen content in L2NO4 correlates with an increase of the cell parameter, Ni oxidation state, work function and electrical conductivity.

The inset in Figure 98a presents the XANES Ni K-edge measurements carried out at beamline BM25A, where a clear shift of the Ni K-edge towards higher photon energies is observed when the samples are treated in increasingly oxidising atmospheres. From the energy position, the Ni oxidation state was calculated, allowing the estimation of the oxygen off-stoichiometry by charge-balance. As shown in Figure 98b, increasing the oxygen content (δ) leads to the expansion of its c lattice parameter (from θ-2θ XRD measurements), in agreement with the incorporation of interstitial oxygen ions