Nanoscale-Resistive Switching in Forming-Free Zinc Oxide Memristive Structures

Abstract

This article presents the results of experimental studies of the impact of electrode material and the effect of nanoscale film thickness on the resistive switching in forming-free nanocrystalline ZnO films grown by pulsed laser deposition. It was demonstrated that the nanocrystalline ZnO film with TiN, Pt, ZnO:In, and ZnO:Pd bottom electrodes exhibits a nonlinear bipolar effect of forming-free resistive switching. The sample with Pt showed the highest resistance values RHRS and RLRS and the highest value of Uset = 2.7 ± 0.4 V. The samples with the ZnO:In and ZnO:Pd bottom electrode showed the lowest Uset and Ures values. An increase in the number of laser pulses from 1000 to 5000 was shown to lead to an increase in the thickness of the nanocrystalline ZnO film from 7.2 ± 2.5 nm to 53.6 ± 18.3 nm. The dependence of electrophysical parameters (electron concentration, electron mobility, and resistivity) on the thickness of the forming-free nanocrystalline ZnO film for the TiN/ZnO/W structure was investigated. The endurance test and homogeneity test for TiN/ZnO/W structures were performed. The structure Al2O3/TiN/ZnO/W with a nanocrystalline ZnO thickness 41.2 ± 9.7 nm was shown to be preferable for the manufacture of ReRAM and memristive neuromorphic systems due to the highest value of RHRS/RLRS = 2307.8 ± 166.4 and low values of Uset = 1.9 ± 0.2 V and Ures = −1.3 ± 0.5 V. It was demonstrated that the use of the TiN top electrode in the Al2O3/TiN/ZnO memristor structure allowed for the reduction in Uset and Ures and the increase in the RHRS/RLRS ratio. The results obtained can be used in the manufacturing of resistive-switching nanoscale devices for neuromorphic computing based on the forming-free nanocrystalline ZnO oxide films.

Forming‐Free Resistive Switching Memory Crosspoint Arrays for In‐Memory Machine Learning

“…Figure 1d shows the measured initial leakage current before forming and the average forming characteristics are collected. As pointed out in previous works, [ 17–20 ] the forming voltage decreases for decreasing oxide thickness, with the thinnest layers showing forming‐free characteristics. We explain this behavior as the combination of a substochiometric composition of HfO 2 and a high concentration of defects in the thin film.…”

“…[20,21] Metals with high work function (such as Pt or TiN) are usually adopted as Schottky-type bottom electrode materials as they are inert with respect to the oxide interface. [18,20] On the other hand, an active metal with good oxygen affinity, such as Ta, Ti, or Hf, [20][21][22] can act as top electrode material for oxygen scavenging, thus leading to the formation of a thin vacancy-rich oxygen exchange layer. By applying a positive voltage to the top electrode, the oxygen vacancies can migrate and reallocate inside the oxide layer with a consequent change of the electrical properties, where the formed oxygen vacancy-based conductive channel dictates a low-resistance state (LRS).…”

“…[ 13,20 ] Within a crosspoint array, a high forming voltage may cause disturbs to the RRAM [ 25 ] devices sharing the same row or column of the selected device. As a result, many efforts have been devoted to reducing the forming voltage, [ 17–19 ] mainly tuning the oxidized interface between oxide and scavenger metal [ 20 ] or changing the oxide stoichiometry. [ 26 ] Ideally, one should aim at the elimination of the forming operation, by fabricating the device in the LRS instead of the more common HRS.…”

“…In this work, we present a novel forming‐free RRAM [ 17–19 ] based on HfO 2 . Thanks to the small thickness of the HfO 2 layer, the RRAM shows a large initial conductance without the need for any forming operation.…”

Effect of carbon on the co-presence of metallic tungsten as a nucleation agent and Eu2+ in glass: crystallization of CaO–Al2O3–SiO2 glass probed with Eu2+ luminescence

“…10 This interpretation ts well with the lines in Ellingham diagrams decreasing in the order W/WO 3 , EuO/ Eu 3 O 4 , Eu 3 O 4 /Eu 2 O as reported in previous studies. 48,49 Although the co-presence of metallic W and Eu 2+ is evident in the glass specimen prepared by the conventional melting method, the reduction efficiency of this method is lower than that of the improved glass raw material addition method used in the present study, as evaluated on the bases of the coloration and the uorescence spectra of glass specimens of W-Eu-Glass-4.0C, -0.40C, and -normal (Fig. 2 and 4).…”