Multilevel Resistive Switching with Oxygen Vacancy Filaments in Pt/TaOx/Cu and Pt/TaOx/Pt Devices
Authors: Kang Yuhong , Ghosh Gargi , Orlowski Marius K 
Published: February 2020

Paper demonstrates that multilevel switching of oxygen vacancy (VO) conductive filaments (CF) in Cu/TaOx/Pt devices is characterized by the relation Ron=A/Icc
n between ON-state resistance (Ron) and compliance current (Icc). However, in contrast to metallic (Cu, Ag) CFs, where the exponent n for various devices is found universally to be n≈1, we find for VO CFs to be n=1.36 with A=0.022.  This constitutes a significant departure from n=1 found for all metallic CFs. As a consequence of n>1, the constant A can no longer be interpreted as the universal minimum switching voltage. In fact, our experiments confirmed that no minimum Vset for Vo could be ascertained. Locally, for a small Icc intervals, one can still fit the data with n=1 and extract following values for the constant A: for Icc
≈ 10nA A=1.7V, for Icc
≈ 0.1mA A=0.52 V, and for Icc=10mA A=0.06V. One concludes that at low Icc currents the minimum set voltage is high and at high Icc currents very low. We have measured the Vset of Vo CF as a function of voltage sweep rate v. We find that, as in the case of metallic CFs, the SET voltage decreases linearly with the log(v). However, in contrast to metallic CFs, the Vset for Vo CF does not saturate at a minimum set voltage, even at such slow ramp rates as v=1x10-5 V/s. We argue that an exponent n>1 implies two distinct mechanisms responsible for the formation of VO CFs. 1st mechanism is similar to the formation mechanism for metallic CFs and accounts for the unity part of the exponent, while the 2nd mechanism is responsible for the remainder (n-1). There is a consensus that at the interface between the active electrode (Cu) and the metal oxide dielectric a redox reaction is taking place: Cu ←→ Cu++e-. When negative bias is applied to Cu electrode, the redox reaction provides an efficient surface injection mechanism of electrons into the oxide while Cu+ ions are returned to the Cu electrode. The electrons are injected and may charge and dislodge a negative oxygen ion from the metal oxide matrix leaving behind a neutral oxygen vacancy Vo according to the reaction:
TaOx+2e-→TaOx-1+O2-=TaOxVo+O2-.
Since the redox reaction takes place at the Cu/TaOx interface and produces a large number of electrons, the creation of Vo will take place predominantly at the interface. Since the oxygen vacancy is known to provide a local Fermi level close to the conduction band, the vacancy will extend the active electrode into the oxide and act as a conduit for more electrons. Thus it is likely that another Vo will be formed below the first one extending thus the partial Vo filament as an extension of the electrode deeper into the oxide. This leads to an increased electric field between the end of the partial Vo CF and the inert electrode Pt.  The self-accelerating mechanism is similar to the formation of Cu CF at positive bias.  The 2nd mechanism for creation of oxygen vacancies consists in electron migration into the oxide, at random, and at some point creating a (Vo,O2-) pair but now in the bulk rather than at the surface. Over the stress time, more and more of vacancies will be generated in the same way. The increased density of vacancies will reach a critical concentration of vacancies allowing for a conductive percolation path between the two electrodes. The 2nd mechanism is verified on Pt/TaOx/Pt devices where the 1st surface electron injection mechanism is eliminated. We measure Vset for Vo CF for Pt/TaOx/Pt devices to be significantly higher, Vset≈-8V than for Cu/TaOx/Pt with Vset≈-2V. Also for Pt/TaOx/Pt devices the relation Ron=A/Icc
n  cannot be fully constructed: Only for Icc>1mA stable non-volatile Vo CFs could be formed. At higher set currents the cell could be set and reset but less than 10-20 times before the device is permanently damaged. To investigate the two Vo mechanisms further, we fabricated and characterized Cu/SiO2/Pt and Pt/SiO2/Pt devices. The thickness of SiO2 layer is dSiO2=32,16, and 8nm. SiO2 is known to be very stable and not susceptible to oxide damage induced by Vo. For both kind of devices with dSiO2=32nm no Vo CF could be formed even at voltages as high as 15V. For dSiO2=16nm devices Vo CF could be formed but not reliably set and reset. For dSiO2=8nm the Vo CF could be readily formed, set and reset.  This behavior is explained in terms of critical size of Vo cluster and compared for ALDTa2O5, TaOx, and SiO2 dielectrics, and complements the two mechanisms discussed above.