The demand for neuromorphic computing is increasing, and resistive random access memory devices (ReRAM) are intriguing candidates for synaptic applications. We studied a Ti/HfOx/Au ReRAM device with this potential in view, and fabricated a Ti/HfOx/Pt device for comparison. Both devices exhibited bipolar switching characteristics. In response to voltage pulse trains, gradual resistance change was observed in the Ti/HfOx/Au device for both the SET and RESET processes, indicating its suitability for artificial synapse application. In contrast, an abrupt resistance change was observed in the SET process of the Ti/HfOx/Pt device. A significant diffusion of Au atoms occurred in the HfOx layer of the Ti/HfOx/Au device, and the Au atoms were oxidized at the interface. This led to an increase in the O vacancy concentration, which assisted the achievement of the gradual resistance change. The present study indicates that the Ti/HfOx/Au device demonstrates good potential for use as an artificial synaptic device.

1. Introduction Recently the application of ReRAM to artificial synapse has been paid much attention in the field of neuromorphic computing([1],[2]). For the implementation of neuromorphic synaptic devices, gradual resistance change response by applying voltage pulse trains is required. We investigated the condition of Ti/HfOx/Au-ReRAM to possess gradual resistance change by changing sputtering conditions of HfOx layer. 2. Sample preparation and experiments Figure 1 shows the structure of Ti/HfOx/Au device. For deposition of HfOx film, we used reactive sputtering with Ar and O2 mixture gases. At this time, we adopted two types of Ar/O2 flow ratios, which is (1)7.8 : 15.6 sccm and (2)7.8 : 2.0 sccm. These devices have a bipolar memory characteristic. SET occurs when a positive voltage is applied, and RESET occurs when a negative voltage is applied. We measured resistance change of RESET process of the ReRAM device with applying DC voltage pulses. 3. Results and discussion Figure 2 shows resistance change behavior with applying voltage pulses at RESET process. Pulse width is 1.0 uA and pulse period is 2.0 uA. Figure 2(a) is the resistance change behavior of device (1) with constant amplitude voltage pulses. Resistance change was rarely observed when pulse amplitude is -1.5 V, however, gradual resistance change more than 1 digit was observed when pulse amplitude is -1.6 V. Moreover, when we applied -1.7 V, gradual resistance change more than 2 digits within 40 pulses was observed. In the case of this device, resistance change was ideal for the use of neuromorphic synaptic device. Figure 2(b) is the results of device (2). Gradual resistance change was observed after 60 pulses when pulse width is -1.5 V. However resistance increased more than 3 digits after a few pulses were applied when pulse amplitudes were -1.6 V and -1.7 V. In the case of device (2), resistance change is almost binary and it is not suitable for use of artificial synapse device. The difference of resistance change behavior may come from spatial variation of oxygen vacancies. For further understanding of the mechanisms, we will evaluate the difference of atomic composition of HfOx layer by XPS and XRD.

In the coming years, threshold switching based on insulator-metal phase transition (IMT) devices is expected to be applied in selector devices for reducing sneak currents and building blocks for neuromorphic computing. In this work, we fabricated IMT devices using NbOx as an insulator layer and compared the device performance for two cases with metal electrodes: an asymmetric electrode device of stacked Pt/Ti/TiN/NbOx/Pt films, and a symmetric electrode device of Pt/Ti/TiN/NbOx/TiN/Ti/Pt. We changed the atomic ratio of Nb and O in NbOx films by controlling the argon-to-oxygen flow ratio during reactive sputtering. In the case of the asymmetric device, we observed a clear hysteresis loop in the current-voltage characteristics, indicating threshold switching only when a positive voltage was applied to the top electrode. We proposed a model in which a domain of the NbO2 phase is present in contact with a filament with oxygen vacancies, with its size changing depending on the direction of the electric field. On the other hand, in the case of the symmetrical device, nearly symmetric hysteresis loops were observed for both positive and negative voltage sweeps.