Resistance Change Behavior Under Voltage Pulses at Reset Process of Ti/HfO2/Au-Reram with Various HfO2 Layer Conditions

Abstract

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.