Study on Oxide Thickness Dependence of Current-Voltage Characteristics for HfOx Based ReRAM Device

A resistance random access memory (ReRAM) is expected to be a next-generation nonvolatile memory because of its fast switching speed, and low power consumption. The device structure is composed of an oxide layer sandwiched with two metal electrodes. ReRAM exhibits repetitive transition between high resistance state (HRS) and low resistance state (LRS). It is considered that resistive switching phenomenon is caused by the formation and rupture of the conductive filament formed in the oxide layer. In this study, we investigated HfO_x thickness dependence of current-voltage characteristics for Ni/HfO_x/Pt ReRAM device.    We fabricated Ni/HfO_x/Pt devices with a simple crossbar structure. HfO_x layer was deposited on the Pt bottom electrode by DC reactive sputtering. The I-Vcharacteristics were measured by two-probe method at room temperature.

   Red line in Figure 1 shows  I-V characteristics of the Ni/ HfO_x/Pt device with oxide thickness of 5.6 nm. Initial state of resistance is HRS. SET process (from HRS to LRS) took place at 2.4 V with a current compliance of 0.1 mA. RESET process (from LRS to HRS) occurred at -1.6 V.  The resistance ratio of HRS to LRS is about 10⁷ with 0.1 V readout bias. On the other hand, blue line shows the I-V characteristics of the Ni/ HfO₂/Pt device with oxide thickness of 4.0 nm. Initial state of resistance is LRS. Forming process was not observed in this device. SET process took place at 1.0 V. RESET process was observed at -0.6 V. Switching voltage of this device is smaller than the former device with the thicker oxide layer. The resistance ratio of HRS/LRS is about 10¹ with 0.1 V readout bias. There was a significant difference in switching properties between these devices whose oxide thicknesses differ only 1.6 nm.

 It is suggested that leakage current increases when the oxide thickness decreases, thereby the on-state occurred initially for the device with 4.0 nm oxide.  It should be noted that I-V characteristic is symmetrical for the device with 4.0 nm oxide, while it is asymmetrical for the device with 5.6 nm oxide. This suggests the mechanism of conductive filament formation and rupture is completely different between these devices. Further discussions and analysis will be presented in the symposium.