Impact of the electronic band structure on the reliability of triple layer a-VMCO devices

Among selector-less RRAM devices, a-VMCO have recently attracted large interest as suitable candidates for high-density memory application. In this paper, we demonstrate that the reliability of triple-layer a-VMCO devices is strongly influenced by the asymmetry of the electronic band structure, causing the presence of a built-in field which energetically favors the SET operation. In particular, the RESET pulse must be >500x longer than the SET one to maximize the endurance lifetime, and the HRS state spontaneously drift toward the LRS state with time. We prove that, by combining the HRS drift over time with the accumulation of the RESET effect during cycling, the endurance characteristics can be drastically improved. We finally show that, by biasing the devices, thus countering the built-in voltage, the HRS drift can be suppressed, demonstrating that a symmetric band structure must be aimed in order to boost the reliability of these devices.