Fatigue of NbO_x-Based Locally Active Memristors-Part II: Mechanisms and Modeling

NbOx materials have shown considerable potential for the applications of artificial neurons due to their volatile threshold switching (TS) behavior, fast switching speed, and low power consumption. The TS can be attributed to its negative differential resistance (NDR) effect associated with heat accumulation. Experimental data show that the degradation of a NbOx-based neuron device is manifested by the shift in transition voltages, including threshold and hold voltages, and shrinkage in the voltage window. This problem strictly undermines the device reliability, and meanwhile, the mechanism is yet to be confirmed. In this work, a physical model involving the role of oxygen vacancies (Vos) is developed to further elucidate the TS behavior in NbOx-based devices. For the first time, a continuous drift of the operation voltages upon unipolar biases is simulated and attributed to the redistribution of Vos. We propose and experimentally demonstrate an efficient method that can mitigate the Vo migration problem, leading to a great improvement in the device endurance. Our developed model is not only closer to the real nature of the TS process in NbOx but can also provide more insight into the performance optimization of this important class of devices.