Reset Condition Effects on the Analog Pulsing of HfO_x-Based Neuromorphic Devices

Filamentary adaptive oxide neuromorphic devices are emerging as a candidate for on-chip, bio-inspired circuits. These devices consist of a metal-insulator-metal structure with typical insulators being dioxides, such as, HfO₂, ZrO₂, SiO₂, etc. When electrically biased, oxygen vacancies are created within the oxide forming a nanoscale conductive bridge, known as the filament. Once formed, the resistance of the filament can be increased or decreased by applying voltages of either positive or negative polarity. For “in-memory” computing based filamentary adaptive oxides, ideally the device’s resistance should linearly increase with the number of applied voltage pulses and decrease linearly with a voltage of opposite polarity. Many oxides have shown promise in achieving these resistance changes, however, comparing their performance in neuromorphic applications is not straightforward. Many studies do not report the specifics of the forming step or any pre-stabilization/ conditioning of the filament before the pulsing experiment. This work demonstrates how the reset condition during conditioning leading up to analog pulsing influences the resistance response with identical pulsing schemes.<br/><br/>Neuromorphic devices consisting of a titanium oxygen reservoir, HfO_x active layer, and gold electrodes are fabricated with photolithography. After forming all the devices with a current limit of 0.1 mA, devices are initially reset with either -1 V, -1.3 V, or -1.5 V. Following the initial reset, 10 consecutive set/reset cycles are conducted. All devices are set to the low resistance state with a voltage sweep from 0 to 1.2 V and a current limit of 0.5 mA, however, the reset sweep is varied between the three reset conditions. Devices from each of the reset conditions are subject to -0.7 V, -0.85 V, and -1 V pulses with 1 μs width. Even though the starting resistance, the device structure, and the analog voltages pulses are identical for all three conditions, the rate of change of the resistance and the overall resistance change is lower for the devices that are pre-conditioned with a lower magnitude voltage. For example, after 30 pulses with -0.7 V amplitude and 1 μs width, the devices pre-conditioned at -1 V have an 83% change in resistance compared to devices pre-conditioned at -1.5 V having a 193% change. One possibility for the varied analog response is that the devices reset with a higher magnitude voltage have a larger filament, suggesting that the geometry of the filament is not solely determined from the forming step. Temperature-calibrated, scanning thermal microscopy (SThM) is used to measure the temperature rise at the top electrode during the pre-conditioning voltage sweeps and during analog pulsing. This temperature rise is then correlated to the filament temperature using a COMSOL Multiphysics® model. It is believed that devices with a larger filament would reach a higher overall temperature and thus exhibit the largest resistance change during pulsing. This work suggests that the current-voltage history of the filament affects the resistance vs. pulse number trend, increasing the difficulty to predict the pulsing scheme needed to program a device’s resistance during neuromorphic circuit training.<br/>This work was supported by the AFOSR MURI under Award No. FA9550-18-1-0024. In part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the NNCI, which is supported by the NSF (ECCS-2025462). This material is based upon the work supported by the NSF GRFP under Grant No. DGE-1650044.