Describing the analog resistance change of HfO_x-based neuromorphic synapses using a compact series trap-assisted tunneling and Ohmic conduction model

Changes in the resistance of Ti/HfOx synapses are known to be governed by a thin-oxide barrier associated with the oxidation/reduction of a Hf-rich conducting filament (CF). However, experimental characterization of the CF is challenging. Critical physical properties and processes, such as the barrier location, time-dependent thickness during analog pulsing, and the temperature-effect on current, need to be better established. In this work, a compact model based on Trap-Assisted-Tunneling and Ohmic transport is utilized to analyze the analog switching of HfOx synapses. The model agrees well with the experimentally observed current-voltage relation and its temperature dependence. The extracted barrier heights during analog pulsing are consistent with a barrier situated near the reset anode; the electrode is opposite to the Ti oxygen-reservoir layer. A Finite Element Analysis simulation, which incorporates oxygen-vacancy migration, independently supports this conclusion. The model further permits extraction of the barrier thickness in relation to the analog pulses.