Superlattice-like PCM synaptic characteristics. (a) Long term potentiation (LTP) properties of superlattice-like PCM synaptic with fixed SET pulse scheme. (b) Long term depression (LTD) properties of superlattice-like PCM synaptic with fixed RESET pulse scheme. (c) Gradual SET of the device is implemented by using pulses with increasing amplitude of 1.8–3.3 V range with 150 mV voltage steps. Gradual reset of the cell resistance is achieved by using stair-case pulses from 2.8–4 V range with 80 mV voltage steps. There are 2 pulses for each voltage step during the SET and RESET operations. (d) An excessively high maximum conductance state may result in a decrease in the effective number of intermediate states. We limit the working range of the conductance to maximize the effective number of intermediate states.