Fig. 1. (a) Fabrication process, (b) schematic illustration displaying the crossbar array structure, (c) TEM image, and (d) depth profile analysis of the W/TiO2/HfO2/TaN memristive device.

Fig. 2. XPS spectra of the W/TiO2/HfO2/TaN memristive device, (a) Ti 2p, and (b) O 1 s core level spectrum from the TiO2 layer, (c) Hf 4f, and (d) O 1 s core level spectrum from the HfO2 layer.

Fig. 3. (a, b) Typical bipolar switching characteristics of W/HfO2/TaN and W/TiO2/HfO2/TaN memristive devices for 50 consecutive cycles in DC double logarithmic modes. Left insets of (a-b) show I-V switching curves of the forming process of W/HfO2/TaN and W/TiO2/HfO2/TaN memristive devices occurring at ~5.0 and 5.50 V, respectively. Arrows indicate switching (set and reset) directions of both memristive devices. (c, d) DC endurance characteristics for 103 cycles of W/HfO2/TaN and W/TiO2/HfO2/TaN memristive devices. (e) Excellent pulse endurance characteristics for over 107 switching cycles attained with 10 µs programming pulses (set: 1.0 V, reset: -1.0 V) of W/TiO2/HfO2/TaN memristive devices. (d) Retention tests made at LRS and HRS show no evident change for over 104 s when W/TiO2/HfO2/TaN crossbar array memristive device is backed at 125 °C.

Fig. 4. (a) Schematic illustrations of biological synapse (left) and crossbar array memristive device (right), which is sandwiched between presynaptic and postsynaptic spikes. (b) Change in response current over consecutive potentiating or depressing voltage pulses. When 50 positive pulses of width 10 μs with amplitude of 1.0 V and subsequently similar 50 negative pulses were applied to the memristive device, the response current gradually increased and then decreased with application of positive and negative pulses, respectively. (c) Synaptic currents of the memristor triggered by a pair of pulses. PPF index is defined as I2/I1 plotted as a function of pulse interval, Δt, between two successive pulses. Synaptic currents of the memristor triggered by a pair of pluses. (d) Characteristics of PPF index as a function of pulse interval (Δt,weight, decrease) with two fixed pulses of 1.0 V and pulse interval of 10 μs. Cyan line represents fitting following an exponential model, i.e., $\text{PPF}={{C}_{1}}\exp \left( -t/{{\tau }_{1}} \right)+{{C}_{2}}\left( -t/{{\tau }_{2}} \right)$. Asymmetric STDPs of (e) Hebbian learning rule and (f) anti-Hebbian learning rule are shown. The change of synapse weight (ΔW) is plotted as a function of the time difference (Δt) between pre- and post-spikes applied to top and bottom electrodes, respectively.

Fig. 5. Schematic of energy band diagram for the W/TiO2/HfO2/TaN memristive device (a) at zero bias, (b) forward bias, and (c) reverse bias. Schematic illustration of the resistive switching mechanism based on the formation/disruption of a conductive filament involving oxygen vacancies. (d) Pristine-state, (e) set-process, where memristive devices present a transition from initial state to low resistance state (LRS), and (f) reset-process, where memristive devices illustrate the switching from LRS to high resistance state (HRS), are shown.