(Invited) Current and Voltage Control of Intermediate States in Bipolar Rram Devices for Neuristor Applications
Authors: García Héctor , Ossorio Óscar G. , Dueñas Salvador , Castán Helena 
Published: May 2020

Nowadays, there is an increasing interest in the fabrication of neuristors in which memristors are used to create a neuron-like behavior (1). Memristors based on resistive switching memories (RRAM) are promising candidates to implement artificial synaptic devices for their use in neuromorphic systems, due to their high number or reachable conductance levels.
In a previous work (2) we show that TiN/Ti/ HfO2/W capacitors exhibit resistive switching behavior and that intermediate conductance states can be obtained by varying the voltage applied to the device (Voltage-control mode, VCM). We demonstrated that the conductance values vary near linearly when applying voltage depression pulses. During depression process, the conductance state depends on the amplitude and the duration of the voltage pulses length. Moreover, this process is accumulative: the conductance decreases when applying successive pulses of same amplitude and length. In contrast, the potentiation characteristic is not linear, as for other synaptic devices, as PRAMs: the transitions from high resistance state (HRS) to low resistance state (LRS) is very sharp and intermediate conductance state cannot be controlled. Decreasing the voltage pulse length or amplitude was not a choice. Additionally, we tried to use ramps where the voltage linearly increases and once again the characteristics remain very nonlinear. In summary, a very poor control of the potentiation state is obtained when using voltage as synapse stimulus.
In the present work, we demonstrate that the potentiation process can be linearly controlled when using current as the synapse stimulus (Current-control mode, CCM).
The devices used in this work are TiN/Ti/10-nm HfO2/W MIM capacitors. The high-k dielectric was deposited by the atomic layer deposition (ALD) technique at 225 ºC using TDMAH and water as hafnium and oxygen precursors respectively. Nitrogen was used as carrier and purge gas. The bottom electrode consists of a 200 nm W layer, and the top electrode consists of a 200 nm TiN layer and a 10 nm Ti layer. Metal electrodes were deposited by magnetron sputtering.
An HP 4155B Semiconductor Parameter Analyzer was used to perform the voltage and the current measurements. In order to perform the pulsed measurements, we used an Agilent 33500B Series waveform generator which provides voltage pulses and ramps. The voltage waveforms are converted to mimic current waveforms using a home-made voltage to current converter. All the measurements were carried out at room temperature and the entire experimental setup was computer controlled.
In figure 1 we show that the conductive path formation (electroforming) requires a very few nA current value. As the filament is formed, the devices reaches the low resistance state (LRS) and the voltage falls to a very low value. Once the filament is formed, we have measured the voltage-current (V-I) characteristic instead of the more usual current-voltage (I-V) characteristic. Figure 2 shows 50 consecutive bipolar switching V-I loops. It is important to point that in the CCM mode, a voltage compliance in the reset transition has to be used. Otherwise, irreversible oxide breakdown takes place due to the huge increase of the structure dissipated power. Departing from the HRS state we have applied current pulses of increasing amplitude. In Fig. 3 we plot the conductance of the device measured at 0.1 mA as a function of the amplitude of the current pulse previously applied. We see that the synaptic potentiation linearly depends on the pulse amplitude in the CCM mode. In Figure 4 we demonstrate that potentiation occurs at the very first current pulse. Successive pulses do not modify the conductance value. In summary, the potentiation process in CCM mode is very fast, non-accumulative and nearly linear. To confirm that this process is very fast we have obtained current transients in the VCM mode (Figure 5) which confirms that when a positive voltage is applied the current increases occurs at very short times (lower than the experimental setup time response).
In contrast, the depression process is better controlled by applying voltage pulses (VCM mode) as described in ref. 2. Moreover, the depression process is slower and accumulative. That can be observed in Fig. 6 in which a train of -0.8 V voltage pulses is applied. The conductance decreases from pulse to pulse and show very long time rates (tens of ms for the showed voltage)
In conclusion, CCM mode for potentiation and VCM mode for depression characteristic is the optimal combination to control the synaptic behavior of the studied bipolar RRAM devices.
References

Timmer, Nature Materials, 2012. DOI: 10.1038/NMAT3510

García et al., Micr. Eng. 215, 110984 (2019).

Figure 1