Analog resistance change depending on the external voltage stress such as the number or height of the voltage pulse is accumulating considerable attention. This is because such characteristics are quite analogues to the signal transfer at the synapses between neurons and therefore it can be applied to the energetically conservative human-brain inspired information processing. Resistive switching device using the oxide material is one of the strong candidates due to the non-volatility of the resistance states and low power and high-speed resistance switching process. Previously, those excellent characteristics have been mainly applied to the digital-type non-volatile memory ReRAM (resistive random access memory) technology [1]. In this contribution, we successfully demonstrated the high-speed reproducible analog resistance change in the device using the TiN electrode and TaO_x thin film synthesized by the reactive sputtering process.

The present analog resistance change device has the stacking structure of TiN(TE)/TaO_x-L/TaO_x-H/TiN(BE). Here, TE and BE respectively denote top and bottom electrodes. TaO_x-L and TaO_x-H are both the reactive sputtering films of TaO_x having different resistivity. The value of resistivity in TaO_x-L is lower than that in TaO_x-H. Henceforce, we represent our device as TaO_x-RAND (resistive analog neuromorphic device). Fig. 1(a) and 1(b) are the optical microscope and cross-sectional transmission electron microscope (TEM) images of TaO_x-RAND. As shown in Fig. 1(a), the load resistor (LR) in series with TaO_x-RAND was fabricated using TiN. Two kinds of LR having different resistance values, 1 and 3 kOhm, were fabricated by adjusting the length of the TiN wire. The role of LR is to suppress the transient current during the rapid resistance decrease process. The diameter of the concave region in Fig. 1(b) is 100 nm (designed value), which corresponds to the device size. The current-voltage (I-V) curve and response to the voltage pulse were measured by applying the external voltage to TiN(TE) with the TiN(BE) grounded.

Figure 2(a) and 2(b) are the I-V curves for TaO_x-RAND with LR = 1 and 3 kOhm respectively. In order to change the resistance of TaO_x-RAND after the reset process, the value of the current compliance (CC) was varied from 75 to 200 uA. In all the cases, the resistance decrease process (set process) is observed when the positive voltage is applied to TE. On the other hands, the resistance increase process (reset process) is observed when the negative voltage is applied to TE. This voltage polarity dependence of the resistance change is quite reasonable when we consider the oxygen ion movement and consequent redox reaction inside the device. It should be noted that the negative differential resistance (NDR) during the reset process is unclear in the present TaO_x-RAND compared with the previously reported resistance switching devices with precious metal electrodes such as Pt. The unclear NDR is expected to cause the gradual resistance increase. Also, the reset process is influenced by the relation between the resistance values of TaO_x-RAND and LR. By connecting the larger LR in series with TaO_x-RAND, the maximum current during the reset operation decreases. The resistance-increase-starting voltage (V_RIS) values during the reset process indicated by the arrows in Figs. 2(a) and 2(b) are strongly influenced by LR. The larger value of LR results in the increase of V_RIS when we compare the data obtained under the same CC condition. On the contrary, corresponding current at V_RIS is almost independent on LR. These results indicate that a certain amount of current is required for the present reset process.

The reset and set voltage pulse width dependences of the resistance in TaO_x-RAND are plotted in Figs. 3(a) and 3(b), respectively. Although the magnitude of the resistance change induced by the voltage pulse is smaller than that induced by the DC voltage stress in Figs. 2, the analog resistance change behaviors in both the reset and set processes are clearly observed. Figure 4 is the voltage pulse number dependence of the resistance change in TaO_x-RAND. In this case, 20 sets of reset and set voltage pulses are alternatively applied to the device. The analog control of the resistance by voltage pulse number was successfully demonstrated. It should be emphasized that the analog resistance changes were realized by the considerably high-speed voltage pulse having the pulse width of 500 ns. This switching speed is much faster than the actual signal transfer process at the synapses [2].

[1] H. Akinaga, and H. Shima, Proceedings of the IEEE 98, 2237 (2010).

[2] H. Jo et al, Nano Lett. 10, 1297-1301 (2010).