Compliance-Current Manipulation of Dual-Filament Switching in a Ta/Ta2O5/In−Sn−

Direct Imaging of Ion Migration in Amorphous Oxide Electronic Synapses with Intrinsic Analog Switching Characteristics

“…In the measurements, positive DC voltages were applied to the film with various V maxs of +1.0–+5.0 V after inducing the initial set switching at I c = 2.0 μA, by which a robust conduction path can be formed without large structural deformation of a-TaO x (middle panels of Figure d). Gradual reset switching and significant V max dependence of the switched resistance were observed in the I – V characteristics with V max = +1.5–+2.5 V. Therefore, the (2) V max -dependent analog reset phenomenon ,,,, was demonstrated in this V max range with input powers of 0.2–0.4 mW, which are equivalent to those in practical devices. ,, Through voltage application with V max ≤ +1.5 V, the a-TaO x thin film first formed a volcano-like structure with a maximum height of 1.1 nm in the conductive region (left panels of Figures c and S5 for the height information in this type of switching), which suggests the occurrence of upward migration of the internal cations (Ta 5+ and Ta 4+ ). , However, this ion migration did not cause significant reset switching in the a-TaO x film, as indicated by the I – V characteristics and current image. At V max = +1.5–+2.5 V, where the analog reset was demonstrated, instead, gradual volume increases of a-TaO x were observed in the conductive regions.…”

“…Unlike the analog-type switching observed under the voltage polarity sequence of +2.5 → −1.7 V (Figure 2b), very abrupt resistive switching was induced under the sequence of −3.5 → +3.0 V with the defined switching voltages of −3.2 V and +2.5 V, and this type of I−V characteristic is also typical in a-TaO x -based resistive switching devices. 3,5,6,13 In this voltage sweeping sequence, structural depression was first caused at −1.5 V by the downward migration of Ta 5+ and Ta 4+ in the conduction path (Figure S6), while the deformation did not cause significant reset switching. The reset switching started to be induced after a small resistance jump at −1.8 V (Figure 3a) and then progressed with the generation of spiking current noise.…”

“…Continuous resistance changes induced by electric field application, i.e., analog (or memristive) resistive switching phenomena, have been demonstrated in nanostructured amorphous metal oxides (TaO x , HfO x , etc. ), − with advantages for applications (high neuromorphic functionality, low power consumption, and high scalability). The hardware development of oxide-based electronic synapses, however, has been hampered by the poorly understood physical mechanisms, which significantly limits function control.…”

“…The hardware development of oxide-based electronic synapses, however, has been hampered by the poorly understood physical mechanisms, which significantly limits function control. Recent research efforts have proposed phenomenological models of the mechanisms based on transport measurements and numerical simulations, ,,,− ,− and the involvement of field-driven migration of constituent ions has been suggested by the models, while the details of the ion migration and origin of the switching continuity remain elusive.…”

“…We selected a-TaO x as the target material in this study because it is currently considered a key material in resistive switching applications. A variety of neuromorphic functions, including high-performance analog resistive switching, − ,, spike timing-dependent plasticity, , and second-order memristor characteristics, have been demonstrated in thin-film devices of a-TaO x , in addition to the fact that it has been widely used as a practical material for commercial resistive random access memory (ReRAM). ,, In amorphous metal oxides, three types of analog switching phenomena have been demonstrated depending on the control parameters: (1) analog set (resistance decrease) controlled by the compliance current ( I c ), ,,, (2) analog reset (resistance increase) controlled by the maximum applied voltage ( V max ), ,,,,, and (3) analog set and reset by multiple voltage applications (i.e., controlled by the voltage application time). ,,, In our measurements, all three types of phenomena were directly demonstrated by C-AFM, and the involved ionic migration was observed in the ångström scale. The observations revealed that the analog resistive switching of a-TaO x originates from complementary migration of the tantalum and oxide ions and suggested that its dominant cause distinctively varies with the control parameters and stages of switching, as exemplified by the electric drift, thermal diffusion, and continuous redox reactions (which are characteristics of a-TaO x ).…”