2.2. Tunable ion dynamics and plasticity of the ion-modulated memtransistors
Figure 2 a shows a schematic diagram of an ion-modulated memtransistor, in which amorphous NbOx acts as the channel material. As a strongly correlated transition metal oxide, NbOx has an intrinsic low conductance level and sensitivity to ion doping, allowing the system to be easily modulated under ion doping with lower power consumption.[33-36] Figure 2b shows the top view of the device under an optical microscope and the cross-sectional transmission electron microscope (TEM) images can be seen in Figure 2c. The detailed process of device fabrication is presented in the Experimental Section. The spatial mapping of dominant elements using energy-dispersive X-ray spectroscopy (EDS) is shown in Figure 2d, including O, P, N, Nb, Si, Ti and Au elements. Also, an EDS line scan through the red arrow in Figure 2d is conducted, which is illustrated in Figure 2e. To fabricate the amorphous LiPON ion conductor, the Li3PO4 target was deposited by the RF magnetron sputtering under N2 flow of 10 sccm, where the reaction between Li3PO4 and N2 will happen, enabling nitrogen atoms to get into the Li3PO4 to form LiPON. In the element characterization, we can see that nitrogen elements are evenly dispersed within the electrolyte layer, forming an amorphous LiPON fast ion conductor. Specific doping modification processes can be seen in Figure 2f. With the introduction of nitrogen atoms into the initial lithium phosphate crystals, the oxygen atoms in the original lattice will be replaced, including both bridge oxygen atoms and non-bridge oxygen atoms, so that the reticular crosslinking structure in the system increases significantly and enable lithium ions to move more easily in the electrolyte.
To measure the memory effect of the ion-modulated memtransistor, forth and back sweeps were conducted on gate bias from -6 V to 4 V, with the small drain-source voltage kept at 0.1 V, anticlockwise hysteresis for the channel current can be observed in Figure 2g, the average on/off ration at 0 V gate bias for the thirty sweeps is approximately 125. The insets in Figure 2g show the ion dynamics while under different stimuli ranges, as Li ions in the LiPON electrolyte are driven to approach or get away from the interface between the electrolyte and the channel under the positive or negative gate bias, which could reversibly modulate the channel conductance. For instance, under the positive gate bias, the electric field will drive the cationic Li ions to migrate to the channel interface, which can form the electric double layer; if under the strong electric field, the moved ions maybe intercalate the deficient sites within the amorphous NbOx, the electrochemical reaction will take place in that, which will lead the reduction of Nb5+ to Nb4+ and the generation for the extra defect energy level. The different sites for the Li ions gathered with respect to the channel correspond to different ionic temporal dynamics, which will be explored in the following experiments. Figure 2h collects the statistical drain current at the on-state or off-state for 0 V gate bias, and both fluctuations are at relatively lower levels. Also, as shown in Figure 2i, the gate leakage current is limited under the sub-nA, which keeps the programming energy ultra-low. Figure 2j and 2k present the transfer curves at different dynamic ranges and sweeping rates, respectively, and the corresponding gate leakage current can be seen in Figure S1 and S2, Supporting Information, as the larger sweeping bias and slower sweeping rate can induce the more prominent increase of the drain-source current.
To explore the relationship between the amplitude of the gate bias and the temporal scale of ion dynamics, a train of gate pulses with different amplitudes (1 V to 5 V) was applied while keeping both the pulse width and interval at 100 ms. As shown in Figure 3 a, increased amplitudes raise more significant channel current and the changes are more likely to be retained after the gate pulses removing, which implies that the memory effect transferred from short-term to long-term. To investigate the short-term memory effect, different pulse widths with fixed amplitude at 2 V, and different pulse amplitudes with fixed widths at 200 ms were applied to the device. The results are shown in Figures 3b and 3c, with more intense stimuli, the channel current reaches a higher level. In addition to the single pulse testing, we also applied a pair of positive pulses with different intervals on the gate to explore the paired-pulse facilitation (PPF) effect of the ion-modulated memtransistor. As illustrated in Figure 3d, the accumulative effect is represented by the ratio of the peaks of drain-source current change induced by the applied pulse pairs (PPF ratio), the relevance between the PPF ratio can be described by a double exponential decay function: 1+C1e-Δt/τ1+C2e-Δt/τ2, the two time constants after fitting are 999.78 ms and 163.62 ms respectively. Similarly, spike-rate-dependent plasticity (SRDP) was investigated in Figure 3e, ten pulses with frequencies of 25, 10, 5, 2 and 1 Hz (2.5 V, 100 ms) were applied to the device, and a more substantial cumulative effect could be observed in the pulse train with higher frequency.
As for long-term ion dynamics investigation, as shown in Figure 3f, different numbers of more strong pulse stimuli (5 V, 200 ms) were applied, and the channel current was continuously monitored by the constant small bias (0.2 V) at the drain side. It can be seen that the channel current change could be retained under strong stimuli, which is different from that channel current decaying back to the initial state in the short-term memory. In Figure 3g, we fixed the number of strong gate pulses while varying the amplitude, and the final retained channel current change could be more prominent under the larger gate pulses. In Figure 3h, eight distinguished states were selected to test their retention for 100 s after the removal of gate pulses with 0.2 V bias applied on the drain terminal. To quantitative describe the device state retention ability, we define a coefficient by the relative drain-source current change concerning the initial level between the current at 0 s and 100 s and all the coefficients about the current change maintained at the lower level. Besides, longer duration of channel conductance of two states with a ratio of 60 was measured, which is shown in Figure 3i. Then 50 distinguished states were shown in Figure 3j, implying that the device state can be tuned to much more levels. And the long-term channel conductance modulation by the gate pulse is shown in Figure 3k, 50 identical positive and negative gate pulses (8 V/-6 V, 100 ms) and 10 cycles of bidirectional analog switching were sequentially applied, and 50-level reproducible switching can be achieved.
The above results show that ion-modulated memtransistors both possess short-term and long-term ion dynamics and can be modulated flexibly by adjusting the amplitude of stimuli, which makes the devices vital blocks for different parts of the artificial neuromorphic vision systems.