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.