The neuromorphic computing system is attracting significant interest to simulate the human brain due to its high fault tolerance and excellent power efficiency[
Journal of Semiconductors, Volume. 42, Issue 1, 014102(2021)
Voltage-dependent plasticity and image Boolean operations realized in a WO x-based memristive synapse
The development of electronic devices that possess the functionality of biological synapses is a crucial step towards neuromorphic computing. In this work, we present a WOx-based memristive device that can emulate voltage-dependent synaptic plasticity. By adjusting the amplitude of the applied voltage, we were able to reproduce short-term plasticity (STP) and the transition from STP to long-term potentiation. The stimulation with high intensity induced long-term enhancement of conductance without any decay process, thus representing a permanent memory behavior. Moreover, the image Boolean operations (including intersection, subtraction, and union) were also demonstrated in the memristive synapse array based on the above voltage-dependent plasticity. The experimental achievements of this study provide a new insight into the successful mimicry of essential characteristics of synaptic behaviors.
1. Introduction
The neuromorphic computing system is attracting significant interest to simulate the human brain due to its high fault tolerance and excellent power efficiency[
In this paper, we report voltage-dependent STP in an Au/WOx/Ti memristive device. The transition from STP to LTP was achieved by increasing the amplitude of the applied voltage, and the sustained high-intensity stimulation induced permanent enhancement of conductance without any decay process. Moreover, the image Boolean operations were also demonstrated in a memristive synapse array based on voltage-dependent plasticity.
2. Experimental section
2.1. Fabrication of the device
The fabrication process of the Au/WOx/Ti device was as follows: first, Ti bottom electrodes (BEs) were deposited on a SiO2/Si substrate (SiO2 thickness 300 nm) by magnetron sputtering at a pressure of 1 Pa; second, a WOx layer was deposited on the Ti BEs by magnetron sputtering using a metal W target at room temperature. The film was prepared using mixed argon and oxygen (at a ratio of 1 : 1) at a pressure of 1 Pa. Finally, Au top electrodes (TEs) were thermally evaporated on the surface of the WOx film.
2.2. Electrical measurements of the device
The current–voltage (I–V) measurements were taken using a Keithley 2636A source meter, and pulse measurements were obtained using an arbitrary function generator (3390, Keithley) and a digital oscilloscope (Keysight DSOS404A). The positive direction of the bias voltage was defined such that the current flowed from the top to the bottom electrode. A cross-sectional image of the device was obtained using an SEM (Nova Nano SEM 450).
3. Results and discussion
Fig. 1(a) shows a schematic of a synapse showing the pre-synapse, post-synapse, and synaptic cleft structure. The strength of a synapse can be dynamically tuned by external stimulation, which represents the neurochemical basis for learning and memory functions. In this study, the two-terminal memristor was designed to simulate a synapse (Fig. 1(b)). The electrical spike transferred from TEs to BEs, which corresponds to the transmission of biological signals from the pre-synaptic neuron to the post-synaptic neuron. Fig. 1(c) shows the as-prepared Au/WOx/Ti memristive device, which was prepared in crossbar arrays using the sputtering deposition technique. Our memristive device has a typical metal–insulation–metal (MIM) structure. As shown in the cross-sectional scanning electron microscope (SEM) image in Fig. 1(d), the 80-nm-thick WOx layer between the Au and Ti electrodes can be clearly seen, which demonstrates the two-terminal structure of this device.
Figure 1.(Color online) The structure and characterization of the Au/WOx/Ti memristive device. (a) Schematic illustration of the biological synapse connecting pre-synaptic and post-synaptic neurons. (b) Schematic illustration of the device, including Au top electrodes, WO
The I–V characteristics of the as-fabricated device were examined under a continuous voltage sweep, as shown in Figs. 2(a) and 2(b). The distinctive history-dependent memristive behavior was observed, whereby the device conductance could be continuously increased (decreased) with positive (negative) voltage sweeps. We found that the rectifying effect was observed during this process, which may be ascribed to the different Fermi levels between the Au electrode and WOx films. According to our previous study, the modulation of Au/WOx Schottky barrier height can account for analog resistive switching (A-RS) that occurs as a result of the migration and accumulation of oxygen ions[
Figure 2.(Color online) Demonstration of spike-intensity-dependent synaptic plasticity in a Au/WO
As shown in Fig. 2(d), the PPF value depends on the time interval between two spikes (the amplitude varied from 0.4 to 2 V, the intervals varied from 1 to 200 ms and the duration was is fixed at 50 ms). The second spike enhanced the maximum current when it closely followed the first spike. Herein, the dependence of the PPF on the pulse interval (Δt) can be fitted by a double exponential function, which shows two-phase behavior:
where t is the pulse interval time, C1 and C2 are the initial facilitation magnitudes of the respective phases, and τ1 and τ2 are the characteristic relaxation times of the respective phases. In the case of voltage amplitude with 2 V, τ1 and τ2 are 23 and 257 ms, respectively. The timescales of our memristive synapse are comparable to those of a biological synapse[
STP can be transformed into long-term potentiation (LTP) through a rehearsal process, which was demonstrated in the memristor by using repeated high-frequency stimulation or increasing the number of stimulations[
Figure 3.(Color online) The transition from STP to LTP by adjusting spike intensity. (a) The device received input stimuli with different features, including a spike train with an amplitude of 0.2 V, weak stimuli with an amplitude of 1.5 V, and strong stimuli with an amplitude of 3 V. (b) Memorization of the image “T” to demonstrate the transition from STM to LTM. Case 1: the conductance of the memristive array before stimulation, after 1.5-V stimulation, and after stimulation for 60 s; Case 2: the conductance of the memristive array before stimulation, after 3-V stimulation, and after stimulation for 60 s. The different color levels represent different magnitude conductance values.
The mechanisms underlying the transition from STP to LTP are relevant to the short-term memory (STM) and long-term memory (LTM) models proposed by Atkinson and Shiffrin, in which the transition from STM to LTM occurs through a process of repetition[
It is interesting to note that the voltage-dependent plasticity allowed our devices to realize important logical operations. Boolean operations are key logical algorithms that are widely used in image processing[
In this work, we successfully reproduced Boolean operations in our memristive synapse array. As shown in Fig. 4(a), 30 spike stimuli with an amplitude of 1.5 V (−1.5 V) and a duration of 50 ms were input from the pre-synapse (post-synapse) to implement the intersection operation, representing the input of image “X” (“Y”). Thus, the memristors in intersection part between “X” and “Y” undergo spike stimuli with an amplitude of 3 V. Fig. 4(b) shows the Boolean intersection operation: when training either the image “X” or “Y” in the memristive synapse array no image was obtained after 60 s due to the STP effect. When the “X” and “Y” images were input simultaneously, only the intersection part received stimuli with a high amplitude. The obtained image “V” could be memorized due to the LTP effect, which indicates the realization of the Boolean intersection operation.
Figure 4.(Color online) Demonstration of image Boolean intersection operation in the memristive synapse array. (a) The stimulation condition for inputting the images “X” and “Y”. (b) The conductance states of the devices under different inputting conditions.
Moreover, to reproduce Boolean subtraction and union operations, the amplitude of input stimuli was modified. Fig. 5(a) shows the input condition and the results of the subtraction operation. The spike stimuli with an amplitude of 3 V (1.5 V) were input from the pre-synapse (post-synapse); thus the intersection between “X” and “Y” received spike stimuli with an amplitude of 1.5 V. When the “X” and “Y” images were input simultaneously, only the part of “X” that was outside “Y” received 3 V stimuli and could be memorized after 60 s, which indicates that the Boolean subtraction operation was realized. Similarly, the spike stimuli with an amplitude of 3 V (−3 V) were input from the pre-synapse (post-synapse) to implement the union operation. As can be seen in Fig. 5(b), all the parts received high-intensity stimuli (above 3 V) and were memorized after 60 s, which indicates that the Boolean union operation was realized.
Figure 5.(Color online) Demonstration of the image. (a) Boolean subtraction and (b) Boolean union operations in the memristive synapse array.
4. Conclusion
In summary, we demonstrated voltage-dependent synaptic plasticity in a WOx-based memristor, in which a higher voltage induced a larger enhancement of PPF. By increasing the spike amplitude, STP could be transformed to LTP, and sustained high-intensity stimulation induced permanent enhancement of conductance without any decay process. The voltage-dependent characteristic allows the memristor to reproduce image Boolean operations in the memristive synapse array. This work promotes the development of accurate and thorough simulation of synaptic behaviors that could have future neuromorphic applications.
Acknowledgements
This work was supported by the fund from Ministry of Science and Technology of China (Nos. 2018YFE0118300 and 2019YFB2205100), the NSFC Program (Nos. 11974072, 51701037, 51732003, 51872043, 51902048, 61774031, 61574031 and U19A2091), the “111” Project (No. B13013), the fund from Ministry of Education of China (No. 6141A02033414), and. The fund from China Postdoctoral Science Foundation (No. 2019M661185). The Fundamental Research Funds for the Central Universities (No. 2412019QD015), and the Fund from Jilin Province (JJKH20201163KJ).
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Jiajuan Shi, Ya Lin, Tao Zeng, Zhongqiang Wang, Xiaoning Zhao, Haiyang Xu, Yichun Liu. Voltage-dependent plasticity and image Boolean operations realized in a WO x-based memristive synapse[J]. Journal of Semiconductors, 2021, 42(1): 014102
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Received: May. 21, 2020
Accepted: --
Published Online: Mar. 19, 2021
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