Characteristics of the typical photoresponsive memristors.
Optoelectronic resistive switching devices, utilizing optical and electrical hybrid methods to control the resistance states, offer several advantages of both photons and electrons for high-performance information detecting, demodulating, processing, and memorizing. In the past decades, optoelectronic resistive switching devices have been widely discussed and studied due to the potential for parallel information transmission and processing. In this chapter, recent progresses on the optoelectronic resistive switching mechanism, materials, and devices will be introduced. Then, their performance such as photoresponsivity, on/off ratio, as well as retention will be investigated. Furthermore, possible applications of the optoelectronic resistive switching considering logic, memory, neuromorphic, and image-processing devices will be summarized. In the end, the challenges and possible solutions of optoelectronic resistive switching devices for the next-generation information technology will be discussed and prospected.
- resistive switching
The great success of transistor-based integrated circuits for modern computing provided exponential development of the way we live, work, and communicate. In the past decades, device scaling-down was an effective approach to maintain the continuous development of computing capabilities. However, the current semiconductor technology is facing the physical scale limitation and bandwidth bottleneck between memory and central processing unit (CPU) in modern-computing architectures, as shown in Figure 1a. Therefore, new devices, new architectures, and even new computing principles are eagerly desired to further enhance computing efficiency and capability in the post-Moore era.
Optoelectronic interconnection is a feasible method to overcome the bottleneck by transmitting data between memory and CPU via photons, which have a much faster transporting speed, a higher bandwidth, no Joule heat, and no interference over electrons [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. As shown in Figure 1b, optical communication between CPU and memory will fully meet the demands of modern-computing architecture in high-frequency data communicating and processing [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. On the other hand, memristor (memory + resistor), a simple two-terminal electrical switch with nonvolatile reconfigurable resistance states, has been considered as one of the promising approaches to construct new architecture with the novel in-memory computing (Figure 1c) [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. Due to the arbitrary nonvolatile resistance states, memristor-based architectures provide the high potential for integrating processor and memory together, which eliminates the bottleneck between memory and processor in the modern von Neumann-computing architecture, thus allowing high parallel data processing [21, 22, 23, 24, 25, 26]. However, the electronic memristor-based architecture still has bandwidth limitation in high-frequency data communicating between different modulators.
The combination of photons and memristor-based circuits will integrate the advantages of photons in a high-speed data transmission and memristors in parallel in-memory computing and may open up a new era for future computing owing to the high bandwidth and low-power consumption. Moreover, the optoelectronic-based memristive system will extend the application of memristor-based architecture to image or visual information processing. As shown in Figure 2, with the increasing demands of mobile computing in human daily life, wearable devices, health-care devices, and human-machine interacting devices including intelligent image sensor, video monitor, invisible touch screen, electronic eyes, wearable heart or blood monitoring and display devices, smart processor, and even brain-implantable devices will be more and more important for an efficient and comfortable way for our better life, body health, daily work, and communication [27, 28, 29, 30, 31, 32]. The optoelectronic memristor-based architecture, wherein both photons and electrons are used for parallel data processing and communicating between input-output (I/O) devices and the in-memory processor, may provide such a platform for the mobile computing.
In this chapter, recent developments on optoelectronic memristor materials and devices, and applications including logic, memory, memristor, and neuromorphic devices will be summarized, then their capability in future optoelectronic on-chip interconnection, in-memory computing, brain-inspired computing, and visual information processing will be evaluated. These optoelectronic approaches may help to overcome the limitations and bottleneck in the current computing devices and architectures, as well as contribute to efficient data analysis, cognitive computation, and artificial intelligence.
2. Optoelectronic resistive switching materials and devices
Resistive switching memory (memristor) is a simple two-terminal device with three-layered structure—two electrode layers for electrical signals input and output, and a “memory” layer in between. The resistance of the “memory” layer can be dynamically reconfigured by voltage- or current-induced ion implantation, interfacial charge accumulation, and so on. Usually, the electrodes are metal or oxide, such as Ag, Au, Pt, Ta, Ir, Cu, ITO, and so on, which will affect the resistive switching behavior by their different work function, electron affinity, electrochemical energy, and so on [33, 34]. For the “memory” layer, various kinds of materials have been used, including binary oxide, nitride, perovskite, low-dimensional materials, and organic materials [35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45]. Due to the simple structure and excellent performance including high-density integration, high-speed, and low-cost fabrication for logic, memory, and neuromorphic applications, memristor has been considered as one of the most promising next-generation information technologies [19, 20, 46].
Meanwhile, optical interconnection with high data transmission speed and no Joule heat or interference has proved to be another effective approach to overcome the data transmission bottleneck between memory and processor. Moreover, combining the advantages of photons and electrons in an optoelectronic memristor will allow highly parallel data transmission and processing, as well as extending the application for image recognition and visual information processing. In the optoelectronic memristor (Figure 3), both optical and electrical signals are able to modulate the resistance states of a “memory” layer, which is sensitive to both electrical and optical signals. Therefore, photosensitive semiconductor materials will meet the demands of the “memory” layer, such as silicon, cerium oxide, zinc oxide, perovskite, low-dimensional materials and organic materials, and so on [47, 48, 49, 50, 51, 52].
In 2012, Ungureanu et al. reported a reversible light-controlled resistive switching memory in metal/Al2O3/SiO2/Si structure, with the photogenerated electrons injection from Si to Al2O3 at sufficient positive bias and charge removal at negative bias . In this work, the light pulses with different wavelength and intensity were utilized to study the photoresponsive behavior of the memory, as well as to achieve multilevel and multifunctional optoelectronic resistive switching memory. This work proved that the metal/Al2O3/SiO2/Si-structured device can work as a multifunctional optoelectronic device including information storage and light sensing, meanwhile, offering the possibility for extra optical degree to be exploited in resistive switching memory devices.
Besides Si, ZnO is another kind of typical photosensitive semiconductor and has been widely studied for UV detectors, as well as resistive switching memories [35, 49]. Based on the semiconducting and photosensing properties of ZnO, Bera et al. combined the resistive switching and persistent photoconductivity (PPC) together in ZnO/NSTO Schottky junction in 2013 . The photo-induced interfacial positive oxygen vacancies movement under electric field can lower the interfacial barrier and modulate the nonvolatile multilevel resistance states, thus realizing persistent photoconductivity and resistive switching. Annealing process was able to erase the PPC to its initial state. The results in this work provided a general route to achieve multifunctional devices by integrating functional materials.
Most recently, two-dimensional materials, such as graphene and MoS2, have drawn much more attention due to their promising properties for the next-generation information technology. In 2014, Roy et al. reported a multifunctional photoresponsive memory based on graphene-MoS2 hybrid three-terminal structure, wherein a gate was used to maintain and erase the trapped charges, thus achieving erasable persistent photoconductivity for optoelectronic memory devices (Figure 4) . In addition, different gate amplitudes and light intensities were used to achieve multilevel memory. This work showed that the novel two-dimensional materials are promising for multifunctional optoelectronic memory.
Besides, organic materials have many advantages including low-cost fabrication, flexibility, and bandgap tunability, which are promising for optoelectronic memory devices. In 2012, Ye reported an optoelectronic multilevel resistive switching memory shown in Figure 5 using a metal-conjugated donor-bridge-acceptor (DBA) molecule, which is responsive to both optical and electrical stimuli . Under dark condition, the Al/metal-conjugated DBA/ITO-structured devices showed a bi-stable resistive switching behavior. When illuminated by UV light, a middle state was induced by the sub-step charge transfer process through the cooperation of UV light and electric field. This work may open up new opportunities of organic materials for designing multifunctional optoelectronic memory.
In a short summary, photo-induced charge trapping and detrapping play the key role in optoelectronic resistive switching effect. Other than the materials stated earlier, carbon nanotube, complex oxide, perovskite, quantum dots, and so on, also show optoelectronic resistive switching effects [57, 58, 59, 60]. The applications of optoelectronic memristor including logic, memory, neuromorphic devices, and their performance will be discussed in the following sections.
3. Multifunctional optoelectronic logic and memory
Optoelectronic memristors, allowing electrical and optical signals to modulate the states, are capable of functioning as sensor, decoder, arithmetic unite, logic, and memory devices for data communication, integrated photonics, in-memory computing, and brain-like computing. In this section, optoelectronic logic and memory devices will be discussed and their performance will be evaluated.
Logic is one of basic and most important functions in the integrated circuits [61, 62, 63, 64, 65]. Therefore, the realization of optical and electrical-mixed logic is the first step for optoelectronic circuits. For example, Kim et al. reported a series of optoelectronic logic devices based on single-walled carbon nanotube (SWNTs)/silicon junctions . As shown in Figure 6a, in the SWNTs/Si junction, voltage and light pulse are inputs and currents are outputs. Due to the photoresponse under voltage bias, the output current is much higher (logical “1”) when both voltage and light are applied to the junction than the current when only one or neither is applied (logical “0”), thus functioning as an “AND” gate (Figure 6b). Similarly, “OR” gate and 2-bit adder functions were also achieved using two junctions. Furthermore, utilizing four junctions with specialized active areas, a 4-bit digital-to-analog converter (DAC) was demonstrated, as shown in Figure 6c and d.
Memory is another basic and important function in integrated circuits. If an optoelectronic memory device can integrate other specific functions such as photo signal detection, demodulation, arithmetic, and even logic in a single device, where the processed data can be stored in situ, then it will lower the complexity of integrated circuits and allow highly parallel computation. To achieve this goal, in 2015, we studied the ITO/CeO2−x/AlOy/Al-structured photoresponsible junction and designed a multifunctional optoelectronic resistive switching memory (OE-memory) with integrated photodetection, demodulation, and arithmetic . As shown in Figure 7, CeO2−x works as a photon-absorbing layer, and the electrons trapped in the defects in the CeO2−x layer near the interface can be excited by photons and leaving positively charged oxygen vacancies, which will lower the effective barrier and decrease the resistance of the junction persistently with a thinner barrier. At the CeO2−x and Al interfaces, a 5-nm native oxide layer formed and acted as an insulating layer to decrease the dark current. ITO works as a transparent and conductive top electrode and Al is the bottom electrode. In this simple two-terminal structure, persistent photoconductivity was observed as shown in Figure 8. The nonvolatile resistance states can be reversibly switched between high resistance state (HRS) and low resistance state (LRS) by visible light pulse and voltage, thus acting as an optical-write and electrical-erase memory.
Further study of this optoelectronic memory showed the wavelength and intensity-dependent photoresponse, which was utilized to design an optical signal detector and demodulator. As shown in Figure 9, with two-digit information (wavelength and intensity) per light pulse, four pulses with blue or green and 4 or 6 pW/μm2 are capable of carrying 8-digit information, which is one letter according to the ASCII code, and can be demodulated and stored into four resistance states. Therefore, optical signal detecting and demodulating functions have been achieved in this simple optoelectronic memory. Furthermore, the photoconductance is increasing linearly with light pulse number (Figure 10a). Based on this linear relationship between photocurrent and the number of light pulses, counter and adder of the number of light pulses can be realized and the details are shown in Figure 10b. Meanwhile, the output results after counting or adding the pulse number can be stored as resistance states, thus allowing the integration of simple arithmetic and memory functions in a single cell. To evaluate the memory retention performance, multilevel resistance states were measured continuously, and after 104 s, the resistance states remain distinguishable (Figure 11). Above all, the simple optoelectronic structure is capable of functioning as optical signal detection, demodulation, arithmetic, and storage. These results demonstrate the possibility of using multifunctional optoelectronic devices for future integrated photonics, parallel processing, and in-memory computing .
In our recent work, to further exploit the integration of logic functions into the memory , electrically resistive switching and the persistent photoconductivity are combined to modulate the resistance states (Figure 12) .
Using one light pulse and one voltage pulse as logical inputs and current as logical output, an “AND” gate was achieved. Only when both a light pulse and a voltage pulse are present, the output value is larger than 150 nA, which is logical “1,” otherwise, the output current is lower than 150 nA, which is logical “0.” Furthermore, the “AND” gate can be reconfigured to “OR” gate when a light-write pulse applied before logic operations, thus allowing the OE-memory functioning as an optoelectronic nonvolatile reconfigurable logic gate (Figure 13). Besides the reconfigurable “AND” and “OR” logic functions, “NOT” operation and a complicated logic operation were achieved by introducing another electrical-erase pulse, as well as optical adder and digital-to-analog converter (DAC) functions were achieved by using two light pulses as inputs. We name this nonvolatile reconfigurable logic gate as “memlogic,” memory of logic operations and outputs.
Based on the optoelectronic reconfigurable logic functions, a proof-of-concept reconfigurable image-processing and memorizing functions were demonstrated, as shown in Figure 14. Two images with “X” and “Y” shapes of visible light pulses are used as inputs into a 5 by 5 memlogic array and current map as output (Figure 14a). When all the cells were reset to “AND” logic state, the processor can find the same part in different images and we named it SAME FINDER (Figure 14b). When all the cells were set to “OR” states, the processor can find the parts either in ‘X” or “Y” images and we named it as ALL FINDER (Figure 14c). Therefore, the single memlogic array can realize two different image processing, as well as image storage functions.
Above all, the memlogic is capable of performing different reconfigurable logic functions and nonvolatile memory and may provide a new method for reconfigurable in-memory computing, integrated photonics, and artificial intelligence .
In summary, for information technology application, these optoelectronic resistive switching devices need to be optimized to improve their performance including the photoresponsivity, speed, memory window, retention time, endurance, fabrication cost, reliability, yield, and so on. Here, the summary of some basic parameters is shown in Table 1. The photoresponsivity measures the input–output gain in a photoresponsive system and means the electrical output per optical input. On/off means the memory window. Speed represents how fast the device can work. For memory or logic applications, the speed needs to be at ns level. Here, the speed of these optoelectronic resistive switching devices is not enough and needs to be improved exponentially. Possible methods will be increasing the build-in electric field to accelerate the photo-induced charge separation by using electrodes with a higher work function or increasing the defects concentration. Retention means how long the state can maintain. From this table, we can see that the performance especially the speed should be improved immediately.
4. Optoelectronic neuromorphic devices
Artificial neural network is one of the most promising ways to achieve brain-like computing and has been an attractive area especially for memristors as the similar ionics-based mechanisms and stochastic dynamics with neurons [69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87]. Optoelectronic memristors provide another degree of freedom by light to control their states, which means light pulse can be utilized as stimuli to control artificial synapse weight and proceed with synaptic behaviors simulation. Moreover, optoelectronic memristor-based networks allow optical communications among nodes and will reduce power consumption, increase communicating speed, and efficiency. Recently, several research groups have proposed optoelectronic artificial synaptic devices based on persistent photoconductivity for neuromorphic computing. In 2010, Agnus et al. reported an adaptive architecture based on optically gated carbon nanotube transistors, which allow optical write and electrical program processes to control the resistance states . The programmed states are nonvolatile and can be used to store synapse weight in adaptive architectures. More specific synaptic behaviors simulation such as short-term memory (STM), long-term memory (LTM), facilitation, and spike-timing-dependent plasticity (STDP) have been achieved in IGZO-based persistent photoconductive devices by Lee in 2017. They also showed the photoenergy and frequency-dependent plasticity. These basic synaptic behaviors simulation will promote the development of optoelectronic artificial neuro-network .
As we all know, human eyes are the most important media to receive visual information from surroundings and provide humans with the ability to recognize objects. In human visual system, the eyes receive light signal and convert it to electrical pulses signal, which can be detected by visual neuron and analyzed or memorized by visual cortex in the brain. In general, two parts for sensing and processing of visual information form the visual system. In recent years, inspired by human visual system, several kinds of artificial visual systems have been proposed to simulate eyes for visual information processing, retinopathy therapy, and reproduction of visual system [27, 89, 90, 91]. Usually, photodetector array acts as the retina to sense the light that converts it to electrical signal for image processor, visual neuron, and cortex. For example, as shown in Figure 15, Mathieson et al. reported a photovoltaic retinal prosthesis, which is capable of detecting light signal, imaging on silicon photodiode array, and processing the detected image. The demonstration of fully integrated wireless implant may reproduce the vision to the patients with retina diseases . However, this photodiode-based system lacks the function of memorizing of image information. Therefore, memristor may be applicable for image memory. In 2017, Chen et al. reported an artificial visual memory system consisting of an imager and a memory. The UV image can be detected by In2O3-based imaging system and stored by Al2O3-based processing and memory system . The design of this artificial visual system provides a novel approach to integrate different devices to achieve more multifunctional bioinspired systems.
Thanks to the reported works, these novel promising optoelectronic synaptic devices have been proposed and achieved and will be the first step for optoelectronic neural networks. Optoelectronic artificial neural networks provide the additional advantage of optical communication into memristor-based neuromorphic architectures. Based on the previous research, the next plan should focus on improving their performance, exploiting optoelectronic artificial neural networks, and optogenetic neural networks.
5. Other approaches for multifunctional resistive switching
Besides the optical-based resistive switching memories, electrical- and magnetic-based resistive switching devices are also interesting and attractive due to their important and promising physical properties and applications .
Recently in our group, a nanochannel device was designed in V2O5-based selector via electric field-induced ion migration . As shown in Figure 16, this electric field-induced nanochannel demonstrates a sharp and reliable metal–insulator transition (MIT) with a 17 ns switching speed, an 8 pJ energy consumption, and less than 4.3% variability. By combining with one HfOx-based memristor, the 1S1R structure device can ensure the correct reading of the memory states continuously for 107 cycles, therefore demonstrating its great possibility to overcome the crosstalk problem in high-density crossbar memory. This work proved that electric-field-induced ion migration at the nanoscale is an effective approach to optimize device performance or design novel conceptual devices.
With the electric field-induced ion migration method, our group also achieved nanoscale magnetization reversal in cobalt ferrite (CFO) thin film (Figure 17) [95, 96]. The electric field-induced migration and redistribution of Co2+ between Fe vacancies induced the unidirectional magnetic anisotropy of the sample along the <110> directions. The reported magnetization reversal is nonvolatile and reversible, which can be controlled by tuning the electric field polarity and amplitude. Such a nanoscale magnetization modulation by nanoionics may provide a novel approach to manipulate the magnetization of magnetic materials for low-power magnetic memory and spintronics.
6. Challenges and possible approaches
Thanks to the efforts of scientific researchers and engineers from all over the world, the optoelectronic resistive switching has achieved great progress. However, there are still some challenges in the way of further development of optoelectronic memristors currently as follows:
Mechanism: the widely accepted mechanism of PPC is the photo-induced charge trapping and detrapping in defects. However, due to the technique limitation of directly mapping photo-generated carriers’ distribution under light illumination, it is hard to clarify the PPC at electron scale.
Materials and performances: many materials are photoresponsive and resistance switchable. Therefore, finding a durable, reliable, and CMOS-compatible material with excellent optoelectronic resistive switching performances is still a challenge, and the performance especially the speed is more challenge due to defects-related process.
Spatial design of the optoelectronic neural networks: optoelectronic platform will need to integrate light source to the integrated circuits. Therefore, the devices’ surface and physical position should be accessible for light source. Then the best way to route the light source to devices’ surface is a critical issue.
Possible approaches include the following:
Mechanism: light-integrated TEM is a possible method to observe the light-controlled physical properties. In my current group in Finland, an in situ TEM under external electric field and light sources is ready, and there may be some promising results about optoelectronics in the following years .
Materials and performance: the direct and effective way to find a suitable material is trying and trying. One promising material has to be tested, optimized, and improved repeatedly before the product, and what we can do is to just be patient. The performance requirement depends on the application, so performance is negotiable .
Spatial design of the optoelectronic neural networks: with the development of nanophotonics and nanolasers, in the near future, integrated photonics will provide efficient and feasible approach to address the spatial design of optoelectronic artificial neural networks .
7. Conclusions and prospects
In this chapter, recent progress on optoelectronic resistive switching are introduced and summarized. Due to the photosensitive media in memristor, light can be used as another degree of freedom to control the resistive switching behavior for multifunctional optoelectronic devices. Here, the main mechanism is the photo-induced carriers trapping and de-trapping in defects in the semiconductor “memory” layer. To further promote the development of the multifunctional optoelectronic devices, their performance should be improved to meet different applications considering image sensor, memory, logic, and neuromorphic devices.
Optoelectronic memristor-based neuromorphic devices, integrating the functions of information storage, processing, image detection, and memory together, may provide high potential for intelligent image sensor and optoelectronic in-memory computation. Based on the integrated photonics [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], optoelectronics [47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60], and optogenetics [98, 99, 100], an optoelectronic artificial neural network may play a key role in future human-machine interactive devices.
This work was supported by the National Key R&D Program of China (2017YFB0405604), the National Natural Science Foundation of China (61722407, 61674152, 51525103, 11474295), the Natural Science Foundation of Zhejiang Province (LR17E020001), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2014261).
Conflict of interest
The authors declare no conflict of interest.