Persistent Photoconductivity Effect Research Articles

Photoresponse Design in Metal Oxide Semiconductor TFTs toward Diverse Applications: Display Drivers, Photodetectors, and Optoelectronic Synapses.

Different application domains impose diverse and often conflicting requirements on the optoelectronic performance of metal oxide semiconductor (MOS) thin-film transistors (TFTs). These varying demands present substantial challenges in the selection of TFT materials and the optimization of device performance. This study begins by examining three primary application areas for TFTs: display drivers, photodetectors, and optoelectronic synapses. A comparative analysis of the optoelectronic properties is conducted among various MOS TFTs fabricated by magnetron sputtering, including indium-gallium-zinc-oxide (IGZO, In/Ga/Zn = 1:2:1), indium-gallium-oxide (IGO, In/Ga = 1:1), indium-zinc-oxide (IZO, In/Zn = 1:1), indium oxide (In2O3), and zinc oxide (ZnO). The investigation reveals the significant impact of material selection on key performance metrics essential for these applications, such as photoresponse, the decay rate of photocurrent (Iph), and negative bias illumination stress (NBIS). Additionally, a comprehensive summary of the applicable domains for each type of TFT is provided. The study also explores the correlation between activation energy (Ea) and TFT performance, indicating that higher Ea is associated with a stronger persistent photoconductivity (PPC) effect but poorer stability. Furthermore, the content of oxygen vacancy (VO) shows a positive correlation with the decay rate of Iph. Lastly, the photogenerated carrier lifetime (τ) is derived and compared among the five MOS materials, revealing the potential applications and performance characteristics of each in optoelectronic devices. The findings offer a nuanced understanding of the intrinsic relationships between material properties, defect states, and photoelectric performance, thereby guiding the selection and optimization of channel layer materials for specific application requirements.

Tailoring Dynamic Synaptic Plasticity in FeTFT Optoelectronic Synapse for Associative Learning

AbstractNeuromorphic hardware with dynamic synaptic plasticity presents fascinating applications in advanced artificial intelligence. However, the development of low‐cost, CMOS (Complementary Metal‐Oxide‐Semiconductor)‐compatible, and dynamically tunable synaptic devices is still nascent. Notably, the spontaneous polarization of hafnium oxide‐based ferroelectric materials, combined with the persistent photoconductivity effect of indium‐gallium‐zinc‐oxide (IGZO) semiconductors, provide a potential solution. In this paper, a novel optoelectronic synaptic device based on ferroelectric thin‐film transistors (FeTFTs) is proposed to achieve dynamic synaptic plasticity through the co‐modulation of light and electrical signals, which can effectively adjust the dynamic range of synaptic weights and emulate complex biological behaviors. The effective dynamic synaptic plasticity of FeTFTs is quantified under different light power intensities and verified through the emulation of complex biological behavior, such as classical conditioning experiments and environmental adaptive behavior. Furthermore, a 3 × 3 FeTFT array is constructed to demonstrate its potential applications in memory functions. This CMOS‐compatible optoelectronic synaptic device with dynamic synaptic plasticity provides a robust hardware foundation for the future development of artificial intelligence, enabling it to adapt to more complex environments and perform tasks efficiently.

An Aqueous Processed Photosensitive Bilayer van der Waals Thin Film for Flexible Neuromorphic Vision

AbstractPrinting colloidal ink of semiconductor nanocrystals, particularly 2D nanosheets, facilitates the generation of van der Waal's thin films (vdWTFs) with inherent flexibility and optoelectronic properties, ideal for wearable intelligence devices, such as bionic vision. Several intricate strategies, such as mechanical/electrochemical exfoliation and pulsed laser/chemical vapor, have been devised for producing solvent‐dispersed nanosheets and corresponding vdWTFs; achieving efficient preparation via more environmentally friendly processes remains a challenge. Here, an ecofriendly aqueous approach for crafting vdWTF, relying on a gentle self‐assembly C8‐BTBT nanosheet aqueous ink, is demonstrated. The resulting vdWTF features a bilayer heterostructure, wherein a crystalline C8‐BTBT nanosheet layer is staggeringly stacked atop a graphene oxide (GO) film. This configuration allows the sliding of the staggered 2D nanosheets, accommodating local tension and compression, thereby averting film breakage and ensuring desirable flexibility. Further, by harnessing the natural photosensitivity of C8‐BTBT and the charge‐trapping capabilities of GO component, the heterostructured bilayer vdWTFs confer exceptional photoperception and intrinsic persistent photoconductivity effects upon their flexible planar devices. These attributes enable the emulation of bio‐visual behaviors such as short/long‐term memory, light adaptation, reinforcement learning, and image recognition.

A Violet‐Light‐Responsive ReRAM Based on Zn2SnO4/Ga2O3 Heterojunction as an Artificial Synapse for Visual Sensory and In‐Memory Computing

AbstractDue to the imitation of the neural functionalities of the human brain via optical modulation of resistance states, photoelectric resistive random access memory (ReRAM) devices attract extensive attraction for synaptic electronics and in‐memory computing applications. In this work, a photoelectric synaptic ReRAM (PSR) of the structure of ITO/Zn2SnO4/Ga2O3/ITO/glass with a simple fabrication process is reported to imitate brain plasticity. Electrically induced long‐term potentiation/depression (LTP/D) behavior indicates the fulfillment of the fundamental requirement of artificial neuron devices. Classification of three‐channeled images corrupted with different levels (0.15–0.9) of Gaussian noise is achieved by simulating a convolutional neural network (CNN). The violet light (405 nm) illumination generates excitatory post synaptic current (EPSC), which is influenced by the persistent photoconductivity (PPC) effect after discontinuing the optical excitation. As an artificial neuron device, PSR is able to imitate some basic neural functions such as multi‐levels of photoelectric memory with linearly increasing trend, and learning‐forgetting‐relearning behavior. The same device also shows the emulation of visual persistency of optic nerve and skin‐damage warning. This device executes high‐pass filtering function and demonstrates its potential in the image‐sharpening process. These findings provide an avenue to develop oxide semiconductor‐based multifunctional synaptic devices for advanced in‐memory photoelectric systems.

Optically and electrically modulated artificial synapses based on MoS2/PZT ferroelectric field-effect transistor for neuromorphic computing system

To present an advanced device scheme of high-performance optoelectronic synapses, herein, we demonstrated the electrically- and/or optically-drivable multifaceted synaptic capabilities on the 2D semiconductor channel-based ferroelectric field-effect transistor (FeFET) architecture. The device was fabricated in the form of the MoS2/PZT FeFET, and its synaptic weights were effectively controlled by dual stimuli (i.e., both electrical and optical pulses simultaneously) as well as single stimuli (i.e., either electrical or optical pulses alone). This could be attributed to the electrical pulse-tunable strong ferroelectric polarization in PbZrxTi1−xO3 (PZT) as well as the polarization field-enhanced persistent photoconductivity effect in MoS2. Additionally, it was confirmed that the proposed device possesses substantial activity, achieving approximately 95 % pattern recognition accuracy. The results substantiate the great potential of the 2D semiconductor channel-based FeFET device as a high-performance optoelectronic synaptic platform, marking a pivotal stride towards the realization of advanced neuromorphic computing systems.

Investigation on room-temperature persistent photoconductivity in γ-InSe layered semiconductor

We have presented the room-temperature persistent photoconductivity (PPC) effect in γ-InSe layered semiconductors. The PPC effect can be observed in the temperature range from room temperature down to 200 K, and the PPC decay is well described by a stretch-exponential function. The temperature-dependent PPC and performance of PPC decay indicate that the random local-potential fluctuations (RLPF) mechanism is responsible for PPC effect inγ-InSe. In addition, the PPC behavior at different temperature regions can be well explained by the percolation model associated with the RLPF mechanism.

Defect-engineering-based titanium dioxide nanowires for artificial optoelectronic synapses

Artificial optoelectronic synapses, as a typical neuromorphic device, have garnered significant attention due to their concurrent capabilities in processing visual perception and memory. In this study, we propose an artificial optoelectronic synapse with a dual-terminal metal-semiconductor-metal (Au/TiO2/Cr/Au) structure. By modulating the concentration of oxygen vacancy defects in TiO2, the device exhibits a significant persistent photoconductivity (PPC) effect. The devices successfully simulated the synaptic behaviors, including paired-pulse facilitation (PPF), short-term plasticity (STP), long-term plasticity (LTP), and the learning-forgetting process. In addition, the device array has the capability to recognize optical signals and exhibits an obvious response gradient under different optical powers. This demonstrates its ability to adaptively detect optical signals and simulate human visual memory.

High-performance Ga2O3 solar-blind UV photodetectors via film growth regulation and surface state engineering

Gallium oxide (Ga2O3) with a wide bandgap is emerging as one of the most promising candidates for solar-blind photodetectors (SBPDs). However, the practical application of Ga2O3-based SBPDs is impeded by substantial defects-related leakage currents, sluggish response speed, and high-cost, making their practical applications face great challenges. Here, we proposed strategic films growth regulation and surface state engineering to grow high-quality yet economical β-Ga2O3 films, thereby achieving high-performance SBPDs. High-resolution transmission electron microscopy revealed the presence of high-quality films with an extremely regular arrangement of β-Ga2O3 (2‾01) planes. The full width at half maximum and root mean square roughness values are 0.07° and 0.716 nm, respectively. The β-Ga2O3 films demonstrated a significant reduction in VO concentrations after O2/N2 plasma treatment. This process can effectively minimize thermal damage to the sample surface while retaining highly reactive free radicals. The photocurrents of devices can be further enhanced, leading to a larger PDCRs of 8.6 × 107, through N2 plasma treatment. Moreover, the films effectually suppress the persistent photoconductivity effect with τr and τd values of 5.31/0.97 s and 0.49/0.87 s before and after N2 plasma treatment, respectively. We have demonstrated the influence mechanism of O2/N2 plasma treatment on photoelectronic performance. This study provides a practical approach to growing high-quality β-Ga2O3 films and preparing high-performance but low-cost Ga2O3-based SBPDs.

MoS2/ZnO-heterostructured optoelectronic synapse for multiwavelength optical information-based sensing, memory, and processing

The rapid development of artificial intelligence and humanoid robots requires hardware-level revolution of synapses-based bionic vision system. However, current optoelectronic synapses featuring multiwavelength sensing and multifunctional processing necessitate intricate integrations of multi-materials and floating gate configurations, which induces complex fabrication process and limited integration density. Here, we propose a novel two-terminal optoelectronic synaptic device based on MoS2/ZnO heterostructure which integrates sense and memory functions and realizes multifunctional optical information processing while circumvents the need of complex configurations. By simultaneously exploiting the photo-adsorption oxygen dissociation property of ZnO and the persistent photoconductivity effect of MoS2, the device demonstrates synaptic capability of four-color perception with a single synaptic event consuming a mere 2.55×10−9 J. Different from the traditional power-density sensitive nociceptor, for the first time photon energy-sensitive nociceptors is developed, which can mimic the process of UV irradiation to the retina leading to fear of visible light. Diverse information processing ability, such as photonic information encryption, associative learning, and logic calculation, was demonstrated in a single device, indicating a practical strategy for facile design for future high integration intensity neuromorphic devices.

Reservoir Computing Based on Oxygen-Vacancy-Mediated X-ray Optical Synaptic Device for Medical CT Bone Diagnosis.

Recognition and judgment of X-ray computed tomography (CT) images play a crucial role in medical diagnosis and disease prevention. However, the storage and calculation of the X-ray imaging system applied in the traditional CT diagnosis is separate, and the pathological judgment is based on doctors' experience, which will affect the timeliness and accuracy of decision-making. In this paper, a simple-structured reservoir computing network (RC) is proposed based on Ga2O3 X-ray optical synaptic devices to recognize medical skeletal CT images with high accuracy. Through oxygen vacancy engineering, Ga2O3 X-ray optical synaptic devices with adjustable photocurrent gain and a persistent photoconductivity effect were obtained. By using the Ga2O3 X-ray optical synaptic device as a reservoir, we constructed an RC network for medical skeletal CT diagnosis and verified its image recognition capability using the MNIST data set with an accuracy of 78.08%. In the elbow skeletal CT image recognition task, the recognition rate is as high as 100%. This work constructs a simple-structured RC network for X-ray image recognition, which is of great significance in applications in medical fields.

Interface‐Engineering Induced Swift and Controllable Solar‐Blind Photoresponse in Ga2O3/SiC Heterojunction Based on Unconventional Rectification Characteristics

AbstractGa2O3 photodetectors with demonstrated high sensitivity provide a potential subversive scheme for solar‐blind photodetection. However, the planar structure and the relatively slow response speed of the device have restricted the integration and application of Ga2O3 photodetectors. Herein, a narrow SiO2 barrier layer is introduced on the commercial SiC substrate, and a vertical photodetector is realized based on β‐Ga2O3/SiO2/SiC heterostructure with controllable tunneling effect by tailoring the band structure at the interface. The developed device exhibits unconventional rectification characteristics and photoresponse performance in different biasing modes owing to the controllable tunneling effect at the interface. In particular, the photodetector achieves a high responsivity (186.8 A W−1) under solar‐blind illumination at reverse bias, while exhibiting obvious advantages in dark current (3 pA), photo‐to‐dark current ratio (8.8 × 106), linear dynamic range (138.8 dB), and specific detectivity (1.4 × 1015 Jones) at forward bias. The photodetector also demonstrates excellent photoresponse stability after 6 months in air without encapsulation. In addition, an alternating biasing strategy, inspired by its bidirectional operability, is proposed to suppress the persistent photoconductivity effect and increase the decay speed by 1.23 × 105 times. This work provides a referable strategy for the further development of high‐performance Ga2O3‐based photoelectronics.

2D Ca2Nb3O10 Optoelectronic Neuromorphic Device for Ultrasensitive UV‐C Vision and Encrypted Communication

AbstractNeuromorphic device that percepts ultraviolet (UV) radiations is essential for supplementing human visual perception. However, UV‐responsive synapses with high sensitivity and wavelength selectivity have not yet been developed so far. Herein, 2D optoelectronic synapses that are able to distinguish between UV‐A, UV‐,B and UV‐C radiations using a single material are demonstrated. The synapses use Ca2Nb3O10 nanosheets with wide bandgap and persistent photoconductivity effect (PPC) as UV‐responsive layer. The devices can detect UV‐C rays at ultralow intensity of 70 nW cm−2, which is the lowest record among 2D UV synaptic devices. Moreover, such a device demonstrates UV image vision, recognition, and memorization functions based on its diverse synaptic behaviors. The atomically thin UV synapses provide implications for enhancing human visual perception capabilities with spectral selectivity, ultrahigh sensitivity, and low‐energy consumption.

An Optical Artificial Synapse Based on Single‐Crystal Se‐Vacancy Bi2O2Se

AbstractThe optical synaptic device is a novel nonvolatile memory device that combines optical sensing function with synaptic plasticity to simulate the basic biomimetic behaviors of the human visual system. It shows a great potential application in artificial vision. However, most of the current optical synaptic devices are either structurally special or complex, which are quite difficult to prepare especially for mass production. In this work, a submillimeter single‐crystal Bi2O2Se flake with selenium vacancies (Bi2O2Se‐VSe) grown by physical vapor deposition is presented. Upon the Bi2O2Se‐VSe flake, an optical synaptic device is designed with the persistent photoconductivity (PPC) effect induced by selenium vacancies. The device can simulate the biological synapses with an obvious synaptic plasticity. Moreover, an artificial vision system consisting of a 3 × 4 array of optical synaptic devices has been fabricated. The intensity of the image pattern can keep at a high memory level of 54.64% and 19.31% for 532 and 1060 nm waiting for 400 s after illumination of 100 s, which demonstrates the devices exhibit excellent image sensing, learning, and memory storage. This work opens up a new route for fabricating the optical synaptic device with a simple structure and provides new ideas for studying artificial vision systems.

Energy-Efficient ReS2-Based Optoelectronic Synapse for 3D Object Reconstruction and Recognition.

The neuromorphic vision system (NVS) equipped with optoelectronic synapses integrates perception, storage, and processing and is expected to address the issues of traditional machine vision. However, owing to their lack of stereo vision, existing NVSs focus on 2D image processing, which makes it difficult to solve problems such as spatial cognition errors and low-precision interpretation. Consequently, inspired by the human visual system, an NVS with stereo vision is developed to achieve 3D object recognition, depending on the prepared ReS2 optoelectronic synapse with 12.12 fJ ultralow power consumption. This device exhibits excellent optical synaptic plasticity derived from the persistent photoconductivity effect. As the cornerstone for 3D vision, color planar information is successfully discriminated and stored in situ at the sensor end, benefiting from its wavelength-dependent plasticity in the visible region. Importantly, the dependence of the channel conductance on the target distance is experimentally revealed, implying that the structure information on the object can be directly captured and stored by the synapse. The 3D image of the object is successfully reconstructed via fusion of its planar and depth images. Therefore, the proposed 3D-NVS based on ReS2 synapses for 3D objects achieves a recognition accuracy of 97.0%, which is much higher than that for 2D objects (32.6%), demonstrating its strong ability to prevent 2D-photo spoofing in applications such as face payment, entrance guard systems, and others.

Piezo-phototronic effect modulated optoelectronic artificial synapse based on a-Ga2O3/ZnO heterojunction

Amorphous gallium oxide (a-Ga2O3) based optoelectronic devices responds to deep ultraviolet (DUV), which triggers their potential applications for simulating biological vision system. However, there is still lack of research on the influence of multiple stimuli on regulating synaptic plasticity in artificial synapse. Herein, we reported a flexible a-Ga2O3/ZnO film heterostructure with tunable DUV photoresponse, serving as a promising platform for artificial optoelectronic synapse. Based on persistent photoconductivity effect under 265 nm illumination, basic synaptic functions have been mimicked. Notably, the strain-induced piezo-phototronic effect of the heterojunction enables the modulation of synaptic plasticity, playing a role analogous to neuromodulator interaction with biological synapse activity. Strain-evoked an additional degree of freedom controls synapse weight-update rate during learning process and synapse weight-decay during forgetting process. The underlying mechanism is attributed to energy band bending tailored by strain-induced piezo-potential that determines carrier separation/transport and the recombination reaction of ionized oxygen vacancy. The recognition accuracy of MNIST images is as high as 92.31% in the artificial neural network constructed by the heterojunction devices. This work not only exhibits the application prospect of ultra-wide bandgap semiconductor-based heterojunction in artificial optoelectronic synapse, but also opens up a new avenue for multiple stimuli modulating synaptic plasticity.

Light-Stimulated IGZO Transistors with Tunable Synaptic Plasticity Based on Casein Electrolyte Electric Double Layer for Neuromorphic Systems.

In this study, optoelectronic synaptic transistors based on indium-gallium-zinc oxide (IGZO) with a casein electrolyte-based electric double layer (EDL) were examined. The casein electrolyte played a crucial role in modulating synaptic plasticity through an internal proton-induced EDL effect. Thus, important synaptic behaviors, such as excitatory post-synaptic current, paired-pulse facilitation, and spike rate-dependent and spike number-dependent plasticity, were successfully implemented by utilizing the persistent photoconductivity effect of the IGZO channel stimulated by light. The synergy between the light stimulation and the EDL effect allowed the effective modulation of synaptic plasticity, enabling the control of memory levels, including the conversion of short-term memory to long-term memory. Furthermore, a Modified National Institute of Standards and Technology digit recognition simulation was performed using a three-layer artificial neural network model, achieving a high recognition rate of 90.5%. These results demonstrated a high application potential of the proposed optoelectronic synaptic transistors in neuromorphic visual systems.

Ultrahigh UV Responsivity Quasi-Two-Dimensional BixSn1-xO2 Films Achieved through Surface Reaction.

In this study, quasi-two-dimensional BixSn1-xO2 (BTO) thin films were fabricated using a liquid metal transfer method. The ultraviolet (UV) photodetector based on BTO thin films was constructed, and the ultrahigh responsivity of 589 A/W was observed at 300 nm UV light illumination. Interestingly, by dropping ethanol during light-off period, the recovery time induced by the persistent photoconductivity (PPC) effect is reduced from 1.65 × 103 s to 5.71 s. Furthermore, the recovery time can also be reduced by dropping methanol, propylene glycol, NaNO2, and Na2SO3 after light termination. The working mechanisms are attributed to the rapid consumption of holes stored in BTO thin films by reaction with those solutions. This work demonstrates that the BTO thin films have potential applications in high-performance UV detectors and present an innovation route to weaken the PPC effects in semiconductors by introducing chemical liquids on their surface.

Coherent Phonon Manipulation via Electron-Phonon Interaction for Facilitated Relaxation of Metastable Centers in ZnO.

Methods that allow versatile manipulation of metastable centers in semiconductors are highly important owing to their potential for quantum information processing and computations. In this study, we demonstrate that the electron-phonon interaction enables phonon participation to promote relaxation of metastable centers in ZnO, which is known for its persistent photoconductivity (PPC) effect. Experimentally, we show that continuous infrared (IR) radiation (1064 nm, ∼30 mW/cm2) promotes longitudinal optical phonons via the Fröhlich interaction and increases the PPC relaxation rate by ∼4 folds. More importantly, we discover that coherent phonons activated by an ultrashort pulse IR laser of the same power increased the relaxation rate by ∼1200-fold, as confirmed by ultrafast transient spectroscopy to be correlated to the excitation of coherent acoustic phonons via the inverse piezoelectric effect. We expect this study to provide valuable guidance for the development of novel quantum and photoactive devices.

Highly-rectified hybrid p-MAPbBr3/n-GaN heterojunction ultraviolet photodetector featuring high-photoresponse

GaN-based photodetectors have demonstrated promising performance and find wide applications in ultraviolet sensing. In recent times, perovskite materials have also attracted significant attention due to their intriguing photoelectrical properties. In this study, we constructed a heterojunction photodetector by combining MAPbBr3 and GaN, and examined its detection capabilities. We conducted measurements of the current–voltage and current–time characteristics of the device. Our results showcased superior detection metrics, including a rectification ratio of 2.57 × 106 and a photo-to-dark current ratio of 1.52 × 105. Meanwhile, a responsivity of 1.02 A W−1, a detectivity of 4.81 × 1013 Jones and an external quantum efficiency of 346.8% under −5 V bias were achieved. Responsivity, detectivity and external quantum efficiency are significantly improved in the forward condition at the expense of low photo-to-dark current ratio and high dark current. The self-driven characteristics, non-linear photocurrent, and the persistent photoconductive effect observed in the photodetector were also discussed. Additionally, we provided guidance for further enhancements of the hybrid perovskite/GaN ultraviolet photodetector.

Nearly Panoramic Neuromorphic Vision with Transparent Photosynapses.

Neuromorphic vision based on photonic synapses has the ability to mimic sensitivity, adaptivity, and sophistication of bio-visual systems. Significant advances in artificial photosynapses are achieved recently. However, conventional photosyanptic devices normally employ opaque metal conductors and vertical device configuration, performing a limited hemispherical field of view. Here, a transparent planar photonic synapse (TPPS) is presented that offers dual-side photosensitive capability for nearly panoramic neuromorphic vision. The TPPS consisting of all two dimensional (2D) carbon-based derivatives exhibits ultra-broadband photodetecting (365-970nm) and ≈360° omnidirectional viewing angle. With its intrinsic persistent photoconductivity effect, the detector possesses bio-synaptic behaviors such as short/long-term memory, experience learning, light adaptation, and a 171% pair-pulse-facilitation index, enabling the synapse array to achieve image recognition enhancement (92%) and moving object detection.