Optoelectronic Synapse Based on 2D Electron Gas in Stoichiometry-Controlled Oxide Heterostructures.

The LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface has been one of the topics studied most during the past few years due to its many intriguing properties such as the two-dimensional electron gas, transient photoconductivity (PC), persistent photoconductivity (PPC), and the coexistence of the PC and PPC. Of them, the PPC effect is the most interesting because of its potential application in exploring the photoelectric memory devices. Until now, tuning of the PPC of the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface under the external stimuli, such as electric or magnetic field is less addressed, while the relevant knowledge is of great value for exploring the memory devices with multifunctionality. In this paper, we report on an electric field control of the persistent PPC at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface. Our LaAlO<sub>3</sub>/SrTiO<sub>3</sub> heterojunction is fabricated by growing the LaAlO<sub>3</sub> film on the SrTiO<sub>3</sub> substrates through using pulsed laser deposition. The substrate temperature is kept at 750 ℃ and the partial pressure of oxygen is maintained at 3.3 × 10<sup>–5</sup> Torr (1 Torr = 1.33322 × 10<sup>2</sup> Pa) during the deposition. The thickness of LaAlO<sub>3</sub> film is controlled to be about 2 nm by setting an appropriate deposition time. The X-ray diffraction experiment confirms that the LAO film is well epitaxial and of single phase. To guarantee the good electric contacts, Al electrodes are soldered at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface and the back side of the SrTiO<sub>3</sub> respectively by ultrasonic welding. We find that the PPC at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface can be significantly reinforced and modulated by the light-enhanced gating effects: that is, after a negative back gate voltage processing combined with a simultaneous light illumination, the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface can exhibit a notable PPC effect. And the PPC effect increases as the negative gate voltage increases, and then attains a maximum at a back gate voltage of about –70 V. Further increase of the negative gate voltage can cause the PPC to decrease. Additionally, the PPC is also found to increase monotonically with increasing the gating time. The present result can be understood in terms of the migration of the oxygen vacancies under the influence of photoelectric synergetic effect. This field enhanced PPC effects at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface may find their applications in designing the photoelectric memory devices with electric tunability.

This thesis presents a comprehensive study of tailored semiconductor devices that bridge conventional photodetection with neuromorphic photomemory applications through strategic engineering of response time characteristics. The fundamental insight underlying this work is that response time represents the critical parameter determining whether an optoelectronic device functions as a conventional photodetector or enables synaptic functionality for brain-inspired computing systems. The research demonstrates that the same physical mechanisms, such as photon absorption, carrier generation, charge transport, and recombination, can be optimized for dramatically different temporal characteristics, enabling devices that adapt their functionality based on operational requirements. Fast response times in the microsecond to millisecond range enable high-speed photodetection for telecommunications and sensing applications, while prolonged response times through persistent photoconductivity (PPC) spanning seconds to days enable the memory and learning functions essential for artificial synapses. The research contributions are organized into four interconnected studies that collectively demonstrate this dual functionality principle. Chapters 2 and 3 focus on optimizing devices for fast photodetection applications. Chapter 2 presents flexible germanium/titanium nitride (Ge/TiN) photodetectors that achieve enhanced near infrared responsivity through plasmonic enhancement and strain engineering, demonstrating how structural modifications can improve conventional photodetection performance while maintaining fast response characteristics. Chapter 3 investigates self powered silicon metal-semiconductor-metal (MSM) photodetectors utilizing asymmetric electrode configurations, achieving efficient photodetection without external bias through engineered Schottky barrier heights. Chapters 4 and 5 shift focus on exploiting extended response times for neuromorphic applications. Chapter 4 demonstrates silicon/indium gallium zinc oxide (Si/IGZO) photomemory devices with persistent photoconductivity exceeding nine days, achieved through substrate conductivity engineering that optimizes band alignment to trap photocarriers and prevent recombination. Chapter 5 presents transparent quartz/IGZO tracking devices that combine optical transparency with neuromorphic functionality, enabling covert trajectory monitoring through synaptic plasticity behaviors. The connection between these studies lies in the systematic exploration of how response time engineering enables the same semiconductor materials to serve two different purposes. The Ge/TiN and Si photodetectors in Chapters 2-3 are optimized for response times in the millisecond range, suitable for high-speed optical communication and imaging applications. In contrast, the IGZO-based devices in Chapters 4-5 exploit response times spanning minutes to days, enabling synaptic behaviors including PPF, learning-forgetting-relearning cycles, and long-term memory retention. All four device systems are based on MSM-type architectures that share the same fundamental band-to band excitation and carrier transport principles, and flexibility, self-powered operation, photomemory, and trajectory tracking are employed as representative manifestations of fast and slow response-time engineering within this common framework. The research provides new insights into the mechanisms controlling persistent photoconductivity, demonstrates practical strategies for substrate and interface engineering, and establishes design principles for next-generation neuromorphic optoelectronic systems. The significance of this research extends beyond individual device performance improvements to establish a fundamental framework for engineering semiconductor devices with tunable temporal characteristics. By demonstrating that the same materials and fabrication processes can yield devices optimized for either fast photodetection or extended photomemory functions, this work opens new possibilities for semiconductor devices with tunable temporal characteristics and design-level flexibility in their functional role.

Various hippocampal and neocortical synapses of mammalian brain show both short-term plasticity and long-term plasticity, which are considered to underlie learning and memory by the brain. According to Hebb’s postulate, synaptic plasticity encodes memory traces of past experiences into cell assemblies in cortical circuits. However, it remains unclear how the various forms of long-term and short-term synaptic plasticity cooperatively create and reorganize such cell assemblies. Here, we investigate the mechanism in which the three forms of synaptic plasticity known in cortical circuits, i.e., spike-timing-dependent plasticity (STDP), short-term depression (STD) and homeostatic plasticity, cooperatively generate, retain and reorganize cell assemblies in a recurrent neuronal network model. We show that multiple cell assemblies generated by external stimuli can survive noisy spontaneous network activity for an adequate range of the strength of STD. Furthermore, our model predicts that a symmetric temporal window of STDP, such as observed in dopaminergic modulations on hippocampal neurons, is crucial for the retention and integration of multiple cell assemblies. These results may have implications for the understanding of cortical memory processes.

Optoelectronic synaptic devices outperform electrical synapses in speed, energy efficiency, and integration density. Recent progress in visual sensing and optogenetics has led to the integration of light-sensitive materials in these devices, promising unmatched speed, connectivity, and bandwidth. Here, we present a copper phthalocyanine (CuPc) based optoelectronic synaptic device boasting femto Joule power consumption stable at room temperature. The optoelectronic synapse can be operated with energy consumption as low as 430.4 fJ which is very attractive from the point of view of low-power neuromorphic devices. By modulating light pulses, the neuromorphic behavior can be emulated including excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), transitioning from short-term plasticity (STP) to long-term plasticity (LTP), spike-rate dependent plasticity (SRDP) and spike-number dependent plasticity (SNDP), etc. Optical potentiation and electrical depression are observed with combined optical and electrical stimulation, proving the multi-functionality of the synapse. Furthermore, the device demonstrates classical associative learning behaviors like Pavlovian conditioning using optical and electrical stimuli. We have established the pain conditioning processes such as hyperalgesic response and pain extinction effects with varying optical pulse amplitudes. These results render the CuPc-based devices as multifunctional and highly versatile artificial synaptic devices for future computing applications, offering unprecedented efficiency and functionality in neuromorphic systems.

The complex oxide heterostructures such as LaAlO3/SrTiO3 (LAO/STO) interface are paradigmatic platforms to explore emerging multi-degrees of freedom coupling and the associated exotic phenomena. In this study, we reveal the effects of multiorbital and magnetic ordering on Rashba spin-orbit coupling (SOC) at the LAO/STO (001) interface. Based on first-principles calculations, we show that the Rashba spin splitting near the conduction band edge can be tuned substantially by the interfacial insulator-metal transition due to the multiorbital effect of the lowest t_2g bands. We further unravel a competition between Rashba SOC and intrinsic magnetism, in which the Rashba SOC induced spin polarization is suppressed by the interfacial magnetic ordering. These results deepen our understanding of intricate electronic and magnetic reconstruction at the perovskite oxide interfaces and shed light on the engineering of oxide heterostructures for all-oxides-based spintronic devices.

In recent years, reversible control over metal-insulator transition has been shown, at the nanoscale, in a two-dimensional electron gas (2DEG) formed at the interface between two complex oxides. These materials have thus been suggested as possible platforms for developing ultrahigh-density oxide nanoelectronics. A prerequisite for the development of these new technologies is the integration with existing semiconductor electronics platforms. Here, we demonstrate room-temperature conductivity switching of 2DEG nanowires formed at atomically sharp LaAlO(3)/SrTiO(3) (LAO/STO) heterointerfaces grown directly on (001) Silicon (Si) substrates. The room-temperature electrical transport properties of LAO/STO heterointerfaces on Si are comparable with those formed from a SrTiO(3) bulk single crystal. The ability to form reversible conducting nanostructures directly on Si wafers opens new opportunities to incorporate ultrahigh-density oxide nanoelectronic memory and logic elements into well-established Si-based platforms.

Perovskite oxide heterostructure possesses attractive magnetic, optical and electric properties, such as superconducting interface between two insulators, two-dimensional electron gas, positive giant magnetoresistance, photoelectric response characteristic, magnetocaloric effect, and coexistent different magnetic structures. Especially for the photoelectric response behaviors of A1-xAxMnO3 (A=La, Pr etc.; A = Sr, Ca etc.) perovskite manganese oxide heterostructure, one has made a systematic study on the photoelectric conversion efficiency, the photovoltaic response speed, and the in-plane lateral photovoltage. Besides A1-xAxMnO3 structure, manganese oxides can also exhibit the double layered perovskite structure A2-2xA1+2xMn2O7. Double layered perovskite structure can be regarded as the layers of perovskite and rock salt which are alternately stacked. This double layered perovskite manganese oxide (such as La2-2xSr1+2xMn2O7) is a natural structure of the tunnel structure: ferromagnetic metal layer-insulating layer-ferromagnetic metal layer. Double layered perovskite manganese oxide has not only the characteristics of giant magnetoresistance, but also the novel physical properties, such as persistent photoconductivity, etc. However, there are few reports on the physical properties of the double layered perovskite manganite oxides, heterostructures, especially the photovoltaic properties. In this work, the La1.3Sr1.7Mn2O7 (LSMO) film is deposited on an n-type SrTiO3-Nb (NSTO) single crystal substrate by a pulsed laser deposition method. Additionally, we study the transporting properties of LSMO/NSTO heterostructure and its photovoltaic effect. The heterostructure exhibits benign rectifying and palpable photovoltaic effect. Under the 532 nm laser irradiation, the photovoltage first increases and then decreases with temperature rising. The maximal photovoltage reaches 400 mV at 150 K which is consistent with the metal-insulator transition temperature of LSMO film. It is indicated that the photovoltaic effect of the heterostructure is regulated by the inner transporting characteristics of LSMO film. The dynamical process of the heterostructure, photovoltaic response, is analyzed. Meanwhile, by analyzing the relationship between the photovoltage and time, it is found that the rising edge fits to the first order exponential function, which is related to the migration of carriers. While the falling edge of second-order exponential function indicates that the compound of carriers has two different physical processes: 1 corresponds to the neutralization process of the carriers aggregated on both junction sides through the external circuit, and 2 corresponds to the annihilation process of non-equilibrium carriers. The carrier lifetime of our p-n junction is longer, on the order of ms, than those of other manganese oxides p-n junctions. Remarkably, the time constants of both the rising edge and falling edge first increase and then decrease as temperature increases, and the maximum values occur at the metal-insulator transition temperature of LSMO film.

Non-BCS-type superconductivity and critical thickness of SrTiO3/LaAlO3/SrTiO3 trilayer interface system

Developing optoelectronic synaptic devices with low energy consumption is of critical importance for neuromorphic computing and visualization systems. In this work, amorphous‐Ga2O3 (a‐Ga2O3) is adopted to realize a low‐power optoelectronic synapse considering its distinguished features of ultra‐wide bandgap, high responsiveness to light stimulation, and strong persistent photoconductivity effect. The basic synaptic functions such as short‐term plasticity (STP) and long‐term plasticity (LTP) have been successfully mimicked. A noise suppression capability is achieved as well, similar to the visual cells. Strikingly, the total energy consumption for triggering an LTP synaptic event is only 136 fJ, approaching the one of a biological synapse. The underlying mechanism for the a‐Ga2O3 synaptic performance is explored with a combined research of X‐ray photoelectron spectroscopy, Kelvin probe force microscopy, electric tests, and Technology Computer‐Aided Design simulations. The consistent results suggest the key role of the synergetic modulation effect of optical and electric fields on the dynamic behaviors of oxygen vacancy (VO) defects. This work indicates the potential applications of a‐Ga2O3 in low‐power optoelectronic synapses.

Imaging sensors with inbuilt processing capability are expected to form the backbone of low‐latency and highly energy efficient artificial vision systems. A range of emerging atomically thin materials provide opportunities to exploit their electrical and optical properties for human vision and brain inspired functions. This work reports atomically thin nanosheets of β‐In2S3 which exhibit inherent persistent photoconductivity (PPC) under ultraviolet and visible wavelengths. This PPC effect enables β‐In2S3‐based optoelectronic devices to optically mimic the dynamics of biological synapses. Based on the material characterizations, the PPC effect is attributed to the intrinsic defects in the synthesized β‐In2S3 nanosheet. Furthermore, the feasibility of adopting these atomically thin synaptic devices for optoelectronic neuromorphic hardware is demonstrated by implementing a convolutional neural network for image classification. As such, the demonstrated atomically thin nanosheets and optoelectronic synaptic devices provide a platform for scaling up complex vision‐sensory neural networks, which can find many promising applications for multispectral imaging and neuromorphic computation.

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

Current artificial neuromorphic systems are mainly constructed using solid‐state optoelectronic synaptic devices, limiting their potential artificial intelligence applications in liquid medium. Here, colloidal CuInGaSe/ZnSe quantum dots (QDs) with environment‐benign feature and broad visible light absorption are prepared to fabricate a photoelectrochemical (PEC)‐type optoelectronic synaptic device operating in aqueous electrolyte, enabling unique biological synaptic behaviors including paired‐pulse facilitation (PPF), short‐term plasticity (STP), long‐term plasticity (LTP) and learning‐forgetting‐relearning process when simulated by optical pulses with various wavelength, pulse number, frequency, power density, and applied voltage. These results demonstrate the feasibility of building PEC‐type optoelectronic synapses using colloidal QDs, paving the way to realize available optoelectronic synaptic devices for prospective underwater artificial intelligence technology.

Neuromorphic computing, an emerging computational paradigm, aims to overcome the bottlenecks of the traditional von Neumann architecture. Two-dimensional materials serve as ideal platforms for constructing artificial synaptic devices, yet existing devices based on these materials face challenges such as insufficient stability. Indium selenide (InSe), a two-dimensional semiconductor with unique properties, demonstrates significant potential in the field of neuromorphic devices, though its application research remains in the initial stage. This study presents an artificial synaptic device based on the InSe/Charge Trapping Layer (CTL)/h-BN heterojunction. By applying oxygen plasma treatment to h-BN to form a controllable charge-trapping layer, efficient regulation of carriers in the InSe channel is achieved. The device successfully emulates fundamental synaptic behaviors including paired-pulse facilitation and long-term potentiation/inhibition, exhibiting excellent reproducibility and stability. Through investigating the influence of electrical pulse parameters on synaptic weights, a structure–activity relationship between device performance and structural parameters is established. Experimental results show that the device features outstanding linearity and symmetry, realizing the simulation of key synaptic behaviors such as dynamic conversion between short-term and long-term plasticity. It possesses a high dynamic range ratio of 7.12 and robust multi-level conductance tuning capability, with stability verified through 64 pulse cycle tests. This research provides experimental evidence for understanding interfacial charge storage mechanisms, paves the way for developing high-performance neuromorphic computing devices, and holds broad application prospects in brain-inspired computing and artificial intelligence hardware.

The accumulation and extrusion of Ca2+ ions in the pre- and post-synaptic terminals play crucial roles in initiating short- and long-term plasticity (STP and LTP) in biological synapses, respectively. Mimicking these synaptic behaviors by electronic devices represents a vital step toward realization of neuromorphic computing. However, the majority of reported synaptic devices usually focus on the emulation of qualitatively synaptic behaviors; devices that can truly emulate the physical behavior of the synaptic Ca2+ ion dynamics in STP and LTP are rarely reported. In this work, Ag/Ag:Ta2O5/Pt self-doping memristors were developed to equivalently emulate the Ca2+ ion dynamics of biological synapses. With conductive filaments from double sources, these memristors produced unique double-switching behavior under voltage sweeps and demonstrated several essential synaptic behaviors under pulse stimuli, including STP, LTP, STP to LTP transition, and spike-rate-dependent plasticity. Experimental results and nanoparticle dynamic simulations both showed that Ag atoms from double sources could mimic Ca2+ dynamics in the pre- and post-synaptic terminals under stimuli. A perceptron network with an STP to LTP transition layer based on the self-doping memristors was also introduced and evaluated; simulations showed that this network could solve noisy figure recognition tasks efficiently. All of these results indicate that the self-doping memristors are promising components for future hardware creation of neuromorphic systems and emulate the characteristics of the brain.

All-in-one metal-oxide heterojunction artificial synapses for visual sensory and neuromorphic computing systems