Integrating ultraviolet sensing and memory functions in gallium nitride-based optoelectronic devices.

Optoelectronic devices present a promising avenue for emulating the human visual system. However, existing devices struggle to maintain optical image information after removing external stimuli, preventing the integration of image perception and memory. The development of optoelectronic memory devices offers a feasible solution to bridge this gap. Simultaneously, the artificial vision for perceiving and storing ultraviolet (UV) images is particularly important because UV light carries information imperceptible to the naked eye. This study introduces a multi-level UV optoelectronic memory based on gallium nitride (GaN), seamlessly integrating UV sensing and memory functions within a single device. The embedded SiO2 side-gates around source and drain regions effectively extend the lifetime of photo-generated carriers, enabling dual-mode storage of UV signals in terms of threshold voltage and ON-state current. The optoelectronic memory demonstrates excellent robustness with the retention time exceeding 4 × 104 s and programming/erasing cycles surpassing 1 × 105. Adjusting the gate voltage achieves five distinct storage states, each characterized by excellent retention, and efficiently modulates erasure times for rapid erasure. Furthermore, the integration of the GaN optoelectronic memory array successfully captures and stably stores specific UV images for over 7 days. The study marks a significant stride in optoelectronic memories, showcasing their potential in applications requiring prolonged retention.

Computational-fitting method for mobility extraction in GaN HEMT.

The third-generation semiconductor gallium nitride (GaN) has drawn wide attention due to its high electron mobility property. However, the classical mobility calculation methods such as Hall effect and transfer length method have limitations in accurately extracting the mobility of GaN High Electron Mobility Transistor (HEMT) due to their inability to consider the resistance in non-gate region or their high fabrication costs. This work proposes an effective yet accurate computational-fitting method for extracting the mobility of GaN HEMT. The method consists of measuring the total resistance between source and drain at different gate voltages over a very small range of overdrive voltage variations, when the sum of the transconductance and capacitance of the device is regarded as constants, and fitting a unique function of the total resistance with respect to the overdrive voltage to determine the carrier mobility and the non-gate resistance. The feasibility and reliability of the method has been also verified.

1.3 kV Vertical GaN-Based Trench MOSFETs on 4-Inch Free Standing GaN Wafer.

In this work, a vertical gallium nitride (GaN)-based trench MOSFET on 4-inch free-standing GaN substrate is presented with threshold voltage of 3.15 V, specific on-resistance of 1.93 mΩ·cm2, breakdown voltage of 1306 V, and figure of merit of 0.88 GW/cm2. High-quality and stable MOS interface is obtained through two-step process, including simple acid cleaning and a following (NH4)2S passivation. Based on the calibration with experiment, the simulation results of physical model are consistent well with the experiment data in transfer, output, and breakdown characteristic curves, which demonstrate the validity of the simulation data obtained by Silvaco technology computer aided design (Silvaco TCAD). The mechanisms of on-state and breakdown are thoroughly studied using Silvaco TCAD physical model. The device parameters, including n--GaN drift layer, p-GaN channel layer and gate dielectric layer, are systematically designed for optimization. This comprehensive analysis and optimization on the vertical GaN-based trench MOSFETs provide significant guide for vertical GaN-based high power applications.

Bifunctional homologous alkali-metal artificial synapse with regenerative ability and mechanism imitation of voltage-gated ion channels.

As a key component responsible for information processing in the brain, the development of a bionic synapse possessing digital and analog bifunctionality is vital for the hardware implementation of a neuro-system. Here, inspired by the key role of sodium and potassium in synaptic transmission, the alkali metal element lithium (Li) belonging to the same family is adopted in designing a bifunctional artificial synapse. The incorporation of Li endows the electronic devices with versatile synaptic functions. An artificial neural network based on experimental data exhibits a high performance approaching near-ideal accuracy. In addition, the regenerative ability allows synaptic functional recovery through low-frequency stimuli to be emulated, facilitating the prevention of permanent damage due to intensive neural activities and ensuring the long-term stability of the entire neural system. What is more striking for an Li-based bionic synapse is that it can not only emulate a biological synapse at a behavioral level but realize mechanism emulation based on artificial voltage-gated "ion channels". Concurrent digital and analog features lead to versatile synaptic functions in Li-doped artificial synapses, which operate in a mode similar to the human brain with its two hemispheres excelling at processing imaginative and analytical information, respectively.

Robust Simulations of Nanoscale Phase Change Memory: Dynamics and Retention.

A robust simulation framework was developed for nanoscale phase change memory (PCM) cells. Starting from the reaction rate theory, the dynamic nucleation was simulated to capture the evolution of the cluster population. To accommodate the non-uniform critical sizes of nuclei due to the non-isothermal conditions during PCM cell programming, an improved crystallization model was proposed that goes beyond the classical nucleation and growth model. With the above, the incubation period in which the cluster distributions reached their equilibrium was captured beyond the capability of simulations with a steady-state nucleation rate. The implications of the developed simulation method are discussed regarding PCM fast SET programming and retention. This work provides the possibility for further improvement of PCM and integration with CMOS technology.

Achieving complementary resistive switching and multi-bit storage goals by modulating the dual-ion reaction through supercritical fluid-assisted ammoniation.

Complementary resistive switching (CRS) is a core requirement in memristor crossbar array construction for neuromorphic computing in view of its capability to avoid the sneak path current. However, previous approaches for implementing CRS are generally based on a complex device structure design and fabrication process or a meticulous current-limiting measurement procedure. In this study, a supercritical fluid-assisted ammoniation (SFA) process is reported to achieve CRS in a single device by endowing the original ordinary switching materials with dual-ion operation. In addition to self-compliant CRS behavior, a multi-bit storage function has also been achieved through the SFA process accompanied by superior retention and reliability. These substantial evolved resistive phenomena are elucidated attentively by our chemical reaction model and physical mechanism model corroborated by the material analysis and current conduction fitting analysis results. The findings in this research present the most efficient way to achieve CRS through only one chemical procedure with significantly improved device performance. Moreover, the supercritical fluid approach envisions tremendous possibilities for further development of materials and electric devices by a low-temperature process, with semiconductor fabrication compatibility and environmental friendliness.

Supercritical Ammoniation-Enabled Interfacial Polarization for Function-Mode Transformation and Overall Optimization of Thin-Film Transistors.

Thin-film transistors (TFTs) have drawn widespread applications in the increasingly sophisticated display field. Despite the mature process of fabricating enhancement-mode TFTs, lack of facile methods to realize depletion-mode TFTs restrains the implementation of complementary-type circuits, which in turn leads to relatively high power. Here, the supercritical fluid technique is introduced to elaborately design and tune the interface, providing the opportunity for function-mode transformation of TFTs. By harnessing supercritical-assisted ammoniation (SCA) treatment, interfacial polarization induces negative shift of threshold voltage (from 0.2 to -9.8 V), which allows TFTs to remain normally on-state in the absence of complex capacitor-integrated circuits. This convenient technique, without an additional manufacturing process to achieve function-mode transformation, can thus enable the fabrication of comprehensive-mode TFTs under the same process. Furthermore, comprehensive optimizations in the mobility (increases from 2.08 to 17.12 cm2 V-1 s-1), leakage current (reduces from 1.33 × 10-11 to 2.22 × 10-12 A), hysteresis (reduces from 11.2 to 0.2 V), and on/off current ratio (increases from 9.65 × 104 to 7.98 × 106) are achieved simultaneously. Based on conjoint analysis of electrical and material characterization, a reaction model is established for a clearer understanding of the interfacial polarization process. Overall, this low-temperature SCA treatment offers an environmentally benign strategy to modulate the function mode of electronic devices via interfacial engineering and optimize device performance at the same time, exhibiting promise in promoting the implementation of complementary, low-power circuit.

Low-temperature supercritical dehydroxylation for achieving an ultra-low subthreshold swing of thin-film transistors.

Thin-film transistors (TFTs) have been widely used in the increasingly advanced field of displays. However, it remains a challenge for TFTs to overcome the poor subthreshold swing in the fast switching and high-speed applications. Here, we provide a solution to the above-mentioned challenge via supercritical dehydroxylation, which combines a low temperature, environmentally friendly supercritical fluid technology with a CaCl2 treatment. An embedded structure of amorphous indium gallium zinc oxide (a-IGZO) TFTs with double-layer high-k dielectric containing Ta2O5 and SiO2 layers was first manufactured. The subthreshold swing of the fabricated TFTs treated with supercritical dehydroxylation was optimized to an ultra-low value of 72.7 mV dec-1. Moreover, other key figures of merits including threshold voltage, on/off ratio and field effect mobility all improved after the supercritical dehydroxylation. The bandgap of the gate dielectric material increased due to the supercritical dehydroxylation verified by the current conduction mechanism. Besides, numerous material analyses further confirmed that owing to the supercritical dehydroxylation the dominant dehydration reactions can effectively repair the defects introduced in the device manufacture. The ultra-low subthreshold swing with optimized electrical performances can be achieved via the low-temperature supercritical dehydroxylation treatment, enabling its promising potential in realizing ultra-fast and low power electronics.

Unveiling the influence of surrounding materials and realization of multi-level storage in resistive switching memory.

Considerable efforts have been made to obtain better control of the switching behavior of resistive random access memory (RRAM) devices, such as using modified or multilayer switching materials. Although considerable progress has been made, the reliability and stability of the devices greatly deteriorate due to dispersed electric field caused by low permittivity surrounding materials. By introducing surrounding materials with a relatively higher dielectric constant, the RRAM devices become promising for cost-effective applications by achieving multilevel storage functionality and improved scalability. A device designed by this principle exhibits multiple distinct and non-volatile conductance states. Moreover, the issue of the increasing forming voltage during device scaling is also solved, improving the capacity of the chips and reducing the power dissipation in the process of the device miniaturization. The COMSOL simulation helps to reveal that the enhanced performance is correlated with a more concentrated electric field around the conductive filament, which is favorable for controlling the connection and rupture of the resistive filament.

The study of bending properties of monolayer MoS2 in non-collinear electrodes using first principles theory.

In this work, we report the theoretical maximum bending angle of MoS2 devices using the creative non-collinear electrodes method based on the first principles theory. The results show that the device with 1T-phase MoS2 electrodes sandwiching N-type MoS2 in a zigzag direction has a better conducting behavior as compared with P-type in an armchair direction. The conductance decreases less than 15% when the angle between the two electrodes is less than 45° in both the equilibrium state and non-equilibrium state because of the continuous resonant response between the two electrodes and the little deformed band structure. This work provides guidance and a physical mechanism for achieving flexible MoS2 transistors that are reliable at a sub-nm bending radius.

Variable-temperature activation energy extraction to clarify the physical and chemical mechanisms of the resistive switching process.

This study investigates the physical and chemical mechanisms during the resistive switching process by means of obtaining the activation energy in the reaction procedure. From the electrochemical and electrical measurement analysis results of HfO2-based resistive random access memory (RRAM), it can be observed that the chemical reaction during the reset process is consistent with the first-order reaction law. The activation energy, Ea, is determined from the reaction rate constant k under a varying-temperature environment in the reset process. The whole reset chemical reaction process can be divided into five phases involving N-O bond breaking, O-O bond breaking and triple-step oxygen ion migration. The methodology of the activation energy determination carried out in this study showcases a distinct approach to elucidate the resistive switching mechanism of RRAM and offers insight into RRAM design for future potential application.

An indirect way to achieve comprehensive performance improvement of resistive memory: when hafnium meets ITO in an electrode.

Emerging resistive random access memory has attracted extensive research enthusiasm. In this study, an indirect way to improve resistive random access memory (RRAM) comprehensive performance through electrode material re-design without intensive switching layer engineering is presented by adopting a hafnium-indium-tin-oxide composite. Working parameters of the device can be effectively improved: not only are low operation power consumption and high working stability achieved, but the memory window is significantly enlarged, accompanied by an automatic self-current-compliance function. The correlation between hafnium incorporation and performance improvements and the corresponding current conduction mechanisms have been thoroughly investigated to clarify the resistive switching behavior and to explain the oxygen absorption buffer effect. The hafnium atom, with large atomic radius, is surrounded by soft electron clouds and has high chemical activity to attract oxygen ions. It facilitates the accumulation of more oxygen ions around the interface of the top electrode and the resistive switching layer, leading to lower current and Schottky conduction. This study presents an important strategy for designing and developing electrode materials to improve the characteristics of RRAM and offers an indirect method to modify device working behaviors, also unveiling a promising prospect for its potential future application in low-power information storage and calculation technology.

Hafnium nanocrystals observed in a HfTiO compound film bring about excellent performance of flexible selectors in memory integration.

In this study, a HfTiO compound film on polyethylene naphthalate (PEN) has been investigated and designed as the selective layer material to fabricate flexible selector devices, since a selector is considered as a promising candidate for solving the sneak current issues in high-density memory integration. According to material analysis, hafnium nanocrystals observed in the HfTiO film play a key role in the performance improvement of the selector. The correlation between the HfTiO material and the corresponding current conduction mechanisms and the proposed physical mechanism model with hafnium nanocrystals have been thoroughly investigated to clarify and explain the enhanced selective behavior including high uniformity, excellent endurance and fast operation speed. Moreover, the selector with the HfTiO film exhibits superior bending reliability with no working performance degradation under a bending radius ranging from 50 mm to 30 mm, indicating the excellent flexibility and applicability of the selector in flexible application scenarios. These achievements of the Pt/HfTiO/ITO selector induced by the HfTiO film with hafnium crystals offer great potential for material and interface design in future memory integration and flexible applications.

Black Phosphorus Quantum Dots for Hole Extraction of Typical Planar Hybrid Perovskite Solar Cells.

Black phosphorus, famous as two-dimensional (2D) materials, shows such excellent properties for optoelectronic devices such as tunable direct band gap, extremely high hole mobility (300-1000 cm2/(V s)), and so forth. In this Letter, facile processed black phosphorus quantum dots (BPQDs) were successfully applied to enhance hole extraction at the anode side of the typical p-i-n planar hybrid perovskite solar cells, which remarkably improved the performance of devices with photon conversion efficiency ramping up from 14.10 to 16.69%. Moreover, more detailed investigations by c-AFM, SKPM, SEM, hole-only devices, and photon physics measurements discover further the hole extraction effect and work mechanism of the BPQDs, such as nucleation assistance for the growth of large grain size perovskite crystals, fast hole extraction, more efficient hole transfer, and suppression of energy-loss recombination at the anode interface. This work definitely paves the way for discovering more and more 2D materials with high electronic properties to be used in photovoltaics and optoelectronics.

Impact of random dopant fluctuation effect on surrounding gate MOSFETs: from atomic level simulation to circuit performance evaluation.

This paper investigates the impact of random dopant fluctuation effect on surrounding gate MOSFET, from atomic statistical simulation of device to circuit performance evaluation. The doping profile is generated by an analysis of each lattice atom and then the threshold voltage variation is obtained by device Drift-Diffusion simulation. Then the circuit performance evaluation is performed by feeding the result into a surrounding-gate MOSFET model. It is shown that a significant fluctuation in threshold voltage is due to the decreasing volume. The circuit simulation results also reveal that a surrounding gate MOSFET based 6-T SRAM presents a promising resistibility to noise disturbance.

Numerical study on heterodyne terahertz detection in field effect transistor.

Numerical method on the heterodyne terahertz detection characteristics of field effect characteristics of field effect transistors is studied in this paper which is based on the hydrodynamic equations which govern the terahertz signal transport in field effect transistors (FETs). A modification is made in an existed numerical tool established by our group by coupling the heterodyne characteristics. This modified numerical tool work well in all operation regions of FETs from sub-threshold to strong inversion and from linear to saturation. And the results are used to demonstrate the potential for using MOS transistors as THz detectors and investigate the optimization of the device structure.