High-enhancement photoluminescence of monolayer MoS2 in hybrid plasmonic systems.

Monolayer molybdenum disulfide (M o S 2) has a weak light-matter interaction due to ultrathin thickness, which limits its potential application in lasing action. In this study, we propose a hybrid structure consisting of a nanocavity and Au nanoparticles to enhance the photon emission efficiency of monolayer M o S 2. Numerical simulations show that photoluminescence (PL) emission is significantly enhanced by introducing localized surface plasmon resonance (LSPR) to the proposed structure. Furthermore, an exciton energy band system is proposed to elucidate the physical mechanism of the PL process. By optimizing the spacer thickness, a high Purcell enhancement factor of 95 can be achieved. The results provided by this work pave the way to improve the PL efficiency of two-dimensional (2D) material, which constitutes a significant step towards the development of nanodevices such as nanolasers and sensors.

Steep-slope vertical-transport transistors built from sub-5 nm Thin van der Waals heterostructures.

Two-dimensional (2D) semiconductor-based vertical-transport field-effect transistors (VTFETs) - in which the current flows perpendicularly to the substrate surface direction - are in the drive to surmount the stringent downscaling constraints faced by the conventional planar FETs. However, low-power device operation with a sub-60 mV/dec subthreshold swing (SS) at room temperature along with an ultra-scaled channel length remains challenging for 2D semiconductor-based VTFETs. Here, we report steep-slope VTFETs that combine a gate-controllable van der Waals heterojunction and a metal-filamentary threshold switch (TS), featuring a vertical transport channel thinner than 5 nm and sub-thermionic turn-on characteristics. The integrated TS-VTFETs were realised with efficient current switching behaviours, exhibiting a current modulation ratio exceeding 1 × 108 and an average sub-60 mV/dec SS over 6 decades of drain current. The proposed TS-VTFETs with excellent area- and energy-efficiency could help to tackle the performance degradation-device downscaling dilemma faced by logic transistor technologies.

HfO2-ZrO2 Ferroelectric Capacitors with Superlattice Structure: Improving Fatigue Stability, Fatigue Recovery, and Switching Speed.

HfO2-ZrO2 ferroelectric films have recently gained considerable attention from integrated circuit researchers due to their excellent ferroelectric properties over a wide doping range and low deposition temperature. In this work, different HfO2-ZrO2 superlattice (SL) FE films with varying periodicity of HfO2 (5 cycles)-ZrO2 (5 cycles) (SL5), HfO2 (10 cycles)-ZrO2 (10 cycles) (SL10), and HfO2 (15 cycles)-ZrO2 (15 cycles) (SL15) were studied systematically. The HfZrOx (HZO) alloy was used as a comparison device. The SL5 film demonstrated improved ferroelectric properties compared to the HZO film, with the 2 times remnant polarization (2Pr) values increasing from 41.4 to 48.6 µC/cm2 at an applied voltage of 3 V/10 kHz. Furthermore, the first-order reversal curve diagrams of different SL and HZO capacitors at different states (initial, wake-up, fatigue, and recovery) were measured. The SL capacitors were found to effectively suppress the diffusion of defects during P-V cycling, resulting in improved fatigue stability characteristics and fatigue recovery capability compared to the HZO capacitor. Moreover, an improved switching speed of the SL films compared to the HZO capacitor was concluded based on the inhomogeneous field mechanism (IFM) model. These results indicate that the SL structure has a high potential in future high-speed ferroelectric memory applications with excellent stability and recovery capability.

Proximity Effect-Induced Magnetoresistance Enhancement in a Fe3GeTe2/NbSe2/Fe3GeTe2 Magnetic Tunnel Junction.

The coupling between van der Waals-layered magnetic and superconducting materials holds the possibility of revealing novel physical mechanisms and realizing spintronic devices with new functionalities. Here, we report on the realization and investigation of a maximum ∼17-fold magnetoresistance (MR) enhancement based on a vertical magnetic tunnel junction of Fe3GeTe2 (FGT)/NbSe2/FGT near the NbSe2 layer's superconducting critical temperature (TC) of 6.8 K. This enhancement is attributed to the band splitting in the atomically thin NbSe2 spacer layer induced by the magnetic proximity effect on the material interfaces. However, the band splitting is strongly suppressed by the interlayer coupling in the thick NbSe2 layer. Correspondingly, the device with a thick NbSe2 layer displays no MR increase near TC but a current dependent on transport properties at extremely low temperatures. This work carefully investigates the mechanism of MR enhancement, paving an efficient way for the modulation of spintronics' properties and the achievement of spin-based integrated circuits.

Impact of multi-domain effect on the effective carrier mobility of ferroelectric field-effect transistor.

HfO2-based ferroelectric field-effect transistors (FeFETs) are a promising candidate for multilevel memory manipulation and brain-like computing due to the multi-domain properties of the HfO2FE films based polycrystalline structure. Although there have been many reports on the working mechanism of the HfO2-based FeFET and improving its reliability, the impact of multi-domain effect on the effective carrier mobility (µchannel) has not been carried out yet. The effectiveµchanneldetermines the level of readout current and affects the accuracy of the precision of peripheral circuit. In this work, FeFETs with HfZrOxFE gate dielectric were fabricated, and the effect of write (or erase) pulses with linear gradient variation on the effectiveµchannelwas studied. For the multiple downward polarization under write pulses, theµchanneldegrades as the domains gradually switch to downward. This is mainly due to the enhancement of the scattering effect induced by the positive charges (e.g. oxygen vacanciesVO2+) trapping and the increase of channel carrier density. For the erase pulses, theµchannelincreases as the domains gradually reverse to upward, which is mainly due to the reduction of the scattering effect induced by the detrapping of positive charges and the decrease of channel carrier density. In addition, the modulation effect of multilevel polarization states onµchannelis verified by numerical simulation. This effect provides a new idea and solution for the development of low power HfO2-based FeFETs in neuromorphic computing.

Realization of Robust and Ambient-Stable Room-Temperature Ferromagnetism in Wide Bandgap Semiconductor 2D Carbon Nitride Sheets.

Due to their weak intrinsic spin-orbit coupling and a distinct bandgap of 3.06 eV, 2D carbon nitride (CN) flakes are promising materials for next-generation spintronic devices. However, achieving strong room-temperature (RT) and ambient-stable ferromagnetism (FM) remains a huge challenge. Here, we demonstrate that the strong RT FM with a high Curie temperature (TC) up to ∼400 K and saturation magnetization (Ms) of 2.91 emu/g can be achieved. Besides, the RT FM exhibits excellent air stability, with Ms remaining stable for over 6 months. Through the magneto-optic Kerr effect, Hall device, X-ray magnetic circular dichroism, and magnetic force microscopy measurements, we acquired clear evidence of magnetic behavior and magnetic domain evolutions at room temperature. Electrical and optical measurements confirm that the Co-doped CN retains its semiconductor properties. Detailed structural characterizations confirm that the single-atom Co coordination and nitrogen defects as well as C-C covalent bonds are simultaneously introduced into CN. Density functional theory calculations reveal that introducing C-C bonds causes carrier spin polarization, and spin-polarized carrier-mediated magnetic exchange between adjacent Co atoms leads to long-range magnetic ordering in CN. We believe that our findings provide a strong experimental foundation for the enormous potential of 2D wide bandgap semiconductor spintronic devices.

Tunable asymmetric magnetoresistance in an Fe3GeTe2/graphite/Fe3GeTe2 lateral spin valve.

van der Waals (vdW) ferromagnetic heterojunctions, characterized by an ultraclean device interface and the absence of lattice matching, have emerged as indispensable and efficient building blocks for future spintronic devices. In this study, we present a seldom observed antisymmetric magnetoresistance (MR) behavior with three distinctive resistance states in a lateral van der Waals (vdW) structure comprising Fe3GeTe2 (FGT)/graphite/FGT. In contrast to traditional spin valves governed by the magnetization configurations of ferromagnetic electrodes (FEs), this distinct feature can be attributed to the interaction between FGT and the FGT/graphite interface, which is primarily influenced by the internal spin-momentum locking effect. Furthermore, modulation of the MR behavior is accomplished by employing the coupling between antiferromagnetic and ferromagnetic materials to adjust the coercive fields of two FEs subsequent to the in situ growth of an FGT oxide layer on FGT. This study elucidates the device physics and mechanism of property modulation in lateral spin valves and holds the potential for advancing the development of gate-tunable spintronic devices and next-generation integrated circuits.

Above-Room-Temperature Strong Ferromagnetism in 2D MnB Nanosheet.

Two-dimensional (2D) room-temperature (RT) ferromagnetic materials have amassed considerable interest in the field of fundamental physics for applications in next-generation spintronic devices owing to their physical properties. However, realizing strong RT ferromagnetism and a high Curie temperature (TC) in these 2D magnetic materials remains challenging. Herein, we develop a 2D MnB nanosheet for known 2D ferromagnets that demonstrates strong RT ferromagnetism and a record-high above-RT TC of ∼585.9 K. Through magnetic force microscopy, clear evidence of ferromagnetic behavior is observed at room temperature. Structural characterization and density functional theory calculations further reveal that (i) after exfoliation of bulk, -OH functional groups were introduced in addition to Mn-B bonds being formed, which increases MnB nanosheet TC to 585.9 K and (ii) the d3↑ spin configuration of Mn mainly contributed to the magnetic moment of MnB, and the hybridization between the dxz (dyz) and dz2 orbitals of the Mn atom provides a large contribution to magnetic anisotropy, which stabilizes the magnetic property of MnB. Our findings establish a strong experimental foundation for 2D MnB nanosheets as ideal materials for the construction of spintronic devices.

BP-based supervised learning algorithm for multilayer photonic spiking neural network and hardware implementation.

We introduce a supervised learning algorithm for photonic spiking neural network (SNN) based on back propagation. For the supervised learning algorithm, the information is encoded into spike trains with different strength, and the SNN is trained according to different patterns composed of different spike numbers of the output neurons. Furthermore, the classification task is performed numerically and experimentally based on the supervised learning algorithm in the SNN. The SNN is composed of photonic spiking neuron based on vertical-cavity surface-emitting laser which is functionally similar to leaky-integrate and fire neuron. The results prove the demonstration of the algorithm implementation on hardware. To seek ultra-low power consumption and ultra-low delay, it is great significance to design and implement a hardware-friendly learning algorithm of photonic neural networks and realize hardware-algorithm collaborative computing.

Ultrahigh thermal-efficient all-optical silicon photonic crystal nanobeam cavity modulator with TPA-induced thermo-optic effect.

We propose and experimentally demonstrate an on-chip all-optical silicon photonic crystal nanobeam cavity (PCNBC) modulator. With the advantages of the strong two-photon absorption (TPA)-induced thermo-optic (TO) effect, ultrahigh thermal-efficient tuning with π phase shift temperature difference ΔTπ of 0.77°C and power Pπ of 0.26 mW is implemented. Moreover, the all-optical modulation is carried out by a pulsed pump light with an average switching power of 0.11 mW. The response times for the rising and falling edges are 7.6 µs and 7.4 µs, respectively. Such a thermal-efficient modulator is poised to be the enabling device for large-scale integration optical signal control systems.

GaN nanowires prepared by Cu-assisted photoelectron-chemical etching.

A novel Cu-assisted photoelectron-chemical etching is proposed to fabricate GaN nanowires. The functional mechanism of assisted metals, etchant concentrations, and the addition of H2O2 was investigated based on theoretical analysis and experiments. The low-cost metal-assisted etchant (CuSO4) proved more favorable than the conventional noble one (AgNO3) for the preparation of GaN nanowires in this work. The formed Ag dendrite blocked the etching when adopting the Ag-assisted etchant, while the Cu-assisted one did not. Moreover, the etchant consisting of 0.01 M CuSO4 and 5 M HF was demonstrated to realize a relatively good surface morphology and fast etching rate. In addition, the common oxidant H2O2 introduced a quasi-stable configuration between the Cu deposition and dissolution, slowing down the formation of the GaN nanowires. The proposed Cu-assisted photoelectron-chemical etching with the advantages of low cost, room temperature, and controllability could offer a new way to fabricate GaN nano-devices.

Compact non-volatile ferroelectric electrostatic doping optical memory based on the epsilon-near-zero effect.

With the booming development of optoelectronic hybrid integrated circuits, the footprint and power consumption of photonic devices have become the most constraining factors for development. To solve these problems, this paper proposes a compact, extremely low-energy and non-volatile optical readout memory based on ferroelectric electrostatic doping and the epsilon-near-zero (ENZ) effect. The writing/erasing state of an optical circuit is controlled by electrical pulses and can remain non-volatile. The device works on the principle that residual polarization charges of ferroelectric film, which is compatible with CMOS processes, are utilized to electrostatically dope indium tin oxide to achieve the ENZ state. Simulation results show that a significant modulation depth of 10.4 dB can be achieved for a device length of 60 µm with an energy consumption below 1 pJ.

Performance improvement of a tunnel junction memristor with amorphous insulator film.

This study theoretically demonstrated the oxygen vacancy (VO2+)-based modulation of a tunneling junction memristor (TJM) with a high and tunable tunneling electroresistance (TER) ratio. The tunneling barrier height and width are modulated by the VO2+-related dipoles, and the ON and OFF-state of the device are achieved by the accumulation of VO2+ and negative charges near the semiconductor electrode, respectively. Furthemore, the TER ratio of TJMs can be tuned by varying the density of the ion dipoles (Ndipole), thicknesses of ferroelectric-like film (TFE) and SiO2 (Tox), doping concentration (Nd) of the semiconductor electrode, and the workfunction of the top electrode (TE). An optimized TER ratio can be achieved with high oxygen vacancy density, relatively thick TFE, thin Tox, small Nd, and moderate TE workfunction.

Electro-Optical Modulation in High Q Metasurface Enhanced with Liquid Crystal Integration.

Electro-optical tuning metasurfaces are particularly attractive since they open up routes for dynamic reconfiguration. The electro-optic (EO) modulation strength essentially depends on the sensitivity to the EO-induced refractive index changes. In this paper, lithium niobate (LiNbO3) metasurfaces integrated with liquid crystals (LCs) are theoretically investigated. Cylinder arrays are proposed to support quasi-bound states in the continuum (quasi-BICs). The quasi-BIC resonances can significantly enhance the lifetime of photons and the local field, contributing to the EO-refractive index changes. By integrating metasurfaces with LCs, the combined influence of the LC reorientation and the Pockels electro-optic effect of LiNbO3 is leveraged to tune the transmitted wavelength and phase spectrum around the quasi-BIC wavelength, resulting in an outstanding tuning sensitivity up to Δλ/ΔV ≈ 0.6 nm/V and relieving the need of high voltage. Furthermore, the proposed structure can alleviate the negative influence of sidewall tilt on device performance. The results presented in this work can foster wide application and prospects for the implementation of tunable displays, light detection and ranging (LiDAR), and spatial light modulators (SLMs).

Mechanical Polarization Switching in Hf0.5Zr0.5O2 Thin Film.

HfO2-based films with high compatibility with Si and complementary metal-oxide semiconductors (CMOS) have been widely explored in recent years. In addition to ferroelectricity and antiferroelectricity, flexoelectricity, the coupling between polarization and a strain gradient, is rarely reported in HfO2-based films. Here, we demonstrate that the mechanically written out-of-plane domains are obtained in 10 nm Hf0.5Zr0.5O2 (HZO) ferroelectric film at room temperature by generating the stress gradient via the tip of an atomic force microscope. The results of scanning Kelvin force microscopy (SKPM) exclude the possibility of flexoelectric-like mechanisms and prove that charge injection could be avoided by mechanical writing and thus reveal the true polarization state, promoting wider flexoelectric applications and ultrahigh-density storage of HZO thin films.

Lateral waveguide scanner integration on surface-emitting mid-infrared lasers.

In this paper, a novel, to the best of our knowledge, monolithic non-mechanical semiconductor laser scanner in the mid-infrared (MIR) spectrum is proposed. A deflector above the active region at the substrate side is used for coupling the vertical light into a lateral substrate waveguide, which creates a chain of coherent emitters such as optical phased arrays (OPAs) for beam steering. The numerical simulation reveals that GaSb-based surface-emitting interband cascade lasers (SE-ICLs) are an excellent platform for waveguide scanner integration. Due to the hundreds of micrometers of optical path difference and the narrow gap between each emitter, an extremely high angle tuning coefficient of 0.84°/nm covering the whole 28.6° steering range is obtained. This work theoretically verifies the feasibility of integrating an OPA scanner into the GaSb-based SE-ICLs, providing a practical solution to fabricate compact steerable MIR laser sources. Note that this substrate OPA concept has strong adaptation potential to extend to even longer wavelength devices such as InP and GaAs-based quantum cascade lasers.

Energy-efficient non-volatile ferroelectric based electrostatic doping multilevel optical readout memory.

Non-volatile multilevel optical memory is an urgent needed artificial component in neuromorphic computing. In this paper, based on ferroelectric based electrostatic doping (Fe-ED) and optical readout due to plasma dispersion effect, we propose an electrically programmable, multi-level non-volatile photonics memory cell, which can be fabricated by standard complementary-metal-oxide-semiconductor (CMOS) compatible processes. Hf0.5Zr0.5O2 (HZO) film is chosen as the ferroelectric ED layer and combines with polysilicon layers for an enhanced amplitude modulation between the carrier accumulation and the confined optical field. Insertion loss below 0.4 dB in erasing state and the maximum recording depth of 9.8 dB are obtained, meanwhile maintaining an extremely low dynamic energy consumption as 1.0-8.4 pJ/level. Those features make this memory a promising candidate for artificial optical synapse in neuromorphic photonics and parallel computing.

Evolution of the Interfacial Layer and Its Impact on Electric-Field-Cycling Behaviors in Ferroelectric Hf1-xZrxO2.

Doped HfO2 thin films, which exhibit robust ferroelectricity even with aggressive thickness scaling, could potentially enable ultralow-power logic and memory devices. The ferroelectric properties of such materials are strongly intertwined with the voltage-cycling-induced electrical and structural changes, leading to wake-up and fatigue effects. Such field-cycling-dependent behaviors are crucial to evaluate the reliability of HfO2-based functional devices; however, its genuine nature remains elusive. Herein, we demonstrate the coupling mechanism between the dynamic change of the interfacial layer and wake-up/fatigue phenomena in ferroelectric Hf1-xZrxO2 (HZO) thin films. Comprehensive atomic-resolution microscopy studies have revealed that the interfacial layer between the HZO and neighboring nonoxide electrode experienced a thickness/composition evolution during electrical cycling. Two theoretical models associated with the depolarization field are adopted, giving consistent results with the thickening of the interfacial layer during electrical cycling. Furthermore, we found that the electrical properties of the HZO devices can be manipulated by controlling the interface properties, e.g., through the choice of electrode match and hybrid cycling process. Our results unambiguously reveal the relationship between the interfacial layer and field-cycling behaviors in HZO, which would further permit the reliability improvement in HZO-based ferroelectric devices through interface engineering.

Dual-Ferroelectric-Coupling-Engineered Two-Dimensional Transistors for Multifunctional In-Memory Computing.

In-memory computing featuring a radical departure from the von Neumann architecture is promising to substantially reduce the energy and time consumption for data-intensive computation. With the increasing challenges facing silicon complementary metal-oxide-semiconductor (CMOS) technology, developing in-memory computing hardware would require a different platform to deliver significantly enhanced functionalities at the material and device level. Here, we explore a dual-gate two-dimensional ferroelectric field-effect transistor (2D FeFET) as a basic device to form both nonvolatile logic gates and artificial synapses, addressing in-memory computing simultaneously in digital and analog spaces. Through diversifying the electrostatic behaviors in 2D transistors with the dual-ferroelectric-coupling effect, rich logic functionalities including linear (AND, OR) and nonlinear (XNOR) gates were obtained in unipolar (MoS2) and ambipolar (MoTe2) FeFETs. Combining both types of 2D FeFETs in a heterogeneous platform, an important computation circuit, i.e., a half-adder, was successfully constructed with an area-efficient two-transistor structure. Furthermore, with the same device structure, several key synaptic functions are shown at the device level, and an artificial neural network is simulated at the system level, manifesting its potential for neuromorphic computing. These findings highlight the prospects of dual-gate 2D FeFETs for the development of multifunctional in-memory computing hardware capable of both digital and analog computation.

Synaptic Behaviors in Ferroelectric-Like Field-Effect Transistors with Ultrathin Amorphous HfO2 Film.

We demonstrate a non-volatile field-effect transistor (NVFET) with a 3-nm amorphous HfO2 dielectric that can simulate the synaptic functions under the difference and repetition of gate voltage (VG) pulses. Under 100 ns write/erase (W/E) pulse, a memory window greater than 0.56 V and cycling endurance above 106 are obtained. The storied information as short-term plasticity (STP) in the device has a spiking post-synaptic drain current (ID) that is a response to the VG input pulse and spontaneous decay of ID. A refractory period after the stimuli is observed, during which the ID hardly varies with the VG well-emulating the bio-synapse behavior. Short-term memory to long-term memory transition, paired-pulse facilitation, and post-tetanic potentiation are realized by adjusting the VG pulse waveform and number. The experimental results indicate that the amorphous HfO2 NVFET is a potential candidate for artificial bio-synapse applications.