Photonic synapses with ultralow energy consumption for artificial visual perception and brain storage

The human visual system,dependent on retinal cells,can be regarded as a complex combination of optical system and nervous system.Artificial retinal system could mimic the sensing and processing function of human eyes.Optically stimulated synaptic devices could serve as the building blocks for artificial retinas and subsequent information transmission system to brain.Herein,photonic synaptic transistors based on polycrystalline MoS_(2),which could simulate human visual perception and brain storage,are presented.Moreover,the photodetection range from visible light to near-infrared light of MoS_(2) multilayer could extend human eyes’vision limitation to near-infrared light.Additionally,the photonic synaptic transistor shows an ultrafast speed within 5μs and ultralow power consumption under optical stimuli about 40 aJ,several orders of magnitude lower than biological synapses(50 ms and 10 fJ).Furthermore,the backgate control could act as emotional modulation of the artificial brain to enhance or suppress memory function,i.e.the intensity of photoresponse.The proposed carrier trapping/detrapping as the main working mechanism is presented for the device.In addition,synaptic functionalities including short synaptic plasticity,long synaptic plasticity and paired-pulse facilitation could be successfully simulated based on the prepared device.Furthermore,the large difference between short synaptic plasticity and long synaptic plasticity reveals the better image pre-processing function of the prepared photonic synapses.The classical Pavlovian conditioning associated with the associative learning is successfully implemented as well.Therefore,the efficient and rich functionalities demonstrate the potential of the MoS_(2) synaptic device that integrates sensing-memory-preprocessing capabilities for realizing artificial neural networks with different emotions that mimic human retina and brain.

A Broadband 630-720 GHz Schottky Based Sub-Harmonic Mixer Using Intrinsic Resonances of Hammer-Head Filter

An room temperature low noise anti-parallel Schottky diode based 630-720 GHz sub-harmonic mixer(SHM) is designed, built and measured. Intrinsic resonances in lowpass hammer-head filter have been adopted to prevent the LO and RF power leak from the IF channel, while greatly minimizing the transmission line size. The mixer consists of 15 um quartz terahertz circuit and 127 um Al2 O3 IF transformer circuit. An improved lumped element equivalent noise model of SBDs guarantees the accuracy of simulation. The measurement indicates that with local oscillating(LO)signal of 2-8 mW, the lowest double sideband(DSB) conversion loss is 8.2 dB at 645 GHz,and the best DSB noise temperature is 2800 K at 657 GHz. The 3 dB bandwidth of conversion loss is 75 GHz from 638 to 715 GHz. The work IF frequency band is above 20 GHz ranging from 1 to 20 GHz with-10 dB return loss.

Giant spin injection into semiconductor and THz pulse emission

Spintronics, which use the spin of electrons rather than their direct motion to carry information, has emerged as one of the leading alternatives to traditional electronics, promising faster information processing and lower energy consumption. Efficient spin injection into semiconductor is a crucial first step to realize useful spintronic devices, which remains an elusive challenge up to now. Recently, a joint group from Nanyang Technological University, the National University of Singapore, and the Agency for Science, Technology and Research (A*STAR) has achieved a breakthrough in the speed and efficiency of spin injection into semiconductor.

Positive Bias Temperature Instability Degradation of Buried InGaAs Channel nMOSFETs with InGaP Barrier Layer and Al2O3 Dielectric

Positive bias temperature instability stress induced interface trap density in a buried InGaAs channel metaloxide-semiconductor field-effect transistor with a InCaP barrier layer and Al2O3 dielectric is investigated. Well behaved split C-V characteristics with small capacitance frequency dispersion are confirmed after the insertion of the InCaP barrier layer. The direct-current Id-Vg measurements show both degradations of positive gate voltage shift and sub-threshold swing in the sub-threshold region, and degradation of positive △Vg in the oncurrent region. The Id-Vg degradation during the positive bias temperature instability tests is mainly contributed by the generation of near interface acceptor traps under stress. Specifically, the stress induced aeceptor traps contain both permanent and recoverable traps. Compared with surface channel InCaAs devices, stress induced recoverable donor traps are negligible in the buried channel ones.

0.14 THz imaging system for security and surveillance

Active terahertz-wave imaging systems play a significant role in security and surveillance applications.A 0.14 THz near-field imaging system is described,which consists of a signal generator and acquisition unit,a transceiver front end,a digital signal process unit and a motor control unit.Based on the two-dimensional synthetic aperture technique and the image reconstruction algorithm,this system is capable of producing three-dimensional images of 2 mm lateral resolution and 3 cm range resolution in concealed weapon detection experiments.

Passivation and dissociation of P_(b)-type defects at a-SiO_(2)/Si interface

It is well known that in the process of thermal oxidation of silicon,there are P_(b)-type defects at amorphous silicon dioxide/silicon(a-SiO_(2)/Si)interface due to strain.These defects have a very important impact on the performance and reliability of semiconductor devices.In the process of passivation,hydrogen is usually used to inactivate P_(b)-type defects by the reaction P_(b)+H_(2)→P_(b)H+H.At the same time,P_(b)H centers dissociate according to the chemical reaction P_(b)H→P_(b)+H.Therefore,it is of great significance to study the balance of the passivation and dissociation.In this work,the reaction mechanisms of passivation and dissociation of the P_(b)-type defects are investigated by first-principles calculations.The reaction rates of the passivation and dissociation are calculated by the climbing image-nudged elastic band(CI-NEB)method and harmonic transition state theory(HTST).By coupling the rate equations of the passivation and dissociation reactions,the equilibrium density ratio of the saturated interfacial dangling bonds and interfacial defects(P_(b),P_(b)0,and P_(b)1)at different temperatures is calculated.

Giant Broadband One Way Transmission Based on Directional Mie Scattering and Asymmetric Grating Diffraction Effects

High performance optical diode-like devices are highly desired in future practical nano-photonic devices with strong directional selectivity.We demonstrate a kind of giant broadband reciprocity optical diode-like devices by simultaneously using the directional Mie scattering effect and the asymmetric grating diffraction effect.The maximum asymmetric subtraction and the asymmetric transmission ratio can reach nearly 100%and 40dB at specified wavelength,respectively.In a wide waveband from 500nm to 800nm,the asymmetric subtraction and the ratio keep larger than 80%and 3.5 dB,respectively,even under oblique incidence.To the best of our knowledge,this is the best one-way-transmission effect observed in the reciprocity optical diode-like devices.In addition,we further demonstrate that this one-way-transmission effect can bring an effective absorption enhancement on gold films.The giant,broadband and angle-insensitive one-way-transmission effect demonstrated here is far beyond the well-known anti-reflection effect in the light-trapping devices and will bring new design philosophy for nano-photonic devices.

First-principles calculations of F-, Cl-, and N-related defects of amorphous SiO_(2) and their impacts on carrier trapping and proton release

The first-principles calculations based on density functional theory are performed to study F-,Cl-,and N-related defects of amorphous SiO_(2)(a-SiO_(2)) and their impacts on carrier trapping and proton release.The possible geometric configurations of the impurity-related defects,the formation energies,the hole or electron trapping of the neutral defects,and the mechanisms to suppress proton diffusion by doping N are investigated.It is demonstrated by the calculations that the impurity atoms can interact with the oxygen vacancies and result in impurity-related defects.The reactions can be utilized to saturate oxygen vacancies that will cause ionization damage to the semiconducting devices.Moreover,the calculated formation energy indicates that the F-or Cl-related oxygen vacancy defect is a deep hole trap,which can trap holes and prevent them from diffusing to the a-SiO_(2)/Si interface.However,three N-related defects,namely N(2)o-H,N(2)o=O,and N(3)o-Vo,tend to act as shallow hole traps to facilitate hole transportation during device operation.The N(2)o and N(3)o configurations can be negatively charged as deep electron traps during the oxide charge buildup after ionization radiation.In addition,the nudged elastic band(NEB) calculations show that four N-related defects,namely N(2)o,N(2)o-H,N(2)o=O,and N(3)o are capable of capturing protons and preventing them from diffusing to and de-passivating the interface.This research reveals the fundamental properties of the F-,Cl-,and N-related defects in amorphous silica and the details of the reactions of the carrier trapping and proton release.The findings help to understand the microscopic mechanisms that alleviate ionization damage of semiconducting devices by doping a-SiO_(2).

Growth and transport properties of topological insulator Bi2Se3 thin film on a ferromagnetic insulating substrate

Exchange coupling between topological insulator and ferromagnetic insulator through proximity effect is strongly attractive for both fundamental physics and technological applications. Here we report a comprehensive investigation on the growth behaviors of prototype topological insulator Bi2Se3 thin film on a single-crystalline LaCoO3 thin film on SrTiO3 substrate, which is a strain-induced ferromagnetic insulator. Different from the growth on other substrates, the Bi2Se3 films with highest quality on LaCoO3 favor a relatively low substrate temperature during growth. As a result, an inverse dependence of carrier mobility with the substrate temperature is found. Moreover, the magnetoresistance and coherence length of weak antilocalization also have a similar inverse dependence with the substrate temperature, as revealed by the magnetotransport measurements. Our experiments elucidate the special behaviors in Bi2Se3/LaCoO3 heterostructures, which provide a good platform for exploring related novel quantum phenomena, and are inspiring for device applications.

First-principles study of non-radiative carrier capture by defects at amorphous-SiO_(2)/Si(100)interface

Defects have a significant impact on the performance of semiconductor devices.Using the first-principles combined with one-dimensional static coupling theory approach,we have calculated the variation of carrier capture coefficients with temperature for the interfacial defects P_(b0) and P_(b1) in amorphous-SiO_(2)/Si(100)interface.It is found that the geometrical shapes of P_(b0) and P_(b1) defects undergo large deformations after capturing carriers to form charged defects,especially for the Si atoms containing a dangling bond.The hole capture coefficients of neutral P_(b0) and P_(b1) defects are largest than the other capture coefficients,indicating that these defects have a higher probability of forming positively charged centres.Meanwhile,the calculated results of non-radiative recombination coefficient of these defects show that both P_(b0) and P_(b1) defects are the dominant non-radiative recombination centers in the interface of a-SiO_(2)/Si(100).

Evaluating the quantum confinement effects modulating exciton and electronic band structures of two-dimensional layered MoSSe films and their photodetection potentials

Emerging two-dimensional ternary transition metal dichalcogenide alloys have attracted much attention for their unique optical and optoelectronic properties,making them ideal candidates for optoelectronic applications.However,a comprehensive understanding of their quantum confinement effects and photoelectronic response characteristics remains crucial for device optimization and performance enhancement.In this study,we employed various spectroscopic techniques to investigate the optical properties and electronic band structures of molybdenum sulfide selenide(MoSSe)films with different layer numbers(4–11 layers).Our results revealed the splitting of Raman modes and shifting of phonon vibrational frequencies with increasing thickness,suggesting that MoSSe has strong interactions within the lattice.The A1g and E2g 1 modes were mainly shifted by internal strain and dielectric screening effect versus thickness,respectively.The redshift phenomenon of A and B excitons with increasing thickness was attributed to the leading effect of quantum confinement on exciton properties and optical band gaps.We observed a strong decrease in the direct bandgap spectral weight in photoluminescence(PL)when the layer number increased from 4 to 5.In addition,we have fabricated MoSSe photodetectors that exhibit a broadband response in the visible wavelength band of 350–800 nm.Furthermore,the observed enhancement in photocurrent and responsivity with increasing film thickness underscored the potential of MoSSebased devices for practical optoelectronic applications.This research contributes to advancing our fundamental understanding of MoSSe materials and paves the way for the design and development of high-performance optoelectronic devices.

Sustainable development of electroencephalography materials and technology

Electroencephalogram(EEG)is one of the most important bioelectrical signals related to brain activity and plays a crucial role in clinical medicine.Driven by continuously expanding applications,the development of EEG materials and technology has attracted considerable attention.However,systematic analysis of the sustainable development of EEG materials and technology is still lacking.This review discusses the sustainable development of EEG materials and technology.First,the developing course of EEG is introduced to reveal its significance,particularly in clinical medicine.Then,the sustainability of the EEG materials and technology is discussed from two main aspects:integrated systems and EEG electrodes.For integrated systems,sustainability has been focused on the developing trend toward mobile EEG systems and big-data monitoring/analyzing of EEG signals.Sustainability is related to miniaturized,wireless,portable,and wearable systems that are integrated with big-data modeling techniques.For EEG electrodes and materials,sustainability has been comprehensively analyzed from three perspectives:performance of different material/structural categories,sustainablematerials for EEGelectrodes,and sustainable manufacturing technologies.In addition,sustainable applications of EEG have been presented.Finally,the sustainable development of EEG materials and technology in recent decades is summarized,revealing future possible research directions as well as urgent challenges.

Enhancing performance of GaN-based LDs by using GaN/InGaN asymmetric lower waveguide layers

The effects of Ga N/In Ga N asymmetric lower waveguide(LWG)layers on photoelectrical properties of In Ga N multiple quantum well laser diodes(LDs)with an emission wavelength of around 416 nm are theoretically investigated by tuning the thickness and the indium content of In Ga N insertion layer(In Ga N-IL)between the Ga N lower waveguide layer and the quantum wells,which is achieved with the Crosslight Device Simulation Software(PIC3D,Crosslight Software Inc.).The optimal thickness and the indium content of the In Ga N-IL in lower waveguide layers are found to be 300 nm and 4%,respectively.The thickness of In Ga N-IL predominantly affects the output power and the optical field distribution in comparison with the indium content,and the highest output power is achieved to be 1.25 times that of the reference structure(symmetric Ga N waveguide),which is attributed to the reduced optical absorption loss as well as the concentrated optical field nearby quantum wells.Furthermore,when the thickness and indium content of In Ga N-IL both reach a higher level,the performance of asymmetric quantum wells LDs will be weakened rapidly due to the obvious decrease of optical confinement factor(OCF)related to the concentrated optical field in the lower waveguide.

A Semi-Blind Method to Estimate the I/Q Imbalanc of THz Orthogonal Modulator

In terahertz communication,the direct frequency conversion structure in which orthogonal mixer is the main frequency conversion unit,makes engineers get into trouble of in-phase(I)branch and quadrature(Q)branch imbalance,carrier wave leakage,etc.These damages result in system performance tremendous degrades.We proposed a semiblind method to estimate the I/Q imbalance of THz orthogonal modulator,based on predefined preamble and pilot symbols for quadrature amplitude modulation(QAM).In this paper,a transmitter with Y band quadrature mixer and 20Gbps base-band signal has been tested.The bandwidth of the baseband signal was 7GHz,and the modulation type was 16QAM.By this method,7dB improvement of the system’s symbol Mean Square Error(MSE)has been got.That means the proposed method can be used to eliminate the I/Q imbalance effectively.

Molecular dynamics simulation of atomic hydrogen diffusion in strained amorphous silica

Understanding hydrogen diffusion in amorphous SiO2(a-SiO2),especially under strain,is of prominent importance for improving the reliability of semiconducting devices,such as metal-oxide-semiconductor field effect transistors.In this work,the diffusion of hydrogen atom in a-SiO2 under strain is simulated by using molecular dynamics(MD)with the ReaxFF force field.A defect-free a-SiO2 atomic model,of which the local structure parameters accord well with the experimental results,is established.Strain is applied by using the uniaxial tensile method,and the values of maximum strain,ultimate strength,and Young's modulus of the a-SiO2 model under different tensile rates are calculated.The diffusion of hydrogen atom is simulated by MD with the ReaxFF,and its pathway is identified to be a series of hops among local energy minima.Moreover,the calculated diffusivity and activation energy show their dependence on strain.The diffusivity is substantially enhanced by the tensile strain at a low temperature(below 500 K),but reduced at a high temperature(above 500 K).The activation energy decreases as strain increases.Our research shows that the tensile strain can have an influence on hydrogen transportation in a-SiO2,which may be utilized to improve the reliability of semiconducting devices.

Ab initio calculations on oxygen vacancy defects in strained amorphous silica

The effects of uniaxial tensile strain on the structural and electronic properties of positively charged oxygen vacancy defects in amorphous silica(a-SiO2)are systematically investigated using ab-initio calculation based on density functional theory.Four types of positively charged oxygen vacancy defects,namely the dimer,unpuckered,and puckered four-fold(4×),and puckered five-fold(5×)configurations have been investigated.It is shown by the calculations that applying uniaxial tensile strain can lead to irreversible transitions of defect structures,which can be identified from the fluctuations of the curves of relative total energy versus strain.Driven by strain,a positively charged dimer configuration may relax into a puckered 5×configuration,and an unpuckered configuration may relax into either a puckered 4×configuration or a forward-oriented configuration.Accordingly,the Fermi contacts of the defects remarkably increase and the defect levels shift under strain.The Fermi contacts of the puckered configurations also increase under strain to the values close to that of Eα′center in a-SiO2.In addition,it is shown by the calculations that the relaxation channels of the puckered configurations after electron recombination are sensitive to strain,that is,those configurations are more likely to relax into a two-fold coordinated Si structure or to hold a puckered structure under strain,both of which may raise up the thermodynamic charge-state transition levels of the defects into Si band gap.As strain induces more puckered configurations with the transition levels in Si band gap,it may facilitate directly the development of oxide charge accumulation and indirectly that of interface charge accumulation by promoting proton generation under ionization radiation.This work sheds a light on understanding the strain effect on ionization damage at an atomic scale.

Transport properties in monolayer–bilayer–monolayer graphene planar junctions

The transport study of graphene based junctions has become one of the focuses in graphene research. There are two stacking configurations for monolayer–bilayer–monolayer graphene planar junctions. One is the two monolayer graphene contacting the same side of the bilayer graphene, and the other is the two-monolayer graphene contacting the different layers of the bilayer graphene. In this paper, according to the Landauer–Büttiker formula, we study the transport properties of these two configurations. The influences of the local gate potential in each part, the bias potential in bilayer graphene,the disorder and external magnetic field on conductance are obtained. We find the conductances of the two configurations can be manipulated by all of these effects. Especially, one can distinguish the two stacking configurations by introducing the bias potential into the bilayer graphene. The strong disorder and the external magnetic field will make the two stacking configurations indistinguishable in the transport experiment.

Preliminary study of 0.14 THz water surface clutter reflectivity at near vertical incidence angle

As the development of radar systems developed for the terahertz and higher millimeter region continues to grow, the interest in the water surface and land clutter at terahertz and higher millimeter frequencies continues to increase. An empirical model of sea clutter reflectivity is described firstly, which is valid at near vertical incidence and has small average absolute deviation compared to other empirical models. Simulated results of 0.14 THz sea clutter at near vertical incidence with thismodel are shown. An indoor test-bed is constructed in order to measure 0.14 THz water surface clutter reflectivity and an experiment is carried out. Initial experimental results are presented and compared to the simulation results, which partially verifies that the empirical sea clutter model still works at 0.14 THz.

First-principles investigations of proton generation inα-quartz

Proton plays a key role in the interface-trap formation that is one of the primary reliability concerns, thus learning how it behaves is key to understand the radiation response of microelectronic devices. The first-principles calculations have been applied to explore the defects and their reactions associated with the proton release in a-quartz, the well-known crystalline isomer of amorphous silica. When a high concentration of molecular hydrogen （H2） is present, the proton generation can be enhanced by cracking the He molecules at the positively charged oxygen vacancies in dimer configuration. If the concentration of molecular hydrogen is low, the proton generation mainly depends on the proton dissociation of the doubly- hydrogenated defects. In particular, a fully passivated E~ center can dissociate to release a proton barrierlessly by structure relaxation once trapping a hole. This research provides a microscopic insight into the proton release in silicon dioxide, the critical step associated with the interface-trap formation under radiation in microelectronic devices.

First-principles calculations of the hole-induced depassivation of SiO2/Si interface defects

The holes induced by ionizing radiation or carrier injection can depassivate saturated interface defects.The depassivation of these defects suggests that the deep levels associated with the defects are reactivated,affecting the performance of devices.This work simulates the depassivation reactions between holes and passivated amorphous-SiO_(2)/Si interface defects(HP_(b)+h→P_(b)+H^(+)).The climbing image nudged elastic band method is used to calculate the reaction curves and the barriers.In addition,the atomic charges of the initial and final structures are analyzed by the Bader charge method.It is shown that more than one hole is trapped by the defects,which is implied by the reduction in the total number of valence electrons on the active atoms.The results indicate that the depassivation of the defects by the holes actually occurs in three steps.In the first step,a hole is captured by the passivated defect,resulting in the stretching of the Si-H bond.In the second step,the defect captures one more hole,which may contribute to the breaking of the Si-H bond.The H atom is released as a proton and the Si atom is three-coordinated and positively charged.In the third step,an electron is captured by the Si atom,and the Si atom becomes neutral.In this step,a Pb-type defect is reactivated.