High responsivity photodetectors based on graphene/WSe_(2) heterostructure by photogating effect

Graphene, with its zero-bandgap electronic structure, is a highly promising ultra-broadband light absorbing material.However, the performance of graphene-based photodetectors is limited by weak absorption efficiency and rapid recombination of photoexcited carriers, leading to poor photodetection performance. Here, inspired by the photogating effect, we demonstrated a highly sensitive photodetector based on graphene/WSe_(2) vertical heterostructure where the WSe_(2) layer acts as both the light absorption layer and the localized grating layer. The graphene conductive channel is induced to produce more carriers by capacitive coupling. Due to the strong light absorption and high external quantum efficiency of multilayer WSe_(2), as well as the high carrier mobility of graphene, a high photocurrent is generated in the vertical heterostructure. As a result, the photodetector exhibits ultra-high responsivity of 3.85×10~4A/W and external quantum efficiency of 1.3 × 10~7%.This finding demonstrates that photogating structures can effectively enhance the sensitivity of graphene-based photodetectors and may have great potential applications in future optoelectronic devices.

Effects of gallium surfactant on AlN thin films by microwave plasma chemical vapor deposition

In this work, AlN films were grown using gallium (Ga) as surfactant on 4° off-axis 4H-SiC substrates via microwave plasma chemical vapor deposition (MPCVD). We have found that AlN growth rate can be greatly improved due to the catalytic effect of trimethyl-gallium (TMGa), but AlN crystal structure and composition are not affected. When the proportion of TMGa in gas phase was low, crystal quality of AlN can be improved and three-dimensional growth mode of AlN was enhanced with the increase of Ga source. When the proportion of TMGa in gas phase was high, two-dimensional growth mode of AlN was presented, with the increase of Ga source results in the deterioration of AlN crystal quality. Finally, employing a two-step growth approach, involving the initial growth of Ga-free AlN nucleation layer followed by Ga-assisted AlN growth, high quality of AlN film with flat surface was obtained and the full width at half maximum (FWHM) values of 415 nm AlN (002) and (102) planes were 465 and 597 arcsec.

Current-collapse suppression and leakage-current decrease in AlGaN/GaN HEMT by sputter-TaN gate-dielectric layer

In this paper, we explore the electrical characteristics of high-electron-mobility transistors(HEMTs) using a TaN/AlGaN/GaN metal insulating semiconductor(MIS) structure. The high-resistance tantalum nitride(TaN) film prepared by magnetron sputtering as the gate dielectric layer of the device achieved an effective reduction of electronic states at the TaN/AlGaN interface, and reducing the gate leakage current of the MIS HEMT, its performance was enhanced. The HEMT exhibited a low gate leakage current of 2.15 × 10^(-7) mA/mm and a breakdown voltage of 1180 V. Furthermore, the MIS HEMT displayed exceptional operational stability during dynamic tests, with dynamic resistance remaining only 1.39 times even under 400 V stress.

Ultra-high photoresponsive photodetector based on ReS_(2)/SnS_(2)heterostructure

Photodetectors based on two-dimensional materials have attracted much attention because of their unique structure and outstanding performance.The response speed of single ReS_(2)photodetector is slow exceptionally,the heterostructure could improves the response speed of ReS_(2)-based photodetector,but the photodetectors responsivity is reduced greatly,which restricts the development of ReS_(2).In this paper,a vertically structured ReS_(2)/SnS_(2)van der Waals heterostructure photodetectors is prepared,using ReS_(2)as the transport layer and SnS_(2)as the light absorbing layer to regulate the channel current.The device has an ultra-high photoconductive gain of 10^(10),which exhibits an ultra-high responsivity of4706 A/W under 365-nm illumination and response speed in seconds,and has an ultra-high external quantum efficiency of1.602×10^(6)%and a high detectivity of 5.29×10^(12)jones.The study for ReS_(2)-based photodetector displays great potential for developing future optoelectronic devices.

Study of enhancement-mode GaN pFET with H plasma treated gate recess

This letter showcases the successful fabrication of an enhancement-mode(E-mode)buried p-channel GaN fieldeffect-transistor on a standard p-GaN/AlGaN/GaN-on-Si power HEMT substrate.The transistor exhibits a threshold voltage(VTH)of−3.8 V,a maximum ON-state current(ION)of 1.12 mA/mm,and an impressive ION/IOFF ratio of 10^(7).To achieve these remarkable results,an H plasma treatment was strategically applied to the gated p-GaN region,where a relatively thick GaN layer(i.e.,70 nm)was kept intact without aggressive gate recess.Through this treatment,the top portion of the GaN layer was converted to be hole-free,leaving only the bottom portion p-type and spatially separated from the etched GaN surface and gateoxide/GaN interface.This approach allows for E-mode operation while retaining high-quality p-channel characteristics.

Homoepitaxial growth of (100) Si-doped β-Ga_(2)O_(3) films via MOCVD

Homoepitaxial growth of Si-doped β-Ga_(2)O_(3) films on semi-insulating(100) β-Ga_(2)O_(3) substrates by metalorganic chemical vapor deposition(MOCVD) is studied in this work. By appropriately optimizing the growth conditions, an increasing diffusion length of Ga adatoms is realized, suppressing 3D island growth patterns prevalent in(100) β-Ga_(2)O_(3) films and optimizing the surface morphology with [010] oriented stripe features. The slightly Si-doped β-Ga_(2)O_(3) film shows smooth and flat surface morphology with a root-mean-square roughness of 1.3 nm. Rocking curves of the(400) diffraction peak also demonstrate the high crystal quality of the Si-doped films. According to the capacitance–voltage characteristics, the effective net doping concentrations of the films are 5.41 × 10~(15) – 1.74 × 10~(20) cm~(-3). Hall measurements demonstrate a high electron mobility value of 51cm~2/(V·s), corresponding to a carrier concentration of 7.19 × 10~(18) cm~(-3) and a high activation efficiency of up to 61.5%. Transmission line model(TLM) measurement shows excellent Ohmic contacts and a low specific contact resistance of 1.29 × 10~(-4) Ω·cm~2 for the Si-doped film, which is comparable to the Si-implanted film with a concentration of 5.0 × 10~(19) cm~(-3), confirming the effective Si doing in the MOCVD epitaxy.

Spin torque oscillator based on magnetic tunnel junction with MgO cap layer for radio-frequency-oriented neuromorphic computing

Recently,it has been proposed that spin torque oscillators(STOs)and spin torque diodes could be used as artificial neurons and synapses to directly process microwave signals,which could lower latency and power consumption greatly.However,one critical challenge is to make the microwave emission frequency of the STO stay constant with a varying input current.In this work,we study the microwave emission characteristics of STOs based on magnetic tunnel junction with MgO cap layer.By applying a small magnetic field,we realize the invariability of the microwave emission frequency of the STO,making it qualified to act as artificial neuron.Furthermore,we have simulated an artificial neural network using STO neuron to recognize the handwritten digits in the Mixed National Institute of Standards and Technology database,and obtained a high accuracy of 92.28%.Our work paves the way for the development of radio-frequency-oriented neuromorphic computing systems.

Diffractive Deep Neural Networks at Visible Wavelengths

Optical deep learning based on diffractive optical elements offers unique advantages for parallel processing,computational speed,and power efficiency.One landmark method is the diffractive deep neural network(D^(2) NN)based on three-dimensional printing technology operated in the terahertz spectral range.Since the terahertz bandwidth involves limited interparticle coupling and material losses,this paper extends D^(2) NN to visible wavelengths.A general theory including a revised formula is proposed to solve any contradictions between wavelength,neuron size,and fabrication limitations.A novel visible light D^(2) NN classifier is used to recognize unchanged targets(handwritten digits ranging from 0 to 9)and targets that have been changed(i.e.,targets that have been covered or altered)at a visible wavelength of 632.8 nm.The obtained experimental classification accuracy(84%)and numerical classification accuracy(91.57%)quantify the match between the theoretical design and fabricated system performance.The presented framework can be used to apply a D^(2) NN to various practical applications and design other new applications.

The origin of distorted intensity pattern sensed by a lens and antenna coupled AlGaN/GaN-HEMT terahertz detector

Antenna-coupled field-effect-transistors(FETs) offer high sensitivity for terahertz detection. Both the magnitude and the polarity of the response signal are sensitive to the localized terahertz field under the gate. The ability of accurate sensing the intensity pattern is required for terahertz imaging systems. Here, we report artefacts in the intensity pattern of a focused terahertz beam around 1 THz by scanning a silicon-lens and antenna coupled AlGaN/GaN high-electron-mobility-transistor(HEMT) detector. The origin of the image distortion is found to be connected with one of the antenna blocks by probing the localized photocurrents as a function of the beam location and the frequency. Although the exact distortion is found with our specific antenna design, we believe similar artefacts could be commonplace in antenna-coupled FET terahertz detectors when the beam spot becomes comparable with the antenna size. To eliminate such artefacts, new antenna designs are welcomed to achieve strong asymmetry in the terahertz field distribution under the gate while maintaining a more symmetric radiation pattern for the whole antenna.

Vertical Schottky ultraviolet photodetector based on graphene and top–down fabricated GaN nanorod arrays

GaN has been widely used in the fabrication of ultraviolet photodetectors because of its outstanding properties.In this paper,we report a graphene–GaN nanorod heterostructure photodetector with fast photoresponse in the UV range.GaN nanorods were fabricated by a combination mode of dry etching and wet etching.Furthermore,a graphene–GaN nanorod heterostructure ultraviolet detector was fabricated and its photoelectric properties were measured.The device exhibits a fast photoresponse in the UV range.The rising time and falling time of the transient response were 13 and 8 ms,respectively.A high photovoltaic responsivity up to 13.9 A/W and external quantum efficiency up to 479%were realized at the UV range.The specific detectivity D*=1.44×10^(10) Jones was obtained at–1 V bias in ambient conditions.The spectral response was measured and the highest response was observed at the 360 nm band.

Fabrication of plasmonic nanopyramidal array as flexible SERS substrate for biosensing application

The proposed work aims to develop a sensitive surface-enhanced Raman spectroscopy(SERS)nano-biosensor.The inverted nano pyramid array on silicon substrate fabricated using electron beam lithography(EBL)was utilised as a master template and the mold was later replicated via nanoimprinting process to prepare gold-coated polymer nanopyramid three-dimensional(3D)SERS substrate.The fast and versatile replication process using nanoimprinting lithography(NIL)can produce polymer nanopyramids in a low-cost and reproducible fashion.Also,the proposed fabrication protocol can be easily upscale for large scale fabrication.The intense electric field confinement at nanotips and four edges of gold-coated polymer nanopyramid enhanced the Raman signal of probe molecules,i.e.,Rhodamine 6G with a limit of detection down to 3.277×10−9 M was achieved.This work also underlines the efficiency of gold-coated polymer nanopyramid arrays in the spectral detection of hemoglobin proteins at low concentrations.The Raman signal enhancement mechanism was further studied through the electromagnetic simulation using COMSOL Multiphysics.In addition,bending test experiments were performed to understand the effect of flexibility on SERS signal response.The fabricated gold-coated polymer nanopyramids arrays could pave the way for the development of low-cost SERS platforms for the detection of hazardous biological and chemical compounds at ultra-low concentrations in practical applications.