Vertical Devices Research Articles

A 3D paper microfluidic device for enzyme-linked assays: Application to DNA analysis.

A paper microfluidic device capable of conducting enzyme-linked assays is presented: a microfluidic enzyme-linked paper analytical device (μEL-PAD). The system exploits a wash-free sandwich coupling to form beads/analyte/enzyme complexes, which are subsequently added to the vertical flow device composed of wax-printed paper, waxed nitrocellulose membrane and absorbent/barrier layers. The nitrocellulose retains the bead complexes without disrupting the flow, enabling for an efficient washing step. The entrapped complexes then interact with the chromogenic substrate stored on the detection paper, generating a color change on it, quantified with an open-source smartphone software. This is a universal paper-based technology suitable for high-sensitivity quantification of many analytes, such as proteins or nucleic acids, with different enzyme-linked formats. Here, the potential of the μEL-PAD is demonstrated to detect DNA from Staphylococcus epidermidis. After generation of isothermally amplified genomic DNA from bacteria, Biotin/FITC-labeled products were analyzed with the μEL-PAD, exploiting streptavidin-coated beads and antiFITC-horseradish peroxidase. The μEL-PAD achieved a limit of detection (LOD) and quantification<10 genome copies/μL, these being at least 70- and 1000-fold lower, respectively, than a traditional lateral flow assay (LFA) exploiting immobilized streptavidin and antiFITC-gold nanoparticles. It is envisaged that the device will be a good option for low-cost, simple, quantitative, and sensitive paper-based point-of-care testing.

General optimization of breakdown voltage and resistivity on power components in terms of doping level and thickness

The emergence of wide band-gap (WBG) materials promises improved performance in semiconductor devices, but also presents significant technical challenges. Developing this immature technology and adapting market-ready silicon-based technology to new materials requires a thorough understanding of the relationship between material properties and the final electrical performance of the component. In this work, we propose a model that relates generic material properties and device-specific parameters (such as doping level or thickness) to the final breakdown voltage and performance of unipolar vertical diodes. Our approach enables optimized device design and evaluation by connecting the electrical performance target with the optimal drift layer characteristics. This model can be applied universally to any material with known impact ionization coefficients, carrier mobility, and thermal activation energy. We focus on diamond as a paradigm for WBG materials, illustrating specific characteristics such as incomplete carrier ionization or elevated doping-dependent critical electric fields. Our model predicts that diamond-based vertical devices with 70 μm layers and doping levels of 1.1015 cm−3 can sustain 20 kV.

Physical Modelling of Charge Trapping Effects in SiC MOSFETs

In the recent past, lots of efforts have been put into further developing SiC power MOSFETs. In addition to optimization of device geometry, i.e., vertical device structure, various post-oxidation anneals have been studied to improve carrier mobility by reducing trap density. Nevertheless, a considerable number of traps remain, which are the central origin for dynamic changes in the threshold voltage of up to several volts during DC and AC operation. To explain the threshold voltage instability, an effective two-state defect model has been recently applied. In this work, we give an overview of modeling efforts to explain the impact of defects on the device threshold voltage and discuss the hysteresis of voltage sweep and bias temperature instabilities in SiC transistors. Based on the combination of measurements and computer simulations, a list of potential defect candidates responsible for the observed threshold voltage instabilities is discussed.

Downsizing the Channel Length of Vertical Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) are promising building blocks for bioelectronic devices such as sensors and neural interfaces. While the majority of OECTs use simple planar geometry, there is interest in exploring how these devices operate with much shorter channels on the submicron scale. Here, we show a practical route toward the minimization of the channel length of the transistor using traditional photolithography, enabling large-scale utilization. We describe the fabrication of such transistors using two types of conducting polymers. First, commercial solution-processed poly(dioxyethylenethiophene):poly(styrene sulfonate), PEDOT:PSS. Next, we also exploit the short channel length to support easy in situ electropolymerization of poly(dioxyethylenethiophene):tetrabutyl ammonium hexafluorophosphate, PEDOT:PF6. Both variants show different promising features, leading the way in terms of transconductance (gm), with the measured peak gm up to 68 mS for relatively thin (280 nm) channel layers on devices with the channel length of 350 nm and with widths of 50, 100, and 200 μm. This result suggests that the use of electropolymerized semiconductors, which can be easily customized, is viable with vertical geometry, as uniform and thin layers can be created. Spin-coated PEDOT:PSS lags behind with the lower values of gm; however, it excels in terms of the speed of the device and also has a comparably lower off current (300 nA), leading to unusually high on/off ratio, with values up to 8.6 × 104. Our approach to vertical gap devices is simple, scalable, and can be extended to other applications where small electrochemical channels are desired.

Fully bulk CMOS compatible Key Shape Floating Body Memory (KFBM)

This paper presents a capacitorless memory cell with bulk CMOS compatibility, consisting of a MOSFET with a virtual floating body formed by the trench. The name Key shape Floating Body Memory (KFBM) is derived from the resemblance of the structure to the shape of an antique key. The carrier concentration in the vertical device beneath the MOSFET results in over more than 5 orders of magnitude of the on–off cell current ratio with off-current less than 100pA/cell. The device achieves a retention time of about 1 s at 85C and over 10 s at 27C all the while maintaining high density and scalability. On the basis of TCAD simulation we have demonstrated high tolerance to disturbance (more than 1000 times with all types of signals), which has been an issue with DRAM memories. KFBM can incorporate both dynamic RAM and flash features.

Fabrication of Quasi-Vertical GaN-On-SiC Trench MOSFETs

We demonstrate quasi-vertical GaN MOSFETs fabricated on SiC substrates. The GaN epitaxial layers were grown via MOCVD on 100 mm 4H-SiC wafers, with the device structure consisting of a 2.5 μm drift layer and a Mg doped p-GaN body. The fabricated transistors exhibit normally-off characteristics, with low off-state leakage behavior and an on/off ratio of over . The specific on-resistance was measured to be which compares favorably to devices fabricated on other foreign substrates. Our results demonstrate an alternative substrate for realizing vertical GaN devices, which potentially offers better material quality and thermal properties compared with other foreign substrate choices.

(Invited) Application of Synchrotron X-Ray Topography Techniques to the Analysis of Defect Microstructures in Bulk GaN Substrates and Epilayers for Power Devices

The study of defects and strain in GaN substrates and epilayers for vertical device development using synchrotron X-ray topography techniques is presented. Synchrotron monochromatic beam X-ray topography (SMBXT) studies show that ammonothermal-grown GaN substrate wafers have the lowest dislocation densities while patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities and HVPE GaN substrates contain high dislocation densities. Epitaxial growth does not nucleate new defects but interactions of dislocations with point defects can cause changes in dislocation configurations. Rocking curve Analysis by Dynamical Simulation (RADS) is shown to be effective in analysis of implantation by correlating the depth profile of strain with the doping profile. Removal of lattice damage induced by implantation requires annealing with proper capping or under high pressures to limit loss of material and lattice distortion. Inductively coupled plasma (ICP) etching may introduce strain in GaN material that can be partially recovered by TBCl etching. Diffusion doping does not introduce new strains in GaN material while neutron transmutation doping at low levels can lead to curing of defects while higher levels lead to irradiation damage and nucleation of new defects.

Atomic resolution interface structure and vertical current injection in highly uniform MoS2 heterojunctions with bulk GaN

The integration of two-dimensional MoS2 with GaN recently attracted significant interest for future electronic/optoelectronic applications. However, the reported studies have been mainly carried out using heteroepitaxial GaN templates on sapphire substrates, whereas the growth of MoS2 on low-dislocation-density bulk GaN can be strategic for the realization of “truly” vertical devices. In this paper, we report the growth of ultrathin MoS2 films, mostly composed by single-layers (1L), onto homoepitaxial n−-GaN on n+ bulk substrates by sulfurization of a pre-deposited MoOx film. Highly uniform and conformal coverage of the GaN surface was demonstrated by atomic force microscopy, while very low tensile strain (∼0.05%) and a significant p+-type doping (∼4.5 × 1012 cm−2) of 1L-MoS2 was evaluated by Raman mapping. Atomic resolution structural and compositional analyses by aberration-corrected electron microscopy revealed a nearly-ideal van der Waals interface between MoS2 and the Ga-terminated GaN crystal, where only the topmost Ga atoms are affected by oxidation. Furthermore, the relevant lattice parameters of the MoS2/GaN heterojunction, such as the van der Waals gap, were measured with high precision. Finally, the vertical current injection across this 2D/3D heterojunction has been investigated by nanoscale current-voltage analyses performed by conductive atomic force microscopy, showing a rectifying behavior with an average turn-on voltage Von = 1.7 V under forward bias, consistent with the expected band alignment at the interface between p+ doped 1L-MoS2 and n-GaN.

Highly Sensitive X‐Ray Detector Made of Large Lead‐Free Perovskite Cs3Bi2I9 Single Crystals with Anisotropic Response

AbstractDirect X‐ray detectors require high‐quality and large‐size materials to reduce radiation hazards. As an all‐inorganic lead‐free perovskite, Cs3Bi2I9 has good prospects for research and applications in the field of X‐ray detection. A refined evaporation method is proposed for the growth of Cs3Bi2I9 single crystals (SCs), and a high‐quality SC with a size of 18 × 20.5 × 3 mm3 is obtained. Due to the anisotropy of Cs3Bi2I9 SCs, two different device structures, vertical and coplanar, are designed. The coplanar device has a higher sensitivity of 59 464.4 µC Gyair−1 cm−2, while the vertical device performs better on the limit of detection (LoD), with an LodD as low as 1.27 nGyair s−1.

Eight Traffic Calming “Easy Pieces” to Shape the Everyday Pedestrian Realm

The need for safe pedestrian movement implies subtracting and modifying space dedicated to vehicles, especially in urban areas. Traffic control measures aim to reduce or modify the width of the carriageway and force the correct use of the space by pedestrians through two approaches: the former is hard and includes physical barriers and the latter is soft and induces psychological fashion effects on the drivers. This paper presents vertical and horizontal devices integrated by landscaping, planting, or other similar works to slow motor vehicle speed, narrow traffic lanes, and/or create smaller distances for pedestrian crossings. Mobility and boundary issues are considered to discuss their warrants and potential impacts. Indeed, the effects of speed or volume treatments should be investigated through a comprehensive multicriteria analysis without overlooking pedestrian level of service, access and connectivity to residents and emergency vehicles, drainage and snow issues, loss of on-street parking lots, and environmental goals in terms of noise and emissions to air reduction.

High-performance ε-Ga2O3 solar-blind ultraviolet photodetectors on Si (100) substrate with molybdenum buffer layer

ε-Ga2O3 (002) thin films were successfully grown on Si (100) substrate by metalorganic chemical vapor deposition (MOCVD). The crystal structure, chemical composition, and surface morphology of ε-Ga2O3 (002) thin films were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The lattice mismatch between Si and ε-Ga2O3 was effectively reduced by inserting Molybdenum (Mo) buffer layer. Quasi-vertical ε-Ga2O3 Schottky barrier diode (SBD) solar-blind photodetectors grown on Si (100) substrate with the Mo buffer layer were fabricated and characterized. The device has a photo-to-dark current ratio (PDCR) of 1.57 × 103, a responsivity (R) of 243.14 A·W-1, a detectivity (D*) of 1.83 × 1017 Jones (cm·Hz1/2 W−1), and an excellent response time due to improved crystal quality and the vertical device structure. This work provides an important reference for the heteroepitaxial growth of ε-Ga2O3 on Si, which is beneficial to the development of low-cost, high-performance ε-Ga2O3 optoelectronic and power electronic devices.

Gate‐Tunable Photovoltaic Efficiency in Graphene‐Sandwiched PdSe2 Photodetectors with Restrained Carrier Recombination

AbstractThe exploration of various two‐dimensional (2D) materials provides a promising platform for constructing van der Waals heterojunction (vdWH) photovoltaic devices. The photovoltaic efficiency of such devices is still a challenge due to the recombination of photogenerated carriers resulting from the intrinsic doping property of materials. Here, PdSe2‐based graphene‐sandwiched vertical devices with high photovoltaic efficiency are constructed. As the graphene‐sandwiched structure limits the diffusion length to tens of nanometers, the carrier recombination during diffusion is spontaneously reduced. Moreover, due to the low‐resistance contact of graphene/PdSe2, the Fermi level and carrier density of PdSe2 can still be significantly tuned by gate voltage, which leads to a sharp enhancement of built‐in field and photovoltaic effect. As a result, the Gr/PdSe2/MoS2/Gr vdWH photovoltaic device shows a high power conversion efficiency of 3.9% and an open circuit voltage of 0.6 V under 650 nm laser illumination at room temperature. The present work provides an efficient scheme for the design of high‐efficiency 2D material photovoltaic devices.

Silver nanowire electrodes for transparent light emitting devices based on WS2 monolayers

Transition metal dichalcogenide (TMDC) monolayers with their direct band gap in the visible to near-infrared spectral range have emerged over the past years as highly promising semiconducting materials for optoelectronic applications. Progress in scalable fabrication methods for TMDCs like metal-organic chemical vapor deposition (MOCVD) and the ambition to exploit specific material properties, such as mechanical flexibility or high transparency, highlight the importance of suitable device concepts and processing techniques. In this work, we make use of the high transparency of TMDC monolayers to fabricate transparent light-emitting devices (LEDs). MOCVD-grown WS2 is embedded as the active material in a scalable vertical device architecture and combined with a silver nanowire (AgNW) network as a transparent top electrode. The AgNW network was deposited onto the device by a spin-coating process, providing contacts with a sheet resistance below 10 Ω sq−1 and a transmittance of nearly 80%. As an electron transport layer we employed a continuous 40 nm thick zinc oxide (ZnO) layer, which was grown by atmospheric pressure spatial atomic layer deposition (AP-SALD), a precise tool for scalable deposition of oxides with defined thickness. With this, LEDs with an average transmittance over 60% in the visible spectral range, emissive areas of several mm2 and a turn-on voltage of around 3 V are obtained.

Microfluidic‐Assisted Growth of Perovskite Single Crystals for Photodetectors

AbstractThe organometal halide perovskites (OMHP) are promising candidates for fast, sensitive, and large area photodetectors. In the last decade, several techniques have been developed with complementary advantages. Film devices are thin and can be scaled to large area, but have a large number of grain‐boundaries related defects. Single bulk crystals instead have higher purity, but are thicker and can not be easily produced on large areas. In this work, a microfluidics‐assisted technique to realize a controlled growth of OMHP single crystals, in the form of microwires, directly on a conductive patterned substrates, is presented. This technique enables the realization of vertical devices with a pixelated sensor layer. The resulting devices present gain, a responsivity up to 200 AW−1and a fast rise time down to 35 µs. This is the first demonstration of a OMHP vertical device realized on a patterned substrate using microfluidics‐assisted techniques.

High Current Density Trench CAVET on Bulk GaN Substrates with Low-Temperature GaN Suppressing Mg Diffusion

We report that, for the first time, a low-temperature GaN (LT-GaN) layer prepared by metal–organic chemical vapor deposition (MOCVD) regrowth was used as a Mg stopping layer (MSL) for a GaN trench current–aperture vertical electron transistor (CAVET) with p-GaN as a carrier blocking layer (CBL). Inserting LT-GaN on top of the p-GaN effectively suppresses Mg out-diffusion into the regrown AlGaN/GaN channel, contributing to the high current capability of GaN vertical devices with a p-GaN CBL. With different MOCVD growth conditions, MSLs inserted in trench CAVETs were comprehensively investigated for the influence of MSL regrowth temperature and thickness on device performance. With the best on-state current performance obtained in this study, the trench CAVET with a 100 nm thick MSL regrown at 750 °C shows a high drain current of 3.2 kA/cm2 and a low on-state resistance of 1.2 mΩ∙cm2. The secondary ion mass spectrometry (SIMS) depth profiles show that the trench CAVET with the 100 nm thick MSL regrown at 750 °C has a dramatically decreased Mg diffusion decay rate (~39 nm/decade) in AlGaN/GaN channel, compared to that of the CAVET without a MSL (~104 nm/decade). In developing GaN vertical devices embedded with a Mg-doped p-type layer, the LT-GaN as the MSL demonstrates a promising approach to effectively isolate Mg from the subsequently grown layers.

Investigation of carbon incorporation in laser-assisted MOCVD of GaN

Background carbon (C) impurity incorporation in metalorganic chemical vapor deposition (MOCVD) grown gallium nitride (GaN) represents one of the major issues in further improving GaN vertical power device performance. This work presents a laser-assisted MOCVD (LA-MOCVD) technique to address the high-C issue in MOCVD homoepitaxial GaN under different growth rate (Rg) regimes and studies the correlations between [C] and Rg. [C] in LA-MOCVD GaN is reduced by 50%–90% as compared to the conventional MOCVD GaN for a wide growth rate range between 1 and 16 μm/h. A mass-transport based model is developed to understand the C incorporation at different Rg regimes. The results obtained from the developed model are in good agreement with experimental data. The model further reveals that LA-MOCVD effectively suppresses C incorporation by reducing the active C species in the gas phase. Moreover, high step velocity in step flow growth mode can facilitate C incorporation at fast Rg, exhibiting steeper C increase. The theoretical model indicates that [C] can be suppressed below 1016 cm−3 with a fast growth rate (Rg) of 10 μm/h by utilizing higher power LA-MOCVD and freestanding GaN substrates with larger off-cut angles.

Design and Experimentation of a Longitudinal Axial Flow Sunflower Oil Threshing Device

To address the problems of threshing loss and high impurity rate during sunflower oil harvesting, a vertical axial flow sunflower oil threshing device was designed. To reduce severe breakage of the sunflower plate and high entrainment loss rate when threshing by the traditional grating gravure sieve, a circular tube-type gravure screen was designed, and a contact model describing the grain rod, sunflower pan, and gravure screen was analyzed. The results show that reducing the diameter of the gravure screen round tubes can effectively reduce breakage of the sunflower pan. The range of the threshing gap, drum speed, and feed amount were determined by a single-factor test. Design-Expert software was used to design a response surface experiment: threshing gap, drum rotation speed, and feed amount were used as test factors, and the threshing loss rate of grains and the grain mass ratio of undersize grains were used as evaluation indicators. A regression model between test factors and evaluation indexes was established by variance analysis of the test results. A software-based numerical optimization function was used to reduce the loss rate of grains and increase the grain mass ratio of undersize grains. The optimal parameters of the threshing device were obtained by multi-objective optimization of all factors: the separation gap was 24.90 mm, drum speed was 244.14 r/min, feed amount was 2.95 kg/s, the threshing loss rate grains was 2.35%, and the grain mass ratio of undersized grains was 81.34%. This study can provide a reference for the design of a combined sunflower oil harvester threshing device.

Microscopic Quantum Transport Processes of Out‐of‐Plane Charge Flow in 2D Semiconductors Analyzed by a Fowler–Nordheim Tunneling Probe

AbstractWeak interlayer couplings at 2D van der Waals (vdW) interfaces fundamentally distinguish out‐of‐plane charge flow, the information carrier in vdW‐assembled vertical electronic and optical devices, from the in‐plane band transport processes. Here, the out‐of‐plane charge transport behavior in 2D vdW semiconducting transition metal dichalcogenides (SCTMD) is reported. The measurements demonstrate that, in the high electric field regime, especially at low temperatures, either electron or hole carrier Fowler–Nordheim (FN) tunneling becomes the dominant quantum transport process in ultrathin SCTMDs, down to monolayers. For few‐layer SCTMDs, sequential layer‐by‐layer FN tunneling is observed to dominate the charge flow, thus serving as a material characterization probe for addressing the Fermi level positions and the layer numbers of the SCTMD films. Furthermore, it is shown that the physical confinement of the electron or hole carrier wave packets inside the sub‐nm thick semiconducting layers reduces the vertical quantum tunneling probability, leading to an enhanced effective mass of tunneling carriers.

Dose-sparing effect of Sabin-derived inactivated polio vaccine produced in Japan by intradermal injection device for rats

The live-attenuated oral polio vaccine has long been used as the standard for polio prevention, but in order to minimize the emergence of pathogenic revertants, the inactivated polio vaccine (IPV), which is administered intramuscularly or subcutaneously, is being increasingly demanded worldwide. However, there is a global shortage of IPV, and its cost is an obstacle in developing countries. Therefore, dose-sparing with intradermal administration of IPV has been investigated. In this study, rats were immunized by intradermal (ID) and intramuscular (IM) administration of Sabin-derived inactivated polio vaccine (sIPV) produced in Japan, and the immune responses were evaluated. The results showed that one-fifth (1/5)-dose of ID administration yielded neutralizing antibody titers comparable to the full-dose IM administration, whereas 1/5-dose of IM administration was less effective than the full dose. Furthermore, a vertical puncture-type ID injection device (Immucise) that was originally developed for humans was modified for rats, resulting in successful and stable ID administration into the thin skin of rats. Based on these results, the ID administration of sIPV using Immucise in clinical use is expected to offer benefits such as reduced amounts of vaccine per dose, cost-effectiveness, and thereby the feasibility of vaccination for more people.

Surface Defect Suppression for High Color Purity Light‐Emitting Diode of Free‐Standing Single‐Crystal Perovskite Film

AbstractHigh color purity is one of the important features for single‐crystal metal halide perovskite light‐emitting diode (LED). Despite single‐crystal perovskite showing low bulk defect concentration, single‐crystal perovskite LEDs do not exhibit high color purity advantage due to the absence of effective surface defect passivation. Herein, one fully wrapped structure is proposed to passivate the surface of the free‐standing CsPbBr3 single‐crystal films. The surface of CsPbBr3 single‐crystal films is wrapped by ultra‐thin polymethyl methacrylate, precisely controlling the thickness. The single‐crystal perovskite film device can achieve high color purity with a full width at half maximum (FWHM) of 15.8 nm) and a large luminescent area of 2.25 mm2. It is observed that surface passivation is due to interaction of CO bond in polymer chains with the Lewis acid PbBr2. The passivated perovskite single‐crystal films significantly improve carrier lifetime and suppress surface defects. It is noteworthy that the passivated free‐standing single‐crystal perovskite films are feasible to build up a vertical LED device structure, avoiding the edge glowing and short‐circuiting of the LED device. This study demonstrates the large luminescent area of the high‐quality millimetre‐scale free‐standing single‐crystal films for wide color gamut display and vertical optoelectronic devices.