Applied Bias Voltage Research Articles

The Impact of Trap-Assisted Tunneling and Poole–Frenkel Emission on Synaptic Potentiation in an α-Fe2O3/p-Si Memristive Device

A signature of synaptic potentiation conductance has been observed in an α-Fe2O3/p-Si device fabricated using spin coating. The conductance of the device in dark conditions and illumination with a white light source was characterized as a function of the application of a periodic bias (voltage) with a triangular profile. The conductance of the device increases with the number of voltage cycles applied and plateaus to its maximum value of 0.70 μS under dark conditions and 12.00 μS under illumination, and this mimics the analog synaptic weight change with the action potential of a neuron. In the range of applied voltage from 0 V to 0.7 V, the conduction mechanism corresponds to trap-assisted tunneling (TAT) and in the range of 0.7–5 V it corresponds to the Poole–Frenkel emission (PFE). The conductance as a function of electrical pulses was fitted with a Hill function, which is a measure of cooperation in biological systems. In this case, it allows one to determine the turn-on threshold (K) of the device in terms of the number of voltage pulses, which are found to be 3 and 166 under dark and illumination conditions, respectively. The gradual conductance change and activation after a certain number of pulses perfectly mimics the synaptic potentiation of neurons. In addition, the threshold parameter extracted from the Hill equation fit, acting as the number of pulses for synaptic activation, is found to have programmability with the intensity of the light illumination.

Scaling with MgO thickness of the proximity exchange field in a Fe/MgO/Si contact

The scaling of the proximity exchange field, which ferromagnetic Fe exerts on the spin accumulation at the MgO/Si interface of Fe/MgO/Si tunnel contacts, was investigated using the inverted Hanle effect. Data as a function of the applied bias voltage shows that the exchange field varies with bias in the same way as the square of the tunnel spin polarization (TSP). The decay of the exchange field with increasing MgO thickness ${t}_{\mathrm{MgO}}$ is rather weak and nonexponential, which we argue to be related to the improved epitaxial quality of the MgO, and the, consequently, improved spin-filtering properties at larger MgO thickness. Using the TSP data to correct for this yields an exponential decay of the exchange field with ${t}_{\mathrm{MgO}}$ with a decay length of $\ensuremath{\lambda}=\ensuremath{-}0.42$ nm. Comparing this to the decay length for the tunnel conductance ($\ensuremath{\lambda}=\ensuremath{-}0.21$ nm) suggests that the exchange interaction is dominated by two-step tunneling processes.

Kinetic simulations of ignited mode cesium vapor thermionic converters

Cesium vapor thermionic converters are an attractive method of converting high-temperature heat directly to electricity, but theoretical descriptions of the systems have been difficult due to the multi-step ionization of Cs through inelastic electron–neutral collisions. This work presents particle-in-cell simulations of these converters, using a direct simulation Monte Carlo collision model to track 52 excited states of Cs. These simulations show the dominant role of multi-step ionization, which also varies significantly based on both the applied voltage bias and pressure. The electron energy distribution functions are shown to be highly non-Maxwellian in the cases analyzed here. A comparison with previous approaches is presented, and large differences are found in ionization rates due especially to the fact that previous approaches have assumed Maxwellian electron distributions. Finally, an open question regarding the nature of the plasma sheaths in the obstructed regime is discussed. The one-dimensional simulations did not produce stable obstructed regime operation and thereby do not support the double-sheath hypothesis.

A 16-channel programmable SiPM bias supply system for Cherenkov telescope imaging camera

In recent years, the Silicon Photomultipliers (SiPMs) have emerged as a promising photodetector in various applications such as high energy physics, nuclear physics and medical instruments. One such application is in the pixelated camera of Imaging Atmospheric Cherenkov Telescopes (IACTs). IACTs are used to detect very high-energy celestial gamma rays using Atmospheric Cherenkov Technique. The SiPM gain is proportional to the overvoltage which is the difference between applied bias voltage and breakdown voltage. As the SiPM breakdown voltage increases with temperature, the overvoltage and hence the gain decreases proportionally (2-3%/°C) at a constant applied bias voltage. Also, the actual bias voltage across the SiPM changes with load current due to voltage drop across a series resistor in SiPM bias circuit, thus causing a change in overvoltage and gain. To maintain a constant gain of the SiPM, the applied bias voltage need to be adjusted to compensate for the changes in temperature and load. We are developing a 256-pixel imaging camera with SiPM as its photo-sensor. The camera will be placed at the focal point of telescope and will be operated during night in outdoor environment at Hanle, Ladakh, India. The night temperature at Hanle typically varies by ∼ 10°C overnight and ∼ 40°C (-20°C to +20°C) over the year. Thus, gain of SiPMs, exposed to the environment, may vary considerably during observations. A prototype 8-channel bias supply board with real time temperature & load compensation is developed to operate the camera SiPMs at fixed gain throughout the observation night.The voltage range of the bias supply for each channel is from 10 V to 80 V with 5 mV resolution and current upto 4 mA. The output voltage and current can be monitored with a resolution of ∼ 5 mV and ∼ 0.3 μA respectively. The single board computer, Raspberry Pi, is connected to the bias supply board over a 7-wire Customized Serial Peripheral Interface (CSPI) for control and monitoring. A multi-channel SiPM bias supply system is realized by daisy-chaining 8-channel boards through CSPI to cater bias voltage to all the pixels of the camera. The system is operated remotely using Ethernet interface of Raspberry-Pi. This paper discusses the design, development and performance of the prototype 16-channel closed-loop, programmable bias supply system with built-in compensation for changes in temperature and load.

Terahertz refractive index sensor based on tunable patterned graphene absorber

We introduce an ultrasensitive refractive-index sensor based on a patterned graphene dual-band absorber operating in the terahertz band. COMSOL Multiphysics software was employed to model and quantitatively assess the suggested micro-nano structure. The structure exhibits two perfect absorption peaks in the 1–9 THz band with theoretical absorptions of 99.4 % and 99.4 %. According to the properties of graphene, the positions of the absorption peaks can be adjusted using the applied bias voltage. Owing to the high symmetry of the proposed structure, it is insensitive to polarization and incident angle variations. Moreover, this study explored the effect of different parameters on the absorption peak by varying the structural parameters. Based on the simulation results, the refractive index sensor has a maximum sensitivity of 3 THz/RIU with a figure of merit (FOM) and can detect small refractive index changes. Therefore, we believe that this structure has promising applications in marine refractive index change determination. • The structure exhibits two perfect absorption peaks in the 1–9 THz band. • The positions of the absorption peaks can be adjusted using the applied bias voltage. • It is insensitive to changes in polarization and angle of incidence. • It has better sensitivity and FOM than most sensors of the same type.

The effects of the electric-magnetic barrier on the spin transmission in the multibarrier semiconductor heterostructures

In the present paper, we study spin transmission in the multibarrier semiconductor heterostructures based on single particle effective mass approximation. These structures are double-barrier and triple-barrier semiconductor hetero-structures that a metallic ferromagnetic is deposited on them. Using Airy function and magnetic barriers approximated by delta function, we calculate transmission coefficient of tunneling electrons and spin polarization. Our results have shown that the parameters as the height and width of the electrical potential barrier, wave vector parallel to the barrier, applied bias voltage and magnetic field are effective parameters in determination of the transmission coefficient.

Tunable, reversible resistive switching behavior of PVA-zirconia nanocomposite films and validation of the trap-assisted switching mechanism by the selective application of external bias voltages.

Flexible, free-standing polyvinyl alcohol (PVA)-zirconia (mean particle size ∼24 nm) nanocomposite films have been synthesized and their performance as a potential next-generation resistive switching device material has been assessed in this report. The nanocomposite films switch from a high resistive state (HRS) to a low resistive state (LRS) at the SET potential and from LRS to HRS at the RESET potential within the voltage window of 5 V. The origination of trap-assisted SET/RESET potentials has been experimentally validated by analyzing the experimental data and invoking various theoretical models. The impregnation of zirconium dioxide (ZrO2) nanoparticles considerably enhances the interfacial charges facilitated by the formation of dangling bonds. The current (I)-voltage (V) characteristics elucidate how the alteration of free volumetric space in the nanocomposites can modify the SET-RESET potential. This leads to tunable SET/RESET potential, good resistance ratio (∼80), and extensive cycling ability of these PVA-ZrO2 organic flexible nanocomposite films. Herein, we have also investigated the effect of applying external bias voltage (equal to the RESET potential) for possible energy bandgap modification and polymer chain orientation. The impedance spectra differ considerably when the sample is subjected to SET, RESET, and zero voltage bias. The observations have been correlated with the UV-vis absorption spectra and electrical studies. The adopted analysis method and obtained results can open up new avenues for designing and analyzing resistive switching-based random-access memory devices.

Investigations on exciton recombination and annihilation in TmPyPB-ETL OLEDs using magnetic field effects.

Although the effect of the electron transport layer (ETL) material TmPyPb on the electroluminescence performance of organic light-emitting diodes (OLEDs) has been extensively studied, the process of TmPyPb regulating exciton recombination and annihilation within the device is still unclear. Here, we fabricated devices of various TmPyPb thicknesses with and without ETL. Subsequently, we measured the magneto-electroluminescence (MEL) of these devices. Specifically, at the same luminance, the triplet-charge annihilation (TQA) process is more likely to occur as the thickness of TmPyPb increases, resulting in a decrease in the maximum luminance of devices. Due to electron leakage and exciton recombination region moving towards the cathode, leading to a decrease in luminance efficiency at first and then an enhancement with an increase in the thickness of TmPyPb. Furthermore, at room temperature, the application of a large bias voltage suppresses singlet fission (SF) processes by modulating the dissociation of singlet polaron pairs (PPS) and the concentration of triplet exciton (T1). This leads to the conversion of SF to the TQA process. At low temperatures, the bias voltage and temperature can regulate the concentration and lifetime of PPS and T1. Therefore, as the temperature decreases, the transition of SF → TQA → triplet-triplet annihilation (TTA) and TQA coexistence → TTA process occurs. Moreover, MEL responses of the TmPyPb-ETL device show a W-linear pattern owing to the combined effect of the hyperfine interaction (HFI) and Zeeman splitting at 145 K. Accordingly, we explored the electroluminescence (EL) performance of TmPyPB-ETL OLEDs and investigated the evolution of SF, TQA, and TTA processes using MEL. Our study revealed the effect of exciton recombination and annihilation in OLEDs with varying thicknesses of TmPyPb.

Interfacial electric field of BiVO4/WO3 photoanode-induced S-scheme charge transfer for enhanced photoelectrochemical performance

A BiVO4/WO3 heterojunction photoanode with an internal electric field can induce a step-scheme charge transfer mechanism, which promotes the separation of photogenerated charge and reduces the applied bias voltage.

Design and analysis of a 2D grapheneplus (G+)-based gas sensor for the detection of multiple organic gases.

A new member of the 2D carbon family, grapheneplus (G+), has demonstrated excellent properties, such as Dirac cones and high surface area. In this study, the electronic transport properties of G+, NG+, and BG+ monolayers in which the NG+/BG+ can be obtained by replacing the center sp3 hybrid carbon atoms of the G+ with N/B atoms, were studied and compared using density functional theory and the non-equilibrium Green's function method. The results revealed that G+ is a semi-metal with two Dirac cones, which becomes metallic upon doping with N or B atoms. Based on the electronic structures, the conductivities of the 2D G+, NG+ and BG+-based nanodevices were analyzed deeply. It was found that the currents of all the designed devices increased with increasing the applied bias voltage, showing obvious quasi-linear current-voltage characteristics. IG+ was significantly higher than ING+ and IBG+ at the same bias voltage, and IG+ was almost twice IBG+, indicating that the electron mobility of G+ can be controlled by B/N doping. Additionally, the gas sensitivities of G+, NG+, and BG+-based gas sensors in detecting C2H4, CH2O, CH4O, and CH4 organic gases were studied. All the considered sensors can chemically adsorb C2H4 and CH2O, but there were only weak van der Waals interactions with CH4O and CH4. For chemical adsorption, the gas sensitivities of these sensors were considerably high and steady, and the sensitivity of NG+ to adsorb C2H4 and CH2O was greater as compared to G+ and BG+ at higher bias voltages. Interestingly, the maximum sensitivity difference for BG+ toward C2H4 and CH2O was 17%, which is better as compared to G+ and NG+. The high sensitivity and different response signals of these sensors were analyzed by transmission spectra and scattering state separation at the Fermi level. Gas sensors based on G+ monolayers can effectively detect organic gases such as C2H4 and CH2O, triggering their broad potential application prospects in the field of gas sensing.

Scanning probe lithography on Ge(111)-c(2×8) surface

The paper describes nanometer scale lithography on atomically clean Ge(111)-c(2×8) surface performed in the ultra-high vacuum scanning tunneling microscope operating at 300 K. Using a standard Pt80Ir20 probe tip and applying bias voltages between 0.5 and 3 V, the Ge surface could be reliably imaged with atomic resolution without any modification of the sample. However, surface modification in highly localized area under the probe tip was observed at the bias voltages from 4 to 5 V. Such modification could occur in the form of the deposition of the tip material onto the scanned area of the sample, extraction of the sample material or generation of defects in the sample crystalline structure. Possible physical mechanisms of the processes outlined above as well as the strategies to achieve reliable scanning probe nanolithography are discussed.

Effects of High-Temperature Growth of Dislocation Filter Layers in GaAs-on-Si

GaAs-on-Si templates with two different dislocation filter layers (DFLs) were grown at 550 °C low-temperature (LT)-DFL and 660 °C high-temperature (HT)-DFL using metal organic vapor-phase epitaxy and the effects of the growth temperature were studied. The threading dislocation density (TDD) values of LT-DFL and HT-DFL were 5.2 × 107 cm−2 and 1.5 × 107 cm−2, respectively. The 1.5 × 107 cm−2 of TDD in HT-DFL is reduced by almost one order compared to the 1.2 × 108 cm−2 of that in the control sample without DFLs. The annihilation process was mainly observed in the HT-DFL by a transmission electron microscope, resulting in a lower TDD. The 500-nm-thick GaAs bulk layer and InAs QDs were regrown on GaAs-on-Si templates and the optical properties were also evaluated by photoluminescence (PL). The highest PL peak intensity of the HT-DFL indicates that less non-radiative recombination in both the GaAs bulk and QDs occurred due to the reduced TDD. The GaAs p–i–n diodes were also fabricated to analyze the bulk leakage (JB) and the surface leakage current. The JB of HT-DFL shows the lowest value of 3.625 × 10–7 A/cm−2 at applied bias voltage of 1 V, which is 20 times lower than the JB of the control sample without DFLs. This supports that the high-temperature growth of DFL can make a good performance GaAs device on Si.

Highly efficient tunable broadband terahertz polarizers based on a graphene metasurface

Ultrathin tunable broadband terahertz transmission mode linear-to-circular polarizers are proposed and numerically validated. Each structure consists of a dielectric substrate with slotted graphene patches on one or both sides of the substrate. Both polarizers yield tunable broadband conversion with large transmission coefficients in terahertz frequency range. Compared to the previous polarizers, the proposed structure with a single-layer of graphene patches exhibits a 120% larger fractional bandwidth and 60% larger transmittance. The polarizer with two layers of graphene patches features a 138% larger fractional bandwidth and 25% larger transmittance, compared to the previous polarizers, with excellent axial ratio. The tuning frequency range for the proposed single- and double-layer structures is (2.4 to 4.9) and (1.8 to 5.8) THz, respectively. The tuning is achieved by varying the graphene Fermi energy (chemical potential) from 0.2 to 1 eV, by the application of a DC bias voltage. Moreover, the polarization conversion performance is well maintained under oblique incidence condition, over a wide range of incident angles up to 70°. The size of the square-shaped unit cell is λ0/10, much smaller than the free-space wavelength of the incident waves, λ 0 , while the thickness is λ0/22. With the above features, the present work offers a further step in developing controllable polarization converters with potential applications in imaging, sensing, and communications.

Simulation on Secondary Electron Multiplication Behavior of the Microchannel Plate under DC Mode

In this study, a three-dimensional microchannel model of a single hollow-core glass fiber was constructed and the Finite Integral Technique and Monte Carlo method were combined to comprehensively simulate the electron multiplication process in a single channel under DC mode. The electron dynamic trajectory in DC electron emission mode was achieved. The effects of different structural parameters and applied bias voltage on the electron gain and the most probable exit energy at the output end of MCP were investigated. The results show that the electrons with a certain initial current can be continuously and stably multiplied in the channel under DC mode and eventually reach a stable value because of the space charge effect; additionally, the electron gain increases with the increase in the bias angle and DC bias voltage and decreases with the increase in the penetration depth of the MCP output electrode. The electron gain at the output end of the MCP increases with the length-to-diameter ratio under the normalized voltage but shows a maximum value under the constant voltage. The simulation results are consistent with the reported experimental trend and theoretical analyses. The method provides data support for the optimal structural design of the microchannel plate.

Stochastic Switching in a Magnetic-Tunnel-Junction Neuron and a Bias-Dependent Néel-Arrhenius Model

Back hopping or telegraphic switching describes stochastic bistate oscillation in magnetic tunnel junctions (MTJ) at high bias, which significantly increases the write error rate of memory storage. Nevertheless, this unfavorable stochastic switching could be utilized to construct extremely compact spiking neuron circuits, where both the spike frequency and duty cycle are proportional to the applied bias voltage. This MTJ neuron is the fundamental building block of future all-spin neural networks. This paper analyzes the mechanism of stochastic switching in the MTJ neuron in detail. The self-heating effect at high bias is identified to induce thermal perturbation through a reduced energy barrier of a weakened perpendicular magnetic anisotropy. The high spike frequency is measured up to 10 MHz, which is mainly limited by the transition time between states and the multistate switching when approaching the Curie temperature of the ferromagnetic phase. Finally, to quantitatively describe the stochastic switching phenomenon, we establish a bias-dependent N\'eel-Arrhenius compact model, which calibrates well with experimental data. Based on this model, the potential and design guideline for realizing MTJ neurons with a spike frequency toward the gigahertz range are also discussed.

Enhanced charge carrier density of a p-n BiOCl/BiVO4 heterostructure by Ni doping for photoelectrochemical applications

Pure and doped p-n BiOCl/BiVO4 heterostructures with various Ni contents were studied for photoanode applications in photoelectrochemical cells. All materials were synthesized by the coprecipitation method. The properties of the pure and doped materials were characterized by various techniques, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UVvisible spectroscopy, and electrochemical impedance spectroscopy. Furthermore, density function theory calculations were employed to investigate the underlying physical mechanism of the Ni substitution. The materials were applied as the photoanode at an applied bias voltage of +1.9 V (vs. RHE) and irradiated over 4200 s under the solar light simulator. The pure p-n BiOCl/BiVO4 heterostructure showed the highest photocurrent of 1 mA/cm2. After 1% Ni doping, the photocurrent density was enhanced, and the highest photocurrent density was approximately 1.7 mA/cm2. From the theoretical calculation aspect, the role of Ni substitution into the parent p-n BiVO4 and BiOCl materials resulted in a surface state under the conduction band of both parent materials. The result was in agreement with the Mott-Schottky analysis, which showed that the charge carrier density increased as Ni atoms were added to the p-n BiOCl/BiVO4 heterostructure. Therefore, the additional state and the increase in the charge carrier density led to the improved photocatalytic activity of the Ni-doped p-n BiOCl/BiVO4 heterostructure.

Confining Fe2O3 in Silicalite-1 for effective catalytic activity in bias-assisted photo-Fenton system for nitrobenzene degradation

Removal of refractory organic pollutants by photo-Fenton process is of great significance to cleaner production. Low yield and utilization of photogenerated carriers limit the application of photo-Fenton process. Positive bias voltage is introduced into the photo-Fenton system for the first time so that the photogenerated electrons no longer flow to the external circuit but participate in the redox cycle of the photo-Fenton reaction. Under the applied bias voltage of +0.3 V, the efficiency of the system to degrade nitrobenzene reaches 95.4% in 60 min and still has 90.3% of nitrobenzene degradation efficiency after five runs while the photo-Fenton system without bias voltage is 80.0%. The bias voltage makes the band bending so that the band gap narrow, leading to easier generation of photogenerated electrons. Then the separated photogenerated electrons combine with Fe3+, which promotes the rapid redox cycle of Fe3+/Fe2+ in the photo-Fenton reaction. Both photogenerated electrons and holes are efficiently utilized in this work, which provides a new method for chemical treatment of refractory organic pollutants.

Reconfigurable transmissive metasurface synergizing dynamic and geometric phase for versatile polarization and wavefront manipulations

Recently, reconfigurable metasurfaces have facilitated dynamic manipulation of electromagnetic (EM) waves, opening new opportunities for fascinating meta-devices. So far, most devices are working in reflection region with limited polarization states. Here, we demonstrate a reconfigurable multifunctional transmissive metasurface for both linearly and circularly polarized waves manipulations. Specifically, the designed meta-atom contains dual-layer metallic cross-strips and one-layer circular slotted patch. Eight positive-intrinsic-negative diodes are loaded between each strip structure through via hole, with each two of diodes as a group along x- and y-direction, respectively, and thus supporting four switchable states via different applied bias voltages. Numerical results indicate that the designed meta-atom achieves high-rate transmission of 0.9 and individual phase manipulation in four independent transmission channels at 7.4 GHz. As a proof-of-concept demonstration, we numerically and experimentally show different versatile functionalities, including anomalous deflections with steered beams in range of 0° ∼ 40°, orbital angular momentum generation with mode of l = 1, and beam focusing among four polarization-encoded channels at 7.4 GHz. Our metasurface owns the advantages of multi-polarization EM wave manipulations in transmission mode, which completely distinguishes from previously reported works with single polarization in reflection mode.

Fast and sensitive terahertz detection with a current-driven epitaxial-graphene asymmetric dual-grating-gate field-effect transistor structure

We designed and fabricated an epitaxial-graphene-channel field-effect transistor (EG-FET) featuring an asymmetric dual-grating-gate (ADGG) structure working as a current-driven terahertz detector and experimentally demonstrated a 10 ps-order fast response time and a high responsivity of 0.3 mA/W to 0.95 Terahertz (THz) radiation incidence at room temperature. The ADGG and drain–source bias dependencies of the measured photoresponse showed a clear transition between plasmonic detection under periodic electron density modulation conditions with depleted regions and photothermoelectric (PTE) detection under entirely highly doped conditions without depleted regions. We identified the PTE detection that we observed as a new type of unipolar mechanism in which only electrons or holes contribute to rectifying THz radiation under current-driven conditions. These two detection mechanisms coexisted over a certain wide transcendent range of the applied bias voltages. The temporal photoresponses of the plasmonic and PTE detections were clearly shown to be comparably fast on the order of 10 ps, whereas the maximal photoresponsivity of the PTE detection was almost twice as high as that of the plasmonic detection under applied bias conditions. These results suggest that the ADGG-EG-FET THz detector will be promising for use in 6G- and 7G-class high-speed wireless communication systems.

Remotely tuned multistate resistive switching in MoS2/NiMnIn thin film heterostructure for highly flexible ReRAM application

In this article, the Cu/MoS2/NiMnIn memory structure was fabricated over stainless steel substrate for flexible electronics applications. The bipolar resistive switching characteristics were observed in two different fabricated devices (i) without sulfur vacancies (D1) and (ii) with sulfur vacancies (D2) in MoS2 thin film. Three different resistance states such as the high resistance state (HRS), intermediate resistance state (IRS) and low resistance state (LRS) have been detected for the D2 memory device. It could be ascribed to the two filamentary models based on the movement of sulfur vacancies and the formation of Cu metallic filament with applied bias voltage. Moreover, the Ohmic and modified space charge limited conduction mechanisms clearly explain the current conduction in different resistance states of device D2. The fabricated MoS2 thin film-based memory structure exhibits stable resistive switching behavior with a high OFF/ON ratio of ∼3.6 × 103, good consistency of ∼3600 endurance cycles and excellent data retention capability up to 3000 s. Moreover, the remote tuning of device D2 was thoroughly investigated and an appreciable change in SET voltage was detected with external temperature and magnetic field. Additionally, the external magnetic field altered the switching states and enhanced the multi-bit data storage capability of the memory device. This can be ascribed to the effect of the Lorentz force on ionic movement in the presence of the external magnetic field. The mechanical flexibility of the memory structure was tested for 1000 bending cycles at various bending angles in both the tensile and compressive bending modes. Hence, the present study opens up new ways for the futuristic flexible device for high data storage and neuromorphic computing applications.