Showing papers on "Biasing published in 2018"

High-Performance Silicon-Compatible Large-Area UV-to-Visible Broadband Photodetector Based on Integrated Lattice-Matched Type II Se/n-Si Heterojunctions.

Abstract: A gold-induced NH4Cl-assisted vapor-based route is proposed and developed to achieve vertically aligned submicron Se crystals on lattice-matched (111)-oriented silicon substrates, based on which a high-performance large-area silicon-compatible photodetector is constructed. Thanks to the energy band structure and the strongly asymmetrical depletion region, the fabricated Se/Si device maintains a similar wavelength cutoff to that of selenium devices before the IR region, along with a high-performance broadband photoresponse in the UV-to-visible region. The large-area photodetector maintains a very low leakage current under a −2 V bias, and a high on/off ratio of 103–104 is obtained with a high photocurrent of 62 nA at 500 nm. A photoresponse is clearly observed when the bias voltage is removed. The pulse response precisely provides a high response speed (τrise + τfall ≈ 1.975 ms), exceeding the fastest Se-based photodetectors in current reports. The enhanced photoelectric properties and the self-power photo...

Strong Depletion in Hybrid Perovskite p-n Junctions Induced by Local Electronic Doping.

Abstract: A semiconductor p-n junction typically has a doping-induced carrier depletion region, where the doping level positively correlates with the built-in potential and negatively correlates with the depletion layer width. In conventional bulk and atomically thin junctions, this correlation challenges the synergy of the internal field and its spatial extent in carrier generation/transport. Organic-inorganic hybrid perovskites, a class of crystalline ionic semiconductors, are promising alternatives because of their direct badgap, long diffusion length, and large dielectric constant. Here, strong depletion in a lateral p-n junction induced by local electronic doping at the surface of individual CH3 NH3 PbI3 perovskite nanosheets is reported. Unlike conventional surface doping with a weak van der Waals adsorption, covalent bonding and hydrogen bonding between a MoO3 dopant and the perovskite are theoretically predicted and experimentally verified. The strong hybridization-induced electronic coupling leads to an enhanced built-in electric field. The large electric permittivity arising from the ionic polarizability further contributes to the formation of an unusually broad depletion region up to 10 µm in the junction. Under visible optical excitation without electrical bias, the lateral diode demonstrates unprecedented photovoltaic conversion with an external quantum efficiency of 3.93% and a photodetection responsivity of 1.42 A W-1 .

A Reconfigurable Broadband Polarization Converter Based on an Active Metasurface

Abstract: We propose an active metasurface whose functionalities can be dynamically switched among linear-to-linear, linear-to-elliptical, and linear-to-circular polarization conversions in a wideband. The active metasurface is composed of butterfly-shaped unit cells embedded with voltage-controlled varactor diodes. By controlling the bias voltage of the varactor diodes, the electromagnetic responses of the proposed metasurface can be tailored, leading to reconfigurable polarization conversions. The simulation results reveal that with no bias voltage, the proposed metasurface is able to reflect linear-polarization waves to cross-polarization waves in the frequency range from 3.9 to 7.9 GHz, with a polarization conversion ratio of over 80%; however, at the bias voltage of −19 V, the metasurface is tuned to be a circular polarization converter in a wideband from 4.9 to 8.2 GHz. Moreover, two equivalent circuits along the $x$ - and $y$ -directions are developed to elucidate the tunable mechanism. The experimental results are in a good agreement with the simulation results obtained from commercial software and from the equivalent circuit model.

Electrically enhanced hot hole driven oxidation catalysis at the interface of a plasmon-exciton hybrid.

Abstract: In this work, an electro-optical device based on a graphene–Ag nanoparticle hybrid is fabricated as the substrate of graphene mediated surface enhanced Raman scattering (G-SERS) manipulated by the gate and bias voltages. Plasmon–exciton coupling promotes co-driven surface catalytic reactions, where the density of states (DOS) of holes and electrons on graphene is well controlled by the gate voltage, and the kinetic energy of holes and electrons is driven by the bias voltage (or current). Our experimental results reveal that the hot holes on graphene mainly contribute to plasmon–exciton co-driven oxidation reactions. The contribution of hot electrons to oxidation reactions is less important. Our novel electro-optical device can be potentially applied in controlling plasmon–exciton co-driven oxidation or reduction reactions by tuning the gate and bias voltages.

Stability of Halide Perovskite Solar Cell Devices: In Situ Observation of Oxygen Diffusion under Biasing.

Abstract: Using in situ electrical biasing transmission electron microscopy, structural and chemical modification to n-i-p-type MAPbI3 solar cells are examined with a TiO2 electron-transporting layer caused by bias in the absence of other stimuli known to affect the physical integrity of MAPbI3 such as moisture, oxygen, light, and thermal stress. Electron energy loss spectroscopy (EELS) measurements reveal that oxygen ions are released from the TiO2 and migrate into the MAPbI3 under a forward bias. The injection of oxygen is accompanied by significant structural transformation; a single-crystalline MAPbI3 grain becomes amorphous with the appearance of PbI2 . Withdrawal of oxygen back to the TiO2 , and some restoration of the crystallinity of the MAPbI3 , is observed after the storage in dark under no bias. A subsequent application of a reverse bias further removes more oxygen ions from the MAPbI3 . Light current-voltage measurements of perovskite solar cells exhibit poorer performance after elongated forward biasing; recovery of the performance, though not complete, is achieved by subsequently applying a negative bias. The results indicate negative impacts on the device performance caused by the oxygen migration to the MAPbI3 under a forward bias. This study identifies a new degradation mechanism intrinsic to n-i-p MAPbI3 devices with TiO2 .

An Improved Multifunctional Active Frequency Selective Surface

Abstract: An improved dual-polarized multifunctional active frequency selective surface (MAFSS) with parallel-feed configuration is proposed in this paper. The designed structure is composed of two metallic layers separated by a thin dielectric layer. Three independent and controllable functions of electromagnetic switching, polarization selection, and frequency tuning are integrated together into the proposed structure via introducing PIN diodes and varactor diodes. To reduce the negative effects of extra feed lines, frequency selective surface (FSS) elements themselves play a significant role in connecting the active components and allowing the direct currents to flow through. A special biasing configuration is designed to provide parallel bias voltages for the two different types of active devices. Finally, an FSS prototype is fabricated and measured, and the measurement results as well as the simulation results both demonstrate great performance of the MAFSS structure. In addition, the proposed structure also performs well under circumstances of oblique incidence and different polarizations.

A honeycomb multilevel structure Bi2O3 with highly efficient catalytic activity driven by bias voltage and oxygen defect

Abstract: In this work, we report a bismuth oxide film electrode with oxygen defects and honeycomb multilevel structure prepared by one-step hydrothermal method. The control of raw materials enables to control the thickness and morphology of BiO electrodes. The photoelectrocatalytic of prepared electrodes were tested in various conditions. Under visible light irradiation (λ ≥ 420 nm) and 3 V bias, the sample BiO-2 has the highest photoelectrocatalytic activity, which is 4.95 times higher than the photocatalytic activity and 9.86 times higher than the electrocatalytic activity. The oxygen defects of bismuth oxide were confirmed by EPR and DFT calculation. The morphology and structure of prepared samples were tested by XRD, scanning electronic microscope (SEM), high-resolution transmission electron microscope (HRTEM) and X-ray photoelectron spectroscopy (XPS). The enhanced photoelectrocatalytic activity is attributed to the proper bias voltage, oxygen defects, and honeycomb multilevel structure. The results of tapping experiments showed that the main active species during the photoelectrocatalytic progress is superoxide radical and the mechanism of the photoelectrocatalysis was proposed.

Outdoor photoluminescence imaging of photovoltaic modules with sunlight excitation

Abstract: To operate photovoltaic power plants at maximum capacity, it is desirable to identify cell or module failures in the field at the earliest possible stage. Currently used field inspection methods cannot detect many of the electronic defects that can be revealed with luminescence-based techniques. In this work, photoluminescence images are acquired using the sun as the sole illumination source by separating the weak luminescence signal from the much stronger ambient sunlight signal. This is done by using an appropriate choice of optical filtering and modulation of the cells' bias between the normal operating point and open circuit condition. The switching is achieved by periodically changing the optical generation rate of at least one cell within the module. This changes the biasing condition of all other cells that are connected to the same bypass diode. This method has the advantage that it can deliver high quality images revealing electrical defects in individual cells and entire modules, without requiring any changes to the electrical connections of the photovoltaic system.

Parametric Variation Analysis of Symmetric Double Gate Charge Plasma JLTFET for Biosensor Application

Abstract: Nanoscale devices have great potential for providing a platform for detecting biomolecules. There are a number of difficulties observed during the fabrication process of these devices, such as random dopant fluctuation, thermal budget, and so on. To cut down these problems, charge-plasma-based concept is introduced. This paper proposes the implementation of charge-plasma-based gate underlap dielectric modulated junctionless tunnel field-effect transistor (CPB DM-JLTFET) for the label free electrical recognition of biomolecules like cell, enzyme, deoxyribonucleic acid, protein, and so on via incorporating the dielectric modulation technique. A gate underlap region is formed in the device via etching oxide material. Label free detection of biomolecules depends upon the electrical property of biomolecules. The effect of device parameters such as cavity length and cavity thickness on surface potential, subthreshold slope, $I_{\mathrm{ON}}/I_{\mathrm{OFF}}$ ratio, and their sensitivity have been discussed. This paper investigates the performance of CPB DM-JLTFET for biomolecule sensing applications while varying dielectric constant, surface density, cavity length, and cavity thickness at different biasing conditions. The device proposed is implemented and simulated by using an ATLAS device simulator.

Bias-switchable negative and positive photoconductivity in 2D FePS3 ultraviolet photodetectors

Abstract: Metal-phosphorus-trichalcogenides (MPTs), represented by NiPS3, FePS3, etc, are newly developed 2D wide-bandgap semiconductors and have been proposed as excellent candidates for ultraviolet (UV) optoelectronics. In spite of having superior advantages for solar-blind UV photodetectors, including those free of surface trap states, being highly compatible with versatile integrations as well as having an appropriate band gap, to date relevant study is rare. In this work, the photoresponse characteristic of UV detectors based on few-layer FePS3 has been comprehensively investigated. The responsivity of the photodetector, which is observed to be determined by bias gate voltage, may achieve as high as 171.6 mAW-1 under the illumination of 254 nm weak light, which is comparable to most commercial UV detectors. Notably, both negative and positive photoconductivities exist in the FePS3 photodetectors and can be controllably switched with bias voltage. The eminent and novel photoresponse property paves the way for the further development and practical use of 2D MPTs in high-performance UV photodetections.

Bright Electroluminescence from Single Graphene Nanoribbon Junctions.

Abstract: Thanks to their highly tunable band gaps, graphene nanoribbons (GNRs) with atomically precise edges are emerging as mechanically and chemically robust candidates for nanoscale light emitting devices of modulable emission color. While their optical properties have been addressed theoretically in depth, only few experimental studies exist, limited to ensemble measurements and without any attempt to integrate them in an electronic-like circuit. Here we report on the electroluminescence of individual GNRs suspended between the tip of a scanning tunneling microscope (STM) and a Au(111) substrate, constituting thus a realistic optoelectronic circuit. Emission spectra of such GNR junctions reveal a bright and narrow band emission of red light, whose energy can be tuned with the bias voltage applied to the junction, but always lying below the gap of infinite GNRs. Comparison with ab initio calculations indicates that the emission involves electronic states localized at the GNR termini. Our results shed light on u...

Dynamically Tunable Substrate-Integrated-Waveguide Attenuator Using Graphene

Abstract: In this paper, a dynamically tunable substrate-integrated-waveguide (SIW) attenuator using graphene is first presented. Two graphene sandwich structures are placed inside an SIW, which serve as conductivity-tunable E-plane septum. By applying biased voltage to the graphene, the attenuation can be dynamically changed; meanwhile, the reflection can still keep at a relatively low level. A closed form of the attenuation is also proposed, and the mechanism of the attenuator is theoretically analyzed in detail. Furthermore, a prototype operating from 7 to 14.5 GHz has been fabricated and measured. Both the theoretical calculation and the numerical simulation results have a good agreement with the experimental results, which present the attenuation ranges from 2 to 15 dB with a stable 7.5-GHz wideband while the bias voltage changing from 0 to 4 V.

Tuning Material Properties of Oxides and Nitrides by Substrate Biasing during Plasma-Enhanced Atomic Layer Deposition on Planar and 3D Substrate Topographies.

Abstract: Oxide and nitride thin-films of Ti, Hf, and Si serve numerous applications owing to the diverse range of their material properties. It is therefore imperative to have proper control over these properties during materials processing. Ion-surface interactions during plasma processing techniques can influence the properties of a growing film. In this work, we investigated the effects of controlling ion characteristics (energy, dose) on the properties of the aforementioned materials during plasma-enhanced atomic layer deposition (PEALD) on planar and 3D substrate topographies. We used a 200 mm remote PEALD system equipped with substrate biasing to control the energy and dose of ions by varying the magnitude and duration of the applied bias, respectively, during plasma exposure. Implementing substrate biasing in these forms enhanced PEALD process capability by providing two additional parameters for tuning a wide range of material properties. Below the regimes of ion-induced degradation, enhancing ion energies...

High-Performance, Ultra-Broadband, Ultraviolet to Terahertz Photodetectors Based on Suspended Carbon Nanotube Films.

Abstract: Ultra-broad spectral detection is critical for several technological applications in imaging, sensing, spectroscopy, and communication. Carbon nanotube (CNT) films are a promising material for ultra-broadband photodetectors because their absorption spectra cover the entire ultraviolet to the terahertz range. However, because of the high binding energy of excitons, photodetectors based on CNT films always require a strong electric field, asymmetric electrical contacts, or hybrid structures with other materials. Here, we report an ultra-broadband bolometric photodetector based on a suspended CNT film. With an abundant distribution of tube diameters and an appropriate morphology (spider web-like), the CNT films display a strong absorption spectrum from the ultraviolet up to the terahertz region. Under illumination, heat generated from the electron-photon interaction dominates the photoresponse of our devices. For small changes in temperature, the photocurrent shows a convincing linear dependence with the absorbed light's power across 3 orders of magnitude. When the channel length is reduced to 100 μm, the device demonstrates a high performance with an ultraviolet responsivity of up to 0.58 A/W with a bias voltage of 0.2 V and a short response time of ∼150 μs in vacuum, which is better than that of many other photodetectors based on CNTs. Moreover, this performance could be further enhanced by optimization.

A 12.8 k Current-Mode Velocity-Saturation ISFET Array for On-Chip Real-Time DNA Detection

Abstract: This paper presents a large-scale CMOS chemical-sensing array operating in current mode for real-time ion imaging and detection of DNA amplification We show that the current-mode operation of ion-sensitive field-effect transistors in velocity saturation devices can be exploited to achieve an almost perfect linearity in their input–output characteristics (pH-current), which are aligned with the continuous scaling trend of transistors in CMOS The array is implemented in a 035- $\mu$ m process and includes 128 k sensors configured in a 2T per pixel topology We characterize the array by taking into account nonideal effects observed with floating gate devices, such as increased pixel mismatch due to trapped charge and attenuation of the input signal due to the passivation capacitance, and show that the selected biasing regime allows for a sufficiently large linear range that ensures a linear pH to current despite the increased mismatch The proposed system achieves a sensitivity of 103 $\mu$ A/pH with a pH resolution of 0101 pH and is suitable for the real-time detection of the NDM carbapenemase gene in E Coli using a loop-mediated isothermal amplification

Electrically tunable terahertz dual-band metamaterial absorber based on a liquid crystal

Abstract: In this paper, a liquid crystal (LC) based tunable metamaterial absorber with dual-band absorption is presented. The proposed absorber is analysed both numerically and experimentally. The analysis shows that the two absorption peaks, originating from the new resonant structure, are experimentally detected at 269.8 GHz and 301.4 GHz when no bias voltage is applied to the LC layer. In order to understand the absorption mechanisms, simulation results for the surface current and power loss distributions are presented. Since liquid crystals are used as the dielectric layer to realize the electrically tunable absorber, a frequency tunability of 2.45% and 3.65% for the two absorption peaks is experimentally demonstrated by changing the bias voltage of the LC layer from 0 V to 12 V. Furthermore, the absorber is polarization independent and a high absorption for a wide range of oblique incidence is achieved. The designed absorber provides a way forward for the realization of tunable metamaterial devices that can be applied in multi-band detection and imaging.

Effect of substrate bias on microstructure and mechanical properties of WC-DLC coatings deposited by HiPIMS

Abstract: The hardness and friction coefficient of the molding tools are the two key factors in influencing their performance during the cutting process. In this regard deposited by physical vapor deposition (PVD), WC-DLC nanocomposite hard coatings featuring high hardness and low friction coefficient are highly preferred to be used as protective coatings. As a newly developed PVD technology, high power impulse magnetron sputtering (HiPIMS) is quite advantageous in terms of the deposition of hard coatings. Substrate bias voltage exerts significant influences on the discharge characteristic of HiPIMS, plasma energy, chemical composition and the microstructure of the deposited coatings, which subsequently affect the coating's mechanical properties and performance in production. This article aims at investigating the effects of substrate bias on the plasma discharge characteristic of HiPIMS and on the mechanical properties, surface morphology, deposition rate, cross-sectional morphology, element concentration, crystal phase composition and tribological properties of the deposited coatings. The results show that the peak discharge current rises up from 57 A to 76 A with the increase of substrate bias from −40 V to −200 V. By controlling the bias voltage, WC-DLC coatings with different microstructures, mechanical properties and tribological properties have been produced. Meanwhile, the C concentrations of deposited coatings decline and the composed phase of the coating is transformed from hexagonal α-C at low bias voltage to equiaxial β-WC1−x and then hexagonal β-W2C accompanied by the rising bias voltage. The deposited WC-DLC coatings exhibit a decrease in surface roughness from Ra 16.1 nm to Ra 9.2 nm. Crystal phase evolutions also play a part in addition to the biased voltage upon the grain size and the hardness of the coating. It is found that the minimum grain size of 6 nm and the maximum hardness of 40.1 GPa appear at −160 V bias voltage when the coating is composed mainly of equiaxial β-WC1−x phase and mixture of β-WC1−x and β-W2C. The friction coefficient of the WC-DLC coating measured by ball-on-disk test increased correspondingly with the increase of the bias voltage except that the wear rate reaches the lowest at −120 V bias voltage, indicating that the wear rate is related not only to friction coefficient but also to coating hardness.

Enhanced Corrosion Resistance and Interfacial Conductivity of TiCx/a-C Nanolayered Coatings via Synergy of Substrate Bias Voltage for Bipolar Plates Applications in PEMFCs

Abstract: Proton-exchange membrane fuel cells are one kind of renewable and clean energy conversion device, whose metallic bipolar plates are one of the key components. However, high interfacial contact resistance and poor corrosion resistance are still great challenges for the commercialization of metallic bipolar plates. In this study, we demonstrated a novel strategy for depositing TiCx/amorphous carbon (a-C) nanolayered coatings by synergy of 60 and 300 V bias voltage to enhance corrosion resistance and interfacial conductivity. The synergistic effects of bias voltage on the composition, microstructure, surface roughness, electrochemical corrosion behaviors, and interfacial conductivity of TiCx/a-C coatings were explored. The results revealed that the columnar structures in the inner layer were suppressed and the surface became rougher with the 300 V a-C layer outside. The composition analysis indicated that the sp2 content increased with an increase of 300 V sputtering time. Due to the synergy strategy of bias...

Effects of DC Bias Tuning on Air-Coupled PZT Piezoelectric Micromachined Ultrasonic Transducers

Abstract: In this paper, we report an air-coupled piezoelectric ultrasonic micromachined transducer (PMUT) using a lead-zirconate-titanate (PZT) piezoelectric layer. A dc bias voltage applied to the PZT film controls its polarization and intrinsic stress, tuning the frequencies of two closely-spaced resonance modes of the rectangular shaped PMUT. At an optimal bias voltage of approximately 5 V, the modes nearly overlap at 230 kHz, increasing the bandwidth by a factor of 8 relative to the zero dc biased state. Measurements of the electromechanical coupling coefficient of the PZT film show that it is maximized at 5-6 V, agreeing with device performance experiments. Acoustic transmission and reception were demonstrated using two identical PMUTs by adding the optimum dc bias of 6 V to the 1.4 V peak-to-peak ac voltage which is maximum within the linear displacement regime. The signal detectable range of the transmit and receive measurement with optimum dc bias tuning was 4 cm to 19 cm in air. [2017-0223]

Resonant Transport in Single Diketopyrrolopyrrole Junctions

Abstract: We study the single-molecule transport properties of small bandgap diketopyrrolopyrrole oligomers (DPPn, n = 1–4) with lengths varying from 1 to 5 nm. At a low bias voltage, the conductance decays exponentially as a function of length indicative of nonresonant transport. However, at a high bias voltage, we observe a remarkably high conductance close to 10–2 G0 with currents reaching over 0.1 μA across all four oligomers. These unique transport properties, together with density functional theory-based transport calculations, suggest a mechanism of resonant transport across the highly delocalized DPP backbones in the high bias regime. This study thus demonstrates the unique properties of diketopyrrolopyrrole derivatives in achieving highly efficient long-range charge transport in single-molecule devices.

Directly modulated 1.3 μm quantum dot lasers epitaxially grown on silicon.

Abstract: We report the first demonstration of direct modulation of InAs/GaAs quantum dot (QD) lasers grown on on-axis (001) Si substrate. A low threading dislocation density GaAs buffer layer enables us to grow a high quality 5-layered QD active region on on-axis Si substrate. The active layer has p-modulation doped GaAs barrier layers with a hole concentration of 5 × 1017 cm−3to suppress gain saturation. Small-signal measurement on a 3 × 580 μm2 Fabry-Perot laser showed a 3dB bandwidth of 6.5 GHz at a bias current of 116 mA. A 12.5 Gbit/s non-return-to-zero signal modulation was achieved by directly probing the chip. Open eyes with an extinction ration of 3.3dB was observed at room temperature. The bit-error-rate (BER) curve showed no error-floor up to BER of 1 × 10−13. 12 km single-mode fiber transmission experiments using the QD laser on Si showed a low power penalty of 1 dB at 5 Gbit/s. These results demonstrate the potential for QD lasers epitaxially grown on Si to be used as a low-cost light source for optical communication systems.

Room temperature microwave oscillations in GaN/AlN resonant tunneling diodes with peak current densities up to 220 kA/cm2

Abstract: We report the generation of room temperature microwave oscillations from GaN/AlN resonant tunneling diodes, which exhibit record-high peak current densities. The tunneling heterostructure grown by molecular beam epitaxy on freestanding GaN substrates comprises a thin GaN quantum well embedded between two AlN tunneling barriers. The room temperature current-voltage characteristics exhibit a record-high maximum peak current density of ∼220 kA/cm2. When biased within the negative differential conductance region, microwave oscillations are measured with a fundamental frequency of ∼0.94 GHz, generating an output power of ∼3.0 μW. Both the fundamental frequency and the output power of the oscillator are limited by the external biasing circuit. Using a small-signal equivalent circuit model, the maximum intrinsic frequency of oscillation for these diodes is predicted to be ∼200 GHz. This work represents a significant step towards microwave power generation enabled by resonant tunneling transport, an ultra-fast pr...

Low temperature-processed ZnO thin films for p–n junction-based visible-blind ultraviolet photodetectors

Abstract: Ultraviolet (UV) photodetectors have drawn extensive attention due to their numerous applications in both civilian and military areas including flame detection, UV sterilization, aerospace UV monitoring, missile early warning, and ultraviolet imaging. Zinc oxide (ZnO)-based UV detectors exhibit remarkable performance; however, many of them are not visible-blind, and the fabrication techniques involve a high-temperature annealing step. Here, we fabricated a p–n junction photodiode based on annealing-free ZnO thin films prepared from ZnO nanoparticles and N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB). NPB was chosen due to its transparent nature in the visible region and high hole mobility. The ZnO nanoparticles and thin films were characterized by UV-visible absorption spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), dynamic light scattering (DLS) particle size analysis, Fourier-transform infrared (FTIR) spectroscopy, photoluminescence spectroscopy, XRD and profilometry. The device exhibited responsivity of 0.037 A/W and an external quantum efficiency (EQE) of 12.86% at 5 V bias under 360 nm illumination. In addition, with no biasing, the device exhibited an on–off ratio of more than 103 and a linear dynamic range (LDR) of 63 dB. A high built-in potential at the ZnO/NPB interface could be the reason for this performance at zero bias. The rise and fall times were 156 ms and 319 ms, respectively. The results suggest that a visible-blind UV photodetector with acceptable performance can be fabricated using annealing-free ZnO films, which may lead to the realization of flexible detectors due to the low-temperature processes involved.

Electrical and impedance properties of MPS structure based on (Cu 2 O–CuO–PVA) interfacial layer

Abstract: In this study, the electrical characteristics of the prepared Au/(Cu2O–CuO–PVA)/n-Si (MPS) structures have been investigated in detail by using the frequency dependent C–V and G/ω–V measurements by taking into account the interfacial polymer layer, surface states (Nss), polarization and series resistance (Rs). The electric parameters such as the diffusion potential (VD), the concentration of donor atoms (ND), and barrier height (BH) values were obtained from C−2–V plots for each frequency and they were found as 0.33 eV, 7.60 × 1013cm−3, 0.65 eV at 10 kHz and 0.70 eV, 6.99 × 1013cm−3, 1.02 eV at 3 MHz. The energy dependent profiles of Nss and their relaxation time (τ) were found by using admittance method and they ranged from 1.09 × 1011 to 1.60 × 1011 eV−1 cm−2 and 7.75 × 10−6 to 9.93 × 10−5 s, respectively. These low values of Nss are indicated that the (Cu2O–CuO)-doped PVA interfacial layer considerably enhances the performance of the Au/n-Si (MS) structure and so it can be successfully utilized instead of the traditional insulator/dielectric layer due to its passivized the surface states. The Rs versus V plot was obtained from the C–V and G/ω–V data using Nicollian and Brews method and it shows a distinctive peak, while the magnitude of the peak decreases with increasing frequency, its position shift towards lower or negative bias voltage with decreasing frequency due to the reordering and restructuring of surface states and their relaxation time under applied bias voltage. The impedance measurements were also performed in the wide range of frequency (100 Hz–1 MHz) at room temperature. The equivalent circuit model parameters such as parallel resistor (Rp), capacitor (Cp) and a series resistance (Rs) were calculated from Cole–Cole plots. The values of Rs, Rp and Cp decrease with increases dc voltage. The decrease of Rp is because of the increasing in the number of injected charge carriers into the device.

Bias dependent conduction and relaxation mechanism study of Cu 5 FeS 4 film and its significance in signal transport network

Abstract: Here we have reported the electric and dielectric properties of a Bornite (Cu5FeS4) film using non-destructive complex impedance spectroscopy (CIS) in the frequency range 40 Hz–20 MHz and bias voltage range 0–0.8 V. A typical hydrothermal reaction was carried out to obtain the Cu5FeS4 material. Thereafter, we fabricated the film of the synthesized material and analysed its various properties as a function of frequency and dc bias voltages. The CIS analysis at room temperature (303 K) for different forward dc bias voltages shows that the bulk resistance plays a predominant role in conduction mechanism. The complex impedance (Nyquist) plots are well modelled with grain and grain boundary resistance by introducing proper ac equivalent circuit. The impedance loss spectra showed that the relaxation peak shifts towards the higher frequency with increasing bias voltages, which implies the possibility of a charge hopping between the localized charge states. Furthermore, we find that dielectric constant ( $${\varepsilon ^\prime }$$ ), dielectric loss ( $${\varepsilon ^{\prime \prime }}$$ ), electrical modulus ( $${{\text{M}}^*}$$ ) and ac/dc conductivity of the Cu5FeS4 film are strongly frequency dependent. The values of $${\varepsilon ^\prime }$$ and $${\varepsilon ^{\prime \prime }}$$ decreases whereas ac conductivity increases with increasing frequency. High frequency and low frequency dielectric constant of the material are found to be 3.48 and in the order of 104 respectively. Electrical modulus study is introduced for better explanation of conductivity relaxation phenomenon. Finally, we investigated the variation of ac/dc conductivity with forward dc bias voltages. This confirmed the presence of a hopping mechanism for electrical transport in our system, which can be best explained on the basis of jump relaxation model. Overall, this extensive study based on complex impedance spectroscopy demonstrates the dielectric relaxation behaviour of Cu5FeS4 film and also shed light on hopping induced conduction mechanism.

A Pulse-Biasing Small-Signal Measurement Technique Enabling 40 MHz Operation of Vertical Organic Transistors.

Abstract: Organic/polymer transistors can enable the fabrication of large-area flexible circuits. However, these devices are inherently temperature sensitive due to the strong temperature dependence of charge carrier mobility, suffer from low thermal conductivity of plastic substrates, and are slow due to the low mobility and long channel length (L). Here we report a new, advanced characterization circuit that within around ten microseconds simultaneously applies an accurate large-signal pulse bias and a small-signal sinusoidal excitation to the transistor and measures many high-frequency parameters. This significantly reduces the self-heating and therefore provides data at a known junction temperature more accurate for fitting model parameters to the results, enables small-signal characterization over >10 times wider bias I–V range, with ~105 times less bias-stress effects. Fully thermally-evaporated vertical permeable-base transistors with physical L = 200 nm fabricated using C60 fullerene semiconductor are characterized. Intrinsic gain up to 35 dB, and record transit frequency (unity current-gain cutoff frequency, fT) of 40 MHz at 8.6 V are achieved. Interestingly, no saturation in fT − I and transconductance (gm − I) is observed at high currents. This paves the way for the integration of high-frequency functionalities into organic circuits, such as long-distance wireless communication and switching power converters.

Influencing Sources for Dark Current Transport and Avalanche Mechanisms in Planar and Mesa HgCdTe p-i-n Electron-Avalanche Photodiodes

Abstract: In this paper, both planar and mesa homojunction p-i-n HgCdTe electron-avalanche photodiodes (e-APDs) are fabricated and investigated to better understand the dark current transport and electron-avalanche mechanisms of the devices and optimize the structures. The experiment results are agreed well by simulated ${I}$ – ${V}$ characteristics based on established numerical models. Our results show that the multiplication region fabrication process leads to enormous characteristic difference between planar and mesa diodes. Shockley–Read–Hall and trap-assisted tunneling current are the main components of dark current for the planar/mesa junction under low bias voltage, and dark current is mainly influenced by band-to-band tunneling and avalanche current when higher reverse bias is added. In addition, we found that the difference between the uniformity of the electric field distributions in multiplication regions is the primary reason for the differences of the dark current. It was also proved that the dark current of planar e-APDs is dramatically affected by the junction corners, and mesa e-APDs dark current is found to be greatly dependent on the multiplication region thickness. Our work provides a great deal of theoretical basis for dark current formation and the avalanche mechanism of HgCdTe e-APDs.