Direct Current Bias Voltage Research Articles

Enhancement of broadband ultraviolet visible photodetection by boron nitride nanoparticles in bulk graphene oxide layer

A proposed boron nitride/graphene oxide (BN/GO) composite photodetector (PD) was prepared using a drop cast method for photodetection in the ultraviolet (UV) and visible light ranges. Morphology of the GO, distribution of BN nanoparticles and composition of B, N, C and O elements were confirmed by field emission scanning electron microscope (FESEM) analysis and energy dispersive x-ray (EDX) spectroscopy results. Raman scattering indicates high frequency intrinsic E2g vibrations at 1349 cm−1 and low intense peaks at 803 cm−1 from hexagonal BN (h-BN) structures in the bulk GO layer, while an overlap in the D and G bands at 1349 and 1581 cm−1 respectively indicates the presence of the GO layer. Excellent photoconduction of 380 and 405 nm UV sources and a 650 nm red laser source showed that the measured photocurrent is dependent on power regardless of the signal wavelength. Variations in sensitivity allow the device to be operated at selected direct current (DC) bias voltages, while high frequency modulation at 1000 Hz showed a profound rise and fall time about 13.6 and 245.6 μs respectively. Characteristics as self-powered PD was observed, has an enhancement about 3450% at its zero bias voltage.

High responsivity, self-powered carbon–zinc oxide hybrid thin film based photodetector

A self-powered n-Si/C–ZnO/SiO2/p-Si heterojunction photodetector (PD) which comprises of carbon (C) and zinc oxide (ZnO) nanostructures on a n-type silicon (n-Si) substrate was prepared via vapor phase transport method. Excellent photodetection under 468 nm light illumination for powers ranging from 2.78 to 2910 µW delivered a quick response of about 9.5 µs. A high photoresponsivity of 2.082/AW and external quantum efficiency about 551% were obtained. The mechanism involved for the generation of a photocurrent at a zero bias voltage was discussed for future energy efficient optoelectronics devices. The morphology and composition of C, Zn and O were confirmed by field emission scanning electron microscope, energy dispersive X-ray and Raman scattering analysis. The formation of ZnO nanowires range from 10 to 100 nm, aided by the photoconduction mechanism. The Raman E2high mode of 437/cm of ZnO and the presence of D and G bands show the formation of a hybrid C–ZnO thin film. The calculated rectifying ratio is found to shift as the direct current bias voltage and light power increased. The deposition of C particles on the ZnO surface creates point defects and sub-energy levels in the ZnO bandgap which favour fast responsivity in the PD.

Hydrothermally synthesized zinc oxide nanoparticle based photodetector for blue spectrum detection

A photodetector, based on zinc oxide nanoparticles (ZnO-NPs) is designed and fabricated using the drop casting method. Two glass slides coated with Indium tin oxide (ITO) thin films approximately 2.5 μm thick are used as source and drain electrodes, together with Schottky contacts fabricated with ZnO-NPs for the efficient transfer of photo-generated charge carriers. The proposed photodetectors showed significant sensitivity under white light emitting diode (LED) illumination at 468 nm, which is near the blue light spectrum. The current–voltage (I–V) characteristics of the proposed photodetectors indicate the generation of electron-hole pairs under white LED illumination at 265 μW, and stable photocurrent is achieved at high direct current (DC) bias voltages. Photodector response and recovery times of approximately 66.52 and 134.36 μs respectively are obtained at LED frequency modulation of 100 Hz at a bias voltage of 5 V. This represents an enhancement of the capabilities of ZnO-NPs, which is typically used for detection in the UV region only.

Powder free PECVD epitaxial silicon by plasma pulsing or increasing the growth temperature

Crystalline silicon thin films are promising candidates for low cost and flexible photovoltaics. Among various synthesis techniques, epitaxial growth via low temperature plasma-enhanced chemical vapor deposition is an interesting choice because of two low temperature related benefits: low thermal budget and better doping profile control. However, increasing the growth rate is a tricky issue because the agglomeration of clusters required for epitaxy leads to powder formation in the plasma. In this work, we have measured precisely the time evolution of the self-bias voltage in silane/hydrogen plasmas at millisecond time scale, for different values of the direct-current bias voltage applied to the radio frequency (RF) electrode and growth temperatures. We demonstrate that the decisive factor to increase the epitaxial growth rate, i.e. the inhibition of the agglomeration of plasma-born clusters, can be obtained by decreasing the RF OFF time or increasing the growth temperature. The influence of these two parameters on the growth rate and epitaxial film quality is also presented.

Varactor‐Tunable Frequency Selective Surface With an Appropriate Embedded Bias Network

AbstractIn this work, a varactor‐tunable frequency selective surface (FSS) is proposed; it is based on a common FSS composed of annular ring slot cells, in which four varactors are loaded on each annular ring slot to dynamically tune its resonant frequency in real time. In addition, an appropriate embedded bias network is added to introduce a same direct current bias voltage for each varactor. Numerical simulations demonstrate that the resonant‐frequency of the proposed FSS can be tuned over a 77.2% wide frequency range from 2.73 to 6.16 GHz by altering the capacitance of the varactors from 1.0 to 0.1 pF; moreover, the embedded bias network has little effect on the band‐pass response, and the minimum of the reflection coefficients is less than −24 dB at all times. Finally, one effective experimental validation is carried out. The proposed tunable FSS possesses the merits of wide tuning range and high transmittance; thus, it has great potential applications in the design of tunable radomes or adaptive screening of unwanted wireless transmissions.

The correlation between optical and mechanical properties of amorphous diamond-like carbon films prepared by pulsed filtered cathodic vacuum arc deposition

Diamond-like carbon (DLC) films were deposited by pulsed filtered cathodic vacuum arc (FCVA) deposition. The microstructure, mechanical and optical properties of DLC films have been investigated as functions of the variation in the substrate negative direct current bias voltage (Vbias) from 0.0 to 1.5 kV. Raman and X-ray photoelectron spectroscopy results show a correlation between the ID/IG ratio and the microstructure in terms of the sp3/sp2 ratio to Vbias. This fact shows that a significant change in the sp3 content, which is attributed to the transformation from graphite-like to diamond-like, is accompanied by a decreasing ID/IG ratio. The relatively high mass density of the films in the range of 2.51 to 2.79 g/cm3 can be obtained with biasing. The mechanical properties, i.e., the hardness and elastic modulus, were 13–25 and 114–145 GPa, respectively. The improvement of the mechanical properties is due to the formation of the compressive residual stress and the local density depending on Vbias. The relationship between the refractive index and the ID/IG ratio agree well with the microstructure and mechanical properties of DLC films. All of these results indicate a vital role of Vbias in determining the DLC properties.

Conductometric Gas Sensing Based on ZnO Thin Films: An Impedance Spectroscopy Study

Zinc oxide (ZnO) thin films have been successfully prepared via a reactive radio frequency (RF) sputtering process. The characterizations of the crystal structure and the morphology of the surface revealed a highly oriented c-axis hexagonal structure with almost homogenous distribution of the grains for the prepared ZnO thin films. The direct current (DC) bias voltage and the effect of acetone vapors on the complex impedance characteristics of the ZnO thin films has been studied. The complex impedance spectroscopy of the prepared ZnO sensor shows a single semicircle which tends to shrink under the effect of both the DC bias voltage and the presence of the acetone vapor. The increase in acetone concentrations and DC applied voltage will result in decreased total sensor resistance and increased capacitance.

Characterization of graphene oxide/silicon dioxide/p-type silicon heterojunction photodetector towards infrared 974 nm illumination

Graphene oxide (GO) based heterojunction photodetector was fabricated using a simple drop casting method which was further heated in a furnace at 450 °C. The physical characterization of the fabricated photodetector revealed non-homogeneous GO layer and presence of D and G band of graphene respectively at 1357 and 1596 cm−1. The photoresponsivity and external quantum efficiency of the photodetector were evaluated and optimized using infrared (IR) illumination at a wavelength of 974 nm. Current–voltage (I–V) characteristics found to be sturdily dependent on the increased laser power as it showed threshold voltage (Vth) at 0.855 V. High photoresponsivity 9.577 mAW−1 was detected for laser power at 2.405 mW at direct-current (DC) bias 13.358 V. Rise and fall time found to be varying with frequency modulated laser and the applied DC bias voltage across the photodetector. High frequency modulation and low DC bias voltage showed profound rise and fall time at 48 and 2210 µs respectively. The photodetector was highly sensitive to 974 nm IR illumination for modulated laser frequency ranging from 1000 to 5000 Hz. Besides that, the fabricated heterojunction photodetector able to function in good condition at low DC bias voltage.

Heterojunction photodetector based on graphene oxide sandwiched between ITO and p-Si

The drop casting method is utilized on indium tin oxide (ITO)-coated glass in order to prepare a sandwiched ITO/graphene oxide (ITO/GO) with silicon dioxide/p-type silicon (SiO2/p-Si) heterojunction photodetector. The partially sandwiched GO layer with SiO2/p-Si substrate exhibits dual characteristics as it showed good sensitivity towards the illumination of infrared (IR) laser at wavelength of 974 nm. Excellent photoconduction is also observed for current–voltage (I–V) characteristics at various laser powers. An external quantum efficiency greater than 1 for a direct current bias voltage of 0 and 3 V reveals significant photoresponsivity of the photodetector at various laser frequency modulation at 1, 5 and 9 Hz. The rise times are found to be 75, 72 and 70 μs for 1, 5 and 9 Hz while high fall times 455, 448 and 426 are measured for the respective frequency modulation. The fabricated ITO/GO-SiO2/p-Si sandwiched heterojunction photodetector can be considered as a good candidate for applications in the IR regions that do not require a high-speed response.

MEMS fabrication and frequency sweep for suspending beam and plate electrode in electrostatic capacitor

We report a MEMS fabrication and frequency sweep for a high-order mode suspending beam and plate layer in electrostatic micro-gap semiconductor capacitor. This suspended beam and plate was designed with silicon oxide (SiO2) film which was fabricated using bulk silicon micromachining technology on both side of a silicon substrate. The designed semiconductor capacitors were driven by a bias direct current (DC) and a sweep frequency alternative current (AC) in a room temperature for an electrical response test. Finite element calculating software was used to evaluate the deformation mode around its high-order response frequency. Compared a single capacitor with a high-order response frequency (0.42 MHz) and a 1 × 2 array parallel capacitor, we found that the 1 × 2 array parallel capacitor had a broader high-order response range. And it concluded that a DC bias voltage can be used to modulate a high-order response frequency for both a single and 1 × 2 array parallel capacitors.

A Novel Radio-Over-Fiber System Based on Carrier Suppressed Frequency Eightfold Millimeter Wave Generation

We propose a novel and simple scheme for photonic frequency eightfold millimeter wave (mm-wave) generation with optical carrier suppression based on only one single-drive Mach–Zehnder modulator (MZM). According to our theoretical analysis and experimental demonstration, by adopting designed direct current bias voltage of MZM and the amplitude voltage of the radio frequency (RF) drive signal, two fourth-order optical subcarriers are generated via a single-drive MZM. Furthermore, the corresponding optical central carrier and undesired sidebands are suppressed simultaneously. Based on our proposed scheme, the generation of 72-GHz optical mm-wave by an RF signal of 9 GHz without any optical filtering is experimentally demonstrated. The radio-over-fiber system with 3.5-Gb/s OOK downstream link based on this generated 72-GHz mm-wave is also experimentally demonstrated. As we know, it is the first time to realize frequency eightfold mm-wave signal generation with simultaneous carrier suppression by using only one single-drive MZM in experiment.

The influence of aqueous sodium dodecyl sulphate solution in the photoresponsivity of nitrogen doped graphene oxide photodetector

The homogeneity and agglomeration in graphene based photodetector plays an important role in the photoconduction. The influence of sodium dodecyl sulphate (SDS) solution in nitrogen doped graphene oxide (N-GO) is studied based on the photoresponsivity behaviour. One step hydrothermal method and drop casting technique are utilized to obtain N-GO photodetectors. High photoresponsivity about 1000 folds is achieved in N-GO prepared with SDS solution (N-GO/SDS) compared to the N-GO prepared without SDS solution. Raman spectrum also revealed a high intense D and G band as well as a slightly broaden 2D band due to sp2 hybridization. The fabricated device has exhibited wide range of responsivity to infrared (IR) laser 974 nm pulse and illumination. Real time current measurement in N-GO and current–voltage (I-V) characteristics in N-GO/SDS showed a significant photoconduction due to laser 974 nm illumination. The external quantum efficiency (EQE) in the N-GO/SDS solution is about 394830% compared to N-GO prepared without SDS, which is only 272%. The evaluated fall time at frequency modulation of 0.1 and 1 Hz for direct current (dc) bias voltage of 5.0 V found to be shorter compared to that of 2.5 V, whereas the fall time at high frequency modulation at 5000 Hz exhibited similar time around 77 μs.

Effects of electric field and bias voltage on corrosion behavior of tin under a thin electrolyte layer

The effects of electric field and bias voltage on corrosion behavior of tin under a thin electrolyte layer (TEL) were investigated by electrochemical and surface techniques. The results indicated that the order of the corrosion rate of tin under different electric fields with the same electric field gradient was as follows, square wave alternating current electric field (ACEF)>sinusoidal wave ACEF>direct current electric field (DCEF)>blank. Moreover, the corrosion rate of tin increased with increased direct current bias voltage under different electric fields. The contributions of electric field gradient and leakage current to the corrosion rate of tin were quantified under different electric fields with different bias voltages.

Epitaxial growth of Cu(001) thin films onto Si(001) using a single-step HiPIMS process

We report on a new route to grow epitaxial copper (Cu) ultra-thin films (up to 150 nm thick) at ambient temperature on Si(001) wafers covered with native oxide without any prior chemical etching or plasma cleaning of the substrate. It consists of a single-step deposition process using high power impulse magnetron sputtering (HiPIMS) and substrate biasing. For a direct current (DC) substrate bias voltage of −130 V, Cu/Si heteroepitaxial growth is achieved by HiPIMS following the Cu(001) [100]//Si(001) [110] orientation, while under the same average deposition conditions, but using conventional DC magnetron sputtering, polycrystalline Cu films with [111] preferred orientation are deposited. In addition, the intrinsic stress has been measured in situ during growth by real-time monitoring of the wafer curvature. For this particular HiPIMS case, the stress is slightly compressive (−0.1 GPa), but almost fully relaxes after growth is terminated. As a result of epitaxy, the Cu surface morphology exhibits a regular pattern consisting of square-shaped mounds with a lateral size of typically 150 nm. For all samples, X-ray diffraction pole figures and scanning/transmission electron microscopy reveal the formation of extensive twinning of the Cu {111} planes.

Tunable Bandpass Filter Using a Switched Tunable Diplexer Technique

In this paper, a new technique using two p-i-n diodes to switch between two channels of a tunable diplexer with constant return loss is proposed to design a tunable bandpass filter (BPF) with wide frequency tuning range (FTR). The used third-order tunable diplexer with two independently tuned channels consists of one common varactor-tuned lumped-element dual-resonance resonator and two varactor-tuned LC resonators. The ports of its two channels are connected by a single-pole double-throw switch, so that one port of the tunable BPF is constituted. And its common port serves as the other port of the tunable BPF. In the switched tunable diplexer design, its direct current (dc) bias voltages are also appropriately selected to improve the in-band and out-of-band performances of the tunable BPF. As examples, a third-order tunable diplexer with constant return loss of 15 dB is first designed. The measured results show that the 3 dB cutoff frequency of its lower channel can be tuned from 0.5 to 1.02 GHz, while the 3 dB cutoff frequency of its upper channel can be tuned from 1.01 to 1.78 GHz. Then, a tunable BPF is designed, and its 3 dB cutoff frequency covers from 0.485 to 1.82 GHz. The proposed tunable BPF exhibits good return loss, wide FTR, and compact size.

Spatial profiles of interelectrode electron density in direct current superposed dual-frequency capacitively coupled plasmas

We present experimentally determined spatial profiles of the interelectrode electron density (ne) in dual-frequency capacitively coupled plasmas in which the negative direct current (dc) bias voltage (Vdc) is superposed; in the experiment, 13 MHz (Plow) was applied to the lower electrode and 60 MHz (Phigh) to the upper electrode. The bulk ne increased substantially with increases in the external power, Phigh, Plow, and with increases in Vdc. When Plow was insufficient, the bulk ne decreased as the Vdc bias increased. The bulk ne increased due to its dependence on Vdc, especially for |Vdc| > 500 V. This may correspond to the sheath voltages (Vs) of the lower electrode. The ne values in front of the upper electrode were coupled with the Vdc: the Vdc dependence first decreased and then increased. The dc currents (Idc) of the upper electrode were collected when a large Plow was applied. The value of Idc at the threshold value of Vdc ≈ Vs (e.g. −500 V) increased with an increase in ne. When |Vdc| exceeded the threshold, the spatial ne profile and the Idc dependence were changed relative to the electrical characteristics of the dc superposition; this led to a change in the location of the maximum ne, the width of the area of ne depletion in front of the electrodes, and a transition in the electron heating modes.

Performance Analysis of Hybrid Metal–Graphene Frequency Reconfigurable Antennas in the Microwave Regime

This paper analyzes the potential application of graphene in microwave frequency reconfigurable antennas. Two multiband designs, one for wireless fidelity (WIFI), covering the frequency bands of 2.4, 3.6, and 5 GHz, and another for the cellular long-term evolution system, operating the frequency bands of 1.8, 2.5, 2.6, and 3.6 GHz, are evaluated to demonstrate the working principle and the performance tradeoffs. The designs are made mostly of copper with some parts made of graphene to enable reconfigurable behavior. The graphene material’s surface impedance is tuned by applying a direct current bias voltage, which allows to obtain one of two extreme values that emulate the ON and OFF states of common switches, such as microelectromechanical systems (MEMSs), field-effect transistor, and p-type insulator n-type diode, or continuous values to mimic reconfigurable antennas loaded by varactors. This material switching modifies the electrical length of the current propagating through the antenna and consequently switches the resonant frequencies. Additionally, results show that hybrid metal–graphene frequency reconfigurable antennas can, at the same time, provide a tunable bandwidth and antenna matching.

Electrical and dielectric characterization of Au/ZnO/n–Si device depending frequency and voltage

Au/ZnO/n-type Si device is obtained using atomic layer deposition (ALD) for ZnO layer, and some main electrical parameters are investigated, such as surface/interface state , barrier height , series resistance , donor concentration , and dielectric characterization depending on frequency or voltage. These parameters are acquired by use of impedance spectroscopy measurements at frequencies ranging from 10 kHz to 1 MHz and the direct current (DC) bias voltages in a range from −2 V to +2 V at room temperature are used. The main electrical parameters and dielectric parameters, such as dielectric constant , dielectric loss , loss tangent , the real and imaginary parts of electric modulus ( and ), and alternating current (AC) electrical conductivity are affected by changing voltage and frequency. The characterizations show that some main electrical parameters usually decrease with increasing frequency because charge carriers at surface states have not enough time to fallow an external AC signal at high frequencies, and all dielectric parameters strongly depend on the voltage and frequency especially in the depletion and accumulation regions. Consequently, it can be concluded that interfacial polarization and interface charges can easily follow AC signal at low frequencies.

In-Plane MEMS Shallow Arch Beam for Mechanical Memory.

We demonstrate a memory device based on the nonlinear dynamics of an in-plane microelectromechanical systems (MEMS) clamped–clamped beam resonator, which is deliberately fabricated as a shallow arch. The arch beam is made of silicon, and is electrostatically actuated. The concept relies on the inherent quadratic nonlinearity originating from the arch curvature, which results in a softening behavior that creates hysteresis and co-existing states of motion. Since it is independent of the electrostatic force, this nonlinearity gives more flexibility in the operating conditions and allows for lower actuation voltages. Experimental results are generated through electrical characterization setup. Results are shown demonstrating the switching between the two vibrational states with the change of the direct current (DC) bias voltage, thereby proving the memory concept.

Particle size characterization using a high resolution planar resonator sensor in a lossy medium

In this work, a microwave planar resonator sensor with boosted quality factor is proposed for the non-contact detection and classification of particles of different diameters in gaseous and aquatic environments. This approach can be applied to rapid particle counting, particle size sensing and classification in lossy and non-lossy medium. The presented sensor operates based on variations in the dielectric properties of materials in the vicinity of a planar open-ended microstrip resonator device. Finite Element Method (FEM) analysis is performed to study the effect of particle size on the transmission response profile of the sensor. Verification of the established FEM model and the functionality of the sensor are investigated by experiment. The resolution of the sensor may be adjusted by changing the quality factor of the resonator, which is directly related to the direct current bias voltage of the active device in the local feedback loop. The initial results show the sensor to be capable of detecting the resonant frequency variations of 1kHz over 1.5GHz, which corresponds to a detectable frequency resolution better than 1 part per million (PPM). The reported sensor operates at 1.42GHz and demonstrates particle classification capability within an overall 500kHz resonant-frequency shift for different bead-diameter sizes (40–400μm) at a distance of 5mm, in both air and water (ε’=78 and tanδ=0.15) using petroleum coke beads as a test material.