External Bias Voltage Research Articles

A transversal approach to predict surface charge compensation in piezoelectric force microscopy

Piezoelectric force microscopy (PFM) has demonstrated to be a powerful tool to characterize ferroelectric materials. However, extrinsic effects, most notably, those resulting from surface charges, often mask or mirror genuine piezoelectric response, challenging PFM data understanding. The contribution of surface charges to PFM signal is commonly compensated by using appropriate external bias voltage, which is ad-hoc selected and sample dependent. Here, we determine the compensating voltage in thin films of different ferroelectric materials and we compare with the corresponding I-V characteristics recorded using suitable electrodes. It turns out that the sign and magnitude of the bias voltage required to compensate the surface charges are related to the asymmetry of the I-V characteristics. We propose that this relation results from the fact that the semiconducting properties of the material determine both the I-V dependence, and the sign of charged adsorbates. We show how to make use of this correlation to predict the required compensation voltage of a non-ferroelectric material and we show that spurious piezoelectric-like contributions are largely cancelled. The results provide guidelines to mitigate common extrinsic contributions in PFM imaging.

Electrically tunable metasurface for dual-band spatial light modulation using the epsilon-near-zero effect.

Electro-tunable metasurfaces have attracted much attention for the active control of incident light at the nanoscale by engineering sub-wavelength meta-atoms. In this Letter, for the first time, to the best of our knowledge, a grating-assisted dual-band metasurface for spatial light modulation is reported that can operate in two crucial telecommunication wavelength bands, i.e., C-band and O-band. The proposed device consists of a silicon-nitride nanograting on top of a silicon-indium-tin-oxide (ITO)-alumina-gold stack. Effective medium theory combined with a modal analysis is used to study the guided-mode resonance dips at 1.55 µm and 1.31 µm in the reflectance spectra. We leverage the epsilon-near-zero effect of ITO by applying an external bias voltage to introduce large modal loss, which leads to the disappearance of the resonance dips at those wavelengths. We obtain a high modulation depth of ∼22.3 dB at 1.55 µm and ∼19.5 dB at 1.31 µm with an applied bias of -4 V and -5 V, respectively. Thus, the proposed metasurface may help to realize dual-band active nanophotonic devices.

Ultra-broadband and completely modulated absorption enhancement of monolayer graphene in a near-infrared region.

Achieving ultra-broadband and completely modulated absorption enhancement of monolayer graphene in near-infrared region is practically important to design graphene-based optoelectronic devices, however, which remains a challenge. In this work, by spectrally designing multiple magnetic plasmon resonance modes in metamaterials to be adjacent to each other, near-infrared light absorption in monolayer graphene is greatly improved to have an averaged absorption efficiency exceeding 50% in a very broad absorption bandwidth of about 800 nm. Moreover, by exerting an external bias voltage on graphene to change Fermi energy of graphene, the ultra-broadband absorption enhancement of monolayer graphene exhibits an excellent tunability, which has a nearly 100% modulation depth and an electrical switching property. This work is promising for applications in near-infrared photodetectors, amplitude modulators of electromagnetic waves, etc.

Spin-current in a Magnetic Semiconductor Tunnel Junction: Effect of External Bias Voltage

This paper investigates spin-current transport in a GaMnAs/GaAs/GaMnAs magnetic semiconductor tunnel junctions under applied bias voltages. The 30-band k.p approach is used to describe the materials within the heterostructure, incorporating both spin-orbit and exchange interactions. We use the transfer-matrix formalism to derive numerical solutions for the wave functions. At specific bias values, we calculate the polarization of the spin-current component along the z direction of the structure. We show oscillations of the two spin-current components perpendicular to the magnetization with equal polarization amplitude and characteristic period. The polarization amplitude varies around 10%, reflecting the typical polarization in such type of material. The oscillation period - which relates to the Larmor frequency for spin precession - increases with the bias voltage values.

High-performance self-driven photodetectors based on self-polarized Bi0.9Eu0.1FeO3/Nb-doped SrTiO3 p-n heterojunctions

Ferroelectric (FE) materials, owing to their remarkable photovoltaic effects, are one class of the most promising materials candidates for self-powered photodetectors. Here, with the pronounced rectification ratio as large as 103 at electric bias of± 1.0 V and a low dark current of 12 pA, the self-polarized Bi0.9Eu0.1FeO3 (BEFO)/Nb-doped SrTiO3 (NSTO) p-n heterojunctions are synthesized. It is revealed that the BEFO/NSTO p-n heterojunctions achieve the excellent photovoltaic effect with a short-circuit current as high as 0.23 μA and an open-circuit voltage of 0.41 V under the light illumination of 405 nm in wavelength and intensity of 151.7 mW/cm2. More importantly, the built-in potential in the as-prepared heterojunctions offers the self-powered photo-detecting functionality, with the high light-on/off current ratio of 2.0 × 104, high responsivity of 0.64 mA/W and sensitive detectivity D* of 1.63 × 1010 Jones, and fast response speed of 50/44 ms without external bias voltage. In particular, the super-performance with R ~ 1.97 mA/W and D*~ 5.0 × 1010 Jones in the weak incident light intensity (1.4 mW/cm2) are demonstrated, better than most previously reported FE-based self-powered photodetectors. Moreover, the devices possess good ambient stability and reproducibility. This work provides a demo for optically driven high-performance electronic devices based on self-polarized ferroelectric materials.

Structural properties and polarization switching of epitaxial Bi2FeCrO6 thin films grown on La2/3Sr1/3MnO3/SrTiO3 (111) substrates

• High quality ferroelectric Bi 2 FeCrO 6 films were prepared by pulsed laser deposition. • The films exhibit a grained structure with an in-plane grain size of about 200 nm. • The as-grown film show a preferential upward polarization direction. • An asymmetric polarization switching is observed due to interface effects. • A mutual influence between polarization and interface bias field is evidenced. Ferroelectric Bi 2 FeCrO 6 (BFCO) double perovskite thin films were grown by pulsed laser deposition on SrTiO 3 (111) covered with a 20 nm thick La 2/3 Sr 1/3 MnO 3 buffer layer. X-ray diffraction measurements performed in θ-2θ and φ scans modes reveal an epitaxial growth of the BFCO with a compressive in-plane strained structure and no spurious phases. The presence of superstructure diffraction peaks indicates an Fe-Cr cationic ordering along the growth axis. Local measurements realized by piezo-response force microscopy show an asymmetric polarization switching with respect to the external bias voltage suggesting the presence of a bias field. This agrees with a preferential ferroelectric polarization observed upon local poling of the BFCO film in opposite directions. The findings can be of importance for understanding the relationship between the atomic structure and polarization dynamics in ferroelectric thin-films.

Two-dimensional SiC Schottky junctions with symmetrical and asymmetrical metal electrode contacts

Four symmetrical and six asymmetrical Schottky contacts in the metal–semiconductor-metal structures were constructed by using two-dimensional (2D) SiC material as channel and four metal materials (Ag, Al, Au, Pd) as electrodes. On the basis of density functional theory and ab initio calculation method, devices’ transmission spectra, current–voltage characteristics, local density of states, rectification ratio, current–temperature characteristics and negative differential thermoelectric resistance were studied. The results show that Schottky contact is formed between the four metal materials and the 2D SiC. Partially asymmetrical metal electrode structure devices have excellent rectification characteristics. The device has the optimal rectification characteristics with a rectification ratio of 747 when Pd and Au are used as asymmetrical electrodes. Moreover, the temperature dependence of thermionic excitation transport has a key influence on the rectification properties of the device. The rectification ratio reaches to 2100 around 200 K in the junction of Pd–SiC–Au. A key finding is that the negative differential thermoelectric resistance can be generated in Pd–SiC–Au device when a temperature difference applied between two electrodes without external bias voltage. The results of this study can provide theoretical reference for the subsequent design of transistors and other device based on 2D SiC material.

Obtaining wide bandwidth with higher-order TM modes merging in a graphene-based logarithmic antenna for THz sensing applications

This paper introduces a wideband graphene material-based logarithmic antenna for THz sensing applications. The antenna consists of ten log-periodic elements which are made up of graphene material. The log-periodic antenna is excited through a silver nanostrip feedline coupled through a vias to the log-periodic elements. The proposed antenna provides wide bandwidth as well as high directivity. The antenna is operating at 1.62 THz, 2.05 THz, 2.48 THz, and 2.84 THz with wide bandwidth of 75.49 % (1.41–3.12 THz). The directivity and front-to-back ratio (FBR) are achieved as 10.16 dB and 15.12 dB, respectively. Since the bandwidth of antenna is wide which generate large number of mode. Therefore, the antenna generated higher order TM4δ (2 ≤δ≤ 8) mode. In addition, the parametric analysis has been done to obtain the impedance matching and frequency tuning by changing the external bias voltage of log-periodic graphene elements. The proposed antenna with the achieved response can be applied for THz sensing applications.

Determining dielectric properties of nematic liquid crystals at microwave frequencies using inverted microstrip lines

ABSTRACT In this paper, we have investigated the dielectric properties of nematic liquid crystals (NLCs) by measuring the resonant frequencies of inverted microstrip lines (IMSLs) of NLC being the substrate material at the microwave frequency band. The designed IMSL devices consist of a cavity for NLC mixture between the inverted microstrip line and the ground plane. The anisotropy values of the NLCs were determined by observing the change of the resonant frequency when the strength of external electric fields applied on the NLCs varied. The experiments demonstrated that the effective dielectric constant of the NLC can be manipulated by the proposed devices via external bias voltage for tuning. The accuracy of the method for determining the dielectric constants of JC-M-LC-E7 (JC-E7) and QYPD-036 LCs was analysed with several prototypes based on two separate designs. The high accuracy of 91% in permittivity determination was demonstrated, and the design and fabrication techniques are beneficial for making microwave components of high tunability.

Broadband, wide-incident-angle, and polarization-insensitive high-efficiency absorption of monolayer graphene with nearly 100% modulation depth at communication wavelength

We theoretically study how to greatly improve the near-infrared light absorption of monolayer graphene through the excitation of magnetic resonance in metamaterials. The absorption maximum of monolayer graphene is able to reach up to about 77% at the communication wavelength of 1550 nm. The conventionally defined absorption bandwidth, i.e., the full width at half maximum (FWHM), is nearly 160 nm. Thanks to the localization nature of magnetic resonance, the broadband high-efficiency absorption of monolayer graphene is insensitive to the incident angle and the light polarization. The absorption maximum, the absorption bandwidth, and the resonance position all have almost no change, when the incident angle is increased to even 60 degrees for both p and s polarizations. By applying an external bias voltage to change Fermi energy, the absorption in graphene can be completely modulated, with a nearly 100% modulation depth. Furthermore, the graphene absorption has an abrupt and large change around the interband transition, which exhibits an electrical switching property. Our work may find potential applications in graphene-based optoelectronic devices such as photodetector and modulator.

Tunable multi-peak perfect absorbers based on borophene for high-performance near-infrared refractive index sensing

Borophene has recently attracted significant attention as an emerging two-dimensional monoelemental material for refractive index sensing because of its ultra-high surface-to-volume ratio and outstanding surface sensitivity. However, current research mainly focuses on designing borophene-based sensors with single-peak absorption, which lacks reliability compared to the performance of multi-peak sensors. In this paper, high-performance borophene refractive index sensors with multiple nearly perfect absorption peaks are proposed. The geometric parameters of the metadevices are optimized using finite-difference time-domain (FDTD) simulations to obtain three strong absorption peaks in the near-infrared (near-IR) regime with intensities of 99.72%, 99.18%, and 98.13%. Further calculations show that the sensitivities of the three strong peaks reach 339.1 nm shift per refractive index units (nm/RIU), 410.1 nm/RIU, and 738.1 nm/RIU, and their corresponding figure of merits (FOMs) reach up to 15.41/RIU, 14.65/RIU and 10.85/RIU, respectively, exhibiting great potential for near-IR refractive index sensing. Moreover, the three absorption peaks can be easily tuned by adjusting the carrier density of borophene, which can be realized by applying different external electric bias voltages. These results can provide theoretical guidance for the development of multi-peak near-IR dynamic integrated photonic devices.

A noticeable improvement in opto-electronic properties of nebulizer sprayed In2S3 thin films for stable-photodetector applications

In the present work, the versatile Indium sulfide thin film (In2S3) are deposited on glass substrates by using nebulizer assisted spray pyrolysis technique at 350 °C for ultra violet (UV) light detection application. The effect of the of indium and sulfur molar concentrations on the structure, morphology, optical characteristics, and photo-sensing capability of the prepared In2S3 films were systematically examined in the range of 0.01 M–0.03 M. The structural analysis of the 0.03 M sample showed a maximum crystallite size of 36 nm. The morphological image from the scanning electron microscope showed a homogeneous, large size grained surface of In2S3 completely covering the substrate. As the molar concentration was raised from 0.01 to 0.03 M, the indium to the sulfur ratio in the prepared films ranged from 1.02 to 1.16. All the prepared films showed good absorption in the ultraviolet region of the electromagnetic spectrum. The sample prepared at 0.03 M was found to have a minimum band gap of 2.38 eV and a maximum PL intensity at 680 nm. The I–V characteristics show a photocurrent of 1.4 × 10−5 A under UV light of wavelength 365 nm for an external bias voltage of ±5 V. A maximum responsivity of 2.74 A/W, detectivity of 2.65 × 1010 Jones was exhibited by 0.03 M sample with a high EQE of 63.9%. The prepared In2S3 thin film can be used as active material in photodetectors.

Bias-free terahertz generation from a silicon-compatible photoconductive emitter operating at telecommunication wavelengths

We present a telecommunication-compatible bias-free photoconductive terahertz emitter composed of a bilayer InAs structure directly grown on a high-resistivity silicon substrate. The bilayer InAs structure includes p+-doped and undoped InAs layers, inducing a strong built-in electric field that enables terahertz generation without requiring any external bias voltage. A large-area plasmonic nanoantenna array is used to enhance and confine optical generation inside the photoconductive region with the highest built-in electric field, leading to the generation of a strong ultrafast photocurrent and broadband terahertz radiation. Thanks to a higher terahertz transmission through the silicon substrate and a shorter carrier lifetime in the InAs layers grown on silicon, higher signal-to-noise ratios are achieved at high terahertz frequencies compared with previously demonstrated bias-free terahertz emitters realized on GaAs. In addition to compatibility with silicon integrated optoelectronic platforms, the presented bias-free photoconductive emitter provides more than a 6 THz radiation bandwidth with more than 100 dB dynamic range when used in a terahertz time-domain spectroscopy system.

Electrical control of Förster resonant energy transfer across single-layer graphene

Abstract In artificial structures of molecular or quantum dot emitters in contact with single-layer graphene (SLG) Förster-type resonant energy transfer (FRET) can occur unconditionally due to the gapless band structure of SLG. A significant breakthrough for applications, however, would be the electrical modulation of FRET between arbitrary FRET pairs, using the SLG to control this process and taking advantage of the particular band structure and the monatomic thickness of SLG, far below the typical Förster radius of a few nanometers. For a proof of concept, we have therefore designed a Sandwich device where the SLG was transferred onto holey Si3N4 membranes and organic molecules were deposited on either side of the SLG. The relative photoluminescence (PL) intensities of donor and acceptor molecules changed continuously and reversibly with the external bias voltage, and a variation of about 6% of FRET efficiency has been achieved. We ascribe the origin of the electrical modulation of FRET to important doping-dependent nonlocal optical effects in the near field of SLG in the visible range.

Quantum Spin Hall Phases in Graphene and the Atomically Thin Transition Metal Dichalcogenide Family

AbstractThe quantum‐spin‐Hall (QSH) phase with helical edge spin states, in which opposite‐moment electron spins counter‐propagate within a topologically preserved time reversal symmetry, is attracting considerable attention. The helical edge spin currents allow zero‐emission energy spintronic‐devices with small error rates by modulating with external bias voltages. Recently, QSH phases and possible 2D topological insulating states have been reported in various atomically thin layers (such as graphene and the transition metal dichalcogenide family). Particularly, (twisted) bi‐layer graphene on heavy substrates is becoming a novel candidate for robust QSH phases (including Chern insulating phases), nevertheless, graphene itself lacks spin‐orbit coupling owing to the light carbon atoms. Here, research of QSH phases focusing on graphene is reviewed, particularly for using heavy adatom decoration and heavy substrates. They will open the door to next‐generation spintronic architectures.

Design of an Aerospace Langmuir Probe Volt-ampere Characteristic Load Simulator

The development process of space Langmuir probe needs to complete a large number of tests and calibration tests on the ground. Due to the long calibration period, complexity and high cost of plasma environment calibration, calibration tests are usually carried out after the instrument has been developed, in order to conduct preliminary tests on Langmuir probe during the instrument development process to verify the performance, as well as to save the cost of calibration tests and increase the reliability of the instrument, An advanced spatial Langmuir probe volt-ampere(I-V) load simulator was designed in this paper. Based on the positive and negative polarity of the external bias voltage, the I-V characteristic curve of the Langmuir probe was divided into positive and negative characteristic curves, which were realized by the combination of the output characteristic curves of NPN and PNP transistors, and the diode switching selectivity. The laboratory test results were consistent with the theoretical curve, verifying the validity and compliance of the design, which can play an important role in supporting the development of the space Langmuir probe and the calibration of the plasma environment.

Active continuous control of terahertz wave based on a reflectarray element-liquid crystal-grating electrode hybrid structure.

In this work, a new and efficient terahertz reflective phase shifter is proposed. The phase shifter is composed of a metal-dielectric-metal structure with a double dipole patch array, as well as copper grating electrodes immersed within the nematic liquid crystal. More specifically, the employed copper grating electrodes consist of two sets of cross-distributed comb grids, whereas at each set of comb grids can be applied an external bias voltage separately. On top of that, the electric field in the liquid crystal (LC) layer can be continuously changed by enforcing an innovative technique. Consequently, the orientation of the LC molecules was fully controlled by the applied electric field, since the dielectric constant of the LC is controlled by the biased voltage. The phase of the reflective electromagnetic wave can be continuously manipulated. Under this direction, the experimental results show that the phase shift exceeds the value of 180° in the range of 102.5 GHz-104.3 GHz, where the maximum phase shift is 249° at 103 GHz. The proposed work provides a new regulation concept for the implementation of LC-based terahertz devices and the respective applications in the terahertz reconfigurable antennas field.

Manipulation and readout of spin states of a single-molecule magnet by a spin-polarized current

Single-molecule memory device based on a single-molecule magnet (SMM) is one of the ultimate goals of semiconductor nanofabrication technologies. Here, we study how to manipulate and readout the SMM's two spin-state of stored information that characterized by the maximum and minimum average value of the $Z$-component of the total spin of the SMM and the conduction-electron, which are recognized as the information bits "$1$" and "$0$". We demonstrate that the switching time depends on both the sequential tunneling gap $\varepsilon_{se}$ and the spin-selection-rule allowed transition-energy $\varepsilon_{trans}$, which can be tuned by the gate voltage. In particular, when the external bias voltage is turned off, in the cases of the unoccupied and doubly-occupied ground eigenstates, the time derivative of the transport current can be used to read out the SMM's two spin-state of stored information. Moreover, the tunneling strength of and the asymmetry of the SMM-electrode coupling have a strong influence on the switching time, but that have a slight influence on the readout time that being on the order of nanoseconds. Our results suggest a SMM-based memory device, and provide fundamental insight into the electrical controllable manipulation and readout of the SMM's two spin-state of stored information.

Neural oscillation of single silicon nanowire neuron device with no external bias voltage

In this study, we perform simulations to demonstrate neural oscillations in a single silicon nanowire neuron device comprising a gated p–n–p–n diode structure with no external bias lines. The neuron device emulates a biological neuron using interlinked positive and negative feedback loops, enabling neural oscillations with a high firing frequency of ~ 8 MHz and a low energy consumption of ~ 4.5 × 10−15 J. The neuron device provides a high integration density and low energy consumption for neuromorphic hardware. The periodic and aperiodic patterns of the neural oscillations depend on the amplitudes of the analog and digital input signals. Furthermore, the device characteristics, energy band diagram, and leaky integrate-and-fire operation of the neuron device are discussed.

Effect of Diode Field on Time-Resolved Photoluminescence of CdTe-Based Solar Cells

Time-resolved photoluminescence (TRPL) measurements with externally applied voltage bias were performed on a variety of thin-film CdTe-based solar cells. These measurements revealed a substantial variation in luminescence decays with the application of external voltage bias in many of the samples. Experimental results were replicated with a MATLAB-based modeling tool. Decay variations are attributed to the changing strength of internal p-n junction electric fields as bias is modulated; this exercise, therefore, illustrates whether or not field effects influence a particular TRPL measurement and how. It is found via modeling that the field influence on TRPL decays is dependent on the cell's bulk minority carrier lifetime, mobility, and carrier concentration, as well as the laser excitation power during a measurement. While nearly insignificant in poorer quality legacy cells, the field effects are substantial in state-of-the-art CdTe cells with high lifetime, mobility, and doping, complicating determination of bulk lifetime. It is found that forward bias is effective in eliminating junction fields. It is also shown that fields can be effectively suppressed by increasing excitation power, but doing so is imprecise and can also introduce additional errors unrelated to fields. It is therefore concluded that lifetime is most accurately measured in completed cells under forward bias at low excitation intensity.