Biased Electrode Research Articles

Saturated cavity ringdown spectroscopy at Balmer-α lineof atomic hydrogen for estimating sheath electric field in plasma

We applied saturated cavity ringdown spectroscopy (CRDS) to the measurement of the Doppler-free absorption spectrum of the Balmer-α line of atomic hydrogen in an inductively coupled hydrogen plasma. The spectrum was used for estimating the electric field in the sheath region in the vicinity to a biased electrode. The absorption frequency (the absorption coefficient multiplied by the speed of light) and the saturation parameter were estimated by fitting the experimental ringdown curve with the theory reported by Giusfredi and coworkers (Phys. Rev. Lett. 104 (2010) 110801). We detected the Lamb dip corresponding to the 2p 2 P o 3/2 - 3d 2 D 5/2 transition in the absorption spectrum in the field-free condition. We observed the change in the Lamb dip spectrum when we repeated the measurement in the sheath, and we succeeded in estimating the sheath electric field of 220–230 V/cm at a distance of 1.8 mm from the electrode which was biased at -100 V with respect to the ground potential. The experimental results indicate the potential of saturated CRDS for the measurement of sheath electric fields in plasmas.

Graphene‐Enhanced Single Ion Detectors for Deterministic Near‐Surface Dopant Implantation in Diamond

AbstractDiamond color centers with applications to single photon sources, quantum computation, and magnetic field sensing down to the nanoscale have been investigated using ensembles of near‐surface implanted atoms. Deterministic ion implantation for ions stopping between 30 and 130 nm deep is demonstrated by configuring an electronic‐grade diamond substrate with a biased surface graphene electrode connected to charge sensitive electronics. The thin graphene electrode has a negligible surface dead layer, so implantation events are signaled from the drift of electron–hole pairs induced by the dissipation of ion kinetic energy in the substrate. Ion beam induced charge maps from a scanned 1 MeV He microbeam show the graphene electrode is highly effective with a charge collection efficiency up to 86% compared to an O‐terminated diamond surface of 30%. For lower energy ions, applicable to near surface implantation, single ion detection confidence is >99(1)% for 19.5 keV H and 85(2)% for 24 keV N ions to allow maps of device surface features by employing a focused ion beam microscope or an atomic force microscope cantilever incorporating a nanostencil beam collimator. In the near‐term, this system can be used to investigate the annealing dynamics for the conversion of single implanted ions into diamond color centers.

Near-Field Photodetection in Direction Tunable Surface Plasmon Polaritons Waveguides Embedded with Graphene.

2D materials have manifested themselves as key components toward compact integrated circuits. Because of their capability to circumvent the diffraction limit, light manipulation using surface plasmon polaritons (SPPs) is highly-valued. In this study, plasmonic photodetection using graphene as a 2D material is investigated. Non-scattering near-field detection of SPPs is implemented via monolayer graphene stacked under an SPP waveguide with a symmetric antenna. Energy conversion between radiation power and electrical signals is utilized for the photovoltaic and photoconductive processes of the gold-graphene interface and biased electrodes, measuring a maximum photoresponsivity of 29.2mAW-1 . The generated photocurrent is altered under the polarization state of the input light, producing a 400% contrast between the maximum and minimum signals. This result is universally applicable to all on-chip optoelectronic circuits.

Plasma sputtering of biased electrodes in an oblique magnetic field

The surface erosion of biased electrodes immersed in a radio frequency (RF) plasma was investigated in oblique magnetic fields ranging from 0.1 T to 3.5 T. The plasma potential and density in the vicinity of the biased electrode have been measured using an RF-compensated cylindrical Langmuir probe moved by a three-dimensional motorised arm in the ALINE facility at 0.1 T. The erosion profiles were determined for different gases (argon and helium), magnetic field strength and plasma volume for the same electrode surface area at an angle of 5∘ with the magnetic field in three different experimental geometries. The radial electric field (perpendicular to the magnetic field) in the plasma column has been deduced from plasma potential measurements. This radial field makes the ions converge to the core of the plasma column at grazing angles (30∘) and is reversed at larger angles. The ion flux on the sample increases with the magnetic field magnitude leading to a strong erosion inhomogeneity. At lower plasma volume, the erosion rate is sensitively enhanced. Local redeposition was identified using x-ray photoelectron spectroscopy on the surfaces exhibiting preferential deposition directions. Finally, the asymmetric erosion patterns obtained from a molybdenum layer etching are explained by the similar asymmetric density maps obtained with the probe and by the drift and the local sheath acceleration.

First H- beam extracted from the non-caesiated external RF-coil ion source at ISIS

A high power, high duty-cycle, non-caesiated negative hydrogen ion source is in development at the ISIS pulsed spallation neutron and muon facility. Stable operation of an inductively-coupled plasma pulsing at 50 Hz, 1 ms, 70 kW has been perfected with no apparent lifetime issues. Novel features compared to similar H- sources include a very low power microwave ignition gun, an adjustable permanent magnet filter field and multiple large 3D-printed components. This paper details the initial commissioning efforts required to generate a beam. An H- beam current of 10 mA was extracted on the first try in pure volume-production mode. Parameter sweeps and optimisation to reach the required 35 mA were put on hold to address ancillary equipment problems. Mitigation of RF pickup required major mechanical changes such as additional screening and magnetic shielding. Extraction voltage holding capabilities were investigated to cope with co-extracted electron current loading in excess of 0.7 A. Preliminary studies of the biased plasma electrode current showed that co-extracted electrons could be suppressed by increasing the magnetic filter field and gas flow.

Improvement of photoresist etching by impedance control of a bias electrode in an inductive discharge

To improve the photoresist (PR) etching in inductively coupled plasma (ICP) without the use of bias power, an impedance control unit is connected in series to the bias electrode of the ICP. This impedance control unit consists of an inductor and a variable capacitor to control the impedance of the bias electrode. With the series resonance of the sheath capacitance and inductance of the bias electrode impedance, the current flowing through the bias electrode increases dramatically, resulting in a higher voltage across the sheath and larger ion energy loss. The effect of series resonance on the decrease of electron density, as observed by a Langmuir probe, is illustrated by a global power balance model. As the capacitance approaches the series resonance condition, the larger ion energy leads to an enhanced PR etch rate, and the more uniform profile of the radio frequency (RF) plasma potential results in more uniform distributions of ion energy and PR etch rate. Additionally, the variation of the RF plasma potential profile is attributed to the enhanced capacitive electrical field in the ICP. These variation are analysed using an electromagnetic simulation.

Transient Photocurrent Response in a Perovskite Single Crystal‐Based Photodetector: A Case Study on the Role of Electrode Spacing and Bias

AbstractTransient photocurrent is a widely applied characterization technique to probe the charge‐carrier photogeneration and extraction dynamics in perovskite optoelectronic devices. Despite the large number of studies on the properties of perovskite single‐crystals (SCs) photodetectors (PDs), the underlying mechanism that governs the spectral line shape of transient photocurrent is not fully understood. Here, methylammonium lead bromide (MAPbBr3)SC based PDs are used to study the effect of different electrode spacing and bias on the transient photocurrent response under blue and green light irradiation. The observed differences in the spectral line shape of the transient photocurrent are explained using three‐step carrier transport model, which reveals the occurrence of carrier trapping and a recombination process in MAPbBr3 SC. The findings are further corroborated by intensity‐dependent photocurrent and impedance spectroscopy analysis of the resulting PDs. This work provides a basic insight into the origin of the different behavior of transient photocurrent response under variable electrode distance, bias, and irradiance light, which is expected to help to further understand and optimize the performance of perovskite based PDs.

Investigation of the dual-frequency bias effect on inductively coupled Cl2 plasmas by hybrid simulation

In the etching process, a bias source is usually applied to the bottom electrode in inductively coupled plasmas (ICPs) to achieve independent control of the ion flux and ion energy. In this work, a hybrid model, which consists of a global model combined bi-directionally with a fluid sheath model, is employed to investigate the dual-frequency (DF) bias effect on the inductively coupled Cl2 plasmas under different pressures. The results indicate that the DC self-bias voltage developed on the biased electrode is approximately a linear function of the phase shift between the fundamental frequency and its second harmonic, and the value only varies slightly with pressure. Therefore, the ion energy on the bottom electrode can be modulated efficiently by the bias voltage waveform, i.e. the fluctuation of the ion energy with phase shift is about 40% for all pressures investigated. Besides, the ion energy and angular distribution functions (IEADFs) in DF biased inductive discharges is complicated, i.e. the IEADFs exhibits a four-peak structure under certain phase shift values. Although the species densities and ion fluxes also evolve with phase shift, the fluctuations are less obvious, especially for Cl2 + ions at low pressure. In conclusion, the independent control of the ion energy and ion flux are realized in DF biased ICPs, and the results obtained in this work are of significant importance for improving the etching process.

Negative-to-Positive Tunnel Magnetoresistance in van der Waals Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 Junctions

The emergent van der Waals magnetic material is a promising component for spintronic devices with novel functionalities. Here, we report a transition of negative-to-positive magnetoresistance in Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 van der Waals all-magnetic tunnel junctions with increasing the applied bias voltage. A negative magnetoresistance is observed first in Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 tunnel junctions, where the resistance with antiparallel aligned magnetization of two Fe3GeTe2 electrodes is lower than that with parallel alignment, which is due to the opposite spin polarizations of two Fe3GeTe2 electrodes. With the bias voltage increasing, the spin polarization of the biased Fe3GeTe2 electrode is changed so that the spin orientations of two Fe3GeTe2 electrodes are the same. Our experimental observations are supported by the calculated spin-dependent density of states for Fe3GeTe2 electrodes under a finite bias. The significantly bias voltage-dependent spin transport properties in van der Waals magnetic tunnel junctions open a promising route for designing electrical controllable spintronic devices based on van der Waals magnets.

Test of ITk 3D sensor pre-production modules with ITkPixV1.1 chip

ITk detector, the new ATLAS tracking system at High Luminosity LHC, will be equipped with 3D pixel sensor modules in the innermost layer (L0). The pixel cell dimensions will be either 25 × 100 μm2 (barrel) or 50 × 50 μm2 (endcap), with one read-out electrode at the centre of a pixel and four bias electrodes at the corners. Sensors from pre-production wafers (50 × 50 μm2) produced by FBK have been bump bonded to ITkPixV1.1 chips at IZM. Bare modules have been assembled in Genoa on Single Chip Cards and characterized in laboratory and on beam.

Limitations of the independent control of ion flux and energy distribution function in high-density inductively coupled chlorine plasmas

Using a self-consistent plasma model coupled with Maxwell's equations, the limitations of independent control of ion fluxes and their energy distribution functions extracted from the high-density inductively coupled chlorine plasma are studied. Two extreme cases of discharge power are considered: 100 W and 1 kW. We find that in the low-power case, plasma is mainly generated by electromagnetic waves while the radio-frequency biased electrode primarily enables plasma ion extraction. Therefore, the ion fluxes and distribution functions are controlled independently. For the high-power case of 1 kW, the bias electrode significantly contributes to plasma generation but has only a small effect on sheath voltage. As a consequence, independent control of ion fluxes and distribution functions becomes impossible. Namely, the increase in the power driving the radio-frequency electrode leads to the increase in the ion fluxes but has little effect on their energy and angular distributions.

Analysis of ion orbits in front of a negative planar electrode immersed in an oblique magnetic field

The orbital motion approach is used to analyze the ion impact on a negatively biased planar wall immersed in a strongly magnetized plasma. It is assumed that the given homogeneous magnetic field forms a small angle with a planar negatively biased electrode, while the inhomogeneous electric field is perpendicular to the electrode. Spatial dependence of the electric field is modeled in such a way that the electric field exhibits two-scale behavior, which is characteristic of plasma sheath problems. The equation of motion of a singly charged deuterium ion is solved for a large variety of parameters and initial conditions. The effects of electrode bias, magnetic flux density, magnetic field angle, initial velocity, electric field scaling, and electrode bias are investigated. It is found that the impact angles of ions are distributed over a surprisingly wide range, and in a vast majority of cases, the angle of impact is several times larger than the magnetic field angle. The study is relevant for the analysis of ion flow to the electrodes in fusion plasmas, i.e., divertors of tokamaks.

Design of a W-Band Dielectric Phase Shifter Based on Liquid Crystal

In this paper, a W band (75–110 GHz) phase shifter based on the liquid crystal of continuous tunability and the three-electrode bias network of excellent electrical manipulation is designed. Taking Rexolite 1422 as the transmission dielectric, in which slots are dug and filled with liquid crystal. The voltage is regulated by the electrode bias network, and then the liquid crystal dielectric constant is changed to realize phase shift. The measured results show that the phase shift of the liquid crystal dielectric phase shifter can be up to 440° through the electrical control of the bias electrode network, which surpasses many designed phase shifters. In the whole W band, the insert loss is about 3 dB and the return loss is almost over 10 dB, emerging excellent matching.

Electron Scraping and Electron Temperature Reduction by Bias Electrode at the Extraction Region of a Large Negative Ion Source

The neutral beam test facility (NBTF), built in Consorzio RFX (Padua, Italy), aims at optimizing the ITER neutral beam injectors through dedicated devices. The facility hosts the full-size prototype of the ion source of the ITER injectors, source for the production of ions of deuterium extracted from a radiofrequency plasma (SPIDER), a radio frequency-based, cesium-assisted negative ion source. The negative ion beam is extracted by a triode with total acceleration voltage up to 108 kV; each grid is provided with 1280 apertures. On the plasma side, the plasma electrode [plasma grid (PG)] is partially covered by an additional electrode [bias plate (BP)] featuring wide openings of the same size as the beamlet groups. The PG and BP can be independently biased with respect to the ion source, influencing the uniformity of the beam and the amount of coextracted electrons. This contribution aims at studying the scraping effect of the electrons by the BP along the direction parallel to the filter field through numerical simulations. The possible role of the BP bias in reducing both the electron density and temperature in front of the extraction apertures and the correlation of the electron temperature with the bias of the electrodes are also discussed.

Effect of negatively biased electrode on two ion species plasma–wall transition, ion drag force, and levitation of dust particle

The effect of negatively biased electrodes on two ion species (argon and xenon) magnetized plasma–wall transition characteristics and the levitation of an isolated dust particle in the sheath region has been investigated using the kinetic trajectory simulation method based on a kinetic theory. It is found that the electrode biasing affects the transition parameters: space charge density, sheath potential, evolution of phase-space, and particle flux toward the electrode. The scale length of the Debye sheath region becomes widened for the increase in negative biasing and the presence of magnetic field as well. The biasing voltage and size of the dust particle have significant effect on the evolution of the dust charge, ion drag force, and levitation of a charged dust grain in the transition region. The dust particle is negatively charged at the particle injection side, and it acquires a positive charge for higher biasing voltage close to the electrode owing to electron depletion in that region. The distance of stable levitation from the electrode increases with the increase in the negative voltage applied to the electrode. Furthermore, the volumetric composition of two species of positive ions influence the dust charging process with the negative charge of the dust particle increasing as the concentration of xenon ions increases.

Development of photo-diodes for Pohang-Accelerator-Laboratory X-ray free-electron laser

The Pohang-Accelerator-Laboratory X-ray free-electron laser (PAL-XFEL) provides intense ultra-short X-ray pulses via a self-amplified spontaneous emission (SASE) process. Since fluctuations are induced by the intrinsic characteristics of the SASE process, we designed and fabricated photo-diodes (PDs) to be used in quadrant beam position and pop-in monitoring systems for measuring intensity and position, and beam size, respectively. Six 500μm-thick high-resistivity silicon wafers were used for different fabrication processes to vary the thicknesses of bias electrode and anti-reflective coating layers optimized for the quantum efficiency (QE) at typical wavelengths of 266 and 400 nm used for pump–probe experiments at the PAL-XFEL. In this paper, we present the electrical characteristics, QE, signal-to-noise ratio, energy resolution of the fabricated PDs, and compared them with those of commercial PD.

Suppression of runaway current by electrode biasing and limiter biasing on J-TEXT

The avoidance of runaway electrons (REs) generated during plasma disruption is of great concern for the safe operation of tokamak devices. Experimental study on the suppression of runaway current by electrode biasing (EB) and limiter biasing (LB) has been performed on the J-TEXT tokamak, which could be an alternative way to suppress the runaway current. The experimental results show that the higher the voltage value, the smaller the runaway current in both EB and LB experiments. The runaway current can be completely suppressed at an electrode biased voltage of +450 V and a limiter biased voltage of +300 V. The comparison of the energy spectra during the runaway plateau phase shows that the maximum energy max (E RE) and radiation temperature T HXR of hard x-rays (HXRs) are significantly reduced after the application of +200 V limiter biased voltage. The electric field generated by the biased voltage may be the key factor to suppress the runaway current, and the measured radial electric field increases obviously after the voltage is applied. This may result in an increase in the loss of REs to realize the suppression of runaway current.

(Invited) Cold Atmospheric Plasma Jet Sources with a Flexible Matrix

Plasma is an ionized gas containing free electrons, ions, neutral species (atoms, molecules, radicals), photons from UV to infrared wavelengths. Its composition makes it a rich environment for reactive process, showing a collective response to external perturbations. Low temperature plasmas are hot electron gas (1-10 eV) which are not in thermal equilibrium with the other cold part of the atomic or molecular species (0.02 eV). These cold plasmas are then suitable for applications to delicate surfaces (skin) without damage, especially since RONS (Reactive Oxygen and Nitrogen Species) show healing properties. Biomedicine is facing new challenges like decontamination, wound healing, and oncotherapy. Plasma medical treatments are selective, instantaneous, tunable in comparison to lasers and chemicals. There is a big need for developing new plasma sources that can treat complex surfaces (with asperities, sharp edges, cavities, complex accessibility)For this purpose, two types of flexible Cold Atmospheric Plasma Jet (CAPJ) sources with different geometries (planar and radial) were made and characterized through: multijet length and flow regime, electrical parameters, Optical Emission Spectroscopy (OES), performance after bending the aerogel. He (99.995%) was used in both configurations. Finite Element Method simulations were run to predict and understand the electromagnetic process in this plasma jet configuration.In planar geometry, DBD (Dielectric Barrier Discharge) was constituted of a silica aerogel placed between two biased electrodes. Silica aerogel has a very low gas permeability and is mechanically stable (highly cross-linked fiber glass network). Plasma jets constituted of 3 aligned jets, a few mm long above the DBD region whose length is proportional to the He flow rate, flowing in laminar regime, were produced by Helium flow (flow rate: 1-4 slm) subjected to sinusoidal voltage of 5kV and 15kHz frequency. Higher flow rate produced turbulent flows. The ratio between gas velocity and discharge voltage can be adjusted: short jets (flow limited operation) are suitable for near surface treatments, longer jets (power limited operation) are appropriate for greater distance treatments. OES showed strong N2 emission. Planar DBD was studied in flat configuration then its flexibility was characterized in bent (concave, convex) configurations. Compliance of the plasma source is limited by aerogel size and thickness. There is possibility to extend the device from 1D to 2D (matrix of plasma jet lines).In radial geometry, DBD was constituted of a soft hollow cylinder (about 5 cm external diameter, 2.5 cm internal diameter and 10 cm long), made of polyethylene (PE) foam, material selected for its flexibility and low permeability. An aluminum/dielectric coated rod electrode was placed in the core of the cylinder, copper circular electrodes were disposed on the external cylinder wall at 15 mm distance from each other surrounding 1mm holes punched in the PE cylinder, where the CAPJs were formed. In this geometry, two different operations modes were apprehended: comb mode was represented by four equidistant axial CAPJs and round brush mode was experienced when three CAPJs were aligned at constant azimuthal angle. In this case, discharges were triggered at 15 kHz AC voltage and a lower voltage than in planar configuration (3.8 kV). The jets were uniform in length and intensity. OES were recorded in the active region (inside the PE housing) and in the afterglow region. In the afterglow region, the spectrum was similar to the one obtained in planar geometry. A preponderance of hydroxyl groups (OH), nitrogen molecules (N2, N2 +), He atoms was found. The source remains stable after deformation of the PE housing.The feasibility of these flexible plasma sources in radial and axial configurations is promising for treating any kind of surface with special topography.

Plasmoelectric Potential in Plasmon-Mediated Electrochemistry.

Plasmon-mediated chemical reactions have attracted intensive research interest as a means of achieving desirable reaction yields and selectivity. The energetic charge carriers and elevated local temperature induced by the nonradiative decay of surface plasmons are thought to be responsible for improving reaction outcomes. This study reports that the plasmoelectric potential is another key contributor in plasmon-mediated electrochemistry. Additionally, we disclose a convenient and reliable method for quantifying the specific contributions of the plasmoelectric potential, hot electrons, and photothermal heating to the electroreduction of oxygen at the plasmonic Ag electrode, revealing that the plasmoelectric potential is the dominating nonthermal factor under short-wavelength illumination and moderate electrode bias. This work elucidates novel mechanistic understandings of plasmon-mediated electrochemistry, facilitating high-performance plasmonic electrocatalyst design optimization.

Tomographic and centroid reconstructions of plasma emission on C-2W via enhanced 300-channel bolometry system.

The C-2W experimental device at TAE Technologies utilizes neutral beam injection and edge biasing to sustain long-lived, stable field reversed configuration (FRC) plasma. An ongoing effort is under way to optimize the electrode biasing system, which provides boundary control to stabilize the FRC. To this end, tomography offers a powerful and non-invasive technique as tomographic reconstruction of the FRC emission profile provides an important assessment of global stability. Recently, a new signal acquisition system was implemented on a bolometer array dedicated to tomography on C-2W, significantly enhancing the signal-to-noise of the collected data. The array consists of 300 simultaneously digitized photodiode channels that respond to a broad range of wavelengths, from soft x-ray to near-infrared, as well as energetic particles, yielding 180 unique lines of sight that intersect a toroidal plane of the FRC near the mid-plane. Utilizing the collected photo-signals from a set of plasma discharges in which the electrode biasing was intentionally terminated mid-shot, time-resolved reconstruction of the plasma emissivity is achieved via pixel-based 1D and 2D tomographic algorithms, revealing sharply annular profiles with a clear magnetohydrodynamic (MHD) mode structure. In addition, reconstruction of the plasma center-of-emission trajectories via a centroid algorithm applied to the same set of discharges demonstrates a cyclical plasma wobble. Crucially, both the tomography reconstruction and centroid reconstruction indicate an n = 1 toroidal mode that reverses from the electron diamagnetic direction to the ion diamagnetic direction and grows in amplitude after bias termination, qualitatively consistent with the expected stabilizing effect of electrodes.