RF Plasma Source Research Articles

Growth and Photoelectrochemical Characterization of Epitaxial ZnTe Photocathodes for Carbon Dioxide Reduction

We report on the growth and photoelectrochemical (PEC) characterization of single crystal, epitaxial zinc telluride (ZnTe) thin film photocathodes. ZnTe is a II-VI semiconductor that has promising photocathode characteristics, including its suitable band gap alignments to CO2 reduction potentials, chemical stability, strong p-type character, and is theorized to have surface sites suitable for CO2 reduction. To gain a deeper understanding of the fundamental charge transport of the reactivity mechanism for ZnTe, we utilized molecular beam epitaxy (MBE) to grow single crystal thin film photocathodes. This work focuses largely on the synthesis and characterization of degenerately-doped ZnTe films on three different gallium arsenide (GaAs) substrate orientations: (100), (110), and (111)A. ZnTe thin films with a thickness of 300 nm, grown in the temperature range of 340–360ºC, were doped with nitrogen via an in situ RF plasma source and characterized by RHEED, XRD, AFM, and Hall effect measurements. The epitaxial films exhibited p-type characteristics with doping concentrations of 1018 to 1020 cm-3. Photoelectrochemical stability and catalytic selectivity of the ZnTe photocathodes were investigated in a three-electrode compression cell in a CO2-saturated electrolyte (0.1 M KHCO3). Utilizing degenerately doped (100) ZnTe, we have demonstrated high (60%) selectivity for CO2 reduction to CO in the absence of cocatalysts. Overall, we showed the correlation between the CO2R selectivity and ZnTe orientation and this work can be leveraged for developing catalyst-free tandem ZnTe-based photocathodes for carbon dioxide reduction.

Effect of gas injection pattern on magnetically expanding rf plasma source

Abstract Argon gas is injected from a back plate having either a radial center hole or shower-patterned eight holes into a 13.3-cm-diameter and 25-cm-long radio frequency (rf) plasma source attached to a 43.7-cm-diameter and 65cm-long diffusion chamber under an expanding magnetic field, which resembles the magnetic nozzle rf plasma thruster. The source has a double-turn loop antenna powered by a 13.56 MHz rf generator at a maximum power level of ~2.8 kW in low-pressure argon, providing a plasma density of about 1018 m−3 in the source. A high plasma density and a slightly low electron temperature are obtained for the shower-pattered case in both the source tube and the diffusion chamber, compared with the center hole case, suggesting that the neutral density profile significantly affects the plasma density profile. This result will provide an improvement in the thruster performance by the gas injection pattern.

Plasma Cleaning of Hydrocarbon and Carbon Contaminated Surfaces of Accelerator Components

To achieve the vacuum quality required for the operation of particle accelerators, the surface of the vacuum vessels must be clean from hydrocarbons. This is usually achieved by wet chemistry processes, e.g., degreasing chemical baths that, in case of radioactive vessels, must be disposed accordingly. An alternative way exploits the oxygen plasma produced by a downstream RF plasma source. This technique offers the possibility of operating in-situ, which is an advantageous option to avoid the handling of voluminous and/or fragile components and a more sustainable alternative to large volume disposable baths. In this work, we test a commercial plasma source in dedicated vacuum systems equipped with quartz crystal microbalances (QCMs). The evolution of the etching rates of amorphous carbon (a-C) thin films deposited on the QCMs to mimic contamination are studied as function of operating parameters. We present the results of the plasma cleaning process applied to the real case of a hydrocarbons-contaminated large vacuum vessel. The studies are complemented by transport simulations and surface contamination monitoring by X-ray photoelectron spectroscopy (XPS) analysis. The evaluation of the vessel cleanliness, which is performed via residual gas analysis (RGA) measurements, is based on CERN’s outgassing acceptance criteria and agrees with both simulations and XPS results.

INFLUENCE OF DOPING WITH CARBON AND NITROGEN ON THE PHOTOACTIVITY OF TiO2 THIN FILMS OBTAINED WITH MePIIID

Titanium dioxide is well known as a photoactive material activated under ultraviolet irradiation. It is either employed as a photocatalyst or it exhibits superhydrophilic behavior after obtaining reduced surface energy under illumination used for self-cleaning or anti-fogging surfaces. For increasing the reactivity of thin films under solar illumination, a reduced band gap is desired. Doping with transition metals or nitrogen has been reported in the literature. However, the incorporation of nitrogen into a growing film is a much more complex process that is presently not completely understood. TiO2 thin layers were produced by metal plasma immersion ion implantation and deposition at room temperature at a pulse voltage of 0–5 kV and a duty cycle of 9 % for an apparently amorphous layer. An auxiliary rf plasma source was employed to increase the growth rate at low gas flow ratios. By adjusting the geometry between the incident ion beam, sputter target and substrate independently of the primary ion energy and species, a controlled deposition of samples was possible. Conventional ion implantation was employed to implant either carbon or nitrogen ions below the surface for bandgap engineering. The resulting thin films were subsequently investigated for optical properties, stoichiometry, structural properties, surface topography and photoactivity.

Effect of rf driving frequency on peripheral high energy electrons in a magnetically expanding plasma reactor

Two operational rf driving frequencies of 2 and 13.56 MHz are employed in a 14-cm-diameter radio frequency (rf) plasma source under an expanding magnetic field. The changes in the radial profiles of the ion saturation current and the electron temperature are observed in the magnetically expanding plasma when changing the driving frequency. Peripheral high temperature electrons are detected for the higher frequency case, which is consistent with previous studies, implying a localized electron heating in the radially outer region near the antenna and a transport along the magnetic field. However, it disappears when lowering the rf driving frequency, which would be due to an increase in a skin depth. Therefore, the present results demonstrate that the rf power would be absorbed in radially outer and entire regions of the discharge tube for the higher and lower rf driving frequency cases, respectively. As a result of the ionization induced by the peripheral high temperature electrons in the expanding magnetic field, the density in the expanding magnetic field for the 13.56 MHz case is higher than the 2 MHz case, resulting in the larger thrust as measured by a pendulum target technique.

Physical Properties of a Low-Power Helicon Source Operating on a High-Frequency Discharge with a Capacitive Component

The results of an experimental study of a low-power RF plasma source placed in a longitudinal magnetic field (helicon thruster), when it operates on a capacitive RF discharge and inductive RF discharges with a capacitive component, are presented. A significant dependence of the characteristics of the ion and electron fluxes of the source on the induction of a constant magnetic field is shown. The fundamental applicability of capacitive RF discharge as a working process in the studied plasma source is demonstrated. It is shown that the increase in the average energy of ions in the flow at the outlet of the source with the appearance of the capacitive component of the discharge is slight.

Experimental demonstration of the strong impact of plasma excitation frequency range on electronic properties of silicon nitride/GaAs interfaces

Passivation/encapsulation of III-V electronic and photonic circuits and devices is often performed using plasma-enhanced chemical vapor deposited SixNy with a standard high frequency excitation of 13.56 MHz. The hypothesis of a possible higher passivation process efficiency for low frequency SixNy deposition on GaAs has been suggested by comparison with published work on high frequency. In this work, we report a direct experimental demonstration of the effect of the RF plasma source frequency on electronic properties of GaAs/SixNy interfaces during a passivation process. GaAs substrates from the same wafer are processed in the same reactor using the same plasma conditions except for the plasma excitation frequency and MIS structures are fabricated to probe interface electronic properties. Comparing interfaces obtained with a plasma source frequency below or over the ion transit frequency (∼ 2 MHz) shows drastic difference in electrical behavior. While Fermi level is pinned when a high frequency plasma (13.56 MHz) is used, a good modulation of surface potential is observed with a low frequency plasma (380 kHz), indicating an interface states density lower by orders of magnitude. X-ray photoelectron spectra analysis is performed in order to investigate GaAs/SixNy interface formation in relation with plasma excitation frequency. We show that a short exposure to the low frequency plasma results in the almost complete removal of native oxides species. To the contrary, a significant amount of oxides remains on structures treated by the high frequency technique. The low frequency plasma technique could be a better alternative to the standard high frequency, as it is less reliant on wet chemical treatments like ammonium sulfide passivation and could allow for processing at lower temperatures.

Initial Properties of Steady State RF Plasma Source by Two Turn Flat Loop Antenna for DEMO Relevant Divertor Simulation Experiment

Initial Properties of Steady State RF Plasma Source by Two Turn Flat Loop Antenna for DEMO Relevant Divertor Simulation Experiment

Controlling the size and density of InN quantum dots formed on sapphire substrate by droplet epitaxy

The growth of InN quantum dots (QDs) on c-plane sapphire by droplet epitaxy (DE) using radio frequency plasma-assisted molecular beam epitaxy is reported here. The QD growth process from liquid In droplets to the InN QDs is described with a focus on the effect ambient nitrogen from an active RF-plasma source has on the formation of In droplets as a function of substrate temperatures. The variation in the shape and size of InN QDs is explained in terms of the In atom surface diffusion and the migration of droplets. Additionally, two nitridation procedures were used to investigate the crystallization of In droplets. The droplet formation was determined to follow well known principles of nucleation theory with ripening. The resulting activation energy for In surface diffusion on sapphire was found to be 0.62 ± 0.07 eV in ultra-high vacuum, ∼10−10 Torr, and 0.57 ± 0.08 eV in ambient N2, ∼10−5 Torr. The growth of InN QDs using the DE method has many advantages over the classical Stranski–Krastanov technique, including the ability to control a wide range of QD shapes, sizes, and densities.

A RF plasma source with focused magnetic field for material treatment

A RF plasma source with focused magnetic field for material treatment

Thirty percent conversion efficiency from radiofrequency power to thrust energy in a magnetic nozzle plasma thruster

Innovations for terrestrial transportation technologies, e.g., cars, aircraft, and so on, have driven historical industries so far, and a similar breakthrough is now occurring in space owing to the successful development of electric propulsion devices such as gridded ion and Hall effect thrusters, where solar power is converted into the momentum of the propellant via acceleration of the ionized gases, resulting in a high specific impulse. A magnetic nozzle (MN) radiofrequency (rf) plasma thruster consisting of a low-pressure rf plasma source and a MN is an attractive candidate for a high-power electric propulsion device for spacecraft, as it will provide a long lifetime operation at a high-power level due to the absence of an electrode exposed to the plasma and a high thrust density. The high-density plasma produced in the source is transported along the magnetic field lines toward the open-source exit and the plasma is then spontaneously accelerated in the MN. By ejecting the plasma flow from the system, the reaction forces are exerted to the thruster structure including the source and the MN, and the spacecraft is resultantly propelled. The thruster will open the next door for space technologies, while the performance of the MN rf plasma thruster has been lower than those of the mature electric propulsion devices due to the energy loss to the physical walls. Here the thruster efficiency of about 30%, being the highest to date in this type of thruster, is successfully obtained in the MN rf plasma thruster by locating a cusp magnetic field inside the source, which acts as a virtual magnetic wall isolating the plasma from the source wall. The increase in the thrust by the cusp can be explained by considering the reductions of the loss area and the plasma volume in a thrust analysis combining a global source model and a one-dimensional MN model.

Characterization of a 2 MHz-radiofrequency-driven magnetically expanding plasma source

A radio frequency (rf) plasma source is operated at 2 MHz under an expanding magnetic field. The source consists of a 14-cm-diameter and 25-cm-long source tube wound by a seven-turn rf loop antenna powered via an impedance matching circuit. The efficient rf power transfer to the plasma is demonstrated, and the high-density plasma above 1012 cm−3 is successfully obtained for a kW level of the rf power. The radial density profile having a center peak is observed both in the source and in the expanding magnetic field; it does not show the conical density structure previously observed in 13.56 MHz rf sources. This implies that the electron heating layer is changed by lowering the driving frequency. Even in the source operated at 2 MHz, the measurement of the ion energy distribution shows the presence of the supersonic ion beam as well as the sources operated at higher frequencies, e.g., 13.56 MHz in the previous studies.

Ion flux–energy distributions across grounded grids in an RF plasma source with DC-grounded electrodes

We present an experimental investigation of the ion flux–energy distribution functions (IFEDFs) obtained across grounded grids in an asymmetric capacitively coupled RF source using a helium discharge. The powered electrode in the RF source is DC-grounded via a λ/4 filter, which lifts its DC potential to zero. Grids of different dimensions (hole width, thickness, and geometric transparency) were used to confine the plasma, while the IFEDF of the ion beam departing the grid and reaching the reactor walls was studied using a retarding field energy analyser. The IFEDF obtained was double-peaked, indicating the presence of fast ions arriving from the plasma source, and cold ions generated upon charge exchange collisions between the fast ions and neutrals. The flux, as well as the peak energies of the two ion groups, depended significantly on the process parameters: RF power, He pressure, the distance between grids and walls, and the dimensions of the grids. The results indicate that confining plasma with grids can reduce the ion flux at the walls by over 60%, significantly lowering the wall sputtering rate. This was confirmed with a dedicated long-exposure plasma discharge with a gridded plasma reactor, wherein less than 1 nm of Cu deposition was found on the DC-grounded powered electrode, and the surface reflectivity was preserved to pristine values. In contrast, a similar experiment in a gridless reactor resulted in Cu deposition of 35 nm with a drastic drop in surface reflectivity. These studies are of great importance for the application of similar RF plasma sources with in-situ cleaning of diagnostic mirrors in fusion devices, as well as in a variety of plasma processing applications.

Quantifying erosion and retention of silicon carbide due to D plasma irradiation in a high-flux linear plasma device

Abstract Silicon carbide (SiC) may be a viable option for future plasma-facing components (PFCs) due to its low hydrogenic diffusivity, high temperature strength, and mechanical resilience to neutron damage (Causey et al., 1978). The erosion and retention properties of SiC were quantified via deuterium plasma exposures in the PISCES-E RF plasma source on SiC-coated graphite samples at impact energies between 20 eV and 90 eV, surface temperatures of 500 K and 950 K, and fluences between 0.4 and 1.0 × 1024 m−2. The chemical sputtering yield of carbon from SiC was estimated by optical spectroscopy, varying between 0.0012 and 0.0083 depending on the deuterium impact energy. Chemical sputtering yields from graphite were 4× higher, on average, than yields from SiC and were largely consistent with previous analytic formulations. Chemical erosion of silicon atoms from SiC was not detected from the SiD molecular band, but the lack of Si surface enrichment at low Ei suggests that a non-collisional Si erosion source may be present. The retention of implanted deuterium in SiC was ~2× higher than that in tungsten at 500 K. Most D retained in SiC was desorbed at a peak temperature ~1000 K, and the desorption rate only varied slightly with impact energy and surface temperature. Fundamental differences in desorption behavior between Si, graphite, and SiC samples suggested that the SiC cubic lattice possessed unique trapping sites that cannot solely be attributed to Si-D or C-D bonds. New questions regarding preferential erosion and uncharacterized defects motivate expanded testing in linear and toroidal devices.

Magnetic nozzle radiofrequency plasma thruster approaching twenty percent thruster efficiency

Development of a magnetic nozzle radiofrequency (rf) plasma thruster has been one of challenging topics in space electric propulsion technologies. The thruster typically consists of an rf plasma source and a magnetic nozzle, where the plasma produced inside the source is transported along the magnetic field and expands in the magnetic nozzle. An imparted thrust is significantly affected by the rf power coupling for the plasma production, the plasma transport, the plasma loss to the wall, and the plasma acceleration process in the magnetic nozzle. The rf power transfer efficiency and the imparted thrust are assessed for two types of rf antennas exciting azimuthal mode number of m=+1 and m=0, where propellant argon gas is introduced from the upstream of the thruster source tube. The rf power transfer efficiency and the density measured at the radial center for the m=+1 mode antenna are higher than those for the m=0 mode antenna, while a larger thrust is obtained for the m=0 mode antenna. Two-dimensional plume characterization suggests that the lowered performance for the m=+1 mode case is due to the plasma production at the radial center, where contribution on a thrust exerted to the magnetic nozzle is weak due to the absence of the radial magnetic field. Subsequently, the configuration is modified so as to introduce the propellant gas near the thruster exit for the m=0 mode configuration and the thruster efficiency approaching twenty percent is successfully obtained, being highest to date in the kW-class magnetic nozzle rf plasma thrusters.

Vertical silicon etching by using an automatically and fast-controlled frequency tunable rf plasma source

An automatically and fast-controlled frequency tunable radiofrequency (rf) system is employed to a plasma etching device, where the rf system contains two rf amplifiers operational in 37 MHz–43 MHz for a plasma source and a wafer stage. Both impedance matching circuits for the source and the wafer stage have no variable capacitors, leading a compact design of the rf system; the power reflection can be minimized by adjusting the frequencies. The rf frequency and the output power are automatically controlled so as to minimize a reflection coefficient and to maintain a constant net power corresponding to a forward power minus a reflected power for both the source and the stage. The source is operated with SF6 and C4F8 gases for silicon etching and passivation in the Bosch process, respectively. For both the cases, the impedance tuning can be accomplished within several ms and the net power is maintained at a constant level. By alternatively switching the SF6 and C4F8 plasmas with pulse widths of 5 s and 2 s, respectively, a vertical silicon etching is performed, where a scallop structure is clearly formed on the etching side wall. By shortening the pulse widths down to 1 s and 0.4 s for the SF6 and C4F8 plasmas, the size of the scallop structure is significantly reduced; the usability of the automatically and fast-controlled rf plasma source for the Bosch process is demonstrated.

Study on the laser collisional induced fluorescence spectrum of helium plasmas

• Laser induced fluorescence and laser collisional induced fluorescence spectra of helium plasma were measured. • Peak heights of spectral lines for the 33S-23P, 33P-23S transitions were decreased despite of the laser irradiation. • The behaviors of laser collisional induced fluorescence spectra were discussed using helium collisional-radiative model. Helium (He) RF plasma source was configured to study laser collisional induced fluorescence (LCIF) spectrum of He plasma. When laser beam with wavelength of 396.5 nm resonant to the 2 1 S − 4 1 P transition line was incident on He plasmas LCIF spectral lines of singlet state transitions as well as triplet state transitions were measured. The peak heights of spectral lines for the 3 3 S − 2 3 P , 3 3 P − 2 3 S , and 3 1 P − 2 1 S transitions decreased despite of laser irradiation. The change in He LCIF spectra by the singlet-to-triplet transition were discussed using collisional-radiative model for LCIF process.

Low-loss superconducting titanium nitride grown using plasma-assisted molecular beam epitaxy

Titanium nitride (TiN) is a known superconducting material that is attractive for use as passive components in superconducting circuits for both conventional and quantum information devices. In contrast to conventional synthesis techniques, here, plasma-assisted molecular beam epitaxy is reported to produce high-quality TiN on bare silicon wafers. Using a rf-plasma source to crack the nitrogen molecules and a conventional high-temperature effusion cell for titanium, TiN growth is completed under nitrogen-rich conditions. The growth and nucleation is monitored in situ, while the structure and composition are characterized using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and scanning transmission electron microscopy. The stoichiometric TiN (111) films sit on an amorphous nitride layer with low impurity concentrations. The films superconduct with Tc=5.4 K, and coplanar waveguide resonators are fabricated with a small center width of 6 μm that demonstrate single-photon quality factors approaching 1M and high-power quality factors over 5M without observing saturation.

Fast and Automatic Control of a Frequency-Tuned Radiofrequency Plasma Source

A frequency-tuned radiofrequency (rf) plasma source is automatically controlled to minimize the rf power reflection and to maintain the constant net rf power corresponding to the forward power minus the reflected power. The experiment is performed with a power amplifier operational for the frequency of 37 - 43 MHz and a compact helicon source consisting of a loop antenna, a solenoid, and an insulator tube. The rf power is supplied to the rf antenna via an impedance matching circuit consisting of only fixed small ceramic capacitors. It is demonstrated that the reflection coefficient of the rf power is minimized within ~10 msec and the net power is successfully maintained at constant.

Minimal multi-target plasma sputtering tool

Abstract A multi-target plasma sputtering tool is developed for a recently progressed new concept of a semiconductor fab system called ‘Minimal Fab’. A frequency-tuning compact helicon plasma source is attached on the side wall of the compact vacuum chamber and the plasma is transported along curved magnetic field lines to a target surface, which is mounted on a rotational target holder and negatively biased by a dc power supply. The target material is sputtered by the ions accelerated by the sheath potential drop and deposited on the substrate facing the target. The target material can be changed by rotating the holder; thereby the multi-layered metallic film can be formed in the single chamber. Substrate heating induced by secondary electrons emitted from the target surface and accelerated by the sheath is successfully inhibited by a magnetic filter near the substrate. The performance of the sputtering reactor designed as a laboratory model is shown here. The sputtering reactor and all of the components required for the sputtering tool are successfully contained within a compact frame of Minimal Fab (29.5 cm in width, 45 cm in depth, and 144 cm in height) via the development of the compact reactor, especially the compact rf plasma source.