Hydrogen Plasma Research Articles

Generation of Short-scale Electrostatic Fields in the Solar Atmosphere and the Role of Helium Ions

Abstract Theoretical models are presented to show that expansion of plasma in the radial direction from a denser solar surface to a rarefied upper atmosphere with short-scale inhomogeneous field-aligned flows and currents in the form of thin threads itself is an important source of electrostatic instabilities. Multifluid theory shows that the shear flow–driven purely growing electric fields appear in the transition region. On the other hand, plasma kinetic theory predicts that the short-scale current sheets (or filaments) produce current-driven electrostatic ion acoustic (CDEIA) waves in the hydrogen plasma of the transition region that damp out in the system through wave–particle interactions and increase the temperature. Similar processes take place in the solar corona and act positively for increasing the temperature further and maintaining it. The shear flow–driven instabilities and CDEIA waves have short perpendicular wavelengths of the order of 1 m and low frequencies of the order of 1 or several Hz when the ions’ shear flow scale length is considered to be of the order of 1 km. It is pointed out that the purely growing fluid instabilities turn into oscillatory instabilities and the growth rates of kinetic CDEIA wave instabilities are reduced when the dynamics of 10% helium ions is taken into account along with 90% hydrogen ions. Therefore, the role of helium ions should not be ignored in the study of wave dynamics in solar plasma.

Plasma radiation behavior approaching high-radiation scenarios in W7-X

The W7-X stellarator has so far performed experiments under both limiter and divertor conditions. The plasma is mostly generated by ECR-heating with powers up to 6.5 MW, and the plasma density is usually limited by the radiation losses from low-Z impurities (such as carbon and oxygen) released mainly from the graphite targets. The present work first summarizes the radiation loss fractions f rad achieved in quasi-stationary hydrogen plasmas in both operational phases, and then shows how impurity radiation behaves differently with the two different boundary conditions as the plasma density increases. The divertor operation is emphasized and some beneficial effects (with respect to impurity radiation) are highlighted: (1) intensive radiation is located at the edge (r/a > 0.8) even at high radiation loss fractions, (2) the plasma remains stable up to f rad approaching unity, (3) the reduction in the stored energy is about 10% for high f rad scenarios. Moreover, effects of wall boronisation on impurity radiation profiles are also presented.

Boron-Doped Diamond/GaN Heterojunction-The Influence of the Low-Temperature Deposition.

We report a method of growing a boron-doped diamond film by plasma-assisted chemical vapour deposition utilizing a pre-treatment of GaN substrate to give a high density of nucleation. CVD diamond was deposited on GaN substrate grown epitaxially via the molecular-beam epitaxy process. To obtain a continuous diamond film with the presence of well-developed grains, the GaN substrates are exposed to hydrogen plasma prior to deposition. The diamond/GaN heterojunction was deposited in methane ratio, chamber pressure, temperature, and microwave power at 1%, 50 Torr, 500 °C, and 1100 W, respectively. Two samples with different doping were prepared 2000 ppm and 7000 [B/C] in the gas phase. SEM and AFM analyses revealed the presence of well-developed grains with an average size of 100 nm. The epitaxial GaN substrate-induced preferential formation of (111)-facetted diamond was revealed by AFM and XRD. After the deposition process, the signal of the GaN substrate is still visible in Raman spectroscopy (showing three main GaN bands located at 565, 640 and 735 cm−1) as well as in typical XRD patterns. Analysis of the current–voltage characteristics as a function of temperature yielded activation energy equal to 93.8 meV.

Characterization of isotope effect on ion internal transport barrier and its parameter dependence in the Large Helical Device

In this paper, the background physics of the isotope effects in the ion internal transport barrier (ITB) are discussed in detail. An heuristic criterion for the ITB strength is defined based on the nonlinear dependence of the ion thermal diffusivity on the local ion temperature in the L-mode phase. Comparing deuterium plasmas and hydrogen plasmas, two isotope effects on the ion ITB are clarified: stronger ITBs formed in the deuterium plasmas and an ITB concomitant edge confinement degradation in the hydrogen plasmas. Principal component analysis reveals that the ion ITB becomes strong when a high input power normalized by the line averaged electron density is applied and electron density profile is peaked. A gyrokinetic simulation suggests that the ITB profile is determined by the ion temperature gradient driven turbulence, while the way the profile saturates in L-mode plasmas is unknown. In the electron density turbulence behavior, a branch transition is observed, where the increasing trend in turbulence amplitude against the ITB strength is flipped to a decreasing trend across the ITB formation. The radial electric field structure is measured by the charge exchange recombination spectroscopy system. It is found that the radial electric field shear plays a minor role in determining the ITB strength.

L–H transition threshold studies in helium plasmas at JET

We present a study of the power threshold for L–H transitions (P LH) in almost pure helium plasmas, obtained in recent experiments at JET with an ITER-like wall (Be wall and W divertor). The most notable new result is that the density at which P LH is minimum, , is considerably higher for helium than for deuterium and hydrogen plasmas. We discuss the possible implications for ITER in its pre-fusion operating power phase.

Hydrogenated Amorphous TiO2-x and Its High Visible Light Photoactivity.

Hydrogenated crystalline TiO2 with oxygen vacancy (OV) defect has been broadly investigated in recent years. Different from crystalline TiO2, hydrogenated amorphous TiO2−x for advanced photocatalytic applications is scarcely reported. In this work, we prepared hydrogenated amorphous TiO2−x (HA-TiO2−x) using a unique liquid plasma hydrogenation strategy, and demonstrated its highly visible-light photoactivity. Density functional theory combined with comprehensive analyses was to gain fundamental understanding of the correlation among the OV concentration, electronic band structure, photon capturing, reactive oxygen species (ROS) generation, and photocatalytic activity. One important finding was that the narrower the bandgap HA-TiO2−x possessed, the higher photocatalytic efficiency it exhibited. Given the narrow bandgap and extraordinary visible-light absorption, HA-TiO2−x showed excellent visible-light photodegradation in rhodamine B (98.7%), methylene blue (99.85%), and theophylline (99.87) within two hours, as well as long-term stability. The total organic carbon (TOC) removal rates of rhodamine B, methylene blue, and theophylline were measured to 55%, 61.8%, and 50.7%, respectively, which indicated that HA-TiO2−x exhibited high wastewater purification performance. This study provided a direct and effective hydrogenation method to produce reduced amorphous TiO2−x which has great potential in practical environmental remediation.

(Invited) Chemical Approaches to Etch and Pattern Copper Films

Microelectronic device fabrication has traditionally been performed by selective removal or etching of material from deposited films to create patterns. Plasma etching has been the mainstay of this process since the early 1970s. The introduction of copper (Cu) metallization (~1997) by IBM resulted in the development of additive approaches (e.g., damascene process) due to the inability to plasma etch Cu. Despite attempts to plasma etch Cu as early as 1980, the only viable plasma-based methods in this time frame were inert gas sputtering, where selectivity to underlying layers and low etch rate were problematic, or chlorinated vapors that required elevated temperatures (>200 C) or significant ion bombardment energies to volatilize Cu chlorides. Both approaches required a hard mask rather than photoresist layers to ensure mask stability and precise pattern transfer. Due to the inability to form high volatility Cu halides that could be removed readily by plasma processes, other non-plasma or partial plasma approaches to Cu etching at lower temperatures were investigated. Addition of a UV lamp to facilitate excitation and desorption of volatile Cu chlorides in plasma etching was invoked. Formation and volatilization of copper by oxidation followed by complexation with organic ligands at lower (~150 C) temperatures was first reported in the mid-1990s, but this processes generated isotropic etch patterns; more recent approaches have reported anisotropy. Plasma chlorination followed by a liquid etch to remove the chlorinated Cu was also reported; due to the directionality of the plasma chlorination step, anisotropic profiles resulted. Hydrogen-based etching of Cu films was reported in 2010, initially with a two-step process consisting of plasma chlorination followed by a hydrogen plasma step to remove volatile products. This effort subsequently demonstrated that etching by pure hydrogen in a one-step process was possible albeit with low etch rates; etch rate increases were observed with vapor additives. Pure hydrogen plasmas etched photoresist layers, thereby requiring hard masks, but additives allowed employment of photoresist as a mask layer. Anisotropic etch profiles from these hydrogen-based processes were possible, in part due to additives protecting sidewalls and photoresist from etching and to ion bombardment during etching.This presentation will describe the development of Cu etching and patterning methods beginning in the 1970s. Mechanistic considerations of the various processes reported will be discussed and the current needs and limitations of these approaches indicated.

(Invited) Bayesian Optimization of Passivating Contacts for Crystalline Silicon Solar Cells

We attempted to utilize Bayesian optimization to realize high-performance crystalline silicon solar cells with passivating contacts, and demonstrates successful application to optimize structural parameter and fabrication process of titanium oxide/silicon oxide/crystalline silicon heterostructure.Bayesian optimization works by constructing a posterior distribution of unknown objective functions based on prior observations. By appropriate choice of the strategy and algorithm, Bayesian optimization could permit reaching the maximum (or minimum) of the unknown objective function by a small number of iterations. This approach is useful when the number of parameters is large and the cost of experiments is high. Formation of titanium oxide/silicon oxide/crystalline silicon heterostructure, which is a promising candidate for passivating contacts, satisfies such a condition since we need to optimize pre-deposition treatment parameters, deposition parameters and post-process parameters.In fact, we optimized three kinds of chemical solution to form silicon oxide interlayer, atomic layer deposition cycle to control the titanium oxide thickness, and process parameters for hydrogen plasma treatment. The parameters include the temperature, time, hydrogen pressure, hydrogen flow rate, RF power and electrode distance. The carrier selectivity, S10, was chosen as the objective function to be optimized. As a consequence, we could reach S10 of 13.63 by only 12 iterations by Bayesian optimization and 10 initial experiments with randomly chosen conditions. These results certifies that Bayesian optimization is useful for accelerating optimization of the practical process conditions in multidimensional parameter space. The optimized conditions could contribute to realization of high efficiency silicon-based heterojunction solar cells.

Applicability of Atmospheric Pressure Plasma Jet (APPJ) Discharge for the Reduction in Graphene Oxide Films and Synthesis of Carbon Nanomaterials

Atmospheric pressure plasma jets (APPJ) are widely used in industry for surface cleaning and chemical modification. In the recent past, they have gained more scientific attention especially in the processing of carbon nanomaterials. In this work, a novel power generation technique was applied to realize the stable discharge in N2 (10 vol.% H2) forming gas in ambient conditions. This APPJ was used to reduce solution-processed graphene oxide (GO) thin films and the result was compared with an established and optimized reduction process in a low–pressure capacitively coupled (CCP) radiofrequency (RF) hydrogen (H2) plasma. The reduced GO (rGO) films were investigated by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Effective deoxygenation of GO was observed after a quick 2 s treatment by AAPJ. Further deoxygenation at longer exposure times was found to proceed with the expense of GO–structure integrity. By adding acetylene gas into the same APPJ, carbon nanomaterials on various substrates were synthesized. The carbon materials were characterized by Raman spectroscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) analyses. Fullerene-like particles and graphitic carbon with short carbon nanotubes were detected on Si and Ag surfaces, respectively. We demonstrate that the APPJ tool has obvious potential for the versatile processing of carbon nanomaterials.

Contribution of leading edge shape to a damaging of castellated tungsten targets exposed to repetitive QSPA plasma loads

The castellated tungsten monoblock is considered one of the preferred reference designs of components faced to plasma in the divertor of a fusion reactor. The behavior of such components during the development of transient events (disruptions or/and of Edge Localized Modes (ELMs)) is still among the most critical issues for future thermonuclear devices. Experimental studies of castellated tungsten surface erosion have been performed within the powerful quasi-stationary plasma accelerator QSPA Kh-50 in conditions relevant to unmitigated ITER Type I ELMs. The surface energy load from the impacting hydrogen plasma streams was 0.9 MJ m−2 during 0.25 ms to achieve pronounced melting of the target surface. The studies of solid and liquid particle generation were performed at sequentially oblique plasma exposure of the different chosen edges of the castellated structure. It was determined that most of the W droplets are ejected from the sharp leading edge of the target due to the development of instabilities in the plasma-liquid metal interface. At the same time, the formation of solid dust particles is mainly attributed to the cracking during the surface cooling after the plasma pulse. Particle velocities typically achieve tens m s−1. Both the re-solidified droplets as well as dust particles have been collected on the special plate near the exposed surface. The maximal size of collected particles reached tens of micrometers. Such particles could either fly away with the next plasma pulses or mix with the material of the collecting plate in the course of repetitive plasma pulses. Thin (submicron) layers of re-deposited tungsten also formed on the surface of the collecting plate.

Low defect and high electrical conductivity of graphene through plasma graphene healing treatment monitored with in situ optical emission spectroscopy

Fundamental studies on graphene (Gr) and its real device applications have been affected by unavoidable defects and impurities which are usually present in synthesized Gr. Therefore, post treatment methods on Gr have been an important subject of research followed by the community. Here, we demonstrate a post-treatment of cm-sized CVD-grown graphene in a Radio Frequency-generated low-pressure plasma of methane and hydrogen to remove oxygen functional groups and heal the structural defects. The optimum plasma treatment parameters, such as pressure, plasma power, and the ratio of the gases, are optimized using in-situ optical emission spectroscopy. This way we present an optimal healing condition monitored with in situ OES. A twofold increase in the conductivity of plasma-treated Gr samples was obtained. Plasma treatment conditions give insights into the possible underlying mechanisms, and the method presents an effective way to obtain improved Gr quality.

Virial expansion of the electrical conductivity of hydrogen plasmas.

The low-density limit of the electrical conductivity σ(n,T) of hydrogen as the simplest ionic plasma is presented as a function of the temperature T and mass density n in the form of a virial expansion of the resistivity. Quantum statistical methods yield exact values for the lowest virial coefficients which serve as a benchmark for analytical approaches to the electrical conductivity as well as for numerical results obtained from density functional theory-based molecular dynamics simulations (DFT-MD) or path-integral Monte Carlo simulations. While these simulations are well suited to calculate σ(n,T) in a wide range of density and temperature, in particular, for the warm dense matter region, they become computationally expensive in the low-density limit, and virial expansions can be utilized to balance this drawback. We present new results of DFT-MD simulations in that regime and discuss the account of electron-electron collisions by comparison with the virial expansion.

The isotope effects in RFP magnetic configuration

The comparison between deuterium and hydrogen plasmas at high current in the RFX-mod experiment proves that the reversed field pinch configuration experiences the isotope effect in a way analogous to tokamak experiments. Both energy and particle confinement exhibit a favourable scaling with the isotope mass M i , respectively, as and as . This effect is mainly due to the mitigation of transport at the edge, which is driven by electrostatic fluctuations, although a mitigation of the stochastic transport in the plasma core is also observed. As in tokamaks, the MHD properties are modified when the gas mass is changed. The most evident modification regards the spectrum of tearing modes that are more stable in deuterium, so that the duration and the purity of quasi single helicity (QSH) phases are enhanced. The thermal structures that develop during the QSH phases enclose a plasma volume that is wider in deuterium discharges than in hydrogen ones.

RRAM Devices with Plasma Treated HfO2 with Ru as Top Electrode for In-Memory Computing Hardware

This work investigates the role of extra oxygen vacancies, introduced by a hydrogen plasma at midpoint of deposition of a 6 nm thick HfO2 to reduce the switching power consumption in a RRAM device. Initially TiN, which is a commonly used metal in CMOS technology, was used as the top electrode for treated HfO2. Subsequently Ru and TaN as top electrodes were explored enhance the switching behavior and power consumption. A range of compliance currents from 1 nA to 1 mA were used to evaluate the switching characteristics. The role of both TaN and Ru as bottom metal was also evaluated. With Ru as top metal the device switched at a compliance current of 1 nA and higher. Whereas when Ru was used as bottom electrode, devices were unable to switch below a compliance current of 50 mA. For TaN as top metal electrode, devices switched at and above 1 mA CC whereas with TaN as bottom metal the initial switching was at CC of 2 mA. It was observed that use of Ru as a top metal significantly reduced the switching energy of the plasma treated HfO2 RRAM device but was ineffective when used as a bottom metal.

Negative ion of hydrogen in dense semi-classical plasmas: Stability and zero-energy resonances

The effects of dense semi-classical plasma (DSCP) on the ground state of the negative ion of hydrogen (H−) and on the dynamics of electron–hydrogen scattering have been investigated. DSCP is described by an effective potential which takes care of the collective effects of the plasma at large distances as well as the quantum mechanical effects of diffraction at small distances. An elaborative wave function is employed in the Rayleigh–Ritz variational method to compute the ground state energy of H− for various values of the plasma parameters. In particular, critical values of the plasma parameters are calculated accurately to make a detailed study on the stability of the ion embedded in DSCP. Furthermore, parameters related to the ground state of H− and H are used in the effective range theory to study the effects of DSCP on the dynamics of low-energy e − H(1s) scattering. Special emphasis is given to investigate the phenomenon of zero-energy resonances by computing the singlet scattering length near the critical values of the plasma parameters.

ECR discharge sustained by millimeter waves as a source of dense plasma flux

The results of the investigation of the dense ECR discharge hydrogen plasma flux formation in the single solenoid magnetic field are presented in this work. The transversal flux profile obtained at the optimal system parameters is shown. The possibility of the formation of homogeneous plasma fluxes with density of 750 mA/cm2 and total current of 5 A is demonstrated. The results of the first experiments of the hydrogen ion beam extraction from the ECR discharge plasma in the single magnetic coil are presented. The record values of the ion current density higher than 1.5 A/cm2 were obtained. The results of the research presented in this paper show the prospects of the proposed system for applications of the neutral beam injector development for the plasma heating in the controlled thermonuclear fusion installations.

Isotope dependence of beta-induced Alfvén eigenmode (BAE) and low frequency mode (LFM) stability in DIII-D

The stability of beta-induced Alfvén eigenmodes (BAE) and the low frequency modes (LFMs) that were formerly called beta-induced Alfvén-acoustic eigenmodes is discussed. After a brief summary of previous publications on the stability in DIII-D beam-heated, reversed-shear, deuterium plasmas with deuterium neutral beam injection (NBI), new observations in mixed hydrogen and deuterium plasmas are reported. With deuterium NBI, BAEs are at least as unstable in mixed-species plasmas as in deuterium plasmas; however, with hydrogen NBI, the BAEs are stable. In contrast, the LFMs are unaffected by changes in beam species, consistent with the previous observation that LFMs are not driven by high-energy beam ions. As predicted by theory, the LFMs appear more unstable in mixed species plasmas than in pure deuterium discharges.

Supporting plasma processes for fabrication of n-doped nano-crystalline silicon thin film on low-cost glass substrates

Nano-crystalline silicon is an excellent material for applications in the optoelectronics industry. This intermediate material combines both the advantages of amorphous and crystalline silicon. However, the typical fabrication process of nano-crystalline silicon previously required high-quality substrates such as quartz or silicon wafers, which limited its potential applications. Recently, plasma treatments have been recognized as the most efficient and customizable methods for transforming materials. In this study, corona plasma was used to treat the low-cost glass substrates to improve their adhesion to silicon thin films. It allowed the film to become more uniform with a surface roughness of less than 10% at the thickness of about 1 μm. The conventional thermal annealing process was replaced by a high-power hydrogen plasma process, which does not cause harmful effects on the glass substrates and stimulates the amorphous-crystalline phase transition. The increasing of the crystalline fraction, which is known as the order of arrangement of silicon atoms network, affects the doped concentration as well as the electrical properties of the nc-Si:H thin films. The resulting n-doped nano-crystalline silicon thin film has a crystalline fraction of 32.8%, corresponding to a sheet resistance of 104 Ω/square.

Tin reduction from fluorine doped tin oxide for silicon nanowire-based solar energy harvesting and storage.

Hydrogen plasma reduction of fluorine doped tin oxide is a beneficial method to form tin nanodroplets on the sample surface directly in the plasma-enhanced chemical vapor deposition reactor. The formation of catalyst droplets is a crucial initial step for vapor-liquid-solid growth of silicon nanowires for radial junction solar cells and solar fuel cell technology. We present an original optical model which allows us to trace the formation process on fluorine doped tin oxide on soda-lime glass substrate from the in situ data and is in a good agreement with the spectroscopic ellipsometry data measured before and during the reduction process. The model reproduces well the phase shift introduced by a transition double layer in fluorine doped tin oxide which acts as a barrier against the sodium diffusion. Furthermore, we study the process of tin reduction from fluorine doped tin oxide in a real time and compare estimated amount of produced metallic tin with images from scanning electron microscopy.The proposed approach is very important for in situ real-time monitoring of the one-pump-down fabrication process used to grow nanowires and form radial junction devices.

Predictive estimation of vacuum ultraviolet emission intensity in a low-pressure inductively coupled hydrogen plasma based on the branching ratio technique

Vacuum ultraviolet (VUV) emission has recently attracted attention in low pressure processing plasmas because of the possibility of high-energy photon damages on the substrates. To quantify the VUV induced damages during the plasma processes, it is need to use of a VUV spectrometer equipped with vacuum systems not readily available in industries. In this work, therefore, we report a simple method to estimate the VUV emission intensity of hydrogen plasmas utilizing a conventional visible spectrometer widely used in plasma processes. From the measurement of hydrogen emission spectra in the visible wavelength region, the VUV emission line (Lyman-β) was calculated using the branching ratio technique and enabled the estimation of Lyman-α emission intensity based on the Boltzmann relation with given plasma parameters. In addition, it was found that the method could also predict the VUV emission intensity for high density hydrogen plasma cases by considering the self-absorption effect by hydrogen atoms. • VUV emission intensity measurement and predictive estimation. • Calculation of VUV Lyman-α (121 nm) emission based on Branching ratio technique. • Correlation between predicted and measured VUV Lyman-α (121 nm) emission intensity. • Self-absorption effect of hydrogen atoms on VUV emission intensity estimation.