Ion-electron Emission Research Articles

The adsorption and diffusion properties of scandium atom on the surfaces of tungsten and noble metals

As for the scandate cathode, the performance of Sc atom, including the adsorbability and diffusivity, plays an important role in the emission property and emission stability of the scandate cathode. W is the most widely used matrix metal in thermionic scandate cathodes, however, the cathode suffers from the recovery of scandium after ion bombardment. Herein, the adsorption properties and diffusion performance of Sc atom on the surfaces of W, Ir, Os, Re and Ru are systematically studied with the density functional theory (DFT). It is found that the adsorbabilities and diffusivities of Sc atom on the surface of W, Ir, Os, Re and Ru are in the order as W(0 1 1) < Re(0 0 0 1) < Ru(0 0 0 1) < Os(0 0 0 1) < Ir(112¯1) and Ir(112¯1) < W(0 1 1) ∼ Os(0 0 0 1) < Ru(0 0 0 1) < Re(0 0 0 1), respectively. The adsorbabilities and the diffusivities of Sc atom on the surface of Re and Ru are higher than those on the surface of W, which suggests that Re and Ru are the potential materials to be the matrices or the coatings of scandate cathodes to improve the ion bombarding insensitivity and electron emission recovery of the cathode.

Secondary emission of neutral and charged particles from intermetallic single-crystal

Sputtering, secondary ion and secondary electron emission were studied experimentally for an intermetallic Ni4Mo compound with high corrosion resistance and great hardness, which is used as material for constructing components of rockets and nuclear reactors. The process of sputtering was analyzed by molecular dynamics simulations of Ni4Mo (111) face for unchanged and changed composition (with segregation) of topmost layers of a disordered and ordered crystal. Predominant exit of Ni and Mo atoms in close-packed directions was observed and explained by correlated collisions. Both nickel and molybdenum atoms are ejected in the same crystallographic directions for a disordered crystal. For an ordered crystal the nickel atoms deviate from these directions. The origin of sputtering and number of ejected particles generated has been calculated. For secondary electron emission from the (111) Ni4Mo face the maxima in close-packed directions were obtained experimentally and explained by scattering of primary and secondary electrons on atoms located in the lateral sides of open channels.

Electron emission in ionization of bromouracil by fast bare carbon ions

SynopsisDouble differential cross sections (DDCS) of the secondary electrons emitted from a vapor jet of bromouracil in collision with 42 MeV bare carbon ions have been measured for different forward and backward emission angles over a wide energy range. The DDCS for bromouracil were compared with that of uracil measure earlier. An unusually large increase in electron emission cross section have been observed for bromouracil.

High-power sputtering employed for film deposition

The features of high-power magnetron sputtering employed for the films’ deposition are reviewed. The main physical phenomena accompanying high-power sputtering including ion-electron emission, gas rarefaction, ionization of sputtered atoms, self-sputtering, ion sound waves and the impact of the target heating are described.

Influence of the Field Electron Emission from the Cathode with an Insulating Film on the Normal Glow Discharge Characteristics

A theoretical model of the cathode sheath of a normal glow discharge for the cathode with a thin insulating film is developed. In addition to the ion-electron emission, the field emission of electrons from the metal cathode substrate into the film under the action of the strong electric field generated in the insulator is taken into account in the model. It is established that the influence of the field electron emission on the glow discharge characteristics is determined by the emission efficiency of the film, equal to the fraction of electrons going out of it into the discharge volume. It is demonstrated that the calculated normal cathode voltage drop in the discharge in argon for the cathode with a barium oxide film coincides with its measured value for the emission efficiency of the film of the order of 0.1, which is in agreement with its experimental estimations.

Kinetics of charged particles in a high-voltage gas discharge in a nonuniform electrostatic field

A high-voltage gas discharge is of interest as a possible means of generating directed flows of low-temperature plasma in the off-electrode space distinguished by its original features [1–4]. We propose a model for calculating the trajectories of charges particles in a high-voltage gas discharge in nitrogen at a pressure of 0.15 Torr existing in a nonuniform electrostatic field and the strength of this field. Based on the results of our calculations, we supplement and refine the extensive experimental data concerning the investigation of such a discharge published in [1, 2, 5–8]; good agreement between the theory and experiment has been achieved. The discharge burning is initiated and maintained through bulk electron-impact ionization and ion–electron emission. We have determined the sizes of the cathode surface regions responsible for these processes, including the sizes of the axial zone involved in the discharge generation. The main effect determining the kinetics of charged particles consists in a sharp decrease in the strength of the field under consideration outside the interelectrode space, which allows a free motion of charges with specific energies and trajectories to be generated in it. The simulation results confirm that complex electrode systems that allow directed plasma flows to be generated at a discharge current of hundreds or thousands of milliamperes and a voltage on the electrodes of 0.3–1 kV can be implemented in practice [3, 9, 10].

Investigation of ion-electron emission in the process of reactive ion-beam etching of dielectric thin film heterostructures

This work presents a series of experimental studies to confirm the main theoretical aspects of ionelectron emission and it searches for the possibility of the practical implementation of the operative control method of reactive ion-beam etching processes of different dielectric thin film materials of electronic engineering. The series of experiments was carried out to study the electron emission on the specially formed thinfilm multilayer heterocompositions of Si3N4/Si, Ta2O5/Al/Si, and Al/TiO2/Si. The evaluation of the effect of the induced surface potential in the dielectric film on the integral signal of the secondary electrons at reactive ion-beam etching was carried out. The dependence of the emission properties of thin dielectric films on the electric field generated in the dielectric by the surface potential induced by ion beam during reactive ionbeam etching was established. It is noted that the current level of secondary electrons from the surface of dielectric films deposited on the substrates of different materials differ in magnitude; i.e., it is determined by the substrate emission properties. It is shown that the electric field strength arising in the dielectric film under the influence of the induced potential creates the conditions for the emergence of Malter’s emission determined by the properties of its own dielectric and substrate.

Current evolution and plasma density space distribution in the reflex discharge with ring cathodes

In this paper the numerical model of direct current gas discharge in drift-diffusion approximation is considered. For two-component plasma the processes of the gas discharge development in the reflex geometry with ring cathodes at a helium pressure of 35 mTorr are studied. We investigate the influence of: (a) the boundary conditions on the dielectric, (b) the electron temperature and (c) the coefficient of the secondary ion-electron emission on the I–U curve of the discharge. In a magnetic field of 50 Gauss the impact of the discharge voltage U = 300–700 V on the evolutionary process of the discharge is examined. The effect of diffusion on maintaining steady state discharge is researched. The parameters of the existence of a high-current (tens of μA) and low voltage (tens of mA) discharge modes are defined.

Thermodynamic model for electron emission and negative- and positive-ion formation in keV molecular collisions

A statistical-type model is developed to describe the ion production and electron emission in collisions of (molecular) ions with atoms. The model is based on the Boltzmann population of the bound electronic energy levels of the quasi molecule formed in the collision and the discretized continuum. The discretization of the continuum is implemented by a free electron gas in a box model assuming an effective square potential of the quasi molecule. The temperature of the electron gas is calculated by taking into account a thermodynamically adiabatic process due to the change of the effective volume of the quasi molecule as the system evolves. The system may undergo a transition with a small probability from the discretized continuum to the states of the complementary continuum. It is assumed that these states are decoupled from the thermodynamic time development. The decoupled states overwhelmingly determine the yield of the asymptotically observed fragment ions. The main motivation of this work is to describe the recently observed $H^-$ ion production in $OH^+ + Ar$ collisions. The obtained differential cross sections for $H^-$ formation, cation production and electron emission are close to the experimental ones. Calculations for the atomic systems $O^+ + Ar$ and $H^+ + Ar$ are also in reasonable agreement with the experiments indicating that the model can be applied to a wide class of collisions.

Modeling of the effect of temperature and field-induced electron emission from the cathode with a thin insulating film on the Townsend discharge ignition voltage in argon–mercury mixture

A model of the low-current (Townsend) discharge in argon–mercury mixture in the presence of a thin insulating oxide film on the cathode is developed. It takes into account the cathode ion-electron emission and the electron emission from the cathode metal substrate under the strong electric field generated in the film by the ion surface charge. An influence of the mixture temperature and the oxide film on the discharge ignition voltage is estimated and it is shown that formation of a thin insulating film on the cathode surface can facilitate the discharge ignition in mercury-containing gas discharge devices at low temperatures.

Enhancement of emission currents in plasma electron sources based on a low-pressure arc discharge

The paper reports on a theoretical and experimental study of the discharge plasma generation with an enhanced electron emission current in a plasma electron source based on a low-pressure arc discharge with a grid-stabilized plasma emission boundary. The source operates at a pressure in the working chamber of p = 0.02-0.05 Pa (Ar), accelerating voltage of up to Ua = 10 kV, and longitudinal magnetic field for electron beam transport of up to Bz = 0.1 T. The experiments show that in the mode of electron emission from the plasma, the voltage Ud between the cathode and grid electrode changes its sign. The numerical simulation demonstrates that the plasma potential and voltage Ud depend on the electric field penetrating from the acceleration gap into the discharge region through the grid meshes, and on the discharge current, gas pressure, geometric transparency of the grid, and gas kind. It is shown that the main mechanisms responsible for the increase in the discharge current and electron emission current from the plasma are associated with secondary ion-electron emission from the emission electrode and with positive feedback between the region of cathode plasma generation and the channel of electron beam transport.

Double differential electron emission in ionization of Coronene by fast ions

SynopsisWe present the absolute differential electron emission cross section of coronene under fast bare O-ion impact. The results show some features which cannot be explained by simple ion-atom type collision result. The difference of the energy and angular distribution data with that for CH4 and Ne may indirectly indicate the influence of collective excitation. We have performed the CB1 calculation for comparison.

Electron emission in collisions between atoms and dressed projectiles

SynopsisDouble-differential cross sections (DDCS) for electron emission in ionization under the impact of dressed projectiles has been calculated within the framework of three-Coulomb wave (3CW) model. The interaction between the dressed projectile ion and the active electron is approximated by a model potential. In order to take into account of the effect of the passive electrons, we have also constructed a model potential which satisfies initial condition and the corresponding wave function has been calculated from the model hamiltonian of the active electron in the target.

Differential electron emission from multi-electronic targets under highly harged bare ion impact

SynopsisWe report a combined experimental and theoretical study of double differential electron emission in ionization of different multi-electronic atoms under fast highly charged bare ion impact. The main aim is to un-derstand the electron dynamics for these targets which is quite different from one or two electron systems.

Properties of Ion–Electron Emission from the Surface of Semiconductor Material During Reactive Ion–Beam Etching

The existing methods of diagnosis of solid surfaces in ion − plasma processes have been analyzed. We found that the most efficient method of estimating surface condition, determining the transition of the etching process from one layer to another and determining the end of the etching process is the registration of ion − electronic emission during ion − beam etching. Results on secondary electron current for ion beam etching of various semiconductors have been reported. We show the experimental setup and describe the electric circuit for the detection of secondary electrons. An experimental study has been carried out to determine the dependence of secondary electron current on the band gap E g and the height of the potential barrier (electron affinity) χ of Ge, Si, GaAs, GaP and SiC semiconductor materials. We found no clearly expressed dependence of integral signal of ion − electronic emission on E g and χ . We show that under the conditions of ion beam etching under the influence of the surface potential the electric field penetrates in the semiconductor volume, leading to a shift in the energy levels of electrons in the surface layer and a change in the secondary electron current due to the appearance of autoelectronic emission. We found that the signal of ion − electronic emission in n − type silicon is higher than in p − type silicon. A model of ion − electronic emission from the surface of semiconductors is presented for the conditions of ion − beam etching, consisting of: emission with the participation of conductivity band electrons, emission due to the direct transition of electrons in the ion – atom system, and autoelectronic emission under the influence of surface potential.

Formation of TiO2 nano-platelets using plasma induced by a pulse KrF-laser

TiO2 Thin films were deposited by RF magnetron sputtering on silicon dioxide substrates and irradiated using a KrF excimer laser. The effect of laser energy on the film has been systemically investigated by X-ray diffractometry, Raman spectral, X-ray photoelectron spectroscopy, and scanning electron microscopy. When the laser fluence was not higher than 0.3J/cm2, an obvious thermal effect of laser irradiation was observed. And with the laser fluence increasing to 1J/cm2, novel TiO2 nano-platelets were found on the surface because the laser induced plasma caused electron–ion emission from the TiO2 film surface. By increasing the laser fluence continually, the nano-platelets become sparse, which was due to the ion density in the plasma induced by laser.

Influence of a cathode surface oxide film on the energy distributions of ions and fast atoms in a glow discharge

A model of the cathode sheath of a glow discharge in the presence of a thin dielectric oxide film on the cathode is formulated. It takes into account cathode ion-electron emission and the emission of electrons from the cathode metal substrate under a strong electric field generated in the film by the ion surface charge. The influence of the oxide film on the discharge parameters, the ion and fast atom energy spectra near the cathode surface, and the intensity of its sputtering is estimated.

Electron emission in ionization of He and Ne by fast dressed oxygen ions and projectile-charge-state dependence

The double-differential cross sections (DDCS) of low-energy electrons emitted at forward, backward, and perpendicular directions are reported for collisions of 3.75 MeV/u ${\mathrm{O}}^{q+}$ ($q=5$, 6, 7, 8) projectiles with He and Ne targets. The measured DDCS are found to be deviating from the ${q}^{2}$ dependence throughout the entire energy region. The effect of projectile electrons, for the dressed ions, as a function of the impact parameter is clearly noticeable for large as well as low-impact parameter collisions. We also present a theoretical calculation based on the prior form of the continuum distorted wave--eikonal initial state approximation, in which the projectile-active electron interaction is modeled with the Green-Sellin-Zachor potential. This particular representation of the potential has been proven to give good qualitative results for projectiles with residual electrons. In addition to the total DDCS, the individual contributions from target ionization, projectile electron loss, and simultaneous ionization processes are also calculated. The total DDCS obtained from these calculations are shown to be in excellent agreement with the experimental observations.

Surface hardening by means of plasma immersion ion implantation and nitriding in glow discharge with electrostatic confinement of electrons

At argon or nitrogen pressure 0.005–5 Pa a metal substrate is immersed in plasma of glow discharge filling a vacuum chamber and negatively biased to U = 1−10 kV. Emitted by substrate secondary electrons bombard the chamber walls and it results in growth of electron emission chamber walls and rise of gas ionization intensity inside chamber. At pressure p< 0.05 Pa nonuniformity of plasma density does not exceed ~10%. Measurement results of sheath width d between homogeneous plasma and a lengthy flat substrate were used to calculate dependence of the ion current density ji on its surface and ion-electron emission coefficient γi. The dependence allows evaluation of average dose of ion implantation on any complex-shaped product of the same material.

Energy and angular distribution of electrons in ionization of He and Ne by 6-MeV/u bare carbon ions: Comparison with continuum-distorted-wave eikonal-initial-state calculations in prior and post forms

We have measured the absolute double-differential cross sections (DDCS) for electron emission in ionization of He and Ne atoms under the impact of 6-MeV/u ${\mathrm{C}}^{6+}$ ions. Data were collected between 1 and 500 eV for He, while for Ne this range was extended up to 1000 eV. The angular ranges covered in the experiment are ${30}^{\ensuremath{\circ}}$ to ${150}^{\ensuremath{\circ}}$ and ${20}^{\ensuremath{\circ}}$ to ${160}^{\ensuremath{\circ}}$ for He and Ne, respectively. The DDCS spectra are compared with the prior and the post forms of the state-of-the-art continuum-distorted-wave eikonal-initial-state model. Both the theoretical models show very good agreement with the energy and angular distributions of the DDCS in the case of He. For Ne, at low energies both are going together and matching very well with the data. In the high-energy region, at extreme forward and backward angles, although both the forms are underestimating the experimental data to some extent, the prior form shows much better agreement compared to the post form. This post-prior discrepancy is attributed to the influence of dynamic screening, on the ionized one, produced by the electrons remaining in the target. The single differential cross sections (SDCS) in emission angle $(\frac{d\ensuremath{\sigma}}{d{\ensuremath{\Omega}}_{e}})$ and electron energy $(\frac{d\ensuremath{\sigma}}{d{\ensuremath{\epsilon}}_{e}})$ are deduced by integrating the electron DDCS spectra. While excellent agreement is obtained for the $\frac{d\ensuremath{\sigma}}{d{\ensuremath{\epsilon}}_{e}}$ spectrum, the $\frac{d\ensuremath{\sigma}}{d{\ensuremath{\Omega}}_{e}}$ provides a further sensitive test to the adequacy of the theoretical model employed. The total cross section obtained from the SDCS spectra is about 11% higher than the prior model for He and about 6% lower for Ne. To get the quantitative picture of the two-center effect, the forward-backward angular asymmetry parameter has been deduced as a function of velocity of the ejected electrons. For both the targets, it is very well reproduced by both the forms of the theory. For the Ne target, $K$-LL Auger angular distribution has also been studied, which shows small asymmetry caused by multiple vacancies in the $L$ shell along with the $K$-shell vacancy.