Electron Emission Coefficient Research Articles

Domain formation on the nonpolar lithium niobate surfaces under electron-beam irradiation: A review

In this review, our recent results on the electron-beam domain writing (EBDW) on the nonpolar surfaces of LiNbO3 crystals of different compositions are presented. Under EB irradiation of the nonpolar surfaces, domains nucleated in irradiation points grow frontally along the polar [Formula: see text]-direction in a thin (of microns in thickness) surface layer; the driving force is the tangential component of space-charge fields induced by EB in irradiation points. This geometry of the experiment provides a possibility of three-dimensional (3D) characterization of domain patterns using the combination of atomic force microscopy (AFM) and second harmonic generation (SHG) confocal microscopy methods. The obtained results permitted us to relate the main characteristics of domain formation (the domain sizes and velocity [Formula: see text] of the frontal motion) to the irradiation conditions (the accelerating voltage [Formula: see text] of scanning electron microscopy (SEM), EB current [Formula: see text], the inserted charge [Formula: see text]). The domain depth [Formula: see text] is controlled by [Formula: see text] via the electron penetration depth; the domain length [Formula: see text] increases linearly with [Formula: see text] owing to the domain frontal growth by the viscous friction law. The electron emission coefficient [Formula: see text] affects the domain formation due to the fundamental dependence of [Formula: see text] on [Formula: see text]. In the framework of current approach to EB charging of insulators, the effect of an enhanced conductance on EBDW characteristics is analyzed. The difference between EBDW characteristics observed in varied LiNbO3 compositions is discussed in the framework of the intrinsic defect structure of LiNbO3. The obtained results extend the possibility of EBDW application to a wider range of crystals.

HPHT single crystal diamond type IIB growth sector influence on the secondary electron emission phenomenon

Secondary electron emission (SEE) coefficient correlation with (100) synthetic HPHT-grown single crystal diamonds type IIa and type IIb growth sectors was investigated. SEE coefficient quantitative measurements were carried out for various growth sectors: the highest magnitudes were revealed for (100), (311) and {110} growth sectors, but {111} growth sectors have SEE coefficient 4‒6 times lower in comparison with mentioned above for both investigated crystal types. The SEE coefficient dependence of primary beam energy is non-standard for all growth sectors: SEE coefficient goes up with primary beam energy increasing. These results are very important for diamond microelectronics as various growth sectors utilization considerably changes output characteristics.

Effect of total emitted electron velocity distribution function on the plasma sheath near a floating wall

Electrons emitted from a solid surface can noticeably affect characteristics of plasma sheath surrounding that surface by modifying current balance at wall, charge separation in sheath region and Bohm criterion at sheath edge. We establish a static sheath model with kinetic electrons and cold ions to emphasize the effect of different total emitted electron velocity distribution functions (EEVDFs) on classic sheath solution and its structure transition. Four total EEVDFs with same average energy are considered separately. It is found that total EEVDFs influence the sheath solution and the threshold of total electron emission coefficient (EEC) for classic sheath dramatically, and can cause no solution for critical space-charge limited (SCL) sheath. These results indicate that, as EEC increases from zero gradually, the sheath will not transit from classic sheath to SCL sheath structure for some special total EEVDFs.

Electron beam recording of microdomains on the nonpolar LiNbO3 crystal surface at different SEM accelerating voltages

The effect of the accelerating voltage U of the SEM electron beam on the characteristics of microdomains recorded by the electron beam method on the nonpolar Y-surface of LiNbO3 crystals has been studied. The thickness Td of domains in the Y direction is determined by the depth range Re of primary electrons, which depends on U. This makes it possible to define Td in the range of 0.2–4 μm at U = 5–25 kV, respectively. The electron emission coefficient σ is estimated for different values of U exceeding the second equilibrium point U2 (σ = 1) in the σ(U) diagram. These data are used to construct the dependence σ(U) for LiNbO3. Based on the exposure characteristics of the length Ld of domains growing along the polar Z axis, the dependence of the space charge field controlling the domain planar growth along Z on the surface electron emission σ is found.

Electron emission from the zirconium coated suspension plasma sprayed bioglass

The suspension plasma sprayed bioglass coatings were covered with electrically conducting zirconium film obtained by sputtering. The microstructure of the coatings, observed by scanning electron microscope (SEM) shows the spherical droplets and some acicular grains formed at the bioglass recrystallization. The coatings were submitted to the electron emission investigations. The measurements enabled to find out the critical electric field and electron emission coefficient. The values of the critical field as low as Eth=12V/μm and of the field enhancement coefficient being as high as β=280 are comparable to the values reached for carbon nanotubes. The possible application of the electron field emitting coatings is the large-area emission source used as e.g. information display.

Sputtering Voltage in the Growth of Vanadium Oxide Thin Films

Vanadium oxide thin films have been deposited by reactive DC magnetron sputtering. The results showed that the sputtering voltage has a special change behavior in Ar condition and Ar/O2 mixed conditions. With O2 gas flow input instantaneously, the sputtering voltage climbs quickly, and then tends to be stable shortly. Also, the sputtering voltage evolution appears a hysteretic loop characteristics with O2 gas supplied increasing and decreasing. The sputtering voltage behavior observed is related to the ion induced secondary electron emission (ISEE) coefficient of the target material. The ISEE coefficient of the oxidized vanadium on target surface is lower than the ISEE coefficient of the pure vanadium metal, which is not in agreement with the generally accepted idea that the ISEE coefficient of oxides is much larger than for metals.

Numerical Simulation for the Effect of Wall Material on Near Wall Conductivity in Hall Thrusters

The wall material plays an important role for the electron current due to near wall conductivity in Hall Thrusters. A Monte Carlo method combined with a one dimensional steady sheath model is presented and is applied to simulate the electron conductive current due to near wall conductivity for the different channel wall materials of Hall thruster. The simulation results show that the higher the secondary electron emission (SEE) coefficient of the channel wall material is, the greater the electron conductive current is. Based on the simulation, a physical explanation is given from the viewpoint of near wall conductivity. For the channel wall material with low SEE coefficient, the secondary electrons taking part in the near wall conductivity becomes less. In addition, the absolute potential drop in the sheath near the wall increases, which means that the sheath can stop more electrons from colliding with the channel wall. And consequently the electron conductive current due to near wall conductivity is much less. The situation is vice verse for the channel wall material with high SEE coefficient. The simulation results are qualitatively in accordance with the experiments. The results can help to choose and design the wall material of the Hall Thrusters with a high performance.

Characterization of periodic domain structures in lithium niobate crystals by scanning electron microscopy and X-ray diffraction analysis

Congruent lithium niobate crystals with periodic domain structures formed by the method of thermoelectric postgrowth treatment have been investigated. Periodic domain structures in the samples of polar ZY-, YZ-, and YX-cuts were characterized by scanning electron microscopy, high resolution x-ray diffractometry, and topography. The evaluation and comparison of the secondary electron (SE) signal cycling, electron emission coefficient, and charge value to equalize the differences in the SE signal have allowed us to specify the features of ZY-and YZ-cut structures. A correlation between the crystal lattice distortions near domain boundaries and the type of domain walls has been found. The domain walls separating the areas with the tail-to-tail Ps vectors not only cause stronger crystal lattice distortions near them, but also are charged less quickly under electron irradiation.

Experimental study of the influence of the processes in the ion-beam drift space on beam current measurements

A method is proposed for separate measurements of the current produced by slow charge-exchange ions and that produced by the ions generated due to gas ionization by the beam ions and fast secondary electrons in the beam drift space. The method is based on an analysis of the current distribution over the electrodes of a modified Faraday cup with nonequipotential electrodes and allows one to determine the coefficient of ion-induced electron emission from the ion collector and the charge-exchange cross section of the accelerated ions. The method has been employed to measure the current of an argon ion beam with an ion energy from a few electronvolts to several tens of kiloelectronvolts and to study the processes in the beam drift space at pressures of 0.03–0.15 Pa.

Gas desorption and electron emission from 1 MeV potassium ion bombardment of stainless steel

Gas desorption and electron emission coefficients were measured for 1 MeV potassium ions incident on stainless steel at grazing angles (between 80\ifmmode^\circ\else\textdegree\fi{} and 88\ifmmode^\circ\else\textdegree\fi{} from normal incidence) using a new gas-electron source diagnostic (GESD). Issues addressed in design and commissioning of the GESD include effects from backscattering of ions at the surface, space-charge limited emission current, and reproducibility of desorption measurements. We find that electron emission coefficients ${\ensuremath{\gamma}}_{e}$ scale as $1/\mathrm{cos}(\ensuremath{\theta})$ up to angles of 86\ifmmode^\circ\else\textdegree\fi{}, where ${\ensuremath{\gamma}}_{e}=90$. Nearer grazing incidence, ${\ensuremath{\gamma}}_{e}$ is reduced below the $1/\mathrm{cos}(\ensuremath{\theta})$ scaling by nuclear scattering of ions through large angles, reaching ${\ensuremath{\gamma}}_{e}=135$ at 88\ifmmode^\circ\else\textdegree\fi{}. Electrons were emitted with a measured temperature of $\ensuremath{\sim}30\text{ }\text{ }\mathrm{e}\mathrm{V}$. Gas desorption coefficients ${\ensuremath{\gamma}}_{0}$ were much larger, of order ${\ensuremath{\gamma}}_{0}={10}^{4}$. They also varied with angle, but much more slowly than $1/\mathrm{cos}(\ensuremath{\theta})$. From this we conclude that the desorption was not entirely from adsorbed layers of gas on the surface. Two mitigation techniques were investigated: rough surfaces reduced electron emission by a factor of 10 and gas desorption by a factor of 2; a mild bake to $\ensuremath{\sim}220\ifmmode^\circ\else\textdegree\fi{}$ had no effect on electron emission, but decreased gas desorption by 15% near grazing incidence. We propose that gas desorption is due to electronic sputtering.

Fast and slow decay of ion beam induced electron emission in boron doped hydrogenated CVD diamond films

The potential use of CVD diamond films as a cold cathode material is limited by several factors associated with fast and slow material degradation processes induced by interaction that gives rise to secondary electron (SE) emission. In particular, the negative electron affinity (NEA) loss which has been attributed to Hydrogen detachment from the film surface, happens during ion or electron bombardment and seriously affects SE emission. Using the published results of NEA temperature dependence and connecting them with SE low-energy spectra at different temperatures, we obtained a linear approximation between NEA and surface Hydrogen coverage. The fast ion dose-dependent changes in SE yield are explained by high production rate of radiation defects near their projected range, and the experimentally obtained cross-section of that process is assessed. As a way to improve SE emission durability, we show here the possibility of surface hydrogenation performed at relatively low temperature of 350 °C to cause complete hydrogen termination of CVD diamond film surface. Subsequent low dose proton bombardment causes significant decrease in ion induced electron emission (IIEE) coefficient γ. Repeated ‘cold’ and ‘hot’ re-hydrogenation only partially recovers the electron emission properties of the film, implying the existence of residual Boron-defect complexes.

Ceramic secondary electron emission and surface charge measurements

The secondary electron emission (SEE) coefficient is strongly dependent on material type due to the fact that secondary electrons come primarily from inelastic electron collisions and intrinsic lattice losses. Other macroscopic factors affecting the emission process are related to surface finish, surface coatings, ion implantation, and surface preparation and cleaning procedures. SEE plays a key role in most proposed models for insulator surface flashover development. We have measured total surface electron yield from a number of materials as well as from similar materials with different surface treatments. The experiments were performed using both a continuous wave and a pulsed electron gun with a hemispherical electron energy spectrometer at vacuum levels in the range of 10/sup -8/ torr. Electron spectroscopy reveals substantial difference in total electron yield due to minor changes in surface finish. The results of SEE measurements and the secondary electron energy distribution, using both continuous wave and pulsed electron beams, are presented. Quasimetalized surfaces show distinct SEE reduction over nontreated samples. Another measurement taken, characterized insulator surface charge as a function of pulse length, number of pulses, and total electron beam current. Material surface charge characteristics using electron pulses from 100 /spl mu/s and up to 1 ms are presented. Results indicate radically different surface charge behavior between single pulse, repetitive pulse, and continuous wave experiments.

Sputtering and ion-induced electron emission of graphite under high-dose nitrogen bombardment

The dependence of the sputtering yield Y and the electron emission coefficient γ of isotropic graphites (POCO-AXF-5Q and Russian MPG-LT) on ion fluence and ion incidence angle θ at near room temperatures and the dependence of γ on target temperature under high dose 30 keV N 2 + ion irradiation were measured. It was found that Y and γ are stabilized at fluences F⩾1×10 19 N/cm 2. A specific target surface topography develops. At steady-state conditions, the N concentration in MPG-LT is 19 at.% and in POCO16 at.%. In the angular range θ=0–80°, Y and γ increase and the angular dependence of Y is slightly stronger than that of γ. Sputtering yields of POCO are 1.5 times higher than those of MPG-LT. The reasons of the difference between the experimental and calculated sputtering yields using the TRIM.SP code are discussed. The dependence of γ on the target temperature manifests a step-like increase at ≃250 °C which may be due to radiation induced structure transformation in the modified surface layer.

Secondary Electron Emission from an Organic Layer with a Surface Charge

Using secondary electron emission (SEE) techniques, conditions for the traveling of electrons near a charged surface were studied. A simple analytical expression was found to relate the effective coefficient of secondary electron emission from the charged surface of an organic liquid layer with the primary-electron current. At low currents, the relationship is close to a root law, the pattern of the dependence does not change with the varying conductivity of the liquid, its thickness, and the charge spot area. This finding suggests that the effective secondary electron emission coefficient and, hence, the conditions of electron motion near a surface charge depend on the only parameter, the current density of incident electrons. According to the estimates of the dielectric permittivity of a liquid, its resistivity, and ion mobility, the effective SEE coefficient at low charging currents is formed in the ohmic mode of current flow through the liquid.

The effective coefficient of secondary electron emission in plasma display panel

The effective secondary electron emission coefficient (SEEC) in a plasma display panel (PDP) is estimated by comparing the Paschen breakdown curves from simulations with the experiment. It is found that the effective SEEC in PDP is dependent on the ratio of electric field to pressure. The estimated values are 0.59/spl sim/0.79 for the pure Ne and 0.1/spl sim/0.13 for the Ne-Xe (96/4) mixture, respectively.

Damage in thin SiO 2–Si structures induced by RIE-mode nitrogen and oxygen plasma

The damage induced in thin SiO 2–Si systems after an exposure to O 2 and N 2 plasma working in reactive ion etching (RIE) mode has been studied by means of C– V and I– V techniques including gate oxide breakdown. A generation of large density of positive oxide charge (in bulk traps as well as in slow states) even during the first 15 s of the exposure has been established and for this exposure time the dependence on the remaining plasma conditions is found to be weak. The RIE damage effects become highly process dependent as the plasma time increases — the fixed oxide charge first increases and then slows down or even turns around depending on discharge conditions. It is suggested that the relative contribution of the two main plasma components (ion bombardment and vacuum UV photons) at different discharge regimes is the reason for the appearance or the absence of the “turn-around” effect and the damage level is determined by the plasma conditions rather than by the type of oxidation (dry or wet). The damage leads to an excess leakage current and degrades the breakdown field. The results reveal that a strong linear correlation between leakage current and plasma created positive charge exists. The electron emission coefficient is found to be dependent on the initial oxide parameters.

Boundary conditions including sheath effects at a plasma-facing surface

It has recently been reported that space-charge saturation occurs adjacent to almost every commonly used plasma-facing material for plasma temperatures above {approximately}50 eV. An electron emission coefficient, defined as the average number of surface-emitted electrons per incident plasma electron, which is near or above unity is responsible for the occurrence of space-charge saturation. With this motivation, a fully kinetic self-consistent theory capable of describing the plasma sheath under conditions of space-charge saturation is developed. The theory is then used to obtain boundary conditions which are suitable for incorporation into computer programs which simulate plasmas. {copyright} {ital 1997} {ital The American Physical Society}

Reduction of the power transmission factor and limitation of the secondary-electron inflow at the divertor sheath

A simple model of the collisional divertor plasma indicates that neutral gas collisions should be important in determining the power transmission factor at the transition layer. Inflow of low-energy electrons created by ionization reduces the total potential drop at the transition layer, and the reduction of the power transmission factor is remarkable when the particle amplification factor is much larger than 1. The secondary-electron inflow is effectively diminished due to deflection of the electron by the Lorentz force when the electron cyclotron frequency is much larger than the plasma frequency. The electron emission coefficient under the space-charge limitation is reduced to a value much smaller than 1 due to formation of the magnetic presheath when the magnetic angle approaches 90°.

Fully kinetic plasma-sheath theory for a cold-electron emitting surface

The fully kinetic, one-dimensional, plasma-sheath theory by Schwager and Birdsall [Phys. Fluids B 2, 1057 (1990)] is further developed. A cold-electron emitting surface is included and a three-dimensional plasma is considered. The sheath potential is not assumed to equal the floating potential so that the theory applies to a current-carrying sheath. Appropriate values are found for higher-order moments of the velocity distribution which depend on the three-dimensional velocity distribution width. Distribution functions in terms of energy and angle are derived. The (effective) temperature, the total energy flux, and the heat flux are evaluated in terms of exact analytic functions. The normalized magnitude of the floating potential for a deuterium plasma with equal ion and electron temperatures is calculated to be ψf=3.2 for δ=0 and ψf=1.8 for δ=0.75 where δ is the electron emission coefficient. The normalized magnitude of the sheath potential for the same plasma (with δ=0) is calculated to be ψs=3.9 for γ=0.02 and ψs=2.8 for γ=−0.02 where γ is the normalized current density. A self-consistent integral solution for the electrostatic potential profile within the sheath is derived.

Cathodic voltage drop in a non-self-sustained discharge

The non-self-sustained discharge, maintained by an electron beam, is widely used in gas lasers. The cathodic space-charge layer has a determining effect on the current flow between the electrodes. The calculation of the parameters of the layer reduces to the solution of a system of nonlinear continuity and Poisson equations with the appropriate boundary conditions. This paper describes a mathematical formulation for calculating the cathodic voltage drop under these conditions in a model which incorporates the electron, ion, and beam current densities, the rate of non-self-sustained ionization, the thickness of the space-charge layer at the cathode, the first Townsend coefficient, and the coefficient of secondary electron emission.