Enhancement Of Secondary Electron Emission Research Articles

Electric field and higher harmonics of RF plasma slit jet measured by antennas and VI probes

The cold atmospheric plasma jets change their character when interacting with the different surfaces. Since such interaction is the primary area of plasma jet applications, it is essential to monitor the process. The non-linearity of the RF plasma slit jet (PSJ) was analyzed using the VI probes and a novel method, the non-intrusive antenna measurements. Regardless of the experimental setup and gas mixture (Ar, Ar/O2, Ar/N2), the PSJ frequency spectrum consisted of the following main features: dominant fundamental frequency peak, relatively strong odd harmonics, and significantly weaker even harmonics. The lowest degree of non-linearity was recorded for the Ar PSJ ignited against a grounded target. Admixing a molecular gas increased the discharge non-linearity. It was attributed to the enhancement of secondary electron emission from the dielectric surfaces. In addition to the non-linearity analysis, the antenna spectra were for the first time used to determine the semi-quantitative values of the PSJ-radiated electric field. The electric fields decreased by a factor of 2 after the admixing of nitrogen and oxygen molecular gases. Out of the studied targets, the highest electric fields were observed when plasma impinged on the grounded targets, followed by the floating target (2× lower) and the PSJ ignited in the open space configuration (4× lower than in the grounded target configuration).

Secondary electron emission enhancement of MgO-Au composite film by SiO2 doping in MgO surface layer

The secondary electron emission (SEE) properties of MgO-Au composite film deposited by magnetron sputtering was improved by SiO2 doping with a Si/Mg ratio of 14 % in its MgO surface layer. Compared to the conventional electronic material MgO/(MgO-Au) film, the SiO2-doped MgO/(MgO-Au) film achieved a secondary electron yield (SEY) of 7.69 with an increase of 9.2 % at the primary electron energy (Ep) of 300 eV and the maximum SEY of 11.4 eV with an increase of 12.9 % at the Ep of 700 eV, and it kept relatively high SEY consistently under the continuous bombardment of the incident electrons with the Ep of 200 eV. This SEE enhancement is attributed to the reductions of both the band gap and work function of MgO crystal as well as the grain enlargement and surface roughness decrease of MgO surface layer induced by SiO2 doping. This scheme of SiO2 doping in the MgO surface layer of MgO-Au film provides a new idea for the preparation of high performance MgO-based SEE material.

Temperature enhancement of secondary electron emission from hydrogenated diamond films

The effect of temperature on the stability of the secondary electron emission (SEE) yield from ∼100-nm-thick continuous diamond films is reported. At room temperature, the SEE yield was found to decay as a function of electron irradiation dose. The SEE yield is observed to increase significantly upon heating of the diamond surface. Furthermore, by employing moderate temperatures, the decay of the SEE yield observed at room temperature is inhibited, showing a nearly constant yield with electron dose at 200 °C. The results are explained in terms of the temperature dependence of the electron beam-induced hydrogen desorption from the diamond surface and surface band bending. These findings demonstrate that the longevity of diamond films in practical applications of SEE can be increased by moderate heating.

Tuning the Electron Affinity and Secondary Electron Emission of Diamond (100) Surfaces by Diels−Alder Reaction

The tuning of electron affinity and secondary electron emission on diamond (100) surfaces due to cycloaddition with 1,3-butadiene is investigated by photoemission experiments and density functional theory (DFT) calculations. A significant reduction in electron affinity up to 0.7 eV and enhancement of secondary electron emission were observed after 1,3-butadiene adsorption. The lowering of vacuum level via 1,3-butadiene adsorption is supported by DFT calculations. The C-H bonds in the covalently bonded organics on diamond contribute to the enhanced secondary electron emission and reduced electron affinity in a mechanism similar to that of C-H bonds on hydrogenated diamond surfaces. This combination of strong secondary emission and low electron affinity by the organic functionalization of diamond has potential applications in diamond-based molecular electronic devices.

In situ electric-field-induced contrast imaging of electronic transport pathways in nanotube-polymer composites

An electric-field-induced contrast mechanism for scanning electron microscopy is reported which permits the visualization of embedded nanomaterials inside various matrices with high contrast and high definition. The high contrast is proposed to result from localized enhancement of secondary electron emission from the nanomaterials due to electric-field-induced changes in their work functions. By utilizing a stage that allows in situ current-voltage measurements inside a scanning electron microscope, single-walled carbon nanotubes embedded within polymethyl methacrylate films were visualized directly. In addition to the rapid assessment of nanotube dispersion within polymers, electric-field-induced contrast imaging enables the determination of percolation pathways. From the contrast in the images, the relative voltage at all points in the electron micrograph can be determined, providing a new mechanism to understand electronic percolation through nanoscale networks.

Sub-bandgap photoenhancement of electron emission and discharging of hydrogenated and hydrogen-free diamond surfaces

In this paper we report on the photoinduced enhancement of secondary electron emission and discharging of hydrogenated and hydrogen-free diamond surfaces subjected to continuous low energy electron irradiation. Diamon surfaces subjected to electron irradiation trap charges resulting in upwards surface band bending and consequent decrease of secondary electron emission. It is demonstrated that simultaneous electron irradiation and illumination of the diamond surfaces with sub-bandgap photons in the visible range enhances the secondary electron emission yield of diamond. For the hydrogenated diamond surfaces it is suggested that these effects are associated to a surface photovoltage effect which results in unpinning of the surface bands by photoexcitation of carriers and compensation of trapped surface charges. For the hydrogen-free diamond, discharging is most likely associated with photoexcitation of the trapped charges in midgap electronic states associated with surface reconstruction.

Desorption of nitrogen from amorphous carbon under electron and swift heavy ion bombardment

Desorption of nitrogen adsorbates from amorphous carbon (a-C) surfaces due to swift ion ( Z=6–73, q=6–54, energy 6–13 MeV/u) and electron impact (5 keV) was studied by secondary electron emission (SEE) measurements. The fluence dependence of ion induced electron emission obeys an exponential decay law as the coverage rate is in the sub-mono-layer range. The desorption yield σ varies approximately as σ∼(d E/d x) 3/2 or σ∼ q 3 as a function of the electronic energy loss d E/d x and the projectile charge q, respectively. We discuss the possible desorption mechanisms connected to electronic excitation. Furthermore, we present an ESCA investigation on the mechanism for the influence of nitrogen adsorbates on SEE yields. The mechanism for the SEE enhancement effect is connected to the surface termination: nitrogen adsorbates are believed to provide surface states which generate a downwards energy band bending, which on relative scale pushes up the Fermi level towards the conduction band minimum, reducing the work function.

Influence of adsorbates on electron emission from amorphous carbon under electron and swift heavy ion bombardment

Secondary Electron Emission (SEE) yield measurements have been used to investigate the desorption of nitrogen adsorbates from amorphous carbon (a-C) surfaces due to swift heavy ion impact. The fluence dependence of ion induced electron emission obeys an exponential decay law when the coverage rate is in the sub-mono-layer range. This reveals the relative contribution of the both emitting zones, the “clean” one (having the SEE properties of carbon) and the covered one (with a higher SEE coefficient) to the total measured yield. The mechanism for the SEE enhancement effect is connected to the surface termination. Nitrogen adsorbates are believed to provide surface states which generate a downwards energy band bending, which on relative scale pushes up the Fermi level towards the conduction band minimum, reducing the work function and increasing the SEE yield subsequently. A shoulder observed on the rising edge of the peak of the true SE peak is ascribe to the fraction of hydrogenated termination of a-C which exhibits a lower electron affinity.

Enhancement of secondary electron emission by annealing and microwave hydrogen plasma treatment of ion-beam-damaged diamond films

In this work we investigate the influence of annealing and microwave (MW) hydrogen plasma exposure of ion-beam-irradiated diamond film surfaces. In particular, we are interested in the recovery of secondary electron emission (SEE) and negative electron affinity (NEA) by removal of the damaged layer. To this aim, we correlate the SEE of variously treated Xe+ ion-damaged diamond films with their bonding structure in the near-surface region, as identified by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy. The 30 keV Xe+ ion bombardment of hydrogenated polycrystalline diamond films to a dose of 2×1015 cm−2 results in the transformation of the near-surface region of a diamond film to sp2-bonded amorphous carbon, increased oxygen adsorption, shift of the electron affinity from negative to positive, and strong degradation of its electron emission properties, although it does not induce a pronounced depletion of hydrogen. Exposure of the ion-bombarded films to MW hydrogen plasma treatment for 30 min produces NEA diamond surfaces, but only partially regenerates SEE properties, retains some imperfection in the near-surface atomic layers, as determined by NEXAFS, and the concentration of oxygen remains relatively high. Subsequent annealing to 610 °C produces oxygen-free diamond films and somewhat increases their SEE. Annealing to 1000 °C results in desorption of the surface hydrogen, formation of positive electron affinity surfaces, and drastically degrades their electron emission properties. Prolonged (up to three hours) MW hydrogen plasma treatment of as-implanted diamond films gradually improves their crystal quality and results in a further increase of SEE intensity. The SEE intensity after three hours MW hydrogen plasma exposure of the ion-beam-irradiated films was found to be ∼50% above the value obtained for the as-deposited diamond films. This treatment does not, however, substantially reduce the concentration of oxygen in the previously damaged diamond, indicating its bulk diffusion during or after ion bombardment. Our results show that removal of damage from a highly disordered diamond surface and recovery of its electron emission properties are possible by MW hydrogen plasma. However, it is a slow process. This is most likely due to the very low etching rate of the low-level damage at the end of the ion beam range.

Heat flows through plasma sheaths

Plasma heat flow to material surfaces through sheaths is studied, taking several key physics factors into account. Electron emission from the surface, which breaks a thermal insulation of the sheath, is studied in both thermoelectron emission (TEE) and secondary electron emission (SEE), in which a correct expression under space charge limited condition is given for arbitrary sheath voltages. Nonlinear thermal bifurcation induced by electron emission is analyzed in the experiment and the theory. The local heat flow was found to be enhanced by a thermal contraction induced by cross-field potential variation in a plasma. An enhancement of SEE of hydrogen-absorbed graphite, and a suppression of SEE by the gyromotion of emitted electrons in obliquely incident magnetic field are identified. The effects of ion reflection on the surface and ponderomotive force are also discussed in terms of energy transmission factor δ. An anomaly of δ in detached recombining plasmas is discussed.

Enhancement of secondary electron emission from heavily Si-implanted and Si-doped GaAs

The enhancement of secondary electron emission was observed for both heavily Si-implanted and heavily Si-doped GaAs after annealing at 950 °C. The enhancement was found to be related to the generation of Ga vacancy during annealing. The assessment of electrical properties of the enhanced area revealed that the enhancement arises from the secondary electron production process in the bulk region, not from the surface effect including band bending at the wafer surface. We suggested that the excitons produced by primary electrons would ionize the lattice defects introduced in ion implantation and/or annealing processes.

The role of atomic hydrogen and its influence on the enhancement of secondary electron emission from C(001) surfaces

The role of chemisorbed hydrogen in the enhancement of low-energy electron emission from natural type IIb C(001) diamond surfaces has been investigated. A hydrogen induced low-energy emission peak, whose intensity was found to be a linear function of surface coverage, was observed. The direct observation of emission from vacuum level states in the photoemission spectra has determined a negative electron affinity of ∼0.4 eV for the hydrogenated C(001)-1×1 surface. Constant initial states photoemission has unambiguously identified the electron emission process with the escape of electrons from bulk electron states at the conduction-band minimum.

Secondary electron emission enhancement and defect contrast from diamond following exposure to atomic hydrogen

Polished nominal (100) surfaces of four types of diamonds were exposed to atomic hydrogen by hot filament cracking of H2 gas or by immersion in a H2 plasma discharge. Both types IIa and IIb (100) diamond surfaces exhibited the following characteristic changes: (a) secondary electron (SE) yield increased by a factor of ∼30 as measured in a scanning electron microscope (SEM), (b) near-surface, nontopographical defects were observable directly using the conventional SE mode of the SEM, (c) surface conductance increased by up to 10 orders of magnitude. These changes were observed only weakly in nitrogen-containing types Ia and Ib diamonds.

Adsorbate induced enhancement of secondary electron emission from the layered compound VSe 2

It is demonstrated how adsorbates may drastically enhance the photoemission yield at low kinetic energies from VSe 2 surfaces. The reason for this enhancement seems to be that the adsorbate by reducing the work function φ creates a condition closely resembling negative electron affinity (NBA), i.e. the vacuum level is pulled down into an absolute band-gap. In contrast to true NBA systems, there are empty states (predominantly of V3d character) available below the vacuum level, but due to low probability for scattering into these states, the NEA-like behaviour prevails. Since the involved band minimum is located close to the K symmetry point of the Brillouin zone, adsorbate induced diffuse scattering is vital to the observed enhancement.

Adsorbate induced enhancement of secondary electron emission from the layered compound VSe2

Adsorbate induced enhancement of secondary electron emission from the layered compound VSe2

Secondary electron emission enhancement by metal monolayer adsorption

Changes in secondary electron emission (SEE) during absorption of a metal vapour can be followed by measuring the changes in the net current icc flowing into the specimen crystal. Plots of icc as a function of deposition time t (for a fixed primary beam energy) are presented for five different adsorbate-substrate systems (Pb or Sn on different Cu single crystals). Comparison is made with results from Auger spectroscopy. Adsorption up to the complete monolayer coverage usually causes significant enhancement of emission. Sharp breaks in the icc-t plots permit an accurate identification of different stages of adsorption. The plots are particularly sensitive to disorder of the adlayer. It is suggested that the observed effects are not due only to work function changes but that there are electron scattering events at the adsorbate-substrate interface which can enhance ionisation in the surface region. Such interfacial effects may be important for quantitative surface electron spectroscopies. The acronym SEECC is proposed for this simple method for surface characterisation.