Low Energy Electron Bombardment Research Articles

Monte Carlo Simulation of Free-standing Thin Films under Low Energy Electron Bombardment: Electron Inelastic Mean Free Path (IMFP) Determination Using Elastic Peak of the Transmitted Electrons

Abstract: The electron energy spectra of transmitted scattered electrons from free-standing films are simulated using a Monte Carlo computational approach. Elastic scattering is simulated using Mott cross-sections and inelastic scattering via discrete processes determined from dielectric function data. This allows one to simulate the secondary electrons as well as the loss peaks near the elastic (zero-loss) peak. The current study suggested a directed approach for determining the electron inelastic mean free path (IMFP) of materials at low primary electron energies. The IMFP of the reference material is not necessary for the suggested technique. The suggested technique uses the ratio between the transmitted elastic peak intensity and the background intensity of backscattered electrons. Free-standing films of Si, Cu, and Au were studied with thicknesses varying from 2 to 12 nm. Primary electron energies of 1, 3, and 5 keV were applied. The results appeared very good, with the percentage error range being between 5% and 25%. We also investigated the proportion of the first and second plasmon peak intensities to the elastic peak intensity. We believe that the latter could provide a directed method of measuring the IMFP of materials. Keywords: Inelastic mean free path, Monte Carlo simulation, Geant4, Free-standing film, Zero-loss peak.

Исследование влияния облучения ионами азота на состояние поверхности лития

A comparison of the surface of solid lithium in the atomically pure state, after contact with oxygen, nitrogen and after irradiation with N+ ions with an energy of 0.6 keV at a current density of 2 µA/mm2 was carried out. The analysis of surface characteristics was carried out using electron Auger spectroscopy. To obtain an atomically pure surface, low-energy electron bombardment was used. It is shown that the contact of an atomically pure Li surface with oxygen leads to the formation of lithium oxide, and with nitrogen with a partial pressure of 10-6 Pa, lithium oxide and oxynitride in a mixture with free lithium atoms. Irradiation of lithium with nitrogen ions leads to the formation of the surface compound Li3N. It is noted that the resulting compound is unstable in ultrahigh vacuum and eventually decays to form lithium oxide due to interaction with the residual oxygen of the spectrometer chamber.

Investigation of the effect of nitrogen ion irradiation on the lithium surface condition

A comparison of the surface of solid lithium in the atomically pure state, after contact with oxygen, nitrogen and after irradiation with N+ ions with an energy of 0.6 keV at a current density of 2 μA/mm2 was carried out. The analysis of surface characteristics was carried out using electron Auger spectroscopy. To obtain an atomically pure surface, low-energy electron bombardment was used. It is shown that the contact of an atomically pure Li surface with oxygen leads to the formation of lithium oxide, and with nitrogen with a partial pressure of 10-6 Pa, lithium oxide and oxynitride in a mixture with free lithium atoms. Irradiation of lithium with nitrogen ions leads to the formation of the surface compound Li3N. It is noted that the resulting compound is unstable in ultrahigh vacuum and eventually decays to form lithium oxide due to interaction with the residual oxygen of the spectrometer chamber. Keywords: lithium, vacuum, ions, electrons, surface.

Interface induced diffusion

Interface induced diffusion had been identified in a thin film system damaged by electron bombardment. This new phenomenon was observed in Al2O3 (some nm thick)/Si substrate system, which was subjected to low energy (5 keV) electron bombardment producing defects in the Al2O3 layer. The defects produced partially relaxed. The rate of relaxation is, however, was different in the vicinity of the interface and in the "bulk" parts of the Al2O3 layer. This difference creates an oxygen concentration gradient and consequently oxygen diffusion, resulting in an altered layer which grows from the Al2O3/Si substrate interface. The relative rate of the diffusion and relaxation is strongly temperature dependent, resulting in various altered layer compositions, SiO2 (at room temperature), Al2O3 + AlOx + Si (at 500 °C), Al2O3 + Si (at 700 °C), as the temperature during irradiation varies. Utilizing this finding it is possible to produce area selective interface patterning.

Electron-Promoted Desorption from Water Ice Surfaces: Neutral Gas-Phase Products

Electron-promoted desorption (EPD) from compact amorphous solid water (c-ASW) has been studied. Low-energy electron bombardment with 200–300 eV electrons leads to H2O depletion, as monitored by reflection–absorption infrared spectroscopy (RAIRS) of the remaining c-ASW film. Cross sections for H2O depletion were calculated to be in the range from 1.6 ± 1.0 × 10–16 to 5.2 ± 3.0 × 10–16 cm2. However, mass spectrometric measurements identify a major component of the desorbing material as H2, which appears with kinetics similar to those for H2O loss. Molecular H2O is observed as a minor desorption product in the gas phase.

ENHANCEMENT OF THE 6.4 keV LINE IN THE INNER GALACTIC RIDGE: PROTON-INDUCED FLUORESCENCE?

A common idea for the origin of the Galactic diffuse X-ray emission, particularly that of the iron lines from neutral and highly ionized atoms, is a superposition of many cataclysmic variables and coronally active binaries. In this scenario, the flux should symmetrically distribute between the east and west on the plane with respect to Sagittarius A$^*$ because the stellar mass distribution determined by infrared observations is nearly symmetric. This symmetry is confirmed for the highly ionized iron line as well as the continuum emission. However, a clear excess of the neutral iron line in the near east of the Galactic center compared to the near-west side is found. The flux distribution of the excess emission well correlates with molecular column density. The X-ray spectrum of the excess emission is described by a power-law continuum plus a 6.4 keV line with the large equivalent width of $\sim1.3$ keV, which is hardly explained by the low-energy electron bombardment scenario. The longitudinal and latitudinal distribution of the excess emission disfavors the X-ray irradiation, neither by Sagittarius A$^*$ nor by nearby X-ray binaries. Then the low-energy proton bombardment is the most probable origin although the high energy density $\sim 80$ eV cm$^{-3}$ in 0.1-1000 MeV is required and there is no conventional proton source in the vicinity.

Switching molecular orientation of individual fullerene at room temperature.

Reversible molecular switches with molecular orientation as the information carrier have been achieved on individual fullerene molecules adsorbed on Si (111) surface at room temperature. Scanning tunneling microscopy imaging directly demonstrates that the orientation of individual fullerene with an adsorption geometry of 5-6 bond is rotated by integral times as 30 degree after a pulse bias is applied between the STM tip and the molecule. Dependences of the molecular rotation probability on the voltage and the process of applied bias reveal that the rotation of a fullerene molecule takes place in two successive steps: the bonding between the fullerene and the Si surface is firstly weakened via electronic excitation and then low energy electron bombardment causes the molecule to rotate by certain degree.

Stabilization of a spike in excitonic light reflection spectra of CdSe crystals subjected to low-energy electron bombardment

In the low-temperature (4.2 K) excitonic light reflection spectra of CdSe crystals subjected to the low-energy (several keV) electron irradiation, an effect of the stop of a “rotation” at the longitudinal exciton frequency is found. The connection of this effect with Fermi level pinning on the surface of the semiconductor is assumed.

Formation of aluminum island films under electron irradiation of a sapphire surface

The present paper is devoted to experimental investigation of electron-stimulated desorption from a single-crystal sapphire surface. Using Auger electron spectroscopy metallization of the sapphire surface, the character of which is dependent on the electron beam parameters, is observed under low-energy electron bombardment,. Using atomic-force microscopy, images of metal island films with a diameter of 50–100 nm are obtained.

Magnetron Sputtering for II-VI Solar Cells: Thinning the CdTe

AbstractMagnetron sputtering (MS) of CdTe and related II-VI materials facilitates low energy ion and electron bombardment that promotes good film growth at substrate temperatures well below those needed for other physical vapor deposition methods. MS also provides good control of deposition rates while allowing scale-up to large areas. In this paper we review the use of MS for deposition of polycrystalline thin films of CdS, CdTe and related materials for solar cells with a focus on reducing the thickness. We relate the deposition conditions and plasma properties determined by Langmuir probe measurements to some of the materials properties of the films through spectroscopic ellipsometry and high resolution electron microscopy. For cells with CdTe layers from 0.35 to 2.5 μm, we have done a first-order optimization of chloride treatment conditions and back contact structure. We discuss the influence of CdTe thickness and post-deposition processing on the efficiency, open-circuit voltage, short-circuit current, and fill factor and show that 10% efficient cells can be fabricated with 0.5 μm of CdTe.

Luminescence and crystallinity enhancement using nano-oxide

The seeded nano-TiO 2 particles in the hydrothermal (HT) solution assist phosphor crystallite nucleation and growth, facilitate formation of cubic grain shape, but limit grain growth during post-annealing so as to strongly enhance the cathodoluminescence (CL) intensity. Nano-crystallites of SrTiO 3:Pr, Al phosphors approximately 80 nm with specific shape are successfully obtained and red cathodoluminescence is easily excited at low accelerating voltage of 1 kV. The obtained phosphors after annealing at 1000 °C 1 h −1 exhibit the highest CL intensity than those obtained under other annealing conditions and those direct-pyrolyzed phosphors as well as mixed-oxide (MO) phosphors having composition similar to hydrothermal ones. The smaller phosphor crystal possesses larger surface and homogeneous activators distribution to facilitate the efficient excitation of low-energy electron bombardment of 1 keV.

Surface photo-voltage effect of hydrogenated diamond subjected to low energy electron irradiation

In this paper we report on the influence of sub-band gap photon illumination on the electron emission of hydrogenated diamond surfaces subjected to continuous low energy electron irradiation. Hydrogenated diamond surface traps charge by resonance electron attachment (REA) and formation of C–H (ads) − anionic transient surface species with a maximum cross section at an incident electron energy of ∼ 9 eV. The steady state population of these anionic surface species results in up-wards surface band bending and consequent decrease of low energy electron emission. It is demonstrated that concurrent low energy electron bombardment and illumination of the diamond surface with sub-band gap photons enhances the secondary electron emission yield of diamond. It is argued that these effects are associated to a surface photo-voltage effect, which results in unpinning of the surface band.

Infrared absorption spectroscopy of diacetylene ions trapped in solid argon

The C4H2+ diacetylene radical cation has been generated in a pulsed jet electrical discharge through both a diacetylene/argon mixture and an acetylene/argon mixture. The product mixture was trapped on a 12 K cryostat window and studied via Fourier transform infrared absorption spectroscopy. The diacetylene cation was also produced via low energy electron bombardment of an effusive C4H2/Ar beam. Two new infrared bands at 3201.6 and 1827.9 cm(-1) have been identified as vibrations of the diacetylene cation, viz the v4 (sigmau) (C-H stretching) and the v5(sigmau) (C[triple bond]C stretching) modes, respectively. Geometry optimization and harmonic frequency calculations, carried out at various spin unrestricted levels (B3LYP, CCSD(T)) for spin doublet structures, indicate that, in its electronic ground state X2pi(g), the C4H2+ cation is linear. Three additional new bands at 2957.5, 1693.8 and 594.5 cm(-1) have been tentatively assigned to the C-H stretching, C[triple bond]C stretching and C[triple bond]C-H (in-plane) bending modes, respectively, of the nonlinear diacetylene anion (C4H2-, X2B(u)).

Reactions induced by low energy electrons in cryogenic films (Review)

We review recent research on reactions (including dissociation) initiated by low-energy electron bombardment of monolayer and multilayer molecular solids at cryogenic temperatures. With incident electrons of energies below 20 eV, dissociation is observed by the electron stimulated desorption (ESD) of anions from target films and is attributed to the processes of dissociative electron attachment (DEA) and to dipolar dissociation. It is shown that DEA to condensed molecules is sensitive to environmental factors such as the identity of co-adsorbed species and film morphology. The effects of image-charge induced polarization on cross sections for DEA to CH3Cl are also discussed. Taking as example, the electron-induced production of CO within multilayer films of methanol and acetone, it is shown that the detection of electronic excited states by high-resolution electron energy loss spectroscopy can be used to monitor electron beam damage. In particular, the incident energy dependence of the CO indicates that below 19 eV, dissociation proceeds via the decay of transient negative ions (TNI) into electronically excited dissociative states. The electron-induced dissociation of biomolecular targets is also considered, taking as examples the ribose analog tetrahydrofuran and DNA bases adenine and thymine, cytosine and guanine. The ESD of anions from such films also show dissociation via the formation of TNI. In multilayer molecular solids, fragment species resulting from dissociation, may react with neighboring molecules, as is demonstrated in anion ESD measurements from films containing O2 and various hydrocarbon molecules. X-ray photoelectron spectroscopy measurements reported for electron-irradiated monolayers of H2O and CF4 on a Si–H passivated surface further show that DEA is an important initial step in the electron-induced chemisorption of fragment species.

The role of electron-stimulated production of O 2 from water ice in the radiation processing of outer solar system surfaces

We present measurements of the electron energy threshold, fluence, and temperature dependence for O 2 production during low-energy (5–100 eV) electron bombardment of thin (∼40 bilayer) amorphous and crystalline D 2O ice films deposited on a Pt(1 1 1) substrate in vacuum. The electron energy threshold (referenced to the vacuum level) for O 2 formation is approximately 10±2 eV. This threshold is due to valence level electronic excitations or ionization of condensed water molecules. Comparison of the fluence dependence with kinetic models shows that the formation of O 2 is dominated by direct excitation and dissociation of a stable precursor molecule, and not by diffusion and recombination of oxygen atoms. The O 2 yield is also strongly dependent upon the temperature of ice, is different for crystalline and amorphous ice films, and is indicative of surface and bulk structural transitions. We demonstrate that electron-stimulated production of O 2 is important in radiation processing of low-temperature icy surfaces present on outer solar system bodies. This is particularly true in regions with strong magnetospheres such as the Saturnine and Jovian systems.

Modification of PEDOT–PSS by low-energy electrons

The stability of conjugated organic materials under electron transport is of great importance for the lifetime of devices such as polymer light-emitting diodes (PLEDs). Here, the modification of thin films of poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate) (known as PEDOT–PSS, often used in the fabrication of PLEDs) by low-energy electrons has been studied using X-ray photo-electron spectroscopy. Thin films of PSSH and molecular solid films of EDOT molecules also have been studied. We find that electrons with kinetic energies as low as 3 eV result in significant modification of the chemical structure of the materials. For thin films of PSSH, the electron bombardment leads to a strong loss of oxygen and a smaller loss of sulfur. In addition, a large amount of the sulfur atoms that remain in the films exhibits a different binding energy because of scissions of the bonds to oxygen atoms. For condensed molecular solid films of EDOT molecules, we find that the carbon atoms bonded to oxygen react and form additional bonds, as evidenced by a new component in the C(1s) peak at a higher binding energy. In the PEDOT–PSS blend, we find both effects. The importance of these observations for light-emitting diodes incorporating PEDOT–PSS films is discussed. This work demonstrates that the combination of in situ low-energy electron bombardment in combination with photo-electron spectroscopy is a powerful method to simulate and study certain processes, associated with low-energy electrons, occurring in organic based devices, which cannot be studied directly otherwise.

Preresonance Raman Spectrum of the C13H9 Fluorene-like Radical

The neutral open-shell species C13H9 formed from fluorene, C13H10, by low-energy electron bombardment and by ultraviolet photolysis in an argon matrix at 12 K has been studied via preresonance Raman, infrared, and ultraviolet/visible spectroscopy. Density functional theory calculations (B3LYP/6-31G(d,p)) of the CH bond energies of neutral fluorene showed that the most probable position for the hydrogen loss is the sp3 carbon in the five-membered ring. Calculations of the C13H9 harmonic vibrational frequencies are shown to match the experimental Raman (and infrared) bands well. A new electronic transition is identified at 283.1 nm (4.38 eV). Its position agrees with earlier time-dependent density functional theory calculations. Oscillator strengths for this transition and three others are estimated. The electronic transitions in the dehydrogenated species, C13H9, are strongly red-shifted compared to fluorene.

CF 3 I on a silicon surface: Adsorption, temperature-programmed desorption, and electron-stimulated desorption

CF 3 I adsorption on a silicon surface and the effect of low-energy electron bombardment of a CF3I-covered silicon surface are relevant to plasma etching. Dissociative chemisorption of CF3I on Si(100) surface is observed at 370 K. Uptake measurements corroborated by work-function change measurements and temperature-programmed desorption (TPD) gives a sticking probability of at least 0.34±0.05. Molecular orbital calculations yield an adsorption energy greater than 3 eV for dissociative chemisorption of CF3X (X=F, Cl, Br, and I) on Si(100) (modeled by Si9H12) with X transferred to a silicon atom. We conclude that the variation in the sticking probability across the CF3X family is a consequence of the activation energy barrier for C–X bond cleavage. In TPD, SiF4 desorbs at 370 and 840 K, SiF3 radical at 770 K, and atomic iodine at 790 K. The parent CF3I does not desorb. Electron-stimulated desorption (ESD) yields F+, F− and a trace of I+. The threshold for the appearance of F+ is 20 eV and for F− is 29 eV. Dissociative ionization and dipolar dissociation are possible mechanisms for ESD of F+. Dipolar dissociation and harpooning are possible mechanisms for ESD of F−. There is evidence that iodine on the surface quenches the electronically excited states that lead to desorption of F−.

Selectivity of bond-breaking in electron-induced dissociation of hydrocarbon films on Au surfaces

Electron-induced dissociation (EID) processes in organic and polymer films are important in a variety of applications. One scientific issue concerns the selectivity of low-energy electron bombardment to cleave a single C–H bond in hydrocarbon condensed films or adsorbed layers. We have undertaken combined TPD and FT-IRAS studies to identify and determine the purity of the surface species produced by EID of cyclohexane and benzene films on Au(1 1 1). Experimental evidence strongly supports a conclusion that EID of cyclohexane and benzene monolayer and multilayer films using incident electrons with an energy of 30 eV can be used to cleave a single C–H bond and produce a single chemisorbed product with high selectivity. An important consequence of these observations is that a variety of adsorbed hydrocarbon intermediates on Au(1 1 1) and other metal surfaces can be prepared in this manner, spectroscopically characterized, and used in studies to advance the understanding of chemical bonding, reactions and catalysis at surfaces.

Light emission from small copper particles excited by current passage or by low-energy electron bombardment

The light emission from small Cu particles was investigated by passing electrical current through them as well as during bombardment by low-energy electrons (⩽400 eV). Measurements were performed in the spectral range 1.18–6.2 eV. The shape of the spectra was dependent on the voltage applied to the tunnel-coupled small Cu particles. As the voltage increases, some features arise in the high-energy part of the spectra. For different modes of particles’ excitation spectra differ in peaks intensities, but their position on the energy scale remains constant. Thus, a conclusion can be drawn that the nature of the light emission obtained at various excitation modes is caused by similar physical origins.