Secondary electron emission in scanning Ga ion, He ion and electron microscopes

The secondary electron (SE) emission for Ga ion, He ion and electron impact have been calculated using Monte-Carlo simulations, in which the trajectories of all the collision partners (i.e., primary ions, recoiled target atoms, and excited electrons (electron cascade) along which SEs are excited) have been simulated. The SE yields for Ga ion impact show a gradual decrease with increasing Z₂ which is opposite to that found for electron impact. The main reasons for the decrease in SE yield for Ga are the decrease in number and energy of electrons excited by the primary ions and the decrease in contribution of the recoiled atoms to the SE yield with increasing Z₂. For electron impact, both primary electrons and backscattered electrons (BSEs) excite the SEs. The additional SE excitation created by the electron cascade by BSEs is enhanced for high-Z₂ metals especially at E > a few keV. For He ion impact, the Z₂-dependence is between that for the Ga ions and the electrons and is weak because the He ion is light but still much heavier than an electron. As to the lateral resolution, the electron excitations by trapped He ions dominate the SE yield, so that the SE excitation volume is narrower than for electron and Ga impacts. This small contribution of BS He ions to the SE yield does not increase the information depth determined by the trapped He ions, in contrast with the large contribution of BSEs to the SE yields for SEM imaging. The simulated incident-angle dependence in SE yields shows that the topography contrast for He-SIM imaging is clearer than that obtained by SEM and Ga-SIM imaging.     

Secondary ion emission from Ti and Si targets induced by medium energy Ar ion bombardment - Experiment and computer simulation

The paper presents experimental results of secondary ion energy distributions obtained for Ti and Si targets bombarded by 20-30 keV monoisotope Ar⁺ ion beam. The influence of the extraction voltages between target and a slit of the electrostatic energy analyzer entrance on the energy distributions of secondary ions was investigated. After optimization of the secondary ion extraction system, the mass spectra of secondary ions were also measured. The investigations were done using recently built experimental system. Experimental data are compared with the computer simulation results obtained using TRQR and SATVAL codes.     

Secondary electron emission from nanocomposite carbon films

The secondary electron emission (SEE) properties of sulfur-incorporated nanocomposite carbon (n-C) films were studied. Maximum SEE yield (δ _max) values obtained ranged from 3.27 to 6.98, which are between those for graphite (δ _max ~ 1) and high purity diamond films in their as-grown condition (δ _max ~ 9), and are consistent with the composite nature of the films. It was found that δ _max values of n-C films are mostly determined by the atomic oxygen concentration on the surface of the films, which appears to control the probability of escape of the secondary electrons from the surface of the films, as inferred by employing Ascarelli’s model for SEE (J Appl Phys 89:689, 2001). Also, mean escape depth values for the secondary electrons were obtained using this model, and their significance as bulk parameters for the films is discussed.     

Nitrogen ion induced nitridation of Si(111) surface: Energy and fluence dependence

We present the surface modification of Si(111) into silicon nitride by exposure to energetic N₂⁺ ions. In-situ UHV experiments have been performed to optimize the energy and fluence of the N₂⁺ ions to form silicon nitride at room temperature (RT) and characterized in-situ by X-ray photoelectron spectroscopy. We have used N₂⁺ ion beams in the energy range of 0.2–5.0 keV of different fluence to induce surface reactions, which lead to the formation of Si_xN_y on the Si(111) surface. The XPS core level spectra of Si(2p) and N(1s) have been deconvoluted into different oxidation states to extract qualitative information, while survey scans have been used for quantifying of the silicon nitride formation, valence band spectra show that as the N₂⁺ ion fluence increases, there is an increase in the band gap. The secondary electron emission spectra region of photoemission is used to evaluate the change in the work function during the nitridation process. The results show that surface nitridation initially increases rapidly with ion fluence and then saturates.     

Thermal ion emission from alkali-metal vanadium bronzes

The ion emission properties of M _x V₂O₅ (M = Li, Na, K, Cs) bronzes were studied at temperatures from 480 to 700‡C in applied fields of up to 20 kV/cm. The results were used to evaluate the activation energies of alkali-metal-ion emission from the vanadium bronzes. The emission properties of the bronzes were found to correlate with their diffusion properties. The mechanisms of ion emission from vanadium bronzes are discussed. Deceased.     

Spectral narrowing phenomena in the emission from a conjugated polymer

We report a study of the morphology dependent emission properties of a liquid crystalline polyfluorene in solid thin films. Spectral narrowing (SN) is observed in spin coated films, but after thermal treatment of the same films the SN is no longer observed. For films which are spin coated on a rubbed polyimide alignment layer, the situation is different. There is then no influence of thermal treatment on the observation of SN. However, specific combinations of the pump polarization direction, film alignment direction and the long axis of the rectangular excitation area now play an important role. The spectral location of SN peaks also varies with film morphology and for different regions of the same sample. The occurrence of SN is strongly dependent on film morphology and the excitation configuration.     

The field-induced ion evaporation from electrolyte solutions

It is demonstrated that, in a mass spectrometer with electrohydrodynamic injection of analyzed substances, the field-induced ion evaporation provides for the effective formation of an ion-cluster beam at an electric field strength of ∼1 V/nm at a charged liquid surface.     

Excited atom emission from metals under ion bombardment

In this paper the available models of excited atom emission from metals under ion bombardment are analysed and a new approach to solving the problem of finding the sputtered atom excitation probability is offered. At distances between a metal and an atom less than some critical distance the ‘metal-sputtered atom’ system is considered adiabatically. At the larger distances interaction between the metal and the atom is considered in terms of a weak perturbation which can excite the valence electron of the atom. The calculations of the probability of sputtered Al and Cu atom excitation and the radiation intensity for a number of spectral lines of these elements have been carried out.     

Anisotropy of secondary ion emission from single crystals

The fact that secondary ion emission (SIE) exhibits some features in case of single crystal bombardment should be taken into account when secondary ions are used to diagnose the surfaces of various substances. The features are most noticeable in the space and energy distributions of secondary ions and in the dependence of the SIE yield on the direction of primary ion incidence (the orientation effect).     

Negative ion emission accompanying the field electron emission from amorphous hydrogenated carbon

It was found that the field electron emission from the surface of amorphous hydrogenated carbon is accompanied by the emission of negative ions of hydrogen and hydrocarbons. This ion emission is explained by the formation and degradation of the local surface emission centers.     

Secondary ion emission of Fe-Ni alloys in the temperature range including the Curie point

The temperature dependence of secondary ion emission was investigated for Fe-Ni ferromagnetic alloys with different Curie points T_c and elemental composition: 35% Ni 65% Fe (T_c=240°C), 40% Ni 60% Fe (T_c=360°C), and 50% Ni 50% Fe (T_c=530°C). The alloy 79% Ni 16% Fe 5% Mo (T_c=345°C) was also studied. The spatial distribution of Ni⁺ and Fe⁺ secondary ions emitted from the (1 1 1) face of invar and permalloy single crystals was shown to be anisotropic with pronounced ion-yield maximum for both components in the 〈1 1 0〉 directions. The shape of the energy distribution of Ni⁺ and Fe⁺ ions was found to be virtually identical for all the alloys under investigation with a most probable energy at 7 eV and a width at half-maximum of 12 eV. The temperature dependence of the Ni⁺ and Fe⁺ emission has a maximum near the Curie point of the investigated alloys and another maximum at the Curie point of nickel which may indicate the precipitation of nickel into microscopic islands on the surface as a result of heating and sputtering. Auger analysis of the surface composition in the surface layers showed a variation in concentration of oxygen and carbon atoms when Fe-Ni alloys pass from the ferromagnetic to the paramagnetic state and this must affect also the secondary ion emission of alloy components.     

Exoelectron emission and critical phenomena in CuCl crystals

The temperature dependence of thermostimulated exoelectron emission (TSEE) and the specific heat c _p of CuCl single crystals were studied in a broad temperature range. It is suggested that the TSEE peaks observed in the region of 375 and 420°C are due to the polytype formation and the structural transition from a sphalerite to wurtzite polymorph; the peak at 170°C is assigned to Cu²⁺ ions present in the CuCl crystals studied.     

Secondary electron emission yields from the J-PARC RCS vacuum components

The J-PARC 3 GeV rapid-cycling synchrotron (RCS) is required to provide 1 MW pulsed protons to the spallation neutron target and the 50 GeV main ring. Since the acceleration period is set at a repetition rate of 25 Hz, the eddy current effect due to such rapid repetition magnetic field is a big issue (i.e., the perturbation of the magnetic field and the heating owing to the ohmic loss) in the metal duct. Therefore, we choose the ceramics duct in the magnets in order to avoid the eddy current effect. But the total secondary electron emission yield (SEY) from the ceramics surface is larger than the metal one and there is a possibility of bringing about the beam instability by those secondary electrons. In order to reduce these undesirable electron emission from the chamber surface, titanium nitride (TiN) is coated on the inside surface of the chambers. We measured SEY from TiN coating surface with various conditions. First, we varied the nitrogen density and crystal face in the coating. Second, we exposed the samples to the oxygen and the water vapour with electron beam bombardment in order to simulate the actual conditions. We further investigate diamond-like carbon coating to search for the possibility of another coating. The results of these measurements are shown.     

Acoustic emission: some problems,tasks and solutions

Recent studies of acoustic emission in metal samples and structures undertaken at CNIITMASH are reviewed. General features of AE are discussed. Some results from AE investigations are presented, including evaluation, sensitivity, AE signal models, AE during plastic deformation. AE from fatigue cracks; also aspects of AE source classification and criteria for object rejection are considered.     

Modelling of cluster emission from metal surfaces under ion impact

Using the molecular-dynamics technique, cluster emission for 5 keV Ar bombardment of a Cu (111) surface has been investigated using a many-body (tight binding) potential for the Cu-Cu interaction. The calculations allow us to analyse the basic processes underlying cluster emission. It is found that two distinct processes can be distinguished which lead to cluster emission under energetic ion bombardment. The first process causes the emission of small clusters, which are emitted by a collective motion during the development of the collision cascade within the first picosecond after impact. Thus, emission times of such clusters agree with the emission times of atoms in sputtering. Such a process can be envisioned if, for example, a few layers below the surface, an energetic recoil causes the development of a subcascade. Energy transferred by this event to the surface is strongly directional and can lead to the simultaneous emission of a group of neighbouring surface atoms, which in some cases will remain bounded and form a cluster after emission. Typically, clusters emitted by this mechanism consist of atoms, which are neighbouring in the target and are almost exclusively surface atoms, similar to all sputtered atoms. Emission of large clusters (cluster sizes of 10 or more atoms), as observed experimentally, is a puzzling phenomenon. From our calculations we conclude that the emission of such large clusters does not occur during the collisional phase of sputtering, but happens much later (5-10 ps after ion impact). Emission can occur for spike events, where all the energy of the impinging ion is deposited locally in a small volume near to the surface, and the sputtering yield is 3-5 times the average yield. Such events are rare, but we have found a few cases in our calculations where stable clusters consisting of more than 20 atoms were emitted. Melting of the spike volume occurs, and the high temperatures and pressures produced can cause emission of large fragments during the thermal phase. The composition of such large clusters is quite different from that of small clusters. They consist of atoms from different layers and the constituents are also generally not next-neighbour atoms. This change in origin of the cluster atoms reflects the mixing and diffusion processes occurring in the melted zone before emission. The calculations indicate that hydrodynamical phenomena might play a role in the emission of large fragments. Additional calculations, where the energy was distributed 'thermally' in a three-dimensional volume under the surface for 500 fs, give very similar results, even in such cases where the kinetic phase of the collision-cascade development was absent.     

Angular dependence of the ion energy,charge, and mass distributions in surface discharge plasma

Previously, the ion energy, charge, and mass distributions in vacuum discharge plasma at the surface of polymeric insulators were studied using an analyzer oriented perpendicularly to the surface, and both multiply charged and high-energy ions were found in the plasma. This Letter presents the first experimental results on the dependence of the ion energy, charge, and mass distributions in the plasma on the particle take-off angle relative to the insulator surface.     

Angular dependence of Auger electron emission from a single crystal specimen

An experimental study of the angular dependence of Auger electron emission from the (111) surface of a single crystal of copper has shown marked crystallographic effects. The equipment, a LEED/AES system incorporating an energy analyzing Faraday cup, is described. The results are discussed in terms of diffraction and inherent angular dependence in the Auger emission.