Development of secondary ion mass spectroscopy using medium energy C60 ion impact

In this paper, we report time-of-flight (TOF) secondary ion mass spectroscopy using primary C₆₀ ions with an energy range from several tens of keV to several hundreds of keV. Application of the spectroscopy to the analysis of a poly(amino acid) film revealed that characteristic peaks, necessary for identification of the amino acid in proteins, show higher intensities for medium energy C₆₀ (120 keV and 540 keV ) impacts than those for low energy C₆₀ (30 keV ) impacts. This finding demonstrates that medium energy C₆₀ ion impacts are useful for highly sensitive characterization of amino acids.     

Comparison of secondary ion emission yields for poly-tyrosine between cluster and heavy ion impacts

Emission yields of secondary ions necessary for the identification of poly-tyrosine were compared for incident ion impacts of energetic cluster ions (0.8 MeV , 2.4 MeV , and 4.0 MeV ) and swift heavy monoatomic molybdenum ions (4.0 MeV Mo⁺ and 14 MeV Mo⁴⁺) with similar mass to that of the cluster by time-of-flight secondary ion mass analysis combined with secondary ion electric current measurements. The comparison revealed that (1) secondary ion emission yields per impact increase with increasing incident energy within the energy range examined, (2) the 4.0 MeV impact provides higher emission yields than the impact of the monoatomic Mo ion with the same incident energy (4.0 MeV Mo⁺), and (3) the 2.4 MeV impact exhibits comparable emission yields to that for the Mo ion impact with higher incident energy (14 MeV Mo⁴⁺). Energetic cluster ion impacts effectively produce the characteristic secondary ions for poly-tyrosine, which is advantageous for highly sensitive amino acid detection in proteins using time-of-flight secondary ion mass analysis.     

Study of density effect of large gas cluster impact by molecular dynamics simulations

Large gas cluster impacts cause unique surface modification effects because a large number of target atoms are moved simultaneously due to high-density particle collisions between cluster and surface atoms. Molecular dynamics (MD) simulations of large gas cluster impacts on solid targets were carried out in order to investigate the effect of high-density irradiation with a cluster ion beam from the viewpoint of crater formation and sputtering. An Ar cluster with the size of 2000 was accelerated with 20 keV (10 eV for each constituent atom) and irradiated on a Si(1 0 0) solid target consisting of 2 000 000 atoms. The radius of the Ar cluster was scaled by ranging from 2.3 nm (corresponding to the solid state of Ar) to 9.2 nm (64× lower density than solid state). When the Ar cluster was as dense as solid state, the incident cluster penetrated the target surface and generated crater-like damage. On the other hand, as the cluster radius increased and the irradiation particle density decreased, the depth of crater caused by cluster impact was reduced. MD results also revealed that crater depth was mainly dominated by the horizontal scaling rather than vertical scaling. A high sputtering yield of more than several tens of Si atoms per impact was observed with clusters of 4-20× lower volume density than solid state.     

The morphology and structure of one-dimensional carbon-carbon composite under high-fluence ion irradiation

The temperature dependences of the ion-induced electron emission yield γ(T), the crystal structure, and the morphology of a surface layer of the one-dimensional carbon fiber composite KUP-VM (1D) under high-fluence (10¹⁸-10¹⁹ ion/cm²) irradiation with 30 keV ions at normal incidence both perpendicular and parallel to the fiber directions have been studied. The target temperature has been varied during continuous irradiation from T = −180 to 400 °C. The surface analysis has been performed by the RHEED, SEM and RBS techniques. The surface microgeometry was studied using laser goniophotometry (LGP). It has been found that ion irradiation results in a loss of anisotropy of the surface layer structure because of amorphization at room temperature or recrystallization at a temperature higher than the ion-induced annealing temperature. The fiber morphology anisotropy remains under ion irradiation.     

Molecular dynamics simulations to explore the effect of chemical bonding in the keV bombardment of Si with C60, Ne60 and Ne60 projectiles

Depth profiling experiments using secondary ion spectrometry (SIMS) have shown effects that are characteristic to the pairing of the projectile with a Si target. Previous molecular dynamics simulations demonstrate that this unusual behavior is due to the fact that strong covalent bonds are formed between the C atoms in the projectile and the Si atoms in the target, which result in the implantation of carbon into the solid. The focus of this paper is to understand how the formation of chemical bonds affects the net sputtered yield. The results of molecular dynamics simulations of the keV bombardment of Si with C₆₀, Ne₆₀ and ¹²Ne₆₀ at normal incidence are compared over a range of incident kinetic energies from 5 to 20 keV. The net yields with Ne₆₀ and ¹²Ne₆₀ are significantly greater than with C₆₀ at all incident kinetic energies, with ¹²Ne₆₀ having the largest values. Application of the mesoscale energy deposition footprint (MEDF) model shows that the initial deposition of energy into the substrate is similar with all three projectiles. Snapshots of the initial pathway of the projectile atoms through the substrate show a similar lateral and vertical distribution that is centered in the region of the energy footprint. Therefore, the reason for the reduced yield with C₆₀ is that the C atoms form bonds with the Si atoms, which causes them to remain in the substrate instead of being sputtered.     

The investigation of energy loss and damage in polycarbonate induced by MeV carbon clusters

Carbon cluster ions (n = 1-5) and Cl⁺, Ti⁺, Ni⁺ ions were used to bombard polycarbonate (PC) films. By comparing the electronic energy loss and the number of chromophores at a fixed wavelength, we obtained the electronic energy loss S_e of carbon cluster ions in PC.     

Cesium/xenon co-sputtering at different energies during ToF-SIMS depth profiling

In this paper, ToF-SIMS dual beam depth profiles of H-terminated silicon wafers were performed with cesium primary ions and for different beam energies. The aim of this study was to investigate the influence of the cesium beam energy on the secondary ion yields during ToF-SIMS dual beam depth profiling. For this purpose, both the cesium beam energy and the cesium surface concentration were varied but the analysis conditions were kept identical for all depth profiles (i.e. Ga⁺ at 25 keV, 45°). For each sputter beam energy (i.e. 250 eV, 750 eV and 2000 eV), the cesium surface concentration was varied by diluting the cesium sputtering beam by xenon ions. This technique allows performing ToF-SIMS depth profiles with cesium surface concentration varying from zero (for pure xenon beam) to a maximum value (for pure Cs beam), depending on the bombardment conditions. For all the beam energies, the Si⁺ signals were found to decrease with the increasing cesium coverage and the lower the energy, the faster the decrease. The Cs⁺, the SiCs⁺ and the signals were found to exhibit a maximum for well defined Cs/Xe mixtures, which were found to depend on the secondary ion species and on the beam energy. Moreover, the maxima were found to shift to higher Cs beam content with the increasing energy. This effect is due to the variation of the cesium surface concentration with the varying beam energy. XPS analysis of the Cs/Xe craters and DYNTRIM computer simulations allowed us to convert the cesium beam scale to a cesium surface concentration scale and to interpret our results.     

Comparison of secondary electron emission in helium ion microscope with gallium ion and electron microscopes

To evaluate secondary electron (SE) image characteristics in helium ion microscope, Si surfaces with a rod and step structures is scanned by 30 keV He and Ga ion beams and 1 keV electron beam. The topographic sensitivity of He ions is in principle higher than that for scanning electron microscope (SEM) because of the stronger dependency of SE yield versus incident angle for He ions. As shrinking to sub nm patterns, the pseudo-images constructed from line profile of SE intensity by the electron beam lose their sharpness, however, the images for the He and Ga ion beams keep clearness due to darkening the bottom corners of the pattern. Here, the sputter erosion for Ga ions must be considered. Furthermore, trajectories of emitted SEs are simulated for a rectangular Al surface scanned by the beams to study voltage contrast, where positive and negative voltages are applied to the small area of the sample. Both less high energy component in the energy distribution of SEs and dominant contribution of direct SE excitation by a projectile He ion keep a high voltage contrast down to a sub nm sized area positively biased against the zero-potential surroundings.     

Fabrication and nuclear analysis of isotopic N and N ion-implanted silicon standards

The fabrication of reliable isotopic nitrogen standards is achieved in Si through ¹⁴N and ¹⁵N ion implantation. 60 keV and ions were implanted at 400 °C up to ∼60% peak atomic concentration, yielding nitrogen-saturated silicon layers as measured using resonant nuclear reaction analysis. No isotopic effect has been observed. The nitrogen standards are validated by measurements of stability under ion irradiation. No significant desorption of nitrogen is observed either under a ⁴He⁺ ion fluence of 3.36 × 10¹⁶ cm⁻² or under a ¹H⁺ ion fluence of 8.60 × 10¹⁷ cm⁻², giving strong evidence that isotopic nitrogen standards can be achieved.     

Density effects on the guided transmission of 3 keV Ne ions through PET nanocapillaries

Experiments for guided transmission of 3 keV Ne⁷⁺ ions through nanocapillaries in insulating PET polymers are reported. The ion guiding was studied for a two types of PET samples which consist of 200 nm capillaries with densities of and . The width of the emission profile and the fraction of transmitted ions were measured as a function of the capillary tilt angle. For the high capillary density the profile width of the transmitted ions is independent of the tilt angle in agreement previous studies. However, for the low-density sample the profile width was found to increase by a factor of 2 as the tilt angle increases from 0° to 8°. The results for the fraction of transmitted ions are used to evaluate the guiding angle, which specifies the guiding power of the material. The guiding powers were found to be equal for the two samples. The present results are discussed in terms of scaling laws, which have recently been established.     

Temperature-dependent surface modification of InSb(0 0 1) crystal by low-energy ion bombardment

Structural and compositional modification of InSb(0 0 1) single crystal surfaces induced by oblique incidence 2-5 keV Ar and Xe ion irradiation have been investigated by means of scanning tunneling and atomic force microscopies, and time-of-flight mass spectroscopy of secondary ion emission. In general, ion-induced patterns (networks of nanowires, or ripples) are angle of incidence- and fluence-dependent. Temperature dependences (from 300 to 600 K) of the RMS roughness and of the ripple wavelength have been determined for the samples bombarded with various fluences. Secondary ion emission from an InSb(0 0 1) surface exposed to 4.5 keV Ar⁺ ions has been investigated with a linear TOF spectrometer working in a static mode. Mass spectra of the sputtered In⁺, Sb⁺ and In₂⁺ secondary ions have been measured both for the non-bombarded (0 0 1) surface and for the surface previously exposed to a fluence of 10¹⁶ ions/cm². In⁺ and In₂⁺ intensities for the irradiated sample are much higher in comparison to the non-bombarded one, whereas Sb⁺ ions show a reversed tendency. This behavior suggests a significant In-enrichment at the InSb(0 0 1) surface caused by the ion bombardment.     

Desorption of gold nanoclusters (2-30 nm) under low excitation of their electronic subsystem by Ar ions (14 keV/nm)

Gold nanodispersed targets with islands-grains sized 2-30 nm were irradiated by Ar⁷⁺ ions with the energy of 45.5 MeV and (dE/dx)_e = 14.2 keV/nm in gold. The desorbed gold nanoclusters were studied by TEM method. For all the targets desorption of intact gold nanoclusters is observed. However, for inelastic stopping of monatomic Ar ions in gold of 14.2 keV/nm desorption of nanoclusters is observed only up to ∼25 nm. The yield of the desorbed nanoclusters considerably decreases from 3 to 0.02 cluster/ion with the increase of the mean size of the desorbed nanoclusters from 3 to 14.2 nm. The results are discussed.     

Calculated energy loss of a swift fullerene ion beam in InP

Bombardment of semiconductors with fullerene has been used to induce the formation of tracks. It is now accepted that target electronic excitation and ionization, which gives rise to the slowing down of the projectile is essential to calculate the track diameter. In the case of cluster beams, like fullerenes, the electronic excitation induced by each of the cluster constituents is enhanced, for certain projectile energies and target depths, by the so-called vicinage effects. Here we use a simulation code to calculate the energy lost by a swift fullerene ion beam in InP, paying special attention to the vicinage effects where they are significative. The code describes classically the movement of each cluster constituent under the influence of the self-retarding force, the Coulomb repulsion among molecular fragments, the wake forces responsible for the vicinage effects and the multiple scattering with the target nuclei. The simulation code also takes into account the possibility that the molecular fragments can also capture or loss electrons from the target, changing its charge state in their travel through the solid.Our simulations show that the energy deposited by the atomic ions that constitute the C₆₀ ion is clearly higher than the energy deposited by the same atomic ions but isolated. This difference being larger as the incident energy increases. We have predicted that track diameters of can be obtained in an InP target when using C₆₀ ions with an initial energy of 300 MeV.     

Ion implantation in 4H-SiC

Silicon carbide offers unique applications as a wide bandgap semiconductor. This paper reviews various aspects of ion implantation in 4H-SiC studied with a view to optimise ion implantation in silicon carbide. Al, P and Si ions with keV energies were used. Channelling effects were studied in both a-axis and c-axis crystals as a function of tilts along major orthogonal planes and off the major orthogonal planes. Major axes such as [0 0 0 1] and the and minor axis like the showed long channelling tails and optimum tilts for minimising channelling are recommended. TEM analyses of the samples showed the formation of (0 0 0 1) prismatic loops and the loops as well,in both a and c-cut crystals. We also note the presence of voids only in P implanted samples implanted with amorphising doses. The competing process between damage accumulation and dynamic annealing was studied by determining the critical temperature for the transition between crystalline and amorphous SiC and an activation energy of 1.3 eV is extracted.     

Secondary ion emission from Cu(1 0 0) surfaces with atomic adsorbates (N, O, Cl, S and Br)

Several targets that consist of atomic species X (X = N, O, Cl, S, Br) adsorbed at hollow sites on the Cu(1 0 0) surface have been examined with low-fluence secondary ion mass spectrometry (SIMS). The positive and negative secondary ion (SI) abundance distributions, which show a range of characteristics, have been discussed with the aid of thermochemical data derived from ab initio calculations. In positive SIMS, CuX⁺ is never observed, while the only heteronuclear (mixed-atom) SI that is observed for all five systems is Cu₂X⁺. In negative SIMS, the dominant heteronuclear species for all systems is , except for N/Cu(1 0 0), which produces no , ions. Cu⁻ emission is observed only for O/Cu(1 0 0). By analogy with results from laser ablation studies of O/Cu targets, it is conjectured that Cu⁻ is a daughter product of the gas-phase dissociation of polyatomic Cu-O anion clusters.     

Mixing of Cr and Si atoms induced by noble gas ions irradiation of Cr/Si bilayers

Cr/Si bilayers were irradiated at room temperature with 120 keV Ar, 140 keV Kr and 350 keV Xe ions to fluences ranging from 10¹⁵ to 2 × 10¹⁶ ions/cm². The thickness of Cr layer evaporated on Si substrate was about 400 Å. Rutherford backscattering spectrometry (RBS) was used to investigate the atomic mixing induced at the Cr-Si interface as function of the incident ion mass and fluence. We observed that for the samples irradiated with Ar ions, RBS yields from both Cr layer and Si substrate are the same as before the irradiation. There is no mixing of Cr and Si atoms, even at the fluence of 2 × 10¹⁶ ions/cm². For the samples irradiated with Kr ions, a slight broadening of the Cr and Si interfacial edges was produced from the fluence of 5 × 10¹⁵ ions/cm². The broadening of the Cr and Si interfacial edges is more pronounced with Xe ions particularly to the fluence of 10¹⁶ ions/cm². The interface broadening was found to depend linearly on the ion fluence and suggests that the mixing is like a diffusion controlled process. The experimental mixing rates were determined and compared with values predicted by ballistic and thermal spike models. Our experimental data were well reproduced by the thermal spikes model.     

Search for short-time phase effects in the electronic damage evolution - A case study with silicon

This work focusses on the production and decay properties of inner-shell vacancies and valence-band excitations induced by swift highly charged ions interacting with amorphous and crystalline Si. High resolution electron spectra have been taken for fast heavy ions at 1.78-5 MeV/u as well as for electrons of similar velocity incident on atomically clean Si targets of well defined phase. Various Auger-electron structures are analyzed concerning their width, their intensity and exact peak position. All measured peaks show a small shift towards lower energy when the charge of the projectile is increased. This finding is an indication for a nuclear-track potential inside the ion track. A detailed analysis of the Auger-electron spectra for amorphous Si and crystalline Si(1 1 1) 7 × 7 points to a small but significant phase effect in the short-time dynamics of ion tracks.     

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γ_T, induced by impact of the fast ions Ne^q+ (q = 2-8) and Ar^q+ (q = 3-12) on Si and Ne^q+ (q = 2-8) on W targets have been measured. It was observed that for a given impact energy, γ_T increases with the charge of projectile ion. By plotting γ_T as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Ne^q+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.