Cluster Ion Irradiation Research Articles

Surface modification of NiTi alloy by ion and gas cluster ion irradiation. The role of chemical segregation

Nitinol is an alloy of roughly equiatomic nickel and titanium, having shape memory and widely used in many applications, such as aerospace, automotive, biomedical and others. One of techniques for its treatment is plasma or ion beam irradiation. In the work we investigated surface composition and topography of the alloy formed by ion and cluster ion bombardment. A strong enrichment of the surface with nickel is detected, which does not agree with contemporary ideas about preferential sputtering. The results were compared with other known results in the area, and the processes occurring under ion irradiation of compound materials were discussed. The role of ambient oxygen in the steady state surface composition establishing is described.

Effect of closeness between a few atoms forming a cluster ion passing through water on the motion of secondary electrons and radial dose

The effect of electric field generated by cluster ion irradiation on the motion and energy deposition of secondary electrons is studied, focusing on the effect of the closeness (EOC) between the atoms in a cluster ion passing through water. This electric field is created by the target molecular ions produced by impact ionization of these atoms. The probabilities of secondary electrons escaping this electric field and the radial dose are simulated to elucidate the effects of not only the number of these atoms and the distances between them but also of their impact ionization cross-sections.

Interplay of the disorder and strain in gallium oxide

Ion irradiation is a powerful tool to tune properties of semiconductors and, in particular, of gallium oxide (Ga2O3) which is a promising ultra-wide bandgap semiconductor exhibiting phase instability for high enough strain/disorder levels. In the present paper we observed an interesting interplay between the disorder and strain in monoclinic β-Ga2O3 single crystals by comparing atomic and cluster ion irradiations as well as atomic ions co-implants. The results obtained by a combination of the channeling technique, X-ray diffraction and theoretical calculations show that the disorder accumulation in β-Ga2O3 exhibits superlinear behavior as a function of the collision cascade density. Moreover, the level of strain in the implanted region can be engineered by changing the disorder conditions in the near surface layer. The results can be used for better understanding of the radiation effects in β-Ga2O3 and imply that disorder/strain interplay provides an additional degree of freedom to maintain desirable strain in Ga2O3, potentially applicable to modify the rate of the polymorphic transitions in this material.

On the origin of nanoripples on silicon by gas cluster-ion irradiation

The first stage of nanoripples formation on silicon surface under inclined gas cluster ion beam irradiation is investigated. It was shown that ordered ripples appear as the result of emerging and self-organization of single standing protrusions spontaneously developed on the surface. The shapes of protrusions and ripples depending on the ion fluence and the angle of incidence are discussed.

Ion tracks in silicon formed by much lower energy deposition than the track formation threshold

Damaged regions of cylindrical shapes called ion tracks, typically in nano-meters wide and tens micro-meters long, are formed along the ion trajectories in many insulators, when high energy ions in the electronic stopping regime are injected. In most cases, the ion tracks were assumed as consequences of dense electronic energy deposition from the high energy ions, except some cases where the synergy effect with the nuclear energy deposition plays an important role. In crystalline Si (c-Si), no tracks have been observed with any monomer ions up to GeV. Tracks are formed in c-Si under 40 MeV fullerene (C60) cluster ion irradiation, which provides much higher energy deposition than monomer ions. The track diameter decreases with decreasing the ion energy until they disappear at an extrapolated value of ~ 17 MeV. However, here we report the track formation of 10 nm in diameter under C60 ion irradiation of 6 MeV, i.e., much lower than the extrapolated threshold. The diameters of 10 nm were comparable to those under 40 MeV C60 irradiation. Furthermore, the tracks formed by 6 MeV C60 irradiation consisted of damaged crystalline, while those formed by 40 MeV C60 irradiation were amorphous. The track formation was observed down to 1 MeV and probably lower with decreasing the track diameters. The track lengths were much shorter than those expected from the drop of Se below the threshold. These track formations at such low energies cannot be explained by the conventional purely electronic energy deposition mechanism, indicating another origin, e.g., the synergy effect between the electronic and nuclear energy depositions, or dual transitions of transient melting and boiling.

Энергетические распределения вторичных заряженных частиц при распылении газовыми кластерными ионами

We present an experimental investigation of the energy spectra of the charged particles emitted from polycrystalline copper under argon and xenon cluster ion irradiation. Positive secondary particles have significantly lower energies than in the case of sputtering with atomic argon. The spectra of the charged particles emitted under xenon cluster bombardment are systematically narrower than the corresponding ones obtained under argon cluster bombardment. The observed regularities can be explained in terms of energy transfer form a cluster to the target.

(Invited) Reactions on Metal Surface and Diketone Induced By Gas Cluster Ions during Atomic Layer Etching

Gas cluster ion beams (GCIB), which consists of several thousands of gaseous atoms or molecules, have been used as novel low-energy ion beams for surface processing and analysis. Since thousands of low-energy (several eV) atoms bombard a surface at the same time and the same position, the bombarded area experiences transient high-temperature and high-pressure conditions, which enhance sputtering of atoms or enhance surface reactions. In recent years, we have been investigating to apply GCIB as energetic ions for atomic layer etching. We have reported ALE of transition metals (Cu and Ni) with GCIB irradiations and diketone (acetylacetone and hexafluoroacetone). We separated each etching step as following; (1) adsorption of diketone molecules on metal oxide, (2) evacuation of residual vapor, (3) irradiation of oxygen GCIB to remove metal oxide. By using 5 keV oxygen GCIB (~ 2eV/molecules), thin layer of nickel oxide with adsorbed acetylacetone is removed. There is no physical sputtering at this energy region, which realizes self-limiting ALE process. From in-situ XPS study, Ni oxide with adsorbed diketone molecules can be removed by Ar-GCIB irradiation. It means that other oxidation method can be used to form metal oxide, and Ar-GCIB can be used for the removal steps. In this talk, we will report surface reactions between metals and diketone induced by gas cluster ion irradiations, and etching characteristics of various metals using GCIB irradiation and diketone.

Finishing Surface Treatment of Potassium Titanyl Phosphate Single Crystals by Argon Cluster Ions

It is demonstrated that a argon cluster ion beam can be used for the finishing treatment of ultrasmooth surfaces (with initial root-mean-square roughness below 0.3 nm) of potassium titanyl phosphate single crystals. Analysis of the power spectral density function of surface roughness showed that cluster-ion irradiation leads to removal of residual chemical contaminations from the subsurface layer and produces additional smoothing of the crystal surface in a broad range of spatial frequencies (ν = 0.2–100 μm–1).

Финишная обработка поверхности монокристаллов титанил-фосфата калия кластерными ионами аргона

AbstractIt is demonstrated that a argon cluster ion beam can be used for the finishing treatment of ultrasmooth surfaces (with initial root-mean-square roughness below 0.3 nm) of potassium titanyl phosphate single crystals. Analysis of the power spectral density function of surface roughness showed that cluster-ion irradiation leads to removal of residual chemical contaminations from the subsurface layer and produces additional smoothing of the crystal surface in a broad range of spatial frequencies (ν = 0.2–100 μm^–1).

Tuning the electronic properties of LaAlO3 / SrTiO3 interfaces by irradiating the LaAlO3 surface with low-energy cluster ion beams

We have investigated the effects of low-energy ion beam irradiations using argon clusters on the chemical and electronic properties of LaAlO3/SrTiO3 (LAO/STO) heterointerfaces by combining X-ray photoelectron spectroscopy (XPS) and electrical transport measurements. Due to its unique features, we show that a short-time cluster ion irradiation of the LAO surface induces indirect modifications in the chemical properties of the buried STO substrate, with (1) a lowering of Ti atoms oxidation states (from Ti4+ to Ti3+ and Ti2+) correlated to the formation of oxygen vacancies at the LAO surface and (2) the creation of new surface states for Sr atoms. Contrary to what is observed by using higher energy ion beam techniques, this leads to an increase of the electrical conductivity at the LAO/STO interface. Our XPS data clearly reveal the existence of dynamical processes on the titanium and strontium atoms, which compete with the effect of the cluster ion beam irradiation. These relaxation effects are in part attributed to the diffusion of the ion-induced oxygen vacancies in the entire heterostructure, since an increase of the interfacial metallicity is also evidenced far from the irradiated area. These results demonstrate that a local perturbation of the LAO surface can induce new properties at the interface and in the entire heterostructure. This study highlights the possibility of tuning the electronic properties of LAO/STO interfaces by surface engineering, confirming experimentally the intimate connection between LAO surface chemistry and electronic properties of LAO/STO interfaces.

Irradiation effect on magnetic properties of FeRh thin films with energetic C60 cluster ion beam

Effect of energetic C60 cluster ion irradiation on magnetic properties in FeRh thin films was examined by SQUID and XMCD measurements. The XMCD signal for the C60 irradiated samples is larger than that for the C1 samples in the initial stage of the irradiation. In contrast, as for the irradiation with larger ion fluence, the XMCD spectrum for the sample irradiated with C1 ion is larger than that for the sample irradiated with C60 cluster ion. Although the magnetization for the C60 irradiation samples continuously decreases with increasing in the ion fluence, the saturation magnetization for the C1 irradiation samples increases with increasing in the ion fluence. Considering these different behaviors in the irradiation induced ferromagnetism, the cluster ion can effectively deposit the apparent elastic collision energy at the surface region in the film, which effectively causes the ferromagnetic state at the surface of the films.

Fibrous structure in GaSb surfaces irradiated with fast Cu cluster ions

The effect of fast cluster irradiation on the formation of fibrous structures is investigated for single crystal GaSb surfaces irradiated by Cun+ ions (n = 1–3) with an energy of 0.4 MeV/atom at ion fluences up to 5 × 1015 cm−2. We study the cluster size dependence on the growth of fibrous network structures. With increasing cluster size, the shape of the fiber changed from rod-like to spherical. To quantitatively evaluate this cluster effect, a fiber diameter d in rod or spherical portion is examined as a function of ion fluence Φ and cluster size n. We find that the fiber diameter nonlinearly increases and follows the relation d∝nα×Φ, with α≈2. This evidently implies that the amount of defects generated by n-sized cluster bombardments varies as n2 for n≤3. Cluster ion irradiation enhances the defect generation owing to the overlap between cascades of individual cluster constituents and is therefore effective for the growth of nanofibers.

Fabrication of optically smooth surface on the cleavage of porous silicon by gas cluster ion irradiation

Electrochemical etching of Si is a promising method of fabricating multilayer photonic structures with hundreds of layers. Nevertheless, the natural cleavage of the porous silicon structure has roughness and then high optical scattering that embarrasses the usage of porous silicon in optical devices. In this study, gas cluster ion beam irradiation was suggested as a polishing technique. The bombardment results in surface smoothing and consequent enhancement of light reflection without significant difference in sputtering rate of layers with different porosity. Increasing irradiation dose results in different behavior of porous layers: densification of low-porosity structures and nano-rods formation in high-porosity layers.

Cluster ion beam assisted fabrication of metallic nanostructures for plasmonic applications

We report a high-throughput, single-step method for fabricating rippled plasmonic nanostructure arrays via self-assembly induced by oblique angle cluster ion beam irradiation of metal surfaces. This approach does not require lithographic or chemical processes and has the prominent advantage of possible large surface area coverage and applicability to different starting materials. The polarization dependent plasmonic property of the gold nano-ripple is due to their one dimension structure. The localized plasmon resonance frequency of synthesized nano-ripple arrays is tunable by changing nano-ripple dimensions that can be engineered by changing the cluster ion beam irradiation parameters. In this specific case presented, using 30keV Ar-gas cluster ion beam, we fabricate gold nano-ripple arrays that show localized plasmon resonance in the visible range through near IR range, tunable by varying cluster ion irradiation fluence.

Graphene, Graphene Oxide and Silicon Irradiation by Cluster Ions of Argon and Highly Charged Ions

ABSTRACTDefect formation in the samples of graphene, graphene oxide and silicon irradiated with Ar cluster and highly-charged ion irradiations were studied. Ar cluster ions, with acceleration energy E = 30 kV (Exogenesis nAccel00, Boston, USA) and total Ar cluster ion fluences ranged from 1x109cm-2to 1x1013cm-2were directed toward various surfaces. Highly-charged ions (HCI) bombardment on surfaces with highly charged Xeq+(q= 22) was employed at Eurasian National University, Kazakhstan, using a DC-60 cyclotron accelerator. Multi-layer graphene oxide, single-layer graphene- (SLG), few-layer of graphene (FLG) and polished Si are used for irradiation experiments. The study of irradiated samples was conducted by Raman spectroscopy, atomic force microscopy (AFM). Uniformly distributed defects and craters were observed on the surfaces of graphene, graphene oxide and silicon irradiated with cluster and HCI beams in our experiments. Ab-initio density-functional theory (DFT) was used to study point defects and molecular-dynamics (MD) simulations were used for studying formation of craters due to gas cluster ion impacts in graphene. The results of simulations were compared with experimental craters and surface shape.

Defect creation in Ge and GaAs semiconductor crystals by huge electronic excitations: A thermal spike description

Irradiation of semiconductor crystals with swift heavy ions or cluster beams is one way of obtaining information about both the defect characteristics in the investigated material and the defect formation mechanisms. In this work, to explain the formation of defects (latent tracks) observed experimentally to be effects of cluster–ion irradiations in Ge and GaAs, we have used the thermal spike model. The part of the energy used for thermal spike formation is determined by the electron–phonon coupling constant, g, which is unknown for semiconductors. The thermal spike calculations performed with an electron–phonon coupling constant, g(300 K) = 1.8 × 1012 W cm−3K−1 for Ge and g(300 K) = 3.2 × 1012 Wcm−3K−1 for GaAs nicely reproduce the size of amorphous tracks observed in both materials irradiated with C60 clusters, assuming the quenching of a vapor phase as the criterion of track formation. Our results also show that g decreases when the electronic stopping power threshold increases. The expected propensity, electron–phonon coupling increasing with the band gap energy, is confirmed.

Surface modification using ionic liquid ion beams

We developed an ionic liquid (IL) ion source using 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) and produced IL ion beams by applying a high electric field between the tip and the extractor. Time-of-flight measurements showed that small cluster and fragment ions were contained in the positive and negative ion beams. The positive and negative cluster ions were deposited on Si(100) substrates. X-ray photoelectron spectroscopy measurements showed that the composition of the deposited layers was similar to that of an IL solvent. This suggests that a cation (A+) or an anion (B−) was attached to an IL cluster (AB)n, resulting in the formation of positive cluster ions (AB)nA+ or negative cluster ions (AB)nB−, respectively. The surfaces of the IL layers deposited on Si(100) substrates were flat at an atomic level for positive and negative cluster ion irradiation. Moreover, the contact angles of the deposited layers were similar to that of the IL solvent. Thus, surface modification of Si(100) substrates was successfully demonstrated with BMIM-PF6 cluster ion beams.

Ripple formations by gas cluster ion irradiation in reactive gas environments

Effects of background gas (N2 or SF6) on ripple formation by oblique incidence gas cluster ion beam (GCIB) irradiation were investigated. When N2 gas was introduced as background gas, both ripple structures and the sputtering yield of Si and SiO2 did not change by Ar-GCIB irradiation at 60° incidence. However, wavelength of ripple became larger when SF6 gas was introduced as background gas. Increase of the sputtering yield in SF6 environment correlates with the change of ripple structures. It is assumed that SF6 molecules adsorbed on ripple structures, and Si or SiO2 ripples were etched preferentially.

Change in magnetic and structural properties of FeRh thin films by gold cluster ion beam irradiation with the energy of 1.67 MeV/atom

The effect of energetic cluster ion beam irradiation on magnetic and structural properties of FeRh thin films have been investigated. The cluster ions used in the present studies consist of a few gold atoms with the energy of 1.67 MeV/gold atom. Saturation magnetization of the sample irradiated with Au3 cluster ion beam (280 emu/cc) is larger than that for the irradiated sample with Au1 ion beam (240 emu/cc) for the same irradiation ion fluence. These results can also be confirmed by the X-ray magnetic circular dichroism (XMCD) measurement; the XMCD signal for Au3 cluster ion irradiation is larger than that for Au1 ion irradiation. Since the ion beam irradiation induced magnetization of FeRh is significantly correlated with the amount of the lattice defects in the samples, cluster ion beam irradiation can be considered to effectively introduce the lattice defects in B2-type FeRh rather than the single ion beam. Consequently, cluster ion irradiation is better than single ion irradiation for the viewpoint of saturation magnetization, even if the same irradiation energy is deposited in the samples.

Irradiation effect of gas-hydrate cluster ions on solid surfaces

In our newly developed gas-hydrate cluster ion source, a vapor of water bubbling with carbon dioxide (CO2) gas was ejected through a nozzle into a vacuum region, and mixed beams of water clusters and carbon dioxide-hydrate clusters were produced by adiabatic expansion. According to time-of-flight measurements, the largest water clusters consisted of approximately 2800 molecules at a vapor pressure of 0.3MPa. Also, the largest mixed clusters contained approximately 2000 molecules. Copper and silicon substrates were irradiated by the water cluster ions as well as carbon dioxide-hydrate cluster ions. X-ray photoelectron spectroscopy measurements showed that carbon was included in the Cu and Si substrates irradiated by the carbon dioxide-hydrate cluster ions, and a chemical shift owing to the formation of carboxyl radicals occurred on the Cu surface. Furthermore, the Cu surface was sputtered, and the sputtering depth was larger than the distance penetrated by the water cluster ion irradiation. Therefore, the formation of carboxyl radicals played an important role in the sputtering of the Cu surface, which occurred effectively in carbon dioxide-hydrate cluster ion irradiation.