Ion sputtering as methods for generation of cluster particles

: The investigations of emission and fragmentation of silicon oxide clusters sputtered from Si surface have been performed. It has been shown that the processes of formation of these clusters can be qualitatively described within the framework of modern concepts, and the main channels of their formation are determined in accordance with the mechanism of combinatorial synthesis.


Introduction
The sputtering of targets by beams of accelerated primary ions is one of the most productive methods for generating cluster [1].It is known that some of the sputtered particles correspond to cluster ions.Cluster ions are formed in vibrationally excited states and decay on their way from the target to the detector [2].The discovery of fragmentation of sputtered clusters [2] has significantly complicated the understanding of the cluster emission mechanism, since their decays change the measured mass spectra, kinetic energy spectra and the distribution of their internal energies.Investigation of the processes of cluster decay makes it possible to obtain information both on the emission of clusters and on the chemical and physical properties of these particles.Physical and chemical properties of a cluster, varying from the properties of atoms to the properties of a solid and depending on the size and structure of the cluster, can serve as a basis for the synthesis of crystals with unique properties that promising for solving actual problems of modern nanotechnology [3].Recently studies, considerable attention is paid to clusters of metal oxides, which play an essential role in modern technology of micro-and nanoelectronics, surface chemistry, catalysis processes, as well as the search for methods of their synthesis and study of fundamental properties.The study of silicon oxide, which plays an important role in these fields is especially important.
The purpose of this work is an experimental study by secondary ion mass spectrometry (SIMS) of the emission and fragmentation of heteronuclear silicon oxide clusters under ion sputtering and analysis of the obtained results from the point of view of existing theoretical concepts.These results are important to understand the nature of the formation of sputtered clusters.From a practical point of view, the obtained results can be essential for solving the problems of the power industry to obtain cheap and environmentally friendly hydrogen fuel.

Experimental research
We used the unique capabilities of an ion microanalyzer with double focusing and reverse geometry of Niro -Johnson type (Fig. 1) [4] to study the formation and fundamental properties of stable hydrogenated silicon oxide clusters Si n O m H x -.
Fig. The clusters were synthesized by ion sputtering of the silicon surface by O 2 + ions while simultaneously exposing the surface to the atmosphere of water wapour.The energy of primary beams was 18,5 keV.To study the emission of Si n O m H k clusters, we used the technique to one described earlier in [5,6], which consists in introducing oxygen or H 2 O vapor onto the bombarded target.The dissociation of Н 2 О upon interaction with the silicon surface with the formation of atomic hydrogen will make it possible to synthesize the silicon oxide clusters and hydrogen atoms in the ion impact zone and generate stable cluster configurations Si n O m H k -.In our experiments, H 2 O vapors were injected through a specially designed injection system directly into the bombardment area.The pressure range varied from P=2*10 -6 Pa (residual vacuum) to P=4-5*10 -3 Pa (the maximum allowable pressure of our vacuum system) The method of investigation of fragmentation processes of sputtered cluster ions in field-free zones L 1 and L 2 is described in [4].

Research results
The results of study of yields of Si n O 2n+1 -clusters sputtered from the Si surface by O 2 + ions (Fig. 2,3) at different pressures of oxygen and water vapor in the bombardment chamber, showed that Si n O 2n and Si n O 2n+1 -have an increased intensity in the mass spectra for all methods of their generation in agreement with [5,6].-and Si n O 2n+1 -clusters.When operating in the dynamic matching mode [4], the time of re-arrival of the primary beam at a given point of the sputtered surface is approximately 0.02 sec.Obviously, at a pressure in the bombardment chamber P=4*10 -3 Pa, this time is quite sufficient for the formation of at least one monolayer of gas molecules on the surface.In the case of water entering the surface, the decomposition of H 2 O molecules on the radiationdamaged target surface into hydrogen and a hydroxyl group is also possible.Later, with the development of the sputtering cascade caused by the incident ion, oxygen atoms located on the surface are captured into the resulting cluster structures, and a sufficient number of them ensures an increase in the yields of Si n O 2n -and Si n O 2n+1 -clusters.It is also interesting to note that the total decrease in the yields of homonuclear Si n -and heteronuclear Si n O m -clusters with m<2n in each cluster series n with an accuracy of several tens of percent (i.e., practically with an accuracy determined by the measurement technique) is equal to the corresponding increase in the yields of Si n O 2n -and Si n O 2n+1 -clusters with the same number of silicon atoms n.Thus, an increase in the yield of Si n O 2n -and Si n O 2n+1 -clusters at an optimal concentration of oxygen atoms on the bombarded surface indicates an increased stability of these clusters.In the case of water being injected onto the bombarded target, hydrogen atoms apparently saturate the existing free bonds in the cluster and stabilize the resulting cluster structures, leading to the effective formation of hydrogenated clusters Si n O 2n+1 H k -(k=1-3) (Fig. 3.).
The values of the dissociation energies for the Si n O 2n+1 -cluster with n = (2-7), obtained by us according to the above-described method from the experiment, are in the range 2.8-4.8eV [5].Based on these data, the calculated values of the excitation energies were obtained, which lie in the range 3.68-17.58eV for clusters with n=2-7 and 0.26-0.35eV, respectively, per one oscillator.Analisys of ion yields illustrated that 1) the yields of the Si n O 2n+1 -magic cluster with H 2 O and O 2 inject are increased compared to the spectrum without injecing, 2) hydrated oxides of the type Si n O 2n+1 H -, Si n O 2n+1 H 2 -, Si n O 2n+1 H 2 -, and the latter begins to appear only in magic clusters with n≥2.In magic clusters, the peaks of one hydrated (hydride) oxide are somewhat higher than the previous peak of Si n O 2n+1 -oxide.Hydrogenation is a simple method for stabilizing the silicon surface against oxidation and thus is important in microelectronics.Reaching an inactive silicon surface, ideally ending in hydrogen (hydrogen), has been accomplished in the last 40 years.However, oxidation still takes place even on hydrogen terminated atomistic flat silicon surfaces.Electrochemical hydrogen passivation for easily emitting silicon pores is an essential but unstable process.Recently silicon nanowires have been produced for large scale synthesis.An essential requirement for their widespread use has been fulfilled.This is both technological and scientific importance of finding ways to stabilize them, so as to skillfully avoid the problems of degradation and low photoluminescence.To achieve this goal, the study of hydrogenated silicon clusters in relation to their certain local structural stability should be urgent, but so far, such information is not attainable.In this experiment, we studied the reaction of an aqueous molecule on different hydrogenated silicon clusters in order to relate stability to the local hydrogen configuration, and to shed light on the way to achieve stability, non-reactivity of hydrogenated silicon structures.Since size effects often appear for properties such as energy gap when the size of cluster structures reaches nanometers, we focus our study on the effect of size on oxidation and thus on stability.In [7], models of hydrogenated surfaces Si (001) and Si (111) -Si 9 H 14 -SiH 2 and Si 10 H 15 -SiH 3 were obtained, as well as smaller Si 2 H 6 -SiH 2 and Si 5 H 10 -SiH 2 , SiH 3 -SiH 3 and Si 4 H 9 -SiH 3 .These reactions were studied to develop a dimensional dependence of reactivity with water and the stability of local structures.The reaction starts from both sides and then a transition state (TS) is formed.The reaction is eventually completed by an H 2 molecule that is released and forms OH attached to the silicon cluster.It turns out that the reaction can go through: Si 9 H 14 -SiH 2 + Н 2 О→IC→TS→ Si 9 H 14 -SiHOH + H 2 The appearance of a weak intermediate complex of a similar reaction -IC, which is 1.4 kcal/mol lower than the reaction.In this case, the O-Si1 distance = 4.054 Å.Thus, it is similar to the fact that the forces for this weak chelator are the dipole-dipole interaction.The system then has to go through a transient state TS in order to reach the product.In the case of ТS with the О-Si1 bond, the distance between the attached О of the Н 2 О atom and the attached silicone atom on the surface of 1.855Å is slightly longer than in the product; accordingly, the formed Н 1 -Н 2 bond is much longer than the regular value in the product.Meanwhile, one of the two hydrogen atoms in water, H1, moves away from the oxygen atom, resulting in an H 1 -O distance of 1.110Å, which is drawn longer than IC.The hydrogen atom H 2 is 1.842110Å, away from the silicon atom.The energy barrier to this reaction is 44.5 kcal/mol in relation to the reactants.From the point of view of thermodynamics, this reaction is beneficial, because it is exothermic at 14.9 kcal/mol.For other complexes -similar reactions.It is noticed that each of these reactions includes an intermediate complex with an energy of almost 2.3 kcal/mol.With an expansion in the bunch size of the response of each sort of silicon-hydrogen arrangement, the energy hindrance diminishes, everything shows an increment in the reactivity of the framework.Responses with SiH3 show a more modest energy obstruction than with SiH 2 .To give more positive help for the reactivity pattern, rate constants were determined at 1 atm.for a watery response inside different hydrogenated setups.For reactions on a dihydride, the calculated reaction rate for medium clusters shows a much larger increase in relation to small clusters, but can only increase slightly with increasing cluster size.Confirmation of the above may also appear from the analysis of their boundary orbitals.It has previously been found that the overlap between the highest occupied molecular orbitals (HOMO) of a molecule and the lowest unoccupied molecular orbitals (LUMO) can be determined by the nature of the chemical reaction.A smaller energy difference between HOMO of one molecule (electron donor) and LUMO of another (electron acceptor) might show a more preferable reaction.Thus, the tendency is the close ratio of HOMO and LUMO of individual hydrogen-silicon clusters, for which, with an increase in the cluster size, their HOMO generally moves forward and LUMO falls, as a result of which the energy gap (gap) decreases.Since the energy barrier size effect is well known for silicon clusters when dimensional changes are in the nanometer range, the correlation of reactivity with the well documented energy barrier size effect could provide an important implication for the size dependent reactivity found here.Based on small silicon clusters, it is expected that the reactivity and rate constants for large clusters will also stabilize for a given temperature and pressure.

Conclusion
The performed experiments indicate that ion sputtering is an effective method for generating heteronuclear oxide clusters Si n O m -and Si n O m H k -of various sizes.Qualitatively, the processes of formation of these heteronuclear clusters can be described in terms of modern concepts [8], and the main channels for their formation are determined in accordance with the mechanism of combinatorial synthesis [6,[9][10][11].

Fig. 2 .
Fig.2.Diagram of yields of Si n O m -clusters (n = 1-3, m = 1… 2n + 1) obtained at a current of O 2 + I=100 nA on the surface of Si target; I -residual vacuum P=6.5 * 10 -6 Pa, the surface was cleaned with an ion beam after H 2 O was injected into the chamber; II -inlet H 2 O into the chamber up to P=2.5 * 10 -3 Pa.

Fig. 3 .
Fig.3.Diagram of yields of Si n O m -and Si n O m H -clusters under bombardment of Si target by O 2 + ions with H 2 O vapor in chamber.Observation of the change in yields of "magic"[5,6] Si n O 2n+1 -clusters showed that with increasing pressure their intensities increase, and at pressure P=4-5*10 -3 Pa the yields of these clusters are maximum.In this case, the absolute values of their intensities when oxygen and water vapor are admitted into the chamber at the same pressure differ little.The most significant difference is that when H 2 O is injected, additional intense peaks of Si n O 2n H k -and Si n O 2n+1 H k -(k=1-3) clusters appear in the mass spectrum of sputtered clusters.Fig.2 shows the overall change in the yield of Si n -and Si n O 2m -clusters (n=1-3) when water vapor is admitted into the bombardment chamber at a current of primary O 2 + ions I 0 =100 nA.As can be seen from Fig.2, the presence of additional oxygen atoms on the sputtered surface significantly changes the mass spectrum of emitted clusters.The peak intensities of both homonuclear Sin-and heteronuclear Si n O m -clusters with m<2n decrease significantly (by one or two orders of magnitude) when H 2 O is injected to the surface.The yields of Si n O 2n -and Si n O 2n+1 -clusters also decrease several times.The increase in the yields of Si n O 2n -and Si n O 2n+1 -with H 2 O vapors can be associated, firstly, with an increase in the concentration of oxygen atoms in the ion bombardment zone, and secondly, with an increased stability of Si n O 2n-and Si n O 2n+1 -clusters.When operating in the dynamic matching mode[4], the time of re-arrival of the primary beam at a given point of the sputtered surface is approximately 0.02 sec.Obviously, at a pressure in the bombardment chamber P=4*10 -3 Pa, this time is quite sufficient for the formation of at least one monolayer of gas molecules on the surface.In the case of water entering the surface, the decomposition of H 2 O molecules on the radiationdamaged target surface into hydrogen and a hydroxyl group is also possible.Later, with the development of the sputtering cascade caused by the incident ion, oxygen atoms located on the surface are captured into