**Ion Bombardment-Induced Surface Effects in Materials**

Farid F. Umarov and Abdiravuf A. Dzhurakhalov

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62731

#### **Abstract**

This chapter deals with the experimental research and computer simulation of low- and medium-energy (*E* 0 = 1-30 keV) ion collisions on the surface of a solid and of the accompanying effects, namely scattering, sputtering, and surface implantation. Experimental and computer simulation studies of low-energy (*Е* 0 = 80–500 eV) Cs+ ions scattering on Ta, W, Re target surfaces and K+ ions scattering on Ti, V, Cr target surfaces have been performed for more accurate definition of mechanism of scattering, with a purpose of evaluation of use of slow ions scattering as a tool for surface layer analy‐ sis. The peculiarities of the process of correlated small angle scattering of 5–15 keV He, Ne, Ar, Kr, Xe, and Rn ions by the Cu(100), Ni(100), and V(100) single-crystal surfaces have been investigated by computer simulation. It has been shown that under these conditions the inelastic energy losses become predominant over the elastic ones. The anomalous energy losses observed experimentally at the grazing ion scattering by the single-crystal surface were explained. It has been shown by computer simulation that the peculiarities of the chain effect at direct and reverse relation of masses of colliding particles and rainbow effect at quasi-single and quasi-double scattering of ions, heavier than adatoms, lead to the appearance of characteristic peaks in the energy and angular distributions of scattered ions. Analysis of these peaks and comparison with experi‐ ment give an opportunity to control the initial stages of adsorption and identification of adsorption structures with the help of low-energy ion scattering. It has been shown that from the correlation of the experimental and calculated energy distributions of the scattered particles, one may determine a spatial extension of the isolated atomic steps on the single-crystal surface damaged by the ion bombardment. Results obtained can be also used to study short-range order in alloys undergoing ordering. Grazing ionsputtering processes of Si(001), SiC(001), and Cu3Au(001) surfaces at 0.5–5 keV Ne+ bombardment have been studied by computer simulations. A preferential emission of Cu atoms in the case of Cu3Au (001) surface sputtering is observed. It was shown that in the case of grazing ion bombardment, the layer-by-layer sputtering is possible, and its optimum is observed within the small angle range of the glancing angles near the threshold sputtering angle. The peculiarities of trajectories, ranges, and energy losses of low-energy different-mass ions channeling in thin single crystals of metals and semiconductors have been thoroughly studied by computer simulation. It has been

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found that in the case of light ions, even at low energy, the main contribution to energy loss is made by inelastic energy losses, whereas for heavy ions, already at *E* < 10 keV, elastic energy losses exceed inelastic ones. Profiles of the distribution of channeled ions have been calculated depending on the crystal lattice type, kind ofions, and their energy. It has been shown that the channeling of low-energy ions through thin single-crystal metal films can be used to determine the sort and adsorption site of light atoms adsorbed on a clean rear surface.

**Keywords:** ion scattering, sputtering yield, layer-by-layer sputtering, surface implan‐ tation, surface channeling, computer simulation, grazing ion bombardment, elastic and inelastic energy losses, vacancy and atomic steps defects on the surface, method of layer-by-layer analysis of single-crystal surface, *PACS codes:* 79.20.Rf; 79.20.Ap

#### **1. Introduction**

Theionscattering,sputtering,andimplantationprocesseshavebeenthesubjectofbothscientific investigations for a long time and recent rapid developing thin-film technologies and nano‐ technologies. These processes underlie such well-known methods of surface science as Ion Scattering Spectroscopy (ISS), Ion Beam Analysis (IBA), Secondary Ion Mass Spectrometry (SIMS), and Ion Beam Modification of Materials (IBMM). Physically, the energy range under consideration is characterized by the dominance of elastic over inelastic energy losses, and by the possibility of considering classical binary collisions using single-center potentials and disregarding the binding energy of the scattering ion in the crystal lattice. The first of these factorsdetermines theupper, andthe second,the lowerboundaryofthe energyrange. Speaking about the surface, one should bear in mind that the bulk of the solid also participates in the process of ion scattering. The scattering depth is greater, as the ions are lighter and their energy is higher. In this energy range, heavy low-energy ions are scattered practically by one or two atomic layers. This is a great asset since it offers a possibility of using simple models of single and double scattering, and under grazing incidence, of calculating scattering produced only by surface atomic rows and the semichannels,formed by them. The possibility of probing only one surface atomic layer by heavy-ion scattering is also unique and does not have analogs in the other methods of the surface diagnostics of solids. Particle bombardment of a clean and adsorption-coveredsolidsurfaces leads toradiation-inducedvacancydefects,atomic steps,and defect clusters, as well as to an atomic scale relief (*<*100 Å) formation. The concentration and the type of the radiation defects being formed depend upon the experimental conditions and significantly influence the particles' trajectories and their angular and energy distributions, as well as the number of scattered particles. Moreover, there is a correlation between the defect type, the blocking angles of the reflected beam and the energy distributions of the scattered particles, which allows the determination of the defect type and its surface concentration [1–5]. For the analysis of the first one or two atomic top layers of a solid, noble gas ions with pri‐ mary energies between about 0.5 and 10 keV are very well suited. This is due to their compara‐ tively large scattering cross sections (of the order of 1018 cm2 */*sr) and due to the effective neutralizationofionsthatpenetrateintothesample.Thus,thedetectionofscatteredionsprovides a powerful tool for surface analysis that is exclusively sensitive to the outermost atomic layers. The method is known in the literature as "ion scattering spectroscopy" (ISS) [6].

found that in the case of light ions, even at low energy, the main contribution to energy loss is made by inelastic energy losses, whereas for heavy ions, already at *E* < 10 keV, elastic energy losses exceed inelastic ones. Profiles of the distribution of channeled ions have been calculated depending on the crystal lattice type, kind ofions, and their energy. It has been shown that the channeling of low-energy ions through thin single-crystal metal films can be used to determine the sort and adsorption site of light atoms adsorbed

**Keywords:** ion scattering, sputtering yield, layer-by-layer sputtering, surface implan‐ tation, surface channeling, computer simulation, grazing ion bombardment, elastic and inelastic energy losses, vacancy and atomic steps defects on the surface, method of layer-by-layer analysis of single-crystal surface, *PACS codes:* 79.20.Rf; 79.20.Ap

Theionscattering,sputtering,andimplantationprocesseshavebeenthesubjectofbothscientific investigations for a long time and recent rapid developing thin-film technologies and nano‐ technologies. These processes underlie such well-known methods of surface science as Ion Scattering Spectroscopy (ISS), Ion Beam Analysis (IBA), Secondary Ion Mass Spectrometry (SIMS), and Ion Beam Modification of Materials (IBMM). Physically, the energy range under consideration is characterized by the dominance of elastic over inelastic energy losses, and by the possibility of considering classical binary collisions using single-center potentials and disregarding the binding energy of the scattering ion in the crystal lattice. The first of these factorsdetermines theupper, andthe second,the lowerboundaryofthe energyrange. Speaking about the surface, one should bear in mind that the bulk of the solid also participates in the process of ion scattering. The scattering depth is greater, as the ions are lighter and their energy is higher. In this energy range, heavy low-energy ions are scattered practically by one or two atomic layers. This is a great asset since it offers a possibility of using simple models of single and double scattering, and under grazing incidence, of calculating scattering produced only by surface atomic rows and the semichannels,formed by them. The possibility of probing only one surface atomic layer by heavy-ion scattering is also unique and does not have analogs in the other methods of the surface diagnostics of solids. Particle bombardment of a clean and adsorption-coveredsolidsurfaces leads toradiation-inducedvacancydefects,atomic steps,and defect clusters, as well as to an atomic scale relief (*<*100 Å) formation. The concentration and the type of the radiation defects being formed depend upon the experimental conditions and significantly influence the particles' trajectories and their angular and energy distributions, as well as the number of scattered particles. Moreover, there is a correlation between the defect type, the blocking angles of the reflected beam and the energy distributions of the scattered particles, which allows the determination of the defect type and its surface concentration [1–5]. For the analysis of the first one or two atomic top layers of a solid, noble gas ions with pri‐ mary energies between about 0.5 and 10 keV are very well suited. This is due to their compara‐ tively large scattering cross sections (of the order of 1018 cm2 */*sr) and due to the effective neutralizationofionsthatpenetrateintothesample.Thus,thedetectionofscatteredionsprovides

on a clean rear surface.

**1. Introduction**

360 Radiation Effects in Materials

The sputtering process has been the subject of both scientific investigations for a long time and recent rapid developing micro- and nanotechnologies. Processes such as plasma etching and sputter deposition that involve ion bombardment at relatively low (~100 eV) ion energies are widely used in semiconductor processing [7]. Though sputtering and surface modifications of single crystals are widely studied, there are not sufficient data in the case of grazing incidence. However, using glancing-angle ion bombardment for surface modification rather than conventional near-normal incidence ions allows expanding the energy range up to ~10 keV, and has the advantages of reducing damage (such as crater formation) and preferentially removing surface asperities [8] leading to flat surfaces. This is due to the peculiarities of sputtering processes at grazing incidence [9]. Si and SiC crystals have a great importance because of their use in semiconductor technologies. Especially, silicon carbide exhibits a large band gap, a higher breakdown field, a higher thermal conductivity, and a higher saturation velocity, compared to widely used silicon. Besides, SiC is a promising shielding material in nuclear fusion systems such as limiters in Tokamak devices, where the surface erosion is also important [10, 11]. In ref. [12], atomically clean and flat Si(100) surfaces suitable for nanoscale device fabrication were prepared by wet-chemical etching followed by 0.3–1.5 keV Ar ion sputtering. It was found that wet-chemical etching alone cannot produce a clean and flat Si(100) surface which can be achieved by subsequent 300 eV Ar ion sputtering at room temperature followed by a 700°C annealing. Application of grazing angles of incidence of ions on the solid surface opens new perspectives in the investigation of composition, structure, and topography of real surfaces and their modification and polishing by ion beams. Sputtering yields of crystalline silicon carbide and silicon have been experimentally determined, and the results have been compared with Monte Carlo simulations for Ne+ , Ar+ , and Xe+ ion bombardment in the energy range of 0.5–5 keV under 60° sputtering with respect to the surface normal [13]. The simulation results depend strongly on the input parameters which are not well known, especially for SiC. The TRIM simulation fits the experimental results very well. The evolution of surface morphology during ion beam erosion of Si(111) at 500 eV Ar+ ion bombardment (60° from normal, 0.75 mA/cm2 collimated beam current) was studied over a temperature range of 500–730°C [14]. Keeping ion flux, incident angle, and energy fixed, it was found that onedimensional sputter ripples with wave vector oriented perpendicular to the projected ion beam direction formed during sputtering at the lower end of the temperature range. For tempera‐ tures above approximately 690°C, growth modes both parallel and perpendicular to the projected ion beam direction contribute to the surface morphological evolution. Thus, though sputtering and surface modifications of single crystals are widely studied, there are not sufficient data in the case of grazing incidence.

Ion implantation has become a very important technique for modifying surface and impurity doping of semiconductors [15–17]. The ion implantation processes lead to the change of a profile of composition and structure of the subsurface layers. Using glancing-angle ion implantation for surface modification rather than conventional near-normal incidence ions allows expanding the energy range up to ~10 keV and has the advantages of reducing damage (such as crater formation) and preferentially removing surface asperities leading to flat surfaces. Channeling of low-energy ions in metal and semiconductor single crystals offers the opportunity to create the method of local ion implantation in ultrathin film nanotechnology and surface nano-engineering. Therefore, ranges, energy losses, and profiles of distribution of low-energy ions channeling in crystals have received considerable experimental and theoret‐ ical interest [18–20]. For small crystal depths, the approaches which are used in the analytical theory of orientation effects on the large depths become unacceptable, and a computer simulation method for the channeling process modeling appears to be the most preferable [21, 22]. So, the theoretical investigation of atomic collision processes in crystals caused by particle irradiation and deposition is usually done using computer simulation, because real physical conditions (e.g., complicated interatomic interaction potential, surfaces, interfaces, defects, etc.) can be taken into account much easier than it is possible by using analytical methods [17, 21–23].
