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Secondary emission of neutral and charged particles from intermetallic single-crystal
Vacuum 148 (2018) 106e112
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Secondary emission of neutral and charged particles from intermetallic single-crystal K.A. Tolpin*, K.F. Minnebaev, V.E. Yurasova Moscow State University, Moscow 119991, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2017 Received in revised form 3 November 2017 Accepted 5 November 2017 Available online 7 November 2017
Sputtering, secondary ion and secondary electron emission were studied experimentally for an intermetallic Ni4Mo compound with high corrosion resistance and great hardness, which is used as material for constructing components of rockets and nuclear reactors. The process of sputtering was analyzed by molecular dynamics simulations of Ni4Mo (111) face for unchanged and changed composition (with segregation) of topmost layers of a disordered and ordered crystal. Predominant exit of Ni and Mo atoms in close-packed directions was observed and explained by correlated collisions. Both nickel and molybdenum atoms are ejected in the same crystallographic directions for a disordered crystal. For an ordered crystal the nickel atoms deviate from these directions. The origin of sputtering and number of ejected particles generated has been calculated. For secondary electron emission from the (111) Ni4Mo face the maxima in close-packed directions were obtained experimentally and explained by scattering of primary and secondary electrons on atoms located in the lateral sides of open channels. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Secondary ion emission Sputtering Secondary electron emission Scattering Intermetallic compounds Molecular dynamics
1. Introduction Research on neutral and charged particle emission under ion bombardment of Ni4Mo single-crystals is important for understanding the mechanisms of particle interaction with a surface of intermetallic compound and for practical applications, e.g. in secondary ion mass spectrometry, catalysis, and in production of hard refractory coatings [1]. Intermetallic compounds with a significant difference of component masses, such as Ni4Mo, are characterized by the difference between the surface and bulk structures, compositions, and properties [1e4]. Among them are such properties as surface segregation, surface ordering, and reconstruction [5]. The processes of ordering in such crystals were studied experimentally [6e9] and by numerical simulation [5,9]. In general, these compounds are thermally treated to obtain an ordered structure [6,7]. The intermetallic crystals can be also ordered using electron [6,7], neutron [7], and ion irradiation [8,9]. In these cases, as shown in Ref. [9], radiation-enhanced ordering is not associated with additional heating during irradiation. It was shown that the segregation leads to a change of surface
* Corresponding author. E-mail address: [email protected] (K.A. Tolpin). https://doi.org/10.1016/j.vacuum.2017.11.005 0042-207X/© 2017 Elsevier Ltd. All rights reserved.
composition of the three top layers of intermetallic compounds, which is different for different faces of crystals. For instance, the top layer of the (001) NiPd face contains 80% of Pd, whereas the second layer has no Pd and the third layer has 64% of Pd [5]. Such changes in composition of surface layers are caused by the interplay of two competing factors: the mechanical stress (that leads to the segregation of the heavy component of the compound on a surface) and the various chemical interactions (that result in short-scale ordering) [5e7]. The sputtering and secondary ion emission from Ni4Mo singlecrystal was first studied in Ref. [10] for ion irradiation of the (001) face. The preferential yield of nickel (light component) was observed which is facilitated by the lower binding energy of Ni in comparison with Mo. Different angular distributions of emitted secondary ions for ordered and disordered (i.e. for randomly placed of Ni and Mo atoms) single-crystals were obtained. However, the experimental setup in Ref. [10] did not allow independent variations of observation angles for primary and secondary ions. In the present work, we eliminated this disadvantage, so that correct studies of azimuthal and polar angle distributions of secondary Ni and Mo ions from the Ni4Mo single-crystal could be made. The process of sputtering was studied also by molecular dynamics simulations. Polar and azimuthal angular distribution of sputtering under irradiation Ni4Mo (111) face by 10 keV Arþ ions at normal incidence (a ¼ 0 ) for unchanged and changed composition
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(i.e. with segregation of topmost surface layers of disordered and ordered crystal) were analyzed. 2. MD simulation Simulation of sputtering was performed for the (111) face of ordered and disordered Ni4Mo single-crystal with tetragonal lattice. The crystalline structure of ordered Ni4Mo single-crystal is illustrated in Fig. 1. The sputtering of Ni4Mo crystals was calculated using the molecular dynamics model with a moving single-crystal block of atoms [11e13]. At fixed moments in time, the interaction of a moving (active) particle with target atoms was considered. The nearest atoms form a block whose radius is equal to the radius of the fifth coordination sphere. In the resulting structure, an incident ion and target atoms can move and interact with each other. When an active particle moved, the block moved too and formed the sphere around the particle. If the energy acquired by interacting target atoms was higher than the threshold level (equal to the binding energy), the positions and momenta of these atoms were recorded and, subsequently, their further motion was calculated. The block of atoms had no time to exhibit instability, because the sputtering of an atom is a short-time process (its duration is approximately 1013 s [14] from the moment of impact of ion). This model made it possible to trace the particle trajectory at large (hundreds of angstroms) distances from the place of the ion incidence. The equations of motion were integrated using a modified Euler predictor-corrector scheme that is stable [14]. Inelastic losses were computed according to the Firsov formula [15]. Thermal oscillations of atoms were assumed to be uncorrelated. Lattice distance a, c and binding energy Eb were chosen from experimental data: a ¼ 5.720 Å, c ¼ 3.564 Å and Eb ¼ 4.2 eV [4]. The interaction potential was determined in the following form [16]: U(r) ¼ Аbm$exp (-r/abm) þ (Аb/r)$exp (-2r/abm), where Abm ¼ 52 (Z1Z2)3/4, abm ¼ 0.219 Å, Ab ¼ k (Z1Z2e2), Z1 and Z2 e are the atomic numbers of the ion and target atom, respectively; r is the vector radius; and k is an adjustable parameter of the order of unity. In the computation, the following parameters were determined: the sputtered atom's momentum, the length of the sputtering cascade path, the sputtering time, the sputtered atom's generation number L, and the depth of the origin x0 of sputtering. 3. Experiment The patterns of sputtering the (111) Ni4Mo face was obtained using the system shown schematically in Fig. 2. The setup for sputtering contains the ion source of Ardenne [17], improved as discussed in Ref. [18], which allowed well-focused beams of argon ions with a current density of 1 mА/sm2 at energy E0 ¼ 10 keV to be obtained. The ion beam, after acceleration
Fig. 1. A structure of ordered Ni4Mo single-crystal with tetragonal lattice. Large and small circles correspond to Mo and Ni atoms, respectively. Semi-closed circles are atoms arranged within the lattice.
Fig. 2. Sputtering system. 1 e ion source, 2 e insulator, 3 e electrostatic lens, 4 e container, 5 e window, 6 e quartz screen, 7 and 10 e glass collectors, 8 e specimen, 9 e holder.
and focusing by a single electrostatic lens, passed through a hole in a quartz screen and bombarded the sample on the area with diameter of 2 mm. The dose of irradiation was measured by current integrator. A glass collector for sputtered particles was placed parallel to the studied surface in front of the sample in the distance of 15 mm. Spot pattern of sputtered particles was used to determine the orientation of single-crystal samples. For studying the secondary ion emission it is important to simultaneously determine the angular and energy distributions of mass-separated secondary species. In our work, these measurements have been performed by a specially designed and constructed setup with a mobile 180 spherical energy analyzer linked to an immobile quadrupole mass spectrometer as shown in Fig. 3. The secondary ion flux was focused in two directions, which ensured a high transmission coefficient and allowed axissymmetric optics to be used. The energy resolution was about 0.5 eV (at transmission energy of 20 eV) and the ion mass range was 1e350 amu. The sample chamber was evacuated by a magnetic discharge pump to a residual pressure on the order of 109 mbar. The primary ions were incident on the target at an angle a ¼ 0 relative to the normal to the surface. The polar angle q of observation of the secondary ion emission could be varied within 90 by rotating the energy analyzer. The yield of secondary ions Iþ was determined either by integrating the mass peak at the certain ion energy or by integrating the energy spectra of ions. The secondary electron emission was measured by the scanning electron microscope (LEO 14XX (VP)). We use a passing of 1 nA current through a crystal. Some times we measured also backscattered and secondary electrons using a Faraday cup. The energies of primary electrons were 10 keV. The topography of surface has been studied by the scanning electron microscope (LEO-1455 (Carl Zeiss)) and by the atomic force
Fig. 3. Schematic diagram of experimental arrangement.
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microscope (ATC FemtoScan Online). 4. Results and discussion 4.1. MD modelling Simulation of sputtering was carried out for the natural state of a (111) Ni4Mo face e with segregation, (when the three topmost layers had 82%, 24% and 50% of Mo, respectively) and also, for comparison, without segregation (hypothetic case). The results of calculations of sputtering for disordered Ni4Mo crystal are shown in sections 4.1.1-4.1.6. 4.1.1. Spot pattern The spot pattern, arising on the plate collector after sputtering of the (111) Ni4Mo face, are shown at Fig. 4. They correspond to sputtering in the directions: [110], [111], [113], and [001]. Formation of spots in these directions for a disordered single crystal is explained by the correlated collisions in accidentally created short chains of atoms in the top layers of the (111) face. It is seen, that, for the surface without segregation, Ni sputtered more than Mo. When we include the segregation, then, on the contrary, Mo sputtered more then Ni. This is due to complicated transmission of Ni through molybdenum atoms in the first layer. 4.1.2. Polar distribution of sputtered atoms The distribution of nickel and molybdenum atoms sputtered from the (111) face of single-crystal Ni4Mo on the polar angle q of output in the ð112Þ plane is shown in Fig. 5. The overall coefficient of sputtering for Ni4Mo (111) face Y ¼ 6.7 atoms/ion for the studied case. This value for the coefficient Y of sputtering for Ni4Mo compound is a little smaller than Y for the individual components of Ni and Mo. This result coincides with experimental data for sputtering of binary compounds, obtained in Ref. [19]. Segregation does not change the main shapes of Y (q) curves, but leads to noticeable growth of Mo sputtering from the first layer with respect to Ni (Fig. 5b).
Fig. 5. Angular dependence of Ni (solid line) and Mo (dotted line) sputtering from (111) face of disordered Ni4Mo crystal on polar angle q of exit in ð112Þ plane for surface without (a) and with (b) segregation of three upper layers.
The maxima in polar distributions of sputtering (Fig. 5a and b) are explained by correlated collisions in the upper layers of the Ni4Mo (111) face. It is known that preferential sputtering of single-crystals is observed in the directions of close-packed chains by direct focusing collisions and by additional focused e assisted collisions [20e26]. According to our calculations for sputtering Ni4Mo (111) face the direct focusing occurs in <110> and <111> chains and the assisting focusing e in <113> and <001> chains, for both Ni and Mo atoms. The maxima in the <110> direction are the greatest, thanks to the fact that atoms in these chains are located more closely to each other, than in remaining chains. Therefore, a process of focusing in <110> directions is the best and the coefficient of sputtering is the greatest. 4.1.3. Azimuthal distribution of sputtered atoms The azimuthal distribution of sputtered particles from the (111) face of Ni4Mo is shown in Fig. 6a and b. The main maxima correspond to <110> directions and the others e to <113> directions. This result can be explained as well as the data in Fig. 5 because the distance between atoms in <110> chains is less, than in <113> chains, therefore focusing, and sputtering in <110> directions is better. The segregation does not change the shape of azimuthal dependence of sputtering, but leads to growth of molybdenum sputtering with respect to nickel (Fig. 6b).
Fig. 4. Spot pattern for sputtered Ni (a) and Mo (b) atoms from (111) face of disordered Ni4Mo single-crystal for the surface without (a,b) and with (c,d) segregation.
4.1.4. Depth of source of sputtering We have analyzed the trajectories of atoms moving in the Ni4Mo single-crystal and obtained the dependence of the sputtering coefficient Y on the source depth position (Fig. 7a and b). It is seen that the source depth x0 for most Ni and Mo atoms is
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Fig. 8. Coefficient of sputtering for Ni4Mo (111) face versus the numbers of generations L for a surface without (a) and with (b) segregation of three upper layers; solid curves are for the sputtered Ni atoms, and dotted curves are for Mo atoms.
Fig. 6. Distribution of sputtering coefficient for Ni (solid line) and Mo (dotted line) atoms versus the azimuthal angle 4 of exit from (111) face of disordered Ni4Mo crystal without (a) and with (b) segregation of three upper layers. Polar angle of observation q ¼ 35 .
4.1.6. Spatial distribution and energy of sputtered particles The spatial distribution and the energy E1 of Ni and Mo particles sputtered from the (111) Ni4Mo face is shown in Fig. 9a and b. Sputtering of an unchanged (without segregation) surface is slightly higher than for a changed one owing to preferential sputtering of Ni (a light component). For the surface with segregation, the spot pattern become a somewhat distinct (Fig. 9b) due to additional focusing of the sputtered particles leaving through the top layer of Mo atoms. For the unchanged surface the fast-moving Ni atoms dominate; for the surface with segregation an exit of slow Mo atoms increased. Distinction in the speed of Ni and Mo atoms is more considerable for the surface without change of composition of top layers.
situated on the fourth (main maximum) and deeper layers when the conditions of focusing collisions work well. For the case with segregation, the source depth is located closer to the surface, than without segregation. The output of most atoms occur originates from the first layer of the (111) face.
4.1.7. Ordered Ni4Mo crystal The spot pattern and the angular distribution of sputtered particles from the (111) face of ordered single-crystal Ni4Mo was studied by MD simulation. The results of calculations are shown in Figs. 10 and 11. It was found that for ordered Ni4Mo single-crystal there is a shift of maxima of issue of Mo concerning Ni, unlike what is observed for a disordered crystal where spots of Ni and Mo are in the same directions (compare the positions of spots in Figs. 4 and 10). The same it could be seen from azimuthal distributions of ordered and disordered Ni4Mo crystal. Indeed for disordered crystal (Fig. 6) the maxima of emission are observed for Ni and Mo in the same directions. However, for ordered crystal (Fig. 11) the Ni emission main maxima correspond to the <110> directions, when for Mo emission main maxima are observed in the <113> directions both with segregation and without it. To explain this result, we consider the atomic arrangement in the (111) Ni4Mo plane, shown in Fig. 12. Here, large and small circles correspond to Mo and Ni atoms, respectively. Arrows indicate the directions of preferential emission of secondary ions. The preferential yield of particles sputtered from single crystals of a single-element and some binary compounds occurs in the
4.1.5. Numbers of generations that lead to sputtering We calculated to what generation L the sputtered atom belongs. According to definition, atom of the first generation (L ¼ 1) is the atom sputtered by an ion; this is a recoil atom. L ¼ 2 e for the atom sputtered by a recoil atom, L ¼ 3 e for the atom sputtered by a secondary recoil atom, etc. The results are shown in Fig. 8a and b. From the Fig. 8a and b it follows that most of sputtered the Ni and Mo atoms are the third generation, i.e. they are sputtered by the secondary recoils. For the surface with segregation the maxima of both components correspond to L ¼ 4 (Fig. 8b). This moving of L to the greatest values occurs probably because the heavy atoms of molybdenum hinder an output of particles from the layers of a crystal that are nearer to the surface.
Fig. 9. Spot pattern for sputtering of NiþMo (the horizontal plane) and the energy of sputtered Ni and Mo atoms (the vertical planes) for the Ni4Mo (111) face of disordered crystal without (a) and with (b) segregation of three upper layers.
Fig. 7. Dependence of sputtering coefficient Y on a source depth x0 under ion irradiation of the Ni4Mo (111) face without (a) and with (b) segregation of three upper layers; solid curves are for the output of Ni atoms, and dotted curves are for Mo atoms.
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Fig. 12. Atomic arrangement in the (111) plane of the Ni4Mo single-crystal. Large and small circles correspond to Mo and Ni atoms, respectively. Arrows indicate directions of preferential emission of sputtered atoms.
Fig. 10. Spot pattern for sputtered Ni (a) and Mo (b) atoms from the (111) face of ordered Ni4Mo single-crystal for the surface without (a,b) and with (c,d) segregation.
single crystals. In ordered Ni4Mo single-crystal there are chains in <001> directions (Fig. 12) causing the assisted focusing to increase. Along the <001> chains both Ni and Mo particles, can be emitted. Meanwhile, Mo atoms with a larger effective interaction radius have more favorable conditions of emission in this direction due focusing collisions. As a result, maxima in <001> directions are observed in the azimuthal angle's distribution of the Mo atoms output (Fig. 11). 4.2. Experiment 4.2.1. Sputtering and secondary ion emission The experimental results on sputtering correlate with those obtained by calculations. Indeed, the spot pattern in Fig. 13a is like the calculated pattern in Fig. 4c. It is interesting that the azimuthal distribution in Fig. 13b for Niþ and Moþ ions emitted from the (111) Ni4Mo face is like those obtained by MD simulations for sputtering of a disordered crystal (Fig. 6): the emission maxima is observed through each 60 in <110> (first and third maxima) and in <113> directions (second and fourth maxima) and the emission in <110> is greater than in <113> directions.
Fig. 11. Distribution of sputtering coefficient for Ni (solid line) and Mo (dotted line) atoms versus the azimuthal angle 4 of exit from the (111) face of ordered Ni4Mo singlecrystal without (a) and with (b) segregation of three upper layers. Polar angle of observation q ¼ 35 .
closest packing directions due to correlated (focused) collisions. For the Ni4Mo crystal, chains of atoms with the closest packing are in the <110> directions. They consist of sequences of four nickel atoms and one molybdenum atom (Fig. 12). In the case when such a chain comes to an end by a surface Ni atom, favorable conditions are created for propagation of focused collisions in several upper lattice layers. Thus, the appearance of maxima in the azimuthal distribution is explained by preferential sputtering of nickel due to focused collisions in the close-packed <110> directions of Ni4Mo
4.2.2. Secondary electron emission A study of secondary electron emission is important not only for receiving fundamental knowledge, but also for practical applications. Really, secondary electron emission is widely applied, for example, in electronic multipliers, in various electron-beam and high-frequency devices, at electron-beam melting and welding, and in the spectroscopic analysis of a solid body [27e29]. In research on secondary electron emission much attention is paid to interaction of electrons with single-crystals. Very interesting phenomena e the anisotropy of secondary and scattered electrons has been found first by Dekker et al. [30]. On the angular dependence of the coefficient of secondary electron emission s(a) small maxima appear and the thin structure of the s(a) curves arises. These maxima occur whenever the primary beam falls along one of the low-index axes of the crystal. The magnitudes of these maxima vary with the energy of the incident electrons, but their angular position does not change. The fine structure of angular distribution of secondary electrons for the single-crystals of metals and semiconductors was obtained earlier in Refs. [31e36]. It was shown that anisotropy of s(a) dependence is defined by two factors. First, the intensity of secondary electron emission from the single-crystal surface depends on the orientation of primary electron beam relative to the crystal
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Fig. 13. Spot pattern on a glass collector for sputtering the (111) Ni4Mo face (a). Dependence of coefficient Iþ of secondary ion emission on azimuthal angle 4 of exit for Niþ ions from Ni4Mo (111) face at the polar angle of observation q ¼ 35 (b). Mass spectrum for ion-irradiated (111) Ni4Mo face (c).
axes (anisotropy of excitation of secondary electrons). Second, the anisotropy in angular distributions of secondary electron exit takes place (anisotropy of emission of secondary electrons). In the present work the secondary-electron emission from the ordered Ni4Mo (111) face versus the angle a of the primary electrons in the ð112Þ plane for initial and ion-irradiated surface was studied. The results are shown in Fig. 14a and b together with the surface relief after sputtering. From Fig. 14 we can draw the following conclusions: 1. The secondary electron emission is less for the disturbed surface (lower curve in Fig. 14a) than for the initial one (upper curve). That is due to absorption of secondary and scattered electrons on the lateral faces of hills on the ion-irradiated surface. The small dose of ion irradiation of Ni4Mo (111) face in our case has led to formation of relief of small height. It has caused a small difference of emission for the disturbed and undisturbed surface and to smaller change of the s(a) curve for the ion-irradiated surface. A reduction of secondary electron emission for the case of specially prepared, artificial surface roughness with triangular grooves (Fig. 15) was studied in Ref. [37] and explained as follows. A primary electron, whose trajectory is shown as continuous line, hits the surface at point A and produces secondary electrons shown with dashed lines. Some of the secondary electrons can escape the groove and move away from the surface. Other secondary electrons would hit an inner side of the groove. With some probability they will be absorbed, or they can generate further secondary electrons, whose trajectories are shown as a dotted line. The process may repeat several times until the energy of higher generations becomes too low and they are absorbed by the surface.
Fig. 14. Dependence of the secondary electron emission coefficient s on angle a of the primary electrons in ð112Þ plain for initial (upper curve) and ion-irradiated (lower curve) Ni4Mo (111) face (a). Surface structure after ion irradiation of Ni4Mo (111) face; Arþ, E0 ¼ 10 keV, dose ¼ 1019 ion/cm2; the height of hills ~30 nm (b).
Fig. 15. Scheme for explaining of changing the secondary ion emission from rough surface.
2. The clear maxima on the s(a) curve at a ¼ 32 and 55 arise for an undisturbed Ni4Mo (111) face (Fig. 14a). They can be explained by additional scattering of primary and secondary electrons on atoms located in the lateral sides of open channels in the close-packed [113] and [001] directions. 3. The shape of the s(a) curve changes for the ion-irradiated surface with developed topography (Fig. 14b). In this case, the additional maxima in secondary electron emission (see also Ref. [38e40]) will appear. They appear when the primary electrons fall onto the lateral faces of hills at different angles, which, in particular, may coincide with the angles of open channels. A similar dependence of reduction in the output of secondary particles from disturbed surface was observed also for the processes of secondary ion emission and sputtering [41,42]. 5. Conclusion Sputtering, secondary ion and secondary electron emission were studied experimentally for the intermetallic Ni4Mo compound with high corrosion resistance and great hardness, which is used as material for constructing components of rockets and nuclear reactors. The process of sputtering was studied also by molecular dynamics simulations. Polar and azimuthal angular distribution of particles sputtered by 10 keV Arþ ions at normal incidence (a ¼ 0 ) on the Ni4Mo (111) face for unchanged and changed composition (with segregation) of topmost surface layers of disordered and ordered crystal was analyzed. The existence of anisotropy of sputtering Ni4Mo single-crystal; i.e. the preferential sputtering in [110], [111], [113], and [001] directions, was established. For disordered single-crystal the main maximum of Ni and Mo sputtering arises in the [110] direction due to direct focusing. For ordered crystal the main Mo maxima shift to the [001] direction, where the assisted focusing is observed. The segregation reduces the role of focusing processes and leads to decreases of sputtering coefficient Y. The reduction of sputtering coefficient for Ni4Mo (111) face with
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respect to that of Mo and Ni has been observed. A dependence of sputtering coefficient Y on a source depth x0 under ion irradiation of the Ni4Mo (111) face without and with segregation of three upper layers is calculated. The same for the numbers of generations L, leads to sputtering, was analyzed. It was found that the ordering of Ni4Mo single-crystal causes a shift of maxima of Mo sputtering with respect to Ni. The experimental results on sputtering correlate with those obtained by calculations: a spot pattern is like the calculated pattern; the azimuthal distribution for Niþ and Moþ ions emitted from the (111) Ni4Mo face is like those obtained by MD simulations for sputtering. A secondary electron emission from the ordered Ni4Mo (111) face versus the angle a of the primary electrons in the ð112Þ plane for initial and ion-irradiated surface was studied experimentally. The secondary electron emission coefficient s has been shown to be lower for disturbed surfaces than for undisturbed ones, a result explained by the absorption of secondary and scattered electrons on the lateral faces of the hills on ion-irradiated surfaces. Maxima in the s(a) dependence arise when primary electrons get into open channels of the crystal lattice, both for the undisturbed surface and for the surface after ion irradiation. These maxima correspond to the close-packed directions, and arise due to additional scattering of primary and secondary electrons on the atoms in open channels. The obtained regularities should be taken into account when exploring the mechanisms of sputtering, secondary ion and secondary electron emission, and designing devices where these emission processes are used. Acknowledgments The authors are grateful to Russian Foundation for Basic Research (grant N 15-02-07819-A) for the financial support and to M.V.Gomoyunova, Yu.V.Martynenko and A.I.Titov for consultations and fruitful discussions. References [1] M. Nastasi, J.W. Mayer, J.K. Hirvonen, Ion-Solid Interactions. Fundamentals and Applications, Cambridge University Press, 1996, p. 540. [2] V.E. Yurasova, Interaction of Ions with the Surface, Prima B, Moscow, 1999, p. 640. [3] R. Behrisch, W. Eckstein (Eds.), Sputtering by Particle Bombardment.IV, SpringereVerlag, Berlin, Heidelberg, 2007. [4] W.A. Harrison, Electronic Structure and the Properties of Solids: the Physics of the Chemical Bond, Freeman, San Francisco, 1980, p. 586. [5] G.N. Derry, R. Wan, F. Strauch, C. English, Segregation and interlayer relaxation at the NiPd (111) surface, J. Vac. Sci. Tech. A29 (2011) 011015. [6] K. Kimura, S. Hata, S. Matsumura, T. Horiuchi, Dark-field transition electron microscopy for a tilt series of ordering alloys: toward electron tomography, J. Electron Microsc. 54 (4) (2005) 373e377. [7] G. Van Tendeloo, J. Van Landuyt, S. Amelinckx, Radiation ordering in quenched alloys observed in situ in the high voltage microscope, Rad. Eff. 41 (1979) 179e184. [8] G. Martin, A. Barbu, Phase stability under irradiation, in: Microscopie Electronique a Haute Tention, 1975, pp. 70e77. [9] C. Abromeit, H. Wollenberger, S. Matsumura, C. Kinoshita, Stability of ordered phases under irradiation, J. Nucl. Mater. 276 (2000) 104e113. [10] S.L. Antonov, I.N. Ivanov, A.A. Orlikovskii, V.Yu. Vasil'chenko, V.E. Yurasova, Sputtering of Ni4Mo single crystals: computer simulation and experiment, Nucl. Instr. Meth. B48 (1990) 553e556. [11] A.A. Promokhov, V.A. Eltekov, V.E. Yurasova, J.S. Colligon, A.S. Mosunov, Computer calculations of single crystal sputtering by low energy ions, Nucl. Instr. Meth. B115 (1996) 544e548. [12] V.E. Yurasova, The influence of models on the results of computer calculation of ion scattering from single crystals, in: Invited Paper of the 7th Summer School on Physics of Ionized Gases, 1974, pp. 427e476. Rovinj, Yugoslavia.
[13] A.S. Mosunov, L.B. Shelyakin, V.E. Yurasova, D. Ciric, B. Perovich, I. Terzic, Computer simulation of ion scattering by polycrystals, Rad. Eff. 52 (1980) 85e90. [14] A.A. Samarskii, A.V. Gulin, Numerical Methods, Nauka, Moscow, 1989, p. 432 [in Russian]. [15] O.B. Firsov, Calculation of the interaction potential of atoms, JETP 36 (1957) 696e699 [in Russian]. [16] H.H. Andersen, P. Sigmund, On the Determination of Interatomic Potentials in Metals by Electron Irradiation Experiments, Ris Reports. N103, 1965, pp. 1e20. [17] М. Ardеnne, Duoplasmatron, Technik 11 (1956) 65. [18] I.V. Orfanov, V.A. Teplyakov, The ionic tube on 50 keV, Prib.Tekh. Eksp. 2 (1960) 150e152 [in Russian]. [19] T.P. Martynenko, Sputtering of diatomic compounds by Liþ ions with small energies, Phys. Solid State 10 (9) (1968) 2876e2878 [in Russian]. [20] G.K. Wehner, Sputtering of metal single crystals by ion bombardment, J. Appl. Phys. 26 (1955) 1056e1057. [21] V.E. Yurasova, Modern theories of cathode sputtering and micro relief of the sputtered metal surface, Abstract of 8th All-Union workshop on the cathode electronics, Leningrad, 1957, p.15; J. Tech. Phys. 28 (9) (1958) 1966e1970 [in Russian]. [22] M.W. Thompson, The ejection of atoms from gold crystals during proton irradiation, Philos. Mag. 4 (1959) 139e141. [23] V.E. Yurasova, N.V. Pleshivtsev, I.V. Orfanov, Directed emission of particles from a copper single crystal sputtered by bombardment with ions up to 50 keV energy, JETP 10 (1960) 689e693. [24] V.E. Yurasova, The interaction of medium energy ions with monocrystals of metals and binary compounds, in: Abstract of the Symposium on Atomic Collision Cascades in Radiation Damages, 1964, pp. 8e9. Harwell, England. [25] R.S. Nelson, M.W. Thompson, Atomic collision sequences in crystals of copper, silver and gold revealed by sputtering in energetic ion beams, Proc. R. Soc. A. Lond. 259 (1961) 458e464. [26] M.W. Thompson, Defects and Radiation Damage in Metals, Cambridge University Press, Cambridge, 1969, p. 442. [27] H. Bruining, Physics and Applications of Secondary Electron Emission, Pergamon Press, New York, London, 1954, p. 178. [28] A.R. Shulman, S.A. Fridrikhov, Secondary-emission Methods of Solid Bodies Studied, Nauka, Moscow, 1977, p. 551 [in Russian]. [29] A. Howie, Diffraction channelling of fast electrons and positrons in crystals, Phil. Mag. 14 (1966) 223e237. [30] A.J. Dekker, R.W. Soshen, Fine structure of secondary emission vs angle of incidence of the primary beam on titanium single crystals, Phys. Rev. 121 (1961) 1362e1369. [31] A.P. Komar, Yu.S. Korobochko, B.D. Grachev, B.I. Mineev, About the mechanism of the fine structure of the angular dependence of the coefficient of secondary electron emission of single crystals, Phys. Solid State 10 (1968) 1547e1548 [in Russian]. [32] I.A. Abroyan, A.I. Titov, Influence of the ion bombardment temperature on the angular dependences of the secondary electron emission of a germanium single crystal, JETP 10 (1968) 3432e3434 [in Russian]. [33] E.D. Wolf, Т.Е. Everhart, E.D. Wolf, T.E. Everhart, Electron beam channelling in single crystal silicon by scanning electron microscopy, Appl. Phys. Lett. 14 (1969) 299e300. [34] A.R. Shulman, V.V. Korablev, Yu.A. Morozov, Secondary electron emission of molybdenum single crystals, JETP 12 (1970) 758e762 [in Russian]. [35] M.V. Gomoyunova, Anisotropy of natural secondary electron emission, Phys. Solid State 14 (1972) 3498e3500 [in Russian]. [36] M.V. Gomoyunova, S.L. Zaslavskii, I.I. Pronin, Anisotropy of elastic reflection of electrons from molybdenum single-crystal, Phys. Solid State 20 (1978) 1586e1589 [in Russian]. [37] M. Pivi, F. King, R. Kirby, T. Ranbenheimer, G. Stupakov, F. Le Pimpec, Sharp reduction of the secondary electron emission yield from grooved surfaces, J. Appl. Phys. 104 (10) (2008) 104904. [38] K.F. Minnebaev, A.A. Khaidarov, I.P. Ivanenko, D.K. Minnebaev, V.E. Yurasova, Features of angular dependence of secondary electron emission from metal single crystals, Nucl. Instr. Meth B 406 (2017) 482e486. [39] K.F. Minnebaev, V.V. Khvostov, E.Yu. Zykova, K.A. Tolpin, J. Colligon, V.E. Yurasova, Secondary particle emission from sapphire single crystal, NIMB 354 (2015) 159e162. [40] E.I. Rau, A.A. Tatarintsev, V.V. Khvostov, V.E. Yurasova, Secondary electron emission and charging characteristics of ion-irradiated sapphire, Vacuum 129 (2016) 142e147. [41] K.A. Tolpin, K.F. Minnebaev, V.E. Yurasova, Detection by sputtering of deformed areas hidden under a surface, Vacuum 138 (2017) 139e145. [42] V.E. Yurasova, V.T. Cherepin, Yu.A. Ryzhov, Structure effects in secondary ion emission of metals and alloys, J. Surf. Invest. X-ray Synchrotron Neutron Tech. 5 (3) (2011) 465e483.
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Secondary emission of neutral and charged particles from intermetallic single-crystal K.A. Tolpin*, K.F. Minnebaev, V.E. Yurasova Moscow State University, Moscow 119991, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2017 Received in revised form 3 November 2017 Accepted 5 November 2017 Available online 7 November 2017
Sputtering, secondary ion and secondary electron emission were studied experimentally for an intermetallic Ni4Mo compound with high corrosion resistance and great hardness, which is used as material for constructing components of rockets and nuclear reactors. The process of sputtering was analyzed by molecular dynamics simulations of Ni4Mo (111) face for unchanged and changed composition (with segregation) of topmost layers of a disordered and ordered crystal. Predominant exit of Ni and Mo atoms in close-packed directions was observed and explained by correlated collisions. Both nickel and molybdenum atoms are ejected in the same crystallographic directions for a disordered crystal. For an ordered crystal the nickel atoms deviate from these directions. The origin of sputtering and number of ejected particles generated has been calculated. For secondary electron emission from the (111) Ni4Mo face the maxima in close-packed directions were obtained experimentally and explained by scattering of primary and secondary electrons on atoms located in the lateral sides of open channels. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Secondary ion emission Sputtering Secondary electron emission Scattering Intermetallic compounds Molecular dynamics
1. Introduction Research on neutral and charged particle emission under ion bombardment of Ni4Mo single-crystals is important for understanding the mechanisms of particle interaction with a surface of intermetallic compound and for practical applications, e.g. in secondary ion mass spectrometry, catalysis, and in production of hard refractory coatings [1]. Intermetallic compounds with a significant difference of component masses, such as Ni4Mo, are characterized by the difference between the surface and bulk structures, compositions, and properties [1e4]. Among them are such properties as surface segregation, surface ordering, and reconstruction [5]. The processes of ordering in such crystals were studied experimentally [6e9] and by numerical simulation [5,9]. In general, these compounds are thermally treated to obtain an ordered structure [6,7]. The intermetallic crystals can be also ordered using electron [6,7], neutron [7], and ion irradiation [8,9]. In these cases, as shown in Ref. [9], radiation-enhanced ordering is not associated with additional heating during irradiation. It was shown that the segregation leads to a change of surface
* Corresponding author. E-mail address: [email protected] (K.A. Tolpin). https://doi.org/10.1016/j.vacuum.2017.11.005 0042-207X/© 2017 Elsevier Ltd. All rights reserved.
composition of the three top layers of intermetallic compounds, which is different for different faces of crystals. For instance, the top layer of the (001) NiPd face contains 80% of Pd, whereas the second layer has no Pd and the third layer has 64% of Pd [5]. Such changes in composition of surface layers are caused by the interplay of two competing factors: the mechanical stress (that leads to the segregation of the heavy component of the compound on a surface) and the various chemical interactions (that result in short-scale ordering) [5e7]. The sputtering and secondary ion emission from Ni4Mo singlecrystal was first studied in Ref. [10] for ion irradiation of the (001) face. The preferential yield of nickel (light component) was observed which is facilitated by the lower binding energy of Ni in comparison with Mo. Different angular distributions of emitted secondary ions for ordered and disordered (i.e. for randomly placed of Ni and Mo atoms) single-crystals were obtained. However, the experimental setup in Ref. [10] did not allow independent variations of observation angles for primary and secondary ions. In the present work, we eliminated this disadvantage, so that correct studies of azimuthal and polar angle distributions of secondary Ni and Mo ions from the Ni4Mo single-crystal could be made. The process of sputtering was studied also by molecular dynamics simulations. Polar and azimuthal angular distribution of sputtering under irradiation Ni4Mo (111) face by 10 keV Arþ ions at normal incidence (a ¼ 0 ) for unchanged and changed composition
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(i.e. with segregation of topmost surface layers of disordered and ordered crystal) were analyzed. 2. MD simulation Simulation of sputtering was performed for the (111) face of ordered and disordered Ni4Mo single-crystal with tetragonal lattice. The crystalline structure of ordered Ni4Mo single-crystal is illustrated in Fig. 1. The sputtering of Ni4Mo crystals was calculated using the molecular dynamics model with a moving single-crystal block of atoms [11e13]. At fixed moments in time, the interaction of a moving (active) particle with target atoms was considered. The nearest atoms form a block whose radius is equal to the radius of the fifth coordination sphere. In the resulting structure, an incident ion and target atoms can move and interact with each other. When an active particle moved, the block moved too and formed the sphere around the particle. If the energy acquired by interacting target atoms was higher than the threshold level (equal to the binding energy), the positions and momenta of these atoms were recorded and, subsequently, their further motion was calculated. The block of atoms had no time to exhibit instability, because the sputtering of an atom is a short-time process (its duration is approximately 1013 s [14] from the moment of impact of ion). This model made it possible to trace the particle trajectory at large (hundreds of angstroms) distances from the place of the ion incidence. The equations of motion were integrated using a modified Euler predictor-corrector scheme that is stable [14]. Inelastic losses were computed according to the Firsov formula [15]. Thermal oscillations of atoms were assumed to be uncorrelated. Lattice distance a, c and binding energy Eb were chosen from experimental data: a ¼ 5.720 Å, c ¼ 3.564 Å and Eb ¼ 4.2 eV [4]. The interaction potential was determined in the following form [16]: U(r) ¼ Аbm$exp (-r/abm) þ (Аb/r)$exp (-2r/abm), where Abm ¼ 52 (Z1Z2)3/4, abm ¼ 0.219 Å, Ab ¼ k (Z1Z2e2), Z1 and Z2 e are the atomic numbers of the ion and target atom, respectively; r is the vector radius; and k is an adjustable parameter of the order of unity. In the computation, the following parameters were determined: the sputtered atom's momentum, the length of the sputtering cascade path, the sputtering time, the sputtered atom's generation number L, and the depth of the origin x0 of sputtering. 3. Experiment The patterns of sputtering the (111) Ni4Mo face was obtained using the system shown schematically in Fig. 2. The setup for sputtering contains the ion source of Ardenne [17], improved as discussed in Ref. [18], which allowed well-focused beams of argon ions with a current density of 1 mА/sm2 at energy E0 ¼ 10 keV to be obtained. The ion beam, after acceleration
Fig. 1. A structure of ordered Ni4Mo single-crystal with tetragonal lattice. Large and small circles correspond to Mo and Ni atoms, respectively. Semi-closed circles are atoms arranged within the lattice.
Fig. 2. Sputtering system. 1 e ion source, 2 e insulator, 3 e electrostatic lens, 4 e container, 5 e window, 6 e quartz screen, 7 and 10 e glass collectors, 8 e specimen, 9 e holder.
and focusing by a single electrostatic lens, passed through a hole in a quartz screen and bombarded the sample on the area with diameter of 2 mm. The dose of irradiation was measured by current integrator. A glass collector for sputtered particles was placed parallel to the studied surface in front of the sample in the distance of 15 mm. Spot pattern of sputtered particles was used to determine the orientation of single-crystal samples. For studying the secondary ion emission it is important to simultaneously determine the angular and energy distributions of mass-separated secondary species. In our work, these measurements have been performed by a specially designed and constructed setup with a mobile 180 spherical energy analyzer linked to an immobile quadrupole mass spectrometer as shown in Fig. 3. The secondary ion flux was focused in two directions, which ensured a high transmission coefficient and allowed axissymmetric optics to be used. The energy resolution was about 0.5 eV (at transmission energy of 20 eV) and the ion mass range was 1e350 amu. The sample chamber was evacuated by a magnetic discharge pump to a residual pressure on the order of 109 mbar. The primary ions were incident on the target at an angle a ¼ 0 relative to the normal to the surface. The polar angle q of observation of the secondary ion emission could be varied within 90 by rotating the energy analyzer. The yield of secondary ions Iþ was determined either by integrating the mass peak at the certain ion energy or by integrating the energy spectra of ions. The secondary electron emission was measured by the scanning electron microscope (LEO 14XX (VP)). We use a passing of 1 nA current through a crystal. Some times we measured also backscattered and secondary electrons using a Faraday cup. The energies of primary electrons were 10 keV. The topography of surface has been studied by the scanning electron microscope (LEO-1455 (Carl Zeiss)) and by the atomic force
Fig. 3. Schematic diagram of experimental arrangement.
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microscope (ATC FemtoScan Online). 4. Results and discussion 4.1. MD modelling Simulation of sputtering was carried out for the natural state of a (111) Ni4Mo face e with segregation, (when the three topmost layers had 82%, 24% and 50% of Mo, respectively) and also, for comparison, without segregation (hypothetic case). The results of calculations of sputtering for disordered Ni4Mo crystal are shown in sections 4.1.1-4.1.6. 4.1.1. Spot pattern The spot pattern, arising on the plate collector after sputtering of the (111) Ni4Mo face, are shown at Fig. 4. They correspond to sputtering in the directions: [110], [111], [113], and [001]. Formation of spots in these directions for a disordered single crystal is explained by the correlated collisions in accidentally created short chains of atoms in the top layers of the (111) face. It is seen, that, for the surface without segregation, Ni sputtered more than Mo. When we include the segregation, then, on the contrary, Mo sputtered more then Ni. This is due to complicated transmission of Ni through molybdenum atoms in the first layer. 4.1.2. Polar distribution of sputtered atoms The distribution of nickel and molybdenum atoms sputtered from the (111) face of single-crystal Ni4Mo on the polar angle q of output in the ð112Þ plane is shown in Fig. 5. The overall coefficient of sputtering for Ni4Mo (111) face Y ¼ 6.7 atoms/ion for the studied case. This value for the coefficient Y of sputtering for Ni4Mo compound is a little smaller than Y for the individual components of Ni and Mo. This result coincides with experimental data for sputtering of binary compounds, obtained in Ref. [19]. Segregation does not change the main shapes of Y (q) curves, but leads to noticeable growth of Mo sputtering from the first layer with respect to Ni (Fig. 5b).
Fig. 5. Angular dependence of Ni (solid line) and Mo (dotted line) sputtering from (111) face of disordered Ni4Mo crystal on polar angle q of exit in ð112Þ plane for surface without (a) and with (b) segregation of three upper layers.
The maxima in polar distributions of sputtering (Fig. 5a and b) are explained by correlated collisions in the upper layers of the Ni4Mo (111) face. It is known that preferential sputtering of single-crystals is observed in the directions of close-packed chains by direct focusing collisions and by additional focused e assisted collisions [20e26]. According to our calculations for sputtering Ni4Mo (111) face the direct focusing occurs in <110> and <111> chains and the assisting focusing e in <113> and <001> chains, for both Ni and Mo atoms. The maxima in the <110> direction are the greatest, thanks to the fact that atoms in these chains are located more closely to each other, than in remaining chains. Therefore, a process of focusing in <110> directions is the best and the coefficient of sputtering is the greatest. 4.1.3. Azimuthal distribution of sputtered atoms The azimuthal distribution of sputtered particles from the (111) face of Ni4Mo is shown in Fig. 6a and b. The main maxima correspond to <110> directions and the others e to <113> directions. This result can be explained as well as the data in Fig. 5 because the distance between atoms in <110> chains is less, than in <113> chains, therefore focusing, and sputtering in <110> directions is better. The segregation does not change the shape of azimuthal dependence of sputtering, but leads to growth of molybdenum sputtering with respect to nickel (Fig. 6b).
Fig. 4. Spot pattern for sputtered Ni (a) and Mo (b) atoms from (111) face of disordered Ni4Mo single-crystal for the surface without (a,b) and with (c,d) segregation.
4.1.4. Depth of source of sputtering We have analyzed the trajectories of atoms moving in the Ni4Mo single-crystal and obtained the dependence of the sputtering coefficient Y on the source depth position (Fig. 7a and b). It is seen that the source depth x0 for most Ni and Mo atoms is
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Fig. 8. Coefficient of sputtering for Ni4Mo (111) face versus the numbers of generations L for a surface without (a) and with (b) segregation of three upper layers; solid curves are for the sputtered Ni atoms, and dotted curves are for Mo atoms.
Fig. 6. Distribution of sputtering coefficient for Ni (solid line) and Mo (dotted line) atoms versus the azimuthal angle 4 of exit from (111) face of disordered Ni4Mo crystal without (a) and with (b) segregation of three upper layers. Polar angle of observation q ¼ 35 .
4.1.6. Spatial distribution and energy of sputtered particles The spatial distribution and the energy E1 of Ni and Mo particles sputtered from the (111) Ni4Mo face is shown in Fig. 9a and b. Sputtering of an unchanged (without segregation) surface is slightly higher than for a changed one owing to preferential sputtering of Ni (a light component). For the surface with segregation, the spot pattern become a somewhat distinct (Fig. 9b) due to additional focusing of the sputtered particles leaving through the top layer of Mo atoms. For the unchanged surface the fast-moving Ni atoms dominate; for the surface with segregation an exit of slow Mo atoms increased. Distinction in the speed of Ni and Mo atoms is more considerable for the surface without change of composition of top layers.
situated on the fourth (main maximum) and deeper layers when the conditions of focusing collisions work well. For the case with segregation, the source depth is located closer to the surface, than without segregation. The output of most atoms occur originates from the first layer of the (111) face.
4.1.7. Ordered Ni4Mo crystal The spot pattern and the angular distribution of sputtered particles from the (111) face of ordered single-crystal Ni4Mo was studied by MD simulation. The results of calculations are shown in Figs. 10 and 11. It was found that for ordered Ni4Mo single-crystal there is a shift of maxima of issue of Mo concerning Ni, unlike what is observed for a disordered crystal where spots of Ni and Mo are in the same directions (compare the positions of spots in Figs. 4 and 10). The same it could be seen from azimuthal distributions of ordered and disordered Ni4Mo crystal. Indeed for disordered crystal (Fig. 6) the maxima of emission are observed for Ni and Mo in the same directions. However, for ordered crystal (Fig. 11) the Ni emission main maxima correspond to the <110> directions, when for Mo emission main maxima are observed in the <113> directions both with segregation and without it. To explain this result, we consider the atomic arrangement in the (111) Ni4Mo plane, shown in Fig. 12. Here, large and small circles correspond to Mo and Ni atoms, respectively. Arrows indicate the directions of preferential emission of secondary ions. The preferential yield of particles sputtered from single crystals of a single-element and some binary compounds occurs in the
4.1.5. Numbers of generations that lead to sputtering We calculated to what generation L the sputtered atom belongs. According to definition, atom of the first generation (L ¼ 1) is the atom sputtered by an ion; this is a recoil atom. L ¼ 2 e for the atom sputtered by a recoil atom, L ¼ 3 e for the atom sputtered by a secondary recoil atom, etc. The results are shown in Fig. 8a and b. From the Fig. 8a and b it follows that most of sputtered the Ni and Mo atoms are the third generation, i.e. they are sputtered by the secondary recoils. For the surface with segregation the maxima of both components correspond to L ¼ 4 (Fig. 8b). This moving of L to the greatest values occurs probably because the heavy atoms of molybdenum hinder an output of particles from the layers of a crystal that are nearer to the surface.
Fig. 9. Spot pattern for sputtering of NiþMo (the horizontal plane) and the energy of sputtered Ni and Mo atoms (the vertical planes) for the Ni4Mo (111) face of disordered crystal without (a) and with (b) segregation of three upper layers.
Fig. 7. Dependence of sputtering coefficient Y on a source depth x0 under ion irradiation of the Ni4Mo (111) face without (a) and with (b) segregation of three upper layers; solid curves are for the output of Ni atoms, and dotted curves are for Mo atoms.
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Fig. 12. Atomic arrangement in the (111) plane of the Ni4Mo single-crystal. Large and small circles correspond to Mo and Ni atoms, respectively. Arrows indicate directions of preferential emission of sputtered atoms.
Fig. 10. Spot pattern for sputtered Ni (a) and Mo (b) atoms from the (111) face of ordered Ni4Mo single-crystal for the surface without (a,b) and with (c,d) segregation.
single crystals. In ordered Ni4Mo single-crystal there are chains in <001> directions (Fig. 12) causing the assisted focusing to increase. Along the <001> chains both Ni and Mo particles, can be emitted. Meanwhile, Mo atoms with a larger effective interaction radius have more favorable conditions of emission in this direction due focusing collisions. As a result, maxima in <001> directions are observed in the azimuthal angle's distribution of the Mo atoms output (Fig. 11). 4.2. Experiment 4.2.1. Sputtering and secondary ion emission The experimental results on sputtering correlate with those obtained by calculations. Indeed, the spot pattern in Fig. 13a is like the calculated pattern in Fig. 4c. It is interesting that the azimuthal distribution in Fig. 13b for Niþ and Moþ ions emitted from the (111) Ni4Mo face is like those obtained by MD simulations for sputtering of a disordered crystal (Fig. 6): the emission maxima is observed through each 60 in <110> (first and third maxima) and in <113> directions (second and fourth maxima) and the emission in <110> is greater than in <113> directions.
Fig. 11. Distribution of sputtering coefficient for Ni (solid line) and Mo (dotted line) atoms versus the azimuthal angle 4 of exit from the (111) face of ordered Ni4Mo singlecrystal without (a) and with (b) segregation of three upper layers. Polar angle of observation q ¼ 35 .
closest packing directions due to correlated (focused) collisions. For the Ni4Mo crystal, chains of atoms with the closest packing are in the <110> directions. They consist of sequences of four nickel atoms and one molybdenum atom (Fig. 12). In the case when such a chain comes to an end by a surface Ni atom, favorable conditions are created for propagation of focused collisions in several upper lattice layers. Thus, the appearance of maxima in the azimuthal distribution is explained by preferential sputtering of nickel due to focused collisions in the close-packed <110> directions of Ni4Mo
4.2.2. Secondary electron emission A study of secondary electron emission is important not only for receiving fundamental knowledge, but also for practical applications. Really, secondary electron emission is widely applied, for example, in electronic multipliers, in various electron-beam and high-frequency devices, at electron-beam melting and welding, and in the spectroscopic analysis of a solid body [27e29]. In research on secondary electron emission much attention is paid to interaction of electrons with single-crystals. Very interesting phenomena e the anisotropy of secondary and scattered electrons has been found first by Dekker et al. [30]. On the angular dependence of the coefficient of secondary electron emission s(a) small maxima appear and the thin structure of the s(a) curves arises. These maxima occur whenever the primary beam falls along one of the low-index axes of the crystal. The magnitudes of these maxima vary with the energy of the incident electrons, but their angular position does not change. The fine structure of angular distribution of secondary electrons for the single-crystals of metals and semiconductors was obtained earlier in Refs. [31e36]. It was shown that anisotropy of s(a) dependence is defined by two factors. First, the intensity of secondary electron emission from the single-crystal surface depends on the orientation of primary electron beam relative to the crystal
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Fig. 13. Spot pattern on a glass collector for sputtering the (111) Ni4Mo face (a). Dependence of coefficient Iþ of secondary ion emission on azimuthal angle 4 of exit for Niþ ions from Ni4Mo (111) face at the polar angle of observation q ¼ 35 (b). Mass spectrum for ion-irradiated (111) Ni4Mo face (c).
axes (anisotropy of excitation of secondary electrons). Second, the anisotropy in angular distributions of secondary electron exit takes place (anisotropy of emission of secondary electrons). In the present work the secondary-electron emission from the ordered Ni4Mo (111) face versus the angle a of the primary electrons in the ð112Þ plane for initial and ion-irradiated surface was studied. The results are shown in Fig. 14a and b together with the surface relief after sputtering. From Fig. 14 we can draw the following conclusions: 1. The secondary electron emission is less for the disturbed surface (lower curve in Fig. 14a) than for the initial one (upper curve). That is due to absorption of secondary and scattered electrons on the lateral faces of hills on the ion-irradiated surface. The small dose of ion irradiation of Ni4Mo (111) face in our case has led to formation of relief of small height. It has caused a small difference of emission for the disturbed and undisturbed surface and to smaller change of the s(a) curve for the ion-irradiated surface. A reduction of secondary electron emission for the case of specially prepared, artificial surface roughness with triangular grooves (Fig. 15) was studied in Ref. [37] and explained as follows. A primary electron, whose trajectory is shown as continuous line, hits the surface at point A and produces secondary electrons shown with dashed lines. Some of the secondary electrons can escape the groove and move away from the surface. Other secondary electrons would hit an inner side of the groove. With some probability they will be absorbed, or they can generate further secondary electrons, whose trajectories are shown as a dotted line. The process may repeat several times until the energy of higher generations becomes too low and they are absorbed by the surface.
Fig. 14. Dependence of the secondary electron emission coefficient s on angle a of the primary electrons in ð112Þ plain for initial (upper curve) and ion-irradiated (lower curve) Ni4Mo (111) face (a). Surface structure after ion irradiation of Ni4Mo (111) face; Arþ, E0 ¼ 10 keV, dose ¼ 1019 ion/cm2; the height of hills ~30 nm (b).
Fig. 15. Scheme for explaining of changing the secondary ion emission from rough surface.
2. The clear maxima on the s(a) curve at a ¼ 32 and 55 arise for an undisturbed Ni4Mo (111) face (Fig. 14a). They can be explained by additional scattering of primary and secondary electrons on atoms located in the lateral sides of open channels in the close-packed [113] and [001] directions. 3. The shape of the s(a) curve changes for the ion-irradiated surface with developed topography (Fig. 14b). In this case, the additional maxima in secondary electron emission (see also Ref. [38e40]) will appear. They appear when the primary electrons fall onto the lateral faces of hills at different angles, which, in particular, may coincide with the angles of open channels. A similar dependence of reduction in the output of secondary particles from disturbed surface was observed also for the processes of secondary ion emission and sputtering [41,42]. 5. Conclusion Sputtering, secondary ion and secondary electron emission were studied experimentally for the intermetallic Ni4Mo compound with high corrosion resistance and great hardness, which is used as material for constructing components of rockets and nuclear reactors. The process of sputtering was studied also by molecular dynamics simulations. Polar and azimuthal angular distribution of particles sputtered by 10 keV Arþ ions at normal incidence (a ¼ 0 ) on the Ni4Mo (111) face for unchanged and changed composition (with segregation) of topmost surface layers of disordered and ordered crystal was analyzed. The existence of anisotropy of sputtering Ni4Mo single-crystal; i.e. the preferential sputtering in [110], [111], [113], and [001] directions, was established. For disordered single-crystal the main maximum of Ni and Mo sputtering arises in the [110] direction due to direct focusing. For ordered crystal the main Mo maxima shift to the [001] direction, where the assisted focusing is observed. The segregation reduces the role of focusing processes and leads to decreases of sputtering coefficient Y. The reduction of sputtering coefficient for Ni4Mo (111) face with
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respect to that of Mo and Ni has been observed. A dependence of sputtering coefficient Y on a source depth x0 under ion irradiation of the Ni4Mo (111) face without and with segregation of three upper layers is calculated. The same for the numbers of generations L, leads to sputtering, was analyzed. It was found that the ordering of Ni4Mo single-crystal causes a shift of maxima of Mo sputtering with respect to Ni. The experimental results on sputtering correlate with those obtained by calculations: a spot pattern is like the calculated pattern; the azimuthal distribution for Niþ and Moþ ions emitted from the (111) Ni4Mo face is like those obtained by MD simulations for sputtering. A secondary electron emission from the ordered Ni4Mo (111) face versus the angle a of the primary electrons in the ð112Þ plane for initial and ion-irradiated surface was studied experimentally. The secondary electron emission coefficient s has been shown to be lower for disturbed surfaces than for undisturbed ones, a result explained by the absorption of secondary and scattered electrons on the lateral faces of the hills on ion-irradiated surfaces. Maxima in the s(a) dependence arise when primary electrons get into open channels of the crystal lattice, both for the undisturbed surface and for the surface after ion irradiation. These maxima correspond to the close-packed directions, and arise due to additional scattering of primary and secondary electrons on the atoms in open channels. The obtained regularities should be taken into account when exploring the mechanisms of sputtering, secondary ion and secondary electron emission, and designing devices where these emission processes are used. Acknowledgments The authors are grateful to Russian Foundation for Basic Research (grant N 15-02-07819-A) for the financial support and to M.V.Gomoyunova, Yu.V.Martynenko and A.I.Titov for consultations and fruitful discussions. References [1] M. Nastasi, J.W. Mayer, J.K. Hirvonen, Ion-Solid Interactions. Fundamentals and Applications, Cambridge University Press, 1996, p. 540. [2] V.E. Yurasova, Interaction of Ions with the Surface, Prima B, Moscow, 1999, p. 640. [3] R. Behrisch, W. Eckstein (Eds.), Sputtering by Particle Bombardment.IV, SpringereVerlag, Berlin, Heidelberg, 2007. [4] W.A. Harrison, Electronic Structure and the Properties of Solids: the Physics of the Chemical Bond, Freeman, San Francisco, 1980, p. 586. [5] G.N. Derry, R. Wan, F. Strauch, C. English, Segregation and interlayer relaxation at the NiPd (111) surface, J. Vac. Sci. Tech. A29 (2011) 011015. [6] K. Kimura, S. Hata, S. Matsumura, T. Horiuchi, Dark-field transition electron microscopy for a tilt series of ordering alloys: toward electron tomography, J. Electron Microsc. 54 (4) (2005) 373e377. [7] G. Van Tendeloo, J. Van Landuyt, S. Amelinckx, Radiation ordering in quenched alloys observed in situ in the high voltage microscope, Rad. Eff. 41 (1979) 179e184. [8] G. Martin, A. Barbu, Phase stability under irradiation, in: Microscopie Electronique a Haute Tention, 1975, pp. 70e77. [9] C. Abromeit, H. Wollenberger, S. Matsumura, C. Kinoshita, Stability of ordered phases under irradiation, J. Nucl. Mater. 276 (2000) 104e113. [10] S.L. Antonov, I.N. Ivanov, A.A. Orlikovskii, V.Yu. Vasil'chenko, V.E. Yurasova, Sputtering of Ni4Mo single crystals: computer simulation and experiment, Nucl. Instr. Meth. B48 (1990) 553e556. [11] A.A. Promokhov, V.A. Eltekov, V.E. Yurasova, J.S. Colligon, A.S. Mosunov, Computer calculations of single crystal sputtering by low energy ions, Nucl. Instr. Meth. B115 (1996) 544e548. [12] V.E. Yurasova, The influence of models on the results of computer calculation of ion scattering from single crystals, in: Invited Paper of the 7th Summer School on Physics of Ionized Gases, 1974, pp. 427e476. Rovinj, Yugoslavia.
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