- Email: [email protected]
Effects of argon-ion bombardment on the properties of the surface layer in spinel crystals of different composition
ARTICLE IN PRESS
Vacuum 82 (2008) 888–894 www.elsevier.com/locate/vacuum
Effects of argon-ion bombardment on the properties of the surface layer in spinel crystals of different composition V.T. Gritsynaa,b,,1, V.V. Bobkova,c,2, S.P. Gokova,d,3, V.V. Gritsynaa,b,1, D.I. Shevchenkoa,e,4 a
V.N. Karazin Kharkiv National University, Svoboda Sq., 4, 61077 Kharkiv, Ukraine b Kurchatov Ave. 21, Apt. 49, Kharkiv 61108, Ukraine c Kurchatov Ave. 21, Apt. 31, Kharkiv 61108, Ukraine d Garibaldi Str. 3, Apt. 15, Kharkiv 61142, Ukraine e Karpatskaya Str. 48, Kharkiv 61118, Ukraine
Abstract Ion-induced photon emission (IPE) during bombardment of magnesium aluminate spinel crystals MgO nAl2O3 by 20 keV Ar+ ions was studied. The dependence of the yield of particles in specific excited states on the fluence of incident ions in the range of (0.1–1.6) 1017 ions/cm2 was measured. It was shown that yield of magnesium and aluminum atoms and ions in most excited states do not depend (or slightly depend) on the fluence of ion bombardment. An exception was found for yields of Mg+ ions in the 4s 2S excited state and Al atoms in the 5p 2P0 excited state leading to emission lines at 292.8 and 669.6 nm, respectively. The yield of particles in these excited states drastically decreases at the start of ion bombardment. Analysis of these results and published data on the bombardmentinduced surface modification of spinel crystals allows to elucidate the role of crystal structure and chemical bonding in the formation of some excited states. The dependence of excited state yield (except of Mg and Al indicated above) from spinel crystals of different composition MgO nAl2O3 (n ¼ 1.0, 1.5, 2.0, and 2.5) does not reflect quantitatively the variation of the calculated bulk concentration of constituent atoms in these targets. r 2008 Elsevier Ltd. All rights reserved. Keywords: Ar+-ion bombardment; Ion–photon emission; Magnesium aluminate spinel
1. Introduction Magnesium aluminate spinel (MgAl2O4 or MgO nAl2O3) is highly resistant to radiation damage. Due to the possible use of this material in thermo-nuclear devices, the modification of surface properties under ion bombardment has been studied intensively during the last few decades. The physical properties (including surface properCorresponding author at: V.N. Karazin Kharkiv National University, Kurchatov Ave. 21, Apt. 49, Kharkiv 61108, Ukraine. Tel.: +380 57 335 3559; fax: +380 57 335 1563. E-mail addresses: [email protected] (V.T. Gritsyna), [email protected] (V.V. Bobkov), [email protected] (S.P. Gokov), [email protected] (D.I. Shevchenko). 1 Tel.: +380 57 335 3108. 2 Tel.: +380 57 335 4101. 3 Tel.: +380 572 66 2453. 4 Tel.: +380 57 711 7607.
0042-207X/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.12.002
ties) of the oxide system MgO nAl2O3 are defined mainly by its structure and composition. The single-phase spinel structure for this oxide system exists over a wide range of constituent oxides from stoichiometric composition at n ¼ 1.0 to a large excess of aluminum oxide up to n ¼ 7. There have been few investigations about the change of spinel surface properties resulting from ion bombardment. It was established that bombardment of stoichiometric spinel MgO 1.0Al2O3 by 8 keV Ar0 atoms leads to a major loss of MgO and formation of a surface layer of nonstoichiometric composition MgO 1.3Al2O3 [1]. Also it was shown that the virgin surface of the spinel had an excess of Al, while the bombarded surface had excess Mg, indicating bombardment-induced segregation of magnesium oxides in the spinel. The bombardment of spinel crystals of different composition MgO 1.0Al2O3 and MgO 2.0Al2O3 with H+ and Ar+ ions of 1.2 keV at different temperature results in depletion of magnesium oxide in the spinel surface [2].
ARTICLE IN PRESS V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
The irradiation with high-energy ions leads to an accumulation of point defects mainly in the anionic sub-lattice [3], to an interchange of Mg2+ and Al3+ cations among oxygen tetrahedrons and octahedrons and at high doses, the implanted layer undergoes transformation from a spinel structure to a meta-stable phase and, finally, to an amorphous state [4]. Under the ion bombardment, photon radiation occurs from ejected particles in excited states, known as ioninduced photon emission (IPE) [5]. The investigation of the main parameters of IPE, such as the spectral composition, the quantum yield, and the spatial distribution of emission at a given wavelength, can give important information on the processes of particle ejection, their excitation, and conservation of this excitation as it leaves the surface of solids [6]. Because these phenomena depend on the composition, structure and binding energy of constituent atoms (ions) in the target, the properties of the target could be identified from the yield of sputtered particles in definite excited states. Indeed, the quantum yield of atoms and ions measured during Ar+-ion bombardment of Al metal and magnesium aluminate spinel MgO 1.0Al2O3 and MgO 2.0Al2O3 show pronounced dependences of IPE on chemical state and the chemical composition of targets [6]. The formation of a magnesium-deficient layer under bombardment of spinel crystals with 60 keV Cu ions was detected based on the dose dependence of intensities of Al and Mg atomic lines originating from excited particles [7]. The mechanism of formation of sputtered particles in excited states during Ar+-ion bombardment of oxide targets was described by Bobkov et al. [8]. In this paper, we present the results of investigations of the dependence of the yield of excited particles on the fluence of Ar+-ion bombardment of spinel single crystals of different compositions MgO nAl2O3, where n ¼ 1.0,
889
1.5, 2.0, and 2.5, in an attempt to get in-situ information on the modification of surface properties of complex oxides under ion bombardment. Using the dependences of the IPE parameters on the dose of irradiation and composition of targets, we derive the influence of the surface and bulk properties of complex oxides on the yield of particles in different excited states. 2. Experimental details This investigation was provided using an experimental system producing a mass-separated Ar+-ion beam at an energy of 20 keV. The incident angle of the ion beam to the target plane was 451, which is near to the maximum value of the sputtering coefficient. The vacuum in the chamber was about 105 Pa (partial oxygen pressure was 5 106 Pa) and was obtain by using oil pumps and a liquid nitrogen trap. The photon emission of excited particles was measured in the direction perpendicular to the plane, created by the bombarding beam and the normal to the target surface. The spectra of emission was analyzed using a monochromator MDR-3 (Optical–Mechanical Association, Leningrad) in the wavelength range of 250–800 nm and registered with a cooled photomultiplier FEU-106 (Electro-Lamp Plant, Moscow) by photon counting. The total number of photons emitted by the excited particles was determined by using the method of collecting the total radiation as described in a previous paper [8]. Magnesium aluminate spinel single crystals of different compositions MgO nAl2O3 (n ¼ 1.0, 1.5, 2.0, and 2.5) grown using the Verneuil method were used as targets. Samples with dimensions of 12 10 mm2 and 0.7 mm in thickness were cut from single crystals and polished on both sides to an optical finish. In this experiment, we provided a prolonged Ar+-ion bombardment at 10 mA/cm2 density. Bombardment fluence
Fig. 1. A typical spectrum of IPE taken during bombardment of a spinel target with 20 keV Ar+ ions.
ARTICLE IN PRESS 890
V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
was varied in the range (0.1–1.6) 1017 ion/cm2 and the change in intensity of emission lines from ejected particles in different excited states was measured. The yield of particles in a given excited state (Yexc) was defined as the flux of excited particles divided by the flux of incident ions. The flux of excited sputtered particles was derived from the quantum yield of the emission line from the upper excited level and the lifetime of the given excited states [9]. Ejected particles represent the excited Mg and Al atoms, Mg+, Al+, and Al2+ ions, the spontaneous decay of which leads to optical radiation.
Table 1 Particles registered in excited states, electronic transitions and spectral composition of IPE, and electronic affinity wi of the upper excited states Atom, ion (type of spectrum)
Transition
l (nm)
Mg (Mg(I))
7s 3S-3p 3P0 3p2 3P-3p 3P0 3p2 3P-3p 3P0 3p 1P0-3s 1S 5s 3S-3p 3P0
277.6 277.9 278.2 285.2 332.9 333.2 333.6 382.9; 3,2 383.8 470.2 518.3 552.8
3d 3D-3p 3P0
3. Results The typical emission spectrum of IPE taken from the spinel target during bombardment by Ar+ ions is shown in Fig. 1. The monochromator registers light from first and second order of reflection, which allows simultaneous scanning of the spectra in the wavelength range from 250 to 800 nm. In this, the observed wavelengths and intensities (for a given spectrum) of all figure-observed lines are shown and identified with emitting particles. The spectra are not corrected for the spectral sensitivity of photomultiplier. Table 1 contains a list of sputtered particles, corresponding electronic transitions leading to emission of photon, wavelength of emitted lines, and electronic affinity of the upper excited states. First, we measured the dependence of the yield of particles in a given excited state of constituent atoms and ions on Ar+-ion fluence for spinel crystals of different composition. From Table 1, the largest value of Yexc was observed for magnesium atoms in exited states 5d 3D (emission at 285.2 nm, Mg(I)) and 4s 3S (emission at 518.3 nm, Mg(I)). The dose dependences of the yield of particles Yexc excited in the states indicated for different spinel crystals are shown in Fig. 2. For the stoichiometric spinel crystal MgO 1.0Al2O3, the Yexc of magnesium atoms slightly increases with the bombardment ion fluence (0.1–1.6) 1017 ion/cm2, but for the crystal MgO 1.5Al2O3, it somewhat decreases. For the other two spinel crystals, the Yexc is practically independent of dose of ion bombardment (Fig. 2a). The sputtered Mg+ ions are formed in three excited states 3p 2P0, 4s 2S and 4f 2F, the transitions from which result in three doublets: 279.5 and 280.2 nm Mg(II), 292.8 and 293.6 nm Mg(II), and a single line of 448.1 nm Mg(II). It should be noted that the higher electronic affinity of upper excited states leads to a lower quantum yield of transition. The behavior of Yexc in the first and third excited states on bombardment dose is very similar to that of excited magnesium atoms (Fig. 2b). The dose dependence of the Yexc of Mg+ in the excited state 4s 2S is completely different from the Yexc for excited magnesium atoms and ions discussed above (Fig. 2c). From very start of ion bombardment, the Yexc drops sharply with increasing dose for all spinel crystals leading to a steady-state condition.
5d 1D-3p 1P0 4s 3S-3p 3P0 4d 1D-3p 1P0 Mg+ (Mg(II))
3p 2P0-3s 2S 4s 2S-3p 2P0 4f 2F0-3d 2D
Al (Al(I))
2
2 0
4d D-3p P 4s0 4P0-3p2 4P 3d 2D-3p 2P0 4s 2S-3p 2P0 5p 2P0-4s 2S
Affinity, wi (eV) 0.466 0.476 0.476 3.296 1.21
1.696 0.666 2.536 1.056
279.5 280.2 292.8 293.6 448.1
10.604 10.614 6.385
257.5 305.0 305.7 308.2 309.2 394.4 396.1 669.6 669.8
1.155 1.685a
3.404
1.97 2.15 0.995
Al+ (Al(II))
4s 1S-3p 1P0 4f 3P0-3d 3D
281.6 358.6 358.7
7.007 3.527
Al2+ (Al(III))
4p 2P0-3d 2D
360.1 361.2 451.2 452.8 452.9
10.627 10.637 7.897
4d 2D-4p 2P0
The bold-type wavelengths indicate the emission where the yield depends on the dose of ion bombardment. a The level of shifted term 4s0 4P0 (two electron excitation) is located higher than the vacuum level.
The dependence of yield of aluminum atoms in different excited states on the fluence of ion bombardment was traced by investigation of quantum yield of all emission lines indicated in Table 1. In Fig. 3a, the dose dependences of the most intense emission line at 309.2 nm Al(I) is shown which arises from a transition from the 3d 2D excited state in Al atoms. One can see that the yield of aluminum atoms in this excited state practically does not depend on the dose of ion bombardment. The dose dependences of atoms excited to the level 5p 2P0 leading to an emission line at 669.6 nm, Al(I) is shown in Fig. 3b. As in the case of Mg+ ions excited in the 4s 2S state, here we also observed a sharp decrease of Yexc of aluminum atoms in 5p 2P0 excited state with increasing ion fluence.
ARTICLE IN PRESS V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
60
891
120
50 100
40 30
80
20
MgO·nAl2O3
60
10
MgO·nAl2O3
40 Yexc (10-5 particle/ion)
Yexc (10-5 particle/ion)
0 2.0 1.5 1.0 0.5
20 0 7
0.0
5
150
4
100
n=1.0
n=1.5
n=2.0
n=2.5
n=1.0
6
n=1.5 n=2.0 n=2.5
3 2
50
1 0
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.0
Fig. 2. The dependences of yield of Mg atoms and Mg+ ions in excited states leading to emission of different lines on the dose of Ar+-ion bombardment of spinel crystals having the composition indicated as: (a) l ¼ 518.3 nm, Mg(I), 4s 2S; (b) l ¼ 448.1 nm, Mg(II), 4f 2F0; and (c) l ¼ 292.8 nm, Mg(II), 4s 2S.
The dependence of the yield of aluminum ions in different excited states on the ion fluence was investigated by registration of the emission lines indicated in Table 1. In fact, the intensities of emission lines from aluminum ions were about two orders of magnitude lower compared with those of excited aluminum atoms. Also Al+ and Al2+ ions (Fig. 4) demonstrate some decrease of the Yexc of excited aluminum ions at the beginning of ion bombardment in the case of MgO 1.5Al2O3 spinel crystals. Data from Figs. 2–4 allow us to derive the dependences of the yield of particles in different excited states on the composition of the targets. The pristine MgO nAl2O3 spinel targets have the same structure but different bulk concentration of magnesium and aluminum ions depending on the molar ratio n. From the figures, one can conclude that no prominent dependence of Yexc on spinel composition (molar ratio n) was observed for all sputtered particles in different excited states except for magnesium ions in excited state 4s 2S leading to emission of the line at a wavelength of 292.8 nm, Mg(II) and aluminum atoms in the excited state 5p 2P0 with emission line at 669.6 nm (Al(I)). The relative yields of particles in the indicated excited states together with the bulk concentration of
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Fluence (1017ion·cm-2)
Fluence (1017ion·cm-2)
Fig. 3. The dependences of yield of Al atoms in excited states leading to emission of different lines on the dose of Ar+-ion bombardment of spinel crystals having the composition indicated as: (a) l ¼ 309.2 nm, Al(I), 3d 2 D and (b) l ¼ 669.6 nm, Al(I), 5p 2P0.
constituent atoms [10] in spinels having different compositions are depicted in Fig. 5. The relative yields were taken at the beginning of ion bombardment (low dose) and at the end of bombardment (high dose). One can see that despite the qualitative similarity of these dependences quantitatively, the sputtering yield of magnesium and aluminum particles in the indicated excited states does not reflect the variation of concentration of constituent ions in spinel crystals of different composition. 4. Discussion The investigation of dose dependences of yield of excited particles Yexc of magnesium and aluminum at the bombardment of magnesium aluminate spinel reveal the existence of three groups of particles: (a) particles in the most excited states, the yield of which does not depend on the fluence of ion bombardment (dose-independent particles); (b) particles in some excited states slightly depend on dose (slightly dose-dependent particles); and (c) Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state, the yield of which drastically
ARTICLE IN PRESS V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
892
1.5
a
MgII 292.8 nm (Low dose) MgII 292.8 nm (High dose) C(Mg)
1.0
MgO·nAl2O3
0.8 1.0
0.6 0.4
0.0
b
Yexc (10-5 particle/ion)
Yexc (10-5 particle/ion)
0.5
n=1.0 n=1.5 n=2.0 n=2.5
2.0
0.2 MgO·nAl2O3 0.0 1.0
0.8 1.5 0.6
1.0
0.4
0.5
0.0
Al I 669.6 nm (Low dose) Al I 669.6 nm (High dose) C(Al)
0.2 0.0
0.2
0.4
0.6
0.8
Fluence (10
1.0 17
1.2
1.4
1.6
ion·cm-2)
Fig. 4. The dependences of yield of Al+ and Al2+ ions in excited states leading to emission of different lines on the dose of Ar+-ion bombardment of spinel crystals having the composition indicated as: (a) l ¼ 358.7 nm, Al(II), 4f 3P0 and (b) l ¼ 452.9 nm, Al(III), 4d 2D.
decreases with increase in fluence (dose-dependent particles). The main mechanisms for creation of ejected particles at the investigated Ar+ ions energy are linear cascades and multiple collisions. In the case of linear cascades, the outgoing particles have low velocities (energy up to 100 eV) and the formation of excited particles is realized primarily through electron exchange at the surface of the target [6]. In the case of multiple collisions, the ejected particles have high velocity (energy up to 1 keV) and the excitation of particles takes place most probably at the surface of the target by the rupture of the molecular complexes [11]. Previous investigations of the characteristics of IPE for dose-independent and slightly dependent excited particles (space distribution of emission lines) leads to conclusion that excited atoms of both Mg and Al are formed in processes involving the multiple collisions (fast particles) implying has developed of linear cascades (slow particles). Formation of the excited Mg+ ions occurred predominantly during the process of multiple collisions (fast particles) [8]. The obtained data, in this work, on the dependences of yield of excited particles on the ion fluence indicate that
1.0
1.5 2.0 Molar ratio (n)
2.5
Fig. 5. The correlation of relative yield of Mg+ ions in 4s 2S excited state (a) and Al atoms in 5p 2P0 excited state (b) to relative bulk concentration of corresponding constituent ions C(Mg) and C(Al) in spinel crystals of different composition (Curve 1: low dose, 0.1 1017 ion/cm2; Curve 2: high dose, 1.4 1017 ion/cm2).
dose-independent excited particles predominantly are formed from linear cascades. The linear cascades take place in the near-surface layer of thickness, which corresponds to the projectile range of the incident ions. The yield of these ejected particles is defined by the concentration of the constituent atoms and their diffusion from bulk of target. The slightly dose-dependent particles also originated in linear cascades, but because this mechanism includes the momentum transfer along of the atomic chains, the structural transformation could have an influence on the yield of some excited particles. We can assume that the appearance of slightly dose-dependent particles is a consequence of structural modification of targets during ion bombardment which includes defect formation (primarily in the anionic sub-lattice), the appearance of cationic inversion due to systematic interchange of Mg and Al among oxygen polyhedrons, and due to a phase transformation. Even more, for spinel crystals having high excess of aluminum oxide MgO 2.0Al2O3 and MgO 2.5Al2O3, the yield of all excited particles (except Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state) practically do not show dependence on dose of ion bombardment (Figs. 2–4). Structural
ARTICLE IN PRESS V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
investigations (including RBS) indicate the decrease in short-range ordering in the spinel structure of crystals of the compositions used which would prevent momentum transfer along atomic chains. Now let us discuss the origin of dose-dependent excited particles. It was shown that Mg+ ions in the 4s 2S excited state and Al atoms in 5p 2P0 excited state, the yield of which drastically decreases with the fluence, are fast particles. It means that they are formed mainly at the surface of target in a process involving multiple collisions. Previous investigations show that, after ion bombardment, the surface is depleted in magnesium and oxygen [1,2]. The yield of excited particles during bombardment of Al metal with Ar+ ions is an order of magnitude lower compared with that of oxides (Al2O3 and MgO nAl2O3) [5]. The reason for enhancement of the yield of excited particles in oxides is the appearance of chemical bonds between the metal and oxygen. We may conclude that the process of formation of dose-dependent excited particles under ion bombardment of magnesium aluminate spinel takes place by the breaking off of the molecular complexes Mg–O or Al–O at the surface of the target [11] where the concentration of oxygen decreases with increasing ion fluence. The dependences of yield of these excited particles on target composition support this conclusion because qualitatively it corresponds to a variation of Mg–O or Al–O complexes on the surface of spinel crystals of different composition. The re-irradiation of the target after measuring the dependences of IPE on dose over the full range of ion fluences (during about 2 h) shows a low yield of Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state (corresponding to the end of the previous irradiation) irrespective of the fact that the target was kept in the vacuum chamber or was exposed to air. This indicates that the initial surface oxygen present at the vacuum pressure of 5 106 Pa does not influence the secondary yields at the start of the bombardment. But the annealing of the target in air at 1000 K for 2 h restores the initial value of yields of these excited particles and the subsequent dependence on the fluence of ion bombardment. Therefore, by annealing, we appear to restore the concentration of oxygen on the target surface and the associated enhancement in yield of Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state due to breaking-off the bonds of Mg–O and Al–O molecular complexes. The absence of proper dependences of yields of particles in different excited states on the composition of spinel crystals can be explained by the specific structure of this oxide. During a variation in content of constituent aluminum oxide from n ¼ 1.0–2.5 in spinel MgO nAl2O3, its structure remains the same. However, for non-stoichiometric crystals (n41.0), the additional Al3+ ions occupy only tetrahedral sites forming additional cationic vacancies in the octahedral position to ensure charge compensation. The formation of additional vacancies enhances the radiation-induced diffusion of Mg and Al particles from
893
the bulk to the surface of the target. Hence, one possible explanation for the weak dependence of sputtering yield of magnesium particles in the most excited states on molar composition may not only arise from effects of decreasing the magnesium concentration but also be the result of enhanced diffusion. The different dependences of yield of Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state on the spinel composition can be explained by another mechanism of formation of excited particles in these excited states. Because such particles arise from breaking-off the bonds of Mg–O and Al–O molecular complexes, the yield of such particles depends on both the metal and oxygen concentrations. The qualitative correlation between dependences of yield of the dose-dependent excited particles and that of the bulk concentration of constituent ions on target composition is due to the low diffusion coefficients of oxygen ions compared with diffusion of cations. 5. Conclusion The ion–photon emission phenomenon has been studied for bombardment of the surface of magnesium aluminate spinel crystals with 20 keV Ar+ ions. It was shown that the yield of Mg and Al atoms and ions in most excited states does not depend (or only slightly depends) on the fluence of ion bombardment in the range (0.1–1.6) 1017 ion/cm2. The exception was found for the sputtering yield of Mg+ ions and Al atoms excited into states 4s 2S and 5p 2P0 leading to emission of lines at 292.8 nm Mg(II) and 669.6 nm Al(I), respectively. The yield of these particles in excited states drastically decreases with the increase in fluence of ion bombardment. The dependences of yield of all particles in different excited states on the composition of spinel crystals quantitatively do not reflect the variation of the calculated bulk concentration of constituent ions in the targets. In general, we found two types of particles excited in definite states, the yield of which depends on the dose of ion bombardment and composition of targets and these could be used as in-situ indicators for modification of surface properties of complex oxides during ion bombardment. Acknowledgments This research was made possible in part by Grant nos. 07-13-06 and 11-13-06 of the Ministry of Education and Science, Ukraine. References [1] Marletta G, Iacona F, Kelly R. Nucl Instr and Meth B 1992;65:181–5. [2] Gritsyna VT, Kasatkina NA, Pershin VF. Nucl Instr and Meth B 1997;127/128:612–5. [3] Afanasyev-Charkin IV, Cooke DW, Gritsyna VT, Ishimaru M, Sickafus KE. Vacuum 2000;58:2–9.
ARTICLE IN PRESS 894
V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
[4] Ishimaru M, Hirotsu Y, Afanasyev-Charkin IV, Sickafus KE. J Phys Condens Matter 2002;14:1237–47. [5] Gritsyna VV. Radiotechnika I Elektronika 1992;37:1874–82. [6] Bobkov VV, Gokov SP, Gritsyna VV, Gritsyna VT, Shevchenko DI. Nucl Instr and Meth B 2004;218:46–52. [7] Bandurko V, Umeda N, Kishimoto N. Nucl Instr and Meth B 2002;190:146–50.
[8] Bobkov VV, Gokov SP, Gritsyna VV, Gritsyna VT, Shevchenko DI, Alimov SS. Nucl Instr and Meth B 2007;256:501–5. [9] Gritsyna VV, Koval AG, Koppe VT, Gokov SP. Optika i Spectroscopiya 1995;78:212–7. [10] Ibarra A, Vila R, Jimenez de Castro M. Philos Mag Lett 1991; 64:45–8. [11] Williams P. Surf Sci 1979;48:161–75.
Vacuum 82 (2008) 888–894 www.elsevier.com/locate/vacuum
Effects of argon-ion bombardment on the properties of the surface layer in spinel crystals of different composition V.T. Gritsynaa,b,,1, V.V. Bobkova,c,2, S.P. Gokova,d,3, V.V. Gritsynaa,b,1, D.I. Shevchenkoa,e,4 a
V.N. Karazin Kharkiv National University, Svoboda Sq., 4, 61077 Kharkiv, Ukraine b Kurchatov Ave. 21, Apt. 49, Kharkiv 61108, Ukraine c Kurchatov Ave. 21, Apt. 31, Kharkiv 61108, Ukraine d Garibaldi Str. 3, Apt. 15, Kharkiv 61142, Ukraine e Karpatskaya Str. 48, Kharkiv 61118, Ukraine
Abstract Ion-induced photon emission (IPE) during bombardment of magnesium aluminate spinel crystals MgO nAl2O3 by 20 keV Ar+ ions was studied. The dependence of the yield of particles in specific excited states on the fluence of incident ions in the range of (0.1–1.6) 1017 ions/cm2 was measured. It was shown that yield of magnesium and aluminum atoms and ions in most excited states do not depend (or slightly depend) on the fluence of ion bombardment. An exception was found for yields of Mg+ ions in the 4s 2S excited state and Al atoms in the 5p 2P0 excited state leading to emission lines at 292.8 and 669.6 nm, respectively. The yield of particles in these excited states drastically decreases at the start of ion bombardment. Analysis of these results and published data on the bombardmentinduced surface modification of spinel crystals allows to elucidate the role of crystal structure and chemical bonding in the formation of some excited states. The dependence of excited state yield (except of Mg and Al indicated above) from spinel crystals of different composition MgO nAl2O3 (n ¼ 1.0, 1.5, 2.0, and 2.5) does not reflect quantitatively the variation of the calculated bulk concentration of constituent atoms in these targets. r 2008 Elsevier Ltd. All rights reserved. Keywords: Ar+-ion bombardment; Ion–photon emission; Magnesium aluminate spinel
1. Introduction Magnesium aluminate spinel (MgAl2O4 or MgO nAl2O3) is highly resistant to radiation damage. Due to the possible use of this material in thermo-nuclear devices, the modification of surface properties under ion bombardment has been studied intensively during the last few decades. The physical properties (including surface properCorresponding author at: V.N. Karazin Kharkiv National University, Kurchatov Ave. 21, Apt. 49, Kharkiv 61108, Ukraine. Tel.: +380 57 335 3559; fax: +380 57 335 1563. E-mail addresses: [email protected] (V.T. Gritsyna), [email protected] (V.V. Bobkov), [email protected] (S.P. Gokov), [email protected] (D.I. Shevchenko). 1 Tel.: +380 57 335 3108. 2 Tel.: +380 57 335 4101. 3 Tel.: +380 572 66 2453. 4 Tel.: +380 57 711 7607.
0042-207X/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.12.002
ties) of the oxide system MgO nAl2O3 are defined mainly by its structure and composition. The single-phase spinel structure for this oxide system exists over a wide range of constituent oxides from stoichiometric composition at n ¼ 1.0 to a large excess of aluminum oxide up to n ¼ 7. There have been few investigations about the change of spinel surface properties resulting from ion bombardment. It was established that bombardment of stoichiometric spinel MgO 1.0Al2O3 by 8 keV Ar0 atoms leads to a major loss of MgO and formation of a surface layer of nonstoichiometric composition MgO 1.3Al2O3 [1]. Also it was shown that the virgin surface of the spinel had an excess of Al, while the bombarded surface had excess Mg, indicating bombardment-induced segregation of magnesium oxides in the spinel. The bombardment of spinel crystals of different composition MgO 1.0Al2O3 and MgO 2.0Al2O3 with H+ and Ar+ ions of 1.2 keV at different temperature results in depletion of magnesium oxide in the spinel surface [2].
ARTICLE IN PRESS V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
The irradiation with high-energy ions leads to an accumulation of point defects mainly in the anionic sub-lattice [3], to an interchange of Mg2+ and Al3+ cations among oxygen tetrahedrons and octahedrons and at high doses, the implanted layer undergoes transformation from a spinel structure to a meta-stable phase and, finally, to an amorphous state [4]. Under the ion bombardment, photon radiation occurs from ejected particles in excited states, known as ioninduced photon emission (IPE) [5]. The investigation of the main parameters of IPE, such as the spectral composition, the quantum yield, and the spatial distribution of emission at a given wavelength, can give important information on the processes of particle ejection, their excitation, and conservation of this excitation as it leaves the surface of solids [6]. Because these phenomena depend on the composition, structure and binding energy of constituent atoms (ions) in the target, the properties of the target could be identified from the yield of sputtered particles in definite excited states. Indeed, the quantum yield of atoms and ions measured during Ar+-ion bombardment of Al metal and magnesium aluminate spinel MgO 1.0Al2O3 and MgO 2.0Al2O3 show pronounced dependences of IPE on chemical state and the chemical composition of targets [6]. The formation of a magnesium-deficient layer under bombardment of spinel crystals with 60 keV Cu ions was detected based on the dose dependence of intensities of Al and Mg atomic lines originating from excited particles [7]. The mechanism of formation of sputtered particles in excited states during Ar+-ion bombardment of oxide targets was described by Bobkov et al. [8]. In this paper, we present the results of investigations of the dependence of the yield of excited particles on the fluence of Ar+-ion bombardment of spinel single crystals of different compositions MgO nAl2O3, where n ¼ 1.0,
889
1.5, 2.0, and 2.5, in an attempt to get in-situ information on the modification of surface properties of complex oxides under ion bombardment. Using the dependences of the IPE parameters on the dose of irradiation and composition of targets, we derive the influence of the surface and bulk properties of complex oxides on the yield of particles in different excited states. 2. Experimental details This investigation was provided using an experimental system producing a mass-separated Ar+-ion beam at an energy of 20 keV. The incident angle of the ion beam to the target plane was 451, which is near to the maximum value of the sputtering coefficient. The vacuum in the chamber was about 105 Pa (partial oxygen pressure was 5 106 Pa) and was obtain by using oil pumps and a liquid nitrogen trap. The photon emission of excited particles was measured in the direction perpendicular to the plane, created by the bombarding beam and the normal to the target surface. The spectra of emission was analyzed using a monochromator MDR-3 (Optical–Mechanical Association, Leningrad) in the wavelength range of 250–800 nm and registered with a cooled photomultiplier FEU-106 (Electro-Lamp Plant, Moscow) by photon counting. The total number of photons emitted by the excited particles was determined by using the method of collecting the total radiation as described in a previous paper [8]. Magnesium aluminate spinel single crystals of different compositions MgO nAl2O3 (n ¼ 1.0, 1.5, 2.0, and 2.5) grown using the Verneuil method were used as targets. Samples with dimensions of 12 10 mm2 and 0.7 mm in thickness were cut from single crystals and polished on both sides to an optical finish. In this experiment, we provided a prolonged Ar+-ion bombardment at 10 mA/cm2 density. Bombardment fluence
Fig. 1. A typical spectrum of IPE taken during bombardment of a spinel target with 20 keV Ar+ ions.
ARTICLE IN PRESS 890
V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
was varied in the range (0.1–1.6) 1017 ion/cm2 and the change in intensity of emission lines from ejected particles in different excited states was measured. The yield of particles in a given excited state (Yexc) was defined as the flux of excited particles divided by the flux of incident ions. The flux of excited sputtered particles was derived from the quantum yield of the emission line from the upper excited level and the lifetime of the given excited states [9]. Ejected particles represent the excited Mg and Al atoms, Mg+, Al+, and Al2+ ions, the spontaneous decay of which leads to optical radiation.
Table 1 Particles registered in excited states, electronic transitions and spectral composition of IPE, and electronic affinity wi of the upper excited states Atom, ion (type of spectrum)
Transition
l (nm)
Mg (Mg(I))
7s 3S-3p 3P0 3p2 3P-3p 3P0 3p2 3P-3p 3P0 3p 1P0-3s 1S 5s 3S-3p 3P0
277.6 277.9 278.2 285.2 332.9 333.2 333.6 382.9; 3,2 383.8 470.2 518.3 552.8
3d 3D-3p 3P0
3. Results The typical emission spectrum of IPE taken from the spinel target during bombardment by Ar+ ions is shown in Fig. 1. The monochromator registers light from first and second order of reflection, which allows simultaneous scanning of the spectra in the wavelength range from 250 to 800 nm. In this, the observed wavelengths and intensities (for a given spectrum) of all figure-observed lines are shown and identified with emitting particles. The spectra are not corrected for the spectral sensitivity of photomultiplier. Table 1 contains a list of sputtered particles, corresponding electronic transitions leading to emission of photon, wavelength of emitted lines, and electronic affinity of the upper excited states. First, we measured the dependence of the yield of particles in a given excited state of constituent atoms and ions on Ar+-ion fluence for spinel crystals of different composition. From Table 1, the largest value of Yexc was observed for magnesium atoms in exited states 5d 3D (emission at 285.2 nm, Mg(I)) and 4s 3S (emission at 518.3 nm, Mg(I)). The dose dependences of the yield of particles Yexc excited in the states indicated for different spinel crystals are shown in Fig. 2. For the stoichiometric spinel crystal MgO 1.0Al2O3, the Yexc of magnesium atoms slightly increases with the bombardment ion fluence (0.1–1.6) 1017 ion/cm2, but for the crystal MgO 1.5Al2O3, it somewhat decreases. For the other two spinel crystals, the Yexc is practically independent of dose of ion bombardment (Fig. 2a). The sputtered Mg+ ions are formed in three excited states 3p 2P0, 4s 2S and 4f 2F, the transitions from which result in three doublets: 279.5 and 280.2 nm Mg(II), 292.8 and 293.6 nm Mg(II), and a single line of 448.1 nm Mg(II). It should be noted that the higher electronic affinity of upper excited states leads to a lower quantum yield of transition. The behavior of Yexc in the first and third excited states on bombardment dose is very similar to that of excited magnesium atoms (Fig. 2b). The dose dependence of the Yexc of Mg+ in the excited state 4s 2S is completely different from the Yexc for excited magnesium atoms and ions discussed above (Fig. 2c). From very start of ion bombardment, the Yexc drops sharply with increasing dose for all spinel crystals leading to a steady-state condition.
5d 1D-3p 1P0 4s 3S-3p 3P0 4d 1D-3p 1P0 Mg+ (Mg(II))
3p 2P0-3s 2S 4s 2S-3p 2P0 4f 2F0-3d 2D
Al (Al(I))
2
2 0
4d D-3p P 4s0 4P0-3p2 4P 3d 2D-3p 2P0 4s 2S-3p 2P0 5p 2P0-4s 2S
Affinity, wi (eV) 0.466 0.476 0.476 3.296 1.21
1.696 0.666 2.536 1.056
279.5 280.2 292.8 293.6 448.1
10.604 10.614 6.385
257.5 305.0 305.7 308.2 309.2 394.4 396.1 669.6 669.8
1.155 1.685a
3.404
1.97 2.15 0.995
Al+ (Al(II))
4s 1S-3p 1P0 4f 3P0-3d 3D
281.6 358.6 358.7
7.007 3.527
Al2+ (Al(III))
4p 2P0-3d 2D
360.1 361.2 451.2 452.8 452.9
10.627 10.637 7.897
4d 2D-4p 2P0
The bold-type wavelengths indicate the emission where the yield depends on the dose of ion bombardment. a The level of shifted term 4s0 4P0 (two electron excitation) is located higher than the vacuum level.
The dependence of yield of aluminum atoms in different excited states on the fluence of ion bombardment was traced by investigation of quantum yield of all emission lines indicated in Table 1. In Fig. 3a, the dose dependences of the most intense emission line at 309.2 nm Al(I) is shown which arises from a transition from the 3d 2D excited state in Al atoms. One can see that the yield of aluminum atoms in this excited state practically does not depend on the dose of ion bombardment. The dose dependences of atoms excited to the level 5p 2P0 leading to an emission line at 669.6 nm, Al(I) is shown in Fig. 3b. As in the case of Mg+ ions excited in the 4s 2S state, here we also observed a sharp decrease of Yexc of aluminum atoms in 5p 2P0 excited state with increasing ion fluence.
ARTICLE IN PRESS V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
60
891
120
50 100
40 30
80
20
MgO·nAl2O3
60
10
MgO·nAl2O3
40 Yexc (10-5 particle/ion)
Yexc (10-5 particle/ion)
0 2.0 1.5 1.0 0.5
20 0 7
0.0
5
150
4
100
n=1.0
n=1.5
n=2.0
n=2.5
n=1.0
6
n=1.5 n=2.0 n=2.5
3 2
50
1 0
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.0
Fig. 2. The dependences of yield of Mg atoms and Mg+ ions in excited states leading to emission of different lines on the dose of Ar+-ion bombardment of spinel crystals having the composition indicated as: (a) l ¼ 518.3 nm, Mg(I), 4s 2S; (b) l ¼ 448.1 nm, Mg(II), 4f 2F0; and (c) l ¼ 292.8 nm, Mg(II), 4s 2S.
The dependence of the yield of aluminum ions in different excited states on the ion fluence was investigated by registration of the emission lines indicated in Table 1. In fact, the intensities of emission lines from aluminum ions were about two orders of magnitude lower compared with those of excited aluminum atoms. Also Al+ and Al2+ ions (Fig. 4) demonstrate some decrease of the Yexc of excited aluminum ions at the beginning of ion bombardment in the case of MgO 1.5Al2O3 spinel crystals. Data from Figs. 2–4 allow us to derive the dependences of the yield of particles in different excited states on the composition of the targets. The pristine MgO nAl2O3 spinel targets have the same structure but different bulk concentration of magnesium and aluminum ions depending on the molar ratio n. From the figures, one can conclude that no prominent dependence of Yexc on spinel composition (molar ratio n) was observed for all sputtered particles in different excited states except for magnesium ions in excited state 4s 2S leading to emission of the line at a wavelength of 292.8 nm, Mg(II) and aluminum atoms in the excited state 5p 2P0 with emission line at 669.6 nm (Al(I)). The relative yields of particles in the indicated excited states together with the bulk concentration of
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Fluence (1017ion·cm-2)
Fluence (1017ion·cm-2)
Fig. 3. The dependences of yield of Al atoms in excited states leading to emission of different lines on the dose of Ar+-ion bombardment of spinel crystals having the composition indicated as: (a) l ¼ 309.2 nm, Al(I), 3d 2 D and (b) l ¼ 669.6 nm, Al(I), 5p 2P0.
constituent atoms [10] in spinels having different compositions are depicted in Fig. 5. The relative yields were taken at the beginning of ion bombardment (low dose) and at the end of bombardment (high dose). One can see that despite the qualitative similarity of these dependences quantitatively, the sputtering yield of magnesium and aluminum particles in the indicated excited states does not reflect the variation of concentration of constituent ions in spinel crystals of different composition. 4. Discussion The investigation of dose dependences of yield of excited particles Yexc of magnesium and aluminum at the bombardment of magnesium aluminate spinel reveal the existence of three groups of particles: (a) particles in the most excited states, the yield of which does not depend on the fluence of ion bombardment (dose-independent particles); (b) particles in some excited states slightly depend on dose (slightly dose-dependent particles); and (c) Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state, the yield of which drastically
ARTICLE IN PRESS V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
892
1.5
a
MgII 292.8 nm (Low dose) MgII 292.8 nm (High dose) C(Mg)
1.0
MgO·nAl2O3
0.8 1.0
0.6 0.4
0.0
b
Yexc (10-5 particle/ion)
Yexc (10-5 particle/ion)
0.5
n=1.0 n=1.5 n=2.0 n=2.5
2.0
0.2 MgO·nAl2O3 0.0 1.0
0.8 1.5 0.6
1.0
0.4
0.5
0.0
Al I 669.6 nm (Low dose) Al I 669.6 nm (High dose) C(Al)
0.2 0.0
0.2
0.4
0.6
0.8
Fluence (10
1.0 17
1.2
1.4
1.6
ion·cm-2)
Fig. 4. The dependences of yield of Al+ and Al2+ ions in excited states leading to emission of different lines on the dose of Ar+-ion bombardment of spinel crystals having the composition indicated as: (a) l ¼ 358.7 nm, Al(II), 4f 3P0 and (b) l ¼ 452.9 nm, Al(III), 4d 2D.
decreases with increase in fluence (dose-dependent particles). The main mechanisms for creation of ejected particles at the investigated Ar+ ions energy are linear cascades and multiple collisions. In the case of linear cascades, the outgoing particles have low velocities (energy up to 100 eV) and the formation of excited particles is realized primarily through electron exchange at the surface of the target [6]. In the case of multiple collisions, the ejected particles have high velocity (energy up to 1 keV) and the excitation of particles takes place most probably at the surface of the target by the rupture of the molecular complexes [11]. Previous investigations of the characteristics of IPE for dose-independent and slightly dependent excited particles (space distribution of emission lines) leads to conclusion that excited atoms of both Mg and Al are formed in processes involving the multiple collisions (fast particles) implying has developed of linear cascades (slow particles). Formation of the excited Mg+ ions occurred predominantly during the process of multiple collisions (fast particles) [8]. The obtained data, in this work, on the dependences of yield of excited particles on the ion fluence indicate that
1.0
1.5 2.0 Molar ratio (n)
2.5
Fig. 5. The correlation of relative yield of Mg+ ions in 4s 2S excited state (a) and Al atoms in 5p 2P0 excited state (b) to relative bulk concentration of corresponding constituent ions C(Mg) and C(Al) in spinel crystals of different composition (Curve 1: low dose, 0.1 1017 ion/cm2; Curve 2: high dose, 1.4 1017 ion/cm2).
dose-independent excited particles predominantly are formed from linear cascades. The linear cascades take place in the near-surface layer of thickness, which corresponds to the projectile range of the incident ions. The yield of these ejected particles is defined by the concentration of the constituent atoms and their diffusion from bulk of target. The slightly dose-dependent particles also originated in linear cascades, but because this mechanism includes the momentum transfer along of the atomic chains, the structural transformation could have an influence on the yield of some excited particles. We can assume that the appearance of slightly dose-dependent particles is a consequence of structural modification of targets during ion bombardment which includes defect formation (primarily in the anionic sub-lattice), the appearance of cationic inversion due to systematic interchange of Mg and Al among oxygen polyhedrons, and due to a phase transformation. Even more, for spinel crystals having high excess of aluminum oxide MgO 2.0Al2O3 and MgO 2.5Al2O3, the yield of all excited particles (except Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state) practically do not show dependence on dose of ion bombardment (Figs. 2–4). Structural
ARTICLE IN PRESS V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
investigations (including RBS) indicate the decrease in short-range ordering in the spinel structure of crystals of the compositions used which would prevent momentum transfer along atomic chains. Now let us discuss the origin of dose-dependent excited particles. It was shown that Mg+ ions in the 4s 2S excited state and Al atoms in 5p 2P0 excited state, the yield of which drastically decreases with the fluence, are fast particles. It means that they are formed mainly at the surface of target in a process involving multiple collisions. Previous investigations show that, after ion bombardment, the surface is depleted in magnesium and oxygen [1,2]. The yield of excited particles during bombardment of Al metal with Ar+ ions is an order of magnitude lower compared with that of oxides (Al2O3 and MgO nAl2O3) [5]. The reason for enhancement of the yield of excited particles in oxides is the appearance of chemical bonds between the metal and oxygen. We may conclude that the process of formation of dose-dependent excited particles under ion bombardment of magnesium aluminate spinel takes place by the breaking off of the molecular complexes Mg–O or Al–O at the surface of the target [11] where the concentration of oxygen decreases with increasing ion fluence. The dependences of yield of these excited particles on target composition support this conclusion because qualitatively it corresponds to a variation of Mg–O or Al–O complexes on the surface of spinel crystals of different composition. The re-irradiation of the target after measuring the dependences of IPE on dose over the full range of ion fluences (during about 2 h) shows a low yield of Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state (corresponding to the end of the previous irradiation) irrespective of the fact that the target was kept in the vacuum chamber or was exposed to air. This indicates that the initial surface oxygen present at the vacuum pressure of 5 106 Pa does not influence the secondary yields at the start of the bombardment. But the annealing of the target in air at 1000 K for 2 h restores the initial value of yields of these excited particles and the subsequent dependence on the fluence of ion bombardment. Therefore, by annealing, we appear to restore the concentration of oxygen on the target surface and the associated enhancement in yield of Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state due to breaking-off the bonds of Mg–O and Al–O molecular complexes. The absence of proper dependences of yields of particles in different excited states on the composition of spinel crystals can be explained by the specific structure of this oxide. During a variation in content of constituent aluminum oxide from n ¼ 1.0–2.5 in spinel MgO nAl2O3, its structure remains the same. However, for non-stoichiometric crystals (n41.0), the additional Al3+ ions occupy only tetrahedral sites forming additional cationic vacancies in the octahedral position to ensure charge compensation. The formation of additional vacancies enhances the radiation-induced diffusion of Mg and Al particles from
893
the bulk to the surface of the target. Hence, one possible explanation for the weak dependence of sputtering yield of magnesium particles in the most excited states on molar composition may not only arise from effects of decreasing the magnesium concentration but also be the result of enhanced diffusion. The different dependences of yield of Mg+ ions in 4s 2S excited state and Al atoms in 5p 2P0 excited state on the spinel composition can be explained by another mechanism of formation of excited particles in these excited states. Because such particles arise from breaking-off the bonds of Mg–O and Al–O molecular complexes, the yield of such particles depends on both the metal and oxygen concentrations. The qualitative correlation between dependences of yield of the dose-dependent excited particles and that of the bulk concentration of constituent ions on target composition is due to the low diffusion coefficients of oxygen ions compared with diffusion of cations. 5. Conclusion The ion–photon emission phenomenon has been studied for bombardment of the surface of magnesium aluminate spinel crystals with 20 keV Ar+ ions. It was shown that the yield of Mg and Al atoms and ions in most excited states does not depend (or only slightly depends) on the fluence of ion bombardment in the range (0.1–1.6) 1017 ion/cm2. The exception was found for the sputtering yield of Mg+ ions and Al atoms excited into states 4s 2S and 5p 2P0 leading to emission of lines at 292.8 nm Mg(II) and 669.6 nm Al(I), respectively. The yield of these particles in excited states drastically decreases with the increase in fluence of ion bombardment. The dependences of yield of all particles in different excited states on the composition of spinel crystals quantitatively do not reflect the variation of the calculated bulk concentration of constituent ions in the targets. In general, we found two types of particles excited in definite states, the yield of which depends on the dose of ion bombardment and composition of targets and these could be used as in-situ indicators for modification of surface properties of complex oxides during ion bombardment. Acknowledgments This research was made possible in part by Grant nos. 07-13-06 and 11-13-06 of the Ministry of Education and Science, Ukraine. References [1] Marletta G, Iacona F, Kelly R. Nucl Instr and Meth B 1992;65:181–5. [2] Gritsyna VT, Kasatkina NA, Pershin VF. Nucl Instr and Meth B 1997;127/128:612–5. [3] Afanasyev-Charkin IV, Cooke DW, Gritsyna VT, Ishimaru M, Sickafus KE. Vacuum 2000;58:2–9.
ARTICLE IN PRESS 894
V.T. Gritsyna et al. / Vacuum 82 (2008) 888–894
[4] Ishimaru M, Hirotsu Y, Afanasyev-Charkin IV, Sickafus KE. J Phys Condens Matter 2002;14:1237–47. [5] Gritsyna VV. Radiotechnika I Elektronika 1992;37:1874–82. [6] Bobkov VV, Gokov SP, Gritsyna VV, Gritsyna VT, Shevchenko DI. Nucl Instr and Meth B 2004;218:46–52. [7] Bandurko V, Umeda N, Kishimoto N. Nucl Instr and Meth B 2002;190:146–50.
[8] Bobkov VV, Gokov SP, Gritsyna VV, Gritsyna VT, Shevchenko DI, Alimov SS. Nucl Instr and Meth B 2007;256:501–5. [9] Gritsyna VV, Koval AG, Koppe VT, Gokov SP. Optika i Spectroscopiya 1995;78:212–7. [10] Ibarra A, Vila R, Jimenez de Castro M. Philos Mag Lett 1991; 64:45–8. [11] Williams P. Surf Sci 1979;48:161–75.