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Secondary particle emission from semiconductor crystals
Vacuum 100 (2014) 84e86
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
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Secondary particle emission from semiconductor crystals V.V. Khvostov, I.K. Khrustachev, K.F. Minnebaev, E.Yu. Zykova*, V.E. Yurasova Moscow State University, 119991 Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2013 Accepted 9 July 2013
Sputtering and secondary ion emission (SIE) from semiconductors with different values of the band gap have been studied experimentally and by means of computer simulation. The particular oscillations of secondary ion energy distribution for graphite, sapphire, and silicon single crystals were observed under irradiation by 1 and 10 keV Arþ ions. This result cannot be interpreted using existing theories of SIE. We have explained them by the interplay of the charge-exchange processes between emitted and incident particles. Comparing the evidence from our experiments and computer simulation, we have drawn the conclusion that secondary ions emerge as a result of charge exchange with Arþ ions not only when exiting the target, but also in the bulk. This is very different from what happens in metals, and needs to be taken into account when describing the processes in SIE, as well as in practical applications in mass spectrometry of secondary ions. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Secondary ion emission Graphite Sapphire Silicon Charge exchange Computer modeling
To date, much attention is given now to experimental and theoretical research on secondary ion emission (SIE). This is related to both the interest in the physical basics of the process and its application in secondary ion mass spectrometry (SIMS); a highly sensitive method for analysis of the surface composition of solids [1]. An essential feature of SIE is the energy spectrum (ES) of secondary ions (SI) which helps to understand the mechanisms of secondary ion emission [2e10]. This paper deals with experimental research of SIE from the (0001) faces of graphite and sapphire single crystals and from the silicon (111) face. In addition, we carried out a computer simulation of sputtering and charge exchange processes for moving particles. The paper discusses special conditions of SIE from semiconductor crystals. The experiments were performed using a high-vacuum unit (r w 6 109 mbar) [11]. Primary Arþ ions of 1 or 10 keV irradiated the targets at angles a ¼ 0 or 45 . The SI were recorded at various SI emission angles q measuring from the normal to the surface. The numerical computation of sputtering was carried out using the pair collision model, or a molecular dynamic model with a mobile singlecrystal block of atoms [10]. The charge exchange of moving particles was computed using the Anderson-Newns Hamiltonian [12]. The energy spectrum of secondary carbon ions was studied for ion bombardment of (0001) faces of highly oriented
* Corresponding author. Tel.: þ7 903 6868078. E-mail addresses: [email protected], [email protected] (E.Yu. Zykova). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.07.009
pyrolytic graphite (HOPG). The ES recorded for Cþ ions are shown in Fig. 1. Apart from the main peak, which shifts with the growth of q (E1 w 25 and 50 eV at q ¼ 45 and 75 , respectively), the descending part of the curves bears small maxima at E1 w 30, 40, 70, 80, and 100 eV; peculiar kind of oscillations (at all angles q) whose emergence cannot be forecast by the currently existing SIE theories. The presence of oscillations in Cþ ES could be due to the processes of charge exchange of sputtered particles with surface atoms. To clear up the issue, we performed a computer simulation of sputtering of graphite (0001) faces and charge exchange of argon ions with the first layer of graphite. The result is presented in Fig. 2 for emission of particles from an elemental graphite block. It was established that, to explain the experimental results obtained, it is necessary to assume the existence of particle recharge in a target, and not just after a departure from it. The assumption is reasonable for SIE from semiconductors and it is confirmed by computations (see Fig. 2b). It turns out that the sputtering of C atoms from the first layer can occur only after successive collisions Arþ/C/C, where C atoms may exit within two limits of emission angle q. (Fig. 2a). If carbon atoms are at the distance of 1.42 Ǻ, they exit at angles q between 45 and 75 (the trajectories are marked as bold lines), and energy E1 of outgoing C atoms varies from 30 to 80 eV. If carbon atoms are at the distance of 2.84 Ǻ, the outgoing C atoms are within angles 30 to 60 (the trajectories are marked as dashed lines), and their energy E1 ¼ 5e50 eV. Curve 1 corresponds to Arþ travelingtraveling without deviation in the centre of a hexagonal graphite cell. In this
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85
Fig. 3. Energy spectra of Siþ ions emitted at angles q ¼ 0 (1), 10 (2), and 45 (3) under ion bombardment of silicon (111) face; Arþ, E0 ¼ 10 keV, a ¼ 45 . Fig. 1. Energy distribution of Cþ secondary ions under irradiation of graphite (0001) face; Arþ, E0 ¼ 10 keV, a ¼ 0 ; emission angles (1) q ¼ 45 (2) q ¼ 75 .
case, electron exchange between Arþ ion and C atoms of the first graphite layer begins at a distance of 1.2 Ǻ above the surface (this distance was determined in calculations). In this case the maximum Po ¼ 0.25 is reached when the Arþ ion is crossing the layer of the bombarded surface. When an Arþ ion is between the graphite layers, the electron exchange stops (Po 0.05). Curve 2 corresponds to such impact parameters for which an Arþ ion interacts with a C atom of the first layer and transfers the charge to it so that a Cþ ion appears. Now the Cþ ion travels into the crystal bulk and, being at distance of 1.2 Ǻ under the first layer of graphite, transfers its charge and energy to another C atom of the first layer which can be sputtered. In this event, the Arþ neutralization probability Po is w0.9. At the same time a charge state of sputtered particles is determined by the time of their interaction with surface, the time depending on the particle energy and the exit angle. It is essential to note that sputtered C atoms are ionized both upon their exit from the crystal and inside the crystal bulk. As a result the oscillations of argon ion neutralization probability Po (and respectively, of carbon sputtered atom ionization probability Pþ) appear (Fig. 2b). The energy spectrum of secondary silicon ions is shown in Fig. 3. The ES has oscillations on the descending part of curves Iþ(E1), at every emission angle q, qualitatively similar to the ones in graphite. The position of the ES principal maximum E1max(q) is observed for Siþ at an energy lower than for Cþ, at q ¼ 45 , E1max(q) w 35 eV and 50 eV for Siþ and Cþ ions, respectively. This difference is associated with considerably greater bonding energy of electron in graphite atom (4.5 eV) as compared with that of silicon atom (1.6 eV).
Fig. 2. Simulation results for particle trajectories and for charge exchange process under ion bombardment of graphite (0001) face; Arþ, E0 ¼ 10 keV, a ¼ 0 . a) Trajectories of C particles in the (0110) plane. b) Probabilities Po of Arþ neutralizing at the first layer of the graphite grid and the associated probabilities of ionizing the sputtered C atoms; curves 1 and 2 refer, respectively, to the interaction with atoms 1 and 2 in (a).
While studying the ES of Siþ secondary ions, we observed the relief that can be produced in ion bombardment of the silicon single crystal (111) face. It was interesting to find out whether or not the surface topography bears an impact on the shape of SI energy distributions. A structure of Si surface was observed after long-term ion bombardment by 10 keV Ar (a ¼ 45 , dose w 1017 ions,cm2). Clearly conical figures grow on the (111) Si face, similar to the ones seen earlier for metal single crystals [13]. It was shown therefore that even such a developed surface with well-oriented features does not bear any impact on the shape of the silicon SI energy spectra. Secondary particle emission from sapphire has been studied separately for oxygen atoms and alumium ions. Oxygen sputtering from Al2O3 (0001) face was carried out experimentally and by numerical simulations at a primary ion energy E0 ¼ 1 keV for angles a ¼ q ¼ 45 . The ES of sputtered O atoms were acquired with two intensity maxima. The lower energy maximum corresponds to sputtered particle energies of E1 w 15e18 eV. This maximum can be related to the cascade sputtering mechanism and its position corresponds to a value equal to about half the atomic binding energy in the lattice (Eb w 30 eV). The higher energy maximum is at the energy of E1 w 45 eV; it corresponds to O atoms sputtered by Arþ ions by direct knocking-out. Computations of sputtered O atoms charge exchange were carried out in the range of their energies 5e75 eV. It was found that the ionization probability of O atoms was of an oscillating nature. The oscillation frequency rises with a drop of primary ion energy, which can be accounted for by a decrease in the time of Ar ion travelling through the area where charge exchange is feasible. The results for Aluminum secondary ion emission from the sapphire (0001) face are presented in Fig. 4. We revealed that the energy spectra of Alþ ions possess two principal maxima and small peaks at the descending part of the ES
Fig. 4. Energy distributions of Alþ secondary ions under irradiation of sapphire (0001) face, Arþ, E0 ¼ 10 keV, a ¼ 0 . Emission angles q ¼ 40, 45, 55, 65, and 75 for curves 1, 2, 3, 4, and 5, respectively.
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for every SI emission angle q. These small peaks are similar to those observed in SIE from graphite or silicon and can be explained by charge exchange. The principal maxima in Alþ ES from sapphire can be found at energies E1 w 5 and w40e60 eV. The greater energy maximum was found to shift with a rise in q towards higher energy E1 of secondary ions. At the same time, the maximum position at lower energy practically does not shift with q, which indicates a different mechanism of its origin: it can be related to cascade sputtering (as in emission of O atoms from sapphire discussed above). An assumption was made that the maximum of energy spectra at lower energies can have a different nature: Alþ ions can be emitted from aluminum islands resulting from predominant oxygen sputtering from the sapphire surface. To verify the assumption, energy spectra were obtained for pure aluminum (Fig. 5). Comparing the ES obtained reveals close agreement between the positions of principal maxima in Alþ ion emission from pure aluminum and those that can be observed for lower energy maxima in SIE of Alþ from sapphire. Hence, this mechanism for the origin of lower energy maxima is also possible in the energy spectra of Alþ SIE from sapphire. The similarities and differences between SIE from single crystals of graphite, silicon and sapphire have been analyzed. For all crystals oscillations in the energy spectra of SI were observed. The oscillations in secondary ion ES were accounted for by the features of charge-exchange between the emitted and incident particles. The principal ES maximum (i.e. the most probable energy of secondary ions) for the semiconductors studied has been found at different energy E1. Thus, for a ¼ q ¼ 45 , the principal ES maximum for SI from silicon, graphite and sapphire is observed at E1max w 35, 50, and 52 eV, respectively. Note that the band-gap width Eg varies for silicon and sapphire in the same sequence (Eg ¼ 1.12 and 9.5 eV for silicon and sapphire, respectively). Inasmuch as the band-gap is very small in graphite [14], the determining factor for SIE of graphite is the bond energy between the target atoms. The same has been observed for SIE from metals: the most probable ion energy is determined by the work-function [4]. For example, in the aforementioned case of ion emission from pure aluminum and from aluminum islands on sapphire, the most probable ion energy is E w 12 eV; the same as was observed in our experiment. Thus, experiments and computer simulations were performed to research of sputtering and secondary ion emission of graphite, silicon and sapphire single crystals. Correlation was noticed between the most probable energy of secondary ions in the semiconductors under study and their band-gap width. For all targets the oscillations of SI energy distributions were found, the emergence of which did not follow from the current SIE theories; they were explained by the features of charge exchange between the emitted and incident particles. The SIE of aluminum and oxygen from sapphire (0001) face has been studied for the first time. It was revealed that the energy spectra of Alþ ions possess two principal maxima and small oscillations on the descending part of the ES for every SI emission angle. It was shown that aluminum sputtering occurred according to a cascade mechanism and the main portion of the sputtered oxygen
Fig. 5. Energy spectra of Alþ secondary ions from aluminum crystal, Arþ, E0 ¼ 10 keV, a ¼ 0 . Emission angles q ¼ 40, 45, 55, 65, 75, and 80 for curves 1, 2, 3, 4, 5, and 6, respectively.
atoms resulted from their direct knock-out. A close agreement was found between the most probable energy of secondary Alþ ions from pure aluminum and for lower energy maxima in SIE of Alþ from sapphire; this indicates a possibility of Alþ secondary ion emission from aluminum islands observed in a number of cases on the sapphire surface. It was established that the silicon single crystal surface relief produced by long-term ion bombardment does not bear an impact on the shape of SI energy spectra. Comparing the experimental and calculated results for the semiconductor crystals under study revealed that secondary sputtering ions were produced due to charge exchange with Arþ ions both upon exit from the target and in its bulk. This is essentially different from what occurs in metals, and has to be taken into account while considering the SIE mechanisms, as well as in practical applications of mass spectrometry of secondary ions. This study was supported by the Russian Foundation for Basic Research, project no. 10-02-01500-a. References [1] Benninghoven A, Rudenauer FG, Werner HW. Secondary ion mass spectrometry. New York: Wiley; 1987. [2] Sroubek Z. Phys Rev Lett B 1975;25:6046. [3] Yu ML, Lang ND. Phys Rev 1983;50:127. [4] Veksler VI. Secondary ion emission in metals. Moscow: Nauka; 1978 [in Russian]. [5] Behrisch R, Wittmaack K, editors. Sputtering bu particle bombardment. Springer Series, vol. 64; 1991. Berlin; Heidelberg. [6] Yurasova VE. Interaction of ions with surface. Moscow: PrimaV; 1999 [in Russian]. [7] Klushin DV, Gusev MYu, Lysenko SA, Urazgildin IF. Phys Rev 1996;54:7062. [8] Makarenko VN, Popov AB, Shergin AP. Izv Akad Nauk SSSR, Ser Fiz 1990;54: 1331. [9] Matulevich YuT, Khrustachev IK, Minnebaev KF, Urazgildin IF, Yurasova VE. Izv Akad Nauk, Ser Fiz 2000;64:665. [10] Mosunov AS, Promokhov AA, Colligon JS, Yurasova VE. Bull of Russ Acad Sci Phys 1998;62(4):556. [11] Khvostov VV, Minnebaev KF, Yurasova VE. Surf Invest X-ray, Synchrotron Neutron Tech 2013;7(1):61. [12] W.Harrison. Electronic structure and the properties of solids, vol. 1. San Francisco: Freeman; 1980. [13] Yurasova VE. J Vac Sci Technol 1977;14:285. [14] AlZahrani AZ, Srivastava GP. Braz J Phys 2009;(4):694.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
Secondary particle emission from semiconductor crystals V.V. Khvostov, I.K. Khrustachev, K.F. Minnebaev, E.Yu. Zykova*, V.E. Yurasova Moscow State University, 119991 Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2013 Accepted 9 July 2013
Sputtering and secondary ion emission (SIE) from semiconductors with different values of the band gap have been studied experimentally and by means of computer simulation. The particular oscillations of secondary ion energy distribution for graphite, sapphire, and silicon single crystals were observed under irradiation by 1 and 10 keV Arþ ions. This result cannot be interpreted using existing theories of SIE. We have explained them by the interplay of the charge-exchange processes between emitted and incident particles. Comparing the evidence from our experiments and computer simulation, we have drawn the conclusion that secondary ions emerge as a result of charge exchange with Arþ ions not only when exiting the target, but also in the bulk. This is very different from what happens in metals, and needs to be taken into account when describing the processes in SIE, as well as in practical applications in mass spectrometry of secondary ions. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Secondary ion emission Graphite Sapphire Silicon Charge exchange Computer modeling
To date, much attention is given now to experimental and theoretical research on secondary ion emission (SIE). This is related to both the interest in the physical basics of the process and its application in secondary ion mass spectrometry (SIMS); a highly sensitive method for analysis of the surface composition of solids [1]. An essential feature of SIE is the energy spectrum (ES) of secondary ions (SI) which helps to understand the mechanisms of secondary ion emission [2e10]. This paper deals with experimental research of SIE from the (0001) faces of graphite and sapphire single crystals and from the silicon (111) face. In addition, we carried out a computer simulation of sputtering and charge exchange processes for moving particles. The paper discusses special conditions of SIE from semiconductor crystals. The experiments were performed using a high-vacuum unit (r w 6 109 mbar) [11]. Primary Arþ ions of 1 or 10 keV irradiated the targets at angles a ¼ 0 or 45 . The SI were recorded at various SI emission angles q measuring from the normal to the surface. The numerical computation of sputtering was carried out using the pair collision model, or a molecular dynamic model with a mobile singlecrystal block of atoms [10]. The charge exchange of moving particles was computed using the Anderson-Newns Hamiltonian [12]. The energy spectrum of secondary carbon ions was studied for ion bombardment of (0001) faces of highly oriented
* Corresponding author. Tel.: þ7 903 6868078. E-mail addresses: [email protected], [email protected] (E.Yu. Zykova). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.07.009
pyrolytic graphite (HOPG). The ES recorded for Cþ ions are shown in Fig. 1. Apart from the main peak, which shifts with the growth of q (E1 w 25 and 50 eV at q ¼ 45 and 75 , respectively), the descending part of the curves bears small maxima at E1 w 30, 40, 70, 80, and 100 eV; peculiar kind of oscillations (at all angles q) whose emergence cannot be forecast by the currently existing SIE theories. The presence of oscillations in Cþ ES could be due to the processes of charge exchange of sputtered particles with surface atoms. To clear up the issue, we performed a computer simulation of sputtering of graphite (0001) faces and charge exchange of argon ions with the first layer of graphite. The result is presented in Fig. 2 for emission of particles from an elemental graphite block. It was established that, to explain the experimental results obtained, it is necessary to assume the existence of particle recharge in a target, and not just after a departure from it. The assumption is reasonable for SIE from semiconductors and it is confirmed by computations (see Fig. 2b). It turns out that the sputtering of C atoms from the first layer can occur only after successive collisions Arþ/C/C, where C atoms may exit within two limits of emission angle q. (Fig. 2a). If carbon atoms are at the distance of 1.42 Ǻ, they exit at angles q between 45 and 75 (the trajectories are marked as bold lines), and energy E1 of outgoing C atoms varies from 30 to 80 eV. If carbon atoms are at the distance of 2.84 Ǻ, the outgoing C atoms are within angles 30 to 60 (the trajectories are marked as dashed lines), and their energy E1 ¼ 5e50 eV. Curve 1 corresponds to Arþ travelingtraveling without deviation in the centre of a hexagonal graphite cell. In this
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85
Fig. 3. Energy spectra of Siþ ions emitted at angles q ¼ 0 (1), 10 (2), and 45 (3) under ion bombardment of silicon (111) face; Arþ, E0 ¼ 10 keV, a ¼ 45 . Fig. 1. Energy distribution of Cþ secondary ions under irradiation of graphite (0001) face; Arþ, E0 ¼ 10 keV, a ¼ 0 ; emission angles (1) q ¼ 45 (2) q ¼ 75 .
case, electron exchange between Arþ ion and C atoms of the first graphite layer begins at a distance of 1.2 Ǻ above the surface (this distance was determined in calculations). In this case the maximum Po ¼ 0.25 is reached when the Arþ ion is crossing the layer of the bombarded surface. When an Arþ ion is between the graphite layers, the electron exchange stops (Po 0.05). Curve 2 corresponds to such impact parameters for which an Arþ ion interacts with a C atom of the first layer and transfers the charge to it so that a Cþ ion appears. Now the Cþ ion travels into the crystal bulk and, being at distance of 1.2 Ǻ under the first layer of graphite, transfers its charge and energy to another C atom of the first layer which can be sputtered. In this event, the Arþ neutralization probability Po is w0.9. At the same time a charge state of sputtered particles is determined by the time of their interaction with surface, the time depending on the particle energy and the exit angle. It is essential to note that sputtered C atoms are ionized both upon their exit from the crystal and inside the crystal bulk. As a result the oscillations of argon ion neutralization probability Po (and respectively, of carbon sputtered atom ionization probability Pþ) appear (Fig. 2b). The energy spectrum of secondary silicon ions is shown in Fig. 3. The ES has oscillations on the descending part of curves Iþ(E1), at every emission angle q, qualitatively similar to the ones in graphite. The position of the ES principal maximum E1max(q) is observed for Siþ at an energy lower than for Cþ, at q ¼ 45 , E1max(q) w 35 eV and 50 eV for Siþ and Cþ ions, respectively. This difference is associated with considerably greater bonding energy of electron in graphite atom (4.5 eV) as compared with that of silicon atom (1.6 eV).
Fig. 2. Simulation results for particle trajectories and for charge exchange process under ion bombardment of graphite (0001) face; Arþ, E0 ¼ 10 keV, a ¼ 0 . a) Trajectories of C particles in the (0110) plane. b) Probabilities Po of Arþ neutralizing at the first layer of the graphite grid and the associated probabilities of ionizing the sputtered C atoms; curves 1 and 2 refer, respectively, to the interaction with atoms 1 and 2 in (a).
While studying the ES of Siþ secondary ions, we observed the relief that can be produced in ion bombardment of the silicon single crystal (111) face. It was interesting to find out whether or not the surface topography bears an impact on the shape of SI energy distributions. A structure of Si surface was observed after long-term ion bombardment by 10 keV Ar (a ¼ 45 , dose w 1017 ions,cm2). Clearly conical figures grow on the (111) Si face, similar to the ones seen earlier for metal single crystals [13]. It was shown therefore that even such a developed surface with well-oriented features does not bear any impact on the shape of the silicon SI energy spectra. Secondary particle emission from sapphire has been studied separately for oxygen atoms and alumium ions. Oxygen sputtering from Al2O3 (0001) face was carried out experimentally and by numerical simulations at a primary ion energy E0 ¼ 1 keV for angles a ¼ q ¼ 45 . The ES of sputtered O atoms were acquired with two intensity maxima. The lower energy maximum corresponds to sputtered particle energies of E1 w 15e18 eV. This maximum can be related to the cascade sputtering mechanism and its position corresponds to a value equal to about half the atomic binding energy in the lattice (Eb w 30 eV). The higher energy maximum is at the energy of E1 w 45 eV; it corresponds to O atoms sputtered by Arþ ions by direct knocking-out. Computations of sputtered O atoms charge exchange were carried out in the range of their energies 5e75 eV. It was found that the ionization probability of O atoms was of an oscillating nature. The oscillation frequency rises with a drop of primary ion energy, which can be accounted for by a decrease in the time of Ar ion travelling through the area where charge exchange is feasible. The results for Aluminum secondary ion emission from the sapphire (0001) face are presented in Fig. 4. We revealed that the energy spectra of Alþ ions possess two principal maxima and small peaks at the descending part of the ES
Fig. 4. Energy distributions of Alþ secondary ions under irradiation of sapphire (0001) face, Arþ, E0 ¼ 10 keV, a ¼ 0 . Emission angles q ¼ 40, 45, 55, 65, and 75 for curves 1, 2, 3, 4, and 5, respectively.
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for every SI emission angle q. These small peaks are similar to those observed in SIE from graphite or silicon and can be explained by charge exchange. The principal maxima in Alþ ES from sapphire can be found at energies E1 w 5 and w40e60 eV. The greater energy maximum was found to shift with a rise in q towards higher energy E1 of secondary ions. At the same time, the maximum position at lower energy practically does not shift with q, which indicates a different mechanism of its origin: it can be related to cascade sputtering (as in emission of O atoms from sapphire discussed above). An assumption was made that the maximum of energy spectra at lower energies can have a different nature: Alþ ions can be emitted from aluminum islands resulting from predominant oxygen sputtering from the sapphire surface. To verify the assumption, energy spectra were obtained for pure aluminum (Fig. 5). Comparing the ES obtained reveals close agreement between the positions of principal maxima in Alþ ion emission from pure aluminum and those that can be observed for lower energy maxima in SIE of Alþ from sapphire. Hence, this mechanism for the origin of lower energy maxima is also possible in the energy spectra of Alþ SIE from sapphire. The similarities and differences between SIE from single crystals of graphite, silicon and sapphire have been analyzed. For all crystals oscillations in the energy spectra of SI were observed. The oscillations in secondary ion ES were accounted for by the features of charge-exchange between the emitted and incident particles. The principal ES maximum (i.e. the most probable energy of secondary ions) for the semiconductors studied has been found at different energy E1. Thus, for a ¼ q ¼ 45 , the principal ES maximum for SI from silicon, graphite and sapphire is observed at E1max w 35, 50, and 52 eV, respectively. Note that the band-gap width Eg varies for silicon and sapphire in the same sequence (Eg ¼ 1.12 and 9.5 eV for silicon and sapphire, respectively). Inasmuch as the band-gap is very small in graphite [14], the determining factor for SIE of graphite is the bond energy between the target atoms. The same has been observed for SIE from metals: the most probable ion energy is determined by the work-function [4]. For example, in the aforementioned case of ion emission from pure aluminum and from aluminum islands on sapphire, the most probable ion energy is E w 12 eV; the same as was observed in our experiment. Thus, experiments and computer simulations were performed to research of sputtering and secondary ion emission of graphite, silicon and sapphire single crystals. Correlation was noticed between the most probable energy of secondary ions in the semiconductors under study and their band-gap width. For all targets the oscillations of SI energy distributions were found, the emergence of which did not follow from the current SIE theories; they were explained by the features of charge exchange between the emitted and incident particles. The SIE of aluminum and oxygen from sapphire (0001) face has been studied for the first time. It was revealed that the energy spectra of Alþ ions possess two principal maxima and small oscillations on the descending part of the ES for every SI emission angle. It was shown that aluminum sputtering occurred according to a cascade mechanism and the main portion of the sputtered oxygen
Fig. 5. Energy spectra of Alþ secondary ions from aluminum crystal, Arþ, E0 ¼ 10 keV, a ¼ 0 . Emission angles q ¼ 40, 45, 55, 65, 75, and 80 for curves 1, 2, 3, 4, 5, and 6, respectively.
atoms resulted from their direct knock-out. A close agreement was found between the most probable energy of secondary Alþ ions from pure aluminum and for lower energy maxima in SIE of Alþ from sapphire; this indicates a possibility of Alþ secondary ion emission from aluminum islands observed in a number of cases on the sapphire surface. It was established that the silicon single crystal surface relief produced by long-term ion bombardment does not bear an impact on the shape of SI energy spectra. Comparing the experimental and calculated results for the semiconductor crystals under study revealed that secondary sputtering ions were produced due to charge exchange with Arþ ions both upon exit from the target and in its bulk. This is essentially different from what occurs in metals, and has to be taken into account while considering the SIE mechanisms, as well as in practical applications of mass spectrometry of secondary ions. This study was supported by the Russian Foundation for Basic Research, project no. 10-02-01500-a. References [1] Benninghoven A, Rudenauer FG, Werner HW. Secondary ion mass spectrometry. New York: Wiley; 1987. [2] Sroubek Z. Phys Rev Lett B 1975;25:6046. [3] Yu ML, Lang ND. Phys Rev 1983;50:127. [4] Veksler VI. Secondary ion emission in metals. Moscow: Nauka; 1978 [in Russian]. [5] Behrisch R, Wittmaack K, editors. Sputtering bu particle bombardment. Springer Series, vol. 64; 1991. Berlin; Heidelberg. [6] Yurasova VE. Interaction of ions with surface. Moscow: PrimaV; 1999 [in Russian]. [7] Klushin DV, Gusev MYu, Lysenko SA, Urazgildin IF. Phys Rev 1996;54:7062. [8] Makarenko VN, Popov AB, Shergin AP. Izv Akad Nauk SSSR, Ser Fiz 1990;54: 1331. [9] Matulevich YuT, Khrustachev IK, Minnebaev KF, Urazgildin IF, Yurasova VE. Izv Akad Nauk, Ser Fiz 2000;64:665. [10] Mosunov AS, Promokhov AA, Colligon JS, Yurasova VE. Bull of Russ Acad Sci Phys 1998;62(4):556. [11] Khvostov VV, Minnebaev KF, Yurasova VE. Surf Invest X-ray, Synchrotron Neutron Tech 2013;7(1):61. [12] W.Harrison. Electronic structure and the properties of solids, vol. 1. San Francisco: Freeman; 1980. [13] Yurasova VE. J Vac Sci Technol 1977;14:285. [14] AlZahrani AZ, Srivastava GP. Braz J Phys 2009;(4):694.