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Relief evolution of HOPG under high-fluence 30 keV argon ion irradiation
Nuclear Instruments and Methods in Physics Research B 354 (2015) 146–150
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Relief evolution of HOPG under high-fluence 30 keV argon ion irradiation N.N. Andrianova a,b, A.M. Borisov a,b, E.S. Mashkova a,⇑, A.A. Shemukhin a, V.I. Shulga a, Yu.S. Virgiliev c a
Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia MATI – Russian State Technology University, Moscow, Russia c NIIgraphite, Moscow, Russia b
a r t i c l e
i n f o
Article history: Received 15 July 2014 Received in revised form 13 November 2014 Accepted 19 November 2014 Available online 5 December 2014 Keywords: High-fluence ion irradiation Highly oriented pyrolytic graphite (HOPG) Ion-induced relief Sputtering yield
a b s t r a c t The results of the experimental study of sputtering and erosion of the basal plane of HOPG under irradiation with 30-keV Ar+ in the range from RT to 400 °C are presented. It has been found that developed at elevated (P250 °C) temperatures needle-like microscopic relief results in twofold sputtering yield increase (Y 2) in comparison with sputtering of a surface with an etch pits microscopic relief at the temperatures less than the ion-induced texture transition temperature Tt 150 °C. The effects of ioninduced graphite relief on high-dose sputtering have been studied using binary-collision computer simulation. The relief was modeled as a sine function surface along two mutually perpendicular surface axes. The simulation has shown that at some parameters of the relief the essential part of the bombarding ions undergoes inclined incidence on the walls of surface hillocks, which increases the density of ion-atom collisions near the surface and, correspondingly, the ejection of atoms. This effect leads to non-monotonic behavior of the sputtering yield on the relief aspect ratio (amplitude/period). The sputtering yield decreases upon reaching the maximum at aspect ratio of 4, and becomes lower than that for a flat surface. The simulation permits to estimate the relation of amplitude to period of relief at T < Tt. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The interaction of accelerated ions with carbon-based materials is of great interest for both technical applications and for refinement of the concept of the physical processes occurring upon ion bombardment [1–4]. Measurements of the sputtering yield Y for polycrystalline graphite at room temperature (RT) and under high fluences with 30-keV Ar+ and N+2 ions showed that the obtained values were approximately two fold higher than the sputtering yields obtained via the computer simulation of a flat target surface [5,6]. Deviations are usually explained by the effect of the development of microscopic relief upon ion-beam erosion of the surface, when it is assumed that the sputtering yield is a function of only the local surface gradient and is independent of its initial curvature. The assumption that the sputtering yield is independent of surface curvature is a reasonable approximation in a case where the curvature radius at a given surface point is much larger than the ion path [7– 9]. During last two decades there were a great number of publications where the ion-induced relief on the target surface was studied theoretically and experimentally (e.g. [3–11]). Most of theoretical works were performed analytically or by computer simulation.
⇑ Corresponding author. Tel.: +7 495 939 4167; fax: +7 495 939 0896. E-mail address: [email protected] (E.S. Mashkova). http://dx.doi.org/10.1016/j.nimb.2014.11.071 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
The experimental works refer mainly to materials important for technical applications like silicon and carbon-based materials including highly oriented pyrolytic graphite (HOPG). HOPG stands out with its distinctive properties among carbon materials [12]. It is the closest to the natural graphite monocrystal. Its mosaic structure is characterized by axis texture in the [0 0 1] direction normal to the plate surface; disorientation of the basal planes does not exceed 500 . The pronounced effect of temperature on ion-induced structural, emission, and morphological changes different from the respective ones for polycrystalline graphites is observed for HOPG upon high-fluence ion irradiation with Ar+ and N+2 ions with energies of tens of keV [3,4,13–18]. In particular, it has been found that the sputtering yield is practically twofold lower upon sputtering of the basal plane of UPV-1T with 30-keV Ar+ ions in comparison with Y values upon the sputtering of polycrystalline graphites in a wide range of ion incidence angles [17]. Analysis of the specific ion-induced surface topography of UPV-1T suggests that there is a morphological reason for the suppression effect. Evolution of the ion-induced surface relief is angle-, energy- and temperature-dependent. At off-normal incidence many authors note the development a rippled topography on the surface with spatial periodicity in the range from 5 nm to 1 lm. At small (near-normal) incidence angles, ripples with wave vector parallel the beam are formed [10]. At normal incidence, the surface may be covered by a three-dimensional (3D) relief consisting of grid
N.N. Andrianova et al. / Nuclear Instruments and Methods in Physics Research B 354 (2015) 146–150
of hillocks and depressions – needle-like or etch pits structures [8– 10]. Although ripple formation under ion bombardment of solids has received considerable interest in recent years (see, for example, the reviews [10,11]), nanopatterning of surface at normal incidence of ions is much less studied. The aim of this work is the experimental and computational study of sputtering and erosion of the basal plane (0 0 1) of highly oriented pyrolytic graphite UPV-1T at high fluences of 30-keV Ar+ ion irradiation at normal incidence on the target surface at different temperatures and, correspondingly, different parameters of hillock-and-valley relief. Emphasis is laid on the influence of surface relief on the sputtering yield. 2. Experimental Samples of highly oriented pyrolytic graphite UPV-1T (NIIgraphite production) were used as the targets. The thickness of the 20 20 mm UPV-1T plates was 3–5 mm. Sample irradiation was conducted at normal surface incidence (h = 0°) using a massmonochromator of the Institute of Nuclear Physics, Moscow State University [19] according to a similar method used in [6,20]. The ion-current density was 0.3–0.4 mA/cm2 with a beam cross-section of 0.3 cm2, and irradiation fluences ut = 1018–1019 ion/cm2 (u is the incident-ion flux density; t is the time of irradiation 10– 100 min). The target temperature was measured using a chromel–alumel thermocouple with its alloy attached to the irradiated side of the target away from the irradiated area. Ion irradiation was monitored via the periodic recording of the ion current and secondary electrons to determine the irradiation fluence and the yield of ion–electron emission c, which was determined as the ratio of the electron and ion current. The measurement error did not exceed 2.5%. The sputtering yield Y was determined by the target weight loss and irradiation fluence. The procedure of sputtering yield determination also included stabilization of the UPV-1T weight in air after ion irradiation. Measurements for each fixed temperature of the target with an initially mirror surface obtained by the removal of a surface layer with sticky tape were performed several times until the sputtering yield Y was independent of the number of irradiations, and, hence, corresponded to the conditions of dynamic equilibrium. The surface morphology before and after irradiation was investigated with a Lyra 3 TESCAN scanning-electron microscope. Quantitative analysis of the microgeometry was conducted by the goniophotometric assessment of the light reflected from a rough surface according to [17,21]. 3. Computer simulation Computer code OKSANA [22,23] used in this work is based on the binary-collision approximation and takes into account the weak simultaneous collisions at large distances. A disordered (amorphous) target was modeled by the rotation of a corresponding monocrystalline atomic block; the rotation procedure was repeated for each new collision. The screened Coulomb potential of Ziegler– Biersack–Littmark (ZBL) [24] was used as the interatomic potential. For a flat surface, at normal incidence the dependence of the sputtering yield on the interatomic potential is rather weak [25]. This is related to the fact that a simultaneous variation of the potentials for the ion–atom and atom–atom pairs results in a decrease or increase of the sputtering powers (dE/dx)ion and (dE/dx)atom whereas the ratio (dE/dx)ion/(dE/dx)atom changes only slightly (the compensation effect [25]). Inelastic energy losses were calculated according to Firsov’s equation [26]. Either a flat potential barrier including the effect of long-range attractive forces, or a spherical (isotropic) which takes into account particle stopping upon leaving the surface, but not trajectory refraction were used for ejected atoms
147
– candidates for sputtering. The 7.41 eV surface binding energies was assumed to be equal to the sublimation energy for graphite, the atomic density is 2.26 g/cm3 [27]. The direction of the incident ion beam was given in Cartesian coordinate system, in which X and Y axis are in the surface plane of the target, and axis Z is directed inside the target. The normal incidence on the target surface is assumed. The OKSANA code allows calculations with consideration of a hillock-and-valley surface relief. The relief is modeled by a sine function in the X–Z and Y–Z planes (the sine periods are 2x and 2y and its amplitude is z/ 2) [28]. Sine periods and amplitudes can vary in a wide range (from units to hundred of nm); z = 0 corresponds to a flat surface. Calculations were performed without the surface relief (z = 0) and with its consideration at various values of x, y and z. Equal values of x and y were assumed (3D relief). Simulations for a 2D relief (a ripple geometry) are also possible. Note that the above (sinusoidal) model of surface relief is similar to that used by Carter [9] in analytical calculations of the effects of surface ripples on sputtering and secondary ion emission.
4. Results and discussion Investigation of the ion-induced structural-morphological states of HOPG upon high fluence irradiation with 30-keV Ar+ ions at temperatures ranging from room temperature to 400 °C revealed the presence of at least four surface modifications varying in both relief and surface-layer crystal structure [29]. Three of them obtained at the temperatures 90, 250, and 400 °C and characterized with different reliefs were selected for this work to investigate UPV-1T sputtering. Some description of the ion-induced UPV-1T-surface changes at a fluence of 1018 ion/cm2 has been presented in [18,29]. The initial mirror surface of the UPV-1T remains relatively smooth only upon ion irradiation at temperatures lower than that of the ion-induced texture transitions Tt 150 °C (Fig. 1a). Systems of nanosized hills and depressions, which are probably caused by the instability of erosion process, are formed on the surface, cf. [7–10]. In the case of oblique ion incidence this process results in a ripple relief [30]. Ion irradiation at T 250 °C results in an extended surface morphology in the form of a quasiperiodic structure of fused ridges with needle-like tops (Fig. 1b). Irradiation at T 400 °C results in a mosaic structure with craters that have a flat bottom with of 1.5–3-lm-diameter, surrounded by walls of fused cones (Fig. 1c). The increase in the fluence by one order of magnitude is accompanied by significant changes in morphology. The enlargement of structural elements and formation of faceted hollows with nanosized roughness of the surface and 620° slope, according to laser goniophotometry data, occurs at T < Tt, see Fig. 1d. The increase in the fluence at T = 250 °C results mainly in an increase in the relief amplitude (Fig. 1e), while at T = 400 °C the mosaic structure is transformed into systems of fused ridges and becomes similar to the relief at T = 250 °C (Fig. 1f). Considering the initial mosaic structure of UPV-1T, it can be suggested that the ridges surrounding the craters at T = 400 °C are formed at the crystallite edges and their sputtering results in ‘‘healing’’ of the craters with a flattened bottom. The described ion-induced structures of the UPV-1T surface were obtained after multiple sputtering cycles with measurement of the sputtering yield Y by the weight method carried out for each cycle (Fig. 2). It was found that the sputtering yield reaches a stable value after t > 10 min (1018 ion/cm2). The yields are Y 1 for temperatures below 90 °C and Y 2 for 250 and 400 °C. Hence, the ion-induced relief developing at elevated temperatures results in a twofold increase in the sputtering yield in comparison with the sputtering of a relatively smooth surface at a temperature T < Tt.
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μ
Fig. 1. SEM images of the UPV-1T surface after bombardment with 30 keV Ar+ ions with a fluence of ut 1018 ion/cm2 (a–c) and ut > 1019 ion/cm2 (d–f) at temperature T < 90 °C (a and d), T = 250 °C (b and e) and T = 400 °C (c and f). The inclination of the sample upon SEM imaging was 30°.
The dependence of the sputtering yield on the incidence angle, Y(h), and distribution of the local ion incidence angles for the rough surface, f(h), found either experimentally or with a particular model of the surface relief are usually considered as a basis for relevant computer modeling [31]. Both the local surface gradient and effect of surface curvature should be considered in calculations related to the sputtering of a surface with a nanosized relief. In other words, consideration of the effects of surface roughness according to the Y(h) dependence and f(h) distribution is not sufficient for surface irregularities which sizes are comparable with the ion path (30 nm for the considered experimental conditions). This is well supported by the results of our OKSANA simulations performed for a 3D-relief graphite surface (Fig. 3a), which reflects the instability of the surface under ion irradiation. It is obvious that, at rather high values of x, an increase in z results in a manifold increase in Y due to the emergence and growth of a surface fraction of sputtering at large local incidence angles, at which the Y(h) dependence has a maximum. The sputtering yield decreases upon reaching the maximum at z/x = 4, and becomes lower than that for a flat surface at
rather high z/x values. The origin of the maximum value of Y is a high sputtering at inclined incidence, which is depressed in some extent by the capture of sputtered atoms by relief structures. It is seen that the maximum value of Y is x-dependent and cannot be higher than 3 atoms/ion, this is higher than the above experimental data (Fig. 2). The dependence Ymax(x) is shown in Fig. 3b. The low values of the sputtering yield at high z/x (Fig. 3a) are due to blocking of the ejection of atoms from narrow and deep channels of relief elements. This phenomenon is considered as one of the methods for improving the erosion resistance of materials [32]. From Fig. 3a, it is also seen that the decrease of x, which is a half of the sine period, results in suppression of the effect of high Y and, at a fixed z/x, is equivalent to disappearing of relief. In this case the parameters of relief become comparable with the ion path in the target. Actually, we observe a kind of transition from micro- to nano-metric scale of roughness. It should be noted that similar conclusions follow from analytical estimates of the effect of roughness cross sections on the angular dependence of sputtering yield for a surface with a nanosized periodic relief [33]. These estimates
N.N. Andrianova et al. / Nuclear Instruments and Methods in Physics Research B 354 (2015) 146–150
149
result from theoretical consideration of the effect of surface curvature on sputtering [7–9]. The above simulated data allow to estimate the ratio of the relief amplitude to its period which results in the minimal value of the sputtering yield (Y 1 atom/ion) in the experiment at a temperature <90 °C. The upper limit of the z/x ratio can be estimated as z/x < 0.4 taking into account the experimental error of the sputtering yield (Figs. 2 and 3a). 5. Conclusions
ϕ Fig. 2. Dependence of the sputtering yield of the UPV-1T surface on the fluence of irradiation with 30 keV Ar+ ions for different temperatures of the target. The simulation results are also presented.
The sputtering yields were measured experimentally and SEM images of the basal (0 0 1) plane of highly oriented pyrolytic graphite UPV-1T were obtained at high-fluence irradiation with 30-keV Ar+ ions at normal incidence in the range from RT to 400 °C. It was found that the ion-induced needle-like relief developing at elevated temperatures caused a twofold increase in the sputtering yield (Y 2) in comparison with sputtering of the surface with etch pits relief at temperatures below Tt 150 °C. Simulation of sputtering of a graphite surface with a nanosized sine relief was carried out using the OKSANA code. It has been shown that the known effect of enhanced sputtering of a rough surface is x-, z- and z/x-dependent, where 2x and z are the period and depth of the relief function. This effect was decreased when the curvature radius of the relief became comparable with the ion path in the target. The comparison of the simulated and experimental data has shown that the ratio z/x does not exceed 0.4 for etch pits relief at temperatures below Tt 150 °C. The values of z/x corresponding to maximum values of Y were found to be about 4. The absolute maximum of the sputtering yield was found to be about 3 atoms/ion, which is some higher than the experimental data. References
Fig. 3. The sputtering yield Y of graphite bombarded with 30 keV Ar ions as a function of the ratio z/x, where x and z are the half period and depth of the relief respectively; calculations using the OKSANA code for different values of x (y = x). The dashed line corresponds to the sputtering yield for a flat surface (z = 0) (a). The maximum sputtering yield Ymax as a function of x (b).
[1] Sputtering by Particle Bombardment, in: R. Behrisch, W. Eckstein (Eds.), Springer-Verlag, 2007. [2] K. Niwase, Int. J. Spectrosc. 2012 (2012) 197609. [3] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, Yu.S. Virgiliev, J. Spacecraft Rockets 48 (2011) 45. [4] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, E.S. Parilis, Yu.S. Virgiliev, Horizons in World Physics, vol. 280, Nova Science Publishers, NY, USA, 2013, p. 171. [5] F.M. Fayazov, E.S. Mashkova, V.A. Molchanov, A.V. Sidorov, A.I. Tolmachev, Nucl. Instrum. Methods Phys. Res., Sect. B 67 (1992) 523. [6] A.M. Borisov, W. Eckstein, E.S. Mashkova, J. Nucl. Mater. 304 (1) (2002) 15. [7] P. Sigmund, J. Mater. Sci. 8 (1973) 1545. [8] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol., A 6 (1988) 2390. [9] G. Carter, J. Phys. D Appl. Phys. 34 (2001) R1. [10] W.L. Chan, E. Chason, J. Appl. Phys. 101 (2007) 12301. [11] A. Keller, S. Facsko, Materials 3 (2010) 4811. [12] T.D. Burchell, MRS Bull. 22 (4) (1997) 29. [13] A.M. Borisov, E.S. Mashkova, Nucl. Instrum. Methods Phys. Res., Sect. B 258 (2007) 109. [14] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, E.S. Parilis, E.A. Pitirimova, M.A. Timofeev, Vacuum 84 (2010) 1033. [15] A.M. Borisov, E.S. Mashkova, A.S. Nemov, Yu.S. Virgiliev, Nucl. Instrum. Methods Phys. Res., Sect. B 256 (2007) 363. [16] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, E.S. Parilis, Yu.S. Virgiliev, Nucl. Instrum. Methods Phys. Res., Sect. B 267 (2009) 2761. [17] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, J. Surf. Invest. 3 (2009) 322. [18] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, V.S. Sevostyanova, Yu.S. Virgiliev, Nucl. Instrum. Methods Phys. Res., Sect. B 315 (2013) 117. [19] E.S. Mashkova, V.A. Molchanov, Medium energy ion reflection from solids, North-Holland, Amsterdam, 1985. [20] N.N. Andrianova, V.S. Avilkina, A.M. Borisov, E.S. Mashkova, E.S. Parilis, Vacuum 86 (2012) 1630. [21] V.S. Avilkina, N.N. Andrianova, A.M. Borisov, Yu.S. Virgil’ev, E.S. Mashkova, J. Surf. Invest. 6 (2012) 631. [22] V.I. Shulga, Nucl. Instrum. Methods Phys. Res., Sect. B 164–165 (2000) 733. [23] V.I. Shulga, W. Eckstein, Nucl. Instrum. Methods Phys. Res., Sect. B 245 (1998) 492. [24] J.F. Ziegler, J.P. Biersack, U. Littmark, in: J.F. Ziegler (Ed.), The Stopping and Range of Ions in Solids, vol. I, Pergamon, New York, 1985. [25] V.I. Shulga, Nucl. Instrum. Methods Phys. Res., Sect. B 254 (2007) 200.
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[26] O.B. Firsov, Zh. Eksper. Teor. Phys. 36 (1959) 1517. [27] W. Eckstein, Computer Simulation of Ion-Solid Interactions, Springer-Verlag, Berlin, 1991. [28] V.I. Shulga, Nucl. Instrum. Methods Phys. Res., Sect. B 339 (2014) 8. [29] N.N. Andrianova, V.S. Avilkina, A.M. Borisov, E.S. Mashkova, Nucl. Instrum. Methods Phys. Res., Sect. B 273 (2012) 58.
[30] K. Takahiro, K. Zhang, F. Rotter, D. Schwen, C. Ronning, H. Hofsass, J. Krauser, Nucl. Instrum. Methods Phys. Res., Sect. B 256 (2007) 378. [31] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, Yu.S. Virgiliev, Nucl. Instrum. Methods Phys. Res., Sect. B 269 (2012) 58. [32] J.F. Ziegler, J.J. Cuomo, J. Roth, Appl. Phys. Lett. 30 (1977) 268. [33] Q. Wei, M. Eddy, K.-D. Li, L. Wang, J. Phys. D Appl. Phys. 42 (2009) 165304.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Relief evolution of HOPG under high-fluence 30 keV argon ion irradiation N.N. Andrianova a,b, A.M. Borisov a,b, E.S. Mashkova a,⇑, A.A. Shemukhin a, V.I. Shulga a, Yu.S. Virgiliev c a
Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia MATI – Russian State Technology University, Moscow, Russia c NIIgraphite, Moscow, Russia b
a r t i c l e
i n f o
Article history: Received 15 July 2014 Received in revised form 13 November 2014 Accepted 19 November 2014 Available online 5 December 2014 Keywords: High-fluence ion irradiation Highly oriented pyrolytic graphite (HOPG) Ion-induced relief Sputtering yield
a b s t r a c t The results of the experimental study of sputtering and erosion of the basal plane of HOPG under irradiation with 30-keV Ar+ in the range from RT to 400 °C are presented. It has been found that developed at elevated (P250 °C) temperatures needle-like microscopic relief results in twofold sputtering yield increase (Y 2) in comparison with sputtering of a surface with an etch pits microscopic relief at the temperatures less than the ion-induced texture transition temperature Tt 150 °C. The effects of ioninduced graphite relief on high-dose sputtering have been studied using binary-collision computer simulation. The relief was modeled as a sine function surface along two mutually perpendicular surface axes. The simulation has shown that at some parameters of the relief the essential part of the bombarding ions undergoes inclined incidence on the walls of surface hillocks, which increases the density of ion-atom collisions near the surface and, correspondingly, the ejection of atoms. This effect leads to non-monotonic behavior of the sputtering yield on the relief aspect ratio (amplitude/period). The sputtering yield decreases upon reaching the maximum at aspect ratio of 4, and becomes lower than that for a flat surface. The simulation permits to estimate the relation of amplitude to period of relief at T < Tt. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The interaction of accelerated ions with carbon-based materials is of great interest for both technical applications and for refinement of the concept of the physical processes occurring upon ion bombardment [1–4]. Measurements of the sputtering yield Y for polycrystalline graphite at room temperature (RT) and under high fluences with 30-keV Ar+ and N+2 ions showed that the obtained values were approximately two fold higher than the sputtering yields obtained via the computer simulation of a flat target surface [5,6]. Deviations are usually explained by the effect of the development of microscopic relief upon ion-beam erosion of the surface, when it is assumed that the sputtering yield is a function of only the local surface gradient and is independent of its initial curvature. The assumption that the sputtering yield is independent of surface curvature is a reasonable approximation in a case where the curvature radius at a given surface point is much larger than the ion path [7– 9]. During last two decades there were a great number of publications where the ion-induced relief on the target surface was studied theoretically and experimentally (e.g. [3–11]). Most of theoretical works were performed analytically or by computer simulation.
⇑ Corresponding author. Tel.: +7 495 939 4167; fax: +7 495 939 0896. E-mail address: [email protected] (E.S. Mashkova). http://dx.doi.org/10.1016/j.nimb.2014.11.071 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
The experimental works refer mainly to materials important for technical applications like silicon and carbon-based materials including highly oriented pyrolytic graphite (HOPG). HOPG stands out with its distinctive properties among carbon materials [12]. It is the closest to the natural graphite monocrystal. Its mosaic structure is characterized by axis texture in the [0 0 1] direction normal to the plate surface; disorientation of the basal planes does not exceed 500 . The pronounced effect of temperature on ion-induced structural, emission, and morphological changes different from the respective ones for polycrystalline graphites is observed for HOPG upon high-fluence ion irradiation with Ar+ and N+2 ions with energies of tens of keV [3,4,13–18]. In particular, it has been found that the sputtering yield is practically twofold lower upon sputtering of the basal plane of UPV-1T with 30-keV Ar+ ions in comparison with Y values upon the sputtering of polycrystalline graphites in a wide range of ion incidence angles [17]. Analysis of the specific ion-induced surface topography of UPV-1T suggests that there is a morphological reason for the suppression effect. Evolution of the ion-induced surface relief is angle-, energy- and temperature-dependent. At off-normal incidence many authors note the development a rippled topography on the surface with spatial periodicity in the range from 5 nm to 1 lm. At small (near-normal) incidence angles, ripples with wave vector parallel the beam are formed [10]. At normal incidence, the surface may be covered by a three-dimensional (3D) relief consisting of grid
N.N. Andrianova et al. / Nuclear Instruments and Methods in Physics Research B 354 (2015) 146–150
of hillocks and depressions – needle-like or etch pits structures [8– 10]. Although ripple formation under ion bombardment of solids has received considerable interest in recent years (see, for example, the reviews [10,11]), nanopatterning of surface at normal incidence of ions is much less studied. The aim of this work is the experimental and computational study of sputtering and erosion of the basal plane (0 0 1) of highly oriented pyrolytic graphite UPV-1T at high fluences of 30-keV Ar+ ion irradiation at normal incidence on the target surface at different temperatures and, correspondingly, different parameters of hillock-and-valley relief. Emphasis is laid on the influence of surface relief on the sputtering yield. 2. Experimental Samples of highly oriented pyrolytic graphite UPV-1T (NIIgraphite production) were used as the targets. The thickness of the 20 20 mm UPV-1T plates was 3–5 mm. Sample irradiation was conducted at normal surface incidence (h = 0°) using a massmonochromator of the Institute of Nuclear Physics, Moscow State University [19] according to a similar method used in [6,20]. The ion-current density was 0.3–0.4 mA/cm2 with a beam cross-section of 0.3 cm2, and irradiation fluences ut = 1018–1019 ion/cm2 (u is the incident-ion flux density; t is the time of irradiation 10– 100 min). The target temperature was measured using a chromel–alumel thermocouple with its alloy attached to the irradiated side of the target away from the irradiated area. Ion irradiation was monitored via the periodic recording of the ion current and secondary electrons to determine the irradiation fluence and the yield of ion–electron emission c, which was determined as the ratio of the electron and ion current. The measurement error did not exceed 2.5%. The sputtering yield Y was determined by the target weight loss and irradiation fluence. The procedure of sputtering yield determination also included stabilization of the UPV-1T weight in air after ion irradiation. Measurements for each fixed temperature of the target with an initially mirror surface obtained by the removal of a surface layer with sticky tape were performed several times until the sputtering yield Y was independent of the number of irradiations, and, hence, corresponded to the conditions of dynamic equilibrium. The surface morphology before and after irradiation was investigated with a Lyra 3 TESCAN scanning-electron microscope. Quantitative analysis of the microgeometry was conducted by the goniophotometric assessment of the light reflected from a rough surface according to [17,21]. 3. Computer simulation Computer code OKSANA [22,23] used in this work is based on the binary-collision approximation and takes into account the weak simultaneous collisions at large distances. A disordered (amorphous) target was modeled by the rotation of a corresponding monocrystalline atomic block; the rotation procedure was repeated for each new collision. The screened Coulomb potential of Ziegler– Biersack–Littmark (ZBL) [24] was used as the interatomic potential. For a flat surface, at normal incidence the dependence of the sputtering yield on the interatomic potential is rather weak [25]. This is related to the fact that a simultaneous variation of the potentials for the ion–atom and atom–atom pairs results in a decrease or increase of the sputtering powers (dE/dx)ion and (dE/dx)atom whereas the ratio (dE/dx)ion/(dE/dx)atom changes only slightly (the compensation effect [25]). Inelastic energy losses were calculated according to Firsov’s equation [26]. Either a flat potential barrier including the effect of long-range attractive forces, or a spherical (isotropic) which takes into account particle stopping upon leaving the surface, but not trajectory refraction were used for ejected atoms
147
– candidates for sputtering. The 7.41 eV surface binding energies was assumed to be equal to the sublimation energy for graphite, the atomic density is 2.26 g/cm3 [27]. The direction of the incident ion beam was given in Cartesian coordinate system, in which X and Y axis are in the surface plane of the target, and axis Z is directed inside the target. The normal incidence on the target surface is assumed. The OKSANA code allows calculations with consideration of a hillock-and-valley surface relief. The relief is modeled by a sine function in the X–Z and Y–Z planes (the sine periods are 2x and 2y and its amplitude is z/ 2) [28]. Sine periods and amplitudes can vary in a wide range (from units to hundred of nm); z = 0 corresponds to a flat surface. Calculations were performed without the surface relief (z = 0) and with its consideration at various values of x, y and z. Equal values of x and y were assumed (3D relief). Simulations for a 2D relief (a ripple geometry) are also possible. Note that the above (sinusoidal) model of surface relief is similar to that used by Carter [9] in analytical calculations of the effects of surface ripples on sputtering and secondary ion emission.
4. Results and discussion Investigation of the ion-induced structural-morphological states of HOPG upon high fluence irradiation with 30-keV Ar+ ions at temperatures ranging from room temperature to 400 °C revealed the presence of at least four surface modifications varying in both relief and surface-layer crystal structure [29]. Three of them obtained at the temperatures 90, 250, and 400 °C and characterized with different reliefs were selected for this work to investigate UPV-1T sputtering. Some description of the ion-induced UPV-1T-surface changes at a fluence of 1018 ion/cm2 has been presented in [18,29]. The initial mirror surface of the UPV-1T remains relatively smooth only upon ion irradiation at temperatures lower than that of the ion-induced texture transitions Tt 150 °C (Fig. 1a). Systems of nanosized hills and depressions, which are probably caused by the instability of erosion process, are formed on the surface, cf. [7–10]. In the case of oblique ion incidence this process results in a ripple relief [30]. Ion irradiation at T 250 °C results in an extended surface morphology in the form of a quasiperiodic structure of fused ridges with needle-like tops (Fig. 1b). Irradiation at T 400 °C results in a mosaic structure with craters that have a flat bottom with of 1.5–3-lm-diameter, surrounded by walls of fused cones (Fig. 1c). The increase in the fluence by one order of magnitude is accompanied by significant changes in morphology. The enlargement of structural elements and formation of faceted hollows with nanosized roughness of the surface and 620° slope, according to laser goniophotometry data, occurs at T < Tt, see Fig. 1d. The increase in the fluence at T = 250 °C results mainly in an increase in the relief amplitude (Fig. 1e), while at T = 400 °C the mosaic structure is transformed into systems of fused ridges and becomes similar to the relief at T = 250 °C (Fig. 1f). Considering the initial mosaic structure of UPV-1T, it can be suggested that the ridges surrounding the craters at T = 400 °C are formed at the crystallite edges and their sputtering results in ‘‘healing’’ of the craters with a flattened bottom. The described ion-induced structures of the UPV-1T surface were obtained after multiple sputtering cycles with measurement of the sputtering yield Y by the weight method carried out for each cycle (Fig. 2). It was found that the sputtering yield reaches a stable value after t > 10 min (1018 ion/cm2). The yields are Y 1 for temperatures below 90 °C and Y 2 for 250 and 400 °C. Hence, the ion-induced relief developing at elevated temperatures results in a twofold increase in the sputtering yield in comparison with the sputtering of a relatively smooth surface at a temperature T < Tt.
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μ
Fig. 1. SEM images of the UPV-1T surface after bombardment with 30 keV Ar+ ions with a fluence of ut 1018 ion/cm2 (a–c) and ut > 1019 ion/cm2 (d–f) at temperature T < 90 °C (a and d), T = 250 °C (b and e) and T = 400 °C (c and f). The inclination of the sample upon SEM imaging was 30°.
The dependence of the sputtering yield on the incidence angle, Y(h), and distribution of the local ion incidence angles for the rough surface, f(h), found either experimentally or with a particular model of the surface relief are usually considered as a basis for relevant computer modeling [31]. Both the local surface gradient and effect of surface curvature should be considered in calculations related to the sputtering of a surface with a nanosized relief. In other words, consideration of the effects of surface roughness according to the Y(h) dependence and f(h) distribution is not sufficient for surface irregularities which sizes are comparable with the ion path (30 nm for the considered experimental conditions). This is well supported by the results of our OKSANA simulations performed for a 3D-relief graphite surface (Fig. 3a), which reflects the instability of the surface under ion irradiation. It is obvious that, at rather high values of x, an increase in z results in a manifold increase in Y due to the emergence and growth of a surface fraction of sputtering at large local incidence angles, at which the Y(h) dependence has a maximum. The sputtering yield decreases upon reaching the maximum at z/x = 4, and becomes lower than that for a flat surface at
rather high z/x values. The origin of the maximum value of Y is a high sputtering at inclined incidence, which is depressed in some extent by the capture of sputtered atoms by relief structures. It is seen that the maximum value of Y is x-dependent and cannot be higher than 3 atoms/ion, this is higher than the above experimental data (Fig. 2). The dependence Ymax(x) is shown in Fig. 3b. The low values of the sputtering yield at high z/x (Fig. 3a) are due to blocking of the ejection of atoms from narrow and deep channels of relief elements. This phenomenon is considered as one of the methods for improving the erosion resistance of materials [32]. From Fig. 3a, it is also seen that the decrease of x, which is a half of the sine period, results in suppression of the effect of high Y and, at a fixed z/x, is equivalent to disappearing of relief. In this case the parameters of relief become comparable with the ion path in the target. Actually, we observe a kind of transition from micro- to nano-metric scale of roughness. It should be noted that similar conclusions follow from analytical estimates of the effect of roughness cross sections on the angular dependence of sputtering yield for a surface with a nanosized periodic relief [33]. These estimates
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result from theoretical consideration of the effect of surface curvature on sputtering [7–9]. The above simulated data allow to estimate the ratio of the relief amplitude to its period which results in the minimal value of the sputtering yield (Y 1 atom/ion) in the experiment at a temperature <90 °C. The upper limit of the z/x ratio can be estimated as z/x < 0.4 taking into account the experimental error of the sputtering yield (Figs. 2 and 3a). 5. Conclusions
ϕ Fig. 2. Dependence of the sputtering yield of the UPV-1T surface on the fluence of irradiation with 30 keV Ar+ ions for different temperatures of the target. The simulation results are also presented.
The sputtering yields were measured experimentally and SEM images of the basal (0 0 1) plane of highly oriented pyrolytic graphite UPV-1T were obtained at high-fluence irradiation with 30-keV Ar+ ions at normal incidence in the range from RT to 400 °C. It was found that the ion-induced needle-like relief developing at elevated temperatures caused a twofold increase in the sputtering yield (Y 2) in comparison with sputtering of the surface with etch pits relief at temperatures below Tt 150 °C. Simulation of sputtering of a graphite surface with a nanosized sine relief was carried out using the OKSANA code. It has been shown that the known effect of enhanced sputtering of a rough surface is x-, z- and z/x-dependent, where 2x and z are the period and depth of the relief function. This effect was decreased when the curvature radius of the relief became comparable with the ion path in the target. The comparison of the simulated and experimental data has shown that the ratio z/x does not exceed 0.4 for etch pits relief at temperatures below Tt 150 °C. The values of z/x corresponding to maximum values of Y were found to be about 4. The absolute maximum of the sputtering yield was found to be about 3 atoms/ion, which is some higher than the experimental data. References
Fig. 3. The sputtering yield Y of graphite bombarded with 30 keV Ar ions as a function of the ratio z/x, where x and z are the half period and depth of the relief respectively; calculations using the OKSANA code for different values of x (y = x). The dashed line corresponds to the sputtering yield for a flat surface (z = 0) (a). The maximum sputtering yield Ymax as a function of x (b).
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