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Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation
Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation M. Ranjan ⇑, P. Joshi, S. Mukherjee FCIPT, Institute for Plasma Research, Gandhinagar (Gujarat), India
a r t i c l e
i n f o
Article history: Received 3 November 2015 Received in revised form 27 February 2016 Accepted 27 February 2016 Available online xxxx Keywords: Graphite Surfactant Nanopatterns Ion irradiation Sputtering yield
a b s t r a c t Low energy ions beam often develop periodic patterns on surfaces under normal or off-normal incidence. Formation of such periodic patterns depends on the substrate material, the ion beam parameters, and the processing conditions. Processing conditions introduce unwanted contaminant atoms, which also play strong role in pattern formation by changing the effective sputtering yield of the material. In this work we have analysed the effect of Cu, Fe and Al impurities introduced during low energy Ar+ ion irradiation on HOPG substrate. It is observed that by changing the species of foreign atoms the surface topography changes drastically. The observed surface topography is co-related with the modified sputtering yield of HOPG. Presence of Cu and Fe amplify the effective sputtering yield of HOPG, so that the required threshold for the pattern formation is achieved with the given fluence, whereas Al does not lead to any significant change in the effective yield and hence no pattern formation occurs. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Graphite is widely used in various important scientific fields. It is used as a plasma facing component (First wall, Divertor) in tokomaks, and in space applications as electrode materials in plasma torches [1–4]. In all such applications graphite interacts with plasma of very low density (106 cm 3) to very high density (1018 cm 3). Also the energies of interacting ions varies from 10 s of eV to keVs. Properties of graphite degrades (thermal conductivity, thermal expansion coefficient) during erosion and redeposition of various impurities from the reactor wall. Various studies are performed for the plasma wall interaction with graphite for both physical and chemical sputtering [5]. All these results only present macroscopic topographies observed under various bombarding conditions and considering a pure system i.e. excluding the effects of impurities. Such structures formed on the graphite surface retain large amount of tritium, which is an undesirable condition. Since graphite erosion investigation is an important issue, in our present study we aim to investigate the erosion rate of the surfaces, utilising the steady state coverage of around one monolayer. Depending upon the chemical bonding with the substrate atoms, the surfactant atoms may drive the surface to form nanosized patterning [6,7]. This will help not only to investigate the fundamental ⇑ Corresponding author. E-mail address: [email protected] (M. Ranjan).
physics behind formation of various nanoscale surface topographies but also formation of its prevention. In the present study Ar ions are used to bombard Highly-Oriented Pyrolytic Graphite (HOPG) surface at a lower energy of 200–400 eV. Fluence and ion energies chosen are relevant to fusion research. Systematically various impurities like Cu, Al and Fe are introduced during ion bombardment and their effect on the sputtering yield and surface topography is reported. 2. Experimentation A UHV system of vacuum level up to 10 8 mbar was used for the experiments and an operating pressure of 2 10 4 mbar was maintained during experiment. Ar ion beam of energy range 200 eV–400 eV produced by Kaufman ion source was used to irradiate the HOPG samples. Fluence of 5.8 1018 cm 2 was incident at angles of 0° and 67° with respect to the surface normal. Such a large fluence is normally expected in the fusion devices [5]. When plasma interacts with a floating electrode, it makes a homogeneous sheath of typically few mm in the surrounding of the sample. High voltage sheath ensures the normal incidence of the ions on the surface. To simulate the similar effect we have also chosen the normal incidence of the ions. In fusion devices ions also move in the presence of magnetic field, at that point they are transported towards the diverter region, there is a possibility of bombardment of ions at some angles. Simulating such an event we have chosen angle of incidence also as 67° with respect to the surface normal.
http://dx.doi.org/10.1016/j.nimb.2016.02.062 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062
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M. Ranjan et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
To introduce the impurity during ion irradiation, mask of different materials (Cu, Al, and Fe) were used as shown in the Fig. 1. HOPG sample of size 10 10 mm was sandwiched between the sample holder and the mask of having circular opening. Mask was prepared in such a way so as to fully cover the sample holder to ensure that no other impurity was introduced during the ion irradiation. In such a case typically monolayer of impurities are introduced during irradiation for the fluence mentioned above as confirmed by RBS measurements [8]. Temperature of the sample holder was kept constant at room temperature to avoid any temperature induced effect. Surface topography was observed using non-contact mode AFM. The images were analysed using WSXM software. A flat plane was subtracted from all images to avoid any tilting or curved nature arises in the measured AFM images.
3. Results and discussion Fig. 2(a) shows AFM image of the bare HOPG sample. Atomic terrace are visible with different contrast. Water molecules are also observed sitting along the terrace sides [9,10]. The sample surface is homogeneously flat with r.m.s roughness of 0.2 nm. Eventhough roughness shown in Fig. 2(a) is higher than 0.2 nm, this rise is due to the existing impurities or trapped molecules available on the surface. When the sample is exposed with 200 eV Ar ions and fluence of 5.8 1018 cm 2 no surface damage is observed (Fig. 2 (b)). Only a slight variation in the surface roughness was observed, very small pits are formed reflecting ion bombardment and origin of some structure formation. Even though in this case Cu mask was used for metal incorporation, but it seems that at such a low energy yield of both graphite and Cu is very low as confirmed by TRIDYN code that threshold of the required metal incorporation was not achieved to form any structure on the surface. Typical r. m.s roughness in this case was found to be 2 nm (average height 6 nm) (as shown in Fig. 2(b)). The same experiment was performed with ion energy of 400 eV. Now a drastic change in the surface topography is observed. Tilted standing nano-pillars are formed with an average height of 125 nm as shown in Fig. 2(c). When the masking material is changed from Cu to Al (Fig. 2(d)), surface remains smooth and very small pits are observed on the surface. The pits size is only slightly higher 6–8 nm height as compared to 200 eV ion bombardments. This experiment clearly demonstrates that simply by changing the species of foreign atoms the surface topography changes drastically. Similarly when the masking material was again changed to Fe, different types of bunches of pillars were observed with an average height of even much higher (73 nm) compared to the case of Cu mask (Fig. 2(e)). Even in the case of no mask, the image looks like Fig. 2(e), since our sample holder is made of SS and contamination from the sample holder sputtered on the sample. In real Tokomak conditions the experimental situation may be even more
Direcon of ion Beam Mask
Graphite Fig. 1. Schematic of the sample mounting arrangement.
complex as species of different material may get simultaneously exposed to the surface and lead to some different structure formation. When angle of incidence was changed from 0° to 67° instead of pillars ordered ripple like structures are formed on the surface. AFM scan and FFT of the surface shown in Fig. 2(f) clearly show the long range order of these ripple patterns. These ripple like patterns have 20 nm amplitude and 100 nm periodicity. Line scanned image at a random place of the Fig. 2(f) is also shows the ripple modulations and the periodicity. Formation of such ripple like pattern is well document in literature [11–16]. TRIDYN code was used to estimate the sputtering yield of graphite both in pure Ar ion bombardment and Ar ion bombardment in presence of contaminants [17,18]. In TRIDYN simulation, the path of the individual moving particles and their collisions are modelled by means of the binary collision approximation for an amorphous substance, using a screened Coulomb potential for nuclear collisions and local or non-local free-electron–gas approximations for the electronic energy loss. TRIDYN can track recoil atoms from previous incident projectiles and simulate changes in local density for subsequent projectile tracking [18]. Therefore it is a better code to compare sputtering yield of graphite in presence of impurities as compared to TRIM/SRIM [6]. Fig. 3(a) shows the variation of ion energies on the sputtering yield (SPYL) at normal incidence. It is clearly visible that yields change drastically with ion energies. Fig. 3(b) shows the plots of sputtering yield variation with incident ion angle from 0° to 90°. It can be seen that there is an increment in the yield up to 70°. Beyond this limit ions may reflect back from the surface, consequently sputtering yield decreases. Later bombardment of 400 eV Ar ion on pure graphite, 99.9% graphite and fraction of 0–0.1% Cu, 99.9% graphite and fraction of 0–0.1% Al and fraction of 99.9% graphite and fraction of 0–0.1% Fe, respectively were simulated to observe the change in the yield in presence of metal incorporation. Plots of the variation in sputtering yield are shown in Fig. 4. In the simulation binding energies of different carbide formation combinations C–Cu, C–Al, C–Fe and C–C, Cu–Cu, Fe–Fe etc. were used and calculated using in build code in TRIDYN software. It can be observed that different surfactants have different influence on the graphite sputtering yield. In the case of Cu and Fe effective yield is much higher compared to Al on the higher fraction side of around 0.06–0.09 %. Presence of Al even show totally different variation, even though sputtering yield of pure Al is much higher than Cu and Fe (Fig. 4(b)). Graphite yield overall increased as compare to its pure form, for instance it is 0.10 at 400 normal incidence bombardment but increases to 0.18 with a Cu fraction of 0.09%. Above calculations clearly show that different surfactant atoms have different effect on the sputtering yield of the substrate material and consequently will have a role in pattern formation. Amplification in yield will enhance the erosion mechanism much more as compared to the normal diffusion process. This additional contribution in Bradly-Harper curvature dependent yield [6,12–14] helps to develop these hillocks like structure even at much lower ion energies, which are normally observed after the bombardment of 10 s of keV ion due to much lower sputtering yield of graphite [5]. Simultaneous erosion of graphite and surfactant atoms will locally make a very complex diffusion process due to different diffusion velocities of both atoms and collision among themselves. This will allow nanostructures features to grow faster rather than smoothen. Furthermore, depending of surfactants forming monolayer of atoms or clusters on the surface or chemically reactive with surface, the erosion process will be decided. Other theories like Bradly-Shipman [13,14] are more accurate to predict dot like structure formation considering two composite materials with different yields. This theory considers binary materials like GaSb or GaAs etc. Due to yield variation of the two materials and preferential sputtering roughness get amplified. This is not the case like in
Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062
M. Ranjan et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
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Fig. 2. (a) AFM image of the bare HOPG sample, (b) ion energy: 200 eV/Cu mask, (c) Cu mask, (d) Al mask, (e) Fe mask, (f) angle of incidence: 67° with respect to the surface normal/Cu mask. Ion energy of 400 eV and Fluence: 5.8 1018 cm 2 was maintained in all the experiments from (b–f). Also angle of incidence was normal incidence in the samples (b–e).
Fig. 3. (A) Energy dependent sputtering yield (SPYL) for graphite (carbon) at normal incidence of Ar ions, (b) angle dependent SPYL for graphite (carbon) at 400 eV energy of Ar ions.
the presence of surfactant, as in this a compound formation and segregation process decide the overall process [8,19–26]. In our case all the samples were measured at a random position away from mask edge and close to centre. It is extremely difficult to land the AFM tip exactly at the centre of the sample. Depending
on the edge to sample centre distance different kinds of patterns are reported to be evolved on the surface [8,20–26]. This is essentially because of the amount of impurity varies from the sample corner to the centre and generate inhomogeneity of contaminant. As is shown in the Fig. 4(a) and (b) depending upon the fraction
Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062
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M. Ranjan et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
Fig. 4. (a) Sputtering yield as a function of steady state coverage of different contaminants for graphite (carbon) substrate using TRIDYN. (b) Sputtering yield of pure metal with their increased content in the experiment.
of the contaminant available the sputtering yield of the material changes drastically. Sputtering yield plays role in pattern formation and it will definitely varies locally depending on the amount of contaminant. Ions bombard both the sample and the mask simultaneously and sputtered material from mask will also fall on the samples. The angle of contaminant falling on the sample surface is not clear and also their collision or interaction with the bombarding ion is also not clear. It is overall a very complex situation. Pattern formation during ion beam erosion and in the presence of impurities developed due to the in homogeneous distribution of impurities on the surface through ion induced phase separation and height fluctuation [8,20–26]. Since erosion rate of Cu/Fe carbide is different than graphite, height difference evolves. Due to conductive nature of graphite diffusion process will also be dominating, especially in the bottom of images Fig 2(c) and (e), where it will difficult for the ions to reach or erode. As mentioned above probably our AFM measurements are slightly off-centred. Due to inhomogeneous distribution of contaminant at off-centred position such tilted pillar like structure may arise due to inhomogeneous erosion from one side to another side. In the presence of metal incorporation, preferential sputtering, tensile stress-induced surface mass transportation, ion-induced segregation, and the change in collision cascade shape are the potential mechanisms [8,20]. Ion-induced segregation explains the nanopatterning process on GaSb for example. Hofsäss et al. [6,23,24] reported that surfactant atoms cause a change in the substrate sputtering yield. Depending on the behaviour of the surfactant atoms (clustering, island formation, attachment to surface defects and grain boundaries, etc.) and their chemical interaction with the substrate (alloy and compound formation) the erosion process becomes inhomogeneous on a local atomic scale. As a result, the pattern formation during sputter erosion is strongly influenced by surfactants leading to a wide variety of new surface nanopatterns. Norris et al. [26] discusses cases of impurity induced pattern formation where instability is driven by a morphological instability that does not require any concentration modulations in phase with the morphology. However, Engler et al. [25] reported that pattern formation requires a competition of un-stabilisation and smoothening with different dependencies on the wavelength. Un-stabilisation is due to the effect of height and composition fluctuations that amplify each other via effects of preferential erosion and geometry. The composition fluctuations are amplified by phase separation and the dependence of the local deposition flux on the local slope. These un-stabilising effects can be suppressed by ion
beam that effect composition fluctuations and ballistic mass drift destroying height fluctuations [25]. Since we have not yet performed detail elemental analysis, so it is hard to conclude at this stage that which phenomenon is prominent in the case of HOPG pattern formation. Overall above mentioned effects are very carefully looked into the fusion devices where all kinds of impurities are possible after plasma material interaction and this essentially means that local sputtering yield of inner wall or divertors may change and different such patterns may evolve on the surface, which is an undesirable condition as such nanostructure may retain the fuel and frequent cleaning of the surface will be required. Also phase separation may change properties of metal like thermal expansion or thermal conductivity of fusion plasma facing materials. 4. Conclusion The pattern morphology, amplitude as well as sputtering yield are quantitatively analysed with different amount of steady state coverage of intentionally introduced surfactant atoms. The role of surfactant atoms in the pattern formation is clearly demonstrated, which is important for any kind of plasma process where energetic ions are used. Surfactant atoms modify the effective sputtering yield of the atoms and this amplification in yield is dominant on the diffusion process and results in free standing nanostructures. Acknowledgement Financial support from DST–Fast track young scientist scheme. References [1] C.H. Skinner, C.A. Gentile, M.M. Menon, R.E. Barry, Nucl. Fusion 39 (1999) 1081. [2] J.P. Coad, N. Bekris, J.D. Elder, S.K. Erents, D.E. Hole, K.D. Lawson, J. Nucl. Mater. 290 (2001) 224. [3] Y. Gotoh, J. Yagyu, K. Masaki, K. Kizu, A. Kaminaga, K. Kodama, J. Nucl. Mater. 313 (2003) 370. [4] H. Yoshida, Y. Yamauchi, Y. Hirohata, T. Arai, S. Suzuki, M. Akiba, J. Nucl. Mater. 337 (2005) 604. [5] A.M. Borisov, E.S. Mashkova, A.S. Nemov, Yu.S. Virgiliev, Nucl. Instr. Meth. Phys. Res. B 256 (2007) 363. [6] H. Hofsass, K. Zhang, Appl. Phys. A 92 (3) (2008) 517. [7] K. Zhang, M. Brotzmann, H. Hofsass, New J. Phys. 13 (2011) 013033. [8] J. Zhou, S. Facsko, M. Lu, W. Moller, J. Appl. Phys. 109 (2011) 104315. [9] Y.H. Lu, C.W. Yang, C.K. Fang, I.S. Hwang, Langmuir 28 (35) (2012) 12691. [10] Y.H. Lu, C.W. Yang, C.K. Fang, H.C. Ko, I.S. Hwang, Sci. Rep. 4 (2014) 7189.
Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062
M. Ranjan et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx [11] A. Keller, S. Roßbach, S. Facsko, W. Möller, Nanotechnology 19 (13) (2008) 135303. [12] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol. A 6 (1988) 2390. [13] R.M. Bradley, P.D. Shipman, Appl. Surf. Sci. 258 (2012) 4161. [14] R.M. Bradley, Phys. Rev. B 87 (2013) 205408. [15] W. Lun Chan, E. Chason, J. Appl. Phys. 101 (2007) 121301. [16] M.A. Makeev, Albert-László Barabásia, Appl. Phys. Lett. 71 (1997) 19. [17] P. Sigmund, Phys. Rev. 184 (2) (1969) 383. [18] W. Möller, W. Eckstein, J.P. Biersack, Comput. Phys. Commun. 51 (1988) 355. [19] R. Kree, Nucl. Instr. Meth. B (2009). [20] S. Macko, F. Frost, B. Ziberi, D.F. Forster, T. Michely, Nanotechnology 21 (2010) 085301.
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[21] T. Basu, D.P. Datta, T. Som, Nanoscale Res. Lett. 8 (2013) 289. [22] S. Macko, J. Grenzer, F. Frost, M. Engler, D. Hirsch, M. Fritzsche, A. Mucklich, T. Michely, New J. Phys. 14 (2012) 073003. [23] H. Hofsäss, K. Zhang, A. Pape, O. Bobes, M. Brötzmann, Appl. Phys. A Mater. Sci. Process. 111 (2012) 653. [24] K. Zhang, O. Bobes, H. Hofsäss, Nanotechnology 25 (2014) 085301. [25] M. Engler, F. Frost, S. Müller, S. Macko, M. Will, R. Feder, D. Spemann, R. Hübner, S. Facsko, T. Michely, Nanotechnology 25 (2014) 115303. [26] S.A. Norris, J. Appl. Phys. 114 (2013) 204303.
Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation M. Ranjan ⇑, P. Joshi, S. Mukherjee FCIPT, Institute for Plasma Research, Gandhinagar (Gujarat), India
a r t i c l e
i n f o
Article history: Received 3 November 2015 Received in revised form 27 February 2016 Accepted 27 February 2016 Available online xxxx Keywords: Graphite Surfactant Nanopatterns Ion irradiation Sputtering yield
a b s t r a c t Low energy ions beam often develop periodic patterns on surfaces under normal or off-normal incidence. Formation of such periodic patterns depends on the substrate material, the ion beam parameters, and the processing conditions. Processing conditions introduce unwanted contaminant atoms, which also play strong role in pattern formation by changing the effective sputtering yield of the material. In this work we have analysed the effect of Cu, Fe and Al impurities introduced during low energy Ar+ ion irradiation on HOPG substrate. It is observed that by changing the species of foreign atoms the surface topography changes drastically. The observed surface topography is co-related with the modified sputtering yield of HOPG. Presence of Cu and Fe amplify the effective sputtering yield of HOPG, so that the required threshold for the pattern formation is achieved with the given fluence, whereas Al does not lead to any significant change in the effective yield and hence no pattern formation occurs. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Graphite is widely used in various important scientific fields. It is used as a plasma facing component (First wall, Divertor) in tokomaks, and in space applications as electrode materials in plasma torches [1–4]. In all such applications graphite interacts with plasma of very low density (106 cm 3) to very high density (1018 cm 3). Also the energies of interacting ions varies from 10 s of eV to keVs. Properties of graphite degrades (thermal conductivity, thermal expansion coefficient) during erosion and redeposition of various impurities from the reactor wall. Various studies are performed for the plasma wall interaction with graphite for both physical and chemical sputtering [5]. All these results only present macroscopic topographies observed under various bombarding conditions and considering a pure system i.e. excluding the effects of impurities. Such structures formed on the graphite surface retain large amount of tritium, which is an undesirable condition. Since graphite erosion investigation is an important issue, in our present study we aim to investigate the erosion rate of the surfaces, utilising the steady state coverage of around one monolayer. Depending upon the chemical bonding with the substrate atoms, the surfactant atoms may drive the surface to form nanosized patterning [6,7]. This will help not only to investigate the fundamental ⇑ Corresponding author. E-mail address: [email protected] (M. Ranjan).
physics behind formation of various nanoscale surface topographies but also formation of its prevention. In the present study Ar ions are used to bombard Highly-Oriented Pyrolytic Graphite (HOPG) surface at a lower energy of 200–400 eV. Fluence and ion energies chosen are relevant to fusion research. Systematically various impurities like Cu, Al and Fe are introduced during ion bombardment and their effect on the sputtering yield and surface topography is reported. 2. Experimentation A UHV system of vacuum level up to 10 8 mbar was used for the experiments and an operating pressure of 2 10 4 mbar was maintained during experiment. Ar ion beam of energy range 200 eV–400 eV produced by Kaufman ion source was used to irradiate the HOPG samples. Fluence of 5.8 1018 cm 2 was incident at angles of 0° and 67° with respect to the surface normal. Such a large fluence is normally expected in the fusion devices [5]. When plasma interacts with a floating electrode, it makes a homogeneous sheath of typically few mm in the surrounding of the sample. High voltage sheath ensures the normal incidence of the ions on the surface. To simulate the similar effect we have also chosen the normal incidence of the ions. In fusion devices ions also move in the presence of magnetic field, at that point they are transported towards the diverter region, there is a possibility of bombardment of ions at some angles. Simulating such an event we have chosen angle of incidence also as 67° with respect to the surface normal.
http://dx.doi.org/10.1016/j.nimb.2016.02.062 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062
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M. Ranjan et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
To introduce the impurity during ion irradiation, mask of different materials (Cu, Al, and Fe) were used as shown in the Fig. 1. HOPG sample of size 10 10 mm was sandwiched between the sample holder and the mask of having circular opening. Mask was prepared in such a way so as to fully cover the sample holder to ensure that no other impurity was introduced during the ion irradiation. In such a case typically monolayer of impurities are introduced during irradiation for the fluence mentioned above as confirmed by RBS measurements [8]. Temperature of the sample holder was kept constant at room temperature to avoid any temperature induced effect. Surface topography was observed using non-contact mode AFM. The images were analysed using WSXM software. A flat plane was subtracted from all images to avoid any tilting or curved nature arises in the measured AFM images.
3. Results and discussion Fig. 2(a) shows AFM image of the bare HOPG sample. Atomic terrace are visible with different contrast. Water molecules are also observed sitting along the terrace sides [9,10]. The sample surface is homogeneously flat with r.m.s roughness of 0.2 nm. Eventhough roughness shown in Fig. 2(a) is higher than 0.2 nm, this rise is due to the existing impurities or trapped molecules available on the surface. When the sample is exposed with 200 eV Ar ions and fluence of 5.8 1018 cm 2 no surface damage is observed (Fig. 2 (b)). Only a slight variation in the surface roughness was observed, very small pits are formed reflecting ion bombardment and origin of some structure formation. Even though in this case Cu mask was used for metal incorporation, but it seems that at such a low energy yield of both graphite and Cu is very low as confirmed by TRIDYN code that threshold of the required metal incorporation was not achieved to form any structure on the surface. Typical r. m.s roughness in this case was found to be 2 nm (average height 6 nm) (as shown in Fig. 2(b)). The same experiment was performed with ion energy of 400 eV. Now a drastic change in the surface topography is observed. Tilted standing nano-pillars are formed with an average height of 125 nm as shown in Fig. 2(c). When the masking material is changed from Cu to Al (Fig. 2(d)), surface remains smooth and very small pits are observed on the surface. The pits size is only slightly higher 6–8 nm height as compared to 200 eV ion bombardments. This experiment clearly demonstrates that simply by changing the species of foreign atoms the surface topography changes drastically. Similarly when the masking material was again changed to Fe, different types of bunches of pillars were observed with an average height of even much higher (73 nm) compared to the case of Cu mask (Fig. 2(e)). Even in the case of no mask, the image looks like Fig. 2(e), since our sample holder is made of SS and contamination from the sample holder sputtered on the sample. In real Tokomak conditions the experimental situation may be even more
Direcon of ion Beam Mask
Graphite Fig. 1. Schematic of the sample mounting arrangement.
complex as species of different material may get simultaneously exposed to the surface and lead to some different structure formation. When angle of incidence was changed from 0° to 67° instead of pillars ordered ripple like structures are formed on the surface. AFM scan and FFT of the surface shown in Fig. 2(f) clearly show the long range order of these ripple patterns. These ripple like patterns have 20 nm amplitude and 100 nm periodicity. Line scanned image at a random place of the Fig. 2(f) is also shows the ripple modulations and the periodicity. Formation of such ripple like pattern is well document in literature [11–16]. TRIDYN code was used to estimate the sputtering yield of graphite both in pure Ar ion bombardment and Ar ion bombardment in presence of contaminants [17,18]. In TRIDYN simulation, the path of the individual moving particles and their collisions are modelled by means of the binary collision approximation for an amorphous substance, using a screened Coulomb potential for nuclear collisions and local or non-local free-electron–gas approximations for the electronic energy loss. TRIDYN can track recoil atoms from previous incident projectiles and simulate changes in local density for subsequent projectile tracking [18]. Therefore it is a better code to compare sputtering yield of graphite in presence of impurities as compared to TRIM/SRIM [6]. Fig. 3(a) shows the variation of ion energies on the sputtering yield (SPYL) at normal incidence. It is clearly visible that yields change drastically with ion energies. Fig. 3(b) shows the plots of sputtering yield variation with incident ion angle from 0° to 90°. It can be seen that there is an increment in the yield up to 70°. Beyond this limit ions may reflect back from the surface, consequently sputtering yield decreases. Later bombardment of 400 eV Ar ion on pure graphite, 99.9% graphite and fraction of 0–0.1% Cu, 99.9% graphite and fraction of 0–0.1% Al and fraction of 99.9% graphite and fraction of 0–0.1% Fe, respectively were simulated to observe the change in the yield in presence of metal incorporation. Plots of the variation in sputtering yield are shown in Fig. 4. In the simulation binding energies of different carbide formation combinations C–Cu, C–Al, C–Fe and C–C, Cu–Cu, Fe–Fe etc. were used and calculated using in build code in TRIDYN software. It can be observed that different surfactants have different influence on the graphite sputtering yield. In the case of Cu and Fe effective yield is much higher compared to Al on the higher fraction side of around 0.06–0.09 %. Presence of Al even show totally different variation, even though sputtering yield of pure Al is much higher than Cu and Fe (Fig. 4(b)). Graphite yield overall increased as compare to its pure form, for instance it is 0.10 at 400 normal incidence bombardment but increases to 0.18 with a Cu fraction of 0.09%. Above calculations clearly show that different surfactant atoms have different effect on the sputtering yield of the substrate material and consequently will have a role in pattern formation. Amplification in yield will enhance the erosion mechanism much more as compared to the normal diffusion process. This additional contribution in Bradly-Harper curvature dependent yield [6,12–14] helps to develop these hillocks like structure even at much lower ion energies, which are normally observed after the bombardment of 10 s of keV ion due to much lower sputtering yield of graphite [5]. Simultaneous erosion of graphite and surfactant atoms will locally make a very complex diffusion process due to different diffusion velocities of both atoms and collision among themselves. This will allow nanostructures features to grow faster rather than smoothen. Furthermore, depending of surfactants forming monolayer of atoms or clusters on the surface or chemically reactive with surface, the erosion process will be decided. Other theories like Bradly-Shipman [13,14] are more accurate to predict dot like structure formation considering two composite materials with different yields. This theory considers binary materials like GaSb or GaAs etc. Due to yield variation of the two materials and preferential sputtering roughness get amplified. This is not the case like in
Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062
M. Ranjan et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
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Fig. 2. (a) AFM image of the bare HOPG sample, (b) ion energy: 200 eV/Cu mask, (c) Cu mask, (d) Al mask, (e) Fe mask, (f) angle of incidence: 67° with respect to the surface normal/Cu mask. Ion energy of 400 eV and Fluence: 5.8 1018 cm 2 was maintained in all the experiments from (b–f). Also angle of incidence was normal incidence in the samples (b–e).
Fig. 3. (A) Energy dependent sputtering yield (SPYL) for graphite (carbon) at normal incidence of Ar ions, (b) angle dependent SPYL for graphite (carbon) at 400 eV energy of Ar ions.
the presence of surfactant, as in this a compound formation and segregation process decide the overall process [8,19–26]. In our case all the samples were measured at a random position away from mask edge and close to centre. It is extremely difficult to land the AFM tip exactly at the centre of the sample. Depending
on the edge to sample centre distance different kinds of patterns are reported to be evolved on the surface [8,20–26]. This is essentially because of the amount of impurity varies from the sample corner to the centre and generate inhomogeneity of contaminant. As is shown in the Fig. 4(a) and (b) depending upon the fraction
Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062
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M. Ranjan et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
Fig. 4. (a) Sputtering yield as a function of steady state coverage of different contaminants for graphite (carbon) substrate using TRIDYN. (b) Sputtering yield of pure metal with their increased content in the experiment.
of the contaminant available the sputtering yield of the material changes drastically. Sputtering yield plays role in pattern formation and it will definitely varies locally depending on the amount of contaminant. Ions bombard both the sample and the mask simultaneously and sputtered material from mask will also fall on the samples. The angle of contaminant falling on the sample surface is not clear and also their collision or interaction with the bombarding ion is also not clear. It is overall a very complex situation. Pattern formation during ion beam erosion and in the presence of impurities developed due to the in homogeneous distribution of impurities on the surface through ion induced phase separation and height fluctuation [8,20–26]. Since erosion rate of Cu/Fe carbide is different than graphite, height difference evolves. Due to conductive nature of graphite diffusion process will also be dominating, especially in the bottom of images Fig 2(c) and (e), where it will difficult for the ions to reach or erode. As mentioned above probably our AFM measurements are slightly off-centred. Due to inhomogeneous distribution of contaminant at off-centred position such tilted pillar like structure may arise due to inhomogeneous erosion from one side to another side. In the presence of metal incorporation, preferential sputtering, tensile stress-induced surface mass transportation, ion-induced segregation, and the change in collision cascade shape are the potential mechanisms [8,20]. Ion-induced segregation explains the nanopatterning process on GaSb for example. Hofsäss et al. [6,23,24] reported that surfactant atoms cause a change in the substrate sputtering yield. Depending on the behaviour of the surfactant atoms (clustering, island formation, attachment to surface defects and grain boundaries, etc.) and their chemical interaction with the substrate (alloy and compound formation) the erosion process becomes inhomogeneous on a local atomic scale. As a result, the pattern formation during sputter erosion is strongly influenced by surfactants leading to a wide variety of new surface nanopatterns. Norris et al. [26] discusses cases of impurity induced pattern formation where instability is driven by a morphological instability that does not require any concentration modulations in phase with the morphology. However, Engler et al. [25] reported that pattern formation requires a competition of un-stabilisation and smoothening with different dependencies on the wavelength. Un-stabilisation is due to the effect of height and composition fluctuations that amplify each other via effects of preferential erosion and geometry. The composition fluctuations are amplified by phase separation and the dependence of the local deposition flux on the local slope. These un-stabilising effects can be suppressed by ion
beam that effect composition fluctuations and ballistic mass drift destroying height fluctuations [25]. Since we have not yet performed detail elemental analysis, so it is hard to conclude at this stage that which phenomenon is prominent in the case of HOPG pattern formation. Overall above mentioned effects are very carefully looked into the fusion devices where all kinds of impurities are possible after plasma material interaction and this essentially means that local sputtering yield of inner wall or divertors may change and different such patterns may evolve on the surface, which is an undesirable condition as such nanostructure may retain the fuel and frequent cleaning of the surface will be required. Also phase separation may change properties of metal like thermal expansion or thermal conductivity of fusion plasma facing materials. 4. Conclusion The pattern morphology, amplitude as well as sputtering yield are quantitatively analysed with different amount of steady state coverage of intentionally introduced surfactant atoms. The role of surfactant atoms in the pattern formation is clearly demonstrated, which is important for any kind of plasma process where energetic ions are used. Surfactant atoms modify the effective sputtering yield of the atoms and this amplification in yield is dominant on the diffusion process and results in free standing nanostructures. Acknowledgement Financial support from DST–Fast track young scientist scheme. References [1] C.H. Skinner, C.A. Gentile, M.M. Menon, R.E. Barry, Nucl. Fusion 39 (1999) 1081. [2] J.P. Coad, N. Bekris, J.D. Elder, S.K. Erents, D.E. Hole, K.D. Lawson, J. Nucl. Mater. 290 (2001) 224. [3] Y. Gotoh, J. Yagyu, K. Masaki, K. Kizu, A. Kaminaga, K. Kodama, J. Nucl. Mater. 313 (2003) 370. [4] H. Yoshida, Y. Yamauchi, Y. Hirohata, T. Arai, S. Suzuki, M. Akiba, J. Nucl. Mater. 337 (2005) 604. [5] A.M. Borisov, E.S. Mashkova, A.S. Nemov, Yu.S. Virgiliev, Nucl. Instr. Meth. Phys. Res. B 256 (2007) 363. [6] H. Hofsass, K. Zhang, Appl. Phys. A 92 (3) (2008) 517. [7] K. Zhang, M. Brotzmann, H. Hofsass, New J. Phys. 13 (2011) 013033. [8] J. Zhou, S. Facsko, M. Lu, W. Moller, J. Appl. Phys. 109 (2011) 104315. [9] Y.H. Lu, C.W. Yang, C.K. Fang, I.S. Hwang, Langmuir 28 (35) (2012) 12691. [10] Y.H. Lu, C.W. Yang, C.K. Fang, H.C. Ko, I.S. Hwang, Sci. Rep. 4 (2014) 7189.
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Please cite this article in press as: M. Ranjan et al., Formation of nanostructures on HOPG surface in presence of surfactant atom during low energy ion irradiation, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.02.062