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Co bilayers during ion beam mixing
Nuclear Instruments and Methods in Physics Research B 313 (2013) 60–63
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The mechanism of phase formation in Pt/Co bilayers during ion beam mixing S. Balaji a,⇑, B.K. Panigrahi a, S. Amirthapandian a, S. Kalavathi a, Ajay Gupta b, K.G.M. Nair a a b
Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India
a r t i c l e
i n f o
Article history: Received 19 June 2013 Received in revised form 5 August 2013 Available online 14 August 2013 Keywords: Ion beam mixing CoPt Co3Pt Phase synthesis
a b s t r a c t Ion beam mixing of Pt/Co/Si (Substrate) bilayers were carried out with a 4 MeV Si+ ion beam at two different temperatures – room temperature and 300 °C. When the mixing is carried out at 300 °C, diffusion of Co atoms to the top Pt layer is observed thereby synthesizing CoPt and Co3Pt crystalline phases. When the irradiation is carried out at room temperature, Co and Pt atoms diffuse inwards to the silicon substrate and no new crystalline phase formation is observed. We propose that the direction of movement of the atoms and the resulting concentration profile in the Pt/Co/Si bilayers during ion beam mixing decides the nature of phase synthesized. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Intermetallic cobalt platinum alloys with the chemically ordered L1o structure have been extensively studied due to their hard ferromagnetic properties. In the ordered state, the material has a very high uniaxial magnetocrystalline anisotropy resulting in a high coercivity and this property makes this alloy an attractive candidate for potential technological applications in high density magnetic recording media [1,2]. Transformation from disordered CoPt phase to ordered phase is observed, when the material is annealed at high temperatures [3,4]. However such high temperature annealing is undesirable for manufacturing process [5]. Ion beam annealing is an efficient tool to tailor the magnetic properties of these alloys [6]. With additional energy provided by the ion beams to create defects, diffusion in the material is enhanced, thereby promoting phase transformation at lower temperatures. During ion beam mixing where the interface is bombarded with energetic ions, the atoms get displaced across the interface and the atoms may have enough mobility to migrate and form new alloy phases. Due to high energy transfer by the incident ions, the temperature of the material can rise up to 103 to 104 K locally in a small region of the material and quenched to room temperature within a time period of 10 10 to 10 9 s [7]. The technique of ion irradiation thus results in the formation of metastable compounds irrespective of the kind of elements and atomic stoichiometry [8]. In particular, phases which cannot be synthesized by thermodynamic means can be very well obtained ⇑ Corresponding author. Tel.: +91 44 27480085; fax: +91 44 27480081. E-mail address: [email protected] (S. Balaji). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.08.005
by ion beam mixing process. Owing to the application of ion beam mixing in synthesizing technologically important materials, many models were proposed, to predict the dominant moving species during the mixing process, as the dominant moving species decides the mixing rates and phase formation. Ding et al. [9] suggested that during ion beam mixing, in the radiation enhanced diffusion (RED) regime, the atomic motion of the atoms with smaller atomic radii is enhanced as compared to atoms with larger atomic radii. Tao et al. [10,11] has examined the direction of atomic motion in thin films during ion irradiation and has concluded that atoms from low damage density region tend to move into higher damage density region via inverse Kirkendall effect. To overcome the shortcomings of atomic size effect proposed by Ding et al. [9] and radiation damage density effect proposed by Tao et al. [10] a new model was developed by Jung et al. [12] that predicted the dominant moving species during ion beam mixing. Subsequent ion irradiation experiments performed on bilayer film [13,14] successfully verified the model proposed by Jung et al. [12]. The ion beam mixing of Co/Pt multilayer film by Ar+ ion irradiation in an external magnetic field leads to the formation of CoPt phase with expanded volume due to ultrafast quenching during the cooling phase of the collision cascade [15]. Ion beam mixing carried out on Co/Pt mulilayers [16] and bilayers [17] has resulted in CoPt and CoPt3 phase synthesis. We have reported the formation of ordered CoPt phase by carrying out ion beam mixing in Pt/Co/Si bilayer films [18] and we have also demonstrated that the ion beam mixing in the Pt/Co/Si bilayers using Pt+ ions is diffusion controlled rather than reaction controlled [19]. In this paper, we suggest a mechanism of phase formation during ion beam mixing
S. Balaji et al. / Nuclear Instruments and Methods in Physics Research B 313 (2013) 60–63
in Pt/Co/Si(substrate) bilayers. We propose that, in ion beam mixing of Pt/Co/Si(substrate) bilayers, the direction of movement of atoms and the resulting concentration profile decides the nature of phase synthesized.
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The crystal structure of the phases in the as-prepared and ion irradiated samples were identified with GIXRD and the results are shown in Fig. 1. The diffraction peaks of Pt (fcc) and Co (hcp) phases are clearly observed in the as-prepared sample. The GIXRD pattern of the bilayer sample irradiated with 4 MeV Si+ ions at 300 °C shows the presence of diffraction peaks corresponding to CoPt and Co3Pt phases. When the bilayer sample is irradiated with 4 MeV Si+ ions at room temperature, a decrease in intensity of the diffraction peaks of Pt and Co is observed and there is no other phase identified. Compared to as-prepared sample the peak positions are shifted in the bilayer sample irradiated with 4 MeV Si+ ions at room temperature and the full width at half maximum (FWHM) of the diffraction peaks has increased after irradiation.
The shift in peak position might be due to strain introduced by the ion beam in the lattice and increase in FWHM most likely as a result of decrease in the particle size. The RBS spectra of the as – prepared and ion irradiated samples are shown in Fig. 2. The thickness and composition of the as prepared sample was estimated from the spectra using the RUMP code [21]. The thickness of the platinum and cobalt layers seen in the spectra was found to be 40 nm and 70 nm respectively. In the ion irradiated samples, the mixing at the Pt/Co and Co/Si interfaces is evident from the tailing of the low energy edge of the platinum and cobalt and high energy edges of the cobalt and silicon respectively. The RBS spectra of the bilayer film irradiated with 4 MeV Si+ ion beam at 300 °C was analyzed with RUMP code and the extracted depth profile is shown in Fig. 3. In the region marked 1 in Fig. 3, which is up to 50 nm thickness, the atomic fraction of Pt and Co are 0.41 and 0.59, respectively. While 40 nm thick top Pt layer was present in the as – prepared sample, the atomic fraction of Co has increased up to 0.59 in the same layer after 4 MeV Si+ ion irradiation at 300 °C signifying major out diffusion of Co atoms. In regions marked 2, 3, up to a depth of 100 nm, the atomic fraction of cobalt is nearly 3 times that of the Pt atomic fraction. The region marked 4, 5, 6, 7 etc. corresponds to silicon substrate in the as prepared sample and the substrate after irradiation contains significant concentration of Co, Pt nearly up to the depth of 230 nm from the substrate surface. The RBS spectra of the bilayer sample irradiated with 4 MeV Si+ ion beam at room temperature was analyzed with RUMP code and the extracted depth profile is shown in Fig. 4. Irradiation of the bilayer with 4 MeV Si+ ion beam at room temperature has stimulated inward diffusion of cobalt and platinum atoms up to a depth of 330 nm (Fig. 4) into the silicon substrate reducing the silicon concentration around 1000 keV of back scattered energy in Fig. 2. The 4 MeV Si+ ion irradiation at room temperature does not induce outward diffusion of Co, while a small hump at backscattered energy of 1121 keV in Fig. 2 implies the diffusion of substrate silicon atoms towards top of the film. Ion irradiation of the bilayers at 300 °C with 4 MeV Si+ ion beam promotes the synthesis of CoPt and Co3Pt phases, even if cobalt and platinum silicides are known to have large negative heat of formation as compared to CoPt, Co3Pt [22]. This result is quite intriguing. The reduced role of heat of formation in phase synthesis suggests that, a more fundamental parameter decides the phase formation during ion beam mixing. Effective heat of formation model
Fig. 1. The GIXRD pattern of the (a) as-prepared Pt/Co bilayer, (b) Pt/Co bilayer irradiated with 4 MeV Si+ ions at 300 °C upto a ion fluence of 2 1016 ions/cm2 (c) Pt/Co bilayer irradiated with 4 MeV Si+ ions at room temperature upto a ion fluence of 2 1016 ions/cm2.
Fig. 2. The RBS spectra of the as-prepared Pt/Co bilayer, Pt/Co bilayer irradiated with 4 MeV Si+ ions at 300 °C upto a ion fluence of 2 1016 ions/cm2, Pt/Co bilayer irradiated with 4 MeV Si+ ions at room temperature upto a ion fluence of 2 1016 ions/cm2.
2. Experiment The Pt/Co/Si(1 0 0) [40 nm/70 nm/substrate] bilayer films were deposited by electron beam evaporation at a rate of 0.1 Å/s in a UHV chamber held at a base pressure of 1.07 10 9 mbar. The samples were irradiated with 4 MeV Si+ ions from 1.7 MV Tandetron accelerator up to a ion fluence of 2 1016 ions/cm2 at room temperature and at 300 °C, and the base pressure of the target chamber during ion irradiation was 1.07 10 6 mbar. The range of penetration of 4 MeV Si+ ions in the sample is found to be 2.35 lm using Monte–Carlo simulation SRIM 2003 [20] and hence the implanted Si is deep inside the Si(1 0 0) substrate. Rutherford backscattering (RBS) measurements were carried out using 2 MeV alpha particles from 1.7 MV Tandetron accelerator on asprepared and ion irradiated samples under normal incidence. The backscattered alpha particles were detected using surface barrier silicon detector with a scattering angle of 165° from the incident beam direction. The grazing incidence X-ray diffraction (GIXRD) studies were performed using a STOE GMBH, Germany made X-ray diffractometer and a glancing angle of 0.2° was kept for Cu-Ka (k = 1.541 Å) radiation for all samples. 3. Results and discussion
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S. Balaji et al. / Nuclear Instruments and Methods in Physics Research B 313 (2013) 60–63
Fig. 3. The (a) Pt, (b) Co, (c) Si depth profiles extracted from RUMP code for Pt/Co bilayer irradiated with 4 MeV Si+ ions at 300 °C upto a ion fluence of 2 1016 ions/ cm2.
Fig. 4. The (a) Pt, (b) Co, (c) Si depth profiles extracted from RUMP code for Pt/Co bilayer irradiated with 4 MeV Si+ ions at room temperature upto a ion fluence of 2 1016 ions/cm2.
introduced by Xia et al. [23] successfully predicted the sequence of phase formation in the cobalt silicide system, by proposing that, the nature of phase synthesized during ion beam mixing depends on the heat of formation, and atomic availability at the thin film interface. In our Si+ ion irradiation experiment at 300 °C, at the Co/Si interface, no cobalt silicide phase is synthesized irrespective of the more negative heat of formation for the silicide phases and atomic availability at the interface. Instead CoPt, Co3Pt phases are synthesized in the bilayers. RBS observations suggest that the enhanced Co atomic motion towards top Pt layer synthesizes CoPt, Co3Pt phases without forming silicide phases. The enhanced Co atomic motion towards top platinum layer at 300 °C was predicted by the model put forward by Jung et al. [12] and has been experimentally verified for Co/Pt bilayer film [13]. Two distinct regimes were identified in the model, the atomic transport in the collision cascade regime and the thermal spike regime, were suggested to be in the athermal regime and that in the radiation enhanced diffusion (RED) regime to be in the temperature dependent regime. As per the model, at the Co/Pt interface, in the RED regime, the diffusion of Co atoms into the Pt layer is enhanced, while in the athermal regime the diffusion of Pt atoms and the Co atoms across the interface is almost equal, due to the difference in their cohesive energies [13,14]. However, the model is suitable only for metallic systems and not suitable for semiconductors [13].
The critical temperature for radiation enhanced diffusion to take place at the Pt/Co bilayer interface is about 214 °C [18] and hence, for 4 MeV Si+ ion irradiation at 300 °C, the bilayer interface is subjected to radiation enhanced diffusion. For the top platinum, and substrate silicon atoms, diffusion in only one direction is possible. While top Pt atoms can diffuse into Co and then into Si substrate, substrate Si atoms can diffuse into Co and top Pt layer and this one way diffusion of top Pt and substrate Si atoms is clearly observed in Figs. 2 and 3. But for the case of Co sandwiched between substrate Si and top Pt layer, diffusion in either side is possible. Co atoms can either diffuse into Si substrate to form cobalt silicides or towards top Pt layer to form CoPt, Co3Pt phases. As per the model proposed by Jung et al. [12] the atomic transport at the interface between two metallic thin films depends on their atomic radius, deposited energy by nuclear stopping in the metallic targets and their cohesive energy. For the case of Pt/Co bilayer, there is enhanced motion of Co atoms into Pt layer at 300 °C because of difference in cohesive energy between Co (4.39 eV) and Pt (5.84 eV) and it is experimentally verified [13,14]. The cohesive energy effect is clearly visible in our results also, as we observe a major fraction of Co atoms diffuse into top Pt layer. Subsequent segregation of cobalt atoms in the platinum layer leads to the formation of the CoPt, Co3Pt phases. In the region marked 1 in Fig. 3, the phase corresponds to CoPt while the region marked 2, 3 in the same figure corresponds to Co3Pt phase. Diffraction peaks corresponding to CoPt, Co3Pt phases are thus clearly seen in GIXRD patterns. Although minor fraction of cobalt, platinum atoms diffuse into silicon substrate, we did not observe any XRD peaks for silicide phases and the reason for this result is explained below. In the case of room temperature irradiation, RED is absent. The SRIM results on Pt/Co/Si bilayers (40 nm/70 nm/substrate) (with Ed for Pt, 36 eV, Co, 22 eV and Si, 20 eV) show that Pt recoils into cobalt layer and subsequently into silicon substrate, Co recoils into top Pt layer and Si substrate, and substrate Si recoils into Co and Pt layers when the bilayers are irradiated with 4 MeV Si+ ion beam. At low temperatures – the athermal regime, the higher cohesive energy species preferentially transport into low cohesive energy species at the bilayer metallic interface [13,14]. At the Pt/Co interface, diffusion of Pt atoms and the Co atoms across the interface is almost equal, due the difference in their cohesive energies [13,14]. Thus, Pt atoms diffuse into cobalt layer and reach silicon substrate because of their cohesive energy difference and recoils produced by 4 MeV Si+ ion beam on the Pt/Co bilayer at room temperature, as clearly observed in RUMP simulation shown in Fig. 4. Thermal diffusion coefficient of silicon into cobalt [24] and cobalt into silicon [25] at 300 °C and room temperature is negligibly small. However during ion irradiation, diffusion coefficient can be greatly enhanced and can be very high [26] than that of thermal diffusion coefficient. At the Co/Si interface, the critical temperature (Tc) for radiation enhanced diffusion (RED) process is calculated by using the empirical relation Tc = 95.2 DHcoh (eV/atom) [27,28] and is found to be 156 °C. Hence, at the Co/Si interface, during room temperature irradiation, RED is absent and penetration of silicon into cobalt and platinum, cobalt into silicon, is thus ascribed to ion induced recoils as revealed by SRIM results and it is consistent with our present experiments. Also due to inward displacement of platinum and cobalt, silicon is displaced outside during room temperature irradiation as seen in our RBS results. However for irradiation at 300 °C, the Co/Si interface is subjected to radiation enhanced diffusion. Hence in addition to recoils, the atomic transport across the Co/Si interface possibly could be influenced by RED. During both high temperature and room temperature irradiation, Co and Pt atoms diffuse into Si substrate. However, the GIXRD observation did not show any silicide phases in the bilayers for both room temperature and high temperature irradiation. Since ternary phase diagram for the Co–Pt–Si [29] is not available, it is
S. Balaji et al. / Nuclear Instruments and Methods in Physics Research B 313 (2013) 60–63
difficult to predict the phases with Co, Pt, Si elemental mixture. However the mixture with Co, Pt, Si elements in our bilayers may contain partially intermixed Pt and Co nano-crystals with inter-diffused Si, some crystalline disorder, possibly solid solution and/or amorphous mixture after irradiation. It is quite interesting to note that, we get almost similar atomic transport results qualitatively at the Pt/Co interface for the case of 4 MeV Si+ ion irradiation on our bilayers as in the case of 80 keV argon ions on the Pt/Co bilayers [13,14]. In the athermal regime, the atomic transport approximately depends on cohesive energy of the elements at the Pt/Co interface [13,14] and hence is independent of the ion beam parameters. In the RED regime-the thermal regime, as in the case of 80 keV argon irradiation on the Pt/Co bilayers [13,14], we note that (Fd)Pt (Fd)Co (11.68 eV/Å/ion) at the bilayer interface for 4 MeV Si+ ion irradiation for our bilayers as per SRIM simulation. Hence, the governing equations given for atomic transport at the Pt/Co interface in references [13,14] give similar results qualitatively, irrespective of the change in ion beam parameters. The only difference to be observed in the present case is that atoms in the bilayer diffuse into silicon substrate during 4 MeV Si+ ion irradiation on the bilayers. As Chang et al. [13] and Son et al. [14] have selected the argon ion beam energy such that the argon ion beam does not reach their substrate, no significant diffusion of atoms in the bilayer, into substrate were reported in their experiments. In our case, the 4 MeV Si+ ion beam penetrates deep inside silicon substrate producing recoils along the path and hence atoms in the bilayer diffuse into silicon substrate. 4. Conclusions The mechanism of phase formation during ion beam mixing in Pt/Co/Si (substrate) bilayers can then be summarized as follows. The direction of movement of the atoms during ion beam mixing is decided by number of factors – temperature, cohesive energy, atomic size, displacement energy and deposited energy by nuclear stopping. In our case, during 4 MeV Si+ ion irradiation on Pt /Co bilayers at 300 °C, due to difference in cohesive energy the Co diffusion towards Pt is enhanced which finally leads to CoPt, Co3Pt phase formation. On the other hand, during 4 MeV Si+ ion irradiation on Pt /Co bilayers at room temperature, cobalt and platinum diffuse into silicon substrate and no new crystalline phase formation is observed. In conclusion, we propose that during ion beam mixing of Pt/Co/Si (substrate) bilayers, the direction of atomic movement and the resultant concentration profile, decides the nature of phase synthesized. We suspect that the above conclusion is true for all ion beam mixing experiments irrespective of kind of
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elements in the bilayers and the results need to be tested on different bilayer systems. References [1] C.W. White, S.P. Withrow, J.D. Budai, D.K. Thomas, J.M. Williams, A. Meldrum, K.D. Sorge, J.R. Thompson, G.W. Ownby, J.F. Wendelken, L.A. Boatner, J. Appl. Phys. 98 (2005) 114311. [2] J.A. Christodoulides, Y. Huang, Y. Zhang, G.C. Hadjipanayis, I. Panagiotopoulos, D. Niarchos, J. Appl. Phys. 87 (2000) 6938. [3] X.C. Sun, Z.Y. Jia, Y.H. Huang, J.W. Harrell, D.E. Nikles, K. Sun, L.M. Wang, J. Appl. Phys. 95 (2004) 6747. [4] Q.F. Xiao, E. Bruck, Z.D. Zhang, F.R. De Boer, K.H.J. Buschow, J. Appl. Phys. 91 (2002) 8819. [5] T. Yokoto, L. Gao, S.H. Liou, M.L. Yan, D.J. Sellmyer, J. Appl. Phys. 95 (2004) 7270. [6] J. Fassbender, D. Revelosona, Y. Samson, J. Phys. D: Appl. Phys. 37 (2004) R179. [7] B.X. Liu, W.S. Lai, Q. Zhang Mat, Sci. Eng. 29 (2000) 1. [8] K.H. Chae, Y.S. Lee, S.M. Jung, Y. Jeon, M. Croft, C.N. Whong, Nucl. Instrum. Methods B 106 (1995) 60. [9] F.R. Ding, R.S. Averback, H. Hahn, J. Appl. Phys. 64 (1988) 1785. [10] K. Tao, C.A. Hewett, S.S. Lau, Ch. Buchal, D.B. Poker, Appl. Phys. Lett. 50 (1987) 1343. [11] K. Tao, C.A. Hewett, S.S. Lau, Ch. Buchal, D.B. Poker, Nucl. Instrum. Methods B 19–20 (1987) 753. [12] S.M. Jung, G.S. Chang, J.H. Song, K.H. Chae, K. Jeong, J.J. Woo, C.N. Whang, J. Kor, Phys. Soc. 28 (1995) 481. [13] G.S. Chang, J.H. Son, T.G. Kim, K.H. Chae, C.N. Whang, J.I. Jeong, Y.P. Lee, Thin Solid Films 341 (1999) 234. [14] J.H. Son, T.G. Kim, G.S. Chang, C.N. Whang, J.H. Song, K.H. Chae, Curr. Appl. Phys. 2 (2002) 117. [15] G.S. Chang, Y.P. Lee, J.Y. Rhee, J. Lee, K. Jeong, C.N. Whang, Phys. Rev. Lett. 87 (2001) 67208. [16] S. Kavita, V. Raghavendra Reddy, S. Amirthapandian, Ajay Gupta, B.K. Panigrahi, J. Phys.: Condens. Matter 21 (2009) 96003. [17] Sanjukta Ghosh, M. Mader, R. Grotzschel, A. Gupta, T. Som, Appl. Phys. Lett. 89 (2006) 104104. [18] S. Balaji, S. Amirthapandian, B.K. Panigrahi, S. Kalavathi, Ajay Gupta, K.G.M. Nair, J. Phys.: Condens. Matter 19 (2007) 356211. [19] S. Balaji, S. Amirthapandian, B.K. Panigrahi, S. Kalavathi, G. Mangamma, Ajay Gupta, K.G.M. Nair, A.K. Tyagi, Nucl. Instrum. Methods B 266 (2008) 1692. [20] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in solids, Pergamon, New York, 1985. [21] R.L. Doolittle, Nucl. Instrum. Methods B 12 (1985) 505. [22] F.R. de Boer, R.Boom, W.C.M. Mattens, A.R. Miedema, and A.K. Niessen, Cohesion in Metals, edited by F.R. de Boer and D.Pettifor (North-Holland, Amsterdam) 1988. [23] W. Xia, M. Fernandes, C.A. Hewett, S.S. Lau, D.B. Poker, J. Appl. Phys. 65 (1989) 2300. [24] P.J. Grundy, P.J. Noland, J. Mat. Sci. 7 (1972) 1082. [25] Hajime. Kitagawa, Kimio Hashimoto, Jpn. J. Appl. Phys. 16 (1977) 173. [26] Gary Was, Fundamentals of Radiation Material Science, Springer, 2007. [27] Y.T. Cheng, X.A. Zhao, T. Banwell, T.W. Workman, M.A. Nicolet, W.L. Johnson, J. Appl. Phys. 60 (1986) 2615. [28] F. Rossi, M. Natasi, M. Cohen, C. Olsen, J.C. Tesmer, C. Egen, J. Mat. Res. 6 (1991) 1175. [29] Juliette Tuaillon – Combes, Estela Bernstein, Olivier Boisron and Patrice Melinon J. Phys. Chem. C 114 (2010) 13168.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The mechanism of phase formation in Pt/Co bilayers during ion beam mixing S. Balaji a,⇑, B.K. Panigrahi a, S. Amirthapandian a, S. Kalavathi a, Ajay Gupta b, K.G.M. Nair a a b
Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India
a r t i c l e
i n f o
Article history: Received 19 June 2013 Received in revised form 5 August 2013 Available online 14 August 2013 Keywords: Ion beam mixing CoPt Co3Pt Phase synthesis
a b s t r a c t Ion beam mixing of Pt/Co/Si (Substrate) bilayers were carried out with a 4 MeV Si+ ion beam at two different temperatures – room temperature and 300 °C. When the mixing is carried out at 300 °C, diffusion of Co atoms to the top Pt layer is observed thereby synthesizing CoPt and Co3Pt crystalline phases. When the irradiation is carried out at room temperature, Co and Pt atoms diffuse inwards to the silicon substrate and no new crystalline phase formation is observed. We propose that the direction of movement of the atoms and the resulting concentration profile in the Pt/Co/Si bilayers during ion beam mixing decides the nature of phase synthesized. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Intermetallic cobalt platinum alloys with the chemically ordered L1o structure have been extensively studied due to their hard ferromagnetic properties. In the ordered state, the material has a very high uniaxial magnetocrystalline anisotropy resulting in a high coercivity and this property makes this alloy an attractive candidate for potential technological applications in high density magnetic recording media [1,2]. Transformation from disordered CoPt phase to ordered phase is observed, when the material is annealed at high temperatures [3,4]. However such high temperature annealing is undesirable for manufacturing process [5]. Ion beam annealing is an efficient tool to tailor the magnetic properties of these alloys [6]. With additional energy provided by the ion beams to create defects, diffusion in the material is enhanced, thereby promoting phase transformation at lower temperatures. During ion beam mixing where the interface is bombarded with energetic ions, the atoms get displaced across the interface and the atoms may have enough mobility to migrate and form new alloy phases. Due to high energy transfer by the incident ions, the temperature of the material can rise up to 103 to 104 K locally in a small region of the material and quenched to room temperature within a time period of 10 10 to 10 9 s [7]. The technique of ion irradiation thus results in the formation of metastable compounds irrespective of the kind of elements and atomic stoichiometry [8]. In particular, phases which cannot be synthesized by thermodynamic means can be very well obtained ⇑ Corresponding author. Tel.: +91 44 27480085; fax: +91 44 27480081. E-mail address: [email protected] (S. Balaji). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.08.005
by ion beam mixing process. Owing to the application of ion beam mixing in synthesizing technologically important materials, many models were proposed, to predict the dominant moving species during the mixing process, as the dominant moving species decides the mixing rates and phase formation. Ding et al. [9] suggested that during ion beam mixing, in the radiation enhanced diffusion (RED) regime, the atomic motion of the atoms with smaller atomic radii is enhanced as compared to atoms with larger atomic radii. Tao et al. [10,11] has examined the direction of atomic motion in thin films during ion irradiation and has concluded that atoms from low damage density region tend to move into higher damage density region via inverse Kirkendall effect. To overcome the shortcomings of atomic size effect proposed by Ding et al. [9] and radiation damage density effect proposed by Tao et al. [10] a new model was developed by Jung et al. [12] that predicted the dominant moving species during ion beam mixing. Subsequent ion irradiation experiments performed on bilayer film [13,14] successfully verified the model proposed by Jung et al. [12]. The ion beam mixing of Co/Pt multilayer film by Ar+ ion irradiation in an external magnetic field leads to the formation of CoPt phase with expanded volume due to ultrafast quenching during the cooling phase of the collision cascade [15]. Ion beam mixing carried out on Co/Pt mulilayers [16] and bilayers [17] has resulted in CoPt and CoPt3 phase synthesis. We have reported the formation of ordered CoPt phase by carrying out ion beam mixing in Pt/Co/Si bilayer films [18] and we have also demonstrated that the ion beam mixing in the Pt/Co/Si bilayers using Pt+ ions is diffusion controlled rather than reaction controlled [19]. In this paper, we suggest a mechanism of phase formation during ion beam mixing
S. Balaji et al. / Nuclear Instruments and Methods in Physics Research B 313 (2013) 60–63
in Pt/Co/Si(substrate) bilayers. We propose that, in ion beam mixing of Pt/Co/Si(substrate) bilayers, the direction of movement of atoms and the resulting concentration profile decides the nature of phase synthesized.
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The crystal structure of the phases in the as-prepared and ion irradiated samples were identified with GIXRD and the results are shown in Fig. 1. The diffraction peaks of Pt (fcc) and Co (hcp) phases are clearly observed in the as-prepared sample. The GIXRD pattern of the bilayer sample irradiated with 4 MeV Si+ ions at 300 °C shows the presence of diffraction peaks corresponding to CoPt and Co3Pt phases. When the bilayer sample is irradiated with 4 MeV Si+ ions at room temperature, a decrease in intensity of the diffraction peaks of Pt and Co is observed and there is no other phase identified. Compared to as-prepared sample the peak positions are shifted in the bilayer sample irradiated with 4 MeV Si+ ions at room temperature and the full width at half maximum (FWHM) of the diffraction peaks has increased after irradiation.
The shift in peak position might be due to strain introduced by the ion beam in the lattice and increase in FWHM most likely as a result of decrease in the particle size. The RBS spectra of the as – prepared and ion irradiated samples are shown in Fig. 2. The thickness and composition of the as prepared sample was estimated from the spectra using the RUMP code [21]. The thickness of the platinum and cobalt layers seen in the spectra was found to be 40 nm and 70 nm respectively. In the ion irradiated samples, the mixing at the Pt/Co and Co/Si interfaces is evident from the tailing of the low energy edge of the platinum and cobalt and high energy edges of the cobalt and silicon respectively. The RBS spectra of the bilayer film irradiated with 4 MeV Si+ ion beam at 300 °C was analyzed with RUMP code and the extracted depth profile is shown in Fig. 3. In the region marked 1 in Fig. 3, which is up to 50 nm thickness, the atomic fraction of Pt and Co are 0.41 and 0.59, respectively. While 40 nm thick top Pt layer was present in the as – prepared sample, the atomic fraction of Co has increased up to 0.59 in the same layer after 4 MeV Si+ ion irradiation at 300 °C signifying major out diffusion of Co atoms. In regions marked 2, 3, up to a depth of 100 nm, the atomic fraction of cobalt is nearly 3 times that of the Pt atomic fraction. The region marked 4, 5, 6, 7 etc. corresponds to silicon substrate in the as prepared sample and the substrate after irradiation contains significant concentration of Co, Pt nearly up to the depth of 230 nm from the substrate surface. The RBS spectra of the bilayer sample irradiated with 4 MeV Si+ ion beam at room temperature was analyzed with RUMP code and the extracted depth profile is shown in Fig. 4. Irradiation of the bilayer with 4 MeV Si+ ion beam at room temperature has stimulated inward diffusion of cobalt and platinum atoms up to a depth of 330 nm (Fig. 4) into the silicon substrate reducing the silicon concentration around 1000 keV of back scattered energy in Fig. 2. The 4 MeV Si+ ion irradiation at room temperature does not induce outward diffusion of Co, while a small hump at backscattered energy of 1121 keV in Fig. 2 implies the diffusion of substrate silicon atoms towards top of the film. Ion irradiation of the bilayers at 300 °C with 4 MeV Si+ ion beam promotes the synthesis of CoPt and Co3Pt phases, even if cobalt and platinum silicides are known to have large negative heat of formation as compared to CoPt, Co3Pt [22]. This result is quite intriguing. The reduced role of heat of formation in phase synthesis suggests that, a more fundamental parameter decides the phase formation during ion beam mixing. Effective heat of formation model
Fig. 1. The GIXRD pattern of the (a) as-prepared Pt/Co bilayer, (b) Pt/Co bilayer irradiated with 4 MeV Si+ ions at 300 °C upto a ion fluence of 2 1016 ions/cm2 (c) Pt/Co bilayer irradiated with 4 MeV Si+ ions at room temperature upto a ion fluence of 2 1016 ions/cm2.
Fig. 2. The RBS spectra of the as-prepared Pt/Co bilayer, Pt/Co bilayer irradiated with 4 MeV Si+ ions at 300 °C upto a ion fluence of 2 1016 ions/cm2, Pt/Co bilayer irradiated with 4 MeV Si+ ions at room temperature upto a ion fluence of 2 1016 ions/cm2.
2. Experiment The Pt/Co/Si(1 0 0) [40 nm/70 nm/substrate] bilayer films were deposited by electron beam evaporation at a rate of 0.1 Å/s in a UHV chamber held at a base pressure of 1.07 10 9 mbar. The samples were irradiated with 4 MeV Si+ ions from 1.7 MV Tandetron accelerator up to a ion fluence of 2 1016 ions/cm2 at room temperature and at 300 °C, and the base pressure of the target chamber during ion irradiation was 1.07 10 6 mbar. The range of penetration of 4 MeV Si+ ions in the sample is found to be 2.35 lm using Monte–Carlo simulation SRIM 2003 [20] and hence the implanted Si is deep inside the Si(1 0 0) substrate. Rutherford backscattering (RBS) measurements were carried out using 2 MeV alpha particles from 1.7 MV Tandetron accelerator on asprepared and ion irradiated samples under normal incidence. The backscattered alpha particles were detected using surface barrier silicon detector with a scattering angle of 165° from the incident beam direction. The grazing incidence X-ray diffraction (GIXRD) studies were performed using a STOE GMBH, Germany made X-ray diffractometer and a glancing angle of 0.2° was kept for Cu-Ka (k = 1.541 Å) radiation for all samples. 3. Results and discussion
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Fig. 3. The (a) Pt, (b) Co, (c) Si depth profiles extracted from RUMP code for Pt/Co bilayer irradiated with 4 MeV Si+ ions at 300 °C upto a ion fluence of 2 1016 ions/ cm2.
Fig. 4. The (a) Pt, (b) Co, (c) Si depth profiles extracted from RUMP code for Pt/Co bilayer irradiated with 4 MeV Si+ ions at room temperature upto a ion fluence of 2 1016 ions/cm2.
introduced by Xia et al. [23] successfully predicted the sequence of phase formation in the cobalt silicide system, by proposing that, the nature of phase synthesized during ion beam mixing depends on the heat of formation, and atomic availability at the thin film interface. In our Si+ ion irradiation experiment at 300 °C, at the Co/Si interface, no cobalt silicide phase is synthesized irrespective of the more negative heat of formation for the silicide phases and atomic availability at the interface. Instead CoPt, Co3Pt phases are synthesized in the bilayers. RBS observations suggest that the enhanced Co atomic motion towards top Pt layer synthesizes CoPt, Co3Pt phases without forming silicide phases. The enhanced Co atomic motion towards top platinum layer at 300 °C was predicted by the model put forward by Jung et al. [12] and has been experimentally verified for Co/Pt bilayer film [13]. Two distinct regimes were identified in the model, the atomic transport in the collision cascade regime and the thermal spike regime, were suggested to be in the athermal regime and that in the radiation enhanced diffusion (RED) regime to be in the temperature dependent regime. As per the model, at the Co/Pt interface, in the RED regime, the diffusion of Co atoms into the Pt layer is enhanced, while in the athermal regime the diffusion of Pt atoms and the Co atoms across the interface is almost equal, due to the difference in their cohesive energies [13,14]. However, the model is suitable only for metallic systems and not suitable for semiconductors [13].
The critical temperature for radiation enhanced diffusion to take place at the Pt/Co bilayer interface is about 214 °C [18] and hence, for 4 MeV Si+ ion irradiation at 300 °C, the bilayer interface is subjected to radiation enhanced diffusion. For the top platinum, and substrate silicon atoms, diffusion in only one direction is possible. While top Pt atoms can diffuse into Co and then into Si substrate, substrate Si atoms can diffuse into Co and top Pt layer and this one way diffusion of top Pt and substrate Si atoms is clearly observed in Figs. 2 and 3. But for the case of Co sandwiched between substrate Si and top Pt layer, diffusion in either side is possible. Co atoms can either diffuse into Si substrate to form cobalt silicides or towards top Pt layer to form CoPt, Co3Pt phases. As per the model proposed by Jung et al. [12] the atomic transport at the interface between two metallic thin films depends on their atomic radius, deposited energy by nuclear stopping in the metallic targets and their cohesive energy. For the case of Pt/Co bilayer, there is enhanced motion of Co atoms into Pt layer at 300 °C because of difference in cohesive energy between Co (4.39 eV) and Pt (5.84 eV) and it is experimentally verified [13,14]. The cohesive energy effect is clearly visible in our results also, as we observe a major fraction of Co atoms diffuse into top Pt layer. Subsequent segregation of cobalt atoms in the platinum layer leads to the formation of the CoPt, Co3Pt phases. In the region marked 1 in Fig. 3, the phase corresponds to CoPt while the region marked 2, 3 in the same figure corresponds to Co3Pt phase. Diffraction peaks corresponding to CoPt, Co3Pt phases are thus clearly seen in GIXRD patterns. Although minor fraction of cobalt, platinum atoms diffuse into silicon substrate, we did not observe any XRD peaks for silicide phases and the reason for this result is explained below. In the case of room temperature irradiation, RED is absent. The SRIM results on Pt/Co/Si bilayers (40 nm/70 nm/substrate) (with Ed for Pt, 36 eV, Co, 22 eV and Si, 20 eV) show that Pt recoils into cobalt layer and subsequently into silicon substrate, Co recoils into top Pt layer and Si substrate, and substrate Si recoils into Co and Pt layers when the bilayers are irradiated with 4 MeV Si+ ion beam. At low temperatures – the athermal regime, the higher cohesive energy species preferentially transport into low cohesive energy species at the bilayer metallic interface [13,14]. At the Pt/Co interface, diffusion of Pt atoms and the Co atoms across the interface is almost equal, due the difference in their cohesive energies [13,14]. Thus, Pt atoms diffuse into cobalt layer and reach silicon substrate because of their cohesive energy difference and recoils produced by 4 MeV Si+ ion beam on the Pt/Co bilayer at room temperature, as clearly observed in RUMP simulation shown in Fig. 4. Thermal diffusion coefficient of silicon into cobalt [24] and cobalt into silicon [25] at 300 °C and room temperature is negligibly small. However during ion irradiation, diffusion coefficient can be greatly enhanced and can be very high [26] than that of thermal diffusion coefficient. At the Co/Si interface, the critical temperature (Tc) for radiation enhanced diffusion (RED) process is calculated by using the empirical relation Tc = 95.2 DHcoh (eV/atom) [27,28] and is found to be 156 °C. Hence, at the Co/Si interface, during room temperature irradiation, RED is absent and penetration of silicon into cobalt and platinum, cobalt into silicon, is thus ascribed to ion induced recoils as revealed by SRIM results and it is consistent with our present experiments. Also due to inward displacement of platinum and cobalt, silicon is displaced outside during room temperature irradiation as seen in our RBS results. However for irradiation at 300 °C, the Co/Si interface is subjected to radiation enhanced diffusion. Hence in addition to recoils, the atomic transport across the Co/Si interface possibly could be influenced by RED. During both high temperature and room temperature irradiation, Co and Pt atoms diffuse into Si substrate. However, the GIXRD observation did not show any silicide phases in the bilayers for both room temperature and high temperature irradiation. Since ternary phase diagram for the Co–Pt–Si [29] is not available, it is
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difficult to predict the phases with Co, Pt, Si elemental mixture. However the mixture with Co, Pt, Si elements in our bilayers may contain partially intermixed Pt and Co nano-crystals with inter-diffused Si, some crystalline disorder, possibly solid solution and/or amorphous mixture after irradiation. It is quite interesting to note that, we get almost similar atomic transport results qualitatively at the Pt/Co interface for the case of 4 MeV Si+ ion irradiation on our bilayers as in the case of 80 keV argon ions on the Pt/Co bilayers [13,14]. In the athermal regime, the atomic transport approximately depends on cohesive energy of the elements at the Pt/Co interface [13,14] and hence is independent of the ion beam parameters. In the RED regime-the thermal regime, as in the case of 80 keV argon irradiation on the Pt/Co bilayers [13,14], we note that (Fd)Pt (Fd)Co (11.68 eV/Å/ion) at the bilayer interface for 4 MeV Si+ ion irradiation for our bilayers as per SRIM simulation. Hence, the governing equations given for atomic transport at the Pt/Co interface in references [13,14] give similar results qualitatively, irrespective of the change in ion beam parameters. The only difference to be observed in the present case is that atoms in the bilayer diffuse into silicon substrate during 4 MeV Si+ ion irradiation on the bilayers. As Chang et al. [13] and Son et al. [14] have selected the argon ion beam energy such that the argon ion beam does not reach their substrate, no significant diffusion of atoms in the bilayer, into substrate were reported in their experiments. In our case, the 4 MeV Si+ ion beam penetrates deep inside silicon substrate producing recoils along the path and hence atoms in the bilayer diffuse into silicon substrate. 4. Conclusions The mechanism of phase formation during ion beam mixing in Pt/Co/Si (substrate) bilayers can then be summarized as follows. The direction of movement of the atoms during ion beam mixing is decided by number of factors – temperature, cohesive energy, atomic size, displacement energy and deposited energy by nuclear stopping. In our case, during 4 MeV Si+ ion irradiation on Pt /Co bilayers at 300 °C, due to difference in cohesive energy the Co diffusion towards Pt is enhanced which finally leads to CoPt, Co3Pt phase formation. On the other hand, during 4 MeV Si+ ion irradiation on Pt /Co bilayers at room temperature, cobalt and platinum diffuse into silicon substrate and no new crystalline phase formation is observed. In conclusion, we propose that during ion beam mixing of Pt/Co/Si (substrate) bilayers, the direction of atomic movement and the resultant concentration profile, decides the nature of phase synthesized. We suspect that the above conclusion is true for all ion beam mixing experiments irrespective of kind of
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