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Stoichiometric MgB2 layers produced by multi-energy implantation of boron into magnesium
Surface & Coatings Technology 203 (2009) 2712–2716
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Stoichiometric MgB2 layers produced by multi-energy implantation of boron into magnesium Z. Werner a,b,⁎, W. Szymczyk a, J. Piekoszewski a,c, M.P. Seah d, R. Ratajczak a, L. Nowicki a, M. Barlak a, E. Richter e a
The Andrzej Sołtan Institute for Nuclear Studies, 05-400, Otwock/Świerk, Poland Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-145 Warsaw, Poland d National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom e Forschungszentrum Rossendorf e.V. Institute für Ionenstrahlphysik und Materialforschung, Postfach 510119, D-01314 Dresden, Germany b c
a r t i c l e
i n f o
Available online 9 March 2009 Keywords: Sputtering yield Ion implantation RBS Magnesium diboride Implantation profiles
a b s t r a c t Ion implantation manufacture of superconducting magnesium diboride films of the MgB2 stoichiometry (B: Mg= 2:1 composition) by boron implantation in Mg wafers requires a precise knowledge of the implantation process properties, in particular of the partial sputtering yields of Mg atoms by B ions. To verify these yields experimentally we deposited thin Mg films on glassy carbon platelets and implanted them with high fluences of 40, 60, and 80 keV B+ ions. He-backscattering (RBS) spectrometry was used to determine before- and afterimplantation depth profiles of Mg and B. The sputtering yields turned out to be small enough (<0.1 atoms per ion) to neglect sputtering in simulations of the implanted profiles. The results of the simulations have been compared to RBS spectra recorded on samples treated with 3 energies/fluencies optimised for a wide plateau of the B:Mg = 2:1 stoichiometric composition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ion implantation is one of a number of possible methods of fabricating superconducting MgB2 films. Effectiveness of this technique has been verified experimentally by several authors [1–4]. To our experience a single energy implantation process is not an adequate solution: even attaining the B:Mg = 2:1 composition ratio at a single depth within the implanted ion range does not guarantee continuity (percolation) of the superconducting MgB2 layer. A multi-energy implantation sequence should be used to extend the stoichiometric region over a greater depth. To select the correct energies and fluences needed in subsequent steps of a multi-energy implantation treatment, depth profile modelling is required. The substrate sputtering phenomenon must be included in such modelling. The maximum retained atomic fraction Cmax of the implanted ions (ratio of the retained number of the implanted atoms to the total number of atoms in the implanted layer, i.e. the number of target atoms plus the number of implanted ions) is a function of the projected range Rp and the range straggling ΔRp. For Rp > 3 ΔRp, i.e. when implantation is not too shallow, we have [5]: Cmax =
1 1 + YS
ð1Þ
⁎ Corresponding author. The Andrzej Sołtan Institute for Nuclear Studies, 05-400, Otwock/Świerk, Poland Fax: +48 227793481. E-mail address: [email protected] (Z. Werner). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.103
The problem of sputtering yields was recently re-examined by Seah [6], who improved the previous approaches of Matsunami et al. [7] and Yamamura and Tawara [8], and presented a revised semi-empirical equation for sputtering yields. That work was based on published data for Ne+, Ar+ and Xe+ and was fitted to the best available theory. For the inert gases, the results are estimated to be accurate to within ±10%. Extrapolation to the case of B+ ions implanted into pure Mg substrates and interpolation to the case of Mg+ ions implanted into pure B targets results in the predictions given in Table 1. These data suggest that the B:Mg = 2:1 composition can be obtained for both approaches (implantation of B+ ions into Mg targets and vice versa) although with a rather little margin for B+ ions implanted into Mg targets and only for implantation energies exceeding 20 keV. High-fluence implantations may often lead to accumulation of the implanted ions near the surface. The above calculations may be extended to targets that are composed of a fraction of B and a complementary fraction of Mg in order to calculate the partial sputtering yields. This is done using linear interpolation for the material density and the surface binding energy. Partial sputtering yields of Mg by B+ ions are shown in Fig. 1 for several B+ ion beam energies. Fig. 1 shows the rapid fall in the Mg removal rate with an increase of the boron content. Boron profiles implanted into Mg targets with an intention to form superconductive MgB2 layers have been investigated by only few authors. Peng et al. [2] implanted 1018 cm− 2 of 80 keV B+ ions into Mg. They concluded that, in the centre of the B distribution (located at a depth of about 250 nm), the B concentration was about 75%, whereas the Mg concentration was below 5%. The same authors [3] implanted
Z. Werner et al. / Surface & Coatings Technology 203 (2009) 2712–2716 Table 1 Yields of sputtering of Mg by B+ ions and of B by Mg+ ions ([6]) and respective Cmax values calculated from Eq. (1). B+ into Mg
Mg+ into B
Energy, keV
Sputter yield
Cmax (%)
Sputter yield
Cmax (%)
20 40 60 80
0.59 0.40 0.31 0.25
63 71 77 80
0.90 0.69 0.55 0.47
53 59 65 68
1018 cm− 2 of B+ ions at 80 keV and 2 · 1018 cm− 2 of B+ ions at 120 keV into Mg and concluded that to obtain the atomic concentration of 6.8 · 1022 cm− 3 of boron atoms in MgB2 in the centre of the profile, the fluence of 80 keV B+ ions is 1.9 · 1018 cm− 2 and reaches the maximum at 400 nm. They did not address the question how the high doses affect the target density. Piekoszewski et al. [4] implanted 5 · 1018 cm− 2 of B+ ions at 80 keV into Mg. They concluded that, directly after implantation, boron is present as a 0.5 μm thick buried layer, containing no less than 75% of B and no more than 25% of Mg in the profile peak. These values significantly exceed the stoichiometric condition. In view of the above data, we decided to confirm the effective sputtering yield in the (Mg, B) system, experimentally, and to validate the reliability of our modelling programs. Since most of the experimental data obtained so far concern implantation of B+ ions into Mg targets, that approach was adopted in this work. We determined the partial sputtering yield YSB → Mg for high fluences of B+ ions implanted into magnesium. With these data we attempted to predict the multienergy, multi-fluence implantation process parameters necessary to form broad layers with the fixed B:Mg = 2:1 concentration ratio. The predictions have been compared with boron profiles deduced from the measured RBS spectra. 2. Experimental The sputtering yield of magnesium by boron ions was determined from the thickness reduction of thin Mg films. For this purpose, 550 nm thick Mg films were vacuum-evaporated onto glassy carbon platelets. This film thickness was selected as a compromise between the layer which must be thin enough to detect its thickness reduction by boroninduced sputtering, even at the expected lower limit of YSB → Mg assumed in advance to be 0.25 and, on the other hand, thick enough to avoid its complete sputtering away, even at the expected upper limit of YSB → Mg assumed (with some safety margin) to be 0.75. The samples were implanted at energies of 40, 60, and 80 keV and fluences of 1.9, 2.4 and 3.0 × 1018 B+ ions/cm2. Next, RBS spectra were recorded using 2 MeV He+ ions. The RBS spectra were analysed using the RUMP code [9]. The code linearly combines stopping powers of individual target atoms (Bragg rule) and calculates the contributions to the RBS spectra of a number of layers defined within the sample by the user in terms of their thickness (expressed in [at. cm− 2] — a unit of area concentration of atoms) and their elemental fractions (all elements taken into account and their content expressed in %). The parameters are varied until the RBS spectrum resulting from the simulations satisfactorily fits that measured one (the trial-and-error method). The boron and magnesium profiles obtained this way were next used to calculate the total content of magnesium left after implantation and the total amount of boron implanted into the sample. These values, in turn allowed us to determine the sputtering yields. The accuracy of the determination of the above values depends of several factors, the most important being: 1. Inaccuracy of the Nprojectiles · Ω product (where Ω is the detector solid angle). This product was determined using an efficiency standard. In our case this was the signal of pure carbon originating from the layer substrate. During the measurements this signal was
2713
consistently constant within ±2%. This number is an upper estimation of inaccuracy and is based upon the results of 7 measurements, which gave the value of standard deviation as 0.54% and the maximum deviation of 1.1%. The possibility of using the C substrate signal to calibrate the detector efficiency appeared very useful since it allowed us both to eliminate casual errors associated with possible current leakage and to determine the accumulated charge of the projectiles. 2. Uncertainty of the scattering cross-section. The RUMP software used for the analysis of RBS spectra ignores deviations from the Rutherford cross-section which may be significant for scattering on light nuclei. In the case of Mg bombarded with 2 MeV He+, no deviation from the Rutherford cross-section was observed [10]. 3. Uncertainty of the stopping power vs. energy relation. The effect of this factor was examined by performing calculations of the aggregated Mg signal as a function of the stopping power. It has been found that a stopping power change by 10% (greater inaccuracy seems unlikely) leads to a change of Mg signal by 1.3%. 4. Uncertainty of fitting the simulated and measured spectra. This factor has been eliminated by multiplying the obtained numbers of Mg and B atoms by a correction factor equal to the ratio of the integrated experimental Mg peak to the integrated simulated one. This factor did not exceed 0.6%. Thus, each of the determined numbers of atoms after correction bears an uncertainty being a sum of the previously discussed three factors and amounts to 2% + 0% + 1.3% = 3.3%. Such accuracy is sufficiently good to determine sputtering yields down to a value of about 0.05 at the adopted B+ ion fluencies. 3. Results 3.1. Sputtering yield Fig. 2 presents RBS spectra of B-implanted Mg thin films compared to RBS spectra of a non-implanted sample. The peak on the right arises from the 550 nm thick Mg layer covering the glassy carbon substrate. The peak of the substrate appears to the left. As the implantation proceeds, boron is deposited into the top of the Mg layer and the layer is seen to grow in depth with the Mg signal minimising at about a third of the original pure Mg one for the 80 keV implant. At the same time, the substrate carbon peak is seen to move further left to greater depths and the boron peak appears to its right forming a shoulder. The results of fitting the above spectra with the RUMP code are presented in Fig. 3. Quantitative results of RUMP analyses are summarised in Table 2 as the amount of magnesium before and after implantation, the nominal
Fig. 1. Partial yields of sputtering of Mg in samples with different Mg atomic fractions for different B+ beam energies.
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Z. Werner et al. / Surface & Coatings Technology 203 (2009) 2712–2716 Table 2 Surface concentration of Mg atoms before and after implantation, and surface concentration of implanted B atoms, as calculated by the RUMP analysis. Energy Surface concentration Nominal B+ [keV] of Mg before fluence implantation [1015 cm− 2] [1015 cm− 2]
Surface concentration Surface of Mg after concentration implantation of B [1015 cm− 2] [1015 cm− 2]
40 60 80
2581 2372 2478
2688 2485 2538
1900 2400 3000
1769 2679 3297
We calculate the effective sputtering yields of Mg by B ions YSB → Mg as: B→Mg
YS Fig. 2. Typical RBS spectra of un-implanted and B-implanted Mg samples at nominal boron fluences of 1.9× 1018 cm− 2 at 40 keV, 2.4 × 1018 cm− 2 at 60 keV and 3.0 × 1018 cm− 2 at 80 keV.
fluence of boron, and the amount of implanted boron. Surface concentrations have been calculated as sums of area concentrations taken over all individual layers defined for the RUMP analyses. The Mg films contain about 5 at.% of oxygen but generally the problem of their oxidation seems to be less severe than expected from the known high reactivity of magnesium. In the 40 and 80 keV ion implanted samples, the oxygen content seems to be reduced after implantation. Taking into account that the Mg content did not decrease significantly even at high boron fluencies, this result may mean that oxygen is selectively removed from the surface layer. In two cases out of three, the retained B fluence is higher than the nominal one, suggesting some inaccuracy of the ion current integrating equipment of our implanter.
=
Mgunimpl − Mgimpl B
ð2Þ
where Mgunimpl and Mgimpl are the Mg contents in the sample before and after implantation, respectively, B is the boron ion fluence. The uncertainty here is dominated by the uncertainties of Mgunimpl and Mgimpl and the uncertainty of B may be ignored. The calculated yields are 0.039, 0.090 and 0.088, at 40, 60, and 80 keV, respectively. Inspection of Fig. 3 reveals that the Mg atomic fractions in the surface zone are approximately 36%, 43% and 66% for 40, 60 and 80 keV implantation energies, respectively. The theoretical magnesium partial sputtering yields (deduced from Fig. 1 for the above Mg contents), together with the experimental data (RUMP-deduced from RBS spectra shown in Fig. 2 and summarized in Table 2) are compared in Fig. 4. The error bars indicated in the figure for the experimental partial sputtering yields represent the upper limit resulting from the uncertainty of the RUMP simulation of the Mg and B content in the layer. The uncertainties of the theoretical values leading to uncertainty of the compositional profile obtained from RBS data amount to approximately ±20% of the deduced sputtering values. 3.2. Boron profiles in magnesium targets Depth profiles of the implanted ions are usually simulated using the well-known SRIM code [11] employing the Monte-Carlo technique. Even if some errors regarding sputtering yields were reported some time ago [12], in view of a negligible sputtering yield in our case it seems that the code may be safely used to calculate stopping powers and ion ranges, i.e. to simulate depth profiles [13]. In our “pure Mg” approach, we ignore effects of the presence of the implanted boron atoms in the Mg lattice. We assume that the lattice does not expand and that boron ions occupy interstitial positions without changing the distances between the magnesium atoms. Next, we add the local boron, CB, and local magnesium, CMg, atomic concentrations (CMg = 4.37 1022 cm− 3 is constant by virtue of the
Fig. 3. B, Mg, C and O profiles in Mg thin-film samples evaporated on carbon substrates calculated from the RBS data using the RUMP code. The results are expressed in RUMPspecific units, in particular the abscissa shows atomic concentration in at./cm− 2 (rather than in µm as used by the SRIM code, see Fig. 6). Top-left: un-implanted sample. The other plots show samples implanted with B+ ions at 40, 60 and 80 keV, respectively.
Fig. 4. The experimental values (RUMP-deduced from RBS spectra, solid circles) and theoretical values (open diamonds) of partial sputtering yield of magnesium atoms by boron ions. Error bars represent the upper limit of RUMP fitting inaccuracy.
Z. Werner et al. / Surface & Coatings Technology 203 (2009) 2712–2716
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Fig. 7. RBS results of the Mg sample implanted as described in the text and subsequently pulse-annealed (thin line), superimposed on the simulated spectrum, which would originate from a pure MgB2 film (bold line).
Fig. 5. Boron profiles simulated for the three different energies and fluencies used in the present experiment. Smooth lines: SRIM simulation, assuming pure Mg targets. Histograms: boron profiles RUMP-deduced from the measured RBS spectra.
assumption that the lattice does not change), and we convert the depth scale expressed in nm into the surface concentration scale expressed in cm− 2. Finally we plot the boron atomic fraction, CB / (CB + CMg), as a function of the surface concentration. Boron profiles simulated by the SRIM program for the three different energies and fluencies used in the present experiment have been compared in Fig. 5 with boron profiles deduced from the measured RBS spectra. All profiles are plotted as fractions of boron atoms vs. surface concentration of atoms. In simulations we assumed
fluencies derived from the RUMP analysis rather than the nominal fluencies. These results allow us to simulate effects of multi-energy ion implantation processes applied to produce flat and broad distributions of boron. For practical reasons we have arbitrarily limited the number of energies used in the processes to 3. The results of one of the simulations performed at energies 80, 60, and 40 keV are shown in Fig. 6. The expected boron plateau (FWHM) should extend from the depth of about 140 nm to a depth of about 440 nm, i.e. should be approximately 300 nm wide. To demonstrate the ability to design implantation treatment leading to stoichiometric MgB2 layers, we present in Fig. 7 the RBS results of the Mg sample implanted as above and subsequently pulseannealed [14] superimposed on a simulated spectrum of a 410 nmthick pure MgB2 film deposited on Mg. We may see that a B:Mg = 2:1 composition layer has indeed been produced. 4. Conclusions The measured values of the effective partial sputtering yield of magnesium by boron ions for energies 40, 60, and 80 keV were 0.039, 0.090 and 0.088, respectively. They agree with the calculations of the partial sputtering yield within the measurement uncertainty. Such small values indicate that the sputtering effects may be ignored in calculations of high-fluence boron profiles in magnesium targets. SRIM simulations, made using the assumption that the implanted boron ions do not affect the target, agree fairly well with profiles deduced from RBS data using the RUMP code. This information is crucial for a reliable optimization of the multi-energy implantation processes parameters required to form thick B:Mg = 2:1 composition layers which are described in our recent publication [15]. Acknowledgments This work was partially supported by the Polish Ministry of Science and Higher Education grant No. 3T08C 01229. Part of the experiment was performed at AIM Rossendorf within the frames of RITA project (Research Infrastructures Transnational Access Contract Number 025646). References
Fig. 6. SRIM-simulated boron profile produced by triple-energy implantation optimized for B:Mg = 2:1 composition in a magnesium target. B+ ions energies were selected as 80, 60, and 40 keV.
[1] H.Y. Zhai, H.M. Christen, C.W. White, J.D. Budai, D.H. Lowndes, Appl. Phys. Lett. 80 (2002) 4786. [2] N. Peng, G. Shao, C. Jeynes, R.P. Webb, R.M. Gwilliam, G. Boudreault, D.M. Astill, W. Y. Liang, Appl. Phys. Lett. 82 (2003) 236. [3] N. Peng, C. Jeynes, R.M. Gwilliam, K.J. Kirkby, R.P. Webb, G. Shao, D.M. Astill, W.Y. Liang, IEEE Trans. Appl. Supercond. 15 (2005) 3265.
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Z. Werner et al. / Surface & Coatings Technology 203 (2009) 2712–2716
[4] J. Piekoszewski, W. Kempinski, B. Andrzejewski, Z. Trybula, L. Piekara-Sady, J. Kaszynski, J. Stankowski, Z. Werner, E. Richter, F. Prokert, J. Stanislawski, M. Barlak, Vacuum 78 (2005) 123. [5] H. Ryssel, H. Glawischnig (Eds.), Ion Implantation Techniques, Springer-Verlag, Berlin, 1982, p. 187, Heidelberg New York. [6] M.P. Seah, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 229 (2005) 348. [7] N. Matsunami, Y. Yamamura, Y. Hikawa, N. Itoh, Y. Kazumata, S. Miyagawa, K. Morita, R. Shimizu, H. Tawara, Atom Data Nucl. Data Tables 31 (1984) 1. [8] Y. Yamamura, H. Tawara, Atom Data Nucl. Data Tables 62 (1996) 149.
[9] RUMP code is commercially available from www.genplot.com. [10] J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Materials Analysis, Materials Research Society, Pittsburg, 1995. [11] SRIM 2006 code available as a freeware at http://www.srim.org. [12] K. Wittmaack, J. Appl. Phys. 96 (2004) 2632. [13] Jim Ziegler, private communication. [14] J. Piekoszewski, J. Langner, Nucl. Instrum. Methods B53 (1991) 148. [15] J. Piekoszewski, W. Kempiński, M. Barlak Z. Werner Sz. Łoś, B. Andrzejewski,. J. Stankowski, L. Piekara-Sady , E. Składnik-Sadowska, A. Kolitsch, R. Grötzschel, this conference.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Stoichiometric MgB2 layers produced by multi-energy implantation of boron into magnesium Z. Werner a,b,⁎, W. Szymczyk a, J. Piekoszewski a,c, M.P. Seah d, R. Ratajczak a, L. Nowicki a, M. Barlak a, E. Richter e a
The Andrzej Sołtan Institute for Nuclear Studies, 05-400, Otwock/Świerk, Poland Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-145 Warsaw, Poland d National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom e Forschungszentrum Rossendorf e.V. Institute für Ionenstrahlphysik und Materialforschung, Postfach 510119, D-01314 Dresden, Germany b c
a r t i c l e
i n f o
Available online 9 March 2009 Keywords: Sputtering yield Ion implantation RBS Magnesium diboride Implantation profiles
a b s t r a c t Ion implantation manufacture of superconducting magnesium diboride films of the MgB2 stoichiometry (B: Mg= 2:1 composition) by boron implantation in Mg wafers requires a precise knowledge of the implantation process properties, in particular of the partial sputtering yields of Mg atoms by B ions. To verify these yields experimentally we deposited thin Mg films on glassy carbon platelets and implanted them with high fluences of 40, 60, and 80 keV B+ ions. He-backscattering (RBS) spectrometry was used to determine before- and afterimplantation depth profiles of Mg and B. The sputtering yields turned out to be small enough (<0.1 atoms per ion) to neglect sputtering in simulations of the implanted profiles. The results of the simulations have been compared to RBS spectra recorded on samples treated with 3 energies/fluencies optimised for a wide plateau of the B:Mg = 2:1 stoichiometric composition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ion implantation is one of a number of possible methods of fabricating superconducting MgB2 films. Effectiveness of this technique has been verified experimentally by several authors [1–4]. To our experience a single energy implantation process is not an adequate solution: even attaining the B:Mg = 2:1 composition ratio at a single depth within the implanted ion range does not guarantee continuity (percolation) of the superconducting MgB2 layer. A multi-energy implantation sequence should be used to extend the stoichiometric region over a greater depth. To select the correct energies and fluences needed in subsequent steps of a multi-energy implantation treatment, depth profile modelling is required. The substrate sputtering phenomenon must be included in such modelling. The maximum retained atomic fraction Cmax of the implanted ions (ratio of the retained number of the implanted atoms to the total number of atoms in the implanted layer, i.e. the number of target atoms plus the number of implanted ions) is a function of the projected range Rp and the range straggling ΔRp. For Rp > 3 ΔRp, i.e. when implantation is not too shallow, we have [5]: Cmax =
1 1 + YS
ð1Þ
⁎ Corresponding author. The Andrzej Sołtan Institute for Nuclear Studies, 05-400, Otwock/Świerk, Poland Fax: +48 227793481. E-mail address: [email protected] (Z. Werner). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.103
The problem of sputtering yields was recently re-examined by Seah [6], who improved the previous approaches of Matsunami et al. [7] and Yamamura and Tawara [8], and presented a revised semi-empirical equation for sputtering yields. That work was based on published data for Ne+, Ar+ and Xe+ and was fitted to the best available theory. For the inert gases, the results are estimated to be accurate to within ±10%. Extrapolation to the case of B+ ions implanted into pure Mg substrates and interpolation to the case of Mg+ ions implanted into pure B targets results in the predictions given in Table 1. These data suggest that the B:Mg = 2:1 composition can be obtained for both approaches (implantation of B+ ions into Mg targets and vice versa) although with a rather little margin for B+ ions implanted into Mg targets and only for implantation energies exceeding 20 keV. High-fluence implantations may often lead to accumulation of the implanted ions near the surface. The above calculations may be extended to targets that are composed of a fraction of B and a complementary fraction of Mg in order to calculate the partial sputtering yields. This is done using linear interpolation for the material density and the surface binding energy. Partial sputtering yields of Mg by B+ ions are shown in Fig. 1 for several B+ ion beam energies. Fig. 1 shows the rapid fall in the Mg removal rate with an increase of the boron content. Boron profiles implanted into Mg targets with an intention to form superconductive MgB2 layers have been investigated by only few authors. Peng et al. [2] implanted 1018 cm− 2 of 80 keV B+ ions into Mg. They concluded that, in the centre of the B distribution (located at a depth of about 250 nm), the B concentration was about 75%, whereas the Mg concentration was below 5%. The same authors [3] implanted
Z. Werner et al. / Surface & Coatings Technology 203 (2009) 2712–2716 Table 1 Yields of sputtering of Mg by B+ ions and of B by Mg+ ions ([6]) and respective Cmax values calculated from Eq. (1). B+ into Mg
Mg+ into B
Energy, keV
Sputter yield
Cmax (%)
Sputter yield
Cmax (%)
20 40 60 80
0.59 0.40 0.31 0.25
63 71 77 80
0.90 0.69 0.55 0.47
53 59 65 68
1018 cm− 2 of B+ ions at 80 keV and 2 · 1018 cm− 2 of B+ ions at 120 keV into Mg and concluded that to obtain the atomic concentration of 6.8 · 1022 cm− 3 of boron atoms in MgB2 in the centre of the profile, the fluence of 80 keV B+ ions is 1.9 · 1018 cm− 2 and reaches the maximum at 400 nm. They did not address the question how the high doses affect the target density. Piekoszewski et al. [4] implanted 5 · 1018 cm− 2 of B+ ions at 80 keV into Mg. They concluded that, directly after implantation, boron is present as a 0.5 μm thick buried layer, containing no less than 75% of B and no more than 25% of Mg in the profile peak. These values significantly exceed the stoichiometric condition. In view of the above data, we decided to confirm the effective sputtering yield in the (Mg, B) system, experimentally, and to validate the reliability of our modelling programs. Since most of the experimental data obtained so far concern implantation of B+ ions into Mg targets, that approach was adopted in this work. We determined the partial sputtering yield YSB → Mg for high fluences of B+ ions implanted into magnesium. With these data we attempted to predict the multienergy, multi-fluence implantation process parameters necessary to form broad layers with the fixed B:Mg = 2:1 concentration ratio. The predictions have been compared with boron profiles deduced from the measured RBS spectra. 2. Experimental The sputtering yield of magnesium by boron ions was determined from the thickness reduction of thin Mg films. For this purpose, 550 nm thick Mg films were vacuum-evaporated onto glassy carbon platelets. This film thickness was selected as a compromise between the layer which must be thin enough to detect its thickness reduction by boroninduced sputtering, even at the expected lower limit of YSB → Mg assumed in advance to be 0.25 and, on the other hand, thick enough to avoid its complete sputtering away, even at the expected upper limit of YSB → Mg assumed (with some safety margin) to be 0.75. The samples were implanted at energies of 40, 60, and 80 keV and fluences of 1.9, 2.4 and 3.0 × 1018 B+ ions/cm2. Next, RBS spectra were recorded using 2 MeV He+ ions. The RBS spectra were analysed using the RUMP code [9]. The code linearly combines stopping powers of individual target atoms (Bragg rule) and calculates the contributions to the RBS spectra of a number of layers defined within the sample by the user in terms of their thickness (expressed in [at. cm− 2] — a unit of area concentration of atoms) and their elemental fractions (all elements taken into account and their content expressed in %). The parameters are varied until the RBS spectrum resulting from the simulations satisfactorily fits that measured one (the trial-and-error method). The boron and magnesium profiles obtained this way were next used to calculate the total content of magnesium left after implantation and the total amount of boron implanted into the sample. These values, in turn allowed us to determine the sputtering yields. The accuracy of the determination of the above values depends of several factors, the most important being: 1. Inaccuracy of the Nprojectiles · Ω product (where Ω is the detector solid angle). This product was determined using an efficiency standard. In our case this was the signal of pure carbon originating from the layer substrate. During the measurements this signal was
2713
consistently constant within ±2%. This number is an upper estimation of inaccuracy and is based upon the results of 7 measurements, which gave the value of standard deviation as 0.54% and the maximum deviation of 1.1%. The possibility of using the C substrate signal to calibrate the detector efficiency appeared very useful since it allowed us both to eliminate casual errors associated with possible current leakage and to determine the accumulated charge of the projectiles. 2. Uncertainty of the scattering cross-section. The RUMP software used for the analysis of RBS spectra ignores deviations from the Rutherford cross-section which may be significant for scattering on light nuclei. In the case of Mg bombarded with 2 MeV He+, no deviation from the Rutherford cross-section was observed [10]. 3. Uncertainty of the stopping power vs. energy relation. The effect of this factor was examined by performing calculations of the aggregated Mg signal as a function of the stopping power. It has been found that a stopping power change by 10% (greater inaccuracy seems unlikely) leads to a change of Mg signal by 1.3%. 4. Uncertainty of fitting the simulated and measured spectra. This factor has been eliminated by multiplying the obtained numbers of Mg and B atoms by a correction factor equal to the ratio of the integrated experimental Mg peak to the integrated simulated one. This factor did not exceed 0.6%. Thus, each of the determined numbers of atoms after correction bears an uncertainty being a sum of the previously discussed three factors and amounts to 2% + 0% + 1.3% = 3.3%. Such accuracy is sufficiently good to determine sputtering yields down to a value of about 0.05 at the adopted B+ ion fluencies. 3. Results 3.1. Sputtering yield Fig. 2 presents RBS spectra of B-implanted Mg thin films compared to RBS spectra of a non-implanted sample. The peak on the right arises from the 550 nm thick Mg layer covering the glassy carbon substrate. The peak of the substrate appears to the left. As the implantation proceeds, boron is deposited into the top of the Mg layer and the layer is seen to grow in depth with the Mg signal minimising at about a third of the original pure Mg one for the 80 keV implant. At the same time, the substrate carbon peak is seen to move further left to greater depths and the boron peak appears to its right forming a shoulder. The results of fitting the above spectra with the RUMP code are presented in Fig. 3. Quantitative results of RUMP analyses are summarised in Table 2 as the amount of magnesium before and after implantation, the nominal
Fig. 1. Partial yields of sputtering of Mg in samples with different Mg atomic fractions for different B+ beam energies.
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Z. Werner et al. / Surface & Coatings Technology 203 (2009) 2712–2716 Table 2 Surface concentration of Mg atoms before and after implantation, and surface concentration of implanted B atoms, as calculated by the RUMP analysis. Energy Surface concentration Nominal B+ [keV] of Mg before fluence implantation [1015 cm− 2] [1015 cm− 2]
Surface concentration Surface of Mg after concentration implantation of B [1015 cm− 2] [1015 cm− 2]
40 60 80
2581 2372 2478
2688 2485 2538
1900 2400 3000
1769 2679 3297
We calculate the effective sputtering yields of Mg by B ions YSB → Mg as: B→Mg
YS Fig. 2. Typical RBS spectra of un-implanted and B-implanted Mg samples at nominal boron fluences of 1.9× 1018 cm− 2 at 40 keV, 2.4 × 1018 cm− 2 at 60 keV and 3.0 × 1018 cm− 2 at 80 keV.
fluence of boron, and the amount of implanted boron. Surface concentrations have been calculated as sums of area concentrations taken over all individual layers defined for the RUMP analyses. The Mg films contain about 5 at.% of oxygen but generally the problem of their oxidation seems to be less severe than expected from the known high reactivity of magnesium. In the 40 and 80 keV ion implanted samples, the oxygen content seems to be reduced after implantation. Taking into account that the Mg content did not decrease significantly even at high boron fluencies, this result may mean that oxygen is selectively removed from the surface layer. In two cases out of three, the retained B fluence is higher than the nominal one, suggesting some inaccuracy of the ion current integrating equipment of our implanter.
=
Mgunimpl − Mgimpl B
ð2Þ
where Mgunimpl and Mgimpl are the Mg contents in the sample before and after implantation, respectively, B is the boron ion fluence. The uncertainty here is dominated by the uncertainties of Mgunimpl and Mgimpl and the uncertainty of B may be ignored. The calculated yields are 0.039, 0.090 and 0.088, at 40, 60, and 80 keV, respectively. Inspection of Fig. 3 reveals that the Mg atomic fractions in the surface zone are approximately 36%, 43% and 66% for 40, 60 and 80 keV implantation energies, respectively. The theoretical magnesium partial sputtering yields (deduced from Fig. 1 for the above Mg contents), together with the experimental data (RUMP-deduced from RBS spectra shown in Fig. 2 and summarized in Table 2) are compared in Fig. 4. The error bars indicated in the figure for the experimental partial sputtering yields represent the upper limit resulting from the uncertainty of the RUMP simulation of the Mg and B content in the layer. The uncertainties of the theoretical values leading to uncertainty of the compositional profile obtained from RBS data amount to approximately ±20% of the deduced sputtering values. 3.2. Boron profiles in magnesium targets Depth profiles of the implanted ions are usually simulated using the well-known SRIM code [11] employing the Monte-Carlo technique. Even if some errors regarding sputtering yields were reported some time ago [12], in view of a negligible sputtering yield in our case it seems that the code may be safely used to calculate stopping powers and ion ranges, i.e. to simulate depth profiles [13]. In our “pure Mg” approach, we ignore effects of the presence of the implanted boron atoms in the Mg lattice. We assume that the lattice does not expand and that boron ions occupy interstitial positions without changing the distances between the magnesium atoms. Next, we add the local boron, CB, and local magnesium, CMg, atomic concentrations (CMg = 4.37 1022 cm− 3 is constant by virtue of the
Fig. 3. B, Mg, C and O profiles in Mg thin-film samples evaporated on carbon substrates calculated from the RBS data using the RUMP code. The results are expressed in RUMPspecific units, in particular the abscissa shows atomic concentration in at./cm− 2 (rather than in µm as used by the SRIM code, see Fig. 6). Top-left: un-implanted sample. The other plots show samples implanted with B+ ions at 40, 60 and 80 keV, respectively.
Fig. 4. The experimental values (RUMP-deduced from RBS spectra, solid circles) and theoretical values (open diamonds) of partial sputtering yield of magnesium atoms by boron ions. Error bars represent the upper limit of RUMP fitting inaccuracy.
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Fig. 7. RBS results of the Mg sample implanted as described in the text and subsequently pulse-annealed (thin line), superimposed on the simulated spectrum, which would originate from a pure MgB2 film (bold line).
Fig. 5. Boron profiles simulated for the three different energies and fluencies used in the present experiment. Smooth lines: SRIM simulation, assuming pure Mg targets. Histograms: boron profiles RUMP-deduced from the measured RBS spectra.
assumption that the lattice does not change), and we convert the depth scale expressed in nm into the surface concentration scale expressed in cm− 2. Finally we plot the boron atomic fraction, CB / (CB + CMg), as a function of the surface concentration. Boron profiles simulated by the SRIM program for the three different energies and fluencies used in the present experiment have been compared in Fig. 5 with boron profiles deduced from the measured RBS spectra. All profiles are plotted as fractions of boron atoms vs. surface concentration of atoms. In simulations we assumed
fluencies derived from the RUMP analysis rather than the nominal fluencies. These results allow us to simulate effects of multi-energy ion implantation processes applied to produce flat and broad distributions of boron. For practical reasons we have arbitrarily limited the number of energies used in the processes to 3. The results of one of the simulations performed at energies 80, 60, and 40 keV are shown in Fig. 6. The expected boron plateau (FWHM) should extend from the depth of about 140 nm to a depth of about 440 nm, i.e. should be approximately 300 nm wide. To demonstrate the ability to design implantation treatment leading to stoichiometric MgB2 layers, we present in Fig. 7 the RBS results of the Mg sample implanted as above and subsequently pulseannealed [14] superimposed on a simulated spectrum of a 410 nmthick pure MgB2 film deposited on Mg. We may see that a B:Mg = 2:1 composition layer has indeed been produced. 4. Conclusions The measured values of the effective partial sputtering yield of magnesium by boron ions for energies 40, 60, and 80 keV were 0.039, 0.090 and 0.088, respectively. They agree with the calculations of the partial sputtering yield within the measurement uncertainty. Such small values indicate that the sputtering effects may be ignored in calculations of high-fluence boron profiles in magnesium targets. SRIM simulations, made using the assumption that the implanted boron ions do not affect the target, agree fairly well with profiles deduced from RBS data using the RUMP code. This information is crucial for a reliable optimization of the multi-energy implantation processes parameters required to form thick B:Mg = 2:1 composition layers which are described in our recent publication [15]. Acknowledgments This work was partially supported by the Polish Ministry of Science and Higher Education grant No. 3T08C 01229. Part of the experiment was performed at AIM Rossendorf within the frames of RITA project (Research Infrastructures Transnational Access Contract Number 025646). References
Fig. 6. SRIM-simulated boron profile produced by triple-energy implantation optimized for B:Mg = 2:1 composition in a magnesium target. B+ ions energies were selected as 80, 60, and 40 keV.
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