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Depth profiling of nickel using the nuclear resonant 58Ni(p,γ)59Cu reaction: Application to NiFe mixed bilayers and Ni-implanted iron
182
Nuclear Instruments and Methods in Physics Research B52 (1990) 182-186 North-Holland
Depth profiling of nickel using the nuclear resonant 58Ni(p,y)59C~ reaction: application to Ni-Fe mixed bilayers and Ni-implanted iron M.A. El Khakani, G. Marest, N. Moncoffre and J. Tousset Institut de Physique NuclPaire de Lyon, INZM-CNRS et Universitk Claude Bernard, 43, Bd du II Novembre 1918 - F-69622 Villeurbanne Cedex, France Received 13 July 1990
The nuclear resonant 58Ni(p,y)59Cu reaction at Ep =1424 keV was used to determine nickel depth distributions in iron-nickel based alloys. y radiations were detected by using a high efficiency Ge detector. For quantifying the reaction yield, the y transitions of EY = 4.82 MeV, 4.33 MeV and 0.491 MeV energies were considered. This technique allows nickel to be profiled with a depth resolution of about 10 nm at the surface of iron matrix. Applications of the method to Ni-Fe ion beam mixed bilayers and N&implanted iron are presented.
1. Introduction Nickel is an alloying element of high technological interest. When an attempt is made to change the superficial properties of materials, ion implantation is a suitable technique. For example Ni-Al superficial alloy was obtained without any adhesion problems, by implanting nickel into aluminium [3]. Ion beam mixing of Ni-Ag, Ni-Sn and Fe-Ni binary systems was studied [1,4,5] in order to understand ion beam bombardment inducing new phase formation. In these studies, the determination of the nickel depth profiles or of the near surface nickel composition is of prime importance to understanding the physical processes. For this purpose, Auger electron spectroscopy was used to profile nickel with some difficulties in converting sputtering time into depth scale [4,5]. However, when samples are to be saved for further investigations only non-destructive techniques can be used. Backscattering spectrometry (RBS) has been recently used to follow the ion mixing of Fe-Ni bilayers after argon or krypton bombardment [l]. Due to the small mass difference between iron and nickel and to the presence of their isotopes in natural Ni-Fe samples, RBS is not adapted to provide quantitative measurements. However in the case of the 64Ni isotope implanted into monoisotopic 57Fe thin films, 64Ni depth profiles were obtained using 5.7 MeV energy He*+ articles [6]. This high energy was required to separate ‘Ni and 57Fe signals in the RBS spectrum allowing quantitative measurements with reduced depth resolution. 0168-583X/90/$03.50
A nickel profiling technique using the 58Ni(p,y)59Cu resonant nuclear reaction at 1424 keV is proposed. Gossett has already reported the possibility to profile nickel using this resonant proton capture reaction, with the restriction to quantify the reaction yield with the first excited state transition [2] (see fig. 1). In this paper, it is shown that with a high efficiency Ge detector it is possible to quantify the yield with all the transitions (4.82, 4.33 and 0.491 MeV y-rays) involved in the reaction, leading to reduced acquisition time. Angular distributions were carried out in order to optimize the detection geometry. Applications to nickel profiling are shown in the case of mixed Ni-Fe bilayers and 58Ni-implanted into iron.
Energy
Fig.1.
Level 4.82
Simplified scheme of y transitions involved in the 58Ni(p,y)59Cu resonant reaction at E, = 1424 keV.
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
MA El Khakani et ai. / Depth ~r~~iing of nickel
Resonant proton capture reactions (p,y), successfully used for profiling low 2 elements, are also useful to profile some elements up to the transition region [2]. The existence of analogue resonances (AR) has allowed to profile titanium into steels using the 48Ti(p,y)49V AR at 1362 keV [2,10]. Due to its high 2, nickel is probably the upper limit element which can be profiled using proton capture reaction. A relatively high proton energy is also required to find an isolated and intense resonance [2]. The excitation function established by Bulter and Gossett shows a lot of resonances in the region studied [7]. For 58Ni the lowest one appears at 855 keV proton energy but the well isolated and most intense resonance is located at 1424 keV, between two less intense resonances at 1376 and 1522 keV. The 1424 keV resonance corresponds to a state in 5gCu at 4.817 MeV. The upper limit of the intrinsic resonance width is 45 eV 171.The simplified decay scheme (extracted from [7,9]) is shown in fig 1. The three y-rays of 4.82 MeV, 4.33 MeV and 491 keV energies are the unique signature of this reaction. These transitions do not interfer with the y transitions arising from $,y) reactions on $her nickel isotopes [p~i~larly Ni (26.1%) and Ni (3.6%)]. Indeed, the excitation curves of ~Ni(p,y)61Cu and 62Ni(p,y)63Cu reactions present many close-set resonances between 1414 keV and 1489 keV proton energy which are one order of magnitude weaker than the 1424 keV one on 58Ni. Moreover y-transitions (6.2 MeV and 7.5 MeV respectively) of high energy emitted by these two reactions decay directly to the ground state [7,8] and thus do not prevent the 58Ni(p,y)59Cu yield from being quantified. The resonance width being very narrow, the depth resolution is determined by the energy spread of the proton beam. The accelerator used in this study delivers a proton beam whose energy spread is - 1 keV at 1424 keV proton energy. Therefore the surface depth resolution in iron is about 10 nm. Considering the relatively low cross section of the reaction and the complexity of the y-ray spectrum when nickel is embedded into an iron based matrix, a Ge detector of high efficiency and good energy resolution must be used.
183
45 “, 60 o and 90 ” ) between the beam axis and the revolution axis of the Ge crystal were summed up separately for different y-rays regions (4.82 MeV, 4.33 MeV and 0.491 MeV). The results show that for the 4.82 MeV transition, the counting rate decreases gradually by a factor of - 2.2 when B increases from 0” up to 90 O. The same tendency was observed for the 419 keV transition (factor of - 1.2 between the two extreme positions), whereas the 4.33 MeV transition had shown the reverse trend. Data for 4.82 MeV and 4.33 MeV transitions were fitted considering W, = PO+ 0.6Pz and W2 = Pa - o.32P2 distributions respectively, PO and Pz are the zero-order and second-order Legendre polynomials respectively (fig. 2A). The total counting rate (adding up the three y yields) decreases when the angle varies from 0 o to 90 O, with a factor of 1.15 between the two extreme positions (fig. 2B). Thus alignment of the detector with the beam axis and the summation of the three y-rays intensities provide a good counting rate. Nickel target of natural composition was analyzed in order to determine the absolute concentration of 58Ni in
4aoo Pi
3. Experimental procedure and results Experiments were ear&d out using the proton beam delivered by the 2.5 MeV Van de Graaff accelerator of the Institut de Physique Nucleaire de Lyon (IPNL). The y-rays were detected using a Ge detector with relative efficiency of 35%. Angular distributions were carried out on pure nickel target at 1429 keV proton energy. Spectra registered at different angles B (8 = O”, 30 “,
Fig. 2. An@ar ~st~butions of the transitions emitted by the 1424 keV Ni(p,y)“Cu resonauce. [A] 4.82 MeV and 4.33 MeV y-transitions. [B] Variation of the total yield (the three transitions were taken into account) versus the detection angle.
184
M.A. El Khakani
et al. / Depth profiling of nickel
[I A
Mev
I.491
11
I I
7
! ;
5
50 ,491 Mev
[I
i-
1’
Jl, L
4
A.82
B
0.6
Mev -I
1
2.0
1.2 GAMMA
ENERGY
6.0
4.0
(Me”)
Fig. 3. y-ray spectra obtained on natural pure nickel using a high efficiency Ge detector. The full-energy and its two related escape peaks for the 4.82, 4.33 and 6.13 MeV transitions are noticed, as well as for the 0.491 MeV one. [A] Before the 1424 keV resonance penetration into the nickel sample EP < 1424 keV). [B] After the 1424 keV resonance penetration (E, B 1424 keV).
profiling measurements. y-ray spectra for energies below and above the 1424 keV resonance energy are displayed in fig. 3. Narrow regions of interest around the y peaks and their escape peaks are chosen in order to discriminate against y peaks from other reactions. Fig. 3 shows that all peaks involved in the 1424 keV resonance are identified. The y peak located at 6.13 MeV energy was present even for a bombardin proton 6!. energy lower than 1424 keV. This is due to the Nt(p,y) reaction at 1347 keV proton energy and the fact that the nickel sample is so thick that this proton energy is reached. As well a contribution due to the (p,ay) reactions on fluorine impurities cannot be excluded. Their extremely intense cross sections give rise to a characteristic 6.13 MeV y-ray [ll]. The net surfaces under the 4.82 MeV with 4.33 MeV and 419 keV peaks together with their related escape peaks are, separately, plotted in fig. 4 as a function of the proton energy. The two curves show a significant rise between 1422 and 1425 keV proton energy, indicating the penetration of the
,
1500 I
1
444444 4 4 4
d 1000
4
500
!
4
4 4
+
t t
b $+,,,+
4’
+ + t
t’rayys detected
o 491 KeV :
. 4.8.? l 433
Yet’
A Iwd o,“.-~.=t.I”.‘:““:““:.‘..: 1420 1415
14,?5
14.90
1435
PTotm
emvgy
(KeV)
1440
1445
Fig. 4. Yield curve of the 58Ni(p,y) reaction on a Ni-target of natural composition. Counts under each y-ray peak and its related escape peaks were summedup.
185
M.A. El Khakani et al. / Depth profiling of nickel
paration and irradiation of samples were carried out in Duisburg (FRG) and Trento (Italy) Universities respectively. In this paper we restrict the study to the application of our technique to profile nickel in such alloys. The invar behavior of the mixed bilayers will be presented elsewhere [14]. The as-evaporated and mixed samples were analysed in order to determine nickel depth concentration since invar effect is closely dependent on nickel concentration in Fe-Ni alloys [l]. Owing to the fact that the analysis was achieved on thin samples, the straggling effect, well known when energetic particles penetrate into matter, must be taken into account. With regard to the thickness of the samples, the Bohr formalism is not applicable. We have used the Vavilov formalism to estimate the full width at half-maximum (FWHM) of the proton energy distribution at a given depth [12]. For example, at a depth of 50 nm in nickel matrix, the proton energy distribution has 3.8 keV of FWHM, whereas at the surface the FWHM is - 1 keV for the proton beam of 1.4 MeV energy. Thus the spectra [counts = f (proton energy)] were treated in the following way: A theoretical spectrum is deduced from a presumed profile. This theoretical spectrum is convoluted with a Vavilov energy distribution from which FWHM varies with depth. The resulting spectrum is compared with the experimental one. The presumed profile is modified until the fit is satisfactory. Fig. 5 displays the obtained nickel depth distributions for the as-evaporated, 6 X 1016 Ar/cm2 (100 keV)
1424 keV 58Ni(p,y)5gCu resonance in the sample. The relatively high and constant background of the 491 keV yield below 1424 keV proton energy is due to the y rays produced in thick nickel samples by the 58Ni(p,y) reaction at 1376 keV resonance. This resonance is one order of magnitude weaker than the 1424 keV one and corresponds to the 4.77 MeV state in 5gCu (branch of 35% to the 491 keV state). For nickel films thinner than 420 nm this background will disappear. As the high and low energy y rays translate faithfully the signature of the 1424 keV resonance, the use of the total yield is justified. Thus it is possible to achieve measurements within reduced acquisition time, and to avoid heating of samples. In the conditions described above, the number of ‘*Ni y rays counts was found to be about 1550 counts/10-4 C in pure nickel. With a beam current of 200 n4, for a nickel concentration of 20 at.% about ten minutes are required to achieve the measurement with about 5% statistical error. Two examples of the application of the 58Ni (p,y)“Cu profiling method are presented below. 3.1. Ion beam mixing of Ni-Fe
thin firms
Ni-Fe bilayers were prepared by successive evaporation of iron and natural nickel on a silicon substrate (see details in fig. 5). The samples were irradiated by 100 keV Ar ions energy or Kr ions at 192 keV incident energy for a fluence of 6 X lOi ion/cm*, in order to produce Ni-Fe invar alloy at the mixed interface. Pre-
‘0,
20 --
‘9 i,
‘1
’
0
20
,+
40
Depth Fig. 5. 58Ni depth distributions
:
s
‘0. ’
’
60
_ ’
3’
’
s s :
80
(lg.cm-2)
obtained
on ion beam Ni-Fe
mixed bilayers.
100
186
M.A. El Khakani et al. / Depth profiling of nickel 50-C ,Q
40;
30
.-.
r
A Experimental
--+.
1.’ \
n
1. ’
Calculated
&’ ‘\‘\.
quantify the 58Ni(p,y)59Cu yield leading to reduce acquisition time, - the optimum counting direction is the axis beam (e=o”), - the depth resolution of the technique near the sample surface is limited only by the energy spread of the proton beam (about 10 nm at the surface of an iron sample). The degradation of the resolution with depth is minimized taking into account the straggling effect.
Acknowledgements
Fig. 6. ‘sNi profile obtained by the 58Ni(p,y)59Cu technique for 2X10” s8Ni/cm* implanted into iron at 80 keV. The
experimentalprofile is compared to a theoretical one. and 6 X lOi Kr/cm2 (192 keV) mixed Ni-Fe samples. The effect of ion beam mixing is clearly observed at the interface. Comparison of the total amount of nickel in the as-evaporated and Ar mixed sample allows to determine the nickel sputtering yield under Ar (100 keV) bombardment [Y(Ar/Ni) = 1.2 in a good agreement with TRIM code prediction (Yr,,,(Ar/Ni) = l.O)]. Taking into account the sputtering effect at the surface, the profile of the mixed sample can be shifted. Thus a direct comparison of the mixing efficiency at the interface can be achieved [14]. 3.2. Nickel implantation
into iron
In order to compare Ni-Fe alloys obtained by ion beam mixing and direct ion implantation, iron samples were implanted with 2 X 10” 58Ni/cm2 at 80 keV energy using the isotope separator of the IPNL. The proton energy was varied by steps of 1 keV in the energy range covering the implanted region. The obtained profile (fig. 6) shows that there is evidence of sputtering effect at the surface. Indeed, the obtained profile was fitted by a theoretical one calculated using an adapted TRIM code version [13]. The good agreement was obtained for an iron sputtering yield of 1.8. The maximum nickel concentration reached was about 44 at.% in the near surface region.
4. Conclusion It is shown in this paper that: _ with a high efficiency Ge detector, all y transitions involved (4.82, 4.33 and 0.491 MeV) can be used to
The authors are grateful to Prof. W. Keune and Dr. C. Tosello for providing and mixing Ni-Fe thin films and thank A. Plantier for performing the implantation. This work was financially supported by the Commission of the European Communities within the CODEST program [contract X1-0024-C(A)].
References PI L.M. Gratton, A Gupta, W. Keune, S. Lo Russo, J. Parellada, G. Principi and C. Tosello, Mater. Sci. and Eng. A 115 (1989) 161. P-1 C.R. Gossett, Nucl. lnstr. and Meth. 168 (1980) 217. [31 M.F. Denanot, P. Popoola and P. Moine, Mater. Sci. Eng. All5 (1989) 145. 141 L. Calliari, L.M. Gratton, L. Guzman, G. Principi and C. Tosello, Mater. Res. Sot. Symp. Proc. vol. 27 (1984) 85. 151 L. Guzman and I. Scotoni, Surface Engineering, Surface Modifications of Materials, NATO-ASI, Serie E: Applied Sciences-NO85 1984 (Martinus Nijhoff Publ.). 161 G. Marest and M.A. El Khakani, to be presented at the Seventh Int. Conf. on Ion Beam Modifications of Materials 1990 (IBMM 1990) Knoxville, TN, USA, (9-14 September 1990). [71 J.W. Bulter and C.R. Gossett, Phys. Rev. 108 (1957) 1473. PI P.A. Treado, C.R. Gossett and L.S. August, Nucl. Phys. A 112 (1968) 32. 191 J.P. Trentehnan, B.E. Cooke, J.R. Leslie, W. MC Latchie and B.C. Robertson, Nucl. Phys. A246 (1975) 457. [101 M.A. El Khakani, H. Jaffrezic, G. Marest, N. Moncoffre and J. Tousset, Mater. Sci. Eng. A 115 (1989) 37. 1111 L.C. Feldman and ST. Picraux, Ion Beam Handbook for Materials Analysis (Academic Press, 1977) p. 205. WI G. Deconninck, Introduction to Radioanalytical Physics (Elsevier, 1978) p. 64. 1131 M.A. El Khakani, H. Jaffrezic, G. Marest, N. Moncoffre and J. Tousset, Nucl. Instr. and Meth. B50 (1990) 406. 1141 C. Tosello, L. Gratton, W. Keune, S. Lo Russo, G. Marest, M.A. El Khakani, J. Parellada, J.M. Fernandez and G. Principi, to be presented at the Seventh Int. Conf. on Ion Beam Modifications of Materials 1990 (IBMM 1990) Knoxville, TN, USA (9-14 September 1990).
Nuclear Instruments and Methods in Physics Research B52 (1990) 182-186 North-Holland
Depth profiling of nickel using the nuclear resonant 58Ni(p,y)59C~ reaction: application to Ni-Fe mixed bilayers and Ni-implanted iron M.A. El Khakani, G. Marest, N. Moncoffre and J. Tousset Institut de Physique NuclPaire de Lyon, INZM-CNRS et Universitk Claude Bernard, 43, Bd du II Novembre 1918 - F-69622 Villeurbanne Cedex, France Received 13 July 1990
The nuclear resonant 58Ni(p,y)59Cu reaction at Ep =1424 keV was used to determine nickel depth distributions in iron-nickel based alloys. y radiations were detected by using a high efficiency Ge detector. For quantifying the reaction yield, the y transitions of EY = 4.82 MeV, 4.33 MeV and 0.491 MeV energies were considered. This technique allows nickel to be profiled with a depth resolution of about 10 nm at the surface of iron matrix. Applications of the method to Ni-Fe ion beam mixed bilayers and N&implanted iron are presented.
1. Introduction Nickel is an alloying element of high technological interest. When an attempt is made to change the superficial properties of materials, ion implantation is a suitable technique. For example Ni-Al superficial alloy was obtained without any adhesion problems, by implanting nickel into aluminium [3]. Ion beam mixing of Ni-Ag, Ni-Sn and Fe-Ni binary systems was studied [1,4,5] in order to understand ion beam bombardment inducing new phase formation. In these studies, the determination of the nickel depth profiles or of the near surface nickel composition is of prime importance to understanding the physical processes. For this purpose, Auger electron spectroscopy was used to profile nickel with some difficulties in converting sputtering time into depth scale [4,5]. However, when samples are to be saved for further investigations only non-destructive techniques can be used. Backscattering spectrometry (RBS) has been recently used to follow the ion mixing of Fe-Ni bilayers after argon or krypton bombardment [l]. Due to the small mass difference between iron and nickel and to the presence of their isotopes in natural Ni-Fe samples, RBS is not adapted to provide quantitative measurements. However in the case of the 64Ni isotope implanted into monoisotopic 57Fe thin films, 64Ni depth profiles were obtained using 5.7 MeV energy He*+ articles [6]. This high energy was required to separate ‘Ni and 57Fe signals in the RBS spectrum allowing quantitative measurements with reduced depth resolution. 0168-583X/90/$03.50
A nickel profiling technique using the 58Ni(p,y)59Cu resonant nuclear reaction at 1424 keV is proposed. Gossett has already reported the possibility to profile nickel using this resonant proton capture reaction, with the restriction to quantify the reaction yield with the first excited state transition [2] (see fig. 1). In this paper, it is shown that with a high efficiency Ge detector it is possible to quantify the yield with all the transitions (4.82, 4.33 and 0.491 MeV y-rays) involved in the reaction, leading to reduced acquisition time. Angular distributions were carried out in order to optimize the detection geometry. Applications to nickel profiling are shown in the case of mixed Ni-Fe bilayers and 58Ni-implanted into iron.
Energy
Fig.1.
Level 4.82
Simplified scheme of y transitions involved in the 58Ni(p,y)59Cu resonant reaction at E, = 1424 keV.
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
MA El Khakani et ai. / Depth ~r~~iing of nickel
Resonant proton capture reactions (p,y), successfully used for profiling low 2 elements, are also useful to profile some elements up to the transition region [2]. The existence of analogue resonances (AR) has allowed to profile titanium into steels using the 48Ti(p,y)49V AR at 1362 keV [2,10]. Due to its high 2, nickel is probably the upper limit element which can be profiled using proton capture reaction. A relatively high proton energy is also required to find an isolated and intense resonance [2]. The excitation function established by Bulter and Gossett shows a lot of resonances in the region studied [7]. For 58Ni the lowest one appears at 855 keV proton energy but the well isolated and most intense resonance is located at 1424 keV, between two less intense resonances at 1376 and 1522 keV. The 1424 keV resonance corresponds to a state in 5gCu at 4.817 MeV. The upper limit of the intrinsic resonance width is 45 eV 171.The simplified decay scheme (extracted from [7,9]) is shown in fig 1. The three y-rays of 4.82 MeV, 4.33 MeV and 491 keV energies are the unique signature of this reaction. These transitions do not interfer with the y transitions arising from $,y) reactions on $her nickel isotopes [p~i~larly Ni (26.1%) and Ni (3.6%)]. Indeed, the excitation curves of ~Ni(p,y)61Cu and 62Ni(p,y)63Cu reactions present many close-set resonances between 1414 keV and 1489 keV proton energy which are one order of magnitude weaker than the 1424 keV one on 58Ni. Moreover y-transitions (6.2 MeV and 7.5 MeV respectively) of high energy emitted by these two reactions decay directly to the ground state [7,8] and thus do not prevent the 58Ni(p,y)59Cu yield from being quantified. The resonance width being very narrow, the depth resolution is determined by the energy spread of the proton beam. The accelerator used in this study delivers a proton beam whose energy spread is - 1 keV at 1424 keV proton energy. Therefore the surface depth resolution in iron is about 10 nm. Considering the relatively low cross section of the reaction and the complexity of the y-ray spectrum when nickel is embedded into an iron based matrix, a Ge detector of high efficiency and good energy resolution must be used.
183
45 “, 60 o and 90 ” ) between the beam axis and the revolution axis of the Ge crystal were summed up separately for different y-rays regions (4.82 MeV, 4.33 MeV and 0.491 MeV). The results show that for the 4.82 MeV transition, the counting rate decreases gradually by a factor of - 2.2 when B increases from 0” up to 90 O. The same tendency was observed for the 419 keV transition (factor of - 1.2 between the two extreme positions), whereas the 4.33 MeV transition had shown the reverse trend. Data for 4.82 MeV and 4.33 MeV transitions were fitted considering W, = PO+ 0.6Pz and W2 = Pa - o.32P2 distributions respectively, PO and Pz are the zero-order and second-order Legendre polynomials respectively (fig. 2A). The total counting rate (adding up the three y yields) decreases when the angle varies from 0 o to 90 O, with a factor of 1.15 between the two extreme positions (fig. 2B). Thus alignment of the detector with the beam axis and the summation of the three y-rays intensities provide a good counting rate. Nickel target of natural composition was analyzed in order to determine the absolute concentration of 58Ni in
4aoo Pi
3. Experimental procedure and results Experiments were ear&d out using the proton beam delivered by the 2.5 MeV Van de Graaff accelerator of the Institut de Physique Nucleaire de Lyon (IPNL). The y-rays were detected using a Ge detector with relative efficiency of 35%. Angular distributions were carried out on pure nickel target at 1429 keV proton energy. Spectra registered at different angles B (8 = O”, 30 “,
Fig. 2. An@ar ~st~butions of the transitions emitted by the 1424 keV Ni(p,y)“Cu resonauce. [A] 4.82 MeV and 4.33 MeV y-transitions. [B] Variation of the total yield (the three transitions were taken into account) versus the detection angle.
184
M.A. El Khakani
et al. / Depth profiling of nickel
[I A
Mev
I.491
11
I I
7
! ;
5
50 ,491 Mev
[I
i-
1’
Jl, L
4
A.82
B
0.6
Mev -I
1
2.0
1.2 GAMMA
ENERGY
6.0
4.0
(Me”)
Fig. 3. y-ray spectra obtained on natural pure nickel using a high efficiency Ge detector. The full-energy and its two related escape peaks for the 4.82, 4.33 and 6.13 MeV transitions are noticed, as well as for the 0.491 MeV one. [A] Before the 1424 keV resonance penetration into the nickel sample EP < 1424 keV). [B] After the 1424 keV resonance penetration (E, B 1424 keV).
profiling measurements. y-ray spectra for energies below and above the 1424 keV resonance energy are displayed in fig. 3. Narrow regions of interest around the y peaks and their escape peaks are chosen in order to discriminate against y peaks from other reactions. Fig. 3 shows that all peaks involved in the 1424 keV resonance are identified. The y peak located at 6.13 MeV energy was present even for a bombardin proton 6!. energy lower than 1424 keV. This is due to the Nt(p,y) reaction at 1347 keV proton energy and the fact that the nickel sample is so thick that this proton energy is reached. As well a contribution due to the (p,ay) reactions on fluorine impurities cannot be excluded. Their extremely intense cross sections give rise to a characteristic 6.13 MeV y-ray [ll]. The net surfaces under the 4.82 MeV with 4.33 MeV and 419 keV peaks together with their related escape peaks are, separately, plotted in fig. 4 as a function of the proton energy. The two curves show a significant rise between 1422 and 1425 keV proton energy, indicating the penetration of the
,
1500 I
1
444444 4 4 4
d 1000
4
500
!
4
4 4
+
t t
b $+,,,+
4’
+ + t
t’rayys detected
o 491 KeV :
. 4.8.? l 433
Yet’
A Iwd o,“.-~.=t.I”.‘:““:““:.‘..: 1420 1415
14,?5
14.90
1435
PTotm
emvgy
(KeV)
1440
1445
Fig. 4. Yield curve of the 58Ni(p,y) reaction on a Ni-target of natural composition. Counts under each y-ray peak and its related escape peaks were summedup.
185
M.A. El Khakani et al. / Depth profiling of nickel
paration and irradiation of samples were carried out in Duisburg (FRG) and Trento (Italy) Universities respectively. In this paper we restrict the study to the application of our technique to profile nickel in such alloys. The invar behavior of the mixed bilayers will be presented elsewhere [14]. The as-evaporated and mixed samples were analysed in order to determine nickel depth concentration since invar effect is closely dependent on nickel concentration in Fe-Ni alloys [l]. Owing to the fact that the analysis was achieved on thin samples, the straggling effect, well known when energetic particles penetrate into matter, must be taken into account. With regard to the thickness of the samples, the Bohr formalism is not applicable. We have used the Vavilov formalism to estimate the full width at half-maximum (FWHM) of the proton energy distribution at a given depth [12]. For example, at a depth of 50 nm in nickel matrix, the proton energy distribution has 3.8 keV of FWHM, whereas at the surface the FWHM is - 1 keV for the proton beam of 1.4 MeV energy. Thus the spectra [counts = f (proton energy)] were treated in the following way: A theoretical spectrum is deduced from a presumed profile. This theoretical spectrum is convoluted with a Vavilov energy distribution from which FWHM varies with depth. The resulting spectrum is compared with the experimental one. The presumed profile is modified until the fit is satisfactory. Fig. 5 displays the obtained nickel depth distributions for the as-evaporated, 6 X 1016 Ar/cm2 (100 keV)
1424 keV 58Ni(p,y)5gCu resonance in the sample. The relatively high and constant background of the 491 keV yield below 1424 keV proton energy is due to the y rays produced in thick nickel samples by the 58Ni(p,y) reaction at 1376 keV resonance. This resonance is one order of magnitude weaker than the 1424 keV one and corresponds to the 4.77 MeV state in 5gCu (branch of 35% to the 491 keV state). For nickel films thinner than 420 nm this background will disappear. As the high and low energy y rays translate faithfully the signature of the 1424 keV resonance, the use of the total yield is justified. Thus it is possible to achieve measurements within reduced acquisition time, and to avoid heating of samples. In the conditions described above, the number of ‘*Ni y rays counts was found to be about 1550 counts/10-4 C in pure nickel. With a beam current of 200 n4, for a nickel concentration of 20 at.% about ten minutes are required to achieve the measurement with about 5% statistical error. Two examples of the application of the 58Ni (p,y)“Cu profiling method are presented below. 3.1. Ion beam mixing of Ni-Fe
thin firms
Ni-Fe bilayers were prepared by successive evaporation of iron and natural nickel on a silicon substrate (see details in fig. 5). The samples were irradiated by 100 keV Ar ions energy or Kr ions at 192 keV incident energy for a fluence of 6 X lOi ion/cm*, in order to produce Ni-Fe invar alloy at the mixed interface. Pre-
‘0,
20 --
‘9 i,
‘1
’
0
20
,+
40
Depth Fig. 5. 58Ni depth distributions
:
s
‘0. ’
’
60
_ ’
3’
’
s s :
80
(lg.cm-2)
obtained
on ion beam Ni-Fe
mixed bilayers.
100
186
M.A. El Khakani et al. / Depth profiling of nickel 50-C ,Q
40;
30
.-.
r
A Experimental
--+.
1.’ \
n
1. ’
Calculated
&’ ‘\‘\.
quantify the 58Ni(p,y)59Cu yield leading to reduce acquisition time, - the optimum counting direction is the axis beam (e=o”), - the depth resolution of the technique near the sample surface is limited only by the energy spread of the proton beam (about 10 nm at the surface of an iron sample). The degradation of the resolution with depth is minimized taking into account the straggling effect.
Acknowledgements
Fig. 6. ‘sNi profile obtained by the 58Ni(p,y)59Cu technique for 2X10” s8Ni/cm* implanted into iron at 80 keV. The
experimentalprofile is compared to a theoretical one. and 6 X lOi Kr/cm2 (192 keV) mixed Ni-Fe samples. The effect of ion beam mixing is clearly observed at the interface. Comparison of the total amount of nickel in the as-evaporated and Ar mixed sample allows to determine the nickel sputtering yield under Ar (100 keV) bombardment [Y(Ar/Ni) = 1.2 in a good agreement with TRIM code prediction (Yr,,,(Ar/Ni) = l.O)]. Taking into account the sputtering effect at the surface, the profile of the mixed sample can be shifted. Thus a direct comparison of the mixing efficiency at the interface can be achieved [14]. 3.2. Nickel implantation
into iron
In order to compare Ni-Fe alloys obtained by ion beam mixing and direct ion implantation, iron samples were implanted with 2 X 10” 58Ni/cm2 at 80 keV energy using the isotope separator of the IPNL. The proton energy was varied by steps of 1 keV in the energy range covering the implanted region. The obtained profile (fig. 6) shows that there is evidence of sputtering effect at the surface. Indeed, the obtained profile was fitted by a theoretical one calculated using an adapted TRIM code version [13]. The good agreement was obtained for an iron sputtering yield of 1.8. The maximum nickel concentration reached was about 44 at.% in the near surface region.
4. Conclusion It is shown in this paper that: _ with a high efficiency Ge detector, all y transitions involved (4.82, 4.33 and 0.491 MeV) can be used to
The authors are grateful to Prof. W. Keune and Dr. C. Tosello for providing and mixing Ni-Fe thin films and thank A. Plantier for performing the implantation. This work was financially supported by the Commission of the European Communities within the CODEST program [contract X1-0024-C(A)].
References PI L.M. Gratton, A Gupta, W. Keune, S. Lo Russo, J. Parellada, G. Principi and C. Tosello, Mater. Sci. and Eng. A 115 (1989) 161. P-1 C.R. Gossett, Nucl. lnstr. and Meth. 168 (1980) 217. [31 M.F. Denanot, P. Popoola and P. Moine, Mater. Sci. Eng. All5 (1989) 145. 141 L. Calliari, L.M. Gratton, L. Guzman, G. Principi and C. Tosello, Mater. Res. Sot. Symp. Proc. vol. 27 (1984) 85. 151 L. Guzman and I. Scotoni, Surface Engineering, Surface Modifications of Materials, NATO-ASI, Serie E: Applied Sciences-NO85 1984 (Martinus Nijhoff Publ.). 161 G. Marest and M.A. El Khakani, to be presented at the Seventh Int. Conf. on Ion Beam Modifications of Materials 1990 (IBMM 1990) Knoxville, TN, USA, (9-14 September 1990). [71 J.W. Bulter and C.R. Gossett, Phys. Rev. 108 (1957) 1473. PI P.A. Treado, C.R. Gossett and L.S. August, Nucl. Phys. A 112 (1968) 32. 191 J.P. Trentehnan, B.E. Cooke, J.R. Leslie, W. MC Latchie and B.C. Robertson, Nucl. Phys. A246 (1975) 457. [101 M.A. El Khakani, H. Jaffrezic, G. Marest, N. Moncoffre and J. Tousset, Mater. Sci. Eng. A 115 (1989) 37. 1111 L.C. Feldman and ST. Picraux, Ion Beam Handbook for Materials Analysis (Academic Press, 1977) p. 205. WI G. Deconninck, Introduction to Radioanalytical Physics (Elsevier, 1978) p. 64. 1131 M.A. El Khakani, H. Jaffrezic, G. Marest, N. Moncoffre and J. Tousset, Nucl. Instr. and Meth. B50 (1990) 406. 1141 C. Tosello, L. Gratton, W. Keune, S. Lo Russo, G. Marest, M.A. El Khakani, J. Parellada, J.M. Fernandez and G. Principi, to be presented at the Seventh Int. Conf. on Ion Beam Modifications of Materials 1990 (IBMM 1990) Knoxville, TN, USA (9-14 September 1990).