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Interstitial trapping in ion implanted Al(Ag)
340
Nuclear Instruments and Methods in Physics Research B36 (1989) 340-344 North-Holland, Amsterdam
INTERSTITIAL I. KHUBEIS
TRAPPING
IN ION IMPLANTED
AI
* and 0. MEYER
Received 26 September 1988
The formation of mixed dumbbells in as-implanted AI alloys is strongly suppressed due to the presence of vacancies and other deep traps for self-interstitial atoms (SIAs}. After anneating the as-implanted systems EO373 and 473 K, mixed dumbbells are formed by irradiation with 1 MeV He-ions at 77 K. The production rate and the maximum concentration are smaller than values obtained for dilute solid solutions of AI under similar radiation conditions. The defect agglomeration and annealing for the as-implanted and for the irradiated implanted alloys was monitored by the dechanneling yield.
1. Introduction In the course of a systematic study of the lattice-site occupation of metallic elements implanted into Al single crystals, high substitutional fractions were observed for implanted systems (e.g. Al(Ag), Al(Cr), Al(Mn)) having small negative values for the heat of solution and for the size mismatch energy [I]. Similar dilute solid solutions in the same concentration range as the implanted alloys revealed the formation of mixed dumbbells during ion irradiation at low temperatures, which rises the question why such impurity-defect complexes are not observed for the ion implanted systems. The formation of impurity-defect complexes due to the trapping of self-interstiti~ atoms (SIAs) and vacancies by solute atoms in dilute solid solutions has been widely studied using a number of selective methods (for a survey see e.g. ref. [2]). The application of the channeling spectroscopy to materials analysis and especially to the analysis of impurity-point defect complexes is well documented [2-41. The technique is based on the fact that the impurity atoms are well screened by the lattice atoms from the analyzing ion beam which is directed along close packed atomic rows and planes as long as these atoms are on substitutional lattice sites, while they become visible if they are displaced due to interactions with point defects to form mixed d~bbells or impurity vacancy cluster. The latter technique was used to measure the formation of mixed dumbbells in Al-O.10 at.% Ag single crystals by irradiating these dilute solid solutions with He- and Kr-ions at temperatures of 70 K and 150 K, where the SIAs are mobile 15-71. From these studies a number of important results could be de* Permanent address: Department of Physics, University of Jordan, Amman, Jordan. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
termined concerning the production rates of SIAs and their kinetics, further the trapping probabi~~es of SIAs by solute atoms and vacancies as well as the thermal stability of mixed dumbbells and their structure. One piece of information which is important for the comparison with ion implanted alloys is the fact that the maximum displaced Ag-atom fraction was about 0.4 to 0.45. This is in contrast to results observed after implanting about the same amount of Ag-atoms ( - 0.1 at.%) at 77 and 293 K [S]. In these experiments high substitutional fractions between 0.88 and 0.95 were observed, irrespective of various implantation and annealing temperatures used. A high substitutional fraction was expected from our systematic studies [1,9] as the system Al(Ag) has rather small values for the size mismatch energy and for the negative value of the heat of solution. It may be speculated that the trapping of SIAs in ion implanted systems is strongly suppressed by the presence of competing trapping centers. A further possibility could be due to the dissociation of impu~ty-defect complexes in the cascade. Such an effect has been observed for vacancy-impurity complexes formed in ion implanted alloys at temperatures where the vacancies are mobile. During irradiation at temperatures below stage III these complexes dissociate and lead to an increase of the substitutional fraction (for more details see ref. 191). The following study has been performed to test the assumption made above and to provide some information on the irradiation stability of ion implanted alloys. 2. Experimental and analysis The experimental arrangement and the preparation of the Al single crystals have been described previously [lo]. The samples are mounted on a liquid helium (4.2
I. Khubeis, 0. Meyer / Interstitial
K) or liquid nitrogen (77 K) cooled goniometer [ll] which is coupled to a 350 keV ion implanter for implantation and postirradiation and to a Van de Graaff accelerator for in situ ion channeling analysis at 2.0 MeV. The peak concentration of the implanted Ag-ions was about 0.22 at%. Some of the implanted Al single crystals were annealed in UHV to temperatures above 293 K. The energy of the He-ions used for postirradiation was 1 MeV, the same energy as used before for the irradiation experiments of solid solutions [5]. Angular scans were taken to separate the true substitutional component from other configurations of high or low symmetry. Such angular yield curves are characterized by the minimum yield, xmin, which is the yield obtained for perfect alignment of the incident beam with the crystal direction normalized to the yield for random alignment, and the critical angle, I/Q, which is half of the tilt angle at half height of the angular yield curve. The substitutional fraction f, is defined as: f, = (1 -XL)/@-
XlhiJ.
(1)
As discussed in ref. [3] this formula is only a first order estimate of the substitutional component and would be rigorously valid only, if the nonsubstitutional fraction occupied random lattice sites. If the lattice site of the nonsubstitutional component can be determined by the channeling experiment then the amount of impurities on various lattice sites can be analyzed with high accuracy by comparing with Monte Carlo calculation simulations. The modified KinchinPease formula D = (0.8 1pF,)/(2 E,N) with the atomic density (N) of the host and a threshold displacement energy (E,) of 18 eV 1121 was used to convert the employed ion fluences ($J) to a displacement per atom scale (0). The program TRIM2 was used to determine the energy density deposited into nuclear collisions, e.g. in eV/At (F,, +/N) [13].
trapping in ion implanted Al(Ag)
60 ,
341
Al
0
Aa
022at.%Ag
in Al TI=T,=77K
1
2MeVHe -RANDOM n+ ALIGNEO
10 0
150
200
250
300
350
400
450
Channel Number
Fig. 1. Random and (110) aligned spectra of 2 MeV He-ions backscattered from an Al single crystal implanted with 0.22 at.% Ag (peak concentration) at 77 K. (110) aligned spectra are shown (a) after implantation and (b) after additional irradiation with 5.6 X 1Ol5 He+/&‘, 1 MeV at 77 K. and 465 was used as an energy window for silver and the region between 270 and 290 for aluminium. The latter region corresponds to the depth region were Ag is located in the crystal. The results of the angular scan measurement are shown in fig. 2. From the fact that the criticle angle for Al and Ag are of the same size it can be concluded that most of the Ag atoms are on perfect substitutional sites. From the differences of the minimum yield values for Al and Ag a substitutional fraction of 0.88 can be estimated using eq. (1). This fraction is slightly lower than a value 0.94 obtained previously for Ag implanted in Al at 77 K using a peak concentration of 0.07 at% [8]. Slight structures in the angular
‘5,::.li;
3. Results Ag-ions with energies of 300 keV are implanted under random directions in Al single crystals at 77 K, choosing peak concentrations of 0.22 at%. Backscattering and channeling analysis has been performed in situ using a 2 MeV He-ion beam. The random and (110) aligned backscattering spectra are shown in fig. 1. Ag is well separated from Al. The Al surface peak of the aligned spectra (a) is mainly due to oxide formation as an oxygen surface peak is clearly seen. From the large difference of the Ag peak areas of the random spectrum and the (110) aligned spectra (a) of the as-implanted sample a large substitutional fraction can be estimated. In order to get more detailed information on the lattice occupation of Ag in Al, angular scan measurements have been performed. The region between channels 445
-1
-.5
0
.5
TILT ANGLE tdeg)
Fig. 2. Angular yield curves of 2 MeV He-ions backscattered from an Al single crystal implanted with Ag-ions. Implantation and channeling
analysis are performed
at 77 K.
342
I. Khubeis, 0. Meyer / Interstitial
yield curve of Ag, which were more pronounced for samples containing a lower Ag peak concentration [S], indicate that 5 to 10% of the Ag atoms are not on random lattice sites probably due to their interactions with point defects to form unique impurity defect clusters. The as-implanted Al single crystal was irradiated with 5.6 x 1015 Het/cm2, 1 MeV at 77 K. For such a fluence, energy and irradiation temperature the concentration of mixed dumbbells reached a maximum during irradiation experiments of solid solution [7]. As can be seen from the (110) aligned spectra (b) in fig. 1, which was taken after the irradiation experiment, there are only minor changes in the dechanneling yield at channel numbers below 250. The substitutional fraction is not changed in this irradiation experiment. In order to see if the formation of mixed dumbbells is suppressed by competing trapping centers e.g. vacancies or more complex defect clusters present in the implanted region which trap freely mobile SIAs, the implanted samples were annealed to 293 K and above and then irradiated at 77 K. The most important features observed during annealing are shown in fig. 3. Warming up the as-implanted samples from 77 to 293 K leads to a large increase of the dechanneling yield due to a change of the defect structure as can be seen by comparing the (110) aligned spectrum (a) in fig. 3 with spectrum (b) shown in fig. 1. Although the peak area of Ag increases, the substitutional fraction as determined using eq. (1) is still 88%. Annealing to 473 K for 1 h (spectrum (b) in fig. 3) leads to a strong decrease of the dechanneling yield and to a slight increase of the substitutional fraction (f, = 0.93). Annealing to 673” C causes out-diffusion of Ag. Only a few Ag atoms are still located at the surface of the Al single crystal. Samples annealed to 373 and 473 K were cooled
6,
0
Al 022at%
5
Ag I" Al
Ag 1 I_
T,=77K, T,=293K z
2MeV
4
He
s RANDOM + - ALIGNED
b f3
1
3"
1
0 150
200
250
300 Channel
350
400
450
Number
Fig. 3. Random and (110) aligned spectra of 2 MeV He-ions backscattered from an Al single crystal implanted with 0.22 at% Ag (peak concentration) at 77 K. (llO)-aligned spectra are shown (a) at 293 K after implantation of Ag at 77 K, and (b) at 293 K after annealing the sample to 493 K.
trapping in ion implanted AI
0.0-l 0
5 He-FLUENCE
10 (10’5ions/cmz)
15
I
20
Fig. 4. The nonsubstitutional fraction as a function of the 1 MeV He-ion fluence during irradiation at 77 K. The results are shown (a) for an Al +O.l at% Ag solid solution [7], (b) for an implanted system annealed to 393 K (c) for an implanted system annealed to 493 K and (d) for a sample as implanted at 77 K.
down to 77 K and then irradiated with 1 MeV He-ions. The substitutional fraction was determined using 2 MeV He-ions at 77 K. The results are shown in fig. 4 were the fraction of Ag-atoms on nonsubstitutional sites is presented as a function of the He-ion fluence. With increasing He-ion fluence the nonsubstitutional fraction increases and reaches a maximum value of about 34% at a fluence of about 6 X 1015 He+/cm* for the sample annealed to 373 K (curve (b)). For the sample annealed to 473 K, the maximum value is slightly smaller (31%) and is reached at larger fluences (curve (c)). For the unannealed, as implanted sample only a very small increase of the nonsubstitutional component from 10% to 11% was observed in this fluence range (curve (d)). The results for irradiations of a solid solution [7] under similar conditions are shown for comparison (curve (a)). From angular yield measurements [7] the formation of mixed dumbbells could be determined from pronounced peaking of the Ag atom yield in the low fluence region while in the high fluence region peaking was no longer observed. Similar angular yield measurements have been performed here using the annealed implanted alloys irradiated with He-ions at 77 K. The results are shown in fig. 5 for Al and Ag after irradiation with 1.7 X lOi He+/cm2 (0, n), 6.2 X 1015 He+/cm2 (A, A) and 13.0 X 1015 He+/cm2 (0, 0). While the increase
of the minimum
yield for Ag with increas-
seen, there is no peaking of the Ag yield near zero tilt angle. In dilute Al solid solutions mixed dumbbells are stable up to stage III (- 200 K). Complete recovery was observed due to annihilation with vacancies which become mobile at about 200 K [7]. Similar observations were made here for the irradiated ion implanted samples. After annealing to 293 K the substitutional fraction increased from 0.68 to 0.94. It is interesting to note that in contrast to the annealing behaviour of as-implanted samples, the annealed and postirradiated saming He-ion
fluence
is clearly
I: Khubeis, 0. Meyer / Interstitial 1.0
. \
9 ? i 3 2
11
.
4
\
\
0.5-
4.
J
k
z::
4 1
lL.4 I -1.0
AL
.
0
i 1
I I -0.5 0 to.5 TILT ANGLE (deg.1
+1.0
Fig. 5. Angular yield curves of 2 MeV He-ions backscattered from an Al single crystal at II K. The crystal was implanted with Ag ions at 293 K and annealed to 393 K. The results are shown after irradiating the system with various fluences of He-ions (1 MeV) at 77 K (0, w: 1.7X lOI He+/cm2; A, A: 6.2 x 1015 He+/cm2; o, 0: 13 x lOi He+/cm2).
ples did not reveal a pronounced increase of the dechanneling yield during warming up to 293 K.
4. Discussion The results presented above clearly reveal why mixed dumbbells are not formed during ion implantation at temperatures below 150 K. During ion irradiation of dilute solid solutions it was shown that at low fluences every free SIA which escapes the recombination will be trapped by a Ag-atom so that from the initial slope of curve (a) in fig. 4 the production rate of mixed dumbbells can be extracted 151. With increasing fluence the trapping rate decreases due to the increased probability that SIAs are annihilated at vacancies or other competing traps. For a fluence of about 6 X lO1’ He+/cm2, where a maximum concentration of mixed dumbbells is reached, the energy density deposited in nuclear collisions is about 0.21 eV/At, which is about 3 orders of magnitude smaller than the energy density deposited during implantation. This indicates that during implantation of Ag the production rate of mixed dumbbells decreases to zero already for a fluence of about 1 X lOI Ag’/cm2. From this it is assumed that only a rather small fraction of mixed dumbbells (- 10m4 at.%) is produced as compared to the total implanted Ag concentration (- 0.22 at.%). The loss of mobile SIA to the surface during implantation in surface near regions is generally observed [14,15] and a high concentration of vacancies
trapping in ion implanted AI
343
will survive in the implanted regions. From these arguments it is clear that further postirradiation of the as-implanted alloys should not change the substitutional fraction as if proven by experiment. A further test of these assumptions is provided by the annealing and postirradiation experiments. First the competing trapping centers are annealed and then the irradiation experiments are repeated at low temperatures. The annealing behaviour of the dechanneling yield provides some hints on the structure of these centers. The annealing is similar to that of quenched Al single crystals where a clustering of vacancies in stage III is followed by a coarsening into faulted loops at the end of stage III. These processes have been seen by electron microscopy [15] as well as by positron annihilation studies [16]. Especially the formation of double faulted loops will lead to a large increase of the dechanneling yield in agreement with the observed results (curve (a) in fig. 4). The annealing of loops occurs in stage V at about 400 K [15]. This is in good agreement with the large reduction of the dechanneling yield as shown for curve (b) in fig. 3. In general it can be stated that the annealing of the as-implanted Al single crystal as monitored by the dechanneling yield resembles that of a quenched Al single crystal. During postirradiation of implanted alloys annealed to 393 K and 493 K and 77 K the formation of a SIA-Ag defect complex, probably a mixed dumbbell, is observed, as expected. However, the production rate as well as the maximum concentration are smaller than the values observed for irradiated dilute solid solutions [5]. The saturation value is reached at about the same value for the deposited energy density, while the slight decrease of the mixed dumbbell concentration with increasing He-ion fluence is not observed (fig. 4). Irradiation of solid solutions at low fluences produces (100) mixed dumbbells with a Ag atom displacement of 0.14 nm from the lattice site 1171. This displacement amplitude was extracted from the peaking of the Ag yield in a (110) angular scan. Such peaking was not observed after irradiation at higher fluences. This observation and the slight decrease of the concentration was attributed to a change of the trapping configuration when perhaps more than one SIA is trapped at an Ag atom which is also reflected in a more complex annealing behaviour [7]. From simulation calculations it is known that a reduction of the displacement amplitude from 0.14 to 0.12 nm is sufficient to move Ag out of a region of high ion flux and thus to avoid peaking [17]. For mixed dumbbells produced here in annealed ion implanted alloys, peaking of the Ag yield has never been observed. This may indicate that for ion implanted alloys more complex trapping configurations prevail. The rather high substitutional fraction of 0.94, which is observed after warming up to 293 K indicates however that Ag-SIA complexes are formed during irradiation
344
I. Khubeis, 0. Meyer / Interstitial
trapping in ion implanted AI
which completely recover by annihilation with mobile vacancies.
port the model proposed to explain the irradiation stability of as-implanted alloys.
5. Conclusions
The authors want to thank R. Gerber for valuable discussion and for carefully reading the manuscript and B. Strehlau for his experimental support.
In our systematic study on the basic mechanisms of the lattice site occupations of ions implanted into metals it was stated that the study of impurities having negative heats of solution would be of great interest as this type of impurities is usually undersized and tends to form interstitial-impurity complexes by irradiation at low temperatures [9]. The results obtained for ion implanted systems can then be compared with numerous results on impurity-point defect interactions obtained by irradiating diluted Al alloys [2,4]. Ion implanted alloys with small negative values for the heat of solution and for the size mismatch energy reveal large substitutional fractions (- 0.95) although ion irradiation of solid solutions with impurity concentrations similar to those of the ion implanted alloys (- 0.1 at.%) produces large fractions of mixed dumbbells (30% to 60%). For the system Al(Ag) studied here it is reasonable to assume that the production rate of mixed dumbbells decreases to zero already in the low fluence region during implantation at Ag concentration levels (- 10m4 at.%) which are small compared to the total implanted concentration. The implanted region is enriched with vacancies and small vacancy clusters as the mobile SIAs partly recombine with vacancies and partly anneal at the sample surface. Small vacancy clusters contribute to the dechanneling yield at large depths, while the implanted region is not strongly damaged. The annealing behaviour as reflected by the change of the dechanneling yield indicates the agglomeration of the vacancies and the formation and annihilation of faulted dislocation loops. Irradiation of the annealed ion implanted alloys produces the formation of mixed dumbbells in the same region of deposited energy density as observed previously for dilute solid solutions. These results sup-
References [l] R. Gerber, 0. Meyer, and G.C. Xiong, Nucl. Instr. and Meth. B31 (1988) 402. [2] M.L. Swanson, in: Advanced Techniques for Characterizing Microstructures, eds. F.W. Wiffen and J.A. Spitznagel (Metal]. Sot. of AIME, New York, 1982) p. 305. [3] L.C. Feldmann, J.W. Mayer, and ST. Picraux, Materials Analysis by Ion Channeling (Academic Press, New York, 1982). [4] M.L. Swanson and L.M. Howe, Nucl. Instr. and Meth. 218 (1983) 613. [5] M.L. Swanson and L.M. Howe, Radiat. Eff. 41 (1979) 129. [6] M.L. Swanson, L.M. Howe, J.A. Moore, and A.F. QuenneviIle, Nucl. Instr. and Meth. 209/210 (1983) 1029. [7] L.M. Howe M.L. Swanson, and A.F. Quenneville, Nucl. Instr. and Meth. B7/8 (1985) 91. [S] G.C. Xiong, A. Azzam, R. Gerber, and 0. Meyer, Nucl. Instr. and Meth. B29 (1988) 643. [9] 0. Meyer and A. Turos, Materials Science Reports 2(S) (1987) 371. [lo] M.K. Kloska and 0. Meyer, Nucl. Instr. and Meth. B14 (1986) 268. [ll] R. Kaufmann, J. Geerk, and F. Ratzel, Nucl. Instr. and Meth. 205 (1983) 293. [12] H.H. Neely and W. Batter, Phys. Rev. 149 (1966) 535. [13] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [14] H. Bernas, M.O. Ruault, and B. Jouffrey, Phys. Rev. Lett. 27 (1971) 859. [15] B.L. Eyre, J. Phys. F3 (1973) 422 and refs. therein. [16] W.R. Wampler and W.B. Gauster, J. Phys. F8 (1978) Ll. [17] N. Matsunami, M.L. Swanson, and L.M. Howe, Can. J. Phys. 56 (1978) 1057.
Nuclear Instruments and Methods in Physics Research B36 (1989) 340-344 North-Holland, Amsterdam
INTERSTITIAL I. KHUBEIS
TRAPPING
IN ION IMPLANTED
AI
* and 0. MEYER
Received 26 September 1988
The formation of mixed dumbbells in as-implanted AI alloys is strongly suppressed due to the presence of vacancies and other deep traps for self-interstitial atoms (SIAs}. After anneating the as-implanted systems EO373 and 473 K, mixed dumbbells are formed by irradiation with 1 MeV He-ions at 77 K. The production rate and the maximum concentration are smaller than values obtained for dilute solid solutions of AI under similar radiation conditions. The defect agglomeration and annealing for the as-implanted and for the irradiated implanted alloys was monitored by the dechanneling yield.
1. Introduction In the course of a systematic study of the lattice-site occupation of metallic elements implanted into Al single crystals, high substitutional fractions were observed for implanted systems (e.g. Al(Ag), Al(Cr), Al(Mn)) having small negative values for the heat of solution and for the size mismatch energy [I]. Similar dilute solid solutions in the same concentration range as the implanted alloys revealed the formation of mixed dumbbells during ion irradiation at low temperatures, which rises the question why such impurity-defect complexes are not observed for the ion implanted systems. The formation of impurity-defect complexes due to the trapping of self-interstiti~ atoms (SIAs) and vacancies by solute atoms in dilute solid solutions has been widely studied using a number of selective methods (for a survey see e.g. ref. [2]). The application of the channeling spectroscopy to materials analysis and especially to the analysis of impurity-point defect complexes is well documented [2-41. The technique is based on the fact that the impurity atoms are well screened by the lattice atoms from the analyzing ion beam which is directed along close packed atomic rows and planes as long as these atoms are on substitutional lattice sites, while they become visible if they are displaced due to interactions with point defects to form mixed d~bbells or impurity vacancy cluster. The latter technique was used to measure the formation of mixed dumbbells in Al-O.10 at.% Ag single crystals by irradiating these dilute solid solutions with He- and Kr-ions at temperatures of 70 K and 150 K, where the SIAs are mobile 15-71. From these studies a number of important results could be de* Permanent address: Department of Physics, University of Jordan, Amman, Jordan. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
termined concerning the production rates of SIAs and their kinetics, further the trapping probabi~~es of SIAs by solute atoms and vacancies as well as the thermal stability of mixed dumbbells and their structure. One piece of information which is important for the comparison with ion implanted alloys is the fact that the maximum displaced Ag-atom fraction was about 0.4 to 0.45. This is in contrast to results observed after implanting about the same amount of Ag-atoms ( - 0.1 at.%) at 77 and 293 K [S]. In these experiments high substitutional fractions between 0.88 and 0.95 were observed, irrespective of various implantation and annealing temperatures used. A high substitutional fraction was expected from our systematic studies [1,9] as the system Al(Ag) has rather small values for the size mismatch energy and for the negative value of the heat of solution. It may be speculated that the trapping of SIAs in ion implanted systems is strongly suppressed by the presence of competing trapping centers. A further possibility could be due to the dissociation of impu~ty-defect complexes in the cascade. Such an effect has been observed for vacancy-impurity complexes formed in ion implanted alloys at temperatures where the vacancies are mobile. During irradiation at temperatures below stage III these complexes dissociate and lead to an increase of the substitutional fraction (for more details see ref. 191). The following study has been performed to test the assumption made above and to provide some information on the irradiation stability of ion implanted alloys. 2. Experimental and analysis The experimental arrangement and the preparation of the Al single crystals have been described previously [lo]. The samples are mounted on a liquid helium (4.2
I. Khubeis, 0. Meyer / Interstitial
K) or liquid nitrogen (77 K) cooled goniometer [ll] which is coupled to a 350 keV ion implanter for implantation and postirradiation and to a Van de Graaff accelerator for in situ ion channeling analysis at 2.0 MeV. The peak concentration of the implanted Ag-ions was about 0.22 at%. Some of the implanted Al single crystals were annealed in UHV to temperatures above 293 K. The energy of the He-ions used for postirradiation was 1 MeV, the same energy as used before for the irradiation experiments of solid solutions [5]. Angular scans were taken to separate the true substitutional component from other configurations of high or low symmetry. Such angular yield curves are characterized by the minimum yield, xmin, which is the yield obtained for perfect alignment of the incident beam with the crystal direction normalized to the yield for random alignment, and the critical angle, I/Q, which is half of the tilt angle at half height of the angular yield curve. The substitutional fraction f, is defined as: f, = (1 -XL)/@-
XlhiJ.
(1)
As discussed in ref. [3] this formula is only a first order estimate of the substitutional component and would be rigorously valid only, if the nonsubstitutional fraction occupied random lattice sites. If the lattice site of the nonsubstitutional component can be determined by the channeling experiment then the amount of impurities on various lattice sites can be analyzed with high accuracy by comparing with Monte Carlo calculation simulations. The modified KinchinPease formula D = (0.8 1pF,)/(2 E,N) with the atomic density (N) of the host and a threshold displacement energy (E,) of 18 eV 1121 was used to convert the employed ion fluences ($J) to a displacement per atom scale (0). The program TRIM2 was used to determine the energy density deposited into nuclear collisions, e.g. in eV/At (F,, +/N) [13].
trapping in ion implanted Al(Ag)
60 ,
341
Al
0
Aa
022at.%Ag
in Al TI=T,=77K
1
2MeVHe -RANDOM n+
10 0
150
200
250
300
350
400
450
Channel Number
Fig. 1. Random and (110) aligned spectra of 2 MeV He-ions backscattered from an Al single crystal implanted with 0.22 at.% Ag (peak concentration) at 77 K. (110) aligned spectra are shown (a) after implantation and (b) after additional irradiation with 5.6 X 1Ol5 He+/&‘, 1 MeV at 77 K. and 465 was used as an energy window for silver and the region between 270 and 290 for aluminium. The latter region corresponds to the depth region were Ag is located in the crystal. The results of the angular scan measurement are shown in fig. 2. From the fact that the criticle angle for Al and Ag are of the same size it can be concluded that most of the Ag atoms are on perfect substitutional sites. From the differences of the minimum yield values for Al and Ag a substitutional fraction of 0.88 can be estimated using eq. (1). This fraction is slightly lower than a value 0.94 obtained previously for Ag implanted in Al at 77 K using a peak concentration of 0.07 at% [8]. Slight structures in the angular
‘5,::.li;
3. Results Ag-ions with energies of 300 keV are implanted under random directions in Al single crystals at 77 K, choosing peak concentrations of 0.22 at%. Backscattering and channeling analysis has been performed in situ using a 2 MeV He-ion beam. The random and (110) aligned backscattering spectra are shown in fig. 1. Ag is well separated from Al. The Al surface peak of the aligned spectra (a) is mainly due to oxide formation as an oxygen surface peak is clearly seen. From the large difference of the Ag peak areas of the random spectrum and the (110) aligned spectra (a) of the as-implanted sample a large substitutional fraction can be estimated. In order to get more detailed information on the lattice occupation of Ag in Al, angular scan measurements have been performed. The region between channels 445
-1
-.5
0
.5
TILT ANGLE tdeg)
Fig. 2. Angular yield curves of 2 MeV He-ions backscattered from an Al single crystal implanted with Ag-ions. Implantation and channeling
analysis are performed
at 77 K.
342
I. Khubeis, 0. Meyer / Interstitial
yield curve of Ag, which were more pronounced for samples containing a lower Ag peak concentration [S], indicate that 5 to 10% of the Ag atoms are not on random lattice sites probably due to their interactions with point defects to form unique impurity defect clusters. The as-implanted Al single crystal was irradiated with 5.6 x 1015 Het/cm2, 1 MeV at 77 K. For such a fluence, energy and irradiation temperature the concentration of mixed dumbbells reached a maximum during irradiation experiments of solid solution [7]. As can be seen from the (110) aligned spectra (b) in fig. 1, which was taken after the irradiation experiment, there are only minor changes in the dechanneling yield at channel numbers below 250. The substitutional fraction is not changed in this irradiation experiment. In order to see if the formation of mixed dumbbells is suppressed by competing trapping centers e.g. vacancies or more complex defect clusters present in the implanted region which trap freely mobile SIAs, the implanted samples were annealed to 293 K and above and then irradiated at 77 K. The most important features observed during annealing are shown in fig. 3. Warming up the as-implanted samples from 77 to 293 K leads to a large increase of the dechanneling yield due to a change of the defect structure as can be seen by comparing the (110) aligned spectrum (a) in fig. 3 with spectrum (b) shown in fig. 1. Although the peak area of Ag increases, the substitutional fraction as determined using eq. (1) is still 88%. Annealing to 473 K for 1 h (spectrum (b) in fig. 3) leads to a strong decrease of the dechanneling yield and to a slight increase of the substitutional fraction (f, = 0.93). Annealing to 673” C causes out-diffusion of Ag. Only a few Ag atoms are still located at the surface of the Al single crystal. Samples annealed to 373 and 473 K were cooled
6,
0
Al 022at%
5
Ag I" Al
Ag 1 I_
T,=77K, T,=293K z
2MeV
4
He
s RANDOM + -
b f3
1
3"
1
0 150
200
250
300 Channel
350
400
450
Number
Fig. 3. Random and (110) aligned spectra of 2 MeV He-ions backscattered from an Al single crystal implanted with 0.22 at% Ag (peak concentration) at 77 K. (llO)-aligned spectra are shown (a) at 293 K after implantation of Ag at 77 K, and (b) at 293 K after annealing the sample to 493 K.
trapping in ion implanted AI
0.0-l 0
5 He-FLUENCE
10 (10’5ions/cmz)
15
I
20
Fig. 4. The nonsubstitutional fraction as a function of the 1 MeV He-ion fluence during irradiation at 77 K. The results are shown (a) for an Al +O.l at% Ag solid solution [7], (b) for an implanted system annealed to 393 K (c) for an implanted system annealed to 493 K and (d) for a sample as implanted at 77 K.
down to 77 K and then irradiated with 1 MeV He-ions. The substitutional fraction was determined using 2 MeV He-ions at 77 K. The results are shown in fig. 4 were the fraction of Ag-atoms on nonsubstitutional sites is presented as a function of the He-ion fluence. With increasing He-ion fluence the nonsubstitutional fraction increases and reaches a maximum value of about 34% at a fluence of about 6 X 1015 He+/cm* for the sample annealed to 373 K (curve (b)). For the sample annealed to 473 K, the maximum value is slightly smaller (31%) and is reached at larger fluences (curve (c)). For the unannealed, as implanted sample only a very small increase of the nonsubstitutional component from 10% to 11% was observed in this fluence range (curve (d)). The results for irradiations of a solid solution [7] under similar conditions are shown for comparison (curve (a)). From angular yield measurements [7] the formation of mixed dumbbells could be determined from pronounced peaking of the Ag atom yield in the low fluence region while in the high fluence region peaking was no longer observed. Similar angular yield measurements have been performed here using the annealed implanted alloys irradiated with He-ions at 77 K. The results are shown in fig. 5 for Al and Ag after irradiation with 1.7 X lOi He+/cm2 (0, n), 6.2 X 1015 He+/cm2 (A, A) and 13.0 X 1015 He+/cm2 (0, 0). While the increase
of the minimum
yield for Ag with increas-
seen, there is no peaking of the Ag yield near zero tilt angle. In dilute Al solid solutions mixed dumbbells are stable up to stage III (- 200 K). Complete recovery was observed due to annihilation with vacancies which become mobile at about 200 K [7]. Similar observations were made here for the irradiated ion implanted samples. After annealing to 293 K the substitutional fraction increased from 0.68 to 0.94. It is interesting to note that in contrast to the annealing behaviour of as-implanted samples, the annealed and postirradiated saming He-ion
fluence
is clearly
I: Khubeis, 0. Meyer / Interstitial 1.0
. \
9 ? i 3 2
11
.
4
\
\
0.5-
4.
J
k
z::
4 1
lL.4 I -1.0
AL
.
0
i 1
I I -0.5 0 to.5 TILT ANGLE (deg.1
+1.0
Fig. 5. Angular yield curves of 2 MeV He-ions backscattered from an Al single crystal at II K. The crystal was implanted with Ag ions at 293 K and annealed to 393 K. The results are shown after irradiating the system with various fluences of He-ions (1 MeV) at 77 K (0, w: 1.7X lOI He+/cm2; A, A: 6.2 x 1015 He+/cm2; o, 0: 13 x lOi He+/cm2).
ples did not reveal a pronounced increase of the dechanneling yield during warming up to 293 K.
4. Discussion The results presented above clearly reveal why mixed dumbbells are not formed during ion implantation at temperatures below 150 K. During ion irradiation of dilute solid solutions it was shown that at low fluences every free SIA which escapes the recombination will be trapped by a Ag-atom so that from the initial slope of curve (a) in fig. 4 the production rate of mixed dumbbells can be extracted 151. With increasing fluence the trapping rate decreases due to the increased probability that SIAs are annihilated at vacancies or other competing traps. For a fluence of about 6 X lO1’ He+/cm2, where a maximum concentration of mixed dumbbells is reached, the energy density deposited in nuclear collisions is about 0.21 eV/At, which is about 3 orders of magnitude smaller than the energy density deposited during implantation. This indicates that during implantation of Ag the production rate of mixed dumbbells decreases to zero already for a fluence of about 1 X lOI Ag’/cm2. From this it is assumed that only a rather small fraction of mixed dumbbells (- 10m4 at.%) is produced as compared to the total implanted Ag concentration (- 0.22 at.%). The loss of mobile SIA to the surface during implantation in surface near regions is generally observed [14,15] and a high concentration of vacancies
trapping in ion implanted AI
343
will survive in the implanted regions. From these arguments it is clear that further postirradiation of the as-implanted alloys should not change the substitutional fraction as if proven by experiment. A further test of these assumptions is provided by the annealing and postirradiation experiments. First the competing trapping centers are annealed and then the irradiation experiments are repeated at low temperatures. The annealing behaviour of the dechanneling yield provides some hints on the structure of these centers. The annealing is similar to that of quenched Al single crystals where a clustering of vacancies in stage III is followed by a coarsening into faulted loops at the end of stage III. These processes have been seen by electron microscopy [15] as well as by positron annihilation studies [16]. Especially the formation of double faulted loops will lead to a large increase of the dechanneling yield in agreement with the observed results (curve (a) in fig. 4). The annealing of loops occurs in stage V at about 400 K [15]. This is in good agreement with the large reduction of the dechanneling yield as shown for curve (b) in fig. 3. In general it can be stated that the annealing of the as-implanted Al single crystal as monitored by the dechanneling yield resembles that of a quenched Al single crystal. During postirradiation of implanted alloys annealed to 393 K and 493 K and 77 K the formation of a SIA-Ag defect complex, probably a mixed dumbbell, is observed, as expected. However, the production rate as well as the maximum concentration are smaller than the values observed for irradiated dilute solid solutions [5]. The saturation value is reached at about the same value for the deposited energy density, while the slight decrease of the mixed dumbbell concentration with increasing He-ion fluence is not observed (fig. 4). Irradiation of solid solutions at low fluences produces (100) mixed dumbbells with a Ag atom displacement of 0.14 nm from the lattice site 1171. This displacement amplitude was extracted from the peaking of the Ag yield in a (110) angular scan. Such peaking was not observed after irradiation at higher fluences. This observation and the slight decrease of the concentration was attributed to a change of the trapping configuration when perhaps more than one SIA is trapped at an Ag atom which is also reflected in a more complex annealing behaviour [7]. From simulation calculations it is known that a reduction of the displacement amplitude from 0.14 to 0.12 nm is sufficient to move Ag out of a region of high ion flux and thus to avoid peaking [17]. For mixed dumbbells produced here in annealed ion implanted alloys, peaking of the Ag yield has never been observed. This may indicate that for ion implanted alloys more complex trapping configurations prevail. The rather high substitutional fraction of 0.94, which is observed after warming up to 293 K indicates however that Ag-SIA complexes are formed during irradiation
344
I. Khubeis, 0. Meyer / Interstitial
trapping in ion implanted AI
which completely recover by annihilation with mobile vacancies.
port the model proposed to explain the irradiation stability of as-implanted alloys.
5. Conclusions
The authors want to thank R. Gerber for valuable discussion and for carefully reading the manuscript and B. Strehlau for his experimental support.
In our systematic study on the basic mechanisms of the lattice site occupations of ions implanted into metals it was stated that the study of impurities having negative heats of solution would be of great interest as this type of impurities is usually undersized and tends to form interstitial-impurity complexes by irradiation at low temperatures [9]. The results obtained for ion implanted systems can then be compared with numerous results on impurity-point defect interactions obtained by irradiating diluted Al alloys [2,4]. Ion implanted alloys with small negative values for the heat of solution and for the size mismatch energy reveal large substitutional fractions (- 0.95) although ion irradiation of solid solutions with impurity concentrations similar to those of the ion implanted alloys (- 0.1 at.%) produces large fractions of mixed dumbbells (30% to 60%). For the system Al(Ag) studied here it is reasonable to assume that the production rate of mixed dumbbells decreases to zero already in the low fluence region during implantation at Ag concentration levels (- 10m4 at.%) which are small compared to the total implanted concentration. The implanted region is enriched with vacancies and small vacancy clusters as the mobile SIAs partly recombine with vacancies and partly anneal at the sample surface. Small vacancy clusters contribute to the dechanneling yield at large depths, while the implanted region is not strongly damaged. The annealing behaviour as reflected by the change of the dechanneling yield indicates the agglomeration of the vacancies and the formation and annihilation of faulted dislocation loops. Irradiation of the annealed ion implanted alloys produces the formation of mixed dumbbells in the same region of deposited energy density as observed previously for dilute solid solutions. These results sup-
References [l] R. Gerber, 0. Meyer, and G.C. Xiong, Nucl. Instr. and Meth. B31 (1988) 402. [2] M.L. Swanson, in: Advanced Techniques for Characterizing Microstructures, eds. F.W. Wiffen and J.A. Spitznagel (Metal]. Sot. of AIME, New York, 1982) p. 305. [3] L.C. Feldmann, J.W. Mayer, and ST. Picraux, Materials Analysis by Ion Channeling (Academic Press, New York, 1982). [4] M.L. Swanson and L.M. Howe, Nucl. Instr. and Meth. 218 (1983) 613. [5] M.L. Swanson and L.M. Howe, Radiat. Eff. 41 (1979) 129. [6] M.L. Swanson, L.M. Howe, J.A. Moore, and A.F. QuenneviIle, Nucl. Instr. and Meth. 209/210 (1983) 1029. [7] L.M. Howe M.L. Swanson, and A.F. Quenneville, Nucl. Instr. and Meth. B7/8 (1985) 91. [S] G.C. Xiong, A. Azzam, R. Gerber, and 0. Meyer, Nucl. Instr. and Meth. B29 (1988) 643. [9] 0. Meyer and A. Turos, Materials Science Reports 2(S) (1987) 371. [lo] M.K. Kloska and 0. Meyer, Nucl. Instr. and Meth. B14 (1986) 268. [ll] R. Kaufmann, J. Geerk, and F. Ratzel, Nucl. Instr. and Meth. 205 (1983) 293. [12] H.H. Neely and W. Batter, Phys. Rev. 149 (1966) 535. [13] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [14] H. Bernas, M.O. Ruault, and B. Jouffrey, Phys. Rev. Lett. 27 (1971) 859. [15] B.L. Eyre, J. Phys. F3 (1973) 422 and refs. therein. [16] W.R. Wampler and W.B. Gauster, J. Phys. F8 (1978) Ll. [17] N. Matsunami, M.L. Swanson, and L.M. Howe, Can. J. Phys. 56 (1978) 1057.