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Lattice site occupation of oversized impurity atoms implanted in aluminium single crystals
140
Nuclear Instruments and Methods in Physics Research B19/20 (1987) 140-144 North-Holland, Amsterdam
L A ' I ' T I C E S I T E O C C U P A T I O N O F O V E R S I Z E D I M P U R I T Y A T O M S I M P L A N T E D IN ALUMINIUM SINGLE CRYSTALS M.K. K L O S K A and O. M E Y E R Kernforschungszentrum Karlsruhe, Institut ]'fir Nukleare Festk6rperphysik, P.O.B. 3640, D-7500 Karlsruhe, FRG
Several oversized elements were implanted in aluminium single crystals. The substitutional fraction obtained by in situ ion channeling analysis is strongly dependent on the implantation temperature• Self-interstitial atoms do not influence the lattice site occupation although they affect the damage component in A1. The nonsubstitutional component consists of impurity atom-vacancy complexes. With increasing heat of solution the substitutional fraction decreases for all implantation temperatures• A part of the impurity-vacancy complexes of the A1Cd, the Alln and the AIPb systems can be recovered by postirradiation at low temperature.
1. Introduction A question of fundamental interest in ion implantation metallurgy is concerned with the lattice position an implanted impurity atom will occupy after it comes to rest. Several more or less successful attempts have been made to determine the physical mechanisms which govern the lattice site occupancy [1-3]. An attractive interaction between point defects and impurity atoms exists and leads to the formation of impurity atom-point defect complexes [4,5]. Impurity atoms which are smaller than the host atoms located on substitutional sites generally contract the lattice. Thus self-interstitial atoms (SIAs) which expand the lattice may be trapped by the impurity atoms and form mixed dumbbells. However, large impurity atoms on substitutional sites expand the lattice. In this case vacancies may be trapped and impurity-vacancy complexes are formed. These oversized atoms possess positive values of the heat of solution (AHsol) [6]. Employing this complex formation model, at least three mechanisms can contribute to the final location of the oversized impurity atom: (i) Spontaneous recombination of the impurity atom with a neighbouring vacancy within the relaxation phase of the collision cascade will bring the impurity on a substitutional lattice site. (ii) Trapping of vacancies within the cooling phase of the cascade at temperatures below the temperature of recovery stage III will displace the impurities from substitutional sites. (iii) Additional trapping of mobile vacancies at temperatures above recovery stage III will also lead to displacements. For the region of positive A/-/so1 values, previous investigations were performed on the lattice site occupation of implanted nonsoluble impurity atoms in V [7,8] and Fe [9] bcc-hosts. Here we extend these studies for AI as 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
fcc-host to determine the substitutional fraction as a function of A//so I. Detailed results on the defect complexes contributing to the nonsubstitutional component will be published elsewhere• To distinguish between the influence of mobile SIA~ and vacancies on the lattice site occupation of impurity atoms, the implantation temperature (TI) has to be varied. Below stage I (15-40 K) [10] point defects are not mobile. Between stage I and III (190-250 K) [10] only SIAs are mobile. Above stage III also vacancies are mobile.
2. Experimental The experimental arrangement and the preparation of the A1 single crystals have been described previously [11]. The samples are mounted on a liquid helium (4.2 K) or liquid nitrogen (77 K) cooled goniometer [12] which is coupled to a 350 keV ion implanter for implantation and postirradiation and to a 2.0 MeV Van de Graaff accelerator for in situ ion channeling analysis. The concentration of the implanted species was between 0.06 and 1.5 at.%. The energies of the ions used for postirradiation were chosen so that they penetrate the implanted region (60-120 nm). In order to avoid sample heating the power dissipated during implantation, postirradiation and channeling measurements was limited to 50 mW, far below the cooling power of the goniometer (2 W at 5 K). The application of channeling measurements to material analysis [13] and the analysis of impurity atom-point defect complexes [14] are well documented in the literature. Angular scans were taken to separate the substitutional component from other configurations of high and low symmetry. For the representation of the results of the postirradiation experiments the increase of the substitutional fraction fs(~)
141
M.K. Kloska, O. Meyer / Oversized impurity atoms in A L single c~. stals 1.5 1.0-1016Ga+ 200 keY in AI ~110, $ 1 . 3 5 a t . % Go
1.0n _J LIJ >r7
___ 0.5._1 <[ no
n AI zx Go
z
77K
00 1.0
'
(3'.0
'
1.0 '
TILT ANGLE ( d e g )
Fig. 1. Angular scan for 2.0 MeV 4He ÷ ions backscattered from an AI single crystal implanted with Ga. Implantation and analysis were performed in situ at 77 K.
with ion fluence ( ~ ) was normalized using the formula fsN ( ~ ) = ( f ~ ( ~ ) - f~(293 K ) ) / ( f~(5 K) - f~(293 K ) ) . The
modified Kinchin-Pease
formula [15] dpa =
( 0 . 8 q ~ F D ) / 2 E D N ) , with the atomic density (N) of the
host and a threshold displacement energy (ED) of 18 eV [16] has been used to convert the employed ion fluences (q~) to a displacement per atom (dpa) scale. To determine the energy density deposited into nuclear collisions (FD) above E D in the depth region where the impurity atoms are located the TRIM2 program [17] has been used.
the angular yield curve of Cd becomes narrower and structured indicating that the Cd atoms are displaced from the substitutional sites at about 220 K [19]. In this temperature region the vacancies are mobile and are trapped by the Cd atoms. At Tx = 293 K the angular yield curve closely resembles that of a random distribution of Cd atoms [19]. It was shown that Cd-vacancy complexes are formed and not Cd precipitates [11,19]. F o r In implanted in Al at 77 and 293 K we obtained f~ = 0.83 and fs = 0.06, respectively. These values are in good agreement with the values reported in ref. [20]. By implanting Pb in A1 at 77 K no concentration dependence in the range from 0.06 to 1.03 at.% Pb on f~ = 0.57 was noted. In fig. 2 the angular yield curves for the A1Pb system are shown. The values of the critical angles ~ l / 2 ( m l ) = 0.67 °, ~bl/E(Pb) = 0.65 ° are very similar. As in the case of AlIn the nonsubstitutional fraction might consist of impurity-vacancy complexes [20]. Pb implanted at 293 K exhibits yield values slightly above 1.0 at tilt angles close to 0.0 o in its angular yield curve which leads to f~ = - 0 . 0 2 and indicates the occurrence of Pb-trivacancy complexes [21]. F o r Rb implanted at 77 and 293 K f~ is near 0.0 without any indication that regular lattice sites are occupied. F o r Cs implanted at 293 K a more pronounced flux peaking was measured which confirms the assumption of complex formation and leads to f~ = - 0.17. In our systematic study the noble gases Kr and Xe constitute in this respect a special position. Due to their filled outer electron shell the chemical interaction with the host atoms can be neglected. But, by trapping a vacancy the noble gas-vacancy complex becomes very mobile.
5.5 1015Pb÷ 300keY in AI c110, - 1.03 ot.% Pb
3. Results After implantation of Ga in an A1 single crystal at 77 K an f~ value of 1.0 is obtained. In fig. 1 the angular yield curves for A1 and Ga match perfectly, the critical angles for A1 and Ga are equal (ffx/2(A1,Ga)= 0.65 ° ). Therefore it can be inferred that the Ga atoms are located on substitutional lattice sites. Implanted at 293 K, f~ is 0.84 and the critical angle for Ga is narrower than that of A1 [18]. Cd implanted into AI at 5 K yields an f~ value of 1.0. Implanted at 77 K an f~ value of 0.94 was obtained in the concentration range from 0.2 to 10.2 at.% Cd [11,19]. The critical angles for AI and Cd have the same value [11]. At T I -- 293 K most of the Cd atoms come to rest on nonregular lattice sites of low symmetry [11]. U p o n warming up a low temperature implant to 293 K
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0
0
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o~
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~ /
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Tilt Angle (deg) Fig. 2. Angular scans of AI single crystals implanted with Pb. Implantation and analysis were performed at 77 and 293 K. II. METALS
M.K. Kiosk.a, O. Meyer / Oversized impurity atoms in A L single crystals
142
Table 1 Substitutional fraction (fs) of oversized impurity atoms implanted at different temperatures (T t = 5, 77 and 293 K) in dependence of the heat of solution (AHsol) and the size mismatch energy [6]. Implanted species
Heat of solution (kJ/mol)
Size mismatch energy (kJ/mol)
fs(5 K)
fs(77 K)
Ga Cd Hg In Kr Xe Pb Rb Cs
3.70 14.23 17.04 30.32 36.03 43.67 48.73 135.12 153.42
3.15 6.08 6.70 19.70 7.27 16.44 40.31 71.05 68.56
1.00 0.49 0.03
1.00 0.94 0.75 0.83 0.47 0.33 0.57 0.02 0.00
Substitutional fraction
0.6
In Kr Bi J J Xeipb
\
- 0.02 0.00 - 0.17
[] sK } o 77K a 293K * 293K/3/
,,K T~=TAI t I I
Rb i
1
I
o
\o/ "" Q2
0.84 0.14 0.06
that the mobility of SlAs plays only a minor role for the lattice site occupation of impurity atoms in A1. The f~ values for samples implanted at T I = 293 K are always lower than for those implanted at T t = 77 K. This result can be explained by an additional trapping of mobile vacancies at 293 K followed by an enhanced formatiola of i m p u r i t y - v a c a n c y complexes. F o r the A1 host in contrast to V and Fe hosts, there seems to be no difference in the defect configuration between a sample
F o r V [7,8] and Fe [9] implanted with impurity atoms it was shown previously that f~ is crucially correlated with A H~oI. Our results for AI as a host are summarized in table i and fig. 3. F r o m table 1 it can be seen, that A//so I and the size mismatch energy [6] are nearly proportional. Further it is seen, that fs decreases with increasing A//so I irrespective of the substrate temperature during implantation. N o significant difference of f~ between T I = 5 and 77 K is observed, indicating
IGa Cd . _ / I SbtHg l.O o,.~
fs(293 K)
\
\
.13
A
*
\
O0
*
- 0.2
0
~ r , /
I
50 Heat of Solution
I
100 AHsol (kJ/mol)
I
150
Fig. 3. Substitutional fraction (fs) of several oversized elements implanted in AI single crystals as a function of the heat of solution (A Hsoj ) and the implantation/analysis temperature (5, 77 and 293 K).
143
M.K. Kloska, O, Meyer / Oversized impurity atoms in AL single co'stals
" o o *
1200 2
J Rondom( } Aligned 5.0 1076H+ 50 keV -~0.03 dpo Aligned postirrodioted 5.01017 H+ 50keYS0.3 dpo Aligned ot 5K with:
~
160
AI
80O
8
-80
q ~
4O0
(..)
0 2OO
230
4&o
300 CHANNEL
470
480
NUMBER
Fig. 4. Random and (110) aligned backscattering spectra from a 8.0 x 1015 Cd+cm2, 200 keV implanted and 50 keV H + postirradiated A1 single crystal. Implantation (at 293 K), postirradiation and analysis (at 5 K) were performed in situ.
implanted at TI = 293 K and a sample implanted at TI _< 77 K and annealed to 293 K. The decrease of f~ during annealing is caused by thermally activated mobile vacancies and possible dissolving of vacancy clusters and subsequent trapping of the migrating vacancies by the impurity atoms. The vacancy clusters have remained from the initial implantation of the impurity atoms at T t _< 77 K. The nonsubstitutional component f~(TI = 77 K ) fs(T~ = 293 K) can be recovered completely by postirradiation at low temperature, even at 5 K [11,19]. In
_
1.0-
~ 08-
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fig. 4 random and (110) aligned backscattering spectra from a Cd implanted and H ÷ postirradiated A1 single crystal are shown. For 0.03 and 0.3 dpa fSN values of 0.21 and 0.70 are obtained, respectively. Applying similar postirradiation fluences at 5 K and at 77 K, a far stronger increase of the damage in the host lattice is noted at 5 K. This result is attributed to the fact that at 5 K the SIAs are immobile and thus enhanced annealing by recombination is suppressed. The effectivity of the recovery process by postirradiation is the same for the A1Cd and the AIPb systems as shown in fig 5. In prevlo'--us studies [11,1--9]it was shown that the effectivity also is independent of the projectile used, the mean transferred energy, the cascade efficiency factor and TI below the temperature of stage III.
Kr+- FLUENCE(dpa)
Fig. 5. Increase of the normalized substitutional fraction (fs~) of Cd (D) and Pb (A) implanted at 293 K in A1 single crystals due to 300 keV Kr + postirradiations at 77 K.
4.
Conclusions
Combined with the results of refs. [11] and [19] the following conclusions can be drawn. The lattice site occupation of implanted oversized impurity atoms is not influenced by mobile SIAs. The nonsubstitutional component consists of impurity-vacancy complexes. This has also been observed for the system AlIn, produced by implanting In in A1 at 77 K, using--~rturbed angular correlation measurements [20]. At TI below the temperature of stage III the process of lattice site occupation is governed by spontaneous recombination of II. METALS
144
M.K. Kloska, O. Meyer / Oversized impurity atoms in AL single crystals
impurity-vacancy pairs and further vacancy trapping within the cooling phase [22] of the collision cascade. At T I above the temperature of stage III additional mobile vacancies are trapped in the delayed cascade regime leading to a significant decrease of f~. The capture radius and the binding energy are strongly correlated with the size mismatch energy and A Hsol, respectively. By trapping a vacancy the value of d/-/soI is reduced [5]. The recovery of the impurity-vacancy complexes due to postirradiation is attributed to the process of spontaneous recombination within the relaxation phase [22] of the displacement spike in the cascade [19]. The fact that the effectivity for recovery is independent of the system allows one to draw the conclusion that the structural configuration of the complexes is the same for all implanted systems. This is underlined by the similarity of the equilibrium phase diagrams [23] which show, except for Ga, a rather low solubility of the impurities in A1 in the solid and liquid phase. We want to thank M. Kraatz and B. Strehlau for performing the implantations and A. Azzam and F. Pleiter for many helpful discussions and suggestions.
References [1] D.K. Brice, Inst. Phys. Conf. Ser. no. 28 (1976) 334. [2] J.M. Poate, J.A. Borders, A.G. Cullis, and J.K. Hirvonen, Appl. Phys. Lett. 30 (1977) 365. [3] D.K. Sood and G. Deamaley, Inst. Phys. Conf. Ser. no. 28 (1976) 196. [4] For a review of the analysis techniques and the results see: M.L. Swanson, in: Advanced Techniques for Characterizing Microstructures, eds., F.W. Wiffen and J.A. Spitznagel, Techniques of Metallurgy, series AIME.
[5] A. Turos, A. Azzam, M.K. Kloska, and O. Meyer, these Proceedings, (IBMM '86) Nucl. Instr. and Meth. B19/20 (1987) 123. [6] A.R. Miedema, P.F. de Ch~tel, and F.R. de Boer, Physica 100B (1980) 1.
[7] A. Azzam and O. Meyer, Phys. Rev. 33 (1986) 3499. [8] A. Azzam and O. Meyer, Nucl. Instr. and Meth. B7/8 (1985) 113. [9] A. Turos and O. Meyer, Phys. Rev. B33 (1986) 8829; O. Meyer and A. Turos, these Proceedings (IBMM '86) Nucl. Instr. and Meth. B19/20 (1987) 136. [10] G. Burger, K. Isebeck, J. Voelkl, W. Schilling, and H. Wenzl, Z. Angew. Phys. 22 (1967) 452. [11] M.K. Kloska and O. Meyer, Nucl. Instr. and Meth. B14 (1986) 268. [12] R. Kaufmann, J. Geerk, and F. Ratzel, Nucl. Instr. and Meth. 205 (1983) 293. [13] L.C. Feldman J.W. Mayer, and S.T. Picraux, Material Analysis by Ion Channeling (Academic Press, New York, 1982). [14] M.L Swanson and L.M. Howe, Nucl. Instr. and Meth. 218(1983) 613. [15] M.J. Norgett, M.T. Robinson, and LM. Torrens, Nucl. Eng. Des. 33 (1975) 50. [16] H.H. Neely and W. Bauer, Phys. Rev. 149 (1966) 535. [17] J.P. Biersack and L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [18] T. Hussain, J. Geerk, F. Ratzel, and G. Linker, Appl. Phys. Lett. 37 (1980) 298. [19] M.K. Kloska and O. Meyer, Phys. Rev. Lett. (1986) accepted for publication. [20] F. Pleiter and K.G. Prasad, Hyperfine Interactions 20 (1984) 221. [21] M.L. Swanson, L.M. Howe, J.A. Moore, and A.F. Quermeville, Can. J. Phys. 62 (1984) 826. [22] M.W. Guinan and J.H. Kinney, J. Nucl. Mater. 103/104 (1981) 1319. [23] W.G. Moffatt, The Handbook of Binary Phase Diagrams (General Electric, New York, 1978).
Nuclear Instruments and Methods in Physics Research B19/20 (1987) 140-144 North-Holland, Amsterdam
L A ' I ' T I C E S I T E O C C U P A T I O N O F O V E R S I Z E D I M P U R I T Y A T O M S I M P L A N T E D IN ALUMINIUM SINGLE CRYSTALS M.K. K L O S K A and O. M E Y E R Kernforschungszentrum Karlsruhe, Institut ]'fir Nukleare Festk6rperphysik, P.O.B. 3640, D-7500 Karlsruhe, FRG
Several oversized elements were implanted in aluminium single crystals. The substitutional fraction obtained by in situ ion channeling analysis is strongly dependent on the implantation temperature• Self-interstitial atoms do not influence the lattice site occupation although they affect the damage component in A1. The nonsubstitutional component consists of impurity atom-vacancy complexes. With increasing heat of solution the substitutional fraction decreases for all implantation temperatures• A part of the impurity-vacancy complexes of the A1Cd, the Alln and the AIPb systems can be recovered by postirradiation at low temperature.
1. Introduction A question of fundamental interest in ion implantation metallurgy is concerned with the lattice position an implanted impurity atom will occupy after it comes to rest. Several more or less successful attempts have been made to determine the physical mechanisms which govern the lattice site occupancy [1-3]. An attractive interaction between point defects and impurity atoms exists and leads to the formation of impurity atom-point defect complexes [4,5]. Impurity atoms which are smaller than the host atoms located on substitutional sites generally contract the lattice. Thus self-interstitial atoms (SIAs) which expand the lattice may be trapped by the impurity atoms and form mixed dumbbells. However, large impurity atoms on substitutional sites expand the lattice. In this case vacancies may be trapped and impurity-vacancy complexes are formed. These oversized atoms possess positive values of the heat of solution (AHsol) [6]. Employing this complex formation model, at least three mechanisms can contribute to the final location of the oversized impurity atom: (i) Spontaneous recombination of the impurity atom with a neighbouring vacancy within the relaxation phase of the collision cascade will bring the impurity on a substitutional lattice site. (ii) Trapping of vacancies within the cooling phase of the cascade at temperatures below the temperature of recovery stage III will displace the impurities from substitutional sites. (iii) Additional trapping of mobile vacancies at temperatures above recovery stage III will also lead to displacements. For the region of positive A/-/so1 values, previous investigations were performed on the lattice site occupation of implanted nonsoluble impurity atoms in V [7,8] and Fe [9] bcc-hosts. Here we extend these studies for AI as 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
fcc-host to determine the substitutional fraction as a function of A//so I. Detailed results on the defect complexes contributing to the nonsubstitutional component will be published elsewhere• To distinguish between the influence of mobile SIA~ and vacancies on the lattice site occupation of impurity atoms, the implantation temperature (TI) has to be varied. Below stage I (15-40 K) [10] point defects are not mobile. Between stage I and III (190-250 K) [10] only SIAs are mobile. Above stage III also vacancies are mobile.
2. Experimental The experimental arrangement and the preparation of the A1 single crystals have been described previously [11]. The samples are mounted on a liquid helium (4.2 K) or liquid nitrogen (77 K) cooled goniometer [12] which is coupled to a 350 keV ion implanter for implantation and postirradiation and to a 2.0 MeV Van de Graaff accelerator for in situ ion channeling analysis. The concentration of the implanted species was between 0.06 and 1.5 at.%. The energies of the ions used for postirradiation were chosen so that they penetrate the implanted region (60-120 nm). In order to avoid sample heating the power dissipated during implantation, postirradiation and channeling measurements was limited to 50 mW, far below the cooling power of the goniometer (2 W at 5 K). The application of channeling measurements to material analysis [13] and the analysis of impurity atom-point defect complexes [14] are well documented in the literature. Angular scans were taken to separate the substitutional component from other configurations of high and low symmetry. For the representation of the results of the postirradiation experiments the increase of the substitutional fraction fs(~)
141
M.K. Kloska, O. Meyer / Oversized impurity atoms in A L single c~. stals 1.5 1.0-1016Ga+ 200 keY in AI ~110, $ 1 . 3 5 a t . % Go
1.0n _J LIJ >r7
___ 0.5._1 <[ no
n AI zx Go
z
77K
00 1.0
'
(3'.0
'
1.0 '
TILT ANGLE ( d e g )
Fig. 1. Angular scan for 2.0 MeV 4He ÷ ions backscattered from an AI single crystal implanted with Ga. Implantation and analysis were performed in situ at 77 K.
with ion fluence ( ~ ) was normalized using the formula fsN ( ~ ) = ( f ~ ( ~ ) - f~(293 K ) ) / ( f~(5 K) - f~(293 K ) ) . The
modified Kinchin-Pease
formula [15] dpa =
( 0 . 8 q ~ F D ) / 2 E D N ) , with the atomic density (N) of the
host and a threshold displacement energy (ED) of 18 eV [16] has been used to convert the employed ion fluences (q~) to a displacement per atom (dpa) scale. To determine the energy density deposited into nuclear collisions (FD) above E D in the depth region where the impurity atoms are located the TRIM2 program [17] has been used.
the angular yield curve of Cd becomes narrower and structured indicating that the Cd atoms are displaced from the substitutional sites at about 220 K [19]. In this temperature region the vacancies are mobile and are trapped by the Cd atoms. At Tx = 293 K the angular yield curve closely resembles that of a random distribution of Cd atoms [19]. It was shown that Cd-vacancy complexes are formed and not Cd precipitates [11,19]. F o r In implanted in Al at 77 and 293 K we obtained f~ = 0.83 and fs = 0.06, respectively. These values are in good agreement with the values reported in ref. [20]. By implanting Pb in A1 at 77 K no concentration dependence in the range from 0.06 to 1.03 at.% Pb on f~ = 0.57 was noted. In fig. 2 the angular yield curves for the A1Pb system are shown. The values of the critical angles ~ l / 2 ( m l ) = 0.67 °, ~bl/E(Pb) = 0.65 ° are very similar. As in the case of AlIn the nonsubstitutional fraction might consist of impurity-vacancy complexes [20]. Pb implanted at 293 K exhibits yield values slightly above 1.0 at tilt angles close to 0.0 o in its angular yield curve which leads to f~ = - 0 . 0 2 and indicates the occurrence of Pb-trivacancy complexes [21]. F o r Rb implanted at 77 and 293 K f~ is near 0.0 without any indication that regular lattice sites are occupied. F o r Cs implanted at 293 K a more pronounced flux peaking was measured which confirms the assumption of complex formation and leads to f~ = - 0.17. In our systematic study the noble gases Kr and Xe constitute in this respect a special position. Due to their filled outer electron shell the chemical interaction with the host atoms can be neglected. But, by trapping a vacancy the noble gas-vacancy complex becomes very mobile.
5.5 1015Pb÷ 300keY in AI c110, - 1.03 ot.% Pb
3. Results After implantation of Ga in an A1 single crystal at 77 K an f~ value of 1.0 is obtained. In fig. 1 the angular yield curves for A1 and Ga match perfectly, the critical angles for A1 and Ga are equal (ffx/2(A1,Ga)= 0.65 ° ). Therefore it can be inferred that the Ga atoms are located on substitutional lattice sites. Implanted at 293 K, f~ is 0.84 and the critical angle for Ga is narrower than that of A1 [18]. Cd implanted into AI at 5 K yields an f~ value of 1.0. Implanted at 77 K an f~ value of 0.94 was obtained in the concentration range from 0.2 to 10.2 at.% Cd [11,19]. The critical angles for AI and Cd have the same value [11]. At T I -- 293 K most of the Cd atoms come to rest on nonregular lattice sites of low symmetry [11]. U p o n warming up a low temperature implant to 293 K
"e 1.0 .e
0
0
0000000000
>-
o~
"(3
~ /
N O
E o
z 0.5
o Pb} 293K 0.0
-1;
0'0
lb
Tilt Angle (deg) Fig. 2. Angular scans of AI single crystals implanted with Pb. Implantation and analysis were performed at 77 and 293 K. II. METALS
M.K. Kiosk.a, O. Meyer / Oversized impurity atoms in A L single crystals
142
Table 1 Substitutional fraction (fs) of oversized impurity atoms implanted at different temperatures (T t = 5, 77 and 293 K) in dependence of the heat of solution (AHsol) and the size mismatch energy [6]. Implanted species
Heat of solution (kJ/mol)
Size mismatch energy (kJ/mol)
fs(5 K)
fs(77 K)
Ga Cd Hg In Kr Xe Pb Rb Cs
3.70 14.23 17.04 30.32 36.03 43.67 48.73 135.12 153.42
3.15 6.08 6.70 19.70 7.27 16.44 40.31 71.05 68.56
1.00 0.49 0.03
1.00 0.94 0.75 0.83 0.47 0.33 0.57 0.02 0.00
Substitutional fraction
0.6
In Kr Bi J J Xeipb
\
- 0.02 0.00 - 0.17
[] sK } o 77K a 293K * 293K/3/
,,K T~=TAI t I I
Rb i
1
I
o
\o/ "" Q2
0.84 0.14 0.06
that the mobility of SlAs plays only a minor role for the lattice site occupation of impurity atoms in A1. The f~ values for samples implanted at T I = 293 K are always lower than for those implanted at T t = 77 K. This result can be explained by an additional trapping of mobile vacancies at 293 K followed by an enhanced formatiola of i m p u r i t y - v a c a n c y complexes. F o r the A1 host in contrast to V and Fe hosts, there seems to be no difference in the defect configuration between a sample
F o r V [7,8] and Fe [9] implanted with impurity atoms it was shown previously that f~ is crucially correlated with A H~oI. Our results for AI as a host are summarized in table i and fig. 3. F r o m table 1 it can be seen, that A//so I and the size mismatch energy [6] are nearly proportional. Further it is seen, that fs decreases with increasing A//so I irrespective of the substrate temperature during implantation. N o significant difference of f~ between T I = 5 and 77 K is observed, indicating
IGa Cd . _ / I SbtHg l.O o,.~
fs(293 K)
\
\
.13
A
*
\
O0
*
- 0.2
0
~ r , /
I
50 Heat of Solution
I
100 AHsol (kJ/mol)
I
150
Fig. 3. Substitutional fraction (fs) of several oversized elements implanted in AI single crystals as a function of the heat of solution (A Hsoj ) and the implantation/analysis temperature (5, 77 and 293 K).
143
M.K. Kloska, O, Meyer / Oversized impurity atoms in AL single co'stals
" o o *
1200 2
J Rondom( } Aligned 5.0 1076H+ 50 keV -~0.03 dpo Aligned postirrodioted 5.01017 H+ 50keYS0.3 dpo Aligned ot 5K with:
~
160
AI
80O
8
-80
q ~
4O0
(..)
0 2OO
230
4&o
300 CHANNEL
470
480
NUMBER
Fig. 4. Random and (110) aligned backscattering spectra from a 8.0 x 1015 Cd+cm2, 200 keV implanted and 50 keV H + postirradiated A1 single crystal. Implantation (at 293 K), postirradiation and analysis (at 5 K) were performed in situ.
implanted at TI = 293 K and a sample implanted at TI _< 77 K and annealed to 293 K. The decrease of f~ during annealing is caused by thermally activated mobile vacancies and possible dissolving of vacancy clusters and subsequent trapping of the migrating vacancies by the impurity atoms. The vacancy clusters have remained from the initial implantation of the impurity atoms at T t _< 77 K. The nonsubstitutional component f~(TI = 77 K ) fs(T~ = 293 K) can be recovered completely by postirradiation at low temperature, even at 5 K [11,19]. In
_
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~ 08-
0
A I Cd
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m
n,0 Z
fig. 4 random and (110) aligned backscattering spectra from a Cd implanted and H ÷ postirradiated A1 single crystal are shown. For 0.03 and 0.3 dpa fSN values of 0.21 and 0.70 are obtained, respectively. Applying similar postirradiation fluences at 5 K and at 77 K, a far stronger increase of the damage in the host lattice is noted at 5 K. This result is attributed to the fact that at 5 K the SIAs are immobile and thus enhanced annealing by recombination is suppressed. The effectivity of the recovery process by postirradiation is the same for the A1Cd and the AIPb systems as shown in fig 5. In prevlo'--us studies [11,1--9]it was shown that the effectivity also is independent of the projectile used, the mean transferred energy, the cascade efficiency factor and TI below the temperature of stage III.
Kr+- FLUENCE(dpa)
Fig. 5. Increase of the normalized substitutional fraction (fs~) of Cd (D) and Pb (A) implanted at 293 K in A1 single crystals due to 300 keV Kr + postirradiations at 77 K.
4.
Conclusions
Combined with the results of refs. [11] and [19] the following conclusions can be drawn. The lattice site occupation of implanted oversized impurity atoms is not influenced by mobile SIAs. The nonsubstitutional component consists of impurity-vacancy complexes. This has also been observed for the system AlIn, produced by implanting In in A1 at 77 K, using--~rturbed angular correlation measurements [20]. At TI below the temperature of stage III the process of lattice site occupation is governed by spontaneous recombination of II. METALS
144
M.K. Kloska, O. Meyer / Oversized impurity atoms in AL single crystals
impurity-vacancy pairs and further vacancy trapping within the cooling phase [22] of the collision cascade. At T I above the temperature of stage III additional mobile vacancies are trapped in the delayed cascade regime leading to a significant decrease of f~. The capture radius and the binding energy are strongly correlated with the size mismatch energy and A Hsol, respectively. By trapping a vacancy the value of d/-/soI is reduced [5]. The recovery of the impurity-vacancy complexes due to postirradiation is attributed to the process of spontaneous recombination within the relaxation phase [22] of the displacement spike in the cascade [19]. The fact that the effectivity for recovery is independent of the system allows one to draw the conclusion that the structural configuration of the complexes is the same for all implanted systems. This is underlined by the similarity of the equilibrium phase diagrams [23] which show, except for Ga, a rather low solubility of the impurities in A1 in the solid and liquid phase. We want to thank M. Kraatz and B. Strehlau for performing the implantations and A. Azzam and F. Pleiter for many helpful discussions and suggestions.
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