- Email: [email protected]
Investigation of ion beam synthesized iron silicide by RBS, XRD, and Mössbauer spectroscopy (CEMS)
NuclearInstruments and Methods in Physics Research B68 (1992) 241-244 North-Holland
MON B
Beam lnteraetions with NatedslsiAtoms
Investigation of ion beam synthesized iron silicide by RBS, XRD, and M6ssbauer spectroscopy (CEMS) H . Reuther, E. Wieser, D . Panknin, R. Gr6tzschel and W. Skorupa Central Institute for Nuclear Research, Rossendorf, PF 19, 0-8051 Dresden, Germany
G . Querner
Technical University Dresden, Mommsenstrasse 13, 0-8027 Dresden, Germany
The formation of FeSi Z by implantation of 4 and 5X1017 Fe/C.2 (300 keV) at 350°C into (100) silicon and subsequent annealing in the temperature range between 600 and 1050°C is studied . Analysis of the phase composition by conversion electron Mössbauer spectroscopy (CEMS) and X-raydiffraction shows an admixture of Fesi beside FeSiZ. The redistribution of iron due to the phase transition from 0- to a-FeSi Z and vice versa is investigated by Rutherford backscattering spectrometry.
1. Introduction The formation of buried insulating, semiconducting or metallic layers by ion implantation is of growing interest in micro-electronics and technology of microsystems. With this respect FeSi Z is an attractive compound because there exist both a semiconducting low temperature phase (ß-FeSiZ) with a composition of 67.7 at.% silicon which has an orthorombic structure and a high temperature phase ((x-FeSi Z) in the composition range of 69.6-72.1 at.% silicon which is tetragonal. The transition from ß- to a-FeSi Z occurs at 967°C [1] for bulk silicide . The formation of iron silicide by high dose implantation into silicon and subsequent annealing at different temperatures has been investigated by several authors [2-6]. A coexistence of FeSi Z and FeSi has been observed in the as-implanted state [3,4] and after annealing [4]. First results concerning the phase transformation of FeSi Z were obtained using RBS and electrical measurements [4,5]. The aim of this work is to study - the phase composition of a silicide layer formed by implantation of Fe with a dose at which the iron concentration in the maximum of the implantation profile corresponds to nearly stoichiometric FeSi Z, and - changes in the structure of the implanted layer by post-implantation annealing, especially the phase transformation from ß- to a-FeSi2 and vice versa .
2. Experimental (100) Si was implanted with doses of 4 and 5 x 1017 Fe +/cmZ (300 keV), respectively, at 350°C. For M6ssbauer spectroscopy the sample was additionally implanted with the M6ssbauer isotope 57Fe (1 .6 x 10 16 57 Fe/cmZ, 300 keV at 350°C). The temperature treatment in N2 gas was carried out in several steps in each case with a first annealing at 600°C, 60 min. The following annealing steps were performed at 850°C (stabilization of P-FeSi Z), 1050°C (transition from ß- to a-FeSi Z) and again at 850°C (retransformation from ato ß-FeSi Z). The iron distribution was studied by Rutherford backscattering spectrometry (RBS) using 1.7 MeV He' ions. Informations on the phase composition of the implanted layer were obtained by conversion electron M6ssbauer spectroscopy (CEMS) and by X-ray diffraction (XRD). While RBS and XRD give informations about the whole implanted layer the main information of CEMS is provided by the near-surface part of the silicide layer because of the strong absorption of the electrons in the material. 3. Results and discussion 3.1. Phase composition Fig. 1 represents the RBS spectra of an as-implanted sample and of samples annealed at 850 and
0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
V. BACKSCATTERING
242
H. Reuther et al. /Ion beam synthesized FeSi2
Table 1 Value R of the average atomic concentration ratio Si/Fe at the depth of highest iron concentration and phase composition in dependence on the annealing conditions; sample 1: 4x1017/cm2, sample 2 : 5x10 17/cm2 Annealing
Sample
R
As-implanted
1
600°C, 60 min
1 2 1
2.00 1.85 2 .05 1 .98
2
1 2
2.16 2.10 2.66 2.46
Phase composition (XRD) 0-FeSi 2, FeSi ß-FeSi 2, FeSi ß-FeSi 2, FeSi ß-Fe`2, FeSi P-FeSi 2, FeSi 0-FeSi 2, FeSi a-FeSi 2, FeSi o-FeSi 2, FeSi
1
2.40
a-FeSi 2, FeSi
1
2.26
ß-FeSi 2, FeSi (a-FeSi2)
600°C, 60 min
+850°C, 30 min 600°C,60 min + 1050°C, 30 min 600°C,60 min +850°C, 30 min + 1050°C, 30 s 600°C,60 min + 1050°C, 30 min +850°C, 300 min
2
1050°C, respectively. The silicide layer is buried below a defective Si top layer of about 20 num . The silicon signal corresponding to this top layer shows the decreasing defect density with increasing annealing temperature by comparison of the aligned spectra with the random spectrum. With increasing annealing temperature the edges of the Fe profiles become steeper. The maximum Fe concentration in the plateau-like part of the 'depth distribution is remarkably lower for the sample annealed at 1050°C compared to the 850°C annealing. The average atomic concentration ratio Si/Fe = R of the silicide layer (calculated from random spectra)
increases with increasing temperature from 1 .85 to 2.46 for the implantation of 5 x 10 17 :,m -2 (table 1). The R values obtained by 850 and 1050°C annealing are in very good agreement with the stoichiometric ratio for ß-FeSi 2 and a-FeSi 2, respectively . The XRD diffraction pattern shows up to an annealing temperature of 850°C peaks of the phases ß-FeSi 2 and FeSi and after the 1050°C annealing peaks of the phases a-FeSi 2 and also FeSi (table 1). From the diffraction results it is not possible to get an information about the relative amount of FeSi and FeSi 2 because of the strong texture of the silicide layer. In fig. 2 M6ssbauer spectra are shown for the as-implanted state and for samples annealed at 850 and 1050°C. All CEMS spectra show an asymmetrical doublet which can be resolved into two symmetrical quadrupole doublets, in the case of fig. 2d even into three doublets. The hyperfine parameters of these doublets are given in table 2, the fractions of the subspectra in table 3. The hyperfine parameters of the doublets I and II correspond to those of ß-FeSi 2 and FeSi as given in ref. [3]. Annealing up to 850°C results in a slight increase of the ß-FeSi 2 fraction and a slight decrease of the FeSi fraction. However, by annealing at 1050°C doublet I (ß-FeSi2) disappears and a new component (doublet III) arises. In agreement with the ß to a phase transition proved by XRD doublet III can be ascribed to a-FeSi 2. The fraction of FeSi detected by CEMS increases by the ß to a phase transition of FeSi 2 and decreases during the retransformation by a subsequent long time annealing at 850°C. The occurrence of FeSi already in the as-implanted state means that the formation of this phase is caused by the implantation process. The good agreement of the average Si/Fe ratio determined from the RBS spectra with the values reported for bulk 0- and a-FeSi2
300CHANNEL N0 .
Fig. 1. Annealing dependence of the RBS spectra of a sample implanted with 4x 10 17 Fe/cm 2, 1 - as-implanted, 2 - 600°C, 60 min+850°C, 30 min, 3 - 600°C, 60 min+1050°C, 30 min. (Measuring geometry aligned (100), for 1 also random.)
H. Reuther et al. / Ion beam synthesized FeSi 2
243
Table 2 Isomer shift (IS) and quadrupole splitting (EQS) of the doublets I-111 of the Müssbauer spectra (number in brackets denotes the standard deviation)
z
IS [mm s'] EQS [mm s -1 ]
Fw
Doublet I (ß-FeSi2 ) 0.02(6) 0.43(6)
Il (FeSi) 0.17(5) 0.50(5)
III (a-FeSiz) 0.30(7) 0.74(9)
w a
z 0 w â W
-2
.
Ü VELOCITY (mm/s)
+2
Fig. 2. Annealing dependence of conversion electron M6ssbauer spectra of a sample implanted with 3.8x1017 56 Fe/cm2 and 1 .6x106 s7 Fe/cm 2, (a) as implanted; (b) 600°C, 60 min +850°C, 30 min; (c)as (b) +1050°C, 36 rain ; (d) as (c) +850°C, 30 min; (e) as (d) +850°C,300min. indicates that the amount of the FeSi admixture within the silicide layer should be small. Contrary to the RBS result aratio of the fraction of FeSi and FeSi 2 of about 1 :1 has been observed by CEMS . This can be explained by the assumption that the FeSi precipitates are mainly located at the surface near edge of the Fe distribution which has the highest weight forthe CEMS signal .
3.2. Phase transformation The RBS spectra of fig. 1 indicate the phase transformation from the orthorhombic ß-FeSi 2 to the te-
tragonal a-FeSi z by a distinct decrease of the average Fe concentration. Therefore the width of the silicide layer increases from about 100 nm to about 130 nm (4 x 10" cm -2 ). The transformation is already complete after annealing at 1050°C for 30 s as proved also by XRD (see table 1) . The RBS spectra of fig. 3 demonstrate the iron redistribution due to the phase retransformation from a- to ß-FeSi2 caused by additional annealing at 850°C, 300 min, subsequent to the 1050°C annealing . The width of the Fe profile becomes smaller and the maximum iron concentration is enhanced. The average ratio Si/Fe decreases from 2.66 to 2.26 (4 x 10" Fe/cm2) by increase of the Fe concentration and simultaneous decrease of the Si concentration . The value of 2.26 is still higher than the stoichiometric value for ß-FeSi 2 indicating that the retransformation is not yet finished after annealing at 850°C, 300 min. This is confirmed by XRD and CEMS. The ß-FeSi2 peaks in the XRD pattern increase with increasing time. The complete retransformation requires for an 850°C annealing a time of about 7 h. The Müssbauer spectra measured after the retransformation annealing at 850°C for 30 and 300 min are shown in figs . 2d and 2e . The CEMS spectrum after 30 min annealing consists of a superposition of the doublets I, II, and III (ß-FeSi2, FeSi, and a-FeSi 2). On the other hand in the CEMS spectrum measured after 300 min annealing no FeSi fraction could be detected . The ß to a transition proceeds fast . Because a-FeSi 2 has a lower Fe concentration than ß-FeSi 2 the excess of Fe diffuses to the interfaces of the silicide layer and forms additional a-FeSi 2 as shown in the RBS spectra Table 3 Area fraction of the doublets in the CEM spectra [%]. (a) 600°C, 60 min; (b) as (a) +850°C, 30 min; (c) as (b) +1050°C, 30 min; (d) as (c) +850°C, 30 min; (e)as (d) +850°C, 300 min Doublet as-implanted 1 II
III
48 52
-
Annealing a b 49 53 51 47 -
-
c
70 30
d 39 26 35
e 52 48
244
H. Reuther et a!. / Ion beam synthesized FeSi2
Fig. 3 . Change of the RBS spectrum of the sample implanted with 4x 10 17 Fe/cm2 due. to the a to ß retransformation annealing. I - 600"C, 60 min + 1050°C, 30 min; 2 - as I +850°C, 300 min . (Measuring geometry aligned (100), for 1 also random.) by the increasing width of the silicide (fig . 1). The enhanced FeSi fraction detected by GEMS after the phase transformation (table 3) indicates that a part of the diffusing Fe is transferred into this phase as d:seussed in ref. [7]. The reverse transformation a to ß is a long-time process. A retransformation annealing at 850°C, 30 min produces no significant effect measurable by RBS or XRD . On the other hand the CEMS spectra show a clear decrease of the FeSi fraction in the surface near range . After annealing at 850°C for 300 min all FeSi in the surface region is dissolved according to CEMS whereas XRD shows still FeSi peaks also after this long-time annealing. Taking into account the redistribution of Fe observed by RBS as a result of the a to ß retransformation it has to be assumed that Fe from dissolved FeSi as well as a-FeSi 2 contributes to the enhanced Fe concentration in the ß-FeSi2 phase. 4. Conclusions The formation of Fe silicide by high dose ion implantation at 350°C is connected with the formation of a small amount of FeSi, mainly at the surface near edge of the Fe distribution. This FeSi part is also present in the annealed samples. The FeSi fraction increases as a result of the ß to oa phase transition and it is decreased by the opposite transformation of the of to the ß phase. The phase transformation from ß- to a-FeSi 2 is connected with a decrease of the average Fe
concentration as expected from the a-FeSi 2 structure containing vacancies within the Fe sublattice . This leads to an increasing width of the silicide layer. The retransformation from a- to ß-FeSi 2 needs much longer time because Fe diffuses against the average concentration gradient . The change in the Fe concentration of FeSi 2 due to the phase transitions is partly compensated by formation or dissolution of FeSi .
References [11 C. Le Corre and J .M. Genin, Phys . Status Solidi B51 (1972) K85. [2] F.H. Sanchez, F. Namavar, J.1 . Budnik, A. Fasikudin and H .C. +iayden, Mater . Res. Soc . Symp . Proc . 5 1 (1986) 439 . [3] G . Müller, G . Klingelh6fer, P. Schwalbach and E. Kankeleit, Nucl. Instr . and Meth . B50 (1990) 348. [41 D. Panknin, E. Wieser, R. Grbtzschel, W. Skorupa, D. Baither, H . Bartsch, G. Querner and A. Danzig, Europ. Mater. Res. Soc ., Spring Meeting, 1991, to appear in Mater. Sci . Eng . B12 (1991) . [5] K. Rademacher, S. Mantl, R. Apetz, Ch. Dieker and H. Liith, Europ. Mater . Res . Soc ., Spring Meeting, 1991, to appear in Mater . Sci . Eng. B12 (1991); and Appl. Phys. Lett. 59 (1991) 2145. [6] D .J. Oostra, D.E.W. Vandenhoudt and C .W.T. BulleLieuwma, Europ. Mater. Res. Soc., Spring Meeting, 1991, to appear in Mater. Sci. Eng. B12 (1991); and Appl. Phys . Lett. 59 (1991) 1737. [7] J . Derrien, J . Chevrier, V, Le Thanh and J.E . Mahan, Proc. of ICFSI-3, Rome, May 6-10, 1991, to appear in Appl. Surf. Science.
MON B
Beam lnteraetions with NatedslsiAtoms
Investigation of ion beam synthesized iron silicide by RBS, XRD, and M6ssbauer spectroscopy (CEMS) H . Reuther, E. Wieser, D . Panknin, R. Gr6tzschel and W. Skorupa Central Institute for Nuclear Research, Rossendorf, PF 19, 0-8051 Dresden, Germany
G . Querner
Technical University Dresden, Mommsenstrasse 13, 0-8027 Dresden, Germany
The formation of FeSi Z by implantation of 4 and 5X1017 Fe/C.2 (300 keV) at 350°C into (100) silicon and subsequent annealing in the temperature range between 600 and 1050°C is studied . Analysis of the phase composition by conversion electron Mössbauer spectroscopy (CEMS) and X-raydiffraction shows an admixture of Fesi beside FeSiZ. The redistribution of iron due to the phase transition from 0- to a-FeSi Z and vice versa is investigated by Rutherford backscattering spectrometry.
1. Introduction The formation of buried insulating, semiconducting or metallic layers by ion implantation is of growing interest in micro-electronics and technology of microsystems. With this respect FeSi Z is an attractive compound because there exist both a semiconducting low temperature phase (ß-FeSiZ) with a composition of 67.7 at.% silicon which has an orthorombic structure and a high temperature phase ((x-FeSi Z) in the composition range of 69.6-72.1 at.% silicon which is tetragonal. The transition from ß- to a-FeSi Z occurs at 967°C [1] for bulk silicide . The formation of iron silicide by high dose implantation into silicon and subsequent annealing at different temperatures has been investigated by several authors [2-6]. A coexistence of FeSi Z and FeSi has been observed in the as-implanted state [3,4] and after annealing [4]. First results concerning the phase transformation of FeSi Z were obtained using RBS and electrical measurements [4,5]. The aim of this work is to study - the phase composition of a silicide layer formed by implantation of Fe with a dose at which the iron concentration in the maximum of the implantation profile corresponds to nearly stoichiometric FeSi Z, and - changes in the structure of the implanted layer by post-implantation annealing, especially the phase transformation from ß- to a-FeSi2 and vice versa .
2. Experimental (100) Si was implanted with doses of 4 and 5 x 1017 Fe +/cmZ (300 keV), respectively, at 350°C. For M6ssbauer spectroscopy the sample was additionally implanted with the M6ssbauer isotope 57Fe (1 .6 x 10 16 57 Fe/cmZ, 300 keV at 350°C). The temperature treatment in N2 gas was carried out in several steps in each case with a first annealing at 600°C, 60 min. The following annealing steps were performed at 850°C (stabilization of P-FeSi Z), 1050°C (transition from ß- to a-FeSi Z) and again at 850°C (retransformation from ato ß-FeSi Z). The iron distribution was studied by Rutherford backscattering spectrometry (RBS) using 1.7 MeV He' ions. Informations on the phase composition of the implanted layer were obtained by conversion electron M6ssbauer spectroscopy (CEMS) and by X-ray diffraction (XRD). While RBS and XRD give informations about the whole implanted layer the main information of CEMS is provided by the near-surface part of the silicide layer because of the strong absorption of the electrons in the material. 3. Results and discussion 3.1. Phase composition Fig. 1 represents the RBS spectra of an as-implanted sample and of samples annealed at 850 and
0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
V. BACKSCATTERING
242
H. Reuther et al. /Ion beam synthesized FeSi2
Table 1 Value R of the average atomic concentration ratio Si/Fe at the depth of highest iron concentration and phase composition in dependence on the annealing conditions; sample 1: 4x1017/cm2, sample 2 : 5x10 17/cm2 Annealing
Sample
R
As-implanted
1
600°C, 60 min
1 2 1
2.00 1.85 2 .05 1 .98
2
1 2
2.16 2.10 2.66 2.46
Phase composition (XRD) 0-FeSi 2, FeSi ß-FeSi 2, FeSi ß-FeSi 2, FeSi ß-Fe`2, FeSi P-FeSi 2, FeSi 0-FeSi 2, FeSi a-FeSi 2, FeSi o-FeSi 2, FeSi
1
2.40
a-FeSi 2, FeSi
1
2.26
ß-FeSi 2, FeSi (a-FeSi2)
600°C, 60 min
+850°C, 30 min 600°C,60 min + 1050°C, 30 min 600°C,60 min +850°C, 30 min + 1050°C, 30 s 600°C,60 min + 1050°C, 30 min +850°C, 300 min
2
1050°C, respectively. The silicide layer is buried below a defective Si top layer of about 20 num . The silicon signal corresponding to this top layer shows the decreasing defect density with increasing annealing temperature by comparison of the aligned spectra with the random spectrum. With increasing annealing temperature the edges of the Fe profiles become steeper. The maximum Fe concentration in the plateau-like part of the 'depth distribution is remarkably lower for the sample annealed at 1050°C compared to the 850°C annealing. The average atomic concentration ratio Si/Fe = R of the silicide layer (calculated from random spectra)
increases with increasing temperature from 1 .85 to 2.46 for the implantation of 5 x 10 17 :,m -2 (table 1). The R values obtained by 850 and 1050°C annealing are in very good agreement with the stoichiometric ratio for ß-FeSi 2 and a-FeSi 2, respectively . The XRD diffraction pattern shows up to an annealing temperature of 850°C peaks of the phases ß-FeSi 2 and FeSi and after the 1050°C annealing peaks of the phases a-FeSi 2 and also FeSi (table 1). From the diffraction results it is not possible to get an information about the relative amount of FeSi and FeSi 2 because of the strong texture of the silicide layer. In fig. 2 M6ssbauer spectra are shown for the as-implanted state and for samples annealed at 850 and 1050°C. All CEMS spectra show an asymmetrical doublet which can be resolved into two symmetrical quadrupole doublets, in the case of fig. 2d even into three doublets. The hyperfine parameters of these doublets are given in table 2, the fractions of the subspectra in table 3. The hyperfine parameters of the doublets I and II correspond to those of ß-FeSi 2 and FeSi as given in ref. [3]. Annealing up to 850°C results in a slight increase of the ß-FeSi 2 fraction and a slight decrease of the FeSi fraction. However, by annealing at 1050°C doublet I (ß-FeSi2) disappears and a new component (doublet III) arises. In agreement with the ß to a phase transition proved by XRD doublet III can be ascribed to a-FeSi 2. The fraction of FeSi detected by CEMS increases by the ß to a phase transition of FeSi 2 and decreases during the retransformation by a subsequent long time annealing at 850°C. The occurrence of FeSi already in the as-implanted state means that the formation of this phase is caused by the implantation process. The good agreement of the average Si/Fe ratio determined from the RBS spectra with the values reported for bulk 0- and a-FeSi2
300CHANNEL N0 .
Fig. 1. Annealing dependence of the RBS spectra of a sample implanted with 4x 10 17 Fe/cm 2, 1 - as-implanted, 2 - 600°C, 60 min+850°C, 30 min, 3 - 600°C, 60 min+1050°C, 30 min. (Measuring geometry aligned (100), for 1 also random.)
H. Reuther et al. / Ion beam synthesized FeSi 2
243
Table 2 Isomer shift (IS) and quadrupole splitting (EQS) of the doublets I-111 of the Müssbauer spectra (number in brackets denotes the standard deviation)
z
IS [mm s'] EQS [mm s -1 ]
Fw
Doublet I (ß-FeSi2 ) 0.02(6) 0.43(6)
Il (FeSi) 0.17(5) 0.50(5)
III (a-FeSiz) 0.30(7) 0.74(9)
w a
z 0 w â W
-2
.
Ü VELOCITY (mm/s)
+2
Fig. 2. Annealing dependence of conversion electron M6ssbauer spectra of a sample implanted with 3.8x1017 56 Fe/cm2 and 1 .6x106 s7 Fe/cm 2, (a) as implanted; (b) 600°C, 60 min +850°C, 30 min; (c)as (b) +1050°C, 36 rain ; (d) as (c) +850°C, 30 min; (e) as (d) +850°C,300min. indicates that the amount of the FeSi admixture within the silicide layer should be small. Contrary to the RBS result aratio of the fraction of FeSi and FeSi 2 of about 1 :1 has been observed by CEMS . This can be explained by the assumption that the FeSi precipitates are mainly located at the surface near edge of the Fe distribution which has the highest weight forthe CEMS signal .
3.2. Phase transformation The RBS spectra of fig. 1 indicate the phase transformation from the orthorhombic ß-FeSi 2 to the te-
tragonal a-FeSi z by a distinct decrease of the average Fe concentration. Therefore the width of the silicide layer increases from about 100 nm to about 130 nm (4 x 10" cm -2 ). The transformation is already complete after annealing at 1050°C for 30 s as proved also by XRD (see table 1) . The RBS spectra of fig. 3 demonstrate the iron redistribution due to the phase retransformation from a- to ß-FeSi2 caused by additional annealing at 850°C, 300 min, subsequent to the 1050°C annealing . The width of the Fe profile becomes smaller and the maximum iron concentration is enhanced. The average ratio Si/Fe decreases from 2.66 to 2.26 (4 x 10" Fe/cm2) by increase of the Fe concentration and simultaneous decrease of the Si concentration . The value of 2.26 is still higher than the stoichiometric value for ß-FeSi 2 indicating that the retransformation is not yet finished after annealing at 850°C, 300 min. This is confirmed by XRD and CEMS. The ß-FeSi2 peaks in the XRD pattern increase with increasing time. The complete retransformation requires for an 850°C annealing a time of about 7 h. The Müssbauer spectra measured after the retransformation annealing at 850°C for 30 and 300 min are shown in figs . 2d and 2e . The CEMS spectrum after 30 min annealing consists of a superposition of the doublets I, II, and III (ß-FeSi2, FeSi, and a-FeSi 2). On the other hand in the CEMS spectrum measured after 300 min annealing no FeSi fraction could be detected . The ß to a transition proceeds fast . Because a-FeSi 2 has a lower Fe concentration than ß-FeSi 2 the excess of Fe diffuses to the interfaces of the silicide layer and forms additional a-FeSi 2 as shown in the RBS spectra Table 3 Area fraction of the doublets in the CEM spectra [%]. (a) 600°C, 60 min; (b) as (a) +850°C, 30 min; (c) as (b) +1050°C, 30 min; (d) as (c) +850°C, 30 min; (e)as (d) +850°C, 300 min Doublet as-implanted 1 II
III
48 52
-
Annealing a b 49 53 51 47 -
-
c
70 30
d 39 26 35
e 52 48
244
H. Reuther et a!. / Ion beam synthesized FeSi2
Fig. 3 . Change of the RBS spectrum of the sample implanted with 4x 10 17 Fe/cm2 due. to the a to ß retransformation annealing. I - 600"C, 60 min + 1050°C, 30 min; 2 - as I +850°C, 300 min . (Measuring geometry aligned (100), for 1 also random.) by the increasing width of the silicide (fig . 1). The enhanced FeSi fraction detected by GEMS after the phase transformation (table 3) indicates that a part of the diffusing Fe is transferred into this phase as d:seussed in ref. [7]. The reverse transformation a to ß is a long-time process. A retransformation annealing at 850°C, 30 min produces no significant effect measurable by RBS or XRD . On the other hand the CEMS spectra show a clear decrease of the FeSi fraction in the surface near range . After annealing at 850°C for 300 min all FeSi in the surface region is dissolved according to CEMS whereas XRD shows still FeSi peaks also after this long-time annealing. Taking into account the redistribution of Fe observed by RBS as a result of the a to ß retransformation it has to be assumed that Fe from dissolved FeSi as well as a-FeSi 2 contributes to the enhanced Fe concentration in the ß-FeSi2 phase. 4. Conclusions The formation of Fe silicide by high dose ion implantation at 350°C is connected with the formation of a small amount of FeSi, mainly at the surface near edge of the Fe distribution. This FeSi part is also present in the annealed samples. The FeSi fraction increases as a result of the ß to oa phase transition and it is decreased by the opposite transformation of the of to the ß phase. The phase transformation from ß- to a-FeSi 2 is connected with a decrease of the average Fe
concentration as expected from the a-FeSi 2 structure containing vacancies within the Fe sublattice . This leads to an increasing width of the silicide layer. The retransformation from a- to ß-FeSi 2 needs much longer time because Fe diffuses against the average concentration gradient . The change in the Fe concentration of FeSi 2 due to the phase transitions is partly compensated by formation or dissolution of FeSi .
References [11 C. Le Corre and J .M. Genin, Phys . Status Solidi B51 (1972) K85. [2] F.H. Sanchez, F. Namavar, J.1 . Budnik, A. Fasikudin and H .C. +iayden, Mater . Res. Soc . Symp . Proc . 5 1 (1986) 439 . [3] G . Müller, G . Klingelh6fer, P. Schwalbach and E. Kankeleit, Nucl. Instr . and Meth . B50 (1990) 348. [41 D. Panknin, E. Wieser, R. Grbtzschel, W. Skorupa, D. Baither, H . Bartsch, G. Querner and A. Danzig, Europ. Mater. Res. Soc ., Spring Meeting, 1991, to appear in Mater. Sci . Eng . B12 (1991) . [5] K. Rademacher, S. Mantl, R. Apetz, Ch. Dieker and H. Liith, Europ. Mater . Res . Soc ., Spring Meeting, 1991, to appear in Mater . Sci . Eng. B12 (1991); and Appl. Phys. Lett. 59 (1991) 2145. [6] D .J. Oostra, D.E.W. Vandenhoudt and C .W.T. BulleLieuwma, Europ. Mater. Res. Soc., Spring Meeting, 1991, to appear in Mater. Sci. Eng. B12 (1991); and Appl. Phys . Lett. 59 (1991) 1737. [7] J . Derrien, J . Chevrier, V, Le Thanh and J.E . Mahan, Proc. of ICFSI-3, Rome, May 6-10, 1991, to appear in Appl. Surf. Science.