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Epitaxial ternary Er0.5Y0.5Si1.7 silicide layers formed by channeled ion beam synthesis
Nuclear Instruments and Methods in Physics Research B 148 (1999) 621±625
Epitaxial ternary Er0:5 Y0:5 Si1:7 silicide layers formed by channeled ion beam synthesis S.M. Hogg
a,* , b
A. Vantomme
a,1
, M.F. Wu
a,b
, Shude Yao b, H. Pattyn a, G. Langouche
a
a Instituut voor Kern-en Stralingsfysica, University of Leuven, B-3001 Leuven, Belgium Department of Technical Physics, Peking University, Beijing, 100871, PeopleÕs Republic of China
Abstract The ternary silicide, Er0:5 Y0:5 Si1:7 has been produced by channeled ion beam synthesis (CIBS) in a Si(1 1 1) substrate. The properties of the material have been compared with those of the binary silicides (i.e. ErSi1:7 and YSi1:7 ) produced by the same method. Analysis was carried out using Rutherford backscattering (RBS) and X-ray diraction (XRD). The study revealed that the layer produced is of good crystalline quality, and is indeed composed of the Er0:5 Y0:5 Si1:7 ternary silicide as opposed to a mixture of the two binaries. It is epitaxial with respect to the Si substrate and possesses the same hexagonal (AlB2 ) structure exhibited by both binary silicides. The azimuthal orientation with respect to the substrate is Er0:5 Y0:5 Si1:7 [0 0 0 1]//Si[1 1 1] and Er0:5 Y0:5 Si1:7 {1 1 2 0}//Si{1 1 0}. The epilayer was found to be elastically strained with the perpendicular strain )0.67% and the parallel strain +0.34% similar to the values found for the Er and Y silicides. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.Tv; 68.55.Nq Keywords: Ternary silicide; Channeled ion beam synthesis
1. Introduction Metallic silicides are frequently used in microelectronic applications such as ohmic contacts, low resistivity gates or buried interconnects. Attention has been drawn to rare earth (RE) silicides due to their low Schottky barrier height on n-type silicon (0.3±0.4 eV), which suggests applications in in-
* Corresponding author.Tel.: +32 16 32 7617; fax: +32 16 32 7985; e-mail: [email protected] 1 Postdoctoral Researcher FWO (Fund for Scienti®c Research, Flanders, Belgium).
fra-red detection [1,2], and to their small lattice mismatch with Si(1 1 1) (1%) [3] making epitaxial growth feasible. Attempts to produce high quality RE silicides have been frequently hindered by the high reactivity of the RE metals causing degradation of: the silicon/silicide interface; the surface of the silicide, and the crystalline quality of the silicide. Ion implantation in a channeling orientation followed by annealing, channeled ion beam synthesis (CIBS) [4], produces superior results. It eliminates the problems due to oxidation, as the silicide formation occurs under the surface. The channeling orientation also minimises the exten-
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 2 3 - X
622
S.M. Hogg et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 621±625
sive lattice damage, caused by the heavy RE ions, observed for implantation performed at the conventional 7° angle. Among other examples, high quality ErSi1:7 layers [5,6], and YSi1:7 [7] (commonly considered as a RE silicide) have been successfully synthesised using CIBS. Both YSi1:7 and ErSi1:7 exist in the hexagonal AlB2 structure with 1 vacancy present in every 6 Si sites. The lattice constants of the two compounds are closely matched and RE metals tend to form mixed compounds and hence synthesis of a ternary silicide appears feasible. Such materials are of interest because of the potential for tailoring silicide properties through composition. In this paper we report the CIBS of the ternary compound Er0:5 Y0:5 Si1:7 and compare it with the binary silicides synthesised in the same manner. 2. Experimental The RE silicide layers were produced by ion implantation of single-charged RE ions into a Si(1 1 1) substrate followed by annealing in a vacuum furnace (10ÿ7 Torr). The ions were produced in a Nielsen plasma ion source from the appropriate chloride. In all cases, the ion beam was directed within 1.5° of the sample normal in order to achieve channeled implantation, during which the substrate was maintained at 450°C to allow dynamic annealing, thereby minimising implantation damage. The ion beam current was approximately 3±5 lA. The implantation energy and dose and the annealing conditions for the three samples are presented in Table 1.
Analysis was performed using Rutherford backscattering (RBS) and channeling measurements carried out with a 1.57 MeV He beam, and X-ray diraction (XRD). 3. Results The RBS/channeling spectra of the as-implanted and annealed samples of Er0:5 Y0:5 Si1:7 are presented in Fig. 1(a) and (b), respectively. The implanted Er and Y can be clearly seen in two peaks indicated by an arrow marking the energy corresponding to backscattering from these elements at the surface. The implanted doses of the two RE ions were calculated as 4.3 ´ 1016 and 4.4 ´ 1016 atoms/cm2 for Er and Y, respectively. The channeling minimum yield for the as-implanted sample, vmin 35%, was obtained from the components of the spectrum arising from both the Er and the Y atoms. Table 2 contains a summary of the RBS/channeling results from the three silicide layers. It can be seen that the post-implantation crystalline quality of the ternary silicide is somewhat inferior to that of the YSi1:7 and similar to that of the ErSi1:7 . The comparison with the YSi1:7 case is predictable as the light 89 Y ions cause signi®cantly less lattice damage than the 166 Er ions. However, this reasoning alone suggests that the vmin of the binary silicide should be lower than for the ErSi1:7 (considering that a sample has been produced with an Er dose of 8 ´ 1016 /cm2 and an as-implanted vmin of only 13%). The source of this inconsistency is believed to lie in precise alignment of the beam. MARLOWE simulations [8,9] show that a misalignment in the in-
Table 1 Details of experimental conditions Silicide
Implanted ion and energy
Dose (/cm2 )
Annealing conditions
YSi1:7
89
1.0 ´ 1017
ErSi1:7
166
Er0:5 Y0:5 Si1:7
166 Er 60 keV then 89 Y 45 keV
600°C for 1 hour 1000°C for 0.5 hours 600°C for 1 hour 1100°C for 0.5 hours 600°C for 1 hour 1000°C for 0.5 hours
Y 60 keV Er 90 keV
1.6 ´ 1017 4 ´ 1016 of Er 4 ´ 1016 of Y
S.M. Hogg et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 621±625
Fig. 1. Aligned (+), random (s) and simulated ()) RBS/ channeling spectra of (a) as-implanted and (b) annealed Er0:5 Y0:5 Si1:7 .
cident angle of as little as 1° during channeled implantation can decrease the projected range by around 12%. This causes an increase in the density of vacancies contracted by the substrate as the same number of vacancies is produced, but the depth over which they are spread is reduced. As it is only possible to align our beam to within an accuracy of 1.5°, this is probably the origin of the
623
inconsistency in the extent of the observed damage. In previous work [4], it has been found that the essential condition necessary for recovering a reasonable crystalline quality subsequent to annealing is that the substrate should not be fully amorphised. In all cases this has clearly been satis®ed. The channeling spectrum indicates a drastic improvement in crystalline quality with annealing in both the signals obtained from the RE elements and also in the damaged region of the silicon. vmin is reduced to 5.5% for both Er and Y con®rming that a high quality silicide layer has been synthesised. This noteworthy post-anneal improvement in quality was expected from the behaviour of the binary silicides (see Table 2). The ensuing crystalline quality of the ternary silicide is apparently slightly inferior than in the binary silicides. This could well be due to microscopic inhomogeneity in the ternary silicide. Simulations allow us to produce a depth pro®le (insets of Fig. 1) of the implanted atoms. The average projected range of the Er is slightly larger than that of the Y due to the diculty in precisely predicting channeled implantation pro®les. Annealing causes the local stoichiometry to alter as Er and Y atoms migrate. It is obvious that not only has the RE:Si ratio approached 1:1.7 but the distribution of Er and Y has also become more uniform, approaching the Er:Y:Si 0.5:0.5:1.7 indicated in the depth pro®les (Fig. 1) by the dotted line. The RE:Si ratio 1:1.7 con®rms the presence of vacancies in the lattice also observed for ErSi1:7 and YSi1:7 . From the RBS spectra it is also apparent that some precipitates remain below the layer. This has also been observed for ErSi1:7 and YSi1:7 and is attributed to the low mobility of the RE ions in Si.
Table 2 Summary of RBS results Silicide
Dose (/cm2 )
vmin of RE (as impl.)
vmin of RE (annealed)
Silicide thickness (nm)
Stoichiometry Si:RE
YSi1:7 ErSi1:7 Er0:5 Y0:5 Si1:7
1.0 ´ 1017 1.6 ´ 1017 4.3 ´ 1016 of Er 4.4 ´ 1016 of Y
16% 34% 35%
3.5 1.5 5.5
48 82 48
1.7 [7] 1.7 [5] 1.7 (this work)
624
S.M. Hogg et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 621±625
A more detailed RBS angular scan was also carried out on the ternary silicide (as previously done for the ErSi1:7 [5] and the YSi1:7 [7]). The scans con®rm that the azimuthal orientation of the three epilayers is identical. This shows that the {1 1 2 0} axis of the RE silicide is parallel to the {1 1 0} of the substrate. XRD measurements of the Er0:5 Y0:5 Si1:7 clearly show the (0 0 0 n) peaks of the hexagonal structure whereas no other epitaxial phases appear to be present. The (0 0 0 4) peaks of all three silicides are shown in Fig. 2. The lattice constants of YSi1:7 and ErSi1:7 produced by CIBS (cepi and aepi ) obtained from these measurements are given in Table 3. Although they are in close proximity, the separation of the (0 0 0 4) peaks of the two binary silicides is clearly visible. Fig. 2(c) shows that, following the consecutive implantation of both the RE ions, only one single (0 0 0 4) peak is present. This peak clearly occurs at a 2h angle intermediate to those of the (0 0 0 4) peak of the ErSi1:7 (Fig. 2(a)) and the YSi1:7 (Fig. 2(b)) giving con®rmation that the silicide formed is indeed a single compound and not an alloy of two binary silicides. (It should be noted that to obtain the necessary accuracy to arm that only a single peak was present arising from the ternary silicide, the measurement of the Er0:5 Y0:5 Si1:7 was performed on a system with higher resolution, hence the dierence in the appearance of the Si(3 3 3) peak.) From the XRD scans we can determine the lattice constants of the silicides in the epilayer, cepi . These are compared with the accepted bulk values in Table 3. As would intuitively be expected, the c lattice constant of the ternary silicide lies between that of its two binary components. There is no data available for the lattice constant of bulk Er0:5 Y0:5 Si1:7 but using Vegard's law we can cal-
Fig. 2. The (0 0 0 4) peak from XRD of (a) ErSi1:7 , (b) YSi1:7 , (c) Er0:5 Y0:5 Si1:7 .
culate a theoretical value by averaging the lattice constants of the other two silicides. The perpendicular elastic strain can then be calculated from cepi ÿ cbulk : e? cbulk Table 3 shows that all three silicides are under compressive strain. The perpendicular elastic strain, e? )0.68 0.02%, experienced by the ternary silicide is of a similar magnitude to the binary silicides.
Table 3 Lattice constants and elastic strain of RE silicides
a
Silicide
cbulk (nm) [2]
cepi (nm) 0.0003
e? (%) 0.02
abulk (nm) [2]
aepi (nm) 0.0003
ek (%) 0.08
YSi1:7 ErSi1:7 Er0:5 Y0:5 Si1:7
0.4144 0.4088 0.4116a
0.4116 [7] 0.40496 [6] 0.4088
)0.67 )0.94 )0.68
0.3842 0.3798 0.3820a
0.3882 [7] 0.3845 [6] 0.3833
+1.04 +1.24 +0.34
Calculated from lattice constants of the binary silicides in accordance with Vegard's law.
S.M. Hogg et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 621±625
We can also make an estimate of the parallel elastic strain, ek , in the lattice by determining the aepi lattice parameter from two asymmetric XRD scans (as in [6]). Once again the polarity of the strain is consistent between the three silicide layers. All are under tensile strain though the magnitude is much smaller, +0.34 0.08%, in the case of the ternary silicide. The fact that the aepi lattice constant of the ternary silicide does not lie between those of the binary silicides casts some doubt over the applicability of VegardÕs law in this case. 4. Conclusions We have shown that using CIBS it is possible to produce a ternary Er0:5 Y0:5 Si1:7 layer of high crystalline quality on a silicon substrate. The ternary silicide has the same defective AlB2 lattice structure as that shared by the ErSi1:7 and the YSi1:7 systems with an intermediate c lattice constant. The strain in the epilayer also closely matches that of the binary systems. An interesting extension of this work would be to investigate the vacancy ordering that is observed in all cases for
625
ErSi1:7 [10] but in the YSi1:7 [11] has been shown to be dependent on growth conditions and annealing temperature.
References [1] J.Y. Duboz, P.A. Badoz, F.A. dÕAvitaya, J.A. Chroboczek, Appl. Phys. Lett. 55 (1989) 84. [2] J.A. Knapp, S.T. Picraux, Appl. Phys. Lett. 48 (1986) 466. [3] R.D. Thompson, B.Y. Tsaur, K.N. Tu, Appl. Phys. Lett. 38 (1981) 535. [4] M.F. Wu, A. Vantomme, H. Pattyn, G. Langouche, Appl. Phys. Lett. 67 (1995) 3886. [5] M.F. Wu, A. Vantomme, J. De Wachter, S. Degroote, H. Pattyn, G. Langouche, H. Bender, J. Appl. Phys. 79 (1996) 6920. [6] M.F. Wu, A. Vantomme, H. Pattyn, G. Langouche, Q. Yang, Q. Wang, J. Appl. Phys. 80 (1996) 5713. [7] M.F. Wu, S. Yao, A. Vantomme, S. Hogg, H. Pattyn, G. Langouche, Q. Yang, Q. Wang, J. Vac. Sci. Technol B 16 (1998) 1901. [8] M.T. Robinson, Phys. Rev. B 40 (1989) 10717. [9] U. Wahl, A. Vantomme, M.F. Wu, H. Pattyn, G. Langouche, Nucl. Instr. and Meth. B 127/128 (1997) 311. [10] H. Bender, M.F. Wu, A. Vantomme, J. De Wachter, H. Pattyn, G. Langouche, MRS Symp. Proc. 402 (1996) 499. [11] S. Jin, J.H. Lin, L.J. Chen, W.D. Shi, Z. Zhang, Nucl. Instr. and Meth. B 96 (1995) 347.
Epitaxial ternary Er0:5 Y0:5 Si1:7 silicide layers formed by channeled ion beam synthesis S.M. Hogg
a,* , b
A. Vantomme
a,1
, M.F. Wu
a,b
, Shude Yao b, H. Pattyn a, G. Langouche
a
a Instituut voor Kern-en Stralingsfysica, University of Leuven, B-3001 Leuven, Belgium Department of Technical Physics, Peking University, Beijing, 100871, PeopleÕs Republic of China
Abstract The ternary silicide, Er0:5 Y0:5 Si1:7 has been produced by channeled ion beam synthesis (CIBS) in a Si(1 1 1) substrate. The properties of the material have been compared with those of the binary silicides (i.e. ErSi1:7 and YSi1:7 ) produced by the same method. Analysis was carried out using Rutherford backscattering (RBS) and X-ray diraction (XRD). The study revealed that the layer produced is of good crystalline quality, and is indeed composed of the Er0:5 Y0:5 Si1:7 ternary silicide as opposed to a mixture of the two binaries. It is epitaxial with respect to the Si substrate and possesses the same hexagonal (AlB2 ) structure exhibited by both binary silicides. The azimuthal orientation with respect to the substrate is Er0:5 Y0:5 Si1:7 [0 0 0 1]//Si[1 1 1] and Er0:5 Y0:5 Si1:7 {1 1 2 0}//Si{1 1 0}. The epilayer was found to be elastically strained with the perpendicular strain )0.67% and the parallel strain +0.34% similar to the values found for the Er and Y silicides. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.Tv; 68.55.Nq Keywords: Ternary silicide; Channeled ion beam synthesis
1. Introduction Metallic silicides are frequently used in microelectronic applications such as ohmic contacts, low resistivity gates or buried interconnects. Attention has been drawn to rare earth (RE) silicides due to their low Schottky barrier height on n-type silicon (0.3±0.4 eV), which suggests applications in in-
* Corresponding author.Tel.: +32 16 32 7617; fax: +32 16 32 7985; e-mail: [email protected] 1 Postdoctoral Researcher FWO (Fund for Scienti®c Research, Flanders, Belgium).
fra-red detection [1,2], and to their small lattice mismatch with Si(1 1 1) (1%) [3] making epitaxial growth feasible. Attempts to produce high quality RE silicides have been frequently hindered by the high reactivity of the RE metals causing degradation of: the silicon/silicide interface; the surface of the silicide, and the crystalline quality of the silicide. Ion implantation in a channeling orientation followed by annealing, channeled ion beam synthesis (CIBS) [4], produces superior results. It eliminates the problems due to oxidation, as the silicide formation occurs under the surface. The channeling orientation also minimises the exten-
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 2 3 - X
622
S.M. Hogg et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 621±625
sive lattice damage, caused by the heavy RE ions, observed for implantation performed at the conventional 7° angle. Among other examples, high quality ErSi1:7 layers [5,6], and YSi1:7 [7] (commonly considered as a RE silicide) have been successfully synthesised using CIBS. Both YSi1:7 and ErSi1:7 exist in the hexagonal AlB2 structure with 1 vacancy present in every 6 Si sites. The lattice constants of the two compounds are closely matched and RE metals tend to form mixed compounds and hence synthesis of a ternary silicide appears feasible. Such materials are of interest because of the potential for tailoring silicide properties through composition. In this paper we report the CIBS of the ternary compound Er0:5 Y0:5 Si1:7 and compare it with the binary silicides synthesised in the same manner. 2. Experimental The RE silicide layers were produced by ion implantation of single-charged RE ions into a Si(1 1 1) substrate followed by annealing in a vacuum furnace (10ÿ7 Torr). The ions were produced in a Nielsen plasma ion source from the appropriate chloride. In all cases, the ion beam was directed within 1.5° of the sample normal in order to achieve channeled implantation, during which the substrate was maintained at 450°C to allow dynamic annealing, thereby minimising implantation damage. The ion beam current was approximately 3±5 lA. The implantation energy and dose and the annealing conditions for the three samples are presented in Table 1.
Analysis was performed using Rutherford backscattering (RBS) and channeling measurements carried out with a 1.57 MeV He beam, and X-ray diraction (XRD). 3. Results The RBS/channeling spectra of the as-implanted and annealed samples of Er0:5 Y0:5 Si1:7 are presented in Fig. 1(a) and (b), respectively. The implanted Er and Y can be clearly seen in two peaks indicated by an arrow marking the energy corresponding to backscattering from these elements at the surface. The implanted doses of the two RE ions were calculated as 4.3 ´ 1016 and 4.4 ´ 1016 atoms/cm2 for Er and Y, respectively. The channeling minimum yield for the as-implanted sample, vmin 35%, was obtained from the components of the spectrum arising from both the Er and the Y atoms. Table 2 contains a summary of the RBS/channeling results from the three silicide layers. It can be seen that the post-implantation crystalline quality of the ternary silicide is somewhat inferior to that of the YSi1:7 and similar to that of the ErSi1:7 . The comparison with the YSi1:7 case is predictable as the light 89 Y ions cause signi®cantly less lattice damage than the 166 Er ions. However, this reasoning alone suggests that the vmin of the binary silicide should be lower than for the ErSi1:7 (considering that a sample has been produced with an Er dose of 8 ´ 1016 /cm2 and an as-implanted vmin of only 13%). The source of this inconsistency is believed to lie in precise alignment of the beam. MARLOWE simulations [8,9] show that a misalignment in the in-
Table 1 Details of experimental conditions Silicide
Implanted ion and energy
Dose (/cm2 )
Annealing conditions
YSi1:7
89
1.0 ´ 1017
ErSi1:7
166
Er0:5 Y0:5 Si1:7
166 Er 60 keV then 89 Y 45 keV
600°C for 1 hour 1000°C for 0.5 hours 600°C for 1 hour 1100°C for 0.5 hours 600°C for 1 hour 1000°C for 0.5 hours
Y 60 keV Er 90 keV
1.6 ´ 1017 4 ´ 1016 of Er 4 ´ 1016 of Y
S.M. Hogg et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 621±625
Fig. 1. Aligned (+), random (s) and simulated ()) RBS/ channeling spectra of (a) as-implanted and (b) annealed Er0:5 Y0:5 Si1:7 .
cident angle of as little as 1° during channeled implantation can decrease the projected range by around 12%. This causes an increase in the density of vacancies contracted by the substrate as the same number of vacancies is produced, but the depth over which they are spread is reduced. As it is only possible to align our beam to within an accuracy of 1.5°, this is probably the origin of the
623
inconsistency in the extent of the observed damage. In previous work [4], it has been found that the essential condition necessary for recovering a reasonable crystalline quality subsequent to annealing is that the substrate should not be fully amorphised. In all cases this has clearly been satis®ed. The channeling spectrum indicates a drastic improvement in crystalline quality with annealing in both the signals obtained from the RE elements and also in the damaged region of the silicon. vmin is reduced to 5.5% for both Er and Y con®rming that a high quality silicide layer has been synthesised. This noteworthy post-anneal improvement in quality was expected from the behaviour of the binary silicides (see Table 2). The ensuing crystalline quality of the ternary silicide is apparently slightly inferior than in the binary silicides. This could well be due to microscopic inhomogeneity in the ternary silicide. Simulations allow us to produce a depth pro®le (insets of Fig. 1) of the implanted atoms. The average projected range of the Er is slightly larger than that of the Y due to the diculty in precisely predicting channeled implantation pro®les. Annealing causes the local stoichiometry to alter as Er and Y atoms migrate. It is obvious that not only has the RE:Si ratio approached 1:1.7 but the distribution of Er and Y has also become more uniform, approaching the Er:Y:Si 0.5:0.5:1.7 indicated in the depth pro®les (Fig. 1) by the dotted line. The RE:Si ratio 1:1.7 con®rms the presence of vacancies in the lattice also observed for ErSi1:7 and YSi1:7 . From the RBS spectra it is also apparent that some precipitates remain below the layer. This has also been observed for ErSi1:7 and YSi1:7 and is attributed to the low mobility of the RE ions in Si.
Table 2 Summary of RBS results Silicide
Dose (/cm2 )
vmin of RE (as impl.)
vmin of RE (annealed)
Silicide thickness (nm)
Stoichiometry Si:RE
YSi1:7 ErSi1:7 Er0:5 Y0:5 Si1:7
1.0 ´ 1017 1.6 ´ 1017 4.3 ´ 1016 of Er 4.4 ´ 1016 of Y
16% 34% 35%
3.5 1.5 5.5
48 82 48
1.7 [7] 1.7 [5] 1.7 (this work)
624
S.M. Hogg et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 621±625
A more detailed RBS angular scan was also carried out on the ternary silicide (as previously done for the ErSi1:7 [5] and the YSi1:7 [7]). The scans con®rm that the azimuthal orientation of the three epilayers is identical. This shows that the {1 1 2 0} axis of the RE silicide is parallel to the {1 1 0} of the substrate. XRD measurements of the Er0:5 Y0:5 Si1:7 clearly show the (0 0 0 n) peaks of the hexagonal structure whereas no other epitaxial phases appear to be present. The (0 0 0 4) peaks of all three silicides are shown in Fig. 2. The lattice constants of YSi1:7 and ErSi1:7 produced by CIBS (cepi and aepi ) obtained from these measurements are given in Table 3. Although they are in close proximity, the separation of the (0 0 0 4) peaks of the two binary silicides is clearly visible. Fig. 2(c) shows that, following the consecutive implantation of both the RE ions, only one single (0 0 0 4) peak is present. This peak clearly occurs at a 2h angle intermediate to those of the (0 0 0 4) peak of the ErSi1:7 (Fig. 2(a)) and the YSi1:7 (Fig. 2(b)) giving con®rmation that the silicide formed is indeed a single compound and not an alloy of two binary silicides. (It should be noted that to obtain the necessary accuracy to arm that only a single peak was present arising from the ternary silicide, the measurement of the Er0:5 Y0:5 Si1:7 was performed on a system with higher resolution, hence the dierence in the appearance of the Si(3 3 3) peak.) From the XRD scans we can determine the lattice constants of the silicides in the epilayer, cepi . These are compared with the accepted bulk values in Table 3. As would intuitively be expected, the c lattice constant of the ternary silicide lies between that of its two binary components. There is no data available for the lattice constant of bulk Er0:5 Y0:5 Si1:7 but using Vegard's law we can cal-
Fig. 2. The (0 0 0 4) peak from XRD of (a) ErSi1:7 , (b) YSi1:7 , (c) Er0:5 Y0:5 Si1:7 .
culate a theoretical value by averaging the lattice constants of the other two silicides. The perpendicular elastic strain can then be calculated from cepi ÿ cbulk : e? cbulk Table 3 shows that all three silicides are under compressive strain. The perpendicular elastic strain, e? )0.68 0.02%, experienced by the ternary silicide is of a similar magnitude to the binary silicides.
Table 3 Lattice constants and elastic strain of RE silicides
a
Silicide
cbulk (nm) [2]
cepi (nm) 0.0003
e? (%) 0.02
abulk (nm) [2]
aepi (nm) 0.0003
ek (%) 0.08
YSi1:7 ErSi1:7 Er0:5 Y0:5 Si1:7
0.4144 0.4088 0.4116a
0.4116 [7] 0.40496 [6] 0.4088
)0.67 )0.94 )0.68
0.3842 0.3798 0.3820a
0.3882 [7] 0.3845 [6] 0.3833
+1.04 +1.24 +0.34
Calculated from lattice constants of the binary silicides in accordance with Vegard's law.
S.M. Hogg et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 621±625
We can also make an estimate of the parallel elastic strain, ek , in the lattice by determining the aepi lattice parameter from two asymmetric XRD scans (as in [6]). Once again the polarity of the strain is consistent between the three silicide layers. All are under tensile strain though the magnitude is much smaller, +0.34 0.08%, in the case of the ternary silicide. The fact that the aepi lattice constant of the ternary silicide does not lie between those of the binary silicides casts some doubt over the applicability of VegardÕs law in this case. 4. Conclusions We have shown that using CIBS it is possible to produce a ternary Er0:5 Y0:5 Si1:7 layer of high crystalline quality on a silicon substrate. The ternary silicide has the same defective AlB2 lattice structure as that shared by the ErSi1:7 and the YSi1:7 systems with an intermediate c lattice constant. The strain in the epilayer also closely matches that of the binary systems. An interesting extension of this work would be to investigate the vacancy ordering that is observed in all cases for
625
ErSi1:7 [10] but in the YSi1:7 [11] has been shown to be dependent on growth conditions and annealing temperature.
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