APPLIED PHYSICS LETTERS
VOLUME 76, NUMBER 14
3 APRIL 2000
Metastable phases of cobalt-ironsilicide formed by sequential implantation of Co and Fe in Si „111… I. De´zsi,a) Cs. Fetzer, and M. Kiss KFKI Research Institute for Particle and Nuclear Physics, H-1525 Budapest 114, Hungary
H. Pattyn, A. Vantomme, and G. Langouche Instituut vor Kern- en Stralingsfysika, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
共Received 4 January 2000; accepted for publication 3 February 2000兲 By implanting Co and Fe in sequence into Si 共111兲, metastable ternary Co1⫺x Fex Si2 phases were formed. Mo¨ssbauer effect measurements showed three resonance line components in the spectrum. Comparison of the central shift 共CS兲 values of the components with those appearing in the stable ternary phases indicated that iron atoms are positioned in the substitutional Co site, in the empty cube of the fluorite-type lattice and in CsCl-like B2 structures. It was found that the CS values of two components are in the velocity range of the values obtained for the metastable ␥ -FeSi2 synthesized using various methods. This result suggests the existence of a similar structure. © 2000 American Institute of Physics. 关S0003-6951共00兲01414-5兴
Cobalt and iron disilicides have received increased attention as possible materials for advanced silicon based electronic devices.1–4 CoSi2 has a fluorite structure, whereas the  -FeSi2 being stable at room temperature exhibits an orthorombic structure. Because of their different structures, cobalt and iron do not form a stable single phase in a broad range of concentrations.5 In contrast to the case of the stable phases, it was shown that by using ion implantation, cobalt and iron can form a metastable homogeneous single phase of fluorite structure in broad concentration ranges.6–8 Even iron might form metastable disilicide ( ␥ -FeSi2兲 of fluorite structure if one applies ion beam induced epitaxial crystallization on iron implanted in Si.9,10 CoSi2 and FeSi2 thin layers in a fluorite structure formed when cobalt or iron was deposited and reacted on a silicon surface.11,12 At greater metal thicknesses, the metal atoms gradually filled the empty cubes of the fluorite lattice as a result of which the structure contained a fraction of the CsCl(B2) structure. In order to gain more information on the iron positions in the metastable fluorite type lattices, we performed Mo¨ssbauer studies on ternary Co–Fe–silicides formed after ion implantation of Co and Fe at various dose values in Si共111兲 at 350 °C. For possible comparison, iron implanted CoSi2 and synthesized iron containing bulk CoSi2 samples were studied. Chemically cleaned 共with HF aqueous solution兲 Si共111兲 single crystals were implanted with 80 keV energy at 350 °C sequentially by 59Co and by 57Fe in order to have buried ternary crystalline phases. The maximum dose value for each sample was 9⫻1016 atom/cm2. The 57Fe doses were 3 ⫻1015, 1⫻1016, and 3⫻1016, giving x values 0.03 共sample 1兲, 0.13 共sample 2兲, and 0.3 共sample 3兲, respectively. Pure was implanted with 3 polycrystalline CoSi2 ⫻1015 atom/cm2 57Fe at 350 °C. The average relative concentration of the implanted atoms was calculated by the
transport of ions in matter 共TRIM兲 code.13 We obtained an average x value in the Co(1⫺x) Fex Si2 phase of 0.015. A bulk CoSi2 sample containing 1 at. % 57Fe relative to Co was prepared by melting 5N Co and 57Fe 共in metallic form兲 with very pure Si in a vacuum furnace. This concentration was chosen because the solubility of iron is less than 2 at. % in CoSi2 in thermal equilibrium.5 The conversion-electron Mo¨ssbauer spectra were measured by a low-background small-sized continuous-flow proportional counter. For our single-line source, we used 30 mCi 57Co in a Rh matrix. The measurements were carried out at room temperature. For the analysis of the Mo¨ssbauer spectra a least squares program was used. The central shift 共CS兲 values in mm s⫺1 are given relative to that of ␣- iron at room temperature. The Mo¨ssbauer spectra of the Co(1⫺x) Fex Si2 samples formed after implantation and annealed at different temperatures are shown in Fig. 1; the spectra of the ternary bulk and
a兲
FIG. 1. Mo¨ssbauer spectra of Si.
Also at: KFKI Research Institute for Technical Physics and Materials Science; electronic mail:
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Fe sequentially implanted in Co implanted
© 2000 American Institute of Physics
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FIG. 2. Mo¨ssbauer spectra of bulk CoSi2 containing 1 at. % iron 共a,b兲 and 57 Fe implanted CoSi2 共c,d兲. 57
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Appl. Phys. Lett., Vol. 76, No. 14, 3 April 2000
Fe implanted CoSi2 samples are shown in Figs. 2共a兲 and 2共b兲 and Figs. 2共c兲 and 2共d兲, respectively. The complex spectra of the as-prepared samples in Fig. 1 do not show significant iron concentration dependence. Three peaks can be distinguished for all samples. Also, for the implanted CoSi2 and for the bulk samples three spectral components can be distinguished. After longtime annealing 共240 h兲, the spectrum of the bulk sample with x⫽0.01 changed considerably: The intensity of line 2 and line 3 increased; for the implanted CoSi2, the intensity of line 3 increased significantly. In order to learn whether the lines are single lines or members of quadrupole split doublets, measurements were made at 54° of the ␥ ray direction relative to the sample surface 共close to the magic angle兲. The relative intensity values did not changed for the as-implanted samples nor for the implanted, and subsequently annealed samples. Therefore, the existence of quadrupole doublets could not be proved. Even when choosing pairs of lines to represent doublets, none of the CS and quadrupole interaction values of known phases 共amorphous inclusive兲 of ironsilicides agree with those values. Nevertheless, because of the complex spectra the population of a small fraction of Fe atoms in a disordered surroundings cannot be completely excluded. The CS1⫽⫺0.19共1兲 and CS3⫽0.47共1兲 mm s⫺1 values of the implanted samples are close to those measured for the ternary bulk samples. In external field measurements, it was found that the resonance lines of 57Fe in bulk CoSi2 samples containing up to 2% iron are singlets, thereby, proving the cubic symmetry around iron in the lattice.14 For the bulk samples line 1 was attributed to substitutional Fe atoms, line 3 to Fe atoms positioned in the vacant cube of Si in the CoSi2 lattice.5 The close CS1 and CS3 values of the implanted and bulk samples indicate the same or very similar atomic arrangements in the two samples. This is also supported by the annealing behavior of the implanted samples, e.g., the intensity of line 3 increased at a high temperature indicating that this position is more stable for Fe, similarly, this behavior is similar to that of the bulk and 57Fe implanted CoSi2 samples.
The linewidth 共⌫兲 values of the implanted samples are broader than those for the bulk sample. This broadening can be attributed to effect of implantation creating defects in the lattice and to interface effects on the buried Co1⫺x Fex Si2 particles in the Si host. However, the existence of line 2 makes the situation more complex. The CS2 values are around 0.16 共2兲 mm s⫺1. We attribute this line to an ordered B2 共CsCl-type兲 phase formed together with the fluorite-type structure during the implantation. The CS value is 0.1 mm s⫺1 lower than the value of the metastable FeSi with B2 structure15 but the large Co concentration may decrease the CS value as in the case of ␣- and  -FeSi2 relative to CoSi2. The simultaneous formation of the fluorite- and CsCl-type structures could also be observed for metastable thin layers of Fe silicides11 on Si and for Co1⫺x Fex Si2 epitaxial layers16 with a CoSi2 fluorite-type structure grown on Si共111兲. It is interesting to note that the CS1 and CS3 values are in the velocity ranges of the resonance lines 共between ⫺0.08 and ⫺0.190, and ⫹0.43 and 0.49 mm s⫺1兲 as were measured for ␥ -FeSi2 prepared by different methods.10,17,18 This suggests the formation of similar atomic arrangements in the two fluorite-like structures. Ternary Co1⫺x Fex Si2 silicides formed by sequential ion implantation of Co and Fe in Si were studied by Mo¨ssbauer spectroscopy. Two 57Fe resonance lines with central shift values characteristic of 57Fe dissolved in CoSi2 fluorite-type lattice were observed. The parameters of these two lines could be identified with iron atoms substituting Co atoms and to those positioned in the vacant cubes in the CoSi2 lattice. The third resonance line probably belongs to iron atoms positioned in a CsCl-type Co-rich B2 lattice formed together with the fluorite-type structure. On annealing, the filling of the vacant cubes by iron in the Co1⫺x Fex Si2 phases formed after implantation, in the synthesized bulk Co1⫺x Fex Si2 and in the 57Fe implanted CoSi2 samples showed that this site is the stable site for iron in the fluoritetype lattice. The work was supported by the Grant No. T 20869 共OTKA兲, ERBFMRXCR960029 共EU兲 and under the cooperation agreement between OMFB and the Ministry of the Flemish Community 共No. B14/96兲.
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