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Periodic surface modulation on thin epitaxial FeSi2 layers on Si(0 0 1)
Surface Science 532–535 (2003) 940–945 www.elsevier.com/locate/susc
Periodic surface modulation on thin epitaxial FeSi2 layers on Si(0 0 1) S. Hajjar, G. Garreau, S. Pelletier, P. Bertoncini, P. Wetzel, G. Gewinner, M. Imhoff, C. Pirri * Laboratoire de Physique et de Spectroscopie Electronique, UMR CNRS-7014, Facult e des Sciences et Techniques, 4, rue des Fr eres Lumi ere, F-68093 Mulhouse C edex, France
Abstract The morphology and the structure of thin metastable iron disilicide films grown on Si(0 0 1) are studied by scanning tunneling microscopy and X-ray photoelectron diffraction. It is shown that the FeSi2 silicide has a quadratic crystallographic structure, with the c-axis perpendicular to the sample surface. As to the film morphology, the silicide consists p p of rather flat islands with a 2 2 R45° surface periodicity for a coverage lower than 4 ML. At a nominal Fe coverage of 4 ML, the silicon surface is almost completely covered. The surface exhibits a quite periodic height . modulation of about 2 A Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Surface structure, morphology, roughness, and topography; Photoelectron spectroscopy; Photoelectron diffraction; Scanning tunneling microscopy; Solid phase epitaxy; Metal–semiconductor interfaces; Silicides; Silicon
1. Introduction The integration of magnetic layers in Si-based devices has motivated recent studies of Fe and Co growth on Si(0 0 1) and Si(1 1 1) [1–3]. Of particular interest is the formation of magnetic metal– silicon interfaces for the fabrication of Si-based spin electronics. Nevertheless, a strong intermixing occurs at the Fe/Si interface even at room temperature, and a metallic silicide is formed along with Si diffusion within a large part of the Fe overlayer. The intermixed zone thickness is not controlled and depends on preparation conditions
*
Corresponding author. Tel.: +33-03-89-33-60-87; fax: +3303-89-33-60-83. E-mail address: [email protected] (C. Pirri).
(substrate temperature, substrate surface morphology, deposition rate. . .). At any rate, Si diffusion in the magnetic layer results in the formation of undesirable magnetic dead layers. To balance this reaction, several new approaches have been tentatively used, such as the passivation of silicon surfaces with Sb for instance [4]. Alternatively, a silicide template can be intentionally formed at the Fe/Si interface to avoid Si diffusion, for instance, and/or to promote the Fe epitaxy. In that case, a well-controlled layer would be achieved, with a good control of its thickness and composition. As for the Fe/FeSi2 /Si(1 1 1) and Fe/CoSi2 /Si(1 1 1) interfaces, a thin (about 10 A thick) silicide layer acts as a rather efficient Sidiffusion barrier [1,5]. In the present work, we investigate the morphology and the crystallographic structure of a
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00142-0
S. Hajjar et al. / Surface Science 532–535 (2003) 940–945
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FeSi2 silicide template epitaxially grown on Si(0 0 1), upon annealing a thin Fe overlayer, by means of scanning tunneling microscopy (STM) experiments, X-ray photoelectron diffraction (XPD) experiments and double scattering XPD simulations.
2. Experiment The experimental set-up is composed of three connected ultrahigh vacuum chambers equipped with low energy electron diffraction (LEED), photoemission, STM and deposition facilities. The Si(0 0 1) samples (p-type with a resistivity of about 0.1 X cm) were cleaned by repeated flashes at 1500 K at a pressure below 1 1010 mbar. Thin Fe and Co layers were evaporated onto the Si substrate maintained at room temperature from homemade evaporators and post-annealed at a temperature in the 750–800 K range. The deposition rate was about 0.3 monolayer (ML) per minute at a pressure of about 3 1010 mbar. One Fe (Co) monolayer is defined as the atomic density of a Si(0 0 1) plane, i.e. 6.8 1014 atoms/cm2 . STM measurements were made in a room-temperatureoperating microscope (commercial Omicron STMAFM microscope), in the constant-current mode. Electrochemically etched, in situ cleaned tungsten tips were used. XPD measurements were done using a hemispherical analyzer operating at an angular resolution of 1°. Fe 2p3=2 and Co 2p3=2 lines were excited with an Al Ka (hm ¼ 1486:6 eV) radiation.
3. Results The silicide morphology has been examined after deposition of pure Fe layers in the 2–4 MLs thickness range subsequently annealed at 800 K. In that thickness range, one expects to form Sidiffusion barrier layers with a single composition, FeSi2 . Fig. 1 shows a STM image collected after deposition of 3 ML Fe, post-annealed at 800 K. This image shows the formation of a non-uniform silicide film. Atomic resolution is achieved in this
200 A ) for a 3 ML Fe deposit on Fig. 1. STM image (200 A Si(0 0 1) at room temperature and annealed subsequently at 800 K (Vs ¼ 2 V; It ¼ 0:5 nA).
image and reveals the co-existence of wellordered two-dimensional (2D) silicide islands and uncovered Si regions. These latter regions are characterized by the standard 2 1 dimer-row reconstruction showing that they are rather free of Fe. So, Fep is concentrated in the islands, which p exhibit a 2 2 R45° surface periodicity. This silicide could crystallize in several structures. The most stable, achieved at high annealing temperature is the semiconducting b-FeSi2 phase. Raunau et al. has shown that b-FeSi2 layers are formed upon annealing thin Fe layers (3 or 4 ML Fe) on Si(0 0 1) at 900 K [6]. They have found that the epitaxial relationship is b-FeSi2 (0 0 1) k Si(0 0 1) and b-FeSi2 [0 1 0] k Sih1 1 0i. The relevant STM p images show that the surface lattice has a 2 p 2 R45° periodicity, as in the present experiments. The semiconducting behavior of b-FeSi2 was checked with scanning tunneling spectroscopy, which exhibits a band gap of about 0.9 eV. In that respect, Fig. 2 shows a valence band spectrum collected at normal electron emission. This spectrum is characterized by a broad non-bonding states band at 0.5–0.6 eV and a sizeable density of states at the Fermi level Ef , which suggests that the silicide is metallic. Since
S. Hajjar et al. / Surface Science 532–535 (2003) 940–945
Intensity (kcps/s)
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8 UPS HeI hυ = 21.22 eV 7 Normal electron collection 6 5 4 3 2 1 0 1.5
1.0
0.5
EF
0.0
-0.5
Binding energy (eV) Fig. 2. UPS-spectrum of FeSi2 (4 ML Fe) in epitaxy on Si(0 0 1) at a photon energy of hm ¼ 21:2 eV collected at normal electron emission (h ¼ 0°).
the contribution at the Fermi level is not sensitive to surface contamination, for instance, valence band photoemission rules out the formation of the semiconducting b-FeSi2 phase. Raunau et al. has also reported the formation of silicide grains with could have the metastable c-FeSi2 structure, on Si(0 0 1). This phase is of CaF2 type symmetry, as for stable CoSi2 [7]. Thus, it was shown that this metastable FeSi2 phases observed on Si(1 1 1) could also grow on Si(0 0 1). Nevertheless, the STM image relevant to the c-FeSi2 phase is very different from that shown in Fig. 1. Another FeSi2 phase has been found to grow on Si(1 1 1), namely a a-FeSi2 -type phase [8,9]. This phase is formed either by solid phase epitaxy, by annealing thin Fe layers at about 780–820 K, or by annealing CsCl-type FeSi2 layers grown at room temperature by molecular beam epitaxy. Thin Fe layers undergo several phase transformations upon annealing, with the following sequence: FeSi1þx ! a-FeSi2 ! c-FeSi2 ! b-FeSi2 . Note that a a-FeSi2 -type phase has never been observed to grow on Si(0 0 1). This phase has a quadratic structure derived from the bulk a-FeSi2 one with and c 5:12 A [8,9]. The a-FeSi2 lata 5:39 A tice can be seen as an alternate Si-bilayer and Fe monolayer, piled up along the [0 0 1] direction, as shown in Fig. 3. The structure of a-FeSi2 grown on Si(1 1 1) has been extensively studied by X-ray diffraction [8,9], XPD [10] and extended X-ray absorption fine structure [11]. It has been shown that the unit cell is derived from that of the ge-
Fig. 3. Sketches of the a-FeSi2 -type and CaF2 -type FeSi2 (CoSi2 ) structures and the cross-sectional cuts through the (1 1 0) plane for a-FeSi2 -type with c-axis perpendicular (a) and parallel (b) to the surface plane and for CaF2 -type silicide (c). The open circle and open square indicate the possible position of Si adatoms.
neric a-FeSi2 by the formation of some random Fe vacancies in the Fe plane and insertion of some Fe atoms between the Si-bilayers, but without any superstructure in either {1 0 0} plane [9]. If grown in epitaxy on Si(0 0 1), the FeSi2 grains could exhibit three different orientations, i.e. [0 0 1]silicide k [0 0 1]Si , [0 1 0]silicide k [0 0 1]Si or [1 0 0]silicide k [0 0 1]Si . Since the thin FeSi2 layer formed in the present experiments is metallic and the STM images are very different from that recorded on c-FeSi2 , the 2D silicide layer could be of a-FeSi2 structure. In this respect, some interesting features can be pointed out by the comparison of the XPD profiles measured on FeSi2 grown on Si(0 0 1) for a 3 ML Fe deposit and profiles simulated using double scattering codes. Fig. 4 shows XPD profiles (Co 2p3=2 and Fe 2p3=2 line intensity versus polar angle h) measured in the same (1 1 0) diffraction plane for CoSi2 (which is the reference system with CaF2 structure) and FeSi2 . Within this growth
S. Hajjar et al. / Surface Science 532–535 (2003) 940–945
Fig. 4. Polar angle scans of Co 2p3=2 (open circles) and Fe 2p3=2 (solid circles) photoelectron intensities from CoSi2 and FeSi2 along the [1–10] azimuth using an Al Ka source (1486.6 eV). Calculated Fe 2p3=2 (Ec ¼ 779 eV) profiles for the a-FeSi2 structure with the c-axis perpendicular to the surface plane, without (a) and with Si segregation (b), with the c-axis parallel to the surface plane, without (c) and with Si segregation (d and e) and for the CaF2 -type structure with (f) and without Si segregation (g). For CoSi2 (0 0 1), the Si adatom position is that given by Weiss et al. in Ref. [13].
procedure, CoSi2 is known to form a well-defined p CaF 2 p 2 -type structure on Si(0 0 1), with a 2 R45° surface superstructure. At a nominal coverage of 3 ML Fe, STM images show that about 75% of the surface is covered by the 2D silicide. This suggests that these silicide islands incorporate about 4 ML Fe. Also shown in Fig. 4 are the XPD simulations of the Fe 2p3=2 line profile for a thin FeSi2 (0 0 1) (4 ML Fe) layer with the aFeSi2 structure with the c-axis perpendicular (a,b) and parallel (c–e) to the surface plane and the CaF2 -type structure (f,g). These simulations were performed using multiple scattering codes described in Ref. [12]. The present simulations take into account double-scattering events and the . Metallic silicluster radius was set to 10 A cides often exhibit Si surface segregation when grown on Si(1 1 1) but also Si(0 0 1), as for p on p CoSi2 , which exhibits a 2 2 R45° LEED
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pattern. On CoSi2 (0 0 1), the Si adatoms (1/4 ML) are located in fourfold coordinated hollow sites [13]. The close similarity in surface periodicity is not necessarily related to the same surface reconstruction but it has to be taken into account in the present simulations. Thus, for each compound, the simulation has been performed assuming ideal unreconstructed crystals terminated by a single Si plane, with or without Si segregation as shown in Fig. 3. Finally, the CaF2 -type phase was simulated with Si adatoms in fourfold coordinated Si hollow sites, as for CoSi2 (0 0 1) surface [13]. The fairly good agreement between the experimental profile for CoSi2 and the profile simulated for CaF2 -type FeSi2 shows that the calculations nicely reproduce the experimental intensity modulation (the difference in wavelength and atomic factors for Co and Fe is negligible here). Data in Fig. 4 show that FeSi2 crystallizes in a cubic, or nearly cubic structure on Si(0 0 1), as evidenced by strong forward scattering at polar angles h ¼ 0° and h 55°. In that respect, the experimental spectrum is compatible with all simulated profiles. The main difference between these profiles resides in a selective intensity variation at polar angles between 20° and 40°, depending on the phase structure. In that polar angle range, the intensity enhancement is, in part, due to higher interference orders between the primary and diffracted waves. A qualitatively good agreement between experimental and simulated XPD profile is obtained for a a-FeSi2 structure with the c-axis perpendicular to the (0 0 1) surface plane, with or without Si adatoms. Indeed, the simulation predicts the deep intensity decrease at a polar angle h of about 25–30° and a narrow forward scattering peak at h ¼ 0°, as observed in experimental FeSi2 spectrum. The peak at h ¼ 0° is only 10–12° wide for a a-FeSi2 with c perpendicular to the (0 0 1) surface plane, in contrast with that predicted for the other possible structures (about 25° wide) and experimental profile of cubic CoSi2 (about 22° wide). This narrow peak is associated with a single scatterer at from the h ¼ 0°, at a distance of more than 4 A emitter. Note that the presence of Si adatoms has only minor effects on the calculated XPD profiles measured in the (1 1 0) plane. These results suggest that the 2D silicide film could be of a-FeSi2
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derived structure, with the c-axis perpendicular to the surface plane. As to the surface morphology, silicide islands are also observed for a coverage of 4 ML Fe, but with a different shape. Fig. 5 shows a STM image collected after the reaction of 4 ML Fe on Si(0 0 1) annealed at 850 K. At this stage, the silicide film consists in a 2D layer with some sparse threedimensional islands (less than 10% of the sample surface). A close examination (XPD, atomically resolved images) of the 2D layer shows that it consists of silicide with the a-derived FeSi2 structure. The most surprising feature is that its surface is not flat but exhibits a spatial undulation along the h1 1 0i directions of the Si substrate. The sur high with a pseudoface modulation is about 2 A . This modulation seems too period of about 200 A large to be attributed to an outward relaxation of the atoms of the topmost layer only. It could reflect a strong modification of the silicide film itself, via a selective lateral distribution of Fe atoms in the vacancy plane for instance, which could induce a local modification of the lattice parameter c. Alternatively, this modulation could be due to a periodic distortion of the whole silicide film. Similar undulations were observed for epitaxial layers with lattice parameter larger than that of the
substrate, i.e. that are under compressive strain. In contrast, a surface undulation has rarely been observed on epitaxial layers under tensile strain [14]. a-FeSi2 grown on Si(1 1 1) has a lattice parameter smaller than that of Si (da=a 0:6%) [9] and thus, assuming a smaller lattice parameter than Si if grown on Si(0 0 1), the FeSi2 silicide would also be under tensile strain. Note that this undulation is not observed on b-FeSi2 layers grown on Si(0 0 1) [6]. b-FeSi2 is also under tensile stress due to the lattice mismatch between silicide and the silicon substrate. This stress is relieved by the formation of anti-phase domain boundaries, which are not observed in Fig. 5. Finally, the composition and structure of the small 3D silicide islands between the 2D silicide layer patches is much more difficult to determine. Nevertheless, the temperature at which the islands are formed suggests that they could crystallize in the small gap semiconducting e-FeSi structure. 4. Conclusions The structure as well as morphology of thin FeSi2 (3–4 ML Fe, annealed at 800 K) grown on Si(0 0 1) has been investigated. The silicide film mainly consists in a 2D silicide layer with a a-FeSi2 derived structure. At a 4 ML Fe coverage, this 2D silicide covers about 90% of the sample surface and exhibits a rather periodic undulation. This film deformation could be due to strain effects. References
Fig. 5. A filled-state (Vs ¼ 2 V, It ¼ 0:5 nA) STM image (3000 4000 A ) measured after the reaction of 4 ML Fe on Si(0 0 1) A annealed at 850 K with a detail of an undulated area of the 2D silicide layer.
[1] S. Hong, P. Wetzel, D. Bolmont, G. Gewinner, C. Pirri, J. Appl. Phys. 78 (9) (1995) 5404. [2] R. Kl€asges, C. Carbone, W. Eberhardt, C. Pampuch, O. Rader, J. Kacheland, W. Gutat, Phys. Rev. B 56 (1997) 10801. [3] J. Alvarez, J.J. Hinarejos, E.G. Michel, G.R. Castro, R. Miranda, Phys. Rev. B 45 (1997) 10042. [4] K.H. Park, J.S. Ha, W.S. Yun, Surf. Sci. 492 (2001) 34. [5] H. von K€anel, Mater. Sci. Rep. 8 (1992) 193. [6] W. Raunau, H. Niehus, G. Comsa, Surf. Sci. 284 (1993) L375. [7] N. Onda, J. Henz, E. M€ uller, K.A. M€ader, H. von K€anel, Appl. Surf. Sci. 56–58 (1992) 421. [8] J. Chevrier, P. Stocker, V. Le Thanh, J.M. Gay, J. Derrien, Europhys. Lett. 22 (1993) 449.
S. Hajjar et al. / Surface Science 532–535 (2003) 940–945 [9] N. Jedrecy, A. Waldhauer, M. Sauvage-Simkin, R. Pinchaux, Y. Zheng, Phys. Rev. B 49 (1994) 4725. [10] U. Kafader, C. Pirri, P. Wetzel, G. Gewinner, Appl. Surf. Sci. 64 (1993) 297. [11] U. Kafader, M.-H. Tuilier, C. Pirri, P. Wetzel, G. Gewinner, D. Bolmont, O. Heckmann, D. Chandesris, H. Magnan, Europhys. Lett. 22 (1993) 529.
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[12] C. Pirri, U. Kafader, G. Gewinner, P. Wetzel, Solid State Commun. 89 (1994) 313. [13] W. Weiss, U. Starke, K. Heinz, G. Rangelov, T.H. Fauster, G.R. Castro, Surf. Sci. 347 (1996) 117. [14] P. Politi, G. Grenet, A. Marty, A. Ponchet, J. Vilain, Phys. Rep. 324 (2000) 271.
Periodic surface modulation on thin epitaxial FeSi2 layers on Si(0 0 1) S. Hajjar, G. Garreau, S. Pelletier, P. Bertoncini, P. Wetzel, G. Gewinner, M. Imhoff, C. Pirri * Laboratoire de Physique et de Spectroscopie Electronique, UMR CNRS-7014, Facult e des Sciences et Techniques, 4, rue des Fr eres Lumi ere, F-68093 Mulhouse C edex, France
Abstract The morphology and the structure of thin metastable iron disilicide films grown on Si(0 0 1) are studied by scanning tunneling microscopy and X-ray photoelectron diffraction. It is shown that the FeSi2 silicide has a quadratic crystallographic structure, with the c-axis perpendicular to the sample surface. As to the film morphology, the silicide consists p p of rather flat islands with a 2 2 R45° surface periodicity for a coverage lower than 4 ML. At a nominal Fe coverage of 4 ML, the silicon surface is almost completely covered. The surface exhibits a quite periodic height . modulation of about 2 A Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Surface structure, morphology, roughness, and topography; Photoelectron spectroscopy; Photoelectron diffraction; Scanning tunneling microscopy; Solid phase epitaxy; Metal–semiconductor interfaces; Silicides; Silicon
1. Introduction The integration of magnetic layers in Si-based devices has motivated recent studies of Fe and Co growth on Si(0 0 1) and Si(1 1 1) [1–3]. Of particular interest is the formation of magnetic metal– silicon interfaces for the fabrication of Si-based spin electronics. Nevertheless, a strong intermixing occurs at the Fe/Si interface even at room temperature, and a metallic silicide is formed along with Si diffusion within a large part of the Fe overlayer. The intermixed zone thickness is not controlled and depends on preparation conditions
*
Corresponding author. Tel.: +33-03-89-33-60-87; fax: +3303-89-33-60-83. E-mail address: [email protected] (C. Pirri).
(substrate temperature, substrate surface morphology, deposition rate. . .). At any rate, Si diffusion in the magnetic layer results in the formation of undesirable magnetic dead layers. To balance this reaction, several new approaches have been tentatively used, such as the passivation of silicon surfaces with Sb for instance [4]. Alternatively, a silicide template can be intentionally formed at the Fe/Si interface to avoid Si diffusion, for instance, and/or to promote the Fe epitaxy. In that case, a well-controlled layer would be achieved, with a good control of its thickness and composition. As for the Fe/FeSi2 /Si(1 1 1) and Fe/CoSi2 /Si(1 1 1) interfaces, a thin (about 10 A thick) silicide layer acts as a rather efficient Sidiffusion barrier [1,5]. In the present work, we investigate the morphology and the crystallographic structure of a
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00142-0
S. Hajjar et al. / Surface Science 532–535 (2003) 940–945
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FeSi2 silicide template epitaxially grown on Si(0 0 1), upon annealing a thin Fe overlayer, by means of scanning tunneling microscopy (STM) experiments, X-ray photoelectron diffraction (XPD) experiments and double scattering XPD simulations.
2. Experiment The experimental set-up is composed of three connected ultrahigh vacuum chambers equipped with low energy electron diffraction (LEED), photoemission, STM and deposition facilities. The Si(0 0 1) samples (p-type with a resistivity of about 0.1 X cm) were cleaned by repeated flashes at 1500 K at a pressure below 1 1010 mbar. Thin Fe and Co layers were evaporated onto the Si substrate maintained at room temperature from homemade evaporators and post-annealed at a temperature in the 750–800 K range. The deposition rate was about 0.3 monolayer (ML) per minute at a pressure of about 3 1010 mbar. One Fe (Co) monolayer is defined as the atomic density of a Si(0 0 1) plane, i.e. 6.8 1014 atoms/cm2 . STM measurements were made in a room-temperatureoperating microscope (commercial Omicron STMAFM microscope), in the constant-current mode. Electrochemically etched, in situ cleaned tungsten tips were used. XPD measurements were done using a hemispherical analyzer operating at an angular resolution of 1°. Fe 2p3=2 and Co 2p3=2 lines were excited with an Al Ka (hm ¼ 1486:6 eV) radiation.
3. Results The silicide morphology has been examined after deposition of pure Fe layers in the 2–4 MLs thickness range subsequently annealed at 800 K. In that thickness range, one expects to form Sidiffusion barrier layers with a single composition, FeSi2 . Fig. 1 shows a STM image collected after deposition of 3 ML Fe, post-annealed at 800 K. This image shows the formation of a non-uniform silicide film. Atomic resolution is achieved in this
200 A ) for a 3 ML Fe deposit on Fig. 1. STM image (200 A Si(0 0 1) at room temperature and annealed subsequently at 800 K (Vs ¼ 2 V; It ¼ 0:5 nA).
image and reveals the co-existence of wellordered two-dimensional (2D) silicide islands and uncovered Si regions. These latter regions are characterized by the standard 2 1 dimer-row reconstruction showing that they are rather free of Fe. So, Fep is concentrated in the islands, which p exhibit a 2 2 R45° surface periodicity. This silicide could crystallize in several structures. The most stable, achieved at high annealing temperature is the semiconducting b-FeSi2 phase. Raunau et al. has shown that b-FeSi2 layers are formed upon annealing thin Fe layers (3 or 4 ML Fe) on Si(0 0 1) at 900 K [6]. They have found that the epitaxial relationship is b-FeSi2 (0 0 1) k Si(0 0 1) and b-FeSi2 [0 1 0] k Sih1 1 0i. The relevant STM p images show that the surface lattice has a 2 p 2 R45° periodicity, as in the present experiments. The semiconducting behavior of b-FeSi2 was checked with scanning tunneling spectroscopy, which exhibits a band gap of about 0.9 eV. In that respect, Fig. 2 shows a valence band spectrum collected at normal electron emission. This spectrum is characterized by a broad non-bonding states band at 0.5–0.6 eV and a sizeable density of states at the Fermi level Ef , which suggests that the silicide is metallic. Since
S. Hajjar et al. / Surface Science 532–535 (2003) 940–945
Intensity (kcps/s)
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8 UPS HeI hυ = 21.22 eV 7 Normal electron collection 6 5 4 3 2 1 0 1.5
1.0
0.5
EF
0.0
-0.5
Binding energy (eV) Fig. 2. UPS-spectrum of FeSi2 (4 ML Fe) in epitaxy on Si(0 0 1) at a photon energy of hm ¼ 21:2 eV collected at normal electron emission (h ¼ 0°).
the contribution at the Fermi level is not sensitive to surface contamination, for instance, valence band photoemission rules out the formation of the semiconducting b-FeSi2 phase. Raunau et al. has also reported the formation of silicide grains with could have the metastable c-FeSi2 structure, on Si(0 0 1). This phase is of CaF2 type symmetry, as for stable CoSi2 [7]. Thus, it was shown that this metastable FeSi2 phases observed on Si(1 1 1) could also grow on Si(0 0 1). Nevertheless, the STM image relevant to the c-FeSi2 phase is very different from that shown in Fig. 1. Another FeSi2 phase has been found to grow on Si(1 1 1), namely a a-FeSi2 -type phase [8,9]. This phase is formed either by solid phase epitaxy, by annealing thin Fe layers at about 780–820 K, or by annealing CsCl-type FeSi2 layers grown at room temperature by molecular beam epitaxy. Thin Fe layers undergo several phase transformations upon annealing, with the following sequence: FeSi1þx ! a-FeSi2 ! c-FeSi2 ! b-FeSi2 . Note that a a-FeSi2 -type phase has never been observed to grow on Si(0 0 1). This phase has a quadratic structure derived from the bulk a-FeSi2 one with and c 5:12 A [8,9]. The a-FeSi2 lata 5:39 A tice can be seen as an alternate Si-bilayer and Fe monolayer, piled up along the [0 0 1] direction, as shown in Fig. 3. The structure of a-FeSi2 grown on Si(1 1 1) has been extensively studied by X-ray diffraction [8,9], XPD [10] and extended X-ray absorption fine structure [11]. It has been shown that the unit cell is derived from that of the ge-
Fig. 3. Sketches of the a-FeSi2 -type and CaF2 -type FeSi2 (CoSi2 ) structures and the cross-sectional cuts through the (1 1 0) plane for a-FeSi2 -type with c-axis perpendicular (a) and parallel (b) to the surface plane and for CaF2 -type silicide (c). The open circle and open square indicate the possible position of Si adatoms.
neric a-FeSi2 by the formation of some random Fe vacancies in the Fe plane and insertion of some Fe atoms between the Si-bilayers, but without any superstructure in either {1 0 0} plane [9]. If grown in epitaxy on Si(0 0 1), the FeSi2 grains could exhibit three different orientations, i.e. [0 0 1]silicide k [0 0 1]Si , [0 1 0]silicide k [0 0 1]Si or [1 0 0]silicide k [0 0 1]Si . Since the thin FeSi2 layer formed in the present experiments is metallic and the STM images are very different from that recorded on c-FeSi2 , the 2D silicide layer could be of a-FeSi2 structure. In this respect, some interesting features can be pointed out by the comparison of the XPD profiles measured on FeSi2 grown on Si(0 0 1) for a 3 ML Fe deposit and profiles simulated using double scattering codes. Fig. 4 shows XPD profiles (Co 2p3=2 and Fe 2p3=2 line intensity versus polar angle h) measured in the same (1 1 0) diffraction plane for CoSi2 (which is the reference system with CaF2 structure) and FeSi2 . Within this growth
S. Hajjar et al. / Surface Science 532–535 (2003) 940–945
Fig. 4. Polar angle scans of Co 2p3=2 (open circles) and Fe 2p3=2 (solid circles) photoelectron intensities from CoSi2 and FeSi2 along the [1–10] azimuth using an Al Ka source (1486.6 eV). Calculated Fe 2p3=2 (Ec ¼ 779 eV) profiles for the a-FeSi2 structure with the c-axis perpendicular to the surface plane, without (a) and with Si segregation (b), with the c-axis parallel to the surface plane, without (c) and with Si segregation (d and e) and for the CaF2 -type structure with (f) and without Si segregation (g). For CoSi2 (0 0 1), the Si adatom position is that given by Weiss et al. in Ref. [13].
procedure, CoSi2 is known to form a well-defined p CaF 2 p 2 -type structure on Si(0 0 1), with a 2 R45° surface superstructure. At a nominal coverage of 3 ML Fe, STM images show that about 75% of the surface is covered by the 2D silicide. This suggests that these silicide islands incorporate about 4 ML Fe. Also shown in Fig. 4 are the XPD simulations of the Fe 2p3=2 line profile for a thin FeSi2 (0 0 1) (4 ML Fe) layer with the aFeSi2 structure with the c-axis perpendicular (a,b) and parallel (c–e) to the surface plane and the CaF2 -type structure (f,g). These simulations were performed using multiple scattering codes described in Ref. [12]. The present simulations take into account double-scattering events and the . Metallic silicluster radius was set to 10 A cides often exhibit Si surface segregation when grown on Si(1 1 1) but also Si(0 0 1), as for p on p CoSi2 , which exhibits a 2 2 R45° LEED
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pattern. On CoSi2 (0 0 1), the Si adatoms (1/4 ML) are located in fourfold coordinated hollow sites [13]. The close similarity in surface periodicity is not necessarily related to the same surface reconstruction but it has to be taken into account in the present simulations. Thus, for each compound, the simulation has been performed assuming ideal unreconstructed crystals terminated by a single Si plane, with or without Si segregation as shown in Fig. 3. Finally, the CaF2 -type phase was simulated with Si adatoms in fourfold coordinated Si hollow sites, as for CoSi2 (0 0 1) surface [13]. The fairly good agreement between the experimental profile for CoSi2 and the profile simulated for CaF2 -type FeSi2 shows that the calculations nicely reproduce the experimental intensity modulation (the difference in wavelength and atomic factors for Co and Fe is negligible here). Data in Fig. 4 show that FeSi2 crystallizes in a cubic, or nearly cubic structure on Si(0 0 1), as evidenced by strong forward scattering at polar angles h ¼ 0° and h 55°. In that respect, the experimental spectrum is compatible with all simulated profiles. The main difference between these profiles resides in a selective intensity variation at polar angles between 20° and 40°, depending on the phase structure. In that polar angle range, the intensity enhancement is, in part, due to higher interference orders between the primary and diffracted waves. A qualitatively good agreement between experimental and simulated XPD profile is obtained for a a-FeSi2 structure with the c-axis perpendicular to the (0 0 1) surface plane, with or without Si adatoms. Indeed, the simulation predicts the deep intensity decrease at a polar angle h of about 25–30° and a narrow forward scattering peak at h ¼ 0°, as observed in experimental FeSi2 spectrum. The peak at h ¼ 0° is only 10–12° wide for a a-FeSi2 with c perpendicular to the (0 0 1) surface plane, in contrast with that predicted for the other possible structures (about 25° wide) and experimental profile of cubic CoSi2 (about 22° wide). This narrow peak is associated with a single scatterer at from the h ¼ 0°, at a distance of more than 4 A emitter. Note that the presence of Si adatoms has only minor effects on the calculated XPD profiles measured in the (1 1 0) plane. These results suggest that the 2D silicide film could be of a-FeSi2
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derived structure, with the c-axis perpendicular to the surface plane. As to the surface morphology, silicide islands are also observed for a coverage of 4 ML Fe, but with a different shape. Fig. 5 shows a STM image collected after the reaction of 4 ML Fe on Si(0 0 1) annealed at 850 K. At this stage, the silicide film consists in a 2D layer with some sparse threedimensional islands (less than 10% of the sample surface). A close examination (XPD, atomically resolved images) of the 2D layer shows that it consists of silicide with the a-derived FeSi2 structure. The most surprising feature is that its surface is not flat but exhibits a spatial undulation along the h1 1 0i directions of the Si substrate. The sur high with a pseudoface modulation is about 2 A . This modulation seems too period of about 200 A large to be attributed to an outward relaxation of the atoms of the topmost layer only. It could reflect a strong modification of the silicide film itself, via a selective lateral distribution of Fe atoms in the vacancy plane for instance, which could induce a local modification of the lattice parameter c. Alternatively, this modulation could be due to a periodic distortion of the whole silicide film. Similar undulations were observed for epitaxial layers with lattice parameter larger than that of the
substrate, i.e. that are under compressive strain. In contrast, a surface undulation has rarely been observed on epitaxial layers under tensile strain [14]. a-FeSi2 grown on Si(1 1 1) has a lattice parameter smaller than that of Si (da=a 0:6%) [9] and thus, assuming a smaller lattice parameter than Si if grown on Si(0 0 1), the FeSi2 silicide would also be under tensile strain. Note that this undulation is not observed on b-FeSi2 layers grown on Si(0 0 1) [6]. b-FeSi2 is also under tensile stress due to the lattice mismatch between silicide and the silicon substrate. This stress is relieved by the formation of anti-phase domain boundaries, which are not observed in Fig. 5. Finally, the composition and structure of the small 3D silicide islands between the 2D silicide layer patches is much more difficult to determine. Nevertheless, the temperature at which the islands are formed suggests that they could crystallize in the small gap semiconducting e-FeSi structure. 4. Conclusions The structure as well as morphology of thin FeSi2 (3–4 ML Fe, annealed at 800 K) grown on Si(0 0 1) has been investigated. The silicide film mainly consists in a 2D silicide layer with a a-FeSi2 derived structure. At a 4 ML Fe coverage, this 2D silicide covers about 90% of the sample surface and exhibits a rather periodic undulation. This film deformation could be due to strain effects. References
Fig. 5. A filled-state (Vs ¼ 2 V, It ¼ 0:5 nA) STM image (3000 4000 A ) measured after the reaction of 4 ML Fe on Si(0 0 1) A annealed at 850 K with a detail of an undulated area of the 2D silicide layer.
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