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Evidence of epitaxial growth of bcc Co on Cr(100)
Surface Science Letters 245 (1991 L175-L178 North-Holland
L175
Surface Science Letters
Evidence of epitaxial growth of bcc Co on Cr(100) F. Scheurer a, B. Carri6re a, j.p. Deville a and E. Beaurepaire b a 1PCMS Groupe "Surfaces-Interfaces" h Groupe d'Etude des MatOriaux Mdtalliques, UM380046 du CNRS, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France
Received 11 October 1990; accepted for publication 4 January 1991
We report a LEED/Auger study of the growth of cobalt ultra-thin films on Cr(100) surfaces. We demonstrate that Co can be grown epitaxially at room temperature, probably in a metastable bcc phase on this chromium surface. A 1 x 1 LEED pattern is observed at least up to 20 cobalt monolayers. The Auger data are consistent with an abrupt interface and a layer-by-layer growth mode.
Growth studies on magnetic transition metal ultra-thin films, sandwiches, multilayers and superlattices are of great interest because of possible modifications in the magnetic properties, induced by lattice expansion/contraction or by the stabilization of thermodynamically metastable phases. F r o m a theoretical point of view m a n y results have been obtained on the magnetic properties of bcc C o / C r superlattices [1-3] making the effective realization of such superlattices very attractive. Earlier experimental work [4] demonstrated the existence of a metastable magnetic bcc phase for Co grown on a G a A s ( l l 0 ) substrate, with a lattice constant of 2.827 ,~. This has been confirmed recently [5] by conversion electron X-ray absorption experiments performed on a 357 ~, thick Co film grown on GaAs. The bcc Co phase was also mentioned several years ago in sputtered C o / C r modulated films by Walmsley et al. [6]. However, this result was questioned by Stearns et al. [7] and, in experiments on electron-beam evaporated and sputtered C o / C r multilayers [7,8], Co was shown to crystallize in the usual hcp phase, stable at room-temperature [with the (0001) Co plane parallel to (110) bcc Cr planes in the case of sputtered multilayers and a texture in the case of electronbeam evaporated films]. Generally speaking, it should be pointed out that there is no information
about the structure and composition of the various interfaces in these films at an atomic scale. Ultrathin films grown on single crystals are thus believed to be useful to address these questions, especially the ones related to the growth modes even if the conditions of growth (evaporation flux for example) are quite different. Recently, in a theoretical work based on tightbinding calculations [9] it has been predicted that the conditions for a layer-by-layer growth mode were nearly satisfied for Co/Cr(100) ultra-thin films and that the C o - C r superlattice was probably realisable. In a previous work [10] in which we studied the growth of Co on Cr(110), a F r a n k van der Merwe growth mode was not evidenced. In this case, cobalt was shown to grow in the S t r a n s k i - K r a s t a n o v mode without any evidence of epitaxy: the L E E D pattern was lost between 1 and 2 ML, and the SEXAFS spectra of the cobalt layers ( < 5 ML) deposited at room temperature were quite similar to those of amorphous Co layers [11]. Annealing at 3 0 0 ° C restored the SEXAFS spectral shape of hcp Co, but not the L E E D pattern. We report now the results of a low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) study of the growth of cobalt on a Cr(100) single crystal surface and of a pre-
0039-6028/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
1_176
F. Scheurer et al.
Et'idence q/epila.vml %rowth qf hc{ C'o on ('r(100)
liminary UV photoemission (UPS) experiment performed in order to get information about the magnetic status of these ultra-thin layers. The sample was placed in an ultra-high vacuum chamber (base pressure 10 10 Tort) and cleaned over a period of ten weeks by long series of argon-ion sputtering and annealing at temperatures in the 600-650 °C range. The clean Cr(100), checked by AES, showed a sharp p(1 × 1) LEED pattern. Cobalt was then evaporated from a welloutgassed filament by Joule heating on the Cr(100) substrate held at room temperature, The main result is that a very sharp p(1 × 1) LEED pattern is continuously observed from 60 to 600 eV at every stage of coverage even for the thickest film believed to have more than 20 monolayers (ML). Neither the intensity of maxima and minima of all the diffraction spots nor the symmetry of the diagram are changed on this energy range. Fig. 1 shows the LEED diagrams at 140 eV for the clean Cr(100) surface, 0.5 Co ML and
more than 10 Co ML, respectively, demonstrating the epitaxial growth of cobalt on Cr(100). The lattice constant found to be 2.93 _+ 0.08 A is not significantly different from that of the clean Cr(100) surface which has been determined by Ekelund and Leygraf [12] and is the same as that of the bulk (100) plane (2.88 A). As it is theoretically expected for bcc (100) surfaces [13,14] no in-plane relaxation is evidenced. The value obtained for our cobalt layers would represent a 3% expansion with respect to the extrapolated lattice constant for metastable bcc Co [15]. However, such a result is within the accuracy of our LEED measurement and, as matters stand at present, should not be considered too strictly. It is clear that these results show an epitaxy of Co on Cr(100). The fact that the intensity distribution of all the LEED spots is the same either on clean Cr(100) or oil the epitaxial layer leads us to think that the lattice is coherent with a bcc stacking cubic or tetragonal) of Co.
Fig. 1. LEED pattern for: (a) clean Cr(100) (b) 0.5 Co ML, (c) 10 Co ML (incident encrgv: 140 eV).
E Scheurer et al. / Evidence of epitaxial growth of bcc Co on Cr(100)
-.e_
t
5
10
15
20
25
t (rain)
Fig. 2. Growth kinetics of cobalt on Cr(100) at room temperature as evidenced by AES. Intensities are measured on the low-energy Auger peaks after background subtraction. The straight lines represent a theoretical fit assuming a layer-bylayer growth mode.
This result is supported by the AES observations. The Auger spectra, obtained in the derivative mode with a RIBER MAC2 analyzer, were recorded continuously during the evaporation without moving the Cr sample. Fig. 2 displays the kinetics of growth obtained by plotting the peakto-peak intensities of the MVV Cr and Co Auger peaks (at 36 and 53 eV, respectively) versus evaporation time; a classical background subtraction has been applied to take into account the large secondary electron cascade. The experimental data points can be fitted with a series of straight lines derived with the assumption of a layer-by-layer growth mode; the breaks indicating the completion of layers can be placed approximately at 5, 10, 15 and 20 min. Assuming that the inelastic mean free path for the Auger electrons is 4.6 _+ 0.2 ~, this corresponds to an evaporation rate of 0.3 * / r a i n . These values are in good agreement with measurements of the evaporation rate and electron mean free path performed on the C o / C r ( l l 0 ) system, and described in a previous work [10]. Beyond 3 ML i.e. after an evaporation time of 15 min one notices that the Cr intensities are too weak. This underestimation of the Cr low-energy peak intensity is due to background effects and to its strong attenuation by the cobalt overlayer. Despite the fact that one observes by AES a
L177
layer-by-layer growth mode, of course in the limit of the sensitivity and precision of such an approach, we cannot exclude a very limited interdiffusion in the first layer: such an interdiffusion could be hardly evidenced by LEED or AES as it has been underlined by De Miguel et al. in a study of the C o / C u interface [16]. However we think that the Co/Cr(100) interface is abrupt at room temperature. We tested the stability of the interface by annealing a 3 ML cobalt layer at 200 o C, 250 ° C and 300 o C, respectively. No change in the LEED pattern could be detected and only above 250 ° C one could observe an appreciable effect on the Auger lineshape, revealing a marked interdiffusion of cobalt and chromium. The epitaxial growth of bcc Co layers on Cr(100) is confirmed by a preliminary UPS experiment [17] in which a comparison was made between the experimentally determined valence bands of Co layers deposited on Cr(100) and theoretical density of states (DOS) calculations [18]. It was found that for 1 and 3 ML Co, the experimental DOS is in agreement with the calculated bcc Co DOS rather than the fcc one. Moreover, the energy of the multiplet splitting of the Co3s photoemission line is reduced with respect to that of clean cobalt, suggesting a decrease of the magnetic moment, contrary to the case of iron ultra-thin films deposited on Cr(100). More detailed ARPES experiments will explore the possible modifications in the valence band structures due to magnetism. In summary, we demonstrated the epitaxial homogeneous growth of cobalt on Cr(100) at room temperature up to 20 ML. The lattice constant of the epitaxial Co phase is not significantly different from that of the Cr substrate and from that expected for bcc Co. The interface is abrupt at room temperature and stays thermally stable up to 250 o C.
References [1] F. Herman, P. Lambin and O. Jepsen, J. Appl. Phys. 57 (1985) 3654. [2] F. Herman, P. Lambin and O. Jepsen, Phys. Rev. B 31 (1985) 4394.
L178
F. Scheurer et al. / Evidence of epitaxial growth of bee Co on Cr(lO0)
[3] H. Hasegawa and F. Herman, Phys. Rev. B 38 (1988) 4863. [4] G.A. Prinz, Phys. Rev. Lett. 54 (1985) 1051. [5] Y.U. Idzerda, W.T. Elam, B.T. Jonker and G.A. Prinz, Phys. Rev. Lett. 62 (1989) 2480. [6] R. Walmsley, J. Thompson, D. Friedman, R.M. White and T.H. Geballe, IEEE Trans. Magn. Mag-19 (1983) 1992. [7] M.B. Stearns, C.H. Lee and T.L. Groy, Phys. Rev. B 40 (1989)8256. [8] N. Sato, J. Appl. Phys. 61 (1987) 1979. [9] D. Stoeffler and F. Gautier, ECOSS-11, Surf. Sci., in press. [10] O. Heckman, Thesis, Strasbourg (1989); O. Heckman, E. Beaurepaire, B. Carrid:re, J.P. Deville, P. Panissod, F. Scheurer, D. Chandesris and H. Magnan, in: Conference Proceedings Vol. 25, 2nd European Conference on Progress in X-Ray Radiation Research, Eds. A. Balerno, E. Bernieri and S. Mobilio (SIF, Bologna, 1990) p. 509.
[11] H. Magnan, D. Chandesris, G. Rossi, G. Jezequel, K. Hricovini and J. Lecante, Phys. Rev. B 40 (1989) 9989. [12] S. Ekelund and C. Leygraf, Surf. Sci. 40 (1973) 179. [13] R.A. Johnson, Surf. Sci. 151 (1985) 311. [14] F. Jona and P.M. Marcus, in: The structure of Surfaces II, Vol. 11 of Springer Series in Surface Sciences, Eds. J.F. van der Veen and M.A. Van Hove (Springer, Berlin, 1988) p. 90. [15] W.C. Ellis and E.S. Greiner, Trans. Am. Soc. Met. 29 (1941) 415. [16] J.J. De Miguel, A. Cebollada, J.M. Gallego, S. Ferrer, R. Miranda, C.M. Schneider, P. Bressler, J. Garbe, K. Bethke and J. Kirschner, Surf. Sci. 211/212 (1989) 732. [17] F. Scheurer, E. Beaurepaire, V. Schorsch, C. Boeglin, B. Carri$re, O. Heckman and J.P. Deville, J. Magn. Magn. Mat., in press. [18] P.M. Marcus and V.L. Moruzzi, Solid State Commun. 55 (1985) 971.
L175
Surface Science Letters
Evidence of epitaxial growth of bcc Co on Cr(100) F. Scheurer a, B. Carri6re a, j.p. Deville a and E. Beaurepaire b a 1PCMS Groupe "Surfaces-Interfaces" h Groupe d'Etude des MatOriaux Mdtalliques, UM380046 du CNRS, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France
Received 11 October 1990; accepted for publication 4 January 1991
We report a LEED/Auger study of the growth of cobalt ultra-thin films on Cr(100) surfaces. We demonstrate that Co can be grown epitaxially at room temperature, probably in a metastable bcc phase on this chromium surface. A 1 x 1 LEED pattern is observed at least up to 20 cobalt monolayers. The Auger data are consistent with an abrupt interface and a layer-by-layer growth mode.
Growth studies on magnetic transition metal ultra-thin films, sandwiches, multilayers and superlattices are of great interest because of possible modifications in the magnetic properties, induced by lattice expansion/contraction or by the stabilization of thermodynamically metastable phases. F r o m a theoretical point of view m a n y results have been obtained on the magnetic properties of bcc C o / C r superlattices [1-3] making the effective realization of such superlattices very attractive. Earlier experimental work [4] demonstrated the existence of a metastable magnetic bcc phase for Co grown on a G a A s ( l l 0 ) substrate, with a lattice constant of 2.827 ,~. This has been confirmed recently [5] by conversion electron X-ray absorption experiments performed on a 357 ~, thick Co film grown on GaAs. The bcc Co phase was also mentioned several years ago in sputtered C o / C r modulated films by Walmsley et al. [6]. However, this result was questioned by Stearns et al. [7] and, in experiments on electron-beam evaporated and sputtered C o / C r multilayers [7,8], Co was shown to crystallize in the usual hcp phase, stable at room-temperature [with the (0001) Co plane parallel to (110) bcc Cr planes in the case of sputtered multilayers and a texture in the case of electronbeam evaporated films]. Generally speaking, it should be pointed out that there is no information
about the structure and composition of the various interfaces in these films at an atomic scale. Ultrathin films grown on single crystals are thus believed to be useful to address these questions, especially the ones related to the growth modes even if the conditions of growth (evaporation flux for example) are quite different. Recently, in a theoretical work based on tightbinding calculations [9] it has been predicted that the conditions for a layer-by-layer growth mode were nearly satisfied for Co/Cr(100) ultra-thin films and that the C o - C r superlattice was probably realisable. In a previous work [10] in which we studied the growth of Co on Cr(110), a F r a n k van der Merwe growth mode was not evidenced. In this case, cobalt was shown to grow in the S t r a n s k i - K r a s t a n o v mode without any evidence of epitaxy: the L E E D pattern was lost between 1 and 2 ML, and the SEXAFS spectra of the cobalt layers ( < 5 ML) deposited at room temperature were quite similar to those of amorphous Co layers [11]. Annealing at 3 0 0 ° C restored the SEXAFS spectral shape of hcp Co, but not the L E E D pattern. We report now the results of a low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) study of the growth of cobalt on a Cr(100) single crystal surface and of a pre-
0039-6028/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
1_176
F. Scheurer et al.
Et'idence q/epila.vml %rowth qf hc{ C'o on ('r(100)
liminary UV photoemission (UPS) experiment performed in order to get information about the magnetic status of these ultra-thin layers. The sample was placed in an ultra-high vacuum chamber (base pressure 10 10 Tort) and cleaned over a period of ten weeks by long series of argon-ion sputtering and annealing at temperatures in the 600-650 °C range. The clean Cr(100), checked by AES, showed a sharp p(1 × 1) LEED pattern. Cobalt was then evaporated from a welloutgassed filament by Joule heating on the Cr(100) substrate held at room temperature, The main result is that a very sharp p(1 × 1) LEED pattern is continuously observed from 60 to 600 eV at every stage of coverage even for the thickest film believed to have more than 20 monolayers (ML). Neither the intensity of maxima and minima of all the diffraction spots nor the symmetry of the diagram are changed on this energy range. Fig. 1 shows the LEED diagrams at 140 eV for the clean Cr(100) surface, 0.5 Co ML and
more than 10 Co ML, respectively, demonstrating the epitaxial growth of cobalt on Cr(100). The lattice constant found to be 2.93 _+ 0.08 A is not significantly different from that of the clean Cr(100) surface which has been determined by Ekelund and Leygraf [12] and is the same as that of the bulk (100) plane (2.88 A). As it is theoretically expected for bcc (100) surfaces [13,14] no in-plane relaxation is evidenced. The value obtained for our cobalt layers would represent a 3% expansion with respect to the extrapolated lattice constant for metastable bcc Co [15]. However, such a result is within the accuracy of our LEED measurement and, as matters stand at present, should not be considered too strictly. It is clear that these results show an epitaxy of Co on Cr(100). The fact that the intensity distribution of all the LEED spots is the same either on clean Cr(100) or oil the epitaxial layer leads us to think that the lattice is coherent with a bcc stacking cubic or tetragonal) of Co.
Fig. 1. LEED pattern for: (a) clean Cr(100) (b) 0.5 Co ML, (c) 10 Co ML (incident encrgv: 140 eV).
E Scheurer et al. / Evidence of epitaxial growth of bcc Co on Cr(100)
-.e_
t
5
10
15
20
25
t (rain)
Fig. 2. Growth kinetics of cobalt on Cr(100) at room temperature as evidenced by AES. Intensities are measured on the low-energy Auger peaks after background subtraction. The straight lines represent a theoretical fit assuming a layer-bylayer growth mode.
This result is supported by the AES observations. The Auger spectra, obtained in the derivative mode with a RIBER MAC2 analyzer, were recorded continuously during the evaporation without moving the Cr sample. Fig. 2 displays the kinetics of growth obtained by plotting the peakto-peak intensities of the MVV Cr and Co Auger peaks (at 36 and 53 eV, respectively) versus evaporation time; a classical background subtraction has been applied to take into account the large secondary electron cascade. The experimental data points can be fitted with a series of straight lines derived with the assumption of a layer-by-layer growth mode; the breaks indicating the completion of layers can be placed approximately at 5, 10, 15 and 20 min. Assuming that the inelastic mean free path for the Auger electrons is 4.6 _+ 0.2 ~, this corresponds to an evaporation rate of 0.3 * / r a i n . These values are in good agreement with measurements of the evaporation rate and electron mean free path performed on the C o / C r ( l l 0 ) system, and described in a previous work [10]. Beyond 3 ML i.e. after an evaporation time of 15 min one notices that the Cr intensities are too weak. This underestimation of the Cr low-energy peak intensity is due to background effects and to its strong attenuation by the cobalt overlayer. Despite the fact that one observes by AES a
L177
layer-by-layer growth mode, of course in the limit of the sensitivity and precision of such an approach, we cannot exclude a very limited interdiffusion in the first layer: such an interdiffusion could be hardly evidenced by LEED or AES as it has been underlined by De Miguel et al. in a study of the C o / C u interface [16]. However we think that the Co/Cr(100) interface is abrupt at room temperature. We tested the stability of the interface by annealing a 3 ML cobalt layer at 200 o C, 250 ° C and 300 o C, respectively. No change in the LEED pattern could be detected and only above 250 ° C one could observe an appreciable effect on the Auger lineshape, revealing a marked interdiffusion of cobalt and chromium. The epitaxial growth of bcc Co layers on Cr(100) is confirmed by a preliminary UPS experiment [17] in which a comparison was made between the experimentally determined valence bands of Co layers deposited on Cr(100) and theoretical density of states (DOS) calculations [18]. It was found that for 1 and 3 ML Co, the experimental DOS is in agreement with the calculated bcc Co DOS rather than the fcc one. Moreover, the energy of the multiplet splitting of the Co3s photoemission line is reduced with respect to that of clean cobalt, suggesting a decrease of the magnetic moment, contrary to the case of iron ultra-thin films deposited on Cr(100). More detailed ARPES experiments will explore the possible modifications in the valence band structures due to magnetism. In summary, we demonstrated the epitaxial homogeneous growth of cobalt on Cr(100) at room temperature up to 20 ML. The lattice constant of the epitaxial Co phase is not significantly different from that of the Cr substrate and from that expected for bcc Co. The interface is abrupt at room temperature and stays thermally stable up to 250 o C.
References [1] F. Herman, P. Lambin and O. Jepsen, J. Appl. Phys. 57 (1985) 3654. [2] F. Herman, P. Lambin and O. Jepsen, Phys. Rev. B 31 (1985) 4394.
L178
F. Scheurer et al. / Evidence of epitaxial growth of bee Co on Cr(lO0)
[3] H. Hasegawa and F. Herman, Phys. Rev. B 38 (1988) 4863. [4] G.A. Prinz, Phys. Rev. Lett. 54 (1985) 1051. [5] Y.U. Idzerda, W.T. Elam, B.T. Jonker and G.A. Prinz, Phys. Rev. Lett. 62 (1989) 2480. [6] R. Walmsley, J. Thompson, D. Friedman, R.M. White and T.H. Geballe, IEEE Trans. Magn. Mag-19 (1983) 1992. [7] M.B. Stearns, C.H. Lee and T.L. Groy, Phys. Rev. B 40 (1989)8256. [8] N. Sato, J. Appl. Phys. 61 (1987) 1979. [9] D. Stoeffler and F. Gautier, ECOSS-11, Surf. Sci., in press. [10] O. Heckman, Thesis, Strasbourg (1989); O. Heckman, E. Beaurepaire, B. Carrid:re, J.P. Deville, P. Panissod, F. Scheurer, D. Chandesris and H. Magnan, in: Conference Proceedings Vol. 25, 2nd European Conference on Progress in X-Ray Radiation Research, Eds. A. Balerno, E. Bernieri and S. Mobilio (SIF, Bologna, 1990) p. 509.
[11] H. Magnan, D. Chandesris, G. Rossi, G. Jezequel, K. Hricovini and J. Lecante, Phys. Rev. B 40 (1989) 9989. [12] S. Ekelund and C. Leygraf, Surf. Sci. 40 (1973) 179. [13] R.A. Johnson, Surf. Sci. 151 (1985) 311. [14] F. Jona and P.M. Marcus, in: The structure of Surfaces II, Vol. 11 of Springer Series in Surface Sciences, Eds. J.F. van der Veen and M.A. Van Hove (Springer, Berlin, 1988) p. 90. [15] W.C. Ellis and E.S. Greiner, Trans. Am. Soc. Met. 29 (1941) 415. [16] J.J. De Miguel, A. Cebollada, J.M. Gallego, S. Ferrer, R. Miranda, C.M. Schneider, P. Bressler, J. Garbe, K. Bethke and J. Kirschner, Surf. Sci. 211/212 (1989) 732. [17] F. Scheurer, E. Beaurepaire, V. Schorsch, C. Boeglin, B. Carri$re, O. Heckman and J.P. Deville, J. Magn. Magn. Mat., in press. [18] P.M. Marcus and V.L. Moruzzi, Solid State Commun. 55 (1985) 971.