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Morphology of fcc Co(110) films on Cu(110)
Surface Science 454–456 (2000) 741–745 www.elsevier.nl/locate/susc
Morphology of fcc Co(110) films on Cu(110) C. To¨lkes a,b, R. David a, M.A. Krzyzowski c, P. Zeppenfeld c, * a Institut fu¨r Grenzfla¨chenforschung und Vakuumphysik, Forschungszentrum Ju¨lich, 52426 Ju¨lich, Germany b Sun Microsystems GmbH, 40880 Ratingen, Germany c Institut fu¨r Experimentalphysik, Johannes-Kepler-Universita¨t Linz, 4040 Linz, Austria
Abstract The surface morphology of fcc Co(110) films deposited on Cu(110) using oxygen as a surfactant is characterized by He atom scattering. Interference measurements reveal that the thicker Co films are quite flat and that this flatness is preserved after removal of the oxygen with atomic hydrogen. During growth of the first few layers a rough film morphology is observed which is related to the deconstruction of the Cu(110)-(2×1)O interface and the formation of a (1×2) reconstructed CoMCuMO phase after deposition of the first Co monolayer. With increasing film thickness ˚ characteristic of the Cu(110) surface to (1.21±0.02)A ˚ attributed the measured step height changes from (1.28±0.02)A to the interlayer spacing of pseudomorphic fcc Co(110). © 2000 Elsevier Science B.V. All rights reserved. Keywords: Atom-solid scattering and diffraction – elastic; Chemisorption; Cobalt; Copper; Growth; Hydrogen atom; Low index single crystal surfaces; Metal–metal magnetic thin film structures; Oxygen; Surface structure, morphology, roughness, and topography
1. Introduction Ultrathin layers of ferromagnetic materials exhibit extraordinary magnetic properties which can be exploited for technical applications [1]. In particular, layered structures (so-called superlattices) of ferromagnetic and nonmagnetic materials can show the effect of giant magnetoresistance (GMR), that is, an unusual decrease of the resistance of these systems when an external magnetic field is applied. Among the many systems that show GMR, the Co/Cu system is by far the most intensively studied one due to its high GMR values and low coercive fields [2,3]. Recent research has demonstrated that the interface roughness in these * Corresponding author. Fax: +43-732-2468-509. E-mail address: [email protected] (P. Zeppenfeld)
systems could play a key role for both the optimization of the GMR ratio and a detailed understanding of the underlying physics [4]. Therefore, control of the flatness and cleanliness of the interfaces at the atomic level is of primary interest. Unfortunately, flat and homogeneous thin films or multilayers in the Co/Cu system are the exception rather than the rule [5]. However, the film growth can be improved by using surfactants such as In, Pb, Au or O [6–9]. On the bare Cu(110) surface Co grows in a three-dimensional fashion and interdiffusion between Co and Cu occurs at temperatures even below room temperature [10]. However, it was shown that oxygen can be used to promote layer-by-layer growth of cobalt deposited at 350 K on the Cu(110) surface [11,12]. In this way atomically-flat, clean Co(110) films with thicknesses between six and at least 22 monolayers (ML) on a Cu(110) surface can be prepared.
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C. To¨lkes et al. / Surface Science 454–456 (2000) 741–745
Simultaneously, the oxygen inhibits the interdiffusion between cobalt and copper. Finally, the oxygen remaining on the surface can be reacted away by atomic hydrogen, leaving behind a clean fcc Co(110)-(1×1) film [13]. The quality of the Co films was judged in Refs. [11,12] by the appearance of pronounced growth oscillation in the coverage regime between 6 and 22 ML of Co, indicating a nearly perfect layer-bylayer growth. A more quantitative account of the film morphology and roughness can be obtained from interference measurements which will be presented in this paper. To this end the probing particles (in our case thermal He atoms) are reflected from the surface and the interference pattern as a function of the He wavelength is recorded.
2. Experimental The experimental setup consists of an ultrahigh vacuum ( UHV ) helium scattering apparatus (base pressure <10−10 mbar) which is equipped with a supersonic helium nozzle beam, the usual tools for surface preparation and analysis, a Co evaporator and a home-made source for atomic hydrogen [13,14]. The Cu(110) sample has a misorientation against the (110) direction of <0.1°. It was cleaned in situ by repeating cycles of 1 keV Ar+ ion bombardment followed by annealing at 800–900 K for several minutes. Exposing the Cu(110) surface to oxygen at 600 K leads to the formation of a (2×1)-ordered ‘added-row’ reconstruction [15]. After an exposure of 10 L O (1 L=10−6 2 Torr · s−1), the surface is saturated with oxygen. Cobalt was evaporated onto this surface at a sample temperature of 350 K from a thin highpurity Co rod (2 mm in diameter, impurity concentration <10−4) by electron impact heating. The growth of Co is further improved if a slight oxygen background pressure of 4.3×10−9 mbar (corresponding to an exposure of 0.65 L O per deposited 2 Co monolayer) is supplied during the growth of the first Co monolayer [12]. The stacking of the Co film is fcc and pseudomorphic with respect to the Cu(110) lattice up to at least 22 ML, whereas
bulk Co forms a hcp structure below 693 K. At a Co coverage of 12 ML, the surface exhibits a (3×1) oxygen induced reconstruction [11]. The ordering of the reconstruction pattern can be further improved by annealing at 500 K [12] while the CuMCo interdiffusion appears to be still suppressed. the oxygen remaining at the surface can be completely removed by exposing the sample to atomic hydrogen at room temperature and subsequent annealing to 380 K [13]. As a result, an oxygen free fcc Co(110)-(1×1) film is obtained which remains pseudomorphic with the Cu(110) substrate. A quantitative evaluation of the flatness can be obtained from He interference curves in which the specularly reflected He intensity is recorded as a function of the perpendicular momentum transfer q. In the present case, q is varied by changing the He gas temperature between 60 and 450 K (E= ˚ −1) corresponding to a 13…100 meV, k =5…14 A i variation of the perpendicular wave vector transfer in specular reflection, q=2k cos(h ), between 7 i i ˚ −1 for an angle of incidence and 20 A h =45°. For i a surface displaying terrace levels separated by multiples of a monatomic step height h, constructive and destructive interference is expected if qh=2np and (2n+1)p, respectively, with n being an integer. Hence, from the position of the intensity minima (anti-phase scattering) and maxima (in-phase scattering), the step height can be deduced. Moreover, a simple scattering model described in Section 3 allows to determine the terrace level distribution from the shape of the interference curve.
3. Quantitative analysis of He interference curves The interference curves (i.e. the specular intensity as a function of the perpendicular momentum transfer q) are calculated by adding the scattering phases from each terrace level weighted by its relative area a on the surface [16,17]: i I(q)=I
0
K
K
N ∑ a eijqh . j j=0
(1)
C. To¨lkes et al. / Surface Science 454–456 (2000) 741–745
The layer population {b } is obtained from {a } j j through N b =∑ a j i i=j .For comparison with the experiment, the attenuation of the specular beam intensity with increasing q due to the Debye–Waller effect has to be included: (q)=I(q)e−jT(q2+f), (2) DW where j=3B2/(m k T2 ) with m being the mass s B D,eff s of the surface atoms, k the Boltzmann constant B and T the effective Debye temperature relevant D,eff to He scattering [18]. f=8m D/B2 is the so-called He Beeby correction and accounts for the acceleration of the He atoms towards the surface as a consequence of the attractive He-surface potential with a well depth D (for the Co/Cu case the value of D=6.35 meV for pure Cu was used, see Ref. [19]). Finally I (q) must be convoluted with the instruDW ment function f(q) reflecting the finite energy width DE/E of the He beam. The instrument function has been determined independently and is well approximated by a Gaussian. In addition, a constant offset I is introduced to account for the add
743
diffuse scattering from surface defects and step edges which, therefore, are taken into account only implicitly in the present model. The parameters {a }, h, I and j are obtained j add from a best fit of the final expression I *f(q)+I to the measured interference curve DW add with the additional normalization condition N ∑ a =1. j j=0
I
4. Results Fig. 1 shows He-interference curves recorded at different stages during the growth of Co on the oxygen precovered Cu(110) surface following the same procedure as described in Ref. [11]. The curve in Fig. 1a was measured on the bare Cu(110) surface. Marked intensity oscillations and the broad layer distribution fitted to this curve (shown in the inset) reveal that the surface contains multiple terrace levels over the range of sensitivity ˚ . This is in contrast to other substrates, of ~1000 A such as Pt(111) and Au(111), which are generally much smoother [20]. Upon oxygen adsorption at 600 K, the Cu(110)-(2×1)O added-row phase is
Fig. 1. He interference curves recorded from the clean Cu(110) surface (a), the Cu(110)-(2×1)O surface (b) and after deposition of 1 ML (c) and 18 ML (d) Co on the Cu(110)-(2×1)O surface. $, Experimental data points; —, fits using the model described in the text. The resulting layer distributions are shown in the insets.
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C. To¨lkes et al. / Surface Science 454–456 (2000) 741–745
obtained. The corresponding interference curve (Fig. 1b) shows that the layer distribution is only slightly broadened with respect to the initial Cu(110) substrate. This indicates that despite the considerable mass transport involved in the surface reconstruction, the surface does remain rather smooth. The step heights obtained from the fits to ˚, Fig. 1a and b are (1.28±0.02) and (1.29±0.02)A respectively, indicating that the terraces are, indeed, separated by Cu(110) monatomic steps. Fig. 1c shows the interference curve after deposition of one monolayer of Co at 350 K, where a (1×2) CoMCuMO phase was observed in He-diffraction [11]. As evidenced by the resulting layer distribution, the formation of this phase is accompanied by a significant mass transport and terrace proliferation. Nevertheless, the terrace ˚ suggesting that the mass height is still 1.28 A transport mainly involves Cu atoms displaced from the (2×1) CuMO surface and that the oxygen is not ‘buried’ at the CuMCo interface. This is consistent with previous results showing that the oxygen floats out to the surface and that a Cu(110)-(1×1) interface is eventually restored. The rough surface morphology evidenced in Fig. 1c is also consistent with the variation of the specular intensity at anti-phase conditions during the deposition of Co (see Fig. 1 of Ref. [11]): after deposition of 1 ML Co, only a very weak growth oscillation maximum is observed. At the same time, the diffraction spectra reveal a (1×2) superstructure with broad diffraction peaks along the
[001] direction [11], indicating the formation of ˚ islands with a mean terrace width of only ca. 40 A along this direction. The low specular intensity and, hence, the rough surface morphology persists up to several monolayers of deposited Co. For coverages of about 6 ML, however, the Co film has smoothened significantly and appreciable growth oscillations up to about 22 ML are observed, together with a sharpening of the surface diffraction peaks [11]. The Co films in this thickness range are, indeed, rather flat as evidenced by the interference curve in Fig. 1d recorded from an 18 ML thick Co film grown on Cu(110)-(2×1)O. The much broader interference oscillations, and the fitted layer distribution in the inset show that over extended areas the surface consists of mainly two terrace levels characteristic of a near layer-bylayer growth mode. Note that the positions of the maxima have shifted, corresponding to an altered ˚ (see below). step height h=1.22±0.02 A As shown in Ref. [12] the growth of the Co films can be further improved by exposing the surface to an oxygen pressure of 4.3×10−9 mbar during the growth of the first Co monolayer. Afterwards, the oxygen can be completely removed by atomic hydrogen [13]. Fig. 2 shows interference curves recorded from a so prepared 11 ML thick Co film before ( Fig. 2a) and after ( Fig. 2b) oxygen removal. In both cases the layer distribution is quite narrow and only a few layers are exposed. Interference curves recorded after deposition of Co using smaller or larger amounts of oxygen
Fig. 2. He interference curves recorded from (a) an 11 ML Co film deposited on the Cu(110)-(2×1)O surface with an oxygen background pressure of 4.3×10−9 mbar during the growth of the first Co monolayer and (b) a 12 ML Co film prepared as in (a) but after removal of the residual oxygen with atomic hydrogen. $, Experimental data points; —, fits using the model described in the text. The resulting layer distributions are shown in the insets.
C. To¨lkes et al. / Surface Science 454–456 (2000) 741–745
yield sharper and more pronounced interference oscillations and, consequently, a broader layer distribution. These results corroborate that the optimum growth conditions as judged from the interference curves are, indeed, those which also yield the most pronounced growth oscillations and the sharpest surface diffraction patterns [12]. In fact, the large improvement in the growth oscillations is due the formation of extended (3×1) CoMO terraces if the correct stoichiometric coverage of 2/3 ML of oxygen is provided [12]. From the interference curves in Fig. 2, a step ˚ (Fig. 2a) and 1.19±0.02 A ˚ height of 1.22±0.02 A (Fig. 2b) can be deduced, which is slightly smaller ˚ expected for bulk fcc than the value of 1.26 A Co(110). Since the Co film is pseudomorphic with the Cu(110) substrate it is laterally expanded by 1.8%. The film may compensate this elastic strain by a vertical contraction: to conserve the volume of the unit cell, the interlayer spacing of the Co(110) film must be reduced by 3.8% with respect to the bulk value. The resulting step height of ˚ is clearly in agreement with the observations 1.21 A in Figs. 1d and 2.
5. Conclusion He interference curves have been found to be particularly useful in the characterization of the thin film growth of Co on Cu(110) and in determining the effect of oxygen as a surfactant. The results support the conclusions drawn previously on the basis of growth oscillation and diffraction measurements. In addition, they allow a quantitative evaluation of the surface morphology through the determination of step heights and layer distributions. For the Co/Cu(110) system the optimum growth conditions are shown to yield atomically flat fcc Co(110) films. These films are found to be
745
contracted normal to the surface by ca. 4% with respect to the bulk value. This contraction may compensate for the lateral expansion of the pseudomorphic Co films.
References [1] G.A. Prinz, Science 282 (1998) 1660. [2] S.S.P. Parkin, R. Bhadra, K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [3] S.S.P. Parkin, Z.G. Li, D.J. Smith, Phys. Rev. Lett. 58 (1991) 2710. [4] P. Zahn, J. Binder, I. Mertig, R. Zeller, P.H. Dederichs, Phys. Rev. Lett. 80 (1998) 4309. [5] M.T. Kief, W.F. Egelhoff Jr., Phys. Rev. B 47 (1993) 10785. [6 ] W.F. Egelhoff Jr., P.J. Chen, C.J. Powell, M.D. Stiles, R.D. McMichael, J. Appl. Phys. 79 (1996) 2491. [7] H.D. Chopra, B.J. Hockey, P.J. Chen, W.F. Egelhoff Jr., M. Wuttig, S.Z. Hua, Phys. Rev. B 55 (1997) 8390. [8] W.F. Egelhoff Jr., P.J. Chen, C.J. Powell, M.D. Stiles, R.D. McMichael, C.-L. Lin, J.M. Sivertsen, J.H. Judy, K. Takano, A.E. Berkowitz, J. Appl. Phys. 80 (1996) 5183. [9] W.F. Egelhoff Jr., P.J. Chen, C.J. Powell, M.D. Stiles, R.D. McMichael, J.H. Judy, K. Takano, A.E. Berkowitz, J. Appl. Phys. 82 (1997) 6142. [10] W.F. Egelhoff Jr., D.A. Steigerwald, J. Vac. Sci. Technol. A 7 (1989) 2167. [11] C. To¨lkes, R. Struck, R. David, P. Zeppenfeld, G. Comsa, Phys. Rev. Lett. 80 (1998) 2877. [12] C. To¨lkes, R. Struck, R. David, G. Comsa, P. Zeppenfeld, Appl. Phys. Lett. 73 (1998) 1059. [13] C. To¨lkes, R. David, K.G. Tschersich, G. Comsa, P. Zeppenfeld, Europhys. Lett. 46 (1999) 589. [14] K.G. Tschersich, V. von Bonin, J. Appl. Phys. 84 (1998) 4065. [15] D.J. Coulman, J. Wintterlin, R.J. Behm, G. Ertl, Phys. Rev. Lett. 64 (1990) 1761. [16 ] C.S. Lent, P.I. Cohen, Surf. Sci. 139 (1984) 121. [17] R. Alsinger, H. Busch, M. Horn, M. Henzler, Surf. Sci. 200 (1988) 235. [18] V. Bortolani, V. Celli, A. Franchini, J. Idiodi, G. Santora, K. Kern, B. Poelsema, G. Comsa, Surf. Sci. 208 (1989) 1. [19] J. Perreau, J. Lapujoulade, Surf. Sci. 122 (1982) 341. [20] M.A. Krzyzowski, C. To¨lkes, P. Zeppenfeld (in preparation).
Morphology of fcc Co(110) films on Cu(110) C. To¨lkes a,b, R. David a, M.A. Krzyzowski c, P. Zeppenfeld c, * a Institut fu¨r Grenzfla¨chenforschung und Vakuumphysik, Forschungszentrum Ju¨lich, 52426 Ju¨lich, Germany b Sun Microsystems GmbH, 40880 Ratingen, Germany c Institut fu¨r Experimentalphysik, Johannes-Kepler-Universita¨t Linz, 4040 Linz, Austria
Abstract The surface morphology of fcc Co(110) films deposited on Cu(110) using oxygen as a surfactant is characterized by He atom scattering. Interference measurements reveal that the thicker Co films are quite flat and that this flatness is preserved after removal of the oxygen with atomic hydrogen. During growth of the first few layers a rough film morphology is observed which is related to the deconstruction of the Cu(110)-(2×1)O interface and the formation of a (1×2) reconstructed CoMCuMO phase after deposition of the first Co monolayer. With increasing film thickness ˚ characteristic of the Cu(110) surface to (1.21±0.02)A ˚ attributed the measured step height changes from (1.28±0.02)A to the interlayer spacing of pseudomorphic fcc Co(110). © 2000 Elsevier Science B.V. All rights reserved. Keywords: Atom-solid scattering and diffraction – elastic; Chemisorption; Cobalt; Copper; Growth; Hydrogen atom; Low index single crystal surfaces; Metal–metal magnetic thin film structures; Oxygen; Surface structure, morphology, roughness, and topography
1. Introduction Ultrathin layers of ferromagnetic materials exhibit extraordinary magnetic properties which can be exploited for technical applications [1]. In particular, layered structures (so-called superlattices) of ferromagnetic and nonmagnetic materials can show the effect of giant magnetoresistance (GMR), that is, an unusual decrease of the resistance of these systems when an external magnetic field is applied. Among the many systems that show GMR, the Co/Cu system is by far the most intensively studied one due to its high GMR values and low coercive fields [2,3]. Recent research has demonstrated that the interface roughness in these * Corresponding author. Fax: +43-732-2468-509. E-mail address: [email protected] (P. Zeppenfeld)
systems could play a key role for both the optimization of the GMR ratio and a detailed understanding of the underlying physics [4]. Therefore, control of the flatness and cleanliness of the interfaces at the atomic level is of primary interest. Unfortunately, flat and homogeneous thin films or multilayers in the Co/Cu system are the exception rather than the rule [5]. However, the film growth can be improved by using surfactants such as In, Pb, Au or O [6–9]. On the bare Cu(110) surface Co grows in a three-dimensional fashion and interdiffusion between Co and Cu occurs at temperatures even below room temperature [10]. However, it was shown that oxygen can be used to promote layer-by-layer growth of cobalt deposited at 350 K on the Cu(110) surface [11,12]. In this way atomically-flat, clean Co(110) films with thicknesses between six and at least 22 monolayers (ML) on a Cu(110) surface can be prepared.
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C. To¨lkes et al. / Surface Science 454–456 (2000) 741–745
Simultaneously, the oxygen inhibits the interdiffusion between cobalt and copper. Finally, the oxygen remaining on the surface can be reacted away by atomic hydrogen, leaving behind a clean fcc Co(110)-(1×1) film [13]. The quality of the Co films was judged in Refs. [11,12] by the appearance of pronounced growth oscillation in the coverage regime between 6 and 22 ML of Co, indicating a nearly perfect layer-bylayer growth. A more quantitative account of the film morphology and roughness can be obtained from interference measurements which will be presented in this paper. To this end the probing particles (in our case thermal He atoms) are reflected from the surface and the interference pattern as a function of the He wavelength is recorded.
2. Experimental The experimental setup consists of an ultrahigh vacuum ( UHV ) helium scattering apparatus (base pressure <10−10 mbar) which is equipped with a supersonic helium nozzle beam, the usual tools for surface preparation and analysis, a Co evaporator and a home-made source for atomic hydrogen [13,14]. The Cu(110) sample has a misorientation against the (110) direction of <0.1°. It was cleaned in situ by repeating cycles of 1 keV Ar+ ion bombardment followed by annealing at 800–900 K for several minutes. Exposing the Cu(110) surface to oxygen at 600 K leads to the formation of a (2×1)-ordered ‘added-row’ reconstruction [15]. After an exposure of 10 L O (1 L=10−6 2 Torr · s−1), the surface is saturated with oxygen. Cobalt was evaporated onto this surface at a sample temperature of 350 K from a thin highpurity Co rod (2 mm in diameter, impurity concentration <10−4) by electron impact heating. The growth of Co is further improved if a slight oxygen background pressure of 4.3×10−9 mbar (corresponding to an exposure of 0.65 L O per deposited 2 Co monolayer) is supplied during the growth of the first Co monolayer [12]. The stacking of the Co film is fcc and pseudomorphic with respect to the Cu(110) lattice up to at least 22 ML, whereas
bulk Co forms a hcp structure below 693 K. At a Co coverage of 12 ML, the surface exhibits a (3×1) oxygen induced reconstruction [11]. The ordering of the reconstruction pattern can be further improved by annealing at 500 K [12] while the CuMCo interdiffusion appears to be still suppressed. the oxygen remaining at the surface can be completely removed by exposing the sample to atomic hydrogen at room temperature and subsequent annealing to 380 K [13]. As a result, an oxygen free fcc Co(110)-(1×1) film is obtained which remains pseudomorphic with the Cu(110) substrate. A quantitative evaluation of the flatness can be obtained from He interference curves in which the specularly reflected He intensity is recorded as a function of the perpendicular momentum transfer q. In the present case, q is varied by changing the He gas temperature between 60 and 450 K (E= ˚ −1) corresponding to a 13…100 meV, k =5…14 A i variation of the perpendicular wave vector transfer in specular reflection, q=2k cos(h ), between 7 i i ˚ −1 for an angle of incidence and 20 A h =45°. For i a surface displaying terrace levels separated by multiples of a monatomic step height h, constructive and destructive interference is expected if qh=2np and (2n+1)p, respectively, with n being an integer. Hence, from the position of the intensity minima (anti-phase scattering) and maxima (in-phase scattering), the step height can be deduced. Moreover, a simple scattering model described in Section 3 allows to determine the terrace level distribution from the shape of the interference curve.
3. Quantitative analysis of He interference curves The interference curves (i.e. the specular intensity as a function of the perpendicular momentum transfer q) are calculated by adding the scattering phases from each terrace level weighted by its relative area a on the surface [16,17]: i I(q)=I
0
K
K
N ∑ a eijqh . j j=0
(1)
C. To¨lkes et al. / Surface Science 454–456 (2000) 741–745
The layer population {b } is obtained from {a } j j through N b =∑ a j i i=j .For comparison with the experiment, the attenuation of the specular beam intensity with increasing q due to the Debye–Waller effect has to be included: (q)=I(q)e−jT(q2+f), (2) DW where j=3B2/(m k T2 ) with m being the mass s B D,eff s of the surface atoms, k the Boltzmann constant B and T the effective Debye temperature relevant D,eff to He scattering [18]. f=8m D/B2 is the so-called He Beeby correction and accounts for the acceleration of the He atoms towards the surface as a consequence of the attractive He-surface potential with a well depth D (for the Co/Cu case the value of D=6.35 meV for pure Cu was used, see Ref. [19]). Finally I (q) must be convoluted with the instruDW ment function f(q) reflecting the finite energy width DE/E of the He beam. The instrument function has been determined independently and is well approximated by a Gaussian. In addition, a constant offset I is introduced to account for the add
743
diffuse scattering from surface defects and step edges which, therefore, are taken into account only implicitly in the present model. The parameters {a }, h, I and j are obtained j add from a best fit of the final expression I *f(q)+I to the measured interference curve DW add with the additional normalization condition N ∑ a =1. j j=0
I
4. Results Fig. 1 shows He-interference curves recorded at different stages during the growth of Co on the oxygen precovered Cu(110) surface following the same procedure as described in Ref. [11]. The curve in Fig. 1a was measured on the bare Cu(110) surface. Marked intensity oscillations and the broad layer distribution fitted to this curve (shown in the inset) reveal that the surface contains multiple terrace levels over the range of sensitivity ˚ . This is in contrast to other substrates, of ~1000 A such as Pt(111) and Au(111), which are generally much smoother [20]. Upon oxygen adsorption at 600 K, the Cu(110)-(2×1)O added-row phase is
Fig. 1. He interference curves recorded from the clean Cu(110) surface (a), the Cu(110)-(2×1)O surface (b) and after deposition of 1 ML (c) and 18 ML (d) Co on the Cu(110)-(2×1)O surface. $, Experimental data points; —, fits using the model described in the text. The resulting layer distributions are shown in the insets.
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C. To¨lkes et al. / Surface Science 454–456 (2000) 741–745
obtained. The corresponding interference curve (Fig. 1b) shows that the layer distribution is only slightly broadened with respect to the initial Cu(110) substrate. This indicates that despite the considerable mass transport involved in the surface reconstruction, the surface does remain rather smooth. The step heights obtained from the fits to ˚, Fig. 1a and b are (1.28±0.02) and (1.29±0.02)A respectively, indicating that the terraces are, indeed, separated by Cu(110) monatomic steps. Fig. 1c shows the interference curve after deposition of one monolayer of Co at 350 K, where a (1×2) CoMCuMO phase was observed in He-diffraction [11]. As evidenced by the resulting layer distribution, the formation of this phase is accompanied by a significant mass transport and terrace proliferation. Nevertheless, the terrace ˚ suggesting that the mass height is still 1.28 A transport mainly involves Cu atoms displaced from the (2×1) CuMO surface and that the oxygen is not ‘buried’ at the CuMCo interface. This is consistent with previous results showing that the oxygen floats out to the surface and that a Cu(110)-(1×1) interface is eventually restored. The rough surface morphology evidenced in Fig. 1c is also consistent with the variation of the specular intensity at anti-phase conditions during the deposition of Co (see Fig. 1 of Ref. [11]): after deposition of 1 ML Co, only a very weak growth oscillation maximum is observed. At the same time, the diffraction spectra reveal a (1×2) superstructure with broad diffraction peaks along the
[001] direction [11], indicating the formation of ˚ islands with a mean terrace width of only ca. 40 A along this direction. The low specular intensity and, hence, the rough surface morphology persists up to several monolayers of deposited Co. For coverages of about 6 ML, however, the Co film has smoothened significantly and appreciable growth oscillations up to about 22 ML are observed, together with a sharpening of the surface diffraction peaks [11]. The Co films in this thickness range are, indeed, rather flat as evidenced by the interference curve in Fig. 1d recorded from an 18 ML thick Co film grown on Cu(110)-(2×1)O. The much broader interference oscillations, and the fitted layer distribution in the inset show that over extended areas the surface consists of mainly two terrace levels characteristic of a near layer-bylayer growth mode. Note that the positions of the maxima have shifted, corresponding to an altered ˚ (see below). step height h=1.22±0.02 A As shown in Ref. [12] the growth of the Co films can be further improved by exposing the surface to an oxygen pressure of 4.3×10−9 mbar during the growth of the first Co monolayer. Afterwards, the oxygen can be completely removed by atomic hydrogen [13]. Fig. 2 shows interference curves recorded from a so prepared 11 ML thick Co film before ( Fig. 2a) and after ( Fig. 2b) oxygen removal. In both cases the layer distribution is quite narrow and only a few layers are exposed. Interference curves recorded after deposition of Co using smaller or larger amounts of oxygen
Fig. 2. He interference curves recorded from (a) an 11 ML Co film deposited on the Cu(110)-(2×1)O surface with an oxygen background pressure of 4.3×10−9 mbar during the growth of the first Co monolayer and (b) a 12 ML Co film prepared as in (a) but after removal of the residual oxygen with atomic hydrogen. $, Experimental data points; —, fits using the model described in the text. The resulting layer distributions are shown in the insets.
C. To¨lkes et al. / Surface Science 454–456 (2000) 741–745
yield sharper and more pronounced interference oscillations and, consequently, a broader layer distribution. These results corroborate that the optimum growth conditions as judged from the interference curves are, indeed, those which also yield the most pronounced growth oscillations and the sharpest surface diffraction patterns [12]. In fact, the large improvement in the growth oscillations is due the formation of extended (3×1) CoMO terraces if the correct stoichiometric coverage of 2/3 ML of oxygen is provided [12]. From the interference curves in Fig. 2, a step ˚ (Fig. 2a) and 1.19±0.02 A ˚ height of 1.22±0.02 A (Fig. 2b) can be deduced, which is slightly smaller ˚ expected for bulk fcc than the value of 1.26 A Co(110). Since the Co film is pseudomorphic with the Cu(110) substrate it is laterally expanded by 1.8%. The film may compensate this elastic strain by a vertical contraction: to conserve the volume of the unit cell, the interlayer spacing of the Co(110) film must be reduced by 3.8% with respect to the bulk value. The resulting step height of ˚ is clearly in agreement with the observations 1.21 A in Figs. 1d and 2.
5. Conclusion He interference curves have been found to be particularly useful in the characterization of the thin film growth of Co on Cu(110) and in determining the effect of oxygen as a surfactant. The results support the conclusions drawn previously on the basis of growth oscillation and diffraction measurements. In addition, they allow a quantitative evaluation of the surface morphology through the determination of step heights and layer distributions. For the Co/Cu(110) system the optimum growth conditions are shown to yield atomically flat fcc Co(110) films. These films are found to be
745
contracted normal to the surface by ca. 4% with respect to the bulk value. This contraction may compensate for the lateral expansion of the pseudomorphic Co films.
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