Samarium films on copper single crystals

Surface Science 251/252 (1991) 519-523 North-Holl~d

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Samarium films on copper single crystals B. Jmgensen, M. Christiansen and J. Onsgaard Fysisk Instifuf, Odense Universitef, Campuwej

55, DK-5230 Odense M, Denmark

Received 1 October 1990; accepted for publication 30 November 1990

We have studied samarium films on Cu(lOO), Cu(llO), and Cu(ll1) surfaces. These studies have concentrated on the geometric and electronic structure of the interface. The geometry of the interface was studied by LEED, and the electronic structure was investigated by XPS. Calibration of the coverage was done by measuring the Cu(2p) and Sm(3d) photolines and the Cu(M#V) and Sm(N,G,02,sNs,,) XAES spectra. We have seen some simple reconstructions in LEED e.g. ~(8 x 2) on the (100) surface, ~(6 x 2) on the (110) surface, and fi x fiR30° on the (111) surface. At 400°C slightly distorted hexagonal LEED patterns were seen on both the (111) and (100) surface. These are ascribed to growth of a Sm-Cu compound on top of the copper substrate.

Thin films of rare earth metals have attracted much attention in recent years. This is due to the special valency and magnetic properties of the rare earth metals. Thin films of rare earth metals have been deposited on many different substrates ranging from semiconductors [l] via transition group metals [2] to refractory metals [3]. While the close packed refractory metal surfaces make atomically sharp interfaces, the picture is far from clear when one looks at the transition metal/rare earth interface. Here interdiffusion and compound formation between substrate and overlayer seems to be more the rule than the exception. Sm is a most interesting rare earth metal due to the small energy difference between the divalent ([Xe]4f ‘jc2) (c = conduction band) and trivalent ([Xe]4f ‘c3) state [4]. An implication of this is that the loss of atomic coordination at the surface results in heterogeneous mixed valence i.e. divalent surface and trivalent bulk [5]. The sam~um-copper system has been studied several times before 16-91, but there is some discrepancy both between the results and their interpretation. The increase of Sm valency with in-

creasing coverage on a Cu(100) was first demonstrated by F&ldt and Myers [6]. Based upon XPSSm(3d) and LEED measurements, the authors in ref. [6j concluded that no intermixing between Cu and Sm was seen, and that the valency displayed by Sm was homogeneously mixed. These conclusions were convincingly challenged by the photoemission measurements by Andersen et al. [7]. The result presented there is that Sm displays heterogeneously mixed valency. This implies that the interface consists of a pure divalent Sm layer, on top of a trivalent Sm layer, which has reacted with the Cu crystal. In a thorough study of Sm on Cu(ll1) by Jaffey et al. [S] intermixing and compound formation between Cu and Sm was found at temperatures higher than 300 K. An ordered structure is suggested to have homogeneously mixed valency, but this assumption is again only based on LEED results. A recent photoemission study of Sm on polycrystalline Cu [9] has shown diffusion of Cu into a thick Sm film at room temperature. This behaviour has also been observed for Cu single crystals [lo]. All these facts suggest that the Sm-Cu interface is not sharp but shows diffusion and possibly reaction.

0039-6028/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

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2. Experimental The Cu(lOO), Cu(ll0) and Cu(ll1) crystals were cleaned using repeated cycles of sputtering (2 keV Ar+) and annealing to 500° C. The crystals were heated by conductance from the resistively heated W-support wires. The temperature was measured using an optical pyrometer and a chromel-alumel thermocouple tied to the crystal. Sm metal was evaporated from an indirectly heated Ta tube, which was extensively outgassed. The evaporator was surrounded by water cooled shields, which kept the pressure below 2 X 10~‘” Torr during deposition. A quartz crystal microbalance was used to monitor the evaporation rate. All the photoemission spectra were recorded using unmonochromatized X-rays from an Al anode (hv = 1486.6 eV).

N(E)

N’(E)

3. Growth of Sm on Cu(ll0) The intensity from the XPS and XAES data was obtained by differentiating the N(E) spectra using a Savitzky-Golay convolution array [ll]. and taking the peak to peak value. This method is equivalent to taking the area of the peaks [12], but faster and simpler. The Sm(3d) region is displayed in fig. 1, where both N(E) and N ‘( E ) are shown. This region consists of Cu(2s) at 386 eV and two multiplets belonging to divalent and trivalent Sm(3d) with a spin-orbit splitting of 27 eV. The energy difference between the divalent and trivalent multiplet is 8 eV, with the divalent at higher kinetic energy. Fig. 2 shows the development of the XPS and XAES signals as the growth progresses at room temperature. The kinetic energy of a Cu(2p) electron excited by AlKcv X-rays is 554 eV, which means that the Cu(2p) signal is not very surface sensitive. This explains why the Cu(2p) signal is seen only to be weakly attenuated for small Sm coverages. But as the amount of Sm on the surface increases, the intensity drops off without sharp breaks to one half of the initial intensity. Both the Cu Auger signal with a kinetic energy of 60 eV and the Sm Auger signal with a kinetic energy of 100 eV, have the desired surface sensitivity and this is clearly seen in the fast decay

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Fig. 1. N(E) and N’(E) spectra of the Sm(3d) region for three different Sm depositions on Cu(ll0) at room temperature: (a) clean Cu. (b) around the 1 ML point, (c) thick film.

of the Cu Auger intensity and the growth of the Sm Auger signal. Unfortunately, there is no sharp break in the Cu or Sm Auger signal indicating the 1 monolayer point, which means that the monolayer coverage can not be found from the Auger curves alone. The evolution of first a weak c(6 x 2) and thereafter a c(2 X 2) LEED pattern helps to define the monolayer coverage. The only possible coverages corresponding to the c(2 x 2) LEED pattern is one or one half substrate monolayer. Because the atomic radius of Sm (2.05 A for the divalent species extrapolated from ref. [13]) is considerable bigger than that of copper. the only reasonable

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Fig. 2. The intensity of the different XPS and XAES signals as a function of Sm deposition on Cu(ll0) at room temperature. See text for details.

solution is one half substrate monolayer. This defines 1 ML of Sm to have a density of 5.5 X lOi atoms cm -*. The Sm coverage of the c(6 x 2) is then 5 ML. The c(2 X 2) LEED pattern disappears around 1.2 ML whereafter only weak substrate spots are seen. The divalent and trivalent Sm(3d) signals further support this definition of 1 ML. When the coverage is smaller than 1 ML, only the divalent Sm signal is increasing, showing that isolated Sm atoms at a copper surface are divalent in agreement with the findings in ref. [7]. The small amount of trivalent Sm seen already at the lowest coverages is most likely due to Sm adsorbed at defect sites on the surface. At the monolayer point the trivalent Sm signal starts to rise, because it is not possible to compress the layer of relatively big divalent Sm any more, and a multilayer forms. The reason that the c(2 x 2) LEED pattern does not disappear at once might

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be due to incorporation of Sm into the Cu substrate under the ordered structure. The information depth for the Cu Auger signal at 60 eV is below 2 ML, this means that the ratio of the signals at 10 and 0 ML should be less than that e -5 = 0 .007. This is not the case meaning simple layer-by-layer growth can be excluded. We see two possible explanations for these results. One is Stranski-Krastanov growth [14], the other is interdiffusion of Cu into the Sm film. We favour the latter picture for three reasons, the first being the high mobility of the Sm-Cu system shown by the appearance of ordered structures at room temperature, and the diffusion of Cu seen in thick Sm films [lo]. Our experiment measures the equilibrium situation between Sm and Cu, because small amounts of Sm are deposited between the measurements, and the time to take one measurement in fig. 2 is long compared to the characteristic time involved in the diffusion of Cu into thin layers of Sm. Secondly, as pointed out in ref. [7] Sm-Cu alloys are trivalent. Thirdly, if the only thing we observed was a Sm layer of growing thickness, the growth mode would have to be Stranslc-Krastanov as the substrate signal is not disappearing. If this is the case, the surface area should increase, and thus the divalent Sm signal should go up, as Sm is divalent at the surface. We did not observe this increase after the first ML, meaning that simple Stranski-Krastanov growth of Sm on a rigid Cu(ll0) surface can be discarded. No chemical shift of the Sm(3d) is observed at any point of the growth, but this does not exclude a chemical reaction of Sm and Cu. A detailed discussion and comparison between the growth at the three surfaces is beyond the scope of this paper.

4. LEED results When small amounts of Sm are deposited on the Cu(ll0) surface at room temperature, nothing but increased background is observed in the LEED pattern. Then gradually, as already mentioned, a weak c(6 X 2) LEED pattern develops, which transforms to a c(2 x 2) pattern, when the coverage is increased. These LEED patterns and specu-

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lative real space structures are shown in fig. 3. All the atoms in the c(2 x 2) have to be at equivalent positions on the surface. In fig. 3 the Sm is shown in four-fold hollow sites, it might as well be bridge, on top position or anywhere else. In c(6 x 2) we have 2 Sm atoms in the unit cell which may occupy different sites. One must bear in mind that the Cu(ll0) surface is not at all rigid, but may reconstruct involving large lateral displacements of the atoms e.g. c(6 X 2)-O 1151. The ~(6 x 2) pattern could also be made by annealing a thick film of Sm, and afterwards sputter clean the sample and finally anneal at 400 o C. The ~(6 X 2) structure, made this way, was totally divalent.

~(6x2) on Cu(ll0)

~(8x2) on Cufl 00)

Fig. 3. LEED patterns and speculative real space structures. In the LEED patterns in the left row filled circles represent substrate spots and open circles superstructure spots. In the real space structures on the right the open circles are Cu atoms and shaded circles Sm atoms. The unit cell is also shown.

on copper single uystafs

This suggests that the small amount of trivalent Sm present in the c(6 X 2) made at room temperature is situated at defect sites or in the bulk below the divalent structure. When Sm is dosed at a Cu(ll0) surface at 400°C only a sharp ~(6 x 2) pattern is observed and this persists up to 3 ML of Sm, where the major part is diffused into the bulk of the crystal. On Cu(100) no ordered structures were seen at room temperature. in contrast to refs. [6,7]. The (6 x fi)R27 o reported in ref. 161 was not observed at all. At 400 o C we observed a ~(2 x 8) pattern, this pattern slowly changed to a distorted hexagonal pattern. The slightly distorted hexagonal pattern is reported in refs. [6,7] at room temperature, we have only observed it at 400 ’ C. This pattern has also been seen for Yb layers on Ni [2] where it was interpreted as compound formation. On Cu(lll) a (fixfi)R30” was observed at room temperature in agreement with ref. [S]. The (i -:) LEED structure (reported in ref. [S]) was not observed. This structure should presumably display homogeneous mixed valency. At 400 o C we have seen LEED patterns consisting of (2 x 2) spots, (2 x 2)R30” spots, and some extra weak spots. The (2 X 2) + (2 X 2)R30” pattern resembles the distorted hexagonal pattern on Cu(700). The similarity of the LEED patterns imply that the same Sm-Cu compound may be present at the two smooth surfaces of Cu. The corrugation of the (110) surface, that even might be reconstructed, is simply too big for the Sm-Cu compound to form.

5. Summary We have argued that intermixing between Sm and Cu must occur already at room temperature. Ordered structures at all three surfaces were seen, some at room temperature, others at elevated temperatures. The high mobility of the system is shown by the appearance of LEED patterns at room temperature. The distorted hexagonal pattern appearing on the smooth (100) and (111) surfaces at 400°C is interpreted as a Sm-Cu compound on top of the copper crystal.

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and R.M. Lambert, Surf. Sci. 124 (1989) 407. [91 D.M. Wieliczka and C.G. Olsson, J. Vat. Sci. Technol. A 8 (2) (1990) 891. B. Jorgensen and J. Onsgaard, in preWI M. Christiansen, paration. 1111A. Savitzky and M.J.E. Golay, Anal. Chem. 36 (1964) 1627. WI B. Jorgensen, PhD Thesis, Odense Universitet, Denmark. 1988. u31 B.J. Beaudry and K.A. Gschneider, Jr, in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 1. Eds. K.A. Gschneider, Jr. and L. Eyring (North-Holland, Amsterdam, 1978) p. 173. Rep. v41 J.A. Venables, G.D.T. Spiller and M. Hanbiicken, Prog. Phys. 47 (1984) 399. and I. Stensgaard. Surf. Sci. 133 (1983) v51 R. Feidenhans’l 453.