- Email: [email protected]
Pd(111) interface
Surface
448
FORMATION
OF ALLOYS
E. BEAUREPAIRE, IPCMS,
AT THE Yb/Pd(lll)
Science 211/212 (1989) 448-455 North-Holland, Amsterdam
INTERFACE
B. CARRIERE
4, rue Biaise Pascal, 67070 Strasbourg,
France
P. LEGARE LCCS, 4, rue Blaise Pascal, 67070 Strasbourg, France G. KRILL, Lahoratoire
C. BROUDER
de Physique des Solides,
D. CHANDESRIS LURE, Received
B&ment
lJniuersit6 de Nancy, 54506 Vandoeuure,
France
and J. LECANTE
209d, 91405 Orsay, France
2 July 1988; accepted
for publication
24 October
1988
Experimental data (UV and X-ray photoemission, Yb L,,, edge) on the interface Yb/Pd( 111) for nominal coverages in the range O-20 nm are discussed. These results suggest the formation of a compound in the first stage of the growth (coverages up to 1 nm). For higher coverages, a progressive dilution of Pd in an Yb matrix is observed near the surface.
1. Introduction In addition to their specific interest (related e.g. to magnetic or catalytic properties) rare earth (RE)/metal interfaces may be viewed as model systems. This is due in particular to the good sensitivity of RE atoms to photon and electron spectroscopies. Moreover, a great advantage arises from the valence instability behaviour in systems containing Ce, Sm, Eu, Tm or Yb atoms where two nearly degenerate 4f configurations of the RE may exist. For instance a large amount of works have shown that Sm atoms on the surface of the (trivalent = 4f5 configuration) metal are divalent (i.e. 4f 6 configuration)
PI.
As it will be discussed below, such a valence change is easily detected by valence band or core level spectroscopies due to the localization of the 4f shell, and it can be in a certain way used as a marker of surface or interface atoms. Recent studies of Sm overlayers in the submonolayer range on different metals (Al, Cu, Pd) tried to correlate the possible long range ordering of the surface 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
E. Beaurepaire et al. / Formation of alloys at the Yb/ Pb(l I I) interface
449
(as detected by LEED experiments) with the electronic structure of the RE atoms [2]. However, conflicting interpretations have been proposed, since the number of cristallographic sites for the RE and possible formation of an alloy are questionable [3,4]. In order to address such problems we chose to study the Yb/Pd(lll) interface at room temperature (RT). One major interest of this system is that a previous work has shown the sensitivity of Yb valence to modification of its local surroundings: Yb is found to be trivalent in Pd-rich alloys and compounds, and divalent in the Yb-rich case, with a critical composition around the equiatomic situation [5]. We shall briefly show in the following how Yb valence, as measured by UV photoemission and surface L,,, absorption can be used to propose a description of the interface formation at RT. This description will be then precised by the interpretation of core level XPS and XAES data. The important point is to notice the strong interdiffusion of Pd and Yb that leads to the creation of alloys at the surface and interface.
2. Experimental UV photoemission (UPS) and surface Lit, absorption experiments (total yield measurement of the absorption coefficient) have been performed at LURE (Orsay). Details are given elsewhere [6]. XPS and XAES data have been obtained from an unmonochromatised AlKa X-ray source with an energy resolution estimated at 0.85 eV from the width of the Fermi edges and core levels. The Pd(ll1) substrate is prepared by successive Ar+ bombardment and heating up to 400° C until only peaks attributable to Pd are present in the XPS or Auger spectra. We checked that this procedure allows the observation of the sharp LEED spots expected for an unreconstructed Pd(ll1) surface. Yb was evaporated by heating from a W basket on a clean substrate maintained at RT. The nominal coverage is measured using a quartz microbalance with a reference to fee Yb density with relative (respectively absolute) accuracy estimated to 10% (respectively 30%). During evaporation, the pressure rose typically up to 2 X 10e9 Torr. The sample was kept in an ultra-high vacuum below 2 X lo-” Torr after Yb evaporation. Surface contamination (mainly 0) was periodically checked by recording the corresponding Auger KLL spectra.
3. UV photoemission
and YbL,,,
absorption
A detailed description and discussion of these results can be found in ref. [6]. We only mention here the main conclusions that have been drawn from the analysis of these data.
E. 3e~ur~p~ire et ai. /’ F~r~ution of aliqts at the Yb/ Pd(I I I) interlace
450
a
UPS
h” =
160
15
b
EV
-“-----
1
6910
10
Binding
energy
Fig. 1. UV photoemission
(eV)
8920 Photon
8930 energy
(a) and YbL,,, edges (b) for different Yb coverages temperature. Solid lines are fits to the data.
8940
8950
(eV)
on Pd( 111) at room
Some typical UPS spectra (hv = 160 ev) are reported in fig. la. Due to the “Cooper minimum” of Pd4d states in this photon energy range, the spectra are dominated by the Yb4f lines. The comparison with known experimental and theoretical works allows the following interpretation of the peaks: peaks labelled A (binding energies 5-12 eV) are well reproduced taking into account the theoretical multiplet structure of Yb3.+ ions (4f” -+ 4f” + ee); peaks labelled B, and $ are two shifted doublets attributed to 4f photoemission from Yb2’ ions in the ground state (the 4f l3 final state is splitted into 4f,,,, and 4f,,, by the spin-orbit coupling of 1.2 ev). The photon energy dependence of the relative intensity of these peaks shows that B, peaks are related to Yb atoms closer to the surface; these Yb atoms are therefore obviously divalent even at the lowest studied coverage (0.4 nm), as it is often found in Yb systems. We emphasize the following points: (i) In the coverage range 6’ = 0.4-1.3 nm, the spectra are quite similar and look like those obtained by Don&e et al. [7] for the surface of the intermetallic Yb,Pd, (where Yb is trivalent in the bulk). This strongly suggests the formation of an alloy in this stage of the interface growth, with trivalent Yb
E. Beaurepaire et al. / Formation of alloys at the Yb / Pb(l11) interface
451
(bonded with Pd) in the interface, and divalent Yb just as the surface (due to the smaller number of bonds for these atoms). (ii) For higher coverages the Yb3+ signal decreases, and a bulk divalent Yb component appears around 0 = 2 nm, indicating that the Yb3+-Pd alloy gets covered by Pd poorer alloys, so that the critical concentration for Yb valence change is overpassed. This evolution is qualitatively confirmed by the interpretation of the Yb L,,, edges presented in fig. lb, where peaks attributed to Yb2+ (respectively Yb3’) are observed around 8935 eV (respectively 8942 eV). Taking into account the large sampling depth of this technique (> 10 nm), the variation of the Yb L,,, valence with coverage can be analysed in agreement with the previous description in a stage of growth of an Yb3+-Pd alloy (for 8 < 1.8 nm), which was then covered by Yb in a divalent state when 13 increases. As discussed in ref. [6a], the difference in valence variation as measured by L,,, and UPS can be understood assuming an island growth of this alloy. From these data, the evolution of the interface Yb/Pd(lll) at RT is driven by a strong interdiffusion process that has also been checked by performing experiments at lower temperatures [6].
4. XFY3 and XAES results The Pd 3d,,, line recorded after various Yb coverages is reported on fig. 2a. For low coverages (0 < 19< 1 nm), a broadening of the line is observed. In an intermediate coverage range, a second peak at higher binding energy is clearly resolved, with only a shoulder at the position of the Pd metal line. This shoulder nearly disappears when B > 3 nm. In order to get further information on the relative intensity and position of these lines, the data have been fitted with Doniach-Sunjic lineshapes (full lines in fig. 2a). The spectrum of pure Pd can be well reproduced with a single asymmetric line (asymmetry index a: = 0.21). The Pd 3d 5,2 line corresponding to Yb overlayers on Pd has been least-squares fitted using two Doniach-Sunjic contributions, the low binding energy one (dashed line in fig. 2a) with the same asymmetry and position as in pure Pd, whereas these parameters are allowed to vary for the higher binding energy contribution (dotted line in fig. 2a). We found that this second contribution is characterized by a low asymmetry index (a = 0.06), and a chemical shift with respect to pure Pd ranging between AE = 0.75 &-0.1 eV for 0 < 1 nm and AE = 1.1 eV for 6 > 3 nm with a continuous variation at intermediate coverage. We notice that this chemical shift is quite large and typical of RE intermetallics (for instance AE = 0.55 and 1.3 eV for LaPd, and La,Pd, respectively [8]). Moreover, the low value of the asymmetry index of this line is not surprising since it is an indication of a weak local density of states at the Fermi level, in agreement with the electronic structure of RE-Pd
E. Beaurep~~re et al. / Formation of alloys at the Yb /
452
Pdfl I I) interface
Yb / Pd MVV
-1154
-1150
4-J __----_-_.L -1275 -1315
Kinetic
energy
’
-328
-338
/
feV)
Auger (c) spectra for different coverages on Fig. 2. Pd3d XPS (a), Yb4d XPS (b) and PdMW Pdflll), recorded with AIK anode (hv = 1487 eV). The full lines in a are fits to the data evidencing the splitting into Fd substrate (dashed lines) and alloy {dotted lines) contributions.
_..--
+ ‘\:* l
50
20
i
1
2
Fig. 3. Relative intensity of the various contributions to XPS spectra as a function of Yb coverage. to (0) Total Pd3d,,, line, (4) substrate Pd contribution to the 3d,,a line, (v) alloys contribution to Yb4d line height at B = 20 nm). Dashed line is a the 3d,,z line, (~1) Yb4d line (normalized simulation of the homogeneous deposition of an equi-volumic Yb-Pd alloy, dotted line is a simulation taking into account an island growth (see text). Note the change in scale at 3 nm. In inset: substrate contribution in a log plot.
E. Beaurepaire et al. / Formation of alloys at the Yb / Pb(l11) interface
453
alloys ]7,8]. We therefore indentify the first ~ont~bution in our fits with the Pd substrate signal and the second to those Pd atoms in contact with Yb and forming an alloy. The fit of Pd3d core level spectra also allows to plot the reduced intensity (referenced to the clean Pd(ll1) substrate) of the substrate and alloyed Pd cont~butions versus Yb coverage (fig. 3). Yb 4d core level spectra are represented in fig. 2b. On this figure, the Yb,O, spectrum (obtained after exposure of an Yb film to 100 L 0,) is given as a signature of Yb in the 4f l3 ground state. It corresponds to known data for ~3+/Ni(l10) thin layers [9] and is due to the strong coupling in the final state between the open 4d and 4f shells. A simple spin-orbit doublet is observed for nominal Yb coverages above 3 nm, suggesting that Yb is mainly divalent in this case. For coverages below 3 nm, the 4d lineshape is intermediate between these two extreme situations, in qualitative agreement with the interpretation of UPS and L,,, data. Due to the strong overlap of Yb2+ and Yb3+ contributions we did not analyse further these spectra and simply report in fig. 3 the relative intensity of the 4d line (normalized to its height for the thickest Yb overlayer). Additional information may be obtained from PdMVV Auger spectra of fig. 2c. Two important points are clearly noted concerning the intensity and lineshape of these peaks. First of all, it appears that this Auger signal is not negligible for Yb overlayers as thick as 10 nm (12% of the clean Pd(ll1) Auger intensity). Due to the good sensitivity to the surface of this technique (about 1.5 nm), we conclude that Pd should be present up to the top layers of the interface (more quantitative estimates will be performed in the following section). We then observe a progressive narrowing and shift of the spectra. Since it is known that the final state of the Auger process in Pd mainly consists of two holes in the 4d bands, we can relate these features to the narrowing and filling of Pd4d states in the alloys that form near the surface. This conclusion is in agreement with the low value of Pd3d,,, asymmetry index discussed above, and with valence band UV photoemission reported in ref. [6a]. In fact, the spectra observed for coverages above 4 nm look quite similar to the one reported for an Mg,,Pd,, alloy [lo], confirming the progressive dilution of Pd in Yb near the surface as the Yb coverage increases.
5. Discussion We shall compare in this section the information that can be obtained from XPS data described above with the evolution of the interface that is proposed in section 3. It is clear from the relative intensities of the various contributions to XPS spectra (see fig. 3) that different regimes must be distinguished. In the following discussion, we shall separate coverages below and above 1 nm. We
454
E. Bemrepaire
et al. / Formrrtion of ulloys at the Yh / P&l I I) interface
first observe a similar increase of the Yb4d line and the alloyed Pd contribution to the 3d line. Since the data are normalized, it suggests the growth of an equi-volumic alloy and therefore supports the hypothesis inferred from UPS measurements that a compound like Yb,Pd, is created at the interface (taking into account the difference in atomic sizes). During this stage, the substrate and alloyed Pd intensities roughly follow the respective laws Z, = ee*“’ and I, = 0.5(1 - eezeix) in agreement with this model (X is an effective mean free path = 2 nm in our case), as demonstrated by the dashed line in fig. 3. However, this experiment cannot conclude definitively about the growth mode of the alloy since the development of three dimensional islands (as suggested in ref. [6a]) is also in correct agreement with our data in this coverage range (as an example, the dotted lines in fig. 3 are calculated for 1.8 nm thick islands appearing after 0.3 nm Yb deposition). At higher Yb coverages, there is a maximum, then a slow decrease of the alloyed Pd contribution. This is related with a change of slope of the bulk contribution in a log plot (inset of fig. 3). It can be interpreted by a gradual decrease of the Pd concentration in the surface layers. so that above an alloy containing 8 = 1.8 nm (as deduced from UPS and L,,, experiments) divalent Yb and dilute Pd is growing. The persistence of the PdMW Auger signal above coverages as high as 13 nm shows that the interfacial region thickness may be quite large until pure Yb is growing. Finally, we note that the XPS Pd substrate signal persists at 0 = 6.3 nm, though it should be negligible from an exponential decay. A tentative explanation would be the development of large Yb-rich islands in the final interface stage, so that the substrate would be observable through the compound created first at the interface. An other explanation would be the appearance of small Pd cluster at the surface, such a behaviour agrees with the large Pd mobility (as evidenced by the existence of diffusion process) and the large surface energy of Pd.
6. Conclusion Experimental data on the Yb/Pd(lll) interface have been presented and discussed. These results show that the interdiffusion process leads to the formation of alloys near the surface, even for thick Yb overlayers (> 10 nm). This is for instance demonstrated by: (i) the occurrence of a shifted Pd3d XPS line still observable at 20 nm Yb deposition, (ii) strong modifications in the Pd MW Auger lineshape. We interpret our results by the growth of an alloy Yb”Pd, (x = 1.33) in the early stage of the deposition (nominal coverage below 1 nm). The Pd
E. Beaurepaire et al. / Formation of alloys at the Yb/ Pb(l I I) interface
455
concentration near the surface of the alloy slowly decreases at higher coverage, so that divalent Yb is deposited for 8 > 1.8 nm. Uncertainties are remaining about the growth mode of these alloys and an extensive Auger work is now in progress in order to solve this problem.
References [l] B. Johansson, Phys. Rev. B 19 (1979) 6615. [2] A. Faldt, D.K. Kristenssen and H.P. Myers, Phys. Rev. B 37 (1988) 2682. [3] A. Nilsson, B. Eriksson, N. Martensson, J.N. Andersen and J. Onsgaard, Phys. Rev. B 36 (1987) 9308. [4] J.N. Andersen, I. Chorkendorff, J. Onsgaard, J. Ghijsen, R.L. Johnson and F. Grey, Phys. Rev. B 37 (1988) 4809. [5] D. Malterre et al., to be published. [6] (a) E. Beaurepaire, C. Brouder, B. Carriere, D. Chandesris, G. Krill, L. Lecante and P. Legare, Phys. Rev. B, submitted; (b) E. Beaurepaire, C. Brouder, B. Carriere, D. Chandresris, G. Krill, J. Lecante and P. LegarC, J. Phys. (Paris) C 9 (1988) 939. [7] M. Don&e, C. Laubschat, E.V. Sampathkumaran, M. Prietsch, T. Mandel, G. Kaindl and H.U. Middelmann, Phys. Rev. B 32 (1985) 8002. [8] F.U. Hillebrecht, J.C. Fuggle, P.A. Bennet, Z. Zolnierk and Ch. Freiburg, Phys. Rev. B 27 (1983) 2179, and references therein. [9] 1. Chorkendorff, J. Onsgaard, J. Schmidt-May and R. Nyholm, Surface Sci. 160 (1985) 587. [lo] P. Weightman and P.T. Andrew& J. Phys. C (Solid State Phys.) 13 (1980) L815.
448
FORMATION
OF ALLOYS
E. BEAUREPAIRE, IPCMS,
AT THE Yb/Pd(lll)
Science 211/212 (1989) 448-455 North-Holland, Amsterdam
INTERFACE
B. CARRIERE
4, rue Biaise Pascal, 67070 Strasbourg,
France
P. LEGARE LCCS, 4, rue Blaise Pascal, 67070 Strasbourg, France G. KRILL, Lahoratoire
C. BROUDER
de Physique des Solides,
D. CHANDESRIS LURE, Received
B&ment
lJniuersit6 de Nancy, 54506 Vandoeuure,
France
and J. LECANTE
209d, 91405 Orsay, France
2 July 1988; accepted
for publication
24 October
1988
Experimental data (UV and X-ray photoemission, Yb L,,, edge) on the interface Yb/Pd( 111) for nominal coverages in the range O-20 nm are discussed. These results suggest the formation of a compound in the first stage of the growth (coverages up to 1 nm). For higher coverages, a progressive dilution of Pd in an Yb matrix is observed near the surface.
1. Introduction In addition to their specific interest (related e.g. to magnetic or catalytic properties) rare earth (RE)/metal interfaces may be viewed as model systems. This is due in particular to the good sensitivity of RE atoms to photon and electron spectroscopies. Moreover, a great advantage arises from the valence instability behaviour in systems containing Ce, Sm, Eu, Tm or Yb atoms where two nearly degenerate 4f configurations of the RE may exist. For instance a large amount of works have shown that Sm atoms on the surface of the (trivalent = 4f5 configuration) metal are divalent (i.e. 4f 6 configuration)
PI.
As it will be discussed below, such a valence change is easily detected by valence band or core level spectroscopies due to the localization of the 4f shell, and it can be in a certain way used as a marker of surface or interface atoms. Recent studies of Sm overlayers in the submonolayer range on different metals (Al, Cu, Pd) tried to correlate the possible long range ordering of the surface 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
E. Beaurepaire et al. / Formation of alloys at the Yb/ Pb(l I I) interface
449
(as detected by LEED experiments) with the electronic structure of the RE atoms [2]. However, conflicting interpretations have been proposed, since the number of cristallographic sites for the RE and possible formation of an alloy are questionable [3,4]. In order to address such problems we chose to study the Yb/Pd(lll) interface at room temperature (RT). One major interest of this system is that a previous work has shown the sensitivity of Yb valence to modification of its local surroundings: Yb is found to be trivalent in Pd-rich alloys and compounds, and divalent in the Yb-rich case, with a critical composition around the equiatomic situation [5]. We shall briefly show in the following how Yb valence, as measured by UV photoemission and surface L,,, absorption can be used to propose a description of the interface formation at RT. This description will be then precised by the interpretation of core level XPS and XAES data. The important point is to notice the strong interdiffusion of Pd and Yb that leads to the creation of alloys at the surface and interface.
2. Experimental UV photoemission (UPS) and surface Lit, absorption experiments (total yield measurement of the absorption coefficient) have been performed at LURE (Orsay). Details are given elsewhere [6]. XPS and XAES data have been obtained from an unmonochromatised AlKa X-ray source with an energy resolution estimated at 0.85 eV from the width of the Fermi edges and core levels. The Pd(ll1) substrate is prepared by successive Ar+ bombardment and heating up to 400° C until only peaks attributable to Pd are present in the XPS or Auger spectra. We checked that this procedure allows the observation of the sharp LEED spots expected for an unreconstructed Pd(ll1) surface. Yb was evaporated by heating from a W basket on a clean substrate maintained at RT. The nominal coverage is measured using a quartz microbalance with a reference to fee Yb density with relative (respectively absolute) accuracy estimated to 10% (respectively 30%). During evaporation, the pressure rose typically up to 2 X 10e9 Torr. The sample was kept in an ultra-high vacuum below 2 X lo-” Torr after Yb evaporation. Surface contamination (mainly 0) was periodically checked by recording the corresponding Auger KLL spectra.
3. UV photoemission
and YbL,,,
absorption
A detailed description and discussion of these results can be found in ref. [6]. We only mention here the main conclusions that have been drawn from the analysis of these data.
E. 3e~ur~p~ire et ai. /’ F~r~ution of aliqts at the Yb/ Pd(I I I) interlace
450
a
UPS
h” =
160
15
b
EV
-“-----
1
6910
10
Binding
energy
Fig. 1. UV photoemission
(eV)
8920 Photon
8930 energy
(a) and YbL,,, edges (b) for different Yb coverages temperature. Solid lines are fits to the data.
8940
8950
(eV)
on Pd( 111) at room
Some typical UPS spectra (hv = 160 ev) are reported in fig. la. Due to the “Cooper minimum” of Pd4d states in this photon energy range, the spectra are dominated by the Yb4f lines. The comparison with known experimental and theoretical works allows the following interpretation of the peaks: peaks labelled A (binding energies 5-12 eV) are well reproduced taking into account the theoretical multiplet structure of Yb3.+ ions (4f” -+ 4f” + ee); peaks labelled B, and $ are two shifted doublets attributed to 4f photoemission from Yb2’ ions in the ground state (the 4f l3 final state is splitted into 4f,,,, and 4f,,, by the spin-orbit coupling of 1.2 ev). The photon energy dependence of the relative intensity of these peaks shows that B, peaks are related to Yb atoms closer to the surface; these Yb atoms are therefore obviously divalent even at the lowest studied coverage (0.4 nm), as it is often found in Yb systems. We emphasize the following points: (i) In the coverage range 6’ = 0.4-1.3 nm, the spectra are quite similar and look like those obtained by Don&e et al. [7] for the surface of the intermetallic Yb,Pd, (where Yb is trivalent in the bulk). This strongly suggests the formation of an alloy in this stage of the interface growth, with trivalent Yb
E. Beaurepaire et al. / Formation of alloys at the Yb / Pb(l11) interface
451
(bonded with Pd) in the interface, and divalent Yb just as the surface (due to the smaller number of bonds for these atoms). (ii) For higher coverages the Yb3+ signal decreases, and a bulk divalent Yb component appears around 0 = 2 nm, indicating that the Yb3+-Pd alloy gets covered by Pd poorer alloys, so that the critical concentration for Yb valence change is overpassed. This evolution is qualitatively confirmed by the interpretation of the Yb L,,, edges presented in fig. lb, where peaks attributed to Yb2+ (respectively Yb3’) are observed around 8935 eV (respectively 8942 eV). Taking into account the large sampling depth of this technique (> 10 nm), the variation of the Yb L,,, valence with coverage can be analysed in agreement with the previous description in a stage of growth of an Yb3+-Pd alloy (for 8 < 1.8 nm), which was then covered by Yb in a divalent state when 13 increases. As discussed in ref. [6a], the difference in valence variation as measured by L,,, and UPS can be understood assuming an island growth of this alloy. From these data, the evolution of the interface Yb/Pd(lll) at RT is driven by a strong interdiffusion process that has also been checked by performing experiments at lower temperatures [6].
4. XFY3 and XAES results The Pd 3d,,, line recorded after various Yb coverages is reported on fig. 2a. For low coverages (0 < 19< 1 nm), a broadening of the line is observed. In an intermediate coverage range, a second peak at higher binding energy is clearly resolved, with only a shoulder at the position of the Pd metal line. This shoulder nearly disappears when B > 3 nm. In order to get further information on the relative intensity and position of these lines, the data have been fitted with Doniach-Sunjic lineshapes (full lines in fig. 2a). The spectrum of pure Pd can be well reproduced with a single asymmetric line (asymmetry index a: = 0.21). The Pd 3d 5,2 line corresponding to Yb overlayers on Pd has been least-squares fitted using two Doniach-Sunjic contributions, the low binding energy one (dashed line in fig. 2a) with the same asymmetry and position as in pure Pd, whereas these parameters are allowed to vary for the higher binding energy contribution (dotted line in fig. 2a). We found that this second contribution is characterized by a low asymmetry index (a = 0.06), and a chemical shift with respect to pure Pd ranging between AE = 0.75 &-0.1 eV for 0 < 1 nm and AE = 1.1 eV for 6 > 3 nm with a continuous variation at intermediate coverage. We notice that this chemical shift is quite large and typical of RE intermetallics (for instance AE = 0.55 and 1.3 eV for LaPd, and La,Pd, respectively [8]). Moreover, the low value of the asymmetry index of this line is not surprising since it is an indication of a weak local density of states at the Fermi level, in agreement with the electronic structure of RE-Pd
E. Beaurep~~re et al. / Formation of alloys at the Yb /
452
Pdfl I I) interface
Yb / Pd MVV
-1154
-1150
4-J __----_-_.L -1275 -1315
Kinetic
energy
’
-328
-338
/
feV)
Auger (c) spectra for different coverages on Fig. 2. Pd3d XPS (a), Yb4d XPS (b) and PdMW Pdflll), recorded with AIK anode (hv = 1487 eV). The full lines in a are fits to the data evidencing the splitting into Fd substrate (dashed lines) and alloy {dotted lines) contributions.
_..--
+ ‘\:* l
50
20
i
1
2
Fig. 3. Relative intensity of the various contributions to XPS spectra as a function of Yb coverage. to (0) Total Pd3d,,, line, (4) substrate Pd contribution to the 3d,,a line, (v) alloys contribution to Yb4d line height at B = 20 nm). Dashed line is a the 3d,,z line, (~1) Yb4d line (normalized simulation of the homogeneous deposition of an equi-volumic Yb-Pd alloy, dotted line is a simulation taking into account an island growth (see text). Note the change in scale at 3 nm. In inset: substrate contribution in a log plot.
E. Beaurepaire et al. / Formation of alloys at the Yb / Pb(l11) interface
453
alloys ]7,8]. We therefore indentify the first ~ont~bution in our fits with the Pd substrate signal and the second to those Pd atoms in contact with Yb and forming an alloy. The fit of Pd3d core level spectra also allows to plot the reduced intensity (referenced to the clean Pd(ll1) substrate) of the substrate and alloyed Pd cont~butions versus Yb coverage (fig. 3). Yb 4d core level spectra are represented in fig. 2b. On this figure, the Yb,O, spectrum (obtained after exposure of an Yb film to 100 L 0,) is given as a signature of Yb in the 4f l3 ground state. It corresponds to known data for ~3+/Ni(l10) thin layers [9] and is due to the strong coupling in the final state between the open 4d and 4f shells. A simple spin-orbit doublet is observed for nominal Yb coverages above 3 nm, suggesting that Yb is mainly divalent in this case. For coverages below 3 nm, the 4d lineshape is intermediate between these two extreme situations, in qualitative agreement with the interpretation of UPS and L,,, data. Due to the strong overlap of Yb2+ and Yb3+ contributions we did not analyse further these spectra and simply report in fig. 3 the relative intensity of the 4d line (normalized to its height for the thickest Yb overlayer). Additional information may be obtained from PdMVV Auger spectra of fig. 2c. Two important points are clearly noted concerning the intensity and lineshape of these peaks. First of all, it appears that this Auger signal is not negligible for Yb overlayers as thick as 10 nm (12% of the clean Pd(ll1) Auger intensity). Due to the good sensitivity to the surface of this technique (about 1.5 nm), we conclude that Pd should be present up to the top layers of the interface (more quantitative estimates will be performed in the following section). We then observe a progressive narrowing and shift of the spectra. Since it is known that the final state of the Auger process in Pd mainly consists of two holes in the 4d bands, we can relate these features to the narrowing and filling of Pd4d states in the alloys that form near the surface. This conclusion is in agreement with the low value of Pd3d,,, asymmetry index discussed above, and with valence band UV photoemission reported in ref. [6a]. In fact, the spectra observed for coverages above 4 nm look quite similar to the one reported for an Mg,,Pd,, alloy [lo], confirming the progressive dilution of Pd in Yb near the surface as the Yb coverage increases.
5. Discussion We shall compare in this section the information that can be obtained from XPS data described above with the evolution of the interface that is proposed in section 3. It is clear from the relative intensities of the various contributions to XPS spectra (see fig. 3) that different regimes must be distinguished. In the following discussion, we shall separate coverages below and above 1 nm. We
454
E. Bemrepaire
et al. / Formrrtion of ulloys at the Yh / P&l I I) interface
first observe a similar increase of the Yb4d line and the alloyed Pd contribution to the 3d line. Since the data are normalized, it suggests the growth of an equi-volumic alloy and therefore supports the hypothesis inferred from UPS measurements that a compound like Yb,Pd, is created at the interface (taking into account the difference in atomic sizes). During this stage, the substrate and alloyed Pd intensities roughly follow the respective laws Z, = ee*“’ and I, = 0.5(1 - eezeix) in agreement with this model (X is an effective mean free path = 2 nm in our case), as demonstrated by the dashed line in fig. 3. However, this experiment cannot conclude definitively about the growth mode of the alloy since the development of three dimensional islands (as suggested in ref. [6a]) is also in correct agreement with our data in this coverage range (as an example, the dotted lines in fig. 3 are calculated for 1.8 nm thick islands appearing after 0.3 nm Yb deposition). At higher Yb coverages, there is a maximum, then a slow decrease of the alloyed Pd contribution. This is related with a change of slope of the bulk contribution in a log plot (inset of fig. 3). It can be interpreted by a gradual decrease of the Pd concentration in the surface layers. so that above an alloy containing 8 = 1.8 nm (as deduced from UPS and L,,, experiments) divalent Yb and dilute Pd is growing. The persistence of the PdMW Auger signal above coverages as high as 13 nm shows that the interfacial region thickness may be quite large until pure Yb is growing. Finally, we note that the XPS Pd substrate signal persists at 0 = 6.3 nm, though it should be negligible from an exponential decay. A tentative explanation would be the development of large Yb-rich islands in the final interface stage, so that the substrate would be observable through the compound created first at the interface. An other explanation would be the appearance of small Pd cluster at the surface, such a behaviour agrees with the large Pd mobility (as evidenced by the existence of diffusion process) and the large surface energy of Pd.
6. Conclusion Experimental data on the Yb/Pd(lll) interface have been presented and discussed. These results show that the interdiffusion process leads to the formation of alloys near the surface, even for thick Yb overlayers (> 10 nm). This is for instance demonstrated by: (i) the occurrence of a shifted Pd3d XPS line still observable at 20 nm Yb deposition, (ii) strong modifications in the Pd MW Auger lineshape. We interpret our results by the growth of an alloy Yb”Pd, (x = 1.33) in the early stage of the deposition (nominal coverage below 1 nm). The Pd
E. Beaurepaire et al. / Formation of alloys at the Yb/ Pb(l I I) interface
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concentration near the surface of the alloy slowly decreases at higher coverage, so that divalent Yb is deposited for 8 > 1.8 nm. Uncertainties are remaining about the growth mode of these alloys and an extensive Auger work is now in progress in order to solve this problem.
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