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BIS investigation of PdSi(111)7 × 7 interface formation
Vacuum/volume41/numbers 1-3/pages 702 to 704/1990
0042-207X/90S3.00 + .00 © 1990 Pergamon Pressplc
Printed in Great Britain
BIS investigation of P d - S i ( 1 1 1 ) 7 x 7 interface formation J Y V e u i l l e n , T T Nguyen and R C i n t i , LEPES/CNRS BP 166)(, 38042 Grenoble, France S D ' A d d a t o , S T u r c h i n i and S N a n n a r o n e , Dipartimento di Fisica, Universitas La Sapienza, 00185 Roma,
Italy and A Santaniello and G Rossi, LURE/CNRS, Orsay, France
A combined BIS-XPS study of the Pd-Si(111) 7 x 7 interface formation is presented. A Pd2 Si-like distribution of partial d density of empty states is found starting from the early stages of Si-Pd bond formation.
Introduction We present a brief summary of the experimental results on the empty electronic states evolution upon Pd-Si(l 11)7 x 7 interface formation obtained by Bremsstrahlung Isochromat Spectroscopy (BIS). The Pd-silicon system is one of the most widely studied metal semiconductor interfaces because of its interesting aspects both from the basic and technological point of view'. Up to date, the experimental and theoretical work has been mainly focussed on the properties of the full electronic states. Only quite recently a study of the empty states of the Pd-Si compounds has been undertaken: the Pd2Si has been studied experimentally by Inverse Photoemission Spectroscopy (IPE) 2 and BIS 3 and theoretically4. Theoretical calculations are also available for the early stages of Pd deposition on silicon s. On the other side a lack of experimental results still exists on the empty states during the Pd-silicon interface formation. To fill this gap we carried out a combined BIS and X-ray Photoelectron Spectroscopy (XPS) study of the growing Pd-silicon interface in the Pd 1.5-25 A coverage range.
Experimental The experiment was carried out in a standard uhv experimental chamber manufactured by Vacuum Science Workshop equipped with X- and uv-photon sources, Low Energy Electron Diffraction (LEED), evaporation and clean surfaces preparation facilities and X-ray monochromator tuned on the AI Kct (1486.6) emission line. The clean S i ( l l l ) 7 x 7 substrate was prepared by ion bombardment and annealing. The surface order was checked by LEED. The Pd films were prepared by Pd evaporation by electron bombardment from a Pd wire wrapped around a W ribbon. Evaporation rates of the order of a few tenths o f A m i n - ' were used. The distance between the hot part of Pd (of the order of a few square millimeters) and the sample was of the order of 15 cm. The film thickness was determined by measuring the XPS emission from the Pd 3d and Si 2p core levels and assuming that a Pd2Si stoichiometry was present at any coverage. The BIS gun was a LaB 6 filament electron gun operated at a sample current not higher than 50 #A; higher sample currents caused, as revealed by the Valence Band (VB) XPS, further reaction between silicon and Pd. 702
The BIS spectra were taken by sweeping the electron gun energy in the range 1480-1500eV. In this condition a typical count rate of a few cpm was obtained which implied overall acquisition of the order of 12 h to achieve a good signal to noise ratio. The problem of electron beam induced surface reaction must be very carefully evaluated in such BIS experiments on interfaces. In order to make sure that no surface reaction was stimulated by the exposition to the electron beam during the BIS acquisition a VB-XPS was taken right after and before each BIS run.
The Fermi level of the system was determined by measuring the edge of the BIS spectrum of Pd. The Fermi level was measured before and after each BIS run; the average was taken and associated to the corresponding spectrum with an error of + 0.2 eV. Consequently a BIS run resulted in the following sequence of spectra: XPS-VB, Pd BIS (Fermi edge), BIS of the interface, Pd BIS (Fermi edge) and XPS-VB (final check of VB).
Results and discussion Figures 1 and 2 summarize the experimental results concerning the BIS and XPS of the VB and of the Pd 3d and Si 2p Core Level (CL) binding energies and F W H M .
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BIS
"6
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o o
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Figure 1. VB-XPS and BIS spectra at different Pd thickness. Raw data. (A, clean Si; B, 1.55 A; C, 3.1 A; D, 6.2 A; E, 12.4 A; F, 24.8 A; G, Pd2Si; H, Pd.)
J Y Veuillen et al: Pd-Si(111 )7 x 7 interface formation
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In order to gain more insights into the evolution of empty states at the Pd-Si interface the BIS contribution of clean silicon was subtracted from the experimental spectra. The silicon contribution was properly attenuated by taking into account the film thickness. The resulting spectra are shown in Figure 3. The subtraction causes the first peak to move closer to the Fermi level and the minimum between the two broad structures (well pronounced at high coverages) to appear also at the lowest investigated coverage. Changes in the attenuation factor of the order of -t- 30 % affect only the relative amplitude of the structures but not the energy position of the two maxima and of the minimum. Likewise a relative shift of ___0.5 eV of the Fermi level does not affect the resulting difference spectrum. The BIS spectra taken at different coverages do not show dramatic differences. They all are characterized by two broad structures separated by a minimum at about 5 eV resembling very closely the BIS spectrum of Pd2Si. Given the region
Figure 2. Binding energies ( + ) and FWHM (O) evolution vs Pd coverage of Pd 3ds/2 and Si 2p core levels. The arrows indicate the binding energies in bulk Pd and Pd2Si.
The measured VB spectra for different coverages and that of Pd2Si are in agreement with those reported 6-s. In fact the structures (A and B) corresponding to the emission from Si 3p and Pd 4d hybridized orbitals are visible while the binding energy of the nonbonding states (B) lowers by increasing coverage; this is in agreement with the picture where the non bonding states binding energies are the most perturbed by the Pd-Si bond formation 7.9. The Pd2Si BIS spectrum is in agreement with that in ref9. The two structures, taking into account the energy dependence of the cross section t°, are mainly due to transitions into empty d states arising from Pd 4d (lower energy structure) and Si 3d. As shown by an inspection of Figure 2 the Pd 3ds/2 and Si 2p binding energies evolve in agreemen_t with the findings reported in ref 7 (Si 2p) and ref 11 (Pd 3d) while the F W H M is nearly constant indicating a bonding uniformity in the probed depth.
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Figure 3. XPS and BIS valence band at different Pd coverages (indicated) after silicon substrate contribution subtraction. The spectra are normalized to the maximum. The error (arrow) is shown.
o ~ . _oo. , .oT. oS_.o o.~io -oO------ioo--
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Figure 4. Comparison between BIS spectra of Pd 2Si (full line) and that of
1.5 A of Pd (open dots). The difference between the two spectra, after normalization to the maximum, is shown in the lower part of each figure (full dots). The statistical error is reported; 4(a) spectra aligned at the experimental Fermi level; (b) same as (a) but with the PdzSi spectrum shifted by 0.25 eV in the low energy direction; (c) same as (a) but with the PdzSi spectrum shifted by 0.25 eV in the high energy direction. 703
J Y Veuillen et al: Pd-Si(111 )7 x 7 interface formation
explored that of the empty antibonding states, the present results favour the conclusion that the structure of empty anti-bonding states is established since the very beginnings of the Pd-Si bond formation at the interface. This result bears some analogy with that obtained by synchrotron radiation UPS at the Cooper minimum7 where a negligible dependence of the binding energies of the full bonding states was observed during the early stages of interface bond formation. However in order to better evidentiate possible small differences between Pd2Si BIS spectrum and that of the forming interface the difference between the normalized Pd2Si and 1.55 A spectra were taken. The results are shown in Figure 4; in Figure 4(a) the spectra are aligned at the experimental Fermi level while in Figure 4(b, c) the Pd2Si spectrum was shifted by 0.25 eV in the direction of the low (high) energies to estimate the influence of the Fermi level experimental error. Small differences of the experimental error suggest small differences in the density of states near E v are present in the cases of the Figures 4(a and b). We think, however, that better statistics needed to draw definitive conclusions about this point though the possible presence of a high density of empty states suggested in ref 5 is not observed.
Conclusions We presented a combined XPS-BIS study of the early states of the Pd-Si interface formation. Because of the values of the partial cross section at the used isochromat energy the partial density of empty d-states was
704
obtained at different Pd coverages. No dramatic evolution in the BIS spectra was observed during the interface formation while the expected lowering of the binding energies of the nonbonding d states was observed in the VB by XPS. The measured density of empty states resembles very closely that of Pd2Si. All this evidence leads to the conclusion that the structure of the bonding/antibondingstates at the Pd-Si interface are established from the very beginning of Pd deposition. Possible differences at E r could be found between the Pd-silicon interface and Pd2Si, though more work is needed to firmly establish this point.
References C Calandra, O Bisi and G Ottaviani, Surface Sci Rep, 4, 271 (1985). 2 F J Himpsel and Th Fauster, J Vac Sci Technol, AI2, 815 (1984). 3 D D Sarma, F U Hillbrecht, M Campagna, C Carbone, J Nogami, I Lindau, T W Barbee, L Braicovich, I Abbati and B De Michelis, Z Phys, B59, 159 (1985). 4 0 Bisi, O Jepsen and O K Andersen, Europhys Lett, l, 149 (1986). s O Bisi and K N Tu, Phys Rev Lett, 52, 1633 (1984). 6 p j Grunthaner and R J Grunthaner, Physica, l17B and llSB, 831 (1983). 7 I Abbati, G Rossi, I Lindau and W E Spicer J l/ac Sci Technol, 19, 636 (1981). 8 0 Bisi and C Calandra, J Phys C, 14, 5479 (1981). 9 0 Bisi, O Jepsen and O K Andersen, Europhys Lett, l, 149 (1986). 1o W Speier, J Fuggle, P Durham, R Zeller, R J Blake and P Sterne, J Phys C, 21, 2621 (1988). 11 S Nannarone, A M Fiorello, U del Pennino, C Mariani, M G Betti and M De Crescenzi, J Vac Sci Technol, A5, 1474 (1987).
0042-207X/90S3.00 + .00 © 1990 Pergamon Pressplc
Printed in Great Britain
BIS investigation of P d - S i ( 1 1 1 ) 7 x 7 interface formation J Y V e u i l l e n , T T Nguyen and R C i n t i , LEPES/CNRS BP 166)(, 38042 Grenoble, France S D ' A d d a t o , S T u r c h i n i and S N a n n a r o n e , Dipartimento di Fisica, Universitas La Sapienza, 00185 Roma,
Italy and A Santaniello and G Rossi, LURE/CNRS, Orsay, France
A combined BIS-XPS study of the Pd-Si(111) 7 x 7 interface formation is presented. A Pd2 Si-like distribution of partial d density of empty states is found starting from the early stages of Si-Pd bond formation.
Introduction We present a brief summary of the experimental results on the empty electronic states evolution upon Pd-Si(l 11)7 x 7 interface formation obtained by Bremsstrahlung Isochromat Spectroscopy (BIS). The Pd-silicon system is one of the most widely studied metal semiconductor interfaces because of its interesting aspects both from the basic and technological point of view'. Up to date, the experimental and theoretical work has been mainly focussed on the properties of the full electronic states. Only quite recently a study of the empty states of the Pd-Si compounds has been undertaken: the Pd2Si has been studied experimentally by Inverse Photoemission Spectroscopy (IPE) 2 and BIS 3 and theoretically4. Theoretical calculations are also available for the early stages of Pd deposition on silicon s. On the other side a lack of experimental results still exists on the empty states during the Pd-silicon interface formation. To fill this gap we carried out a combined BIS and X-ray Photoelectron Spectroscopy (XPS) study of the growing Pd-silicon interface in the Pd 1.5-25 A coverage range.
Experimental The experiment was carried out in a standard uhv experimental chamber manufactured by Vacuum Science Workshop equipped with X- and uv-photon sources, Low Energy Electron Diffraction (LEED), evaporation and clean surfaces preparation facilities and X-ray monochromator tuned on the AI Kct (1486.6) emission line. The clean S i ( l l l ) 7 x 7 substrate was prepared by ion bombardment and annealing. The surface order was checked by LEED. The Pd films were prepared by Pd evaporation by electron bombardment from a Pd wire wrapped around a W ribbon. Evaporation rates of the order of a few tenths o f A m i n - ' were used. The distance between the hot part of Pd (of the order of a few square millimeters) and the sample was of the order of 15 cm. The film thickness was determined by measuring the XPS emission from the Pd 3d and Si 2p core levels and assuming that a Pd2Si stoichiometry was present at any coverage. The BIS gun was a LaB 6 filament electron gun operated at a sample current not higher than 50 #A; higher sample currents caused, as revealed by the Valence Band (VB) XPS, further reaction between silicon and Pd. 702
The BIS spectra were taken by sweeping the electron gun energy in the range 1480-1500eV. In this condition a typical count rate of a few cpm was obtained which implied overall acquisition of the order of 12 h to achieve a good signal to noise ratio. The problem of electron beam induced surface reaction must be very carefully evaluated in such BIS experiments on interfaces. In order to make sure that no surface reaction was stimulated by the exposition to the electron beam during the BIS acquisition a VB-XPS was taken right after and before each BIS run.
The Fermi level of the system was determined by measuring the edge of the BIS spectrum of Pd. The Fermi level was measured before and after each BIS run; the average was taken and associated to the corresponding spectrum with an error of + 0.2 eV. Consequently a BIS run resulted in the following sequence of spectra: XPS-VB, Pd BIS (Fermi edge), BIS of the interface, Pd BIS (Fermi edge) and XPS-VB (final check of VB).
Results and discussion Figures 1 and 2 summarize the experimental results concerning the BIS and XPS of the VB and of the Pd 3d and Si 2p Core Level (CL) binding energies and F W H M .
ll/~
XPS
BIS
"6
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o o
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-5
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Figure 1. VB-XPS and BIS spectra at different Pd thickness. Raw data. (A, clean Si; B, 1.55 A; C, 3.1 A; D, 6.2 A; E, 12.4 A; F, 24.8 A; G, Pd2Si; H, Pd.)
J Y Veuillen et al: Pd-Si(111 )7 x 7 interface formation
Pd 3d s~2
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+
+
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.6
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Pd thickness (~,)
In order to gain more insights into the evolution of empty states at the Pd-Si interface the BIS contribution of clean silicon was subtracted from the experimental spectra. The silicon contribution was properly attenuated by taking into account the film thickness. The resulting spectra are shown in Figure 3. The subtraction causes the first peak to move closer to the Fermi level and the minimum between the two broad structures (well pronounced at high coverages) to appear also at the lowest investigated coverage. Changes in the attenuation factor of the order of -t- 30 % affect only the relative amplitude of the structures but not the energy position of the two maxima and of the minimum. Likewise a relative shift of ___0.5 eV of the Fermi level does not affect the resulting difference spectrum. The BIS spectra taken at different coverages do not show dramatic differences. They all are characterized by two broad structures separated by a minimum at about 5 eV resembling very closely the BIS spectrum of Pd2Si. Given the region
Figure 2. Binding energies ( + ) and FWHM (O) evolution vs Pd coverage of Pd 3ds/2 and Si 2p core levels. The arrows indicate the binding energies in bulk Pd and Pd2Si.
The measured VB spectra for different coverages and that of Pd2Si are in agreement with those reported 6-s. In fact the structures (A and B) corresponding to the emission from Si 3p and Pd 4d hybridized orbitals are visible while the binding energy of the nonbonding states (B) lowers by increasing coverage; this is in agreement with the picture where the non bonding states binding energies are the most perturbed by the Pd-Si bond formation 7.9. The Pd2Si BIS spectrum is in agreement with that in ref9. The two structures, taking into account the energy dependence of the cross section t°, are mainly due to transitions into empty d states arising from Pd 4d (lower energy structure) and Si 3d. As shown by an inspection of Figure 2 the Pd 3ds/2 and Si 2p binding energies evolve in agreemen_t with the findings reported in ref 7 (Si 2p) and ref 11 (Pd 3d) while the F W H M is nearly constant indicating a bonding uniformity in the probed depth.
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Figure 3. XPS and BIS valence band at different Pd coverages (indicated) after silicon substrate contribution subtraction. The spectra are normalized to the maximum. The error (arrow) is shown.
o ~ . _oo. , .oT. oS_.o o.~io -oO------ioo--
"O~ I 0
•
. ;°o_T °°.I_.,_
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1
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Figure 4. Comparison between BIS spectra of Pd 2Si (full line) and that of
1.5 A of Pd (open dots). The difference between the two spectra, after normalization to the maximum, is shown in the lower part of each figure (full dots). The statistical error is reported; 4(a) spectra aligned at the experimental Fermi level; (b) same as (a) but with the PdzSi spectrum shifted by 0.25 eV in the low energy direction; (c) same as (a) but with the PdzSi spectrum shifted by 0.25 eV in the high energy direction. 703
J Y Veuillen et al: Pd-Si(111 )7 x 7 interface formation
explored that of the empty antibonding states, the present results favour the conclusion that the structure of empty anti-bonding states is established since the very beginnings of the Pd-Si bond formation at the interface. This result bears some analogy with that obtained by synchrotron radiation UPS at the Cooper minimum7 where a negligible dependence of the binding energies of the full bonding states was observed during the early stages of interface bond formation. However in order to better evidentiate possible small differences between Pd2Si BIS spectrum and that of the forming interface the difference between the normalized Pd2Si and 1.55 A spectra were taken. The results are shown in Figure 4; in Figure 4(a) the spectra are aligned at the experimental Fermi level while in Figure 4(b, c) the Pd2Si spectrum was shifted by 0.25 eV in the direction of the low (high) energies to estimate the influence of the Fermi level experimental error. Small differences of the experimental error suggest small differences in the density of states near E v are present in the cases of the Figures 4(a and b). We think, however, that better statistics needed to draw definitive conclusions about this point though the possible presence of a high density of empty states suggested in ref 5 is not observed.
Conclusions We presented a combined XPS-BIS study of the early states of the Pd-Si interface formation. Because of the values of the partial cross section at the used isochromat energy the partial density of empty d-states was
704
obtained at different Pd coverages. No dramatic evolution in the BIS spectra was observed during the interface formation while the expected lowering of the binding energies of the nonbonding d states was observed in the VB by XPS. The measured density of empty states resembles very closely that of Pd2Si. All this evidence leads to the conclusion that the structure of the bonding/antibondingstates at the Pd-Si interface are established from the very beginning of Pd deposition. Possible differences at E r could be found between the Pd-silicon interface and Pd2Si, though more work is needed to firmly establish this point.
References C Calandra, O Bisi and G Ottaviani, Surface Sci Rep, 4, 271 (1985). 2 F J Himpsel and Th Fauster, J Vac Sci Technol, AI2, 815 (1984). 3 D D Sarma, F U Hillbrecht, M Campagna, C Carbone, J Nogami, I Lindau, T W Barbee, L Braicovich, I Abbati and B De Michelis, Z Phys, B59, 159 (1985). 4 0 Bisi, O Jepsen and O K Andersen, Europhys Lett, l, 149 (1986). s O Bisi and K N Tu, Phys Rev Lett, 52, 1633 (1984). 6 p j Grunthaner and R J Grunthaner, Physica, l17B and llSB, 831 (1983). 7 I Abbati, G Rossi, I Lindau and W E Spicer J l/ac Sci Technol, 19, 636 (1981). 8 0 Bisi and C Calandra, J Phys C, 14, 5479 (1981). 9 0 Bisi, O Jepsen and O K Andersen, Europhys Lett, l, 149 (1986). 1o W Speier, J Fuggle, P Durham, R Zeller, R J Blake and P Sterne, J Phys C, 21, 2621 (1988). 11 S Nannarone, A M Fiorello, U del Pennino, C Mariani, M G Betti and M De Crescenzi, J Vac Sci Technol, A5, 1474 (1987).