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Spectroscopic investigation of the early formation stage of the Si(111)(2×1)-Mo interface
Surface Science 177 (1986) L901-L906 North-Holland, Amsterdam
L901
S U R F A C E SCIENCE LETTERS S P E C T R O S C O P I C I N V E S T I G A T I O N O F T H E EARLY F O R M A T I O N STAGE O F T H E S i ( l l l ) ( 2 × 1 ) - M o INTERFACE I. ABBATI, L. BRAICOVICH, B. DE MICHELIS, A. F A S A N A and A. R I Z Z I Istituto di Fisica del Politecnico, Milano, Italy
Received 19 December 1985; accepted for publication 6 August 1986
The overgrowth of Mo onto Si(lll)(2x 1) at room temperature up to four monolayers is studied with valence band photoemission (hu = 21.2 eV) and with Auger spectroscopy. The interface is reactive and the results suggest an overgrowthwhich takes place via the formation of islands having electron states typical of silicides.
Among Si d-metal interfaces the case of refractory metals is particularly important and has received increasing attention with the main emphasis on the electron states [1]. In this context the S i - M o interface is still an open problem and has specific reasons of interest. In fact synchrotron radiation, photoemission and Auger spectroscopy (excited with an electron beam) have suggested for Si(111)(2 x 1), prepared by cleavage in situ, a considerable S i - M o mixing in the early formation stage of the interface at room temperature [2,3]. On the other hand a recent photoemission and Auger experiment by Balaska et al. [4] has shown a layer-by-layer growth without mixing of Mo onto Si(111)(7 x 7) at room temperature. This calls for a deeper understanding of these problems with the final goal of figuring out how the substrate morphology can influence the interface growth. The first step in this direction is a new spectroscopic investigation of the early growth of Mo onto Si(111) at room temperature. T h e research has been carried out with an apparatus different from that of our previous work [2,3]. In order to eliminate the possibility of a mixing artificially promoted during sample preparation, we have avoided putting energy onto the sample during the Mo deposition and during the measurements. For this reason the Si substrate was very far from the Mo source ( - 40 cm) (this distance was less than 10 cm in refs. [2,3]) and the sample was i'n good thermal contact with the manipulator. For the same reason we have not used electrons as an excitation source in Auger spectroscopy and we have measured with a lock-in the Auger structures present in the spectrum excited with Mg K a radiation. The valence band photoemission was excited with He I (hv--21.2 eV) radiation. All spectra were measured with a double pass 0039-6028/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L902
I. A bbati et al. / The early formation stage of the Si(l 11)(2 × 1)-Mo interface
cylindrical mirror analyzer and the data acquisition was done with a computer. The Si samples were cleaved in situ below I × 10-~° Torr and the pressure remained below 2 × 10 - 9 Torr during Mo evaporation. The layout of the apparatus is that of our previous work [5]. In order to have sufficient surface sensitivity the low energy Auger transitions were measured, i.e. Si(LVV) ( - 90 eV) and Mo(MNV) ( - 185 eV). Some important features of the growth mechanism can be understood on the basis of the Auger results. We will refer to the ratio of the Auger intensities of Mo(MNV) and of Si(EVV) transitions since the ratio is not influenced by small possible variations of sample positioning after successive evaporations. The values of this M o / S i ratio as measured with our apparatus are plotted versus the number of deposited atoms in fig. 1A (solid curve). It is important to note that the values of the ratios at very small coverages (mainly at 0.1 and 0.3 monolayer, i.e., up to n = 0.24 × 10 ~5 a t o m s / c m 2) have little dependence on the nature of the overgrowth mechanism; independent of the morphology, all Mo atoms are seen since the escape depth effect is basically absent for very small coverages while the intensity of Si is determined by the Si escape depth [6]. Thus the measured ratios at low coverages can be used to obtain an internal calibration of the Auger measurements, This is also confirmed by an independent test based on the calculations of the ratios expected at low coverages. This is done by considering the probabilities of photoionization in creating the Si and Mo holes with M g K a radiation [7], the distribution in intensities of the different Auger transitions filling the M hole of Mo [8], the escape depth [6] and the energy dependence of the spectrometer efficiency. The calculated ratios are in agreement within 20% with the measured values and this is a very strong support in favour of our calibration. On the basis of this calibration it is possible to calculate the Auger ratios expected in our experiment in correspondence to different overgrowth models. Two important results are obtained. (i) The layer-by-layer growth without intermixing should give the dashed curve of fig. 1A which is clearly incompatible with the experimental results. Thus this growth mechanism which has been pointed out for the growth onto the (7 x 7) surface [4] must be discarded for the S i ( l l l ) (2 x 1) surface. (ii) By assuming the presence of intermixing with a layer-by-layer growth of a silicide having a uniform composition one obtains the three curves of fig. 1B corresponding to the known stoichiometries of Mo silicides [9]. The figure shows that also this mechanism must be discarded. By assuming the occurrence of intermixing, this shows either that the interface is graded in composition or that more than one silicide is present. The consideration of the valence photoemission spectra is very useful in elucidating the above results. The spectra are given in fig. 2 (solid line) at increasing coverages; for comparison the spectra are superimposed on the spectrum of pure Mo (dashed curve). In order to point out the differences in
L Abbati et al. / The early formation stage of the Si(lll)(2 x 1)-Mo interface
if
B
L903
Mo 3 Si// /"
UosSi
3
Exp.
-"
~r I
A
tMo
I I I
/ I
15
I /
/ I I I
10
I /
I
/ / / / /
1
/
Exp.
I
I
1 2 3 n [10~s at/crn 2]
!
4
Fig. 1, Ratio r of the Mo Auger signal (MNV) to the Si Auger signal (LVV) as a function of the number n (atoms/cm 2) of deposited Mo atoms. Lower panel (A): comparison between the experiment (solid curve) and the expected curve for layer-by-layer growth of pure Mo (dashed curve). Upper panel (B): comparison between the experiment (solid curve) and the expected curves for layer-by-layer growth of the three Mo silicides (dashed lines).
the shape the spectra are normalized to the same maximum. The most relevaht results are the following. (i) The valence photoemission shows clearly the formation of some kind of silicide [1]. This is pointed out by the structures A and B indicated by the arrows in fig. 2. In fact the formation of Si(p)-metal(d) hybrid bonds creates deep bonding states in the region A while a fraction of d-states of nonbonding
L904
L Abbati et al. / The early formation stage of the Si(l l l )(2 × 1)-Mo interface
I
I -
4
i
I -
2
i
i ~~ . ~ v ! ~015 E F
eV
Fig. 2. HeI (hu = 21.2 eV) angle integrated valence photoemission from the S i ( l l l ) ( 2 x 1 ) - M o interface grown at room temperature (solid curves). The spectra are labelled with the coverage expressed in number of deposited atoms per cm 2. The spectra are compared with that of pure Mo taken under the same conditions and normalized to the same m a x i m u m (dashed curves).
nature is left in the Fermi region [1]. This originates the increase of the intensity near E v with respect to the metal with the formation of the shoulder B. This key feature has been pointed out for the S i ( l l l ) - C r interface [10] and for silicides [11]. Thus the present photoemission results are the clear evidence of intermixing. This rules out also the possibility of a Stransky-Krastanov growth without intermixing which, strictly speaking, cannot be ruled out on the basis of the Auger analysis reported in fig. 1A. (ii) The shapes of the valence photoemission spectra are extremely stable when the coverage increases indicating a clear tendency towards the formation of a silicide having a well-defined stoichiometry. This does not exclude that more than one silicide is present as suggested in other systems by Del Giudice et al. [12], but, there is surely a dominant species. This situation is analogous to what suggested for S i ( l l l ) - N i in Rutherford backscattering experiments [13]. Thus the Auger results of fig. 1B and the impossibility of fitting the data with a model based on uniform overgrowth of a silicide cannot be attributed to the formation of a graded interface or to the presence in relevant proportion of more than one silicide. This situation must be regarded as the effect of a growth with the formation of islands. Thus the combined analysis of the Auger and of the photoemission results rules out any overgrowth without intermixing and strongly suggests the formation of islands of silicides having a rather defined stoichiometry. By considering the curves of fig. 1B, since the Si signal
L A bbati et al. / The early formation stage of the Si(111)(2 × 1)- Mo interface
L905
contains also the contribution from the Si between the islands, one could argue that the composition is not silicon rich. The present work is based on spectroscopic results and not on a structural method; thus the analysis of the growth mechanism cannot be pushed too far. In particular it is very hard to assess if the details of the overgrowth are similar to those introduced in the model for S i - N i based on the formation of silicide islands [13]. It is not possible to distinguish in the Auger Si signal the s u b c o m p o n e n t s from the islands and from a skin of Si on top of the islands which could be present in analogy to the model of ref. [13]; this difficulty arises essentially from the fact that the shape of the Auger Si line is not a significant diagnostic of the environment in M o silicides [14] as opposed to the S i - n e a r noble metal case [15]. These difficulties call for further structural work which could complement the present spectroscopic results. Nevertheless it has already been possible to point out the main features of the growth mechanism. In conclusion, the combined analyses of Auger intensities and of valence photoemission have shown that the interface between M o and S i ( l l l ) (2 × 1) is reactive already at r o o m temperature. The data give a clear indication in favour of the formation of silicide islands and a strong support for a preferential stoichiometry in the early growth of the interface. The connection of the results with other available information on related systems has also been pointed out. This work has been entirely supported by the C I S M (Centro Interuniversitario di Struttura della Materia) of the Ministro della Pubblica Istruzione of Italy and by the G N S M ( G r u p p o Nazionale di Struttura della Materia) of the C N R (Consiglio Nazionale delle Ricerche) of Italy.
References [1] C. Calandra, O. Bisi and G. Ottaviani, Surface Sci. Rept. 4 (1985) 273, and references therein. [2} G. Rossi, I. Abbati, L. Braicovich, I. Lindau and W.E. Spicer, J. Vacuum Sci. Technol. 21 (1982) 617. [3] G. Rossi, I. Abbati, L. Braicovich, I. Lindau, W.E. Spicer, U. del Pennino and S. Nannarone, Physics 117-118B (1983) 795. [4] H. Balaska, R.C. Cinti, T.T.A. Nguyen and J. Derrien, Surface Sci. 168 (1986) 225. [5] I. Abbati, L. BraJcovich, A. Fasana, C,M. Bertorti, F. Manghi and C. Calandra, Phys. Rev. B23 (1981) 6448. [6] M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2. [7] J.J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Eds., Handbook of X-Ray Photoelectron Spectroscopy (Perkin Elmer, Eden Prairie, MN). [9] F.A. Shunk, Constitution of Binary Alloys (Second Supplement) (McGraw-Hill, New York, 1969); E.I. Gladyshevskii, Crystallochemistry of Silicides and Germanides (Nauka, Moscow, 1971) (in Russian).
L906
L A bbati et al. / The early formation stage of the Si(l l 1)(2 x 1)-Mo interface
[10] A. Franciosi, D.J. Peterman, J.H. Weaver and V.L. Moruzzi, Phys. Rev. B25 (1981) 4981. [11] A. Franciosi, J.H. Weaver, D.G. O'Neill, Y. Chabal, J,E. Rowe, J.M. Poate, O. Bisi and C. Calandra, J. Vacuum Sci. Technol. 21 (1982) 624. [12] M. Del Giudice, M. Grioni, J.J. Joyce, M.W. Ruckman, S.A. Chambers and J.H. Weaver, Surface Sci. 168 (1986) 309. [13] E.J. van Loenen, J.F. van der Veen and F.K. le Goues, Surface Sci. 157 (1985) 1. [14] J.A. Roth and C.R. Crowell, J. Vacuum Sci. Technol. 15 (1978) 1317. [15] P.S. Ho, G.W. Rubloff, J.E. Leurs, V.L. Moruzzi and A.R. Williams, Phys. Rev. B22 (1980) 4784.
L901
S U R F A C E SCIENCE LETTERS S P E C T R O S C O P I C I N V E S T I G A T I O N O F T H E EARLY F O R M A T I O N STAGE O F T H E S i ( l l l ) ( 2 × 1 ) - M o INTERFACE I. ABBATI, L. BRAICOVICH, B. DE MICHELIS, A. F A S A N A and A. R I Z Z I Istituto di Fisica del Politecnico, Milano, Italy
Received 19 December 1985; accepted for publication 6 August 1986
The overgrowth of Mo onto Si(lll)(2x 1) at room temperature up to four monolayers is studied with valence band photoemission (hu = 21.2 eV) and with Auger spectroscopy. The interface is reactive and the results suggest an overgrowthwhich takes place via the formation of islands having electron states typical of silicides.
Among Si d-metal interfaces the case of refractory metals is particularly important and has received increasing attention with the main emphasis on the electron states [1]. In this context the S i - M o interface is still an open problem and has specific reasons of interest. In fact synchrotron radiation, photoemission and Auger spectroscopy (excited with an electron beam) have suggested for Si(111)(2 x 1), prepared by cleavage in situ, a considerable S i - M o mixing in the early formation stage of the interface at room temperature [2,3]. On the other hand a recent photoemission and Auger experiment by Balaska et al. [4] has shown a layer-by-layer growth without mixing of Mo onto Si(111)(7 x 7) at room temperature. This calls for a deeper understanding of these problems with the final goal of figuring out how the substrate morphology can influence the interface growth. The first step in this direction is a new spectroscopic investigation of the early growth of Mo onto Si(111) at room temperature. T h e research has been carried out with an apparatus different from that of our previous work [2,3]. In order to eliminate the possibility of a mixing artificially promoted during sample preparation, we have avoided putting energy onto the sample during the Mo deposition and during the measurements. For this reason the Si substrate was very far from the Mo source ( - 40 cm) (this distance was less than 10 cm in refs. [2,3]) and the sample was i'n good thermal contact with the manipulator. For the same reason we have not used electrons as an excitation source in Auger spectroscopy and we have measured with a lock-in the Auger structures present in the spectrum excited with Mg K a radiation. The valence band photoemission was excited with He I (hv--21.2 eV) radiation. All spectra were measured with a double pass 0039-6028/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L902
I. A bbati et al. / The early formation stage of the Si(l 11)(2 × 1)-Mo interface
cylindrical mirror analyzer and the data acquisition was done with a computer. The Si samples were cleaved in situ below I × 10-~° Torr and the pressure remained below 2 × 10 - 9 Torr during Mo evaporation. The layout of the apparatus is that of our previous work [5]. In order to have sufficient surface sensitivity the low energy Auger transitions were measured, i.e. Si(LVV) ( - 90 eV) and Mo(MNV) ( - 185 eV). Some important features of the growth mechanism can be understood on the basis of the Auger results. We will refer to the ratio of the Auger intensities of Mo(MNV) and of Si(EVV) transitions since the ratio is not influenced by small possible variations of sample positioning after successive evaporations. The values of this M o / S i ratio as measured with our apparatus are plotted versus the number of deposited atoms in fig. 1A (solid curve). It is important to note that the values of the ratios at very small coverages (mainly at 0.1 and 0.3 monolayer, i.e., up to n = 0.24 × 10 ~5 a t o m s / c m 2) have little dependence on the nature of the overgrowth mechanism; independent of the morphology, all Mo atoms are seen since the escape depth effect is basically absent for very small coverages while the intensity of Si is determined by the Si escape depth [6]. Thus the measured ratios at low coverages can be used to obtain an internal calibration of the Auger measurements, This is also confirmed by an independent test based on the calculations of the ratios expected at low coverages. This is done by considering the probabilities of photoionization in creating the Si and Mo holes with M g K a radiation [7], the distribution in intensities of the different Auger transitions filling the M hole of Mo [8], the escape depth [6] and the energy dependence of the spectrometer efficiency. The calculated ratios are in agreement within 20% with the measured values and this is a very strong support in favour of our calibration. On the basis of this calibration it is possible to calculate the Auger ratios expected in our experiment in correspondence to different overgrowth models. Two important results are obtained. (i) The layer-by-layer growth without intermixing should give the dashed curve of fig. 1A which is clearly incompatible with the experimental results. Thus this growth mechanism which has been pointed out for the growth onto the (7 x 7) surface [4] must be discarded for the S i ( l l l ) (2 x 1) surface. (ii) By assuming the presence of intermixing with a layer-by-layer growth of a silicide having a uniform composition one obtains the three curves of fig. 1B corresponding to the known stoichiometries of Mo silicides [9]. The figure shows that also this mechanism must be discarded. By assuming the occurrence of intermixing, this shows either that the interface is graded in composition or that more than one silicide is present. The consideration of the valence photoemission spectra is very useful in elucidating the above results. The spectra are given in fig. 2 (solid line) at increasing coverages; for comparison the spectra are superimposed on the spectrum of pure Mo (dashed curve). In order to point out the differences in
L Abbati et al. / The early formation stage of the Si(lll)(2 x 1)-Mo interface
if
B
L903
Mo 3 Si// /"
UosSi
3
Exp.
-"
~r I
A
tMo
I I I
/ I
15
I /
/ I I I
10
I /
I
/ / / / /
1
/
Exp.
I
I
1 2 3 n [10~s at/crn 2]
!
4
Fig. 1, Ratio r of the Mo Auger signal (MNV) to the Si Auger signal (LVV) as a function of the number n (atoms/cm 2) of deposited Mo atoms. Lower panel (A): comparison between the experiment (solid curve) and the expected curve for layer-by-layer growth of pure Mo (dashed curve). Upper panel (B): comparison between the experiment (solid curve) and the expected curves for layer-by-layer growth of the three Mo silicides (dashed lines).
the shape the spectra are normalized to the same maximum. The most relevaht results are the following. (i) The valence photoemission shows clearly the formation of some kind of silicide [1]. This is pointed out by the structures A and B indicated by the arrows in fig. 2. In fact the formation of Si(p)-metal(d) hybrid bonds creates deep bonding states in the region A while a fraction of d-states of nonbonding
L904
L Abbati et al. / The early formation stage of the Si(l l l )(2 × 1)-Mo interface
I
I -
4
i
I -
2
i
i ~~ . ~ v ! ~015 E F
eV
Fig. 2. HeI (hu = 21.2 eV) angle integrated valence photoemission from the S i ( l l l ) ( 2 x 1 ) - M o interface grown at room temperature (solid curves). The spectra are labelled with the coverage expressed in number of deposited atoms per cm 2. The spectra are compared with that of pure Mo taken under the same conditions and normalized to the same m a x i m u m (dashed curves).
nature is left in the Fermi region [1]. This originates the increase of the intensity near E v with respect to the metal with the formation of the shoulder B. This key feature has been pointed out for the S i ( l l l ) - C r interface [10] and for silicides [11]. Thus the present photoemission results are the clear evidence of intermixing. This rules out also the possibility of a Stransky-Krastanov growth without intermixing which, strictly speaking, cannot be ruled out on the basis of the Auger analysis reported in fig. 1A. (ii) The shapes of the valence photoemission spectra are extremely stable when the coverage increases indicating a clear tendency towards the formation of a silicide having a well-defined stoichiometry. This does not exclude that more than one silicide is present as suggested in other systems by Del Giudice et al. [12], but, there is surely a dominant species. This situation is analogous to what suggested for S i ( l l l ) - N i in Rutherford backscattering experiments [13]. Thus the Auger results of fig. 1B and the impossibility of fitting the data with a model based on uniform overgrowth of a silicide cannot be attributed to the formation of a graded interface or to the presence in relevant proportion of more than one silicide. This situation must be regarded as the effect of a growth with the formation of islands. Thus the combined analysis of the Auger and of the photoemission results rules out any overgrowth without intermixing and strongly suggests the formation of islands of silicides having a rather defined stoichiometry. By considering the curves of fig. 1B, since the Si signal
L A bbati et al. / The early formation stage of the Si(111)(2 × 1)- Mo interface
L905
contains also the contribution from the Si between the islands, one could argue that the composition is not silicon rich. The present work is based on spectroscopic results and not on a structural method; thus the analysis of the growth mechanism cannot be pushed too far. In particular it is very hard to assess if the details of the overgrowth are similar to those introduced in the model for S i - N i based on the formation of silicide islands [13]. It is not possible to distinguish in the Auger Si signal the s u b c o m p o n e n t s from the islands and from a skin of Si on top of the islands which could be present in analogy to the model of ref. [13]; this difficulty arises essentially from the fact that the shape of the Auger Si line is not a significant diagnostic of the environment in M o silicides [14] as opposed to the S i - n e a r noble metal case [15]. These difficulties call for further structural work which could complement the present spectroscopic results. Nevertheless it has already been possible to point out the main features of the growth mechanism. In conclusion, the combined analyses of Auger intensities and of valence photoemission have shown that the interface between M o and S i ( l l l ) (2 × 1) is reactive already at r o o m temperature. The data give a clear indication in favour of the formation of silicide islands and a strong support for a preferential stoichiometry in the early growth of the interface. The connection of the results with other available information on related systems has also been pointed out. This work has been entirely supported by the C I S M (Centro Interuniversitario di Struttura della Materia) of the Ministro della Pubblica Istruzione of Italy and by the G N S M ( G r u p p o Nazionale di Struttura della Materia) of the C N R (Consiglio Nazionale delle Ricerche) of Italy.
References [1] C. Calandra, O. Bisi and G. Ottaviani, Surface Sci. Rept. 4 (1985) 273, and references therein. [2} G. Rossi, I. Abbati, L. Braicovich, I. Lindau and W.E. Spicer, J. Vacuum Sci. Technol. 21 (1982) 617. [3] G. Rossi, I. Abbati, L. Braicovich, I. Lindau, W.E. Spicer, U. del Pennino and S. Nannarone, Physics 117-118B (1983) 795. [4] H. Balaska, R.C. Cinti, T.T.A. Nguyen and J. Derrien, Surface Sci. 168 (1986) 225. [5] I. Abbati, L. BraJcovich, A. Fasana, C,M. Bertorti, F. Manghi and C. Calandra, Phys. Rev. B23 (1981) 6448. [6] M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2. [7] J.J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Eds., Handbook of X-Ray Photoelectron Spectroscopy (Perkin Elmer, Eden Prairie, MN). [9] F.A. Shunk, Constitution of Binary Alloys (Second Supplement) (McGraw-Hill, New York, 1969); E.I. Gladyshevskii, Crystallochemistry of Silicides and Germanides (Nauka, Moscow, 1971) (in Russian).
L906
L A bbati et al. / The early formation stage of the Si(l l 1)(2 x 1)-Mo interface
[10] A. Franciosi, D.J. Peterman, J.H. Weaver and V.L. Moruzzi, Phys. Rev. B25 (1981) 4981. [11] A. Franciosi, J.H. Weaver, D.G. O'Neill, Y. Chabal, J,E. Rowe, J.M. Poate, O. Bisi and C. Calandra, J. Vacuum Sci. Technol. 21 (1982) 624. [12] M. Del Giudice, M. Grioni, J.J. Joyce, M.W. Ruckman, S.A. Chambers and J.H. Weaver, Surface Sci. 168 (1986) 309. [13] E.J. van Loenen, J.F. van der Veen and F.K. le Goues, Surface Sci. 157 (1985) 1. [14] J.A. Roth and C.R. Crowell, J. Vacuum Sci. Technol. 15 (1978) 1317. [15] P.S. Ho, G.W. Rubloff, J.E. Leurs, V.L. Moruzzi and A.R. Williams, Phys. Rev. B22 (1980) 4784.