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X-ray photoelectron spectroscopy study of the vapor deposition of copper onto a MgO(100) surface
Applied Surface Science 3 3 / 3 4 (1988) 143-151 North-Holland, A m s t e r d a m
143
X-RAY P H O T O E L E C T R O N S P E C T R O S C O P Y STUDY O F T H E VAPOR D E P O S I T I O N O F C O P P E R O N T O A MgO(100) SURFACE Ib A L S T R U P Haldor Topsoe Research Laboratories, DK-2800 Lyngby, Denmark
and Preben J. MCILLER Department of Physical Chemistry, H.C. Orsted Institute, University of Copenhagen, 5 Universitetsparken, DK-2100 Copenhagen O, Denmark Received 23 August 1987; accepted for publication 9 October 1987
The formation of a vapor-deposited Cu overlayer on a MgO(lO0) crystal surface has been investigated using X-ray photoelectron spectroscopy (XPS). The results, in terms of electron energies, Auger parameters, intensity ratios, and Cu peak width as functions of the time of deposition, are consistent with the Stranskii-Krastanov growth model. The copper, deposited at room temperature, is at low coverages in a non-metallic state. During the deposition of Cu, a mixed monolayer of non-metallic and metallic Cu is formed, and finally the deposition is producing on top of it a continuous layer of metallic Cu.
1. Introduction
The study of the interaction between metal particles and oxide surfaces is of great importance in metallurgy, ultra-thin film microelectronics, ceramics and in heterogeneous catalysis. In the latter the interaction plays a crucial role for the activity, selectivity and stability of the catalyst. Oxide-supported metal catalysts have been widely studied in powder form while relative few studies have been carried out on single crystals. It is generally supposed that the active part of a supported metal catalyst is the free surface of the metal particles and that the interaction between the metal particles and the support stabilizes the small metal particles by preventing migration and sintering. In the case of oxide-supported copper catalysts, which are widely used for the water gas shift reaction and for synthesis of methanol, the chemical state of the active sites are still debated. In recent Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) studies on the electronic structure changes during the synthesis of ultra-thin layers of copper on MgO 0169-4332/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
144
1. Alstrup. P.J. Moiler / Vapor deposition of copper onto MgO(lO0)
crystal surfaces [1,2] it was proposed that the copper initially appears in a Cu(I) state, in agreement with a recent ab initio calculation [3]. In the present work the electronic state of the copper as a function of the build-up of an ultra-thin layer on MgO(100) is investigated by XPS so as to further elucidate the process of changes of electronic state with increasing deposits of copper.
2. Experimental procedure Copper deposition was carried out at room temperature in a surface spectrometer (VG Scientific Ltd., UK) equipped with facilities for XPS, UPS, and AES. An attached preparation chamber is equipped with an ion sputter gun (VG Scientific Ltd., AG2) and an evaporator cooled by liquid N 2 during evaporation. The background pressure was about 10 10 Tort (1 Tort = 133.3 Pa) in the analysis chamber and about 5 × 10 l0 Torr in the preparation chamber. A Cu wire of 99.995% purity (Johnson Matthey Ltd., UK) was used as an evaporation source. The MgO crystal (W&C. Spicer & Co. Ltd., UK) was cleaved in air along a (100) plane before introduction into the spectrometer. After baking the spectrometer overnight at 450 K the sample was heated to about 680 K for about 10 h. Upon a preliminary Cu deposition the crystal was cleaned by cycles of Ar + bombardment (2 kV, 3.5 ktA) and heat treatment (610 K in U H V for 10 rain). Subsequently the sample was heated in 10 6 Torr oxygen at about 570 K for 1 h. This procedure is known to give an excellent LEED pattern [2,4] indicating a well-structured surface. The only impurity seen in the XPS spectra was carbon. The carbon peak area corresponded to a carbon coverage of about 0.05 monolayer. The first part of the Cu deposition consisted of six depositions each of an evaporation time of 60 s, aimed at a deposition of 0.2 monolayer each. The second part consisted of four depositions each of 300 s evaporation time. Cu and Mg photoemission and X-ray excited Auger peaks as well as oxygen photoemission peaks were recorded after each evaporation step. AI K a radiation, a constant pass energy of 50 eV and a 4 mm slit width were used for all recordings. The intensities of the peaks were determined as the total counts minus background counts in an energy region bracketing the peak or as in the case of the Cu 2p spectrum - the two peaks. For the Cu 2p peaks a non-linear background was constructed using the method of Shirley [5] while for the other peaks a linear background was used.
3. Results Fig. 1 shows the intensity ratios R ( C u / M g ) = l(Cu 2 p ) / l ( M g ls) and R(MglK ) = l(Mg l s ) / l ( M g KL2.3L2,3) plotted as functions of the deposition
I. Alstrup, P.J. Mailer / Vapor deposition of copper onto MgO(100)
50
2.5
40
2.0
145
A
,~
m t~
---
30
].5 .o m
~
2O c
0.5
10
D
5
T0
15
Deposition time
Fig. 1. XPS intensity ratios:
i
L
i
20
25
3O
0
"r(min)
R(Cu/Mg)=/(Cu2p)/l(Mg]s) and R(Mg~K)=/(MgIs)/
/(Mg KL2,3L2,3)as functions of Cu deposition time during growth upon MgO(100).
time ~-. The binding energies, BE, (not corrected for charging, and relative to the Fermi level) of Cu 2p3/2 and of Mg ls, respectively, are plotted as functions of ~- in fig. 2. The charging-up is seen initially to increase until T = 6 rain. With the following depositions the charging-up rapidly decreases and finally disappears at • = 21 rain, indicating a formation of a continuous copper layer at that time. The differences in energy between a photoemitted electron and its associated Auger electron, the so-called Auger parameters, a c , and aMg, were calculated as the sum of the kinetic energy of the Auger peak
1. Alstrup. P.J. Moller/ Vapor deposition of copper onto MgO(lO0)
146
945 1315
940 --
-
935 ~=
>~
1310
m
1305 -Cr'-
93O 1300
L
l i 10 Disposition time,
0
, 20 T(min)
i 30
Fig. 2. Binding energies, BE, not corrected for charging and relative to the Fermi level, ol C u 2 p ~ / ; and M g l s electrons as functions of Cu deposition time T during growth upon MgO(100).
2485
1852
Cu
© 2484
~ 1851
E
a~ =:e
2483
1850
Cu~0
1849
,
1__ 10
~
_~ 20
2482 3O
Deposition time v(min) Fig. 3. Auger parameters, ~('u and c~Mg, as functions of Cu deposition time during growth upon MgO(lO0). ~(,, and aMg are determined within +0.1 eV.
L Alstrup, P.J. Moiler/ Vapordeposition of copperonto MgO(lO0)
147
= 11 =10
.cz
~L.
v
6
1
I
1
1
1
I
1
O
5
10
15
20
25
30
Deposition time
-r(min)
Fig. 4. Effective peak width AWc. of the Cu 2p3/2 peak as a function of Cu deposition time ~during growth upon MgO(100). AWcu= ~A/h, where A is the peak area and h is the peak height. and the BE of the largest photoemission peaks, i.e. Cu 2p3/2 and Mg ls for Cu and Mg, respectively. The Auger parameters are plotted as a function of ~- in fig. 3. Initially at 1 min deposition, the aCu value is within the range reported for Cu2 ° [6]. aco increases rapidly with ~ and saturates at a value within the range reported for metallic Cu [6] at ~"= 16 min. A n effective peak width, AWcu of the Cu 2p3/2 peak was determined as one half the ratio of the peak area to the peak height of the peak. AWcu increases linearly with ~- in the first 11 min, as shown in fig. 4, after which it decreases during the next period of deposition, reaching a value appropriate for metallic Cu. Some of the peaks due to the Cu L3M4,sM4, 5 Auger electrons are shown in fig. 5. The shape of the Auger peaks for T = 2, 3, 4, 5 and 6 min, which is not shown, does not differ significantly from the peak at T = 1 rain. We note that at ~- = 26 min the shape of the peak is not to be distinguished from the well-established Auger peak of pure Cu. The shape of the other peaks in the spectrum did not change significantly except for the above-mentioned broadening of the Cu 2p peaks (fig. 4).
4. Discussion In figs. 1 - 4 we clearly observe two distinct deposition regions. The intensity ratios, the binding energies and the Auger parameters show one break point at
148
I. Alstrup, P.J. Moiler / Vapor deposition of copper onto MgO(lO0)
time
•jf//•TDepesLtion (min)
26
21 .t=
_,,J
,3
I
I
I
605 597 589 581 573 565 557 ~indin 9 eneTgy, BE (eV)
Fig. 5. X-ray excited Cu L3M4,sM4,s Auger spectra at increasing deposition times of Cu upon MgO(100). approximately 6 min of deposition. These results are thus consistent with the Stranskii-Krastanov model for the kinetic growth mechanism, the initial linear section showing the two-dimensional completion of the first monolayer (ML) followed by growth of nuclei. This growth mechanism for the C u / M g O
L Alstrup, P.J. Moiler / Vapor deposition of copper onto MgO(lO0)
149
system, which was suggested previously on the basis of A E S / E E L S / L E E D experiments [2], is hence confirmed. The value of ac~ for low r indicates, as mentioned above, that the low-coverage deposited Cu atoms are in a Cu(I) state. It should be noted, however, that this interpretation depends on the assumption that a fraction of a M L of C u 2 0 will have essentially the same Cu Auger and core electron energies as bulk Cu20. To our knowledge, this assumption has not yet been substantiated. At higher coverages the increase in the Auger parameter and in the width of the C u 2 0 peaks indicate that increasing amounts of the copper are present in the metallic state. The results for the last three depositions (16, 21 and 26 min, respectively) indicate that metallic Cu now completely covers the sample and that the last two depositions contribute mainly to a thickening of the metallic overlayer. Figs. 1 - 4 further indicate that 11 min of deposition has resulted in an intermediate situation in which a complete metallic overlayer has not yet been formed. It is possible to determine the coverage Ocu on a MgO(100) surface as a function of r in the first part of the deposition sequence from the data of fig. 1, applying the following assumptions: (1) that the inelastic mean free paths, )~,, may be calculated from the formula suggested by Seah and Dench [7] for elements, and (2) that the two sets of intensity ratios give approximately the same coverages. Hence, OCu/"2 p
R ( C u / M g ) = R ( C u / M g ) ~ 1 ~ 0-c~F1s ' 1 - o(.~rls
R ( M g ' K ) = R ( M g ' K ) ~ 1 - OCurKCL '
(1)
(2)
where ~, = 1 - e x p [ - 1 / ( ; k , sin rp)].
(3)
In the present experiments, sin qo = 0.755, where qo is the electron escape angle. R ( C u / M g ) ~ and R ( M g 1K)~ are the bulk intensity ratios. Using ~2p = 4.65 ML, )kls = 2,62 ML, )~KCL= 6.82 ML, solving for Oc,~ in each of eqs. (1) and (2) and equating the two obtained expressions for 0cu we obtain R ( C u / M g ) o o = 4 . 2 2 and R ( M g l K ) ~ = 1.80. Then the coverages Ocu may be determined for all the values of r corresponding to 0co _< 1. Fig. 6 gives the calculated 0c. for each value of ~-. The two sets of intensity ratios have a different origin: one is calculated solely from signals from the substrate, while the other is calculated from signals both from the substrate and from the outermost layer. These two sets give the same, and hence consistent, values for 0cu. The amount of deposited Cu, as judged from the coverage changes A0c., are approximately the same for the second, third and fourth
150
1. Alstrup, P.J. Moller/ Vapor deposition of copper onto MgO(lO0)
×
1.0
x
,3
[]
&
¢xl
[]
05
0
I
O
i
2
1
I
I
I
I
~_
4 6 Deposition time, T(min)
Fig. 6, Coverages Ocu at increasing values of Cu deposition time ,r during growth of Cu upon MgO(100): (H) calculated from R(MgIK); (×)calculated from R(Cu/Mg). deposition while the first deposition apparently yields a considerably larger change. AO(. u after each of the fifth and sixth depositions is smaller and dependent on the intensity ratio used for the calculation. That the first deposition gave a larger AOcu value may be explained by the way ~- was determined. In these experiments significant evaporation was assumed to commence when a red glow could be observed through the output aperture of the evaporator. It is conceivable that it took a longer time in the first deposition to reach this point than in the later ones, and that a significant evaporation took place before the glow was evident in the aperture. The significantly smaller A0cu calculated for the intensity ratios upon the fifth and sixth depositions probably are not due to smaller amounts of deposited copper but rather to the onset of nucleation on top of the non-completed monolayer (Ocu -~ 0.8), causing some deviation from linearity in the Oc,u versus T plot and giving different results for the two intensity ratios. Thus the change in growth mode apparently starts earlier than judged from the break point in figs. 1 and 2. This earlier start may explain the low c~cu value measured after the sixth deposition and is in agreement with the leveling-off of the R(Mg,K ) ratio after the fifth deposition (fig. 1). A similar unambiguous quantitative interpretation of the results for the last four depositions is probably not possible and has not been attempted. The results in this study demonstrate that considerable information is contained in XPS spectra recorded during growth of thin films. In addition to the above results figs. 1-3 indicate further interesting changes in the MgO(100)
L Alstrup, P.J. Moiler / Vapor deposition of copper onto MgO(lO0)
151
surface during deposition. Clearly, significant changes in the electronic structure of the MgO surface take place as soon as the deposition is initiated, and again when deposition on top of the first layer begins. The latter change indicates that the first layer is changed by the deposition on top of it. Much of this information in the spectra, however, cannot be unraveled without results from supplementary techniques applied in situ.
5. Conclusion On the basis of XPS spectra it is indicated that an ultra-thin copper film vapor deposited upon a sputter-cleaned and annealed MgO(100) crystal surface grows according to a Stranskii-Krastanov mechanism. The island growth on top of the first layer starts, however, before completion of the first layer. This first layer is not homogeneous with regard to the electronic state of the deposited copper. At low coverages the copper is in the Cu(I) state while metallic Cu is increasingly contributing to the spectra at the higher coverages. Changes in the Auger parameter for Mg with copper coverage further indicate that changes in the electronic structure of the MgO, as previously indicated by EELS experiments, occur when the deposition starts and again when growth on top of the first layer starts, thus indicating that the electronic changes are induced in the first layer by the deposition of further Cu on top of it.
Acknowledgements We are grateful to Ane Blom for technical assistance and to Jian-Wei He for fruitful discussions.
References [1] [2] [3] [4] [5] [6]
P.J. Moiler and J.-W. He, Nucl. Instr. Methods Phys. Res. B 17 (1986) 137. J.-W. He and P.J. Moiler, Surface Sci. 178 (1986) 934. N.C. Bacalis and A.B. Kunz, Phys. Rev. B 32 (1985) 4857. D.G. Lord and M. Prutton, Thin Solid Films 21 (1974) 341. D.A. Shirley, Phys. Rev. B 5 (1972) 4709. C.D. Wagner, in: Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, Eds. D. Briggs and M.P. Seah (Wiley, New York, 1983) p. 496. [7] M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2.
143
X-RAY P H O T O E L E C T R O N S P E C T R O S C O P Y STUDY O F T H E VAPOR D E P O S I T I O N O F C O P P E R O N T O A MgO(100) SURFACE Ib A L S T R U P Haldor Topsoe Research Laboratories, DK-2800 Lyngby, Denmark
and Preben J. MCILLER Department of Physical Chemistry, H.C. Orsted Institute, University of Copenhagen, 5 Universitetsparken, DK-2100 Copenhagen O, Denmark Received 23 August 1987; accepted for publication 9 October 1987
The formation of a vapor-deposited Cu overlayer on a MgO(lO0) crystal surface has been investigated using X-ray photoelectron spectroscopy (XPS). The results, in terms of electron energies, Auger parameters, intensity ratios, and Cu peak width as functions of the time of deposition, are consistent with the Stranskii-Krastanov growth model. The copper, deposited at room temperature, is at low coverages in a non-metallic state. During the deposition of Cu, a mixed monolayer of non-metallic and metallic Cu is formed, and finally the deposition is producing on top of it a continuous layer of metallic Cu.
1. Introduction
The study of the interaction between metal particles and oxide surfaces is of great importance in metallurgy, ultra-thin film microelectronics, ceramics and in heterogeneous catalysis. In the latter the interaction plays a crucial role for the activity, selectivity and stability of the catalyst. Oxide-supported metal catalysts have been widely studied in powder form while relative few studies have been carried out on single crystals. It is generally supposed that the active part of a supported metal catalyst is the free surface of the metal particles and that the interaction between the metal particles and the support stabilizes the small metal particles by preventing migration and sintering. In the case of oxide-supported copper catalysts, which are widely used for the water gas shift reaction and for synthesis of methanol, the chemical state of the active sites are still debated. In recent Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) studies on the electronic structure changes during the synthesis of ultra-thin layers of copper on MgO 0169-4332/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
144
1. Alstrup. P.J. Moiler / Vapor deposition of copper onto MgO(lO0)
crystal surfaces [1,2] it was proposed that the copper initially appears in a Cu(I) state, in agreement with a recent ab initio calculation [3]. In the present work the electronic state of the copper as a function of the build-up of an ultra-thin layer on MgO(100) is investigated by XPS so as to further elucidate the process of changes of electronic state with increasing deposits of copper.
2. Experimental procedure Copper deposition was carried out at room temperature in a surface spectrometer (VG Scientific Ltd., UK) equipped with facilities for XPS, UPS, and AES. An attached preparation chamber is equipped with an ion sputter gun (VG Scientific Ltd., AG2) and an evaporator cooled by liquid N 2 during evaporation. The background pressure was about 10 10 Tort (1 Tort = 133.3 Pa) in the analysis chamber and about 5 × 10 l0 Torr in the preparation chamber. A Cu wire of 99.995% purity (Johnson Matthey Ltd., UK) was used as an evaporation source. The MgO crystal (W&C. Spicer & Co. Ltd., UK) was cleaved in air along a (100) plane before introduction into the spectrometer. After baking the spectrometer overnight at 450 K the sample was heated to about 680 K for about 10 h. Upon a preliminary Cu deposition the crystal was cleaned by cycles of Ar + bombardment (2 kV, 3.5 ktA) and heat treatment (610 K in U H V for 10 rain). Subsequently the sample was heated in 10 6 Torr oxygen at about 570 K for 1 h. This procedure is known to give an excellent LEED pattern [2,4] indicating a well-structured surface. The only impurity seen in the XPS spectra was carbon. The carbon peak area corresponded to a carbon coverage of about 0.05 monolayer. The first part of the Cu deposition consisted of six depositions each of an evaporation time of 60 s, aimed at a deposition of 0.2 monolayer each. The second part consisted of four depositions each of 300 s evaporation time. Cu and Mg photoemission and X-ray excited Auger peaks as well as oxygen photoemission peaks were recorded after each evaporation step. AI K a radiation, a constant pass energy of 50 eV and a 4 mm slit width were used for all recordings. The intensities of the peaks were determined as the total counts minus background counts in an energy region bracketing the peak or as in the case of the Cu 2p spectrum - the two peaks. For the Cu 2p peaks a non-linear background was constructed using the method of Shirley [5] while for the other peaks a linear background was used.
3. Results Fig. 1 shows the intensity ratios R ( C u / M g ) = l(Cu 2 p ) / l ( M g ls) and R(MglK ) = l(Mg l s ) / l ( M g KL2.3L2,3) plotted as functions of the deposition
I. Alstrup, P.J. Mailer / Vapor deposition of copper onto MgO(100)
50
2.5
40
2.0
145
A
,~
m t~
---
30
].5 .o m
~
2O c
0.5
10
D
5
T0
15
Deposition time
Fig. 1. XPS intensity ratios:
i
L
i
20
25
3O
0
"r(min)
R(Cu/Mg)=/(Cu2p)/l(Mg]s) and R(Mg~K)=/(MgIs)/
/(Mg KL2,3L2,3)as functions of Cu deposition time during growth upon MgO(100).
time ~-. The binding energies, BE, (not corrected for charging, and relative to the Fermi level) of Cu 2p3/2 and of Mg ls, respectively, are plotted as functions of ~- in fig. 2. The charging-up is seen initially to increase until T = 6 rain. With the following depositions the charging-up rapidly decreases and finally disappears at • = 21 rain, indicating a formation of a continuous copper layer at that time. The differences in energy between a photoemitted electron and its associated Auger electron, the so-called Auger parameters, a c , and aMg, were calculated as the sum of the kinetic energy of the Auger peak
1. Alstrup. P.J. Moller/ Vapor deposition of copper onto MgO(lO0)
146
945 1315
940 --
-
935 ~=
>~
1310
m
1305 -Cr'-
93O 1300
L
l i 10 Disposition time,
0
, 20 T(min)
i 30
Fig. 2. Binding energies, BE, not corrected for charging and relative to the Fermi level, ol C u 2 p ~ / ; and M g l s electrons as functions of Cu deposition time T during growth upon MgO(100).
2485
1852
Cu
© 2484
~ 1851
E
a~ =:e
2483
1850
Cu~0
1849
,
1__ 10
~
_~ 20
2482 3O
Deposition time v(min) Fig. 3. Auger parameters, ~('u and c~Mg, as functions of Cu deposition time during growth upon MgO(lO0). ~(,, and aMg are determined within +0.1 eV.
L Alstrup, P.J. Moiler/ Vapordeposition of copperonto MgO(lO0)
147
= 11 =10
.cz
~L.
v
6
1
I
1
1
1
I
1
O
5
10
15
20
25
30
Deposition time
-r(min)
Fig. 4. Effective peak width AWc. of the Cu 2p3/2 peak as a function of Cu deposition time ~during growth upon MgO(100). AWcu= ~A/h, where A is the peak area and h is the peak height. and the BE of the largest photoemission peaks, i.e. Cu 2p3/2 and Mg ls for Cu and Mg, respectively. The Auger parameters are plotted as a function of ~- in fig. 3. Initially at 1 min deposition, the aCu value is within the range reported for Cu2 ° [6]. aco increases rapidly with ~ and saturates at a value within the range reported for metallic Cu [6] at ~"= 16 min. A n effective peak width, AWcu of the Cu 2p3/2 peak was determined as one half the ratio of the peak area to the peak height of the peak. AWcu increases linearly with ~- in the first 11 min, as shown in fig. 4, after which it decreases during the next period of deposition, reaching a value appropriate for metallic Cu. Some of the peaks due to the Cu L3M4,sM4, 5 Auger electrons are shown in fig. 5. The shape of the Auger peaks for T = 2, 3, 4, 5 and 6 min, which is not shown, does not differ significantly from the peak at T = 1 rain. We note that at ~- = 26 min the shape of the peak is not to be distinguished from the well-established Auger peak of pure Cu. The shape of the other peaks in the spectrum did not change significantly except for the above-mentioned broadening of the Cu 2p peaks (fig. 4).
4. Discussion In figs. 1 - 4 we clearly observe two distinct deposition regions. The intensity ratios, the binding energies and the Auger parameters show one break point at
148
I. Alstrup, P.J. Moiler / Vapor deposition of copper onto MgO(lO0)
time
•jf//•TDepesLtion (min)
26
21 .t=
_,,J
,3
I
I
I
605 597 589 581 573 565 557 ~indin 9 eneTgy, BE (eV)
Fig. 5. X-ray excited Cu L3M4,sM4,s Auger spectra at increasing deposition times of Cu upon MgO(100). approximately 6 min of deposition. These results are thus consistent with the Stranskii-Krastanov model for the kinetic growth mechanism, the initial linear section showing the two-dimensional completion of the first monolayer (ML) followed by growth of nuclei. This growth mechanism for the C u / M g O
L Alstrup, P.J. Moiler / Vapor deposition of copper onto MgO(lO0)
149
system, which was suggested previously on the basis of A E S / E E L S / L E E D experiments [2], is hence confirmed. The value of ac~ for low r indicates, as mentioned above, that the low-coverage deposited Cu atoms are in a Cu(I) state. It should be noted, however, that this interpretation depends on the assumption that a fraction of a M L of C u 2 0 will have essentially the same Cu Auger and core electron energies as bulk Cu20. To our knowledge, this assumption has not yet been substantiated. At higher coverages the increase in the Auger parameter and in the width of the C u 2 0 peaks indicate that increasing amounts of the copper are present in the metallic state. The results for the last three depositions (16, 21 and 26 min, respectively) indicate that metallic Cu now completely covers the sample and that the last two depositions contribute mainly to a thickening of the metallic overlayer. Figs. 1 - 4 further indicate that 11 min of deposition has resulted in an intermediate situation in which a complete metallic overlayer has not yet been formed. It is possible to determine the coverage Ocu on a MgO(100) surface as a function of r in the first part of the deposition sequence from the data of fig. 1, applying the following assumptions: (1) that the inelastic mean free paths, )~,, may be calculated from the formula suggested by Seah and Dench [7] for elements, and (2) that the two sets of intensity ratios give approximately the same coverages. Hence, OCu/"2 p
R ( C u / M g ) = R ( C u / M g ) ~ 1 ~ 0-c~F1s ' 1 - o(.~rls
R ( M g ' K ) = R ( M g ' K ) ~ 1 - OCurKCL '
(1)
(2)
where ~, = 1 - e x p [ - 1 / ( ; k , sin rp)].
(3)
In the present experiments, sin qo = 0.755, where qo is the electron escape angle. R ( C u / M g ) ~ and R ( M g 1K)~ are the bulk intensity ratios. Using ~2p = 4.65 ML, )kls = 2,62 ML, )~KCL= 6.82 ML, solving for Oc,~ in each of eqs. (1) and (2) and equating the two obtained expressions for 0cu we obtain R ( C u / M g ) o o = 4 . 2 2 and R ( M g l K ) ~ = 1.80. Then the coverages Ocu may be determined for all the values of r corresponding to 0co _< 1. Fig. 6 gives the calculated 0c. for each value of ~-. The two sets of intensity ratios have a different origin: one is calculated solely from signals from the substrate, while the other is calculated from signals both from the substrate and from the outermost layer. These two sets give the same, and hence consistent, values for 0cu. The amount of deposited Cu, as judged from the coverage changes A0c., are approximately the same for the second, third and fourth
150
1. Alstrup, P.J. Moller/ Vapor deposition of copper onto MgO(lO0)
×
1.0
x
,3
[]
&
¢xl
[]
05
0
I
O
i
2
1
I
I
I
I
~_
4 6 Deposition time, T(min)
Fig. 6, Coverages Ocu at increasing values of Cu deposition time ,r during growth of Cu upon MgO(100): (H) calculated from R(MgIK); (×)calculated from R(Cu/Mg). deposition while the first deposition apparently yields a considerably larger change. AO(. u after each of the fifth and sixth depositions is smaller and dependent on the intensity ratio used for the calculation. That the first deposition gave a larger AOcu value may be explained by the way ~- was determined. In these experiments significant evaporation was assumed to commence when a red glow could be observed through the output aperture of the evaporator. It is conceivable that it took a longer time in the first deposition to reach this point than in the later ones, and that a significant evaporation took place before the glow was evident in the aperture. The significantly smaller A0cu calculated for the intensity ratios upon the fifth and sixth depositions probably are not due to smaller amounts of deposited copper but rather to the onset of nucleation on top of the non-completed monolayer (Ocu -~ 0.8), causing some deviation from linearity in the Oc,u versus T plot and giving different results for the two intensity ratios. Thus the change in growth mode apparently starts earlier than judged from the break point in figs. 1 and 2. This earlier start may explain the low c~cu value measured after the sixth deposition and is in agreement with the leveling-off of the R(Mg,K ) ratio after the fifth deposition (fig. 1). A similar unambiguous quantitative interpretation of the results for the last four depositions is probably not possible and has not been attempted. The results in this study demonstrate that considerable information is contained in XPS spectra recorded during growth of thin films. In addition to the above results figs. 1-3 indicate further interesting changes in the MgO(100)
L Alstrup, P.J. Moiler / Vapor deposition of copper onto MgO(lO0)
151
surface during deposition. Clearly, significant changes in the electronic structure of the MgO surface take place as soon as the deposition is initiated, and again when deposition on top of the first layer begins. The latter change indicates that the first layer is changed by the deposition on top of it. Much of this information in the spectra, however, cannot be unraveled without results from supplementary techniques applied in situ.
5. Conclusion On the basis of XPS spectra it is indicated that an ultra-thin copper film vapor deposited upon a sputter-cleaned and annealed MgO(100) crystal surface grows according to a Stranskii-Krastanov mechanism. The island growth on top of the first layer starts, however, before completion of the first layer. This first layer is not homogeneous with regard to the electronic state of the deposited copper. At low coverages the copper is in the Cu(I) state while metallic Cu is increasingly contributing to the spectra at the higher coverages. Changes in the Auger parameter for Mg with copper coverage further indicate that changes in the electronic structure of the MgO, as previously indicated by EELS experiments, occur when the deposition starts and again when growth on top of the first layer starts, thus indicating that the electronic changes are induced in the first layer by the deposition of further Cu on top of it.
Acknowledgements We are grateful to Ane Blom for technical assistance and to Jian-Wei He for fruitful discussions.
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