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Oxygen adsorption on thin cobalt films. An Auger and work function study
Vacuum/volume
Pergamon
48/number
J-g/pages 851 to 65311997 (0 1997 Elsevier Science Ltd
All riahts reserved. Printed in Great Britain
0042-207X/97 $1 J.OO+.OO
PII: soo42-207X~97ww52-3
Oxygen adsorption on thin cobalt films. An Auger and work function study G Benitez, J M Heras* and L Viscido, University
of La Plata, Institute
of Physical
Chemistry
(INIFTAI,
C.C. 16,
Sue. 4, (19001 La Plata, Argentina accepted in revised form 79 December 1996
The adsorption of oxygen by thin cobalt films supported on oxidized Sit 100)was studied at 300 K using Auger electron spectroscopy (AES) and work function changes (Acpl.The films were prepared in separate UHVsystems either by vapor deposition (PVDI or by pulsed laser ablation (PLA). The oxygen saturation exposure depends on the preparation method: in PVD-films at least 100 L are needed, while in films deposited by PLA only 70 L are required. However, films prepared by PLA, but on cleaved muscovite, showed uptake curves saturating at x 80 L as in evaporated films. The work function changes upon oxygen adsorption showed an is clearly initial increase of 0.2 eV, followed by a steep decrease, saturating at = - 1.2 eV. An oxide formation demonstrated by the AES MVV spectra of Co at exposures above 9 L. 0 1997 Elsevier Science Ltd. All rights reserved
Introduction The interaction of oxygen with Co and the oxide formation on Co has lately been intensively studied.‘-7 Due to the interest in the catalytic properties of Co and Co oxides, the aim of this paper is to report the results of oxygen adsorption and oxide formation on a system that mimics an industrial catalyst, Co/SiO,/Si( IOO), in which the Co was deposited by evaporation (PVD) or by pulsed Laser ablation (PLA). It is known that in PLA the high flux of ablated material onto the substrate produce high quality films at high growth rates. A comparison between oxygen uptake on Co films formed by PVD and PLA would give some information on the influence of a structural factor in the uptake kinetics. Experimental details Experiments were performed in two different UHV-systems operating at a base pressure of -2 x lo-“‘mbar. One equipped with photoelectric work function, Auger and mass spectrometry capabilities, the other (a Riber LDM-32 “) with AES, XPS, RHEED, ellipsometry and PLA capabilities. Substrates were prepared from Si(100) single crystals in which a thin SiO, film was grown either by 0: bombardment9 or by thermal oxidation at 773 K. A coiled filament heated by Joule effect was employed for PVD-films, while those prepared by PLA, a 10 x 10 x 0.6mm3 target sheet was used, (both Co, 99.9998% pure from Goodfellow Metals). The pulses were produced in a KrF laser (Lextra 200 of Lambda Physik) at a rate of 1 Hz, with 450mJ energy. The
*To whom all correspondence
should be addressed.
thickness of the Co layer was monitored by a quartz crystal microbalance (QCM) in PVD layers and in situ ellipsometry (Uvisel of Jobin Yvon) in the case of PLA deposits. Oxygen (Messer Griesheim 4.8) was dosed through a leak valve at pressures between 4 x 10m9to 2 x 10m8mbar. The oxygen uptake was monitored following the Auger signal ratio 105,3/1c0778, and in PVD layers, also the work function changes. These changes in WF were determined by the threshold shifts of the secondary electron energy distribution of the sample biased to - 27 V, upon excitation with the Auger electron gun.”
Results and discussion Deposition of the Co-layers Figure l(a) shows the Auger spectra of the clean Si(lOO), the oxidized surface (through 0: bombardment), and after PVD deposition of 10 monolayers (ML) of Co. This amount, derived from the QCM assuming an hcp-layer, was just enough to suppress the substrate Auger signals. Figure 1(b) shows the Auger spectra of PLA-prepared films onto 1?1uscouite, and onto the SiOl prepared thermally. Thicknesses were elipsometrically measured. Remarkably, considerable smaller thicknesses were required to suppress the substrate Auger signals with PLA deposition than with PVD. In the former case just 3 ML of Co were enough to supress the SiOz Auger signal, indicating very low roughness and the absence of islanding. However, upon annealing at 300 K, PLA deposited films showed a decrease in the ratio Co MVV at 54 eV to Co MVV at 778 eV, the substrate Auger signals simultaneously reappearing [Figure l(b) upper curve]. In the PVD films, practically no change was observed. Moreover, in thick PLA deposited films (x 15 ML) the Co signals ratio approached that of PVD films remaining practically con651
G Benitez et al: Oxygen adsorption
on thin cobalt films 1.2
1’1,
I’
I
I
r n
’
I
0
2.0
1.5
4 ML of Co on Muscovite
co
, 200
I
400
1
I
I
I
I
I
I
600
I
800
Kinetic Energy [eVl Figure 1. Auger spectra of the cobalt layers obtained by (a) vapor deposition (PVD), (b) pulsed laser ablation (PLA) deposition. The upper spectrum of part (b) shows the changes observed on the 3 ML of Co on Si02 after 16 h at 300 K in the UHV-chamber. More details in text.
0
20
40
60
80
100
120
02 exposure [L] stant upon heating up to 500K. Consequently, thin PLA deposited layers (z4ML) are unstable on SiO, and suffer a measurable agglomeration process even at 300 K. Structural studies with electron microscopies are being performed on PLA films. Preliminary results on thick films deposited onto muscouite indicate: (a) with SEM, no surface structure up to amplifications of (b) with STM, a fine 50,000X, (only small droplets =I-5pm); grained morphology, each grain being z 9 nm in diameter.
Figure 2. (a) Oxygen uptake curves for differently prepared Co films: (0) vapor deposited films 10 ML thick; (0) films deposited on to muscoui~eby pulsed laser ablation (PLA), 4 ML thick; (c) PLA films deposited on SiO,, 3 ML thick. More details in text. (b) Correlation between oxygen exposure and coverage on PVD deposited film. The coverage 0 was calculated from the Auger peak-to-peak signals, taking into account the relative sensitivity factor. The initial slope permits the calculation of the sticking coefficient of oxygen at 300 K. More details in text.
Oxygen adsorption. During exposure of the Co sample at 300 K to 02, the Auger spectra should be taken at a high scanning rate (Z 35 eV/s) in order to reduce: (i) beam damage of the surface and (ii) the difference in exposure between the detection of the OKLL_and the CoLMM peaks. At this acquisition speed, a careful study of information retrieval from noisy spectra was necessary. Taking into account that the ratio of the peak-to-peak heights of the OKLL at 5 13 eV to the CoLMM at 778eV was the primary information to construct the uptake curves, some spectra smoothing is necessary in order to minimize errors. This smoothing was successfully achieved by a spline method.“.‘2 The uptake curves derived from the OKLL to CoLMM peak-topeak ratio measured in the smoothed spectra as explained above, are compared in Figure 2(a) for the different preparation methods. Remarkably, on films PLA-deposited onto Si02 above IOL the uptake reduces considerably and practically saturates at 40L with a signal ratio which represents 25% of the value corresponding to evaporated films. This saturation value is comparable to the one reported in single crystals.6 In spite of the different UHV-systems used, the exposures measured on PVD and PLA films do not differ greatly because of a similar geometric arrangement of the leak valve and the ionization gauge. Hence, the oxygen uptake by Co-films PLA-deposited can be compared with those of PVD films.
The oxygen coverage (defined as the number of oxygen adatoms per Co surface atom) can be calculated assuming that the Auger relative sensitivity factor 0 5 13 to Co 778 is similar to that we found in a Co-sheet oxidized at 700K in an O2 atmosphere (aocO = 0.38). The Co Auger signal from the topmost layer was calculated from the exponential attenuation law. The results for adsorption at 300K on PVD deposited film are shown in Figure 2(b). From the initial slope, a sticking coefficient s, is obtained assuming that the surface density for the Co film is that of Co(OOO1): 1.85 x lOI me2. Accordingly, at room temperature s, 20.2. This value may be affected by an uncertainty of z 50% arising from the assumptions made and errors in the exposure measurement. However, within this uncertainty, it agrees very well with the sticking coefficient reported for Co(OOO1) single crystals (s, = 0.27).* Moreover, Figure 2(b) shows that the uptake curve behaves linearly up to ~20 L. This extended constant adsorption rate is interpreted by several authors2,3.6 as a dissociative adsorption through a precursor state of weakly bound molecular oxygen, which diffuses over the occupied sites. Figure 3 shows the true secondary electron distribution curves upon oxygen exposure from which the changes in work function (Acp) are determined by extrapolating the steepest slope of the curves until they cut the energy independent background. Acp showed an initial increase of 0.2 eV, followed by a steep decrease
652
G Benitez et a/: Oxygen adsorption on thin cobalt films
25
26
27
26 29 30 31 32 Kinetic Energy [eV’j
33
Figure 3. True secondary exposure
obtained
electron distribution curves upon oxygen with the CMA biasing the sample to - 27 V.
after = 10 L, saturating at z - 1.2 eV. The work function of clean Co films thoroughly annealed at 373 K is 5.12 f 0.03 eV,” while that of Co oxide is 4.01 eV.14 Hence upon oxidation of the Co film a decrease Acp- 1 eV is expected as experimentally found. An oxide formation at exposures above 9L is clearly demonstrated by AES. As Figure 4 shows, in the MVV spectra of Co, the peak at 54 eV of the clean surface develops upon oxidation a shoulder at 57 eV.
Conclusions Thin polycrystalline Co films are oxidized upon exposure to zz 9 L O,, even at 300 K. The value of the sticking coefficient at 300 K, s, = 0.2, is in agreement with the reported one for a Co(OOO1) single crystal.’ The constancy of s, up to z 20 L can be interpreted assuming that chemisorption is controlled by a molecular precursor. The maximum in the work function change also points to precursor mediated oxidation. The difference in oxygen uptake between PLA and PVD films is possibly due to the lower surface area of the former. Acknowledgements The authors acknowledge the financial support of the CONICET (Argentine Research Council) as well as the donation of equipment by the A von Humboldt, the Volkswagen Werk-Foun-
Figure 4. Changes exposure.
in the MVV Auger More details in text.
spectrum
of Co upon
oxygen
dations (Germany) and the International Program for Physical Sciences (IPPS, Sweden). They are also grateful to R Machorro, J Siqueiros, L Morales, M Farias, G Soto, J Bulicz, J Valenzuela and L Cota of the Institute of Physics, Ensenada Laboratory, Mexico University (UNAM) for skillful assistance in the preparation and characterization of pulsed Laser deposited Co films. References 1. Freund, H.-J. and Hohlneicher, G., Ber Bunsenge’s Phys Chem, 1979, 83, 100. 2. Bridge, M. E. and Lambert, R. M., Surf&i, 1979,82,413. 3. Castro, G. R. and Kiippers, J., Surf Sci, 1982, 123,456. 4. Wang, N-L, Kaiser, U., Ganschow, 0.. Wiedmann, L. and Benninghoven, A., Surf Sci, 1983, 124, 51. 5. Lahtinen, J., Vaari, J., Talo, A., Vehanen, A. and Hautojlrvi, P., Vacuum, 1990,41, 112. 6. Klingenberg, B., Grellner, F., Borgmann, D. and Wedler, G., Surf Sci, 1993,296, 374. I. Smardz, L., KCibler, U. and Zinn, W., J Appl Phys, 1992,71,5199. 8. Pertaining to the Institute of Physics, Ensenada Laboratory, Mexico University (UNAM). 9. Benitez, G., Carelli, J. L., Heras, J. M. and Viscido, L., Surf Interface Analysis, 1994. 22, 214. IO. Bachmann, G., Oechsner, H. and Scholtes, J., Fresenius Z Anal Chem, 1987,329, 195. 11. Hesse, R., Littmark, U. and Staib, P., Appl Phys, 1976, 11, 233. 12. Ramirez Cuesta, A., University of San Luis, Argentina, private communication. 13. Heras, J. M., Acia Cient Venezolana, 1980, 31, 308. 14. International Critical Tables, Vol VI. MacGraw-Hill, New York (1929) p 54.
653
Pergamon
48/number
J-g/pages 851 to 65311997 (0 1997 Elsevier Science Ltd
All riahts reserved. Printed in Great Britain
0042-207X/97 $1 J.OO+.OO
PII: soo42-207X~97ww52-3
Oxygen adsorption on thin cobalt films. An Auger and work function study G Benitez, J M Heras* and L Viscido, University
of La Plata, Institute
of Physical
Chemistry
(INIFTAI,
C.C. 16,
Sue. 4, (19001 La Plata, Argentina accepted in revised form 79 December 1996
The adsorption of oxygen by thin cobalt films supported on oxidized Sit 100)was studied at 300 K using Auger electron spectroscopy (AES) and work function changes (Acpl.The films were prepared in separate UHVsystems either by vapor deposition (PVDI or by pulsed laser ablation (PLA). The oxygen saturation exposure depends on the preparation method: in PVD-films at least 100 L are needed, while in films deposited by PLA only 70 L are required. However, films prepared by PLA, but on cleaved muscovite, showed uptake curves saturating at x 80 L as in evaporated films. The work function changes upon oxygen adsorption showed an is clearly initial increase of 0.2 eV, followed by a steep decrease, saturating at = - 1.2 eV. An oxide formation demonstrated by the AES MVV spectra of Co at exposures above 9 L. 0 1997 Elsevier Science Ltd. All rights reserved
Introduction The interaction of oxygen with Co and the oxide formation on Co has lately been intensively studied.‘-7 Due to the interest in the catalytic properties of Co and Co oxides, the aim of this paper is to report the results of oxygen adsorption and oxide formation on a system that mimics an industrial catalyst, Co/SiO,/Si( IOO), in which the Co was deposited by evaporation (PVD) or by pulsed Laser ablation (PLA). It is known that in PLA the high flux of ablated material onto the substrate produce high quality films at high growth rates. A comparison between oxygen uptake on Co films formed by PVD and PLA would give some information on the influence of a structural factor in the uptake kinetics. Experimental details Experiments were performed in two different UHV-systems operating at a base pressure of -2 x lo-“‘mbar. One equipped with photoelectric work function, Auger and mass spectrometry capabilities, the other (a Riber LDM-32 “) with AES, XPS, RHEED, ellipsometry and PLA capabilities. Substrates were prepared from Si(100) single crystals in which a thin SiO, film was grown either by 0: bombardment9 or by thermal oxidation at 773 K. A coiled filament heated by Joule effect was employed for PVD-films, while those prepared by PLA, a 10 x 10 x 0.6mm3 target sheet was used, (both Co, 99.9998% pure from Goodfellow Metals). The pulses were produced in a KrF laser (Lextra 200 of Lambda Physik) at a rate of 1 Hz, with 450mJ energy. The
*To whom all correspondence
should be addressed.
thickness of the Co layer was monitored by a quartz crystal microbalance (QCM) in PVD layers and in situ ellipsometry (Uvisel of Jobin Yvon) in the case of PLA deposits. Oxygen (Messer Griesheim 4.8) was dosed through a leak valve at pressures between 4 x 10m9to 2 x 10m8mbar. The oxygen uptake was monitored following the Auger signal ratio 105,3/1c0778, and in PVD layers, also the work function changes. These changes in WF were determined by the threshold shifts of the secondary electron energy distribution of the sample biased to - 27 V, upon excitation with the Auger electron gun.”
Results and discussion Deposition of the Co-layers Figure l(a) shows the Auger spectra of the clean Si(lOO), the oxidized surface (through 0: bombardment), and after PVD deposition of 10 monolayers (ML) of Co. This amount, derived from the QCM assuming an hcp-layer, was just enough to suppress the substrate Auger signals. Figure 1(b) shows the Auger spectra of PLA-prepared films onto 1?1uscouite, and onto the SiOl prepared thermally. Thicknesses were elipsometrically measured. Remarkably, considerable smaller thicknesses were required to suppress the substrate Auger signals with PLA deposition than with PVD. In the former case just 3 ML of Co were enough to supress the SiOz Auger signal, indicating very low roughness and the absence of islanding. However, upon annealing at 300 K, PLA deposited films showed a decrease in the ratio Co MVV at 54 eV to Co MVV at 778 eV, the substrate Auger signals simultaneously reappearing [Figure l(b) upper curve]. In the PVD films, practically no change was observed. Moreover, in thick PLA deposited films (x 15 ML) the Co signals ratio approached that of PVD films remaining practically con651
G Benitez et al: Oxygen adsorption
on thin cobalt films 1.2
1’1,
I’
I
I
r n
’
I
0
2.0
1.5
4 ML of Co on Muscovite
co
, 200
I
400
1
I
I
I
I
I
I
600
I
800
Kinetic Energy [eVl Figure 1. Auger spectra of the cobalt layers obtained by (a) vapor deposition (PVD), (b) pulsed laser ablation (PLA) deposition. The upper spectrum of part (b) shows the changes observed on the 3 ML of Co on Si02 after 16 h at 300 K in the UHV-chamber. More details in text.
0
20
40
60
80
100
120
02 exposure [L] stant upon heating up to 500K. Consequently, thin PLA deposited layers (z4ML) are unstable on SiO, and suffer a measurable agglomeration process even at 300 K. Structural studies with electron microscopies are being performed on PLA films. Preliminary results on thick films deposited onto muscouite indicate: (a) with SEM, no surface structure up to amplifications of (b) with STM, a fine 50,000X, (only small droplets =I-5pm); grained morphology, each grain being z 9 nm in diameter.
Figure 2. (a) Oxygen uptake curves for differently prepared Co films: (0) vapor deposited films 10 ML thick; (0) films deposited on to muscoui~eby pulsed laser ablation (PLA), 4 ML thick; (c) PLA films deposited on SiO,, 3 ML thick. More details in text. (b) Correlation between oxygen exposure and coverage on PVD deposited film. The coverage 0 was calculated from the Auger peak-to-peak signals, taking into account the relative sensitivity factor. The initial slope permits the calculation of the sticking coefficient of oxygen at 300 K. More details in text.
Oxygen adsorption. During exposure of the Co sample at 300 K to 02, the Auger spectra should be taken at a high scanning rate (Z 35 eV/s) in order to reduce: (i) beam damage of the surface and (ii) the difference in exposure between the detection of the OKLL_and the CoLMM peaks. At this acquisition speed, a careful study of information retrieval from noisy spectra was necessary. Taking into account that the ratio of the peak-to-peak heights of the OKLL at 5 13 eV to the CoLMM at 778eV was the primary information to construct the uptake curves, some spectra smoothing is necessary in order to minimize errors. This smoothing was successfully achieved by a spline method.“.‘2 The uptake curves derived from the OKLL to CoLMM peak-topeak ratio measured in the smoothed spectra as explained above, are compared in Figure 2(a) for the different preparation methods. Remarkably, on films PLA-deposited onto Si02 above IOL the uptake reduces considerably and practically saturates at 40L with a signal ratio which represents 25% of the value corresponding to evaporated films. This saturation value is comparable to the one reported in single crystals.6 In spite of the different UHV-systems used, the exposures measured on PVD and PLA films do not differ greatly because of a similar geometric arrangement of the leak valve and the ionization gauge. Hence, the oxygen uptake by Co-films PLA-deposited can be compared with those of PVD films.
The oxygen coverage (defined as the number of oxygen adatoms per Co surface atom) can be calculated assuming that the Auger relative sensitivity factor 0 5 13 to Co 778 is similar to that we found in a Co-sheet oxidized at 700K in an O2 atmosphere (aocO = 0.38). The Co Auger signal from the topmost layer was calculated from the exponential attenuation law. The results for adsorption at 300K on PVD deposited film are shown in Figure 2(b). From the initial slope, a sticking coefficient s, is obtained assuming that the surface density for the Co film is that of Co(OOO1): 1.85 x lOI me2. Accordingly, at room temperature s, 20.2. This value may be affected by an uncertainty of z 50% arising from the assumptions made and errors in the exposure measurement. However, within this uncertainty, it agrees very well with the sticking coefficient reported for Co(OOO1) single crystals (s, = 0.27).* Moreover, Figure 2(b) shows that the uptake curve behaves linearly up to ~20 L. This extended constant adsorption rate is interpreted by several authors2,3.6 as a dissociative adsorption through a precursor state of weakly bound molecular oxygen, which diffuses over the occupied sites. Figure 3 shows the true secondary electron distribution curves upon oxygen exposure from which the changes in work function (Acp) are determined by extrapolating the steepest slope of the curves until they cut the energy independent background. Acp showed an initial increase of 0.2 eV, followed by a steep decrease
652
G Benitez et a/: Oxygen adsorption on thin cobalt films
25
26
27
26 29 30 31 32 Kinetic Energy [eV’j
33
Figure 3. True secondary exposure
obtained
electron distribution curves upon oxygen with the CMA biasing the sample to - 27 V.
after = 10 L, saturating at z - 1.2 eV. The work function of clean Co films thoroughly annealed at 373 K is 5.12 f 0.03 eV,” while that of Co oxide is 4.01 eV.14 Hence upon oxidation of the Co film a decrease Acp- 1 eV is expected as experimentally found. An oxide formation at exposures above 9L is clearly demonstrated by AES. As Figure 4 shows, in the MVV spectra of Co, the peak at 54 eV of the clean surface develops upon oxidation a shoulder at 57 eV.
Conclusions Thin polycrystalline Co films are oxidized upon exposure to zz 9 L O,, even at 300 K. The value of the sticking coefficient at 300 K, s, = 0.2, is in agreement with the reported one for a Co(OOO1) single crystal.’ The constancy of s, up to z 20 L can be interpreted assuming that chemisorption is controlled by a molecular precursor. The maximum in the work function change also points to precursor mediated oxidation. The difference in oxygen uptake between PLA and PVD films is possibly due to the lower surface area of the former. Acknowledgements The authors acknowledge the financial support of the CONICET (Argentine Research Council) as well as the donation of equipment by the A von Humboldt, the Volkswagen Werk-Foun-
Figure 4. Changes exposure.
in the MVV Auger More details in text.
spectrum
of Co upon
oxygen
dations (Germany) and the International Program for Physical Sciences (IPPS, Sweden). They are also grateful to R Machorro, J Siqueiros, L Morales, M Farias, G Soto, J Bulicz, J Valenzuela and L Cota of the Institute of Physics, Ensenada Laboratory, Mexico University (UNAM) for skillful assistance in the preparation and characterization of pulsed Laser deposited Co films. References 1. Freund, H.-J. and Hohlneicher, G., Ber Bunsenge’s Phys Chem, 1979, 83, 100. 2. Bridge, M. E. and Lambert, R. M., Surf&i, 1979,82,413. 3. Castro, G. R. and Kiippers, J., Surf Sci, 1982, 123,456. 4. Wang, N-L, Kaiser, U., Ganschow, 0.. Wiedmann, L. and Benninghoven, A., Surf Sci, 1983, 124, 51. 5. Lahtinen, J., Vaari, J., Talo, A., Vehanen, A. and Hautojlrvi, P., Vacuum, 1990,41, 112. 6. Klingenberg, B., Grellner, F., Borgmann, D. and Wedler, G., Surf Sci, 1993,296, 374. I. Smardz, L., KCibler, U. and Zinn, W., J Appl Phys, 1992,71,5199. 8. Pertaining to the Institute of Physics, Ensenada Laboratory, Mexico University (UNAM). 9. Benitez, G., Carelli, J. L., Heras, J. M. and Viscido, L., Surf Interface Analysis, 1994. 22, 214. IO. Bachmann, G., Oechsner, H. and Scholtes, J., Fresenius Z Anal Chem, 1987,329, 195. 11. Hesse, R., Littmark, U. and Staib, P., Appl Phys, 1976, 11, 233. 12. Ramirez Cuesta, A., University of San Luis, Argentina, private communication. 13. Heras, J. M., Acia Cient Venezolana, 1980, 31, 308. 14. International Critical Tables, Vol VI. MacGraw-Hill, New York (1929) p 54.
653