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X-ray photoelectron and auger electron spectra of magnesium thin films
Thin Solid Films, 34 (1976) 99-102
© Elsevier Sequoia S.A., Lausanne-Printed in Switzerland
99
X-RAY PHOTOELECTRON AND AUGER ELECTRON SPECTRA OF MAGNESIUM THIN FILMS*
N. C. HALDER, J. ALONSO, Jr., AND W. E. SWARTZ,Jr. Department o f Physics, University o f South Florida, Tampa, Fla. 33620 (U.S.A.)
(Received August 25, 1975)
1. INTRODUCTION The purpose of this paper is to report our electron spectroscopy study of the oxidation of Mg thin films under a controlled background pressure. In particular we shall be concerned with X-ray photoelectron spectroscopy (XPS) of the valence and core levels and Auger electron spectroscopy (AES) of the KLL transitions. Recently many interesting papers 1-a on XPS and AES have appeared in the literature dealing with various lattice structures of the metal, e.g. single crystal, polycrystal and some alloys of Mg, but apparently a systematic analysis of the XPS and AES spectra of Mg in its two possible oxide phases has not been performed. Metallic Mg is highly reactive to 02 even under a very high vacuum (~10 -a Torr) and therefore thin films prepared under different background pressures must exhibit 9 the surface characteristics of the oxides in their respective XPS and AES results. 2. EXPERIMENTAL Several thin films of Mg of purity 99.999% were evaporated lO onto ultraclean microscope glass slides at pressures of 10-6-10 -8 Torr, with thicknesses ranging from 200 to 25 000 A. XPS measurements were made using the AI Kt~ X-ray line on an ESCA-36 spectrometer maintained at a pressure of about 10 -a Torr. The spectra were calibrated with reference to the valence band minimum of pure Au, which occurs at 5.0 eV, and the C (ls) line of pure C, which occurs at 284.8 eV. The films included in the present investigation identified as fdms 1,2 and 3, with thicknesses of 200, 1000 and 5000 A respectively, were prepared at a pressure of 10 -7 Torr at room temperature. Film 4, with thickness 25 000 A, was prepared at a pressure of 10 -6 Torr at room temperature. Film 5, with thickness about 200 A, was prepared at a pressure of 10 -a Torr at room temperature; the labels "film 5-B" and "film 5-A" refer respectively to spectra taken immediately after deposition and spectra taken 2 h later without breaking the vacuum. The details of the experimental procedure and the exact method of sample preparation are described elsewhere u'12 and the results of the AES of the above films are * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 5-03.
100
N.C. HALDER, J. ALONSO, Jr., W. E. SWARTZ, Jr.
Film
I
METAL
2
\ i
T --
i
Film
4
2
~21
STOICNIOMETRIC ' - -
CHEMISORBED~
E~
/
EV(2s)
/
-
-
(9)
Film (~) 3
i
OXIDES
EV(2p)
/
'
J
-
-
/
-
-
-
I 8
~
E p h GROUNDSTATE
8
i
I Film
I
5-A
(dl) Film
(A)
i
5-e
lSHOLE --yAL
I
2P2 HOLEEM(K~L)
CNEHISORBED STOICHIOHETR]C
(e)
11"
11~
If)
11" 11"
' 1155
' 1185
EK (eV)
Fig. 1. KLL Auger spectra of Mg thin films. Fig. 2. Energy level diagrams of Mg and its oxides: (A) photoelectron emission and X-ray emission involving various core levels; (B) effect of two types of oxidation on the one- and two-hole states of KLL Auger transitions.
shown in Fig. 1. The XPS spectra referred to as film 5-A and film 5-B were averaged since little difference was observed between the two sets of spectra. However, in the AES spectra some distinct differences were noted and therefore no averaging was attempted. In the present analysis the Fermi energy was normalized to zero and the binding energy (in eV) in the valence and core levels was obtained from E b = 1486.6 - 4.5 - Ek In this equation 1486.6 eV is the photon energy for the A1 Ke line, 4.5 eV is the spectrometer work function and Ek is the measured kinetic energy of the ejected photoelectrons. The AES results, however, do not depend on the photon energy 9' 13 and hence they are plotted directly in terms of the kinetic energy Ek of the photoelectrons. 3. RESULTS AND DISCUSSION
3.1. Valence and core electron spectra As mentioned earlier Mg is readily oxidized in the presence of very little 02, and in fact the valence band spectra in films 1-4 were hardly noticeable. The very strong peak of O (2p) at 7 eV is found to dominate the valence band Mg (3s) peak of Mg which is positioned at 2 eV in the pure metal. This peak at 2 eV, however, is found to
X-RAY PHOTOELECTRONAND AUGER ELECTRON SPECTRA OF Mg FILMS
101
shift towards 1 eV on gradual oxidation of the Mg to various Mg oxides. Even under a high vacuum of 10 -a Torr the O (2p) peak is still visible as in fdm 5. For the core electron spectra the shifts are found to occur in the opposite direction on oxidation of the Mg, with shifts of 0.6, 1.1 and 1.5 eV for the Mg (Is), Mg (2s) and Mg (2p)levels, respectively. Ley et aL 1 have studied the XPS of pure Mg films in the pressure range 3 x 10-2°-6 x 10 - n Torr and have observed some additional structure due to plasmon loss peaks. The positions of the peaks in the core electron spectra were 1303.0, 88.55 and 49.4 eV for the Mg (Is), Mg (2s) and Mg (2p) levels, respectively. These results compare closely with those of f'dm 5. Very recently Fuggle et al. 2 have also measured the XPS of pure polycrystalline Mg and some Mg compounds at a pressure of about 10 -1° Torr. The peak shifts they observed for Mg on oxidation were 1.9, 1.4 and 1.7 eV for the Mg (Is), Mg (2s) and Mg (2p) levels, respectively. The difference between the present study and that of Fuggle et al. 2 is that while we found a gradual increase in shift from the ls level to the 2p level for Mg, no such trend was noticeable in the latter study within their quoted error of + 0.2 eV. 3.2. K L L A E S transitions
It is abundantly clear from Fig. 1 that the AES spectra are much more sensitive to oxidation than the XPS spectra. We were able to record all the KLL transitions, including the KL 1L 3 (peak 6b) which is not easily observable 6. The peaks 1,4, 5, 6, 7 and 9 are due to pure Mg, whereas peaks 2, 3, 6b and 8 are due to Mg in MgO. The presence or absence of these peaks in a spectrum gives an indication of the percentage of MgO in Mg while the spectrum is being obtained. Our results for these observed transitions agree reasonably well with the earlier results of Wagner and Biloen 6, Ley et al. 1 and Fuggle et al. 2 However, there are differences here in that we studied films of different thicknesses, whereas Wagner and Biloen 6 studied solid Mg samples polished under controlled conditions and then subjected to a limited exposure to air, and Ley et al. 1 and Fuggle et al. 2 investigated polycrystalline films of a single thickness under a higher vacuum than that employed by us (~10 -8 Torr). 3.3. Interpretation o f the observed peak shifts
To find a possible interpretation for the observed peak shifts we also investigated the 02 spectra in every film. Specifically, the O (Is) line was found to shift from 531.8 to 530.5 eV as we moved from fdm 1 to film 5. This change was neither systematic nor gradual, thus predicting the existence of two different oxide formations 14-17 in the sample: the chemisorbed oxide and the stoichiometric oxide. Further evidence for these double phase oxides can be derived 18 from the two different kinds of shifts noted in the AES spectra of Mg. For example, in films 1--4 the average line shifts for the KL2L3, KL 1L a and KL1Lz transitions are about the same (6.2 eV) but quite different from the 4.7 eV shift found for these transitions in film 5. It is believed 11'1z that while the 6.2 eV shift is due to the formation of a stoichiometric oxide in films 1-4, the smaller shift in film 5 of 4.7 eV is due to the chemisorbed oxide. We represent in Fig. 2 the possible energy level diagrams I for the XPS and AES spectra of pure Mg and chemisorbed and stoichiometric MgO. As shown in the XPS case (Fig. 2(A)), a constant shift in E b was not observed for all the core level spectra because
102
N.C. HALDER, J. ALONSO, Jr., W. E. SWARTZ,Jr.
films 1-4 contained a mixture of both stoichiometric and chemisorbed MgO, the dominating component being stoichiometric MgO. On the other hand, Film 5 contained mostly chemisorbed MgO. This picture is more conspicuous in the AES diagram (Fig. 2(B)). We can then write the work function of the KLL transition energies (in eV) as ES(KLL) = EM(KLL) + • + 6.2 EC(KLL) = EM(KLL) + ~ + 4.7 where E s and E c are the energies for the stoichiometric and chemisorbed MgO respectively, and E M is the energy for the pure metal, defined by
eM(KLL) = eM(K) - eM(LL) A similar interpretation has also been proposed by Ley et al. 1 in their study of XPS and AES spectra of atomic and metallic Mg. REFERENCES 1 L. Ley, F. R. McFeely, S. P. Kowalczyk, J. G. Jenkin and D. A. Shirley, Phys. Rev. B, 11 (1975) 690. 2 J. C. Fuggle, L. M. Watson, D. J. Fabian and S. Afforssman, J. Phys. F, 5 (1975) 375. 3 J. T. Grant, M. P. Hooker, R. W. Springer and T. W. Hass, J. Vac. Sci. Technol., 12 (1975) 481. 4 A. P. Janssen, R. C. Schoonmaker and A. Chambers, Surf. Sci., 47 (1975) 41. 5 J. Tejeda, M. Cardona, N. J. Shevchik, D. W. Langer and E. S. Schonherr, Phys. Status Solidi B, 58 (1973) 189; J. Tejeda, N. J. Shevchik, D. W. Langer and M. Cardona, Phys. Rev. Lett., 30 (1973) 370. 6 C. D. Wagnerand P. Biloen, Surf. ScL, 35 (1973) 82. 7 M. Suleman and E. B. Pattington, Surf. Sci., 35 (1973) 75. 8 L. H. Jenkins and M. F. Chung, Surf. Sci., 33 (1972) 159. 9 L.V. Azaroff, X-ray Spectroscopy, McGraw-Hill,New York, 1974. 10 N.C. Halder and S. H. Hunter, Z. Naturforsch., 29a (1974) 1771. 11 N.C. Halder, J. Alonso, Jr., and W. E. Swartz, Jr., Phys. Rev. B (1976), to be published. 12 N. C. Halder, J. Alonso, Jr., and W. E. Swartz, Jr., Z. Naturforsch (1975), to be published. 13 K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johanson, T. Bergmark, S. Karlson, I. Lindgren and B. Lindberg, ESCA - Atomic, Molecular and Solid State Structure Studied by Means o f Electron Spectroscopy, Almqvist and Wiksell, Stockhohn, 1967. 14 G. Schon, Surf. ScL, 35 (1973) 96. 15 T. Robert, M. Barrel and G. Offergeld, Surf. Sci., 33 (1972) 123. 16 D. H. Loescher, Prec. 9th Rare Earth Conf., Blacksburg, U.S.A., 1971. 17 D. M. Holloway,Appl. Spectros., 27(1973)95. 18 E.J. Fernandez and D. M. Holloway, J. Vac. ScL Technol., 11 (1974) 612.
© Elsevier Sequoia S.A., Lausanne-Printed in Switzerland
99
X-RAY PHOTOELECTRON AND AUGER ELECTRON SPECTRA OF MAGNESIUM THIN FILMS*
N. C. HALDER, J. ALONSO, Jr., AND W. E. SWARTZ,Jr. Department o f Physics, University o f South Florida, Tampa, Fla. 33620 (U.S.A.)
(Received August 25, 1975)
1. INTRODUCTION The purpose of this paper is to report our electron spectroscopy study of the oxidation of Mg thin films under a controlled background pressure. In particular we shall be concerned with X-ray photoelectron spectroscopy (XPS) of the valence and core levels and Auger electron spectroscopy (AES) of the KLL transitions. Recently many interesting papers 1-a on XPS and AES have appeared in the literature dealing with various lattice structures of the metal, e.g. single crystal, polycrystal and some alloys of Mg, but apparently a systematic analysis of the XPS and AES spectra of Mg in its two possible oxide phases has not been performed. Metallic Mg is highly reactive to 02 even under a very high vacuum (~10 -a Torr) and therefore thin films prepared under different background pressures must exhibit 9 the surface characteristics of the oxides in their respective XPS and AES results. 2. EXPERIMENTAL Several thin films of Mg of purity 99.999% were evaporated lO onto ultraclean microscope glass slides at pressures of 10-6-10 -8 Torr, with thicknesses ranging from 200 to 25 000 A. XPS measurements were made using the AI Kt~ X-ray line on an ESCA-36 spectrometer maintained at a pressure of about 10 -a Torr. The spectra were calibrated with reference to the valence band minimum of pure Au, which occurs at 5.0 eV, and the C (ls) line of pure C, which occurs at 284.8 eV. The films included in the present investigation identified as fdms 1,2 and 3, with thicknesses of 200, 1000 and 5000 A respectively, were prepared at a pressure of 10 -7 Torr at room temperature. Film 4, with thickness 25 000 A, was prepared at a pressure of 10 -6 Torr at room temperature. Film 5, with thickness about 200 A, was prepared at a pressure of 10 -a Torr at room temperature; the labels "film 5-B" and "film 5-A" refer respectively to spectra taken immediately after deposition and spectra taken 2 h later without breaking the vacuum. The details of the experimental procedure and the exact method of sample preparation are described elsewhere u'12 and the results of the AES of the above films are * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 5-03.
100
N.C. HALDER, J. ALONSO, Jr., W. E. SWARTZ, Jr.
Film
I
METAL
2
\ i
T --
i
Film
4
2
~21
STOICNIOMETRIC ' - -
CHEMISORBED~
E~
/
EV(2s)
/
-
-
(9)
Film (~) 3
i
OXIDES
EV(2p)
/
'
J
-
-
/
-
-
-
I 8
~
E p h GROUNDSTATE
8
i
I Film
I
5-A
(dl) Film
(A)
i
5-e
lSHOLE --yAL
I
2P2 HOLEEM(K~L)
CNEHISORBED STOICHIOHETR]C
(e)
11"
11~
If)
11" 11"
' 1155
' 1185
EK (eV)
Fig. 1. KLL Auger spectra of Mg thin films. Fig. 2. Energy level diagrams of Mg and its oxides: (A) photoelectron emission and X-ray emission involving various core levels; (B) effect of two types of oxidation on the one- and two-hole states of KLL Auger transitions.
shown in Fig. 1. The XPS spectra referred to as film 5-A and film 5-B were averaged since little difference was observed between the two sets of spectra. However, in the AES spectra some distinct differences were noted and therefore no averaging was attempted. In the present analysis the Fermi energy was normalized to zero and the binding energy (in eV) in the valence and core levels was obtained from E b = 1486.6 - 4.5 - Ek In this equation 1486.6 eV is the photon energy for the A1 Ke line, 4.5 eV is the spectrometer work function and Ek is the measured kinetic energy of the ejected photoelectrons. The AES results, however, do not depend on the photon energy 9' 13 and hence they are plotted directly in terms of the kinetic energy Ek of the photoelectrons. 3. RESULTS AND DISCUSSION
3.1. Valence and core electron spectra As mentioned earlier Mg is readily oxidized in the presence of very little 02, and in fact the valence band spectra in films 1-4 were hardly noticeable. The very strong peak of O (2p) at 7 eV is found to dominate the valence band Mg (3s) peak of Mg which is positioned at 2 eV in the pure metal. This peak at 2 eV, however, is found to
X-RAY PHOTOELECTRONAND AUGER ELECTRON SPECTRA OF Mg FILMS
101
shift towards 1 eV on gradual oxidation of the Mg to various Mg oxides. Even under a high vacuum of 10 -a Torr the O (2p) peak is still visible as in fdm 5. For the core electron spectra the shifts are found to occur in the opposite direction on oxidation of the Mg, with shifts of 0.6, 1.1 and 1.5 eV for the Mg (Is), Mg (2s) and Mg (2p)levels, respectively. Ley et aL 1 have studied the XPS of pure Mg films in the pressure range 3 x 10-2°-6 x 10 - n Torr and have observed some additional structure due to plasmon loss peaks. The positions of the peaks in the core electron spectra were 1303.0, 88.55 and 49.4 eV for the Mg (Is), Mg (2s) and Mg (2p) levels, respectively. These results compare closely with those of f'dm 5. Very recently Fuggle et al. 2 have also measured the XPS of pure polycrystalline Mg and some Mg compounds at a pressure of about 10 -1° Torr. The peak shifts they observed for Mg on oxidation were 1.9, 1.4 and 1.7 eV for the Mg (Is), Mg (2s) and Mg (2p) levels, respectively. The difference between the present study and that of Fuggle et al. 2 is that while we found a gradual increase in shift from the ls level to the 2p level for Mg, no such trend was noticeable in the latter study within their quoted error of + 0.2 eV. 3.2. K L L A E S transitions
It is abundantly clear from Fig. 1 that the AES spectra are much more sensitive to oxidation than the XPS spectra. We were able to record all the KLL transitions, including the KL 1L 3 (peak 6b) which is not easily observable 6. The peaks 1,4, 5, 6, 7 and 9 are due to pure Mg, whereas peaks 2, 3, 6b and 8 are due to Mg in MgO. The presence or absence of these peaks in a spectrum gives an indication of the percentage of MgO in Mg while the spectrum is being obtained. Our results for these observed transitions agree reasonably well with the earlier results of Wagner and Biloen 6, Ley et al. 1 and Fuggle et al. 2 However, there are differences here in that we studied films of different thicknesses, whereas Wagner and Biloen 6 studied solid Mg samples polished under controlled conditions and then subjected to a limited exposure to air, and Ley et al. 1 and Fuggle et al. 2 investigated polycrystalline films of a single thickness under a higher vacuum than that employed by us (~10 -8 Torr). 3.3. Interpretation o f the observed peak shifts
To find a possible interpretation for the observed peak shifts we also investigated the 02 spectra in every film. Specifically, the O (Is) line was found to shift from 531.8 to 530.5 eV as we moved from fdm 1 to film 5. This change was neither systematic nor gradual, thus predicting the existence of two different oxide formations 14-17 in the sample: the chemisorbed oxide and the stoichiometric oxide. Further evidence for these double phase oxides can be derived 18 from the two different kinds of shifts noted in the AES spectra of Mg. For example, in films 1--4 the average line shifts for the KL2L3, KL 1L a and KL1Lz transitions are about the same (6.2 eV) but quite different from the 4.7 eV shift found for these transitions in film 5. It is believed 11'1z that while the 6.2 eV shift is due to the formation of a stoichiometric oxide in films 1-4, the smaller shift in film 5 of 4.7 eV is due to the chemisorbed oxide. We represent in Fig. 2 the possible energy level diagrams I for the XPS and AES spectra of pure Mg and chemisorbed and stoichiometric MgO. As shown in the XPS case (Fig. 2(A)), a constant shift in E b was not observed for all the core level spectra because
102
N.C. HALDER, J. ALONSO, Jr., W. E. SWARTZ,Jr.
films 1-4 contained a mixture of both stoichiometric and chemisorbed MgO, the dominating component being stoichiometric MgO. On the other hand, Film 5 contained mostly chemisorbed MgO. This picture is more conspicuous in the AES diagram (Fig. 2(B)). We can then write the work function of the KLL transition energies (in eV) as ES(KLL) = EM(KLL) + • + 6.2 EC(KLL) = EM(KLL) + ~ + 4.7 where E s and E c are the energies for the stoichiometric and chemisorbed MgO respectively, and E M is the energy for the pure metal, defined by
eM(KLL) = eM(K) - eM(LL) A similar interpretation has also been proposed by Ley et al. 1 in their study of XPS and AES spectra of atomic and metallic Mg. REFERENCES 1 L. Ley, F. R. McFeely, S. P. Kowalczyk, J. G. Jenkin and D. A. Shirley, Phys. Rev. B, 11 (1975) 690. 2 J. C. Fuggle, L. M. Watson, D. J. Fabian and S. Afforssman, J. Phys. F, 5 (1975) 375. 3 J. T. Grant, M. P. Hooker, R. W. Springer and T. W. Hass, J. Vac. Sci. Technol., 12 (1975) 481. 4 A. P. Janssen, R. C. Schoonmaker and A. Chambers, Surf. Sci., 47 (1975) 41. 5 J. Tejeda, M. Cardona, N. J. Shevchik, D. W. Langer and E. S. Schonherr, Phys. Status Solidi B, 58 (1973) 189; J. Tejeda, N. J. Shevchik, D. W. Langer and M. Cardona, Phys. Rev. Lett., 30 (1973) 370. 6 C. D. Wagnerand P. Biloen, Surf. ScL, 35 (1973) 82. 7 M. Suleman and E. B. Pattington, Surf. Sci., 35 (1973) 75. 8 L. H. Jenkins and M. F. Chung, Surf. Sci., 33 (1972) 159. 9 L.V. Azaroff, X-ray Spectroscopy, McGraw-Hill,New York, 1974. 10 N.C. Halder and S. H. Hunter, Z. Naturforsch., 29a (1974) 1771. 11 N.C. Halder, J. Alonso, Jr., and W. E. Swartz, Jr., Phys. Rev. B (1976), to be published. 12 N. C. Halder, J. Alonso, Jr., and W. E. Swartz, Jr., Z. Naturforsch (1975), to be published. 13 K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johanson, T. Bergmark, S. Karlson, I. Lindgren and B. Lindberg, ESCA - Atomic, Molecular and Solid State Structure Studied by Means o f Electron Spectroscopy, Almqvist and Wiksell, Stockhohn, 1967. 14 G. Schon, Surf. ScL, 35 (1973) 96. 15 T. Robert, M. Barrel and G. Offergeld, Surf. Sci., 33 (1972) 123. 16 D. H. Loescher, Prec. 9th Rare Earth Conf., Blacksburg, U.S.A., 1971. 17 D. M. Holloway,Appl. Spectros., 27(1973)95. 18 E.J. Fernandez and D. M. Holloway, J. Vac. ScL Technol., 11 (1974) 612.