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Auger spectra of magnesium and aluminium
SURFACE SCIENCE 32 (1972) 731-734 © North-Holland Publishing Co.
A U G E R SPECTRA OF M A G N E S I U M A N D A L U M I N I U M
Received 14 April 1972 High purety magnesium (99.9995%) and aluminium (99.999%) crystalline specimens have been studied by Auger spectroscopy. A three grid LEED system was used which was entirely built in our laboratory1) and which has been adapted to spectrometry studies by one of the authors2). A retarding-potential method is employed combined with a synchronous detection system and electronic differentiation which provides derived curves of the secondary electron energy distribution 3-5). The spectra were observed by varying the experimental conditions: primary electron energy (Ep = 500-2000 eV), incident beam angle with respect to the crystal, amplitude of the ac modulation Vpw The samples were subjected to successive ion bombardments and annealings (argon ion bombardment of 450 eV is used with a current of 4 IxA for Mg or 2 IxA for AI; annealing is carried out at 620°C for A1 and 350°C for Mg). The magnesium samples, in particular, had to be subjected to a large number of such cycles 6).
Auger spectra of magnesium (fig. 1) The clean Mg spectrum (fig. lc) consists of a main line at 46 eV which can be identified as the L23CC transition. The calculated energy of such a transition, using energy level tables 7, s) and taking into account the electron distribution in the metal 9), is 47 eV. Three further peaks or shoulders are observed on the low energy side of the main peak at 39 eV, 35.5 eV and 25.5 eV. A modulating voltage of larger amplitude Vpp(6 V) also reveals the presence of a structure at about 13 eV on the high energy side of the L23CC peak (fig. ld). The low energy structures can be interpreted respectively as the excitation, by the Auger electrons, of a surface plasmon, a volume plasman, and two volume plasmons. The results are in agreement with the work of Powell and Swan 1°,11) who observed peaks at 7.1 eV and 10.5 eV from the elastic peak in the Mg characteristic loss spectrum and which were identified as a surface plasmon and a volume plasmon. However, the structure observed at 35.5 eV is far more marked than the other two and must consist of both the 731
732
G. DUFOUR, H. GUENNOU
A N D C. B O N N E L L E
L23CC electrons exciting a volume plasmon and the Auger LtL23C electrons. An estimation of the type mentioned for the L23CC transition leads to an energy value of 37 eV. Moreover, it is possible that a slight surface oxidization effect contributes to the 35.5 eV peak intensity. It should be noted that the spectrum of superficially oxidized Mg (fig. la) has a main line at 34.5 eV. This can be interpreted as being due to the L23VVelectrons in the oxide: indeed X-ray spectro(o)
(b)
(c)
(d)
I! f.2.5
7-
i
uA
5
35.5
35.5
fi34"5j~_j~ 30 46
50
30
I
I
I
[
~o
5o
6o
3o
,4
51
I
so
so
30
~0
50
60
Energy(~zV)
Fig. 1. (a, b, c) Auger spectra of magnesium during progressive cleaning (Ep = 1000 V, Vpo= 3 V, glancing incidence); (d) clean Mg (Ep = 1000 V, Vpp= 6 V). scopy results °) give a shift of 4.5 eV for the electron distribution maximum in the valence band of the oxide with respect to that of the metal conduction band: for a more precise estimation of the transition energy, the energy changes in the Lt and L23 levels due to the modification in the outer-electron distribution, should be taken into account. A much weaker structure is observed at 27 eV in the oxidized Mg spectrum which, in a similar way, may be attributed to a transition of the type LtL23V in the oxide. The high energy structure observed for clean Mg could be due to aplasmon gaint2,13). However, the presence of structures either side of the main line at a distance approximately equal to the volume plasmon energy may also suggest an "inverse Raman effect". Such a mechanism could intervene between an Auger electron and the plasma wave the latter being excited mainly by the incident electrons, and would be compatible with the life-times. But we should also take into account possible interactions between the con-
A U G E R SPECTRA OF
MgA N D
A1
733
tinuum and the distribution of electronic states in the unoccupied conductibility band. Shapes and widths of the observed structures will be discussed in a latter paper in comparison with X-ray-spectroscopy results 0). Auger spectra of aluminium (fig. 2) Most of the transitions observed for magnesium were also observed in the aluminium spectra2): the energy of the main line (L23CC) in the clean A1 spectrum is 65 eV; the 51 eV and 37 eV peaks could be partly due to the (o)
(b) Ep=IOOOV
El: ) = 1 4 0 0 V Vpp=4V
Vpp= 5 V xl
c:
52
LaJ
r I I I 30 40 50 60
I
I
I
I
I
30 40 50 60 70 Energy (eV~
Fig. 2. Auger spectra of aluminium. (a) Oxidized A1; (b) clean A1.
excitation of one and two volume plasmons respectively, the latter corresponding also to the L1L23C Auger electrons. Our results are in agreement with other works carried out on this metal 14-16). G. DUFOUR,H. GUENNOU and C. BONNELLE Laboratoire de Chimie Physique Matidre et Rayonnement, Ddpendant de l' Universitd de Paris V1, Associd au C.N.R.S., 11, rue Pierre et Marie Curie, 75 - Paris V, France
734
G. DUFOUR,H. GUENNOU AND C. BONNELLE
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) ll) 12) 13) 14) 15) 16)
L. de Bersuder, Compt. Rend. (Paris)262 (1966) 1055. G. Dufour, Th6se de 36me cycle (June 1970). L. N. Tharp and E. J. Scheibner, Surface Sci. 8 0967) 247. R. E. Weber and W. T. Peria, J. Appl. Phys. 38 (1967) 4355. L. A. Harris, J. Appl. Phys. 39 (1968) 1419. H. Guennou, Th6se de 36me cycle, to be published. Y. Couchois, J. Phys. Radium 16 0955) 253. J. A. Bearden and A. F. Burr, Rev. Mod. Phys. 32 (1967) 125. C. Senemaud, J. Phys. (Paris) 32 (1971) 89. C. J. Powell and J. B. Swan, Phys. Rev. 116 (1959) 81. C. J. Powell and J. B. Swan, Phys. Rev. 118 (1960) 640. L. H. Jenkins and M. F. Chung, Surface Sci. 26 (1971) 151. J. A. D. Matthew and C. M. K. Watts, Phys. Letters 37A (1971) 239. D. T. Quinto and W. D. Robertson, Surface Sci. 27 (1971) 645. M. Suleman and E. B. Pattinson, J. Phys. F (Met. Phys.) 1 (1971) L 21. L. H. Jenkins and M. F. Chung, Surface Sci. 28 (1971) 409.
A U G E R SPECTRA OF M A G N E S I U M A N D A L U M I N I U M
Received 14 April 1972 High purety magnesium (99.9995%) and aluminium (99.999%) crystalline specimens have been studied by Auger spectroscopy. A three grid LEED system was used which was entirely built in our laboratory1) and which has been adapted to spectrometry studies by one of the authors2). A retarding-potential method is employed combined with a synchronous detection system and electronic differentiation which provides derived curves of the secondary electron energy distribution 3-5). The spectra were observed by varying the experimental conditions: primary electron energy (Ep = 500-2000 eV), incident beam angle with respect to the crystal, amplitude of the ac modulation Vpw The samples were subjected to successive ion bombardments and annealings (argon ion bombardment of 450 eV is used with a current of 4 IxA for Mg or 2 IxA for AI; annealing is carried out at 620°C for A1 and 350°C for Mg). The magnesium samples, in particular, had to be subjected to a large number of such cycles 6).
Auger spectra of magnesium (fig. 1) The clean Mg spectrum (fig. lc) consists of a main line at 46 eV which can be identified as the L23CC transition. The calculated energy of such a transition, using energy level tables 7, s) and taking into account the electron distribution in the metal 9), is 47 eV. Three further peaks or shoulders are observed on the low energy side of the main peak at 39 eV, 35.5 eV and 25.5 eV. A modulating voltage of larger amplitude Vpp(6 V) also reveals the presence of a structure at about 13 eV on the high energy side of the L23CC peak (fig. ld). The low energy structures can be interpreted respectively as the excitation, by the Auger electrons, of a surface plasmon, a volume plasman, and two volume plasmons. The results are in agreement with the work of Powell and Swan 1°,11) who observed peaks at 7.1 eV and 10.5 eV from the elastic peak in the Mg characteristic loss spectrum and which were identified as a surface plasmon and a volume plasmon. However, the structure observed at 35.5 eV is far more marked than the other two and must consist of both the 731
732
G. DUFOUR, H. GUENNOU
A N D C. B O N N E L L E
L23CC electrons exciting a volume plasmon and the Auger LtL23C electrons. An estimation of the type mentioned for the L23CC transition leads to an energy value of 37 eV. Moreover, it is possible that a slight surface oxidization effect contributes to the 35.5 eV peak intensity. It should be noted that the spectrum of superficially oxidized Mg (fig. la) has a main line at 34.5 eV. This can be interpreted as being due to the L23VVelectrons in the oxide: indeed X-ray spectro(o)
(b)
(c)
(d)
I! f.2.5
7-
i
uA
5
35.5
35.5
fi34"5j~_j~ 30 46
50
30
I
I
I
[
~o
5o
6o
3o
,4
51
I
so
so
30
~0
50
60
Energy(~zV)
Fig. 1. (a, b, c) Auger spectra of magnesium during progressive cleaning (Ep = 1000 V, Vpo= 3 V, glancing incidence); (d) clean Mg (Ep = 1000 V, Vpp= 6 V). scopy results °) give a shift of 4.5 eV for the electron distribution maximum in the valence band of the oxide with respect to that of the metal conduction band: for a more precise estimation of the transition energy, the energy changes in the Lt and L23 levels due to the modification in the outer-electron distribution, should be taken into account. A much weaker structure is observed at 27 eV in the oxidized Mg spectrum which, in a similar way, may be attributed to a transition of the type LtL23V in the oxide. The high energy structure observed for clean Mg could be due to aplasmon gaint2,13). However, the presence of structures either side of the main line at a distance approximately equal to the volume plasmon energy may also suggest an "inverse Raman effect". Such a mechanism could intervene between an Auger electron and the plasma wave the latter being excited mainly by the incident electrons, and would be compatible with the life-times. But we should also take into account possible interactions between the con-
A U G E R SPECTRA OF
MgA N D
A1
733
tinuum and the distribution of electronic states in the unoccupied conductibility band. Shapes and widths of the observed structures will be discussed in a latter paper in comparison with X-ray-spectroscopy results 0). Auger spectra of aluminium (fig. 2) Most of the transitions observed for magnesium were also observed in the aluminium spectra2): the energy of the main line (L23CC) in the clean A1 spectrum is 65 eV; the 51 eV and 37 eV peaks could be partly due to the (o)
(b) Ep=IOOOV
El: ) = 1 4 0 0 V Vpp=4V
Vpp= 5 V xl
c:
52
LaJ
r I I I 30 40 50 60
I
I
I
I
I
30 40 50 60 70 Energy (eV~
Fig. 2. Auger spectra of aluminium. (a) Oxidized A1; (b) clean A1.
excitation of one and two volume plasmons respectively, the latter corresponding also to the L1L23C Auger electrons. Our results are in agreement with other works carried out on this metal 14-16). G. DUFOUR,H. GUENNOU and C. BONNELLE Laboratoire de Chimie Physique Matidre et Rayonnement, Ddpendant de l' Universitd de Paris V1, Associd au C.N.R.S., 11, rue Pierre et Marie Curie, 75 - Paris V, France
734
G. DUFOUR,H. GUENNOU AND C. BONNELLE
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) ll) 12) 13) 14) 15) 16)
L. de Bersuder, Compt. Rend. (Paris)262 (1966) 1055. G. Dufour, Th6se de 36me cycle (June 1970). L. N. Tharp and E. J. Scheibner, Surface Sci. 8 0967) 247. R. E. Weber and W. T. Peria, J. Appl. Phys. 38 (1967) 4355. L. A. Harris, J. Appl. Phys. 39 (1968) 1419. H. Guennou, Th6se de 36me cycle, to be published. Y. Couchois, J. Phys. Radium 16 0955) 253. J. A. Bearden and A. F. Burr, Rev. Mod. Phys. 32 (1967) 125. C. Senemaud, J. Phys. (Paris) 32 (1971) 89. C. J. Powell and J. B. Swan, Phys. Rev. 116 (1959) 81. C. J. Powell and J. B. Swan, Phys. Rev. 118 (1960) 640. L. H. Jenkins and M. F. Chung, Surface Sci. 26 (1971) 151. J. A. D. Matthew and C. M. K. Watts, Phys. Letters 37A (1971) 239. D. T. Quinto and W. D. Robertson, Surface Sci. 27 (1971) 645. M. Suleman and E. B. Pattinson, J. Phys. F (Met. Phys.) 1 (1971) L 21. L. H. Jenkins and M. F. Chung, Surface Sci. 28 (1971) 409.