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The oxygen O - KLL Auger spectra of cadmium oxide
Vacuum/volume 42/number Printed in Great Britain
15lpages 983 to 986/l 991
The oxygen
0-KLL
0042-207x/91 $3.00+.00 @ 1991 Pergamon Press plc
Auger spectra
of cadmium
oxide
C Jardin, Laboratoire de MirGralogie-Cristallographie (CNRS URA SOS), Universite’ Claude Bernard de Lyon 1, 43 Boulevard du 17 Novembre 1978, 69622 Villeurbanne Ce’dex, France
and M Ghamnir, received
M Bouslama
for publication
22 April
and
B Khelifa,
Laboratoire
d’optique,
Universite’ d’Oran, 37 100 Es-Senia,
Algeria
1991
The particular structure of KLL Auger emission of oxygen in cadmium oxide was investigated. The strong doublet detected at 510 and 515 eV, interpreted in terms of inter-atomic transition KVV adjacent to the intra-atomic line KL23L23, corresponds to an intrinsic behaviour of cadmium oxide under electron excitation. The high intensity of the inter-atomic component at 515 eV results from relaxation and screening effects connected to the presence of Cd-4d holes. This Auger structure is magnified in Cd0 because of the high cross-section for cadmium Auger emission combined to a relative long lifetime of the Cd-4d holes produced by the Auger cascade through the electron structure of CdO. This typical spectrum was observed from bulk Cd0 oxide as well as from the oxidized surface of metallic cadmium. Among the series of 4d metals, cadmium also displays a special oxygen affinity in the presence of electron irradiation.
1. Introduction Cadmium
c-KLL
Owing to their applications in electronic devices, the II-VI and III-V semiconductors have received much attention in the last two decades. Further developments of these materials require deep studies of their surface and interface behaviours. Especially, the structure of the thin interfacial oxide layers is very important in MOS structures, for example. It is the purpose of this paper to investigate the Auger spectra of oxidized metals of atomic number Z = 48-52 (Cd, In, Sn, Sb, Te), and particularly cadmium oxide which displays a typical 0-KLL Auger emission. Measurements were performed in the N(E) mode using a hemispherical analyser’. The sample was excited by a primary beam of tunable energy and current (standard values : E, = 1850 eV, J, = 10-j A cm-‘) and with a 20” grazing incidence. The polycrystalline samples were treated ex situ by a routine chemical cleaning procedure. To remove residual surface contamination, mainly composed of oxygen and carbon as detected by AES, the substrates were sputter cleaned by 500 eV argon ions after their introduction in the uhv system.
t Cd-M45N45N45
0-KLL
(a)
(b)
I 400
I 300
500
Kinetic energy (eV)
+
Figure 1. Auger spectra of polycrystalline cadmium (a) after introduction of the sample in the spectrometer; (b) after removal of the residual surface contamination by argon ion bombardment (energy of the incident Ar+ ions : 500 eV ; current : 3 PA ; sputtering time : 1 h).
2. Oxidation of cadmium After removal of carbon and oxygen impurities by argon ion bombardment, the surface of polycrystalline cadmium is characterized by Auger and loss spectra shown in Figures 1 and 2. Oxidation at room temperature of the cleaned surface occurs at very low oxygen partial pressure (po, = lo-* Pa) under the impact area of the electron beam (energy : 1850 eV ; current density : 10-j A cm- ‘). Comparing with the neighbour elements of atomic number Z = 4652 (Pd, Ag, Cd, In, Sn, Sb, Te) observed under similar experimental conditions, cadmium appears to be very sensitive to oxygen with the stimulation of the primary electron beam ; such particular oxygen affinity of cadmium is not apparent from thermodynamic data, i.e. the heat of formation of the oxides expressed in kJ mole- ’ O2 which may be used
as an indication
of the metal-oxygen
affinity2.
50
40
f
M
20
10
0
Loss energy (eV)
Figure 2. Loss spectra recorded at a primary energy EP = 1000 eV for a cleaned surface of metallic cadmium and bulk Cd0 oxide.
Figure 3 illustrates the evolution of Cd-M,,N,,Nd5 and OKLL Auger spectra of cadmium and oxygen vs the observation time. Comparing with the initial metallic form interpreted in terms of multiplet splitting in the two-holes final state3-‘, the 983
C Jardin
et a/:
Oxygen
0-KLL
Auger
spectra
tively: 1.6,4.0, 3.2,0.2,4.4eV (kO.5 eV) for Cd, In, Sn, Sb, and Te. These values are in general agreement with those given by Sen er alh (1.3, 3.8, 5.5,0.S eV for Cd, In, Sn and Sb), except for tin ; looking at the shift of the main M,NN group, other values were measured for this metal: 2.3 eV as reported by Barlow et al’ and 3.9 eV by Wagner and Biloen8. Such a discrepancy may arise from different experimental conditions of the oxidation process (influence of the thickness and the nature-SnO, SnO,of the oxide). 3. The particular oxygen 0-KLL Auger spectra of cadmium oxide
(b) 360
\ 500
400
380
Kinetic energy (eV) GD
520
Kinetic energy (eV) +
Figure 3. Oxidation at 300 K of the cadmium surface under the impact area of the primary electron beam (Er = 1850 eV; J, = 10-j A cm-*; oxygen partial pressure po, - 10-s Pa). Consecutive (a) CddM,,N,,N,, and (b) 0-KLL Auger spectra were recorded after an observation time of about 5 min. Auger emission from bulk Cd0 is shown for comparison.
M45N45N45 Auger spectra of oxidized cadmium loses fine structure and is shifted to lower kinetic energies. Changes in the structure of Cd-M,,N,,N,, emission are mainly governed by the alteration of the valence levels and relaxation effects during the oxidation process. The shift toward lower energy between MNN Auger spectra of metal and oxide is a general trend for elements of the investigated series (Figure 4). The energy differences, measured from the main Auger peak M,NN, are respec-
The oxygen Auger spectra of oxidized cadmium display a typical structure characterized by two prominent peaks at 510 and 515 eV (Figure 5). From our knowledge, such an unusual 0-KLL structure was not previously investigated and explained. A strong doublet in the 0-KL,,L,, group was only observed in some materials such as nitrate (KNO,), chlorate (KCIO,) and some molecules in the vapour phase’ ’ ‘. Usually, the 0-KLL spectra of metal oxides are of quasi-atomic character with a multiplet structure arising from coupling in the final state: KL,L, (‘S) ; KL,L,, (‘P and ‘P) and the main peak KL,,L,, (‘S combined to the predominant ‘D component). The widths of these lines, their absolute and relative kinetic energies and their relative intensities may be changed with the nature of the oxide (Figure 6) but the overall shape of the spectra is preserved. For instance, of the oxygen ionicity reduces it was shown that enhancement
0-KLL
Cd0
KVV-----------------__i I
I
480
I
Figure oxide
5. Structure
of the oxygen
Oxygen
energy
520
(eV)
0-KLL
0-KLL
I
I
500 Kinetic
+
Auger
spectra
of cadmium
h
Te @) % Z0ev,
(a)
Kinetic energy (eV) + nmeric energy (e v) + Figure 4. Structure of M,,N,,N,, Auger tin and tellurium. (a) Clean polycrystalline surfaces. 984
spectra of cadmium, metals. (b) Partially
indium, oxidized
Figure 6. Oxygen samples.
Auger
spectra
from
pure
MgO,
cc-Alz03 and SiOz
C Jardin
eta/:
Oxygen
0-KLL
Auger spectra
the E(KL,,L,,)-E(KL,L,,) energy difference and increases the I(K2,L,,)/I(KL,L,,) and I(KL,,L,,)/I(KL,L,) relative intensities’.’ *.’ 3; the full width at half maximum A.!?,/, of the main OKL23L23 peak is also decreased“‘. Nevertheless, different mechanisms may be responsible for extra-structure on the 0-KLL spectra.
3.1. Inelastic effects. Auger electrons suffer energy losses during their emission from the solid. A maximum loss intensity due to volume plasmon excitation is observed for most of metal oxides in the range 20-30 eV from the parent peak ; so, 0-KL ,L,, and 0-KL ,L , Auger peaks located at about 20-30 eV from the main component 0-KL,,L,, will be more modified by such an inelastic effect. Furthermore, loss structure in the Auger spectra is easily detected from the comparison with the loss spectra adjacent to the elastic peak. This is demonstrated in Figure 7 in the case of beryllium oxide BeO. The plasmon gain line is also possible if decay of the plasmon occurs during the Auger transition with a transfer of its energy to the ejected electron” 16. Because of the different lifetimes of the processes, there is a low probability for such an event in most cases. So, inelastic effects cannot explain the particular structure observed for CdO. 3.2. Juxtaposition of different oxides. The spectra may be the result of superimposed 0-KLL emissions of different oxides as two distinct phases, bulk oxide and adsorbed or ‘episorbed’ oxygen”. This was observed, for example, in the study of the oxidation of chromium’8 or nickel (Figure 8). In the case of oxidized cadmium, the particular 0-KLL structure is detected for surface oxide (resulting from the oxidation of metallic cadmium as described in the previous section) as well as for bulk Cd0 oxide characterized by electron diffraction. Moreover, the structure of the spectra is not changed by heat treatments in vacuum of these samples. These observations suggest that only one oxide (Cd0 type) is involved, in agreement with previous LEED-AES results of Joyner et al ’ 9. The relative intensity of the oxygen Auger peaks is almost independent of the primary energy, the current density (in the checked ranges: l-2 keV, 10~4-10~2 A cm-*) and the observation time. So, electron beam induced defects-as observed, for instance2’, in other systems such as SiO,-are not detected on the Cd0 surface.
1 Be0
Kinetic energy (eV) Figure 7. Oxygen responding comparison
I
+
Auger spectra of beryllium oxide Be0 with the cor-
loss spectra (recorded at a primary of the inelastic effects.
energy EP = 500 eV) for
180
Kinetic ezrgy Figure 8. Example of superimposition the system
(eV) 2
of two different
0-KLL
spectra in
: oxygen on nickel.
The effect of the initial ion bombardment of the sample on the investigated Auger spectra may be also considered. Argon ion etching of the surface was only used to remove carbon impurity during the cleaning procedure and no special attention has been devoted to the influence of ion dose, ion energy and ion current density on the chemical nature of the cadmium oxide. Nevertheless, it is well known that ion bombardment of an oxide often induces some damages: broken bonds, change of the stoichiometry, reduction of the oxide and increasing surface roughness. Because of the particular oxygen affinity of metallic cadmium (as described in Section I), we may expect that such possible defects are mainly removed after heat treatments of the sample in vacuum and after the first AES observations. 3.3. Contribution of different oxygen sites. For a given oxide, various oxygen sites with different chemical surroundings may contribute to the Auger signal. This situation corresponds to the previous one at the microscopic scale. For cadmium oxide, Cd0 (rock-salt structure), oxygen sites are equivalent except for the top atoms because of the breaking of the three-dimensional solid by its surface. According to the analysis depth of the AES technique, the top surface and some volume atoms are always involved and their distinction is not usually possible from Auger spectra; only a broadening of the line is expected from such different contributions. 3.4. Charging effects and artefacts. Dealing with insulating materials such as oxides, care must be taken with charging phenomena. Although pernicious, charging effects are readily detected from a change of the excitation parameters’” : energy, current, incidence angle or position of the impact point of the primary beam on the sample (Figure 9). This was not observed for the investigated cadmium oxide. Moreover, no extra-structure related to charging or artefact effects is present on Cd-MNN Auger spectra adjacent to 0-KLL. 3.5. Inter-atomic transitions. Inter-atomic or cross-transitions in oxygen Auger spectra of metal oxides are usually observed, if present, as a shoulder on the high energy side of the main intraatomic 0-KL,,L2, peak*‘. They are interpreted in terms of deexcitation of the initial O-Is hole from the metal level of the valence band or from 0-2~ electrons of a nearest neighbour oxygen site. For ionic compounds, they are expected to have a low intensity compared with the dominant quasi-atomic Auger 985
C Jardin e? a/: Oxygen 0-KLL
Auger spectra In conclusion, the structure at the high energy side (at about 5 eV) of the intra-atomic 0-KL2,L2, line is attributed to an interatomic transition 0-KVV involving the previous effects related to the intrinsic behaviour of cadmium oxide under electron irradiation. The transition is inter-atomic in the sense that de-excitation of the inner 0-1.~ hole occurs through delocalized electrons of the valence band, not attached to the excited oxygen site, and resulting from the need of the long-lived Cd-4d holes to be screened and relaxed.
Kinetic energy (eV) + Figure 9. Charging effects on n-Ai,O,: different positions of the beam impact on the sample induce a shift of the Auger 0-KLL spectra.
lines of the anion, as observed for MgO or Be0 oxides (rock-salt structure similar to CdO). An alternative explanation is proposed to account for the strong doublet of the 0-KLL spectra of cadmium oxide, based on the following observations : (I) In ground state, Cd ‘* ion in Cd0 has a vacant s-orbital and the filled p-orbit& of oxygen ions constitute the valence band with a distinct filled 4d level of cadmium. This situation closely corresponds to the electronic structure of the related 3ti metal oxide ZnO” 24. (2) With a common energy of about 2 keV for the primary electron beam, cadmium is known to display a high cross-section for Auger emission and the Auger cascade leads to holes in the upper 4d level. Their lifetimes are governed by the rate of decay processes (delocalization in the valence band, emission of photons, generation of phonons, electron excitation.. .). The (relatively) small line-widths of the Cd-M,,N,,N,, components suggest that the two-hole final state localized in the 4d band has a relatively long lifetime*“. (3) Under the influence of primary electron irradiation, the density of states of the oxide and particularly the local DOS around the excited sites are changed. This depends on the available channels for the decay of the created holes. In some insulating compounds, primary electron excitation leads to the decomposition of the material due to a ‘coulombic explosion’ resulting from the Auger cascadeZh. Such an extrennz case is not observed for Cd0 but we may expect that the presence of longlived holes in the Cd-4d level induces some charge transfer from clcctrons of oxygen ions towards the cadmium sites.
986
References
’ Bui Minh Due, C Jardin, J P Gauthier and P Michel, J f/r~.v E: Sticnt /nslrum, 12,43 (1979). ‘J Moreau, In O.~_rdutiondcs MPtaus (Edited by J Bernard), Vol I. p 239. Gauthier-Vihars. Paris (1962). ‘T E Gallon and J D Nuttall, Surf& Sci, 53, 698 (1975). ‘P Weightman, J Php C: Solid St Phys, 9, I 1 17 ((1976) ‘A C Parry-Jones, P Weightman and P T Andrew% J PhJ’.\ C: So/id St Phn, 12, 1587 (1979). ’ P.Sen, M S Hedge and C N R Rao. Appl Surfucr Sci, IO, 63 (1982).
‘S M Barlow. P Bavat-Mokhtari and T E Gallon. J Phv.v C: 5’nlill , , ._ Iyt. Ph,l:r, 12,557; (1979j. liC D Wagner and P Biloen, Surfhcc~ Sci, 3582 (1973). ‘C D Wagner. D A Zatko and R H Raymond, AnuIyr Chum, 52, 1445 (1980). “‘R R Rye. T E Madey, J E Houston and P H Holloway. J Cheer Ph,,;.r, 69, 1504 (1978). ” W E Moddeman, T A Carlson, M 0 Krause, B P Pullen, W E Bull and G K Schweitzer, J Chem Ph~‘.c,55, 2317 (1971). “P Ascarelli and G Moretti, Surf&e Itzte~/ticcAncrl. 7,8 (1985). “R Weissmann and K Miiller, Su,-fuce Sci Rep, 105, 251 (1981). “C Jardin and D Robert, A,!$ Surfucc~ Sci, 35,495 (198889). ” L H Jenkins and M F Chung, Sur/lrcr Sci, 28,409 (197 I). “‘J C Fuggle R Lasser, 0 Gunnarson and K Schonhammer, Ph,r.v Rrr k/l. 44, lO9d (1980). “P Legare. G Maire, B Carl-i&e and J P Deville, Su&cc Sri. 68, 348 (1977). “C Jardin Bui Minh Due. J P Gauthier and P Michel, J Micro.rc Sy(‘(.I1.OSL.&?ron, 4, 55 (1979). “R W Joyner, M W Roberts and G N Salaita, Surface Sci. 84, L505 (1979). “‘C Jardin. Le Vidc, lcr Couches Minces. S243, 95 (1988). ” M Salmeron. A M Baro and J M Rojo, Phys Rec.. BI3,4348 (1976). “C R Brundle and R I Bickley, J C S Faraduy II, 75, 1030 (1979). ?jL Ley, R A Pollak, F R McFeely. S P Kowalczyk and P A Shirley, Plq..~ Ret:, B9. 600 (1974). ?’ L Fiermans, R HoogewiJs and J Vcnnik, .Sur/ircc, Sci, 47, 1 (1975). ?I P Weightman, P T Andrews and A C Parry-Jones, J Phys C: Solid Sr Ph,v.s, 12, 3635 (1979). “M L Knotek and P Feibelman, Surfucc Sci. 90, 7X (1979).
15lpages 983 to 986/l 991
The oxygen
0-KLL
0042-207x/91 $3.00+.00 @ 1991 Pergamon Press plc
Auger spectra
of cadmium
oxide
C Jardin, Laboratoire de MirGralogie-Cristallographie (CNRS URA SOS), Universite’ Claude Bernard de Lyon 1, 43 Boulevard du 17 Novembre 1978, 69622 Villeurbanne Ce’dex, France
and M Ghamnir, received
M Bouslama
for publication
22 April
and
B Khelifa,
Laboratoire
d’optique,
Universite’ d’Oran, 37 100 Es-Senia,
Algeria
1991
The particular structure of KLL Auger emission of oxygen in cadmium oxide was investigated. The strong doublet detected at 510 and 515 eV, interpreted in terms of inter-atomic transition KVV adjacent to the intra-atomic line KL23L23, corresponds to an intrinsic behaviour of cadmium oxide under electron excitation. The high intensity of the inter-atomic component at 515 eV results from relaxation and screening effects connected to the presence of Cd-4d holes. This Auger structure is magnified in Cd0 because of the high cross-section for cadmium Auger emission combined to a relative long lifetime of the Cd-4d holes produced by the Auger cascade through the electron structure of CdO. This typical spectrum was observed from bulk Cd0 oxide as well as from the oxidized surface of metallic cadmium. Among the series of 4d metals, cadmium also displays a special oxygen affinity in the presence of electron irradiation.
1. Introduction Cadmium
c-KLL
Owing to their applications in electronic devices, the II-VI and III-V semiconductors have received much attention in the last two decades. Further developments of these materials require deep studies of their surface and interface behaviours. Especially, the structure of the thin interfacial oxide layers is very important in MOS structures, for example. It is the purpose of this paper to investigate the Auger spectra of oxidized metals of atomic number Z = 48-52 (Cd, In, Sn, Sb, Te), and particularly cadmium oxide which displays a typical 0-KLL Auger emission. Measurements were performed in the N(E) mode using a hemispherical analyser’. The sample was excited by a primary beam of tunable energy and current (standard values : E, = 1850 eV, J, = 10-j A cm-‘) and with a 20” grazing incidence. The polycrystalline samples were treated ex situ by a routine chemical cleaning procedure. To remove residual surface contamination, mainly composed of oxygen and carbon as detected by AES, the substrates were sputter cleaned by 500 eV argon ions after their introduction in the uhv system.
t Cd-M45N45N45
0-KLL
(a)
(b)
I 400
I 300
500
Kinetic energy (eV)
+
Figure 1. Auger spectra of polycrystalline cadmium (a) after introduction of the sample in the spectrometer; (b) after removal of the residual surface contamination by argon ion bombardment (energy of the incident Ar+ ions : 500 eV ; current : 3 PA ; sputtering time : 1 h).
2. Oxidation of cadmium After removal of carbon and oxygen impurities by argon ion bombardment, the surface of polycrystalline cadmium is characterized by Auger and loss spectra shown in Figures 1 and 2. Oxidation at room temperature of the cleaned surface occurs at very low oxygen partial pressure (po, = lo-* Pa) under the impact area of the electron beam (energy : 1850 eV ; current density : 10-j A cm- ‘). Comparing with the neighbour elements of atomic number Z = 4652 (Pd, Ag, Cd, In, Sn, Sb, Te) observed under similar experimental conditions, cadmium appears to be very sensitive to oxygen with the stimulation of the primary electron beam ; such particular oxygen affinity of cadmium is not apparent from thermodynamic data, i.e. the heat of formation of the oxides expressed in kJ mole- ’ O2 which may be used
as an indication
of the metal-oxygen
affinity2.
50
40
f
M
20
10
0
Loss energy (eV)
Figure 2. Loss spectra recorded at a primary energy EP = 1000 eV for a cleaned surface of metallic cadmium and bulk Cd0 oxide.
Figure 3 illustrates the evolution of Cd-M,,N,,Nd5 and OKLL Auger spectra of cadmium and oxygen vs the observation time. Comparing with the initial metallic form interpreted in terms of multiplet splitting in the two-holes final state3-‘, the 983
C Jardin
et a/:
Oxygen
0-KLL
Auger
spectra
tively: 1.6,4.0, 3.2,0.2,4.4eV (kO.5 eV) for Cd, In, Sn, Sb, and Te. These values are in general agreement with those given by Sen er alh (1.3, 3.8, 5.5,0.S eV for Cd, In, Sn and Sb), except for tin ; looking at the shift of the main M,NN group, other values were measured for this metal: 2.3 eV as reported by Barlow et al’ and 3.9 eV by Wagner and Biloen8. Such a discrepancy may arise from different experimental conditions of the oxidation process (influence of the thickness and the nature-SnO, SnO,of the oxide). 3. The particular oxygen 0-KLL Auger spectra of cadmium oxide
(b) 360
\ 500
400
380
Kinetic energy (eV) GD
520
Kinetic energy (eV) +
Figure 3. Oxidation at 300 K of the cadmium surface under the impact area of the primary electron beam (Er = 1850 eV; J, = 10-j A cm-*; oxygen partial pressure po, - 10-s Pa). Consecutive (a) CddM,,N,,N,, and (b) 0-KLL Auger spectra were recorded after an observation time of about 5 min. Auger emission from bulk Cd0 is shown for comparison.
M45N45N45 Auger spectra of oxidized cadmium loses fine structure and is shifted to lower kinetic energies. Changes in the structure of Cd-M,,N,,N,, emission are mainly governed by the alteration of the valence levels and relaxation effects during the oxidation process. The shift toward lower energy between MNN Auger spectra of metal and oxide is a general trend for elements of the investigated series (Figure 4). The energy differences, measured from the main Auger peak M,NN, are respec-
The oxygen Auger spectra of oxidized cadmium display a typical structure characterized by two prominent peaks at 510 and 515 eV (Figure 5). From our knowledge, such an unusual 0-KLL structure was not previously investigated and explained. A strong doublet in the 0-KL,,L,, group was only observed in some materials such as nitrate (KNO,), chlorate (KCIO,) and some molecules in the vapour phase’ ’ ‘. Usually, the 0-KLL spectra of metal oxides are of quasi-atomic character with a multiplet structure arising from coupling in the final state: KL,L, (‘S) ; KL,L,, (‘P and ‘P) and the main peak KL,,L,, (‘S combined to the predominant ‘D component). The widths of these lines, their absolute and relative kinetic energies and their relative intensities may be changed with the nature of the oxide (Figure 6) but the overall shape of the spectra is preserved. For instance, of the oxygen ionicity reduces it was shown that enhancement
0-KLL
Cd0
KVV-----------------__i I
I
480
I
Figure oxide
5. Structure
of the oxygen
Oxygen
energy
520
(eV)
0-KLL
0-KLL
I
I
500 Kinetic
+
Auger
spectra
of cadmium
h
Te @) % Z0ev,
(a)
Kinetic energy (eV) + nmeric energy (e v) + Figure 4. Structure of M,,N,,N,, Auger tin and tellurium. (a) Clean polycrystalline surfaces. 984
spectra of cadmium, metals. (b) Partially
indium, oxidized
Figure 6. Oxygen samples.
Auger
spectra
from
pure
MgO,
cc-Alz03 and SiOz
C Jardin
eta/:
Oxygen
0-KLL
Auger spectra
the E(KL,,L,,)-E(KL,L,,) energy difference and increases the I(K2,L,,)/I(KL,L,,) and I(KL,,L,,)/I(KL,L,) relative intensities’.’ *.’ 3; the full width at half maximum A.!?,/, of the main OKL23L23 peak is also decreased“‘. Nevertheless, different mechanisms may be responsible for extra-structure on the 0-KLL spectra.
3.1. Inelastic effects. Auger electrons suffer energy losses during their emission from the solid. A maximum loss intensity due to volume plasmon excitation is observed for most of metal oxides in the range 20-30 eV from the parent peak ; so, 0-KL ,L,, and 0-KL ,L , Auger peaks located at about 20-30 eV from the main component 0-KL,,L,, will be more modified by such an inelastic effect. Furthermore, loss structure in the Auger spectra is easily detected from the comparison with the loss spectra adjacent to the elastic peak. This is demonstrated in Figure 7 in the case of beryllium oxide BeO. The plasmon gain line is also possible if decay of the plasmon occurs during the Auger transition with a transfer of its energy to the ejected electron” 16. Because of the different lifetimes of the processes, there is a low probability for such an event in most cases. So, inelastic effects cannot explain the particular structure observed for CdO. 3.2. Juxtaposition of different oxides. The spectra may be the result of superimposed 0-KLL emissions of different oxides as two distinct phases, bulk oxide and adsorbed or ‘episorbed’ oxygen”. This was observed, for example, in the study of the oxidation of chromium’8 or nickel (Figure 8). In the case of oxidized cadmium, the particular 0-KLL structure is detected for surface oxide (resulting from the oxidation of metallic cadmium as described in the previous section) as well as for bulk Cd0 oxide characterized by electron diffraction. Moreover, the structure of the spectra is not changed by heat treatments in vacuum of these samples. These observations suggest that only one oxide (Cd0 type) is involved, in agreement with previous LEED-AES results of Joyner et al ’ 9. The relative intensity of the oxygen Auger peaks is almost independent of the primary energy, the current density (in the checked ranges: l-2 keV, 10~4-10~2 A cm-*) and the observation time. So, electron beam induced defects-as observed, for instance2’, in other systems such as SiO,-are not detected on the Cd0 surface.
1 Be0
Kinetic energy (eV) Figure 7. Oxygen responding comparison
I
+
Auger spectra of beryllium oxide Be0 with the cor-
loss spectra (recorded at a primary of the inelastic effects.
energy EP = 500 eV) for
180
Kinetic ezrgy Figure 8. Example of superimposition the system
(eV) 2
of two different
0-KLL
spectra in
: oxygen on nickel.
The effect of the initial ion bombardment of the sample on the investigated Auger spectra may be also considered. Argon ion etching of the surface was only used to remove carbon impurity during the cleaning procedure and no special attention has been devoted to the influence of ion dose, ion energy and ion current density on the chemical nature of the cadmium oxide. Nevertheless, it is well known that ion bombardment of an oxide often induces some damages: broken bonds, change of the stoichiometry, reduction of the oxide and increasing surface roughness. Because of the particular oxygen affinity of metallic cadmium (as described in Section I), we may expect that such possible defects are mainly removed after heat treatments of the sample in vacuum and after the first AES observations. 3.3. Contribution of different oxygen sites. For a given oxide, various oxygen sites with different chemical surroundings may contribute to the Auger signal. This situation corresponds to the previous one at the microscopic scale. For cadmium oxide, Cd0 (rock-salt structure), oxygen sites are equivalent except for the top atoms because of the breaking of the three-dimensional solid by its surface. According to the analysis depth of the AES technique, the top surface and some volume atoms are always involved and their distinction is not usually possible from Auger spectra; only a broadening of the line is expected from such different contributions. 3.4. Charging effects and artefacts. Dealing with insulating materials such as oxides, care must be taken with charging phenomena. Although pernicious, charging effects are readily detected from a change of the excitation parameters’” : energy, current, incidence angle or position of the impact point of the primary beam on the sample (Figure 9). This was not observed for the investigated cadmium oxide. Moreover, no extra-structure related to charging or artefact effects is present on Cd-MNN Auger spectra adjacent to 0-KLL. 3.5. Inter-atomic transitions. Inter-atomic or cross-transitions in oxygen Auger spectra of metal oxides are usually observed, if present, as a shoulder on the high energy side of the main intraatomic 0-KL,,L2, peak*‘. They are interpreted in terms of deexcitation of the initial O-Is hole from the metal level of the valence band or from 0-2~ electrons of a nearest neighbour oxygen site. For ionic compounds, they are expected to have a low intensity compared with the dominant quasi-atomic Auger 985
C Jardin e? a/: Oxygen 0-KLL
Auger spectra In conclusion, the structure at the high energy side (at about 5 eV) of the intra-atomic 0-KL2,L2, line is attributed to an interatomic transition 0-KVV involving the previous effects related to the intrinsic behaviour of cadmium oxide under electron irradiation. The transition is inter-atomic in the sense that de-excitation of the inner 0-1.~ hole occurs through delocalized electrons of the valence band, not attached to the excited oxygen site, and resulting from the need of the long-lived Cd-4d holes to be screened and relaxed.
Kinetic energy (eV) + Figure 9. Charging effects on n-Ai,O,: different positions of the beam impact on the sample induce a shift of the Auger 0-KLL spectra.
lines of the anion, as observed for MgO or Be0 oxides (rock-salt structure similar to CdO). An alternative explanation is proposed to account for the strong doublet of the 0-KLL spectra of cadmium oxide, based on the following observations : (I) In ground state, Cd ‘* ion in Cd0 has a vacant s-orbital and the filled p-orbit& of oxygen ions constitute the valence band with a distinct filled 4d level of cadmium. This situation closely corresponds to the electronic structure of the related 3ti metal oxide ZnO” 24. (2) With a common energy of about 2 keV for the primary electron beam, cadmium is known to display a high cross-section for Auger emission and the Auger cascade leads to holes in the upper 4d level. Their lifetimes are governed by the rate of decay processes (delocalization in the valence band, emission of photons, generation of phonons, electron excitation.. .). The (relatively) small line-widths of the Cd-M,,N,,N,, components suggest that the two-hole final state localized in the 4d band has a relatively long lifetime*“. (3) Under the influence of primary electron irradiation, the density of states of the oxide and particularly the local DOS around the excited sites are changed. This depends on the available channels for the decay of the created holes. In some insulating compounds, primary electron excitation leads to the decomposition of the material due to a ‘coulombic explosion’ resulting from the Auger cascadeZh. Such an extrennz case is not observed for Cd0 but we may expect that the presence of longlived holes in the Cd-4d level induces some charge transfer from clcctrons of oxygen ions towards the cadmium sites.
986
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