Interatomic Auger analysis of the oxidation of thin Ba films

Applications of Surface Science 16 (1983) 125-138 North-Holland Publishing Company

INTERATOMIC AUGER ANALYSIS OF THE OXIDATION FILMS I. Characterization of the low energy Auger s p e c t r u m G.A. HAAS, C.R.K. MARRIAN

125

OF THIN Ba

a n d A. S H I H

Naval Research Laboratory, Washington, DC 20375, USA

Received 15 October 1982; accepted for publication 15 March 1983

The oxidation states of thin Ba films on Ir have been studied using an analysis of interatomic Auger lines. From published XPS data, some of the low energy AES lines were described in terms of various possible core, core, valence transitions where the core states were Ba states but the valence state could be either due to Ba or O. The magnitude of all AES lines, whose energy indicated an O valence state, was observed to increase with oxidation while those ascribed to Ba valence states decreased. The Ba 4d, 5p, valence transitions in the 65-75 V range appeared relatively free from clutter of other Ba and substrate lines and were the ones primarly used in determining the Ba-O bonding. The increase with oxidation of the interatomic Ba 4d, 5p, O 2p line at - 68 V was found to be correlated with the increase in the O 2p loss peak. Furthermore, shifts in energy of this peak for various film thicknesses varied in accordance with corresponding shifts in O 2p binding energy with respect to the Fermi level. Also, the decrease with oxidation of the Ba 4d, 5p, 6s line at - 73 V was found to be correlated with the decrease in conduction electrons contributing to the surface plasmon peak. The manner in which these lines can be used to determine the extent of electron transfer in the formation of compounds other than BaO (such as BaC 2) is also given.

1. Introduction It has b e e n l o n g r e c o g n i z e d [1] t h a t the l o w w o r k f u n c t i o n o f a t h i n Ba f i l m o n a m e t a l s u r f a c e is s t r o n g l y r e l a t e d n o t o n l y to t h e a m o u n t o f Ba b u t also a p p a r e n t l y to t h e d e g r e e o f o x i d a t i o n o f t h a t film. C a t h o d e s c u r r e n t l y u s e d in microwave tubes are generally composed of an impregnated metal matrix from w h i c h this s a m e t y p e o f f i l m is c o n t i n u o u s l y d i s p e n s e d to the surface. It is t h e r e f o r e v e r y d e s i r a b l e in t h e u n d e r s t a n d i n g o f m o d e m c a t h o d e s n o t o n l y to b e a b l e to c h a r a c t e r i z e t h e t h i c k n e s s b u t also t h e s t a t e o f the o x i d a t i o n o f t h e s e B a s u r f a c e films a n d p o s s i b l y d e t e r m i n e t h e i r r e l a t i o n to the c a t h o d e w o r k function. W h i l e n o s u r f a c e c o m p o s i t i o n a l t e c h n i q u e s existed w h e n t h e s e r e l a t i o n s w e r e first n o t e d , m o d e m t e c h n i q u e s s u c h as A u g e r e l e c t r o n s p e c t r o s c o p y ( A E S ) c a n n o w i d e n t i f y the c o m p o n e n t s o f the s u r f a c e a t o m s . T h e s e tech0378-5963/83/0000-0000/$03.00

© 1983 N o r t h - H o l l a n d

126

G.A. Haas et al. / Oxidation of thin Ba films. I

niques, however, have not been able to provide information on B a - O bonding and have also been hampered by inclusion into the AES signal of oxygen compounds (other than BaO) which normally occur in actual cathodes. Furthermore, the very slight shifts which occur in Ba core level energies during oxidation [2] do not provide a very definite means to characterize this surface layer. Because of observed distinct differences between Ba and BaO in the low energy Auger spectra [3], and the fact that the low energy AES lines are largely due to contribution from valence states, a more detailed study was initiated of this region of the Auger spectra to see if the interatomic nature of some of the core, core, valence transitions could be used to characterize the B a - O bonding of these surfaces. (Similar interatomic effects have been noted for example in MgO [4,5].) In Part I of this work (described in this paper), changes in the low energy Auger spectra were analyzed as a thin ( - 10-layer) Ba film on an Ir substrate was oxidized. From published XPS data, the AES lines were then described in terms of various possible core, core, valence transitions where the core states were Ba states but the valence states could be either due to Ba or O. It will be shown that the magnitude of all the lines ascribed to the O valence states increased with oxidation while those ascribed to Ba valence states decreased. One set of these transitions, the Ba 4d, Ba 5p, valence lines appeared to be relatively free from clutter of other Ba and substrate lines and therefore was used as a measure of the B a - O bonding. Specifically, the increase with oxidation of the interatomic Ba 4d, 5p, O 2p line was found to be correlated with the increase in the O 2p electron loss peak. Also, the decrease with oxidation of the Ba 4d, 5p, 6s line was found to be correlated with the decrease in conduction electrons contributing to the Ba surface plasmon peak. In part II of this work [6], the interatomic analysis was extended from - 10 layers down to fractional monolayer films of Ba and BaO on W surfaces as well as Ir. The work function variation with thickness and degree of oxidation was then determined for both of these substrates. From these calibrations, the exact coverage and degree of oxidation of actual impregnated cathodes was determined. Work function and compositional changes during oxidation, desorption, as well as activation of impregnated cathodes were also studied. Characteristics of surface compositional techniques used in this investigation, as well as descriptions of the low energy electron reflection (LEER) technique recently developed for work function analysis and the techniques and calibrations of B a / B a O deposition have all been reported by the authors elsewhere in this issue [7] as well as in previous issues of this journal, and will not be re-introduced here.

G.A. Haas et aL / Oxidation of thin Ba films. I

127

2. Low energy Auger spectrmn Fig. 1 shows a series of AES plots of a - 10-layer Ba film on Ir(100) as that film is being oxidized. The numbers on the fight of the graph refer to the extent of oxidation and are given in langrnuirs (1 L of 02 -- 104 Torr s or deposition - equivalent to a monolayer of O 2). For convenience in comparison to specific Auger transitions, the arrows on the data generally point to the middle of the differentiated signal (i.e. the peaks of the N(E) curve) rather than the bottom, as is usually done. It is seen that the AES characteristics for the clean Ba surface differ markedly from the oxidized surface. Certain lines appear to get bigger with oxidation while others decrease. For example, the - 8 V and - 20 V lines both increase while the ~ 56 V line decreases. Others, like the - 12 V line, seem to

5p, V, V 5s, 5p, V 5s, V, V

4d, 5s, V

4d, 5p, V

.,.

~

1.2L 3.1L

~

~"~

8.0/

19.1L

I

I

Fig.

l.Lowenergy Auger spectra taken during the oxidation of a -

0

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20

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I

40

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60

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I

80

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I

100

ENERGY (eV)

10-1ayer film of Ba on

Ir.

128

G.A. Haas et al. / Oxidation of thin Ba films. I

decrease with oxidation at low oxygen levels but then increase again at higher oxygen levels, while still others such as the - 40 V line do not seem to change appreciably. The lines at - 68 V and - 72 V are particularly distinctive in the way the former increases with oxidation while the latter decreases with oxidation. Some of the experimental AES lines of fig. 1 are tabulated in table 1 along with the manner in which they change during oxidation. In order to ascribe possible Auger transitions to these low energy AES lines, a list was made up of the binding energies (with respect to the Fermi level) of the upper core levels of Ba (as well as O). These are listed at the bottom of table 1 and represent "rounded-off" values which are generally within about a volt of the published XPS values. The possible Auger transitions that might be responsible for the various AES lines and their calculated energies are given in the third and fourth columns, respectively, of table 1. It is noted that there is a surprisingly good agreement between the experimental and calculated values

Table 1 Experimental AES energy (V) -8 -12

Variation with oxidation

Possible AES transition

Increases

Ba 5s, Ba 5p, O 2p

Decreases (Lo 0 2 )

Ba 5s, Ba 5p, Ba 6s

Calculated AES energy (V) 9.5 13

or

Increases (Hi 02)

Ba 5p, Ba 6s, Ba 6s O 2s, O 2p, O 2p

12 11

Increases

Ba 5s, O 2p, O 2p

20

-40

Remains nearly constant

Ba 4d, Ba 5s, Ba 5p

43

-52

Increases(?)

Ba 4d, Ba 5s, O 2p

53.5

- 56

Decreases

Ba 4d, Ba 5s, Ba 6s

57

68

Increases

Ba 4d, Ba 5p, O 2p

68.5

72

Decreases

Ba 4d, Ba 5p, Ba 6s

72

Increases(?)

Ba 4d, O 2p, O 2p

79

-

-

-

20

- 79

Binding energies a) (eV) Ba 4d Ba 5s O 2s Ba 5p O 2p Ba 6s

90 31 22 16 5.5 2

a) "Rounded-off" values, i.e. within - 1 Doveren [2] and Thomas et al. [9].

eV of published XPS data of Verhoeven and Van

G.A. Haas et al. / Oxidation of thin Ba films. I

129

especially in view of the uncertainty of assuming the XPS values for the binding energy during an actual Auger process. A particularly significant point in table 1 is that all the AES lines showing an increase during oxidation had an O 2p final state while those showing a decrease during oxidation resulted from Ba 6s electron final states. Furthermore, if neither of these appear in the final state (e.g., - 43 V), then there was little change with oxidation. There are also other possible transitions (e.g. in the 58 V region from Ba 4d, 5p, 5p), but the presence of these could not be distinguished from some of the other neighboring lines (which also included the Ir substrate for very thin films). Possible Auger lines originating from levels of higher binding energy (i.e. Ba 4p at 180 V) were not considered since the Auger spectra of fig. 1 were essentially the same at primary beam energies less than 180 V. On the upper portion of fig. 1 are shown the major Auger transitions responsiblef or the various groups * of lines. Because of the multiplicity of inputs complicating the spectra in the 10-20 and 50-60 V region, the majority of the effort was subsequently concentrated on the analysis of the Ba 4d, 5p, valence transition at - 68 and 72 V for characterizing the state of oxidation.

3. C h a r a c t e r i z a t i o n s o f t h e B a 4d, 5p, v a l e n c e t r a n s i t i o n s

3.1. Intensity variation with oxidation

Fig. 2a shows in a very simplistic way how the Ba 4d, 5p, valence transitions occur. The Ba 4d level is ionized by an incoming electron and a Ba 5p electron fills this vacancy. The energy released by this process (i.e. Ba 4d - Ba 5p ~ 74 V) is thus given to another electron in the system. This electron can be either another core electron or it can also be a valence electron such as the Ba 6s. In this latter case the 6s electron emerges with - 72 V of energy. If, however, the Ba has become oxidized, then this - 74 V is given to an electron whose energy is determined by the O 2p energy level in the BaO system. Since the valence electron in the BaO system has been decreased in energy (bound more tightly) by several volts compared to the Ba 6s energy, the electron emerging from the Ba 4d, 5p, valence transition in the interatomic BaO system will also have a similarly lower energy, viz. - 68 V. As is indicated on the right side o f fig. 2a, before oxidation then, one would expect the 72 V contribution to the Auger spectra to predominate. As oxygen is deposited and some of the Ba oxidizes, the - 6 8 V O 2p derived contribution should increase. However, since this

* More complicated spectra than simple doublets are generally present in the core, core, valence Auger transitions because of spin-orbit splitting of some of the states.

G.A. Haas et aL / Oxidation of thin Ba films. I

130

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AES ENERGY (aV)

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Ba4d, Ba5p, VALENCE TRANSITIONS

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4

6 8 10 12 14 16 OXYGEN EXPOSURE (LANGMUIRS)

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18

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20

(b)

Fig. 2. (a) Graphic representation of transitions giving rise to the Ba 4d, 5p, valence Auger lines and how these spectra change during oxidation. (b) Plot of peak heights of these two Auger lines during oxidation of a - 10-layer film.

must occur at the expense of losing valence electrons at the Ba 6s level, the - 72 V contribution should correspondingly decrease. As oxidation continues, the 68 V contribution would eventually dominate the spectrum. This is shown in graphic form in fig. 2b. In subsequent AES data presented in this paper, the standard convention [8] will be used of describing the AES line by the minimum in the d N / d E value rather than the mid-point. This, for example, changes the 72 V Ba 4d 5p, 6s Auger electron to the 73 V Ba AES line [8]. The O 2p contribution will be shown to actually range in energy from 67 to 70 V and for convenience is still referred to as the "68 V" peak.

3.2. Correlation of AES peaks with electron loss data While the O 2p derived peak at "68 V" increased with oxidation, as shown in figs. 1 and 2, electron loss data on this same 10-layer film also showed a

G.A. Haas et al. / Oxidation of thin Ba films. I Ba

131

MONOLAYERONIr

10LAYERSB a 02 EXPOSURE

60 I I I I I

70 ~

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OLO2

Ba ~

.•

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~!a SURFACE/ ,

~

.4L BaO

~~6V

SURFACE

~ SMONS 19L

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LOSS V

I I I I 30 20 10 0 ELS(eV)

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40

20 ELS(eV)

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0

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60

I I II

70 AES(eV)

80

Fig. 3. Variationof electron loss spectra (ELS) during oxidation of a 10-layerBa film on Ir.

Fig.4. Comparison of ELS and AES data during oxidation O 2p loss peak and corresponding rise in "68 V" peak.

of a monolayer Ba film on Ir. Note

rise

in

corresponding increase in the O 2p electron loss peak near - 68 V [9] (fig. 3). This same effect is also observed on monolayer films, as is shown in fig. 4. Here, too, the rise in "68 V" peak is accompanied by a corresponding rise in the O 2p loss peak. Similarly, the decrease of the 73 V (Ba 6s) peak is accompanied by a corresponding decrease in the density of Ba conduction electrons as is indicated by the height of the Ba surface plasmon peak [9]. (Correlations between the 73 V (Ba 6s) AES peak and density of conduction electrons as obtained from energy shifts of the surface plasmon peak have also been reported in ref. [ 10].)

3.3. Energy shifts of "'68 V'" 0 2p AES line While the - 7 3 V Ba 6s and - 7 5 V AES lines are relatively stable in energy, the same was not found for the "68 V" line. One of the factors

132

G.A. Haas et al. / Oxidation of thin Ba films. I

60 II

70 I I I

80 I I I I I

~10 LAYERS Ba OXIDIZED ",,20 L 02 HEATED 2 MIN AT:

vJ y,J

67.4 LAYERS OF OXIDIZED Ba

~

67.8

5 1.0 73

~

770K

950K

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5.0

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1455K

167.8

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I 50

I 60

I 70

AES ENERGY (eV)

I 80

I 90

60

70

80

AES ENERGY (eV)

Fig. 5. Change in low energy Auger spectra for different-thickness films of oxidized Ba on Ir. Note shift to lower energy of "68 V" peak with increasing thickness. Fig. 6. Change in low energy Auger spectra of oxidized 10-layer Ba film on Ir being heated to 1455 K. Note return to higher energy of "68 V" peak as BaO layer is desorbed.

affecting the energy was the layer thickness. This is shown in fig. 5 where it is seen that the "68 V" line shifts from over 69 V for a monolayer film to under 68 V for a 10-layer film. Similarly, as such a thick film is being thermally desorbed (fig. 6), the peak again shifts back to a value above 69 V (i.e. - monolayer) before being finally desorbed. (The stronger binding energy of the monolayer compared to thicker films of BaO is evident for T > 1100 K). In fig. 7 the shift in the "68 V" peak is plotted for various different BaO film thicknesses. A possible contribution to this shift in energy might be the change in binding energy of the filled band (i.e. O 2p states) for different thickness films, as is discussed below.

G.A. Haas et aL / Oxidation of thin Ba films. I

133

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1

2

3

4

5

6

7

8

9

10

LAYERS OF Ba (OXIDIZED)

Fig. 7. Shift in Ba 4d, 5p, O 2p AES peak for different thickness BaO films.

It is well known that the work function of BaO films decreases with thickness even beyod a monolayer and continues to do so up to - 10 layers. This change in work function was separated into changes in Fermi level and electron affinity using the low energy electron reflectivity (LEER) techniques [7], the results of which are given in fig. 8. It is seen that the Fermi-level-toconduction-band energy decreases with thickness. Conversely (if the band gap remains fairly constants), the Fermi-level-to-valence-band energy would correspondingly increase. This increase in binding energy of the BaO valence band with respect to the Fermi level would cause the O 2p levels to exist lower in 1.0

.8 a Z

.<

.6

(~'.,.0.0 o .....

,"*~ ....

-~ -

-.,..oO- - - - -

VACUUM LEVEL

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z o_

CONDUCTION B A N D EDGE

o -.2

a -.4 Z 0 -.6 ~ -.8

FERMI LEVEL

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LAYERS OF BaO

Fig. 8. Change in energy of Fermi level and vacuum level with respect to conduction band edge for different thickness BaO films.

134

G.A. Haas et al. / Oxidation of thin Ba films. I

A

>_ 0.

70

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Ba MONOLAYER EXPOSED TO: O60L H20, 100L AMB. a 5KL CO2

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Z

ERROR

"r

I

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400

200

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600

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800 1 0 0 0 1 2 0 0 1 4 0 0 1600 TEMPERATURE K

Fig. 9. Decrease in energy of "68 V" peak (i.e., return to normal value) during recovery from CO 2 and H 2 0 poisoning.

~

CLEAN Ir .25 LAYERS OF Ba .63

.75

1.25 2.5

5

10

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70 80 AES ENERGY (eV)

I 90

Fig. 10. Build-up of "73 V" and "75 V" AES peaks with increasing Ba film thickness. Note that "75 V" peak (arrow) does not appear to increase beyond a monolayer.

G.A. Haas et al. / Oxidation of thin Ba films. I

135

energy for thicker films and hence cause the "68 V" O 2p peak to also appear lower in energy. (Since the O 510 V peak involves core, valence, valence transitions, a shift to lower energies with thickness might also be expected in that peak.) Shifts in energy of the "68 V" peak were also observed in thin Ba films which had been exposed to larger amounts of H 2 0 or CO 2. Here, the "68 V" peak increased in energy to - 70 V. U p o n heating, however, the peak would shift down to - 6 9 V before desorption of the oxide (fig. 9). This shift in energy may again be related to work function changes, namely those caused by the decomposition of the hydroxide and carbonate [11]. There also seems to be a relation between the position of the "68 V" peak and the substrate used. In thin BaO films on W, for example, the 68 V peak appears lower in energy than on Ir but it does not appear to change as rapidly with thickness as it does on Ir. 3.4. Characteristics o f the "'75 V" p e a k

While the "68 V" AES peak and "73 V" AES peak have been related to O 2p and Ba 6s valence states, no such clear-cut relation has been found for the "75 V" AES peak. Fig. 10 shows that it is related to the deposition of Ba since it and the "73 V" peak increase together up to - 1 monolayer. Beyond that, however, the "73 V" peak increases but, as well as can be determined from the derivative spectra, the "75 V" peak does not. Furthermore, as is seen in fig. 4, the "75 V" peak does not seem to be affected by oxidation. It is possible that this "75 V " line is related to some Ba levels which appear close to the Fermi level at the surface.

4. Effects of different oxidizing agents on Ba films An example of how the Ba 4d, 5p, valence transitions (i.e. "68 V" and "73 V" lines) can be used to understand the oxidizing effects on Ba is shown in fig. l l. Here, the results of exposing a 10-layer Ba film to various different oxidizing agents are illustrated along with a similar spectrum from a 10-layer BaO film. (The entire species of exposures as well as high energy AES data are described in ref. [7]; the example, illustrated here, is that of exposures that describe full oxidation before the build-up of excess layers, viz. the exposure region before the work function starts to rise again.) Is is seen t h a t for H 2 0 , CO2, and 02 the results are very similar to the BaO case indicating full oxidation of the Ba from the reactions: Ba + H 2 0 ~ BaO + n 21',

(1)

Ba

(2)

+ CO 2 ~

BaO + CO 1',

136

G.A. Haas et a L / Oxidation of thin Ba films. 1 ,--,10 LAYERS Ba EXPOSED TO: ~50L H20

- 100L CO2

",,20 L 0 2

"-'500L CO

Ba O (10 LAYERS)

I

I

I

I

I

I

I

20

30

40

50

60

70

80

AES ENERGY (eV)

Fig. 11. Low Energy Auger spectra of - 10-layer Ba films which are oxidized (to minimum work function values) using H 2 0 , CO2, 02 and CO.

Ba + ½ 02 ~ BaO.

(3)

From a comparison of the "68 V" and "73 V" line for the case of CO, however, it appears that, even after 500 L of CO, the Ba is not fully "oxidized". The reason for this is the chemical reaction 3 Ba + 2 CO ~ 2 BaO + BaC 2,

(4)

which indicates that out of three Ba atoms, two form BaO and one forms BaC 2. The remaining "73 V" peak for the CO case in fig. 11 then is very likely not due to metalic Ba (still waiting to be "oxidized") but is probably due to BaC 2 as eq. (4) predicts. (High energy AES data taken on this same film and reported in ref. [7] show that the ratio of BaO to BaC 2 is essentially that shown on the right side of eq. (4).) It appears, therefore, that in the formation of BaC 2, the Ba valence electrons retain their Ba 6s energy state and unlike the case of BaO, do not form interatomic Auger transitions. (Preliminary results

G.A. Haas et al. / Oxidation of thin Ba filrns. I

137

on BaS, on the other hand, indicate that compound does form interatomic Auger transition [7].) It should be noted that some of the CO on the right hand side of eq. (2) may also react with some Ba as given by eq. (4), resulting in the combined equation (assuming all CO reacts) 5 B a + 2 CO 2 ~ 4 BaO + BaC 2,

(5)

where one out of five Ba atoms is converted to BaC 2 by the CO v Since a considerable portion of the CO is expected to escape, however, the actual amount of BaC 2 formed by the exposure of CO 2 on Ba is expected to be considerably less. (Specifically, the high energy AES results [7] suggest that for this case the end products are one BaC 2 for - 6 BaO.) The effect of the slight BaC 2 formation with CO 2 exposure is also detectable in fig. 11 through a careful comparison of the slightly larger "73 V" peak for CO 2 compared to H 2 0 and 02 exposed Ba as well as BaO.

5. Summary The following summarizes the major conclusions reached in this paper: (1) The low energy AES characteristics change appreciably during the oxidation of Ba films. Core, core, valence transitions dominate this low energy spectrum and the majority of those changes during oxidation are the result of changes from a Ba valence state to an oxygen valence state. (2) Because of the relative freedom from other unwanted AES lines in the 65 to 75 V range, the Ba 4d, 5p, valence doublet was considered most promising for characterizing the oxidation. This doublet consisted of a "68 V" AES line attributed to the O 2p valence state and a "73 V" AES line attributed to the Ba 6s valence state. (3) The "68 V" line was observed to increase with oxidation and was also found to be related to the O 2p electron loss peak. The energy of the "68 V" line was furthermore seen to change in accordance with expected changes in O 2p binding energy as noted from work function changes. (4) The "73 V" line was observed to decrease with oxidation and was also found to be related to the free electron density as determined by by Ba surface plasmon studies. (5) It appears that for a given set of conditions (i.e. film thickness, substrates) the relative heights of the "68 V" and "73 V" AES peaks can adequately describe some aspects of the B a - O bonding. Also, it is suggested that when these data are used in conjunction with AES data, they can give information on the electron transfer in other compounds such as BaC 2 and BaS.

138

G.A. Haas et al. / Oxidation of thin Ba films. I

References [1] [2] [3] [4] [5] [6] [7] [8]

A.L. Reimann, Thermionic Emission (Wiley, New York, 1934). J.A.T. Verhoeven and H. van Doveren, Appl. Surface Sci. 5 (1980) 361. R. Forman, Appl. Surface Sci. 2 (1979) 258. A.P. Janssen, R.C. Sehoonmaker, A. Chambers and M. Prutton, Surface Sci. 45 (1974) 45. V.M. Bermudez and V.H. Ritz, Surface Sci. 82 (1979) L601. G.A. Haas, A. Shih and C.R.K. Marrian, Appl. Surface Sci. 16 (1983) 139. A. Shih, G.A. Haas and C.R.K. Martian, Appl. Surface Sci. 16 (1983) 93. L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Spectroscopy, 2nd ed. (Physical Electronics Industries, Eden Prairie, MN, 1976). [9] R.E. Thomas, A. Shih and G.A. Haas, Surface Sci. 75 0978) 239. [10] J.W. Gibson and R.E. Thomas, Appl. Surface Sci. 16 0983) 163. [11] G.A. Haas and A. Shih, Appl. Surface Sci. 8 (1981) 145.