Auger photoelectron coincidence spectroscopy (APECS) a tool for understanding auger emission from solids

Journal of Electron Spectroscopyand RelatedPhenomena72 (1995) 289-297

Auger Photoelectron Coincidence Spectroscopy (APECS) A Tool for Understanding Auger Emission from Solids S.M. Thurgate and C.P. Lund School of Mathematical and Physical Sciences Murdoch University Perth, Western Australia

ABSTRACT The Auger emission process from solids can be very complex, with a variety of processes producing intensity in the final spectra. APECS is a technique that can be used to reduce this complexity. In the APECS experiment, Auger electrons are measured only when the corresponding electron from the ionisation event is also measured. Thus, one can eliminate parts of the Auger spectra by looking only at features within the spectra that are due to the particular ionisation event in question. The LVV Auger spectra of the 3d transition metals show many complicating effects. In order to study these, we have made a systematic study of the following metals: Fe, Co, Ni, Cu and Ga. While Ga is not a member of the 3d transition metals, it does represent the end of the series and shows some limiting behaviour of many of the trends observed in the other materials. We have observed the L23VV lines in coincidence with the 2p3/2 and 2pl/2 photoemission lines in all these materials. We have also examined several alloys of Ni and Fe. From these studies a number of trends are clear.

1. INTRODUCTION Auger spectroscopy is widely used as a qualitative technique for estimating the surface concentration of elements. Although it is understood that Auger lineshapes are sensitive to chemical environment, they are seldom used as the only source of bonding information.0) This is largely due to the complexity of the Auger process itself, making it difficult to easily extract useful information. The Auger process involves at least three electrons, and often more. In the 3d transition metals, the Auger process is complicated by the fact that the Fermi level lies in the incomplete d band. This configuration gives rise to the possibility of shake up/off processes in both the initial and final state configurations. A full understanding of these is essential to a complete interpretation of the observed

lineshapes. Progress in the interpretation of these lines in terms of a complete ab-initio calculation has been slow and is perhaps still some distance away. A number of experiments however have indicated the significance of some of the important processes. There have been a number of synchrotron experiments carried out near various thresholds(2' 3) that have highlighted the complexity of these lines and several coincidence measurements which indicate the sources of these complications. (4' 5) In this work, we present the results of a study of the L23VV spectra of Fe, Co, Ni, Cu and Ga in coincidence with the 2pl/2 and 2p3/2 photoelectron peaks using Auger photoelectron coincidence spectroscopy (APECS). We have examined the pure elements together with several alloys of Ni/Fe. A number of trends with increasing atomic number can be seen in these data.

0368-2048/95 $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02298-4

290 2. EXPERIMENTAL SETUP In the APECS experiment, an Auger electron is only counted if the photoelectron from the ionisation event is also counted. The Auger analyser is generally scanned through the Auger line while the photoelectron analyser is fixed at the energy of the photoelectron line. The APECS experiment is necessarily more complex than regular electron spectroscopy. An electron spectrometer is required for both the Auger electron and photoelectron. Coincidence electronics are needed to measure the true and accidental coincidence counts. The most significant problem encounted in making these style of measurements from surfaces comes from the high intensity of the uncorrelated secondary electrons. (6) The way to make useful APECS measurements is to use electron spectrometers with very good timing resolution to distinguish between true coincidences and random coincidences. We have purpose built spectrometers that have very good timing resolution (1.6 nS for the whole system). These are 127 ° analysers, with multichannel plate detectors placed at an angle to the exit slit in order to compensate for the differences in flight paths through each analyser. This follows the suggestion made by V~lkel and Sandner. Further details of our apparatus can be found in reference (7). The samples were each prepared by ion bombardment and annealing. The Cu and Co samples were prepared from foils. The Fe and Ni samples were cut from single crystals. The Fe/Ni samples were cut from polycrystalline rods. The GaAs samples were prepared from orientated (110) bars. These were prepared by ion bombardment with 1 keV Ar÷ ions. The Ga to As Auger lines indicated that the GaAs was near to stoichometric, though enriched to some extent with Ga. The base pressure of the system was 2 x 10"l° Torr. When the Fe samples were being investigated, the liquid N2 sublimator was used continuously to minimize the 02 and CO partial pressures. The samples were cleaned every 12 hours. Examination of the C and O lines indicated no significant build up of contaminants in that period. The energy resolution of the scanning analyser was 2.6 eV. This rather poor resolution was a consequence of the high pass energy, selected to give optimum count rates.

Despite this, the spectrometer still took a substantial time to acquire a single spectrum. In general, each spectrum took 30 days to generate. As a consequence of the poor energy resolution, we attempted to examine systems where energy resolution was not of prime consideration. In all the data that follows, Mg Kcx radiation was used to produce the primary ionisation. Typically, we used 220 watts of incident power in the x-ray tube. Each graph shows the total number of counts acquired in a period of approximately 30 days. The take-off angle was 45 o to the surface normal for both analysers. 3. RESULTS AND DISCUSSION 3.1. The Elements The results of these measurements of the pure elements together with the Ga line in GaAs are shown in Figures 1 and 2. They show the L23W spectra of the five elements collected in coincidence with the 2p3~ and 2p~/2 photoelectron lines respectively. A number of trends can be seen in these data. In order to simplify consideration, we have divided analysis into consideration of five different effects. These are:

i) ii) iii) iv) v)

the change from band-like to atom-like spectra; the effect of the spectator hole on going from atom-like to band-like; the change in the ratio of the Coster-Kronig (CK) to mainline emission ratio; the presence of initial state and final state shake up/off satellites in the tail of the Auger line; the presence of extrinsic satellites in the spectra.

i) Band-like to atom-like spectra. The spectra in Figure 1 show the L23VV spectra collected in coincidence with the 2p3/2 line. Hence they are free from any influence of any cascade process such as L2 --~L3M45M45 or L 1-~L3M45M45. While the resolution of the measurements are relatively poor, the transition from broad band-like to atom-like spectra is clear. This transition in the 3d metals has been previously documented and is well understood. (s' 9) It is a consequence of the final state energy levels being pulled clear of the band following ionisation in some materials, resulting in these exhibiting atom-like spectra. In those cases where the final state orbitals lie in the band, the spectra are band-like.

291 800 750

A

///.t~ 600 •

500

co o

¢3 0

250

. ~t~/

400 200

J

0 '11

680

700

720

i ~

-2:}0 900

740

,,. ,lrJt,.i,iiT, t 920

940

960

E r e ~ (eV)

820

800 B 600

3 0

o

200

-200 750

J iiiiiilii 770

733

810

400 E 3 0

o

2013

t} 1030

1050

1070

10~0

~(eV) Figure 1. L23w spectra of(A) Fe, (B) Co, (C) Ni, (D) Cu and (E) Ga in coincidence with the 2p3/2. The top dashed curve is the regular singles spectrum, while the solid line is the best fit to a model spectrum.

1500

1000

The coincidence spectra show the same peaks in the atom-like spectra that have been previously assigned to atom-like peaks. Some peaks present in the singles spectra are not present in the coincidence spectra, though none of these have been previously assigned to atom-like peaks.

500

830

~~

600

/

En~(eV)

t--1 O o

E

850

870 Energy(eV)

890

292 125(

A

D t /

75[

/

co

0

250

....i_...'"'

-250 680

O

o

200

;',\,. ..... "-'-',

700

720

740

900

920

F_rergy(eV)

,o0ol

B

1100

1120

E ^

/ t

f

/

',

/

,,e t-

tO

960

Enerw(eV)

6OO

t,.)

940

:D 0

2133

/

500

0

I*iI "p't~' 750

770

790

810

(e~

Figure 2. L23W spectra of (A) Fe, (B) Co, (C) Ni, (D) Cu and (E) Ga in coincidence with the 2pl/2. The top dashed line is the singles spectntm, while the solid line is the best fit to a model spectrum.

1000

830

1080

(eV)

C

0 0

1080

850

870

K"~'~ (e~

890

ii) Band-like to atom-like spectra in the presence of spectator holes. The transition from band-like to atom-like is a consequence of the change in ratio of the interaction energy of the holes in the final state to the valence band width. Across the 3d transition series, the width of the band narrows, so the orbitals in the final state are pushed clear giving rise to the atom-like spectra. in the coincidence spectra with the 2Pu2 photoelectron line, the intensity seen in the L 3 W is due to the cascade process L2-L3W. The CK process produces a hole in the valence band which is a

293 spectator to the Auger decay. This spectator hole has a number of effects. In the case of those elements that have atom-like spectra, the position of the atomic terms can change. The terms are those of the doubly ionised ion, rather than the singly ionised species. This effect is responsible for the 2.5 eV satellite in the Cu spectrum. Th~s interpretation was first proposed by Roberts et al.,( ) and the coincidence experiments of Haak et al. (11) confirmed the interpretation. The spectator hole can also have an effect on the shape of the hand-like spectra. Cobalt normally displays a band-like spectra. However, in coincidence with the 2pl/2, the cobalt line can be seen to narrow and move to lower kinetic energy. 02) This can be seen to be a result of the increase in the interaction energy of the holes in the final state. Ni, Cu and Ga continue to exhibit atom-like spectra in the presence of the additional hole. Fe shows band-like spectra in the presence of the spectator hole, without substantial change in the lineshape. iii) The c h a n g e in the ratio of the Coster-Kronig (CK) to mainline emission ratio. The probability of

CK emission in the 3d metals depends on the separation of the L2-L3 shells and the work function of the materials. The work function increases across the period and the probability of CK processes decrease. This can be seen clearly in the data from the spectra in coincidence with the 2pl/2. The probability of a CK process can be found directly from this data. iv) P r e s e n c e of s h a k e up/off satellites in the tails of the 3d transition metals. The existence of shake

up/off satellites in the tail of Ni has been demonstrated in synchrotron measurements by Whitfield et a/(2) and in the data of M&rtensson, Nyholm and Johannson. (13) They proposed three ways in which a three hole final state could be produced. These are: (i) (ii) (iii)

L 1---}L3M45---~M45M45M45 L2--).L3M45--}M45M45M45 L3M45--}M45M45M45

(i) and (ii) are CK processes while (iii) is a shake up/shake off. In addition to these, there are four ways a four hole final state can be produced. These are: (iv) L 1--}L2M45--}L3M45M45---}M45M45M45M45 (v) L 1M45--~L3M45M45---rM45M45M45M45

(vi) (vii)

L2M45---}L3M45M45---}M45M45M45M45 L3M45M45---}M45M45M45M45

Process (iv) is due to a double CK process. Process (v) has a shake in the initial state followed by a CK into the LI hole to give the four hole final state. Process (vi) is similar, but involves a CK into the L2 hole. Process (vii) involves a double shake in the initial ionisation. These multi-hole processes have relatively large probabilities of occurring because the Fermi level lies in the incomplete d band. The high density of states near the continuum makes the probability of many of these processes high. APECS can be used to discriminate against a number of these processes appearing in the collected spectra. 04) Those processes which result in the photoelectron losing energy so that it does not fall within the acceptance energy of the photoelectron analyser, will not contribute to the Auger signal. Hence, when APECS is used to measure the L 3 W spectra coincidence with the 2p3/2, then none of the processes (i) - (vii) will contribute. If the spectra is collected in coincidence with the 2pl/2, then only process (ii) will contribute to the L3VV peak. In order to determine the size of the contribution to the regular, singles spectra, the L3M45M45 APECS spectra collected in coincidence with the 2pl/2 and 2p3/2 lines were added in the appropriate ratios. These ratios were determined by measuring the L2L3V CK ratio from the L23VV spectra in coincidence with the 2pl/2 and then using the fact that the 2pl/2 level has half the multiplicity of the 2p3/2 level. After the two spectra had been added, the inelastic background was corrected to account for the fact the inelastic mean free path (IMFP) applicable to the coincidence measurement is different from that suitable for the singles s~ctra. It can be shown( ) that for coincidence measurements: 1 1 1 -- - - + -(1) 2c~, 2AEs 2pE The correction to the inealstic background was made by applying the formalism developed by Tougaard, using his "universal" energy-loss curve together with IMFP from the calculations of Tanuma, Powell and Penn. 05)

294 others. The probability of shake up/off occurring depends on the details of the valence band and the position of the Fermi level. We have examined some of these effects in our study of Ni/Fe alloys as we will see later.

1000 A

,', 750

B

.,

750 "E

500

500

250

250

0

O; 690

700

710

760

Energy (eV) 1500 C

770

780

Energy (eV)

,',

1000

40O

50O

2OO

v) The presence of extrinsic satellites. Plasmon satellites have long been known to accompany electron emission in solids. The APECS experiment is able to show that in some materials the mainline can have intensity due plasmons from an emission at a higher energy. This can be seen in the spectra of Ga in GaAs (shown in Figure 4). This is known to have a bulk plasmon at 16.3 eV below the L3w peak. This is not seen in coincidence with either the 2p3/2 or 2pw2 photoelectron lines. A satellite of 11.5 eV below the mainline is seen, however, in coincidence with the 2p3/2. This is the surface plasmon.

0 -500 83O

840

850

Energy (eV)

860

"20~00

010

920

1G

i; 16.3eV~

930

'3 F

Energy (eV)

r

Figure 3. The corrected L23W spectra of Co, Fe, Ni and Cu, including 2p3/2 arid 2pl/2 components compared to the singles spectrum (upper solid lines). After making these corrections, we found that there still were substantial differences in the low energy tails between the L 3 W coincidence spectra and the regular singles spectra, for the 3d transition metals (as can be seen in Figure 3). For Ga in GaAs, there was no significant difference. We also noted that the maximum in the difference between the spectra for Ni occurs at the same energy as that found by Whitfield e t a/(2) to be due to the formation of the four hole final state. We conclude that the processes (i) to (vii) excluding process (ii) are responsible for the additional intensity in the tail. The magniture of these differences makes it unlikely that they are due largely to process (i) though this undoubtedly makes some contribution. This shows that the processes that involve shake in the initial state, processes (iii) and (vi), are important in the L3VV spectra of these materials. In Ga, the Fermi level is clear of the 3d band, and shake into the continuum is much less likely. This also indicates a further problem in analysis of these spectra when the atoms are chemically combined with

1S, '

~E o O o

1

.

1050

1075

1100

1125

Energy (eV)

B

1G A

606

16.3eV;

t-"

o O

1S 306

1060

1075 Er~rgy (eV)

1100

1125

295 contribution to the spectra. The solid lines show the spectra from the alloy. Peaks F and G are analogous to peaks C and D. However, there is no band contribution in this case, and the spectra has become completely atom-like. This effect has been previously reported from high resolution(17) Auger data, though the effect is very obvious in the APECS spectra, even with modest resolution. The change to atom-like spectra reflects changes in the band structure and possibly the final state coulomb interaction energy in the alloy.

1000 C

16.3 eV F

i A

6OO

16.3 eV

11~v11.~b ~

to ¢-

~

1

8 200 ....

-200

---

1050

."¢

1075

1100

1125 1250

Energy(eV) Figure 4. L23MasM45spectra of Ga in coincidence with the 2p3/2 (A) and 2pi/2 (B). The dashed line is the singles spectrum, while the solid line is the best fit to a model spectrum. For comparison, the best fit spectra are shown in panel C as dashed and chained curves, while the regular singles spectrum is shown as a solid line. As previously discussed, the IMFP applicable to the coincidence measurement is necessarily shorter than either that suitable for the Auger electron or the photoelectron. Hence the coincidence measurement has an IMFP closer to the 136.6 eV Ga photoelectron IMFP than the 1068.3 eV Auger electron IMFP and is consequently more surface sensitive. The spectra taken in coincidence with the 2Pl/2 shows that the second surface plasmon from the L2M45M45 lies beneath the L3M45M45. 3.2 Ni/Fe Alloy System Alloy systems are considerably more difficult to study than pure elemental samples in an APECS experiment. 06) This is due to the decreased number density of target atoms. As both the Auger electron and the photoelectron must be collected, the coincidence count rate falls like

[N1/(N l + N2)]

where Nl and N2 are the number density of the element under consideration and the number density of all other elements respectively. We have looked at Nisoo/.Feso% and Ni80o/,Fe20o/°. In Figure 5 we show the Ni L3VV spectra of both the pure material and the 50% alloy, together with an analysis of the recognizable contributions to the peaks. The peaks labelled B, C and D are the components of the pure Ni. Peaks C and D describe the various atomic terms, while B is the band-like

750

h,

E

",

0 250

-250 830

840

850

860

870

Energy (eV)

Figure 5. L23w Spectrum of Ni in pure Ni (dashed curves) and Niso*/,Feso.~(solid line). The peaks B, C and D are components of the pure Ni peak, while F and G are components of the alloy peak as shown in the text. A further effect can be seen in the low energy tail of these spectra. Both spectra were collected in coincidence. There was no appreciable difference in the height of the background in the singles mode, yet the background is substantially decreased in the coincidence measurements of the alloy. This is clear in the coincidence spectra with both the 2pl/2 and 2p3/2 that the background is suppressed in the alloy compared to the pure materials, as can be seen in Figure 6. This conclusion is further supported by the data from the Ni L23VV line of Nigoo/oFe20o~ in coincidence with the 2p3/2. This is shown in Figure 7. This shows that the band-like contribution is absent also in this alloy and that the height of the background is intermediate between the pure material and the Nisoo/.Fesoo,~alloy. We have interpreted these results in terms of final state shake up/off effects. As shown in the previous section, shake up/off occurs frequently as a consequence of the initial ionisation, and it plays a substantial role in the shape of the low energy tail.

296 1500

D

A

4OO

~00 t-

t:3

o

O O

(..)

IT]T '~ - x

840

850

860

= !V

870

t-<1 "

870

880

880

Em~(oV) Figure 6. The L23w spectra of Ni in pure nickel (panels A and B), and Ni in Niso./.Feso~ (panels C and D). Panels A and C are in coincidence with the 2p3/2, while B and D are coincident with the 2pit2.

2OOO 8

1500 1~ 1000 O

o

500 t m

/

,400

O Z

t

v

83O

840

850

860

870

880

E =3

O O

i _JJ

~(ev) -200 800

800

830

860

880

0 (eV)

(-

4OO

Figure 7. L23W spectra of Ni in Niso./.Fe2o.,~ (data points and dashed line), Niso./.Fc5o.,~ (lower chained curve) and pure Ni (upper solid line).

-200

These results on alloys have forced us to speculate that shake up/off can occur with some relatively high probability in the final state. In such an event, an electron would be shaken from the valence band as the Auger electron was emitted. This would shift the energy of the Auger electron to lower energies to be part of the tail. Wc have bccn able to show that the initial state satellite of the Ni 2p3/2 photoclcctron line is more

O

l

840

850

860

Ener~ (eV)

870

880

297 intense in the alloy than in the pure material, (16) whereas the final state shake is more intense in the pure material than in the alloy. Hence both singles spectra appear to be similar. We suspect that the reason for this is that if the initial state ionisation caused a shake up/off, then there is a change in the local electron distribution and potential making a shake up/off in the final state unlikely. If a shake up/off occurs in the initial state, than a shake up/off is unlikely to occur in the final state. In the current case, the alloying of the Ni with Fe changes the likelihood of an initial state shake by changing the hand structure and the position of the Fermi level within the band. As the percentage of Fe increases, the initial state shake becomes more probable. This reduces the probability of a consequent final state shake up/off. The significant point here is that there is no good reason why an initial state shake up/off will produce a satellite that has the same detailed lineshape as a final state shake up/off. While the integrated intensity of both may be the same, it is not reasonable to suppose that the N(E) distributions will be the same. Hence, in order to account for the lineshapes of these materials in detail, it is important to know the distribution of intensities between these different processes. 4. CONCLUSION The L3VV Auger lines of 3d transition metals exhibit a number of complex effects. APECS measurements show that there is substantial intensity in these multi-hole final states. The study of the alloys show that the effect of the chemical environment can be subtle. If our goal is to account fully for the measured lineshapes, then these effects must be taken into account.

.

S. Whitfield, G.B. Armen, R. Car, J.C. Levin and B. Crasemann, Phys. Rev. A, 37(1988) 419.

. D. Sarma, C. Carbone, P. Senn, R. Cimino and W. Gudat, Phys. Rev. Lett. 63 (1989) 656. .

S.M. Thurgate and J. Neale, Surf Sci. Lett. 257 (1991) L605.

. H.W. Haak, G.A. Sawatzky and T.D. Thomas, Phys. Rev. Lett. 41 (1978) 1825. .

S.M. Thurgate, Surf and lnterf. Anal 20 (1993) 627.

.

S. Thurgate, B. Todd, B. Lohmann and A. Stelbovics, Rev. Sci. Inst. 61 (1990) 3733.

.

9.

M. Cini, SoL State Comm. 24 (1977) 681. G. Sawatzky, Phys. Rev. Lett. 39 (1977) 504.

10. E.D. Roberts, P. Weightman and C.E. Johnson, ,I. Phys. C 8 (1975) L301. 11. H.W. Haak, G.A. Sawatzky, L. Ungier, J.K. Gimewski and T.D. Thomas, Rev. Sci. lnstr. 55 (1984) 696. 12. S.M. Thurgate, in: The Structure of Surfaces 111, S.Y. Tong, M.A. Van Hove, K. Takayanagi and X.D. Xie (eds), Springer-Verlag, Berlin (1991), p179. 13. N. Mfirtensson, R. Nyholm and B. Johannson, Phys. Rev. B, 30 (1984) 2245.

5. A C K N O W L E D G E M E N T S

14. C.P. Lund, S.M. Thurgate and A.B. Wedding, Phys. Rev. B, 49 (1994) 11352.

This project was funded by the Australian Research Council. It also supported one of us (CPL) during this project.

15. S. Tanuma, C.J. Powell and D.R. Penn, Surf lnterf Anal. H (1988) 577.

6. REFERENCES

16. S.M. Thurgate, C.P. Lund and A.B. Wedding, Phys. Rev. B. (1994) - In Press.

1.

C.J. Powell, Le Vide, les Couches, Minces, 271 (1994) 127.

17. P.A. Bennett, J.C. Fuggle, F.V. Hillebrecht, A. Lenselink and G.A. Sawatzky, Phys. Rev. B, 27 (1983) 2194.