High resolution Auger electron spectroscopy of metallic copper

JournaEof Electron Spectroscopy and Related Phenomena Ekevier Publishing Company, Amsterdam - Printed in The Netherlands

HIGH RESOLUTION COPPER

GUNNAR

AUGER

ELECTRON

SPECTROSCOPY

OF METALLIC

SCHON

Division of Chemical Technology, Chemical Center, Lurtd (Sweden) (First

received

18 May 1972;

in final form

14 August

1972)

ABSTRACT

Part of the LMM Auger spectrum from metallic copper has been studied in a high resolution X-ray photoelectron spectrometer. Fine structure not eat-her reported has been observed. The main L,M,,,M,,5 p eak is very narrow, 1.O eV, although the valence band is involved in the transition. The agreement between experimental and calculated Auger electron energies is very good. Since fine structure is found to be an intrinsic property in Auger spectra the interpretation of “satellite” peaks as due to electron-plasmon interactions should be used with care. The L3M4,5M4,5 peak is very sensitive to the copper surface conditions. Surface oxygen affects the peak in a characteristic way. INTRODUCTION

Auger electron spectroscopy is a technique very often used to examine the chemical composition of a surface layer. The surface is usually irradiated with electrons and the electron energy analysis is performed with a retarding field analyzer or with an electrostatic cylindrical spectrometer. Two reviews of Auger spectroscopy’, 2 have recently been published. In this work the LMM Auger electrons from copper have been studied in a high resolution electron spectrometer designed for X-ray photoelectron spectroscopy. In Auger electron spectroscopy the differential distribution dN(E)/dE is normally recorded in order to increase the sensitivity. The peak position is then by convention the maximum slope of the high energy tail. In X-ray photoelectron spectroscopy the peaks, including Auger peaks, are directly recorded, and it is possible to determine the peak position within 0.1 eV, which is better than published results from a retarding field analyzer or from an electrostatic cylindrical spectrometer.

J. Electron Spectrosc., 1 (1972/73)

377

EXPERIMENTAL

The experiments were performed with an AEI ES 100 electron spectrometer equipped with a hemispherical electrostatic analyzer. Oil diffusion pumps fitted with cold traps produce a vacuum of 10e7 torr during the experiments. These vacuum conditions do not allow any definitive surface studies. It is possible to heat the sampie to 400°C. All spectra presented here were obtained with Al Ka radiation (1486.6 eV). The instrumental resolution for gold 4f7, 2 electrons is 1.30 eV under the experimental conditions in this work. With a narrower detector slit and Mg Kol radiation a resolution of 1.05 eV is obtained for the same gold peak. The instrument is calibrated so that the difference between photoelectron peaks measured with Mg Ka and Al Ka radiation is 233.0 eV. The Fermi level is taken as the inflection point of the steep high-energy edge of the 4d band from palladium. All energies are measured in relation to the Fermi level. The binding energy of Au 4f,,* electrons then becomes 84.0 eV. The binding energy of the C 1s line due to carbon contamination from the vacuum system is 284.8 eV. In Table 1 measured

TABLE

1

COMPARISON OF ELECTRON FROM BEARDEN

Electron

level CU &W/Z

BINDING

ENERGIES

(THIS

WORK)

AND

X-RAY

Binding energy (e V) This work

Energy d@ereme

932.2 122.4

809.8

811.1

X-ray energies (e V) Beardens

532.2 335.2

197.0

197.0

(e V)

cu

3s1/2

Pd I’d

3&i/2

Ag

Ag 3&/2

3~312

573.0 368.2

204.8

204.8

Pt Pt

4d5/2 4f7i2

314.5 71.1

243.4

243.6

Au Au

4&,/z 4f7/a

335.1 84.0

251.1

251.0

3~~2

DATA

binding energies from different metals are compared with Bearden’s X-ray data. The metals used for calibration were studied as foils at 100°C except Cu, which was studied at 300°C. A fresh surface was produced by mechanically scratching prior to investigation. The measured energies were consistent within 0.1 eV. 37s

1.

Eiectron

Spectrosc.,

1 (1972/73)

EXPERIMENTAL

RESULTS

Figure 1 shows a wide scan spectrum from a metallic copper foil at 400°C. Both photoelectrons and Auger electrons are present in the spectrum. Table 2 gives the binding energies of the copper electrons measured in this work compared to the I

I

1

LMM

I

AUGER

I

I

I

I

I

ELECTRONS

cu3p

Cu 3d

o-

--I600

800

1200

1000

Figure 1. Electron spectrum from a metallic copper foil at 400°C.

TABLE

2

BINDING

ENERGIES

OF COPPER

ELECTRONS

Electron [eve!

Binding energy (eV) This work

Binding energy (eV) Siegbuh et a1.4

Binding energy (e V) Bearden and Burr5

cu cu Cu cu i$ cu Cu

1096.4 952.1 932.2 122.4 77.1 75.2 3.0

1096 951 931 120

1096.6 951.0 931.1 119.8

2s 2p1/2 2pqz 3s 3~~2 3p3/a

36

f & f * & & *

0.3 0.1 0.1 0.1 0.1 0.1 0.1

f j, * &

0.4 0.4 0.4 0.6

74

73.6 jz 0.4

2

1.6 zk 0.4

values reported by Siegbahn et alP and Bearden and Burr’. The unsymmetric valence band (3d electrons) was measured at the peak maximum. In Figures 2 and 3 more detailed spectra of the L2,3 MM Auger electrons are shown. Table 3 summarizes the J. Electron S’ectrosc., 1 (1972/73)

379

11

LsM4,5Mz,3 L3MqzM4.5 L&W%,3 LzMz,sM4,5 plasmon loss

plasmon loss

LdhM4,5

914.7 zt 0.1 916.7 & 0.1 919.0 & 0.1 921.7 f 0.1 935.1 & 0.1 938.7 -_t0.1

858.5 f 0.5 865.5 f 1.0 899.0 4 1.0

832.0 f 0.5 839.5 & 0.5 847.0 f 0.5

938.7

918.4

839.0 845.6 858.9 865.5

799.0

918.9 f 0.5

839.2 i!x 0.5 846.4 f 0.5

959

938

860 865 878

816

790

766.2

775.0 i 0.5 798.5 & 0.5 820.0 f 1.0 777.5 It 5.0

729 745

719.7

719.6

0.5 0.5 1.0 OS

718.0 f 731.5 f 759.0 f 768.0 &

1 2 3 4 5 6 7 8 9 10

Bonzel9

Aksela Calculated et u/a8

This work Measured

Line Assignment number

-

940

920

840 848 859 868 887

770 776 798 814

718 733

940

855

780

950

875

795

920

850

775

Ch~ng McKee Palmberg Quint0 and and and et al.13 Jenkins10 Roberts11 Rhodinlz

COMPARlSUN OF DIFFERENT MEASUREMENTS OF AUGER ELECTRON ENERGIES (eV) FOR COPPER ”

TABLE 3

914

835

768

937 & 2

919 f 2

839 $: 2

769 i 3

Wagnerl4 Yin et a1.l5

I

750

Figure 2. LMM

1

1

I

800

850

900

Ekin

(e’J1

Auger electron spectrum from a metallic copper foil at 400°C.

Auger electron energies measured in this investigation together with earlier reported copper Auger electron energies. The spectrum at 25°C (Fig. 3) was obtained from a mechanically scratched copper surface. The foil was then heated to 400°C and the spectrum recorded again (Fig. 3). Upon decreasing the temperature from 400°C to 25 “C again the spectrum did not change. The change in the L,M,,,M,,, Auger electron spectrum upon raising the temperature is accompanied by a pronounced decrease in the oxygen peak height. Figure 4 shows the oxygen peak at 25 “C and at 400 “C. There is no indication from the Cu 2p electron lines of the presence of copper oxide either at room temperature or at 400 “C. DISCUSSION

Calibration Table 1 shows that the calibration of the efectron spectrometer is consistent with the X-ray energies reported by Bearden for all metals except copper. The copper foil used in this work was not oxidized, because the Cu 2p peaks showed no copper oxide structure 6y 7 and the oxygen signal was very small during the calibration measurements. We have no explanation for the discrepancy of 1.3 eV. In a recent J. E&w-on Specrrosc., 1 (1972/73)

381

a

900

Figure 3. L~M4.5M4.5 400°C (upper curve).

905

Auger

910

Ekin(ev)

electron spectrum from metallic copper at 25 “C (lower curve) and

0 945

950

Ekin(eV)

Figure 4. Oxygen signal from metallic copper at 25°C (lower curve} and 4ooT

382

J.

Elecfron

(upper curve).

Spectrosc.,

1 (1972/73)

study by Baer et a1.l 6, the maximum intensity of the Cu 3d band is found 3.1 eV below the Fermi level. The value obtained here was 3.0 eV.

Consistency

in experimental

data

According to Table 3 there is a large divergence between reported values. However, the agreement between this work, that of Aksela et aL8, Yin et al,15 and Chung and Jenkins” is very good. Aksela et al. ’ and Yin et al.’ 5 recorded the direct spectrum instead of the differentiated spectrum. The latter method was used by Chung and Jenkins”. Yin et al.’ used X-ray exciting radiation as in this work, whereas Aksela et al.* and Chung and Jenkins” used an electron beam. Yin et a1.l5 worked with a hemispherical electrostatic analyzer, Aksela et al. ’ with an electrostatic cylindrical spectrometer and Chung and Jenkins’ ’ with a retarding field analyzer using three-grid LEED optics. Thus it is possible to obtain consistent results with X-ray and electron exciting radiation and with an electrostatic cylindrical spectrometer, an electrostatic hemispherical spectrometer or a retarding field analyzer. This confirms the conception that the mutua1 calibration of the’ three different analyzers is identical and that the same result is obtained whether the exciting radiation is X-rays or electronsey 17. It is also clear that it can be difficult to calibrate the instrument correctly. It is important that reports on electron energy measurements contain information about the reference values used for calibration.

Calculated Auger electron energies The kinetic energy of an Auger electron can be expressed by eqn. (1) according to Burhop’ 8

where Eyczj and E,(,) are binding energies of the atomic electron levels of the neutral atom and E,,,(ZI is the binding energy of atomic level y in a singly ionized atom. Equation (1) was generalized by Bergstrom and Hill’ ’ as

where Eycz+l) is the corresponding electron energy in element z f 1, which is zinc in the present case, and AZ is a parameter which can be adjusted to fit experimental data. The value of the parameter is normally in the region 0.6-1.320. The value 1.0 is adopted in this work, which leads to eqn. (3) E fww

= K(z) - Ex(z) - Ey(r+ 1)

(3)

The notation Evxy refers to the trapping of electron x and the ejection of electron y. If the Auger transition Evyx is considered, where electron y is trapped and eIectron x ejected, the following Auger electron energy is obtained E UYdZJ =

E”(Z) - Eycz, - Ex,z+ 1)

J. Electron Spectrosc., 1 (1972/73)

(4) 383

The two processes have the same initial and final states according to Chung and Jenkins”. If x and y are within the same energy level, eqns. (3) and (4) are identical, but if x and y are different, eqns. (3) and (4) do not give the same energy, which they should according to Chung and Jenkins ” . However, there are different states in the doubly ionized atom13 4, which is also seen here from the peak splittings in the Auger spectrum from copper. Different final states may be obtained depending on whether x or y is ejected. This justifies our attempt to calculate Auger electron energies from both eqn. (3) and (4). The copper electron energies in Table 2 were used for the calculation. The Cu 3p level is taken to be 75.8 eV. In this work zinc electron energies were determined as: Zn 3s = 136.7 eV, Zn 3p = 90.2 eV and Zn 3d = 10.8 eV. Auger electron energies calculated in this way are the energies at the Fermi level. In order to convert the recorded kinetic electron energies to the Fermi level, the work function of the spectrometer must be added. Since the sample and the spectrometer are connected via conduction material their Fermi levels will be equal. The work function of the spectrometer is 9.7 eV, which is calculated From extensive calibration. This is a very high value for a work function and should be regarded as an instrumental constant instead of the true spectrometer work function. For some of the published data in Table 3 it is not clear whether the Auger electron energies are measured relative to the Fermi level or relative to the vacuum level of the sample. Equations (3) and (4) were obtained after crude theoretical approximations, but nevertheless good agreement between experimental and calculated Auger electron energies is obtained in this work (Table 3). For copper L,,sMM Auger electron spectrum most of the main Auger electron energies are very well described by eqns. (3) and (4). Th e expected Auger line splitting due to the two Cu 2p levels, separated by 1.9 eV, may be hidden in the broadness of the Auger peaks involving these levels. Comparison with spectra j+om gaseous compounds We cannot at present explain all the features of the L2,,MM Auger spectrum from copper. However, it is very interesting to compare the results presented here with the LL2,,MM Auger electron spectra from gaseous krypton 21 and from bromine in gaseous bromo-substituted methanesz2. Splittings of the Auger peaks from krypton and bromine are very similar to the splittings presented here for copper. This indicates that in Auger transitions the 3d valence band electrons in metallic copper behave similarly to the 3d electrons in gaseous krypton or bromine. The similarity between gaseous and solid L2 ,,MM Auger spectra is one reason why we have been very restrictive in explaining peaks in the Auger spectrum as due to electron-plasmon interactions. This explanation is frequently usedz3q 24 but in the light of the discussion above, “satellite” peaks are an intrinsic fine structure of the Auger spectrum, and this explanation must also be considered. From the copper photoelectron spectrum, only one plasmon loss of 20 eV is detected. This is the same value as reported by Joshi” 384

J.

Electron

Spectrosc.,

1 (1972/73)

for the plasmon energy in copper. From Table 3 it is seen that only two peaks have been assigned as plasmon loss peaks, both of which have 20 eV lower energy than the two most intense peaks in the Auger spectrum. Auger peak intensities The intensities of copper L,MM Auger peaks are much greater than those of L,MM Auger peaks. The ratio between the number of created vacancies in Cu 2p,,, and bromine in bromo-suband Cu 2~~~~ levels is 1:2. In the studies of krypton’l stituted methanes22 the intensity ratio between L2MM and L,MM Auger peaks was 1:2. The probability for Auger transitions in copper is apparently much lower when the primary vacancy is in the 2p,,, level rather than the 2p3,2 level. The selection rules and transition probabilities in Auger transitions have yet to be elucidated.

/nformation

about

valence

band structure

The fine structure of the L3M,,,M,,, Auger peak (Fig. 3) is interesting. The spectrum at 400 “C has briefly been reported by us earlier6. The very narrow main peak, 1 .O eV, is remarkable, as it is in sharp contrast to the opinion that when the valence band is involved the Auger line will depend on the broadness of the valence band, which is 3.0 eV for copper at half-width. Fruitful information about the densities of states in the valence band could be obtained from the fine structure of this L,M,,,M,,5 Auger peak. Musket and Fortnerz6 studied KLL Auger peaks from beryllium and found two peaks with an energy difference of I1 eV. This was found to be in accordance with twice the experimental difference between the valence band maxima for s symmetry and p symmetry electrons”. The four separated peaks in the L,M,,,M,,, spectra could possibly in the same way reveal the nature of the copper valence band. The same peaks appear at 25 “C and 400°C (Fig. 3), but the relative intensities are different. This is used as a criterion for the assumption that the small peaks at 400 “C are not due to plasmon losses or gains. Upon heating, the oxygen peak decreases (Fig. 4) and when the temperature is again lowered to 25 “C, the peaks maintain the same shape as at 400 “C. The oxygen peak decrease could be due to oxygen desorption, but adsorbed oxygen is also known to be absorbed into copper when the temperature is raised2*’ 29_ The difference between the two Auger spectra is apparently due to different amounts of oxygen on the copper surface at 25°C and at 400°C. This could provide information about the nature of the bonds between adsorbed species and the metal surface atoms. The L3M,,,M,,, Auger peaks from copper oxides are very different from the peaks presented here for metallic copper with or without surface oxygen3 O. The carbon peak in Fig. 1 is present at both 25 “C and 400 “C! but does not seem to affect the copper Auger spectra, since identical Auger spectra have been obtained from surfaces with different amounts of carbon present. J. Electron

Spectrosc.,

1 (1972/73)

385

CONCLUSIONS

In an X-ray photoelectron spectrometer it is possible to study Auger electrons very accurately. The results presented here indicate that this method is superior in terms of resolution and accuracy of energy analysis to ordinary Auger electron spectroscopy where electrons are used as excitation radiation and the analyzer is of the retarding field type or of the electrostatic cylindrical spectrometer type. Very good agreement between experimental and calculated copper L2,3MM Auger electron energies has been obtained. The fine structure of the spectrum is similar to the L,,,MM Auger spectrum from gaseous krypton’l and that from bromine in gaseous bromo-substituted methanesz2. Fine structure is an intrinsic property of Auger spectra. Many satellite peaks interpreted as plasmon losses in the literature may therefore be due to an intrinsic Auger fine structure. The fine structure of the copper and surface LsM,.,M,,, P eak is very sensitive to the copper surface conditions oxygen affects the peak in a characteristic way. ACKNOWLEDGEMENTS

The author is grateful to Professor Sten T. Lundin for his generous support during the course of this work. Collaboration with Docent R. Larsson and Dr. B. Folkesson is greatly appreciated. The work is supported by the Swedish Board for Technical Development and the Bank of Sweden Tercentenary Fund. REFERENCES 1 2 3 4

5 6 7 8 9 10 11 12 13 14 15 16 17

386

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