Core-valence (KLV, KVV, LVV) auger and high resolution valence band XPS spectra of aluminium: a comparison with the results of cluster MO calculations

Journalof Electron Spectroscopy and Related Phenomena 72 (1995) 157-161

Core-valence (KLV, KVV, LVV) Auger and high resolution valence band XPS spectra of aluminium: a comparison with the results of cluster MO calculations Zs. Kovfics, L. K 6 v 6 r , D. V a r g a , P. W e i g h t m a n a, J. Pfilinkfis a n d H. A d a c h i b Institute o f Nuclear Research o f Hungarian A c a d e m y o f Sciences H - 4 0 0 1 D e b r e c e n , P . O . B . 51 H u n g a r y a D e p a r t m e n t o f P h y s i c s a n d I n t e r d i s c i p l i n a r y R e s e a r c h C e n t e r in S u r f a c e S c i e n c e , U n i v e r s i t y o f L i v e r p o o l , P.O. 147, L 6 9 3 B X , U K bDept, o f M e t a l l u r g y , K y o t o U n i v e r s i t y , Y o s h i d a - h o n m a c h i , K y o t o 606, J a p a n

Abstract Core-valence (KLV, KVV, LVV) Auger and valence band XPS spectra of aluminium have been measured by high resolution and high sensitivity electron spectrometers and interpreted by different theoretical models. For core-core-valence transitions both the Green function formalism, applied to account for the distortion of the local density of states due to the presence of the final state core hole, and the cluster MO (DVX~) calculation, using a cluster potential with a core hole, describe well the shape of the measured spectra. In the case of the AI KVV spectrum, a spectral width (FWHM) of 14.5 eV has been observed which is considerably smaller, than predicted by previous experiments. The shape of the core-valence-valence Auger spectra can be interpreted successfully using the ground state density of states obtained from the cluster MO model.

~TRODUCTION Auger spectroscopy can provide unique information on electronic structure of metals in the vicinity of the core hole created [1-3]. Recent efforts to understand the spectra of Auger transitions involving one or two valence electrons in the case of simple metals have provided insight into the site specific screening and correlation processes [4] and revealed the sensitivity of the local density of states to the changes in the atomic environment, especially to alloying. Core-valence Auger lineshapes in simple metals have been used to test the validity and accuracy of the different theoretical models used to calculate the local density of states (LDOS) in the ground state and the core-ionized states [5]. Core-core-valence Auger profiles have been calculated for Na, Mg and A1 from first principle using a self-consistent embedding model [5], indicating the necessity of a correction for the variation of the atomic Auger

transition matrix elements with the energy of the valence electrons. In this report we present KLV, KVV and LVV Auger as well as high resolution valence band XPS spectra of aluminium and compare the experimental spectral shapes to the results of our calculations based on a cluster molecular orbital DVX~ model able to account for the presence of a core hole. For the interpretation of the KLV spectra, we applied a more simple approximation as well, where the proper lineshapes were obtained by distorting the ground state as a consequence of the presence of the core hole. In the case of the core-core-valence Auger spectra, our results are compared with previous measurements [6]. Owing to the better spectrometer resolution, we achieved considerably lower width for the KVV spectra than in the previous measurement [6].

0368-2048/95 $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 6 8 - 2 0 4 8 ( 9 4 ) 0 2 3 3 I-X

158 EXPERIMENTAL

APPARATUS

AND

Auger lineshape [3]:

PROCEDURE A(E)=~Ptl,nt®n t, where @ means convoluHigh purity polycrystalline A1 samples were excited by Mo X-rays and the Auger and photoelectron spectra were measured with a home built, high luminosity electron spectrometer [7,8]. The spectrometer based on a 180° hemispherical analyzer and was used in the fixed retardation ratio (FRR) working mode. The energy resolution in the case of the KLV, KVV and LVV spectra was 0.4, 2.0 and 0.2 eV, respectively. Energy calibration of electron spectra was performed with the aid of the photoelectron and Auger lines from polycrystalline Cu and Ag samples [9]. During the measurements the vacuum was better than 10-7 Pa and the sample surface was cleaned by applying successive Ar + ion sputtering. This way we kept the oxygen and carbon contamination negligible, monitoring the oxidation of the A1 KLL spectra. High resolution valence band XPS spectra were excited by monochromated A1 Ka X-rays whose linewidth was 0.5 eV [10] and the sample surface was cleaned by in-vacuo mechanical scraping.

l,l'

tion, /,l'=0,1,2 are the angular momentum quantum numbers of the final state holes, Pll' the atomic Auger matrix elements and n l the calculated DOS of the cluster. The perturbed DOS in the presence of a l type hole is given by

n~(E) nl ( E ) = (I

-

WlI(E))2 +(Wlnn ~ (E)) 2

where w l are the perturbation parameters and I(E) is the Hilbert transform of the unperturbed DOS nl°(E) [3] i.e. oo

I(E)

=

I n(E')dE' In the case of CVV Auger E-E' " --00

THEORETICAL

FRAMEWORK

Theoretical Auger line shapes were obtained applying the ground state DOS from DVX~x MO calculation [11] using self-consistent-charge (SCC) iteration with Oh (Fm3m) cluster symmetry and a cluster including 19 atoms (from the comparison of calculations with different cluster sizes, we concluded that the size of 19 atoms is sufficient). In DVXct formalism the molecular orbitals are expressed by LCAO scheme. For comparison with experimental XPS spectra, the local densities of states were obtained replacing MO levels by energy distribution functions of Lorentzian shape and 2 eV widths. The core hole screening effects were accounted for in two ways: by applying the Cini-Sawatzky model and considering an atom with a hole as a point defect in a crystal [12,13] and performing cluster type DVXo~ calculations using a potential with a corresponding final state (L t or L 2 3) core hole, respectively. Using the well know~ formula for

transitions in Al the electron-electron correlation is expected to be negligible.

RESULTS

AND DISCUSSION

KLV lineshapes Fig. 1. shows our experimental KLV spectrum. In order to obtain the KL1V and KL23V Auger lineshapes, structures due to plasmon losses were subtracted from the KLV spectra and corrections for the inelastic background were made by using the Shirley method [14]. Our data treatment assumes that the intensity of the plasmon losses follows a Poisson distribution, and we used experimental values [15] for the parameter of the plasmon loss. From each spectra a Gaussian instrumental function with a width of 350 meV was subtracted with deconvolution. The fitting procedures allow for variation of the plasmon widths and asymmetries arising from lifetime and dispersion effects. For the Auger matrix element ratios the experimental values

159 650 3v

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__]

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1440

t480

t460

1500

~irieLio [nergy IeV)

Fig. 1. Experimental aluminium.

KLV

Auger

spectra

of

were taken i.e. KLs/KLp=0.9; KL23s/KL23P=0.25 [16,17]. The KL1V and KL23V Auger lineshapes obtained from DVXcc calculation are based on the following considerations: (i) the KLIV lineshape is determined by the final state DOS taking into account the cluster potential inside a sphere with a

radius of the atom in which the Auger transition takes place, (ii) in the case of KL23V lines the lineshape is determined by the final state DOS calculated with a cluster potential considering the nearest neighbour atoms. Our model has the consequence that in KL1V Auger transition the screening process of s local DOS is essentially an intrinsic process. The theoretical KL1V and KLz3V Auger lineshapes calculated using the MO closter model show a good agreement with the experimental spectra (Figs. 2, 3). This good agreement is attributable to the fact that calculated lineshapes reflect the screened local DOS in presence of the final state core hole.It is clearly indicated in the spectra that the difference in the two lineshapes is mainly due to the increase of the contribution from the s-type partial DOS. The calculations based on the point defect model also show a good agreement with both the experiments and the results from the cluster model (Figs. 2, 3).

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Exp

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1465 1470 1475 !,180 1,185 1490 1,i95 1501;

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i

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:415 1420 1425 1430 14351440 1445 1450 1455

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[(ine',ie Energy (eV)

[(inetie energy (eV]

Fig. 2. Comparison of experimental and theoretical Al KL 1V spectra. a., Experimental and point defect model b., Experimental and cluster MO model

Fig. 3. Comparison of experimental and theoretical A1 KL 2 3 V spectra. a., Experimental a~d point defect model b., Experimental and cluster MO model

160

K V V and v a l e n c e b a n d lineshapes In Fig. 4. we present the measured AI KVV spectrum. 320 ,102 318 316

considerably smaller, than was predicted by previous experiments. This value is close to that estimated by using one-electron-like theory [18]. The high resolution A1 valence band photoelectron spectrum is shown in Fig. 6, in comparison with the result of cluster model calculations, (using the photoionization cross sections of Scofield [19]). The experimental data show rather good agreement with the theory.

314 312 4 310 1500 1510 1520 1530 1540 1550 1560 1570 1580 159,3

~

Exp.

3

Kinetic Energy (eV)

Fig. 4. Measured KVV Auger spectrum of A1. According to the final state rule, the core-valencevalence (KVV and L23VV) Auger and the photoelectron valence baird spectra should reflect the ground state DOS. After background correction, a 2 eV Gaussian instrumental broadening was subtracted from the KVV spectrum. The instrumental function was obtained by convolving a backscattered electron spectrum recorded at FRR ratio 20 with a Gaussian until the obtained distribution fitted the backscattered spectrum recorded at FRR ratio 4. Fig. 5 shows our experimental AIKVV spectrum (after corrections) in comparison with the result of a previous experiment.

! 0

15

l0

0

Fig. 6. High resolution valence band XPS spectrum of A1 [9] compared with the spectrum calculated using the DVXc~ cluster MO model. In Fig. 7 our experimental KVV spectrum is compared with the lineshape obtained from a monochromatized XPS aluminium valence band experiment by self convolution. 800

700 I 6oo ~

800

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Binding Energy (eV)

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Rel. Energy (eV')

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Rel. Energy (eV)

Fig. 5. Comparison of the present experimental (corrected for inelastic background) AIKVV spectrum to that obtained earlier [7]. Our measurements prove that the width (FWHM) of the KVV spectrum (14.5 eV) is

Fig. 7. Comparison of the experimental AI KVV lineshape to that calculated from the measured XPS valence band spectrum by self convolution. The similarity of the two widths futher supports that the width for the KVV spectrum, obtained by us, is the correct value. The experimental AI KVV spectrum corrected for inelastic background, can be seen in Fig. 8, in comparison with our theoretical spectra obtained from the

161

cluster model calculations adding the contributions from plasmon losses. We used the ratios of the angular parts of the KVV Auger matrix elements 0.81:2.5:3.7 for ss:sp:pp [17]. aoo

ii

.

.

_

<

600

.

.

References

',

~:r

t 5oo[

~: ,if4, ~4 300

.MML,'~, L:,-r~:/ (M;

Fig. 8. The experimental KVV Auger spectrum (corrected for inelastic background but showing the plasmon structure) and compared to the results of the DVXoc cluster MO model. The results demonstrate that the cluster MO model describe the core-valence-valence Auger lineshapes quite well in A1 and no correlation effects can be identified from the spectra. fi0q~ 0

2

This work was supported by the research projects: COST/D5/12014 (CEC), OTKA/T007274/1993, OTKA 3011

I

.

~~o IAtFvvl

7oe~'

Acknowledgments

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Fig. 9. Comparison of the experimental KVV and L2,3VV spectra. The KVV and L 2 3VV core-valence-valence spectra are compared in Fig. 9. The comparison shows differences in the structure of the low kinetic energy part. These differences are possibly due to the higher intensity contribution from the bulk (KVV) or surface (L23VV) plasmons. A further cause of the diffel:ences can be the presence of the L IL2sV Coster-Kronig satellite in the L2,3VVspectrum

I. G. Cubiotti, G. Mondio and K. Wandelt 1989 Auger Spectroscopy and Electronic Structure, Proc. of IWASES I (Springer Verlag.). 2. K. Wandelt, C.O. Almbladh and R. Nyholm (Eds), Phys. Scripta T41 (1992), Proc of IWASES II. 3. D.E. Ramaker, Crit. Rev. Solid State Mater Sci. 17 (1991) 211. 4. P. Weightman, J. Electron Spectrosc. Relat. Phenom. 68 (1994) 127. 5. P.S. Fowles, J.E. Inglesfield P. Weightman, J. Phys: Condens. Matter 3 (1991) 641. 6. V. Contini, C. Presilla and F. Sacchetti, Solid State Communications, 70 (1989) 851-853. 7. L. K6v~r, D. Varga, I. Cserny, J. T6th and K. T6k~si, Surface and Interface Anal. 19 (1992) 9. 8. D. Varga, L. K6v6r, I. Cserny, J. T6th and K T6k~si, to be published. 9. M.P. Seah, G.C. Smith and M.T. Anthony, Surf. Interface Anal. 15 (1990) 293. 10. P.H. Hannah. PhD Thesis, 1985, The Univ. of Liverpool. 11. T. Tanabe, H. Adachi and S. Imoto, Jap. Journal of Applied Physics, 17 (1978) 49-58. 12. D.E. Ramaker, F.L. Hutson, N . H Turner and W.N. Met, Phys. Rev. B33 (1986) 2574. 13. Yu N. Kucherenko, J. Electron Spectrosc. Relat. Phenom., 53 (1990) 39-50. 14. D.A. Shirley, Phys. Rev. 55 (1972) 4709. 15. B. Feuerbacher, B. Fitton, R.F. Willis, Photoemission and the electronic properties of surfaces (Wiley, N.Y. 1978) p. 122. 16. G. Cubiotti, in Proc. IWASES I see ref [1] p. 51. 17. D.E. Ramaker, F.L. Hutson, N.H. Turner and W.N. Met, Phys. Rev. B33 (1986) 2574. 18. C.O. Almbladh and A.L. Morales, Phys. Rev. B39 (1989) 3503. 19. J.H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 129.