XPS and factor analysis for investigation of sputter-cleaned surfaces of metal (Re, Ir, Cr)–silicon thin films

Applied Surface Science 179 (2001) 316±323

XPS and factor analysis for investigation of sputter-cleaned surfaces of metal (Re, Ir, Cr)±silicon thin ®lms R. Reiche*, S. Oswald, K. Wetzig Institut fuÈr FestkoÈrper- und Werkstofforschung Dresden, Postfach 27 00 16, D-01171 Dresden, Germany

Abstract XPS and factor analysis were used for investigation of the electronic structure of MexSi1 x (Me ˆ Re, Ir or Cr) thin ®lms which show a semiconductor-to-metal transition at a critical (metal) concentration xc. Sputter cleaning applied for surface preparation leads to enrichment of Me and, as proved by TEM investigations, to silicide formation. For RexSi1 x and IrxSi1 x ®lms, this in¯uences the XPS valence band and the core level spectra, signals of silicide bonding are extracted and their contributions to the XPS data are quanti®ed by factor analysis. On that way, an indirect connection to xc for RexSi1 x ®lms is found by quanti®cation of metallic Re contributions. For the CrxSi1 x ®lms, valence band studies are more promising, energy shifts of the valence band edge correlate with the electrical transport properties. # 2001 Elsevier Science B.V. All rights reserved. PACS: 79.60-i; 61.80-x; 68.55.Nq Keywords: X-ray photoelectron spectroscopy; Factor analysis; Metal±silicon thin ®lms; Silicides; Sputtering artifacts

1. Introduction Metal±silicon systems as Re±Si, Ir±Si and Cr±Si have received much interest in recent years because of their high temperature stable, semiconducting silicides which are considered promising for thermoelectric applications [1,2]. As a starting point amorphous thin ®lms of those binary systems were grown. These MexSi1 x (Me ˆ Re, Ir or Cr) ®lms show a semiconductor-to-metal transition at a critical (metal) concentration xc. For example, an xc  0:32 was found by electrical measurements of amorphous

*

Corresponding author. Tel.: ‡49-351-4659-451; fax: ‡49-351-4659-452. E-mail address: [email protected] (R. Reiche).

RexSi1 x ®lms [3]. To our knowledge this is the highest published value of a critical concentration of amorphous metal±semiconductor alloys, it might already be the percolation threshold for the Re atoms that could determine the electrical transport behaviour. One aim of the thin ®lm investigations is to ®nd out correlations between the electrical transport properties, defect structures and the electronic structure. For studies of their electronic structure X-ray photoelectron spectroscopy (XPS) may be gainfully applied [4]. In this paper, we extend our previous XPS investigations of RexSi1 x ®lms [4,5] to a comparative study of three MexSi1 x (Me ˆ Re, Ir or Cr) systems. The surfaces of the thin ®lms were sputter-cleaned before XPS analysis. This might be essentially useful for the amorphous samples, however, ion bombard-

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 0 0 - 2

R. Reiche et al. / Applied Surface Science 179 (2001) 316±323

ment-induced changes of surface composition and structure have to be considered [4±7]. The XP core level spectra of argon ion bombarded MexSi1 x ®lms over the whole composition range 0  x  1 are evaluated systematically by factor analysis (FA) [8]. On that way, it is possible to extract and to quantify the principal components in¯uencing the XP core level peak shapes. The XPS valence band region is of obvious interest for an explanation of the electrical transport behaviour and will complement our FA study of the ®lms. 2. Experimental Homogeneously composited RexSi1 x, IrxSi1 x and CrxSi1 x ®lms (100 nm) were deposited at room temperature in a cryopumped vacuum system by magnetron sputtering onto thermally oxidised silicon wafers. Alternatively some IrxSi1 x ®lms were produced under ultra high vacuum conditions by electron beam evaporation [9]. The ®lm composition was determined by Rutherford backscattering spectroscopy (RBS) and by energy-dispersive X-ray spectroscopy (EDXS). Silicon-rich ®lms (x < 0:5) were shown to be amorphous by X-ray diffraction (XRD) and transmission electron microscopy (TEM) [9,10]. The XPS investigations were performed on sputtercleaned surface regions. For sputter cleaning the following conditions were applied: ion beam of typically 0.4 mA current with Ar‡ ions of 3.5 keV energy at an incidence angle of 308 to the surface normal, the ion beam was scanned over a surface area as large as 10 times the region analysed by XPS. The photoelectron spectra were recorded with a Physical Electronics model PHI 5600 CI electron spectrometer using Mg Ka (1253.6 eV) or monochromated Al Ka excitation (1486.6 eV), electron take-off angle of 458, pass energy of the concentric hemispherical analyser ®xed at 12 eV and data acquisition with 0.1 eV per step. The binding energy scale of the instrument was calibrated against Cu 2p3/2 (932.7 eV) and Au 4f7/2 (84.0 eV) lines with a tolerance of <0.1 eV in the energy difference between the two peaks. The system base pressure was 3  10 8 Pa. Complementary to the XPS measurements, Auger electron spectroscopy (AES) and microscopical studies (SEM and TEM) of the samples were carried out.

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3. XPS data analysis procedures The XPS results that we present in this paper were obtained by a quanti®cation of atomic compositions, by factor analysis of core level spectra and by an evaluation of valence band spectra. The results of the latter two analysis methods again are related to the obtained atomic composition scale. Therefore, some remarks about the composition calculation and, as it is rarely applied with XPS, about our factor analysis procedure are given in the following section. Two approaches were used for quanti®cation of atomic composition. Naturally for our sputter-deposited ®lms the investigated surface region is assumed to be homogeneous, isotropic and smooth. As a semiquantitative approach atomic compositions were ®rst calculated with the atomic sensitivity factors of the XPS system [11]. The second approach is the quanti®cation of our binary MexSi1 x samples by reference to spectra from element standards obtained under identical analysis conditions. XMe ˆ

st IMe =IMe st ‡ F st ; IMe =IMe SiMe ISi =ISi

XMe ‡ XSi ˆ 1

with (1)

In Eq. (1), Ik ; Ikst are the intensities of a photoelectron peak of element k (Me or Si) in the analysed sample and in the element standard, respectively. With the matrix factor   lm …EMe †lstSi …ESi † astMe 3 FSiMe ˆ (2) lm …EA †lstMe …EMe † astSi the different escape depths of the photoelectrons are considered and the different atomic densities of the element standards are accounted for, the mean free path lm(E) and the average volume am of an atom in matrix m are calculated after [12]. In order to extract and to quantify overlapping components in XP core level spectra factor analysis (FA) was used. The FA is a multivariate statistical technique [8] which has been gainfully applied for XPS data analysis of metal±silicon thin ®lms [4±7]. The theory of FA has been addressed extensively in literature, e.g. [8,13,14], and we have reported about our FA procedures previously [7,15,16]. Therefore, only the main features of FA together with some details of our FA procedure for the Me 4f

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(Me ˆ Re or Ir) and the Si 2s spectra are given here. The whole procedure consists of data pre-processing, principal component analysis (PCA) and transformation (PCT). For a clearer FA response we preprocessed the Me 4f photoelectron spectra by a Shirley-type background subtraction [17], the Si 2s spectra by subtraction of a linear background, which comes mainly from the Me 4f electrons [18], and both the background-stripped spectra by concatenation to a joint data matrix. The concatenation of selected spectral windows is useful for revealing correlations among several elements [4±7,16,19,20]. After this pre-processing the variance in the data is analysed by the FA procedure. Basically, it is assumed that each concatenated spectrum represents a linear combination of independent (pure) spectra named as principal components (PCs). In the PCA part, an eigenanalysis of the data matrix is carried out splitting the spectral information into the shape of orthogonal eigenvectors and their contribution to the signals (loadings). For decision about the number of signi®cant eigenvectors, i.e. those that are necessary to reproduce the data matrix within experimental error, several test functions based on the eigenvalues (e.g. imbedded error function, factor indicator function) [8] are calculated and give some hints on the requested number, which is checked and worked out afterwards by visual criteria (e.g. negligible loadings of secondary eigenvectors for each concatenated spectrum, noisy appearance of the reconstructed spectra's residuals) [18]. The output of this crucial step in FA are mathematically abstract PCs, which ®t the spectral features, and secondary eigenvectors, which take account for the noise and are neglected. Care has been given to the fact that ``ghost components'' could appear due to mistakes in data pre-processing, especially it was checked that the number of PCs of the no-concatenated spectral regions is in accordance with the concatenated ones [18]. In the PCT part of FA, the PCs are transformed by matrix rotation (target and abstract rotation) [8] into spectroscopically relevant spectra under two boundary conditions: PCs' peak shapes should be as close as possible to the lineshapes of the included element standards and their loadings should be between 0 and 1. Finally, to get results that are easier to interpret, the PCs are normalised to its maximum peak height and afterwards their loadings li are re-calculated according P to: li ˆ 1 for each atomic composition XMe.

4. Results and discussion The ®lms were exposed to atmospheric pressure during their transfer into the spectrometer, and therefore, their surfaces had to be sputter-cleaned before XPS analysis. An ion ¯uence of typically 1  1016 Ar‡/cm2 was necessary for the removal of hydrocarbon contamination and of some oxide. This ion ¯uence corresponds to sputtering of a layer of about 2 nm thickness. On the other hand, it leads to an enrichment of the metallic element in the surface region (Fig. 1). When the samples had been kept under normal ambient conditions for longer times, a higher ion ¯uence was needed to clean their surfaces. As can be seen in Fig. 1 for the RexSi1 x ®lms, the enrichment increases with the ion ¯uence. It is clear that while removing the surface contamination and oxide some sputtering of the MexSi1 x ®lm is occuring. Moreover, the ®lm is in¯uenced in deeper regions (depth  10 nm) by the penetrating argon ions. With this the Me enrichment may be understood in terms of preferential sputtering of the Si atoms at the surface and preferential atomic mixing, e.g. preferential recoil implantation of the Re atoms, leading to non-monotonous

Fig. 1. Atomic composition XRe in the sputter-cleaned surface region via the rhenium fraction x of the as-deposited RexSi1 x ®lms. XRe was calculated with the atomic sensitivity factors ASF of the XPS system (open symbols) and by reference to spectra from element standards (solid symbols) using Eq. (1) with matrix correction (2). For sputter cleaning of RexSi1 x ®lms with x > 0:6 a higher argon ion ¯uence was necessary: these ®lms were exposed to 4 instead of 2  1016 Ar‡/cm2 for the second XPS measurement series.

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composition variations from the surface into the ®lm [18]. However, at the low ion ¯uences used for sputter cleaning, we are still close to the homogeneous situation of the as-deposited ®lm and the atomic composition in the surface region may be approximated by Eq. (1). Using element standards and the matrix correction (2) we obtained somewhat higher atomic fractions of the metallic element than with the semiquantitative approach via the XPS atomic sensitivity factors (Fig. 1). In the following, the matrix-corrected composition values XMe are used for further evaluation of the XPS signals. Binding energy shifts of the Si 2s, Si 2p and Re 4f or Ir 4f core level peaks of different samples and after various ion ¯uences show a systematic functional behaviour in dependence on XMe [4,18]. In order to ®nd out the basic peak shapes, the principal components (PCs), that are responsible for the binding energy shifts factor analysis (FA) was applied. For the RexSi1 x ®lms four PCs are found: beside the two elemental PCs Re and Si, two Re±Si PCs are extracted from the concatenated Si 2s±Re 4f spectra (Fig. 2). The silicon-rich PC ReSiv shows a shift of ‡0.5 eV B.E. in Si 2s relative to the peak position of elemental Si, but there is no B.E. shift in the Re 4f spectrum. In contrast, the rhenium-rich PC RewSi is characterised by a shift of ‡0.3 eV B.E. in Re 4f compared to the peak positions of elemental Re and almost no B.E. shift can be seen in the Si 2s peak. Because of these characteristics, two different types of Re±Si chemical bonding may be assumed. Following this idea with a look at possible phases in the Re±Si phase diagram [21,22], the two Re±Si PCs may be connected with the two silicides ReSi1.75 and Re2Si. Rhenium disilicide, reported as ReSi1.75 [23], is a narrow-gap semiconductor with a band gap of 0.12±0.15 eV [24,25] while Re2Si is a metallic phase [21]. Both silicides were found by TEM investigations of selected RexSi1 x ®lms. Though argon ion thinning had been used in the preparation procedure morphological differences between as-deposited and bombardment-modi®ed samples were clearly visible [18]. Ion bombardment of an amorphous silicon-rich (x ˆ 0:1) ®lm seems to induce ReSi1.75 nanocrystallite (<5 nm) formation. At higher rhenium content (x ˆ 0:33) Re2Si crystallites are formed. And ®nally, a rhenium-rich (x ˆ 0:9) ®lm has already got Re2Si crystallites and contains even more crystallites after

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Fig. 2. Results of XPS factor analysis (FA) for the RexSi1 x ®lms: (a) spectra of the principal components (PCs) obtained from concatenated Si 2s and Re 4f core level data (Mg Ka excitation was used), (b) normalized loadings of the PCs as a function of atomic composition XRe (spline lines to guide the eye), the critical Re fraction xc for the semiconductor-to-metal transition is indicated at the composition scale.

the ion bombardment. In conclusion, the formation of the equilibrium phases of the Re±Si system [21] is supported by sputtering under our argon ion bombardment conditions. Nevertheless, a large amount of amorphous phase is still found after ion bombardment. Therefore, we suggest to interpret our XPS and FA results for the Re±Si PCs in terms of the formation of short range order agglomerates with silicide bonding. In our FA results (Fig. 2), the PC ReSiv has its maximum contribution to the core level spectra (Fig. 2b) at X Re  0:3, which is close to the disilicide composition x ˆ 0:36 [21]. A stronger point for the assignment of PC ReSiv to disilicide bonding comes from XPS measurements of crystalline ReSi1.75 ®lms [25], the core level spectra show a positive Si 2p peak shift and no shift in the Re 4f peaks as in the case of PC

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ReSiv. The PC RewSi (Fig. 2a) should be a signal for the silicide bonding of the rhenium-rich, metallic phase Re2Si. In fact, a very similar FA component was found after ion bombardment-induced Re2Si formation at Re/Si interfaces [6,26]. The Re2Si corresponds to an atomic composition of x ˆ 0:66±0.67 [21], for which the PC RewSi shows a marked loading (Fig. 2b). However, the maximum loading of PC RewSi is found at X Re  0:9. This deviation appears to be mainly caused by the ion bombardment-induced microstructure, the coexistence of different phases, which actually is quanti®ed by the PCs loadings via XRe. Another reason is probably the small oxygen content of the rhenium-richest ®lms (those were sputtered with a higher ion ¯uence in Fig. 1), it leads to a shoulder at the higher B.E. side of the Si 2s peak (Fig. 2a) and could produce a small shift in Re 4f similar to the shift caused by silicide bonding [6,18]. Beside these XPS results on silicide bonding, FA indicates a correlation to the electrical transport properties of the as-deposited ®lms. The loadings of PC Re rise from nearly 0 to 0.35 for atomic compositions XRe passing through the critical composition of the semiconductor-to-metal transition xc  0:32, i.e. metallic Re regions were built-up that might have determined the electrical transport behaviour of the ®lms. One may object that those Re regions were formed by the ion bombardment. In TEM investigations [10], however, the formation of Re micro grains with sizes up to 1 mm was observed for an as-deposited Re0.45Si0.55 ®lm after remaining at room temperature for 0.5 year. In Fig. 3, results of XPS measurements and FA for IrxSi1 x ®lms are shown. The FA reveals ®ve PCs: beside the two elemental PCs Ir and Si, three Ir±Si PCs of different Ir 4f/Si 2s intensity ratio are found (Fig. 3a). All three Ir±Si PCs are positively shifted (‡0.45 to ‡0.6 eV B.E.) in the Si 2s peak relative to elemental Si, but in the Ir 4f peak positions they behave differently: ‡0.35 eV B.E. shift for PC IrSiv, almost no shift for PC IrwSi and 0.3 eV B.E. shift for PC IrmSi with respect to metallic Ir. As for the RexSi1 x ®lms these Ir±Si PCs may be interpreted in terms of silicide bonding, though the iridium-rich side of the Ir±Si phase diagram [22] is not well explored yet making the silicide ascription of the PCs IrwSi and IrmSi hardly possible. For the siliconrich side of Fig. 3b the only Ir±Si PC is IrSiv. There is no correlation of the FA results to the critical atomic

Fig. 3. Results of XPS factor analysis (FA) for the IrxSi1 x ®lms: (a) spectra of the principal components (PCs) obtained from concatenated Si 2s and Ir 4f core level data (mono-chromated Al Ka excitation was used), (b) normalized loadings of the PCs as a function of atomic composition XIr (spline lines to guide the eye), the critical Ir fraction xc for the semiconductor-to-metal transition is indicated.

composition xc  0:22 for IrxSi1 x ®lms [27], but the PC IrSiv can be assigned to the semiconducting silicide Ir3Si5 of the Ir±Si phase system [28]. In XPS measurements of an arc-melted Ir3Si5 standard, whereas for preparation the surface was scraped in situ by a ®le, the same peak shifts were found as for the PC IrSiv. Besides, Ir3Si5 is stoichiometric at an atomic composition x  0:37 [28] which is about the composition XIr of the maximum loading of PC IrSiv in Fig. 3b. Different from the other two MexSi1 x systems, for the CrxSi1 x ®lms and its Si 2s, Si 2p and Cr 3p peaks very small binding energy shifts were found: especially in Si 2s only ‡0.1 eV B.E. relative to elemental Si for 0:14  X Cr  0:4 and in Cr 3p increasing shifts up to ‡0.2 eV B.E. relative to elemental Cr with XCr

R. Reiche et al. / Applied Surface Science 179 (2001) 316±323

decreasing from 0.5 to 0.2 were measured. By FA of the concatenated Si 2s±Cr 3p spectra a silicon-rich PC is extracted that dominates the core level spectra for 0:1  X Cr  0:4, but does not show a particularly marked loading within this composition range. On basis of these results it is dif®cult to draw some conclusions on the electronic interaction of the Cr and Si atoms. Disilicide bonding may play a role as for the other two MexSi1 x systems, but in that case its in¯uence on the CrxSi1 x core level spectra is much smaller. In Fig. 4a, we present selected XPS valence band spectra of IrxSi1 x ®lms at different atomic composition XIr. Valence band changes appear in correspondence to the FA results (Fig. 3b): up to X Ir ˆ 0:37 just one broad peak around 4 eV B.E. is visible rising in intensity corresponding to an increase of the loadings of PC IrSiv, for higher XIr more Ir±Si components contribute and the valence band splits up into two distinct peaks which start to shift towards lower B.E. for X Ir > 0:6 corresponding to increasing contributions of PC IrmSi. The valence band spectra themselves are determined by intensity and distribution changes of Ir 5d states because of the photoionisation cross-section ratio Ir 5d:Ir 6s:Si 3p ˆ 94:3:1 [29]. The splitting-up of the XPS valence band is a general feature for silicides [2,30] and was also found for iridium silicides [31]. Basically the valence band structure is made of non-bonding d states near the Fermi level (peak marked with n in Fig. 4a) and bonding d-Si 3p hybridised states (around peak b in Fig. 4a) which determine the stability of the silicide extending to about 6 eV below the Fermi level. For X Ir ˆ 0:37 the splitting-up of the valence band is hardly visible, Ir3Si5 contributions may be covered due to the prevailing amorphous structure. In spite of this, the formation of Ir3Si5 could have an in¯uence on the valence band edge position because of its band gap of 1.57 eV energy [32] which is greater than the band gap of silicon. For metals the valence band edge position is at the Fermi energy at 0 eV B.E. and can be determined by calculating the ®rst derivative of the valence band spectrum [33]. A deviation from 0 eV B.E. for the ®rst maximum in the valence band derivative as shown in Fig. 4b is a deviation from metallic behaviour. With this idea, the energy shifts via XMe in Fig. 4c for the XPS valence band edge of IrxSi1 x, RexSi1 x and CrxSi1 x ®lms can be evalu-

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Fig. 4. Valence band investigations of sputter-cleaned MexSi1 x ®lms (Me ˆ Ir, Re, Cr) after monochromated Al Ka excitation (hn ˆ 1486:6 eV): (a) selected valence band spectra of IrxSi1 x ®lms at different atomic composition XIr, (b) ®rst derivative (®vepoint-method) of the valence band edge of some spectra in (a) to show deviations from metallic behaviour, (c) energy shifts of the ®rst maximum in the derivative of the valence band edge of IrxSi1 x, RexSi1 x and CrxSi1 x ®lms as a function of atomic composition XMe (spline lines to guide the eye), the critical fraction xc for the semiconductor-to-metal transition in the as-deposited ®lms is indicated with a vertical dotted line to the respective curve.

ated. In general, a decreasing energy shift is observed with increasing XMe. There is a good correlation to the electrical transport properties of the as-deposited CrxSi1 x ®lms which have an xc  0:12 for the semiconductor-to-metal transition [34,35]. For the IrxSi1 x ®lms, no such correlation can be found, after argon ion bombardment just the pure Ir ®lm appears to be metallic. For 0:3  X Ir  0:43 the valence band edge shift even increases, the formation of Ir3Si5 seems likely. On the other hand, this is in correspondence with the PC IrSiv dominating the photoelectron spectra (Fig. 3b), which agrees again to the ascription of this

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PC to the semiconducting phase Ir3Si5. Likewise for the RexSi1 x ®lms, the formation of semiconducting ReSi1.75 seems to determine the shift of the valence band edge and its contribution is quantitatively recorded in the loadings of PC ReSiv (Fig. 2b). Moreover, it is interesting to note that similar changes of the valence band shape as described above for IrxSi1 x are observed for RexSi1 x [4,18]. Finally, for the CrxSi1 x ®lms, the formation of semiconducting CrSi2 [2] appears negligible, otherwise it would certainly in¯uence the valence band edge as CrSi2 crystallisation clearly changes the transport properties of the ®lms [36]. 5. Conclusions Sputter cleaning with argon ions is not a good tool to prepare surfaces of RexSi1 x and IrxSi1 x ®lms for XPS investigations of their electronic structure. Enrichment of the metallic element and silicide formation are supported and in¯uence the XPS spectra. On the other hand, factor analysis is gainfully applied to extract signals of silicide bonding and to quantify their contributions to the XPS data, the coexistence of different phases can be quantitatively evaluated in dependence on the atomic composition XMe. Obviously, this is an advantage that will make in situ-XPS investigations of these metal-silicon materials very useful. For the CrxSi1 x ®lms only an ion bombardment-induced Cr enrichment was found, silicide formation is negligible or of little in¯uence on the XPS spectra. This is promising for the evaluation of the electronic structure around the semiconductor-to-metal transition. However, photoelectron spectroscopy of CrxSi1 x ®lms of very low Cr content (X Cr < 0:12) has still to be carried out. Acknowledgements The authors would like to thank Dr. J. Schumann and Dr. R. Kurt for producing the metal±silicon thin ®lms, Dr. J. Thomas and D. Hofman for performing TEM investigations and Dr. D.B. Migas for carrying out LMTO bandstructure calculations on some Re±Si unit cells. All of them, Dr. H. Vinzelberg and Prof. Dr. A. Heinrich we thank for helpful discussions.

References [1] C.B. Vining, in: Proceedings of the 9th International Conference on Thermoelectrics, Pasadena, California Institute of Technology, California, 1990, p. 249. [2] H. Lange, Phys. Stat. Sol. (b) 201 (1997) 3. [3] H. Vinzelberg, A. Heinrich, C. Metz, J. Schumann, in: Proceedings of the 6th International Conference On Hopping and Related Phenomena, Jerusalem, 1995, p. 224. [4] R. Reiche, S. Oswald, H. Vinzelberg, C. Metz, J. Schumann, A. Heinrich, K. Wetzig, Fresenius J. Anal. Chem. 358 (1997) 329. [5] R. Reiche, S. Oswald, J. Thomas, H. Reuther, M. Dobler, J. Schumann, K. Wetzig, in: Proceedings of the ECASIA 1997 Ð Seventh European Conference on Applications of Surface and Interface Analysis, Wiley, Chichester, 1997, p. 456. [6] R. Reiche, S. Oswald, D. Hofman, J. Thomas, K. Wetzig, Fresenius J. Anal. Chem. 365 (1999) 76. [7] R. Reiche, S. Oswald, M. Dobler, H. Reuther, M. Walterfang, K. Wetzig, Nucl. Instr. Meth. B 160 (2000) 397. [8] E.R. Malinowski, D.G. Howery, Factor Analysis in Chemistry, Wiley, New York, 1980. [9] R. Kurt, W. Pitschke, A. Heinrich, J. Schumann, K. Wetzig, in: Proceedings of the 16th International Conference on Thermoelectrics, IEEE 1997th 8291, Dresden, 1998, p. 303. [10] J. Thomas, J. Schumann, W. Pitschke, Fresenius J. Anal. Chem. 358 (1997) 325. [11] C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.H. Raymond, L.H. Gale, Surf. Interface Anal. 3 (1981) 211. [12] M.P. Seah, W.A. Dench, Surf. Interface Anal. 1 (1979) 2. [13] M.F. Koenig, J.T. Grant, J. Electron Spectrosc. Phenom. 41 (1986) 145. [14] B.J. Kooi, M.A.J. Somers, Surf. Interface Anal. 21 (1994) 501. [15] S. Oswald, S. Baunack, Surf. Interface Anal. 25 (1997) 942. [16] R. Reiche, R. Thielsch, S. Oswald, K. Wetzig, J. Electron Spectrosc. Relat. Phenom. 104 (1999) 161. [17] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [18] R. Reiche, PhD Thesis, Technische UniversitaÈt Dresden, Germany, 2000. [19] H. Bubert, H. Jenett, Fresenius Z. Anal. Chem. 335 (1989) 643. [20] H. Bubert, M. Korte, R. Garten, E. Grallath, M. Wielunski, Anal. Chim. Acta 297 (1994) 187. [21] J.L. Jorda, M. Ishikawa, J. Muller, J. Less-Common Met. 85 (1982) 27. [22] T.B. Massalski, Binary Alloy Phase Diagrams, Vol. 2, 2nd Edition, W.W. Scott, 1992. [23] U. Gottlieb, B. Lambert-Andron, F. Nava, et al., J. Appl. Phys. 78 (1995) 3902. [24] C. Krontiras, L. GroÈnberg, I. Suni, Thin Solid Films 161 (1988) 197. [25] T.A. Nguyen Tan, J.Y. Veuillen, P. Muret, S. Kennou, A. Siokou, S. Ladas, F.L. Raza®ndramisa, M. Brunel, J. Appl. Phys. 77 (1995) 2514. [26] S. Oswald, R. Reiche, Appl. Surf. Sci. 179 (2001) 308±316.

R. Reiche et al. / Applied Surface Science 179 (2001) 316±323 [27] R. Kurt, PhD Thesis, Technische UniversitaÈt Dresden, Germany, 1999. [28] C.E. Allevato, C.B. Vining, J. Alloys Comp. 200 (1993) 99. [29] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [30] M. Azizan, R. Baptist, T.A. Nguyen Tan, J.Y. Veuillen, Appl. Surf. Sci. 38 (1989) 117. [31] Y.M. Yarmoshenko, S.N. Shamin, L.V. Elokhina, V.E. Dolgih, E.Z. Kurmaev, et al., J. Phys. Condens. Matter 9 (1997) 9403.

323

[32] J. Schumann, D. Elefant, C. Gladun, A. Heinrich, W. Pitschke, H. Lange, W. Henrion, R. GroÈtzschel, Phys. Stat. Sol. (a) 145 (1994) 429. [33] M.T. Anthony, M.P. Seah, Surf. Interface Anal. 6 (1984) 95. [34] A. MoÈbius, Phys. Stat. Sol. (b) 144 (1987) 759. [35] D. Elefant, C. Gladun, A. Heinrich, J. Schumann, H. Vinzelberg, Phys. Stat. Sol. (b) 151 (1989) K155. [36] C. Gladun, A. Heinrich, J. Schumann, W. Pitschke, H. Vinzelberg, Int. J. Electron. 77 (1994) 301.