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The intrinsic asymmetry of photoelectron peaks: dependence on chemical state and role in curve fitting.
ELSPEC 3491
Journal of Electron Spectroscopy and Related Phenomena 95 (1998) 45–56
The intrinsic asymmetry of photoelectron peaks: dependence on chemical state and role in curve fitting. A.M. Salvia a, James E. Castle b,* a
University of Basilicata, Potenza 85100, Italy University of Surrey, Guildford GU2 5XH, UK
b
Received 16 December 1997; revised 23 April 1998; accepted 12 May 1998
Abstract Correct estimation of peak and background in XP-spectra is necessary for quantitative analysis. The problem is made difficult because neither the peak nor the background have a known shape and their separation requires the photoemission process to be treated as a sequence of well separated events. Background removal can be undertaken using the Tougaard method which is based on a careful analysis of the electron transport taking place in a solid but this procedure is designed to remove only the extrinsic component. There is a further component of the background which is intrinsic to the peak and associated with the photoemission event at the atomic/molecular level. In this paper we build on previous papers, which were concerned with metallic elements, to assess the importance of oxidation on the intrinsic asymmetric shape of the peak and thus make this available for curve fitting. This was done, using a programme, Tryfit, based on the Shirley type algorithm modified by Proctor to quote separately the peak and background intensity in XP-spectra. The programme allows the shape parameter, k, to be extracted which helps in defining the ‘intrinsic’ shape of a photoelectron peak. The shape parameter, k, is found to be independent of instrumental effects and intrinsically related to atomic number and, as now shown in this paper, to a certain extent to chemical state. It is demonstrated in this paper that its use can help in quantifying individual peak contributions to a multicomponent XPS spectrum. q 1998 Elsevier Science B.V. All rights reserved Keywords: XPS quantification; Electron energy loss; XPS peak shape; Curve fitting
1. Introduction In recent papers [1,2] we showed that the part of the ‘Shirley’ background within the range of a typical photoelectron peak can be accounted for by electron energy losses generated during photoemission and is almost independent of the extrinsic effects related to the transport of the electron through the solid described by Tougaard [3]. This background gives the peak an intrinsic asymmetry which can be defined by a single parameter, referred to as the shape * Corresponding author.
parameter, k. For the pure elements, k varies with atomic number and was shown to be independent of the variables associated with spectrum acquisition and similar in value for different orbitals of a given element. In this paper we report the results of a study of the dependence of peak asymmetry, as defined by k, on the chemical state of an element and we show how knowledge of the shape parameter can help in deriving the correct fitting choices for the interpretation of XPS data. The method by which the parameter is defined and normalised to the peak area can be understood using Fig. 1 and its caption. This figure illustrates the fitting
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of the data by a Gaussian–Lorentzian (G/L) peak in the form of a Voigt function. The Tryfit programme as developed by Proctor [4] considers that the Voigt function can be combined with a parameter ‘Tail 1’ to allow for the introduction of the asymmetry arising, particularly in metallic states, from conduction electron/hole interactions and with a parameter ‘Tail 2’ which represents the total background arising from scattering on the basis of the Shirley algorithm [5]. The use of a tail to include the Shirley background in the curve-fitting procedure has been previously discussed by Sherwood [6]. The background used in this fitting extends into the data points on the low binding energy side of the peak and Proctor has modified the Sherwood tail through the use of a polynomial, P, to give Tail 2 different shapes in order to match the ‘actual’ spectrum profile, where: P = B1 + B2 p D=W + B3 p (D=W )2 + B4 p (D=W )3
Fig. 1. Illustration of Cu 3s curve fitting as used in this paper: (a) raw spectrum; (b) construction of a background and peak fitting using Tryfit. A Voigt function peak shape is added to a Shirley background convoluted with a polynomial tail (Tail 2). The total background rise is defined by the point ‘h’ which, in turn, is fixed by the chosen value of ‘k’. In this illustration the value of k used was that derived from fitting the Cu 2p spectrum. Tail 1 was fixed at zero; (c) the raw data after background removal.
and W = Full Width Half Maximum, FWHM in channels, D = X − X 0, where X 0 is the peak centre and X − X 0 expresses the distance in channels from the peak centre. When B1 = 1 and B2, B3 and B4 are set to zero the family of tails take the form of straight lines having a common origin at the peak centre. We have used the height of this origin, h, divided by the area of the G/L peak to define the value of the shape parameter, kappa, k, which was shown to have ‘intrinsic’ properties [1,2]. In practice, the position of the origin is obtained from the ‘Tail 2’ parameter given by the program as a ratio to peak height, H; see Fig. 1. We use the value of the shape parameter only to fix the magnitude of the integral background beneath the main part of the peak itself. The tail extending from the peak gradually changes from intrinsic to extrinsic in character. Tougaard and Ignatiev [7] defined this tail using a parameter, D, which was measured at a point distant from the peak so that it was mainly determined by extrinsic losses. In our case we have taken the inverse value but, crucially, defined it at the peak centre so that it represents intrinsic losses. Two oxide systems were chosen for examination of chemical effects on k: chromium, a typical transition element and one for which we already have considerable experience in peak fitting [8] and copper, which gives easy access to two well defined chemical states of a single element.
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2. Experimental procedure 2.1. X-ray photoelectron spectroscopy XPS spectra of the chromium and copper compounds were recorded by an ESCA 3 spectrometer (VG Scientific Ltd, UK) using Al Ka excitation (1486.6 eV) operated at 20 mA and 13 kV. The base vacuum during acquisition was always better than 10 −9 torr. The analyser was operated at a constant pass energy of 50 eV and at a slit width of 4 mm, giving a resolution of 1.1 eV on the silver 3d 5/2 line. A take-off angle of 458 was used throughout this work. When necessary, specimens were bombarded with Ar ions to eliminate surface contamination. Spectra were acquired using a VG 3040 datasystem attached to the ESCA 3 and then transferred to the PC computer on which the fitting software was run. None of the BE values reported in the XPS figures and tables are corrected for surface charging. 2.2. Specimen preparation 2.2.1. Chromium Chromium metal (0.05 mm thick, 99.99%, polyester backed foil, Goodfellow Metals, Cambridge) was analysed ‘as received’ and after sputtering with Ar ions by a VG-AG2 gun in the analysis chamber of the ESCA 3. During ion etching, the energy of argon ions was 6.0 keV with a focusing voltage of 3.0 kV and the residual pressure in the analyser chamber was 10 −6 torr. Specimens of pure chromium (III) oxide (Puratronic, Ventron GMBH) were prepared by pressing the powder onto Indium foil. This allowed a safe introduction into the preparation chamber. The spectra were recorded only after the wide scan showed that no features from Indium foil were present. 2.2.2. Copper Copper was chosen for this test primarily because it is easily oxidized in situ from the clean metal surface through the two forms of oxide. The oxides also have clear differences in Auger parameters [9] and different satellite structures both of which point to significant final state interactions with neighbouring atoms which may give rise to differing background asymmetries.
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Copper was obtained as wire (OD, 0.25 mm) from Goodfellow Metals, Cambridge (99.99% purity). To produce evaporated films the copper wire, wound on tungsten filaments (OD, 0.5 mm) was evaporated onto a stainless steel sample holder substrate in the VG ESCA3 preparation chamber. Before evaporation, the substrate sample holder, copper wire and tungsten filament were ultrasonically cleaned in methanol. The sample-holder was further cleaned by argon etching in conjunction with heating before starting deposition. Outgassing of the tungsten filament and metal wire was also performed before positioning the substrate opposite the evaporator. Copper was then oxidized in air dried using a liquid nitrogen trap. Oxidation time, temperatures and air pressures were chosen as appropriate to the metal for each oxidation stage [10]. Cuprous oxide was obtained by vacuum annealing partly oxidized copper so that any cupric oxide was reduced by back reaction with copper. Wide and detailed spectra were acquired for each intermediate oxidation state until the Cu (II) oxide formed was sufficiently thick to show its characteristic features and to obscure the underlying metal. The sequence of spectra in Fig. 2 clearly shows the appearance of a well defined peak-position shift and shake-up satellites according to the oxidation state.
3. Results 3.1. Copper The value of the shape parameter, k, derived from the 2p spectra, Fig. 2, of the Cu (Fig. 2(a)), Cu 2O (Fig. 2(d)), and CuO (Fig. 2(e)) samples (Table 1) shows that it has a small dependence on chemical state although the Cu (0) and Cu (II) kappa values are found to be nearly identical and the largest change is for Cu (I). This behaviour is consistent with the fact that the Auger parameters of Cu (0) and Cu (II) are also closer than the Auger parameters of Cu (0) and Cu (I) [9] and hence there may be more closely spaced final states in the former pair. 3.2. Chromium The chromium 2p region of the metal and of its trivalent oxide (Figs. 3 and 4) were fitted and gave
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Fig. 2. Cu2p Spectra sequence of Copper metal ‘in situ’ oxidized using dry air: (a) copper metal ‘as deposited’; (b) air . 1 torr 258C, 40 min; (c) air . 1 torr, 300–2008C, 30 min; (d) as (c) plus 30 min 4508C, vacuum: Cu 2O 100%; (e) air . 1 torr, 4008C, 20 min: CuO 100%.
the values of k recorded in Table 1. The shape parameter, k, for chromium metal and the oxide, Cr 2O 3, were derived as a mean value from at least three different spectra acquired under the same experimental conditions. Again it is seen that there is a small but significant dependence on chemical state. However, comparison of k for chromium metal and chromium (III) oxide shows that the intensity increase across the body of the main peaks is very similar. However, Cr 2O 3 has a complex peak structure (Fig. 4) with shake up satellites and a discrete energy loss giving a step in the background peculiar to the oxide, but the influence of all of these
features is negligible for a small energy range in the vicinity of the peak. The long range energy losses which have been discussed previously by us [8] do not contribute to the value of k. 3.3. Oxygen The value of k was calculated from the oxygen 1s peaks of cuprous and cupric oxide with the interesting result that k, and hence the intensity of the intrinsic background, is seen (Table 1) to vary from one oxide to another. This implies that intrinsic losses are generated by photoexcitation of an anion as a result
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Table 1 The shape parameter, k, obtained, for the examined elements, by using Tryfit curve fitting results Core level
k a (eV −1)
Spectrometer/ Pass energy (eV)
X-ray
Sample
Cu 2p Cu 2p O 1s
0.039 6 3.7% 0.038 0.050 0.050
VgEsca3/50 Leybold/50 VgEsca3/50
Al Ka Mg Ka Al Ka
Cu film Metal foil b Cu 2O
Cu 2p O 1s
0.040 0.030
VgEsca3/50
Al Ka
CuO
Cr 2p Cr 2p
0.073 6 11.0% 0.063 6 10.4%
VGEsca3/50 VgEsca3/50
Al Ka Al Ka
Evaporated film Cr 2O 3
a The k-values associated with relative % error are expressed as a mean of at least, three replicates. bCopper metal foil was acquired, for comparison, with a Leybold LH X1 instrument at the University of Basilicata, Potenza, Italy.
of final state variations in the associated cation. Just from a qualitative point of view, considering what is reported about the chemical structure of the above compounds, the finding of the same value of kappa for copper (I) and oxygen suggests that in monovalent copper the d-levels are not core-like and are in the same energy regions as the anion valence levels. Different values of k are expected for Cu (II) and oxygen in cupric oxide since the d-orbitals are reported [11] to retain their atomic identity in divalent compounds of copper.
4. Discussion 4.1. The use of the shape parameter in XPS curve fitting The variation in k for the two metals and their oxides examined in this study is relatively small. The dependence of the value of the shape parameter for oxygen on the cation, with which it is associated, indicates that photoexcitation of oxygen ions leads to final states that involve excited states in the cation. Similar effects are seen by way of the Auger parameter [9] and have been shown to stem from the large polarisability of oxygen ions [12]. Thus, we anticipate that there will not always be a unique value of k for a given valence state but for some elements, such as oxygen, there will be a series of values depending on the local chemical environment.
From a practical point of view, however, we return to the fact that k, being related to intrinsic processes, should not depend on sample thickness, photon energy or the presence of an overlayer, as it would be if related to ‘extrinsic’ processes. This is recognised in the Tougaard algorithm which removes practically no intensity from the peak itself. The value of k obtained for reference homogeneous samples can therefore be applied to define a peak shape for a given element whatever its distribution in depth when present in complex layered specimens. We will demonstrate the validity of this argument using two examples. In the first example we consider the spectrum from a chromium metal foil carrying a thin oxide layer on the surface. In order to generate a sample with a known source of extrinsic background, titanium was evaporated onto the surface. As a test for success in the outcome of curve fitting we have used the metal/ oxide area ratio, determined using the areas of the G/L peaks for Cr (0) and Cr (III) respectively. As we show in Appendix A, this should remain independent of the overlayer thickness [13]. Fig. 5(a) shows the curve fitted Cr 2p region of the chromium substrate before deposition. In fitting this spectrum we started with the parameters obtained from the reference spectra and the programme was allowed to adjust them with the condition that k met and k ox should remain as close as possible to their reference values. This was done interactively by adjusting the peak and Tail 2 parameters of both metal and oxide components as Tryfit was not
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Fig. 3. Ion-etched cleaned Chromium metal Cr 2p curve fitted region.
designed to have k as a fitting parameter which could be automatically held at a constant value [4]. As can be seen from the figure, a good fit could be obtained by using constant tails (as for the reference spectra) and by adding a broad peak to account for the residual intensity arising from extrinsic losses. The most important thing is that for the first time the oxide/metal structure of the peak has been fitted using individual shapes for oxide and metal without having to make any assumption as to the nature of extrinsic losses in the close vicinity of the peak.
The fit used above was repeated on the spectrum obtained when the titanium overlayer had been added, restraining peak and k parameters to the values used for fitting Fig. 5(a). Fig. 5(b) shows the result. It can be seen, by comparing Fig. 5(a) and (b) and the fitting parameters given in their caption, that the same metal/oxide area ratio is obtained. This is as it should be since the titanium was evaporated in situ onto the chromium, which was not cleaned or etched prior to this deposition The difference between the spectra is entirely confined to the extrinsic
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Fig. 4. Chromium oxide (Cr 2O 3) Cr 2p curve fitted region.
background which becomes much larger after the titanium is deposited. In the second demonstration, we fit a Cr/Cr 2O 3 spectrum in which the oxide thickness was great enough to give significant uncertainty in the apportionment of background between the component peaks. In this test we show that, when the value of k is fixed, the ratio of the peak areas is independent of the shape of the extrinsic tail added to the G/L peak being that the common origin of the tails, as defined by kappa (i.e. intrinsic losses), is the same. The spectrum obtained from chromium carrying a relatively thick oxide is shown in Fig. 6. This spectrum has been fitted
using a constant Tail 2 plus a peak-like energy loss feature (Fig. 6(a)), as normally done for chromium spectra and also with a sloping Tail 2 (Fig. 6(b)) by choosing non-zero B-coefficients. The non-zero coefficients allow full fitting of the peak region without recourse to a Tougaard background as they mix elements of extrinsic background into the intrinsic background. The value of k used and the metal/ oxide area ratio obtained, are given in the captions to Fig. 6. Again we see that when the shape parameter is fixed, then the area ratio is maintained, independently of decisions made concerning the ‘extrinsic’ background shape.
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Fig. 5. a) Cr 2p region of an ‘as received’ chromium metal carrying a thin oxide overlayer; (b) the same region after a titanium overlayer had been evaporated onto the surface. Relevant fitting parameters: (a) Metal peak: centres; 574.4, 583.6, FWHM; 2.00, 2.12, G/L mix; 48%, Tail 1; 0.210, k; 0.072. Oxide peak: centres; 577.4, 586.9, FWHM; 3.19, G/L mix; 48%, Tail 1; 0.0, k; 0.063. Additional small peaks are used for satellites. Metal/oxide area ratio (2p 3/2 peak) = 4.03. (b) Metal peak: centres; 574.9, 584.0, FWHM; 2.00, 2.14, G/L mix; 50%, Tail 1; 0.181, k; 0.074. Oxide peak: centres; 578.0, 587.2, FWHM; 3.18, G/L mix; 50%, Tail 1; 0.0, k; 0.060. Additional small peaks are used for satellites. Metal/oxide area ratio (2p 3/2 peak) = 4.05.
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Fig. 6. Chromium metal carrying a thick oxide overlayer. Curve fitted Cr 2p region using TRYFIT. Relevant fitting parameters: (a) Constant Tail 2. Metal peak: centres; 574.4, 583.6, FWHM; 1.90, 1.90, G/L mix; 45%, Tail 1; 0.210, k; 0.072. Oxide peak: centres; 577.0, 586.4, FWHM; 3.50, G/L mix; 45%, Tail 1; 0.0, k; 0.063. Additional small peaks are used for satellites. Metal/oxide area ratio (2p 3/2 peak) = 0.97. (b) Sloping Tail 2. Metal peak: centres; 574.4, 583.6, FWHM; 1.80, 1.94, G/L mix; 45%, Tail 1; 0.220, k; 0.073. Oxide peak: centres; 577.0, 586.5, FWHM; 3.5, G/L mix; 45%, Tail 1; 0.0, k; 0.065. Additional small peaks are used for satellites. Metal/oxide area ratio (2p 3/2 peak) = 0.96.
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4.2. General discussion In earlier attempts [13–15] to devise and use a curve-fitting procedure which includes a background component in the fit for two or more chemical states, an impasse was reached because of the impossibility of deciding how the total background should be shared between the component peaks. In the case of a metal oxide, equally good fits can be obtained with all the background on the metal or on the oxide or for
any given mixture. The two extremes for chromium/ oxide are illustrated in Fig. 7. Clearly a better understanding of the permissible range of loss structures is necessary before the method can be used to analyse unknown surface structures and a protocol is required to remove subjective elements in the fitting procedure. The use of k as a shape parameter suits this requirement since it provides an integration constant for the Shirley background function appropriate for each peak and which is independent of extrinsic loss
Fig. 7. Chromium with a titanium overlayer. When the user is free to add background as desired, equally good fits can be obtained if the background is ascribed in any proportion to either peak. These show the extreme options of all background added to the oxide peak, or all added to the metal peak (from Ref. [13]).
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structure as defined by the Tougaard function. The peak parameters plus an intrinsic background characterised by the single parameter, k, together define a shape which can by used in the fitting routine within the narrow window of a high resolution peak. The intrinsic nature of the shape parameter, k, makes it suitable as a fitting parameter assisting the assignment of the correct proportion of background rise to each component in the envelope of overlapping peaks. We have shown here that it does depend on the chemical state of the element and different values will, in general, be needed for the different chemical states present in a multicomponent peak. In some cases, such as for the oxides of the transition metals examined here, the variation is not large. In other cases, especially when there are conducting and non-conducting compounds of the same element the variation can be very significant. Certain other elements, notably oxygen, have a behaviour which shows that their intrinsic background is influenced by that of the other elements with which they are combined. This finding indicates that photoexcitation of one atom can lead to a variety of final state electron levels in neighbouring atoms, and hence appearing as energy loss contributions to the photoelectron peak. Use of the shape parameter as a fitting parameter enables background removal from the peak structure as the sum of individual components of intrinsic background from each component peak together with a generalised extrinsic background of the Tougaard form. We have shown that when this is done, modifications to the extrinsic background, arising from the layered structure of the near surface, do not influence the outcome of the curve fitting. The success of this approach to curve fitting raises the possibility of developing fitting programs in which the peak clusters are fitted using k to define an intrinsic background and fitting the residue with a Tougaard background to define the position of the elements within a surface layer structure. When, as is often the case, the acquired window of the spectrum is too small for a full Tougaard to be fitted, then the B coefficients, as used in this work, could be utilised to give an indication of the likely layer structure. For these outcomes to be achieved it will be necessary for the program developers to devise a means by which the shape parameter can be entered and held constant as a fitting parameter.
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Acknowledgements We are grateful to S. Tougaard for the inspiration drawn from his scientific production and to A. Proctor for TRYFIT and much helpful discussion. We are pleased to acknowledge the contribution made to this line of work by the initial studies of I. AbuTalib. This work was assisted by the ‘Ministero della Universita’ e della Ricerca Scientifica e Tecnologica’ in collaboration with the ‘Conferenza dei Rettori delle Universita’ Italiane’ and of the ‘British Council’ (UK) under the ‘British-Italian Collaboration in Research and Higher Education’-1998- project.
Appendix A From Ref. [13] Using the usual mean free path for inelastic scattering, l, we can calculate the thickness, d ox, of any oxide present on the surface from the relationship dox = l sin v ln{(ICrIII =ICr0 ) + 1}
(A1)
where v = the take-off angle and I is the intensity of the oxide or metal component of the peak, as indicated by the subscript. The assumption is made in deriving this relationship that the standard intensities of chromium in metal and oxide are equal for very thick samples. Similarly the thickness, d Ti, of a titanium layer attenuating the oxide signal is given by dTi = l sin v ln{ICrIII =I9CrIII }
(A2)
where I9 is the reduced signal in the presence of titanium. The effect of both oxide and titanium can be seen on the metal peak, namely: 0 =I9Cr0 } dox + dTi = l sin v ln{ICr
(A3)
where I 0 refers to the intensity from a perfectly clean chromium surface. In the present case: 0 = ICr0 exp(dox =l sin v) ICr0
(A4)
Eq. (A3) can be used to obtain the combined thickness of oxide and titanium at any stage of evaporation and Eq. (A2) can be used independently to obtain the total thickness of titanium at any stage. Alternatively, these equations can be used with each individual stage of evaporation to yield the thickness added.
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The apparent chromium metal/oxide ratio at any thickness of titanium overlayer is given by I9CrIII =I9Cr0 = R
(A5)
where, as before, the prime indicates the intensity of the chromium signal in the presence of titanium. From the relationship used above it follows that R = exp − (dTi =l sin v)=exp − (dTi + dox )=l sin v = exp(dox =l sin v) − 1
(A6)
which is independent of d Ti.
References [1] A.M. Salvi, J.E. Castle, ECASIA 97, J. Wiley and Sons, Chichester, UK, 1997, pp. 809–812. [2] A.M. Salvi, J.E. Castle, J. Electron Spectroscopy and Related Phenom., accepted. [3] S. Tougaard, Surface and Interface Analysis (SIA) 11 (1988) 453.
[4] A. Proctor, Tryifit and Googly Manuals supplied at the University of Surrey. J.N. Fiedor, A. Proctor, M. Houalla, D.M. Hercules, SIA 23 (1995) 204. [5] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [6] P.M.A. Sherwood, in: Briggs, Seah (Eds.), Practical Surface Analysis, Appendix 3, John Wiley and Son Ltd, New York. [7] Tougaard and A. Ignatiev, Surf. Sci. 129 (1983) 335. [8] A.M. Salvi, J.E. Castle, E. Desimoni, J.F. Watts, Appl. Surf. Sci. 90 (1995) 333. [9] C.D. Wagner, in: Briggs, Seah (Eds.), Practical Surface Analysis, Appendix 5, John Wiley and Son Ltd, New York, 1990. [10] J.E. Castle, D. Epler, Proc. R. Soc. Lond. A 339 (1974) 49. [11] P.A. Cox, The Electronic Structure and Chemistry of Solids, Oxford University Press, Oxford, 1991. [12] J.E. Castle, R.H. West, J. Electr. Spectr. and Relat. Phenom. 16 (1979) 195. [13] I. Abu-Talib, PhD thesis, University of Surrey, UK, 1985. [14] J.E. Castle, I Abu-Talib, S.A. Richardson, in Mat. Res. Soc. Symp. Proc., Pittsburgh, USA, 1985, pp. 471–479. [15] J.E. Castle, R. Ke, J.F. Watts, Corrosion Science 30 (1990) 771–778.
Journal of Electron Spectroscopy and Related Phenomena 95 (1998) 45–56
The intrinsic asymmetry of photoelectron peaks: dependence on chemical state and role in curve fitting. A.M. Salvia a, James E. Castle b,* a
University of Basilicata, Potenza 85100, Italy University of Surrey, Guildford GU2 5XH, UK
b
Received 16 December 1997; revised 23 April 1998; accepted 12 May 1998
Abstract Correct estimation of peak and background in XP-spectra is necessary for quantitative analysis. The problem is made difficult because neither the peak nor the background have a known shape and their separation requires the photoemission process to be treated as a sequence of well separated events. Background removal can be undertaken using the Tougaard method which is based on a careful analysis of the electron transport taking place in a solid but this procedure is designed to remove only the extrinsic component. There is a further component of the background which is intrinsic to the peak and associated with the photoemission event at the atomic/molecular level. In this paper we build on previous papers, which were concerned with metallic elements, to assess the importance of oxidation on the intrinsic asymmetric shape of the peak and thus make this available for curve fitting. This was done, using a programme, Tryfit, based on the Shirley type algorithm modified by Proctor to quote separately the peak and background intensity in XP-spectra. The programme allows the shape parameter, k, to be extracted which helps in defining the ‘intrinsic’ shape of a photoelectron peak. The shape parameter, k, is found to be independent of instrumental effects and intrinsically related to atomic number and, as now shown in this paper, to a certain extent to chemical state. It is demonstrated in this paper that its use can help in quantifying individual peak contributions to a multicomponent XPS spectrum. q 1998 Elsevier Science B.V. All rights reserved Keywords: XPS quantification; Electron energy loss; XPS peak shape; Curve fitting
1. Introduction In recent papers [1,2] we showed that the part of the ‘Shirley’ background within the range of a typical photoelectron peak can be accounted for by electron energy losses generated during photoemission and is almost independent of the extrinsic effects related to the transport of the electron through the solid described by Tougaard [3]. This background gives the peak an intrinsic asymmetry which can be defined by a single parameter, referred to as the shape * Corresponding author.
parameter, k. For the pure elements, k varies with atomic number and was shown to be independent of the variables associated with spectrum acquisition and similar in value for different orbitals of a given element. In this paper we report the results of a study of the dependence of peak asymmetry, as defined by k, on the chemical state of an element and we show how knowledge of the shape parameter can help in deriving the correct fitting choices for the interpretation of XPS data. The method by which the parameter is defined and normalised to the peak area can be understood using Fig. 1 and its caption. This figure illustrates the fitting
0368-2048/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 36 8- 2 04 8 (9 8 )0 0 20 5 -9
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of the data by a Gaussian–Lorentzian (G/L) peak in the form of a Voigt function. The Tryfit programme as developed by Proctor [4] considers that the Voigt function can be combined with a parameter ‘Tail 1’ to allow for the introduction of the asymmetry arising, particularly in metallic states, from conduction electron/hole interactions and with a parameter ‘Tail 2’ which represents the total background arising from scattering on the basis of the Shirley algorithm [5]. The use of a tail to include the Shirley background in the curve-fitting procedure has been previously discussed by Sherwood [6]. The background used in this fitting extends into the data points on the low binding energy side of the peak and Proctor has modified the Sherwood tail through the use of a polynomial, P, to give Tail 2 different shapes in order to match the ‘actual’ spectrum profile, where: P = B1 + B2 p D=W + B3 p (D=W )2 + B4 p (D=W )3
Fig. 1. Illustration of Cu 3s curve fitting as used in this paper: (a) raw spectrum; (b) construction of a background and peak fitting using Tryfit. A Voigt function peak shape is added to a Shirley background convoluted with a polynomial tail (Tail 2). The total background rise is defined by the point ‘h’ which, in turn, is fixed by the chosen value of ‘k’. In this illustration the value of k used was that derived from fitting the Cu 2p spectrum. Tail 1 was fixed at zero; (c) the raw data after background removal.
and W = Full Width Half Maximum, FWHM in channels, D = X − X 0, where X 0 is the peak centre and X − X 0 expresses the distance in channels from the peak centre. When B1 = 1 and B2, B3 and B4 are set to zero the family of tails take the form of straight lines having a common origin at the peak centre. We have used the height of this origin, h, divided by the area of the G/L peak to define the value of the shape parameter, kappa, k, which was shown to have ‘intrinsic’ properties [1,2]. In practice, the position of the origin is obtained from the ‘Tail 2’ parameter given by the program as a ratio to peak height, H; see Fig. 1. We use the value of the shape parameter only to fix the magnitude of the integral background beneath the main part of the peak itself. The tail extending from the peak gradually changes from intrinsic to extrinsic in character. Tougaard and Ignatiev [7] defined this tail using a parameter, D, which was measured at a point distant from the peak so that it was mainly determined by extrinsic losses. In our case we have taken the inverse value but, crucially, defined it at the peak centre so that it represents intrinsic losses. Two oxide systems were chosen for examination of chemical effects on k: chromium, a typical transition element and one for which we already have considerable experience in peak fitting [8] and copper, which gives easy access to two well defined chemical states of a single element.
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2. Experimental procedure 2.1. X-ray photoelectron spectroscopy XPS spectra of the chromium and copper compounds were recorded by an ESCA 3 spectrometer (VG Scientific Ltd, UK) using Al Ka excitation (1486.6 eV) operated at 20 mA and 13 kV. The base vacuum during acquisition was always better than 10 −9 torr. The analyser was operated at a constant pass energy of 50 eV and at a slit width of 4 mm, giving a resolution of 1.1 eV on the silver 3d 5/2 line. A take-off angle of 458 was used throughout this work. When necessary, specimens were bombarded with Ar ions to eliminate surface contamination. Spectra were acquired using a VG 3040 datasystem attached to the ESCA 3 and then transferred to the PC computer on which the fitting software was run. None of the BE values reported in the XPS figures and tables are corrected for surface charging. 2.2. Specimen preparation 2.2.1. Chromium Chromium metal (0.05 mm thick, 99.99%, polyester backed foil, Goodfellow Metals, Cambridge) was analysed ‘as received’ and after sputtering with Ar ions by a VG-AG2 gun in the analysis chamber of the ESCA 3. During ion etching, the energy of argon ions was 6.0 keV with a focusing voltage of 3.0 kV and the residual pressure in the analyser chamber was 10 −6 torr. Specimens of pure chromium (III) oxide (Puratronic, Ventron GMBH) were prepared by pressing the powder onto Indium foil. This allowed a safe introduction into the preparation chamber. The spectra were recorded only after the wide scan showed that no features from Indium foil were present. 2.2.2. Copper Copper was chosen for this test primarily because it is easily oxidized in situ from the clean metal surface through the two forms of oxide. The oxides also have clear differences in Auger parameters [9] and different satellite structures both of which point to significant final state interactions with neighbouring atoms which may give rise to differing background asymmetries.
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Copper was obtained as wire (OD, 0.25 mm) from Goodfellow Metals, Cambridge (99.99% purity). To produce evaporated films the copper wire, wound on tungsten filaments (OD, 0.5 mm) was evaporated onto a stainless steel sample holder substrate in the VG ESCA3 preparation chamber. Before evaporation, the substrate sample holder, copper wire and tungsten filament were ultrasonically cleaned in methanol. The sample-holder was further cleaned by argon etching in conjunction with heating before starting deposition. Outgassing of the tungsten filament and metal wire was also performed before positioning the substrate opposite the evaporator. Copper was then oxidized in air dried using a liquid nitrogen trap. Oxidation time, temperatures and air pressures were chosen as appropriate to the metal for each oxidation stage [10]. Cuprous oxide was obtained by vacuum annealing partly oxidized copper so that any cupric oxide was reduced by back reaction with copper. Wide and detailed spectra were acquired for each intermediate oxidation state until the Cu (II) oxide formed was sufficiently thick to show its characteristic features and to obscure the underlying metal. The sequence of spectra in Fig. 2 clearly shows the appearance of a well defined peak-position shift and shake-up satellites according to the oxidation state.
3. Results 3.1. Copper The value of the shape parameter, k, derived from the 2p spectra, Fig. 2, of the Cu (Fig. 2(a)), Cu 2O (Fig. 2(d)), and CuO (Fig. 2(e)) samples (Table 1) shows that it has a small dependence on chemical state although the Cu (0) and Cu (II) kappa values are found to be nearly identical and the largest change is for Cu (I). This behaviour is consistent with the fact that the Auger parameters of Cu (0) and Cu (II) are also closer than the Auger parameters of Cu (0) and Cu (I) [9] and hence there may be more closely spaced final states in the former pair. 3.2. Chromium The chromium 2p region of the metal and of its trivalent oxide (Figs. 3 and 4) were fitted and gave
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Fig. 2. Cu2p Spectra sequence of Copper metal ‘in situ’ oxidized using dry air: (a) copper metal ‘as deposited’; (b) air . 1 torr 258C, 40 min; (c) air . 1 torr, 300–2008C, 30 min; (d) as (c) plus 30 min 4508C, vacuum: Cu 2O 100%; (e) air . 1 torr, 4008C, 20 min: CuO 100%.
the values of k recorded in Table 1. The shape parameter, k, for chromium metal and the oxide, Cr 2O 3, were derived as a mean value from at least three different spectra acquired under the same experimental conditions. Again it is seen that there is a small but significant dependence on chemical state. However, comparison of k for chromium metal and chromium (III) oxide shows that the intensity increase across the body of the main peaks is very similar. However, Cr 2O 3 has a complex peak structure (Fig. 4) with shake up satellites and a discrete energy loss giving a step in the background peculiar to the oxide, but the influence of all of these
features is negligible for a small energy range in the vicinity of the peak. The long range energy losses which have been discussed previously by us [8] do not contribute to the value of k. 3.3. Oxygen The value of k was calculated from the oxygen 1s peaks of cuprous and cupric oxide with the interesting result that k, and hence the intensity of the intrinsic background, is seen (Table 1) to vary from one oxide to another. This implies that intrinsic losses are generated by photoexcitation of an anion as a result
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Table 1 The shape parameter, k, obtained, for the examined elements, by using Tryfit curve fitting results Core level
k a (eV −1)
Spectrometer/ Pass energy (eV)
X-ray
Sample
Cu 2p Cu 2p O 1s
0.039 6 3.7% 0.038 0.050 0.050
VgEsca3/50 Leybold/50 VgEsca3/50
Al Ka Mg Ka Al Ka
Cu film Metal foil b Cu 2O
Cu 2p O 1s
0.040 0.030
VgEsca3/50
Al Ka
CuO
Cr 2p Cr 2p
0.073 6 11.0% 0.063 6 10.4%
VGEsca3/50 VgEsca3/50
Al Ka Al Ka
Evaporated film Cr 2O 3
a The k-values associated with relative % error are expressed as a mean of at least, three replicates. bCopper metal foil was acquired, for comparison, with a Leybold LH X1 instrument at the University of Basilicata, Potenza, Italy.
of final state variations in the associated cation. Just from a qualitative point of view, considering what is reported about the chemical structure of the above compounds, the finding of the same value of kappa for copper (I) and oxygen suggests that in monovalent copper the d-levels are not core-like and are in the same energy regions as the anion valence levels. Different values of k are expected for Cu (II) and oxygen in cupric oxide since the d-orbitals are reported [11] to retain their atomic identity in divalent compounds of copper.
4. Discussion 4.1. The use of the shape parameter in XPS curve fitting The variation in k for the two metals and their oxides examined in this study is relatively small. The dependence of the value of the shape parameter for oxygen on the cation, with which it is associated, indicates that photoexcitation of oxygen ions leads to final states that involve excited states in the cation. Similar effects are seen by way of the Auger parameter [9] and have been shown to stem from the large polarisability of oxygen ions [12]. Thus, we anticipate that there will not always be a unique value of k for a given valence state but for some elements, such as oxygen, there will be a series of values depending on the local chemical environment.
From a practical point of view, however, we return to the fact that k, being related to intrinsic processes, should not depend on sample thickness, photon energy or the presence of an overlayer, as it would be if related to ‘extrinsic’ processes. This is recognised in the Tougaard algorithm which removes practically no intensity from the peak itself. The value of k obtained for reference homogeneous samples can therefore be applied to define a peak shape for a given element whatever its distribution in depth when present in complex layered specimens. We will demonstrate the validity of this argument using two examples. In the first example we consider the spectrum from a chromium metal foil carrying a thin oxide layer on the surface. In order to generate a sample with a known source of extrinsic background, titanium was evaporated onto the surface. As a test for success in the outcome of curve fitting we have used the metal/ oxide area ratio, determined using the areas of the G/L peaks for Cr (0) and Cr (III) respectively. As we show in Appendix A, this should remain independent of the overlayer thickness [13]. Fig. 5(a) shows the curve fitted Cr 2p region of the chromium substrate before deposition. In fitting this spectrum we started with the parameters obtained from the reference spectra and the programme was allowed to adjust them with the condition that k met and k ox should remain as close as possible to their reference values. This was done interactively by adjusting the peak and Tail 2 parameters of both metal and oxide components as Tryfit was not
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Fig. 3. Ion-etched cleaned Chromium metal Cr 2p curve fitted region.
designed to have k as a fitting parameter which could be automatically held at a constant value [4]. As can be seen from the figure, a good fit could be obtained by using constant tails (as for the reference spectra) and by adding a broad peak to account for the residual intensity arising from extrinsic losses. The most important thing is that for the first time the oxide/metal structure of the peak has been fitted using individual shapes for oxide and metal without having to make any assumption as to the nature of extrinsic losses in the close vicinity of the peak.
The fit used above was repeated on the spectrum obtained when the titanium overlayer had been added, restraining peak and k parameters to the values used for fitting Fig. 5(a). Fig. 5(b) shows the result. It can be seen, by comparing Fig. 5(a) and (b) and the fitting parameters given in their caption, that the same metal/oxide area ratio is obtained. This is as it should be since the titanium was evaporated in situ onto the chromium, which was not cleaned or etched prior to this deposition The difference between the spectra is entirely confined to the extrinsic
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Fig. 4. Chromium oxide (Cr 2O 3) Cr 2p curve fitted region.
background which becomes much larger after the titanium is deposited. In the second demonstration, we fit a Cr/Cr 2O 3 spectrum in which the oxide thickness was great enough to give significant uncertainty in the apportionment of background between the component peaks. In this test we show that, when the value of k is fixed, the ratio of the peak areas is independent of the shape of the extrinsic tail added to the G/L peak being that the common origin of the tails, as defined by kappa (i.e. intrinsic losses), is the same. The spectrum obtained from chromium carrying a relatively thick oxide is shown in Fig. 6. This spectrum has been fitted
using a constant Tail 2 plus a peak-like energy loss feature (Fig. 6(a)), as normally done for chromium spectra and also with a sloping Tail 2 (Fig. 6(b)) by choosing non-zero B-coefficients. The non-zero coefficients allow full fitting of the peak region without recourse to a Tougaard background as they mix elements of extrinsic background into the intrinsic background. The value of k used and the metal/ oxide area ratio obtained, are given in the captions to Fig. 6. Again we see that when the shape parameter is fixed, then the area ratio is maintained, independently of decisions made concerning the ‘extrinsic’ background shape.
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Fig. 5. a) Cr 2p region of an ‘as received’ chromium metal carrying a thin oxide overlayer; (b) the same region after a titanium overlayer had been evaporated onto the surface. Relevant fitting parameters: (a) Metal peak: centres; 574.4, 583.6, FWHM; 2.00, 2.12, G/L mix; 48%, Tail 1; 0.210, k; 0.072. Oxide peak: centres; 577.4, 586.9, FWHM; 3.19, G/L mix; 48%, Tail 1; 0.0, k; 0.063. Additional small peaks are used for satellites. Metal/oxide area ratio (2p 3/2 peak) = 4.03. (b) Metal peak: centres; 574.9, 584.0, FWHM; 2.00, 2.14, G/L mix; 50%, Tail 1; 0.181, k; 0.074. Oxide peak: centres; 578.0, 587.2, FWHM; 3.18, G/L mix; 50%, Tail 1; 0.0, k; 0.060. Additional small peaks are used for satellites. Metal/oxide area ratio (2p 3/2 peak) = 4.05.
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Fig. 6. Chromium metal carrying a thick oxide overlayer. Curve fitted Cr 2p region using TRYFIT. Relevant fitting parameters: (a) Constant Tail 2. Metal peak: centres; 574.4, 583.6, FWHM; 1.90, 1.90, G/L mix; 45%, Tail 1; 0.210, k; 0.072. Oxide peak: centres; 577.0, 586.4, FWHM; 3.50, G/L mix; 45%, Tail 1; 0.0, k; 0.063. Additional small peaks are used for satellites. Metal/oxide area ratio (2p 3/2 peak) = 0.97. (b) Sloping Tail 2. Metal peak: centres; 574.4, 583.6, FWHM; 1.80, 1.94, G/L mix; 45%, Tail 1; 0.220, k; 0.073. Oxide peak: centres; 577.0, 586.5, FWHM; 3.5, G/L mix; 45%, Tail 1; 0.0, k; 0.065. Additional small peaks are used for satellites. Metal/oxide area ratio (2p 3/2 peak) = 0.96.
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4.2. General discussion In earlier attempts [13–15] to devise and use a curve-fitting procedure which includes a background component in the fit for two or more chemical states, an impasse was reached because of the impossibility of deciding how the total background should be shared between the component peaks. In the case of a metal oxide, equally good fits can be obtained with all the background on the metal or on the oxide or for
any given mixture. The two extremes for chromium/ oxide are illustrated in Fig. 7. Clearly a better understanding of the permissible range of loss structures is necessary before the method can be used to analyse unknown surface structures and a protocol is required to remove subjective elements in the fitting procedure. The use of k as a shape parameter suits this requirement since it provides an integration constant for the Shirley background function appropriate for each peak and which is independent of extrinsic loss
Fig. 7. Chromium with a titanium overlayer. When the user is free to add background as desired, equally good fits can be obtained if the background is ascribed in any proportion to either peak. These show the extreme options of all background added to the oxide peak, or all added to the metal peak (from Ref. [13]).
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structure as defined by the Tougaard function. The peak parameters plus an intrinsic background characterised by the single parameter, k, together define a shape which can by used in the fitting routine within the narrow window of a high resolution peak. The intrinsic nature of the shape parameter, k, makes it suitable as a fitting parameter assisting the assignment of the correct proportion of background rise to each component in the envelope of overlapping peaks. We have shown here that it does depend on the chemical state of the element and different values will, in general, be needed for the different chemical states present in a multicomponent peak. In some cases, such as for the oxides of the transition metals examined here, the variation is not large. In other cases, especially when there are conducting and non-conducting compounds of the same element the variation can be very significant. Certain other elements, notably oxygen, have a behaviour which shows that their intrinsic background is influenced by that of the other elements with which they are combined. This finding indicates that photoexcitation of one atom can lead to a variety of final state electron levels in neighbouring atoms, and hence appearing as energy loss contributions to the photoelectron peak. Use of the shape parameter as a fitting parameter enables background removal from the peak structure as the sum of individual components of intrinsic background from each component peak together with a generalised extrinsic background of the Tougaard form. We have shown that when this is done, modifications to the extrinsic background, arising from the layered structure of the near surface, do not influence the outcome of the curve fitting. The success of this approach to curve fitting raises the possibility of developing fitting programs in which the peak clusters are fitted using k to define an intrinsic background and fitting the residue with a Tougaard background to define the position of the elements within a surface layer structure. When, as is often the case, the acquired window of the spectrum is too small for a full Tougaard to be fitted, then the B coefficients, as used in this work, could be utilised to give an indication of the likely layer structure. For these outcomes to be achieved it will be necessary for the program developers to devise a means by which the shape parameter can be entered and held constant as a fitting parameter.
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Acknowledgements We are grateful to S. Tougaard for the inspiration drawn from his scientific production and to A. Proctor for TRYFIT and much helpful discussion. We are pleased to acknowledge the contribution made to this line of work by the initial studies of I. AbuTalib. This work was assisted by the ‘Ministero della Universita’ e della Ricerca Scientifica e Tecnologica’ in collaboration with the ‘Conferenza dei Rettori delle Universita’ Italiane’ and of the ‘British Council’ (UK) under the ‘British-Italian Collaboration in Research and Higher Education’-1998- project.
Appendix A From Ref. [13] Using the usual mean free path for inelastic scattering, l, we can calculate the thickness, d ox, of any oxide present on the surface from the relationship dox = l sin v ln{(ICrIII =ICr0 ) + 1}
(A1)
where v = the take-off angle and I is the intensity of the oxide or metal component of the peak, as indicated by the subscript. The assumption is made in deriving this relationship that the standard intensities of chromium in metal and oxide are equal for very thick samples. Similarly the thickness, d Ti, of a titanium layer attenuating the oxide signal is given by dTi = l sin v ln{ICrIII =I9CrIII }
(A2)
where I9 is the reduced signal in the presence of titanium. The effect of both oxide and titanium can be seen on the metal peak, namely: 0 =I9Cr0 } dox + dTi = l sin v ln{ICr
(A3)
where I 0 refers to the intensity from a perfectly clean chromium surface. In the present case: 0 = ICr0 exp(dox =l sin v) ICr0
(A4)
Eq. (A3) can be used to obtain the combined thickness of oxide and titanium at any stage of evaporation and Eq. (A2) can be used independently to obtain the total thickness of titanium at any stage. Alternatively, these equations can be used with each individual stage of evaporation to yield the thickness added.
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The apparent chromium metal/oxide ratio at any thickness of titanium overlayer is given by I9CrIII =I9Cr0 = R
(A5)
where, as before, the prime indicates the intensity of the chromium signal in the presence of titanium. From the relationship used above it follows that R = exp − (dTi =l sin v)=exp − (dTi + dox )=l sin v = exp(dox =l sin v) − 1
(A6)
which is independent of d Ti.
References [1] A.M. Salvi, J.E. Castle, ECASIA 97, J. Wiley and Sons, Chichester, UK, 1997, pp. 809–812. [2] A.M. Salvi, J.E. Castle, J. Electron Spectroscopy and Related Phenom., accepted. [3] S. Tougaard, Surface and Interface Analysis (SIA) 11 (1988) 453.
[4] A. Proctor, Tryifit and Googly Manuals supplied at the University of Surrey. J.N. Fiedor, A. Proctor, M. Houalla, D.M. Hercules, SIA 23 (1995) 204. [5] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [6] P.M.A. Sherwood, in: Briggs, Seah (Eds.), Practical Surface Analysis, Appendix 3, John Wiley and Son Ltd, New York. [7] Tougaard and A. Ignatiev, Surf. Sci. 129 (1983) 335. [8] A.M. Salvi, J.E. Castle, E. Desimoni, J.F. Watts, Appl. Surf. Sci. 90 (1995) 333. [9] C.D. Wagner, in: Briggs, Seah (Eds.), Practical Surface Analysis, Appendix 5, John Wiley and Son Ltd, New York, 1990. [10] J.E. Castle, D. Epler, Proc. R. Soc. Lond. A 339 (1974) 49. [11] P.A. Cox, The Electronic Structure and Chemistry of Solids, Oxford University Press, Oxford, 1991. [12] J.E. Castle, R.H. West, J. Electr. Spectr. and Relat. Phenom. 16 (1979) 195. [13] I. Abu-Talib, PhD thesis, University of Surrey, UK, 1985. [14] J.E. Castle, I Abu-Talib, S.A. Richardson, in Mat. Res. Soc. Symp. Proc., Pittsburgh, USA, 1985, pp. 471–479. [15] J.E. Castle, R. Ke, J.F. Watts, Corrosion Science 30 (1990) 771–778.