84
Anal. Chem. 1980, 52, 84-92
Quantitative Determination of Surface Composition of Sulfur Bearing Anion Mixtures by Auger Electron Spectroscopy N. H. Turner,' J. S. Murday, and D. E. Ramaker Surface Chemistry Branch, Naval Research Laboratory, Washington, D.C. 20375
A method has been examined for the quantitative analysis by Auger Electron Spectroscopy (AES) of the surface composition of sulfur bearing anion mixtures. K2S04-Ag2S and Ag2S04K,S03 salt mixtures have been analyzed using the AES S LVV spectra of SO:-, SO3'-, and S2-. The method employs a computed comparison between the AES S L W lineshapes for pure materials and the measured line shape of the mixture. The results of this procedure have been compared to those obtained by X-ray Photoelectron Spectroscopy and the cation AES spectra. For S042--S2- mixtures in the range of 10-50% sulfide, the S AES predictions were usually within 5 % of the compositions predicted from the cation AES intensity. Lesser accuracy would be anticipated for more extreme composition ratios. The SO4'- concentration in the 20% S042--80% SO-: mixtures was estimated poorly ( -30 % uncertainty) owing to a combination of effects. The sources of possible problems wlth the method are discussed: base-line effects, the existence of standard reference line shapes, electron beam damage, sample charging, and shadowing.
Both Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) have been used for qualitative analysis of surface composition for a number of years. Despite some difficulties in achieving quantification ( I @ , XPS is the spectroscopy of choice for quantitative analysis because of its simpler line shapes as compared to AES. XPS can quantitatively estimate the relative amounts of a given atom in a mixture of oxidation states when the chemical shift is sufficiently large to differentiate the species (7,8). However, there are examples where the difference in binding energy is too small, e.g., Cu and CuzO (9). AES has advantages over XPS for certain analysis situations where information about the elemental environment in the surface region is desired. First, with most available equipment, the area of analysis with AES is less than that of XPS. Second, the mean escape depth of the electrons for many Auger transitions is less than that for most of the principal XPS lines (Mg or Al X-ray source); hence, there is an increased surface sensitivity. However, it should be noted that detecting X P S electrons ejected nearly parallel to the surface will increase the surface sensitivity of XPS. Third, most elements have greater sensitivity with AES than XPS when normal operating conditions are used (10). Fourth, it should be realized that Auger transitions which involve valence electrons, contain information about the density of states (DOS) of individual elements in the valence region. XPS valence band measurements provide only nonlocalized information about the DOS of the valence band. It has been demonstrated that AES can distinguish at least between oxide and nonoxide forms of the elements Mg through S (11). The ability to make such an identification between such species as S2-and SO:- or Mg and MgO is based upon both the energy and the line shape of the observed AES transitions. AES has been used by Chang and Boulin (12) to determine the thickness of oxide layers on Si and A1 by the
observation of the changes in the peak intensities of the KLL transitions of the oxides vs. the elemental form during Ar ion sputtering. They also followed similar changes in the AES Si LVV spectra. Recently Wager and Wilmsen (13) have investigated thinner SiOz layers than those examined by Chang and Boulin. In these two instances, the shifts in the energy of the Auger transitions were large enough to permit a quantitative analysis of the oxide from the element simply by using the peak-to-peak intensities of the two transitions. In other cases the chemical shift in the AES spectra of various states of an element has not been large enough to d o w such determinations, e.g., the molybdenum oxides system (14). However, changes in the line shapes of AES spectra may still provide the possibility of distinguishing elemental states in such cases where the shift in the AES transition energy is difficult to observe. Distinguishing mixture compositions would require obtaining the AES spectra of the well characterized, pure materials before attempting to analyze mixtures. We will report in the sections on K2S04-Ag,S and K2S03Ag,SO, mixtures the results of such a study using mixed salt samples to test the accuracy of using the differences in the AES LVV spectra of various sulfur anions to find the relative amount of each anion in the area of analysis. The cation AES spectra and the anion XPS will serve as standards with which to compare the experimental results. In addition to the examination of quantification, the relative intensity of the AES S LVV transition of SO4*- compared to S2-has been determined. The signal strength and line shape of the AES transition depends upon the amount of the available charge in the valence bands and on its distribution in energy, e.g., the electron DOS. These factors place limits on the range of relative amounts that can be analyzed and will be discussed in the section on Reference Spectra. The alteration of oxide surface composition by irradiation with an electron beam has been carefully studied for SiOz (15) and for Li2S04(16). Beam damage was a serious problem in this work. The effect of the beam damage and some other factors hindering quantitative analyses will be discussed in the section on Factors Influencing Quantitative Measurement.
EXPERIMENTAL The samples were prepared from the following compounds: Ag2S (ICN Pharmaceuticals, purity not stated); K2S04(Baker Analyzed Reagent); KzS03, Na2S04,Na2S03,AgZSO4 (Fisher Scientific Co., ACS Certified);and CdS (Alfa Products, ultrapure). These particular salts were chosen because Na, K, and Ag have strong XPS and AES spectral lines which for the most part do not interfere with each other. From XPS analysis, the Ag2S appeared to be slightly oxidized (12%). Both Ag,S and Ag2S04 were quite resistant to electron beam damage. The alkali sulfides were found unusable owing to what appeared to be very rapid surface reactions upon even a brief exposure to the atmosphere. The method of preparation was similar to that described previously (11) except that in the case of mixtures (Ag,S--K2S04and K2S03-Ag,S04)there was thorough stirring of the reagents before pressing the salts into In substrates. The samples were made in an inert atmosphere; only at the time of sample introduction to the spectrometer was there exposure to the atmosphere (no more than 1 min). The mixtures were quantitatively prepared (&3%)
This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
by weighing; however, the AES and XPS analyses did not agree with the weighings. Visual microscopic examination indicated that there was a relatively uniform distribution across the samples; but the slightly smaller AgzS particles tended to adhere to the surface of the K2S04crystals. This would result in shadowing of the K2S04by the AgzS; the spectroscopic analysis shows the amount of observed Ag,S was in fact greater than that expected from the weighings. The AgzSO, and K2S03particles could not be visually discriminated; it is presumed that the spectroscopic and weight determination of these mixtures disagreed for the same reason. The sequence used to obtain the XPS and AES spectra closely followed that reported previously ( 1 1 ) with the following modifications. The XPS binding energy measurements were referenced to an Au 4filZbinding energy of 83.8 eV. XPS spectra in the region of the individual elements were made with a multichannel signal averager to improve the signal-to-noise ratio; spectra were taken with a Physical Electronics energy analyzer (Model 15-255G) set for an energy resolution of 1.6% and a pass energy of 100 eV. A defocused 1-keV electron beam was used for the AES spectra; thus the analyzer transmission defined the sample area and electron beam damage could be minimized. The electron energy analyzer was run in normal mode with a resolution ( U / E )of 1.6% and with a 3-eV modulation so that adequate signal intensity could be obtained. After each sequence of spectra was obtained, a profile of the electron beam in the horizontal direction was made with a Faraday Cup in all instances when quantitative data was desired. The maximum current levels were observed to be fairly constant over a 1-2 mm range with a fwhm of approximately 4 mm. The current density in the peak of the defocused beam was observed to be approximately 5 bA/cm2. A set of four independent determintions of the composition of each mixture sample can be obtained from the data, Le., the measurement of both the cation and anion intensity by XPS and AES. The best check of the S AES determination should be the cation AES. The X-ray and electron sources were incident on the sample from different, grazing angles. The shadowing phenomenon that rendered the weight comparisons to be of little value could also result in differences between the XPS and AES determinations of the mixture composition. The area under the various XPS peaks was determined by deconvolving the X-ray satellite contribution and by a fitting of the lines to a simple Gaussian function with a linear base line. A more complex function did not appear to be needed owing to the relatively broad peaks; the fitting was done with the MLAB program ( 1 7 ) . The AES derivative spectra for sulfur were digitized in the 80-180 eV region to allow further data reduction. The contributions of the redistributed primary and secondary electrons to the AES spectra, Le., the base-line correction, were accounted for via the procedure given in the Appendix. The AES spectrum of one of the pure compounds was normalized to the spectrum of the other pure compound of the mixture. both with respect to the XPS area of the S 2p peak and the beam current (maximum observed with the Faraday Cup). The normalized spectral intensities are subscripted by “r*’;Le., Ag+, and S2I are normalized Ag+ peak-to-peak height and S2- line shape, respectively. The spectra for the mixtures also were normalized with respect to the beam current in the same manner. The components of the mixture spectra were then determined from Equation 1 with the use of the MLAB program (17).
M = CIA
+ CZB
85
A
(1)
where M is the line shape of the mixture spectrum, A and B are the digitized line shapes of the normalized pure compounds, and Cl and Cz are proportionality coefficients determined by MLAB.
RESULTS AND DISCUSSION Reference Spectra. In Figure 1, the measured AES S LVV spectra of AgzS, Ag2SO4,K2S04,and K2S03are shown along with several other line shapes; the general features of and S*-spectra are similar to those reported prethe Sod2viously for the same anion with different cations (11). There is a n effect on the position ( - 2 - 5 eV) of the observed peaks due to sample charging between different materials. Moreover,
loo
120
140
180
180
KINETIC ENEROY IN *V
Figure 1. AES S L V V spectra of (a) Ag,SO,, (b) Na,SO,, (d) Na,S03, (e) K,SO3, (f) ASPS, and (9) CdS
(c) K,S04,
some differences do appear in the SO4’- and SO3’- spectra in the energy range 135-145 eV that seem t o depend upon the cation; this may be due to differing susceptibility to damage by the electron beam used to excite the observed AES spectra.
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ANALYTICAL CHEMISTRY, VOL. 52, NO 1, JANUARY 1980
Included in Table I are the energies of the various AES and X P S transitions used in this investigation along with some previously reported values. In most cases, after corrections for sample charging, the agreement in the XPS binding energies appears to be reasonably good. A binding energy of 531.6 eV was found for the 0 1s peak in Ag,SO, and there appeared to be little, if any, charging shifts with this compound. This binding energy value agrees reasonably with values determined by Lindberg et al. (18)for a series of Sod2 compounds (532.2 eV) and 531.9 eV for Na2S03. For the other oxygenated sulfur compounds studied, the 0 1s peak position was set at 531.6 eV to compensate for sample charging. The other XPS peaks were shifted by the same amount. This procedure may lead t o some small errors in our reported binding energies, since there can be slight differences in the binding energies of the 0 1s peak of SO4*-with different cations (18);however, the agreement between different determinations of the same compound in this study and comparisons to previous investigations seems to be fairly reasonable. In addition the Auger parameter values that were determined in this study are given in Table I; comparisons with other results (when available) are shown. Since the Auger parameter involves the difference between the X-ray excited AES and XPS lines, sample charging should not affect the result. Part of the discrepancy between this work and that of Wagner (19) could be due to spectrometer calibration. The Cu 2 ~ 3 1 2value in this work was found to be (932.8 eV) vs. that of Wagner's (932.4 eV) (19)while the Au 4f7 line was set a t 83.8 eV in both cases. The X-ray excited Auger transition (XEAES) energies reported in Table I have been corrected for both sample charging and work function. The electron-excited Auger (EEAES) energies are corrected for work function only; this accounts for the large apparent discrepancy seen for the K salts in particular. The X-ray excited AES S LVV spectra were very weak signals superimposed on a steep background; it was difficult to make accurate determinations of the peak maxima. The peak-to-peak intensity of the AES S LVV transition for S2- is substantially different from that observed for SO,' . T o obtain a better understanding of the origin of this difference, the S spectra of Ag,S and Ag2S0, were studied in some detail. With a common Ag+ it was thought that a normalization of the AES S spectra could be made by comparison of the Ag AES signals without the need for more elaborate calibration factors. However, when the cross-section-corrected XPS intensities for the Ag 3d5 and S 2p lines of the Ag2S and Ag2S0, reference samples were calculated, Ag, BSOl and Ag, 4Swere found as the stoichiometries. The Scofield cross sections (20)may not be absolutely correct and this may account for the deviation of one compound from nominal stoichiometry, but a t least one of these compounds is very nonstoichiometric. Recently Aylmer et al. noted differences in the XPS determined stoichiometry vs. that expected for NiS and NiS04 (21). The two reference samples in this work had essentially identical XPS S intensities (within 5% 1; this is evidence that the amounts of S on the two were equal. The ratio of the XPS Ag intensities was Z A g 2 ~ / Z A B 2 b ~ 4 = 1.8; the same ratio was observed for the ratio of the Ag AES peak-to-peak heights from the two samples. On this basis we conclude that equal amounts of S atoms are contributing to the observed Auger lines on the two samples. While the odd stoichiometries make these samples somewhat suspect, the XPS sulfur binding energies for the two samples did confirm their nominal chemical state except for the presence of about 12% of the sulfur in Ag,S that was oxidized to The S AES spectra were integrated with appropriate base-line corrections taken into account and the electron loss features were deconvolved (22). The ratio of the areas of the two AES
-
S LVV spectra was determined to be Zs"/Zso~ 2.3. Damage to these samples due to the electron beam did not appear to be serious; this problem will be discussed later. It is possible to compare the measured Zsz / I s 0 2 with a theoretical estimate. The Auger intensities can be calculated from the theoretical framework of Ramaker (22-25) and local electron densities as derived from X-ray emission spectroscopy (XES), XPS, and molecular orbital theory. The transition intensities are calculated from the expression (2)
C2XnC2YmPKnrn
Ikxq a n,m
where CZXnand CZYmare the populations of the nth and mth atomic orbitals on the central atom in the X,Y molecular orbitals, respectively and PGm is the appropriate atomic Auger matrix element. The sum is only over those orbitals localized on the atoms with the initial core hole. The electron populations deduced for SO,2- and S2 are presented in Table 11. The different total charge state for SO4*-compared to S2 in Table I1 can be compared with the number of valence electrons calculated from the Kn XES spectra by an empirical formula given by Coulson and Zauli (26). Using the measured K n energies for a number of sulfides and sulfates (27)we obtain average values of 6.3 and 4.1 for the valence charge states, respectively. These compare very well with the assumed electron populations in Table 11 if only the s and p electrons are considered. The d electrons are not counted because they are too delocalized to contribute to the K n charge shift. The calculation of the Auger intensities also requires Auger matrix elements; they are assumed to be ss/pp = 0.29, spipp = 0.33, sd/pp = 0,072, pd/pp = 0.88, and dd/pp = 1.66 (23,28). The calculated ratio of the S2- to sulfur Auger intensities is (Zs2 /Iso; ) A E ~= 1.4 or 1.9 if one includes the 3d electrons or deletes them, respectively. Given uncertainties in the population assignments and in the matrix elements, the latter is in reasonable agreement with the measured area ratio of 2.3. The 3d electron intensity contribution may be overestimated; the matrix elements for the transitions involving d electrons were estimated roughly by extrapolation from transition metal oxide data. The ratio of the peak-to-peak heights for the sulfur dN( E ) / d E AES spectra is measured to be (Is?/Zso~-)m,pp 4.3. This is a factor of two greater than the ratio of the areas. The difference is easily accounted for by recognizing that the AES SO4* LVV spectra is disbursed over a larger energy interval; the slopes of the line shape are reduced accordingly. The demonstrated large changes in the peak-to-peak heights for the sulfur LVV line shape as a function of chemical environment need to be considered if one wants to quantify Auger spectra by using scale factors (29, 30). XPS examination of the pure Ag2S materials showed that about 12% of the S 2p intensity occurred at a binding energy consistent with S as SO4*-. The presence of the oxidized species in this study will not affect any comparisons made between the mixture cation and anion AES intensities and the reference intensities, since the oxidized species will be present in both the reference and mixture samples. However, any comparison of the sulfur XPS and AES intensities from a mixture sample must take the oxidized Ag2S into account. K2S04-Ag2SMixtures. The XPS line areas, determined as outlined in the Experimental section and normalized by the Scofield cross sections, led to the stoichiometries reported in Table 111. The difference between the measured and nominal values may be due to several factors: (1) The simplistic scheme for estimating the peak areas; (2) inaccuracy in the Scofield cross section (cross sections deduced from Berthou and Jorgensen (31) would reduce the values in Table I11 by 10-1570);(3) actual nonstoichiometry. Electron inelastic mean free path corrections are relatively small and would serve
-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
I ? ? e.(9 n! f 4 7 r' Y f +I - tl tl tl tl - +I +I tl
0 0 0 0 0 0 0 0 0 0 0 TI
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
Table 11. Sulfur Valence Electron Populations valence electron 3s 3P 3d H,Sa 0.15 4.4 1.8 Li2SOqb I t , , 5t,, l e 0.1 1.5 4t2 2.3 0.05 5a 1 0.4 3t2 0.9 0.15 4a 0.75 I. H.Hillier and V. R. Saunders, Int. J. Quantum Ref. 23. C h e m . , 4, 203 (1970). Table 111. Sample Stoichiometry as Measured by XPS reference
AgxS I 0.1
KYSO,
2.36
2.46
2.6 2.6 2.6
2.4 2.2 2.5 1.9
i
0.05
Mixture 1 2
3 4
2.5
only to increase the difference. For the purposes here, the absolute accuracy is not so important, rather it is the variation from sample to sample which shows up limitations for quantification. In particular, compared to the reference samples, XPS measurements of stoichiometry for the mixtures show up as rich in Ag and poor in K. If the XPS stoichiometry estimates are accurate, then the AES data should show a similar trend. In Figure 2 is shown the AES S LVV spectral region for four mixtures of AgzS04and K2S04; the base line has been corrected by the procedure mentioned in the Appendix. In addition the calculated spectra that result from use of Equation 1are displayed as a dashed line. It was not necessary to shift the reference line shapes on the energy scale to achieve the results in Figure 2; the cation AES kinetic energies of the mixture and reference samples confirm the absence of any charging shifts between the two. As can be seen, the fits between the experimental and the calculated spectra are close over most of the spectral region. In Table IV the coefficients are given that were determined by Equation 1. For comparative purposes, the ratios of the peak-to-peak height of the cations in the mixture to the peak-to-peak height observed in the spectrum of the pure material are shown also. The results from both methods of analyses are in good agreement, in most instances, i.e., within the standard error of estimate in all cases except two. Further, the agreement is as good as we obtained by comparing two different sets of reference samples. The error limits for the cation ratios were determined by taking twice the RMS of the noise in a region of the AES spectra that did not contain spectral information. The larger relative errors are where they would be expected, (the subscripts m and r will be used to denote the mixture and normalized reference spectra, respectively, and intensity is implicit in the use of the work ratio)
120
160
140
KINETIC ENERGY I N eV
Experimental AES S spectra (base-line corrected) of Ag,S-K,SO, mixtures (-), and computed spectra (- - -) Figure 2.
Le., in the Ag+,/Ag+, ratio and in the coefficient C(S042-). Under the experimental conditions of this study, the AES Ag+ signal-to-noise ratio was somewhat less than that for K+. The contribution of SO:- to the total mixture spectrum would be expected to be subject to more error because of the less intense AES SO4'- LVV spectrum compared to that of Sz-,The summation of the values C(S2-) and C(S042-)from a given experiment is less than one in all cases; this is probably due to differences in the packing of the salt particles for the mixtures vs. that for the pure materials (see Experimental section). The ratio of S2-/S042-,as determined by the AES S LVV line shape, is compared to the AES Ag+/K+ ratio and to the XPS S2-/S042-ratio in Figure 3. If each measurement were errorless, the data should fall on a line of slope unity. The agreement between the S XPS and S AES determinations is remarkably good when one considers that the normalization (determined from the two reference samples) requires that the X-ray flux, measured electron flux, and instrument response be the same for two different samples. The scale factors for the S and cation AES measurements have the same beam factors so that those two determinations should track one another provided that the AgzS and K2S04stoichiometries do not change. The agreement found between the AES S and cation data contrasts with the XPS measured stoichiometries, which are shown in Table I11 to be different in the mixtures
Table IV. Ratio of Mixture t o Pure Sample AES Signal Intensities, K,SO,/Ag,S C(P)
0.098 0.152 0.197 0.258 a
I 4
i L
0.002
0.002 0.002 0.003
Ag+,alAg+, 0.088 0.112i 0.194 0.259 F ~
c(so,2-)
K + ~b ~i +r, ~
0.005
0.617
t
0.018
0.634
x
0.004
0.006 0.008 0.009
0.792
i
0.010
0.803
?
0.012
0.600
i i
i
0.014
0.551 i
0.006 0.005 0.004
0.602 0.577
The subscripts m a n d r refer to mixture a n d reference spectra, respectively.
Correction for SO,*- in Ag,S was made.
ANALYTICAL CHEMISTRY, VOL. 52,NO. 1, JANUARY 1980
Table V. Ratio of Mixture to Pure Sample AES Signal Intensities Ag,SO,/K,SO, energy shift SO, sample reference, e V C( SO,,-) Ag+,"/Ag+, a b
a b a b
0.28 c 0.14 i 0.39 ?; 0.18 i
0 0 -0.5
-0.5 -2.5 -2.5
0.70
i
0.05 0.02 0.05 0.02 0.03
0.30
i
0.01
0.42 0.19 0.42 0.19 0.42 0.19
c(so,~-) 2.11 t 0.80 i 1.95 t 0.74 c 1.45 * 0.55 *
89
K+mu;~+r
0.11
1.89 0.74 1.89 0.74 1.89 0.74
0.04 0.11 0.04 0.06
0.02
See footnote a in Table IV.
i d
2r
CIS - 2 1 ciso;
21
plot of AES S intensity ratio vs. XPS S intensity ratio and AES cation intensity ratio. C(S2-) and C(SO,'-) are from Equation 1. For the XPS results (0)R is (S2-/S042-)xps corrected for the S0;I'observed in the Ag,S. For the AES cation ratio, (X) R is (Ag m/ Figure 3. A
Ag+r)/W+m/K+r)
compared with the reference samples. The reason for this discrepancy is not known; one possibility is an unaccounted interference of the K 2s line in our measured Ag 3d5,, intensity. The data discussed above relate to smaller amounts of S2mixed in with a S042-.The differentiation of the two species is aided by the large energy shift between the principal features in the Auger spectra and by the enhanced sensitivity of S2- compared to SO:- (see section on Reference Spectra). For small amounts of SO4'- with S2-the problem is substantially harder, because a weak SO4,- AES line shape is superimposed on the low energy structure of the S2-. If there were a unique S2-line shape to the low energy side, this would be a simple problem of signal to noise. However, as can be seen in Figure 1,f and g, the low energy structure on the S2does change with cation. This difference is true even if the 12% SO4'- contribution is removed from the Ag,S line shape; the CdS had no detectable oxidized species. The line shape changes with cat,ion are not unique to sulfide; similar changes have been observed for silicide (32). K2S03-AgZS04Mixtures. The AES S LVV spectra of S032-and S042--are different as is indicated in Figure 1;this result would be expected also from the differences in the XES spectra between various SO3,- and compounds (33). With this in mind, mixtures of A g 8 0 4 and KzSO3 were studied in a manner similar in procedure to that developed for the S2--S042-mixtures. The XPS S 2p line shape observed for the KzS03is definitely skewed toward higher binding energies in the samples used for these measurements compared to the other XPS S 2p line shapes found in this study. Later data on K2S03 obtained from a different supplier showed a S 2p line shape more characteristic of SO;', but the AES S L W features were not sufficiently different in either set of experiments to change
120
140
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KINETIC ENERGY IN e Y
Figure 4. (a)Experimental AES spectrum (base-linecorrected)for a Ag,SO,-K,SO, mixture (-), and the computed spectrum (---). (b) The experimental spectrum with the Ag,SO, contribution (determined from the cation ratio) subtracted
the conclusions drawn in this subsection. The nature of the AES SO3,- LVV lineshape will be discussed below. The Ag 3d5,, line on three different mixture samples was observed to be at a binding energy of 371.6 with a substantial shoulder near 367.2 eV. Evidently there were a t least two different Ag+ species present on the mixture samples. This suggests that the majority of the Ag2S04was located on top of KzS03(charge shifted by 3-4 eV) and the remainder was in contact with the ground plane. The K 2p binding energy was found to be shifted by 3 eV in accord with this model. The charge shifts were observed also in the S 2p binding energies. The computer fitted line shape for a Ag2SO4-E;,SO3mixture is compared to the experimental lineshape in Figure 4a; the second mixture showed equally poor agreement between the two line shapes. The values of C(S04'-) and C(S032-) are reported in Table V for the base-line corrected data, along with the appropriate cation intensity ratios. The C(SOS2-) values are in reasonable agreement with the K ratios as might be expected since KzS03 predominates, but the C(S04'-) values are quite low compared to the Ag+ ratios. The reasons for the much poorer agreement in this mixture as compared with the AgzS-K2S04mixtures are threefold. First, the SO?' and S042-AES S LVV line shapes are more nearly equivalent (see Figure 1). Both have similar low energy structures. However, there is sufficient difference in line shape and energy such that better agreement might be expected. Second, while the Ag2S04 AES S line shape was very stable under the electron beam, the K2S03 AES S line shape was susceptible to beam damage. If one subtracts out the reference AES SO': line shape, scaled by the Ag+,/Ag+, intensity ratio, from that
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
observed from the mixture, the result is shown in Figure 4b. The difference is somewhat similar to the AES S LVV lineshape observed for Na2S03. This suggests that the K2S03in the mixture was more stable than it was by itself. The C(S042-)value was in error because it represents the smaller signal and the S042-principal feature falls directly in the beam damage induced changes energy region where the SO': were greatest. Third, the sample charging which was observed with the X-ray beam also is apparent for the electron excited AES. In particular the AES Ag energy shows evidence of shifts of -0.5 eV. That is not much, but if the reference Sod2-line shape is shifted to lower energy by 0.5 eV, the value of C ( S O l - ) changes significantly (see Table V). Moreover, in the X-ray study of the mixture, the AgzS04appeared to be partly charge shifted and partly in agreement with the reference. If this also happened for the electron irradiated study, the sulfate reference spectra would no longer be accurate. However, the AES Ag+ line shape did not show any evidence for charging induced broadening. The mixture composition determined from the AES mea= 4.8 and 4.1 for surements of [(K+,/K+,)/(Ag+,/Ag+,)]AES samples 1 and 2, respectively, differs substantially from that = 2.0 and 2.6. measured by XPS [(K+,/K',)/(Ag',/Ag',)]~ We believe this is due to shadowing problems with the XPS, since the X-ray flux incident onto the sample is at a grazing angle. The Ag2S04species, which was suggested to be on top of the K2S03 by virtue of the observed charging, contributed a larger intensity in the XPS measurement because it screens the K2S03from the X-ray flux. Factors Influencing Quantitative Measurement. Base-Line Corrections. Since the correction to the base line due to redistributed primary and secondary electrons involves some effort, the question of its necessity must be addressed. In this particular set of determinations it was observed that the results with and without base-line correction were different. Base-line corrections improved the agreement between cation and S ratios by -2% for the S042--S2-mixtures and -20% for the so42--so32mixtures. In other experiments in this laboratory it has been observed that the base line can be different even for spectra taken one after the other. The prudent course of action would appear to be that the base-line correction should be made unless one is absolutely certain that it is not needed. For a rough check of experimental results such corrections probably are not warranted. Lineshape Differences. The ability to take a totally unknown mixture of S,OYn- species and to estimate their quantity on the basis of the AES S spectra depends on the uniqueness of the reference spectra, i.e., that there is a sulfate signature which is different from sulfite, sulfide, etc. The spectra shown in Figure 1 suggest that this is basically true. The essential differences are independent of cation effects. The S2-line shape is expected to be more cation dependent than the oxygenated species, since a S2-is directly bonded to the cation; however, no pronounced differences were observed for the sulfides used in this study. As was noted above, small amounts of one salt in the other are difficult to measure because there are some environmentally induced changes (beam damage, sample charging, and perhaps crystal structure and cation effects) in the S anion AES which complicate any comparisons. In addition to S042-,S032-,and S2-,sulfur combines with oxygen to form several other stoichiometries, such as S20?'. We attempted to examine the line shape of Na2S203,but we never succeeded in recording a line shape in which we had confidence. This species was quite unstable under our electron beam conditions. The relative intensities of the observed peaks (123, 138, and 147 eV, uncorrected peak minima) were not reproducible. Since the two sulfurs in S2032can "loosely" and one S2-type sulfur, be thought of as containing one SO':
even without beam damage effects it may be difficult to discriminate S203'-in mixtures by its AES S L W lineshape. Electron B e a m Damage. The widespread applicability of the technique described for the determination of different oxide states of the same element by AES would be more useful if the effects of the electron beam upon the sample did not have to be considered. There may be, in fact, systems where it may not be a factor, Le., Po43-and P3- (11). Different appear to have somewhat different behavior cations of Sod2with respect to observed changes in the AES S LVV spectra; e.g., the spectra of Ag2S04does not change as markedly with respect to electron beam exposure as that of KzS04. The susceptibility to electron beam damage under the similar experimental conditions observed in this study was K+ > Na+ Li' > Ag+. The sulfite samples show the same K+ > Na+ ranking. There have been numerous investigations of photon radiation damage in salts (34). For nitrates the damage is thought to be caused by ionization or excitation of the ion followed by its decomposition. For the nitrates, where the influence of the cation has been investigated, it has been postulated that the relative susceptibilities to damage might depend on the field strength of the cation or on the free volume in the crystal (35). For the K', Na+, Li+, and Ag+ sulfates the first ionization potential of the cation is 4.3, 5.1,5.4, and 7.5 eV, respectively; these potentials may reflect on bond ionicities. The room temperature anhydrous sulfates of K, Na (Forms I11 and V), and Ag are orthorhombic; Li2S04 is monoclinic (36). The ranking of free volumes for these crystals can be estimated by calculating the crystal volume per molecular unit and subtracting out the anion volume as estimated from the Pauling radii. This procedure provides the following estimates of the free volume plus sulfate volume in A3: KzS04,87; Na2S04,80; Li2S04,80; and Ag2S04,78. The ranking of both the ionization potentials and the crystal free volumes correspond to the observed damage susceptibility. Na2S04and Na2S03were chosen for a more detailed examination of the beam damage effects on the AES LVV line shape. The Ag2S04showed little damage for the beam dosages used here, while the K salts showed considerable damage. The Ag2S04line shape in Figure 1 is thought to be a relatively good S042-AES S LVV line shape in spite of the stoichiometry problems discussed above. The AES S LVV spectra for Na2S04are shown in Figure 5a for electron beam dosages of C/cm2 and C/cm2. The XPS S 2p lineshape after the lower dosage was not noticeably different from the virgin sample, but noticeable broadening to the low binding energy side is evident for the larger dosage. The only definitive change in the AES line shape is the growth of a peak near 145 eV. The nature of the damage may be a reduction to S032-;for much larger electron dosages (-10-1 C/cm2 and greater) on Li2S04Sasaki et al. (16) suggested the production of SO?-, So, and S2-on the basis of XPS line shape analysis. The growth of a peak a t 145 eV is consistent with the AES line shape of SO?-. Alternatively, the change may be due to loss of oxygen from the sulfate tetrahedron; a similar defect state was proposed by Schmidtal for electron beam damage in Si02 (37). For the S032-our best Auger line shape is represented in Figure 6; XPS analysis of the S 2p shows considerable damage already a t the C/cm2 exposure (see Figure 6a). The general features are believed to be representative because we have observed a similar line shape for N in NO3- from the XEAES (38). The damage induced changes in the SO3'appear in the 14&145 eV region of the AES line shape. The XPS S 2p line seems to shift some intensity to higher binding energies as well as the expected reduced oxidation states. This led us to suspect a sample charging problem; in fact, the
-
-
ANALYTICAL CHEMISTRY, VOL. 52, KINETIC ENERGY I N e V
,
100
120
NO. 1, JANUARY 1980
91
KINETIC E N E R G Y IN e V 140
160 I
140
120
-
160
r-7 1 -
1 1 1065
1075
1085
1065
1075
Flgure 5. (a) AES S L V V spectra of Na2S04after 1.7 min (-), and 20-min exposure to the electron beam (- - -). (b) XPS S 2p spectrum before exposure (-), and after 19-min exposure to the electron beam (- - -1
electron beam exposure caused the XPS 0 1s line to decrease in kinetic energy by 0.9 eV and the Na 1s line to decrease by 0.5 eV. The XPS S 2p line in Figure 6b has been shifted to a lower binding energy by 0.7 eV to compensate for charging. For the lo-*C/cm2 exposure, the higher binding energy intensity begins to decrease and a distribution of lower oxidation starts to increase. Several different approaches were tried to minimize or eliminate the effects of the electron beam upon the AES spectra of Na2S04and K2S04. The energy of the beam was varied from 500--3000 17 with little discernible effect. The sample stage temperature was reduced to approximately liquid nitrogen temperatures; this also did not appear to reduce changes induced by the electron beam. A small partial pressure of O2 has been observed by Thomas ( 1 5 ) and Ramaker et al. (24)to reduce the electron beam effect upon the AES spectra of SiO?. An O2 beam directed by a hypodermic needle a few cm from a Na2S04sample (sufficient to raise the base pressure in the system by -2 x lo4 Torr with continuous pumping by a 200 L/s ion pump) did not materially alter the electron beam interactions as monitored by the AES spectrum. Sample Charging. The energies at which the various AES transitions are observed may vary from sample to sample owing to differences in specimen charging. In this study differential charging between reference and mixture samples has not been observed to be a large effect (1 eV or less). This was monitored by noting the positions of the various AES cation transitions of the standards vs. those in the mixtures. In general, charging shifts could be accounted for by adding into the fitting routines whereby each energy shifts, LE,, reference spectrum could be shifted up or down in energy. However, these additional parameters can lead to problems. For instance, it was shown in the subsection on K2S03-Ag2S0,
1085
KINETIC ENERGY IN eV
KINETIC ENERGY I N eV
(a) AES S L V V spectra of Na2S03after 1.8-min (-) and 12-min (---) exposure to the electron beam. (b) XPS S 2p spectra before exposure (-), after 1.8-min exposure (- - -), and after 13-min Figure 6.
exposure
(.-.-a)
to the electron beam
mixtures that a shift of -0.5 eV for the SO:- reference spectra led to much better agreement between the C values and the cation ratios. The fit of the comput.ed and observed mixture AES, as determined by the sum of the squared residuals, also improved. We found that the sum was minimized (reduced by a factor of 4) if the SO4'- reference spectra were allowed to shift down by -2.5 eV. However, the C values so calculated (see Table V) were in very poor agreement with the cation ratios. This tells us, at least for the case examined here, that a four-parameter fit (energy and gain for each reference spectra) would have provided poor results.
CONCLUSIONS The LVV Auger line shapes of third row elements have characteristic line shapes which identify the presence of oxygen ligands. For the case of the S,O, moieties examined in this work, there appears to be distinct line shapes which can be used to identify a t least the Sod2-,S032-,and S2- species. When these ionic moieties are present as a mixture, it is possible to estimate their relative concentrations by computing the observed line shapes from reference line shapes. However, there are several serious difficulties which limit the accuracy of this approach. The most serious problem is electron beam induced damage, which for electron beam dosages as low as 1 0 ~C/cm2 ~3 can cause changes in the chemical identity of the molecular ions. Furthermore, the susceptibility to damage appears to be cation as well as anion dependent. For the sulfates tested, Ag2S04is the most stable followed by Na+, Li', and K+. Other sources of problems are base-line distortions, sample charging, and shadowing. APPENDIX The effect of the redistributed primary and secondary electrons upon the observed AES spectra was accounted for
Anal. Chem. 1980, 52,92-96
92
by the modification of a previously developed procedure (22). The background function, B ( E ) , is given in Equation A1: B(E) = AI S ( E ) A,P(E) A3 (All where Al, A,, and A3 are linear constants, S is a function that accounts for secondary electrons, and P is a function that involves the redistributed primary electrons. The form of S and P are shown in Equation A2:
+
+
E is the energy of the point under consideration, Eo is the secondary electron peak, 4 is the work function, E, is primary beam energy, Eh is an effective orbital binding energy for the system studied, and m and n are exponents with values of about two and one, respectively. The derivative of EB(E)with respect to E must be used with derivative AES data (the factor E must be included due to the spectrometer response).
Estimates are made of Eo,4, E,, m, and n from experimental or theoretical considerations to reduce linear dependence so that proper convergence of an iterative least squares process can be achieved.
LITERATURE CITED (1) C. J. Powell and P. E. Larson, Appl. Surf. Sci., I , 186 (1978). (2) D. R. Penn, J . Nectron Spectrosc. Relat. Phenom., 9, 29 (1976). (3) C. S. Fadley, in "Progress in Solid State Chemistry", G. Samorjai and J. McCaldin, Eds., Pergamon Press, New York, 1976, p 265. (4) T. A. Carlson, "Photoelectron and Auger Spectroscopy", Plenum Press, New York, 1975. (5) S. H. Hercules and D. M. Hercules, in "Characterization of Solid Surfaces", P. F. Kane and G. B. Larrabee, Eds., Plenum Press, New York, 1974, p 307.
W. M. Riggs and M. J. Parker, in "Methods of Surface Analysis", A. W. Czanderna, Ed., Elsevier, New York, 1975, p 103. D. T. Clark, W. J. Feast, W. K. R. Musgrave, and 1. Ritchie, J . Polym. Sci., Polym. Chem. Ed., 13, 857 (1975). N. L. Carig, A. B. Harker, and T. Novakov, A f m s . Environ.,8, 15 (1974). P. E. Larson, J . Necfron Spectrosc. Relat. Phenom., 4, 213 (1974). L. J. Brillson and G. P. Ceasar, J . Appl. Phys., 47, 4195 (1976). M. K. Bernett, J . S. Murday, and N. H.Turner, J . Necfron Specfrosc. Relat. Phenom., 12, 375 (1977). C. C. Chang and D. M. Bouiin. Surf. Sci., 69, 385 (1977). J. F. Wager and C. W. Wilmsen, in Proc. of Int. Topical Conf., "Physics of SiO, and Its Interfaces", Yorktown Heights, N.Y., March 1978, S. T. Pantelide, Ed., Pergamon Press, New York, 1978, p 373. T. T. Lin and D. Lichtman, J . Vac. Sci. Technol., 15, 1689 (1978). S. Thomas, J . Appl. Phys., 45, 161 (1974). T. Sasaki, R. S.Williams, J. S. Wong, and D. A. Shirley, J . Chem. Phys., 68, 2718 (1978). G. D. Knotl and D. K. Reece, in "Proceedings of the ONLINE 72 International Conference", Online Computer Systems, Ltd., Burnel University, United Kingdom, 1972, p 497. 6.J. Lindberg, K. Hamrin. G. Johansson, ti, Geiius, A. Fahlman, C. Nordling, and K. Siegbahn, Phys. Sci., 1, 286 (1970). C. D. Wagner, Anal. Chem., 47, 1201 (1975). J. H. ScofieM, J . Electron. Spectrosc. Relat. Phenom., 8, 129 (1976). D. M. Aylmer, H. Razzaul, and J. E. Carver, Anal. Chem.. 51, 581 ( 1979). D. E. Ramaker. J. S. Murday, and N. H. Turner, J . Electron Spectrosc. Relat. Phenom., 17, 45 (1979). D. E. Ramaker and J. S.Murday, in preparation. D. E. Ramaker, J. S. Murday, N. H. Turner, G. Moore, M. G. Lagally, and J. Houston, Phys. Rev. 6 , 19, 5375 (1979). D. E. Ramaker and J. S.Murday, J . Vac. Sci. Technol., 16, 510 (1979). C. A. Coulson and C. Zauli, Mol. Phys., 6, 525 (1963). A. Faessler and M. Geohring, Nafurwissenschaften, 39, 169 (1952). E. J. McGuire, Sandia Laboratories Reports SC-RR-69-137 (1969), and SC-RR-710075 (1971). C. C. Chang, in "Characterization of Solid Surfaces", P. F. Kane and G. 6.Larrabee, Eds., Plenum Press, New York, 1974, Chapter 20. L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, "Handbook of Auger Electron Spectroscopy", Physical Electronic Ind., Eden Prairie, Minn., 1976. H. Berthou and C. K. Jorgensen, Anal. Chem.. 47, 482 (1975). J. A. Roth and C. R. Crowell, J . Vac. Sci. Technol., 15, 1317 (1978). Y. Takahashi and K. Yabe, Bull. Chem. SOC. Jpn., 42, 3064 (1969). E. R. Johnson, "The Radiation-Induced Decomposition of Inorganic Molecular Ions", Gordon and Breach, New York, 1970. Ref. 34, p 35. R. W. G. Wyckoff, "Crystal Structures", Interscience, New York, 1960. K. Schmidtal, Surf. Sci., 77, 523 (1978). F. Hutson, J. Ganjei, D. E. Ramaker. and J. S. Murday, unpublished results.
RECEIVED for review June 5 , 1979. Accepted September 27, 1979.
Room Temperature Carbon and Oxygen Determination in SingIe-Crystal SiIicon D. Warren Vidrine Nicolet Instrument Corporation, 5225 Verona Road, Madison, Wisconsin 537 1 7
A more rapid, precise, and sensitive analytical method has been developed for C and 0 in single-crystal Si. The method Is a modification and Improvement of the standard ASTM analytlcal method made possible by the wavenumber scale stability, sensitivity, and photometric accuracy available with current laser-references FT-IR instrumentation.
Analysis of carbon and oxygen content in single-crystal silicon is of great importance to the semiconductor industry. Part-per-million levels of these impurities in the undoped raw 0003-2700/80/0352-0092$0 1.OO/O
silicon used in integrated circuit (IC) manufacture can greatly influence the rejection rate and long-term reliability of these devices. The increased use of IC devices in computers and control systems (e.g., automobiles) now requires the production of high-reliability devices without accompanying costly high rejection rates. Because refined single-crystal silicon typically is a t least 99.9995% pure, the chemical matrix environment of the impurity atoms is quite constant, and excellent Beer's law linearity of infrared absorbance with impurity concentration is observed. However, the standard American Society for Testing Materials (ASTM) infrared methods for determining
c 1979 American Chemical Society
