Angle resolved XPS study of inhomogeneous specimens of polycrystalline silver covered with uniform graphite overlayers

applied

surface science ELSENIER

Applied Surface Science 90 (1995) 241-250

Angle resolved XPS study of inhomogeneous specimens of polycrystalline silver covered with uniform graphite overlayers M. Sreemany, T.B. Ghosh Department

of Physics and Meteorology, Received 6 December

*

Indian Institute of Technology, Kharagpur 1994; accepted for publication

- 721302, India

13 May 1995

Abstract The depth dependence of the XPS peak shape parameter is determined experimentally for inhomogeneous specimens of the type f(n) = 0 for x, > x > 0 and f(x) = constant for x > x0, where f(x) is the electron emitter concentration as a function of depth (x) and x, is the overlayer thickness. In order to do this, specimens with sharp interfaces are prepared by depositing uniform overlayers of graphite on flat surfaces of polycrystalline silver. The peak shape parameter thus determined is found to be a function of the overlayer thickness. The observed dependence has been explained in terms of the existing first order analytical expression for this parameter. Angle resolved studies have also been done to explore the possible contributions arising out of surface excitation losses. Such studies clearly pointed out the presence of such an effect. Experimental results are discussed with a special emphasis on the possible sources of systematic errors which may contribute to the measured values of this parameter.

1. Introduction Monoenergetic electrons, that are emitted from the electron emitters placed underneath a solid surface, undergo energy losses of various kinds. These

energy losses are due to two distinct physical processes, e.g. intrinsic and extrinsic losses. Early theoretical calculations [l-4] have shown that due to electrostatic screening of the core-hole created in the ionization process, any peak in a solid is always

* Corresponding author. Phone: +91 3222 2221; Fax: 3222 2303; E-mail: [email protected].

+91

0169-4332/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00081-X

accompanied by electrons of lower energy. These intrinsic electrons give the primary peak a tail which is seen to extend 2-3 plasmon excitations below the main peak [5,6]. Electrons which lose energy during its transportation to the solid surface are called the extrinsic electrons. These electrons give a rising background structure extending up to several eV on the lower energy side of a primary peak. Based on a physical model for electron transport in solids [7], formulae for separating the background spectra are now available [7,8]. It has been shown analytically that the intensity of the unscattered to the inelastically scattered electrons depends on the path length travelled by the photon excited electrons in the solid

242

M. Sreemany, T.B. Ghosh /Applied

before emission [7,8]. This dependence has been utilized to extract valuable in-depth composition profile information through a simple analysis of the ratio of the peak area to the increase in the background which is defined as the peak shape parameter [9-111. Although the possibility of extracting a depth-dependent composition profile is being predicted, one is frequented with the following problems while experimentally determining this parameter: (i) The resultant spectrum is a convolution of both intrinsic and extrinsic effects and, hence, it is difficult to isolate the two contributions. Many authors have addressed to this difficulty and have proposed various methods for isolating the elastically scattered part of the spectrum. Among these some are empirical [12-141 and some are based on the physical processes of electron transport in solids 171.It is now well understood that the inelastic loss contribution immediately under an elastically scattered peak is negligible [15]. This, probably, is the reason for a close agreement of the quantitative results obtained using various methods of background subtractions 1141. (ii) Till date very limited information is available on the inelastic scattering cross-section at low energies (E < few keV). For those analytical expressions available for the determination of the peak shape parameter assume a dielectric response description of the solid-electron interaction [8,16,17]. This appears to be an over-simplification on the ground that the free-electron-like assumption may not be strictly valid for electron emitters placed under different environmental conditions. (iii) Baird et al. [4] studied the angular dependence of the relative intensities of the bulk and the surface plasmon losses of the Al 2p core level. These authors had observed a six-fold increase in the single surface plasmon intensity, followed by a decrease by 20% in the bulk-plasmon intensity. These observations led Tougaard to assume [15] that surface excitations contribute very little to the overall background intensity. However, the situation is expected to be different when solid surfaces are exposed to different atomic environments. Although overlayer experiments give us a possibility of observing such phenomena, no attempt has so far been made in this regard. This may be due to the practical difficulty in

Surface Science 90 (I 995) 241-250

preparing uniform thin overlayers with sharp interfaces. Recently, Sreemany and Ghosh [18] in their studies with graphite covered surfaces of polycrystalline gold have observed for the first time an enhancement effect on peak shape parameter due to surface excitation. (iv) Although considerable theoretical developments have taken place in recent years on the applicability of XPS peak shape analysis, very little work has so far been done to test the experimental validity of the derived relations. Tougaard et al. [19] used this algorithms to determine the concentration profiles of some atomic distributions. Recently Sreemany and Ghosh have determined the peak shape parameter for some homogeneous and inhomogeneous specimens and have discussed the effect of the choice of background on this parameter [18]. They have also proposed a simple method to test the validity of the existing first order relations for homogeneous and inhomogeneous specimens [18]. (v) Again, elastic electron scattering will cause the electron path to deviate from a straight line [20-231. The influence of elastic scattering on the energy spectra in the near peak region is, however, likely to be smaller than the influence caused by the uncertainty on available data for the inelastic mean free path and elastic scattering cross-sections [24]. For these reasons angular deflection of scattered electrons is not taken into consideration. In view of the growing technological interest in understanding the near surface region of solids, XPS peak shape analysis gives an alternative way of obtaining depth-dependent composition profiles. The advantage of peak shape analysis over other methods lies in the fact that the technique is simple, very fast and non-destructive in nature. The aim of the present investigation is to verify relevant formulae for peak shape parameters for a substrate covered with an overlayer and also to address the above-mentioned uncertainties associated with XPS quantification. As mentioned earlier, a substrate covered with an overlayer is an ideal choice for observing a surface enhanced effect like surface plasmon excitation; to observe such an effect angle resolved studies have also been carried out at various levels of overlayer thickness. For this inhomogeneous samples of a graphite coated surface of polycrystalline silver are prepared. We report here our findings in this regard.

M. Sreenany,

T.B. Ghosh /Applied

2. Theoretical considerations For solids for which electron emitters are located at a depth of x, from the surface such that f(x) = 0 and f(x) = C for x > x, Tougaard for O
cos B)/F(

E) dE (I)

243

Surface Science 90 (1995) 241-250

nique. Prior to deposition the glass substrates were thoroughly cleaned and heated at elevated temperatures to reduce the adsorbed gas content. Graphite coated surfaces were obtained by exposing these films in a graphite arc. Deposition was done for a certain interval of time so, that approximately an overlayer of u 200-300 A was deposited. These films were then argon sputtered in the preparation chamber of VG-ESCALAB MKII to reach an overlayer thickness of about 50 .&. This was achieved by taking the XPS spectra at various stages of the sputtering. Further sputtering was stopped as soon as the Ag 3d signal was obtained.

and B,=C(Acos

e)[h+(x,/cos

Xexp( -x,/h xK(E,

cos 0)

- E)JpF(

E) dE.

E

Assuming scattering parameter D=D,/[l+

3.2. Measurement

e>]

cos (?)I,

(2)

(3)

where D, = l/[ AK(E, - E)] is defined as the universal constant for homogeneous solids [26]. Sreemany and Ghosh [18] verified the validity of relation (3) by studying an inhomogeneous sample of Au covered with graphite overlayers of various thicknesses (x0). A reasonable agreement between the theoretical and experimental results is achieved. However, results of angle resolved studies gave a systematic change in D with the change in emission angle which could not be explained from relation (3). The authors have attributed this to an additional energy loss due to surface excitations. In an attempt to confirm the observed effect, further studies are carried out on graphite covered polycrystalline surface of silver.

3. Experimental details

3.1. Sample preparation Spectroscopic grade silver was deposited glass substrate by electron beam evaporation

thickness

The signal (1,) from a substrate covered by a flat overlayer of thickness x, is given by

a sharp primary peak and for a single event, the inelastic loss or peak shape is expressed as [18] (x,/A

of overlayer

on a tech-

Z,=q

exp[-x,/A(E,)

cos 01,

and that from an overlayer 1, = c[ 1 - exp( -x,/A(

(4)

is given by E,)

cos

cl)],

(5)

where 8 is the electron emission angle with respect to the sample surface normal and C and c correspond to the intensities obtained from pure substrate and pure overlayer elements, respectively; ACE,) and ACE,,) correspond to inelastic mean free path values at the respective energies. From Eqs. (4) and (5) we get W0/~)/(~~/C)] = (1 -ee- x,/ACE,)

sin a)/,-x./h(E,)sin

a, (6)

where (Y is the electron take-off angle given by (~=900-

8.

In the present paper, the inelastic mean free path (A) has been determined using Penn’s relation [27]. As the inelastic mean free path values for graphite at 1201.6 eV (C 1s) and at 1117.9 eV (A& 3d,,,) are very clote to each other (ACE,) = 15.85 A andoh = 15.0 A), an average value of A(A,, = 15.4 A) has been assumed. Under this assumption the overlayer thickness (x0) can then be obtained from xO=Aav[ln[l+$)]

cos 8.

244

M. Sreemany, T.B. Ghosh /Applied

The estimated values of overlayer thicknesses are also compared with the values obtained from etch rate measurements [18]. The thicknesses thus obtained are found to agree within 10%. Errors in the measured values of X, using relation (7) primarily arise due to the uncertainties associated with such overlayer experimentation [28]. These uncertainties arise mainly from the calculated values of A, measured values of line intensities and surface roughness, and in the determined values of the electron

Surface Science 90 (I 995) 241-250

emission angle (0). Although the error sources contributing to the measured values of x, following the above procedure are known, it is difficult to estimate the resultant error in the measured values of x,. Hence, no claim is made with regard to the accuracy on the reported values of x,. However, since an agreement within 10% is achieved when x, is compared with the values obtained from etch rate measurements, the reported X, values can be assumed to be very close to the actual thickness.

a

o

Experimental

Experimental

Fig. 1. Experimentally determined and theoretically calculated (obtained from relation (6)) angultr variation of thf intensity for t@ overlayer/substrate system of graphite covered silver films of overlayer thickness x,. (a) x, = 38.6 A, (b) x, = 23.9 A; (c) x, = 17.5 A, Cd) x, = 8.3 A,.

M. Sreemany, T.B. Ghosh /Applied Surface Science 90 (1995) 241-250

3.3. Spectrometer setting Spectra were recorded by a VG ESCALAB MKII spectrometer using Al Ka radiation operating at 12 kV and 34 mA. Throughout the experiment the analysis chamber pressure was maintained at 5 X lo-” Torr. Each specimen was analyzed by a combination of a 1250 eV survey scan and a 25 eV high resolution scan for all the relevant photoelectron lines and their associated background. All the high resolution spectra were recorded at a constant pass energy (20 eV) giving an instrumental resolution of 0.40 eV. 3.4. Intensity measurements XPS peak intensity measurements were done using the method of linear background subtraction following Ghosh and Sreemany [14]. For homogeneous solids estimated uncertainty is reported to be around 6%. However, for specimens covered with overlayers the uncertainty in the measured intensity is expected to be more because of an increase in the inelastic loss background level. A rough estimation is obtained by comparing the intensity ratios measured for a pair of photoelectron lines emitted from an overlayer substrate system with those obtained from homogeneous specimens. This is found to be within 10%.

4. Results and discussion The aims of the present investigation are firstly to test the validity of relation (3) and secondly to explore the possibility of the surface enhancement effect on the peak shape parameter. Reliability of the experimental test in this regard depends much on the flatness of the overlayer under scrutiny. Therefore, it is felt necessary first to examine the flatness of the inhomogeneous specimens under investigation. An excellent test in this respect is the surface sensitivity enhancement study [29]. It is reported that surface roughness leads to an averaging of the electron exit angles and also a shadowing effect such that in most of the cases the surface enhancement effect is not observed [29]. Hence, this gives a fairly good test for the quality of the flat overlayer under investigation. For surface enhancement studies, at a given value

245

of the overlayer thickness (x,), measurements of the intensities are carried out at various values of 8. For larger values of 8, emission of photoelectrons takes place from shallower depths. Such studies need a reliable estimation of 19in order to know the location of the electron emitters underneath an overlayer. B’s are usually measured from the vernier provided with the sample manipulator system in the spectrometer which has a large backlash error. This requires calibration of the 0 scales while angle resolved studies are performed. Surface enhancement effect provides an opportunity to have a reliable estimation of 8. Figs. la-ld describe the results of the surface sensitivity enhancement studies done on graphite covered polycrystalline silver surfaces. The various overlayer thicknesses are obtained by Ar+ sputtering the film with an initial thickness of - 39 A. The details of the overlayer thickness measurements are discussed in Section 3.2. For a given value of x, and also knowing NE,) and NE,), values of can be calculated for different (Y ‘s, (ZJQ/( ZJC) using relation (6). This serves as calibration plot for the variation of is further etched for a given interval of time keeping the etching conditions identical. Tke second overlayer thickness estimated is - 23.9 A. Results of the surface enhancement study done on this specimen are shown in Fig. lb. The measured values of (Z,,/G)/(Z,/~) for various electron take-off angles are found to differ from an ideal variation with x, = 23.9 w (smooth curve), although the general trend is found to follow the ideal nature. The observed deviation may be due to (i) errors in the intensity measurements; (ii) errors in the 8 measurements; (iii) errors due to sample positioning; (iv) sample surface roughness.

246

IU. Sreemany, T.B. Ghosh /Applied Surface Science 90 (1995) 241-250

As discussed in Section 3.4, the procedure used for intensity measurements may account for an error of the order of N 6%. The observed large deviation of the measured values of /, therefore, cannot be due to the intensity measurements alone. Since sufficient care was taken to expose the same portion of the surface of a given film for XPS measurements, hence the observed deviation does not arise because of sample positioning. The most probable error in these measurements may arise due to the errors in the measured values of 8, which is obtained from the drum readings of the Xm manipulator provided with the spectrometer [18]. Assuming errors due to intensity measurements and sample positioning being negligible, the observed deviations have been attributed to the errors in the measured values of 8. The measured values of 8 are corrected for its further use in the angle resolved studies. For this the calibration plot of the surface sensitivity enhancement study is used. For a measured value of / at a given value of drum reading, corrected values of 8 are obtained by comparing the intensity ratios with the calibration plot. 8 corrections are explained in Fig. lb. A,, A,, A, and A, indicate the corrected 8 positions. Figs. lc and Id show the results of the surface sensitivity enhancement studies ,for graphite overlayer thicknesses 17.5 and 8.3 A, respectively. As discussed above, the measured changes in the values of (Z,/~>/ for various electron take-off angles show a similar trend which is expected from the idea! flat surface of overlayer thicknesses 17.5 and 8.3 A, respectively. For the reasons discussed in the previous paragraph the observed deviations are attributed to the errors in the measured values of 0. The wide scan X-ray photoelectron spectra recorded for a graphite covered silver surface at various depths are shown in Fig. 2. All the wide scan spectra were recorded at identical spectrometer settings. Different overlayer thicknesses were obtained by argon sputtering. At different depths, angle resolved studies were also performed. Table 1 gives the results of such studies. Column 1 gives the measured overlayer thicknesses. The various depths shown in the column are calculated as was described in Section 3.2. Columns 2 and 3 give the measured and corrected values of the electron emission angles that the sample surface normal makes with the ana-

Cls pxl

I

M3P

lb)

Ag 3d

Ag3p

I

=-!!r+x3 x3

tdl ClS

I

I

I

I

I

1

I

I

loo 200 300 400 so0 600 700 Wo Binding Energy (ev I Fig. 2. Wide-scan XPS spectra of graphite covered silyer surfaces at different graphite overlayer thicknesses: (a) - 39 A, (b) - 24 & (c) - 18 zk; (d) - 8 .&.

lyzer entrance. The corresponding depth values are also shown. Inelastic mean free path (A) values in the present analysis are calculated using Petn’s method 1271.These are 15.85, 14.98, and 12.74 A at 1201.6, 1114.9 and 897.6 eV, respectively. Columns 5 and 6 give the values of D measured for Ag 3d and Ag 3p lines. The values of the inelastic loss or

M. Sreemany, T.B. Ghosh /Applied Surface Science 90 (1995) 241-250

247

Table 1 Measured values of D corresponding to Ag 3d and Ag 3p lines of silver covered by graphite overlayers of various thicknesses at various electron emission angles Overlayer thickness (x0)

0 meas (deg.)

8 corr (deg.)

x,/cos

f&,,

DAg3d DAg3P

38.6

0 10 15 30

0 10 13.5 26.5

38.60 39.20 39.70 43.13

13.2 9.8 11.6 11.4

-

23.9

0 15 30 40

11.5 23 26 32

24.39 25.96 26.59 28.18

14.4 14.2 13.9 14.1

14.7 14.1 13.9 -

17.5

5 20 35

15 19 29

18.12 18.51 20.01

14.7 13.9 13.3

15.7 14.3 12.4

0 15 30 40

0 13 36 47

8.30 8.52 10.26 12.17

20.4 19.0 18.0 15.9

23.1 22.8 20.1 18.4

(‘JQ

8.3

peak shape parameters reported are obtained by measuring the intensities following the method suggested by Ghosh and Sreemany 1141. Figs. 3 and 4 give the plots of 1nD versus ln(1 + x,/A cos 8) obtained using relation (3) for the Ag 3d and Ag 3p lines. The calculations are based assuming D, = 75 eV [9]. Experimentally measured values of D for various x,/A cos 0 are also shown in these plots. A best-fit straight line is obtained for all the experimentally determined points. The slopes measured for these lines are found to be N 1.057 and m 1.059, respectively, whereas the corresponding slope obtained from a direct plot of the relation (3) is 1.0. The nearness of the slopes and the linear variation of the experimentally determined D values show that the depth dependence of the inelastic loss parameter predicted by relation (3) is correct for inhomogeneous specimens. However, the intercepts obtained experimentally are found to differ considerably from those predicted by relation (3). Similar results have also been obtained in our study of a gold surface covered with graphite overlayers [18]. The

In

(l+,m)x0

-

Fig. 3. Variation of the inelastic loss parameter of the Ag 3d line signal in an inhomogeneous graphite covered silver film with the change in the graphite overlayer thickness: (a) experimentally determined; (b) obtained from relation (3).

I 0

c

1

I

I

I

1

0.5

1.0

1.5

2.0

In(1+-&1Fig. 4. Variation of the inelastic loss parameter of the Ag 3p line signal in an inhomogeneous graphite covered silver film with the change in the graphite overlayer thickness: (a) experimentally determined; (b) obtained from relation (3).

248

M. Sreemany, T.B. Ghosh/Applied Surface Science 90 (199% 241-250

intercept is obtained by extrapolating the experimentally measured values to x, = 0, from which the value of D,, can be obtained. D, measured experimentally for 3d and 3p lines is 38.5 and 39.0 respectively. Closeness in the measured values of D,, for the Ag 3d and Ag 3p lines indicates that D,, does not depend on the electron energy. Incidentally these values are found to be very close to the reported values of the peak shape parameters for homogeneous samples of silver [18]. However, this is found to be different from that obtained from a plot of relation (3). This deviation appears reasonable as the assumed value of D, for plotting relation (3) is a theoretical estimate [9]. In practice, D, measured for homogeneous samples gives a much lower value [9,18]. These observations point out the fact that in deriving relation (3) for inhomogeneous specimens it has been assumed that the inelastic loss cross-section K(E, - E) of a given element is independent of its environment. The above study clearly shows that such an assumption may not be strictly valid. Although the data, till date, with regard to the matrix dependence of K(E, -El is limited, the present investigation shows a possibility of such measurements. Table 1 also gives the values of D measured for various electron emission angles. From a study of Table 1 it is evident that the values of D for overlayer thicknesses X, > 17.5 w remain the same within the experimental uncertainties. Uncertainties associated with the measured values of D are primarily due to the error in intensity measurements. The presently employed method for intensity measurements gives an uncertainty of the order of N 10% (see Section0 3.4). However, for overlayer thicknesses x, < 17.5 A the measured values of D show a systematic change with an increase of 8. A similar change of D with the electron emission angle has been reported earlier [18]. Fig. 5 gives a plot of In D versus ln[l + (x,/A cos 0>] in an enlarged scale for the Ag 3p line. For comparison this plot also shows the best-fit line obtained earlier. It is evident from the plot that the observed changes in D cannot be explained with the help of relation (3). Attempts were also made to observe such effect in homogeneous polycrystalline films and in the bulk samples of silver. No detectable change in D was observed

A9 3~ XtJ =17.5 1

2.9 I 0

2.1

+

I

1

I

I

0.7

0.8

0.9

I

1.0

1

x0

ln Il+ xcose I Fig. 5. Variation of the inelastic loss parameter of the Ag 3p line signal in an inhomogeneous graphite covered silver film at an overlayer thickness of - 17.5 A with the change in emission angle (enlarged region III of Fig. 4).

with the change in electron emission angles, although observance of such an effect in case of Au has been reported [30]. This may be due to the fact that when only homogeneous samples are considered, electrons are usually collected from all possible depths measured from the surface. In this regard, it is worthwhile to note that overlayer experimentation gives us a unique opportunity to record electron signals from substrate atoms very close to the interface. It is now well understood that electron-electron interactions can excite collective density fluctuations in the electron gas in a solid. This gives rise to two distinct energy loss processes due to excitations of bulk plasmons and surface plasmons. While the physical basis of the loss process in bulk plasmon excitation is well explained, the quantitative knowledge with regard to the surface plasmon excitation is still limited. This is because the surface excitations depend on the surface conditions. The usual observance of W, = is valid for surface-vacuum interfaces where W, and Wp are the energy losses due to surface and bulk plasmons, respectively [31]. Baird et al. [4] have studied the relative intensities of bulkand surface-plasmon loss peaks associated with the Al 2p peak. For electrons emitted at grazing angles, Baird et al. [4] reported a six-fold increase in the

M. Sreemany, T.B. Ghosh /Applied

single surface-plasmon relative intensity, while a single bulk-plasmon relative intensity showed a decrease by 20%. These observations led Tougaard [151 to predict that overall contributions to peak shape parameters may not be of much significance. However, present results of angle resolved studies have clearly demonstrated that the assumptions made by Tougaard [15] in this regard is not strictly valid. To further interpret our results on angle resolved studies we refer to the work of Powell and Swan [32]. In their study on the characteristic electron energy losses in aluminium thin films it has been reported that the low lying energy loss peak (now attributed to surface-plasmon excitations) changed considerably in position (< W,/ a> as well as in intensity relative to the bulk-plasmon loss peak with the decrease in film thickness. This has been interpreted as if the low lying loss is influenced by the surface layers of the specimens. Subsequent studies of Baird et al. [4] confirmed the observations made by Powell and Swan [32]. In the present study the observance of enhanced surface effect may arise because of the presence of a graphite overlayer. The presence of a graphite overlayer changes the boundary conditions to be satisfied by electromagnetic waves for its propagation across the interface. Thus, in the present situation energy loss peaks due to surface excitations may be at an energy location where complete ammlation is not possible. This possibly explains the observed angular variation of the peak shape parameter. It is interesting to note that the enhanced surface effect is obtained only for an overlayer thickness < 17.5 A. No detectable angular effect is observed for an overlayer thickness greater than 17.5 A. This may be due to the prominence of extraneous electron energy losses over the plasmon losses for higher overlayer thicknesses.

5. Conclusions 2WS peak shape parameters for Ag 3d and Ag 3p lines are measured for various graphite overlayer thicknesses. The depth dependence of the peak shape parameter is found to be explained by relation (3). The observed differences in the values of the inter-

Surface Science 90 (1995) 241-250

249

cepts obtained experimentally as well as from a plot of relation (3) have been attributed to the matrix dependence of the inelastic loss cross-section. Peak shape parameters measured for various electron emission angles show a variation which could not be explained by relation (3). The observed angular dependence has been explained in terms of additional energy losses due to surface excitations. The observance of surface excitations is also found to depend on the overlayer thickness. In the preseft study for graphite overlayers of thickness > 17.5 A, no detectable angular effect on the peak shape parameter is observed.

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[28] C.J. Powell, J. Electron Spectrosc. Relat. Phenom. 47 (1988)

197. [29] D. Briggs and J.C. Riviere, in: Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Bds. D. Briggs and M.P. Seah (Wiley, New York, 1983) ch. 3, pp. 87-139.

Surface Science 90 (1995) 241-250

[30] A. Jouaiti, A. Mosser, M. Romeo and S. Sindo, J. Electron Spectrosc. Relat. Phenom. 59 (1992) 327. [31] C. Xittel, Introduction of Solid State Physics, 5th ed. (Wiley, New York, 1976) ch. 10, p. 287. [32] C.J. Powell and J.B. S.van, Phys. Rev. 115 (1959) 869.