Measurement of positron annihilation induced Auger electron emission from Fe, Cu, Pd, and Au

:...:.A:: ,..y.:: ‘).+.:. ..:...).(::. y.:: :<;.;y j:. .y.: 2”

Surface Science North-Holland

264 (1992) 127-134

..,

,...

... .) . ‘. :+,;j: .I : :, ....::..: .:: ,i: :.,::, .:: ::.’ . .:. ; :; .:;

‘kurface sciencg ii.;j.’.::: ,...,.:.:.j ::..:::y.j:.::, jj’:’.;.> :j:j:+:, j,y.j:

Measurement of positron annihilation emission from Fe, Cu, Pd, and Au

induced

Auger electron

K.H. Lee ‘, A.R. Koymen ‘, D. Mehl ‘, K.O. Jensen b and A. Weiss a a Department of Physics, Unil;ersity of Texas at Arlington, P.0. Box 19059, Arlington, TX 76019-0059, h School of Physics, lJnir,ersity of East A&a, Received

20 June

1991; accepted

USA

Norwich NR4 7TJ, UK

for publication

15 October

1991

Positron annihilation induced Auger electron spectroscopy (PAESJ makes use of electron-positron annihilation to create core holes which result in Auger electron emission. This excitation process provides PAES with significantly higher surface selectivity than conventional Auger techniques. To date, PAES measurements have been reported on 5 elements. In this paper we report the first positron annihilation measurements of Pd and Au along with measurements of Cu and Fe. Analysis of the data indicates that the intensities of the low energy Auger signals of Cu, Fe, Pd, and Au are in the proportion of 1.00:0.57: 1.35: 1.17. These measurements should be useful in providing elemental standards for investigations of epitaxial growth and/or intermixing in bimetallic systems. The measured intensities are compared to calculations for core annihilation probabilities. In addition, measurements which demonstrate the sensitivity of PAES to H, chemisorption on Pd are presented.

1. Introduction Positron annihilation induced Auger electron spectroscopy (PAES) provides a unique method for the analysis of the elemental composition of the surface 111. This technique has a number of advantages over conventional electron induced Auger electron spectroscopy (EAES) including higher surface selectivity 121, greatly reduced secondary electron background and energy (for a given signal to noise ratio) [3]. To date, PAES measurements have been reported on only 5 elements. In this paper we present spectra for two elements, Pd and Au, for which PAES results have not been previously reported. In addition the relative Auger intensities obtained from Cu, Fe, Pd and Au, corrected for positronium emission and the effect of surface contamination, are reported and compared with theoretical values of core annihilation probabilities. These measurements should be useful in providing elemental standards and in obtaining estimates of intensi003Y-6028/92/$05.00

0 1992 - Elsevier

Science

Publishers

ties in future PAES studies including these elements. The ability of PAES to determine the contents of the top surface layer stems from the fact that a large fraction of low-energy positrons (< 25 eV> implanted into a metal or semiconductor diffuse back to and become trapped at the surface. A few percent of the positrons trapped at the surface annihilate with core electrons to create core holes which result in Auger electron emissions. Because positrons that contribute to the PAES signal are trapped in a surface state, they annihilate almost exclusively with atoms at the surface. As a result almost all of the Auger electrons originate from the topmost atomic layer [2]. In addition because the core holes are created by matter-antimatter annihilation and not by collisional ionization as in conventional Auger spectroscopy, a low-energy positron beam can be used. As a result, the PAES technique can be used to eliminate the large secondary electron background present in EAES spectra and beam damage to

B.V. All rights

reserved

12x

K. H. I,ee et ccl. / PAES of Fe, Cu, Pd, and Au

the sample both of which are related incident beam energy of EAES.

to the high

polishing, the samples were sputtered and annealed in vacuum (base pressure < 4 x lo- “’ Torr). EAES was used to determine the surface contamination.

2. Experiment The apparatus used in the PAES measurements has been described previously [4-61. It consists of a magnetically guided low-energy positron beam, a UHV sample chamber for surface preparation and analysis, and a trochoidal energy analyzer. A permanent magnet is mounted behind the sample to produce a field gradient which redirects the outgoing Auger electrons along the spectrometer axis. The combination of trochoidal monochromator and permanent magnet permits the measurement of the total kinetic energy of Auger electrons with an effective angular acceptance of - 2n. The FWHM of the spectrometer resolution function was set at - 7 eV. The relatively broad resolution was necessary to obtain reasonable count rates with the positron beam current of - 4 x lO_” A available in our apparatus at the time of the measurements (a higher-resolution and higher-flux system is currently under development in our laboratory). A sputter ion gun and conventional CMA based EAES system were used for sample cleaning and characterization. A microchannel plate (MCP) is used to detect electrons whose energy has been selected by the energy analyzer [4]. Three NaI(T1) crystal coupled to photomultipliers are used to detect the y-rays produced by annihilations of positrons and electrons. When the Auger signal is induced by the annihilation of positrons and core electrons, yrays are emitted in coincidence with the Auger electrons. The annihilation y-ray signal is used to gate the MCP counts. This greatly reduces background due to MCP dark counts [3] and counts from scattered positrons. The PAES measurements were performed on four high-purity (99.99%) metal foil samples of Fe, Cu, Pd and Au. These samples were mounted on a tantalum sample holder connected to a UHV manipulator. The samples were polished mechanically with 1, 0.3 pm alumina and 0.06 km colloidal silica polishing suspension. After

3. Results

and discussion

3. I. PAES spectra The positron annihilation induced Auger electron spectra of Fe, Cu, Pd and Au are plotted in figs. la-ld respectively. The spectra were obtained from the detection of electrons in coincidence with annihilation gamma-rays from the samples. The data is the sum of 10 loops in which the energy range was increased in 1 eV steps with an accumulation time of 30 s per point for each loop (total 300 s per point). The primary peaks of the Fe and Cu spectra correspond to the M,,VV Auger transitions, the peak of the Pd spectrum to the N,,VV transition and the Au spectrum to the O,,VV transition. The Auger peaks from other transitions originating from more tightly bound core levels (e.g., M,VV for Fe and Cu, N,VV for Pd, N,,VV and 0,VV peak for Au) are small relative to the primary peaks. It is interesting to note that the ratios of intensities of Auger peaks are quite different in PAES as compared to EAES due to the difference in the two excitation mechanisms. In general Auger transitions involving core levels of lower ionization energy will be favored in PAES because the annihilation probability decreases rapidly with decreasing electron orbit radius due to the repulsion of the positron from the nucleus [lo]. The sharp rise in counts below - 30 eV observed in the spectra shown in fig. 1 is due to the onset of collisionally excited secondary electrons. The maximum energy of secondary electrons produced through collisional processes can be determined from the following relation [3]: E&Ek=Ep-4-+4+,

(1)

where E, is the kinematic energy edge, E, is the energy of the primary beam, and 4-, 4’ are the electron and positron work function of the sample, respectively.

K.H. Lee et al. / PAES of Fe, Cu, Pd, and Au

3.2. Relative positron Auger intensities The relation between the measured PAES signals and the core level annihilation probabilities obtained from theory can be written as N XYZ

= EXYZ (mT~x)w,YzfssN,~ T

(2)

where Nxyz is the number of detected Auger electrons resulting from annihilation induced XYZ Auger transitions, (or is the probability that a positron trapped in the surface state will annihilate with a core electron in the state T, PT_X is

129

the probability that a core hole in the state T will result in a core hole in a state X, W,,, is the probability that a core hole in a state X will result in an XYZ Auger transition, lXyZ is the probability that an electron resulting from an annihilation induced XYZ transition is detected, f,, is the fraction of incident positrons that annihilate in the surface state and NO is the number of incident positrons. In our experiment the sample was negatively biased (- 5 V) with respect to a grounded grid in front of it to attract back the positrons reemitted from the sample. Previous work [7] has indicated

600 g 500

(4

0

2 2 400 ‘; .3 300 s ;i ,’

200 100

0 Fe

M2aVV

600

Fig. 1. Positron annihilation induced Auger spectra of (a) integrated range. Solid curves represent a fit to a function function [4]. Region (1) is the primary Auger peak area sample bias (- 5

Fe, (b) Cu, (c) Pd and (d) Au, indicating Auger transitions involved in obtained from a convolution of EAES data with the system resolution and region (2) corresponds to the low-energy contribution. Note: the V) shifts the energy by + 5 eV.

130

K.H. Let rt al. / PAES of Fe, Cu, Pd, cud Au

that for clean, single crystal metals the fraction of positrons that annihilate in the bulk is negligibly small at E, I 100 eV. Therefore we can write r,, + f;, = 1)

(3)

where f,, is the fraction of incident positrons that form positronium and then leave the surface. In order to compare our experimental results with theoretical calculations of ur it is convenient to normalize our data to f,, = 1 -f,,,. Dividing both sides of eq. (2), we get

=E

XYZ

1

C’TTBT+XWXYZNI,~ T

(4)

where N>I;,, is now defined as the normalized Auger intensity. The positronium fraction ( fpb)was determined from the annihilation y-ray energy spectra using the following equation [8]:

(5) In this equation K = (T- P)/P, where T is the sum of counts in the NaI(T1) spectra in a range corresponding to 170-585 keV and P is the sum of counts in the range 472-550 keV. The constants R,, R,,, P, and P,, where obtained by inverting eq. (5) for reference spectra taken at room temperature from a clean and a Cssaturated Cu surface for which the positronium fraction was taken to be 50% [9] and 100% [21,22], respectively. Calculated positronium fractions at room temperature are 63.3( * 1.2)0/o, 50.0 (f 1.2)%, 48.1( + 1.2)s and 53.3( + 1.2)‘% for Fe, Cu, Pd and Au, respectively. A comparison of measured and theoretical values of PAES intensities for all samples relative to Cu are shown in table 1. The steps taken in determining the relative PAES intensities (Irelat) from our measurements are outlined as follows: first we integrate the e--y coincidence counts over a range which includes the PAES peaks

Table 1 PAES intensities for the primary Au relative to Cu

transitions

Range

I,,,,, (Cu : Fe : Pd : Au)

of integration

“’

Not including 0 < E < E, Including 0 < E < E, Theory h,

of Cu. Fe. Pd and

1.00:0.37:0.‘)1 :0.x”, 1.00:0.57: 1.35: 1.17 1.oo: 0.93 : 1.32 : I .06

,I’ The integration area within the secondary peak (0 < E < E,) is labeled as region (2) in figs. la-Id. “I Theoretic values are determined by summing the calculated annihilation probabilities [Y] of the relevant levels (see text).

indicated in the Auger spectra shown in fig. 1. Second, a constant background determined from the high-energy portion (91-100 eV for Fe, 116125 eV for Cu, 86-100 eV for Pd and 98-100 eV for Au) of each spectrum was subtracted. We define the integrated intensity after background subtraction as I, (where a = Fe, Cu, Pd, Au). Third, we correct for surface contamination. This was done by plotting the integrated counts of each loop as a function of time since the previous sputter (see fig. 2) and using a straight-line fit to this data to extrapolate 1, back to zero time after sputtering to obtain locu. Next we divide by (1 f,\) yielding: I,;, = L/( Finally

1 -.fpJ

(6)

we calculate:

I,,,,, = GJ16C”

(7)

Our data indicate that there is a significant contribution to the integrated intensity of the low energy Auger lines indicated in fig. 1 from a broad low-energy tail associated with these peaks. Unfortunately the onset of the collisionally excited secondary electron background makes it impossible to directly determine the PAES intensities below E,. We obtain an estimate of the PAES intensity below E, using the area of the rectangles labeled as region (2) in figs. la-ld. This procedure, which was motivated by the fact that the intensity on the low-energy side of Cu and Fe spectra approach a constant value down to E,, probably overestimates the relative PAES

K.H. Lee et ul. / PAES of Fe, Cu, Pd, and Au

intensity from Pd and Au. The relative PAES intensities without this contribution is also listed in table 1 for comparison. The experiments were complemented by theoretical calculations of core annihilation probabilitics. The model employed here is identical to that of ref. [lo]. Hence, only a brief description will be given here. In the calculations an effective positron potential which includes the image potential is first constructed using the corrugated mirror model [13]. The surface electron densities needed in this construction were obtained by a superposition of free atom densities. A ramp potential was added to the Coulomb part of the potential to the correct for the misrepresentation of the surface dipole in the atomic superposition model [10,14]. The height of the ramp was adjusted for each surface to reproduce the correct experimental (when available) [7] or theoretical [16] positron work function. The precise location of the image plane in the corrugated mirror construction is usually adjusted to let the calculation reproduce the experimental value for the positron binding energy to the surface. Of the systems studied here the binding energy has been determined only for Cu(100) [71. However, the binding energy variation between different surfaces is known to be fairly small [7]. Since the results of the calculations depend only weakly on the image plane position, we have therefore assumed the binding energy to be identical for all surfaces when determining the image plane position. After construction of the potential, the positron wave function is found as the numerical solution of the Schrodinger equation. The total annihilation rate is then found from the positron wave function and the electron density using a local density approximation where we take into account that the local annihilation rate in the image potential region vanishes [9,14,17]. The core annihilation probabilities for different core levels are obtained as the ratio between the partial annihilation rates for each level and the total annihilation rate. The annihilation rates for each core level are calculated using the independent particle model. This ignores the enhancement of the annihilation rates due to electron-positron correlations 1183 but is expected to reproduce relative

131

values well, although the absolute numbers will be too small by a factor of about 1.5 [18]. Although theoretical values of Irelat could be obtained from the calculated annihilation probabilities using eq. (4) and summing the values of N’ over the Auger transitions indicated in fig. 1, a simplified procedure was used. The theoretical values of Irelat given in table 1 were estimated by summing the calculated annihilation probabilities of the levels 3s and 3p for Fe, 4s and 4p for Pd, and 4f, 5s and 5p for Au and dividing these probabilities by the corresponding sum for the 3s and 3p levels for Cu. This procedure is justified for the following reasons: (11 The annihilation probabilities for deeper core levels can be neglected since they are at least two orders of magnitude lower than for the levels listed above. (21 Holes in levels more shallow than those listed do not result in Auger electrons in the measured range. (3) The probability that the creation of a hole in one of the listed levels results in at least one Auger electron in the energy range of the PAES measurements has been calculated to exceed 99% [19,201. (4) In those cases where a single core hole can result in the production of two Auger electrons, one of these electrons has an energy below the measured energy range. (5) The escape probability of the Auger electrons is not a strong function of energy in the range measured. The agreement between theory and experimental values of lrelat are good with the exception of Fe. The experimentally determined probability (a,,,> that a positron in a surface state annihilates with a core electron resulting in the emission of an Auger electron in the range measured can be estimated from the results of relative Auger intensities (lr,,,t) in table 1 and the absolute cross section (ffMz, = 5.4% for the M,,VV transition) for Cu(100) as determined by Mehl [ 111. Using:

ut”t =

uMM,,

I I,,,,, = I M&u

)

(8)

= 1.10 is the ratio of measured ztotCu/zM2, PAES intensity (with background subtracted) integrated over the range 50 to 120 eV to the intensity integrated over the range corresponding

where

K.H. Lee et al. / PAES of Fe, CM, Pd, and Au

132

to the M,,VV transition (50 to 78 eV>. The probabilities calculated from eq. (7) are uttotCu= 5.94% for Cu, vtOtFe = 3.39% for Fe, ~,~,,,+,= 8.02% for Pd and @ttotAu= 6.95% for Au. The corresponding values of a,,, calculated by summing the relevant core hole annihilation probabilities are 3.85% for Cu(3s + 3~1, 3.59% for Fe(3s + 3~1, 5.10% for Pd(4s + 4p) and 4.09% for Au (4f + 5s + 5~). In general the experimental values are higher than theoretical. This is reasonable since many-body enhancement of the core annihilation rate has not been included in the theory [lo]. 3.3. H, chemisorption on Pd The PAES signal originates almost exclusively from the topmost atomic layer due to localization

of the positrons at the surface. Thus adsorbates can be expected to have a large effect on PAES intensities. The effect of overlayers on PAES intensities has been demonstrated for S on Cu by Mehl et al. [2]. The PAES intensities of Fe, Cu, Pd and Au plotted as a function of time are shown in fig. 2. These plots indicate that the PAES intensities of all samples decrease as a function of time since the last sputter cleaning of the surface. The PAES intensities recovered when the samples were sputter cleaned. These facts lead to the conclusion that the PAES intensity is decreasing due to the growth of an overlayer of contaminants which is growing with time. These contaminants come either from adsorption of residual gas in our vacuum system (which is mostly -_

6001

600 ,pp

(4

-z 500 s 0 400 / u ; c,

3001

2 : 200c, 2

$

$

$

dl+

loo-

@

OO

d Fe 100 I,

:

7.6

200 1

x

10e4

/, 300

rni!-’

400 /

500

Time(Minutes)

Time(Minutes) 600

I

I

:: 500

2

0 400 u I +g 300c’ cu &200aJ c, E loo-

d AU

Time(Minutes)

:

1.6

x

10e4

ook--mwrw Time(

min-’

”

400

”

500

Minutes)

Fig. 2. The PAES intensity for (a) Fe, (b) Cu, (c) Pd and (d) Au as a function of time since the last sputter cleaning of the surface. This figure shows the PAES intensity integrated over region (1) in fig. 1 for each loop. The solid line was obtained from a least-squares fit to the data. The quantities dFe,Cu,Au.Pdare decay-time coefficients as defined in eq. (9).

K.H. Lee et al. / PAES of Fe, Cu, Pd, and Au

H2), or segregation

to the surface of H, out of the bulk as seems to be the case with Pd. The partial pressure of H, was measured to be 2.9 X lo-” Torr out of the total pressure 3.4 X lo-l0 Torr at the beginning of experiment. Decay-time coefficients were determined from the data shown in fig. 2 using

d= Iml/Yi,,t,

(9)

where d is the decay-time coefficient with unit min-‘, and m and Yint are the slope and y-intercept of a straight-line fit to the data. The decaytime coefficients are indicated in fig. 2. The decay-time coefficient from Pd was significantly larger than the others indicating that the rate at which the surface of Pd becomes contaminated was much more rapid. To demonstrate directly the effect of H, chemisorption on the Pd surface, we exposed the sample to Hz gas by backfilling the chamber. No peaks other than Pd were observed in the EAES spectra. In contrast to the PAES results only a small decrease in the Pd EAES intensity was observed (based on the surface saturation coverage of 1.35 ML of H, [12] we estimate that the decrease in EAES intensity from Pd should be less than 10% at 1000 L) as the sample was exposed to H, gas. Fig. 3 indicates the decrease in Pd PAES intensity upon exposure to H,. The

133

data were corrected for H, induced changes in positronium fraction
4. Conclusion

7 E 0.2 k

0

200 Hydrogen

400

600

Coverage

800

1000

(Langmuirs)

Fig. 3. Experimental demonstration of the effect of H, chemisorption on Pd surface at three different H, exposures. The solid curve was obtained from a cubic spline fit to the data.

The positron annihilation induced low-energy Auger signals for Fe, Cu, Pd and Au are all sufficiently intense to permit PAES studies of these surfaces with present laboratory based positron beams. The relative PAES intensities integrated over the energy range of the principal PAES peaks are given in table 1. These data should be useful as elemental standards in future research work such as in studies of surface segregation, surface alloying and thin film growth. The ratios of Cu, Pd, Au show the same trends as the theoretical estimates which are based on summing the core annihilation probabilities calculated by Jensen [lo] for the core levels involved in the transitions. The fact that the Pd PAES signal decreased after exposure of the Pd surface to H, strongly suggests that PAES is sensitive to the

134

K.H. Let et al. / PAES of‘ Fe, Cu, Pd, and Au

presence of hydrogen on the Pd surface since EAES spectra gave no indication of other contaminants. This sensitivity is most likely due to the positron wave function being pushed away from the Pd ion cores by the overlayer.

Acknowledgements This research was supported in part by the Robert A. Welch Foundation, Texas Advanced Research Program and Nato Grant No. 34-900. K.O. Jensen thanks the Commission of European Community for financial support.

References 111 AIL Weiss, R. Mayer, M. Jihaly. C. Lei, D. Mehl and K.G. Lynn, Phys. Rev. Lett. 61 (1988) 2245. [21 D. Mehl. A.R. Koymen, K.O. Jensen, F. Gotwald and A. Weiss, Phys. Rev. B 41 (1990) 799. f.11A.H. Weiss, D. Mehl, A.R. Koymen, K.H. Lee and Chun Lei. J. Vat. Sci. Technol. A 8 (1990) 2517. [41 Chun lei, D. Mehl, A.R. Koymen, F. Gotwald, M. Jibaly and A. Weiss, Rev. Sci. Instrum. 60 (1989) 3656. of University of Texas at F.51 M. Jihaly, PhD Dissertation Arlington (19X7). [61 Chun Lei, Master Thesis of University of Texas at Arlington (1988).

[71 See reviews by A.P. Mills, in: Positron Solid State Physics. International School of Physics “Enrico Fermi”, Course 83, Eds. W. Brandt and A. Dupasquier (North-Holland. Amsterdam, 1983); P.J. Schultz and K.G. Lynn, Rev. Mod. Phys. 60 (1988) 701, and references therein. 181 A.P. Mills, Jr.. Phys. Rev. Lett. 41 (1978) 1828. I91 A.P. Mills, Jr.. Solid State Commun. 31 (1979) 623. [IO1 K.O. Jensen and A. Weiss, Phys. Rev. B 41 (19YO) 3Y28. of University of Texas at ill1 D. Mehl. PhD Dissertation Arlington (1990). and G. Ertl. Surf. Sci. YY [121 R.J. Behm, K. Christmann (1 YXO)320. and M.J. Puska, Phys. Rev. Lett. SO [131 R.M. Nieminen (1983) 281. and K.O. Jensen, Phya. Rev. B (Rapid [141 R.M. Nieminen Commun.) 38 (1988) 5764. Phys. [lSl R.M. Nieminen, N.J. Puska and M. Manninen. Rev. Lett. 53 (1984) 129X. [IhI M.J. Puska, P. Lanki and R.M. Nieminen, J. Phys. Condensed Matter 1 (1989) 60X1. (171 K.O. Jensen and A.B. Walker, J. Phys. F 1X (1988) L277. J.U. Andersen and D.N. Lowy. Phys Rev. [I81 E. Bonderup, B 20 (1979) 883. [I91 E.J. McGuire, Phys. Rev. A 5 (1972) 1052. Phys. Rev. A 9 (1974) 1X40, and private DO1 E.J. McGuire, communication. [211 D.W. Gidley and A.R. Koymen, Phys. Rev. B 37 (19X8) 2465. [221 A.R. Koymen. K.H. Lee, D. Mehl and A. Weiss. to be published.