PHYSICAL REVIEW B VOLUME 49, NUMBER 20 15 MAY 1994-II

Segregation and dissolution of Ag-rich clusters at the (100) surface of Al-3 at. % Ag

E. Wetli*Institut fiir Angewandte Physik, Eidgenossische Technische Hochschule Zurich, CH 8093-Zurich, Switzerland

M. Erbudak and T. SchulthessLaboratorium fur Festkorperphysik, Eidgenossische Technische Hochschule Ziirich, CH 8093-Ziirich, Switzerland

(Received 20 January 1994)

Silver segregation on a (100) surface of Al-3 at. % Ag, induced by heat treatment, is studied at temper-atures up to 480 C. The surface concentration of silver is monitored with Auger-electron spectroscopy,while its geometry is determined with low-energy electron di8'raction. The measurements show that thesegregation results in a Ag concentration of up to 30 at. % at the surface which forms a disordered over-

layer. The electronic information about the Ag atoms, obtained both by means of x-ray-photoelectronand electron energy-loss spectroscopy, reveals that Ag2A1-like clusters segregate to the surface at tem-

peratures ranging up to 440'C. At higher temperatures, the clusters reversibly dissolve, in agreementwith the bulk phase diagram, without changing the average Ag concentration at the surface.

I. INTRODUCTION

The segregation of a component in an alloy is typicallyinduced by exposure of the surface to adsorbates or byheat treatment. In a solid solution, the kinetics of thesegregation is determined by the diffusion coefficient ofthe segregating atoms in the matrix. In the case ofdecomposing alloys, on the other hand, the existing pre-cipitates can complicate the kinetics of segregation.Then, it remains to be elucidated whether segregationwill occur as in a solid solution where atoms migrate tothe surface and the bulk diffusion coeScient accounts forthe process or whether the precipitates diffuse to the sur-face. It is of fundamental interest to know whethertemperature-dependent surface processes are related tothe phase diagram of the alloy or whether they occur in-dependently from the bulk.

In the following study, some of these questions are ad-dressed for heat-treatment induced Ag segregation at a(100) surface of Al-3 at. % Ag using methods of electronspectroscopy. According to the phase diagram, ataround 400'C, Al-3 at. % Ag shows a transition from thehigh-temperature a phase where Al and Ag coexist as ahomogeneous solid solution to a region where Ag atomsprecipitate in Ag2A1. ' The existence of the stable yzones, consisting of Ag2A1, has been verified experimen-tally. The specimen investigated in this work also con-tained such clusters. We have used Auger-electron spec-troscopy (AES) to determine the concentration of Ag onthe surface at various temperatures. By simultaneouslyperforming low-energy electron diffraction (LEED) x-ray-photoelectron spectroscopy (XPS), and electron-energy-loss spectroscopy (EELS), we show that structuralinformation on the segregated species is also accessible inelectron spectroscopies.

In EELS, measurement of the plasmon energy allowsone to determine the volume density of nearly-free elec-trons. Thus, if the number of electrons remains constant,one can determine the volume in which the electrons are

distributed. In this work, this relationship was used todecide whether the Ag atoms are present in clusters or di-luted uniformly in the Al matrix. Further, XPS data onthe 4d states of Ag at different temperatures are com-pared with the computational results obtained within theframework of a layer Korringa-Kohn-Rostoker (LKKR)formalism using coherent-potential approximation(CPA). The theoretical calculations show a narrowing ofthe 4d bandwidth (5) for Ag as the coordination number(CN) of the Ag atoms is reduced. For this reason,changes in the experimental values for 6 indicate thepresence of Ag-rich clusters. Hence, structural informa-tion about the arrangement of the Ag atoms is obtainedby methods of electron spectroscopy that are in generalmuch faster than the commonly employed structuraltechniques based on x-ray scattering. In the presentstudy, the results apply to only a few atomic layers at thesurface owing to the short mean free path of the emittedelectrons in XPS and AES.

II. EXPERIMENT

The measurements were performed in an ultrahighvacuum (UHV) apparatus with a total pressure in the10 Pa range. An Al-3 at. %%uoAgsingle-crysta 1 surfac ewas oriented with the x-ray Laue method to within +0.5'

for the (100) surface and spark cut. The surface was firsttreated with 5 M NaOH in order to remove the destroyedlayers during spark cutting. Subsequently, diluted HNO3was used to remove the residual Ag. In UHV, the surfacewas cleaned by etching under grazing incidence with Ar(1500 eV, 0.1 pA/mm ) and heating to 150'C until notraces of 0 and C were detectable by AES. After the lastsputtering cycle, no heat treatment is applied in order toavoid Ag segregation. Surfaces prepared in this mannerdisplayed sharp LEED pattern characteristics for the(100) surface. Sample heating was carried out using anoven mounted under the sample and the temperature wasmeasured using a chromel-alumel therrnocouple in directcontact with the surface. An electron gun produced a

0163-1829/94/49(20)/14628(4)/$06. 00 14 628 Q~ 1994 The American Physical Society

SEGREGATION AND DISSOLUTION OF Ag-RICH CLUSTERS. . .

monoenergetic beam with a current of 5 pA at primaryenergies up to 3000 eV for the AES and EELS analyses.For the XPS studies, a nonmonochromatized Mg-K ra-diation was used. The angle of electron emission was 30'with respect to the surface that limited the informationdepth to only a few atomic layers. Emitted electronswere registered in an energy analyzer which consisted ofa focusing input lens and a combination of a two-grid re-tarding stage with a dispersive energy filter. The energyresolution of 0.4 eV was constant over the whole energyrange for the reported spectra. Data collection and ener-

gy sweep of the analyzer were controlled by a microcom-puter.

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III. RESULTS AND DISCUSSION

0.8ZUJI-Z

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FIG. 1. The normalized time-dependent evolution of the AgM4 5N4 5N4 5-Auger signal obtained at the (100) surface of Al-3at. % Ag for three different temperatures. Solid lines representan analytical function whose parameters are used to determinethe activation energy for the silver segregation (Ref. 4).

Figure 1 shows the time-dependent evolution of theAg-Auger signal intensity for three different tempera-tures. Here, the M4 5N4 5N4 5-Auger transition, integrat-ed between 341—358 eV, is taken as the signal intensity.For each temperature, the data are normalized to the sat-uration coverage and were fitted to an analytic functionwhich allows the determination of the Ag diffusioncoefficient in the bulk at different temperatures. Thesevalues are then used to find an activation energy of 106kJ/mol for the bulk diÃusion coefficient. Below, XPSand EELS data are presented for the (100) surface, i.e.,the prepared surface (t =0) as well as for the segregatedsample at T=250 and 480'C after equilibrium has beenreached (t = ao ). The surface concentration at 250 and at480'C is estimated to be 23+1 and 16+1 at. % Ag, re-spectively. Cooling the sample to room temperature(RT) after heat treatment at 250'C leads to an increase ofthe surface concentration to 30 at. %. Except for theprepared surface, no LEED pattern could be observed forthe segregated surfaces.

Electrons inelastically scattered from surfaces carry in-formation on elementary and collective excitations. InAg, spectra predominantly show electronic transitionsbetween the 4d band and unoccupied states while in Al,

FIG. 2. EELS results in an extended energy-loss region of90-165 eV on pure Al and Al-3 at. % Ag alloy as (a) preparedand (b) segregated at 480'C. Data are obtained at a primary-electron energy of 1900 eU. In order to accurately determinethe plasmon energy, the measurements are extended up to thetenth volume-plasmon loss.

collective excitations dominate and multiple plasmons areobserved throughout the spectrum. The plasmon energyhv is given by hv=e(4nn/nt )'~, where e is the unitcharge, m the electron mass, and n the electron density.Hence the measurement of the plasmon energy allowsone to determine the electron density. Figure 2 showsthe volume-plasmon excitations in an extended energy-loss region of 90-165 eV, after subtracting a homogene-

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BINDING ENERGY (eV)

FIG. 3. Photoemission spectra of the conduction bands ofpure Ag as well as Al-3 at. % Ag. Spectra of the alloy arerecorded at (a) RT immediately after preparation and at (b)480'C after equilibrium has been reached. Data are obtainedwith Mg-K radiation and plotted relative to the Fermi level.The spectra are normalized to approximately the same peak in-

tensity. The width (FWHM) of the emission band at 6 eV is 3.6eV for Ag, 2.9 eV for the spectrum (a), and 2.5 eV in the case ofspectrum (b).

14 630 E. WETLI, M. ERBUDAK, AND T. SCHULTHESS 49

ous background. Energy losses up to the tenth plasmonloss are easily detectable. Thus, any change in theplasmon energy would appear ten times enhanced at thetenth plasmon loss. Within the experimental accuracy(better than 0.1 eV in hv), no change in the plasrnon en-

ergy (h v= 15.8+0. 1 eV) is detected between Al and Al-3at. % Ag at temperatures below 440'C. Therefore, onecan exclude any change larger than 1.3% per formulaunit in the density of excitable electrons in going from Alto Al-3 at. % Ag. This observation indicates that the Ag5s electrons do not participate in collective oscillationsand that there is no noticeable volume change in alloyingAl with 3 at. % Ag in the crystalline state at tempera-tures below 440'C. The fact that there is up to 23 at. %%uo

Ag on the surface and 3 at. % in the volume suggests thatAg is present as precipitates isolated from Al and do notdilute the Al matrix by homogeneously dissolving in it.

Measurements performed on the specimen maintainedat 480'C show that the energy of the volume plasmon isreduced to 15.5+0. 1 eV. Clearly, this change in theplasmon energy is induced by the change in n, the densityof excitable electrons. Since Ag electrons do not contrib-ute to plasmon oscillations, the change in the plasmon en-

ergy can only be attributed to a volume change, i.e., silveris dissolved in the aluminum matrix. Using the experi-mentally determined values for the plasmon energy in therelationship between hv and n, we find that there is3.8+1.3% volume change upon heating to 480'C. Sincethis value is very close to the nominal Ag content in thealloy, this observation is taken as evidence that below

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FIG. 4. Muffin-tin density of states at the Ag sites calculatedwith the layer Korringa-Kohn-Rostoker method in thecoherent-potential approximation (Ref. 8) for Ag metal, Al-50at. % Ag, and Al-3 at. %%uoAg . Thespectr aar econvolute dwit h aGaussian function of 1 eV in order to simulate the experimentalbroadening due to the photon and electron energies and plottedwith different magnifications in order to have the same peak in-

tensity. The Fermi level is at the origin of the energy scale.

440'C Ag precipitates in the Ag-3 at. % Ag alloy in clus-ters and is completely dissolved in the Al matrix above440'C.

In the XPS study, primarily the electronic states nearthe Fermi level (EF ) were measured in order to determinethe 5 of the Ag 4d electrons. Three spectra are taken,one at RT immediately after surface preparation, one at250 C, and another at 480 C after equilibrium has beenreached. The results are shown in Fig. 3 together withspectra for the Ag metal in the same energy region. The4d states in pure Ag show up at about 6 eV with a fullwidth at half maximum (FWHM) of 3.6+0. 1 eV. Theband displays a typical crystal-field splitting of about 1.7eV induced by the crystalline environment of the Agatoms in agreement with previously reported spectra.By contrast, in the prepared alloy, the Ag 4d electronshave a width of 2.9+0. 1 eV. With segregation, the lineshape and the binding energy of the 4d states remain un-changed up to 440'C within experimental error whiletheir intensities increase according to the overall Ag con-tent at the surface. However, when the sample is kept at480 C, 6 is reduced to 2.5+0. 1 eV, and, subsequently, itreverts to 2.9+0. 1 eV when the sample is cooled below440'C.

Figure 4 illustrates the theoretical bulk density ofstates at the Ag site for pure Ag and the alloys Al-50at. % Ag and Al-3 at. % Ag. The calculations are per-formed using the LKKR-CPA method reported previous-ly. The spectra are then broadened using a Gaussianfunction with FWHM of 1 eV for a comparison with theexperimental spectra. The calculations both on sandwichstructures, where the Ag distribution is not homogene-ous, and on homogeneous bulk alloys show that b onlydepends on the CN of the Ag atoms, and is independentof their spatial arrangement. The narrowing in 5 withdecreasing Ag concentration in the alloy and hence withdecreasing CN is apparent in the figure.

IV. CONCLUSIONS

The XPS measurements show that the bandwidth ofthe Ag 4d electrons measured at the (100) surface of theprepared Al-3 at. %%uoA g is inclos eagreemen t wit h th etheoretical value for Al-50 at. % Ag and the experimentalone obtained from the hexagonal A140Ag6o. Hence, we

conclude that in Al-3 at. %%uoA g, Agexist s inhexagona1Ag-rich clusters that extend up to a surface regionprobed by XPS. Ag in these clusters has a CN whichcorresponds to that in an alloy with 50—60 at. % Ag.This interpretation is in agreement with the properties ofthe decomposing bulk alloy where y zones (Ag2A1) per-sist at lower temperatures.

As the temperature is increased above RT, the 4d-emission intensity as well as the Ag-AES signal increasewhile the bandwidth of the 4d states remains constant.This observation shows that Ag2A1-like clusters enrichthe surface. The enrichment is limited to 3—4 atomic lay-ers as determined by sputter-profiling experiments. Theactivation energy for this segregation process is approxi-mately the same as that for the diffusion of Ag atoms inthe bulk Al. During segregation, the bandwidth of the

49 SEGREGATION AND DISSOLUTION OF Ag-RICH CLUSTERS. . . 14 631

Ag 4d electrons remains broad at 2.9 eV. Since the pho-toemission process is much faster than a typical diffusionmotion of the Ag atoms, this observation suggests the ab-sence of isolated Ag atoms. Hence, the process of segre-gation must occur in a manner which does not change theaverage CN of Ag atoms, at least in the near-surface re-gion probed by the XPS. The saturation concentration ofAg indicates that up to half of the surface area must beoccupied with clusters that contain close to 60 at. %%uoAg.The fact that no LEED pattern is observed for the segre-gated surface suggests that either there is no long-rangeorder in the clusters or a disordered arrangement of or-dered clusters is present on the surface. The second pos-sibility is more plausible, if we assume that the surfaceprecipitates have a structural order similar to the hexago-nal Ag2A1 in the bulk. Then, the cluster size has to beappreciably smaller than the coherence length of elec-trons used for LEED (typically 100 A).

Heating the sample to temperatures above 440'C re-sults in a narrowing of the 4d bandwidth, indicating astill lower CN for the Ag atoms present in the analyzedvolume at and below the surface. This observation showsthat the clusters homogeneously dissolve in the matrixwithout changing the overall Ag concentration in a limit-ed surface region from where the AES signal originates.

This is a reversible process, and the Ag-rich clusters nu-

cleate back as the temperature is reduced below 440 C.It has to be noted that the change in the Ag concentra-tion in the volume analyzed with AES is negligibly smallbelow and above 440 C. Hence, this process is astructural phase transition in which the CN is the criticalparameter.

In summary, we have found no evidence for segrega-tion of isolated Ag atoms on a (100) surface of Al-3 at. %%uo

Ag. On the contrary, the segregation is initiated by themigration of small Ag-rich, Ag2Al-like clusters to thesurface. There is no long-range order on the segregatedsurface. The dissolution and reprecipitation of the clus-ters reversibly take place at 440'C that is approximatelyequal to the bulk transition temperature.

ACKNOWLEDGMENTS

The authors are grateful to F. Moro for his assistanceduring the measurements, to G. Veligelebi for her criticalevaluation of the manuscript, to G. Kostorz and D. Pes-cia for helpful discussions, and to the SchweizerischerAluminium-Fonds and Schweizerischer Nationalfondszur Forderung der Wissenschaftlichen Forschung forfinancial support.

'Present address: Laboratorium fur Festkorperphysik,Eidgenossische Technische Hochschule Zurich, CH-8093Zurich, Switzerland.

~M. Hansen, in Constitution of Binary Alloys, Metallurgy andMetallurgical Engineering Series (McGraw-Hill, New York,1958); P. Haasen, Physikalische Metallkunde (Springer, Ber-lin, 1984).

2J. M. Howe, H. I. Aaronson, and R. Gronsky, Acta Metall. 33,649 (1985).

Here, CN is used to designate only the number of Ag neighborsof a Ag atom.

4E. Wetli, M. Erbudak, and G. Kostorz, Appl. Surf. Sci. 64, 111(1993).

5The Ag concentration determined by AES is given in units ofat. % rather than in monolayers. The reason is the fact that

Ag forms clusters and that the surface sensitivity is limited toapproximately 7 A owing to the mean free path of the 350 eVAg-Auger electrons.

E. Wetli, Ph.D. thesis, Eidgenossische Technische HochschuleZurich (1993).

7J. C. Fuggle, E. Kallne, L. M. Watson, and D. J. Fabian, Phys.Rev. B 16, 750 (1977).

S. Crampin, R. Monnier, T. Schulthess, G. H. Schadler, and D.D. Vvedensky, Phys. Rev. B 45, 464 (1992).

P. Weightman and P. T. Andrews, J. Phys. C 13, 3529 (1980),have reported on A140Ag60 that is in hexagonal phase. Usinga better energy resolution compared to that used in thepresent work, they have measured Ag 4d electrons with abandwidth of 2.8 eV.