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Surface Science 600 (2006) 4108–4112

Surface segregation in FeSi alloys

M. Vondracek a,*, V. Dudr a, N. Tsud b, P. Lejcek c, V. Chab a,K.C. Prince d, V. Matolın b, O. Schneeweiss e

a Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, CZ-162 53 Praha 6, Czech Republicb Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles University,

V Holesovickach 2, 180 00 Praha 8, Czech Republicc Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-182 21 Praha 8, Czech Republic

d Sincrotrone Trieste S.C.p.A., SS 14 - km 163, 5 in AREA Science Park, 34012 Basovizza, Trieste, Italye Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zizkova 22, CZ-616 62 Brno, Czech Republic

Available online 8 May 2006

Abstract

The surface segregation of a Fe94Si6 and Fe76Si24 alloys was studied using high resolution photoemission spectroscopy with synchro-tron radiation at 150–166 eV photon beam energy. During Ar+ ion sputtering and following heat treatment, a SiO2 layer and segregationof Si atoms in three clearly resolved phases occurred. This indicates formation of silicides, mainly Fe3Si and cubic FeSi superstructures.The photoemission measurements were complemented by 57Fe Mossbauer spectroscopy in different modes which gave information aboutdeeper surface layers (approximately 10–30000 nm) of the samples. The results derived from the Mossbauer spectra support the conclu-sions concerning phase composition of the surface deduced from photoemission spectroscopy.� 2006 Elsevier B.V. All rights reserved.

Keywords: Fe–Si alloy; Photoemission spectroscopy; Mossbauer spectroscopy; Surface; Segregation

1. Introduction

Silicon alloys are very important and widely used mate-rials in electronic and magnetic circuits. Their basic re-search has been focusing on problems of segregation ofsolute elements on surfaces and interfaces with the aim ofachieving a deeper understanding of processes during heattreatment by formation of optimal structure and phasecomposition.

Strong effects of oxidation conditions on external orinternal oxidation in Fe–Si alloys was investigated in [1],where the microscopic features of oxide layers formed onthe (011) surface of an Fe–6 at.%Si alloy were described.The microscopic morphology and elemental distributionin oxide layers strongly depend on oxidation conditions.Corresponding to the morphological changes of the sur-

0039-6028/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2006.01.129

* Corresponding author.E-mail address: vondrac@fzu.cz (M. Vondracek).

face, changes in the distribution of alloying elementsoccurred. A simple phenomenological model of solute seg-regation at partially ordered grain boundaries of Fe–Si al-loys and consistent with present understanding ofequilibrium segregation phenomena was proposed in [2].The model was used to correlate complex concentrationand temperature dependencies of silicon segregation inordered Fe–Si alloys. Segregation at grain boundariesin polycrystalline samples of Fe–Si alloys with 4, 12 and17 at.% Si using emission Mossbauer spectroscopy wasinvestigated in [3]. Comparison of the results derived fromthe emission (grain boundary) spectra and transmission(bulk) spectra showed differences in the chemical composi-tion and atomic ordering between grain interior and grainboundaries. An increase in Si concentration at grainboundaries was found. This increase is more pronouncedfor the alloys with the higher Si content. In the Fe–17 at.% Si alloy, strong site preference of the diffusingatoms was detected. The diffusing atoms prefer the lattice

Fig. 1. Si 2p core level photoemission spectrum from Fe76Si24 alloy at300 �C temperature fitted by four resolved doublets and one broad oxidepeak with Shirley background.

M. Vondracek et al. / Surface Science 600 (2006) 4108–4112 4109

sites which are expected to be occupied by structural vacan-cies in the D03 superstructure.

Detailed study of the surface properties of single crystal-line Fe3Si (100) by AES, UPS, LEED, ISS and XPS mea-surements [4] has shown that the surface may contain pureFe or mixed Fe/Si (1:1). It was concluded from the XPSdata that the thermally equilibrated surface has a largerconcentration of Si in the top layers than in the bulk andthat the segregated overlayer is ordered to a metastablephase with the CsCl structure and stoichiometry FeSi.The (100), (110), and (111) surfaces of Fe3Si have beenstudied by quantitative LEED and AES [5]. Reversiblephase transitions between the D03 and the B2 structureupon annealing, triggered by a substantially reversible sur-face segregation of Si, was observed. On all surfaces, Si ter-mination is preferred either by forming a topmost Si layerwhenever the choice between Si and Fe exists, or by directSi occupation of nominal Fe sites. Recent photoemissionspectra from Fe–6 at.%Si alloy [6] showed several orderedstructures on the surface with significant chemical shiftsof the Si 2p core level. The relative intensities of the shiftedcomponents changed during heat treatment, but a peakassignment of the phases was ambiguous.

This paper compares results from an alloy with low sil-icon content (6 at.%) with one with much higher siliconcontent (24 at.%) to understand processes on the surfacefrom the point of view of segregation and atomic ordering.

2. Experimental

The samples of dimensions 10 · 10 mm were cut fromsheets using spark erosion and subsequently their surfaceswere ground and polished using standard metallographyprocedures. A Fe94Si6 sample was cut from a grain orientedsilicon steel (3 wt.% � 6 at.% Si), Fe76Si24 polycrystallinealloy was prepared by induction melting of pure Fe (3Ngrade) and Si (semiconductor grade) in vacuum (10�3 Pa)and homogenized at 1200 �C for 6 h in hydrogen. High res-olution photoemission spectra were taken on the MaterialsScience Beamline at the synchrotron light source Elettra inTrieste, Italy [7]. The measurements at the Si 2p edge werecarried out using photon energies 150–166 eV in normalemission geometry, and therefore a surface layer approx.1 nm thick was studied. Photoelectrons were collected bya Phoibos 150 electron energy analyser. The monochroma-tor was set to 170 meV photon bandwidth, resolutionincluding electron analyzer was 230 meV. Prior to loadinginto the UHV chamber the samples were cleaned in anultrasound bath of acetone for 10 min. Surface cleaningby repeated Ar+ ion sputtering and heating as well as finalheat treatment were done in situ. The photoemission mea-surements were complemented by 57Fe Mossbauer spec-troscopy in different modes which gave information ondeeper surface layers (�10–30000 nm) of the samples.For the investigation of the surface layer �10 nm deepemission Mossbauer spectroscopy (EMS) was used. Thesample (source) EMS was prepared in the same way as

for the photoemission. Investigation of a surface layer todepths up to 300 nm was carried out using conversion elec-tron Mossbauer spectroscopy (CEMS) in two differentexperimental setups: (i) with a detector based on a channel-tron and the small angle incidence (SAI) geometry (�5�) ofthe c radiation onto the sample surface which yielded spec-tra of the surface from the depth about 50 nm thick and (ii)gas-filled electron detector with normal incidence of thegamma radiation which yielded spectra corresponding toa layer up to 300 nm deep. The Mossbauer spectra mea-sured in scattering geometry (SGMS) with detection of14.4 keV gamma radiation using a 2p proportional countergave information on the bulk of the sample (�30 lm thicksurface layer). A detailed description of Mossbauer spectrameasurement and processing was reported in [3,6,8].

3. Results and discussion

3.1. Core-level photoemission

Si 2p spectra show silicon in several different well-defined states. After initial Ar+ ion sputtering followedby 30 min heating to 500 �C SiO2 with binding energy102.5–103.5 eV is the dominant impurity on both samples.Next the oxide layer was removed by Ar+ sputtering andthe sample showed up to four well resolved doublets (Si2p1/2 and 2p3/2) at 98.6, 98.9, 99.2 and 99.6 eV bindingenergy of 2p3/2 peak (Fig. 1). A broad oxide peak growsagain with increasing temperature, but it is much weaker.Thanks to the high resolution of photoemission with syn-chrotron radiation, it was possible to determine the relative

4110 M. Vondracek et al. / Surface Science 600 (2006) 4108–4112

concentration of silicide phases in the surface layer of bothsamples during sputtering and heat treatment (Figs. 2 and3). We assume that the two doublets represent iron silicides– the Fe3Si (with binding energy 98.9 eV) and cubic FeSi(99.2 eV). The third one is a depleted layer of a-Fe with dis-solved Si (98.6 eV) and the weakest peak probably repre-sents a-FeSi2 (99.6 eV). Results from Fe deposition [9]and Fe+Si co-deposition [10] on Si monocrystals indicatea decreasing binding energy with increasing iron contentand the same result was found earlier on various iron sili-cides [11]. Synchrotron radiation photoemission studieswith high resolution [12] are also not in contradiction withour results. Formation of different Fe–Si compounds,which do not exist simultaneously in equilibrium in bulkFe–Si alloys, can be expected at the surface and it is inagreement with the phase analysis results reported onamorphous thin films, e.g., [13].

Fig. 2. Si 2p core levels from Fe94Si6 alloy after Ar+ ion sputtering andfollowing annealing up to 500 �C with hm = 150 eV. Contribution ofindividual phases is displayed in inset.

Fig. 3. Si 2p core levels from Fe76Si24 alloy after Ar+ ion sputtering andfollowing annealing up to 500 �C with hm = 166 eV. Contribution ofindividual phases is displayed in inset.

The spectral components change their intensities buttheir binding energies are conserved in all steps of the heattreatment and peaks are sharp. Therefore it can be ex-pected that all Si atoms reside in rather well-defined andequivalent sites. The sequence of photoemission spectra re-sults shows that Ar+ sputtering removes the SiO2 layer thenallows us to detect phases in deeper layers like a-Fe(Si).Below the SiO2 layer two silicide phases can be found –Fe3Si and cubic FeSi. The first one dominates in the surfaceof the Fe94Si6 and the second one in Fe76Si24 alloy, which isconsistent with the increased amount of silicon producing asurface phase which is more Si rich. The SiO2 layer is stableover the whole range of annealing temperatures in theFe76Si24 alloy, contrary to the Fe94Si6 where its amountsdecreases after annealing at 500 �C.

Mossbauer spectroscopy. The Mossbauer spectra (Fig. 4)of the Fe94Si6 sample surface were decomposed by fittinginto components (sextets) representing the ferromagnetic

Fig. 4. Conversion electron Mossbauer spectrum (CEMS) – top- andscattering geometry Mossbauer spectrum with detection of gamma rays(SGMS) – bottom – of the Fe94Si6 sample surface.

Fig. 5. Mossbauer spectra of the Fe76Si24 samples: transmission spectrumof the foil using a 57Co in Rh source (top); EMS spectrum from the surface(centre); CEMS spectrum of the layer of 57Fe deposited on the surface(bottom). The components in the EMS spectrum represent: (i) the pureiron surface (the six line pattern filled by dots), (ii) the Fe3Si (the blackfilled six line patterns) and (iii) cubic iron silicides (the single line filled bythe diagonal cross).

M. Vondracek et al. / Surface Science 600 (2006) 4108–4112 4111

a-Fe(Si) phase. They correspond to iron atoms differingin number of silicon atoms (plus impurities and vacancies)in the nearest neighbourhood. The changes between theCEMS and SGMS spectra are due to different scanningdepth. Distributions of the component intensities derivedfrom CEMS-SAI, CEMS, and SGMS are shown inTable 1.

From the spectra shown in Fig. 4 formation of a layer ofa pure iron at the surface can be proposed. It can be ex-pected as a result of disproportionation by oxidation ofthe surface. Silicon forms the SiO2 (as detected by XPS)and a layer enriched in iron remains. A decrease in Si con-centration can be deduced from the increase and decreaseof the (0) and (1) intensities, respectively.

The transmission (bulk) and emission (surface)Mossbauer spectra of the Fe76Si24 sample are shown inFig. 5. For comparison, CEMS spectrum of the pure57Fe deposited on the surface is added there. The transmis-sion spectrum corresponds well to the spectra of the

Table 1Results of the analysis of the Mossbauer spectra of the Fe94Si6 samplesurface after vacuum annealing at 500 �C

Number of nn Si atoms 0 1 2 3

Relative intensity

CEMS-SAI 0.97 0.03 0.00 0.00CEMS 0.85 0.12 0.03 0.00SGMS 0.59 0.28 0.07 0.04

Relative intensity of Mossbauer components corresponding to the differ-ent number of silicon atoms in nearest neighbourhood (0–3 from 8 atomsin nn) are in columns.

Fe–24.4 at.% Si reported in [14]. The emission spectrumdiffers substantially from that of the bulk. The dominatingsextet has similar parameters (d = 0.005 ± 0.005 mm/s,r = 0.00 ± 0.01 mm/s and Bhf = 33.0 ± 0.1 T) as pure ironbut two times broader line width. This indicates that an al-most pure iron layer is formed. However the line broaden-ing reflects also a high density of defects (vacancies) in thebcc structure. Besides the sextet a single line is present here.It can be ascribed to cubic silicides like FeSi or FeSi2. Thecubic structure can be expected because of zero quadrupolesplitting which indicates spherical symmetry in the sur-roundings of the iron atoms. The isomer shift of this com-ponent d = �0.024 ± 0.005 mm/s excludes its assignmentto iron atoms in the SiO2 surface layer or with anothernon-cubic iron silicide. The results derived from Mossbauerspectra are in agreement with our XPS surface analysis butthe lower depth selectivity does not allow more exact con-clusions about the position and amount of the phases inthe surface. Observations of the bcc iron rich phase andthe cubic FeSi phase in the Mossbauer spectra agree withthe results reported in [4].

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4. Conclusions

From the XPS and Mossbauer spectroscopy resultsthe phase composition of the surface layers of the samplesinvestigated can be determined. The topmost layer isformed by SiO2 as expected. It can be removed by Ar+

sputtering but it grows again during UHV annealing insmaller amounts. Below this oxide layer silicides – Fe3Siand cubic FeSi – were recognized. The first one dominatesin the surface of Fe94Si6 and the second one in Fe76Si24

which corresponds with the increased silicon amount inthe sample alloys. The narrow peaks indicate orderedphases. The spectral components change their intensitiesbut their binding energies are constant in all steps of theheat treatment. Therefore it can be expected that all Siatoms reside in rather well-defined and equivalent sites.The last layer before bulk is formed by iron rich a-Fe(Si)with substantially reduced silicon content in comparisonwith the bulk concentration. In the Mossbauer spectrumthe layer dominates and the broad line width indicates ahigh density of vacancies there.

Acknowledgements

This work was supported by the Grant Agency ofthe Academy of Sciences of the Czech Republic (No.IAA1041404) and projects of the AS CR institutesAV0Z20410507. The Materials Science Beamline is sup-ported by the Grant Agency of the AS CR under grantNo. IAA1010143 and by the Ministry of Education ofthe Czech Republic under grant INGO LA 151. N. Tsud

thanks the ICTP for a TRIL fellowship. The authors thankthe staff of Materials Science Beamline, Sincrotrone Triestefor their invaluable help to experimental work.

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