SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 29, 249–254 (2000)

XPS investigation with factor analysis for thestudy of Ge clustering in SiO2†

S. Oswald,1* B. Schmidt2 and K.-H. Heinig2

1 Institut fur Festkorper- und Werkstofforschung, PF 270016, D-01171 Dresden, Germany2 Forschungszentrum Rossendorf, PF 510119, D-01314 Dresden, Germany

The change of the depth profile and chemical bond character of Ge in GeY ion-implanted SiO2 layersduring annealing in an O2 atmosphere has been studied by x-ray photoelectron spectroscopy (XPS). The Gedepth profiles in as-implanted and annealed samples as measured by XPS are in agreement with profilesmeasured by Rutherford backscattering spectroscopy (RBS). At interfaces, XPS gives more informationabout the Ge depth distribution than RBS. Thus, other than RBS, XPS could prove that the fraction ofimplanted Ge, which moves during annealing to the SiO2/Si interface region, resides on the Si side of thisinterface. Additionally, the high- and low-contrast nanoclusters in Ge-implanted samples, which have beenfound recently in cross-section transmission electron microscopy images, could be identified by XPS, incombination with data analysis by factor analysis, to consist mainly of elemental Ge and GeO2, respectively.Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: XPS; depth profiling; factor analysis; Ge; cluster formation; nanocrystals

INTRODUCTION

Ion beam synthesis of elemental Ge and Si nanoclusters inSiO2 has been studied1 – 6 to look for possible future appli-cations because of their charge storage capabilities2,3 (fornon-volatile memory devices) and their luminescence5 (foroptoelectronics devices). With ion implantation, the con-centration and depth distribution of impurities, as well asthe nanocluster size distribution, can be controlled over awide range by dose, energy and temperature variation.7

Depending on the conditions during post-implantationannealing (temperature, time, atmosphere), a redistributionof implanted Ge atoms has been observed.1,2,4 This redistri-bution of Ge is strongly connected with its precipitation.1,4

Different analytical techniques have been applied tostudy precipitation and Ostwald ripening, as well as chem-ical reactions, during post-implantation annealing. Ruther-ford backscattering spectroscopy (RBS) gives the depthdistribution of the implanted material in the samples.1,2,4,6

Details on the microstructure of the precipitated impuri-ties can be obtained by cross-section transmission electronmicroscopy (XTEM).1,4,7

For SiO2 layers studied by RBS after annealing in N2, aredistribution of as-implanted Ge depth profiles into threewell-separated peaks has been found recently.1 A redis-tribution due to self-organization of Ge nanoclusters,8,9

which can result in such a multi-layer depth profile, couldbe excluded due to different length scales of the diffu-sional screening length10 of the Ge nanocluster systemand the distance between the three Ge peaks. It has beenargued that chemical reactions of the implanted Ge with

* Corresponding to: S. Oswald, Institut fur Festkorper- und Werk-stofforschung, Postfach 270016, D-01171 Dresden, Germany.E-mail: s.oswald@ifw-dresden.de

† Paper presented at ECASIA 99, 4–8 October 1999, Seville, Spain.

a very low concentration of impurities (moisture) in theannealing ambient could be the driving force for the decayof the Gaussian-like Ge depth profile into sublayers.1

This assumption is at least consistent with kinetic latticeMonte-Carlo simulations.1,11

In order to obtain more definite experimental condi-tions, GeC-implanted SiO2 layers have been annealed inpure O2. Under such an annealing ambient the implantedGe profile decays also into three peaks. However, ithappens faster and at lower temperatures than duringannealing under an ‘inert’ N2 or Ar ambient.1 Com-puter simulations based on a recently published mean-fieldmodel12 have been performed for this experiment. It hasbeen shown that the Ge depth redistribution (as measuredby RBS) and the Ge nanocluster evolution (as observedby XTEM) are consistent with the assumption that theannealing process is strongly controlled by chemical reac-tions between the implanted Ge and oxygen supplied bythe annealing atmosphere.12

Here we present an XPS study of GeC-implanted andO2-annealed SiO2 layers that have been investigated inRefs 1 and 11 by RBS, TEM and computer simulations.It is the aim of the present paper to improve the knowl-edge about the mechanisms of Ge nanocluster formationand evolution by: depth profiling of Ge by XPS; and XPSanalysis of the depth-dependent chemical bond character-istics of Ge atoms. X-ray photoelectron spectroscopy isa surface analytical technique using characteristic shiftsin the x-ray-induced photoelectron spectra that depend onthe chemical bond character of the emitting atoms. Thepotential of this technique for chemical state identifica-tion using both core-level and valence-band spectroscopyhas been demonstrated by studies of thin Ge films on Si,including cluster formation.13,14

Investigations by XPS can be carried out also for insu-lating layers. Very recently, XPS has been applied to studythe chemical bond character of Ge implanted into SiO2.15

Copyright 2000 John Wiley & Sons, Ltd. Received 10 October 1999Revised 18 December 1999; Accepted 18 December 1999

250 S. OSWALDET AL.

For chemical state investigations of insulating layers ithas to be taken into account that interpretation problemsmay occur due to a shift originating from surface charg-ing and due to bond-breaking by ion sputtering for depthprofiling. However, recently we have demonstrated that areliable phase identification can be achieved for insulatingmaterials by a special data processing technique in con-nection with factor analysis (FA).16,17 In the present paperthis methodology for the data evaluation will be used.

EXPERIMENTAL DETAILS AND DATAANALYSIS

Germanium was incorporated into 480 nm thick, thermallygrown, SiO2 layers at room temperature by ion implanta-tion with a dose of 5ð 1016 cm�2 at 350 keV ion energyand room temperature. This leads to a maximum Ge con-centration of¾4 at.% at a depth of 250 nm, as estimatedby a TRIM simulation and measured by RBS. Isothermalannealing was performed at 950°C in a standard furnaceusing dry oxygen as the ambient atmosphere. Annealingtimes were 15, 30, 45 and 90 min. For all samples, XTEM

micrographs have been recorded with a 300 keV PhilipsCM 300 microscope (0.14 nm line resolution). For theXPS measurements a PHI 5600 CI system was used withthe following specifications: Al K non-monochromatizedx-rays at 400 W and 10° angle of incidence; electronanalysis using hemispherical analyzer, 59 eV pass energy,acceptance area 800µm, acceptance angle 45°; low-energyelectron flood gun for charge compensation; ion sputter-ing with 3.5 keV, 3.5µA ArC ions, 2 mmð 2 mm rasterarea, angle 30°; standard single-element sensitivity factorsfor concentration evaluation.

The data analysis with respect to peak shift and peakshape changes was carried out using a software pack-age for factor analysis based on a MATLAB softwareenvironment.18 Details concerning this program and theprocedures for correcting the residual peak shift due tosurface charging changes are described in Ref. 16.

RESULTS

Germanium depth profiles of as-implanted and annealedsamples determined by XPS (see Figs 1(a) and 1(b),

Figure 1. Comparison of Ge (implanted into 480 nm thick SiO2) depth profiles measured by XPS (a,b) with RBS spectra (c,d) for twosamples: (a,c) as-implanted sample; (b,d) sample annealed at 950 °C for 30 min.

Surf. Interface Anal. 29, 249–254 (2000) Copyright 2000 John Wiley & Sons, Ltd.

XPS AND FACTOR ANALYSIS OF Ge CLUSTERING IN SiO2 251

respectively) are in good agreement with profiles mea-sured by RBS (Figs 1(c) and 1(d), respectively). Bothmethods prove the redistribution of a substantial frac-tion of implanted Ge towards the SiO2/Si interface duringannealing [Figs 1(b) and 1(d)]. However, owing to thedifficulty of the RBS method to relate the energy scaleof backscattered ions to a depth scale, from Fig. 1(d) itcannot be decided reliably whether the Ge at the SiO2/Siinterface resides there in the SiO2, in the Si or directlyat the interface. The depth scale used in Fig. 1(d) has notbeen calculated in a straightforward manner from the RBSdata, but it has been adjusted using additional informa-tion obtained from XTEM and XPS measurements. Otherthan the RBS method, XPS depth profiling of Ge clearlyshows where the Ge resides at the interface [see Fig. 1(b)].The small width of the Ge interface peak, as well as itscoincidence with the vanishing oxygen concentration andthe increasing silicon concentration, proves that the Ge isdissolved in a thin silicon layer at the Si/SiO2 interface.The maximum Ge concentration estimated from the XPSprofile without further calibration lies within 5–6 at.%in the expected range. The Ge accumulation at the inter-face found by XPS seems to be higher than that detectedby RBS.

In Fig. 2, looking at the Ge 2p3/2 peak shapes of thetwo samples considered in Fig. 1, a strong dependence ofthe XPS line shape on depth can be recognized. Thesechanges in the shape and the peak energy are not inducedby differential surface charging because both the Si 2pand O 1s lines are sharp and they show (due to the chargecompensation used by low-energy electron flooding) onlya minimum energy shift during the measurements. Thus,it can be concluded that the bond state of Ge varies overthe SiO2 layer thickness and changes during annealing.

To gain insight into the correlation of the peak shapechanges and the microscopic redistribution process duringannealing, the complete (for all annealing times) mea-sured data sets were analysed with the factor analysis (FA)method as described in Ref. 16. The data of the Ge 2p3/2

peak of all five depth profile measurements were concate-nated to consider the correlation between the samples.Within one sample, depth-dependent peak energy shiftsare small (see Fig. 2). Between different measurements,however, significant peak shifts occur due to charging (theorigins are changes in experimental conditions such asx-ray tube position, sample contacting, measuring positionon the sample, etc.), and these shifts have to be correctedbefore doing FA. As an internal reference peak for thissample series, the O 1s peak of SiO2 has been chosen (inthe SiO2–Si system the Ar 2p photoelectrons are not suit-able for that purpose17). Because the O 1s peak is needed,for the Si substrate the data analysis has to be restrictedto a depth where SiO2 residuals are still seen in the XPSspectra. The influence of suboxides at the interface can beneglected because no peak shape changes in the Siox andO peaks (proved by FA) were found.

Using the FA procedure, three different Ge compo-nents have been found; these component will be denotedPC1, PC2 and PC3 in the following (PCD principalcomponent). Figure 3 shows the line shapes of the threePCs, which describe consistently all spectra (see the FAmethod, Ref. 17) and are normalized to equal peak heightin this plot. As will be shown below, PC1, PC2 and PC3are related to Ge having different chemical bond states. InFig. 4, for all samples the depth variation of the relative

Figure 2. The Ge 2p3/2, Si 2p and O 1s photoelectron spectrarecorded during XPS depth profiling (shown in Fig. 1) for:(a) as-implanted sample; (b) sample annealed at 950 °C for30 min.

portions(loadings)of the threePCs(upperpart) is com-paredwith thecorrespondingXTEM pictures(lowerpart).At this point it shouldbe notedoncemore that the datain Fig. 4 areonly artificially combinedto demonstratetheone-stepdataanalysisprocedure.Theexperimentalresultswereobtainedof coursefrom five differentdepth–profilemeasurements(four XTEM pictures)at five differentsam-ples. The relative intensitiesof the loadings in Fig. 4correspondto thepeakheightsof PC1–PC3.A strongcor-relationbetweenthe depth-dependentrelativechangesofthethreePCsandthemicrostructureof Geprecipitationinthe XTEM imagescanbe detected.After annealing,darkandhigh-contrastfeaturesvaryingin sizeandlocaldensity[Figs 4(a)–(c)] wereobserved,which could be identifiedby lattice-planeimagingasGeprecipitates.Theseprecip-itateswereoxidized from the surface(from left) up to asharpborderin Figs 4(a)–(c) (15–45 min of annealing),resultingin low-contrastfeatures;total oxidationis there-fore to be assumedafter 90 min of annealing[Fig. 4(d)].

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 29, 249–254 (2000)

252 S. OSWALDET AL.

Figure 3. Spectral line shapes of the three principal componentsobtained by factor analysis of concatenated Ge 2p3/2 spectraldata for the calculated depth profiles given in Fig. 4, whichcan be related to: PC1, i.e. elemental Ge (dissolved in Si at theinterface and large Ge nanoclusters in the SiO2 layer); PC2, i.e.oxidized Ge (GeOx : 1 < x < 2); PC3, i.e. partially oxidized Ge ofsmall nanoclusters (GeOx : 0 < x < 1).

The PC1with the lowestbinding energy (BE) valueof1217eV (pleasenotethat theabsoluteBE valuesarecor-rectedslightly with respectto the surfacecontaminationC 1speakat¾285eV) couldnot befoundin as-implantedsamples.In annealedsamples,PC1is foundat theSiO2/Siinterfaceand in depthregionsthat coincidewith regionsof high-contrastnanoclustersin the XTEM images.Ashas beenshown above [Fig. 1(b)], the PC1 fraction atthe SiO2/Si interfacemustbe locatedon the Si side.ThisPC1 fraction cannotbe a thin Ge layer at the interface

becausesucha layerwould give a dark line in theXTEMimages.Thus, the Ge is probablydissolvedin a thin Silayer. Consequently,PC1 of the interfaceregion shouldberelatedto Ge–GeandGe–Si bonds,where,for config-urationalentropyreasons,Ge–Si bondsshoulddominate.The fraction of PC1within the SiO2 hasa similar photo-electronenergy to that of the interfacefraction. Thus, itshouldbe relatedto Ge–Ge and/orto Ge–Si bondstoo.It shouldbe emphasizedthat for annealedsamplesPC3occursonly simultaneouslywith PC1[seeFigs 4(a)–(c)].The depth profiles of PC3 and PC1 are similar, with ahigher concentrationof PC3. For increasingannealingtimes,the ratio betweenPC3andPC1decreases.For thelongestannealingtime of 90 min both PC3andPC1dis-appearin SiO2 [Fig. 4(d)]. Thedisappearanceof PC3andPC1 happensvia a shrinkageof the depthrangeof theiroccurrence[Figs 4(a)–(c)]. During annealingin O2 thedeepertails of the PC1andPC3profilesseemto remainunchanged,whereasat thesurfacesidesharpcutsremovemoreandmorefrom thesedepthprofiles.Principalcom-ponent2 hasbeenfound only within the SiO2 layer andnot at the SiO2/Si interface.In the as-implantedsample[Fig. 4(x)] PC2 occurssimultaneouslywith PC3. After90 min of annealing[Fig. 4(d)] PC2 at the highestBEvalue of 1220 eV hasa profile that is similar to that ofthe as-implantedsample,but after this time no PC3 isleft. At intermediateannealingtimes the PC2 profile isclearly structured[Figs 4(a)–(c)]. There seemsto be aPC2 fraction belonging to the depth rangeof PC1 andPC3occurrence,andanotherPC2fraction on top of thisdepthrange.

DISCUSSION

At first it shouldbeemphasizedthat thechemicalcompo-sition of the sampleasstudiedby XPS depthprofiling is

Figure 4. Depth profiles of the three principal components (PC1 PC3) of Ge (implanted into 480 nm thick SiO2) obtained by factoranalysis from concatenated XPS data of five samples after different annealing treatment (upper row). The PC1 profiles agree nicelywith the depth distribution of the high-contrast Ge precipitates in the XTEM images (lower row); at the interface, PC1 is segregatedGe. Principal components 2 and 3 are fully and partially oxidized Ge, respectively, either homogeneously mixed with SiO2 (no XTEMcontrast) or oxidized Ge clusters (low-contrast precipitates). A more quantitative description is difficult because of the influence of ionsputtering (see text).

Surf. InterfaceAnal. 29, 249–254 (2000) Copyright 2000JohnWiley & Sons,Ltd.

XPS AND FACTOR ANALYSIS OF Ge CLUSTERING IN SiO2 253

influenced by the measuring process itself. The XPS signalcomes from a few nanometres of the SiO2 surface, and justthese few nanometres are modified by ion beam mixingduring sputtering, which is unavoidable for depth profil-ing. For the ArC ions used a TRIM calculation gives aprojected range of¾6 nm in SiO2 and a sputter coefficientof ¾1. Thus, the XPS-studied top layer has experiencedion beam mixing during removal of almost the last 10 nm,for which 1016–1017 ArC cm�2 are needed. The Ge nan-oclusters (2–5 nm) embedded in the top few nanometresof SiO2 in particular are substantially ion beam mixed withthe SiO2 matrix, which has been estimated by TRIM cal-culations too. Consequently, the number of photoelectronscoming from covalently bonded Ge will underestimate theoriginal content of elemental Ge in the sample, becausesputtering increases the number of Ge–O bonds artificially.This underestimation is larger for smaller Ge nanoclusters.Fortunately, ArC ion beam mixing of Ge nanoclusters is farfrom complete. Thus, XPS depth profiles should still con-tain substantial information about the chemical bond-stateevolution of Ge in SiO2 during annealing. On the otherhand, completely oxidized Ge nanoclusters, which shouldbe obtained at long annealing times, can be reduced par-tially by ion sputtering, which leads to broader Ge 2p3/2

peaks (as observed for PC2) for pure GeO2.Although former experimental and theoretical investi-

gations,1,12 using the same samples as in the present XPSstudy give strong indications of the important role of anoxidizing atmosphere for the Ge nanocrystal formationprocess, an experimental verification of the chemical reac-tions predicted in Refs 11 and 12 is still missing. Thisproof is given by the results of the present XPS study.

A comparison in Fig. 4 between the time evolution ofthe PCs with both the Ge depth profiles measured byRBS and the Ge depth profile predicted by the modelof Ref. 12 identifies the PCs consistently if we assumethat PC1, PC2 and PC3 consist of covalently bonded Ge,strongly oxidized Ge (GeOx : 1 � x � 2) and weaklyoxidized Ge (GeOx : 0 < x < 1), respectively. For theas-implanted sample it can be expected that Ge 2p3/2

photoelectrons come mainly from strongly (PC2) andweakly (PC3) oxidized Ge, as is confirmed by Fig. 4(x).This can be understood by taking into account that, owingto GeC ion implantation, each O and Si atom of theSiO2 layer is displaced many times, as confirmed byTRIM calculations. When a displaced O or Si atom or animplanted Ge atom comes to rest, it immediately formsbonds with its neighbours. Owing to the Ge implanted intoSiO2, there will be no longer enough oxygen availablefor complete oxidation of both Si and Ge. The oxygendeficit will be shared by both Si and Ge. Thus, thestatistical probability for an as-implanted Ge atom formingGeO2 is high as long as only a few atomic per centGe are implanted. Taking into account that GeO2 isthermodynamically less stable than SiO2 and allowing fora slightly preferred formation of SiO2 C GeOx instead ofSiOx C GeO2, the slightly elevated concentration of PC3can be understood. In a recently published XPS studyusing Ge 3d5/2 photoelectrons,15 the formation of GeO2and GeOx in SiO2 layers implanted with GeC has beenfound too.

After a long (90 min) annealing in O2 at 950°C it canbe expected that all of the Ge embedded in the SiO2 layerbecomes fully oxidized. This is confirmed by Fig. 4(d),where only PC2 is found within the SiO2 layer, which

should be GeO2 in that case. Additionally, an intense PC1is found in the region of the SiO2/Si interface, whichcorresponds to Ge segregated into Si during annealing andforms mainly Ge–Si bonds (because of Ge segregation;see Refs 12 and 19).

The most interesting evolution of Ge is found afterannealing times<90 min. Whereas for the as-implantedsample [Fig. 4(x)] and the sample with fully oxidized Ge[Fig. 4(d)] the ion beam mixing by ArC bombardment isobviously rather unimportant, changes of Ge bonds dueto ion beam mixing must be considered carefully for thesamples of Figs 4(a)–(c), where Ge nanoclusters are seenin the XTEM images. The comparison of the XPS depthprofiles with the XTEM image in Fig. 4(a) proves thatPC1, i.e. elemental Ge, is found at depths only wherethe XTEM image shows Ge nanocrystals. The relativelyhigh concentration of PC3 (and PC2, i.e. oxidized Ge) inthis nanocluster depth region is mainly due to the ionbeam mixing of Ge nanoclusters. Especially for smallnanoclusters, a large volume fraction of elemental Gewill be mixed into the surrounding SiO2 matrix, resultingin Ge–O bond formation and consequently in a peakshift to higher binding energy (which was also foundin Ref. 20 for small PbS nanocrystals in SiO2). Themixing of elemental Ge from clusters into the SiO2 matrixbecomes less and less dominant with increasing clustersize. Therefore, the ratio between elemental and (partly)oxidized Ge increases with longer annealing times (i.e.larger clusters), as can be seen in Figs 4(b) and 4(c)from the decreasing portion of PC2C PC3 in relation toPC1. The XPS spectra of Figs 4(a)–(c) reveal additionallythat the Ge between the surface and the nanoclusterregion is strongly oxidized (PC2). This is in excellentagreement with the predictions of the model of Ref. 12.This agreement holds also for the evolution of the depthprofiles of elemental and oxidized Ge, as will be discussedin more detail elsewhere.19

CONCLUSIONS

The depth profiles of GeC-implanted SiO2 as measuredby XPS are in agreement with profiles measured recentlyby RBS. In the SiO2/Si interface region, XPS givesmore information about the Ge depth profile than RBS.Thus, other than RBS, XPS could prove that the fractionof implanted Ge, which moves during annealing to theSiO2/Si interface region, resides on the Si side of thisinterface. Additionally, the change of the Ge depth profileand chemical bond character of Ge in GeC ion-implantedSiO2 layers during annealing in an O2 atmosphere hasbeen studied by XPS. By special data analysis (factoranalysis), a deeper insight into the peak shape changesduring annealing is possible. The high- and low-contrastnanoclusters in Ge-implanted samples, which have beenfound recently in XTEM images, could be identifiedby XPS to consist mainly of elemental Ge and GeO2,respectively. For interpretation of the XPS spectra, the ionbeam mixing of nanostructured materials during sputteringfor depth profiling must be taken into account.

Acknowledgements

The authors thank R. Grotzschel for RBS measurements and A. Mark-witz for TEM studies.

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254 S. OSWALDET AL.

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Surf. Interface Anal. 29, 249–254 (2000) Copyright 2000 John Wiley & Sons, Ltd.