xps survey spectra simulation of nano-structured surfaces

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Research ArticleReceived: 13 June 2008 Revised: 30 October 2008 Accepted: 2 December 2008 Published online in Wiley Interscience: 28 January 2009

(www.interscience.wiley.com) DOI 10.1002/sia.3016

XPS survey spectra simulationof nano-structured surfacesE. Ollivier∗ and J. P. Langeron

We have developed a software tool for the generation of survey spectra in X-ray photoelectron spectroscopy (GOSSIP) tosimulate wide spectra in the range 200–1500 eV from nano-structured surfaces. It is based on linear combination of deltalayers spectra with the atomic spectra of the elements or compounds of the surface to be simulated. The set of delta layersto reproduce any model is a 200-file database of thin layers regularly buried up to a depth of 40 nm and has been generatedwith QUASES. The atomic spectra that constitute a second database have themselves been determined with QUASES fromexperimental spectra of the elements or compounds in pure form. The principle of GOSSIP is described. Then the generationprocess is validated by comparison with experimental data for simple rectangular in-depth distribution of elements. Copyrightc© 2009 John Wiley & Sons, Ltd.

Keywords: XPS; QUASES; inelastic background; spectrum simulation; nano-structured surface; GOSSIP

Introduction

X-ray photoelectron spectroscopy is widely used for two basicpurposes:

– The determination of the chemical state of surface atomsbased on the energy chemical shift induced in core levels bychemical bonding.– The determination of surface chemical composition usingpeak area ratios.

The use of peak area ratios for quantification quickly gives a setof atomic percentages, but means that the sample composition isassumed to be constant over the depth probed by XPS.

This is rarely true and it is precisely because samples areinhomogeneous on the nanometre depth scale that surfaceanalysis (XPS or AES) finds its value compared with othersolid materials analytical techniques such as energy dispersivespectrometry (EDS) or X-ray fluorescence spectrometry (XRF). Theassumption that a solid composition is constant in the volume ofmatter probed by XPS or AES can lead to important errors in quan-tification of surfaces[1] and misinterpretation of recorded data.Surface quantification cannot be decoupled from in-depth atomicdistribution, or surface nanostructure determination. Severalmethods have been established to account for or to determine thisnanostructure for reliable surface quantification: peak intensityattenuation,[2] angular-resolved XPS,[2,3] sputter depth profiling.[4]

These methods are only based on peak intensities or variationof these intensities with a parameter (angle or time) and do notexploit the rich information contained in the inelastic backgroundassociated with the peaks.

In that aim, SESSA,[5,6] a new software tool for quantitative AESand XPS has been recently designed and is released by NationalInstitute of Standards and Technology (NIST).[7] SESSA is a databasethat contains all physical data required to perform quantitativeinterpretation of auger-electron or X-ray photoelectron spectrumfor a layered specimen. A simulation module, based on a MonteCarlo algorithm, is also included into SESSA: the simulated spectra,for layer compositions and thicknesses specified by the user, can

be compared with the measured spectra and adjusted to findmaximum consistency.

In a similar way, with generation of survey spectra in x-rayphotoelectron spectroscopy (GOSSIP), depth distribution andconcentrations of species are determined through absolutecomparison of the measured data and the simulated XPS surveyspectrum of the nano-structured surface model. However, acomparison of both software tools is not timely. GOSSIP aimsat simulating wide XPS spectra and not the fine structure ofdiscrete losses found near the photoelectron peaks. Furthermore,in its present version, GOSSIP minimises physical data inputs fromthe user and this could appear as an oversimplification at thelight of recent developments in electron transport theory. On thispoint, ways for improvements may probably be considered fornext versions.

In GOSSIP, the simulation is based on linear combination ofdelta layers spectra generated with QUASES with files of adatabase of atomic spectra of elements or compounds determinedexperimentally. The work of Sven Tougaard,[1] which led to thesoftware QUASES, has shown that surface nanostructure can bequantitatively and rather accurately determined by analysis of thecontinuous inelastic background associated with a photoelectronpeak, as the shape and intensity of this background is dependenton the depth at which the element is buried. QUASES has twomain functions,[8] ANALYZE and GENERATE. ANALYZE is used todetermine the in-depth distribution of one element by removingthe inelastic background from the experimental spectrum in thevicinity of one peak of this element. The true primary excitationspectrum of one atomic layer of this element (or atomic spectrum)is obtained at the end. GENERATE is used to calculate the inelasticbackground associated with a peak for the assumed in-depthdistribution of one element and its atomic spectrum.

∗ Correspondence to: E. Ollivier, EADS Innovation Works, 12 rue Pasteur, France.E-mail: [email protected]

EADS Innovation Works, 12 rue Pasteur, 95152 Suresnes, France

Surf. Interface Anal. 2009, 41, 295–302 Copyright c© 2009 John Wiley & Sons, Ltd.

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E. Ollivier and J. P. Langeron

The inelastic differential cross section K (E, T) used in QUASES

is approximated, for most metals, their oxides and alloys, by auniversal cross section (UCS),[9] which has the form:

λ(E)K(E, T) = BT/(C + T2)2 (1)

where T is the energy loss, λ the inelastic mean free path (IMFP)and B and C are constant parameters. A critical review of theUCS has been published by Tougaard.[10] The fact that the energydependence is entirely included into λ(E) that plays the role ofa scale factor is fundamental for the survey spectrum generationwith GOSSIP. For light elements and polymers, a three-parameterinelastic differential cross section K(E, T) that gives a better fit withexperimental results can be used.[10]

Background

GOSSIP is an extension of the Tougaard method to simulateXPS survey spectra from nano-structured surfaces in the range200–1500 eV.

The thickness and composition of each layer are entered. Thenthe contribution of each element or compound of the modelis calculated and the contributions are added to produce thecomplete simulated spectrum. Linear combination of spectra hasalready been proposed by Langeron[11] as a general method forAuger quantification and more recently by Basile et al. for XPS.[12]

GOSSIP extends this method to the analysis of nano-structuredsamples.

GOSSIP works independently of QUASES. However QUASES

is necessary to generate the files of the databases of GOSSIP.To generate the survey spectrum of a given in-depth distribution

of elements A, B, C, two sets of files are required by GOSSIP. Firsta set of spectra of pure elements A, B, C are recorded from 200 to1500 eV. Then their atomic spectra are obtained by removingthe inelastic background with QUASES ANALYZE functionassuming a homogeneous depth distribution. These spectra mustbe recorded with the same experimental conditions (source,spectrometer transmission, mode and pass energy). This is doneonce to constitute a database of atomic spectra. The procedureis the one described in the manual of QUASES. The spectraare corrected for the transmission function of the spectrometerbefore background analysis. This database is independent ofthe acquisition mode and of the particular instrument used forrecording the experimental data. The value of the IMFP λ0 to beused in QUASES is arbitrary at this step, but must be the samefor all the elements. (So, strictly speaking, atomic spectra in thisarticle do not correspond exactly to atomic spectra in the sense ofS. Tougaard.) The case of compounds with elements not availablein pure state, for example oxygen, is discussed later.

The second set of files required by GOSSIP is a set of deltalayers, buried at regular depths and generated with the functionGENERATE of QUASES, using the sub-function delta layers. Theatomic spectrum used to generate this set is a pulse P of arbitraryheight H at kinetic energy E = 1500 eV and zero elsewhere. Theenergy width of the pulse is equal to the energy step chosen inrecording the experimental spectra (1 eV in our case). The twomain requirements are that the IMFP λ0, B and C coefficients usedfor generating these delta layers must be identical to the onesused in generating the atomic spectra and that the delta thicknessis lower than the IMFP λ0 value. This last condition is required byQUASES when generating delta layers. A set of n delta layers with

Figure 1. Fitting of averaged calculated IMFP for 27 elements and 15compounds (Refs [7,8]) IMFP data have been normalised to 2 nm at KE1000 eV.

thickness 0.2 nm and located at depths d = 0.1 × (2i − 1) nm, withi = 1 to n, has been used in this work for the simulation of spectra.It is shown later that a range 1 < i < 200, giving the deeper deltalayer at a depth of 39.9 nm, correctly reproduces the experimentalpure spectra. So a set of 200 files, easily created with QUASES, issufficient for simulating any model.

Finally, GOSSIP requires a function describing the dependenceof the effective attenuation length λ′ (EAL) with kinetic energy. Itis now known[1,13 – 15] that the EAL λ′ should be used to accountfor the depth of origin of the photoelectrons instead of theIMFP λ. But the EAL is in the general case a complex functionof material, energy and geometry of experiment. However,as a first approximation, it has been assumed that, owingto the experimental conditions used with GOSSIP (emissionangle � = 0◦), the energy dependence of the EAL is roughlycomparable with the IMFP energy dependence.[6,14] Although itis now considered to be a very rough approximation,[15,16] thisdependence has been represented by a function of the kineticenergy of the form λ′ = AEp,[17] where E is the kinetic energyand A and p are constants. The important parameter for thereproduction of inelastic background shapes of inhomogeneoussamples is the p value giving the energy dependence of the EAL. Agood fit of the IMFP averaged data calculated for 27 elements and15 compounds by Tanuma et al.,[18,19] between 200 and 1500 eV,is obtained with p = 0.75 (Fig. 1). A has been averaged to avalue A = 0.1 and a default EAL function 0.1E0.75 has beenused for the simulated spectra in this work. However, in GOSSIP,A and p can be modified by the user, and eventually, a morerepresentative function could be introduced without changingthe general principle of simulation. Furthermore, the currentversion of GOSSIP uses a unique UCS function and a unique EALfunction, but the principle of GOSSIP nanostructure simulationwould allow layered structures simulation with different UCS andEAL functions for each layer. For instance, thin polymer layers onmetallic substrate could be simulated with GOSSIP. This possibilitywill be implemented in next versions of GOSSIP.

To take into account the energy dependence of the EAL forspectral generation, GOSSIP uses the fact that the spectrum of aburied atomic layer is a function of the reduced depth z/λ′ wherez is the depth at which the layer is buried. Let us assume thatwe want to simulate a model with a buried layer of element A,with a constant atomic percentage c(A), located between depthsz1 and z2. For each point j at energy Ej of the atomic spectrum

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XPS survey spectra simulation

Figure 2. Graphical user interface of GOSSIP.

Sat of element A, and for each delta layer located between z1 andz2 corrected by the calculated EAL, the delta layer spectrum isshifted to the kinetic energy corresponding to the energy of pointj, corrected by the ratio c(A) × Sat(j)/H where Sat(j) is the intensityof the atomic spectrum at point j and added. By the procedure,the variation of the EAL with kinetic energy is taken into account.The procedure is repeated for each element A, B, C of the model,and the simulated spectrum is obtained by adding the results.

GOSSIP

Figure 2 shows the appearance of the graphical user interface ofGOSSIP. A window used to generate a rectangular layer has beenopened and is overlaid on the main window. In the current versionthree options for generating rectangular models are implemented:

Option 1: The generation of a rectangular layer model with upto three elements in the top layer and up to three elements inthe substrate. The relative atomic percentage of each element,the thickness of the top layer and the source (mono-Al or dualAl or Mg) must be entered. The layer top position can be set toa value different of zero, so this model also includes the buriedlayer model.Option 2: The generation of a rectangular layer model with upto three elements in the top layer and up to three elements inthe substrate. The absolute atomic concentration N (at/nm3) ofeach element, the top position and thickness of the top layerand the source (mono-Al or dual Al or Mg) must be entered. Adatabase of atomic concentration of pure elements is includedinto GOSSIP.Option 3: The generation of a rectangular two layers modelwith up to three elements in the top layer and in the mid-layer and up to three elements in the substrate. The atomicpercentage of each element, the top and mid-layer thicknessesand the source (mono-Al or dual Al or Mg) must be entered.

Spectra from homogeneous surfaces may be modelled by notentering an element for the top layer, so that the layer thickness isthen set to zero.

Non-rectangular models (exponential profiles, islands) will beimplemented in next versions of GOSSIP, as the principle ofGOSSIP generation applies also to these models. However, somebasic islands models are implicitly included in the rectangularmodels as, for example, a top layer of a compound of A and B, withthickness d and atomic percentages c(A) and c(B), on a substrateB, can also be viewed as islands of element A on B with a coveringratio c(A) and islands height d.

The EAL coefficients A and p can be modified (λ′ = AEp).However, all the presented simulated spectra have been generatedusing the default values A = 0.1 and p = 0.75. Experimentalspectra can be imported and corrected by the transmissionfunction for comparison with generated models. Spectra notrecorded with the same X-ray source power than the data base ofGOSSIP or spectra with low signal can be expanded for comparisonto modelled spectra.

GOSSIP was developed under Microsoft Visual Basic 6 andworks on Windows XP. Generation of a model is typically of tensof seconds.

Experimental

The experimental work has been done with a Kratos AXIS Ultra. Thedata were recorded with the aluminium monochromatic source orwith the aluminium dual-anode source with a 15-kV high-voltageand 5-mA emission current. The Axis Ultra hemispherical mirrorspectrometer is equipped with eight-channel electron multipliers.The individual multiplier voltages and the global dead time wereset following the methods described by Seah et al.[20,21] Thus,the presented data are dead time corrected. The spectra wererecorded with an emission angle � = 0◦ and an X-ray incidence

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angle � = 60◦. The data were recorded with a dwell time of 1 sand an energy step of 1 eV.

The data base of pure elements was recorded with the large areamode (hybrid mode) for the aluminium monochromatic source(∼700 × 350 µm2) and with the large area mode (electrostaticmode) for the dual-anode (∼400 × 400 µm2), with a pass energyof 160 eV.

The absolute transmission functions for these several modeswere determined through comparison with the pure spectra ofsilver, gold and copper recorded with the Metrology Spectrometerand published by Seah and Smith.[22]

These functions were regularly checked during this work,by running periodically a clean silver sample. The transmissionreproducibility was very good and scattered by less than 3% in the6-month period of measurements.

At loading in GOSSIP, the experimental spectra can be automat-ically corrected by the corresponding transmission functions forcomparison with simulated survey spectra.

Pure elements prepared for testing GOSSIP were limited to thefollowing set of pure elements: Ag, Au, Cr, Cu, Fe, Ni, Pd, Pt, Sn, Znand C.

Clean surfaces were easily obtained by careful polishing of purematerial and 1-µm diamond paste finishing. Then the samples werecleaned with acetone and in-situ argon ion etching until carboncontamination and oxygen peak were of negligible intensities.Carbon spectrum from an organic compound (triacontane,C30H62) was also included in the set of pure spectra to simulatecarbon contamination.

Oxides of some of these elements were prepared by thermaloxidation of pure elements in the preparation chamber, followingthe experimental conditions (temperature, oxygen pressure) givenby Seah et al.[23]

Some metallic deposition layers were prepared in situ by thermalevaporation of the substrate element on a silicon wafer to minimisesurface roughness, and thermal evaporation or in-situ sputteringdeposition of the top layer with the sputter ion beam.

Results

Database

Figures 3–6 give examples of database spectra, recorded with themonochromatic aluminium source, for silver, copper, chromiumand nickel. In each case, the spectrum corrected by thetransmission function and the corresponding atomic spectrum aredisplayed. The atomic spectra have been obtained with QUASES,using the values B = 2866 eV2 and C = 1643 eV2 in the two-parameter UCS, and λ0 = 1 nm.

Figure 7 gives a few examples of delta layers spectra usedin GOSSIP. The input pulse intensity was arbitrarily chosen atH = 10 000 counts/s. For convenience, the curves have beenscaled to the no-loss peak of the 2.1-nm delta layer and shiftedon the horizontal axis. These few examples illustrate how themaximum of the inelastic losses shifts to lower kinetic energyas the layer depth increases. At 900 eV kinetic energy (KE), theinelastic background intensity for the 8.1-nm-deep layer is seventimes higher than for the 2.1-nm-deep layer.

Simulation of a pure element spectrum

The first test for GOSSIP validation was to check that the procedurebased on the combination of delta layers is valid in recovering

Figure 3. Transmission-corrected spectra of pure silver recorded withmonochromatic AlKα and its atomic spectrum generated with QUASES

using UCS coefficients B = 2866 eV2 and C = 1643 eV and λ0 = 1 nm.

Figure 4. Transmission-corrected spectra of pure copper recorded withmonochromatic AlKα and its atomic spectrum generated with QUASES


peak intensities and experimental inelastic backgrounds ofsurvey spectra of pure elements homogeneous with depth. Thecomparison of the calculated spectrum generated with GOSSIPwith the experimental spectrum of homogeneous copper is givenon Fig. 8. The simulated spectrum was generated by a combinationof 200 layers, that is up to a depth of 40 nm. The two spectra arevery similar and their relative difference is less than 2% from 300to 1500 eV (Fig. 8). Below 300 eV, the relative difference increasesto reach 3.5% at 200 eV. A similar trend is observed for otherelements such as nickel and chromium.

For elements such as silver or gold, this effect is even moremarked.

Figure 9 shows the simulated spectra of silver using 100, 150,200 and 300 layers. The fit at low kinetic energy is not good for 100layers, even though the simulated peaks intensities fit very wellwith experimental data. Increasing the number of layers to 150or 200 gives a better fit on low energy background. Adding morelayers in the generation process does not completely reproducethe recorded data. The spectrum simulated with 300 layers isalmost identical to the spectrum simulated with 200 layers. Therelative difference between experimental data and simulated datais less than 3.5% from 500 to 1500 eV (Fig. 9). Below 500 eV, therelative difference increases to reach about 6% at 200 eV.


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Figure 5. Transmission-corrected spectra of pure chromium recorded withmonochromatic AlKα and its atomic spectrum generated with QUASES


Reducing the thickness of delta layers did not solve the problem.So this effect may probably be attributed to the calculationof the inelastic background of individual delta layers that veryslightly underestimates the inelastic background far from thephotoelectron peak from which it is issued. This slight differencepiles up in the low energy region where the greatest number ofdelta layers is combined to reproduce the inelastic background.This explains that elements having their main transition at highkinetic energy are more sensitive to this effect.

However, this difference effect is much minimised in simulationof inhomogeneous models with GOSSIP. Indeed, to reducecomputing time, generation of a simulated spectrum from oneelement of the substrate region is optimized by subtracting thecomplementary surface layers from the homogeneous spectrumof this element, and not by combining all the delta layerscorresponding to the substrate.

Simulation of a chromium nickel homogeneous alloy

As a useful option in the one-layer model, GOSSIP allowsthe simulation of homogeneous compounds survey spectra byentering no element in the top layer.

Figure 10 shows the simulation of a chromium nickel homoge-neous alloy compared with experimental data. The experimentalspectrum was recorded with the aluminium monochromaticsource after argon ion cleaning of a polished sample. Small residualpeaks of oxygen and carbon are visible.

Alloy quantification by X-ray fluorescence has given a relativecomposition of 80% of nickel in weight and 20% of chromium inweight, corresponding to 78% of nickel and 22% of Chromium inatomic percentage. Visually, the spectrum simulated with theseatomic percentages fits correctly the experimental data. However,the bottom of Fig. 10 that gives the relative difference betweenboth spectra measured in absolute value shows some differences.They are observed in the chromium peaks Cr2p and in the highenergy range inelastic background. These differences can berespectively explained by the presence of a small amount ofchromium oxide at the top surface, and by the inelastic backgroundcontributions of carbon contamination, embedded argon ions andtraces of titanium present in the alloy.

Simulation of a thin nickel oxide

The simulation of homogeneous samples is not a full test ofGOSSIP as the procedure to account for the variation of the EAL

with energy plays no role in the generation process. The true testis to simulate in-depth inhomogeneous samples.

Obtaining a rectangular deposition layer with two metallicelements for testing GOSSIP appeared to be a rather difficultexperimental task. Besides the practical difficulties of cleanand uniform in-situ material deposition, most metallic elementsdeposited on a metallic substrate do not follow a layer-by-layergrowth mode (Franck Van der Merwe growth) but rather anisland growth mode or a layer plus island growth mode, witheventually interdiffusion or formation of a chemical compound atthe interface.[24] While the test samples were generally well fittedwith GOSSIP using the simple islands models, a rectangular oxideappears as a more convincing test for demonstrating the validityof GOSSIP, with oxide thickness as the unique parameter.

But for simulation of spectra with elements not available in pureform (in this case oxygen) two methods are available:

– To add in the database an averaged oxygen atomic spectrumfrom a series of bulk oxides, taking into account the atomicconcentration of oxygen in these compounds.– To add in the database atomic spectra of oxides extractedwith QUASES from the bulk oxides.

The first method would have the advantage to permitoxides simulation with variable oxygen concentrations, but thedisadvantage that the chemical shift induced in the metal peakwould not be taken into account.

Figures 11–13 illustrate the second method for nickel oxide. Athick oxide was obtained in situ by oxidation of pure nickel at 400 ◦Cfor 2 h. After slight argon etching to reduce the surface carboncontamination to a negligible level, a spectrum was recorded withthe aluminium dual source (Fig. 11). High-resolution spectrum ofthe Ni2p peak confirmed that the metallic substrate was not visibleanymore. Then the ‘atomic spectrum’ of nickel oxide was obtainedby removing the inelastic background with QUASES (Fig. 11) andadded to the database of atomic spectra.

A thin nickel oxide was obtained in situ by oxidation of purenickel at 400 ◦C for 10 min. Peak fitting of high-resolution Ni2ppeaks showed that the metallic substrate was still visible. A smallamount of carbon contamination was visible.

Figure 12 shows the GOSSIP simulation compared with experi-mental data using a rectangular two-layer model:

– A carbon contamination layer of 0.2 nm.– A nickel oxide layer of 1.2 nm on a pure nickel substrate.

The simulated spectrum fits rather well the experimentalspectrum. The relative difference of both spectra is less than 5% onthe whole energy range, except in a few regions. In the oxygen O1sregion, the simulated spectrum underestimates the oxygen peakinelastic background by about 8%. This may indicate that someoxygen atoms are more buried than our simple rectangular modelassumes. In the carbon C1s region, the simulated carbon peakintensity is lower than the experimental intensity by about 15%.As a result, the nickel peaks are slightly overestimated. However,this difference corresponds to only half a monolayer of carboncontamination and indicates that the carbon contamination hasnot a uniform distribution.

Figure 13 confirms the good fit by the superposition of theexperimental and simulated spectra in the Ni2p peak region (topof the figure). The experimental peak shape is well reproduced.The Ni2p peaks from pure nickel and nickel bulk oxide have alsobeen displayed for comparison.


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Figure 6. Transmission-corrected spectra of pure nickel recorded with monochromatic AlKα and its atomic spectrum generated with QUASES usingUCS coefficients B = 2866 eV2 and C = 1643 eV and λ0 = 1 nm.

Figure 7. Delta layers at depth 2.1, 3.1, 5.1 and 8.1 nm, generated withan input pulse of energy E = 1500 eV, height H = 10 000 counts/s andλ0 = 1 nm.

Figure 8. Comparison of pure copper spectrum of Fig. 4 and simulatedspectra with 200 layers (top) – relative difference of both spectra (bottom).

Assuming a homogeneous sample, as usual programs do, themeasurement of relative intensities of peaks (Ni2p, O1s andC1s) corrected by the sensitivity coefficients gives the followingatomic composition: Ni 52.8 at%, O 28.6 at% and C 18.6 at%.Figure 14 shows the simulated homogeneous spectrum usingthese percentages. The three peak heights, as expected, are in

Figure 9. Comparison of pure silver spectrum of Fig. 3 and simulatedspectra with 100, 150, 200 and 300 layers (top) – relative difference of puresilver spectrum and 300 layers spectrum (bottom).

Figure 10. Chromium nickel homogeneous alloy simulation. Comparisonof experimental and simulated data (top) and their relative difference(bottom).

good agreement with experimental peak heights, but the inelasticbackgrounds are completely different. This simply illustrates thatvery different nanostructures can give the same peak intensities


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Figure 11. Thick nickel oxide and its atomic spectrum.

Figure 12. Thin nickel oxide simulation. Comparison of experimental andsimulated data (top) and their relative difference (bottom).

and that only inelastic background analysis allows the truenanostructure to be determined.

Conclusion

This article has described a new software tool, called GOSSIP, forfast and more reliable quantification of XPS spectra, by takinginto account the sample surface nanostructure. Most methods forsurface quantification are only based on no-loss peak intensities,the basic ones even assuming that the composition is constantin the volume of matter probed by XPS. This assumption canlead to important errors in quantification of surfaces or evenmisinterpretation of recorded data. Reliable surface quantificationcannot be decoupled from the in-depth atomic distribution orsurface nanostructure determination.

In that aim, GOSSIP makes use of the whole informationcontained in recorded data that is no-loss photoelectron peaksand their associated inelastic background.

Depth distribution and concentrations of species are deter-mined through absolute comparison of the recorded data andthe simulated XPS survey spectra of the nano-structured surfacemodels. GOSSIP uses a data base of experimental pure elementsand compounds spectra, their atomic spectra and a set of deltalayers generated with QUASES. These spectra are independent

Figure 13. Comparison of experimental and simulated Ni2p peak(top) – pure nickel Ni2p peak and bulk oxide Ni2p peak for compari-son (middle) and relative difference between experimental and simulateddata (bottom).

Figure 14. Comparison of experimental thin nickel oxide spectrum withsimulated spectrum assuming a homogeneous composition (top) and theirrelative difference (bottom).

of the particular instrument used for recording data and GOSSIPcould be used by laboratories that have an instrument with well-characterised transmission functions. In turn, these laboratoriescould enhance GOSSIP data base content.

However, GOSSIP is still in a development stage and theauthors would be grateful for any suggestion to improve it orany proposition of collaboration for further development.

Acknowledgements

We are grateful to Martine Villatte (EADS IW) for having laid thefoundations of this work some years ago.

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xps survey spectra simulation of nano-structured surfaces

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