SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 26, 773È782 (1998) Quantitative Analysis of Overlapping XPS Peaks by Spectrum Reconstruction : Determination of the Thickness and Composition of Thin Iron Oxide Films” Peter Graat* and Marcel A. J. Somers¤ Delft University of Technology, Laboratory of Materials Science, Rotterdamseweg 137, NL-2628 AL Delft, The Netherlands The composition and thickness of thin iron oxide Ðlms on polycrystalline pure iron were evaluated from Fe 2p spectra as measured by x-ray photoelectron spectroscopy. To this end the experimental spectra were reconstructed from reference spectra of the constituents Fe0, Fe2‘ and Fe3‘. The background contributions in the spectra owing to inelastic scattering of signal electrons were calculated from the depth distributions of these constituents and their reference spectra. In the reconstruction procedure the Ðlm thickness and the concentrations of Fe2‘ and Fe3‘ in the oxide Ðlm were used as Ðt parameters. The values obtained for the oxide Ðlm thickness were compared with thickness values determined from the intensity of the corresponding O 1s spectra and with thickness values resulting from ellipsometric analysis. The sensitivity of the reconstruction procedure with regard to Ðlm thickness and Ðlm composition was tested. 1998 John Wiley & Sons, Ltd. ( KEYWORDS : XPS spectra ; overlapping peaks ; oxidation ; Ðlm thickness ; composition accuracy INTRODUCTION The photoelectron spectrum of a sample contains infor- mation on the elements present, their chemical state and their distribution. In principle, all these parameters can be extracted from a spectrum, but the accuracy of quan- tiÐcation relies largely on the procedure of spectral analysis. If peaks originating from various species in a sample overlap, the spectrum has to be unravelled in its components before quantiÐcation. The common pro- cedure comprises subtraction of a Shirley-type background1 from the overall spectrum and subsequent Ðtting of the resulting background-corrected spectrum with a set of (arbitrary) peaks. However, for inhomoge- neous samples, each constituent has its own depth dis- tribution and, consequently, its own contribution of (elastically and) inelastically scattered signal electrons to the background. Therefore, it is not possible to calculate the background straightforwardly from an overall spec- trum that is composed of overlapping spectra of constit- uents with unlike depth distributions. A pragmatic approach to deal with such spectra for the case of an oxidized FeÈCr alloy and of oxidized Cr was presented in Refs 2 and 3. A procedure with a more physical foun- dation, based on the work of Tougaard and co-workers (see e.g. Refs 4È 6), for quantiÐcation of electron energy spectra containing overlapping peaks resulting from a substrateÈÐlm assembly, was described in Refs 7 and 8 * Correspondence to : P. Graat, Max Planck Institute for Metals Research, 92, D-70174 Stuttgart, Germany. Seestra÷e ¤ Present address : IPT Materials Technology, The Technical Uni- versity of Denmark, Building 204, DK-2800 Lyngby, Denmark. ” Dedicated to Prof. Siegfried Hofmann on the occasion of his 60th birthday. and was applied to x-ray photoelectron spectra from passivated FeÈCr alloys in Ref. 9, yielding the Ðlm thickness. In the present work a procedure is provided to quantify both Ðlm thickness and composition by reconstruction of an overall spectrum from reference spectra of pure components. The procedure is applied to XPS Fe 2p spectra of oxidized pure iron. The Ðlm thickness results are compared with values obtained from the intensity of O 1s spectra of the same samples and with values obtained from an ellipsometric analysis of the oxidation of iron. The sensitivity of the recon- struction procedure with regard to Ðlm thickness and Ðlm composition is evaluated. RECONSTRUCTION OF A SPECTRUM FROM REFERENCE SPECTRA In an electron energy spectrum the background owing to inelastic scattering of signal electrons can be calcu- lated with the formalism derived by Tougaard and co- workers (see e.g. Refs 4 È 6). This formalism incorporates the following physical quantities : the concentration depth (z) distribution, of an electron emitter M ; C M (z), the inelastic mean free path (IMFP), j, of the electrons and the cross-section, K(T ), for a loss of kinetic energy, T , due to inelastic scattering of these electrons. Re- cently, TougaardÏs formalism was generalized by taking into account the depth dependence of the IMFP.10 A sample with a known composition, containing only one chemical state of the element of interest, can be used to obtain a reference spectrum for this chemical state. If the reference sample is homogeneous within the volume analysed by the electron spectroscopic tech- nique, the intrinsic spectrum, of one electron j M (E), emitter (atom or ion) M can be obtained from the mea- CCC 0142È2421/98/110773È10 $17.50 Received 2 March 1998 ( 1998 John Wiley & Sons, Ltd. Accepted 12 May 1998