Quantification of surface nano-structures by angle-resolved XPS (ARXPS) is straightforward and works quite reliably for perfectly flat surfaces of amorphous materials. For rough surfaces, the interpretation of ARXPS is, however, complicated because the angular variation of the XPS peak intensity depends on the surface roughness. Even for ideally flat substrates, ARXPS analysis of laterally inhomogeneous surface structures grown on the surface is quite complex. The reason is that neighboring nano-clusters shadow the XPS peak intensity. The effect depends on cluster shape as well as the distribution of clusters on the surface. In addition the effects depend on the flatness of the underlying substrate. The interpretation of ARXPS then becomes quite complex. In the present paper, we have studied this problem by analyzing ZnO nano-clusters grown on substrates of SiO 2 and Al 2 O 3 . Thus we compared the results of quantification by the four techniques: ARXPS, XPS-peak shape analysis, Rutherford backscattering spectroscopy and x-ray fluorescence spectrometry. While the latter three techniques gave consistent results, the results of the ARXPS analysis were way off. This deviation is discussed in terms of the above-mentioned shadowing effect of neighboring clusters as well as roughness of the underlying substrates. Different normalization methods in the ARXPS analysis procedure are compared and it is found that some of the observed problems for the substrate peaks (but not for the peaks from the overlayer film) can be reduced by applying reference samples with similar roughness for normalization of the data. In conclusion, the ARXPS technique is very much dependent on surface roughness as well as on the morphology of the thin films. Thus for reliable quantification with ARXPS it is necessary to have independent knowledge on surface roughness as well as the distribution of islands of the thin films.

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Articles you may be interested inAnalysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy A critical characteristic of nanoparticles and, in general, of nanomaterials, is the overweighing importance of the surface and interface layers relative to the bulk because of the small size of the aggregates ͑in three dimensions͒ or thickness of the layers ͑in the case of two dimensions͒ that constitute the nanomaterial. This article reports the characterization of interface layers of ZnO/MO x (MO x : Al 2 O 3 and SiO 2 ) using x-ray photoelectron spectroscopy ͑XPS͒. Careful experiments consisting of the deposition of ZnO material on SiO 2 and Al 2 O 3 substrates have been performed. Several samples were produced and characterized in situ. The nanostructure of the first stages of growth of the ZnO deposited was determined by Tougaard peak-shape analysis of several photoelectron peaks in both the substrate and overlayer and the growth mechanisms determined were found to be consistent. Thus, the actual nanostructure of the growing ZnO films was carefully determined. In addition, the chemical interaction at the ZnO/MO x interface was monitored by following the variation of the Auger parameter of the Zn atoms as the amount of ZnO deposited was increased. Thus, changes of the Auger parameter of the Zn atoms were correlated with the actual nanostructures formed by the ZnO deposits. From this information, a model is presented that accounts for changes in the electronic parameters determined by XPS as a result of bonding and polarization interaction at the interface.

Quantification on the nanometre scale is a key task in quality control and for the development of new materials in nanotechnology. In this paper we have studied the consistency in the determination of the amount of substance found by XPS peak shape analysis, Rutherford backscattering spectroscopy (RBS) and x-ray fluorescence spectrometry (XRF). To this end, ZnO was deposited by plasma-enhanced chemical vapour deposition on the three substrates. Four different sets of samples were produced, with the amount of ZnO deposited in the range 1-10 nm. From XPS analysis it is found that ZnO grows in the form of islands on all three substrates. For each system, the analysis was done independently with two XPS peaks from the overlayer with widely different kinetic energy and one XPS peak from the substrate. The growth mechanism found from analysis of each of the three peaks was consistent and the total amount of determined ZnO material was identical to within 15%. The root-mean-square deviation from the XPS quantification of the relative AOS was 20% for XRF and 16% for RBS. Because the absolute amount of substance determined from analysis of the three XPS peaks for each sample was consistent, it is concluded that the energy dependence of the applied inelastic mean free paths (taken here from the empirical TPP-2M formula) is correct. It was found that the absolute amounts of substance determined by RBS and XRF are consistently factors of 2.1 and 1.5 lower than the that determined by XPS peak shape analysis. It is suggested that the main reason for this large discrepancy is inaccuracy in the applied 'effective' inelastic electron mean free path.

Cadmium sulfide has been deposited by evaporation on five different substrates: CdTe, ZnO, Ag, TiO2, and partially reduced titanium oxide (i.e., TiO1.73). The deposition rate and the evolution of the Cd∕S ratio on the different substrates have been determined by x-ray photoelectron spectroscopy. The growth mode of the films has been also studied by analyzing the shape of the backgrounds behind the photoemission peaks (peak shape analysis). It has been found that, under completely equivalent conditions, the deposition efficiency (i.e., sticking coefficient) is large on CdTe and TiO1.73, but very small on ZnO and TiO2. Silver constitutes an intermediate situation characterized by a long induction period where the deposition rate is small and a later increase in deposition efficiency comparable to that on CdTe. For the initial stages of deposition, below an equivalent monolayer, it has been also found that the Cd∕S ratio is smaller than unity on TiO1.73 and ZnO but larger than unity on CdTe and Ag substrates. For sufficiently long deposition times the Cd∕S ratio on the surface reaches unity. Except for silver substrate, cadmium appears as Cd2+ and sulfur as S−2 species at the initial stages of deposition. On the silver surface, cadmium adsorbs as Cd0 at low coverage. Peak shape analysis has shown that cadmium sulfide grows according to layer-by-layer mechanism (Frank–van de Merwe model) when the substrates are CdTe and TiO1.73, but large particles are formed that do not cover the surface for ZnO and Ag substrates (Volmer-Weber growth model). These results are consistent with the different chemical affinities of the substrate towards the atoms of cadmium and sulfur produced during the evaporation of the cadmium sulfide.