Determination of growth mechanisms by X-ray photoemission and ion scattering spectroscopies: application to thin iron oxide films deposited on SiO2

We have studied the formation of iron oxides on SiO2 under varying growth conditions by X-ray photoemission spectroscopy (XPS) and ion scattering spectroscopy (ISS). Three different procedures for quantitative analysis of the experimental data were used: (1) traditional analysis of the XPS peak intensity by assuming a layer formation that covers the surface completely; (2) combination of the XPS peak and ISS intensities assuming a Poisson distribution of island heights; and (3) Tougaard method to determine in-depth profiles from analysis of the peak and background. The difference between the three methods lies mainly in the a priori assumptions made on the surface morphology. In contrast to methods (1) and (2), the Tougaard method is free from assumptions about the growth structure of the overlayer. By a critical comparison of the results, it is concluded that the Tougaard method gives the most complete and reliable result. Iron oxides on SiO2 grow with strong island formation, whose height depends on the preparation conditions of the deposit. The growth mechanisms are proposed. Thus, for example, if iron oxide is deposited on SiO2 at room temperature in PO2=2×10−6 mbar, islands of ∼22 Å homogeneous thickness are formed for surface coverages below 20%. If the samples are annealed at 773 K in PO2≈4×10−5 mbar, higher islands are formed (∼32 Å). For iron oxide treated by a plasma of oxygen, ∼40 Å tall islands are formed for surface coverages below 30%. Besides, even ∼35 Å of iron oxide does not fully cover the SiO2 substrate surface. Strong shadowing effects are observed in the ISS signal from the iron oxide deposits on SiO2 due to the tall island formation. As expected, the shadowing effects are stronger when taller islands have been formed.