Determination of growth mechanisms by X-ray photoemission and ion scattering spectroscopies: application to thin iron oxide films deposited on SiO 2

Abstract

We have studied the formation of iron oxides on SiO 2 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 SiO 2 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 SiO 2 at room temperature in P O 2 =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 P O 2 ≈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 SiO 2 substrate surface. Strong shadowing effects are observed in the ISS signal from the iron oxide deposits on SiO 2 due to the tall island formation. As expected, the shadowing effects are stronger when taller islands have been formed.