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Thin films and coatings play a significant role in today’s society, with applications in electronics, optics, mechanics, and biomedicine. Further advancement in the field of surface coatings requires a good understanding of the unique features of ultrathin films and surfaces, which can only be reached with analysis techniques capable of resolving composition and morphology on a sub-nm scale.

Time-of-flight low-energy ion scattering (ToF-LEIS), is such a method, with the surplus of non-destructive analysis. This thesis is devoted to demonstrating the use of in situ ToF-LEIS in the study of ultrathin film synthesis and surface modification. To showcase the possibilities offered by ToF-LEIS, two studies are presented: (1) The study of the phase transition of ultrathin nickel silicides and (2) the study of Ti-based coatings exposed to oxygen at elevated temperatures.

In the first study, the growth and the phase transition sequence of Ni on Si(100) are investigated for initial film thicknesses below 4 nm. A partially ordered film with composition equivalent to Ni₂Si is found for room-temperature deposition. Further, an unprecedented direct phase transition from orthorhombic δ-Ni₂Si to epitaxial NiSi_2-x, skipping the NiSi phase present in thicker films, is observed at 290 °C. This direct phase transition is explained by an enhanced interfacial effect between the substrate and the silicide for ultrathin films.

The second study contains the in situ investigation of effects of low amounts of oxygen and high temperature on four coatings, in situ grown Ti as well as ex situ grown Ti, TiN, and (Ti,Al)N. On the ex situ samples, a persistent surface oxide layer is found, not removable by ion sputtering, limiting the effects of further oxygen exposure compared to the in situ sample. The nitride samples show a high stability towards further oxidation even at elevated temperatures. Additionally, the formation of an Al-rich surface layer is observed on (Ti,Al)N above 750 °C.