Oxidation of Sn Thin Films to SnO₂. Micro-Raman Mapping and X-ray Diffraction Studies

The oxidation of tin layers deposited onto alumina substrates is investigated with the aim to identify the different steps of the process and obtain information on the sample homogeneity, phase segregation, and degree of oxidation. It is shown that at least three phases coexist at 450 °C, Sn, SnO, and SnO₂, and remarkable inhomogeneities, already visible at an optical inspection, are found in the thin film. A micro-Raman mapping of the layer shows that these inhomogeneities are related to the presence of different Sn oxidation states, as evidenced by the inhomogeneous distribution of SnO and SnO_x Raman bands. The thin film becomes homogeneous after annealing treatments above 550 °C, where only the SnO₂ cassiterite phase is detected.