Formation and structure of tin-iron oxide thin film CO sensors

Rheotaxial growth and thermal oxidation (RGTO) for depositing thin films is a recognized technique in preparing gas sensitive semiconducting oxides. This paper presents a study performed by x-ray diffraction and scanning Auger microscopy of the mechanisms of growth and formation of the thin films of the new ternary compound Sn_1−xFe_xO_y with an iron content in the range O < x < 25 at. %. A structural model of this compound, which is found to be stable over a very large range of Sn/Fe ratios, can be derived by partially substituting Fe³⁺ ions in Sn⁴⁺ sites. This is an easy substitution in view of the similar values shown by the ionic radii (Fe₃₊ = 0.64 Å, Sn⁴⁺ = 0.71 Å) and the Pauling electronegativity (Fe³⁺ = 1.8, Sn⁴⁺ = 1.8) of these two ions. Experimental data, showing that this material is an excellent CO sensor, are reported.