Chemical and phase compositionof SnO_x:F films grown by DC reactive sputtering

Using X-ray diffraction and X-ray photoelectron spectroscopy we studied the structural variations of SnO_x:F films grown by DC reactive sputtering. We used a metallic tin target and an Ar-O₂-Freon plasma. We found that a mixture of compounds, i.e. SnO, Sn₃O₄, Sn₂O₃ and SnO₂ were formed in the films. From the analysis of the deconvoluted peaks of the X-ray diffractograms and from the X-ray photoelectron spectroscopy results, we obtained the Sn²⁺/Sn and Sn⁴⁺/Sn molar fractions present in the films. The dependence of the Sn²⁺/Sn and Sn⁴⁺/Sn molar fraction with oxygen content was fitted by applying a combinatory model. The Sn²⁺/Sn and Sn⁴⁺/Sn molar fractions obtained from XRD and XPS were compared with theoretical values and showed good agreement. When the cathode voltage used to grow the films is varied, the Sn⁴⁺/Sn molar fraction shows two well-defined regions, related to the SnO and SnO₂ stoichiometries. This is related to the Sn²⁺ → Sn⁴⁺ transition in the cathode, as described in a previously proposed phase diagram for DC reactive sputter deposition of SnO₄:F films. Sn²⁺ and Sn⁴⁺ are both present in the films at low current densities, and SnO and SnO₂ are the only species found in these conditions. Sn₂O₃ and Sn₃O₄ are formed at current densities over 30 mA cm^-2, when the growth takes place near the transition region in the phase diagram. The growth rates are also affected and closely related with the variations of the cathode voltage in this region.