Chemical and phase composition of SnOx:F films grown by DC reactive sputtering

Using X-ray diffraction and X-ray photoelectron spectroscopy we studied the structural variations of SnOx: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 SnOx: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.