Lattice strain effects on the structural properties and band gap tailoring in columnarly grown Fe-doped SnO₂ films deposited by DC sputtering

In this work, we present the study of undoped and Fe-doped SnO₂ polycrystalline films grown by direct-current sputtering on glass substrates. X-ray diffraction (XRD) analysis revealed that the as-deposited films exhibit an amorphous phase, whereas, after 2 h of thermal annealing at 500 °C, the films showed a rutile-type structure with no evidence of secondary phases. Rietveld refinement analyses of the XRD patterns revealed that the undoped films do not reveal a preferred orientation as is displayed in the bulk system, whilst the Fe-doped films showed a (1 0 1) preferred orientation which is enhanced with the deposition time (film thickness). A detailed analysis revealed a clear dependence of the unit cell volume on the crystallite size. The latter effect was mainly associated with strain effects occurring during the film deposition. Additionally, the growth of columnar-shaped structures was determined by cross-section scanning electron microscopy images. Interesting features of the fundamental absorption were determined via UV-vis spectroscopy. The obtained results revealed a monotonous decrease of the band gap with film thickness, which becomes larger for the Fe-doped SnO₂ films and is associated with a change in the residual strain, in good agreement with the XRD analysis and Raman spectroscopy measurements.