Blue shift of the absorption onset and bandgap bowing in rutile Ge<i>x</i>Sn1−<i>x</i>O2

The ordinary dielectric functions (DFs) of rutile GexSn1−xO2 thin films grown by pulsed laser deposition have been obtained by spectroscopic ellipsometry from the near infrared to the ultraviolet range. Additionally, the Ge content x and the lattice parameters were determined by energy dispersive x-ray spectroscopy and x-ray diffraction, respectively. The lattice parameter deviates partly from Vegard's law due to epitaxial strain. In the DFs, a blue shift of the onset of absorption with increasing Ge content is observed. Evaluation of the DFs yields the characteristic transition energies at the absorption onset, and the corresponding bowing parameter of the dipole allowed direct bandgap of b=(0.70± 0.09) eV.