Growth and Characterization of Epitaxial FeWO4 Thin Films with Controlled Oxygen Stoichiometry

We report the growth of single-phase epitaxial FeWO4 thin films, using plasma-assisted molecular beam epitaxy, and investigate structural, optical, and electronic properties. The FeWO4 films grow in (100) orientation on c-plane sapphire (0001) substrates and exhibit 3 rotational twin variants where FeWO4 [001] is aligned to sapphire [100] equivalent in-plane directions. X-ray diffraction measurements indicate that the epitaxial FeWO4 (100) structure is optimized when 80-100 W of rf power is applied to an atomic oxygen source during growth, yielding films with minimal strain and impurity phases or other orientations. In films grown with 120 W of rf power, FeWO4 crystallites develop inhomogeneous and homogeneous strains and are potentially contaminated with Fe3+ oxide phase impurities. In films grown with 60 W of rf power, FeWO4 crystallites do not form fully epitaxial layers. X-ray photoelectron spectroscopy indicates that the structural changes are correlated with the Fe3+/Fe2+ oxidation state ratio increasing from 0.6-1.4 with rf power from 60-120 W. X-ray fluorescence spectroscopy indicates that the Fe/W composition ratio is also increasing from 1.1-1.8 with rf power from 60-120 W. Ultraviolet and visible optical absorption spectra indicate a 1.8 +- 0.1 eV band gap with an additional interband absorption feature at 3.1 +- 0.1 eV in the 80-100 W films, with similar onsets observed in the 60 W films. In the 120 W films, the higher lying transition is shifted to 2.7 +- 0.1 eV due to the Fe3+ enrichment. Electrical resistivity decreases over 2 orders of magnitude with oxidation from 104-105 ..omega.. cm in 60 W films to 120 +- 10 ..omega.. cm in 120 W films. Thermopower measurements show p-type to n-type conductivity conversion when oxidation states shift from Fe2+ majority in the 100 W films to Fe3+ majority in the 120 W films. We conclude that electron polaron hopping driven by Fe3+ is a dominant transport mechanism and a source of n-type conductivity in overoxidized FeWO4 films.