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 Ω cm in 60 W films to 120 ± 10 Ω 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.