We report the growth of epitaxial FeWO4 thin films on c-plane sapphire via pulsed laser deposition using a Fe2O3/WO3 target ablated in an O2 atmosphere. At a constant O2 pressure of 1 mTorr, x-ray diffraction (XRD) confirms FeWO4 as the major crystalline phase for substrate temperatures from 500 to 800 °C. As temperature increases, x-ray fluorescence (XRF) shows the Fe/W ratio remains nearly constant at 0.90 ± 0.02, while x-ray photoelectron spectroscopy (XPS) shows Fe3+ to Fe2+ conversion increases from 20% to 35%. Morphological analysis reveals phase separation, likely of amorphous Fe3+ oxide from crystalline FeWO4, increasing with Fe3+ conversion. This is attributed to an O-rich laser ablated flux, where conversion is driven by the Arrhenius temperature dependence of Fe3+ → Fe2+ reduction on the film surface. At a constant substrate temperature of 750 °C, XRD confirms FeWO4 formation for O2 pressures from 0.5 to 10 mTorr. As pressure increases, XRF shows the Fe/W ratio decreases from 0.98 to 0.70, while XPS shows Fe3+ conversion rises from 15% to 70%. Morphology shows phase separation decreasing with increasing Fe3+ conversion. This is attributed to scattering, where higher O2 pressure makes the laser ablated flux O-deficient relative to Fe and W, facilitating Fe2+ formation. Films with Fe3+ conversion above ∼30% and Fe/W ratios from 0.86 to 0.96 exhibit FeWO4 optical and electronic properties suitable for photoanode applications.
Hylak et al. (Fri,) studied this question.