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

Iron tungstate (FeWO₄) is a promising new photoanode material for solar fuels devices, with a favorable band gap and chemical stability. However, synthesis of high quality thin films of n-type photoactive FeWO₄ has not been achieved, and is not well understood, due to challenges in achieving phase purity, and the unclear role of oxidation in determining optical absorption and electrical conductivity properties.

We report here the first growth of single phase epitaxial FeWO₄ thin films, using oxygen plasma assisted molecular beam epitaxy, and investigate the film structural, optical, and electronic transport properties. The films are grown on c-plane sapphire (0 0 0 1) at 650 °C substrate temperature by evaporating elemental Fe and molecular WO₃ from effusion cells with atomic O flux provided by an rf atom source. The FeWO₄ films are oriented in (1 0 0) growth direction and exhibit 3 rotational twin variants where FeWO₄ [0 1 0] and [0 0 1] are aligned to sapphire [1 2 0] equivalent and [1 0 0] equivalent in-plane directions, respectively.

Optical absorption measurements revealed a 1.7-1.9 eV fundamental band gap with an additional transition near 3 eV consistent with a high FeWO₄ joint density of states in phase pure, O stoichiometric FeWO₄ films. Electrical conductivity was measured using the Van der Pauw technique with indium ohmic contacts. We observe that resistivity decreases over 2 orders of magnitude from >10000 Ω cm to 100 Ω cm as films are increasingly oxidized. Hall measurements indicate that overoxidized films are n-type with 1 cm² V^-1 s^-1 mobility and 10¹⁶-10¹⁷ cm^-3 carriers. Films with higher resistivity had indeterminate carrier type due to changing sign of the Hall voltage. The resistivity trend with oxidation is likely due to increased Fe³⁺ in the lattice of over-oxidized films facilitating electron polaron hopping. The extremely high resistivity of under-oxidized films suggests oxygen vacancies are not a principal factor for n-type conductivity in FeWO₄.

Epitaxial growth of FeWO₄ is driven by lattice match for a supercell consisting of 3 FeWO₄ unit cells stacked along the [0 1 0] direction. Additionally, matching hexagonal oxygen sublattices is factor guiding the epitaxial growth. Growth of FeWO₄ with high phase purity occurs within an atomic O flux window generated by a plasma sustained with 80-100 W of rf power. This range is specific to our cation flux conditions, which are stoichiometric in Fe and W and are effused at a rate sufficient to grow FeWO₄ at 100 nm/hr. A deficient atomic O flux (60 W rf power) causes epitaxial breakdown based on XRD observation of polycrystalline FeWO₄ with reduced oxide impurities. Excess O flux (120 W rf power) induces growth of hematite (Fe₂O₃) as a competing major phase.

Our initial study of oxygen stoichiometry in FeWO₄ thin films suggests that a limited amount of over-oxidation is beneficial to photoanode synthesis. This is because oxidation decreases the total resistivity, causes n-type conductivity, and retains the sub-2eV optical transition. In the future, we will report on photoelectrochemical measurements of FeWO₄ and also on (1) the relation between overoxidation and the strong sub-2 eV optical transition (2) optimizing cation/anion flux balance in oxide MBE processes to achieve faster growth rates for thicker films, and (3) MBE growth on conductive substrates.