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.<br/><br/>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 FeWO₄films 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.<br/><br/>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 &gt;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^-1s^-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.<br/><br/>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.<br/><br/>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.