Surface oxidation of refractory metals for stable oxide flux generation in molecular beam epitaxy

Evaporating refractory metals such as tungsten (W) to grow their corresponding oxides remains challenging for molecular beam epitaxy (MBE) due to their high melting points and low vapor pressures at conventional effusion-cell temperatures. Conventional approaches using oxide precursors like WO3 often suffer from incongruent evaporation and flux instability caused by oxygen loss. Here, we utilize an in situ surface-oxidation method to generate a volatile oxide flux directly from a refractory W metal source for epitaxial tungsten-oxide thin film growth. In this approach, the W metal source is heated under a controlled oxygen partial pressure, dynamically forming and evaporating a surface oxide layer. This process produces a stable, high-purity, and tunable oxide flux at temperatures well below those required for metal evaporation. By controlling the oxygen partial pressure and electron-beam power, we can selectively generate tungsten-oxide species for epitaxial growth. In the low-flux regime (1–3 × 10−3 Å s−1), the deposition rate scales linearly with oxygen partial pressure and remains constant for over an hour, enabling the reproducible growth of ultrathin epitaxial WO3 films. Extending this approach to thermal-laser epitaxy achieves deposition rates up to ∼1.5 Å s−1 while maintaining excellent crystallinity. This surface-oxidation-assisted synthesis approach offers several advantages including the use of high-purity elemental sources, precise flux control, long-term stability, and selective generation of oxide species during growth, providing an alternative pathway for the MBE growth of high-quality oxides involving refractory metals with improved control and reliability.