Phase and stoichiometry control in superconducting FeSe layers on SrTiO₃

Phase and stoichiometry control are crucial to employ the superconducting properties of FeSe thin films, and with it the previously reported interfacial boost in superconductivity promoted by the SrTiO₃ surface. This work investigates how growth parameters influence the phase and chemical composition in FeSe layers on SrTiO₃(001) substrates by molecular beam epitaxy. In the first part, the influence of substrate surface preparation on the stabilization of the respective FeSe phase and film morphology is evaluated by atomic force microscopy (AFM), reflection high energy-electron diffraction (RHEED), and X-ray diffraction (XRD). Continuous, phase-pure β-FeSe layers were observed on non-ideally prepared substrates only at high growth temperatures, whereas optimized surface preparation yielded similar results at much reduced temperatures. Although RHEED indicated atomically smooth film topography, AFM revealed pronounced island growth. In the second part, the stoichiometry of phase-pure β-FeSe films grown under different growth conditions is evaluated by XRD and structural calculations. Supporting transport measurements identified a narrow growth window to satisfy the stoichiometric requirement for superconducting β-FeSe thin films.