Epitaxial growth and valence control of strained perovskite SrFeO3 films

We have fabricated single-crystalline thin films of perovskite-type iron oxide ${\mathrm{SrFeO}}_3$ which include high valent iron $({\mathrm{Fe}}^{\text{4+}}\mathrm{).}$ The fabrication is accomplished on the lattice-mismatched substrates of ${\mathrm{SrTiO}}_3$ [(STO), 1.4% tensile strain] and $({\mathrm{LaAlO}}_3{)}_{\text{0.3}}({\mathrm{SrAl}}_{\text{0.5}}{\mathrm{Ta}}_{\text{0.5}}{\mathrm{O}}_3{)}_{\text{0.7}}$ [(LSAT), 0.5% tensile strain] by pulsed-laser deposition of ${\mathrm{SrFeO}}_{\text{2.5}}$ with ${\mathrm{Fe}}^{\text{3+}}$ state and subsequent annealing in ozone. The ${\mathrm{SrFeO}}_{\text{2.5}}$ films are grown in a layer-by-layer mode, exhibiting atomically smooth surfaces. The ozone annealed films on LSAT show metallic conductivity as observed in bulk ${\mathrm{SrFeO}}_3,$ whereas those on STO remain semiconducting due to large tensile strain. A pseudomorphically strained epitaxial structure can be maintained by the proper choice of substrates and growth conditions through the huge volume change accompanied by the valence change from ${\mathrm{Fe}}^{\text{3+}}$ to ${\mathrm{Fe}}^{\text{4+}}.$