Epitaxial growth and valence control of strained perovskite SrFeO3 films

We have fabricated single-crystalline thin films of perovskite-type iron oxide SrFeO3 which include high valent iron (Fe4+). The fabrication is accomplished on the lattice-mismatched substrates of SrTiO3 [(STO), 1.4% tensile strain] and (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 [(LSAT), 0.5% tensile strain] by pulsed-laser deposition of SrFeO2.5 with Fe3+ state and subsequent annealing in ozone. The SrFeO2.5 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 SrFeO3, 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 Fe3+ to Fe4+.