Long-range order and thermal stability of thin Co${}_2$FeSi films on GaAs(111)B

Co${}_2$FeSi/GaAs(111)B hybrid structures are grown by molecular-beam epitaxy and characterized by transmission electron microscopy (TEM) and x-ray diffraction. The Co${}_2$FeSi films grow in an island growth mode at substrate temperatures $T_S$ between $T_S$~=~100${}^{\circ }$C and 425${}^{\circ }$C. The structures have a stable interface up to $T_S=275{}^{\circ }$C. The films contain fully ordered $L{2}_1$ and partially ordered $B2$ phases. The spatial distribution of long-range order in Co${}_2$FeSi is characterized using a comparison of TEM images taken with superlattice reflections and the corresponding fundamental reflections. The spatial inhomogeneities of long-range order can be explained by local non-stoichiometry due to lateral segregation or stress relaxation without formation of extended defects.