Iron chalcogenide FeTe1-xSex have attracted much attention, in terms of the novel superconducting state under magnetic field [1,2]. In addition, the strong spin-orbit coupling induces the topologically non-trivial energy dispersion [3], expecting the realization of topological superconductivity (SC). The electron doping such as the Co substitution for Fe site is important to approach the Dirac semimetal band to the Fermi energy, suggested by the ARPES study [3]. Thus, the removal of the excess Fe induced in as-grown crystals is essential for the SC of high quality [4]. Here, we dealt with the oxygen annealing for Fe1+y-xCoxTe0.6Se0.4 (Co-FTS) with x=0 to 0.15. We confirmed that a single-step annealing was partially useful to reduce the content of excess Fe and to improve the SC in Co-FTS, although the formation of stout oxide layer made the complete removal difficult [4]. The Hall effect measurements up to 6 T indicated a tendency that the excess Fe pushed up the Hall coefficient while the Co-doping pushed down it. Toward the complete removal, we also perform a cyclic process consisting of the O2 annealing and the succeeding etching, as suggested by the recent study on Fe(Te,S) [5].

[1] H. Kitano et al., Phys. Rev. B 113, 064513 (2026).
[2] Y. Sun et al., Phys. Rev. B 112, 184503 (2025).
[3] P. Zhang et al., Nat. Phys. 15, 41 (2019).
[4] L Dong et al., Phys. Rev. Mater. 3, 114801 (2019).
[5] R. Kurihara et al., Phys. Rev. Mater. 9, 034802 (2025).