  The effect of vacuum annealing on superconductivity is investigated in
Fe(Se,Te) single crystals. It is found that superconductivity is not enhanced
by annealing under high vacuum (~ 10^(-3) Pa) or by annealing in a sealed
evacuated quartz tube. In a moderate vacuum atmosphere (~ 1 Pa), iron oxide
layers are found to show up on sample surfaces, which would draw excess Fe out
of the crystal. Thus, it is suggested that remanent oxygen effectively works to
remove excess Fe from the matrix of Fe(Se,Te) crystals, resulting improvement
of superconducting transition temperature. Our transport measurements suggest
that the excess Fe scatters the carriers on electron- and hole-type channels in
a different manner. We discuss how the mobility of two types of carriers
correlate with superconductivity. Since both the electron and hole bands are
important for the occurrence of superconductivity, excess Fe would suppress
superconductivity mainly due to strong scattering of electrons.
