  We have systematically investigated the effect of oxygen nonstoichiometry in
a nominally undoped superconductor T'-(La,Y)2CuO4+y. In the experiments, the
reduction condition was changed after the sample growth by MBE. The
superconductivity is very sensitive to the reduction condition. With
systematically increasingly reduced atmospheres, resistivity shows a continuous
drop and no discontinuity is observed even until the appearance of
superconductivity. The absence of the highly insulating state expected for Mott
insulators around y ~ 0 suggests that T'-(La,Y)2CuO4 has intrinsic carriers.
The role of residual apical oxygen, which is detrimental to superconductivity,
is also discussed based on the resistivity-temperature characteristics in
insufficiently reduced samples.
