  In order to realize superconductivity in cuprates with the T'-type structure,
not only chemical substitution (Ce doping) but also post-growth reduction
annealing is necessary. In the case of thin films, however, well-designed
reduction annealing alone without Ce doping can induce superconductivity in the
T'-type cuprates. In order to unveil the origin of superconductivity in the
Ce-undoped T'-type cuprates, we have performed bulk-sensitive hard x-ray
photoemission and soft x-ray absorption spectroscopies on superconducting and
non-superconducting Nd$_{2-x}$Ce$_x$CuO$_4$ ($x=$ 0, 0.15, and 0.19) thin
films. By post-growth annealing, core-level spectra exhibited dramatic changes,
which we attributed to the enhancement of core-hole screening in the CuO$_2$
plane and the shift of chemical potential along with changes in the band
filling. The result suggests that the superconducting Nd$_2$CuO$_4$ film is
doped with electrons and that the electronic states are similar to those of
Ce-doped superconductors.
