  Epitaxial thin films of SrTiO$_3$(100) doped with 6% and 12% Ni are studied
with resonant angle-resolved photoelectron spectroscopy (ARPES) at the Ti and
Ni L2,3-edges. We find that the Ni doping shifts the valence band (VB) of
pristine SrTiO$_3$ towards the Fermi level (p-doping) and reduces its band gap.
This is accompanied by an upward energy shift of the Ti t2g-derived mobile
electron system (MES). Thereby, the in-plane dxy-derived bands reduce the
embedded electron density, as evidenced by progressive reduction of their Fermi
momentum with the Ni concentration, and the out-of-plane dxz/yz-derived bands
depopulate, making the MES purely two-dimensional. Furthermore, the Ti and Ni
L2,3-edge resonant photoemission is used to identify the Ni 3d impurity state
in the vicinity of the valence-band maximum, and decipher the full spectrum of
the VO-induced in-gap states originating from the Ni atoms, Ti atoms, and from
their hybridized orbitals. Our experimental information about the dependence of
the valence bands, MES and in-gap states in Ni-doped SrTiO$_3$ may help
development of this material towards its device applications associated with
the reduced optical band gap.
