  Scanning tunneling microscopy and spectroscopy are utilized to study the
atomic-scale structure and electronic properties of infinite-layer
Sr0.94La0.06CuO2+y films prepared on SrRuO3-buffered SrTiO3(001) substrate by
ozone-assisted molecular beam epitaxy. Incommensurate structural
supermodulation with a period of 24.5{\AA} is identified on the CuO2-terminated
surface, leading to characteristic stripes running along the 45o direction with
respect to the Cu-O-Cu bonds. Spatially resolved tunneling spectra reveal
substantial inhomogeneity on a nanometer length scale and emergence of in-gap
states at sufficient doping. Despite the Fermi level shifting up to 0.7 eV, the
charge-transfer energy gap of the CuO2 planes remains fundamentally unchanged
at different doping levels. The occurrence of the CuO2 superstructure is
constrained in the surface region and its formation is found to link with
oxygen intake that serves as doping agent of holes in the epitaxial films.
