  The spatial distribution of the differential conductance for ultrathin Pb
films grown on Si(111)7x7 substrate is studied by means of low-temperature
scanning tunneling microscopy and spectroscopy. The formation of the
quantum--confined states for conduction electrons and, correspondingly, the
appearance of local maxima of the differential tunneling conductance are
typical for Pb films; the energy of such states is determined mainly by the
local thickness of Pb film. We demonstrate that the magnitude of the tunneling
conductivity within atomically flat terraces can be spatially nonuniform and
the period of the small-scale modulation coincides with the period of
Si(111)7x7 reconstruction. For relatively thick Pb films we observe large-scale
inhomogeneities of the tunneling conductance, which reveal itself as a gradual
shift of the quantized levels at a value of the order of 50 meV at distances of
the order of 100 nm. We believe that such large-scale variations of the
tunneling conductance and, respectively, local density of states in Pb films
can be related to presence of internal defects of crystalline structure, for
instance, local electrical potentials and stresses.
