  Spectra of the differential tunneling conductivity for ultrathin lead films
grown on Si(111)7x7 single crystals with a thickness from 9 to 50 monolayers
have been studied by low-temperature scanning tunneling microscopy and
spectroscopy. The presence of local maxima of the tunneling conductivity is
typical for such systems. The energies of maxima of the differential
conductivity are determined by the spectrum of quantum-confined states of
electrons in a metallic layer and, consequently, the local thickness of the
layer. It has been shown that features of the microstructure of substrates,
such as steps of monatomic height, structural defects, and inclusions of other
materials covered with a lead layer, can be visualized by bias-modulation
scanning tunneling spectroscopy.
