Nonuniform quantum-confined states and visualization of hidden defects in Pb(111) films

The spatial dependence of the differential conductivity of ultrathin Pb films deposited on the Si(111)7$\times$7 surface has been studied by low-temperature scanning tunneling microscopy and spectroscopy. Pb films are characterized by the presence of quantum-confined states for conduction electrons and, correspondingly, maxima of the differential tunneling conductivity. The energy of these states is determined mainly by the local thickness of the Pb layer. It has been found that the tunneling conductivity within atomically smooth terraces can be spatially inhomogeneous, and the period of small-scale modulation coincides with the period of the Si(111)7$\times$7 reconstruction. Large-scale inhomogeneities of the tunneling conductivity have been detected in sufficiently thick Pb films. They are manifested in the gradual shift of the quantum-confined levels by about 50 meV at a distance of about 100 nm. Such inhomogeneities of the tunneling conductivity and, correspondingly, the density of states in Pb films can be attributed to the presence of intrinsic defects of the crystal structure, e.g., local electrical potentials and stresses.