  We investigate Ag(100) thin films epitaxially grown on a Fe(100) substrate
using a low-temperature scanning tunneling microscope. Fabrication of a wedge
structure by evaporating Ag through a shadow mask allows us to observe
systematic evolution of quantum well (QW) states for layer thicknesses varying
from 3 to 16 monolayers (ML). Close inspection of differential conductance
spectra and images reveal significant modulations of QW states in the lateral
directions, presumably due to the local defects at the Ag/Fe interface. The
area where QW states are modulated extends over ~ 5 nm. In clear contrast, near
a surface atomic step, QW states exhibit negligible changes at least up to 1 nm
away from the step, leaving unmixed the two sets of neighboring QW states
belonging to different thicknesses. The results are discussed in terms of a
simple electron wave diffraction model.
