Mapping subsurface structure through atomically thin bismuth films on Si(1 1 1)−(7 × 7) with scanning tunneling microscope

We performed scanning tunneling microscopy/spectroscopy measurements and spatial mapping of dI/dV on a few atomic layers of bismuth (Bi) film on a Si(1 1 1)−(7 × 7) substrate. At a Bi coverage of four monolayers (ML), local thickness variation could be measured due to thickness dependence of the surface states. At the nominal coverage of 6.5 ML, the dI/dV map reveals the subsurface structures, such as substrate step edges and buried Bi islands. The subsurface structures could be observed at specific biases, by both the electronic interference in a Bi film and the variation of the Bi surface states as a function of the film thickness.