QUANTUM-WELL RESONANCES IN SCANNING-TUNNELING-MICROSCOPY

Thin metallic overlayers on a substrate from quantum wells perpendicular to the surface. They can give rise to resonant states, which are observable e.g. by photoemission and LEED. Recent experimental results [J.A. Kubby and W.J. Greene, Phys. Rev. Lett. 68 (1992) 329] indicate that they can also be observed in scanning tunneling microscopy, which extends the range of this extremely surface-sensitive device into the interior of the sample, and makes it possible to image features of a buried interface with the STM. We present the results of model calculations for Ni/Cu(100) and Pd/Ag(100). The variations in the tip height caused by the resonances are of order 0.1 angstrom, which agrees well with the observations. We find that quantum well resonances may be observable if they occur at or above the Fermi energy of the sample, in a band perpendicular to the surface. The lateral resolution of buried steps is estimated using a junction of two free-electron quantum wells. It is found to vary slowly as a function of the overlayer thickness. For a metallic layer 5 angstrom thick, we obtain a resolution in the order of 5 angstrom.