QUANTUM-WELL RESONANCES IN SCANNING-TUNNELING-MICROSCOPY

Thin metallic overlayers on a substrate from quantum wells perpendicul ar to the surface. They can give rise to resonant states, which are ob servable e.g. by photoemission and LEED. Recent experimental results [ J.A. Kubby and W.J. Greene, Phys. Rev. Lett. 68 (1992) 329] indicate t hat 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 b uried interface with the STM. We present the results of model calculat ions for Ni/Cu(100) and Pd/Ag(100). The variations in the tip height c aused by the resonances are of order 0.1 angstrom, which agrees well w ith the observations. We find that quantum well resonances may be obse rvable if they occur at or above the Fermi energy of the sample, in a band perpendicular to the surface. The lateral resolution of buried st eps is estimated using a junction of two free-electron quantum wells. It is found to vary slowly as a function of the overlayer thickness. F or a metallic layer 5 angstrom thick, we obtain a resolution in the or der of 5 angstrom.