Results obtained on aluminum and copper surfaces are used to demonstrate the ability of scanning tunneling microscopy (STM) and spectroscopy (STS) to detect subsurface structures through their influence on the electronic structure. Subsurface Ar bubbles in Al lead to a quantum well bounded by the outer surface and the top of the bubbles. Using z(V) spectroscopy, where the STM feedback loop keeps the current constant while ramping the voltage, it is possible to detect the energy steps between the quantum well states; combined with a one-dimensional model employing a realistic potential for the electrons, this allows an estimate of the thickness of the quantum well, i.e., the depth of the bubbles. Simulated STM images calculated with a three-dimensional scattering theory reproduce many details of the interference pattern, and confirm the size and geometry of the bubbles. Interference patterns attributed to subsurface scatterers have been also detected on Cu(111) and Cu(100). We propose that the patterns observed on Cu(111) are due to focusing of electron waves in certain crystallographic directions, whereas those on Cu(100) are unexplained up to now.
Corresponding author: M. Schmid (schmid).
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