Imaging of dopants in surface and sub-surface layers of the transition metal dichalcogenides WS2 and WSe2 by scanning tunneling microscopy

Abstract

The van-der-Waals surfaces (0001) of the layered structure semiconductors WS2 and WSe2 are known to be free of intrinsic surface states. Therefore, they provide an ideal system for investigations of the influence of individual dopants on the local electronic properties, which can be measured by scanning tunneling microscopy (STM). Individual dopant sites were resolved as topographic depressions superimposed on the atomically resolved lattice. The apparent depth of these depressions showed a discrete statistical distribution and was attributed to the spatial depth of the dopant site. Using an STM-induced electrochemical process, we could locally expose the first and second sub-surface layer to correlate the previously recorded topographic contrast to the location of buried dopants. To our knowledge this is the first direct proof of the capability of STM to detect individual sub-surface dopants. An interpretation of the contrast mechanism is given in terms of tip-induced band-bending effects and current transport mechanisms involving minority charge carrier injection and majority charge carrier extraction.