IMAGING OF DOPANTS IN SURFACE AND SUBSURFACE LAYERS OF THE TRANSITION-METAL DICHALCOGENIDES WS2 AND WSE2 BY SCANNING-TUNNELING-MICROSCOPY

The van-der-Waals surfaces (0001) of the layered structure semiconduct ors WS2 and WSe2 are known to be free of intrinsic surface states. The refore, they provide an ideal system for investigations of the influen ce 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 at omically resolved lattice. The apparent depth of these depressions sho wed a discrete statistical distribution and was attributed to the spat ial depth of the dopant site. Using an STM-induced electrochemical pro cess, we could locally expose the first and second sub-surface layer t o correlate the previously recorded topographic contrast to the locati on 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-indu ced band-bending effects and current transport mechanisms involving mi nority charge carrier injection and majority charge carrier extraction .