Investigation of acceptors in p-type WS by standard and 2 photo-assisted scanning tunneling microscopyrspectroscopy

Current imaging tunneling spectroscopy CITS.and photo-assisted scanning tunneling spectroscopy STS.is used to characterize dopants in p-type WS2 single crystals. While the local charge distribution at ionized acceptor sites give rise to topographic depressions on a nm scale, CITS measurements reveal an additional bright ring. On the base of a one dimensional metal–insulator–semiconductor MIS.model, the topographic contrast and the ring structure are explained by two competing current mechanisms involving tunneling into unoccupied states of the valence and the conduction band. Local surface photovoltage LSPV.imaging allows to directly measure the local potential. We observe a nonoscillating reduction of the tip-induced bandbending in the vicinity of the acceptors and correlate this with the lateral extension of the topographic depressions and the ring structures observed in CITS measurements. In addition, photoinduced tunneling current PITC. measurements do not show an enhanced minority charge carrier recombination at acceptor sites. q1999 Elsevier Science B.V. All rights reserved