Microfour-point probe for studying electronic transport through surface states

Microfour-point probes integrated on silicon chips have been fabricated wit h probe spacings in the range 4-60 mum. They provide a simple robust device for electrical transport measurements at surfaces, bridging the gap betwee n conventional macroscopic four-point probes and scanning tunneling microsc opy. Measurements on Si(111) surfaces in ultrahigh vacuum reveal that the S i(111)-root 3x root3-Ag structure induced by a monolayer of Ag atoms has a four-point resistance two orders of magnitude lower than that of the Si(111 )-7x7 clean surface. We attribute this remarkable difference to direct tran sport through surface states, which is not observed on the macroscopic scal e, presumably due to scattering at atomic steps. (C) 2000 American Institut e of Physics. [S0003-6951(00)02749-2].