NEW POTENTIOMETRY METHOD IN SCANNING-TUNNELING-MICROSCOPY - EXPLOITING THE CORRELATION OF FLUCTUATIONS

We developed a new scanning tunneling microscopy technique to measure the surface potential. The new method exploits the tunneling voltage dependence of the tip-sample separation. The indirect measurement of the potential together with a differential measurement technique makes the new potentiometry insensitive to errors of the electronic setup and provides submicrovolt sensitivity limited by approximately thermal noise of the tunneling resistance. We illustrate the new technique by basic measurements performed under ultrahigh-vacuum conditions. In addition we present the coherence of tunneling current fluctuations and potential fluctuations which underlines the quality of the new technique: the coherence differs significantly from unity. We conclude that the tunneling resistance does not have thermal voltage fluctuations predicted by Nyquist's formula. Possible sources of residual voltage fluctuations such as single electron tunneling effects are discussed. © 1995 American Institute of Physics.