Metal-organic-silicon nanoscale contacts

Abstract

A scanning tunneling microscope tip is used to create nanoscale contacts on degenerately doped ${n}^{+}\text{\ensuremath{-}}\mathrm{Si}(100)$ surfaces. Current-voltage spectra are recorded as the tip transitions from tunneling to point contact on surfaces prepared in three ways: (1) the clean $2\ifmmode\times\else\texttimes\fi{}1$ surface, (2) with a covalently bonded benzene overlayer, and (3) through nanoscale clean silicon windows formed within the benzene film. Contacts to the clean surface are more Ohmic than rectifying and show a surface leakage current that arises from partially occupied ${\ensuremath{\pi}}^{*}$ states. Contacts to the benzene surface do not display a surface current and exhibit significant current rectification. The unpinning of the Si band structure by the organic adsorbate leads to inversion and a limiting minority-carrier tunnel current under reverse bias. Contacts to the windows simulate defects to an overlayer and reveal characteristics of a hybrid junction. Current-voltage spectra through the windows are free of surface leakage and are independent of the cleaned area. The return of majority carrier tunneling under reverse bias demonstrates that the pinning states of the clean substrate are restored to the windows. However, the charge distribution on the windows is significantly different from the clean surface because the restored surface states are isolated within the benzene monolayer.