Conductivity of Si(111) - 7 x 7: the role of a single atomic step

The Si(111) - 7 x 7 surface is one of the most interesting semiconductor surfaces because of its complex reconstruction and fascinating electronic properties. While it is known that the Si - 7 x 7 is a conducting surface, the exact surface conductivity has eluded consensus for decades as measured values differ by 7 orders of magnitude. Here we report a combined STM and transport measurement with ultra-high spatial resolution and minimal interaction with the sample, and quantitatively determine the intrinsic conductivity of the Si - 7 x 7 surface. This is made possible by the capability of measuring transport properties with or without a single atomic step between the measuring probes: we found that even a single step can reduce the surface conductivity by two orders of magnitude. Our first principles quantum transport calculations confirm and lend insight to the experimental observation.