Site-Dependent Ambipolar Charge States Induced by Group V Atoms in a Silicon Surface

We report that solitary bismuth and antimony atoms, incorporated at Si(111) surfaces, induce either positive or negative charge states depending on the site of the surface reconstruction in which they are located. This is in stark contrast to the hydrogenic donors formed by group V atoms in silicon bulk crystal and therefore has strong implications for the design and fabrication of future highly scaled electronic devices. Using scanning tunnelling microscopy (STM) and density functional theory (DFT) we determine the reconstructions formed by different group V atoms in the Si(111)2 x 1 surface. Based on these reconstructions a model is presented that explains the polarity as well as the location of the observed charges in the surface. Using locally resolved scanning tunnelling spectroscopy we are furthermore able to map out the spatial extent over which a donor atom influences the unoccupied surface and bulk electronic states near the Fermi-level. The results presented here therefore not only show that a dopant atom can induce both positive and negative charges but also reveal the nature of the local electronic structure in the region of the silicon surface where an individual donor atom is present.