Origin of nonhydrogenic hole transition in boron-doped silicon: An STM/S study

The origin of the nonhydrogenic hole transition at $\ensuremath{\sim}23\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ in the boron-doped silicon is still under debate. Here we employ the scanning tunneling microscope to uncover that a boron-containing complex on the silicon (111) surface shows three shallow acceptor states. Among them, the first excited state has the typical energy of the nonhydrogenic hole transition. We then show that this energy gap originates from the strain-induced mixing of the valence bands, unraveling a distinct origin for the nonhydrogenic hole transition in the boron-doped silicon.