Towards a single P donor in Si

Individual P donors in Si form the basis of several schemes for realizing solid-state qubits. Technologically, all such schemes rely on the precise positioning of P donors in the Si crystal and fabrication of local gates, only a few tens of nanometers wide. Towards this goal, we have demonstrated that the spatial resolution of STM-lithography allows precise positioning of donors on the sub-nm length scale and also fabrication of \textit{all-epitaxial, planar }quantum dot (QD) architectures, with source, drain, and gate patterns of precisely defined dimensions. Here, we report the STM-lithography fabrication of an ultra-small QD consisting, in the extreme limit, of a single P donor. Transport spectroscopy of the QD-device at mK temperatures shows stable Coulomb oscillations with an addition energy (around zero gate bias) of 44$\pm $2 meV. This value corresponds to the difference in the binding energies of the 1-electron (D$^{0})$ and the 2 --electron (D$^{-})$ states of a P donor in Si. The first two D$^{0 }$excited states have also been identified.