Ultradense phosphorus in germanium delta-doped layers

Phosphorus (P) in germanium (Ge) δ-doped layers are fabricated in ultrahigh vacuum by adsorption of phosphine molecules onto an atomically flat clean Ge(001) surface followed by thermal incorporation of P into the lattice and epitaxial Ge overgrowth by molecular beam epitaxy. Structural and electrical characterizations show that P atoms are confined, with minimal diffusion, into an ultranarrow 2-nm-wide layer with an electrically active sheet carrier concentration of 4×1013 cm−2 at 4.2 K. These results open up the possibility of ultranarrow source/drain regions with unprecedented carrier densities for Ge n-channel field effect transistors.