The in situ n-type doping of Ge thin films epitaxial grown on Si substrates was limited to 1019/cm3 by phosphorous out-diffusion during growth at 600C. By studying the phosphorous diffusion in Ge with different background doping, it was found that the diffusion coefficient was extrinsic and was enhanced 100 times in Ge doped at 1019/cm3 compared to intrinsic diffusivity. To achieve higher phosphorous concentration, delta-doped layers were used as a dopant source for phosphorous in-diffusion. It was shown that  the doping level was a result of the competition between in-diffusion and dopant loss. The high diffusivity at high n-type carrier concentration led to a uniform distribution of phosphorous in Ge with the concentration above 3 x 1019/cm3.

High Phosphorous Doped Germanium: Dopant Diffusion and Modeling. Y.Cai, R.Camacho-Aguilera, J.T.Bessette, L.C.Kimerling, J.Michel: Journal of Applied Physics, 2012, 112[3], 034509