The impact of Si delta layer on phosphorus (P) diffusion in germanium (Ge) is investigated. A Ge cap / in-situ P-doped Ge / Ge buffer layer stack is deposited and post-annealed using a single wafer reduced pressure chemical vapor deposition (RPCVD) tool. The P doping level in Ge is ~5×1019 cm-3. In the case of samples without Si delta layer, P diffusion / segregation and desorption from the Ge surface happened during Ge cap layer deposition at 550°C resulting in dopant dose reduction and profile broadening. By interposing the P-doped Ge layer by Si delta layers, the P diffusion is suppressed. The diffused P is piled-up at the position of the Si delta layers. The P diffusion suppression effect by the Si delta layer is observed after postannealing even at 650°C. This effect is pronounced by increasing the Si dose. XRD-reciprocal space mapping shows that the Si delta layer is pseudomorphic to the Ge lattice, indicating that the Si atoms are on lattice site. After postannealing, Si diffusion is observed but no Ge crystal degradation was evident. Enhanced Si diffusion is observed by presence of P. Based on these results, the P diffusion suppression seems to be caused by the replacement of substitutional Si by diffused P.