The effect of Ge on the P doping has been investigated by the in-situ observation of surface P in the P-doped Si growth using Si2H6 and PH3. If the P-doped Si growth is performed without Ge, PH3 adsorption reaches saturation at the higher P-doping concentrations exceeding 3 x 10(18)/cm(3). With Ge the growing surface, Ge diminishes the self-limiting behavior of the PH3 adsorption on the Si surface and enhances the PH3 adsorption. The enhancement of the P doping in the SiGe growth is caused by an increase of surface P coverage due to the enhancement of PH3 adsorption.