100 ppm PH3 diluted in hydrogen is used as the n-type dopant gas in Si and Si1−xGex epilayers grown by ultrahigh vacuum chemical vapor deposition (UHVCVD) using Si2H6 and GeH4. The phosphorus concentration in Si increases linearly at a small PH3 flow rate and becomes nearly saturated at higher flow rates, while the phosphorus concentration in Si1−xGex only shows a nearly linear behavior with PH3 flow rate. The growth rates of Si and Si1−xGex epilayers decrease seriously (∼50%) and slightly (∼10%) with the increase of PH3 flow rate, respectively. These results can be explained by a model based on the enhancement of hydrogen desorption rate at smaller PH3 flow rates and different levels of the effects of phosphorus blocking of surface-activated sites between Si and Si1−xGex epilayers at higher PH3 flow rates.