PH3 is found to chemisorb on Si(100) and the saturation coverage is strongly temperature dependent. Apparently the desorption of the hydrogen at temperatures above 400 °C can open up more Si sites for PH3 chemisorption. Both PH3 and phosphorus adsorbed on the Si surface can effectively block the coadsorption of silane molecules. The data is consistent with the observed rapid decrease in Si deposition rate during insitu doping with PH3 in a SiH4 reactor.