We report on the growth of germanium–silicon and silicon epitaxial layers by ultrahigh vacuum/chemical vapor deposition. When aninsituclean is used, films grown at 600 °C are defect‐free by planar section transmission electron microscopy and no oxygen is detectable by secondary ion mass spectroscopy at the growth interface.Insitudoping has been studied using B2H6/H2and PH3/H2as source gases. Ge0.13Si0.87films doped with boron up to 5×1019cm−3have been grown and sharp doping transitions have been obtained. Phosphorus doping concentrations in silicon are limited by a decrease in growth rate with increasing phosphorus flow rate which is attributed to phosphorus blocking of reaction sites. A model for the decrease in growth rate with PH3/H2flow is proposed which provides a good fit to the measurements.