Methods of depositing compound semiconductors onto substrates are disclosed, including directing gaseous reactants into a reaction chamber containing the substrates, selectively supplying energy to one of the gaseous reactants in order to impart sufficient energy to activate that reactant but insufficient to decompose the reactant, and then decomposing the reactant at the surface of the substrate in order to react with the other reactants. The preferred energy source is microwave or infrared radiation, and reactors for carrying out these methods are also disclosed.