Synchrotron‐radiation (SR) ‐excited chemical‐vapor deposition of Ge crystals on Si(100) using GeH4has been investigated. When SR is incident parallel to the surface, reaction between a photofragment and a surface hydride yields constant deposition rate. Above 280 °C, thermal growth rate limited by H2desorption at the surface is superimposed. When SR is perpendicularly directed to the surface, the growth is enhanced and the activation barrier is reduced. Electronic excitation of admolecules may open an efficient bond rearrangement pathway to densify the hydrogen‐rich Si film. The Ge film is uniform at high pressures due to surfactant effect of hydrogen atoms, but when islanding occurs with decreasing hydrogen coverage, the activation energy further decreases. Isothermal Si/Ge multilayer growth at 300 °C is demonstrated by alternately feeding GeH4and Si2H6. ©1996 American Institute of Physics.