Precise measurements on the growth rate and the hydrogen coverage on the growing surface of SiGe epitaxial film by silane/germane gas-source molecular beam epitaxy (GSMBE) have been performed for the germane mixing ratio X(g) of 0%-6%. For all X(g), the growth-rate Arrhenius plot showed a distinct separation into the high-temperature region with a lower activation energy and the low-temperature region with a higher activation energy, similar to silane GSMBE. The growth rates in the low-temperature region increased with X(g), while those in the high-temperature region peaked at X(g) = 0.8% and decreased thereafter. As a result the transition temperature shifted towards lower temperatures. The activation energy in the low-temperature region stayed almost unchanged or increased with X(g), which is quite contrary to the previous understandings and id thus discussed. A model is presented to describe the hydrogen desorption process from SiGe surfaces, which explains both the growth rate in the low-temperature region and the temperature-programed-desorption (TPD) spectra obtained from the quenched growing surface. The role of Ge in the low-temperature region is concluded to enhance processes involved in hydrogen desorption from Si atoms.