The present investigation concerns Si-(1-x)Ge-x thin film growth that, at certain deposition conditions, is characterized by nonlinearity in the partial growth rate of germanium versus the increase of the germane (GeH4) content in the reaction gas mixture. A linear increase had previously been understood as caused by a reaction rate limited growth mechanism. The nonlinear increase of the partial growth rate of germanium versus the GeH4 partial pressure will be explained by means of an adsorption controlled growth mechanism based on competitive adsorption of both silane and Transition from one to the other growth mechanism is possible as shown by the discussion of corresponding experimental growth results.