A time-resolved optical emission spectroscopic study identified transient behavior of the excited SiH emission in a parallel plate SiH4 H2 plasma. The transient behavior could be prevented by filling the background gas with H2 prior to plasma ignition. Applying this condition, state-of-the-art microcrystalline silicon (μc-Si:H) could be deposited irrespective of the applied H2 flow, ultimately demonstrated by a 9.5% efficient solar cell deposited from pure SiH4. The results are discussed in terms of SiH4 back diffusion: an initial diffusion flux of SiH4 from the reactor's dead volume back into the plasma. © 2005 American Institute of Physics.