Deposition rates over 10 A/s can be obtained for device-grade microcrystalline silicon with the VHF-GD (Very High Frequency - Glow Discharge) method, as applied to hydrogen-diluted silane plasmas. The morphological phase transition from amorphous to microcrystalline silicon can be controlled by varying e.g. the applied VHF-power or the dilution level. The glow of the plasma associated with this morphological phase transition is monitored by optical emission spectroscopy (OES). Thereby, we find that the decisive criteria for µc-Si:H growth is the OES-ratio between the Hα-line (atomic hydrogen line at 656nm) and the SiH* - line (at 414 nm); in our case, as soon as this ratio becomes lower than 1.7 one obtains microcrystalline growth.