Understanding the amorphous-to-microcrystalline silicon transition in SiF₄/H₂/Ar gas mixtures

We report on the growth of microcrystalline silicon films from the dissociation of SiF₄/H₂/Ar gas mixtures. For this growth chemistry, the formation of HF molecules provides a clear signature of the amorphous to microcrystalline growth transition. Depositing films from silicon tetrafluoride requires the removal of F produced by SiF₄ dissociation, and this removal is promoted by the addition of H₂ which strongly reacts with F to form HF molecules. At low H₂ flow rates, the films grow amorphous as all the available hydrogen is consumed to form HF. Above a critical flow rate, corresponding to the full removal of F, microcrystalline films are produced as there is an excess of atomic hydrogen in the plasma. A simple yet accurate phenomenological model is proposed to explain the SiF₄/H₂ plasma chemistry in accordance with experimental data. This model provides some rules of thumb to achieve high deposition rates for microcrystalline silicon, namely, that increased RF power must be balanced by an increased H₂ flow rate