Growth mechanism of microcrystalline silicon at high pressure conditions

A diagnosis of the plasma process for high rate deposition of microcrystalline silicon by very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) is explored in this article. The Hα/Si∗ intensity ratio measured by optical emission spectroscopy is a fingerprint of the amorphous to crystalline transition. Irrespective of the mode of the plasma (α or γ type, different power, pressure and flow) the transition occurs at the same value of this ratio. Moreover, the depletion condition is not necessary for the crystalline regime of growth (at least for the VHF cases studied here). The intensity of hydrogen treatment per deposited Si species is the most dominant prerequisite for nucleation rather than radical/ionic species. An increase of the deposition rate (also Si∗ intensity) is achieved at high pressures; however, a balance between the increase in dissociation rate due to increased silane partial pressure and lowering of dissociation due to reduced electron temperature limits the optimum deposition rate. The crystallinity is reduced at high pressures. One-dimensional plasma modeling shows that the loss of atomic hydrogen concentration due to abstraction reactions (H + SiH4=SiH3 + H2) is negligible and thus does not explain the loss of crystallinity at high pressures (obtained from the modeling). A monotonous decrease of Hα/Si∗, that explains the loss of crystallinity with increasing pressure is attributed to a decrease in electron temperature.