Mechanisms and rate determining steps in plasma induced high rate CVD of silicon an germanium: similarities and differences

(Received 1st February, accepted 17 April 1992)

Abstract
The recent progress in the understanding of the reaction mechanism of plasma induced CVD of silicon from silane and the identification of the rate determining steps enabled us to significantly increases the deposition rate of device quality a-Si, c-Si and epi-Si. The mechanism is similar to that found for thermal CVD. The first step is the fragmentation of monosilane into SiH ₂ and H ₂ followed by fast insertion and formation of disilane and trisilane. The higher silanes represent the reactive intermediates for the silicon deposition because their reactive sticking coefficients are orders of magnitude larger than that of monosilane. All these features of the mechanism are well documented by experimental data from our and several other laboratories and by theoretical calculations which will be summarized here.

The understanding of plasma CVD of germanium from monogermane is less complete. Our recent data show that there are at least two different kinetic regimes with a different reaction mechanism. The data show that, unlike in the case of silicon, higher germanes are probably not involved in the process. A very preliminary interpretation of the data suggests that divalent fragments and possibly other radical species are the dominant precursors for the deposition of germanium. Significant differences between silicon and germanium are found also regarding the amorphous-to-crystalline transition.