Origin of Pseudo Second Order Reaction in a-Si:H Growth

Abstract

It is generally accepted that a pair of SiH3 radicals are responsible for the film growth reactions of hydrogenated amorphous silicon, namely a first SiH3 radical to pick up surface-terminating H leaving active site on the growing surface, and a second SiH3 to stick there. Since the first reaction step is rate determining, the film growth rate is proportional (1st order) to the SiH3 density. However, in some case, 2nd order dependence on SiH3 density is reported, especially when the film growth rate (and thus the SiH3 density) is quite low. This funny behavior contradicts the general concept that a higher order reaction will take place when the number density of the concerning species is high. In this report, a detailed analysis of these growth reactions based on the method of steady state is given to prevail that the origin of such a pseudo 2nd order dependence is clearly explained by introducing the reverse reaction to the first step that determines the growth rate. In addition, discussion is presented to show that such a reverse reaction is plausible.