Atomistic origin of amorphous-structure-promoted oxidation of silicon

The atomic arrangements in the crystalline silicon (c-Si) and amorphous silicon (a-Si) are distinctly different, which affects the surface restructuring during oxidation and the resulting Si/oxide interfacial structures. Here, based on reactive molecular dynamics simulations, we elucidate that the amorphous-structure-promoted oxidation of silicon is associated with the structural defects in the a-Si. Randomly distributed structural defects on the surface of the a-Si not only induce a selective adsorption of O-2 which facilitates the dissociation, but also cause the oxides to nucleate like islands and combine into the oxide film, in comparison to the layer-by-layer oxide growth on the c-Si surface. Moreover, the defects in the a-Si provide an easy pathway for the penetration of oxygen into the a-Si structure, thus resulting in higher oxidation rate and thicker oxide layer than those in the c-Si. The phase separation and thermal crystallization of the amorphous oxide films at high temperatures are also discussed in depth. This study provides a better understanding of the crucial role of atomic structure in determining the oxidation mechanism, the oxide growing kinetics and the resulting oxide structures on silicon, which suggests the key to fabricate ultra-thin oxide films to fulfill the specific requirements of microelectronic and photovoltaic applications.