Experimental and theoretical study of the crystallization of chemical-vapor-deposited mixed-phase silicon films

The deposition of polycrystalline silicon films by the low-temperature crystallization of as-deposited, mixed-phase silicon films obtained by low-pressure chemical-vapor deposition was studied. It was found that this type of material, consisting of crystalline regions embedded in an amorphous matrix, can crystallize in threefold to fourfold shorter crystallization time than that required for the complete crystallization of as-deposited amorphous films. The faster crystallization was attributed to the combination of the growth of the preexisting crystallites and the faster nucleation and growth of random crystallites in mixed-phase films. In addition, it was shown that by utilizing composite films, consisting of an amorphous layer and a mixed-phase layer, a comparable grain size to that of crystallized amorphous films can be obtained. The key process parameters, controlling the grain size and the crystallization time of composite films, were found to be the layer sequence, the deposition rates of the layers, and the thickness ratio of the layers. A mathematical model, accounting for the crystallization of composite films, was also formulated to explain the experimental trends. The theoretically predicted values by this model, for the grain size of the crystallized films, were found to be in excellent agreement with the experimental observations.