Composition and structure of semi-insulating polycrystalline silicon thin films

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Abstract
The structural properties of semi-insulating polycrystalline silicon (SIPOS) are reviewed. The models derived from physical examinations have been used as the basis for physical models of electrical conduction, and two possibilities have emerged. Either the material is viewed as a continuous network of amorphous silicon with oxide regions interspersed or, vice versa, an oxide network with silicon regions interspersed. In order to establish which model is correct, we have examined SIPOS films containing between 7 and 51 at.% oxygen. Rutherford backscattering and X-ray photoemission are the main tools employed. Both as-deposited films, and films annealed such that changes in crystallinity are observed, have been examined and the following conclusions drawn: SIPOS containing less than about 30 at.% oxygen is mainly amorphous silicon with oxide islands and crystallization of the silicon is evident after annealing at 900°C; SIPOS containing more than 38 at.% oxygen is mainly amorphous silicon oxide containing isolated regions of amorphous silicon; annealing at 1100°C is necessary before crystallisation of the silicon occurs. Commonsense arguments are employed to show that a transition from one structure to the other should occur between these concentrations of oxygen. The nature of the oxide has not been unambiguously determined but, as far as can be shown, it does not appear to change with annealing.