New amorphous Si Sn alloys have been prepared using a dc cathodic sputtering technic '-' for 0 < x < 0.12. Routine characterization measurements were performed, leading to establish for the first time the existence of these new amorphous semiconductors. Both the average and optical gaps decrease with increasing Sn content, with the latter extrapolating to 0 at x % 0.5. The high sensibility of a-Si electronic structure to Sn substitution is discussed in the framework of the tight binding

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... , stressing the importance of the atomic relativistc corrections.