Selective low-power plasma decomposition of silane-methane mixtures for the preparation of methylated amorphous silicon.

Abstract

We report on a systematic study of the deposition and the physical properties of amorphous-silicon--carbon alloys produced by glow discharge from silane-methane mixtures. We define a ``low-power'' regime of preparation in which the chemistry of deposition is separated from the chemistry of the plasma. In this regime there is no primary decay of the methane, which then acts as a buffer gas. The carbon is incorporated in the solid only by reaction of the methane gas with the active species produced by the plasma decomposition of the silane. In this mode of preparation, we find the following. (1) The physical properties (in particular, the optical properties) are insensitive to the preparation conditions, depending mostly on the methane-to-silane ratio. (2) There is a smooth ``chemical'' incorporation of carbon in the amorphous silicon network, giving an alloy with good semiconducting properties. (3) The amount of carbon that can be incorporated is limited to less than 40 at. %, whereas in the high-power regime alloys can be produced with arbitrary [C]/[Si] proportions. (4) In the low-power regime the carbon is incorporated mostly in the form of methyl groups ${\mathrm{CH}}_{3}$, so that the material produced in this condition should be labeled as ``methylated amorphous silicon.''