ALLOYING EFFECTS ON THE OPTICAL ABSORPTION EDGE AND HYDROGEN ABSORPTION OF SPUTTERED a-Si_xGe_{1 - x}:H.

Hydrogenated amorphous silicon germanium alloy films were prepared by co-sputtering from two semi-disc targets. The films are about 1 mu m thick, and 12. 5 cm long. The composition along the length of the film varies continuously from 70 to 5 at. % Si. The optical absorption and photoconductivity as a function of wavelength were measured. The energy gap as evaluated from these measurements decreases linearly from 1. 65 ev for 70 at. % Si to 1. 1 ev for 7 at. % Si. The hydrogen content of the samples varied between 5 at. % for the germanium-rich end to 27 at. % for the silicon-rich end. It is found that hydrogen bonds preferentially to silicon over germanium by a factor of 10-15 except at the silicon-rich end (70 at. % Si) where the factor decreases to about 5. On the whole the characteristics of the authors' sputtered films are comparable to those reported for similar alloys prepared by glow discharge. An important advantage of the sputtered films is, however, that a whole range of alloy concentration can be prepared at one run.