EFFECTS OF SI INCORPORATION ON THE STRUCTURAL-CHANGE OF A-BXSI1-X ALLOY-FILMS

Amorphous BxSi1-x films can be easily prepared by low-pressure chemica l vapor deposition over the whole range of x from 0 to 1. In this arti cle, the structural change of BxSi1-x films (0 less-than-or-equal-to x less-than-or-equal-to 1) was studied by x-ray diffraction and infrare d (IR) absorption experiments. It was found that these two methods are complementary to each other. X-ray results showed that when x is decr eased, there is a gradual transition from the a-B structure, through t he SiB4 structure, to the amorphous silicon structure. The transition to a-Si structure is complete at a surprisingly high boron concentrati on of around 40 at. %. Infrared data also revealed an unexpected resul t-that the presence of boron suppresses the formation of Si-H types of bonds in high silicon content films, in strong contrast to films prep ared by the glow discharge method. Another interesting feature of the IR absorption spectra is the predominance of an absorption band, relat ed to the presence of boronlike icosahedral clusters, in all films wit h boron concentration to as low as 17.6 at. %. Such clusters may be re sponsible for the easy formation of an impurity band lying about 0.2 e V above the valence mobility edge, as observed by transport measuremen ts of boron-doped a-Si films prepared by low-pressure chemical vapor d eposition.