Effects of Si incorporation on the structural change of a-BₓSi₁₋ₓ alloy films

Abstract

Amorphous BₓSi₁₋ₓ films can be easily prepared by low‐pressure chemical vapor deposition over the whole range of x from 0 to 1. In this article, the structural change of BₓSi₁₋ₓ films (0≤x≤1) was studied by x-ray diffraction and infrared (IR) absorption experiments. It was found that these two methods are complementary to each other. X-ray results showed that when x is decreased, there is a gradual transition from the α-B structure, through the SiB₄ structure, to the amorphous silicon structure. The transition to α-Si structure is complete at a surprisingly high boron concentration of around 40 at. %. Infrared data also revealed an unexpected result—that the presence of boron suppresses the formation of Si—H types of bonds in high silicon content films, in strong contrast to films prepared by the glow discharge method. Another interesting feature of the IR absorption spectra is the predominance of an absorption band, related to the presence of boronlike icosahedral clusters, in all films with boron concentration to as low as 17.6 at. %. Such clusters may be responsible for the easy formation of an impurity band lying about 0.2 eV above the valence mobility edge, as observed by transport measurements of boron-doped α-Si films prepared by low-pressure chemical vapor deposition.