The role of carbon on the electrical properties of polycrystalline Si1−yCy and Si0.82−yGe0.18Cy films

Abstract

A comparison is made of the electrical effects of carbon in n-and p-type in situ doped polycrystalline Si 1Ϫy C y and Si 0.82Ϫy Ge 0.18 C y layers. Values of resistivity as a function of temperature, effective carrier concentration and Hall mobility are reported. The n-type polycrystalline Si 1Ϫy C y and Si 0.82Ϫy Ge 0.18 C y films show dramatic increases in resistivity with carbon content, rising from 0.044 ⍀ cm to 450 ⍀ cm ͑0 and 0.8% C͒ and 0.01 ⍀ cm to 2.4 ⍀ cm ͑0 and 0.6% C͒, respectively. In contrast, the increase in B-doped films is much less severe, rising from 0.001 ⍀ cm to 0.939 ⍀ cm ͑0 and 7.9% C͒ and 0.003 ⍀ cm to 0.015 ⍀ cm ͑0 and 4% C͒ for the Si 1Ϫy C y and Si 0.82Ϫy Ge 0.18 C y layers, respectively. The grain boundary energy barrier, determined from the temperature dependence of the resistivity, is found to vary as the square of the C content in the n-type polycrystalline Si 1Ϫy C y and Si 0.82Ϫy Ge 0.18 C y layers, but linearly in the p-type Si 1Ϫy C y layers. The square law dependence seen in the n-type layers for C contents up to 0.9% is explained by an increase in the grain boundary trap density due to the presence of carbon, whereas the linear relationship seen in the p-type layers for C contents between 2% and 8% is explained by a shift in the grain boundary trap energy toward the valence band. Finally, lower values of grain boundary energy barrier are obtained in p-type Si 0.82Ϫy Ge 0.18 C y layers with a C content of 4% than in equivalent Si 1Ϫy C y layers, which could be explained by a larger shift in trap energy toward the valence band.

Boron segregation and electrical properties in polycrystalline Si1−x−yGexCy and Si1−yCy alloys

“…This was seen in one previous article and attributed to an increase in the trap density at the grain boundaries caused by carbon. 20 An increased trap density would trap a much larger fraction of the active carriers, reducing the carrier concentration, and form larger potential barriers, reducing the mobility. For the growth conditions used for these layers, at carbon levels above 1% some of the carbon may be incorporating on nonsubstitutional sites during growth, and perhaps more readily form these traps.…”