The role of carbon on the electrical properties of polycrystalline Si_1-yC_y and Si_0.82-yGe_0.18C_y films

A comparison is made of the electrical effects of carbon in n- and p-type in situ doped polycrystalline Si_1-yC_y and Si_0.82-yGe_0.18C_y layers. Values of resistivity as a function of temperature, effective carrier concentration and Hall mobility are reported. The n-type polycrystalline Si_1-yC_y and Si_0.82-yGe_0.18C_y films show dramatic increases in resistivity with carbon content, rising from 0.044 Ω cm to 450 Ω cm (0 and 0.8% C) and 0.01 Ω cm to 2.4 Ω cm (0 and 0.6% C), respectively. In contrast, the increase in B-doped films is much less severe, rising from 0.001 Ω cm to 0.939 Ω cm (0 and 7.9% C) and 0.003 Ω cm to 0.015 Ω cm (0 and 4% C) for the Si_1-yC_y and Si_0.82-yGe_0.18C_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_1-yC_y and Si_0.82-yGe_0.18C_y layers, but linearly in the p-type Si_1-yC_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_0.82-yGe_0.18C_y layers with a C content of 4% than in equivalent Si_1-yC_y layers, which could be explained by a larger shift in trap energy toward the valence band. © 2001 American Institute of Physics.