Electronic structure of Si1-yCy and Si1-x-yGexCy alloys with low C concentrations

Relaxed Si_1-yC_y and Si-lattice-matched Si_1-x-yGe_xC_y alloys with low C concentrations of 1.6% and 3.1% have been studied from first principles. The bottom of the conduction bands in relaxed Si_1-yC_y lies on the line Δ, as it does in Si. The band gap is linearly dependent on the C concentration and the reduction is as little as -0.9y eV. On the other hand, Si-lattice-matched Si_1-x-yGe_xC_y has a direct gap and the band gap is reduced by -3.9y eV from that in relaxed Si_1-xGe_x. The free energy of Si_1-x-yGe_xC_y shows that the atoms are arranged at random, however, this arrangement does not affect the result for the band gap in the alloy.