Fundamental band gap and direct-indirect crossover in Ge1-x-ySixSny alloys

The Ge-like indirect band gap of Ge1−x−ySixSny alloys has been determined over an extended y > x range using photoluminescence spectroscopy from films grown on Ge-buffered Si substrates. It is found that the compositional dependence of this transition can be fit with a bilinear expression of the form (in eV): Eind=(0.668±0.008)+(0.67±0.15)x−(1.77±0.16)y. These energies are significantly below the prediction from a simple linear interpolation between Si, Ge, and α-Sn, revealing a large negative bowing in the compositional dependence similar to that found earlier for direct transitions. The direct-indirect crossover boundary is found to lie along the compositional line y = (0.062 ± 0.014) + (0.76 ± 0.23)x, which in the limit x → 0 agrees with earlier results for the binary Ge1−ySny alloy.