Near-band-gap photoluminescence of Si-Ge alloys

The low-temperature near-band-gap photoluminescence of Si_1-x{Ge}ₓ$ is studied over the whole composition range 0{≤}x{≤}1. We identify free- and bound-exciton processes and determine the properties of momentum-conserving phonons. From our results we determine the low-temperature band gap of the alloys. Analytical expressions are derived for the X and L bands: EgxX(x) =1.155-0.43x+0.206x² eV; EgxL(x)=2.010-1.270x eV. The intensity and the linewidth of the various excitonic transitions are found to depend only on the statistical alloy fluctuations. No preferential clustering of Si and Ge atoms is detected.