Calculated densities of states (DOS's) and dielectric functions epsilon 2( omega ) of crystalline Si1-xGex alloys (x=0, 1/3, 2/3, 1) with 12 atoms per unit cell and space group P43/212 (simple-tetragonal structure with 12 atoms per unit cell (ST-12)) are presented. DOS results compare favorably with the experimentally known features of the electronic structure of amorphous alloys. For the optical spectra the authors find that the shift to smaller energies of the main absorption edge as compared to those of diamond-type crystals and the shrinking of the 'optical gap' with Ge alloying are quantitatively explained by the model. It is further found that the calculations in the ST-12 structure agree better with the measured data for the amorphous phase in the case of Si than for Ge.
 

Physical Review B, 38 1378-83, 1988.