Origin of the unusually large band-gap bowing and the breakdown of the band-edge distribution rule in the SnxGe1-x alloys

The unusual nonlinear behaviors of the band gaps in SnxGe1-x alloys are investigated using first-principles calculations. We show that the large bowing of the direct band gap is induced by the disordering effect. Moreover, we calculated individual contribution of the band-edge states and found that the bowing of the conduction band edge is much larger than the bowing of the valence band edge, although the natural valence-band offset between Ge and Sn is larger than the natural conduction-band offset. The breakdown of the band-edge distribution rule is explained by the large lattice mismatch between Ge and Sn and the large deformation potential of the band-edge states.