BAND-GAPS IN SOME GROUP-IV MATERIALS - A THEORETICAL-ANALYSIS

We have studied the effects of expansion of the lattice on band-edge levels in Si, Ge, Sn, and SiSn. The variation in the gap in the Si-Ge-Sn series can be explained as moving along a universal curve of the variation of band-edge levels with the combined effect of volume and potential. For all of the materials studied, the gap at low volumes is from GAMMA to X. There is a small range of volumes where the gap is from GAMMA to L, and at high volumes the gap is direct until it closes. Thus, in addition to the usual process of applying postive pressure to close the gap, applying negative pressure also causes band overlap and metallization.