Predicted Modifications in the Direct and Indirect Gaps of Tetrahedral Semiconductors

The charge density of GaAs was studied at various k points and for various bands by the augmented-spherical-wave method, using muffin-tin spheres at the Ga, the As, and, additionally, the interstitial sites. The lowest Xc conduction-band points were found to be unique in having a high "charge" density in the interstitial spheres. It has therefore been predicted and verified that the Xc points move up in energy relative to the Γc point when closed-shell atoms (like He) are substituted at the interstitial sites. The calculations also indicate indirect-direct conversion for SiHe.