Zone-folded direct band gaps and energetics in strained (Si)4/(Ge)4 superlattices

Based on first-principles pseudopotential calculations, we analyze the energetics and the evolution of band energies as a function of Ge-Ge interlayer spacings for the (Si)4/(Ge)4 superlattice pseudomorphic to Si(001). It is demonstrated that the recently proposed occurrence of stretched bulklike Ge-Ge bonds is energetically unfavorable, and yields transition energies in disagreement with electroreflectance and photocurrent data. © 1990 The American Physical Society.