Electronic structure of ordered silicon alloys: Direct-gap systems.

Abstract

Density functional theory (within the local density approximation) is applied to minimize the total energy of a set of Si/(group IV) ordered alloys and to calculate the associated electronic band structures. A series of fully relaxed Si/Sn alloys show direct gaps when constructed on the (100) surface of silicon in cubic-based superlattices, with the band gap of the Si/Sn alloys decreasing nearly linearly with increased atomic Sn fraction. Equivalent calculations performed with Ge and C substitutions are predicted to have direct and indirect gaps, respectively. Ternary alloys of Si/Ge/Sn and Si/Ge/C have band-structure features dominated by either the Sn or C substitutions having band gaps of the respective binary alloys. The magnitude of the conduction-band curvature (the effective mass) at the zone center of these alloys shows a strong variation dependent on the particular substitution. Energy barriers of the minor constituents are computed to estimate the stability of the binary alloys with resulting barriers near 0.25 eV for Sn and Ge atoms in a silicon matrix. \textcopyright{} 1996 The American Physical Society.