Abstract
We have used our one-electron Green function method to study the stability of deep a1 symmetry states associated with the first-row impurities (e.g. oxygen) in Si, GaP and GaAs. We suggest a simple two-centre model which helps to identify the driving mechanism of the instability and present numerical results which shed some light upon the applicability of the Ham mechanism in diamond and zinc-blende structures. We find that—in the limit of small displacements—all a1 levels in the gap are stable irrespective of the details concerning one-eloctron potentials.