Stability ofDXcenters inAlxGa1−xAs alloys

The band-gap dependence of the binding energy of Si-induced DX centers in ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As alloys was determined from an ab initio total-energy approach. Band-structure modifications resulting from changes in alloy composition and hydrostatic pressure were examined. The ‘‘negative-U’’ reaction, 2${\mathit{d}}^0$→${\mathit{d}}^{+}$+${\mathit{DX}}^{\mathrm{-}}$, where d and DX represent fourfold- and threefold-coordinated states of the Si donor, respectively, was found to be exothermic for x≥18% in ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As alloys and for P≥23 kbar in pure GaAs, in good agreement with experimental data.