Energetics ofDX-center formation in GaAs andAlxGa1−xAs alloys

The energetics of the shallow-deep transition of donor states in ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As alloys and the problem of Fermi-level pinning by DX centers in highly doped GaAs are examined via simple theoretical models and ab initio self-consistent pseudopotential total-energy calculations. The atomic displacements responsible for the formation of DX centers in Si- and S-doped GaAs are determined. Defect formation is found to be accompanied by a large bond-rupturing lattice distortion. The results of our pseudopotential calculations indicate that DX is a highly localized and negatively charged defect center. The atomic structure for DX is shown to provide a satisfactory explanation for the large Stokes shift between its optical and thermal ionization energies. An important conclusion of our study is that DX centers are an unavoidable feature of substitutional dopants and that the formation of these defects may be suppressed via the introduction of interstitial donors which are unlikely to undergo similar structural transformations. This implies that the normal doping procedure which relies on group-IV and -VI substitutional impurities needs to be modified. Possible choices for interstitial dopants from group-III and -V elements are examined.