A new type of deep-donor DX center, with orthorhombic C2v symmetry, was identified in III-V semiconductors. The center was predicted to occur only for anion site dopants; especially S. Its atomic structure, as deduced from  ab initio  calculations, was characterized by cation-cation dimer-bond formation. Experimental data on S-doped GaAs and GaSb were shown to support this type of DX structure. Theoretical results for DX centers with orthorhombic and trigonal symmetries, in S-doped, Se-doped and Te-doped GaSb, GaAs and GaAsAl alloys were analyzed. It was concluded that the situation was most favorable when the ionic bonding radius of the impurity was smaller than that of a host anion. Sulfur impurities in these materials were predicted to form this type of cation-cation bonded DX center preferentially. The structural parameters which were deduced from X-ray fine-structure spectroscopic measurements of S-doped GaAs were explained more satisfactorily by the cation-cation bonded DX state than by a trigonal-symmetry broken-bond DX model. It was suggested that the quenching of persistent photo-conductivity in GaSb involved the transfer of electrons from the broken-bond DX state to the cation-cation bonded DX state. The new cation-cation bonded DX center was found to be the most favorable DX state; even for Te impurities in AlGaAs alloys, when the impurity was surrounded by Al atoms.

C.H.Park, D.J.Chadi: Physical Review B, 1996, 54[20], R14246-9