Temperature dependence of hyperfine coupling of the anion antisite in III-V compounds

Temperature-dependent electron-paramagnetic-resonance experiments have been performed on the arsenic antisite defect in electron-irradiated GaAs. The 4.5% decrease of the central hyperfine coupling constant A upon heating from 4 to 250 K is much too large to be attributable to the thermal expansion of the lattice. We analyze the results in a simple molecular model, and attribute the temperature dependence of A to admixing of excited p-like $T_2$ state into the s-like $A_1$ states by the orbit-phonon interaction. This interaction is responsible for a large reduction of the local stiffness coefficient associated with the restoring force after a displacement of the As antisite to an off-center position. These results suggest that, although the isolated As antisite is well behaved, a small perturbation may be sufficient to induce a strong lattice relaxation and metastability, in agreement with a recent model for the EL2 center. The extension of this analysis to the case of the phosphorous antisite defect in GaP suggests that this defect is at the limit of a Jahn-Teller instability.