It was shown that an exciton was self-trapped at the ABC optical center by a lattice relaxation of 0.035eV. The self-trapping arose from the electron component of the exciton, which was highly localized within about 1 atomic spacing of the core of the optical center. The effects of compressive stresses upon the photon energy and polarization of the ABC luminescence band were reported for temperatures of 4.2 and 20K. They could be fitted accurately by expressing the bound exciton states as perturbed band states of Si. The deformation potentials which were required for the hole component of the exciton were very close to those of the perfect lattice. This was consistent with the hole being only weakly localized on the center, while the deformation potential of the electron was modified by its strong localization. The center was considered to be a particularly clear case of a so-called pseudo-acceptor iso-electronic center, with properties that appeared to be highly localized or weakly localized, depending upon the particular measurement. The importance of lattice relaxation in binding the exciton implied that symmetry determinations which were made by means of optical measurements might not reflect the true symmetry of the unrelaxed core of the defect.

G.Davies, M.Z.Iqbal, E.C.Lightowlers: Physical Review B, 1994, 50[16], 11520-30