EXCITON SELF-TRAPPING AT AN ISOELECTRONIC CENTER IN SILICON

We show that an exciton is self-trapped at the well-known ABC optical center in silicon by a lattice relaxation of 35+/-3 meV. The self-trap ping arises from the electron component of the exciton, which is highl y localized within about one atomic spacing of the core of the optical center. The effects of compressive stresses on the photon energy and polarization of the ABC luminescence band are reported for temperature s of 4.2 and 20 K. They can be fitted accurately by expressing the bou nd exciton states as perturbed band states of the silicon. The deforma tion potentials required for the hole component of the exciton are ver y close to those of the perfect lattice, consistent with the hole bein g only weakly localized on the center, while the deformation potential of the electron is modified by its strong localization. The center is thus a particularly clear case of a ''pseudoacceptor'' isoelectronic center, with properties that appear to be highly localized or weakly l ocalized depending on the particular measurement. The importance of th e lattice relaxation in binding the exciton implies that symmetry dete rminations by optical measurements may not measure the symmetry of the unrelaxed core of the defect.