MODELS OF THE SELF-TRAPPED EXCITON AND NEAREST-NEIGHBOR DEFECT PAIR IN SIO2

The adiabatic potential surface for triplet exciton self-trapping and Frenkel-defect pair creation in SiO2 has been studied with use of a periodic supercell model and a self-consistent intermediate neglect of the differential overlap method for open shell. The results obtained confirm completely the self-trapped-exciton (STE) model obtained earlier [A. Shluger, J. Phys. C 21, L431 (1988)], making it possible to interpret in greater detail the nature of its optical absorption and luminescence. It is shown that in the case of the SiO2 structure considered (the ideal -cristobalite) the formation of nearest-neighbor defect pair (NNDP) from the STE state requires surmounting an energy barrier of about 1 eV. The electronic component of a NNDP comprises two E-like centers localized on two silicons surrounding a single oxygen vacancy, one of which is perturbed by the hole component of the -Si-O-O-Si- peroxide bridge. STE and NNDP are shown to belong to the same adiabatic energy surface with the minima corresponding to NNDP to be deeper than that for STE. Calculated spectra of optical transition energies of NNDP include two transitions corresponding to electronic-component excitation (6.1 eV for one E center and 5.6 eV for another E center, perturbed by a peroxide bridge), charge-transfer transition from a E center to the peroxide bridge (Emax=4.9 eV), and hole-component excitation (Emax=1.7 eV). The driving forces of the triplet exciton self-trapping and Frenkel-defect formation are discussed. © 1990 The American Physical Society.