The spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed. These defects, the E′-centers (oxygen vacancies with a trapped hole or 3-fold-coordinated silicons), different variants of diamagnetic `ODCs' (oxygen-deficiency centers), and their Ge-related analogs play a key role in the fiber-optic Bragg grating writing processes. The controversy surrounding the structural models for the Si- and Ge-related ODCs is discussed and the similarity between the bulk and surface point defects in silica is emphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen vacancies and E′-centers are discussed. The effects of glassy disorder have a profound effect on defect properties and interconversion processes in silica.

Optically active oxygen-deficiency-related centers in amorphous silicon dioxide

The defect structure of glasses is discussed primarily in the context of vitreous silicon dioxide (v-SiO2) Our current understanding of the nature of defects in v-SiO2 is reviewed on the basis of the available electron spin resonance data, and a unified picture of radiation-damage processes is formulated for high-purity silicas of both low- and high-OH contents The critical role of hydrogen in the latter case is noted Several presently outstanding questions relating to the optical spectra of defects in fused silica are highlighted with particular emphasis on data obtained in the fiber optic geometry Some potential answers to these questions are suggested, and an attempt is made to chart the directions of the next 20 years' research in glass defect structure

Defect structure of glasses: Some outstanding questions in regard to vitreous silica

Luminescence centers causing optical absorption bands in the 5 eV region in oxygen-deficient pure (‘Si-doped’), as well as in Ge- and Sn-doped glassy SiO2 were studied. The spectral and particularly kinetic data show that in all cases luminescence centers with closely similar properties exist which constitute an isoelectronic series. They are attributed to defects, formed by electrically neutral twofold coordinated Si, Ge, and Sn atoms (T20 centers) in glassy SiO2. Twofold coordinated Si causes the characteristic ‘B2’ optical absorption band at 5.03 eV. Upon reaction with atomic hydrogen, T20 centers convert to the well known ‘H(I)’, ‘H(II)’ and ‘H(III)’ ESR-active defects. The alternative models for these defects are discussed.

Isoelectronic series of twofold coordinated Si, Ge, and Sn atoms in glassy SiO2: a luminescence study

It is shown that the green/blue luminescence which is found in oxidized nanocrystalline and ‘‘porous’’ silicon is associated with the presence of OH groups adsorbed on structural defects in the SiO2 network

Origin of the green/blue luminescence from nanocrystalline silicon

Electron bombardment of thick pure SiO2 induces the buildup of a negative charge which can be observed through a ‘‘mirror’’ effect in a conventional Auger scanning microscope. A mechanism for the creation of this charge is proposed in terms of trapping of an electron in defects due to the irradiating beam. The influence of temperature is studied on amorphous and monocrystalline SiO2. The temperature dependence of the existence of high negative charge shows around 270 °C an anomalous effect which depends on the irradiation time. The role of electronic excitation to produce defects in silica is discussed.

A.B. Pakovich

Papers

A new intrinsic defect in amorphous SiO2: Twofold coordinated silicon