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

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

The current controversy over the nature of the centers giving rise to the 1.9 eV photoluminescence (PL) band (the R-band), the suggested defect models and the relevant experimental data are briefly reviewed. The luminescence emission, excitation and polarization spectra of neutron-irradiated synthetic silica were studied between 6 and 300 K using site-selective dye-laser and Ar ion laser excitation. Resonant zero-phonon lines (ZPL) were observed below 80 K both in luminescence emission and excitation spectra in the 1.9–2.1 eV region. A vibration line in emission spectra 890 cm−1 below the ZPL energy is attributed to the symmetric stretching vibration of the silicon-non-bridging oxygen bond in the ground electronic state of the non-bridging oxygen hole center. A similar line 860 cm−1 above the ZPL in the excitation spectra corresponds to the same vibration in the excited state. The intensities of the resonant ZPLs are dependent on the excitation energy and show a nearly Gaussian distribution with the peak at 1.935 ± 0.01 eV and halfwidth 82 ± 7 meV. In the zero approximation, this distribution describes the concentration distribution of the PL centers with the respective energies of the excited electronic state. The 4.8 eV excitation band of the 1.9 eV PL is complex, due to different electronic transitions. No ZPLs or vibrational structures are observed under excitation with KrF excimer laser or Xe lamp in this band. The optical absorption in this region is due to overlapping bands of several different defect centers. The low-temperature luminescence bands at 2.05–2.1 eV and 2.35–2.4 eV, excited by the green (2.41 eV) and blue (2.71 eV) Ar ion laser lines, have a nature different from the 1.9 eV band. Several different defects contribute to the 2.0 eV optical absorption band in irradiated glassy SiO2.

The origin of the intrinsic 1.9 eV luminescence band in glassy SiO2

An electron spin resonance (ESR) study has been carried out of the influence of postdeposition heating on stacks of (100)Si with nanometer-thick dielectric layers of SiOx and atomic-layer-deposited Al2O3, ZrO2, and HfO2. This reveals upon postdeposition oxidation (PDO) in the range 650–800°C the appearance of various SiO2-characteristic defects, including E′, EX, and a 95-G doublet, in different intensities with increasing anneal temperature, attesting to significant modification and∕or additional SiOx interlayer growth. The revealed defects enable probing of the quality of the interlayer on atomic level. In terms of the E′ criterion, it is found that the SiOx interlayer formed in (100)Si∕SiOx∕ZrO2 is drastically inferior to standard thermal (100)Si∕SiO2, exhibiting over one order of magnitude more O-deficiency (E′) centers. The (100)Si∕Al2O3 and (100)Si∕HfO2 systems appear more robust with better interlayer properties. Particularly in the (100)Si∕HfO2 structure, both the E′ and EX densities remain small ...

A. A. Bobyshev

Papers

Twofold coordinated Si and Ge atoms and “hydrogen” paramagnetic centers in amorphous SiO2 and GeO2