In this paper we discuss recent advances in the theory of intrinsic defects in amorphous SiO2. We pay particular attention to the E' centers, and to the oxygen-related hole traps.

It is shown that defects can be identified in liquids, that they are related to atomic diffusion, and a model is described whereby defects give rise to the observed relaxation behavior of viscous liquids. A review is presented of defects in silicate and fluoroberyllate liquids, as studied by computer simulation. It is found that defects have the form of overcoordinated or undercoordinated atoms, and that atomic diffusion occurs only at the site of defects. The manner in which an Ising model can represent the detect structure of viscous liquids to a first approximation is presented. The results of Monte Carlo simulations of a 2-dimensional kinetic Ising model are described. The behavior of this model is qualitatively the same as that of viscous liquids.

Molecular dynamics computer simulations were used to study surfaces of pure silica glass. The potentials used here were those previously established to model bulk silica and have been extended to study surface relaxation in a perfect vacuum. A large number of surfaces were made using different starting configurations; system sizes, and cooling procedures. Following “fracture”, many broken bonds rearranged in response to the changes in the net forces in the surface region. After this reconstruction, the simulations showed the expected general features observed experimentally, such as a prevalence of oxygen atoms at the outermost surface, non-bridging oxygens, and strained siloxane bonds. Three fold silicons (similar to e’ centers) were initially present in the “fractured” surfaces but most often were incorporated into the network tetrahedrally after reconstruction. Other defects produced during the reconstruction were five coordinated silicons and more importantly, edge sharing tetrahedra, forming the strained siloxane bonds. Bond angles and bond lengths for each defect were determined, showing good agreement with previously published results as well as providing new information. Finally, estimations for silanol concentrations were made which compare well with experimentally determined coverages. The computer simulation technique used here adequately reproduces many of the structural and dynamic characteristics of silica glass surfaces.

A model of CRN in two dimensions is presented, composed of equilateral but not equiangular polygons with a constant coordination number = 3; in a simplified version only pentagons, hexagons and heptagons are present in the network. We introduce the mean potential energy per atom averaged over a typical cell containing three adjacent polygons; we assume that although the thermal equilibrium is not attained for single atoms, it is attained on the level of these cells, so that we can apply the virial theorem for the cells. Then we minimize the free energy which contains the configuration entropy contribution. In terms of two variables P and δ (hexagon frequency and mean bond angle deviation) we get the surfaces of constant energy. Under stress the energy configurations cease to be one-connected, and the 0-th homotopy group is no more trivial. This can give rise to surface singularities (cracks).

It is shown that the elastic properties of covalent networks are very sensitive to coordination. High coordination networks are relatively hard and the elastic constants are determined by the covalent forces. Low coordination networks are relatively soft and the elastic constants are determined by longer range forces.

The principal defects in the stoichiametric chalcogenide glass GeSe2 are identified as hcapolar bonds which break the chemical order of the network. Glass compositional variation of this broken symmetry indicates that the hcmopolar bonds are not randcmly distributed in the network but dress edges of characteristic clusters. The cluster morphology is based on the layered structure of the stoichiametric crystalline ccmpound. The size of molecular clusters in the glass is found to be 50–75 Å

Crystalline SiSe2 and a series of Six Se1−x clear yelloworange glasses (0.33 × 0.40) were synthesized from the elements and were studied by Raman spectroscopy. Spectra of crystals were analysed in terms of the crystal structure of SiSe2 whereas for the glasses a new cross-linked chain-cluster (CLCC) model is proposed and forms the basis of extensive spectral analysis. Extended chain only and random network models are inconsistent with the data. The CLCC model is characterized by well defined chains of edge-sharing (SiSe4) tetrahedra, as in the crystal, which are cross-linked via corner sharing (SiSe 4) tetrahedra to form very large highly viscous inorganic polymers. The structure has considerable medium range order and provides for a self-consistent mechanism for transition to non-stoichiometric glasses in the range SiSe 2 to Si2 Se3 The smooth transition is monitored through spectral changes i the Si-Se vibrational stretching region from 150 to 300 cm and in the regiof of the collective modes at lower frequencies from 0 to 150 cm The frequencies and band contours of the latter dramatically track the introduction of disorder as the compositions move away from the stoichiometric SiSe 2 composition. The CLCC model is quite different from the raft model for GeSe 2 in some ways just as the crystalline structures of SiSe 2 and GeSe 2 differ from each other. Photoactivity in GeSe glass is one consequence of these structural differences in the two vitreous materials.

Six (chalcogen)1−x glasses represent a class of amorphous, binary, semiconducting solids of increasing theoretical and technological importance. In our laboratory, attention has been directed to these systems since they can form the basis of a remarkable series of fast ion conducting glasses. These vitreous materials are being considered as solid electrolytes for advanced electrochemical cells of high specific energy. For example, when stoichiometric SiS 2 is appropriately combined with Na2S, a Na-ion conducting glass results that has a room-temperature Na-ion conductivity that is several orders of magnitude greater than that of its oxide counterpart [1].The structures and defects present in these alkali-modified, ternary glasses are not known [2]. In fact, until recently, there has been a paucity of attention directed to the “simple” binary glass systems Si/S, Si/Se, and Si/Te. Therefore, four years ago, we began a systematic study of the structure and dynamics of Si xS1 SixSe1−x, and SixTe1−x covalent glasses. Of necessity, an interdisciplinary approach was adopted.

Network structures and glass forming tendency of g-(Ge1−ySny)1−x (Se, or S)x are discussed in connection with the percolation transition near the Phillips critical composition xc = 0.8, and local or extended topological transitions due to strains induced by compositional changes or applied pressures.

The ideas and energy diagrams of the valence alternation pair model (also: negative-U model) are introduced and matched to the classical steady-state photoconductivity analysis in terms of two discrete recombination levels in the gap. Data from bulk a-As47Se53 serve as example. Negative effective correlation energies Ueff = -0.72 eV for a-As2Se3, −0.61 eV for a-As2Se4 and -0.58 eV for a-As47Se53 are obtained. The positively charged defects are found to be Se3+ with relaxation energies W+ ≃ 0.1 to 0.2 eV, and the negatively charged defects are As2− with relaxation energies W− ≃ 0.3 to 0.6 eV.

A. A. Lebedev developed a crystallite model for glasses in the 1920's which was supported by the 1930 publication of Randall, Rooksby, and Cooper But, the 1932 publication by Zachariasen entitled “The Atomic Arrangement in Glass” introduced the Warren and Zachariasen continuous random network (CRN) model. In that model, as applied to oxide glasses, oxygen polyhedra surrounding cations (e.g. SiO4 tetrahedra in silicate glasses) are linked together in a random manner to form extensive three-dimensional networks which do not exhibit long-range symmetry or periodicity. Zachariasen did assume that the forces between atoms in glasses and crystals are essentially the same, and noted the same polyhedra in both, but did not discuss any further possible correspondence between glasses and crystals.

Low- and wide-angle X-Ray scattering methods were applied to determine the size of structural inhomogeneities in the surface and internal layers of silicate glass.

The functions of radial and cylindrical distribution of electron density in oxide glass, determined by Fourier-Bessel transformation method, were applied.

The corrected structural parameters (mainly distances and packing coefficients) were obtained enabling preliminary determination of the most probable space model of glass. Structural inhomogeneities appearing due to defects and local distortions of the components of elementary space structure of glass, were determined.

The method was applied in investigation of SiO2 in form of quartz glass and gel as well as the system SiO2-H2 O (hydrated gel). Them, it was transformed to investigation of a complex oxide glass (SiO2 -Na2 O-CaO-MgO-R2 O3).

In result of the analysis we obtained the structural parameters of glass (a1, a2, a3, … , an; Zcl, Zc2, Zc3, …, Zcn) as well as the parameters describing degree of crystallization, concentration and size of inhomogeneities.

Several paramagnetic defects are created by exposure of amorphous SiO2 to sub-band-gap light. The spectra of the dominant centers can be isolated by using their annealing, microwave power, and excitation-photonenergy dependence. The results are discussed in terms of specific models for the intrinsic defects in a-SiO2.

Electron spin resonance measurements have been carried out on samples of Suprasil Wl (dry silica) subjected to ultraviolet laser radiation (λ = 248 nm, E = 5 eV/photon). Studies have been made for fixed irradiation temperature (room) variable accumulated ultraviolet dose and fixed accumulated dose (3000 J/cm2) at various irradiation temperatures in the range 110 K to 335 K. Three principal defect centers are observed. Non-bridging oxygen hole centers are created at all temperatures in the range studied with slightly higher efficiency at room temperature (ration 300 K/150 K ∼ 2.5). Comparison of the dose dependent growth curve of the 4.8 eV absorption and its isochronal annealing curve with those for the oxygen hole center clearly identify the origin of the absorption band with this defect. A threshold temperature ∼ 200 K is found for oxygen vacancy creation consistent with results on single crystalline quartz. Post irradiation annealing at 593 K eliminates the vacancy centers and the peroxy radical resonance appears. Its growth as a function of accumulated ultraviolet dose and irradiation temperature supports the hypothesis that peroxy radicals form by the trapping of diffusing, molecular oxygen at the oxygen vacancy center.

Laser irradiation (λ = 248 nm, E = 5 eV) of thermal, amorphous SiO2 is found to produce thermally reversible, linear compaction reaching a maximum of 16 % (accumulated, incident dose of ∼ 2000 J/cm2 followed by irreversible compaction and photoetching. Assuming the volume fluctuation to be ΔV/V = 3(Δ1/1)/(1+2σ) this corresponds to a maximum volume fluctuation of −36 %. An approximately linear refractive index versus ΔV/V relationship found in the linear compaction regime agrees with the predictions of the Lorentz-Lorenz law. Laser compaction results are found to be consistent with those obtained using hydrostatic pressure. That is argued by comparison with crystalline SiO2 polymporph, that compaction occurs via collapse of a high order ring structure into one having predominately 2 or 3 membered rings. Irreversible compaction/etching occurs when the lattice can no longer support a reduction in ring dimension and dangling bond defects are created. The presence of a large number of defects is sufficient for mechanism of superficial desorption and photoetching to become important.

Gamma- and UV-induced defect centers in germanium and fluorine doped silica have been studied by electron paramagnetic resonance (EPR) spectroscopy. The complex spectrum at g≃2 in γ-irradiated germanium doped glass corresponds to a superposition of resonances from several germanium E′-centers. In UV-irradiated samples, however, the EPR spectrum is dominated by only one type of germanium E′-centers. Significant spectral simplification of γ-irradiated germanium doped silica can be achieved by heating or broadband photoirradiation. Similar results are observed in multimode germanium doped core optical fibers. UV-induced optical loss spectra in the 0.5–1.5 μm wavelength range were also measured in these core fibers as well as the growth kinetics of the UL-induced absorption. Gamma-irradiation of fluorine doped silica generated two different types of silicon E′-centers, Ea1. At lower radiation dose one sees a mixture of Ea1 and Ea2, but at higher radiation dose Ea2 dominates. A spectrum dominated by the Ea2 variant is also observed in LW-irradiated samples and in photobleached low gamma dose samples.

The effects of radiation damage on bulk laser-induced breakdown in Si02 were studied. Samples studied included Spectrasil A, B, and WF (water free). Measurements of laser-induced breakdown were conducted with 532 and 1064 nm laser pulses of approximately 30 nsec duration. Reductions of up to 50% in the laser-ind ced breakdown threshold were observed at 530 nm for samples exposed to 108 rads of γ-radiation.

Point defects in amorphous silicon dioxide (a-SiO2) have been studied for over 30 years by numerous spectroscopic techniques, including optical absorption and emission and electron spin resonance (ESR). While all of these experimental approaches have yielded valuable insights, only ESR has afforded detailed atomic-scale structural characterizations. ESR spectra of two fundamental classes of defects in a-SiO have been knon since the 1956 work of Weeks. However, many crucial detaiis such as the 29Si and 17O hyperfine structures of these centers have been provided only within the past dozen years. From these results, it is known that the so-called E′ center comprises an unpaired electron in a dangling tetrahedral orbital of a single silicon atom at the site of an oxygen vacancy; at least three variants of the E′ center have been distinguished in a-SiO2. Two additional intrinsic defect types, the nonbridging oxygen hole center and the superoxide radical, have also been identified by ESR, these being relatively more prevalent in irradiated silicas of high and low hydroxyl contents, respectively. Recent theoretical developments have proven indispensible to the modeling of the E′ centers and have yielded valuable insights on the superoxide defects.

In the chalcogenide glasses the primary defects are thought to involve under- and over-coordinated chalcogen atoms which are diamagnetic under quasiequilibrium conditions because of the presence of a strong electron-lattice interaction. These defects can be optically, or in some cases thermally, excited into metastable paramagnetic configurations which are well described as chalcogen “dangling bonds.” Alloying the chalcogenide elements with group V or group IV atoms produces additional diamagnetic defects which in their excited paramagnetic configurations also appear to be well described as dangling bonds. Some of these defects may play a role in the photodarkening (PD) process which is a metastable shift of the absorption edge to lower energies after irradiation with light of energy greater than or equal to that of the band gap. The PD process requires the presence of a non-bonding plike valence band. The microscopic description of the PD process may involve the creation of defects or the tunneling of chalcogen atoms.

Great progress has been made in optical device technology over the past decade. This progress brings a host of new services which include voice, data and visual communications. The realization of low loss optical fibers which give rise to long distance optical communication and the growth in information processing through computer technology will lead to an accelerating growth in the utilization of light guides and light wave communication systems. Heavy metal fluoride glasses have proved to be excellent hosts for both rare earth and 3d transition metal ions. In addition, their potential as light guides is at present unexcelled. This glass is especially promising for optical display devices, laser hosts, and electroluminescence panels. Numerous defects and impurities can be incorporated in the glass which absorb or emit light. Through optical studies of rare earth ions such as Er3+, Ho3+, Nd3+ and Pr3+ it is possible to investigate the multiphonon emission rate for optical transitions in the heavy metal fluoride materials. It is found that these particular materials have a much lower multiphonon rate than oxide glasses. This makes them attractive for room temperature devices. When 3d transition metal ions are incorporated into heavy metal fluoride glasses, the optical properties are similar in many cases to those in crystals. Inhomogeneous broadening of the absorption and emission bands occurs in the glass, but the lifetimes and oscillator strengths of the transitions in glass and crystals are of the same magnitude. Radiation damage of glasses can be detrimental to long range optical communication. It is found that the heavy metal fluoride glasses damage by the photochemical mechanism which is also dominant in highly ionic materials. Optical absorption and electron spin resonance measurements have been utilized to identify the types of radiation induced defects in these materials.