David L. Griscom

Electron spin resonance in glasses

Abstract The present paper reviews a quarter of a century of electron spin resonance (ESR) studies in glasses. Specific subtopics addressed include transition-group ions, radiation-induced defect centers, ferromagnetic precipitates, and photo-induced centers. For brevity, most of the examples in the first three categories are confirmed to the familiar oxide glass systems and only the most concrete conclusions are elaborated. The section treating radiation-induced defects is further abridged in light of several reviews of this topic in the current literature. Even within these limitations of scope, a great diversity of material is summarized. Most of the studies cited provide information concerning the atomic scale structure of the host glasses and several have immediate impact on practical problems ranging from glass technology to geochemistry.

Diffusion of radiolytic molecular hydrogen as a mechanism for the post‐irradiation buildup of interface states in SiO2‐on‐Si structures

A review is made of recent literature dealing with radiation‐induced point defects distributed volumetrically in thermally grown SiO2‐on‐Si or superficially at the silicon interface, with particular emphasis on the results of electron‐spin‐resonance experimentation. The observed defect types and their anneal kinetics are then compared with recent advances in the understanding of similar species and processes in irradiated bulk fused silica. It is concluded that radiolytic molecular hydrogen is formed in thermally grown SiO2 layers, just as it is in bulk fused silica, and that the diffusion of this hydrogen determines the temperature and time dependencies of the post‐irradiation interface state buildups.

Oxygen-associated trapped-hole centers in high-purity fused silicas

Abstract Two distinct oxygen-associated trapped-hole centers (OHCs) are identified in samples of room-temperature γ-irradiated, high-purity fused silica. One, which we label the “wet” OHC, predominates in the high-OH-content (wet) silicas while the other, the “dry” OHC, is more prevalent in low-OH (dry) silicas. Excellent computer simulations of the low-temperature electron-spin-resonance spectra are obtained for both wet and dry silicas using only the relative abundance of the “wet” and “dry” OHCs as an adjustable parameter. Analysis of the 17 O-hyperfine structure which occrs in samples of wet silica enriched in 17 O provides direct confirmation that the “wet” OHC is a hole trapped in a single nonbonding 2p-orbital of an oxygen (presumed nonbridging). Correlation of optical absorption and electron spin resonance via isochronal pulse anneals indicates that the “dry” OHC has an optical transition ay 7.6 eV. In addition, it is reported that the “dry” OHC can be induced in the dry silicas by the fiber drawing process. From the present results, an O 2 − molecular ion model appears most attractive for the “dry” OHC.

Radiation effects in glasses used for immobilization of high-level waste and plutonium disposition

This paper is a comprehensive review of the state-of-knowledge in the field of radiation effects in glasses that are to be used for the immobilization of high-level nuclear waste and plutonium disposition. The current status and issues in the area of radiation damage processes, defect generation, microstructure development, theoretical methods and experimental methods are reviewed. Questions of fundamental and technological interest that offer opportunities for research are identified.

Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus‐doped silica glass and optical fibers

Defect centers induced by ionizing radiation (50–100‐keV x rays, 60Co γ rays) in high purity P‐doped silica glass have been observed and elucidated by ESR spectroscopy. Four generic species are well characterized on the basis of the observed 31P hyperfine splittings and g values as defects analogous to PO2−3 (phosphoryl), PO4−4 (phosphoranyl), PO2−2 (phosphinyl), and PO2−4 radicals. The latter species, also termed the phosphorus‐oxygen‐hole center (POHC), is shown to occur in two variants comprising holes trapped on one or two nonbridging oxygens. Radiation‐induced Si E′ centers with and without P next‐nearest‐neighbors were also identified, and a singlet resonance S due to E′ type defects such as (OSi2)Si⋅ and/or (O2Si)Si⋅ was observed to grow in with annealing above ∼800 K, regardless of whether or not the sample was irradiated. The structures, formation mechanisms, and precursors of these defects have been determined or inferred for all centers. Radiation‐induced optical absorption spectra over the ran...

Defect centers in a germanium‐doped silica‐core optical fiber

The radiation‐induced defect centers in a low‐loss Corning germanium‐doped optical fiber have been studied. In addition to silicon E′ centers, four germanium‐related centers, corresponding to electrons trapped at the site of oxygen vacancies in s p3 orbitals of germanium ions with zero to three next‐nearest‐neighbor germaniums, were observed. A model which assumes Gaussian distributions in the excited‐state energy‐level splittings has been successfully used to computer simulate the ESR spectrum of the irradiated fiber.

Post‐irradiation cracking of H2 and formation of interface states in irradiated metal‐oxide‐semiconductor field‐effect transistors

Molecular hydrogen is alternately introduced into and removed from the gate oxide of irradiated metal‐oxide‐semiconductor field‐effect transistors at room temperature by changing the ambient between forming gas (10/90% H2/N2) and nitrogen. Using charge pumping, it is observed that H2 causes a simultaneous buildup of interface states and decrease of trapped positive charge. The results are explained by a reaction sequence in which H2 is cracked to form mobile H+, which under positive bias drifts to the Si/SiO2 interface, and reacts to produce a dangling‐bond defect. The rate limiting step over most of the time domain studied is the cracking process. Two types of cracking sites are modeled by molecular orbital calculations: oxygen vacancies (E’ centers) and broken bond hole traps (BBHTs). Initial‐ and final‐state energies, as well as the activation energies, are calculated. The calculations indicate that the latter is the more likely H2 cracking site. The combined experimental and theoretical results sugges...

The electronic structure of SiO2: A review of recent spectroscopic and theoretical advances

Abstract Experimental and theoretical progress in understanding the electronic structure of SiO 2 is critically reviewed. Relatively basic introductions to both the experimental and theoretical methods are provided.

Defects in amorphous insulators

Abstract Electron spin resonance characterizations are presented for a number of defects in a selection of amorphous insulators, including oxide glasses, oxyhalide glasses, halide glasses, nitrate glasses, oxynitride glasses, and a sulfide glass.

Electron trapping in amorphous SiO2 studied by charge buildup under electron bombardment

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.