Among the common vacancy-related point defects in silicon, the E center is one of the most prominent due to its degrading effect in silicon-based technology. Even though it has been the subject of extensive experimental and theoretical studies, a comprehensive theoretical model capable of reproducing the experimental evidence for all three dopants (P, As, and Sb) is still missing. Guided by a Jahn-Teller model, we are able to reproduce the absorption bands and the transition probability between equivalent geometries of the defect at low temperatures by including many-body-perturbation corrections based on the GW approximation on top of the density functional theory. At higher temperatures, vacancies become mobile centers, enabling the reorientation of the whole defect and contributing to the dopant diffusion. The underlying mechanisms of the vacancy-mediated dopant diffusion are revisited, characterizing the activation energies of such technologically relevant processes and obtaining quantitative results in good agreement with experiment.

Synthetic vitreous silica is currently the preferred material for the production of optical fibres because of the several excellent properties of this glass, e.g. high transmission in the visible and IR domains, high mechanical strength, chemical durability, and ease of doping with various materials. For instance, fiber lasers and amplifiers exploit the light amplification properties provided by rare-earth ions employed as dopants in the core of silica-based optical fibers. The structure and composition of the nearest neighbor shell surrounding rare-earth ions in silica-based optical fibers and amplifiers have been intensively debated in the last decade. To reduce aggregation effects between rare-earth ions, co-dopants such as phosphorus and aluminium are added as structural modifiers; phosphorus-doping, in particular, has proved to be very efficient in dissolving rare-earth ions. In this work, we provide further insights concerning the embedding of P atoms into the silica network, which may be relevant for explaining the ease of formation of a phosphorus pentoxide nearest-neighbor shell around a rare-earth dopant. In particular, by means of first-principles calculations, we discuss alternative models for an irradiation (UV, x–, γ-rays) induced paramagnetic center, i.e. the so called room-temperature phosphorus-oxygen-hole center, and its precursors. We report that the most likely precursor of a room-temperature phosphorus-oxygen-hole center comprises of a micro-cluster of a few (at least two) neighboring phosphate tetrahedra, and correspondingly that the occurrence of isolated [(O-)₂P(=O)₂]⁻ units is unlikely even at low P-doping concentrations. In fact, this work predicts that the symmetric stretching of P=O bonds in isolated [(O-)₂P(=O)₂]⁻ units appears as a Raman band at a frequency of ~1110 cm⁻¹, and only by including at least another corner-sharing phosphate tetrahedron, it is shown to shift to higher frequencies (up to ~40 cm⁻¹) due to the shortening of P=O bonds, thereby leading to an improved agreement with the observed Raman band located at ~1145 cm⁻¹.