Deep Levels and Electron Paramagnetic Resonance Parameters of Substitutional Nitrogen in Silicon from First Principles.

Nitrogen is commonly implanted in silicon to suppress the diffusion of self-interstitials and the formation of voids through the creation of nitrogen-vacancy complexes and nitrogen-nitrogen pairs. Yet, identifying a specific N-related defect via spectroscopic means has proven to be non-trivial. Activation energies obtained from deep-level transient spectroscopy are often assigned to a subset of possible defects that include non-equivalent atomic structures, such as the substitutional nitrogen and the nitrogen-vacancy complex.

v-PO micro-clustering in P-doped silica studied by a first-principles Raman investigation.

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.

Photoactivated processes in optical fibers: generation and conversion mechanisms of twofold coordinated Si and Ge atoms.

In this work we present an extensive investigation of nanoscale physical phenomena related to oxygen-deficient centers (ODCs) in silica and Ge-doped silica by means of first-principles calculations, including nudged-elastic band, electron paramagnetic resonance parameters calculations, and many-body perturbation theory (GW and Bethe-Salpeter equation) techniques. We show that by neutralizing positively charged oxygen monovacancies we can obtain model structures of twofold Si and Ge defects of which the calculated absorption spectra and singlet-to-triplet transitions are in excellent agreement with the experimental optical absorption and photo-luminescence data. In particular we provide an exhaustive analysis of the main exciton peaks related to the presence of twofold defects including long-range correlation effects.

Vibrational properties of vitreous GeSe2 with the Becke-Lee-Yang-Parr density functional.

The vibrational properties of vitreous GeSe(2) are studied within the Becke-Lee-Yang-Parr (BLYP) generalized gradient approximation to density functional theory. For this purpose, we consider two models. The first one is derived from a chemically ordered network, previously generated through classical molecular dynamics.

Vibrational spectra of vitreous SiO and vitreous GeO from first principles.

Using a density-functional approach, we calculate the principal vibrational spectra of vitreous SiO and vitreous GeO and discuss their analogies and differences. For both glasses, we generate model structures consisting of a random network of corner-sharing tetrahedra and differing only by their packing density. The comparison between calculated and measured neutron structure factors supports the validity of our model structures.

Medium-range structural properties of vitreous germania obtained through first-principles analysis of vibrational spectra.

We analyze the principal vibrational spectra of vitreous GeO(2) and derive therefrom structural properties referring to length scales beyond the basic tetrahedral unit. We generate a model structure that yields a neutron structure factor in accord with experiment. The inelastic-neutron, the infrared, and the Raman spectra, calculated within a density-functional approach, also agree with respective experimental spectra.