First principles vibrational spectra of tetrahedrally-bonded glasses: SiO2, GeO2 and GeSe2

Disordered oxides, such as vitreous silica (v-SiO2) and vitreous germania, (v-GeO2) and chalcogenide glasses, such as vitreous germanium diselenide (v-GeSe2) are currently key materials in many technological fields, ranging from Si-based microelectronic to optical fibers and information storage devices production. Thus an increasing interest is nowadays devoted to understanding the structural arrangements of atoms in these materials. Diffraction probes like x-ray and neutron diffraction applied to amorphous materials can only partially characterize their atomic structure, since these materials lack translational symmetry. A complementary class of experimental techniques is constituted by vibrational spectroscopies, since the latter are also sensitive to the underlying structure. Yet, extracting structural information from the associated experimental data is rather difficult and above all it requires an accurate theoretical modeling of the material under investigation. This thesis is dedicated to the first-principles investigation of vibrational spectra of the tetrahedrally bonded glasses v-SiO2, v-GeO2 and v-GeSe2. To this aim, we make use of methods based on density functional theory which allow us to calculate vibrational spectra for a set of selected model structures for each one of these materials. We take advantage of a recent development which consists in applying a finite electric field under the constraint of periodic boundary conditions to calculate coupling factors, and hence to derive the infrared and Raman spectra. Through a comparison with experimental data of several vibrational spectroscopies, and by taking into account the structural differences of our models, we interpret features of the considered vibrational spectra. The extracted structural information represents a refinement of our knowledge of the investigated materials.

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