Density of states and group velocity of electrons in SiO2 calculated from a full band structure

The full band structure of SiO2 has been determined in order to calculate parameters that are necessary for the description of carrier transport. Ab initio calculations of the density of states and group velocity for the conduction bands of SiO2 are worked out as a function of energy. Four different crystal structures of SiO2 are investigated, which are known to be built up by the same fundamental unit, namely, the SiO4 tetrahedron: they are the α- and β-quartz and the α- and β-cristobalite. All of them are polymorphs of silica. The conduction bands are calculated by means of two different techniques: the Hartree-Fock method and density-functional theory. The different features of the two methods are examined. Eight energy bands are used to calculate the density of states and group velocity for the energies of interest. Based on such calculations, the relevant scattering mechanisms have been modeled to determine the microscopic relaxation times. This in turn allows for the solution of the Boltzmann transport equation in the coordinate and energy space, which eventually leads to the calculation of macroscopic quantities such as carrier concentration, average velocity, and average energy. Examples are given of calculated electron mobility and average energy, along with comparisons with experimental results available in the literature.