Defects Density of States Model of Cathodoluminescent Intensity and Spectra of Disordered SiO2

Electron beam measurements show that disordered SiO₂ exhibits electron-induced luminescence and that it varies with incident beam energy and current density, sample temperature, and wavelength. A simple model based on the electronic band structure and defect density of states—initially used to explain electron transport in highly disordered insulating materials—has been extended to predict the relative cathodoluminescent intensity and spectral radiance for disordered SiO₂ as a function of these variables. Due to the large bandgap of insulating SiO₂, thermal excitation from the valence to conduction band is highly improbable; excitation is through collisions of the incident high-energy electrons. For visible and near-infrared light to be emitted, there must be other states within the forbidden bandgap for electrons to occupy. These localized defect or trap states of disordered SiO₂ are due to structural or substitutional chemical defects. Results for both thin-film samples with penetrating radiation and thick bulk samples with nonpenetrating radiation are presented. The data were fit with the proposed model using saturation dose rate and mean shallow trap energy as fitting parameters, which favorably compare with results from independent experiments.