Disordered SiO2 is commonly used for optical instrumentation and coatings. In space telescope applications, these materials can be exposed to low temperature (particularly for IR telescopes) and simultaneous electron fluxes from the space plasma environment. During recent charging tests of this dielectric material, a discernable glow was detected emanating from the surface of the SiO2, indicating that the incident electron beam induced a luminescent effect, termed cathodoluminescence. As the sample cooled from 300 K to 120 K, a change in the intensity and energy spectrum of the glow was observed between 250 nm and 1700 nm, demonstrating that the SiO2 cathodoluminescence is temperature dependent. Cathodoluminescence occurs when a high energy electron excites a valence band electron into the conduction band, then a transition takes place between the extended conduction states and the localized states below the mobility edge resulting from structural defects. This final electron transition is the origin of the emitted photon, hence the luminescence. As sample temperature and the thermal energy of the electrons vary, the trap state population, distribution of accessible trap states, and transitions between states also vary. A dynamic model of electrons in these localized trap states is proposed to explain the temperature dependent experimental cathodoluminescence spectra collected. Using our experimental results in conjunction with literature references, the specific structural defects in SiO2 responsible for distinct features in the cathodoluminescence spectra can be identified. From our experimental results, a simple qualitative model of disordered band theory has been developed to describe the states and electron dynamics in our SiO2 samples. Ultimately, such knowledge is important in the optimal design of space telescope optics.
Available at: http://works.bepress.com/gregory_wilson/3/