X-ray induced luminescence of high-purity, amorphous silicon dioxide

Abstract

A comprehensive study of x-ray stimulated luminescence has been carried out on four types of high-purity, amorphous silica (a-SiO2). Both high OH and low OH as well as oxygen-excess and oxygen-deficient materials were studied. The room-temperature, visible x-radio luminescence (XRL) was measured continuously as a function of x-ray dose from zero to 400 Mrad volume average dose. In addition to the XRL measurements, electron paramagnetic resonance (EPR) was used to determine the concentrations of the two key radiation-induced defects, the E′ center and the nonbridging oxygen hole center (NBOHC). The XRL spectra were deconvolved into four Gaussian components with centers at 1.9, 2.2, 2.6, and 2.75 eV. The same centers and widths could be used to describe the spectra in all four types of a-SiO2, only the intensities varied. The 2.6 and 2.75 eV lines are strongly dose dependent, rising from near zero intensity at zero dose in all four materials. These two lines are strongly correlated with each other; they have essentially the same dependence on dose and sample type. This correlation suggests that these two lines are due to the same radiation-induced defect, or to closely related defects. The dose dependence and sample-to-sample variation of these two lines bear some similarities to the E′ concentrations. In contrast to the 2.6 and 2.75 eV lines, the 1.9 eV line has a high intensity at the lowest doses measurable. A simple phenomenological model is proposed to describe the 1.9 eV XRL line. This model involves two populations of defects; one population is present at zero dose and is assumed to be dose independent, while the second population is dose dependent. Evidence is presented that the dose-dependent defect is the NBOHC. The XRL due to the dose-independent population may be associated with a transient response to the x rays, or to a metastable defect; this population may not be observable in post-irradiation experiments such as EPR and conventional photoluminescence. Similar to the 1.9 eV line, the 2.2 eV line also has relatively high intensity at the lowest measurable x-ray dose. The behavior of this line is in general agreement with the self-trapped exciton model.