Cathodoluminescence and cathodoelectroluminescence of amorphous SiO₂ films

Cathodoluminescence of amorphous SiO₂ films thermally grown on a Silicon substrate has been observed in a scanning electron microscope using wavelength dispersed registration by a charge coupled device (CCD) camera. Spectra have three bands: at 650 nm (red), 460 nm (blue), and 285 nm (UV) whose intensities change during the initial period of electron beam excitation. Luminescence peak dose dependence has been investigated in a wide range of current density (10^-5 to 10^-3 A cm^-2) and temperature (90 to 500 K). An interpretation of the dose-temperature dependence is made by a model of precursor transformation via a metastable interlevel. Application of an electric field during continuous electron excitation (cathodoelectroluminescence) causes an enhancement up to five times of the blue band intensity. On the other hand, the red band decreases in the electric field. Based on these phenomena, the UV and the blue luminescence band are attributed to an internal electron impact excitation within a localized center, probably twofold-coordinated silicon in the SiO₂ network, whereas the red band is ascribed to band-to-band-recombination via localized levels attributed to non-bridging oxygen.