Cathodoluminescence induced in oxides by high-energy electrons: Effects of beam flux, electron energy, and temperature

The cathodoluminescence (CL) induced in four oxide single crystals (alpha-Al2O3, ZrO2: Y or YSZ, MgAl2O4, and TiO2) by high-energy electrons from 400 keV to 1250 keV was studied as a function of beam parameters (flux and energy). The main CL bands are related to F center (oxygen vacancy) formation by elastic collisions above the threshold displacement energy of oxygen atoms. The beam-intensity dependence is interpreted on the basis of a kinetic-rate model involving F-center formation and annihilation. The temperature effect was also followed from 110 K to 300 K. A broad maximum is found for all bands at about 200 K for sapphire, whereas a monotonous increase with temperature is observed for YSZ. The plots of CL intensity versus temperature are mainly interpreted by the interplay between the thermal dependence of thermalized free-carrier trapping rates and luminescence efficiency. Finally, the dependence of CL intensity on the primary electron energy for F centers in YSZ showing a maximum at about 600 keV is explained on the basis of the interplay between point-defect formation and secondary-electron energy spectra production.