Different behaviors of F+ centers due to electron beam irradiations between synthetic sapphire and Be-diffusion-treated natural sapphire

Cathodoluminescence (CL) intensities of F+ centers in Be-diffusion-post-treated natural sapphire (BNS) and synthetic sapphire have been found to substantially increase under continuous irradiations of 15-keV electrons with constant incident currents ranging from 0.1 to 2 μA and to be represented approximately by an exponential saturation function of the electron fluence. The efficiency of F+-center CL emissions at room temperature (RT) was higher in the synthetic sapphire than in the BNS specimen. Whereas the CL intensity of the BNS sample had little temperature dependence, that of the synthetic sapphire decreased by approximately one order of magnitude with decreasing the specimen temperatures from RT to 85 K. In addition, time-resolved photoluminescence data measured for the 420-nm emission evidence a relatively shorter lifetime of the BNS specimen. These facts indicate that the natural sapphire which was successfully post-treated with Be diffusion had a slightly but substantially poorer crystalline quality than synthetic sapphire. Possible origins of these observed differences are discussed.