Electron trapping phenomena at TiN/α-Al2O3 interfaces

The accumulation of charges on TiN coated surfaces of alumina has been considered to be a precursory event of flashovers of the material under high power rf operation. In order to study this effect, we have investigated the surface potential Vs and the luminescence of sapphire windows under electron irradiation within the 1-5 keV energy range. Beyond a critical beam energy, enough for the injection of electrons into the substrate through the thin TiN overlayer, the surface potential of TiN-coated samples becomes negative and a simultaneous decrease of the specimen current is observed. This behaviour results from the trapping of electrons at the TiN/Al₂O₃ interface, stimulated by the attractive effect of the electric images of opposite sign in the TiN layer. On the contrary, Vs remains close to O for a clean substrate. The change in the cathodoluminescence (CL) of sapphire crystals mainly arises from charge transfer between preexisting defects. The CL spectra of the sample after high power rf operation or after high electron irradiation dose are closely related, with a typical behaviour of the 330 nn and 415 nm emissions respectively associated to F⁺ and F centers derived from oxygen vacancies