A spin-dependent recombination study of radiation-induced P_b1 centers at the (001) Si/SiO₂ interface

The generation of interface defects at the Si/SiO₂ boundary is an important aspect of radiation damage in MOS devices. The observations of several independent groups over the last twenty years have demonstrated that Si/SiO₂ interface silicon "dangling bond" defects called P_b centers dominate the interface trap generation process. In the technologically important (001) Si/SiO₂ system, two P_b variants may appear. One variant, the P_b0 center, generally dominates radiation and hot carrier damage; however, a less well studied variant, called P_b1, may play a significant secondary role. The electronic properties of the P_b1 centers are quite controversial. Early work by Gerardi et al. indicated quite strongly that the P_b1 centers, like the other P_b family members, have two levels in the Si bandgap. Several recent publications by Stesmans and Afanas'ev argue that P_b1 centers do not have any levels in the Si bandgap. We present spin-dependent recombination (SDR) measurements which demonstrate that P_b1 centers have levels around the middle of the Si bandgap. Our SDR results strongly suggest that the P_b1 center electron correlation energy is significantly smaller than that of P_b0. Our results may help explain some apparently contradictory studies involving the energy distribution of radiation induced interface states. We also discuss possible relevance of these results to the precision of oxide charge measurements based on the shifts in capacitance versus voltage curves for the Fermi energy at midgap.