Photodarkening and bleaching in amorphous silicon nitride

Abstract

UV-induced enhancement of the subgap optical absorption has been observed in thin films of chemically vapor deposited silicon nitride. The threshold for this process is ∼4 eV. Visible light bleaches out much of the enhanced absorption and 200-400°C thermal anneals remove the rest, leaving the optical properties of the nitride in the "as-grown" state. The characteristics of this creation and bleaching process suggest that UV light "optically dethermalizes" the distribution of carriers in localized band tail states. Correlation of our data with recent electron spin resonance results suggests that many of the trapped carriers may be singly occupied silicon dangling bonds.