Defects induced by He+ irradiation in γ-Si3N4

Abstract

Formation and evolution of defect levels in the electronic structure of silicon nitride with cubic spinel structure, γ-Si3N4, after the irradiation with He+ ions was investigated using spectroscopic techniques. Strong changes of cathodoluminescence (CL), photoluminescence (PL), photoluminescence excitation (PLE) and Raman spectra were detected. In particular, excitonic PL was significantly inhibited and a new near-IR band appeared with the band gap excitation hν≥Eg = 5.05 eV. This was explained by an effective trapping of photoinduced electrons and holes by charged defects. The spectral shift of PL with the excitation photon energy indicated heterogeneous nature of the defect sites. The energetic position of near-IR and visible PL bands correlate, suggesting an interaction with the common cation defect to be an origin. The visible PL of exciton bound to a neutral defect Si× was red shifted, which was attributed to the permutations between empty and occupied octahedral and tetrahedral sites, inherent to the spinel structure, after collisions with He+ ions. The positively charged cation sites in the spinel structure are compensated by VN‴ anion vacancies. The local deformation of the spinel lattice affects PL intensity of the self-trapped exciton at 4.35 eV.